The active trace of Boconó Fault, across Llano Corredor and Mucubají Passes (Highlands of the Cordillera de Mérida, Venezuela): Tectonic landforms and implications
La traza activa de la falla de Boconó, a través de los pasos de Llano Corredor y Mucubají (Altiplano de la Cordillera de Mérida, Venezuela): Accidentes tectónicos e implicaciones
Franck A. Audemard M.1, *
1 Venezuelan Foundation for Seismological Research (FUNVISIS). Prolongación final, Calle Mara, El Llanito, 1070, Caracas, Miranda, Venezuela.
* Corresponding author: (F.A. Audermard) This email address is being protected from spambots. You need JavaScript enabled to view it.
How to cite this article:
Audemard M., F.A., 2025, The active trace of Boconó Fault, across Llano Corredor and Mucubají Passes (Highlands of the Cordillera de Mérida, Venezuela): Tectonic landforms and implications: Boletín de la Sociedad Geológica Mexicana, 77(3), A170625. http://dx.doi.org/10.18268/BSGM2025v77n3a170625
Manuscript received: February 2, 2025. Corrected manuscript received: June 11, 2025. Manuscript accepted: June 16, 2025
ABSTRACT
Two Boconó fault (BF) segments, at the crossing of the Mucubají and Llano Corredor passes (Cordillera de Mérida), have been characterized: the southern portion of Boc-C and its relay with Boc-B. Tectonic landforms were mapped following geomorphologic criteria, specifically for strike-slip faults. This geomorphologic-geologic approach provides reliable and precise location of the (potentially) active tectonic structures in a given region, but also characterizes its tectonic style parameters: type of faulting, sense of slip, slip rate, lateral extent, simplicity/complexity of the fault trace(s), damage zone and fault-zone width. Segment Boc-C in the study area has been subdivided into 4 distinct sections: 2 of them (1 and 4) cut across or bound Last Glacial Maximum (LGM), which are glacial landscapes typical of Páramo conditions, while the others run along the southern margin of large valleys excavated by the Santo Domingo and Aracay rivers, with valley bottom profiles exhibiting 7 to 12% gradients. Each of these sections shows its particularity: section 1 mainly cuts across incompetent LGM moraine complexes, developing an “en echelon” fault pattern with transpressional (pressure ridges) and transtensional jogs or relays (pull-apart basins; PAB), while the BF at other sections is preserved in competent basement rocks. Section 4 juxtaposes LGM deposits resting onto the BF scarp, which are displaced some 200 m dextrally and 40 normally. While the slip rate of Boc-C inside the Apartaderos PAB is still ranging between 7 and 8 mm/a, it jumps to 11.9 mm/a outside the PAB, very comparable to the geodetic rate. On the other hand, the slip rate of Boc-B, at the Apartaderos PAB, remains in the order of 2.3-3.0 mm/a.
Keywords: tectonics, geomorphologic criteria, pull-apart basin, slip rate, strike-slip faulting, Páramo.
RESUMEN
Dos segmentos de la falla de Boconó (FB), entre los páramos de Mucubají y Llano Corredor (Cordillera de Mérida), han sido caracterizados: la porción sur del segmento Boc-C y su relevo con Boc-B. Las geoformas tectónicas han sido cartografiadas usando criterios geomorfológicos, especialmente aquellas de contexto transcurrente. Este enfoque geomorfológico-geológico brinda la ubicación confiable y precisa de los rasgos tectónicos (potencialmente) activos en una región dada, pero también determina parámetros de su estilo tectónico: tipo de fallamiento, sentido de desplazamiento (cinemática), tasa de desplazamiento, extensión lateral, simplicidad/complejidad de la(s) traza(s) de falla, zona dañada y ancho de la zona de falla. El segmento Boc-C en el área de estudio ha sido subdividido en 4 secciones distintivas: 2 de ellas (1 y 4) cortan o limitan paisajes glaciares de Páramo del Último Máximo Glaciar (LGM por sus siglas en inglés), mientras los otros surcan grandes valles en su margen sur, excavados por los ríos Santo Domingo y Aracay, que presentan fondos de valle con gradientes entre 7 y 12 %. Cada una de estas secciones presenta su particularidad: la 1 principalmente corta a través de complejos morrénicos del LGM incompetentes, desarrollando un patrón de fallas dispuestas “en echelon”, con curvaturas o relevos transpresivos (lomos de presión) y transtensivos (cuencas en tracción; CET), mientras la FB en otras secciones está preservada en rocas ígneo-metamórficas competentes del basamento de la cadena. La sección 4 yuxtapone depósitos LGM contra el escarpe de la FB, los cuales están desplazados unos 200 m dextralmente y 40 normalmente. Mientras la tasa de desplazamiento de Boc-C dentro de la CET de Apartaderos aún continúa entre 7 y 8 mm/a, ésta salta a 11.9 mm/a fuera de la CET; muy comparable a la tasa geodésica. Por su parte, la tasa de Boc-B, en la CET de Apartaderos, es de 2.3-3.0 mm/a.
Palabras clave: tectónica, criterios geomorfológicos, cuenca en tracción, tasa de desplazamiento, fallamiento transcurrente, Páramo.
1. Introduction
Mapping coseismic surface breaks of individual earthquakes is a fundamental source for identification and characterization of onshore active faults worldwide. This information could be substantially enhanced if recorded seismologic data of the earthquake itself is also available, allowing thus the definition of reliable seismotectonic associations. In this way, only the outcropping of recently ruptured faults are identifiable and their seismogenic potential assessable. In terms of seismic hazard assessment of a given region, this is not enough because many faults, although also being seismogenically active, reappear infrequently. The return period between its large or destructive earthquakes is much longer than the time covered by the observational (historical + instrumental) window of humankind in most regions of the world.
In other words, these active faults rupture the ground surface every few thousands or tens of thousands years. However, during the thorough field mapping of earthquake ruptures, mainly from the beginning of the second half of the XX century, particular landforms along the faults after individual earthquakes have been recognized. In this respect, the studies carried out along the San Andreas Fault in California at that time (Vedder and Wallace, 1970; Wesson et al., 1975; Slemmons, 1977, among many others) deserve a special mention for strike-slip settings. More recently, similar efforts have been spent in the recognition of blind (near-to-surface) thrust faults (e.g., Schumm, 1986; Audemard, 1999, Guccione et al., 2002; Audemard, 2003b; Ollarves et al., 2006; Audemard et al., 2016). Since faults also tend to rupture repeatedly along finite portions of its entire length (concept of fault segmentation), it has been recognized that individual landforms can grow stepwise through time by the accumulation of individual slips during each earthquake.
In more recent times, it has been evoked that this process can occur over a longer time span (e.g., in geologic time), producing what has been named as “seismic landscape” (Serva and Slemmons, 1995; Serva et al., 1997; Michetti and Hancock, 1997; Michetti et al., 2005). Consequently, these sets of tectonic landforms have become a powerful geomorphological tool for identifying the surface traces of active (or potentially active) faults. In addition, their seismogenic potential can be characterized in terms of the recurrence of its maximum credible earthquakes (e.g., Audemard, 2003a, 2005; Perucca and Audemard, 2021).
In this contribution, we shall present the surface mapping of a typically dextral fault in a highland climatic environment subject to glacial processes in the Late Pleistocene (Mérida Glaciation, as defined by Schubert, 1974) and currently under peri-glacial conditions. The Holocene climate has been responsible for a very slow morpho-dynamic evolution of the landscape. Little amount of rainfall does not induce significant runoff in this region above 3 000 m of altitude, which usually falls during frequent and long drizzles in the rainy season (between April and November). These current Páramo conditions, in combination with a rather fast dextral slip rate of the Boconó Fault of the order of 10 mm/a, leads to an excellent preservation of the fault—or earthquake—related landforms along this stretch of the Boconó Fault (BF). However, in this region, the fault also goes from Páramo conditions into more humid tropical conditions (2 200-3 000 m). This situation allows comparing the tectonic landforms formed under different morpho-climatic conditions along a single major active fault.
2. The Boconó Fault (BF)
The Boconó Fault (BF) is comparable to other large active strike-slip fault systems in the world, such as the Alpine Fault (New Zealand), the Gobi-Altai Fault (Mongolia), the Great Sumatran Fault (Indonesia) or the San Andreas Fault (southwestern USA). The NE-SW trending, dextral BF extends for close to 600 km in Venezuela, between the Colombo-Venezuelan border and the Caribbean coast of Venezuela. A stretch of about 450 km runs down the backbone of the Cordillera de Mérida (CdM), slightly oblique to the chain axis (Figure 1). The CdM exhibits a maximum width in the order of 100 km (Figure 1) and reaches 4 978 m at Bolívar peak, southeast of Mérida city in the central CdM. This relief is bounded on both flanks by the lowlands of the Maracaibo and Llanos basins on the northwest and southeast respectively. The present CdM build-up essentially results from Pliocene-Quaternary transpression due to oblique compression between two discrete continental blocks (Audemard and Audemard, 2002; Audemard, 2003b; 2014a). The present-day oblique compression is responsible for stress partitioning in the CdM, by which both the BF slips dextrally and the CdM shortens in NW-SE direction (Audemard and Audemard, 2002; Audemard, 2003b; Backé et al., 2006). This Plio-Quaternary compression superposes to a previous Miocene compressional phase, which have together inverted Jurassic (half-) grabens, exposing Precambrian and Paleozoic rocks of the South America continental crust at its highest peaks.
To the north, the BF bounds the Caribbean Coast range of northern Venezuela on the west, along the Yaracuy depression. Farther north, at the Caribbean coast, the BF exhibits a 45º clockwise bend that allows prolongation into the east-west striking San Sebastián-El Pilar fault system (Figure 1A). To the south, the Boconó fault connects with the North Andes Sliver Eastern Boundary (NASEB; which contains the Guaicáramo Fault), through the Bramón-Chucarima-Pamplona fault system, after undergoing two opposite right-angle bends (Figure 1A; Audemard et al., 2021); a structure described as the Pamplona indenter by Boinet (1985) and Boinet et al. (1985) or the Pamplona wedge in the sense of Velandia et al. (2020). In turn, this Colombian fault system appears to extend as far south as the Jambelí graben (Guayaquil gulf, offshore Ecuador), through the Algeciras, Pallatanga and Dolores faults (e.g., Audemard et al., 2021). This dextral mega-shear system has been proposed as responsible for splitting the northwestern corner of South America—named as the North Andean Block—from the rest of the continent (Case et al., 1971; Dewey, 1972; Pennington, 1981; Stephan, 1982; Audemard, 1993; Freymueller et al., 1993; Ego et al., 1996; Audemard, 1998; 2014a; Audemard et al., 2021). Due to the lack of well-studied sedimentary records for any of the several pull-apart basins associated with the BF, Audemard (1993; 1998) and Audemard and Audemard (2002) have estimated the age of activation of the BF from the age of opening and filling of the Jambelí graben, interpreted as a pull-apart basin (PAB) mainly filled by Plio-Quaternary marine deposits whose sedimentation started in the Late Miocene (Benítez, 1986); or the southern equivalent of the Peruvian Tumbes basin (Lemgruber-Traby et al., 2020; Peuzin et al., 2023). The Boconó Fault along its full length has been identified, mapped and characterized rather easily by the large number of along-strike tectonic landforms, among which: a continuous series of aligned 1-5 km wide valleys and linear depressions, passes, saddles, trenches, sag ponds, scarps and sharp ridges by many authors (Rod, 1956; Schubert, 1980a; 1980b; 1982; Giraldo, 1985; Soulas, 1985; Soulas et al., 1986; Soulas and Singer, 1987; Casas, 1991; Ferrer, 1991; Singer and Beltrán, 1996; Audemard et al., 1999; Audemard, 2009; Alvarado et al., 2015). Among those, the alignment of valleys associated to the fault is the most conspicuous feature as it makes the fault easily detectable from any remote sensing imagery, such as radar (SLAR; Figure 1B), satellite imagery (e.g., Google Earth®), or aerial photographs at various scales. This chain of aligned valleys results from a more prone material to removal by linear erosion of rivers, due to persistent and repeated BF fracturing and brecciation.
3. Study area
This study focuses on the surface expression of the Boconó fault (BF). Particularly, the fault portion under evaluation extends northeasterly from the Mucubají pass to the Llano Corredor pass, located both at Páramo heights (above 3 000 m high; Figure 2). Most of the 40-km-long fault extent corresponds to the southern portion of fault segment C (Boc-C; Figure 2), after the fault sectioning proposed by Audemard et al. (2000) and the latter seismogenic fault segmentation of Audemard (2014b), but also covers the relay with segment B (Boc-B; Figure 2).
This stepover is transtensional, in which sits the Apartaderos pull-apart basin (Figures 2 and 3). The Apartaderos PAB is 60 km NE of the city of Mérida in the central Cordillera de Mérida and corresponds to a releasing bend of the Boconó Fault (Soulas, 1985; Audemard et al., 1999). BF is well-preserved along a high-altitude (3 500 m) drainage divide that separates the southeasterly flowing streams (Orinoco basin) from the northwesterly flowing streams (Maracaibo basin). This divide is nestled in the area of Laguna Mucubají. The fault comprises two conspicuous sub-parallel strands located at about 1-1.5 km apart.
Schubert (1980b) interpreted the western portion of this area (near Mucuchíes, SW of El Cerrito) due to a large releasing stepover along the fault, whereas Soulas (1985) mapped the section of the fault between Mucuchíes and Los Zerpa as a releasing bend. In fact, this part of BF has a slightly more easterly strike regarding the overall NE-SW trend (Audemard et al., 1999), supporting Soulas (1985)’s interpretation.
Besides, FUNVISIS (1999) has postulated that this local transtension at the convergence of the Valera and Boconó faults, as well as the kinematics of the Tuñame Fault (dominant normal faulting), is partly induced by the clockwise rotation of the block north of BF and bounded by the north-south trending sinistral faults of Valera and Burbusay on the west and east respectively, whose kinematics in turn results from a bookshelf rotation mechanism induced by simple shear between the dextral faults of Oca-Ancón and Boconó. However, although the vertical component of slip appears to be significant at some localities in the PAB (e.g., from SW to NE: near El Cerrito, in the village of Apartaderos, Los Zerpas and near Las Tapias), most of the fault geomorphology is typical of a strike-slip fault, as shall be discussed and profusely illustrated in the next section. In addition, both fault strands offset latest Pleistocene and Holocene deposits, and fault scarps along the southern strand appear much fresher, more conspicuous and more recently formed than on the northern strand.
4. Boconó Fault between Mucubají and Llano Corredor passes
From the morphotectonic interpretation of aerial photographs at different scales, assisted by more modern tools, such as open-access Google Earth® imagery, the BF exhibits four distinctive surface fault behaviours along strike. From SW to NE: A) a SW-NE trending southern portion that cuts across a glacial landscape at the Mucubají pass, mostly coinciding with the relay between segments Boc-B and Bob-C (Apartaderos PAB). The herein delivered mapping extends farther to the SW up to El Cerrito (on Boc-B; Figure 3); B) a N 60° -trending section where the BF runs along the bottom of the Santo Domingo valley, at lower altitudes, between 2 800 and 1 900 m, extending between Las Tapias and the Santo Domingo dam; C) a more northerly trending (≈ N35°E) portion of the fault, running roughly parallel and SE of the Aracay river; and D) a second fault section also running across a glaciated landscape, at the Páramo heights of the Llano Corredor pass. Next, we shall describe in detail the geomorphic expression and geological context of each of these four sections.
4.1. APARTADEROS PAB (EL CERRITO-LAS TAPIAS; SECTION 1)
In this southernmost section, the BF runs across spectacular alpine glaciated landscapes at the Mucubají pass. It corresponds to the Mucubají high-altitude water divide that separates the southeasterly flowing streams (Orinoco Basin) from the northwesterly flowing streams (Maracaibo Basin) in the axis of the central Cordillera de Mérida. This divide is nested in the area of Lake Mucubají (Sierra Nevada National Park). As mentioned earlier, the BF at Mucubají displays two strands, which bound the WSW-ENE trending Apartaderos PAB (Figures 2 and 3): the northern strand (Boc-B) and the southern one (Boc-C). Both active fault strands are preserved across magnificent LGM moraine complexes, sitting between 3 000 and 3 500 m in elevation, under typical Páramo environments. Excellent preservation of the tectonic landforms results from small rainfall, which usually falls during frequent and long drizzles that do not induce significant runoff.
This high-altitude basin is being deeply dissected by both the Chama and Santo Domingo river headwaters, whose drainage divide is exactly at the lake Mucubají Páramo. Even though this depression seems to have accumulated a rather thick sequence of Pleistocene glacial deposits as those preserved at Mesas del Caballo and Julián (see Figure 5 in Audemard et al., 2010). There is no doubt that the deactivation of Holocene sedimentation in this perched PAB is due to generalized chain-wide uplift. This induces parallel-to-chain axis deep incision by the Santo Domingo and Chama rivers along the heavily fractured bedrock or brecciated material generated by the axial BF activity. Therefore, this basin is being destroyed by erosion because generalized chain uplift prevails over local tectonically driven sinking induced by localized transtension due to the BF kinematics. As a matter of fact, the Mucubají pass acts as a huge fault saddle (FS) at regional scale, from which the rivers drain away along the BF trace (Figures 2 and 3). The same configuration is present at the Llano Corredor pass, between the Aracay and Burate rivers, at the NE tip of the studied fault section.
4.1.1. NORTHERN STRAND AT APARTADEROS PAB
The northern fault strand (Boc-B), that bounds the northwest edge of the PAB, runs from the southern flank of Morro de Los Hoyos in the NE to the Lagunillas PAB in the SW, located farther SW of the city of Mérida (Figure 1). It follows the fault segmentation proposed by Audemard (2014b). The portion of the fault between La Toma (El Cerrito; Figure 3) and Tabay –a satellite town to Mérida in the NE- has been previously mapped by Alvarado et al. (2015) using criteria of earthquake-related landforms. The Bob-B section extending from La Toma (El Cerrito) to Morro de Los Hoyos, at the Mucubají Páramo, in Figure 3, will be described here.
Boc-B, in the Apartaderos PAB, mostly runs flanking the northern half of the MA chain on the south slope and rarely does it beneath the slope. This slope drains into the PAB. Therefore, this fault strand commonly displays a set of coexisting landforms resulting from the lateral displacement of small rounded spurs (interfluves) laterally bounded by south-draining poorly incised rills or creeks, particularly at Morro de Los Hoyos, north of lake Mucubají (Figure 3). The dextral lateral offset of interspersed interfluves and rills leads to the formation of small shutter ridges (SR) that may display counterscarp (CS) and trenches (TR) when deprived of sedimentation, dextrally offsetting the drainage network (OD). This can in turn accumulate fine-grained Quaternary deposits (PQ) in small trenches (TR); when complete filling is achieved, well-defined and aligned benches (BE) along (or near to) bedrock-cored slopes develop (Figures 4A to 4D). At this late evolutionary stage, deflected (Diverted; DD) and beheaded drainages (BD) may form (Figure 5A). This configuration of ponded Q fine-grained slope deposits against older-rock shutter ridges, exhibiting a general bench form, was confirmed by an excavated trench at the locality of Morro de Los Hoyos (Audemard et al., 1999). An earlier evolutionary stage is recognizable at La Toma (Figure 5C), where a well-developed trench (TR) forms against a counterscarp (CS) resting against a shutter ridge (SR) flank, because of low sediment accumulation in a setting with significant runoff due to a tilted fault-trench bottom.
On the other hand, at the opposite (NE) end of the shutter ridges to the drainage blockage, the fault shows tectonic scarps that slightly increase the slope angle. Linear drainages (LD), sag ponds (SP), and fault saddles are also present at several locations (Figure 3). Two localities along Boc-B deserve a particular mention, since measured slips have been obtained. At La Toma (El Cerrito in Figure 3; Figure 5D), the alluvial fan of La Toma river, built from Holocene (post LGM) debris flow deposits mainly (based on the size of exposed blocks in large-boulder fields), is offset about 25 m both vertically and dextrally, but the deposit has not been dated yet. Instead, lateral moraines at El Desecho (Figure 3) are dextrally offset in 40 m and vertically in only 6 m (Figure 5B), which have been cosmogenically dated by Angel et al. (2016) at 18-19 ka as time of deglaciation onset. These measured offsets support the transtensional character of BF at the Apartaderos PAB and allows estimating that the tectonic slip vector (striation) at the Apartaderos PAB northern strand ranges from 45° pitch (equal normal and dextral components) to about 10° (dominant strike-slip component). This attests to local transtension within the PAB.
4.1.2. SOUTHERN STRAND AT APARTADEROS PAB
The southern BF strand at the Apartaderos PAB corresponds to the SW tip of Boc-C segment that extends for some 90-100 km, as far NE as the town of Boconó (Figure 1), where it overlaps in a transpressional stepover with Boc-D segment. This follows Audemard (2014b)’s fault segmentation. Except for the Apartaderos PAB northern strand already described, the 4 mentioned sections to be described next all belong to Boc-C (Figure 3).
This first Boc-C section extends across from El Caballo to Las Tapias moraines (Figures 2, 3 and 6), at Páramo heights (between 2 800-3 500 m high) of the Mucubají pass, where a spectacular Last Glacial Maximum (LGM) landscape is preserved (Figure 6).
The BF cuts across the different moraine complexes of El Caballo, Mucubají, La Victoria, Los Zerpa and Las Tapias consecutively from SW to NE (Figures 3 and 6). It never gets in contact with the basement, which is composed of Precambrian rocks of the Iglesias Group outcropping both in the Sierra Nevada and Camacha ranges. In fact, as illustrated by Audemard et al. (2010) in their Figure 5, the surface trace of the southern BF strand of the Apartaderos PAB never leaves the rather thick glacial deposits, with a minimum estimated thickness of some 500 m. This contrasts with the remaye-like feature described by those authors as the crown or scar of a slow north-sliding huge deep-seated gravitational slope deformation (DSGSD; Figures 2 and 6), that does run at the lithological contact between the Precambrian igneous-metamorphic basement and LGM complexes, which had been considered as a splaying branch of the active trace of the BF by Schubert (1980a).
The surface expression of this section 1 of Boc-C (Figure 2) comprises several few-kilometer-long splays disposed “en echelon”, where most relays define small PABs (transtension) but occasionally also few pressure ridges (2, in fact; transpression; Figure 6). In the Mucubají complex (Figure 6), the PAB looks more like rhombic in shape as produced at a releasing relay, while the 3 others—when crossing the high-relief lateral moraines of the La Victoria, Los Zerpa and Las Tapias—show more the shape of PABs produced at a double-fault bend or releasing bend.
The description of this section will be split into two for simplicity and clarity: west and east of lake Mucubají, respectively. West of lake Mucubají (Figures 3 and 6), similarly to Bob-B at the Apartaderos PAB, dextrally offset features by Boc-C, such as lateral moraines and outwash fans, at El Caballo moraine complex and Mesa del Caballo, lead to the formation and coexistence of several landforms (Figures 7A to 7D and 8A to 8C): a couple of meter high shutter ridges (SR) that may display counterscarp (CS) and associated trenches (TR), which dextrally offset the drainage network (OD), and can in turn accumulate fine-grained Quaternary deposits (PQ) in small sag ponds (SP) and trenches (TR). Occasionally, the SR and CS can be as high as the height of a lateral moraine, like at El Caballo moraine complex (Figures 7A and 7B), where post LGM vertical slip of 15 m and dextral offset of 85 m have been estimated. This set of tectonic landforms are also present in the southern flank of Mesa del Caballo (Figures 3, 6 and 7D). The peculiarity here is that the up to 5 m high (up-hill facing; CS) fault scarp displays a free face of 2 to 3 meters in height (Figure 7D). Of particular mention are the formation of two few-hundred-meter-long pressure ridges (PR), as the result of transpressional arrangements of the Boc-C fault traces. The westernmost one occurs in the middle of a large outwash fan at La Cañada (between Mesa del Caballo to the west and El Caballo moraine to the east; Figures 3, 6 and 7C). It is a few tens of meters high.
On top of this, a small GPR survey conducted by Audemard et al. (2006) in 2004 associates this pressure ridge (PR; midground of Figure 7C) with an underlying positive flower structure. The other one, located between El Caballo moraine to the west and the western Mucubají lateral moraine to the east (Figures 3 and 6) is visible in Figure 8A, where BF enters the western lateral moraine of the Mucubaji moraine complex. Within this complex, the BF is clearly visible only when crossing and dextrally offsetting the western lateral moraine (Figures 3, 8A and 8C), and on the SE shore of lake Mucubaji, behind the Park Ranger house, where Boc-C used to be expressed by a fault trench (TR) and a water-filled sag pond (SG; Figures 3 and 8D); today unluckily converted into a trout fishery pool by the own rangers.
East of lake Mucubají, the BF is not easily followed on the field across the eastern Mucubají lateral moraine, which is more widely spread and much less prominent (although more voluminous) than its western counterpart (Figure 6). This is because deformation branches off in different small fault splays and traces. In fact, Boc-C when descending into the Chama valley to the NE, from 3 500 to 2 800 m but still cutting through LGM landscape, exhibits several jumps to the right. These control the formation of few, at least 6, PABs of hundreds m in width and length. The PAB in the eastern Mucubají lateral moraine is the largest of them, the only rhombic in shape and about 500 m wide and over 1 km long (Figure 6). The other PABs respond to a lazy releasing double bend geometry instead (Figures 6 and 9). The active fault mapping through typical landforms of strike-slip faulting shown in Figure 3 is easily reported on freely accessible Google Earth® (satellite) imagery, as exhibited in Figure 9. This can be a useful tool in initial stages of reconnaissance, but definitely does not replace the quality and resolution of data that can be extracted from stereoscopic aerial photography, as that shown in Figure 6.
However, Figure 9 allows to report an additional fifth lazy Z releasing bend PAB, but smaller, just south of the village of Las Tapias. A very close analog to this is present in the last perched LGM remnant, but sits atop of a rounded NE-SW elongated hill. Both small sigmoidal PABs are about 200 m long, measured along the NE-SW trend of BF (maximum opening direction).
Particular attention has been devoted to the crossing of the BF (Boc-C segment) with Los Zerpa and Las Tapias moraine complexes (Figures 6, 9, 10, and 11). A combined aerial photograph interpretation and field reconnaissance of diagnostic landforms of Quaternary and Holocene tectonic activity, as performed all along the whole studied BF section, has led to the mapping, identification, and characterization of the BF sector extending between Las Tapias and Los Zerpa moraines (Figures 6, 9, 10 and 11).
This section of the fault has easy access from the Barinas-Mérida national road that passes by closely (Figures 6, 9, 10 and 11). The BF here essentially exhibits a single fault trace, out of the PBs previously mentioned, where brittle deformation seems to concentrate on a very narrow zone, which hardly exceeds a few tens of meters in width. LGM glacial landforms, such as La Victoria, Los Zerpa, and Las Tapias moraines, are dextrally offset, up to 100 m (Figures 3, 6, 10 and 11). Older moraine deposits, such as NE of La Victoria moraine, (Figure 6), yield larger displacements of the order of 330 m, which would tentatively date them back to around 40 ka (Audemard, 2009), assuming a constant Boc-C slip rate at the Apartaderos PAB of 7-8 mm/a.
The NNW-SSE trending Los Zerpa moraine system is about 1 km long from the outer edge of its frontal moraine (NNW) to the closest Precambrian outcrops (SSE), on which lies the moraine complex. The Los Zerpa moraine complex, as well as its coeval LGM neighbors (Mucubají, La Victoria and Las Tapias) rests on a Precambrian igneous-metamorphic basement assigned to the Iglesias Group (Schubert, 1968). These moraine systems are bounded and locally covered, in their upstream portion, by lateral and frontal moraines developed during a last re-advance: La Canoa and La Canoita (green dotted lines in Figure 6, and Figure 10). Boc-C crosses the moraine complex right behind the LGM frontal moraine, dextrally offsetting it in the order of 100 m (Figures 6 and 10), but also some down-to-N 30 m vertically (Figures 3 and 11C). A 100 m dextral offset can be measured at both crestlines of its lateral moraines and at the stream draining the moraine, between the active drainage course running at the moraine bottom and the abandoned hanging outlet perched on the terminal moraine (Figure 10). The vertical offset is also measured by comparing the lateral moraine crestline heights on both sides of the PAB (Figure 11C). The entire moraine complex is then affected by two moraine-scale processes induced by this normal-dextral fault slip, which are tightly interlinked. The formation of a narrow PAB right behind the terminal moraine (Figures 6, 10 and 11A to 11C), in association with a more easterly short segment of the main strand of the Boconó fault when crossing the moraine (Figures 6 and 10), drives the gravitational collapse of the right lateral moraine (Figures 10 and 11C). The formation of the small PAB results from a transtensional jog or lazy Z double bend along the fault (Figures 6 and 10), still observable at the scale of Figure 3. The void effect introduced by the coseismic PAB sinking is responsible for a set of deformations recorded by the late Pleistocene sedimentary fill of the Los Zerpa moraine, as reported by Carrillo et al. (2006): rotational sliding small-scale faulting, seismically induced tight folding, soft sediment deformations and liquefaction. Also, the creation of this small narrow PAB is responsible for deactivation of the original Los-Zerpa-paleolake spillway (BD in Figure 10), and the presence of down-faulted and staircased late Pleistocene-Holocene(?) alluvial terraces on the right bench of the current moraine stream (Quebrada Los Zerpa) running inside the moraine complex (Figures 6 and 10).
Between Los Zerpa and Las Tapias moraines, the active Boc-C trace also affects a well-preserved outwash alluvial fan breached from a post-LGM re-advance moraine (green moraines in Figure 6). The quantified dextral slip between the capture up-stream drainage (CD) and the beheaded down-stream drainage (BD) ranges between 60-80 m (from Figure 6), and 75 m (from Figure 9), but can be as low as 35 m (from Figure 10). Assuming that the chosen markers are always the same, this variability in measures extracted from aerial photos may result from well-known photo distortion from center to edges of the photograph. This can be resolved by measuring directly on satellite imagery provided by Google Earth®, using the ruler function, where the dextral offset of the main feeding drainage of the outwash fan is of 60 m. This value compares well with the 66 m obtained from Figure 12 (F-F’ offset). The offset of this alluvial fan generates a very visible N-facing fault scarp at the trailing SW edge of the fan as its distal northern half moves east (Figures 6, 9, 10, 11C and 12), while it simultaneously forms a counter-scarp (CS; up-hill facing scarp) at its leading edge (Figures 6, 9, 11B, 11D and 13A). Beneath the N-facing scarp, a small lake sits on top of the downthrown northern block (Figures 9, 10, and 11C). This water body was not visible on aerial photographs of mission A-34 of 1952 reproduced in Figure 6. This lake is not a sag-pond because it is an artificial water reservoir built in the seventies for the Los Frailes Hotel, sitting in the very northern tip of Los Zerpa frontal moraine (Figures 9 and 10).
Next to the NE, Boc-C draws a small ≈10-m-high transpressional jog when nearing the Las Tapias moraine from the SW (PR in Figures 6, 9, 11D and 13A). At this locality, a rather modest GPR survey (Audemard et al., 2006) revealed that such a pressure ridge (PR; visible in midground of Figure 13A) is associated with an underlying pop-up structure.
Las Tapias moraine, as it occurs with Los Zerpa moraine, is also cut by Boc-C, right behind the terminal or frontal moraine (Figures 12, 13B and 13C). In this case, the horse-shoe shape exhibited by most of these LGM moraines, is almost destroyed by the formation of an elongated sigmoidal PAB at the Boc-C crossing with Las Tapias moraine (Figures 12 and 13C). Offset drainages (OD) and crestlines (OC), linear drainages (LD), ponded Quaternary deposits (PD), shutter ridges (SR) and counter-scarp (CS; uphill-facing scarp) are common tectonic landforms at this moraine complex (Figures 12, 13B and 13C). The main drainage of this moraine is offset 140-150 m dextrally (Figures 9 and 12). The OD yields a dextral offset of 130-140 m, using the tools provided by the Google Earth® portal. In Figure 3, an offset (OC and OD) of 100 m had been initially reported. Both lateral moraines of Las Tapias allow estimating an offset of 20 m only, both vertically and horizontally, reported in Figures 3 and 13B. This 20-m slip occurs only on the southern PAB bounding fault (Figures 12 and 13B). In addition, these slip values confirm the normal-dextral slip (45° -pitch slip vector) of this PAB bounding fault, as should have been expected. The geometry of these PABs at the tip of the moraine complexes of Las Tapias and Los Zerpa (Figures 6, 9, 10 and 12), attest to the decompression of the lateral moraines by the dextral offset of the frontal moraine (effect of abutment loss), which is very well expressed in the stretching of their lateral moraines (Figures 11C and 13A). In the same way, the outwash fan located between these 2 moraine complexes also shows decompression above the N-facing scarp due to the same cause (Figure 10). Other values of dextral slip across minor drainages are also reported in Figure 12 (indicated by A-A’ and so on). The largest of the dextral offsets in this network correspond to C-C’ and G-G’ of 85-90 m, when most of them are around 50-60 m. The latter values come from drainages installed on the outwash fans associated with the breaching of late re-advance glacial forms, contemporaneous to La Canoa and Canoita moraines (delineated by green dotted lines in Figure 6). Conversely, the 2 largest values may precede the breaching of the late re-advance moraines and post-date the faulting of the LGM moraines by Boc-C, since these rivers appear to contour or border the outwash fans.
NE of the Las Tapias moraines (Figures 9, 12, and 13D), another outwash fan is mappable as well as two other (older?) moraine complexes. The fan and the northeasternmost complex hold a small PAB each. They are similar in shape and dimension and are about 200 m wide in the Boc-C strike (maximum opening?).
4.2. SANTO DOMINGO VALLEY (LAS TAPIAS-SANTO DOMINGO DAM; SECTION 2)
This second section of Boc-C (Figure 2) extends from Las Tapias to the hydroelectric José Antonio Páez dam, which is the lowest point (around 1900 m in elevation) of both Santo Domingo and Aracay rivers which converge at the dam-site itself with the Pueblo Llano river. This defines the point where several chain axial rivers have merged to put the enough erosional power together to cut across the resistant Precambrian rocks of the Iglesias Group that compose the core of the Sierra Nevada and Camacha ranges on the southern half of the CdM (south of BF).
This stretch of Boc-C runs most of its length at the valley bottom of the Santo Domingo river. The fault trace here is not as conspicuous as in the previous section 1, where Boc-C cuts across glaciated landscape. Higher rainfall levels could be easily held responsible for such a difference in clarity of the surface expression of the active faulting, but it is not the case. Rainfall in the Páramo environments is about 950-1 000 mm/ year, unimodal, with a large peak of precipitation between June and September. In the Santo Domingo valley, rainfall is not much larger, being around 1300-1400 mm/year. Though vegetation is significantly denser and taller below Páramo conditions (more exuberant canopy than the small frailejones in altitude), between 2800 and 1900 m high, the difference in tectonic landform preservation resides in erosion. While the Páramos (valid for both Mucubají and Llano Corredor passes) are of rather smooth and flat topography around 3300-3500 m in elevation and rich in more pervious rocks as Pleistocene tills or diamictons, the Santo Domingo valley runs onto outcropping rocks belonging to the Precambrian igneous-metamorphic MR core complex and has a strong gradient where the river drains from 3500 m (Páramo) to 1900 m (dam-site) in elevation in only 22 km (7% average gradient). Moreover, the Santo Domingo catchment area is much larger than many of the small rivers draining the glaciated landscape above Páramo heights. Thus, erosional processes are dominating the Santo Domingo catchment. Although large volumes of sediments —alluvial, colluvial or mass wasting (rich in debris flows deposits) in origin—are deposited at the valley bottom, such as those where the town of Santo Domingo is settled, they are also rapidly flushed out by the stream power of the current Santo Domingo river.
In fact, Boc-C most of the time runs on the southern margin of and along the Santo Domingo river (Figure 14). The village of Santo Domingo sits on different well-developed alluvial terraces, filling the bottom of this valley floor, of which one stands out for its size. The exposed terrace scarps can be as high as 45 m south of the town of Santo Domingo, whereas the fault scarp in resistant rocks of the Iglesias Group can be 4 folds the terrace thickness (180-200 m high scarps; Figure 14). A set of almost continuous north-facing triangular facets, occasionally trapezoidal in shape, reveal the presence of Boc-C along the rather straight river course (Figure 14). The height of this present-day north-facing fault scarp does not necessarily match the fault vertical throw because it is magnified by linear erosion, due to the Santo Domingo river running along the scarp foot. As a matter of fact, the Boc-C scarp at Santo Domingo is a combination (addition) of tectonic scarp and fault-line scarp (erosional), where at least a fourth of the current exposed Boc-C scarp height is erosional by removal of the terrace deposits eroded at the valley floor (Figure 14).
Boc-C in section 2 only behaves differently when climbing down from the moraine complexes exposed at Las Tapias at 2 800 m to the Santo Domingo trout farming located 200-300 m down, where the BF is expressed as a set of kilometer-scale R shears.
4.3. ARACAY VALLEY (SANTO DOMINGO DAM-LLANO CORREDOR PASS; SECTION 3)
Boc-C in the Aracay valley changes strike counterclockwise from N60°E along section 2, to an almost N 30-35°E in section 3 (Figures 3, 15 and 16). In this section, erosion is even more aggressive than in section 2 because the valley bottom of the Aracay river drops from 3 500 m at Llano Corredor pass to 1 900 m at the dam-site, but in only 12 km in length (13-14% average gradient). This is attested by the solid load transported by the Aracay river, which is deposited at the artificial lake shores as a fan delta, as well as by the onset of large slope instabilities, as large as the one detectable on the Google Earth® image depicted in Figure 15. Therefore, tectonic landforms preservation along this Aracay section is even at more risk. In fact, section 3 of Boc-C is characterized by a set of coexisting landforms. Behind (large) kilometer-scale shutter ridges (SR) in resistant Precambrian rocks of the Iglesias Group (SR1 to SR4 on Figures 16 and 17A to D, on which the Aracay town spreads out), the BF exhibits linear drainages (LD), draining away from large fault saddles (FS), uphill-facing scarps (CS), fault trenches (TR) or sag-ponds (SP) and ponded Quaternary deposits (PQ; Figures 17A to D).
Schubert (1969, 1980a) and Soulas (1985) have proposed an abandoned fault trace of the Boconó Fault along the Aracay River thalweg. This possible linear trace is shown in Figure 16 along the Aracay thalweg. Audemard et al. (2010) state that, if Soulas et al. (1986)’s proposal of an abandoned active trace of the Boconó Fault along the Aracay river happens to be confirmed, they hypothesized that the cumulative activity through thousands of years of La Camacha Range DSGSD would have finally disconnected the upper part of the fault plane. Subsequently, the fault had to cut across the sliding mass to rectify the fault dip (simplify the up-dip fault plane geometry), as illustrated in their Figure 9.
Considering the almost 25°-30° counter-clockwise change in strike of Boc-C between section 2 and 3, we now favor that the La Camacha range is, in fact, a several-kilometer-scale pressure ridge (pop or push-up structure) that overruns the Aracay valley to the NW. In turn, this would have favored the NW flank destabilization under a large sackung (DSGSD) generation (Figure 16), thus pushing off-strike a former trace of the BF (shown in yellow in Figure 15), all exhibiting the general petal shape of a large NW half of a positive flower structure. In the same sense, the more easterly strike of section 2 (N60°E), in comparison with the 3 other sections, would justify the large tectonic scarps of this section along the Santo Domingo thalweg, implying large transtension in section 2 and the formation of a wedge-shaped basin pinching to the north (half graben section in appearance). This basin has progressively collected sediments mostly sourced by the northern slope, which have been finally remobilized by the axial Santo Domingo river out of the CdM.
4.4. LLANO CORREDOR PASS (SECTION 4)
Section 4 of the BF runs across Páramo environments, in a glaciated LGM landscape at the Llano Corredor pass. As for section 1, the Boc-C tectonic landforms are well-preserved mainly due to the low erosion suffered by this elevated (around 3500 m high) rather flat terrain. The earthquake-related landforms are even more spectacular than at the Mucubají pass. They are easily mappable and recognizable from different remote sensing imagery, such as in Google Earth® satellite images (Figure 16), an assembled photomosaic from mission 010455 aerial photos of 1973 (Figure 17), as well as from far away on the field in a clear day (Figure 18).
However, section 4 holds a fundamental difference from section 1. Instead of running across the LGM moraine deposits as along section 1, section 4 juxtaposes the LGM moraine complexes and deposits against the Precambrian igneous-metamorphic rocks of the Iglesias Group (Figures 16 through 19). In fact, the moraine units rest against the basement in perfect fault contact (Figure 17). The contact is a N-facing tectonic scarp of over 10 km in length, roughly trending N45°E (Figures 17 to 19), going back to the regional fault trend. In Figure 19A, it is reported a set of displaced glacial features (A-A’ through G-G’) to estimate the dominant dextral slip, which is in the order of 200 m (extracted directly from Google Earth® satellite images). This dextral slip is accompanied by a vertical throw that can be estimated at as much as a fifth of the dextral component, around 30-40 m. Consequently, the tectonic striation (direction of the total slip) can be estimated at a 9°-12° pitch to the NE at the Llano Corredor pass.
Further NE, Boc-C runs essentially in the Burate river valley bottom, except for few localities such as at the villages of Las Mesitas and Niquitao, where the BF exhibits (counter-)scarps in alluvial deposits of the Burate river, as clearly visible in Google Earth® satellite imagery.
5. Discussion
The mapped portion of the active Boconó Fault (BF) exhibits numerous tectonic landforms of active (Quaternary) faulting, which allow characterizing not only the kinematics of the BF but also quantifying its recent tectonic slip rate on the basis of LGM or younger deposits offsets. From its tectonic style, the studied stretch of Bo-C has been subdivided in 4 very different sections.
Next 4 subchapters discuss separately different relevant fault aspects, such as their morphology, total slip amount, slip rates and implications.
5.1. FAULT MORPHOLOGY
For decades now, the Boconó fault (BF) has been identified, mapped and characterized easily by the large number of along-strike geomorphic features, among which are a continuous series of aligned 1-5 km wide valleys and linear depressions, passes, saddles, trenches, sag ponds, scarps and sharp ridges (e.g., Rod, 1956; Schubert, 1969; 1980a; 1980b; 1982; Giraldo, 1985; Soulas, 1985; Soulas et al., 1986; Soulas and Singer, 1987; Casas, 1991; Ferrer, 1991; Singer and Beltrán, 1996; Audemard et al., 1999; 2008; Audemard, 2003b; 2014b; 2016; Carrillo et al., 2008; Alvarado et al., 2015; Pousse-Beltran et al., 2017). Prior to our studies, both Schubert and Soulas, in an independent manner, have commonly used the approach of mapping the active fault trace of BF on the basis of typical strike-slip landforms or geomorphic evidences, but at chain scale.
Among those earthquake-related landforms, the alignment of independent tens-of-kilometer-long valleys roughly located along the axis of the Cordillera de Mérida (CdM)—drained by different large rivers from SW to NE (Torbes, La Grita, Mocotíes, Chama, Santo Domingo, Aracay, Burate, Boconó, Chabasquén and Turbio rivers)—is the most conspicuous geomorphologic feature at chain-scale, as it makes the fault easily recognizable and mappable in radar (SLAR) images (Figure 1).
In the central CdM, particularly in the portion between the Mucubají and Llano Corredor passes, the fault-related landforms that allow the identification of the BF and characterization of its Quaternary activity exhibit their largest variety and frequency ever observed. Benches (BE), fault scarps, fault counterscarps (CS, or uphill-facing scarps), offset (OD) and deflected drainages (DD), ponded Quaternary sediments (PQ), fault trenches (TR), sag ponds (SP), shutter ridges (SR), linear ridges, offset moraines and offset alluvial fans (OC) are present. Each of the 4 sections of Boc-C herein described exhibit a particular set of these tectonic landforms that distinguish one from the others. For instance, Boc-C section 1 is represented by small slivers or slices of irregular topography mainly dextrally displaced, which on the leading end shows counterscarp (CS), ponded Quaternary deposits (PQ), fault trench/ sag pond/bench (TR/SP/BE), offset drainage (OD) shutter ridges (SR), and deflected (DD) or captured drainage (CD) occasionally. Meanwhile, it is characterized by a valley-facing smoothed scarp, accompanied by an OD, at the trailing end. These landforms are mostly preserved in a glacial landscape, mainly ascribable to the LGM.
Section 2 is mostly mapped on the basis of scarps with different degrees of evolution (trapezoidal scarps, triangular facets, etc.) or fault-line scarps, since the yet large tectonic escarpment is enhanced by linear erosion at the foot of the scarp. This erosional signature is not negligible since it may represent a 1/5th of the total scarp height, where most of these scarps are preserved in Precambrian igneous-metamorphic rocks of the Iglesias Group.
Section 3 of Boc-C instead is evidenced by kilometer-scale shutter ridges in basement rocks, which have associated uphill-facing scarps (CS), fault saddles (FS), linear drainages (LD), trenches (TR) and/or sag ponds (SP) to highlight the fault trace. This section, as well as section 1, is associated with a huge DSGSD (Audemard et al., 2010).
Finally, along section 4, the fault trace is expressed as a rather rectilinear tectonic scarp carved in the Precambrian basement rocks of the Iglesias Group, against which leans the LGM moraine complexes of Llano Corredor. The scarp, although a significant down-to-north normal component is responsible for up to a fifth of the total displacement (some 40 m), the scarp results mostly from a dominant dextral offset of some 200 m.
5.2. FAULT SLIP AMOUNT
The right-lateral sense of slip of this fault has been established using diverse geomorphologic features, being among the most common landforms: crestlines of mountainous ridges, interfluves or moraines, drainage channels, alluvial and glacial deposits and shutter ridges.
Several kilometric-scale pull-apart basins (PABs) along the BF also confirm the right-lateral sense of slip of this major fault. These basins form at the appropriate transtensional geometries (bend or relay). Schubert (1980b, 1982, 1984) has paid particular attention to some of those: Las González-Estanques, Mucuchíes-Las Mesitas and Yaracuy. The region under study belongs to the Mucuchíes-Las Mesitas PAB. A discussion on the application of the pull-apart model to some of these basins has been presented by Audemard (1996). Particularly, Casas and Diederix (1992) and Casas (1995) have debated the PAB origin of the Yaracuy valley, originally proposed by Schubert (1983). However, some other basins have also been postulated by other authors, such as: Los Mirtos-Zumbador (composite PAB west of San Cristobal, Táchira state; Singer and Beltrán, 1996), Cabudare (northeastern tip of the CdM, east Lara state; Giraldo, 1985; Giraldo and Audemard, 1997) and Apartaderos-Mucubají (central CdM, Mérida state; Soulas, 1985, Audemard et al., 1999). Los Mirtos-Zumbador PAB corresponds to the boundary between the most southwestern segment Boc-A proposed by Audemard (2014b) and the cross-border segment COL-VEN defined by Rodríguez (2017). This study has identified several others. For instance, section 2 of Boc-C, the portion of BF running along the Santo Domingo valley bottom, between the Mucubají pass and the Santo Domingo dam-site at Las Piedras, requires being transtensional because this section has a more easterly trend (N 60°E; some 10-15° more to the east) than the NE-SW main BF trend. This seems supported by the fact of: a) the height of the cumulative BF scarp here of some 180-200 m; b) the off-axis position to the SE in the valley bottom of the Santo Domingo river against the foot of the scarp; c) the wedge shape of the Quaternary deposits, thickening considerably to the SE; also responsible for pushing south the course of the Santo Domingo river; and d) the current removal of the Quaternary alluvial deposits by the SE-reclined Santo Domingo river, enhancing the fault scarp height in some additional 40 m. In addition, 6 hectometric-to-kilometric-scale PABs have been mapped between the moraines of Mucubají and a few kilometers north of Las Tapias, all preserved in the periglacial deposits.
Except for the PAB that affects the right lateral moraine of the Mucubají complex, which is rhombic in shape, all 5 others result from lazy z releasing bend geometries. In addition, 3 of them sit where the lateral moraines exhibit significant relief regarding the internal river floor (La Victoria, Los Zerpa and Las Tapias, from SW to NE). Finally, 2 of them are just behind the frontal moraine (Los Zerpa and Las Tapias). Only one PAB forms when BF crosses an (outwash) alluvial fan (at the village of Las Tapias).
In addition, this study has focused on quantifying slip along Boc-C only on deposits ascribed to the LGM or younger. The largest horizontal displacements were measured on section 4, at the Llano Corredor pass. There, several features (Figure 19A) are offset dextrally but also normally with a northern block down. The lateral displacement has been quantified directly on Google Earth® satellite imagery at 200 ± 20 m, using the ruler function. The vertical offset is estimated at a 1/5th (30-40 m) of the dextral offset, allowing to assess the tectonic striation at around 10° E pitch. A similar dextral slip of 200 m has been deduced at 2 small pull-apart basins (PAB), at the NE tip of section 1, affecting a post-LGM outwash alluvial fan and another glacial deposit. The horizontal slip of Boc-C decreases towards the SW along section 1 (Figure 3), from the sector of Las Tapias-Los Zerpa-Victoria moraines, where this study confirms the values of at least 100 m of dextral offset already published in the literature (Audemard et al., 1999; Wesnousky et al., 2012), to 85 m in the Caballo moraine (Audemard et al., 1999). Wesnousky et al. (2012), in reference to this latter locality, cite a similar estimate by Schubert (1980a) of 60 to 80 m for this same feature. In fact, dextral slip in the order of 130-140 m across Las Tapias moraine complex has been estimated using the Google Earth® portal (Figures 9 and 12), at both the right lateral moraine crestline (OC) and the moraine internal stream offsets.
So, it can be summarized that the post-LGM slip along Boc-C, from section 1 to the others, increases from 85 m at El Caballo to 100 m at La Victoria and Los Zerpa, to 130-140 m at Las Tapias moraine and 200 m south of Las Tapias village. This latter value remains the same at section 4 of Boc-C, at the Llano Corredor pass (200 m). Interestingly, as the Boc-B – Boc-C overlap (relay) ends (Figure 3), and dextral slip is transferred from segment Boc-B onto Boc-C, slip on Boc-C, at the 2-strand overlap portion, stays between 85 (El Caballo) and 100 m (Los Zerpa), while it is largest at Boc-B at El Desecho (40 m). However, at the NE end of Boc-B, still under the overlap, the slip increases to 130-140 m at Las Tapias moraine on Boc-C, but jumps substantially as Boc-C does not overlap anymore with Boc-B, at Las Tapias village, where the size of 2 PABs allowed estimating the slip at 200 m.
5.3. FAULT SLIP RATE
The estimation of the BF slip rate for different time intervals and via geologic criteria, has been a matter of interest for many years. The dextral offsets measured along the fault yield a Quaternary slip rate ranging between 3 and 14 mm/a. For quite a while, the BF slip rate was thought to decrease towards both ends. In fact, south of the Apartaderos PAB, where the fault appeared to be the fastest, average slip rate decreases to 5.2 ± 0.9 mm/a between Mérida and San Cristobal (Audemard, 1997) and as little as a 1 mm/a at the Venezuela-Colombia border (Singer and Beltrán, 1996). Audemard et al. (1999) postulated that the apparent lower slip rate of Boc-A at the Colombian border resulted of: a) displacement transfer into the Pamplona indenter convergence; b) slip distribution along at least three active strands of the Boconó fault system in the southern CdM; and c) slip transfer to other sub-parallel active faults, such as the Queniquéa, San Simón, Uribante-Caparo, and Seboruco faults, among others. As a matter of fact, this has been lately confirmed by Reinoza et al. (2024). They calculated that the GPS-derived velocity gradient between DAL0 and PED0 sites—which are sitting on each side across the chain and are separated by Aguas Calientes-San Simón, Boconó, Uribante and Caparo faults—is in the order of 10 mm/a, which is similar to the simple addition (6.7-10.6 mm/a) of the individual geologic slip rates of Aguas Calientes (1.0-1.5 mm/a) and Boconó (5.2±0.9 mm/a at La Grita segment; Audemard, 1997), Uribante (0.5 mm/a), and Caparo (0.9-2.5 mm/a) faults (Audemard et al., 2000; Audemard, 2001 and references therein). This same argument of distributed strain—sub-parallel and branching faulting along the northernmost portion of the Boconó Fault along Boc-E (Audemard et al., 1999)—was conceived as the best explanation then for the apparent slip rate drop (1.5-3 mm/a) to the NE, reported by Casas (1991) along the Yaracuy valley. More recently, Pousse-Beltran et al. (2017) have used two alluvial fans offset by Boc-E (Yaracuy Valley) to quantify slip rates, by combining 10Be cosmogenic (TCN) dating with measurements of tectonic displacements on high-resolution satellite images (Pleiades). Based upon a fan dated at >79 ka and offset of 1 350–1 580 m and a second fan dated at 120–273 ka and offset of 1 236–1 500 m, these authors obtained two Pleistocene rates of 5.0–11.2 and <20.0 mm/a, consistent with the regional geodesy (12 mm/a). For them, this indicates that the Boc-E accommodates 40 to 100% of the deformation between the South American plate and the Maracaibo Block.
Particularly for the region under study, previous studies in the Mucubají area (Schubert, 1980a; Soulas, 1985; Soulas et al., 1986) obtained a first average slip rate of about 5 to 9 mm/a, based on 60 to 100 m of dextral offset (measurement dispersion depends on authors) of the Los Zerpa moraines, which were radiocarbon-dated at a minimum of about 13 ka old (Salgado-Labouriau et al., 1977). Later studies in this area (Audemard et al., 1999), where the BF splays into two sub-parallel bounding strands (Boc-B and Boc-C) of the Apartaderos PAB, have determined 85-100 m of dextral offset across moraines on the southern strand of the fault (Boc-C) at three sites (Figures 3, 9, 10 and 12). Meanwhile, on the northern strand (Boc-B), much smaller glaciers developed on the south-exposed slope of the range and only one tongue of ice at El Desecho advanced far enough south to be displaced by Boc-B. At this locality, there is about 40 m of dextral offset across El Desecho moraine (Figure 5B); all these values were reutilized by Wesnousky et al. (2012) for their later slip-rate estimates. Conceivably, after Audemard et al. (1999), these offsets (85-100 m on Boc-C and 40 m on Boc-B) have accumulated during the past 15 ± 2 ka. In such a case, these data yield late Pleistocene-Holocene slip rates for the northern and southern strands of 2.3-3.0 mm/a and 5.0-7.7 mm/a, respectively. Thus, these authors concluded that the BF shows an overall slip rate between 7.3 and 10.7 mm/a for the past 15 ± 2 ka in the vicinity of Lake Mucubají. In consequence, the southern and northern strands respectively carry about 75% and 25% of the 7-to-10 mm/a net slip rate measured at the Apartaderos PAB. These rates are essentially consistent with those predicted by plate motion models of about 10-12 mm/a, assuming that the BF is part of the main boundary between the Maracaibo Triangular Block and the South America plate (e.g., Molnar and Sykes, 1969; Minster and Jordan, 1978; Soulas, 1986; Freymueller et al., 1993; Audemard et al., 2021; Pousse Beltran et al., 2017; Reinoza et al., 2024).
More recently, Wesnousky et al. (2012) reassessed these estimates by incorporating more dating (TCN for Terrestrial Cosmo-Nuclides) and bracketing more confidently the age of the LGM landforms between Mérida and the Santo Domingo dam-site. Boulders on the La Victoria and Los Zerpa moraines of the Sierra Nevada that mark the extent of the last glacial maximum (LGM) yield 10Be TCN surface exposure ages of 16.7 ± 1.4 ka (averaged from 8 samples), aging the LGM landscape in almost 2 ka. Using the offsets measured by Audemard et al. (1999), Wesnousky et al. (2012) determined that the 10Be TCN based BF slip rate is about ~5.5 to 6.5 mm/a, slower than of Audemard et al. (1999)’s estimates and notably less than the total right-lateral slip of 12±2 mm/a of shear documented across the CdM from geodesy. The new TCN ages obtained by Wesnousky et al. (2012) were further supported by other TCN ages obtained by Carcaillet et al. (2013), who determined that the oldest moraines (Oldest Dryas) in the Mucubají area were between 18.1 ka (for the most advanced moraine) and 16.8 ka (for the largest ridge; 5 ridges are between 18.1 and 15.8 ka); the Older Dryas was dated at 14 ka, while, at La Victoria-Los Zerpa-Las Tapias moraine complexes, the younger Dryas yields an age at 12-13 ka. For Los Zerpa moraine particularly, Carcaillet et al. (2103) determined two TCN ages on 2 boulders: 13.84 ± 1.74 and 12.48 ± 1.39 10Be-ka, with a weighted average of 13.0 ± 1.1 ka. For a dextral slip of 100 m (Audemard et al., 1999 and this study), the slip rate at Los Zerpa is in the range of 7-8 mm/a; similar to the value (5.0-7.7 mm/a) proposed by Audemard et al. (1999) for this southern strand (Boc-C) of the Apartaderos PAB. La Victoria should yield a similar slip rate (similar slip and age). On the other hand, the Las Tapias slip has been reassessed in this study and is in the range of 130-140 m. Assuming that Las Tapias moraine is coeval with Los Zerpa and La Victoria moraines, the slip rate can be recalculated in the range of 9.3-11.6 mm/a (dextral offset of 130-140 m from 13.0 ± 1.1 ka). Out of the Apartaderos PAB, this slip rate still increases, since the slip jumps to 200 m. If the landforms at Llano Corredor are of the same age as the largest ridges at the Mucubají pass (at around 16.8 ka by Carcaillet et al., 2013), based on the degree of development, the slip rate at section 4 of Boc-C can be estimated at around 11.9 mm/a. Finally, these slip rates are consistent with the regional geodesy (12 mm/a; e.g., Pousse Beltran et al., 2017; Reinoza et al., 2024).
5.4. IMPLICATIONS FOR MORE-ADVANCED INVESTIGATIONS: PALEOSEISMOLOGY
As a matter of fact, a previous very thorough and detailed neotectonic mapping is key to the success of complementary paleoseismic investigations by trenching of the direct (on-fault) evidence of Quaternary surface faulting (Audemard and Singer, 1996; 1997; Audemard, 2005; Michetti et al., 2005; Audemard and Michetti, 2011; Perucca and Audemard, 2021). In addition, Audemard and Michetti (2011) emphasize that successful outcomes from trench assessments rely largely on the prior recognition of active or capable fault traces. These results also rest on the understanding of the interaction between tectonics and sedimentation at the chosen trench site, prior to excavation. Therefore, the tectonic style of the 4 distinct mapped sections of the Boconó fault (BF) along segment Boc-C have significant implications for the further refinement of fault parameters in terms of seismic hazard through paleoseismic investigations across the causative fault.
In that sense, Audemard and Michetti (2011) stress that the most common on-fault geomorphic features used as paleoseismic indicators, from a geomorphologic viewpoint, are fault scarps or counterscarps, sag or fault ponds, pop-ups or pressure ridges (at smaller size, they are known as mole tracks), shutter ridges, and open fissures, among others. These authors add that, regardless of the tectonic style (thrust, normal, or strike-slip faulting), fault scarps are definitely the most widely assessed feature due to their common occurrence. Furthermore, they constitute a perturbation at the ground surface, which triggers local morphodynamic processes such as erosion, redeposition, and surface smoothing that have a sedimentary signature being prone to fossilization. So, scarps are the most commonly excavated features for paleoseismic purposes, in which the success of this type of investigation is enhanced if sag—or fault—bounded ponds form against them.
Consequently, on one hand, sections 2 and 3 of Boc-C are almost theoretically ruled out for further paleoseismic investigations because erosion is a prevailing process in the Santo Domingo (section 2) and Aracay (section 3) valleys. This is due to high topographic gradients even along the valley bottoms (7 and 12% in average, respectively), except for very local conditions, making the identification of potential trench sites a major challenge. Under these environmental conditions, the favoring factors (e.g., continuous sedimentary record in low-energy environments, among others; e.g., Audemard, 2003b, 2005) for the record of past earthquakes and its preservation through time can be hardly met. On the other hand, section 4 shows another unsavable difficulty: the Quaternary glacial deposits rest on tectonic contact against a very competent and resistant igneous-metamorphic basement, which makes any (manual or machinery) excavation almost impossible, unless it is accepted in advance to solely dig the down-thrown (NW) fault compartment. Therefore, section 1, which runs across a rather low-relief (almost flat) LGM/ Páramo landscape, is the only one to offer the set of most favorable morphological and geological conditions for performing paleoseismic trenching, between the Mucubají and Llano Corredor passes. In fact, ponded Quaternary (PQ) deposits in a fault trench (TR) or sag-pond (SP), against a counter-scarp (CS; uphill-facing scarp) or shutter ridge (SR; linear ridge) have yielded excellent paleoseismic results across the bounding faults of the Apartaderos PAB, at Morro de Los Hoyos across the NE tip of Boc-B (Audemard et al., 1999) and Mesa del Caballo on Boc-C section 1 (Audemard et al., 2008).
6. Conclusions
The growth of tectonic landforms due to the cumulative effect of repeating earthquake along active faults, typically formed along surface ruptures during individual earthquakes, the “seismic landscape” as named by Michetti et al. (2005) among others, has the power of allowing the identification and characterization of active (Quaternary) faulting. In this study, it has been applied and demonstrated the usefulness and applicability of such a neotectonic approach. On top of that, a thorough and detailed neotectonic mapping is actually key to the further evaluation of the earthquake potential of any fault, fault splay or fault segment prior to conducting paleoseismic exploratory trenches. This approach favors the most assertive selection of the potential trench sites across a given fault, thus substantially ensuring the success of this assessment. The only way of skipping this “unsavable” mapping is to trench a recently ruptured fault across the freshly exposed ground rupture. On the other hand, consistency and persistency of the geomorphic criteria (earthquake-related or tectonic landforms) must be sought at all times because this provides the most reliable location of the active (or potentially active) tectonic structures under study, as well as their tectonic style (type of faulting, sense of slip, slip rate, lateral extent, simplicity/complexity of the fault trace(s), damage zone, fault-zone width, among others).
The main concluding remarks are:
A. From the tectonic landforms mapped over a stretch of 40 km, between Mucuchíes and Las Mesitas, the Boconó fault (BF) offsets dextrally linear morphological features, such as interfluves, lateral moraines and the drainage network of LGM age or younger (Latest Pleistocene and Holocene).
B. BF exhibits a lazy releasing bend, responsible for the formation of the Apartaderos pull-apart basin (PAB), at the overlap or relay between segments Boc-B and Boc-C, where the Mucubají pass sits under Páramo conditions. At a smaller scale, Boc-C along section 1 replicates the same behavior. The fault trace is composed of “en echelon” shorter segments, connected by four kilometer-scale PABs at releasing stepover or jogs that are cutting across the horse-shoe-shaped LGM moraines between lake Mucubají and the village of Las Tapias.
C. The northern strand of BF (Boc-B) at the Apartaderos PAB, exhibits a maximum horizontal offset of 40 m, accompanied by 6 m of down-to-south normal component, at the LGM El Desecho moraine, yielding a slip rate of 2.3-3.0 mm/a. Towards the center of the PAB depocenter (La Toma or El Cerrito), the BF exhibits tectonic striations of 45° pitch, with equal normal and dextral components of 25 m preserved on the debris-flows fan of La Toma creek. This supports the PAB configuration, as well as the dextral slip of BF in post-LGM times.
D. The southwestern portion of Boc-C has been subdivided in four distinct sections from SW to NE. Sections 1 and 4 run across LGM glaciated landscape, whereas sections 2 and 3 mainly are inside the Santo Domingo and Aracay river valleys, flanking their southern slopes. These valleys are erosion-dominated because the valley bottom shows average gradients of 7 and 12% over a distance of 22 and 12 km, respectively. This precludes the preservation of landforms on which to quantify any fault slip.
E. Section 2 is transtensional, because of the most easterly trend of this section with respect to the entire Boc-C, which is supported by a set of evidence (very high north-facing normal fault scarps; wedge-shaped valley fill with sediment sourced from the NW; SE-pushed Santo Domingo river course), while section 3 is clearly transpressional, in which the La Camacha range acts as a pluri-kilometric pop-up (northern-half positive flower) structure, which in turn induced the past abandonment of a former NW-pushed trace of Boc-C and the destabilization of the northern flank of the La Camacha range in the shape of a huge DSGSD in Precambrian competent foliated rocks.
F. New post-LGM slips and slip rates have been calculated using LGM glaciated landforms in sections 1 and 4 of Boc-C. In section 1, it appears that the tectonic slip increases from SW to NE: 85 m at the Caballo moraine; 100 m at both La Victoria and Los Zerpa moraines; 130-140 m at Las Tapias moraine; and 200 m at the village of Las Tapias. Meanwhile, several LGM landforms at Llano Corredor, in section 4, show a consistent dextral offset of 200 ± 20 m, accompanied by 40 m of normal component downthrown to the north. It is interpreted that towards NE, when leaving the PAB bend or relay between Boc-B and Boc-C, BF becomes more dextral and loses its PAB normal component of section 1, clearly attested by 4 PABs of kilometer-size perched on the consecutive Mucubají, La Victoria, Los Zerpa and Las Tapias moraine complexes, from SW to NE.
G. Inside the Apartaderos PAB (section 1), the slip rate of Boc-C is calculated in the range of 7 to 8 mm/a. At the NE tip of this PAB, south of the village of Las Tapias, the slip rate increases to 9.3-11.6 mm/a (dextral offset of 130-140 m from 13.0 ± 1.1 ka to present-day). Northeast of the Apartaderos PAB, this slip rate still increases, since the slip jumps to some 200 m. If Llano Corredor landforms are age equivalent to the largest ridges at the Mucubají pass, the slip rate at section 4 of Boc-C can be estimated at around 11.9 mm/a. These slip rates outside the Apartaderos PAB are consistent with the regional geodetic rates (11-12 mm/a).
Contributions of authors
The author was responsible for the conceptualization of the topic, the preparation of figures and tables, and the analysis.
Financing
Venezuelan Foundation for Seismological Research (FUNVISIS) y el National Science and Technology Fund (FONACIT) de Venezuela.
Acknowledgments
First, I wish to express my gratitude to the Venezuelan Foundation for Seismological Research (FUNVISIS) that financed and supported my more than 20 years of research along the Boconó Fault. As well, to Fondo Nacional de Ciencia y Tecnología –FONACIT- of Venezuela for funding 4 different projects on the study region between 2002 and 2017. My most heartfelt thanks go to all my colleagues who walked along these Boc-B and Boc-C segments of BF; and to those who trenched them with me as well. You are too many to be listed, but the EU PILOTO Project (PI Dr. Daniela Pantosti from INGV-Roma) deserves a special mention for permitting the South America Workshop On Paleoseismology (SAWOP) meeting—the first paleoseismological field school carried out in Venezuela (and South America), at Morro de Los Hoyos-Apartaderos in March 1997. To all local folks who made my life easier and helped on the field one way or the other. My particular thanks go to Laura Patricia Perucca (UNSJ, San Juan, Argentina) and Dr. Antoni Camprubi (Editor in-chief of Boletín de la Sociedad Geológica Mexicana, BSGM), whose requests pushed me to summarize my “almost archived” findings on this portion of Boconó Fault. Last but not least, my appreciation also goes to two colleagues who spent enormous time in improving a first draft of this contribution; particularly to my friend, officemate, field / 1st trench partner (La Grita trench in October 1986), company fellow and wedding godfather: Dr. Carlos María Giraldo Ceballos.
Conflicts of interest
The author declares no conflicts of interest.
Handling editor
Laura P. Perucca.
References
Alvarado, M., Cantos, G., Pérez, E., Audemard, F.A., 2015, Cartografía neotectónica de la Falla de Boconó entre Tabay y La Toma, Mérida – Venezuela: Boletín Geología UIS, 36(2), 47–55.
Angel, I., Audemard, F.A., Carcaillet, J., Carrillo, E., Beck, C., Audin, L., 2016, Deglaciation chronology in the Mérida Andes from cosmogenic 10Be dating (Gavidia valley, Venezuela): Journal of South American Earth Sciences, 71, 235–247. https://doi.org/10.1016/j.jsames.2016.08.001
Audemard, F.A., 1993, Néotectonique, Sismotectonique et Aléa Sismique du Nord-ouest du Vénézuéla (Système de failles d’Oca-Ancón): Montpellier, France, Université Montpellier II, PhD thesis, 369 p.
Audemard, F.A., 1996, Contribución del Dr. Carlos Schubert Paetow (1938-94) al conocimiento de la Neotectónica del Caribe: visión crítica de un colega neotectonista: Boletín Sociedad Venezolana de Geólogos, 21(2), 23–37.
Audemard, F.A., 1997, Holocene and historical earthquakes on the Boconó fault system, southern Venezuelan Andes: trench confirmation: Journal of Geodynamics, 24(1-4), 155–167. https://doi.org/10.1016/S0264-3707(96)00037-3
Audemard, F.A., 1998, Evolution Géodynamique de la Façade Nord Sud-américaine: Nouveaux apports de l’Histoire Géologique du Bassin de Falcón, Vénézuéla, in XIV Caribbean Geological Conference, Trinidad, 1995, 2, 327–340.
Audemard, F.A., 1999, Morpho-structural expression of active thrust fault systems in humid tropical foothills of Colombia and Venezuela: Zeitschrift für Geomorphologie, 118, 1–18.
Audemard, F.A., 2001, Revisión y actualización de los parámetros sismogénicos de las fallas activas o potencialmente activas en Venezuela (Parcial Rpt. to Project INTEVEP 99-162): FUNVISIS’, Unpublished Report for INTEVEP S.A, 24 pp + appendices.
Audemard, F.A., 2003a, Estudios paleosísmicos por trincheras en Venezuela: métodos, alcances, aplicaciones, limitaciones y perspectivas: Revista Geográfica Venezolana, 44(1), 11–46. Audemard, F.A., 2003b, Geomorphic and geologic evidence of ongoing uplift and deformation
in the Mérida Andes, Venezuela: Quaternary International, 101-102, 43–65. https://doi.org/10.1016/S1040-6182(02)00128-3
Audemard, F.A., 2005, Paleoseismology in Venezuela: objectives, methods, applications, limitations and perspectives: Tectonophysics, 408(1-4), 29–61. https://doi.org/10.1016/j.tecto.2005.05.034
Audemard, F.A., 2009, 5.3.8 Falla de Boconó (VE-06b y VE-06c), in Proyecto Mutinacional Andino. Geociencia para las comunidades andinas (eds.), Atlas de deformaciones cuaternarias de Los Andes: Publicación Geológica Multinacional, 7, 259–271.
Audemard, F.A., 2014a, Active block tectonics in and around the Caribbean: A Review, in Schmitz, M., Audemard, F.A., Urbani, F. (eds.), El Límite Noreste de la Placa Suramericana - Estructuras Litosféricas de la Superficie al Manto (The Northeastern Limit of the South American Plate - Lithospheric Structures from Surface to the Mantle): Editorial Innovación Tecnológica, Facultad de Ingeniería-Universidad Central de Venezuela/FUNVISIS.
Audemard, F.A., 2014b, Segmentación sismogenética de la falla de Boconó a partir de investigaciones paleosísmicas por trincheras, Venezuela occidental: ¿Migración de la ruptura hacia el noreste en tiempos históricos?: Revista Asociación Geológica Argentina, 71(2), 247–259. https://revista.geologica.org.ar/raga/article/view/424
Audemard, F.A., 2016, Evaluación paleosísmica del segmento San Felipe de la Falla de Boconó (Valle del Yaracuy, Venezuela noroccidental): ¿Responsable del terremoto del 26 de marzo de 1812?, Boletín Geología UIS, 38(1), 125-149. http://dx.doi.org/10.18273/revbol.v38n1-2016007
Audemard, F.A., Michetti, A.M., 2011, Geological criteria for evaluating seismicity revisited: 40 years of paleoseismic investigations and the natural record of past earthquakes, in Audemard, F.A., Michetti, A.M., Mccalpin, j.(eds.), Geological criteria for evaluating seismicity revisited: 40 years of paleoseismic investigations and the natural record of past earthquakes, GSA Special papers 479, 1-21. https://doi.org/10.1130/2011.2479(00)
Audemard, F.A., Singer, A., 1996, Active Fault Recognition in Northwestern Venezuela and its Seismogenic Characterization: Neotectonic and Paleoseismic approach: Geofísica Internacional, 35(3), 245–255. https://doi.org/10.22201/igeof.00167169p.1996.35.3.460
Audemard, F.A., Singer, A., 1997, La ingeniería de fallas activas en Venezuela: historia y estado del arte en Seminario Internacional de Ingeniería Sísmica: Aniversario del Terremoto de Caracas de 1967: Caracas, Universidad Católica Andrés Bello, 11–27.
Audemard, F.A., Machette, M., Cox, J., Dart, R., Haller, K, 2000, Map and Database of Quaternary Faults in Venezuela and its Offshore Regions: Open-File Report 00-0018, 1:2,000,000: Denver, Colorado, EUA, U.S. Geological Survey, 1 map with text, 78 p.
Audemard, F.A., Ollarves, R., Díaz, G., Bechtold, M., Cataldi, A., 2006, El geo-radar como herramienta para la definición de fallas activas: Aplicación en el sector central de la falla de Boconó, estado Mérida, Venezuela: Revista de la Facultad de Ingeniería de la Universidad Central de Venezuela, 21(4), 57–70.
Audemard, F.A., Ollarves, R., Betchtold, M., Díaz, G., Beck, C., Carrillo, E., Pantosti, D., Diederix, H., 2008, Trench investigation on the main strand of the Boconó fault in its central section, at Mesa del Caballo, Mérida Andes, Venezuela: Tectonophysics, 459(1-4), 38–53. https://doi.org/10.1016/j.tecto.2007.08.020
Audemard, F.A., Beck, C., Carrillo, E., 2010, Deep-seated gravitational slope deformations along the active Boconó fault in the central portion of the Mérida Andes, western Venezuela: Geomorphology, 124(3-4), 164–177. https://doi.org/10.1016/j.geomorph.2010.04.020
Audemard, F.A., Perucca, L.P., Pantano, A., Avila, C., Onorato, M R., Vargas. H.N., Alvarado, P., Viete, H., 2016, Holocene compression in the Acequión valley (Andes Precordillera, San Juan Province, Argentina): Geomorphic, tectonic, and paleoseismic evidence: Journal of South American Earth Sciences, 67, 140–157. https://doi.org/10.1016/j.jsames.2016.02.005
Audemard, F.A., Pantosti, D., Machette, M., Costa, C., Okumura, K., Cowan, H., Diederix, H., Ferrer, C., SAWOP Participants, 1999, Trench investigation along the Merida section of the Boconó fault (central Venezuelan Andes), Venezuela: Tectonophysics, 308(1-2), 1–21. https://doi.org/10.1016/S0040-1951(99)00085-2
Audemard M., F.A., Mora-Páez, H., Fonseca P., H.A., 2021, Net right-lateral slip of the Eastern Frontal Fault System, North Andes Sliver, northwestern South America: Journal of South American Earth Sciences, Special Issue 8th International Symposium on Andean Geodynamics, 109, 103286. https:// doi.org/10.1016/j.jsames.2021.103286
Audemard, F.E., Audemard, F.A., 2002, Structure of the Mérida Andes, Venezuela: relations with the South America-Caribbean geodynamic interaction: Tectonophysics, 345(1-4): 299–327. https://doi.org/10.1016/S0040-1951(01)00218-9
Backé, G., Dhont, D., Hervouët, Y., 2006, Spatial and temporal relationships between compression, strike-slip and extension in the Central Venezuelan Andes: Clues for Plio-Quaternary tectonic escape: Tectonophysics, 425(1-4), 25–53. https://doi.org/10.1016/j.tecto.2006.06.005
Benítez, S., 1986, Síntesis geológica del graben de Jambelí en IV Congreso Ecuatoriano de Geología, Minería y Petróleo, 1, 137-160.
Boinet, T., 1985, La frontière méridionale de la plaque caraïbe aux confins colombo-vénézuéliens (Norte de Santander, Colombie): données géologiques: Paris, France, Université de Paris VI, PhD thesis, 204 p.
Boinet, T., Bourgois, J., Mendoza, H., Vargas R., 1985, Le poinçon de Pamplona (Colombie): un jalon de la frontière méridionale de la plaque caraïbe: Bulletin de la Société Géologique de France, 8, I(3), 403–413.
Carcaillet, J., Angel, I., Carrillo, E., Audemard, F.A., Beck, C., 2013, High resolution ice retreat since the Late Mérida Glacial (Late Glacial Maximum) in the Sierra Nevada of the Mérida Andes: Quaternary Research, 80(3), 482–494. https://doi.org/10.1016/j.yqres.2013.08.001
Carrillo, E., Audemard, F.A., Beck, C., Cousin, M., Jouanne, F., Cano, V., Castilla, R., Melo, L., Villemin, T., 2006, A Late Pleistocene natural seismograph along the Boconó fault (Mérida Andes, Venezuela): the moraine-dammed Los Zerpa paleo-lake: Bulletin de la Societé Géologique de France, 177(1), 3–17.
Carrillo, E., Beck, C., Audemard, F.A., Moreno, E., Ollarves, R., 2008, Disentangling Late Quaternary climatic and seismo-tectonic controls on Lake Mucubají sedimentation (Mérida Andes, Venezuela): Palaeogeography, Palaeoecology and Palaeoclimatology, Special issue, 259(2-3), 284–300. https://doi.org/10.1016/j.palaeo.2007.10.012
Casas, A., 1991, Estudio sismotectónico del valle del Yaracuy: FUNVISIS-Universidad de Zaragoza, unpublished report.
Casas, A., 1995, Geomorphological and sedimentary features along an active right-lateral reverse fault: Zeitschrift für Geomorphologie N. F., 39(3), 363–380.
Casas, A., Diederix, A., 1992, El valle de Yaracuy (límites de la Placa Caribe, Venezuela): ejemplo de una cuenca cuaternaria asociado a una curvatura de falla, en VIII Congreso Latinoamericano de Geología: Salamanca, Sociedad Geológica de España, 4, 269–279.
Case, J.E., Duran, L.G., Lopez, A., Moore, W.R., 1971, Tectonic investigations in western Colombia and eastern Panama: Bulletin of the Geological Society of America, 82, 2685–2712. https://doi.org/10.1130/0016-7606(1971)82[2685:TIIWCA]2.0.CO;2
Dewey, J.W., 1972, Seismicity and tectonics of western Venezuela: Bulletin of the Seismological Society of America, 62(6), 1711–1751. https://doi.org/10.1785/BSSA0620061711
Ego, F., Sébrier, M., Lavenu, A., Yepes, H., Egues, A., 1996, Quaternary state of stress in the Northern Andes and the restraining bend model for the Ecuadorian Andes: Tectonophysics, 259, 101–116.
Ferrer, C., 1991, Características geomorfológicas y neotectónicas de un segmento de la falla de Boconó entre la ciudad de Mérida y la Laguna de Mucubají, Estado Mérida, en Guía de la excursión: Escuela Latinoamericana de Geofísica, 25 p.
Freymueller, J.T., Kellogg, J.N., Vega, V., 1993, Plate motions in the north Andean region: Journal of Geophysical Research, 98(21), 853–863.
FUNVISIS, Audemard, F.A. (coord.), 1999, Estudio de Neotectónica y Geología de Fallas Activas del Triángulo de Fallas de Boconó, Oca-Ancón y Valera (Proyecto INTEVEP 97-018): Funvisis, unpublished report for INTEVEP, S.A., 138 p.
Giraldo, C., 1985, Neotectónica y sismotectónica de la región de El Tocuyo-San Felipe (Venezuela centro-occidental), en VI Congreso Geológico Venezolano: Caracas, Sociedad Venezolana de Geólogos, 4, 2415–2451.
Giraldo, C., Audemard, F.A., 1997, La cuenca de tracción de Cabudare, Venezuela centro-occidental en VIII Congreso Geológico Venezolano: Porlamar, Sociedad Venezolana de Geólogos, 1, 351–357.
Guccione, M.J., Mueller, K., Champion, J., Shepherd, S., Carlson, S.D., Odhiambo, B., Tate, A., Stream response to repeated coseismic folding, Tiptonville dome, New Madrid seismic zone: Geomorphology, 43(3–4), 313–349. https://doi.org/10.1016/S0169-555X(01)00145-3
Lemgruber-Traby, A., Espurt, N., Souque, C., Henry, P., Calderon, Y., Baby, P., Brusset, S., 2020, Thermal structure and source rock maturity of the North Peruvian forearc system: Insights from a subduction-sedimentation integrated petroleum system modeling: Marine and Petroleum Geology, 122, 104664.
Michetti, A.M., Hancock, P.L., 1997, Paleoseismology: understanding past earthquakes using Quaternary geology: Journal of Geodynamics, 24(1-4), 3–10.
Michetti, A.M., Audemard F.A., Marco, S., 2005, Future trends in paleoseismology: Integrated study of the seismic landscape as a vital tool in seismic hazard analyses: Tectonophysics, 408(1-4), 3–21. https://doi.org/10.1016/j.tecto.2005.05.035
Minster, J., Jordan, T., 1978, Present-day plate motions: Journal of Geophysical Research, 83(B11), 5331–5354. https://doi.org/10.1029/JB083iB11p05331
Molnar, P., Sykes, L., 1969, Tectonics of the Caribbean and Middle America Regions from focal mechanisms and Seismicity: Geological Society of America Bulletin, 80(9), 1639–1684. https://doi.org/10.1130/0016-7606(1969)80[1639:TOTCAM]2.0.CO;2
Ollarves, R., Audemard, F.A., López, M., 2006, Morphotectonic Criteria for the Identification of Active Blind Faulting in Alluvial Environments: Case Studies from Venezuela and Colombia: Zeitschrift für Geomorphologie, 145, 81–103.
Pennington, W.D., 1981, Subduction of the Eastern Panama Basin and seismotectonics of northwestern South America: Journal of Geophysical Research Solid Earth, 86(B11), 10753-10770. https://doi.org/10.1029/JB086iB11p10753
Perucca, L.P., Audemard, M., F.A., 2021, De la neotectónica a la amenaza sísmica en América Latina y el Caribe: Boletín de la Sociedad Geológica Mexicana, 73(2), P260221, 1–11. https://doi.org/10.18268/BSGM2021v73n2p260221
Peuzin, A., Saillard, M., Espurt, N., Michaud, F., Bulois, C., Praeg, D., Régnier, M., Calderon, Y., 2023, Gravity-driven large-scale deformation system in the Tumbes-Guayaquil forearc basin, Northern Andes (Northern Peru-Southern Ecuador): Journal of Structural Geology, 173, 104909. https://doi.org/10.1016/j.jsg.2023.104909
Pousse-Beltran, L., Vassallo, R., Audemard, F.A., Jouanne, F., Carcaillet, J., Pathier, E., Volat, M., 2017, Pleistocene slip rates on the Boconó fault along the North Andean Block plate boundary, Venezuela: Tectonics, 36(7), 1207–1231. https://doi.org/10.1002/2016TC004305
Reinoza, C.E., Audemard M., F.A., Pereira, R., Ortega, L., Schmitz, M., Yegres, L.A., 2024, Velocity field for Western Venezuela: Elastic modeling in the Southern Merida Andes: Boletín Geológico SGC, 51(1). https://doi.org/10.32685/0120-1425/bol.geol.51.1.2024.672
Rod, E., 1956, Strike-slip faults of northern Venezuela: American Association of Petroleum Geologists Bulletin, 40(3), 457–
476. https://doi.org/10.1306/5CEAE3E5-16BB-11D7-8645000102C1865D
Rodríguez D.L.M., 2017, Neotectónica y paleosismología en los Andes de Mérida, en la zona limítrofe colombo-venezolana: con énfasis en las fallas de Boconó y Aguas Calientes: Venezuela, Doctorado individualizado del postgrado Ciencias de la Ingeniería, Universidad Central de Venezuela, tesis doctoral, 2 tomos, 256 p + 213 p.
Salgado-Labouriau, M., Schubert, C., Valastro, S., 1977, Paleoecologic analysis of a late Quaternary from Mucubají, Venezuelan Andes: Journal of Biogeography, 4, 313–325. Schubert, C., 1968, Geología de la región de Barinitas-Santo Domingo. Andes Venezolanos Surorientales: Boletín de Geología, 10, 181–161.
Schubert, C., 1969, Geologic structure of a part of the Barinas front, Venezuelan Andes: Geological Society of America Bulletin, 80, 443–458.
Schubert, C., 1974, Late Pleistocene Mérida Glaciation, Venezuelan Andes: Boreas, 3(4), 147–152. https://doi.org/10.1111/j.1502-3885.1974.tb00673.x
Schubert, C., 1980a, Morfología neotectónica de una falla rumbo-deslizante e informe preliminar sobre la falla de Boconó, Andes merideños: Acta Científica Venezolana, 31, 98–111.
Schubert C., 1980b, Late Cenozoic pull-apart basins, Boconó fault zone, Venezuelan Andes: Journal of Structural Geology, 2(4), 463–468. https://doi.org/10.1016/0191-8141(80)90008-5
Schubert C., 1982, Neotectonics of the Boconó fault, western Venezuela: Tectonophysics, 85(3-4), 205–220. https://doi.org/10.1016/0040-1951(82)90103-2
Schubert, C., 1983, La cuenca de Yaracuy: una estructura neotectónica en la región centro-occidental de Venezuela: Geología Norandina, 8, 194–202.
Schubert, C., 1984, Basin formation along Boconó-Morón-El Pilar fault system, Venezuela: Journal of Geophysical Research, 89(B7), 5711–5718.
Schumm, S., 1986, Alluvial river response to active tectonics, in Studies in National Research Council (eds.), Active tectonics: Impact on Society: Washington D.C., National Academy Press, Studies in Geophysics, 80–94. https://doi.org/10.17226/624
Serva, L., Slemmons, D.B., 1995, Perspective in paleoseismology: Seattle, Washington, A.E.g. Special Publication, Peanut Butter Publishing, 6, 138 p.
Serva, L., Vittori, E., Ferreli, L., Michetti, A.M., 1997, Geology and seismic hazard, in Proceedings of the Second France–United States Workshop on Earthquake Hazard Assessment in Intraplate Regions: Centraland Eastern United States and Western Europe October 16, 1995: Nice, France, Ouest Editions, 20–24.
Singer, A., Beltrán, C., 1996, Active faulting in the Southern Venezuelan Andes and Colombian borderland, in 3rd International Symposium on Andean Geodynamics: Saint-Malo, France, 243–246.
Slemmons, D., 1977, Faults and earthquake magnitudes: Vicksburg, U.S. Army Engineer Waterways Experiment Station, Miscellaneous paper S-73-1.
Soulas, J-P., 1985, Neotectónica del flanco occidental de los Andes de Venezuela entre 70º30’ y 71º00’W (Fallas de Boconó, Valera, Piñango y del Piedemonte), en VI Congreso Geológico Venezolano: Caracas, 4, 2690–2711.
Soulas, J-P., 1986, Neotectónica y tectónica activa en Venezuela y regiones vecinas, en VI Congreso Geológico Venezolano, 1985: Caracas, 10, 6639–6656.
Soulas, J-P., Rojas, C., Schubert, C., 1986, Neotectónica de las fallas de Boconó, Valera, Tuñame y Mene Grande. Excursión Nº 4, en VI Congreso Geológico Venezolano, 1985: Caracas, 10, 6961–6999.
Soulas, J.P., Singer, A., 1987, Evidencias de actividad cuaternaria en las fallas, in Soulas, J.P., Singer, A., Lugo, M (eds.), Tectónica cuaternaria, características sismogénicas de las fallas de Boconó, San Simón y del piedemonte occidental andino y efectos geológicos asociados a la sismicidad histórica (Proyecto Sumandes): Venezuela, FUNVISIS’ unpublished report for Maraven S.A., 1 mapa sin escala, 90 p.
Stephan, J., 1982. Evolution géodynamique du domaine Caraibe Andes et Chaine Caraibe sur la transversale de Barquisimeto (Venezuela): Universidad Pierre et Marie Curie, doctoral thesis, 512 p.
Vedder, J., Wallace, R., 1970, Map showing recently active breaks along the San Andreas and related faults between Cholame Valley and Tejon Pass, California, scale 1:24,000.: U.S. Geological Survey Miscellaneous Geologic Investigations, Map I-574.
Velandia, F., García-Delgado, H., Zuluaga, C., López, J., Bermúdez, M.A., Audemard. F.A., 2020, Present-day structural frame of the Santander Massif and Pamplona Wedge: The interaction of the Northern Andes: Journal of Structural Geology, 137, 104087. https://doi.org/10.1016/j.jsg.2020.104087
Wesnousky, S.G., Aranguren, R., Rengifo, M.,Owen, L. A., Caffee, M. W., Murari, M. K., Pérez, O.J., 2012, Toward quantifying geomorphic rates of crustal displacement, landscape development, and the age of glaciation in the Venezuelan Andes: Geomorphology, 141, 99–113. https://doi.org/10.1016/j.geomorph.2011.12.028
Wesson R., Helley, E., Lajoie, K., Wentworth, C., 1975, Faults and future earthquakes: U.S. Geological Survey, Professional Paper P-941A.
Peer Reviewing under the responsibility of Universidad Nacional Autónoma de México.
This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)