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Boletín de la Sociedad Geológica Mexicana
http://dx.doi.org/10.18268/BSGM2005v57n1a4 |
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El sistema de fallas Taxco-San Miguel de Allende y la Faja Volcánica Transmexicana, dos fronteras tectónicas del centro de México activas durante el Cenozoico
Susana A. Alaniz-Álvarez*, y Ángel Francisco Nieto-Samaniego
Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Apartado Postal 1-742, Querétaro, Qro., 76001, México.
* This email address is being protected from spambots. You need JavaScript enabled to view it.
Abstract
The Transmexican Volcanic Belt has been recognized as a major volcanic arc which crosses Mexico from the Pacific coast to the Gulf of Mexico. This volcanic arc had normal faulting and volcanism since Miocene. It has suggested that the volcanic arc is not parallel to the Mesoamerican trench because the subduction angle varies being gentler to the east. Also it has been proposed a preexisting zone of weakness below the Transmexican Volcanic Belt. In this work we present the deformation events that have been recorded north and south of the Transmexican Volcanic Belt in order to identify the time in which this zone began to be a crustal discontinuity and also to determine the deformation field precursor of the emplacement of the volcanic arc. We use as a reference the studies along the Taxco-San Miguel de Allende because it is the only recognized structure that crosses the Transmexican Volcanic Belt. In the Mesa Central, north of the Transmexican Volcanic Belt, the postlaramidic deformation occurred in three extensional events during the Eocene, Oligocene and Miocene to Recent. The three events liberated extension in two horizontal directions and shortening in the vertical direction. The Eocene event was extensional but the directions of the principal extensions are not very well known. The Oligocene event was the most important; it extended 20% in the ~ENE-WSW direction. This event migrated toward west, finishing in the opening of the California Gulf, and also migrated toward south-southeast to the Transmexican Volcanic Belt. The most recent event, active since middle Miocene, has been developed in the Transmexican Volcanic Belt and its northern boundary. In the Sierra Madre Oriental, east of the Mesa Central and north of the Transmexican Volcanic Belt, the Cenozoic deformation has been minimum. South of the Transmexican Volcanic Belt, in the Taxco region, there were two postlaramidic deformation events, mainly liberated by NW-SE and N-S lateral faults. The first one occurred in late Eocene with a NNW-SSE horizontal extension direction and shortening to the ENE-WSW. The second event was early Oligocene with a maximum extension to the NE-SW and shortening to the NW-SE, both migrated toward east.
Most of the volcanic rocks along the Transmexican Volcanic Belt are Miocene, however it has reported Oligocene mafic volcanic rocks in the middle of the Mexico basin. Besides, this volcanic arc is located into tectonic basins bounded by major faults. In the case of the Mexico basin, faulting initiated in late Eocene time. It is concluded that since the Eocene, the deformation style was different in the Mesa Central and in the Sierra Madre del Sur, this implies the presence of a detachment zone between these provinces. From the Oligocene the lateral faults activity in the Sierra Madre del Sur produced shortening in the NW-SE principal direction, generating an oblique-extension with a left-lateral component within the zone now occupied by the central part of the Transmexican Volcanic Belt. The pre-Miocene deformation events formed a detachment zone in which the Mexico basin was developed, also through this zone was canalized the magmas surface forming the central part of the Transmexican Volcanic Belt.
Keywords: Transmexican Volcanic Belt, major fault, Cenozoic, reactivation.
