Abstract

CHAVEZ-CABELLO, Gabriel et al. The San Marcos Fault: A Jurassic multi-reactivated basement structure in northeastern Mexico. Bol. Soc. Geol. Mex [online]. 2005, vol.57, n.1, pp.27-52. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2005v57n1a2.

San Marcos Fault (FSM) is a regional lineament in northeast Mexico with a minimum length of 300 km separating the Coahuila block from the Coahuila Fold Belt; the fault dip NNE and strike WNW. FSM is a basement fault that has been reactivated multiple times, and along its trace there is an stratigraphic and structural evidence of intermittent activity since at least the Late Jurassic to the Pliocene-Quaternary. The oldest structural evidence recognized in this work suggests the FSM accommodated mainly NNE crustal extension in pre-Tithonian and Neocomian time. This extension may have contributed to development and growth of the Sabinas basin to the north; this in turn casts doubts on previously proposed existence of large lateral slip across the fault in Late Jurassic time.

At least four reactivation events have been recognized along the FSM. The first, in the Neocomian, was normal and triggered deposition of the San Marcos Formation. The second reactivation of the FSM involved reverse slip during Paleogene time, and it causes minor folds associated to the FSM. Interpretation of the reactivation event of FSM as a reverse fault is based on: (a) the occurrence of drape folds and minor tectonic transport to the south-southwest along the main trace of the fault; (b) the occurrence of a nearly perpendicular relation between fold axes of different generation in the southwest sector of the Sabinas basin; (c) uplift of progressively older rocks towards the northeast within San Marcos valley (VSM); and, (d) by the existence of near perpendicular directions of tectonic transport determined for different structures within VSM (e.g., faults in the western sector of the valley record tectonic transport to the west and faults in the southwest sector of the valley record tectonic transport to the south-southwest). Minor faults associated with the FSM vary in orientation from nearly E-W to nearly N-S, and are best represented by the El Caballo and El Almagre faults, exposed western Coahuila and southeastern Chihuahua. Reactivation as a reverse fault occurred late, relative to a earlier stage of detachment (locally duplicating the stratigraphic sequence) in localities over the Coahuila platform and Sabinas basin itself. The relative importance and scale of detachment folds needs to be explored with further detail.

The third reactivation event is normal to a left-lateral component (late Miocene-early Pliocene), and the fourth and last event is dominantly normal (Pliocene-Quaternary). The last two reactivations events along the FSM were recognized along the segment of the fault buried by volcanic products of the Camargo Volcanic Field, in southeast Chihuahua state. These late events appear to be present along the FSM in Coahuila, but here they do not affect Cenozoic sequence that would allow to establish the age of faulting.

Keywords : San Marcos Fault; Sabinas basin; Coahuila Fold Belt; Coahuila; Northeastern Mexico.

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