Active faults and state of stress in the central part of the Trans‐Mexican Volcanic Belt, Mexico 1. The Venta de Bravo Fault

Abstract

The central part of the Trans‐Mexican volcanic belt is being deformed by seismically active, west‐east striking normal faults that cause pronounced scarps. Structural and seismic along‐strike discontinuities allow the following segmentation: (1) The western segment, the area between Morelia and the Los Azufres volcanic center, is characterized by ≤ 25‐km‐long fault scarps that define the Cuitzeo graben, and the Santa Ana Maya horst which has a relief ≤ 850 m with respect to the Cuitzeo graben. The faults cut several Quaternary scoria cones as well as alluvium. (2) The central neotectonic segment extends between Maravatio in the west and Altamirano volcano in the east. The most prominent structure of this segment is the 45‐km‐long Venta de Bravo fault with a relief ≤ 300 m. The February 22, 1979, mb = 5.3 Maravatio earthquake occurred on this fault. The late Quaternary vertical slip rate along the easternmost part of the Venta de Bravo fault is estimated at 2 mm/yr. The lateral extension of the segment is defined by the limits of the Venta de Bravo fault and the eastern termination of the Cuitzeo graben. (3) The eastern segment is made up of the Acambay graben. The limit between the central and eastern segments is defined by an extensional jog between the Venta de Bravo and Pastores faults, which indicates a left‐lateral strike‐slip component. The most recent rupture of this segment occurred in the November 19, 1912, Ms = 6.9 Acambay earthquake that caused vertical displacements ≤ 50 cm along the faults flanking the graben. Stress determinations based on Quaternary scoria cone alignments, the focal mechanism of the 1979 earthquake, and the inversion of striation measurements indicate a stress field with Sv>SENE>SNNW. The fault striations as well as the focal mechanism indicate a minor but consistent left‐lateral strike‐slip component. The observed stress and deformation can be explained by the superposition of two sources of stress. Isostatically compensated surface loads related to the high elevation of the Trans‐Mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt. The observed left‐lateral strike‐slip component, on the other hand, can be explained by compressional far‐field stresses caused by loads at the boundary between the Cocos and North American plates; the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left‐lateral slip along the observed faults. Alternatively, the observed left‐lateral strike‐slip component may accommodate trench‐parallel motion of crustal blocks in southwestern Mexico.

Reappraisal of the Geology and Geochemistry of Volcán Zamorano, Central Mexico: Implications for Discriminating the Sierra Madre Occidental and Mexican Volcanic Belt Provinces

“…This ENE-oriented structural system is subparallel to the NE-SW fractures that influenced Volcán Zamorano. Finally, NNW-SSE extension is similar to the Pleistocene extensional deformation affecting the MVB that is still active (Suter et al, 1992), and clearly contrasts to the E-W extension affecting mostly the SMO province.…”

“…The plateau lavas have strong compositional similarities to those representing regional plateau basaltic or andesitic lava flows, dated at about 6.5 Ma for the Meseta Río San Juan (Grupo San Juan), 7.7 Ma for the Pathé area, ~8.1 Ma in the vicinity of Querétaro city, ~7.5 Ma from Volcán La Joya, 6.5 Ma from Volcán La Cruz, and ~9 Ma from Volcán Apartadero (Table 6; also Suter et al, 1992;Verma, 2001aVerma, , 2001b.…”

Petrogénesis ígnea de la Faja Volcánica Transmexicana

“…Sin embargo, también se han reportado evidencias de movimientos cuaternarios en la región de Morelia (Garduño-Monroy et al, 2001). En cambio, la parte oriental del sistema se caracteriza por una estructura tipo graben asimétrico, formado por las fallas Epitacio Huerta y Acambay-Tixmadeje, al norte, y las fallas Venta de Bravo y Pastores, al sur, que exhiben una pequeña componente lateral izquierda (Suter et al, 1992;Suter et al, 1995b). Este sector es el más activo del sistema, y prueba de ello es el sismo de 1912 con magnitud 6.9 y epicentro en Acambay (Urbina y Camacho, 1913;Suter et al, 1995b;Suter et al, 1996), para el cual se ha estimado una recurrencia de ~3600 años durante el Holoceno (Langridge et al, 2000).…”

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

“…Martínez-Reyes y Nieto-Samaniego (1990) cartografiaron las fallas que se encuentran al lado poniente del SFTSMA en la Faja volcánica Transmexicana, las que en conjunto Suter et al (1992Suter et al ( , 1995 denominaron sistema de fallas Chapala-Tula (Figura 2). Las fallas de este sistema son normales con una componente lateral izquierda subordinada.…”