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The trans-Mexican volcanic belt is an active volcanic arc related to subduction along thc Middle America trench. The central part of the belt is being ticformed by the Chapala-Tula fault zone, an approxi•nately 450-kin-long and 50-km-wide zone of active extension. The volcanic arc and the arc-parallel Chapala-Tula fault zone are superposed nearly perpendicularly on the preexisting stress and dcformation province of the Mexican Basin and Range. The Acambay graben, about 40 Fan long and 15 km wide, is located approxi•natcly 100 km northwest of Mexico City and is one of the major troughs within the Chapala-Tula fault zone. The border faults of the Acambay graben, Acambay-Tixmadej6 in the north and Pastores in the south, are separated in the west by stepovers from range-bounding faults of similar orientation, Epitacio Huerta in the north and Venta de Bravo in the south. The stepovers occur at the intersection of these faults with an older system of Basin and Range faults. An early-stage right-lateral component of motion along the Venta de Bravo and Pastores faults is inferred on a map scale froin a left-stepping en echelon array of normal fault segments. The divergence of the en echelon segments froin the general fault trend decreases gradually from west to east, suggesting that the early extension was rotational. The present relative displacement along the southern margin of the system, on the other hand, results in a left-lateral strike-slip component. This is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left-oblique slip. The striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component. This is supported on a map scale by the structure of the right stepover between the Acambay-Tixmadej6 and Epitacio Huerta

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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.

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