The Acambay graben: Active intraarc extension in the trans‐Mexican volcanic belt, Mexico
Authors: Suter Max , Quintero‐Legorreta Odranoel , López‐Martinez Margarita , Aguirre‐Díaz Gerardo , Farrar Edward 
Published: February 2004

The trans‐Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench. The central part of the belt is being deformed by the Chapala‐Tula fault zone, an approximately 450‐km‐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 deformation province of the Mexican Basin and Range. The Acambay graben, about 40 km long and 15 km wide, is located approximately 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‐Tixmadejé 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 from a left‐stepping en echelon array of normal fault segments. The divergence of the en echelon segments from 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‐Tixmadejé and Epitacio Huerta faults, which shows no evidence of local extension or shortening. The divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation. This could explain why no active extension has been observed to the east of the Chapala‐Tula fault zone, in the eastern part of the trans‐Mexican volcanic belt. During the Ms = 6.9 Acambay earthquake of November 19, 1912, surface rupture occurred along both margins of the graben at the base of multiple‐event scarps. Along the Acambay‐Tixmadejé fault, the coseismic rupture is 41 km long. The vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum. Furthermore, the change in the vertical surface offset along the fault is approximately proportional to the change in height of the Acambay‐Tixmadejé multiple‐event fault scarp. The easternmost part of the ground rupture passes through a plain and not at the base of a multiple‐event scarp as farther west. It may therefore correspond partly to an increase in length of the Acambay‐Tixmadejé fault during the 1912 earthquake. The slip rate along the southern border of the Acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40Ar/39Ar age of 0.4±0.1 Ma. This basalt may be displaced up to 15 m by the Pastores fault, which indicates a middle‐late Quaternary slip rate of ≤0.04 mm/yr. Furthermore, based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake, we estimate a recurrence interval of ≥5000 years for major earthquakes along the faults of the Acambay graben.