Quaternary intra-arc extension in the central Trans-Mexican volcanic belt

Abstract

The Trans-Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc-parallel faulting. Major intra-arc basins within the central part of the belt (between long 998W and 1028W) are (from west to east) the Cuitzeo and Acambay grabens, the Aljibes half-graben, and the Mezquital graben. In this region, ;100 eastwest2striking, .2 km long, steeply dipping normal faults, expressed by pronounced multi-event fault scarps, have been mapped. Of the 100 faults, ;65 displace rocks of known Quaternary age (younger than 1.6 Ma), and 22 cut rocks with a documented age younger than 750 ka. Known historical surface ruptures are limited to faults of the Acambay graben. Overall, the faults cause north-south to north-northwest2south-southeast2oriented extension of ,1 km, which is distributed over a 30250-km-wide zone that has 5210 faults in cross section and a relative extension of ,3%. Quaternary vertical slip rates, estimated for 13 of the faults, have a mean of 0.07 mm/yr. They are highest at the southern margin of the Cuitzeo graben, the northern margin of the Acambay graben, and in the Aljibes half-graben, where they measure between 0.16 and 0.18 mm/yr. The north-south to north-northwest2south-southeast2oriented Quaternary bulk extension rate of the system is likely to be 0.2 6 0.05 mm/yr. This fault system is in an initial stage of coalescence. Its western and eastern parts consist mostly of isolated fault segments, whereas in the central part deformation is localized onto a few through-going faults. The longest of these are the Venta de Bravo fault (45 km) and the Acambay-Tixmadeje fault (34 km). The short fault traces have a simple structure, whereas the longer ones are commonly composed of two or more segments. There is no obvious migratory pattern of Quaternary fault activity, which suggests that the entire region is tectonically active.