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

The Trans-Mexican volcanic belt is an active volcanic are related to subduc tion along the Middle America trench and characterized by shallow seismicit y and synvolcanic to postvolcanic extensional are-parallel faulting. Major intra-are basins within the central part of the belt (between long 99 degre esW and 102 degreesW) are (from west to east) the Cuitzeo and Acambay grabe ns, the Aljibes half-graben, and the Mezquital graben, In this region, simi lar to 100 east-west-striking, >2 km long, steeply dipping normal faults, e xpressed by pronounced multi-event fault scarps, have been mapped, Of the 1 00 faults, similar to 65 displace rocks of known Quaternary age (younger th an 1.6 Ma), and 22 cut rocks with a documented age younger than 750 ha, Kno wn historical surface ruptures are limited to faults of the Acambay graben, Overall, the faults cause north-south to north-northwest-south-southeast-o riented extension of <1 km, which is distributed over a 30-50-km-wide zone that has 5-10 faults in cross section and a relative extension of <3%. Quat ernary 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-no rthwest-south-southeast-oriented Quaternary bulk extension rate of the syst em is likely to be 0.2 +/- 0.05 mm/yr, This fault system is in an initial s tage of coalescence. Its western and eastern parts consist mostly of isolat ed fault segments, whereas in the central part deformation is localized ont o a few through-going faults. The longest of these are the Venta de Brave f ault (45 km) and the Acambay-Tixmadeje fault (34 km), The short fault trace s 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 activ e.