THE QUERETARO GRABEN: BASIN AND RANGE FAULTING IN THE CENTRAL-NORTHERN PORTION OF THE MEXICAN VOLCANIC BELT

NNW normal faults intersect with ENE normal faults at the vicinity of Queretaro City, in central Mexico. This intersection produced an orthogonal arrangement of grabens, semi-grabens and horsts that include the Querétaro graben. The NNW faults (NSF) are part of the Taxco-San Miguel de Allende fault system, which is proposed as part of the southernmost Basin and Range province in Mexico. The ENE to EW faults (EWF), are part of the E-W oriented Chapala-Tula fault system, which has been interpreted as an active intra-arc fault system of the Mexican Volcanic Belt. Seventy four normal faults were mapped, from which the largest and with the best morphological expression in the region are the NSF. More numerous, although shorter, are the EWF. Cross cutting relationships between these two fault sets at this region indicate that some faults of the NSF are younger than the EWF. This indicates that the NSF at Queretaro City and vicinity had moved recently as they displaced faults of the relatively young intra-arc fault system. This is confirmed by a seismic sequence of small events (Mw < 3.4) that occurred in January and February of 1998 associated to a NNE normal fault at Sanfandila, a locality 20 km to the SE of Queretaro City, as well as by isoseismal maps of recent earthquakes in nearby regions to the east. Most Basin and Range faults in Mexico are considered old faults, some dated back to the Eocene, but there are a few reports of Basin and Range faults that have moved in the Quaternary or even in historic times. These faults, including the NSF of the Queretaro region may be considered as reactivated Basin and Range faults. This seems possible, since the Basin and Range extensional regime is still active in central-northern Mexico