Structural and kinematics constraints on a recent plate boundary: The northern limit of the Southern Mexico Block

The first part of this work defines the Plio-Quaternary boundaries and kinematics of the Southern Mexico Block (SMB), a microplate uncoupled from North America along the Trans-Mexican Volcanic Belt (TMVB). Structural data combined with analysis of satellite images and seismic profiles show that a major left-lateral strike-slip fault affects the Veracruz Basin and post-5 Ma volcanic rocks of the Los Tuxtlas Volcanic Field. Low seismicity associated to faulted Quaternary markers such as alluvial fans, alluvial terraces and volcanoes argue for active faulting in this area. Plio-Quaternary left-lateral strike-slip faulting along the Veracruz area is important because it connects two important structural provinces: the left-lateral transtensive faulting that affects the central part of the TMVB and the left-lateral Strike-Slip Faults Province of Chiapas to the south. These three active deformation zones accommodate a couterclockwise rotation of the SMB with respect to the North American plate. The model we propose predicts a Plio-Quaternary rotation of 0.45°/Ma with a pole located at 24.2°N and 91.8°W. The second part of this work deals with the tectonic evolution of central Mexico. Structural data and paleomagnetic results for Early Oligocene volcanic units of the Mesa Central are used to investigate the tectonic activity along the N130° trending Tepehuanes-Zimapán fault system (TZFS). In San Luis Potosí and Guanajuato areas structural data and paleomagnetic mean directions that indicate ~10° counterclockwise rotations argue for a left-lateral transtensive motion along the TZFS. The Oligocene intra-arc deformation is similar to the present one that affects the TMVB. This suggests that the SMB could be as old as Early Oligocene. During Late Oligocene and Miocene the boundaries of this crustal block probably migrate jointly with the volcanic arc to reach their present configuration.