Definition of the stress-deformation field and seismotectonics of the Morelia-Acambay fault system, Mexico

Abstract

The Morelia-Acambay Fault System (MAFS) is located in the central-western part of the Trans-Mexican Volcanic Belt (TMVB). The MAFS is a 30 km wide-transtensional shear zone made up of E-W and ENE-WSW oriented faults. Since middle Miocene (7–9 Ma), the seismotectonics of this fault system has controlled the evolution of E-W tectonic depressions (Zacapu, Cuitzeo, Morelia, and Acambay), and the lacustrine environments. This paper describes the spatial variation of the state of recent deformation through the structural analysis of fault populations measured in the field and the definition of the regional stress regime from the analysis of the focal mechanisms of shallow intraplate earthquakes.

Kinematic data indicate that the MAFS formed under a maximum NE-SW oriented horizontal compression (Ey). Today, the NE-SW faults act as left-lateral oblique slip faults, the main W-E faults as left lateral normal faults; and the NE-SW faults are displayed in en échelon setting. A variation in the direction of Ey is deduced from our data: Ey in fact remains NE-SW oriented in the north-western part of the MAFS, whereas it rotates toward the NNE-SSW and E-W directions in the western (Pázcuaro-Zacapu-Morelia-Cuitzeo-Acámbaro) and eastern (Maravatio-Acambay) MAFS sectors, respectively.

Data integration indicates that the main stress axes trajectories remained constant since the Pliocene. Some of the faults of MAFS are presently active, as demonstrated by the distribution of historical epicenters (1858, 1912, 1979 and 2007). These results are of primary relevance for future seismic risk studies in the region.

The most reasonable explanation to account for the transtensional regime occurring in the central part of the TMVB is the oblique convergence between the lithospheric plates and, in particular, the partition of the subduction angle in the northern part of the Mesoamerican subduxiton zone.