Why is the Central Trans-Mexican Volcanic Belt (102°–99°W) in transtensive deformation?

Abstract

Using well-defined focal mechanisms, new poles of rotation, and accurate bathymetry, it will be shown that the N34°E oblique convergence between the Cocos and North America plates, along the N290°E Middle American Trench, in front of Central Mexico (102°–99°W), is associated with slip partitioning. In the same way, it will be shown that the forearc sliver will move rigidly eastward along a trench parallel left-lateral strike-slip fault. By reviewing the literature and analysing seismicity, the E–W Central Trans-Mexican Volcanic Belt (TMVB) is the only active zone in Central Mexico. It is the best candidate to accommodate slip partitioning at the trench where mainly extensive E–W-trending normal active faults with left-lateral component are reported. Fault striae inversions, derived from the literature and our focal mechanisms inversion, confirm that the state of stress in the Central TMVB is transtensive since Middle Quaternary with Shmin ( σ 3) trending NW–SE and Shmax ( σ 1 or σ 2) trending NE–SW. The oblique convergence and slip partitioning at the trench induce this state of stress. The N290°W trench-parallel component, which should be accommodated in the upper plate, is, however, not parallel to the E–W arc-parallel component, but differs by about 20° clockwise. This implies that the former must be partitioned along the E–W faults of the Central TMVB. If we assume that the trench-parallel slip rate of 8 mm year −1 (102°–99°W) is fully accommodated along the Central TMVB, we should expect maximum horizontal extensional and left-lateral slip rates along E–W normal faults of 2 mm year −1 and less than 7 mm year −1, respectively. Strain partitioning may also be accommodated partially by the accretionary wedge or/and by normal faulting that affects the deeper part of the upper plate around 18°N of latitude.