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Analysis of depths and focal mechanisms of 16 small earthquakes in Central Mexico, along with previous data, shows that (a) the subducted Cocos plate becomes subhorizontal between 110 and 275 km from the trench reaching a depth of about 50 km, and (b) the bottom part of the overriding continental plate is in tensional stress regime. Neotectonic structures in Central Mexico and stress orientations (estimated from borehole elongations, cinder cone alignments, and fault‐slip analysis) in the Trans Mexican Volcanic Belt, which lies in the northeastern portion of the area under study, also indicate the same stress regime. Absolute motion of the North American plate in the region has a component normal to the trench of 20 mm/yr which, if true, would result in compressional stress in the upper plate if the trench position is fixed in an absolute frame of reference. If we allow for seaward retreat of the trench then the tensional stress in the overriding plate and the observed geometry of the Benioff zone can be explained. This, however, implies that seaward retreat of a trench is possible even for a young (∼15 m.y. old) subducting slab. Alternatively, tectonic erosion of the leading edge of the continent, which has been proposed to explain truncated igneous and metamorphic continental terrane, could give rise to the tensional stress but would not explain the geometry of the Benioff zone.
Singh et al. (Fri,) studied this question.