DETAILED STRUCTURAL GEOLOGY OF THE MESOZOIC ROCKS OF THE VALLE DE BRAVO AREA (MEXICO STATE), CENTRAL MEXICO

In the Valle de Bravo Area, State of Mexico, is exposed a sequence of metasedimentary rocks of Lower Cretaceous age formed by limestone, calcareous phyllite, black slate, and sericite-chlorite phyllite, which have been intruded by andesitic to dacitic dikes and sills. To the SW these rocks are interlayered with basaltic pillow lavas. The metasedimentary rocks have been assigned to different tectonostrastigraphic units by various authors. Elias et al. (2000) placed these rocks in the Arcelia-Palmar Chico subterrane, while Salinas (1993) considered them to be part of the Teloloapan subterrane of the Guerrero terrane. On the other hand, Freydier et al. (2000) have proposed that these rocks are part of the Arperos basin sequence, which represents the suture between the Guerrero and Sierra Madre terranes.

We have carried out a detailed structural analysis of these rocks based on 1:50000 to 1:10000 scale field mapping, as well as outcrop and thin-section study. Based upon this work we have recognized two phases of folding, later affected by faulting. The first deformation (D1) is represented by a well developed foliation (S1) defined by the alignment of sericite flakes and insoluble residues along planes parallel to the axial surfaces of isoclinal folds of all scales. S1 is subhorizontal or slightly dipping toward the NW. A second deformation (D2) is ubiquitous in the Valle de Bravo and surrounding areas and is related to a subhorizontal shear zone with a vergence to the SW. D2 is characterized by the development of a spaced crenulation cleavage (S2) which is axial planar to asymmetric folds whose short limbs are frequently truncated by inverse faults. SW vergent S-C fabrics are also observed on micro and meso scales. The S fabric is subparallel to S2, dipping 60-80° toward 060-070°, while the C planes parallel the inverse faults which dip 10-25° toward 045-055°. Isoclinal folds associated with D1 are folded by D2.

The metasedimentary sequence is bounded to the W by a NS-striking, E-vergent thrust fault which places volcanic and volcanosedimentary rocks of the Arcelia-Palmar Chico subterrane on top. This thrust, and associated minor structures, is apparently younger than D2, which suggests that it could be associated with a third deformational event.