VEINS AS PROGRESSIVE DEFORMATION MARKERS IN FOLDS OF THE MEXICAN FOLD-THRUST BELT, CENTRAL MEXICO

In the central part of the Mexican fold-thrust belt is exposed a sequence of thinly to medium bedded limestone, locally interbedded with chert and shale, which are an expression of the Cretaceous Zimapan Basin. This basin was shortened while being overthrust from west to east by the El Doctor Platform, during the Laramide orogeny. While the platform rocks formed km-scale, open folds, the basin rocks developed tight, recumbent, NE verging folds. Both the platform strata and the basin strata are cut by veins of different ages which record the detailed strain history of the rocks and show evidence of layer parallel shear.

Special attention was paid to the characterization of fold styles and strain distribution, because these are a potential source of information about the deformation history of rocks, the mechanical conditions during deformation, and the folding mechanisms themselves. In order to understand those mechanisms, we have used the veins as deformation markers and as indicators of progressive deformation, through a systematic analysis of kinematic and cross-cutting relationships.

There are several generations and styles of veins. There are those that developed in the early stages of fold development and that are parallel to the beds. Second and third generation veins were differentiated according to their position within the folds: one group of folded veins concentrated on the fore-limb, and another group of extended veins on the back-limb. Veins of all generations were formed by solution transfer and associated fracturing during fold development. The differences in orientation between folded vs. stretched veins allowed us to define the shortening and extension fields of finite strain. The asymmetry of the angular distribution of the folded and stretched structures suggests that components of both pure shear and simple shear contributed to the bulk deformation of the rock package.