THREE-DIMENSIONAL GEOMETRY AND MECHANICAL STRATIGRAPHY OF A PLUNGING DETACHMENT FOLD IN THE MONTERREY SALIENT, NE MEXICO

We used 1:20,000 scale geologic mapping, digital elevation data, aerial photos, and a series of 2-D balanced cross sections to construct the 3-D structural geometry of a westward-plunging detachment fold complex in the Sierra Madre Oriental near Saltillo, Mexico. The fold complex is the northwesternmost structure of the Monterrey Salient, and it is bounded by the Parras Basin on its northern and western sides. Structural analysis of a 200 square km area suggests that the detachment horizon beneath the fold complex tapers to the north and west causing the abrupt loss of structural elevation and contrasting structural styles between the mountain front and the adjacent basin.

The different structural geometries are related to different mechanical stratigraphy units. The lower mechanical stratigraphy unit shows a fold complex trending NWW-SEE that detaches in the evaporitic Minas Viejas Formation. Folds in this lower unit gradually change geometry from a double-headed cylindrical box-fold in the east to a single north-verging west-plunging fold in the west. Geometry of the lower unit is complicated by hinterland inflation that causes 1000 m excess above the regional elevation of the back limb of the fold complex. Inflation of the back limb can be caused by migration of evaporite from the hinterland. The upper mechanical stratigraphy unit presents south-verging fault-propagation folds trending EW that detach in the Parras Formation. Folds in this upper unit are wider in the west and they tighten and plunge to the east. Coincident with these changes in geometry, we determined that shortening varies gradually from 40 % in the east of the area to 19 % in the west. The changes in shortening, the contrasting trends and plunge directions between folds in the upper and the lower mechanical stratigraphy units show us a transference in displacement from the lower to the upper kinematic unit caused for the tapering to the northwest of the Minas Viejas Formation.