New Mexico Geological Society Annual Spring Meeting — Abstracts

Allochthonous Permian rocks of the Yeso, Glorieta, and San Andres Formations are present at a number of locations along the east side of the Socorro constriction in the central Rio Grande rift. These rocks overlie autochthonous Triassic and Cretaceous sediments (lower Chnle through lower Tres Hermanos) along low-angle faults, and also display decollement structures at several stratigraphic levels within the Permian section itself. Insight into problems concerned with the direction(s) of emplacement, and the mechanics and timing of deformation is obtained by detailed structural mapping and strain analysis of various kinematic indicators and structural features.

Marked differences in the mechanical properties associated with various members of the stratigraphic section have been instrumental in the deformation. The Permian section is essentially a series of thick, competent units (limestones and sandstones), which overlie an interval of relatively incompetent lithologies (evaporites and weak sandstones). Structural features indicate that a sheet comprised of these superincumbent, competent rocks has become detached from the incompetent lower section, and has been thrust over the Mesozoic sediments. Analysis of the deformation shows that a vertical strain profile through a typical allochthon can be characterized as extensional in the upper portions and compressional in the lower regions. This distinctive pattern suggests that the translation of the allochthonous rocks is best described by gravitational detachment along horizons of pronounced competency contrast.

Additional support for this conclusion is obtained from rock-mechanical experiments preformed on the lithologies critical in the deformation. The resultant data on the rheological properties of the rocks have been used to construct Mohr analyses which explain the main structural characteristics observed in the deformation.

Results indicate that the source of these allochthons were uplifts located in the region northeast to southeast of the study area. Based on structural, stratigraphic, and intrusional relationships, the deformation is believed to have taken place during the Paleocene Epoch.