New Mexico Geological Society Annual Spring Meeting — Abstracts

Allochtonous sheets involving Permian rocks of the Yeso, Glorita, and San Andres Formations are found at a number of locations along the east side of the Socorro constriction in the central Rio Grande rift. These rocks, which are commonly observed to overlie autochthonous(?) Triassic and Cretaceous sediments, also display decollement structures at several stratigraphic leveIs within the Permian section itself. On the basis of structural feature and juxtaposed stratigraphy at least three Ievels of allochthons can be recognized. Insight into problems concerned with the direction of emplacement and the mechanics of deformation characteristic of these allochthons is obtained by strain analysis of various kinematic indicators.

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 competent units, such as limestones and sandstones. interbedded with relatively incompetent lithologies, such as evapdrites and shales. The compression, or extension, of such a medium will lead to stress and strain distributions which are dictated by the properties of the layers. Analysis of the deformational features preserved in the rocks 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 overall pattern suggests that the translation of the allochthonous rocks is best described by gravitational detachment along horizons of pronounced viscosity contrast. Preliminary results indicate the source of the allochthons were uplifts located northeast to southeast of the study area.