RICO, LAURA
Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712-1101

Abstract: Geometric and Kinematic Evolution of a Complete Detachment Fold in a Natural Cross-Section

The San Julian Uplift is located within the Sierra Madre Oriental, a Laramide-age thin-skinned fold and thrust belt with correlative stratigraphy and structural style to the Perdido and the Mississippi Fan foldbelts in the deep Gulf of Mexico. Tilting of a detachment fold train in the northeastern flank of the San Julian Uplift produced steeply plunging fold axes, and subsequent erosion created a natural cross section of entire folds, their basal detachment and older units. A fold in this train is used here to test kinematic and geometric implications of current detachment fold models, and to study the relations ip between regional scale folding and macroscopic and microscopic deformation mechanisms. Because kinematically distinct models of detachment fold evolution can produce the same final fold geometry, an analysis of the kinematics that dominate natural fold development is essential to validate these models.

A differential GPS study coupled with field mapping was used to determine the geometry of the fold. Timing and quantification of strain-accommodating mechanisms, such as veins, stylolites, calcite twins, and secondary folds and faults, are being used to constrain the kinematic evolution of the fold. Field observations have shown that vein development was highly localized, forming areas of strain concentration which do not necessarily coincide with present day curvature. This coupled with the small thickness of the evaporitic decollement layer in comparison with the fold amplitude, is best explained by a migrating hinge model.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas