MECHANISMS FOR THE FORMATION OF DOMAL AND BASINAL DETACHMENT FAULTS - A 3-DIMENSIONAL ANALYSIS

Detachment faults (i.e., regional low-angle normal faults) in metamorphic core complexes of the North American Cordillera commonly exhibit doubly plunging antiformal (domal) and synformal (basinal) geometries. These geometries have been previously attributed to the superposition of two processes: (1) the antiforms and synforms with axes parallel to the extension direction originated as primary fault undulations or corrugations developed coevally with fault slip, and (2) the antiforms and synforms with axes perpendicular to the extension direction were produced by isostatic uplift due to tectonic denudation. However, the coaxial relationship between undulations of detachment faults and the sedimentary beds and metamorphic foliations in both the upper and lower plates of some detachment fault systems in the U.S. Cordillera suggests that the synforms and antiforms with axes parallel to the extension direction may have formed as folds, and that the domal and basinal geometry of detachment faults may have formed synchronously by a single process. A three-dimensional, elastic thin plate model is developed to investigate possible mechanisms for the formation of domal and basinal detachment faults. This model explores the interactions among the vertical, horizontal, and basal shearing forces during the formation of warped detachment faults. The model considers the role of compression perpendicular to the extension direction and stress reduction parallel to the extension direction. The results of the model suggest that the stress reduction itself in the extension direction is insufficient to cause buckling of a thin elastic crust. Two mechanisms are most likely to explain the formation of domal and basinal geometries of detachment faults: (1) an upward pushing by undulatory crustal roots or buoyant synextensional plutons beneath the extensional belt, and (2) buckling caused by a compression perpendicular to the extension direction. Both mechanisms are consistent with the geologic constraints during the development of Cordilleran core complexes. In addition to isostatic uplift due to tectonic denudation, upward warping of detachment faults can also be caused by buckling, basal shearing, and buoyant forces.