Influence of mechanical stratigraphy and initial stress state on the formation of two fault propagation folds

摘要

Kinematic and mechanical modeling of the Rip Van Winkle (SE New York, USA) and La Zeta (SW Mendoza, Argentina) anticlines illustrate the influence of mechanical stratigraphy and initial stress state on the kinematics of fault propagation folding. In both anticlines, faults nucleating at distinct stratigraphic levels open upward into triangular zones of folding. Folding intensity and finite strain attenuates with distance from the fault tip. Trishear reproduces the bulk geometry and finite strain of the relatively homogeneous limestone sequence of the Rip Van Winkle anticline and predicts an initial location of the fault tip consistent with the field observations. Folding of the heterogeneous sedimentary section of La Zeta anticline, however, cannot be simulated by the trishear model alone. An additional mode of deformation involving transport of material from the backlimb into the hinge area and extension parallel to the direction of fault propagation is necessary to reproduce the geometry and finite strain of the anticline. Mechanical, distinct element modeling (DEM) of the anticlines indicates that their contrasting kinematics could have resulted from differences in mechanical stratigraphy and initial stress state. Folding of a homogenous, normally consolidated assemblage (initial horizontal to vertical stress ratio, K_o=1) is trishear like and resembles the Rip Van Winkle anticline. Folding of a heterogenous (layered such as La Zeta), over-consolidated assemblage (K_o > 1) departs from the trishear model and resembles La Zeta anticline. Based on the DEM simulations, we postulate that the Rip Van Winkle anticline formed at high depths (high overburden loads and lithostatic stress conditions), and that La Zeta anticline formed at shallow depths, after substantial uplift and erosion of the Andean mountain front (which induced over-consolidation and high K_o).