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The Influence of Pre-Existing Thrust Faults on Normal-Fault Development in Two-Phase Experimental Models

Abstract

The border faults of many rift basins parallel pre-existing thrust faults and other low-angle structures. If the border fault is low-angle, then the resulting basin geometry and internal deformation may differ from that of a typical rift basin, potentially influencing the volume, maturity, and distribution of hydrocarbons within the basin. New experimental (analog) models test whether pre-existing thrust faults influence the amount and distribution of subsequent fault-related extension. Modeling materials are wet clay over silicone polymer. The polymer distributes deformation across the model and, at least, partially decouples the shallow deformation from the basal displacement discontinuity. Two-phase models of shortening followed by extension test the influence of pre-existing thrust faults on subsequent extension. Single-phase extension-only and shortening-only models serve as controls. In extension-only models, a border-fault zone with moderately dipping normal-fault segments forms near the displacement discontinuity and propagates upward, and a densely faulted rollover fold forms in the hanging wall. In shortening-only models, gently dipping thrust faults and backthrusts form in two narrow zones that emanate from the displacement discontinuity. The gently dipping thrust faults in the shortening-only model are offset farther from the underlying displacement discontinuity than the moderately dipping normal faults in the extension-only model. In the two-phase model, the border-fault zone consists of moderately dipping normal faults as in the extension-only model. The fault zone, however, is offset farther from the displacement discontinuity than that in the extension-only model. This likely results from partial reactivation of the low-angle thrust fault at depth, shifting surface faulting away from the displacement discontinuity. The rollover fold in the two-phase model is sparsely faulted compared to that in the extension-only model, likely because the reactivated, low-angle thrust fault can accommodate more extension than the moderately dipping normal fault in the extension-only model. The deformed zone is also wider and shallower in the two-phase model than in the extension-only model. These results match deformation in the Cenozoic rift zone of Thailand that parallels the structural trend of the Sumatra-Andaman subduction zone. Rift basins are wide and shallow, and their border faults are partially reactivated pre-existing low-angle structures.