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Analogue models of inversion tectonics

Summary Inversion of dip-slip fault systems by horizontal recompression has been modelled using sand, sand-mica and sand-clay analogues. Three extension-inversion systems have been tested: uniform basal detachment, simple listric detachment and ramp/flat detachments. Extension above a uniform basal detachment produces a domino or conjugate array of near planar faults. Upon inversion only faults at angles < 60° to the layering and suitably oriented with respect to the compression direction undergo reactivation. With increased contractional strains all faults are rotated to high angles and reactivation ceases. Shortcut faults may develop in the footwall of steeply dipping faults. Extension above a simple listric detachment fault produces a characteristic rollover anticline and associated crestal collapse graben structure. Upon inversion asymmetric uplift is associated with the major detachment. For models with no competency contrast the rollover anticline is accentuated. The crestal collapse graben faults are reactivated and the graben tightens producing a ‘flower-like’ fault system. Reactivation of synthetic faults ceases when they become rotated into angles too steep for contractional deformation. Extension of ramp/flat detachment systems produces characteristic rollover anticlines and crestal collapse graben systems above each concave-up segment of the listric fault system. It also produces a hanging wall syncline associated with the convex-up segment of the detachment. Inversion results in asymmetric uplift at the emergence of the reactivated detachment and above the convex-up portion of the detachment surface. The crestal collapse graben faults become reactivated and rotated into steep angles resulting in flower-like geometries. The rollover anticline is accentuated resulting in nappe-like structures. The results of these analogue studies of inversion structures indicate that considerable further research is needed to understand the geometries and kinematics of inversion fault systems.