Experiments on basin inversion above reactivated domino faults

Abstract

Results are presented from scaled sand box experiments which investigate the structural inversion of half-graben basins generated by the contractional reactivation of extensional domino fault systems. The overburden was deformed above a series of rigid basement fault blocks that were extended in a domino manner. The models were constructed from either (1) sand with mica interlayers (anisotropic behaviour), (2) pure sand (isotropic behaviour), or (3) a sand-clay mixture (behaviour with increased competence). During the extensional phase of deformation, sand or alternating sand/mica layers were deposited in teh half-grabens associated with individual domino faults to simulate a syn-extensional stratigraphic sequence together with a component of overall subsidence. After termination of the extensional phase, sand layers were added to represent a post-rift passive subsidence stratigraphy. Subsequent contraction of the experimental configuration resulted in progressive reactivation of the domino fault system which was accompanied by back-rotation of both the footwall and hanging wall in each of the individual fault blocks. This produced null points along the profile section of the fault planes which separated net contraction above from net extension below. Continued shortening of the system led to further contraction being accommodated along footwall shortcut faults. It is suggested that these shortcuts developed in response to (1) the accommodation of space problems generated by increased bed length in the graben acquired during extension and (2) by more efficient low angle fault trajectories in keeping with Coulomb failure during horizontal to subhorizontal compression during inversion. In the anisotropic model, folding in the footwall partially suppressed the initiation of footwall shortcut faults. Footwall shortcut faulting did not occur in experiments where the dips of the original extensional faults were decreased before the inversion. In all inversion experiments the contractional deformation associated with the shortening of the half-graben was concentrated in the footwall to each domino basin-bounding fault. A control experiment in which an undeformed sand pack was deposited above an extended series of basement fault blocks revealed that the early extensional architecture formed in the other experiments had a profound influence on the resultant inversion geometries. The results show clear similarities to interpreted examples of natural basin inversion structures associated with reactivated domino fault systems, as well as to inferred strike-slip induced flower structures.