Rifting and the formation of conjugate pairs of continental margins

Abstract

Sedimentary rift basins are formed by thinning and progressively subsidence of continental lithosphere during the first phases of extension. If the extension process persists it will finally cause the break of the plate. The greatly thinned continental lithosphere, split in two plates, subsides deep below sea level to form a conjugate pair of rifted continental margins. Despite the fact that rift basins and conjugate margins are ubiquitous structures, the deformation processes that control the deformation from a moderately extended basin to a highly-stretched pair of conjugate margins are still greatly debated. A particularly disputed and unresolved issue is the common report that crustal thinning from the continental shelf to the deep portions of the margin (estimated from seismically-measured crustal thickness) is greater than extension caused by the brittle faulting imaged on seismic records. Traditionally the hypotheses to explain this paradox in fundamental observations fall in three main groups of models. A first model put forward to solve this extension discrepancy advocates the differential stretching of an upper brittle and lower ductile crustal layers, but it does not readily explain the archetypal asymmetric structure of conjugate pairs of rifted margins. Cross sections of conjugate margins show that one margin exhibits gradual crustal thinning that is accompanied by large faults, and the conjugate exhibits abrupt crustal thinning, yet extensional faulting is comparatively less important. A second model involves whole-crust (or whole lithosphere) detachment faults that are active from early in the rifting to full break up. This structure could create both the amount of thinning and margin’s asymmetry, but it has been shown that such a faults are mechanically problematical, particularly when invoked from early in the rift. In addition, the extension discrepancy happens at both conjugate margins, which has given raise to the paradox that both margins seem upper plate (i.e. the hanging-wall) to a detachment. This observation has lead to hypothesize alternatively detachments with opposite dips that are capable to explain the same structures. A third group of models proposes that much of the brittle extension that has occurred is unobserved because of limitations of the seismic methods, caused either by faulting that is below the seismic resolution, undetectable deformation along 100-km-scale detachments that appear as top of the basement and have no syn-tectonic sediment deposits, or to structural complexity created by cross-cutting arrays of several generations of faults. In this seminar we will discuss the virtues and limitations of those models and interrogate a recently proposed new alternative model. This new model has been developed using depth seismic images that have permitted to accurately measure extension on faults and compare it to thinning estimated from seismic measurements of crustal thickness. The new model under discussion has implications that might help explain some of the major currently debated issues: 1) Is crustal-scale thinning is to a first degree caused by simple Andersonian faulting, as earthquakes world-wide seem to support? 2) Are modern seismic methods are capable of unequivocally image the causal faults that control rifting? 3) Is Conjugate margin asymmetry intrinsic to continental extension and strain localization? 4) Are Syn-rift sediment abundance and distribution controlled by the temporal evolution of faulting?