Simple Andersonian faulting explains extension paradox and formation of asymmetry of conjugate non-volcanic margins (Invited)
Abstract
Sedimentary rift basins are created by thinning and progressively subsidence of continental lithosphere during the first phases of extension. If the extension process persists it may ultimately cause the break of the plate. The greatly thinned 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 lead from a moderately extended basin to a highly-stretched pair of conjugate margins are still greatly debated. An unresolved problem is the common report that crustal thinning (estimated from seismically-measured thickness) is greater than extension caused by the brittle faulting imaged on seismic records. A model 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 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 very appealing model involves whole-crust (or whole lithosphere) detachment faults that are active from early in the rifting. 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 (hangingwall) to a detachment, so that opposite dips are proposed for detachments that explain the same structures. A third group of models put forward that much brittle extension is unobserved because of seismic methods limitations, caused either by faulting that is below the seismic resolution, undetectable deformation along 100-km-scale detachments that appear as top of the basement, or structural complexity created by cross-cutting arrays of several generations of faults. We present a new model developed using pre-stack depth-migrated seismic images that have permitted to accurately measure extension on faults and compare it to thinning estimated from seismic measurements of crustal thickness. We have used the seismic observations to create an area-balanced kinematic model of extension during rifting that solves the paradox of the extension discrepancy by generating a fault-controlled crustal thinning that evolves from a rift basin to the asymmetric structure and extreme crustal thinning of conjugate pairs of rifted margins. Our observations support 1) that crustal thinning is to a first degree caused by simple Andersonian faulting, and 2) that the causal faults are unequivocally visible on seismic images.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T32C..05R
- Keywords:
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- 8105 TECTONOPHYSICS / Continental margins: divergent