Stretching and thinning factors viewed through numerical models of continental extension and rifting

Abstract:

Rifted continental margins share a transition from a thickened crust to thinned transitional crust that can span either modest (i.e. 10’s of km) to large (i.e. 100’s of km) distances. A longstanding question surrounding rifted margins concerns how the fault-restored extension in the upper crust from fault-heave measurements does not match that expected from crustal thinning observed in seismic data or thinning factors calculated using uniform pure-shear subsidence models, the so-called “extension discrepancy”. Here, we revisit this issue at the lithosphere scale drawing on recently completed numerical results. We extract thinning profiles from four end-member geodynamic models with varying width and asymmetry, and propose tectonic models that best explain those results. Moreover, we relate the spatial and temporal evolution of crustal thinning to mantle thinning, something difficult to achieve only by using observations due to the lack of geologic relationships that constrain thinning at different stages between continental rift and lithospheric breakup. Our results support the hypothesis that the upper to lower crust extension discrepancy is overestimated because of the difficulty to recognize both polyphase sequential faulting, and detachment faults in seismic data. But, more importantly, the results support that depth-dependent stretching is likely to predominate at specific stages of rift evolution because crustal and mantle thinning distributions are not always spatially coincident, and at times are not even balanced by an equal magnitude of thinning in two-dimensions. Thus, depth-dependent stretching is an important controlling factor on rifted margins evolution, but not in the manner historically proposed, and its implications in subsidence and thermal histories of margins will likely require integrating results of geodynamic models into kinematic subsidence and heat flow studies.