Mechanical analysis of continental rifting and orogenic collapse

Abstract

Extension of the continental lithosphere occurs during orogenic collapse, rifting of continents, and development of back arcs, and is therefore a critical process during the evolution of tectonic plates. The accommodation of extension is a function of mechanical coupling within the continental lithosphere, and research presented here employs 2D numerical modeling to explore extension-driven dynamic interactions that occur between layers of differing rheological properties. Extension rate exerts a strong control on mechanical coupling between brittle faulting and flow within the deep crust, and therefore on the localisation of deformation during extension. This thesis presents 2D numerical experiments with a 100 km thick lithosphere (normal thickness continental crust, 40 km) in which the extension rate (1 to 2 cm/yr), viscosity of the deep crust, and density of the rift basin fill are systematically varied. Experiments under slow extension (1 cm/yr) exhibit homogeneous strain across the lithosphere for 20-25 My followed by rapid localization of deformation into a rifting instability, whereas experiments under fast extension (2 cm yr-1) exhibit development of a rifting instability within 2 My. These results capture a critical transition from instantaneous rifting to prolonged thinning under lithospheric extension. Experimental results illustrate that the viscosity of the deep crust will dramatically impact mechanical coupling within lithosphere, thereby influencing formation of passive margins and exhumation of near-Moho material within gneiss domes. Numerical experiments address the role of the deep crust by systematically varying the viscosity and density of the deep crust in continental lithosphere under extension (40 km and 60 km crust). The low viscosity deep crust end member produces significant exhumation by flow into a metamorphic core complex, while the high viscosity deep crust end member exhibits formation of lithospheric-scale shear zones and minimal deep crustal flow. In order to further explore these results, pressure-temperature-time-deformation paths along cross sections in a subset of experiments (60 km crust), which illustrate that convergent flow within channels at near-Moho depths is not restricted to systems with low viscosity deep crust. Results indicated that the viscosity of the deep crust is a primary control on the accomodation of extension, and highlight a dynamic feedback between deep, ductile deformation and shallow, brittle deformation during core complex development. Finally, I address the competition between vertical mass transport as a result of (1) deposition of sediments and (2) exhumation of the deep crust. Numerical experiments in which deep crust viscosity and basin infill density are systematically varied illustrate that this competition can strongly impact the tempo of deformation localisation in extended continental lithosphere. Experiments exhibit approximately similar timing and extent of deformation localization between sediment-present and sediment-absent experiments. For a basin infill that is lighter then the deep crust, experiments with a strong deep crust exhibit distributed extension in the shallow crust and burial of the exhumed deep crust under basins, and experiments with a weak deep crust exhibit localized deformation and efficient exhumation of the deep crust to the near-surface with limited basin deposition. However, experiments with a density inversion exhibit a gravitational instability with rapid sinking of the basin fill; this instability is accompanied by exhumation of deep crust on the sides of the basin. If the viscosity of the deep crust is sufficiently low, a drip of cool, dense basin infill ponds at the Moho, dramatically impacting the geotherm at the center of the experiment.