Lithospheric extension, deep crustal flow, and basin development
Abstract
Geophysical measurements and seismic imaging in zones of lithospheric extension commonly show increased heat flow and a history of faulting and sedimentary deposition in rift basins. It is increasingly recognized that the deep, ductile continental crust also influences the evolution of rifts, in some cases by providing a weak layer that accommodates faulting in the shallow crust and thereby controls the location of basin depo-centers and rates of subsidence. In cases in which the deep crust is particularly weak, it flows laterally and competes with basin deposition. A suite of 2D numerical models explores how deep crust viscosity and sediment density influence the structures of rifts and deposition in basins. The model (Underworld 2D) is 360 km wide and 160 km thick and includes frictional shallow crust and viscous deep crust; the lithosphere is 100 km thick, and rheologies for crust and mantle are based on experimental flow laws.
Two cases of crustal thickness (60 km orogenic crust, and 40 km crust), and two divergence rates (1 cm/yr and 2 cm/yr) are considered. Results show that lithospheric extension in the case of 60 km crust generates efficient upward flow of the deep crust and, as a result, suppression of basin deposition; deep crust viscosity exerts a major control on the evolution of extension. This is consistent with the observation that metamorphic core complexes are typically overlain by thin supra-detachment basins. Extension of 40 km thick crust at 2 cm/yr displays a transition from rifting to drifting over ~8-12 Myr depending on deep crust viscosity and basin-fill density. Decreasing the divergence rate to 1 cm/yr results in a very different rifting style, including a long phase (~20-30 Myr) of distributed faulting and basin deposition, followed by a rapid transition (<10 Myr) from rifting to drifting. Rapid rifting begins with necking of the lithospheric mantle and is followed by complete attenuation of the continental crust. Crustal thinning is more or less rapid depending again on deep crust viscosity and basin density. This rifting instability is a result of the model internal evolution and can develop rapidly even in the case of slow plate divergence.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T33F0431T
- Keywords:
-
- 8105 Continental margins: divergent;
- TECTONOPHYSICS;
- 8109 Continental tectonics: extensional;
- TECTONOPHYSICS;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS