Deep Seismic Reflections and the Deformational Mechanics of the Continental Lithosphere

Abstract

Deep seismic reflection profiling of the continents shows the existence of significant dipping reflectors within the upper lithosphere which are intepreted as major low angle faults. These low angle faults do not cut or offset the Moho; to date no unequivocal example of a low angle fault passing continuously from the surface through into the mantle has been observed. Instead these major low angle fault reflections either merge into or are obscured by the sub-horizontal reflections of the lower crust. Thermo-rheological models predict the existence of low-strength, low-viscosity layers within and at the base of the lower continental crust related to the vertical changes in composition and rheology between the quartzo-feldspathic crust and the peridotitic mantle, and the quartz dominated upper crust and the plagioclase dominated lower crust. The major low angle faults imaged by deep reflection seismology are interpreted to sole out into horizontal detachments within these low viscosity regions m the lower crust. Lithosphere deformation within these low viscosity regions is most probably achieved by anastomosing networks of sub-horizontal shear zones. Deep seismic reflection and refraction studies show that mountain root Moho topography created during continental collisional tectonics is absent for old mountain belts (e.g., Caledonides and Variscides). Thermo-rheological models suggest that the decay and annihilation of mountain root Moho topography can be achieved within 100 Ma by an outward ductile flow in the low viscosity channel of the lower crust and an inward ductile half space flow in the upper mantle. The ductile flow process within the lower crust and mantle, and surface erosion are isostatically coupled. Moho topography is also generated by crustal thinning during continental extensional tectonics and the formation of extensional sedimentary basins. Thermo-rheological models suggest that whether or not such basin anti-root Moho topography persists or decays by ductile flow depends critically on the lower crustal and upper mantle viscosity structure prior to extension which in turn depends on crustal thickness and geothermal gradient. If the crustal thickness was of average thickness or less (≤ 35 km) prior to extension, Moho topography is predicted to persist. In contrast, if the crust was substantially thickened prior to extension (≥ 45 km) then basin anti-root Moho topography will rapidly be annihilated by ductile flow. These model predictions are consistent with observation of extensional Moho topography from deep seismic reflection profiling in the North Sea basin and the Basin and Range, USA. The decay of Moho topography by ductile flow is predicted to be extremely wavelength dependent. Short and long wavelength components are predicted to persist through geological time with only intermediate wavelength components being annihilated by ductile flow.