The Fluid Crustal Layer and Its Implications for Continental Dynamics

Abstract

Extensional tectonism exposes relatively intact cross-sections of the pre-tectonic crust. The paleodepth of deep portions of the exposed crustal sections in strongly extended structural domains in the Basin and Range province constrains the amount of upper crustal thinning accommodated along extensional structures, which in many cases is in excess of 10 km. Regionally averaged topography in the province is generally lower in the strongly extended domains than in adjacent stable blocks in which the upper crust is not appreciably thinned. The difference in elevation between extended and unextended areas suggests that the differential thinning of the upper crust is probably not accommodated by inflow or outflow of asthenosphere, mantle lithosphere or mafic lower crust. Simple isostatic calculations suggest that the density of the compensating medium is probably within 100–200 kg/m3 of the density of average upper continental crust, indicating that it lies within the crust and may be in large part quartzose. This conclusion is independently supported by laboratory experiments on the strengths of rocks, which suggest that quartzose rocks are substantially weaker than mafic and ultramafic rocks over a broad range of temperatures likely to exist in the deep crust, and with reflection seismograms in deformed regions which suggest that the Moho is subhorizontal beneath areas with large gradients in upper crustal vertical strain. It is suggested that the upper crust floats on a quartzose layer in the mid-crust, which under orogenic conditions appears to behave as a relatively inviscid fluid at geologic timescales (>10,000 a) and subcontinental lengthscales (100–1000 km). A fourfold division of the orogenic lithosphere is therefore proposed: (1) The upper crust, which at geologic timescales has the properties of a solid and is able to support shear stresses of 10’s to 100’s of MPa; (2) a fluid crustal layer (fluid in the sense of the asthenosphere), which flows so as to eliminate horizontal gradients in vertical stress on geologic timescales; (3) a solid lower crust, generally mafic, that may be substantially stronger than the overlying fluid layer; and (4) the mantle part of the lithosphere, which is much stronger than the fluid layer. The thickness of the fluid layer may range from 0 to over 30 km, but is generally in the 15–25 km range.