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.
A model of continental deformation is thus proposed in which localized strain within the mantle lithosphere and solid lower crust is accompanied by regional failure of the fluid layer. Heterogeneously deforming solid upper crustal blocks ride buoyantly on the fluid layer, whose flow field is governed in large part by maintaining flotational equilibrium with solid upper crustal blocks, generally diverging from regions where deformation results in relative thickening of the upper crustal layer. The fluid layer may also accommodate uniform-sense simple shear, coupling the integrated upper crustal strain with discrete areas of increase or decrease of horizontal surface area at the top of the solid lower crust and mantle lithosphere. Intracrustal isostatic compensation during extensional or compressional orogenesis will be most likely to occur in areas of thick crust and high heat flow, as in the mid-Tertiary Basin and Range or the modern-day Tibetan Plateau and Altiplano regions, resulting in broadly distributed crustal deformation and plateau-like topography. Areas of low geotherm or relatively thin crust may tend to accommodate strain in a more localized fashion with isostatic compensation occurring only by asthenospheric flow, accompanied by the development of high local topographic relief. A transition from one regime to the other may be exemplified by the contrast between regions such as the Basin and Range and Afar triangle regions (fluid layer present) and the northern Red Sea (fluid layer absent). Because vertical strains of the upper and fluid crustal layers are complementary, intense localized thickening of the upper crust may be accompanied by strong thinning of the fluid layer, depending on the initial ratio of their thicknesses. Similarly, areas of strongest upper crustal extension may result in overall thickening of the fluid layer beneath them. Outflow of fluid from beneath areas of crustal thickening may serve to thicken crust beneath some cratonic forelands (e.g., the western Great Plains), or conversely inflow toward areas of strong upper crustal thinning may attenuate crust on the margins of rifts (e.g., the western Colorado Plateau).
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Wernicke, B. (1990). The Fluid Crustal Layer and Its Implications for Continental Dynamics. In: Salisbury, M.H., Fountain, D.M. (eds) Exposed Cross-Sections of the Continental Crust. NATO ASI Series, vol 317. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0675-4_21
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