The Fluid Crustal Layer and Its Implications for Continental Dynamics

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