Continental Lower-crustal Flow: Channel Flow and Laminar Flow

Abstract

Numerous geological, geophysical and geochemical investigations and finite element modeling indicate that crustal flow layers exist in the continental crust. Both channel flow model and laminar flow model have been created to explain the flow laws and flow mechanisms. As revealed by the channel flow model, a low-viscosity channel in middle to lower crust in orogen or plateau with thick crust and high elevation would flow outward from mountain root in response to lateral pressure gradient resulted from topographic loading or to denudation. However, according to the laminar flow model proposed based on investigation of the Qinghai-Tibet plateau, circulative movement of crustal lithologies with different rheological properties between basin and orogen would occur, under the driving forces resulted from dehydration and melting of subduction plate on active continental margin and from thermal energy related to upwelling and diapiring of intercontinental mantle plume or its gravitational interactions. Similarly, when driven by gravity, the softened or melted substances of the lower crust in a basin would flow laterally toward adjacent mountain root, which would result in a thinned basin crust and a thickened orogenic crust. Partially melted magma within the thickened orogenic lower crust would cause vertical movement of metamorphic rocks of lower to middle crust due to density inversion, and the vertical main stress induced by thermal underplating of lower crust would in turn lead to formation of metamorphic core complexes and low-angle detachment fault systems. Lateral spreading of uplifting mountain due to gravitation potential would result in thrust fault systems on the border between mountain and basin. Meanwhile, detritus produced synchronously by intense erosion of uplifting mountain would be transported and deposited along the marginal deep depression in the foreland basin dragged by lower crust flow. Channel flow is similar to laminar flow in a variety of aspects, covering continental intraplate deformation, ductile extrusion of middle to lower crust, synchronous transition from orogenic extrusion to extension, ductile deformation and exhumation of deep metamorphic rocks, partial melting, magmatism, etc. However, radical differences exist between the two models, such as in tectonic setting, flow domain, flow surface, flow scale, flow pattern, flow regime, flow direction, flow substance, flow behavior or flow effects, flow time, and flow mechanism. Channel flow can be regarded as part of a spatial-temporal structure of laminar flow, but lower crustal laminar flow is actually driven by thermal energy and gravity and not by surface processes such as denudation or topographic loading. Therefore, from a global viewpoint, laminar flow is only a small part of multi-grade or multi-scale circulative flow system of the earth.