The mechanics of continental extension and sedimentary basin formation: A simple-shear/pure-shear flexural cantilever model

Abstract

Abstract Deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins. Major basement faults are generally planar and extend down to 10–15 km depth corresponding to the base of the seismogenic layer. Beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle. A mathematical model is described of the geometric, thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle. During faulting, upper crustal footwall and hanging-wall blocks behave as two mutually self supporting flexural cantilevers; their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse. For extension on multiple faults, interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics. The flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing, horizontal displacement and polarity. The effects of fault spacing, fault polarity and the amount of extension, as well as erosion of footwall uplift on crustal structure and basin geometry during both syn-rift and post-rift stages of basin evolution are explored. Models of “passive” rifting, involving lithosphere extension in response to the build-up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere, appear to conform to the main geological observations. The role of “active” rifting, driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain.