Characteristics and consequences of flow in the lower crust

In some places, there is strong evidence that the lower continental crust h as flowed so as to smooth out variations in crustal thickness caused by dif ferential crustal extension or shortening. In order to better understand th e processes involved, we investigate the behavior of a fluid layer over a f luid half-space to see how such a system responds to the deformation of its upper and lower boundaries. This simple system can be used to Study both t he decay of crustal thickness contrasts and the behavior of a thin lithosph eric sheet. The changing response of the system to variations in density an d viscosity contrasts and to different boundary conditions imposed on the f luid interface can easily be studied analytically. The most important resul ts are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lat eral variations in crustal thickness cause the fluid to develop a steep fro nt, which may cause a topographic step above it at the Earth's surface. Def ormation within the channel will be principally by simple shear. The clear association of lower crustal flow with regions of thickened crust and magma tic;activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur. The smoothing of crustal thickness cont rasts leads to differential vertical motions, and is thus a method by which substantial tilting can occur without faulting. This differential uplift m ay be responsible for rotating and exhuming some of the detachment faults i n metamorphic core complexes in the Basin and Range province of the western United States. It is also a method of causing structural inversion in basi ns that does not require the reactivation of normal faults as thrusts or re verse faults.