Continental rifting: Progress and outlook



Increased research activity on continental rifts has led to a flood of new data in the last 15 years, but there is little consensus about the basic mechanisms and causes of rifting. The well-known association of taphrogenic features—doming, faulting, and volcanism—is widely regarded as the result of asthenosphere convection and lithosphere thinning. This concept is supported by observations of long-wavelength negative Bouguer anomalies, and of subnormal Pn velocities in the upper mantle, resulting from convective mass and heat transfer into the base of the lithosphere. Doming, and in some instances faulting, is the result of isostatic adjustment to the resulting expansion. The crust of rifts is abnormally thin (70–80% of normal) and contains high-density rocks assumed to be primarily intrusive basic igneous rocks. The collapse of rift lloors may be the consequence of stress-induced lateral extension in the upper crust, possibly assisted by a mass excess of mafic intrusions in the rifted crust. Anomalous electrical conductivity (low) and heat flow (high) are the result of heating and magmatic and hydrothermal activity.