Mechanical aspects of continental rifting

Abstract

Continental rifting is associated with characteristic processes such as crustal extension and vertical movements observed on the surface, and deep-seated phenomena such as lithospheric thinning and upwelling of mantle material. These characteristics are expressed in a variety of geophysical anomalies; increased heat flow, low long wavelength Bouguer gravity with relative gravity high along the rift, delayed body-wave travel times, as well as by volcanic activity with various depth of magma source. We discuss the concepts of rifting by either lateral or vertical transmission of mechanical energy on the basis of several quantitative numerical models. The results suggest that, in particular, plate collision and plate bending at hot spots fail as self supporting mechanisms of rifting because of the mechanical problems of transmitting the necessary energy over the required distances. Lithospheric stretching mechanisms have been rejected on the basis of geothermal arguments. Mantle diapir models are discussed in detail with reference to the initiation and diapir phases of development, and the induced stress regime. The typical wavelength of continental rifts requires the existence of a distinct crustal layer. Even with a low amplitude diapir, the ascending mantle material deforms the upper layers by necking and doming. Modelled rates of mantle diapirism satisfy the available constraints from data. The maximum rate of diapiric ascent coincides with the maximum rate of crustal extension, which is, however, confined to a narrow surface zone. The stress regime becomes drastically altered by the structural changes at the crust-mantle boundary, induced by a crustal mass excess developed during rifting. There is a substantial decrease in the amount of extensive stresses after structural modification during rifting, and possibly even a change to a compressive stress regime. Such changes are related to the development into the so-called fossil grabens beneath basins. The initiation and development of elongated mantle diapirs is the only concept that explains the majority of phenomena associated with continental rifting.