Mechanisms of uplift preceding rifting

Abstract

The feasibility of three mechanisms, that have been proposed to account for the uplifts and heat flow anomalies which might precede rifting, is examined. The conductive heating, following an increase in the heat flux at the base of the lithosphere, could result in large scale uplifts (1000 m for 15 mW/m 2 change in heat flux) on a time scale of 10 8 years. A pulse of additional heat flow could induce a more rapid uplift but not the surface heat flow anomaly which is always delayed. The advection of heat by magma intrusion into the lithosphere could also produce the uplift and heat flow anomaly. If magmas are injected at a steady rate, an uplift of the order of 1000 m can be induced in about 50 · 10 6 years, but it requires the rate of magma injection to be equivalent to 0.3% of the total volume of the lithosphere per 10 6 years. A short intense episode of magma invasion could explain a rapid uplift and change in surface heat flow but it requires the replacement of about 30% of the total volume of the lithosphere. The complete replacement of the lithosphere by the rise of a diapir of lighter and hotter asthenospheric material has been modelled numerically. The model indicates that the rise of the asthenospheric diapir can take place in a very short time (3 · 10 6 years) and produce the surface uplift and the crustal thinning observed in rift systems, provided that, following heating, the lower lithosphere viscosity is reduced to ~ 10 21 Pa s.