Structure and Evolution of Some Continental Rifts--Consequences for Their Thermomechanical Behavior: ABSTRACT

Abstract

Continental rifts are attractive basins for hydrocarbon exploration, because they presently furnish 10% of the world hydrocarbon production, yet cover only 5% of the Earth's surface. The evolution of the rifts where volcanism is of reduced importance will be emphasized through present examples such as the Suez rift (Egypt), the rifts beneath the Bay of Biscay, and other Atlantic-type continental margins. The continental rifts, typically 50-100 km (30-60 mi) in width, are tectonically characterized by the extension and thinning of the continental crust together with subsidence in the central part of the rift. The superficial structure consists mainly of tilted blocks bounded by listric normal faults. Continental extension may reach 50% of the crust's original horizontal length, and even more depending on the amount of dike intrusions from the mantle. The thinning of the crust may reach much higher rates, perhaps 300% or more where oceanic floor is formed in the central part of the rift. The subsidence in the rift trough is characterized by the alternation of rapid downward movements over a period of a few million years and periods of relative stagnation. Meanwhile, the shoulders of the rift may uplift after the beginning of normal faulting in the main troughs. Heat flow is higher generally in the rifts than in the surrounding areas. This evolution is the consequence of an initial thermal perturbation in the deep asthenosphere. The evolution of the lithosphere-asthenosphere system is studied by means of a thermomechanical model with the hypothesis of a non-newtonian rheological behavior of the lithosphere as a function of the depth. The vertical movements of the continental crust, the thinning of the lithosphere, the stresses, and the gravity anomalies in the thermal regime are given as a function of time during rifting. End_of_Article - Last_Page 793------------