Continental rifting as a function of lithosphere mantle strength

Abstract

The role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo-mechanical finite-element code. In the lithosphere, the mantle/crust strength ratio ( S M/ S C) that decreases with increasing Moho temperature T M allows two strength regimes to be defined: mantle dominated ( S M > S C) and crust dominated ( S M < S C). The transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for T M > 700 °C. 2D numerical simulations for different values of S M/ S C show how the uppermost mantle strength controls the style of continental rifting. A high strength mantle leads to strain localisation at lithosphere scale, with two main patterns of narrow rifting: “coupled crust–mantle” at the lowest T M values and “deep crustal décollement” for increasing T M values, typical of some continental rifts and non-volcanic passive margins. The absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust. These numerical results are compared and discussed in relation with series of classical rift examples.