Mantle exhumation at passive margins

Laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent-ocean boundary. Four-layer models were constructed with sand and silicone putty, to represent the brittle and ductile layers, respectively, of both crust and mantle. It is shown that necking of the whole lithosphere model is nearly symmetrical (pure shear) but that asymmetrical structures (simple shear) develop internally, due to heterogeneous boudinage and/or faulting of brittle layers. Relative movements between brittle layers is accommodated by layer-parallel shear in the ductile layers defining a set of conjugate shear zones. With increasing stretching, lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer. The experimental results are used to propose a general model of passive margin formation leading to mantle exhumation. They are also compared to the situation observed on the west Iberia margin. The exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale. Shearing of exhumed mantle rocks does not correspond to detachment faults cross-cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high-strength sub-Moho mantle.