MANTLE EXHUMATION AT PASSIVE MARGINS

Laboratory models of lithosphere necking have been used to study the m odes of passive margin formation and related mantle exhumation at cont inent-ocean boundary. Four-layer models were constructed with sand and silicone putty, to represent the brittle and ductile layers, respecti vely, of both crust and mantle. It is shown that necking of the whole lithosphere model is nearly symmetrical (pure shear) but that asymmetr ical structures (simple shear) develop internally, due to heterogeneou s boudinage and/or faulting of brittle layers. Relative movements betw een brittle layers is accommodated by layer-parallel shear in the duct ile layers defining a set of conjugate shear zones. With increasing st retching, lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of bou dins in the brittle layer. The experimental results are used to propos e a general model of passive margin formation leading to mantle exhuma tion. They are also compared to the situation observed on the west Ibe ria margin. The exhumation of lithosphere mantle is the result of a bu lk pure shear at lithospheric scale. Shearing of exhumed mantle rocks does not correspond to detachment faults cross-cutting the whole litho sphere at the onset of rifting but results from heterogeneous stretchi ng and boudinage of the high-strength sub-Moho mantle.