INITIATION OF ASYMMETRIC EXTENSION IN CONTINENTAL LITHOSPHERE

The physical conditions are investigated under which the lithospheric- scale style of extension is pure shear or simple shear. We focus on th e initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions . Continental collision, magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension. We use a finite element method to model the thermo-mechanic al evolution of continental lithosphere. Experimental flow laws are us ed to model the elastic, brittle, power law creep or diffusion creep r heology of lithospheric rocks. Our results indicate that if in-plane f orces change from compressive to tensile immediately after a rapid mou ntain building phase, initiation of a lithosphere-scale detachment fau lt is possible. We find a strong dependence of the extensional style o n the distribution with depth of residual stresses from the collision phase. This result is consistent with observations of gravitational co llapse in regions, like the Aegean and the Basin and Range Province, w here detachment faults have exhumed lower crustal rocks. The predicted dip direction of the fault also agrees with observations in these are as. Intrusion of magma into continental lithosphere, which is subject to in-plane tensile forces, will cause localization of pure shear defo rmation. The style of deformation resulting from mantle plumes impingi ng to the base of the lithosphere is symmetric. Delamination of lithos pheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase, when in-plane forces change sign from c ompressive to tensile. This result also strongly depends on the assume d residual stress distribution. If delamination occurs during the moun tain building phase, the style of deformation will be pure shear. Anot her interesting outcome from our modeling is that dramatic strain weak ening as a result of a deformation mechanism change from dislocation c reep to diffusion creep, reduces the tendency to strain localization.