A test of the validity of yield strength envelopes with an elastoviscoplastic finite element model

We have generated an elastoviscoplastic (EVP) rheological model of the lith osphere with an extended Maxwell model containing tin series! a linear elas tic component, a creep component based on a flow law for dislocation creep in olivine, and a frictional component simulating Drucker-Prager plasticity based on Byerlee's rule. Finite element analyses for topographic loading o f this oceanic lithosphere were carried out with two separate final loads ( 100 and 150 MPa) that were reached by four different load growth times (0, 0.1, 1, 10 Myr). Our results for the stress state and deformation of loaded lithosphere at 41.7 Myr into the model run are compared to results generat ed by the mechanical response of a time-independent elastic-perfectly plast ic (EP) lithosphere, using a moment-curvature relationship based on the con stant strain-rate yield strength envelope (YSE) and adopting a strain-rate representative of the EVP solution at 41.7 Myr. With identical flexural loa ding and material parameters, the deflection profiles of the EVP and EP sol utions are quite similar, but it is unclear how the EP strain rate could be selected a priori without guidance from the EVP solution. For example, thi s uncertainty translates to about a 5 per cent error per decade of strain r ate in the temperature gradient obtained by matching maximum moment and cur vature in our EP models. The stress distributions of the time-dependent EVP model show deviations from the EP model (as defined by the YSE and an elas tic core) in crystal plastic (macroscopically continuous dislocation creep) regions, where we observe vertical, lateral and temporal variations in the strain rate. At times much greater than the load growth time, the stress d istribution in the lithosphere is independent of the loading rate and depen ds on the load magnitude only in that portion of the lithosphere that yield s to frictional slip. After loading ceases, residual creep zones develop (i n the vicinity of the brittle-plastic transition and the elastic-creep tran sition), driven by high stress in these regions.