An experimental study on semi-brittle and plastic rheology of Panzhihua gabbro

We have carried out a systematic experimental study on semi-brittle and plastic deformation of fine-grained Panzhihua gabbro under dry condition with temperature range of 700—1100℃, confining pressure of 450—500 MPa, and strain rate of 1×10-4—3.1×10-6 s-1, using a triaxial testing system with a Griggs type solid medium pressure vessel. In terms of the parameters in the flow law and microstructure after deformation, the rate-dependent deformation can be categorized into three modes: (i) In temperature range of 700—800℃, the deformation is accommodated by semi-brittle flow, with activation energy Q = 612 12 kJ/mol, and stress exponent n = 14.6. The deformation in microscopic scale corresponds to the dislocation glide accompanied with microfracturing. (ii) In temperature range of 900—950℃, the predominant deformation mechanisms in this phase are mechanical twinning and dislocation glide, with activation energy Q =720 61 kJ/mol, and stress exponent n = 6.4. (iii) In temperature range of 1000—1150℃, the major deformation mechanisms are dislocation glide and dislocation climb with minor processes of partial melting, with activation energy Q = 699 55 kJ/mol and stress exponent n = 4.1. The microstructure and deformation mechanism of our experiments are comparable to the results of clinopyroxene and diabase as observed in previous studies. The flow stress of a mafic lower crust is calculated based on the rheological parameters of dry fine-grained gabbro, which implies that a lower curst with mafic granulite may be brittle, and it is possible to fracture and produce frictional slips. This may be an important implication for earthquake nucleation in the lower crust.