Investigating Constitutive Parameters Of Sliding Instability Of Pyroxenes Under Hydrothermal Conditions And Their Implications On The Mechanism Of Slow Slips In Subduction Zones And Continental Lower Crust

To understand the mechanical properties of mafic rocks related to the tremors in the lower crust and some subduction zones,we focused on the frictional behavior of pyroxene minerals,which are major minerals in mafic rocks,such as gabbro and basalt.We employed augite and diopside(clinopyroxenes)as simulated gouge sample material to run velocity stepping sliding tests under hydrothermal conditions to obtain their ratedependent properties.A previous study on a simulated pyroxene gouge(separated from gabbro)was not able for us to obtain the full set of constitutive parameters,so two technical means in this study are adopted to avoid the previous shortcomings: we employ servo control of confining pressure to eliminate the significant perturbations in normal stress control incurred by slow response and use cm-size augite and diopside single crystals as gouge material to avoid contamination from other minerals.In the framework of rate-and state-dependence friction law,we aim to obtain frictional sliding properties of augite and diopside.Special attention was paid to the degree of velocity weakening.Together with the microstructural features of deformed samples,we studied the deformation mechanisms,especially those responsible for the evolution effect and thus velocity weakening.Thus,on the basis of experimental work on clinopyroxene(augite and diopside),we further investigated the effect on mafic rocks and lower-crust.The simulated augite gouge sample exhibited stable sliding behavior at most tested temperatures.The steady state friction coefficient showed no systematic temperature dependence with average value of ~0.72.Under ~200 effective normal stress,velocity dependence transitioned to velocity weakening at ~215 °C,and the velocity weakening persisted up to 607 °C.The b-a values were revealed to range from 0.0009-0.0014,with inferred average b/a values ranged from 1.15-1.18,both indicating minor velocity weakening at 303-607 °C.Inferred constitutive parameters through numerical fitting to rate-and state-dependent friction law show that the evolution effect of friction has an increasing trend with temperature up to 403 °C,indicating an Arrhenius-type thermally-activated creep mechanism behind the evolution effect.Together with the observed signatures of the ubiquitous precipitated particles(50-100 nm),this implies that intergranular pressure solution(IPS)process is the most likely mechanism operating at the frictional contacts and governing the evolution effect for augite.The simulated diopside gouge sample exhibited stable sliding at 100-200 °C(~100 MPa)and 100-400 °C(~200 MPa).The steady state friction coefficient of diopside showed no systematic temperature dependence with average values of 0.725(~100 MPa)and 0.728(~200 MPa).Under both ~100 MPa and ~200 MPa effective normal stress,velocity dependence transitioned to velocity weakening at ~205 °C(~100 MPa)and ~220 °C(~200 MPa),and the velocity weakening persisted up to ~600 °C.At ~300 °C,diopside showed relatively strong velocity weakening with b-a values between 0.0034-0.0044.At ~400 °C-~600 °C,the degrees of velocity weakening were less affected by temperature,showing minor velocity weakening with b-a values of ~0.0013(~100 MPa and ~200 MPa),which was similar to the result of augite(~0.0012).A special property was found in diopside,showing a transition from velocity weakening to velocity strengthening as slip advanced at 310 °C and 413 °C(~200 MPa).Similar precipitated particles were found in deformed samples at all tested temperatures.With the same comparison in IPS model,it is found that IPS process is the most likely mechanism for diopside as well.The statistic result of cracks in the deformed samples under ~200 MPa effective normal stress show that,the number of cracks increases as the temperature goes up,but the cracks within gouge layer become short and straight.The minor velocity weakening shown by augite and diopside,combined with previous data on plagioclase,is consistent with the modest velocity weakening seen in mafic rocks,such as quart-free oceanic basalt.Such a velocity-weakening property tends to favor slow slip events on the down-dip portion of the subduction interface below currently locked seismogenic zones.