| Previous studies found that most of the focal depths of continental earthquakes are shallower than 20 km, suggesting that the seismogenic layer is mainly in the upper crust. Thus it is generally accepted that the upper crust is brittle and the lower crust is basically ductile. However, recent relocations of earthquakes found that some earthquakes actually occurred in the lower crust. More recently, the discovery of tremors in the lower crust portion of the San Andreas fault has attracted more attention on the mechanical property of the lower crust, and some experimental studies have been carried out to address this issue.Previous experiments have shown that pyroxene and plagioclase minerals (both separated from gabbro) and their mixtures all show velocity-weakening behavior in the earth’s crust temperature range under effective normal stress 200 MPa, which can result in unstable slips. This work is to examine whether the velocity-weakening behavior of plagioclase gouge also applies to a relatively broader range of effective normal stress.On the basis of previous experimental data, this work investigated properties of frictional sliding of plagioclase gouges under different effective normal stresses. The objective was to examine the influence of effective normal stress on the property of frictional sliding. Three groups of experiments were designed:effective normal stress 100 MPa with pore water pressure of 30 MPa and 100 MPa, respectively, and another effective normal stress of 300 MPa with pore water pressure of 30 MPa. Temperatures were applied between 100～600℃, covering most of the lower crust temperature range. The experimental results show the following observations and insights into the focused problem.(1) The friction strength of plagioclase increases first then decreases with the growing temperature. Under effective normal stress of 100 MPa with pore water pressure of 30 MPa, the mechanical behavior of plagioclase exhibits the following features. In the low temperature range, plagioclase gouge shows stable sliding at 100℃ and 300℃, but quasi-static oscillation at 200℃.In the high temperature range from 400℃to 600℃, sliding behavior depends on sliding velocity. At 400℃, stable sliding occurs at low speed (0.122μm/s) but oscillatory slips at higher speed(1.22 μm/s). At 500℃, oscillatory slips appear at both speeds. At 600℃, oscillatory slips continue to occur at low speed (0.122μm/s) but stable sliding at higher speed(1.22 μm/s). All results under this effective normal stress are attributed to velocity-weakening behavior.The features of sliding properties of plagioclase under effective normal stress of 100 MPa with pore water pressure of 100 MPa are summarized as follows:At 100℃ and 200℃, the sliding is stable, while between 300～600℃ it depends on the applied rate. At 300℃, the gouge shows oscillatory slips at low speed (0.122μm/s) but stable sliding at high speed (1.22μm/s). At 400℃ and 500℃, it shows stable sliding at low speed but oscillatory slips at high speed. At 600℃, stable sliding occurs at both speeds. All these results are of velocity -weakening behavior.Under effective normal stress of 300 MPa, the sliding behavior of plagioclase gougeis characterized by stable sliding between 100℃ to 300℃; At 400℃, it shows stable sliding at low speed (0.122μm/s) but oscillatory slips at high speed (1.22μm/s). The sliding behavior is characterized by oscillatory slips at temperatures of 500℃ and 600℃. Except the velocity-strengthening behavior at 200℃, other temperature range corresponds to velocity-weakening behavior.(2) Through numerical fitting of the mechanical curve to the slowness friction law, parameters a, b and dc, in the constitutive law were obtained. It is found that the oscillatory slips are the result of small dc values which make the critical stiffness rise remarkably and approach the system stiffness. The parameter a of the direct rate effect increases with temperature elevation for temperatures from 200～500℃, showing a typical feature of thermally activated processes. But at 600℃, the parameter a falls to a value deviated from the increasing trend, with the evolution effect showing a similar drop.(3) Experimental results under effective normal stress of 100 MPa and 300 MPa suggests that the friction strength (coefficient of friction) of plagioclase changes with temperature, showing an increasing trend between 200～300℃, but a decreasing trend in the high temperature range.(4) The plagioclase gouge at three different effective normal stresses all shows velocity weakening for temperatures greater than 300℃, which decreases with the increase of effective normal stress, suggesting that lower effective normal stress promotes unstable slips.(5) In the high temperature section, the parameters of the plagioclase gouge (direct effect parameters a and evolution effect parameters of b) increases with the rising effective normal stress.To sum up, the changes in the effective normal stress have no significant effect on the sliding behavior of plagioclase, which basically shows velocity weakening. Therefore, a conclusion on the mechanical properties of the earth’s lower crust based on an early experimental result at 200 MPa effective normal stress remain applicable in a wider effective normal pressure range, i.e. unstable sliding can occur in the lower crust. |
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