In this paper, rheological experiments are carried out on natural hornblende under high temperature and high pressure. We used polarizing microscope and scanning electron microscope to analyze the experimental samples' microstructure, investigating the mechanisms of hornblende under the condition of different temperature, pressure and strain rate. The experimental results reveal the following features of the stress-strain curves of deformed samples: As the temperature increases, the stress-strain curve of the samples changes gradually from strengthening to yielding and weakening, sample strength reduces significantly; with the increase of confining pressure, the sample strength increases; and with the decrease of strain rate, the sample strength reduces, and it significantly reduces in the samples with the compression direction heterotropic to foliation. Plenty of transgranular fractures as well as a small amount of cataclastic deformation occur in hornblendite at temperature of 500℃, and the deformed sample is dominated by brittle deformation. At temperature of 600℃, porphyroclast system consisting of residual plaques and cataclastic series grows in the samples, wavy extinction appears in part of hornblende crystals, the deformation is characterized mainly by cataclastic deformation with ductile deformation, locally. At temperature of 700℃, the deformation is mainly dominated by intragranular kink, and dehydration and fine-grained microcrystalline appear locally, containing microcracks. The deformation of the sample is in the brittle-plastic transition phase; At temperature of 800℃, almost no obvious brittle deformation is observed in the deformed samples, the samples are dominated by dynamic recrystallization, and dehydration appears. Therefore, at the temperature conditions of 500℃, 600℃, 700℃ and 800℃, the deformation of hornblende is characterized by brittle fracture, cataclastic flow, crystal kink, dynamic recrystallization and dehydration, which shows the deformation mechanism varying from brittle to brittle-ductile, and to ductile deformation.

The depth distribution of aftershocks of the 1996 Lijiang M_S7.0 earthquake is strongly time-dependent. Events occurring shortly after the main shock had deeper focal depths, and as the time going on, the focal depth of the aftershocks became shallower and shallower, i.e. the cutoff depth of seismicity became shallower and shallower with time. As we know, the lowermost events occur around the depth of brittle-plastic transition, and this depth depends on strain rate. The postseismic deformation model inferred from GPS data show that the major contribution of postseismic strain release comes from the lower layer of the crust. These results suggest that significant afterslip is related to viscous relaxation of lower layer. We estimated the lower bound of the strain rate according to Marone's et al.(1991)afterslip model and the slip data observed at the surface of Xianshuihe Fault. The results show that the strain rate is high after the main shock, and decreases gradually with time. We calculated the strength profile of middle crust based on flow law of wet quartz, estimated strain rate, temperature profile determined using the heat flow data at Lijiang, as well as crustal structure based on P wave velocity. By comparing the cutoff depth of seismicity and the brittle-plastic transition depth of the middle crust, we found the two depths are consistent to each other. We suggest the temporary existence of deeper small events after main shock and the depth distribution of aftershock is due to the changing brittle-plastic transition of the middle crust corresponding to strain rate variation from high to lower values after the main shock, and this kind of change is the manifestation of rheology of the middle crust.