| Low velocity layers in crust may be mechanically weak layers and thus favorable for the occurrence of earthquakes. They also act as a kind of bottom boundary of tectonic blocks in continent. Although previous petrophysical works have displayed that the dehydration of hydrous minerals in amphibolites and the dehydration melting of amphibolites are possible causes for the formation of low velocity layers in mid crust, there are still insufficiencies in these works. It is necessary to perform petrological experiments with rock samples of amphibolites under vapor-absent and closed conditions, and then to discuss the formation cause of some low velocity layers based on geological, petrological, petrophysical, and geophysical data.The contents of this paper comprise the following four parts:(1) Field work in Kangding area, Sichuan province. In the field work, some metapelitic rocks, migmatites and magmatic rocks have been collected, and some typical geological phenomena have been recorded. Later, the rock samples and the recorded phenomena have been analyzed in laboratory.(2) Collections and analyses of a lot of seismic sounding data, geothermal data, and magnetotelluric sounding data.(3) Experimental studies on the plagioclase amphibolite of Kangding group. Based on four petrological experiments, this paper has discussed the conditions, phenomena, and products of the dehydration reaction and the dehydration melting reaction. All the experiments have been run on the piston-cylinder apparatus with solid confining media in Laboratory of Tectonophysics, China Seismological Bureau. In these experiments, pyrophyllite and NaCl composed the solid confining media and the rock samples were sealed in red copper capsules.(4) According to the previous works of experimental petrophysics (mainly rock velocities under high pressure and high temperature) and geothermal data about the tectonic unit, this paper has discussed the in-situ physical properties (mainly the velocities) of granitoid rocks and amphibolites which supposed to be typical rocks in the mid crust in Kang-Dian tectonic zone. The results from the above discussions were then compared with the seismic and magnetotelluric sounding data. At last the formation cause of some low velocity layers in the crust in Kang-Dian tectonic zone was discussed.The conclusions of this paper are:(1) Several geological events, including at least regional metamorphism, intrusion of magma and tectonic movement, had occurred in Kang-Dian tectonic zone. For example, the widespread distribution of metapelitic rocks, i.e. Kangding group, shows that the early regional metamorphism reached amphibolite facies. The existence of migmatites implies the occurrence of anatexis: the partial fusion of crustal rocks and recrystallization of melts are illustrated by the textural features in both rock samples and polished sections of migmatized amphibolites containing coarse-grained blocks; the breccias in the brecciated amphibolite migmatite can be put together into one mass, which implies that the melts produced by the fusion in amphibolites had induced the brittle fracture of the rocks, and that the melts hadn't move far. The intrusion of mafic veins and granitoid bodies into metapelitic rocks and migmatites is the clues of magmatic activities in crust and even mantle of the tectonic area after the regional metamorphism. The later tectonic movement is displayed by a fault dislocating the mafic vein. The above-mentioned geological events indicate that Kang-Dian tectonic zone had been a geologically active area in geological history.(2) The dehydration reaction and the dehydration melting reaction have occurred in rock samples of the plagioclase amphibolite of Kangding group in four petrological experiments. (1) In the experimental samples of Run4(400MPa 800C 7h), Run5(400MPa 700 C 36h), Run6(250MPa ? 700 C , 24h), the optical characteristics of hornblendes and biotites are changed, which implies that dehydration reactions have occurred. And it seems tha...
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