ANALYSIS ON MAGNITUDE CHARACTERISTICS OF THE SHALLOW CRUSTAL TECTONIC STRESS FIELD IN QINGHAI-TIBET PLATEAU AND ITS ADJACENT REGION BASED ON IN-SITU STRESS DATA

Based on the in situ stress measurement data from “Fundamental Database of Crustal Stress Environment in Continental China”, nearly 2000 pieces of data have been properly picked out, covering the geographic space of 21°N–40°N and 73°N–110°N and the depth of 0∼2 km. By excluding the gravity effect on in-situ stress in the target area, and taking the uneven distribution of the sample data along depth into consideration, the effect of tectonic stress field will be analyzed in this paper. In order to exclude the gravity effect, two modes, namely Heim's hypothesis and A. H. Gennik's hypothesis, have been adopted to estimate the upper and lower limits, under which the tectonic stress characteristic of the shallow crust in Qinghai-Tibet Plateau and its periphery have been studied. As shown in the results: (1) The maximum horizontal stress σ_H and minimum horizontal stress σ_h in Tibetan Plateau and its periphery increase linearly with the depth D: σ_H = 22.115D + 5.761, σ_h = 14.893D + 3.269; and the estimated magnitudes of the maximum horizontal tectonic stress σ_T and minimum horizontal tectonic stress σ_t vary as follows respectively: 4.609 < σ_t < 15.522D + 4.609, 3.121 < σ_t < 6.366D + 3.121(D > 0); tectonic difference stress σ_T – σ_t = 7.222D + 2.492, surface value (D = 0 km) is about 2.5 MPa, and increases at a gradient of 7.2 MPa·km^–1 with the depth. (2) Within the measured depth scale, σ_T, σ_t and σ_T – σ_t values of Qinghai-Tibet block and research areas in the northern/middle/southern segments of North-South seismic belt all increase linearly with the burial depth; when D = 1 km, the maximum of σ_T values from statistical regression of all blocks is 30.1 MPa, and the minimum is 17.6 MPa, with the blocks in descending order of values such as: Qinghai-Tibet block, northern segment, middle segment and southern segment of the North-South seismic belt; when D = 1 km, the maximum of σ_T – σ_t values from statistical regression of all blocks is 15.8 MPa, and the minimum is 8.9 MPa, with the blocks in descending order of values as: Qinghai-Tibet block, northern segment, middle segment and southern segment of the North-South seismic belt. Generally, the stress magnitudes in Qinghai-Tibet block are stronger than those in the North-South seismic belt. (3) Compared with the North-South seismic belt, an obvious feature of “stronger tectonism in the deep crust than that in the shallow” is shown in the crust of Qinghai-Tibet block under northward compression.