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The Songpan block in the northeastern Qinghai–Tibet Plateau (QTP) located at the junction of the E–W- and N–S-trending tectonic belts has affected the formation and evolution of the Chinese continent since Mesozoic and is a tectonic node in the central part of the Chinese continent. In this study, the Hezuo-Tangke deep seismic reflection profiling of the 257 km-long was conducted in 2004, which is the first profile across the Songpan block and the West Qinling orogen. We reprocessed the data in 2009, and the results revealed the fine lithosphere structure of the northern margin of the Songpan block and the West Qinling orogen. The results also revealed the connection between the Songpan block and East Kunlun–West Qinling orogeny, indicating that the Songpan block and the West Qinling orogen belong to the same stable continental block and that their lower crusts are both characterized by north-dipping strong reflectors, which provide seismic evidence for subduction of the lower crust of the Songpan block beneath the West Qinling orogen. We found that the sub-horizontal Moho reflectors beneath the West Qinling orogen indicate that it underwent intensive extension after collisional orogeny. The results of this study also indicate that the Kunlun fault did not cut off the Moho. Actually, it terminated at a sub-horizontal decollement in the lowermost crust of Tibet at a depth of ~ 35 km. The data provided through the investigation also are helpful in evaluating prospective oil and gas resources in the Songpan block.

The Sulu–Dabie high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt as the product of subduction-collision between the northern China plate and Yangtze plate underwent a process of formation and evolution from deep subduction→exhumation→extension→slow uplift. The study of its modern tectonic stress field has great significance for a complete understanding of the process of formation and evolution of the HP–UHP metamorphic belt, especially the exhumation and uplift of the belt. Wellbore breakouts are the most visual tectonic phenomenon which can characterize the modern stress action in the main borehole of Chinese Continental Scientific Drilling (CCSD). Ultrasonic borehole televiewer reflection wave data show that wellbore breakouts began to occur at 1216 m depth of the main borehole. A total of 143 borehole televiewer images were collected from 1216 to 5118 m depth (hole completion depth). After data processing and statistics, the average azimuth of the long dimension of the wellbore breakout obtained was 319.5° ± 3.5°, indicating that the average azimuth of the maximum horizontal principal stress causing wellbore breakout initiation was 49.5° ± 3.5°. The maximum and minimum horizontal principal stress values at 52 depths in the interval of 1269 to 5047 m were estimated using the elements of wellbore shapes (wellbore depth and width), combined with the cohesive strength and internal frictional angle of the rock obtained by rock mechanical tests on samples, and the static load stresses at corresponding depths were calculated according to the rock density logging data. The results indicate that: the maximum and minimum horizontal principal stresses are 41.4 and 25.3 MPa at 1269 m depth respectively and 164.7 and 122 MPa at 5047 m depth respectively; the maximum vertical stress is 141.3 MPa at 5047 m depth with a density of 2.8 g/cm 3 ; the in-situ stresses increase nearly linearly with depth. The magnitudes and directions of the three principal stresses reflect that the regional stress field around the CCSD main borehole is mainly in a strike-slip state, which is consistent with the basic features of the regional stress field determined using other methods.