Ultra) high-pressure (HP) rocks can be exhumed rapidly by subduction reversal or divergent plate motion. Recent studies show that subduction reversal can in particular occur in a divergent double subduction zone when the slab pull of one slab exceeds that of the other, shorter one, which then experiences a net upward pull. This recent hypothesis, first proposed for Triassic HP-rocks exposed in the central Qiangtang mélange belt in central Tibet, can explain the exhumation of (ultra) HP rocks through upward slab movement. However, this model lacks the support of kinematic evidence. In this study, based on the recognition of multiple deformational phases, we analyze the kinematics of the HP-bearing mélange in central Qiangtang. Based on new 40 Ar-39 Ar geochronology data and those collected from the literature, we present a temporal framework for the new observations. We recognize a switch in sense of shear between the prograde (D1) and exhumation (D2-3) paths. The change of shear sense reflects the reversal from downward to upward movement of the oceanic slab below. Early D2 represents the early exhumation stage that caused retrograde metamorphism from eclogite to blueschist facies. No magmatism occurred during this period. Continued exhumation from blueschist facies to greenschist facies resulted in D2-D3 structures. Voluminous igneous activity occurred during this stage. We suggest that subduction reversal in a divergent double subduction zone can best explain the kinematic evolution and temporal framework above. This exhumation model may provide a new perspective on the exhumation mechanism for other HP rocks around the world.
The genesis of the subduction mélange in the central Qiangtang terrane has been a long hot debate. However, little research has been conducted on the brittle failure within the accretionary wedge, which is very important to unveil the structural evolution of the mélange. In this study, based on the recognition of multiple deformational phases, we analyse the characteristics and formation history of the vein system in the Gangma Co mélange. Six groups of quartz veins are recognized. Foliation‐parallel extension veins (G1 veins), shear veins (G2 veins) and foliation‐perpendicular extension veins (G4 veins) are supposed to have formed during the subduction of oceanic crust, recording the repeated low‐angle thrust‐sense frictional sliding, tensile fracturing and stress changes generated by subduction‐related earthquakes. Subsequent vertical extension veins (G5 veins) are suggested to be related to the exhumation of the underplated mélange, while the horizontal extension veins (G6 veins) in the last phase represent the final horizontal thrusting. The temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging from 120 to 200°C, coinciding with the temperature conditions of the slow earthquake region where episodic tremors and slow slip occur. This contribution supports that the Gangma Co mélange represents an in situ subduction zone and that its internal vein system is a response to the tectonic evolution of the Longmu Co‐Shuanghu Tethys Ocean.
The Hailar Basin, located in north‐east China, is a typical continental rifted basin that contains oil and gas. The basin formation process comprised several stages of construction and reformation with complex formation mechanisms. The Bayanhushu (BYHS) Sag is a secondary structural unit in the south‐west Hailar Basin with a significant resource potential, but its current poor exploration and insufficient understanding of the structural evolution characteristics are restricting further oil and gas exploration. Therefore, the study of the structural evolution of the BYHS Sag plays a pivotal role in the future exploration and development of oil and gas. There are different hypotheses on the formation mechanisms and structural evolution of the BYHS Sag. To further understand the evolutionary history of the BYHS Sag, a structural physical simulation experiment was used based on the structural interpretation and geometric analysis of a seismic section. Inversion validation was then undertaken by the 2DMove equilibrium profile recovery technology. It was found that the formation process of the BYHS Sag was mainly controlled by the western Adunchulu Fault. Faults on the section developed in succession from top to bottom. The fault plane experienced multiple changes, thus forming a special seat‐shaped structural pattern. Structural inversion occurred twice during the evolution of the sag. The compressive stress during the tectonic inversion mainly acted in a SE direction. It is inferred that this was related to the subduction of the Palaeo‐Pacific Plate under the Eurasian Plate and the intermittent compression caused by the transmission of stress of the arc–continent collision.
The identification of a microcontinent in Bangong-Nujiang suture zone (BNSZ) is important to understanding the tectonic evolution of Bangong-Nujiang Tethys Ocean (BNTO). Subduction-related ophiolite is one of the keys to discriminate a microcontinent in suture zones, which can be generated during the oceanic subduction beneath the microcontinent. Generally, ophiolites in the adjacent area of a microcontinent would show characteristics of a volcanic arc, while ophiolites in adjacent area of an ocean show characteristics of a fore-arc. North Pengco ophiolites (NPOs) and South Pengco ophiolites (SPOs) were collected in the central section of BNSZ, and dated at 115.0-111.5 Ma by SHRIMP II. According to their trace element characteristics, NPOs were determined to have formed at a setting of subduction-related volcanic arc and affected by slab-derived fluids or melts. The diagram based on the Ce versus Ce/Y shows that NPOs originated from the partial melting of spinel peridotites. On the other hand, SPOs were generated from the partial melting of ultra-depleted mantle at a setting of subduction-related fore-arc on the basis of extraordinarily low REEs summation and HFSEs (e.g., Zr, Ti, Nb and Ta), relatively high concentrations of compatible elements (e.g., Cr, Ni), large ratios of CaO/TiO 2 (37.67-437.50) and Al 2 O 3 /TiO 2 (20.78-424.50), which were also affected by slab-derived fluids. Based on the relative location of NPOs and SPOs, it reasonably proposed that a northward subduction of a minor oceanic basin had occurred at the Early Cretaceous, trigged by the Dongkaco microcontinent (DMC). In reverse, these ophiolites provide evidence for the existence of DMC in BNTO. Combining with other subduction-related ophiolites developed in BNSZ, BNTO might have comprised of several minor oceanic basins separated by some blocks like DMC in the central and west sections, rather than a unified ocean basin.
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