Search

Continental‐scale shear zones in Indochina record Cenozoic tectonic processes in SE Asia. Previous extrusion models link these shear zones to northward indentation of the Indian continent and conflict with distributed Oligocene conjugate strike‐slip pairs, formed by E‐W‐directed compression (present coordinates). This paper presents evidence of Oligocene shearing along the Mogok Metamorphic Belt in Myanmar, the western boundary of the Indochina block. The Kyanigan quarry at northern Mogok Metamorphic Belt exposes paragneiss, marble and quartzite schist with E‐W striking, which would be dragged from N‐S striking foliations of the main Mogok Metamorphic Belt by right‐lateral shearing of the brittle Sagaing Fault. After restoration, right‐lateral shearing 'σ' structures, cored with garnet, in paragneiss in the Kyanigan quarry are consistent with 'σ' and 'δ' structures in the Moulmein granitic mylonite to the south. U–Pb ages of metamorphic zircon of a sheared paragneiss and a leucogranite, cross‐cutting shear foliation in the Kyanigan quarry, and a biotite 39Ar‐40Ar age of mylonite at Moulmein constrain dextral shearing to 33–25.4 Ma, coeval with other 31–24.5 Ma shear zones in Indochina. The eleven NW‐striking left‐lateral shear zones and N‐S to NE‐striking right‐lateral shear zones define Oligocene conjugate fault patterns in Indochina. After restoration of ~80–45° clockwise rotation, these Oligocene conjugate shear zones, coupled with the anticlinal configuration of the Xuelongshan and Doi Inthanon‐Doi Suthep domes, reflect approximate N–S‐directed shortening. These structures to the north with coeval Myanmar‐Sumatra magmatic arc to the south demonstrate that they correspond to back‐arc contraction during northward subduction of the Indian Ocean. Oligocene continental‐scale intra‐continental shearing may be triggered by syn‐subduction compression in SE Asia. [ABSTRACT FROM AUTHOR]

Early Palaeozoic evolution of the Kazakhstan orocline in the Central Asian Orogenic Belt (CAOB) is key to reconstructing the Palaeozoic tectonic framework in Central Asia. This study presents field mapping, geochemistry, and geochronological data from the Wulasitai area of northern West Junggar to constrain the subduction polarity and major tectonic events in the Chingiz arc of the Kazakhstan orocline during the Early Palaeozoic. The mapping area outcrops imbricated coherent turbidite slices and mélanges that consist of chert, limestone, silt, sandstone, and conglomerate blocks in mudstone matrix, representing an OPS (Ocean Plate Stratigraphy) mélange in an accretionary complex. Existing fossil ages from these blocks range from the Ordovician to Silurian, while our new detrital zircon U-Pb samples from the turbiditic matrix yield maximum depositional ages (MDTs) from 428 ± 2 Ma to 450 ± 1 Ma. Detrital zircon ages of ~510 to 430 Ma in the mélange suggest the Chingiz arc to the south of the study area as a major sediment source, providing an indirect constraint on the polarity of subduction. We suggest that the northern part of the Chingiz arc may be underlain by southward subduction, with the accretionary complex located on the Wulasitai area of the northern West Junggar. The Wulasitai mélange is overlain depostionally by volcanic rocks carrying depleted HFSE and enriched LILE, LREE. The andesite and tuff are dated to 415 ± 5 Ma, 414 ± 3 Ma, and 402 ± 6 Ma, which we interpret as reflecting the development of the volcanic arc onto the previous subduction complex. This model implies the generation of the arc (accretion arc) as the slab rolling back and trench retreating, which may play an important role in the evolutionary history of CAOB. [ABSTRACT FROM AUTHOR]