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]

Ophiolites in the Zagros orogen represent relics of the Neo‐Tethyan Ocean that existed between the Laurasia and Gondwana continents. There is much debate about the formation and evolution of the Neo‐Tethyan Ocean. Knowledge of its pre‐collisional tectonic evolution is essential in constraining the timing and process of the collision between the Eurasian and Arabian continents. We present a combined analysis of zircon LA–ICP–MS U–Pb ages and Hf–O isotopes of the Sardasht ophiolite from the NW Zagros orogen. Our new data demonstrate that the Sardasht ophiolite formed at ca. 50.6 Ma. Geochemistry of plagiogranite and diabase indicates that the Sardasht ophiolite probably originated in the mantle wedge, influenced by subduction fluids. The mafic rocks of the Sardasht ophiolite include boninites, arc basaltic rocks, and OIBs, which accreted together when the Neo‐Tethys finally closed. Finally, we argue that there was an intra‐oceanic fore‐arc extension and arc system during 50–24 Ma in the southern branch of the Neo‐Tethys in NW Zagros. [ABSTRACT FROM AUTHOR]

As a record of oceanic subduction, an accretionary complex plays a key role in the reconstruction of plate tectonic history. The Kekesentao ophiolitic mélange between the Chinese Altai orogen to the north and the Kazakhstan collage to the south played an important role in the subduction–accretion of the Chinese Altai in the Palaeozoic. This article provides new structural field relations, and petrographic, geochemical, and geochronologic data for the Kekesentao ophiolitic mélange, and detrital zircon analyses of turbidites that are imbricated with the mélange. The Kekesentao mélange consists of ultramafic rocks, meta‐gabbros, pillow basalts, cherts, and imbricated trench turbidites, tuffaceous sandstones and quartz schists (ocean plate stratigraphy). The mélange shows a 'block in matrix' structure in the field, and the general imbricated dip of the thrust system is to the N‐NE, indicating southward accretion. Pillow basalts and meta‐gabbros have a typical N‐MORB geochemical signature, and the U–Pb zircon age of a meta‐gabbro, interpreted as the age of pre‐subduction oceanic crust, is Early Devonian (399–393 Ma). The upper time limit of accretion of the mélange is constrained by a Late Devonian (361 Ma) intrusive granitic pluton. Detrital zircon ages of surrounding turbidites range from 369 to 3,114 Ma, which is typical of magmatic 540–460 Ma zircons from the Chinese Altai, suggesting that the Chinese Altai arc was the source of zircons in the mélange. The Kekesentao ophiolite was accreted to the accretionary complex before the Late Devonian. Our analysis of structures, ocean plate stratigraphy, and zircon isotopic provenance of the Kekesentao ophiolitic mélange and turbidites provides a robust conclusion that southward growth of the accretionary complex in the Kekesentao area of the Chinese Altai was semi‐continuous from the Early Palaeozoic to Devonian. [ABSTRACT FROM AUTHOR]

The Carboniferous tectonic history of the northern West Junggar of NW China is of key importance for understanding the tectonics of the Altaids. This paper presents a systematic study of the emplacement of the E'min ophiolite, the ages of turbidites, and a stitching granite dike in order to constrain the late Carboniferous oceanic evolution of northern West Junggar. Our field investigation reveals that the Tarbagatay subduction–accretion complex formed by imbrication of turbidites and ophiolitic slices in the Western Tarbagatay Mountain, which is considered as the forearc of the Saur arc in northern West Junggar. Field relationships and zircon geochronology suggest that the emplacement of the E'min ophiolite was later than the minimum time of deposition of hanging wall turbidites, which have a zircon age of 324 Ma, and must be earlier than the crystallization time of a 311‐Ma stitching granite dike that intruded the younger turbidites. The turbidites become younger from north to south, with minimum deposition ages varying from 324 to 309 Ma. The 311‐Ma dike has an adakitic affinity suggesting that it may have formed by magma melted from a subduction slab wedge and from accreted material at ~310 Ma. Therefore, we propose an accretionary arc setting for the Saur arc, with southward migration of the trench and magma front. The 311‐Ma dike has a high Al enrichment and arc‐related geochemical signature suggesting that it was generated by melting of accreted trench sediments. This may indicate that subduction of the Saur arc may have lasted to at least the late Carboniferous. The development of such arc accretion suggests that the formation of the southern Altaids was fundamentally similar to that of accretionary orogens in the western Pacific. [ABSTRACT FROM AUTHOR]

There is a broad consensus that the final closure of the Paleo‐Asian Ocean lead to the formation of the southern segment of the Central Asian Orogenic Belt. However, the final closure of the Paleo‐Asian Ocean remains controversial. To address this issue, a systematical study of geochemical and zircon U–Pb on the Kawabulak ophiolite in the southern Eastern Tianshan was carried out. The Kawabulak ophiolite is an accretionary prism that consists of different lithological tectonic slices and marks the suture zone that welded the Central Tianshan composite arc and the northern passive margin of the Tarim Craton. Zircon U–Pb data show that gabbro blocks of the Kawabulak ophiolite formed at 360.9 ± 3.3 Ma in the northern part and 326.0 ± 3.3 Ma in the southern part, respectively. The geochemistry features of basalts and gabbros show OIB‐type, arc/subduction‐related affinities, indicating different tectonic setting origins. A granitic gneiss with an arc‐related geochemical feature yielded a zircon U–Pb age of 477.2 Ma, indicating the earliest subduction event. The youngest deformed arc‐related granitic intrusion (295 Ma) in the Kawabulak prism indicates that the final closure timing of the Paleo‐Asian Ocean in Eastern Tianshan is after the Early Permian. The results of our study shed light to the temporal and spatial evolution of the southern Central Asian Orogenic Belt. [ABSTRACT FROM AUTHOR]

The Tethyan Himalaya Sequence between the Yarlung‐Zangbo Suture and South Tibetan Detachment is a fundamental component of the Himalayan orogenic belt, traditionally considered to belong to the passive continental margin of India. However, the eastern Tethyan Himalaya Sequence is poorly understood, compared with that in the central and western Himalaya. To improve general understanding, we undertook north‐south geological‐structural profiles across all the litho‐tectonic units of the eastern Tethyan Himalaya Sequence and produced new isotopic‐geochemical data from all units, to constrain their mode of occurrence, structure, tectonic setting, and time of formation. The whole Tethyan Himalaya Sequence consists of a thrust‐imbricated mélange that contains blocks of ophiolites (such as Lang County, Yumen, and Lhunze), oceanic islands (e.g., Comei) with ocean island basalts‐type basalts, mafic greenschists, limestone olistoliths, and cherts in a turbiditic matrix of greywackes, siltstones, and slates in south‐directed thrust slices. Our findings suggest that the eastern Tethyan Himalaya Sequence represents the evolution of an accretionary prism, rather than a passive margin of the Indian Plate. The prism was created during northward subduction of Neo‐Tethys between ~130 and ~50 Ma. These data allow us to produce a new tectonic model for the structural development of the eastern Himalayan orogenic belt in relation to the India‐Asia collision. [ABSTRACT FROM AUTHOR]