Midcrustal shearing and doming in aCenozoic compressive setting along theAilaoShan‐RedRiver shear zone

Abstract

The Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan-Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top-to-NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike-slip shear zone. Microfabrics of crystallographicpreferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300-5008C) in the mantling schist to the midcrust (15 km or more, 6508C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn-kinematic mineral assemblages and microstructures, which range from garnet 1 amphibole 1 biotite 1 sillimanite 1 rutile 1 feldspar in the core to garnet 1 staurolite 1 biotite 1 epidote 1 muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top-to-NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse-sense shear along the two schist limbs of the dome during contraction-related doming (D2-D3); (3) sinistral strike-slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural-thermal patterns suggest the antiformal dome formation was roughly coeval with top-to-NE ductile shearing in the midcrust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike-slip deformation.

Paleomagnetic Evidence for 25–15 Ma Crust Fragmentation of North Indochina (23–26°N): Consequence of Collision With Greater India NE Corner?

“…As suggested by Li, Yang, et al (), it is likely that tectonism caused both fluid migration within fractured rock (thus chemical growth of pigmentary hematite) and rotations. Such an age window also overlaps with main age of strike‐slip deformation documented along the major shear zones of Indochina (i.e., the Gaoligong, Chongshan, and ARRSZ, although onset of strike‐slip shear is mostly set at a slightly older 32 Ma age; Leloup et al, , , ; Searle et al, ; Zhang et al, ; Zhang, Zhang, Chang, et al, ; Zhang, Zhang, Zhong, et al, ; Zhang et al, ).…”

“…The ARRSZ is a major shear zone that received considerable attention since the 1980s, and was viewed by some authors as separating deforming and drifting Indochina from the stable South China Block. Ductile top‐to‐the‐east midcrustal shortening took place during the 55–30 Ma window, followed by sinistral strike‐slip shearing at 27–15 Ma, and younger E‐W exhumation (Leloup et al, , , ; Tapponnier et al, ; Zhang et al, ). After about 10 Ma, the ARRSZ was reactivated as a brittle dextral fault yielding a cumulative ≤40 km displacement (Burchfiel & Wang, ; Schoenbohm et al, ; Wang et al, ; Wang, Zhang, et al, ).…”

First Identification of Late Permian Nb‐Enriched Basalts in Ailaoshan Region (SW Yunnan, China): Contribution From Emeishan Plume to Subduction of Eastern Paleotethys

“…The Ailaoshan region comprises a number of tectonic units (e.g., Lai, ; J. L. Liu et al, ; H. Liu et al, ; Yunnan Bureau of Geology and Mineral Resources, ), including (from east to west) the Red River Fault, Ailaoshan metamorphic complex, Ailaoshan Fault, Ailaoshan ophiolitic mélange (e.g., Shuanggou ophiolite) and Paleozoic low‐grade metamorphic rocks, western Ailaoshan volcanic belts and the Lixianjiang‐Amojiang Fault (Figure b). The Ailaoshan‐Red River shear zone is situated between the Ailaoshan and Red River faults (Figure b) and had experienced large‐scale sinistral strike‐slip movements since the Tertiary (e.g., Leloup et al, ; B. Zhang et al, ). The shear sense then reversed to dextral in the Miocene‐Pliocene along the Red River fault (e.g., Allen et al, ).…”

Mineral phase equilibria and zircon geochronology constrain multiple metamorphic events of high‐pressure pelitic granulites in south‐easternTibetan Plateau

“…The collision of the Indian and Eurasian plates resulted in the formation of the world's highest plateau in Tibet, as well as the extrusion of south‐eastern Asia along many planet‐scale strike‐slip faults (Liu et al, ; Tang et al, ; Tapponnier et al, ; Tapponnier, Peltzer, Armijo, Dain, & Cobbold, ; Zhang et al, , ; Zhong et al, ). The Sanjiang (Jinshajiang, Lancangjiang, and Nujiang) tectonic zone is located along the south‐eastern margin of the Tibetan Plateau, where a complete record of the metamorphic and tectonic evolution of the eastern Tethys is preserved.…”

“…This evolution included the opening of the Palaeo‐Tethys Ocean, the subduction of its oceanic plate, and the amalgamation of Gondwana‐derived microcontinental blocks (e.g., South China, Indochina, Baoshan, and Tengchong) with Palaeozoic arc‐related terranes (Deng, Wang, Li, Li, & Wang, ; Metcalfe, , ; Mo, Deng, & Lu, ). Several previous studies have addressed the petrological, microstructural, and geochronological aspects of the magmatic and metamorphic suites in the Sanjiang tectonic zone (Deng, Wang, Li, & Santosh, ; Gilley et al, ; Liu, Wang, Liu, & Liu, ; Metcalfe, , ; Wang, Liu, Liu, Shi, & Cai, ; Zhang et al, ), and three major metamorphic belts (i.e., Diancang Shan [DCS]–Ailao Shan [ALS], Lancangjiang, and Gaoligong) have been recognized as marking the suturing and amalgamation of the microcontinents (Figure a). Among these, the ALS belt separates the Indochina Block to the west from the South China Block (SCB) to the east, and it is an important region for understanding the evolution history of the Palaeo‐Tethys (Deng et al, ; Fan, Wang, Zhang, Zhang, & Zhang, ; Metcalfe, , ).…”