Petrostructural and Geochronological Constraints on Devonian Extension‐Shortening Cycle in the Chinese Altai: Implications for Retreating‐Advancing Subduction

Abstract

Structural and metamorphic effects of a Devonian extension‐shortening cycle in the NW Chinese Altai were investigated using combined structural‐petrological analysis and zircon and monazite U‐Pb geochronology. Structural observations revealed a ubiquitous, sub‐horizontal metamorphic fabric (S1), which was originally sub‐parallel with horizontal bedding (S0) of supracrustal Devonian volcanic‐sedimentary basins. This metamorphic fabric is defined by a sillimanite‐bearing migmatitic gneissosity associated with extensional shear bands and a cordierite‐bearing schistosity in the orogenic lower and middle crust, respectively. Staurolite relics preserved in the S1 fabric are interpreted as relics of older, higher pressure Barrovian‐type metamorphism. These structural‐metamorphic features are correlated with decompression and heating and interpreted as reflecting a crustal‐scale extensional phase. S1 was folded to form N‐S‐oriented upright antiforms cored by migmatites and locally transposed by steep S2 foliation during a ∼W‐E‐directed shortening (in current coordinates). Andalusite and cordierite overgrew the migmatitic S2 foliation attesting to decreasing pressure associated with vertical extrusion of migmatites in cores of F2 antiforms. New U‐Pb age data from subhorizontally foliated migmatites suggest that the D1 extension started at least before ∼410 Ma. Age data of syn‐D2 granite intrusions and dykes constrain the D2 shortening starting soon after D1 extension and ending at ∼378 Ma. Combined with regional data, cycles of extensional and contractional tectonic regimes can be defined, which also show eastward migration along‐strike of the Altai range. Such extension‐shortening cycles could result from alternating retreating and advancing subduction, which governed the evolution of the Altai accretionary system from the Ordovician until the Carboniferous.

Switching From Subduction Zone Advance to Retreat Explains the Late Paleozoic Evolution of the East Junggar System, Central Asian Orogenic Belt

“…The structural data presented in this study, as well as those published from the Chinese Altai, indicate that the orogenic fabrics of both the East Junggar and the Chinese Altai were oriented roughly in the NNE-SSW direction during the late Carboniferous (e.g., Guy et al, 2020;Kong et al, 2022;Xu et al, 2021). Although the orientation of compressive structures in the Chinese Altai was acquired in late Devonian (e.g., Kong et al, 2022), the data of this study and those from equivalent units in Mongolia (Lehmann et al, 2010) indicate a continuous E-Wdirected (current coordinates) shortening during the late Carboniferous (ca. 310-300 Ma).…”

“…270–250 Ma (Guadalupian to Lopingian), also affected the Chinese Altai and reworked precisely dated pegmatite dyke swarms (Jiang et al., 2019; Miao, Zhang, Schulmann, Lexa, et al., 2023; Shu et al., 2022). The D 2 deformation led to the reorientation of the late Devonian to early Carboniferous upright folds into a WNW–ESE direction along the whole southern margin of the Chinese Altai (Kong et al., 2022; Miao, Zhang, Schulmann, Lexa, et al., 2023; Xu et al., 2021). This deformation is also related to the extrusion of granulite‐facies rocks and migmatites in core of large antiforms (Broussolle et al., 2018).…”

Peri‐Siberian Ordovician to Devonian Tectonic Switching in the Olkhon Terrane (Southern Siberia): Structural and Geochronological Constraints

“…We argue that this shortening in the central segment of the Mongolian Collage temporally corresponds to the D3 upright folding related to shortening in the Olkhon Terrane. Finally, the Devonian sinistral shearing reported from the Olkhon Terrane can be at least temporally correlated with upright folding and doming recently reported from the Chinese Altai (Kong et al, 2022;Wang et al, 2021). However, the kinematic correlation of the Olkhon Terrane and Chinese Altai is so far not possible.…”

“…Sub‐horizontal metamorphic fabric in orogens is a common feature and is explained either as related to nappe stacking associated to recumbent folding as is the case in Olkhon Terrane tectonic models (Fedorovsky, 1997 and many others), horizontal channel flow in large hot orogens (e.g., Beaumont et al., 2006), horizontal syn‐extensional flow (e.g., Cosgrove, 1997; Y. D. Jiang et al., 2019; Kong et al., 2022), or extensional collapse of thickened orogens associated with the development of migmatite core complexes (e.g., Vanderhaeghe & Teyssier, 2001). The key element is the metamorphic history associated with the horizontal fabric, which shows HP (high pressure) to MP (middle pressure) metamorphic gradients and typical clockwise P–T loop with P max preceding T max in nappe‐stacking or channel flow models (Jamieson et al., 2002).…”

“…Sub-horizontal metamorphic fabric in orogens is a common feature and is explained either as related to nappe stacking associated to recumbent folding as is the case in Olkhon Terrane tectonic models (Fedorovsky, 1997 and many others), horizontal channel flow in large hot orogens (e.g., Beaumont et al, 2006), horizontal syn-extensional flow (e.g., Cosgrove, 1997;Y. D. Jiang et al, 2019;Kong et al, 2022), or extensional collapse of thickened orogens associated with the development of migmatite core complexes (e.g., Vanderhaeghe & Teyssier, 2001).…”