New structural data on Late Paleozoic tectonics in the Kyrgyz Tien Shan (Central Asian Orogenic Belt)

Abstract

The tectonic evolution of the Central Asian Orogenic Belt (CAOB) is characterized by the successive accretion of lithospheric blocks, leading to different interpretations about the polarity of subductions during the Paleozoic, the number of microplates and oceanic basins and the timing of tectonic events. This is especially the case in the Tien Shan area.In this paper, we propose new structural maps and cross-sections of Middle and South Kyrgyz Tien Shan (MTS and STS respectively). These cross-sections highlight an overall dextral strike-slip shear zone in the MTS at the crustal scale and a North verging structure in the STS. These structures are Carboniferous in age and sealed by a late Carboniferous conglomerate, later overlain by Mesozoic and Cenozoic deposits. The STS exhibits two deformation phases: (1) a top-to-the-South normal shearing that can be related to subduction or exhumation dynamics and (2) a top to the North nappe stacking that we link to the late Paleozoic collisional events between the MTS and the Tarim block.We propose a new interpretation of the tectonic evolution of the Kyrgyz Tien Shan during the Late Paleozoic collision. This model involves a partitioned collisional deformation in Late Carboniferous times, with an orthogonal collision to the south, between the Tarim and MTS, and a strike-slip regime to the north along a dextral E-W zone located between the MTS and the North Tien-Shan/ Kazakh platform, the so-called Nikolaev Line. The docking of the large Tarim Craton against the CAOB corresponds to a collision phase, which ended the longlived Paleozoic subduction history in the CAOB and was followed in the TS region by intense strike-slip deformation during the Permian.

Unravelling a Devonian–Triassic seamount chain in the South Tianshan high‐pressure/ultrahigh‐pressure accretionary complex in the Atbashi area (Kyrgyzstan)

“…However, the subduction polarity of the South Tianshan Ocean has long been controversial. Southward subduction (Charvet et al, ; Jourdon et al, ) and northward subduction (Gao et al, ; Gao et al, ; Sang et al, ; Xiao, Huang, Han, Sun, & Li, ; Xiao et al, ; L. Zhang et al, 2007) have both been proposed. Furthermore, the timing of the final closure of the South Tianshan Ocean is likewise a subject open to debate since.…”

Tectonometamorphic evolution of the Atbashi high‐Punits (KyrgyzCAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

“…Maximum temperature recorded by micaschists and calcschists of the accretionary prism has been estimated in a previous study at ~340°C by RSCM (Jourdon, Petit, et al., ). In addition, chlorite temperatures were estimated for samples KG‐14‐58 and KG‐14‐66, which range between 250–300°C and 230–275°C, respectively (Figure b).…”

“…Therefore, despite the blue appearance of these rocks, due to Mg‐riebeckite (and not to glaucophane), they did not record any typical blueschist facies metamorphic conditions. Calcschists of the same unit recorded consistent T max at ~340°C (Jourdon, Petit, et al., ). For metabasites, the epidote compositional range combined with chlorite thermometry suggests a near isothermal decompression to 3 kbar, at 280–320°C.…”

“…In this paper, we argue for a model which combines a long‐lived (390–340 Ma) north‐dipping subduction of this oceanic lithosphere below MTS, followed by a short‐lived (340–320 Ma) subduction below the Tarim craton. After this long period of subduction, a transition towards a transcurrent tectonic regime throughout the CAOB accommodated rotations of different blocks in the Tien Shan with respect to Siberia and Tarim cratons, during the Late Carboniferous to Early Permian (Allen & Vincent, ; Allen, Windley, & Zhang, ; Jourdon, Petit, et al., ; Laurent‐Charvet, Charvet, Monié, & Shu, ; Rolland et al., ; Wang et al., ). Coeval with this wrench tectonics, widespread intracontinental magmatism affected the Tien Shan belt and the northern margin of the Tarim craton.…”

“…At the top, this unit is bounded by a top‐to‐the‐South detachment underlined by strongly deformed mafic rocks (Loury et al., ). The accretionary prism unit, which appears also in the upper structural position in the Ak‐Talaa area (Figure b,c), is also in contact with the HPMU along the top‐to‐the‐South detachment. This unit underwent low‐grade metamorphism and an intense polyphase deformation resulting in folding and development of a NE–SW schistosity dipping towards the north or south (Jourdon, Petit, et al., ). The uppermost unit is constituted of low‐grade to unmetamorphosed sedimentary rocks, Silurian to Devonian in age (based on the occurrence of Tabulata corals), and is mainly composed of limestones, shales and sandstones (Biske et al., ). In some places, as in the Tash‐Rabat Valley, the limestones are metamorphosed, as suggested by the presence of marbles, but metamorphic conditions are unconstrained.…”