Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia

Abstract

Kinematic significance and time scales of geodynamic processes forming the Altai OrogenicBelt are addressed through structural and petrological analysis combined with zircon and monazite geochronology. The study area is composed of orogenic lower crust represented by a Devonian migmatite-magmatite complex and orogenic middle and upper crust formed by an amphibolite-facies Ordovician sedimentary sequence and a weakly to unmetamorphosed Devonian volcano-sedimentary cover, respectively. The orogenic lower and middle crust were first affected by moderate thickening, which formed subhorizontal Barrovian metamorphic schistosity. This fabric was reworked by deep crustal melting and intrusion of granite sheets during horizontal extension at 400-380 Ma. Soon after, this horizontal fabric was affected by NW-SE shortening generating crustal-scale upright folding associated with subvertical flow of still partially molten orogenic lower crust. During this event, the orogenic lower and middle crust were tightly juxtaposed with upper crustal sedimentary rocks. The last event was related with a NE-SW oriented convergence resulting in large-scale folding and megafold interference pattern in the Permian at 280-273 Ma. Combined with existing regional data, our results allow proposing a Devonian tectonic switching from compression to extension and back to compression, as a response to variations of subduction dynamics between slab advance and retreat in a Pacific-type suprasubduction system. The Permian folding was associated with the progressive northward exhumation of thermally softened crust. This tectonic evolution is in response to the indentation of the rigid Junggar arc domain into the weak Altai wedge.

Revision of the Chinese Altai‐East Junggar Terrane Accretion Model Based on Geophysical and Geological Constraints

“…Synthesizing previous structural studies (Broussolle et al, 2018; Jiang et al, 2015; P. Li et al, 2015; P. Li, Sun, et al, 2016; Qu & Zhang, 1994; Wei et al, 2007; Zhang et al, 2015), Broussolle et al (2019) came to the conclusion that three Paleozoic deformation phases affected the Chinese Altai rocks. Originally horizontal metamorphic fabric S1 associated with extensive Early to Mid‐Devonian syntectonic magmatism was deformed by upright NE/SW‐oriented F2 folds of Late Devonian age cored by migmatites and granites (Jiang et al, 2015, 2019). Then the whole edifice was reworked by Permian upright NW‐SE F3 folds and steep deformation/cleavage zones transposing all previous structures.…”

“…(a) The Habahe Group, north of Buerjin (modified from Xinjiang, BGMR, 1966). (b) Contacts between the Habahe Group and the Altai Formation, north of Altay city (modified from Jiang et al, 2019). (c) The Kalasu area and the Altai Formation (modified from Broussolle et al, 2018).…”

“…Altogether, the modern structural and petrological studies show that the northern part of the Chinese Altai is represented by the weakly metamorphosed Paleozoic sequences in the west and the high‐grade, magma‐rich orogenic core to the east both affected by Devonian folding (Broussolle et al, 2019). In contrast, the southern Chinese Altai seems to reflect mainly Permian deformation, magmatism, and metamorphism with their intensity progressively increasing toward the EZ in the south (Jiang et al, 2019). This general pattern can be interpreted in terms of terrane accretion, or, alternatively, it may have resulted from a metamorphic‐magmatic reworking and a complex structural reorganization of the primarily homogeneous accretionary wedge as suggested by Broussolle et al (2019).…”

“…Tectonics Permian vertical and tabular deformation zone (Figures 12b and 12c) that is associated with extrusion of partially molten lower crust (Broussolle et al, 2018;Jiang et al, 2019). This deformation zone is composed of granulites and migmatites ( Figure 13) where the decomposition of Fe-rich biotite and the formation of magnetite imposed high magnetic susceptibility of these rocks (Martelat et al, 2014).…”

Garnet U-Pb and O isotopic determinations reveal a shear-zone induced hydrothermal system

“…1a), and occurs in the Early Devonian Kangbutiebao Formation 11 . Studies in the past decade have provided clear magmatic, structural, and metamorphic frameworks, improving our understanding of the geodynamic evolution of the Chinese Altai orogen 12–17 . The Kangbutiebao Formation is mainly composed of thick layers of meta-rhyolitic volcanoclastic rocks interlayered with thin layers of meta-sandstone and marble, all of which underwent greenschist to amphibolite facies metamorphism 18 .…”