Episodic provenance changes in Middle Permian to Middle Jurassic foreland sediments in southeastern Central Asian Orogenic Belt: Implications for collisional orogenesis in accretionary orogens

Highlights

• Foreland deposition brings insight into collisional dynamics in accretionary orogens.

• Detrital U-Pb-Hf data reveal episodic changes of sedimentary provenance in Linxi.

• First arrival of northern NCC detritus to the frontal NAO by ca. 270 Ma.

Abstract

The final collisional orogenesis in accretionary orogens is crucial for reconstructing the whole orogenic circle and understanding the material and energy recycling along convergent margins. However, it is always not straightforward to be addressed, particularly when the final suturing following the ocean closure occurred softly, for instance, the Central Asian Orogenic Belt (CAOB). In this study, a comprehensive dataset of detrital zircon U-Pb ages and Hf isotopes for the Middle Permian to Middle Jurassic foreland sediments from the Linxi region, southeastern CAOB, is synthesized. We observed multiple abrupt changes in sedimentary provenance over time from this dataset: (1) the ca. 275 Ma sediments (Upper Zhesi Fm.) were entirely sourced from the local NAO, whereas the ca. 270–258 Ma sediments (Middle-Upper Linxi Fm.) received an additional detritus contribution from the northern NCC, (2) the ca. 257–236 Ma sediments (Lower-Middle Xingfuzhilu Fm.) returned to a pure NAO source, but the slightly postdated ca. 227 Ma sediments (Upper Xingfuzhilu Fm.) show provenance affinities of both NAO and northern NCC, (3) the ca. 171 Ma sediments (Middle Xinmin Fm.) were exclusively derived from the Late Paleozoic to Early Mesozoic magmas in the NAO and northern NCC with no Early Paleozoic and Precambrian detritus. The above episodic shifts in sedimentary provenance observed in the Linxi region, in conjunction with other regional geological evidence, provide key constraints on the pre- to post-collisional evolution in the southeastern CAOB, including the transition from waning subduction to initial collision, subsequent slab break-off and intracontinental contraction, and the post-collisional extension before the superposition of the Paleo-Pacific tectonic regime. Our results show that understanding the spatio-temporal changes of sedimentary provenance of the foreland basins systems during the plate convergence and orogenic evolution can aid in exploring the final collisional dynamics and evolutionary history in ancient accretionary orogens.

Introduction

Reconstructing the terminal collision after the complete consumption of the oceanic lithosphere in accretionary orogens is the prerequisite to understanding material recycling and energy exchange processes along the convergent margins (Condie, 2007; Cawood et al., 2009). However, it could be rather challenging if the final suturing process takes place in a “soft” manner without typical continent-continent collisional signatures (such as large-scale crustal shortening/thickening and high to ultra-high grade metamorphism; Song et al., 2014), as exemplified by the Appalachian orogen in North America (Draut and Clift, 2013). The same is true of the giant Central Asian Orogenic Belt (CAOB) that developed between the North China, Tarim, and Siberian cratons in central-east Asia (Windley et al., 2007; Cawood et al., 2009; Xiao et al., 2015; Song et al., 2015, Song et al., 2021; Eizenhöfer and Zhao, 2018).

A general view is that the CAOB was built by the amalgamation of numerous orogenic components, including arcs, microcontinents, seamounts, oceanic crust fragments, and accretionary complexes during the prolonged subduction of the Paleo-Asian Ocean (PAO) from Neoproterozoic to Early Mesozoic (e.g., Windley et al., 2007; Xiao et al., 2015; Şengör et al., 2018). In the southeastern section of CAOB, the PAO ultimately closed through a soft suturing of the opposing accretionary prisms along the Solonker suture zone (SSZ) (Fig. 1a) (Sengör and Natal'in, 1996; Xiao et al., 2015; Fu et al., 2018; Song et al., 2018, Song et al., 2021; Eizenhöfer and Zhao, 2018). Although the life span of this final collision has been roughly constrained to the Middle Permian to Middle-Late Triassic period (Xiao et al., 2009; Li et al., 2014a; Li et al., 2017a; Eizenhöfer and Zhao, 2018; Song et al., 2021), how it evolved remains enigmatic, especially considering that different deep dynamics, including slab break-off, moderate crustal thickening, and post-orogenic delamination, might be involved (Jian et al., 2010; Li et al., 2014a; Li et al., 2017a). Thus, reconstructing the last episode of the orogenic evolution in the southeastern CAOB requires new solid evidence and a more comprehensive understanding of the available information.

Generally, foreland basin systems associated with the retroarc, peripheral (or collisional), and retreating collisional settings dominate the sedimentary accumulation on convergent margins during the plate convergence and orogenic evolution (Supplementary Fig. S1) (e.g., Garzanti et al., 2007; DeCelles, 2012). Also, major geodynamic processes on convergent margins are usually invoked where abrupt changes in sedimentary provenance emerge (Cawood et al., 2012; Gehrels, 2014). Thus, evaluating the spatio-temporal provenance changes of the foreland sediments on convergent margins can aid in reconstructing the final orogenic evolution in ancient accretionary orogens. For the southeastern CAOB, in association with the pre- to post-collisional evolution, different foreland basin systems had contributed to the clastic deposits in the frontal region of the northern accretionary orogen (NAO), north side of the SSZ. In this paper, we focused on a comprehensive dataset of detrital zircon U-Pb ages and Lu-Hf isotopes of the Middle Permian to Middle Jurassic sediments from the Linxi region of the frontal NAO (Fig. 1b), and identified episodic changes in sedimentary provenance from ca. 275 Ma to ca. 171 Ma. These results, together with other regional geological evidence, provide significant insights into the last episode of orogenic evolution in the southeastern CAOB.