The thickness variation of the continental crust at the convergent margin is intricately linked to the dynamic interaction between tectonic and magmatic processes and is also a direct response to the deep Earth dynamics. The Xar Moron tectonic belt underwent a major tectonic transformation from extension to compression during the Early Triassic, and studies on its deep dynamic process is of significance for understanding the tectonic evolution of the Central Asian Orogenic Belt. In this study, through detailed field geological survey, a series of wide and gentle northeast-trending folds and south-dipping thrust faults were found in the upper crust of the Xar Moron tectonic belt, with deformation patterns characteristic of thin-skinned structures, which shows a unidirectional shortening from northwest to southeast. Balance cross-section restorations suggest that the structural deformation caused similar to 30% of shortening and similar to 4km of upper crustal thickening. A crustal thickness-age curve has been constructed using published geochemical data of Permian to Triassic magmatic and sedimentary rocks in the Linxi area. The data suggests that during the Early to Middle Permian, the crustal thickness was continuously thinned from 49km to 33km, indicating an overall extension period. In the Late Permian to Early Triassic, the crustal thickness was increased by similar to 15km, and reached a peak thickness of similar to 48km. This rapid crustal thickening process may be related to vertical crustal accretion caused by magmatic activities and mountain-building material stacking related to tectonic activitives. Based on the estimated area of magmatic rocks during the convergent period and the thickness of the crust, it is believed that the newly-formed juvenile crust account for similar to 11% of the total crust. The interaction between the deep crust and mantle differed between these two periods. During the Early Permian in the Xar Moron tectonic belt, the epsilon(Hf) (t) of igneous zircons was relatively high (6.1 similar to 19.9; averaged at 10.1), with scattered delta O-18 values (5.1 parts per thousand similar to 8.3 parts per thousand). This indicates the addition of mantle-derived materials during the magma formation process and traces the extension process related to deep mantle upwelling in the Linxi area. In contrast, during the Late Permian to Early Triassic, the eHf (t) values of granite zircons relatively decreased (-1.1 similar to 17.2; averaged at 9.3), while the delta O-18 values remained higher than the mantle source value (>5.9 parts per thousand), revealing a continuous remelting and transformation process of homogenous mantle magma. Based on the characteristics of the sedimentary environments, crustal thickness changes, isotope variations of magmatic rocks, the amount of crustal growth, and the unidirectional compressive shorting background in the upper crust, we propose that the Xar Moron tectonic belt may have undergone a mantle subduction. The upwelling of the asthenosphere during the Early to Middle Permian and the increased density of the lower crust and lithospheric mantle during the Late Permian to Early Triassic may be plausible as the deep Earth dynamic drivers behind the occurrence of mantle subduction.
