Abstract

The Xiaoxia anticline is a basement-cored fold in the northeastern Qinghai–Tibetan Plateau. It formed during the Middle–Late Cenozoic in a NNE-orientation oblique to prevailing WNW-striking thrusts in the surrounding region. The fold lies within the fault-bounded Xining Basin, which has behaved as a clockwise rotating and internally deforming block during the mid–late Tertiary. Proterozoic crystalline basement rocks and Jurassic–Cretaceous–Cenozoic terrestrial sediments exposed within the fold record three separate oblique-slip deformation events as determined by cross-cutting generations of faults and folds. The modern Xiaoxia anticline developed by: (1) outward-directed thrusting on the two fold limbs and, (2) oblique-slip thrusting on closely spaced minor NNE-striking faults that caused distributed uplift and passive shear folding of the sedimentary cover within the fold core. Tilted and thrusted river terraces on the flanks of the Xiaoxia anticline dated by optically stimulated luminescence and electron spin resonance indicate that folding has continued into the Late Quaternary (post-45.5 ± 5.5 ka) and therefore may still be active today. The development of the Xiaoxia anticline appears to be best explained in the context of the kinematic evolution of the Xining Basin's bounding strike-slip faults. Formation of the Xiaoxia anticline demonstrates that partitioned transpression in an evolving transpressional setting can invert intra-montane basins and produce basement-cored folds, thereby contributing to regional plateau uplift.

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