Cenozoic tectonic evolution of the South Ningxia region, northeastern Tibetan Plateau inferred from new structural investigations and fault kinematic analyses

• New structural data suggest a two-stage Cenozoic deformation in NE Tibet.
• The first is featured by basin formation and inversion from ca. 30 to 9.5 Ma.
• The second comprises mountain building and redeformation since ca. 9.5 Ma.
• The structural data show clockwise rotation of σ1 from NE to ENE in Quaternary.

The northeastward growth process of the Tibetan Plateau has been a key issue to be long-termly debated, which is well documented by the South Ningxia region at the northeasternmost periphery of the Tibetan Plateau due to its Cenozoic convex-to-the-northeast arcuate range-and-basin structures. Here new structural measurements and fault kinematic analysis, together with new geochronological results, delineate a two-stage Cenozoic tectonic evolution in the region, providing the new structural evidences to decipher the northeastward growth process of the Tibetan Plateau. The first stage is marked by widespread sedimentary basin formation during the Oligocene to Late Miocene (ca. 30–10.5 Ma) under NW-SE extensional regime, and subsequent basin inversion triggered by NW-SE compression in the Late Miocene (ca. 10.5–9.5 Ma); these events were probably associated mainly with the far-field effect of the northwestward subduction of the Pacific Plate, a little related to the plateau growth. The second stage largely presents an intensive mountain building process and following re-deformation since the Late Miocene (ca. 9.5 Ma), which was predominantly a result of the far-field responses to the northeastward growth of the Tibetan Plateau, and partially to the Pacific subduction. The second stage further comprises three alternating episodes of shortening and extension events. The earliest episode of NE-SW compression caused intensive crustal shortening in a long period from the Late Miocene to Early Pleistocene (ca. 9.5–1.8 Ma), leading to a significant mountain building process in this region. A gentle NE-SW extension subsequently dominated here in the Late Pleistocene (?–ca. 18 ka), as manifested by the widespread appearance of paleo-lakes along the NW-striking boundary faults. Since the end of the Late Pleistocene (ca. 18 ka), a youngest tectonic regime affected this region, characteristic of a tectonic transpression, causing evidently strike-slip activities on the primary faults and the development of small-scale pull-apart basins along the Haiyuan Fault.

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