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Evidence for three Cenozoic phases of upper crustal shortening of the Xiongpo structure in the Longmen Shan fold-and-thrust belt, China: Implications for the eastward growth of the eastern Tibetan Plateau

Abstract:

Cenozoic crustal shortening across the Longmen Shan (LMS)fold-and-thrust belt records the eastward growth of the Tibetan Plateau. Although the structural geometries of individual structures in the region have been studied, the kinematics and rates of deformation across these structures remain unclear. In this study, we construct a geological cross section to derive the subsurface structural architecture of the Xiongpo structure in the LMS piedmont using one seismic reflection profile, and data from two boreholes. The structure displays a superimposed deep fault-bend fold and shallow structural wedge with three splay thrust faults. Using two-dimensional kinematic models, based on fault-bend folding algorithm, we reconstruct three episodes of upper crustal shortening, with 7.0 ± 0.3 km of total shortening. Combining with previously published low-temperature thermochronology data, we further estimate that the structure has experienced an average shortening rate of ∼ 0.2 mm/yr since 40–25 Ma. We infer that the structure acted as the deformation front of the LMS fold-and-thrust belt between 40 and 25 Ma and 15–10 Ma driven by the eastward growth of the Tibetan Plateau. Finally, we review the shortening rates of other five structures within the LMS piedmont, and find that these six structures (including the Xiongpo structure)individually have undergone an average shortening rate < 0.5 mm/yr on each fault, suggesting that strain is partitioned along several structures within the LMS fold-and-thrust belt. We also show that the southern LMS piedmont has absorbed more upper crustal shortening than the central LMS, ∼0.5–0.7 mm/yr, since ∼2.6 Ma.