Tracing the “missing shortening” in fold-and-thrust belts: Insights from structural analyses of the Hotan-Mazatagh transect in the West Kunlun foreland, NW China

Abstract

Use of balanced cross-section methods, which assume length and area conservation in two dimensions, to quantitatively analyze fold-and-thrust belts is vital in structural analyses. However, layer-parallel shortening is beyond this assumption and thus indescribable by balanced cross-section methods. The contribution of layer-parallel shortening in accommodating shortening has rarely been quantitatively determined, especially in sections hundreds of kilometers in scale. In this study, we conducted structural analyses on the ∼230-km-long Hotan-Mazatagh transect along the West Kunlun foreland of the NW Tibetan Plateau, based on interpretations of seismic reflection profiles. The results reveal that the transect forms a long thrust sheet detaching along the basal Cenozoic detachment, with deformation localized in the Hotan, Manan, and Mazatagh belts. Out of the ∼23.82 km of cutoff displacement translating into the thrust sheet, only ∼1.4 km and ∼4.94 km are absorbed by the Hotan anticline and the Mazatagh detachment thrust, respectively. The remaining ∼17.48 km extent of shortening is absorbed by layer-parallel shortening in the ∼180-km-long section between the Hotan and Mazatagh belts. The layer-parallel shortening produces a strain rate of ∼2.43 × 10−8 yr−1 and absorbs ∼77.97% of the shortening of the segment. Our results argue for caution when using balanced cross-section methods to determine deformation magnitudes. We suggest that localized deformation and layer-parallel shortening function together to shape the ∼230-km-long Hotan-Mazatagh transect into a combined discrete-and-distributed deformation style.