Crustal architecture of the Bangong-Nujiang Suture reveals Meso-Tethyan bidirectional subduction: Evidence from Pn-wave receiver function imaging

The Bangong-Nujiang Suture (BNS), a pivotal tectonic boundary on the Tibetan Plateau, is widely recognized as the result of the Meso-Tethys Ocean closure. Magmatic arc rocks and porphyry Cu deposits adjacent to the BNS are clearly associated with oceanic lithosphere subduction; however, the debated subduction polarity remains a critical unresolved issue in reconstructing the Meso-Tethyan evolution. To better constrain the deep architecture beneath this critical suture zone, we conducted Pn-wave receiver function analysis using INDEPTH-III broadband seismic data, generating high-resolution crustal images along the BNS. Our seismic profiling reveals two significant structural characteristics: (1) systematic lateral discontinuities in intracrustal interfaces across the BNS, and (2) a distinctive Moho morphology characterized by a central upward arch (∼180 km width) with its southern and northern extensions reaching into the upper mantle. We interpret this arcuate Moho configuration as preserved remnants of Meso-Tethyan oceanic crust entrapped during terminal convergence between the Lhasa and Qiangtang Blocks. These findings provide unprecedented seismological evidence supporting a bidirectional subduction model, wherein the Meso-Tethyan oceanic lithosphere underwent bidirectional subduction prior to continental collision. This study offers new insights into the complex geodynamic processes associated with the closure of the Meso-Tethys Ocean and the subsequent Tibetan Plateau formation.