Structure of the lower crust beneath the Songpan block and West Qinling orogen and their relation as revealed by deep seismic reflection profiling

Lying at the northeastern Qinghai-Tibet Plateau, the Songpan block is tectonically located at the junction between the E-W- and N-S-trending tectonics of continental China. The Songpan block has long controlled and affected the formation and evolution of continental China owing to its special tectonic environment. Understanding the fine structure of the lithosphere beneath the Songpan block and revealing its relation to the East Kunlun-West Qinling orogen not only laid a foundation for studying the deep processes of plate collision at the northeastern margin of the Qinghai-Tibet Plateau but also are concerned with petroleum prospect evaluation in the Songpan block. A -257-km-long, deep seismic reflection profile across the Zoige basin on the northern margin of the Songpan block and the West Qinling orogen were for the first time surveyed in 2004. The profile has revealed the fine lithospheric structure of the Songpan block and the West Qinling orogen and found that both the Songpan block and the West Qinling orogen belong to the same stable continental block, and that their lower crusts are both characterized by north-dipping strong reflection. The north-dipping reflection constitutes seismic evidence for the northward subduction of the lower crusts of the Songpan block and West Qinling orogen. The nearly level Moho reflections of the Zoige basin and the West Qinling orogen show that they underwent intense extension after coUisional orogeny.