Seismic evidence for the absence of deeply subducted continental slabs in the lower lithosphere beneath the Central Orogenic Belt of China

摘要

A study of teleseismic P-wave tomography was carried out along the Central Orogenic Belt of China (COBC) consisting of the Qinling, Tongbai, Hong'an, Dabie and Sulu orogens that were created by northward subduction of the South China Block beneath the North China Block to subarc depths in the Paleozoic to Mesozoic. The results show the presence of high velocity anomaly underneath the COBC and its adjacent cratonic regions but the absence of high velocity anomaly in the lower lithosphere beneath the COBC. The high velocity anomaly is deep underneath the COBC, possibly representing the relic of the subducted Paleo-Tethyan oceanic slab predating the continental deep subduction of the South China Block. The high velocity anomaly is also deep underneath the southeastern part of the North China Block, possibly being a relic of the subducted Paleo-Pacific slab. However, no relic of the deeply subducted continental slab is seismically detected in the lower lithosphere beneath the Dabie and Sulu orogens, though the occurrence of ultrahigh-pressure metamorphic rocks in the two orogens has clearly demonstrated continental deep subduction to subarc depths in the Triassic. Instead there is only the relatively flat and uniform Moho boundary beneath the COBC region, indicating that the subduction-thickened continental lithosphere along the COBC has lost its root. It is inferred that the orogenic root beneath the COBC region has been foundered into the asthenospheric mantle at the postcollisional stage. The foundering would have taken place in the Early Cretaceous as indicated by the extensive occurrence of magmatic and migmatitic rocks along the COBC. As a consequence, the seismic tomography cannot detect the deeply subducted continental slab remnant from the Triassic period. It is possible that the tomographic visibility of continental subduction in the lower lithosphere is limited to the past 100 Myr.