Shallow Structure of the Crust in the Sulu-Dabie Region,China and its Seismotectonic Implication

The P-wave velocity structure in the shallow crust is investigated in and around the Sulu-Dabie region by using seismic reflection data for deep soundings in 48 survey profiles and from rock velocity determinations.The observed velocity distributions show obvious heterogeneities in this region.The low velocity anomalies are observed mainly in the west of the Dabie region and the East Sea regions.The high velocity anomalies emerge in the shallow crust of the Sulu and Dabie orogeny.These high-velocity anomalies can be attributed to the ultra-high pressure metamorphosed(UHPM)rock formed by exhumation motion of mantle materials during the orogeny.The high-velocity anomalies in the different shallow layers beneath the Sulu region are located to the northeast of the Tan-Lu fault.The high-velocity anomalies beneath the Dabie region are located southwest of the Tan-Lu fault.Such a distribution pattern of velocity anomaly zones may reveal historical motion of a left-lateral strike-slip for the Tan-Lu fault,which differs from the result of a right-lateral strike-slip motion regime known from modern seismology,indicating a more complex tectonic motion along the Tan-Lu fault.

Petrotectonics of Sulu-Dabie metamorphic belt,Central and East China

The Sulu-Dabieshan metamorphic belt separates the North China and Yangtze cratons. This metamorphic belt is further divided into three petrological-tectonic units which are respectively equivalent to a root zone of an Andean-type magmatic arc (northern Dabie terrane), continental collision suture zone (southern Dabie terrane) and metamorphosed cover sediments of the Yangtze continental margin (Susong metamorphic complex). They are regularly located in the space from north to south in Dabieshan and roughly from northwest to southeast in Sulu region in spite of being strongly reconstructed by the Tan-Lu Fault. The occurrence of the Mesozoic granites and alkali dike swarm are also regular in the space. They occur in the Dabie area, especially in the northern Dabie terrane. These facts support a suggestion that there was subduction of the Yangtze Plate under the North China Plate or obduction of the North China Plate over the Yangtze Plate. Up to now, no evident remnant of old occean crust has been identified. Ultrahigh pressure metamorphic (UHPM) rock-bearing southern terrane is possibly a root of continent-continent collision zone representing a suture between the North China and Yangtze Plates. The fact that UHPM lenses were tectonically emplaced into not-high pressure metamorphic host gneisses indicates deeply subducted rocks uplifted to middle-upper crust after continental collision. The UHPM rocks and related rocks occurring on the surface at the moment suffered at least two stages of uplifting and exhumation.

Deep crustal structures of eastern China and adjacent seas revealed by magnetic data

Through reduction to the North Pole and upward continuation of the total field magnetic anomalies, we analyze magnetic patterns and spatial distributions of different tectonic blocks and crustal faults in eastern China and adjacent seas. Depths to the Curie isotherms are further estimated from radially averaged amplitude spectra of magnetic data reduced to the pole. Data reductions effectively enhance boundaries of regional tectonic belts, such as the Dabie ultra-high metamorphic belt, the Tanlu Fault, and the Diaoyudao Uplift. Curie depths are estimated at between 19.6 and 48.9 km, with a mean of 31.7 km. The Subei Basin and the south Yellow Sea Basin in the lower Yangtze block show relatively deep Curie isotherms, up to about 35 km in depth, whereas in the surrounding areas Curie depths are averaged at about 25 km. This implies that the lower Yangtze Block has experienced a unique tectonic evolution and/or has unique basement lithology and structures. From a regional perspective, sedimentary basins, such as the Subei Basin, the south Yellow Sea Basin, and the East China Sea Basin, normally show deeper Curie isotherms than surrounding uplifts such as the Diaoyudao Uplift and the Zhemin Uplifts. Curie isotherms also upwell significantly in volcanically active areas such as the Ryukyu Arc and the Cheju Island, confirming strong magmatic and geothermal activities at depth.