关键词: 华北克拉通破坏, 岩石圈厚度, 地幔转换带, 古老岩石圈薄弱带

Abstract:

 A better understanding of the Phanerozoic tectonic reactivation and destruction of the North China Craton (NCC) demands a detailed knowledge of the deep structural features of the region. We have migrated teleseismic P and Sreceiver functions from dense seismic array data to construct the structure images of the lithosphere and the upper mantle along three profiles that traverse the three constituent parts of the NCC. Our images reveal substantial structural differences among different parts of the craton. In contrast to the Eastern NCC where significantly thinned lithosphere (60100 km) is widespread, the Central and Western NCC are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere. The thick lithosphere (>200 km) is present beneath the stable Ordos Basin and the thinned lithosphere (up to 80 km) is found in the Late Cenozoic YinchuanHetao and ShaanxiShanxi rift areas, with the sharpest change occurring over a lateral distance of ~200 km. Near the boundary between the Eastern and Central NCC, a rapid thickening of the lithosphere by 2040 km over ~100 km laterally is observed, concordant with abrupt changes in surface topography and roughly coincident with the NorthSouth Gravity Lineament. It also corresponds to sharp changes in crustal structure, mantle transition zone thickness and structure of the 660 km discontinuity. Such a shallowdeep structural concordance throughout the whole upper mantle may be indicative of different Phanerozoic lithospheric tectonics and mantle processes in the two domains. The much thinned crust and lithosphere, thick mantle transition zone and complex structure of the 660 km discontinuity in the Eastern NCC may reflect the strong influence that the deep subduction of the Pacific slab and associated processes have had on mantle dynamics and lithospheric reactivation of the region since the Mesozoic time. The presence of thinned lithosphere in the Central and Western NCC together with petrological and geochemical data suggest that lithospheric remobilization and thinning may have affected the NCC much further to the west than previously thought. The rift areas in the region, which are underlain by significantly thinned lithosphere, appear to be related to ancient mechanically weak zones. These preexisting lithospheric structures may have been repeatedly reactivated by successive tectonic events during the longterm evolution of the NCC, and particularly have been weakened and thinned by the Cenozoic IndiaEurasia collision which eventually caused rifting in these areas. On the other hand, the generally relatively thick crust and lithosphere and normaltothin mantle transition zone beneath the Central and Western NCC suggest that multiple tectonic events, including the IndiaEurasia collision, may have had a relatively weak effect on the tectonic evolution of this region compared to that of the Pacific subduction on the Eastern NCC. This is plausibly responsible for the preservation of the cratonic nucleus under the Ordos Basin and the marked lithospheric heterogeneity of the region.

Key words:  destruction of the North China Craton, lithospheric thickness, mantle transition zone, preexisting lithospheric weak zone