Phanerozoic evolution of the lithosphere beneath the Sino-Korean craton

Large areas of northern China are underlain by the Sino-Korean Craton, which was stabilized in Early Proterozoic time. However, the eastern part of this craton has been tectonically active since the late Mesozoic, with the development of large sedimentary basins, high heat flow, extensive seismicity and widespread volcanism of both calc-alkaline and intraplate type. Analysis of garnet and spinel concentrates from diamondiferous kimberlites shows that an Archean lithospheric keel ca 200 km thick, characterized by low heat flow and containing abundant harzburgitic rocks interlayered with depleted lherzolites, existed beneath western Shandong and southern Liaoning Provinces in mid-Ordovician time. In these areas, geophysical data now show a thin lithosphere (60-120 km) and an elevated geotherm. This requires the removal or transformation of 80-140 km of Archean lithosphere since Ordovician time. Mineral-chemical evidence from Mesozoic-Tertiary kimberlites in the Taihang-Luliang and Telling areas, and the compositions of xenoliths in Tertiary basalts, suggest that where the lithosphere now is less than 80-100 km thick, the mantle portion consists of fertile Phanerozoic Iherzolite. Regional variations in sub-Moho seismic velocities, and isotopic data, suggest that some relict buoyant depleted Archean (or locally Proterozoic) mantle may be present beneath areas with thicker lithosphere, and that this is underlain by more fertile Phanerozoic lithosphere. The western boundary of the lithosphere replacement probably coincides roughly with the North-South Gravity Lineament, a prominent gradient zone separating a western domain of large negative Bouguer anomalies from an eastern domain with small positive to negative anomalies. The replacement of cool Archean lithosphere by hot Phanerozoic material can explain the major uplift of the area, with the formation of narrow rift basins, in Jurassic time. Subsequent thermal relaxation would cause the widespread subsidence that produced the broad sedimentary basins with associated positive gravity anomalies, that cover much of the eastern part of the craton. The lithosphere replacement may have involved thermal erosion and lateral displacement related to convective overturn accompanying Jurassic-Cretaceous subduction of the Kula Plate, and Cretaceous-Tertiary subduction of the Pacific plate. Alternatively, the removal of the Archean keel could be related to a Triassic collision between the North China block and the Yangtze craton to the south. However, neither model appears to offer an adequate explanation for the present distribution of thinned lithosphere, as mapped by geophysical data.