Geophysical Consequences of Lithosphere Erosion: Eastern Sino-Korean Craton

W L Griffin (GEMOC, Earth Sciences, Macquarie Univ., Sydney 2109, and CSIRO-EM, Box 136, NSW 2113)

S Y O'Reilly (GEMOC, Earth Sciences, Macquarie Univ., Sydney 2109, Australia), A Zhang (Institute of Mineral Deposits, CAGS, Beijing 100037, China) and C G Ryan (CSIRO-EM, Box 136, NSW 2113, Australia)

The Sino-Korean Craton was stabilized in Early Proterozoic time, but 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 a depleted Archean lithospheric keel ca 200 km thick, with low heat flow, existed beneath Shandong and Liaoning Provinces as late as 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 and from xenoliths in Tertiary basalts, suggest that where the lithosphere now is less than 80-100 km thick, the mantle portion consists of fertile Phanerozoic lherzolite. Regional variations in sub-Moho Vp suggest that some relict buoyant depleted Archean (or locally Proterozoic) mantle may remain beneath areas with thicker lithosphere. The western boundary of the lithosphere replacement probably coincides with the North-South Gravity Lineament, a prominent gradient separating a western domain of thick crust and large negative Bouguer anomalies from the eastern part characterized by small positive to negative anomalies.

The replacement of cool Archean lithosphere by hot Phanerozoic material led initially to major uplift, with the formation of narrow rift basins. Subsequent thermal relaxation and the higher intrinsic density of the Phanerozoic mantle lithosphere caused widespread subsidence, producing the broad sedimentary basins, with associated positive gravity anomalies, that cover most of the eastern part of the craton. Replacement of the Archean depleted mantle probably involved both thermal erosion and lateral displacement related to subduction-induced convective overturn. The first stage of the process may have accompanied subduction of the Kula Plate in Jurassic-Cretaceous time, with a second stage related to Cretaceous-Tertiary subduction of the Pacific plate.