GEMOC ARC National Key Centre

TERTIARY LITHOSPHERE REPLACEMENT BENEATH THE EASTERN SINO-KOREAN CRATON:  DENSITY CONSIDERATIONS

S.Y. O'Reilly, W.L. Griffin and Y. Poudjom Djomani, GEMOC Macquarie University

In the eastern Sino-Korean Craton, Paleozoic kimberlites sampled a harzburgite-rich Archean lithosphere ca 200 km thick, with a very low geotherm.  Tertiary parakimberlites and alkali basalts in the same region sampled a lithosphere 60-110 km thick that consists of fertile lherzolites; xenolith data define a high geotherm characteristic of advective heat transport.  These data imply removal of most, if not all, of the Archean lithosphere beneath the Tertiary sampling sites, and its replacement by fertile Phanerozoic lithosphere, similar to that found beneath other young extensional areas.  However, calculations of mean lithosphere compositions and densities show that the Archean lithosphere was *2% lighter than the asthenosphere, and could not have "delaminated"; this raises the question of how the observed lithosphere replacement occurred.  Detailed seismic tomography (Yuan 1996) suggests that the Archean lithosphere was rifted and dispersed by asthenospheric material that welled up along narrow zones, and spread out below the crust.  This produced a 70-100 km zone of fertile mantle overlying a mixture of ancient and new mantle material.  Calculations show that the heating involved in this process will initially reduce the density of the lithospheric column significantly, leading to rapid uplift, even though the Phanerozoic lithosphere is intrinsically =1.5%  denser than the Archean lithosphere.  The displacement of Archean mantle by hot asthenospheric material also will provide heat for crustal magmatism.  On cooling, the overall density of the lithospheric column will significantly exceed that of the original Archean column, and thermal relaxation will lead to subsidence and basin formation, as observed in the Bohai Gulf area today.  Maps of lithospheric thickness and heat flow suggest that translithospheric fault systems such as the TanLu zone have played a major role in focussing asthenospheric upwelling.  The distribution of large negative Bouguer anomalies to the west of the North-South Gravity Lineament, and small positive to small negative anomalies to the east, implies that the lithosphere replacement is concentrated east of the Lineament.