Abstract:
Two major events in the late Mesozoic that synchronously occurred in the Pacific and periphery are intensive tectono-magmatic pulses in the Circum-Pacific mobile belt and on the Darwin Rise. The geodynamics relating them appears to be the gravity instability between the Pacific-wide head of a super plume and its lithospheric overburden. The dynamics controlling the tectono-magmatic event in the Circum-Pacific mobile belt is the centripetal, inclined upwelling of a thermal plume driven by backwash current at the margin of the overburdened lithosphere. The backwash current causes the outward-decreasing pressure gradient in the plume head, and, coupling with active upwelling through the super plume trunk, swells the lithosphere to form the Darwin Rise with widespread magmatism. Superimposed upon the above geodynamics of axial symmetry is the eastward deflective force acting on buoyant masses ascending through the rotating Earth's interior. The resultant E-W asymmetry is represented by the marked eastward migration of the front of the west Pacific continental margin and by the eastward facing foreland thrust belts and the two eastward convex arcs within continent gaps in the east Pacific continental margin. The Cretaceous to Eocene ophiolites in southeastern Asia and Melanesia may be fragments of the decomposed margin of the west to southwest Darwin Rise, because of their contemporaneity of igneous ages and their geometrical concordance. If the circum-Pacific Phanerozoic multiple ophiolites had origins and decomposing processes similar to those in southeastern Asia and Melanesia, the Ordovician ophiolites, which is oldest among the multiple ophiolites, could mark the birth of the Pacific in the earliest Phanerozoic.