The extreme southeastern part of the Papuan peninsula is composed mainly of middle Eocene submarine basalt which resembles mid-ocean-ridge tholeiite. During middle Miocene time, the basaltic pile was intruded by potassic rocks, with which pronounced gravity and magnetic anomalies are associated. Although continental rocks have not been found in this area, geophysical data indicate that the crust is of near to normal continental thickness. The uplift of the basalt may be due to differentiation and expansion of the underlying mantle under a tensional regime, and the potassic rocks may be the surface expression of diapirs formed during that differentiation.

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