Abstract

The tectonics and the Tertiary structural history of the Central Sumatra Back-arc Basin in the P.T. Caltex Pacific Indonesia area of operations, situated largely between the Bila River to the north and the Kampar River to the south, are discussed. The structural style in Central Sumatra is produced by a tensional regime strongly overprinted by a (later?) dextral wrench component. Compression does not appear to have been a dominant force; most compression observed in the Tertiary section can be ascribed to that component of wrench tectonics.

A newly described type of fold, which appears to occur in other Sunda Basins also, results from specific depositional and tectonic processes related to wrench faulting. It is named the Sunda Fold. Two regional seismic lines across the Basin, and several short seismic lines across individual structures, illustrate tensional, wrench, and drape structuring.

The shallow Pretertiary basement and thin sediments of the Central Sumatra Basin are characterized by a very high geothermal gradient. The average gradient for 58 wells is 3.38°F/100'. The high geothermal gradient results from crustal fractures penetrating to the upper mantle. An important Mesozoic crustal feature, the Kerumutan Line, separates Pretertiary oceanic and continental crust.