Accretion, collision and strike-slip faulting: the Woyla group as a key to the tectonic evolution of North Sumatra

Abstract

The Woyla Group exposed in the Batang Natal River, North Sumatra, is made up of fault-bounded silvers of serpentinite, pillow basalt, ribbon chert, pelagic silstones, mélanges, andesitic volcanics, volcaniclastic turbidites and massive limestones. This assemblage has previously been interpreted as fragments of a collapsed Jurassic-Late Cretaceous marginal basin. In the present study the Woyla Group is re-interpreted as part of an accretionary complex formed from ocean floor materials of Triassic to Early Cretaceous age, incorporating collided sea-mounts, plateaux and volcanic arc fragments accumulated during the subduction of a major ocean, Tethys III, prior to India's collision with Asia. Early Cretaceous fossils and Late Cretaceous intrusions constrain the time of accretion to mid-Cretaceous. The Langsat volcanics, associated volcaniclastics and intrusive granodiorites at the western end of this section, have been dated by the K/Ar method as Late Oligocene, demonstrating that they are unrelated to the rest of the complex and were emplaced along strike-slip faults prior to the mid-Miocene. The Woyla Group is considered in the context of the tectonic evolution of North Sumatra, resulting from the break-up of Gondwanaland and the accumulation of its fragments along the margin of Asia, the collision of India, the opening of the Andaman Sea and the initiation of the present arc and the Sumatran strike-slip fault zone.