TECTONIC DEVELOPMENT OF THE INCIPIENT BANDA ARC-CONTINENT COLLISION: GEOLOGIC AND KINEMATIC EVOLUTION OF SAVU ISLAND, INDONESIA

The island of Savu consists of north- and south-verging thrust sheets of Late Triassic to Middle Jurassic Australian continental margin units rimmed by a discontinuous mélange belt, fragments of forearc basement and synorogenic units. A previously unrecognized pillow basalt unit was also found inter-layered with the Jurassic Wai Luli Formation. North-verging folds are associated with propagation of the Savu thrust system, which moves the back of the accretionary wedge over the southern Savu forearc basin. South-verging thrust sheets make up the bulk of the island and are well-exposed in southern Savu. Composite area-balanced cross sections imply that the detachment for the thrust sheets is in the lower Babulu or upper Aitutu Formations, at around a depth 2600 m. and a maximum age for the initiation of collision of 4.0 Ma without producing excess mass.

Scaled sandbox deformation experiments of the collision produced two separate zones of thrust stacking like that observed in Savu. The upper-most units of the Australian continental margin stack as imbricate thrusts at the toe of the backstop. Lower units form duplexes that stack underneath the backstop. A zone of mélange developed along the top and bottom of the backstop where shear strains were greatest.

Analysis of foraminifera in synorogenic units from Savu indicates that at 1.8 Ma the outer arc was 1.0-1.5 km below sea level, which yields a long-term surface uplift rate of around 1mm/yr. At this rate, it would take a maximum of 3.2-5.0 Ma to uplift these deposits from a pre-collisional submarine depth of 3.5-4.0 km. The surface uplift pattern of the sandbox model mimics that observed in uplifted coral terraces in Savu, with the highest rates above the Savu thrust and duplex zone.

This study demonstrates the fundamental influence of backstop geometry and mechanical stratigraphy in shaping the orogenic wedge during the transition from subduction to collision.