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