TECTONIC ANATOMY OF THE ACTIVE BANDA ARC-CONTINENT COLLISION

Multidisciplinary studies of deformation patterns at a range of temporal scales reveal the incremental tectonic development of the early stages of an active collision zone. New GPS measurements throughout the transition from subduction to collision indicate how strain is progressively distributed away from the trench to forearc and backarc thrust systems, and along transverse faults. The active fault systems divide the collision zone into a series of crustal blocks that have similar directions of motion parallel to the lower plate, but at different rates. Several emergent islands within the collision zone are rimmed with flights of coral terraces that reveal the pattern of deformation over temporal scales of 10⁴ – 10⁵ yrs. Coral terraces on most islands are tilted away from nearby crustal block boundaries, and yield rock uplift rates of 1.0-1.5 mm/yr. Beneath the coral terraces are synorogenic sedimentary deposits with abundant forams, which reveal the age and depth of origin of the deposit. Long-term surface uplift rates, at temporal scales of 10⁶ yrs., obtained from these data are similar to short-term rates and further constrain the age of collision propagation to the west. Erosional windows through synorogenic deposits reveal the structure of the orogenic wedge. Detailed structural field studies in Timor, Rote and Savu demonstrate that Australian continental margin cover sequences accrete along multiple pathways. The upper Cretaceous to Pliocene section stacks along imbricate thrusts above a decollement in thick Jurassic mudstones. The underlying sedimentary units stack up as duplexes beneath this detachment. The leading edge of the forearc acts as an accretionary backstop early in the collision, but later is inserted into and deforms the wedge. Eventually the backstop is detached from its roots to become a hinterland-verging passive roof thrust. The final stages of arc-continent collision involve rapid uplift and out-of-sequence thrusting due to impingement between the continental shelf of the lower plate and the volcanic arc of the upper plate. This stage of collision is manifest in central Timor where the forearc basin is only 30 km wide due to encroachment of the retrowedge, arc volcanism has ceased, rapid uplift rates produce a flight of 25 coral terraces, and a new plate boundary forming along the northern edge of the arc consumes the backarc basin.