THE TECTONIC EVOLUTION AND REGIONAL SIGNIFICANCE OF KISAR ISLAND, INDONESIA

Kisar, a small island in eastern Indonesia, consists of metamorphic rocks encircled by coral terraces and is found exactly where it shouldn't be. It is located northeast of the active arc-continent collision of Timor and between the Outer (non-volcanic) and Inner (volcanic) Banda Arcs, and emerges from a small ridge in the middle of a forearc suture zone 3 km deep. Kisar, noted for decades and mentioned in several regional studies, remains enigmatic. Obvious tectonic questions must be asked regarding its origin, evolution, and present location, and the causes and processes responsible for such. A recent field investigation and laboratory work are now unlocking Kisar's past. Metamorphic rocks, among the youngest exposed in the world, reveal the island's early geologic history, and coral terraces record Quaternary uplift. In a regional context, this study yields insights to active hinterland collisional processes in the Banda Orogen and provides data essential to evaluate the seismic hazard potential of the region.

Morphology of the uplifted terraces was surveyed for spatial data, and temporal data was obtained by U-Th isotope series dating of coral samples. These terraces can be correlated to known sea-level highstands. Combined, these data can estimate the rate of uplift on Kisar for the past 1 Ma. This represents vertical strain in this part of the collision zone. The island's coral terraces are gently warped, indicating the pattern of current deformation. Additionally, loose, relatively “fresh” coral found high on terraces may be strong evidence for large paleotsunamis. Structural, petrographic, and geochemical analyses on metamorphic rocks, ranging in grade from phyllite to amphibolites facies reveal earlier deformational patterns and metamorphic history.

Kisar is at one end of 400 km of young, unstudied arc-continent collision in eastern Indonesia. As a part of this young mountain belt, information gained from the island can be related to hinterland deformational processes in general, which apply to the evolution of most of Earth's mountain systems. Identifying and quantifying orogenic processes is also crucial in assessing seismic hazards in an area that has suffered tremendously in recent years.