Progressive Emergence and Warping of Islands in the Active Banda Arc-Continent Collision As Recorded By Uplifted Coral Terraces: Tectonic and Geohazards Implications

Abstract:

Uplifted coral terraces throughout the Banda-Sunda Arc transition reveal how strain is distributed over the past thousand to million years in the active arc-continent collision, and are key to identifying hazardous active faults. U-series age analysis of the lowest coral terraces yields surface uplift rates that vary in a non-systematic way along strike from 0.2 to 1.5 mm/a over short wavelengths of a few kms. For example, coral terraces are tilted varying degrees northward in Sumba, SSE in Savu, NNW in Rote and generally south along the north coast of Timor. In all of these cases the tilt is away from zones of active thrusting and folding. In Sumba the forearc is ramping up and over the northern edge of the Scott Plateau along what is likely a north dipping thrust. In Savu the coral terraces rise where the back of the accretionary wedge is ramping up over the forearc basin on the south dipping Savu Thrust. In Rote coral terraces form on the front of the accretionary wedge where it is ramping up over the subducting Australian continental margin. The north coast of East Timor is likely uplifting due to internal thrusting and closure of the Wetar Strait. Localized uplift of circular islands is associated with diapirism. The diapiric island of Kisar is cored by syn-collisional metamorphic rocks.

The association of uplift and warping with short wavelength deformational processes argues against the commonly held interpretation that coral terraces in the Banda arc-continent collision manifest the effects of slab tear or some other lithospheric scale process. The pattern of uplift correlates best with proximity to active faults, folds and diapirs.

In terms of geohazards, flights of uplifted coral terraces are the smoking gun for sources of large earthquakes and tsunami. Many of the terraces show signs of co-seismic uplift. Tsunami deposits with young corals are found on some of these terraces as high as 20 m above sea level.