NEW INSIGHTS INTO THE STRATIGRAPHIC AND STRUCTURAL EVOLUTION OF THE ACTIVE BANDA OROGEN

The active Banda arc-continent collision is comprised of Australian passive margin cover sequences and portions of uplifted Banda forearc that are commonly juxtaposed and difficult to sort out. Our field studies and U/Pb age analysis helped us assign most units to either Asian of Australian affinity. Age data from the uplifted Banda forearc units indicate an Asian affinity with a maximum U/Pb age of 80 Ma (Standley and Harris, 2006). In contrast, analysis of detrital zircons from sandstones of Australian continental margin cover sequences have peak ages at 237-353 Ma and 1788-1895 Ma. These age constraints provide a reliable new method for reconstructing the stratigraphic and structural evolution of the collision zone.

Petrographic and provenance analysis of Triassic Australian affinity greywacke units have QFL abundances consistent with a proximal syn-rift intracratonic or recycled orogen source, probably from the north. Structural measurements of these units indicate a north-northwest to south-southeast vergence direction and 30-40% average shortening.

Structural reconstructions of the arc-continent collision commonly fail to accommodate the forearc lithosphere. The Banda forearc, which is 200 km wide north of Savu, has been completely over ridden by retro-wedge thrusting north of East Timor. In order to constrain the shortening of the forearc area-balanced structural models were constructed to test different geometries. The first model maintains the integrity of the forearc slab by delaminating the Australian continental margin and wedging the forearc into the accretionary wedge. The second model internally thickens the forearc through a series of thrusts. A non-area balanced model maintains the shape of the subducting and shortening of the forearc was accomplished through subduction erosion. These three different models are compared to the gravity field to determine the most likely geometry of this active arc-continent collision.