THE BANDA ARC-CONTINENT COLLISION: MODERN ANALOG OF THE EARLY ACCRETION OF WESTERN NORTH AMERICA

The active Banda arc-continent collision of the Timor region is used as a modern analog for many different types of ancient mountain systems, and as the fundamental process for the accretion of continents. However, until recently, only reconnaissance-style geological and geophysical investigations were conducted of the region. Over the past few years, several new detailed investigations of Banda arc-continent collision geodynamics, at a variety of temporal scales, were conducted by students at BYU. These include: detailed structural analyses (Vorkink, 2004; Standley, 2007; Zobell, 2007), long-term uplift rates and depositional histories of synorogenic sediments (Roosmawati, 2005), short-term uplift rates and deformation patterns of coral terraces (Cox et al., 2005), geochronological studies of hinterland metamorphism, uplift and exhumation (Standley, 2007), and geodetic studies of regional strain fields (Nugroho, 2004).

These new data provide the constraints needed to construct numerical models of the active collision zone that test geodynamic issues, such as: 1) Is uplift caused by crustal shortening or lithospheric processes (delamination)? 2) How is strain partitioned during the transition from subduction to collision? 3) How is plate boundary reorganization represented at different temporal scales? 4) How is plate convergence at the trench transferred to the back arc? 5) How is an arc-forearc terrane accreted to a continent?

Construction of serial sections through the oblique collision reveal the progressive transition from subduction, where strain is localized near the trench, to collision, where strain is partitioned to retrowedge thrusts systems as plate coupling increases. Fragments of the Banda forearc are thrust over multiple stacks of shortened Australian continental margin cover sequences. Tectonic affinity is distinguished by very different U/Pb detrital zircon ages. Deep marine synorogenic units rise from 4 km depth to > 1000 m above SL in less than 4 Ma. These units, and the flights of coral terraces deposited on them, yield uplift rates of 1-2 km/Ma. Most uplift is concentrated above retrowedge thrust systems that progressively transfer plate convergence to the backarc through time. Eventually the Banda Arc is accreted to the northern edge of Australia, which has caused an active phase of plate boundary reorganization in the region.