Variable rates of Late Quaternary surface uplift along the Banda Arc-Australian plate collision zone, eastern Indonesia

Abstract

Radiometrically dated emergent coral terraces from southeastern Indonesia provide estimates of differential vertical strain in the Banda Arc-continent collision complex. At Semau island, two samples from the lowest emergent reef (5-7 m) yield 230Th dates that correspond to the 5a (c. 83 ka) sea-level highstand and a low surface uplift rate of 0.2-0.3 m per 1000 years. At Rote island, samples from the lowest emergent reef (c. 1-2m) on both north and south sides of the island yield late Holocene ages and an average short-term uplift rate of c.l-1.5m per 1000 years. Similarity of ages from different samples on the north coast of Rote suggests possible coseismic emergence. Survey data from nine emergent reefs and marine notches up to 170m in altitude on the south side of Rote indicate that uplift rates may have been c.l-1.5m per 1000 years for c.l20000-130000 years. Combined with previous studies, these results indicate that late Quaternary surface uplift rates vary an order of magnitude along the strike of the Banda orogen. Vertical displacement rates are greatest in young parts of the orogen where the shelf-slope break recently has been underthrust beneath the orogenic wedge, as at Rote, and in older parts of the orogen where retroarc thrust faulting occurs, as at Alor island. This paper presents new 230Th ages and associated surface uplift rates for emergent coral reefs on two islands near West Timor in southeastern Indonesia (Fig. 1). On Rote island, no previous work has been done on emergent corals. On Semau island, only one coral sample has yielded a previous age estimate (Jouannic et a!. 1988). Both islands are located in an important zone of transition from subduction to collision along the plate boundary between the Indian-Australian and Eurasian plates. Whereas the transition from subduction to collision is poorly preserved in most other collision zones, it is well preserved in the Timor region. Many lines of evidence indicate that both horizontal and vertical rates of deformation are greater in this area than elsewhere in the orogen. This paper also compiles all previous estimates of surface uplift rates from coral dating on islands throughout southeastern Indonesia. We use this compilation to consider regional variability in vertical strain rates in relation to recent models of tectonic processes in the Banda Arc-Australian continental margin collision zone. Cenozoic deformation in the Banda ArcAustralian continental plate collision zone The Indonesian region contains an active subduction zone, accretionary arc, volcanic island arc, and emergent collision zone. In western Indonesia, oceanic crust of the Indian-Australian plate is subducting northward beneath the Eurasian plate along the Java trench, forming the Sunda volcanic arc and accretionary wedge, which includes the volcanic islands of Sumatra and Java (Fig. 1). In eastern Indonesia, where oceanic crust has been subducted completely, underthrusting of buoyant Australian continenta1lithosphere since about 3 Ma changes the Java Trench into a collisional foredeep and the locally inactive Banda Arc into an arc-continent collision zone along the Timor Trough (Hamilton 1979; Bowin eta!. 1980; Silver eta!. 1983; Breen eta!. 1986; Karig eta!. 1987). Underthrusting of the mature Australian continental margin significantly increases the influx of accretionary material and the frictional resistance to slip along the plate boundary (Harris 1991). Collisional strain MERRITTS, D., EBY, R., HARRIS, R., EDWARDS, R. L. & CHANG, H. 1998. Variable rates of Late Quaternary surface uplift along the Banda Arc-Australian plate collision zone, eastern Indonesia. In: STEWART, I. S. & VITAFINZI, C. (eds) Coastal Tectonics. Geological Society, London, Special Publications, 146, 213-224. 214 D. MERRITTS ET AL. Explanation of Symbols Thrust fault; teeth on upper plate