Abstract

Analog and digital seismic reflection and gravity data from different sources, drill-holes, and seaMARC II (seafloor mapping system) side-scan mozaic data from the Flores Basin, provide an excellent database with which to determine the geologic and tectonic development of the Bali-Flores back arc fold-thrust zone. Remarkable continuity of west-east structural orientation over 800 km along the Eastern Sunda back arc region suggests a uniformity in direction of convergence between the Indo-Australian and Eurasian plates. Westward transition from a well-defined accretionary wedge to fold structural styles indicates a westward decrease in the total amount of shorthening. The phenomena of back arc thrusting north of Flores can be used as a good model of the initial stage of arc reversal polarity in which the oceanic crust of the Flores Basin is being subducted southward beneath the arc, opposite to the northward subduction of the Australian continental crust along the Timor Trough. While the Bali Basin represents an excellent modern analog of the initial stage of a foreland fold-thrust belt formation, here the Sunda Shelf is downbowed to the south resulting in the compressional deformation along the southern margin of the Bali Basin. The back arc region of the Eastern Sunda arc is currently closing and will form a suture zone in a future stage of development.

The seismic and drill-hole data have supported several geologic and tectonic developments of the study area: (1) most of the northern basin margin represents a Paleocene extensional (rifting) tectonic regime; (2) this extensional tectonic environment was then inverted to form typical "Sunda Fold" structural styles; (3) downbowing (flexural) of the southeast Sunda Shield margin (northern basin margin) occurs to the south beneath the volcanic ridge; and (4) the back arc fold-thrust zone formed since Neogene time was associated with both the Australian margin-Banda arc collision as well as subducting of the Roo Rise (oceanic plateau) in the Sunda Trench south of Bali.