Transition from subduction to arc-continent collision: Geologic and neotectonic evolution of Savu Island, Indonesia

Abstract

Field analyses of stratigraphy, structure, and tectonic geomorphology of Savu Island defi ne the age and provenance of accreted Australian continental margin sequences and overlying synorogenic cover, and the structure, kinematics, and uplift history of the transition from subduction to collision in the eastern Sunda-Banda arc. The results highlight the dominant infl uence of lower plate composition and structure in shaping Savu Island and initiating intraforearc shortening. Provenance and biostratigraphic analyses of rocks accreted to the edge of the Sunda-Banda forearc indicate that they mostly consist of Triassic to Cretaceous synrift and postrift successions of the Australian continental margin. These rocks are similar in composition and provenance to Gondwana sequence units found throughout the Banda arc to the east, such as the Triassic Babulu, Jurassic Wai Luli, and Cretaceous Nakfunu Formations of Timor. Previously unrecognized units of pillow basalt are found interlayered with Jurassic beds and incorporated into melange and mud diapirs. These basalt occurrences have major and trace element compositions similar to those of Indian Ocean mid-oceanic ridge basalt and are likely associated with Jurassic development of the Scott Plateau volcanic margin. Southdirected thrusting of these units via a duplex thrust system detached the Middle Triassic section of the underthrust Scott Plateau. The Savu thrust system consists of a series of active north-directed thrust faults found onshore and offshore the north coast of Savu. Thrust faults mapped onshore, penetrated by the Savu #1 well and imaged in vintage seismic refl ection profi les, offset the youngest deposits of Savu. The uplift history and deformation pattern associated with the Savu thrust is investigated at a variety of temporal scales. Foraminifera-rich synorogenic deposits indicate low average surface uplift rates until after 1.9 Ma ago, when pelagic chalk deposits were raised from depths of >2500 m to the surface in fewer than 1 Ma. Island emergence is well documented by uplifted coral terraces that encrust the highest ridges to 338 m elevation. U/Th analysis of uplifted coral yields ages of 122 ka near sea level, indicating slow uplift rates of 0.2 mm/a over the past 400 ka. Most synorogenic deposits are stripped from the south coast, exposing parts of the accretionary wedge. The deeply eroded nature of this part of the island, combined with its steep fi rst-order stream gradients, indicates that it underwent rapid rock uplift and exhumation in the past 1‐2 Ma. However, the south coast region is now subsiding, as evidenced by drowned streams and southtilted, submerging coral terraces. Streams draining north over the Savu thrust system show convex-upward patterns with gradients commonly associated with intermediate uplift rates. Flights of coral terraces also document growth of the island to the north above thrust-related folds. These results inform us that the transition from subduction to collision involves (1) strain partitioning away from the subduction zone into the upper plate, (2) forearc closure, (3) structure inherited from the lower plate, (4) initiation of a crustal suture zone, and (5) uplift and exhumation of the accretionary wedge.