Alternating thin versus thick-skinned decollements, example in a fast tectonic setting: The Misool–Onin–Kumawa Ridge (West Papua)

The Misool–Onin–Kumawa Ridge (Eastern Indonesia) is a broad anticline in the lower plate of the Seram subduction system. In the south it lies between the Seram accretionary wedge and the young Lengguru fold-and-thrust belt (<8 My). A large seismic dataset from recent petroleum exploration in the area allows the ridge to be interpreted as the result of a dual system of thin-skinned and thick-skinned tectonics. A forebulge effect may be superimposed on the emergent sections of the ridge (Onin and Kumawa Domes), where the morphology has been reactivated.The evolution results from what appears to be a continuum of deformation through three major stages: (1) formation of a Messinian thin-skinned fold-and-thrust belt over a shaly–silty Permian–Paleocene unit; (2) a Pliocene thick-skinned event responsible for the uplift of the ridge, possibly induced by the onset of continental subduction; and (3) recent Pleistocene deformation when thin-skinned tectonics resumed in the Seram Trough. Currently, the Seram wedge abuts the ridge, transferring compression northward into the Salawati Basin.The jumps of active detachment levels may be a response to changes in subduction parameters (velocity, rugosity, etc.) during the transition between oceanic and continental subduction, or at least from thinned crust to thicker continental crust.