In Timor, eastern Indonesia, where the northern margin of the Australian continent is colliding with the Banda Arc, Australian continental margin sediments are being incorporated into an imbricate wedge, which passes northward into a foreland fold and thrust belt. Field mapping in Timor has shown that scaly clays, containing irregularly shaped or phacoidal blocks (up to several meters long) and composed of a wide range of lithologies derived from local stratigraphic units, occur in three environments: along wrench faults, as crosscutting shale diapirs, and associated with mud volcanoes. A model is proposed linking these phenomena. Shales become overpressured as a result of overthrusting; this overpressure is released along vertical wrench faults, which cut through the ove thrust units; overpressured shales containing blocks of consolidated units rise along the fault zones as shale diapirs; and escaping water, oil, and gas construct mud volcanoes at the surface.

An extensive melange deposit in Timor, the Bobonaro Scaly Clay, has been interpreted by Audley-Charles and subsequent workers as an olistostrome. Our study interprets the Bobonaro Scaly Clay as the product of shale diapirism. Shale diapirs are likely to be generated wherever water-saturated sediments are incorporated into accretionary complexes. Diapirs have not, however, been recorded as occurring in ancient accretionary complexes, although melanges, usually interpreted as olistostromes, are commonly reported. Criteria are proposed by which the products of shale diapirism might be recognized, including: (1) shape of the diapiric body; (2) its relationship to the surrounding rocks; (3) nature of the matrix and of the enclosed blocks; (4) shapes of the blocks and their relationship to he matrix; and (5) effects of deformation. The importance of shale diapirism in the formation of melanges has not been fully appreciated, and the products of shale diapirism constitute a major component of ancient and present-day accretionary complexes.