Melange production and the importance of shale diapirism in accretionary terrains

Abstract

Although melanges are widely believed to form in subduction zones, there is little agreement as to the mechanism by which the characteristic chaotic aspect of a melange develops1,2. Debate centres on the relative effects of gravity sliding down the inner trench slope and shearing along thrust faults during offscraping or underplating2,3. The results of deep sea drilling and mapping of modern accretionary terrains4 combined with geophysical investigation of outer arc regions favour the thrust or fold-packet model1,4–8 of accretion, although some melange deposits exposed on land appear to fit an olistostrome model better1,9. Shale diapirs and mud volcanoes are abundant in many modern accretionary terrains10–12, and probably form as a result of rapid overloading caused by tectonic thickening4,13. An investigation of extensive melange zones in northern Irian Jaya has now shown that they are the product of shale diapirism, which suggests that shale diapirism is an important process in the tectonic mixing of accretionary terrains.