Melange production and the importance of shale diapirism in accretionary terrains

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 develops 1,2 . Debate centres on the relative effects of gravity sliding down the inner trench slope and shearing along thrust faults during offscraping or underplating 2,3 . The results of deep sea drilling and mapping of modern accretionary terrains 4 combined with geophysical investigation of outer arc regions favour the thrust or fold-packet model 1,4–8 of accretion, although some melange deposits exposed on land appear to fit an olistostrome model better 1,9 . Shale diapirs and mud volcanoes are abundant in many modern accretionary terrains 10–12 , and probably form as a result of rapid overloading caused by tectonic thickening 4,13 . 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.