background of the project
When continental crust enters a subduction zone and arc-continent collision begins, the work of the “subduction factory” is disrupted and the thermal state of the subduction zone changes dramatically. Nevertheless, in both fossil and active settings the remnant volcanic arc, the forearc basin, the accretionary complex, and foreland and suture basins provide a wide range of information about the evolution of the subduction zone and final arc-continent collision. For example, Tertiary boninites in the western Pacific are restricted to the forearc region of island arcs, and are thought to record processes that occur during the subduction of young, hot oceanic lithosphere at the onset of subduction. Boninites in active and fossil arc-continent collision settings can be interpreted in a similar manner. With increased subduction there is a change in the composition of the arc volcanism from arc-tholeiite to island arc calc-alkaline suites that mark the evolution to a mature island-arc settings. The effect of the entrance of the continental crust into the subduction zone on the arc volcanic composition is not always easily determined, although some clues, such as Pb isotope ratios or Sm and Nd ratios may provide an indication of a continental crustal source. In areas of active arc-continent collision such as Papua New Guinea, Timor, and Taiwan, arc volcanism waned and stopped in the accreted arc 1–3 m.y. after the entry of the continental crust into the subduction zone, although in all cases volcanism still occurs outboard of the accreted arc. In the fossil setting of the Southern Urals, magmatic activity in the accreted arc appears to have waned shortly after the entrance of the continental crust into the subduction zone, and to have stopped within about ~5 m.y..
In both active and fossil settings arc-continent collision is accompanied by the development of an accretionary complex that incorporates deeply subducted and exhumed material derived from the continental margin, shallowly offscraped sediments, ultramafic fragments derived from the suprasubduction zone, and by the development of a foreland basin. Studies in both active and fossil settings indicate that the rigid forearc forms either an arcward- or trenchward-dipping backstop. During the early stages of arc-continent collision, the mechanically weak sediments that cover the upper part of the continental crust or continent-ocean transition zone may be subducted to shallow levels where they undergo low grade metamorphism and deformation. Those metasediments are progressively detached and transferred to the upper plate to form part of the accretionary complex. More and more evidence is accumulating that suggests that not only can continental crust be subducted to depths greater than 200 km, but that there is also a significant flux of both continental crust and oceanic lithosphere in the subduction channel. The mechanism by which exhumation of deeply subducted material takes place is still a matter of active research, although numerical and analoque modelling suggests that buoyancy may be the dominant process. Whatever the process, the cycle of subduction, high pressure metamorphism, and exhumation appears to take place within a relatively few million years, bringing high pressure rocks to the surface early in the arc-continent collision event. In the Southern Urals, for example, high pressure rocks of the Maksutovo Complex were supplying sediments to suture forearc basin within 10 million years of the peak metamorphic conditions. Ultramafic fragments and ophiolites within the accretionary complex are generally derived from the forearc.
In Papua New Guinea, Timor, and Taiwan accretionary wedge and arc uplift has been taking place at rates of 0.8–10 mm yr-1 since the arrival of the continental crust at the subduction zone. In Timor and Taiwan the accretionary wedge is being uplifted and eroded, providing the major sediment source for the accretionary complex. In Papua New Guinea, however, it is the forearc that is now being uplifted and that supplies the sediments to the accretionary complex. The change from a mixed source to an exclusively volcanic source in Papua New Guinea has been interpreted to mark the arrival of the Australian crust at the subduction zone. A similar process appears to have been active in the Southern Urals, where the change to a volcanic arc sediment source has been interpreted to mark uplift and erosion of the arc due to the arrival of the continental crust at the subduction zone. The widespread occurrence of syn-sedimentary deformation and olistostromes in the Uralian forearc, intra-arc and suture basin sediments indicates a system-wide instability that is thought to record seismic activity related to the entry of the full thickness of the continental crust into the subduction zone. This interpretation is corroborated by active seismicity studies in Papua New Guinea, Timor, and Taiwan that indicate that continental crust can continue to underthrust the arc for at least 3–5 m.y. after its entry into the subduction zone, providing a mechanism for triggering soft sediment deformation and olistostrome formation in poorly consolidated forearc and accretionary complex sediments.
Ore formation is an integral part of the geodynamic evolution of the Earth’s crust and mantle. Many tectonic settings, including that of arc-continent collision, create conditions conducive to the generation of water-rich magma, but the generation of ore deposits appears to be restricted to locations and short periods of change in temperature and stress imposed by transitory plate motions. Crustal influence is evident in the strong structural controls on the location and morphology of many ore deposits. Crustal-scale fault–fracture systems provide the fabric for major plumbing systems within the colliding arc. Rapid uplift and hydraulic fracturing can generate or focus magmatic–hydrothermal fluid flow that may be active for time spans significantly less than a million years. Once a hydrologically stable flow is established, ore formation is strongly dependent on the steep temperature and pressure gradients experienced by the fluid, particularly within the upper crust.
The study of geological risks is currently high on the agenda in most research funding agents around the world, including the US and Europe. This emphasis is reflected in the recent publication of seismic hazard maps such as the Global Seismic Hazard Assessment Program of the International Lithosphere Program, or the USGS Seismic Hazard Map program.