MODERN AND ANCIENT FOREARCS AS ARCHIVES OF ARC SEDIMENTARY AND TECTONIC PROCESSES

Oceanic island arc are the building blocks of the continental crust and their forearc volcaniclastic strata can be a record of how an arc has evolved through time in response to different tectonic regimes. However, interpreting such records can be complicated because of preferential preservation of certain parts of the arc edifice over longer periods of geologic time. Steady-state subduction can be destructive to the arc crust because subduction tectonic erosion is common in oceanic settings. Final collision may also cause loss of arc crust, although this depends on the polarity of collision. ‘Forward-facing’ collision can accrete an oceanic arc on to either a passive or an active continental margin, with the arc facing the continent and colliding trench- and forearc-side first. In a ‘backward-facing’ collision, involving two subduction zones with similar polarity, the arc collides backarc-first with an active continental margin. The best preserved oceanic arcs, such as Kohistan or Talkeetna, were initially accreted in ‘backward-facing’ mode. Arc systems are most likely to undergo accretion shortly before forward-facing collision with a continent because of the thick sediments on the continental margin. Thus, most forearc and accretionary-prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity. Forward-facing collision involves substantial recycling, melting, and fractionation of continent-derived material during and after collision, and so produces melts rich in silica and incompatible trace elements. As a result, forward-facing collision can drive the composition of accreted arc crust toward that of average continental crust. The example of Taiwan suggests that not much crust of lost during accretion and that this is an efficient way to build continental crust.