A general model of arc-continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides

The collision of the Luzon Arc with southern China represents the best example of arc--continent collision in the modern oceans, and compares closely with the Early Ordovician accretion of the Lough Nafooey arc of Connemara, Ireland, to the passive margin of Laurentia. We propose a general model for steady-state arc--continent collision in which arc crust is progressively added to a passive margin during a process of compression, metamorphism and magmatism lasting 3-10 Ma at any one location on the margin. Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion can be more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events. During the collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2-3 Ma) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab. Arc-cont inent collision is an important part of the Wilson cycle by which intra-oceanic arc crust is added to the margins of continents, and is probably the principle method by which the cont inental crust is built, at least during the Phanerozoic (e.g. Taylor & McLennan 1985; Rudnick & Fountain 1995). The recognition in average continental crust of a depletion in Nb relative to other trace elements, a feature long associated with subduction magmatism (Pearce 1982), represents compel l ing evidence to suggest a subduction-related origin to the continental crust. However, the link between arc and continental crustal genesis is not a simple one, because intra-oceanic arc crust is too mafic and too light rare earth e lement (LREE)-deple ted to be a direct analogue for continental crust (e.g. Kay 1985; El lam & Hawkeswor th 1988). Al though some oceanic arcs do seem to produce melts with more LREE-enr iched chemistries (e.g. Yogodzinski & Kelemen 1998), these are typically geographically restricted and are often in areas of unusual tectonics, such as the oblique convergence in the western Aleutians. Active continental margins The role of active cont inental , ra ther than oceanic, margins in generating new continental crust is presently unclear. Al though isotopic and geochemica l evidence from such arcs does suggest that much of the output involves reworking existing continental crust (e.g. Rogers & Hawkesworth 1989), there must be some component of new melt extraction from the mantle. However, high rates of tectonic erosion limit the amount of new material added to the continent by magmatism at many active margins. Subduction accretion is limited to regions where there is major influx of clastic sediments from the continents. Reymer & Schubert (1984) estimated that globally only 23-33 km 3 of new melt were added every Ma per km of active margin during the Phanerozoic, but Holbrook et al. (1999) estimated higher rates of 55-82 km 3 Ma -1 km -1 for the Aleutians. Similarly, Suyehiro et al. (1996) indicated long-term, average accretion rates of 66 km 3 Ma -1 km -1 in the Izu arc. In comparison, es t imated longterm rates of crustal erosion From: LARTER, R.D. & LEAT, P.T. 2003. Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes. Geological Society, London, Special Publications, 219, 81-98. 0305-8719/03/$15.00 9 The Geological Society of London 2003. 82 PETER D. CLIFT, HANS SCHOUTEN & AMY E. DRAUT reach 70 km 3 Ma -1 km -1 in Tonga (Cliff & MacLeod 1999), 96 km 3 Ma -1 km -1 in Costa Rica (Vannucchi et al. 2001), 96 km 3 Ma -1 km -1 in Honshu (von Huene & Lallemand 1990), 37-54 km 3 Ma -1 km -1 in nor thern Chile (von Huene et a/. 1999) and 109 km 3 Ma -1 km -1 along the Peruvian margin (Cliff et aL 2003b). These rates imply net loss of continental crust in such settings. In contrast, the accretion of an oceanic arc to a continental margin represents a clear net gain in crust, and may be the most efficient mechanism for producing new continental crust, at least since the Precambrian. A r c crustal compos i t i on If oceanic arcs are, in fact, the building blocks of the continental crust then our inability to relate the compositions of the two might reflect a misunderstanding of the bulk composition of one or the other. One possible solution is that the bulk crust of oceanic arcs is actually much more silicic and LREE-enr iched than is apparent from the volcanic cover. This hypothesis was supported by the identification of lower seismic velocity, high silica, middle crust in the Izu arc (Suyehiro et al. 1996). However, subsequent seismic refraction experiments in other oceanic arcs have failed to identify similar siliceous crustal units (e.g. Aleutians; Holbrook et al. 1999). Accreted island arc crustal sections in orogenic belts also argue against significant volumes of siliceous mid crust in oceanic arcs (e.g. Kohistan: Miller & Christensen 1994; Talkeetna, Alaska: Pearcy et al. 1990). Al though the volcaniclastic apron surrounding the arc volcanic front may be up to 6 km thick and dominated by silica-rich tephra (e.g. Tonga: Tappin et al. 1994), these sediments do not show L R E E enrichment (Clift 1995) and cannot shift the bulk composi t ion of the accreted crust in the manner required to transform it into LREE-enr iched continental crust. The signi f icance o f a rc -con t inen t coll is ion If oceanic arc crust does not have the same composition as the continental crust then it is possible that the process of its accretion to the continent is instrumental in effecting a transformation. Draut & Clift (2001) showed that during collision of the oceanic Lough Nafooey arc, now exposed in the Caledonides of western Ireland, to the passive margin of Laurentia during the early Ordovician (c. 480-470 Ma), volcanism underwent a transition from mafic and LREE-depleted to siliceous and LREE-enriched. This syncollisional volcanism (represented by the Tourmakeady Group in South Mayo, Ireland) is more 110" 18" Meta . . . . . . . marg