Pacific-type Orogens

Abstract

Pacific-type orogeny (PTO) has long been recognized as a contrasting accretionary alternative to continent-continent collisional orogeny. However, since the original concept was proposed, there have many new developments, which make it timely to produce a new reevaluated model, in which we emphasize the following new aspects. First, substantial growth of Tonarite-Trondhjemite-Granite (TTG) crust, and second the reductive effect of tectonic erosion. The modern analog of a Pacific-type orogen developed through six stages of growth exemplified by specific regions; initial stage 1: the southern end of the Andes; stage 2: exhumation to the midcrustal level at Indonesia outer arc; stage 3: the Barrovian hydration stage at Kii Peninsula, SW Japan; stage 4: the initial stage of surface exposure of the high-P / T regional metamorphic belt at Olympic Peninsula, south of Seattle, USA; stage 5: exposure of the orogenic core at the surface at the Shimanto metamorphic belt, SW Japan; and stage 6: post-orogenic processes including tectonic erosion at the Mariana and Japan trench and the Nankai trough. The fundamental framework of a Pacific-type orogen is an accretionary complex, which includes limited ocean floor material, much terrigenous trench sediment, plus island arc, oceanic plateau, and intra-oceanic basaltic material from the ocean. The classic concept of a PTO stresses the importance of the addition within accreted rocks of new subduction-generated arcs and TTGs, which were added along the continental margins particularly during the Cretaceous. Besides the above additional or positive aspects of a PTO, here we emphasize the negative effects of previously little-considered tectonic erosion caused by subduction over time. The evaluation of such extensive tectonic erosion leads a prospect of the presence of huge quantities of TTG material in the lower transition zone, where many subducted slabs have ponded, as illustrated by mantle tomography. This is confirmed by density profiles of the mantle, which show that TTGs are abundant only along the bottom of the upper mantle accompanied by slab peridotite,

Asia: a frontier for a future supercontinent Amasia

“…The rough estimates of 'second continents' at the MTZ over (Maruyama et al 2011). This suggests that the source of continental breakup might have come from a 'second' continent.…”

“…The convergent boundaries are subduction zones, which are, on the one hand, the place of the most active surface interaction of oceanic and continental plates/microplates and, on the other hand, the place of juvenile continental crust formation through arc calc-alkaline andesite magmatism. The process of continental growth in Asia consequently includes magmatic processes in the form of TTGtype arc magmatism and tectonic processes in the form of accretion (Maruyama et al , 2011Safonova et al 2011). A striking and remarkable characteristic of Asia growth is the ubiquitous occurrence of tectonic erosion and arc subduction, which takes place in Pacific-type orogens only (Yamamoto et al 2009;Isozaki et al 2010;Stern and Scholl 2010;Aoki et al 2012).…”

“…The hydrous plumes could be generated by a reaction of dehydration of DHMSs to yield dry olivine plus water at a depth of 410 km. That water formed at high temperatures can reduce the melting temperature of many silicates enough to form a magma-bearing plume (Litasov 2011;Maruyama et al 2011Maruyama et al , 2013. Several hydrous plumes were identified under East Asia (Huang and Zhao 2006;Zhao 2009), whose tomographic models trace those plumes down to a depth of 410 km, but not much deeper.…”

“…By this mechanism, P-type belts supply huge volumes of TTG-type crust to the upper mantle. The eroded TTG-type material can be subducted to depths of 440-660 km and accumulate at the mantle transition zone (MTZ) to form a kind of 'second continent' Maruyama et al 2011;Kawai et al 2013; see Section 4).…”

“…P-type belts are especially important sites of juvenile crust formation through tonalite-trondhjemite-granodiorite type (TTG-type) granitoid magmatism, i.e. calc-alkaline andesitic volcanism and M-and I-type granitoid magmatism (Maruyama et al 1996(Maruyama et al , 2011. On the other hand, P-type belts are sites of strong plate interactions accompanied by tectonic erosion and arc subduction (Scholl and von Huene 2007;Yamamoto et al 2009;Stern and Scholl 2010;Isozaki et al 2010).…”