HIGHWAY TO THE SUTURE ZONE: A COMPARATIVE STUDY OF TERRANE ACCRETION BETWEEN THE MESOZOIC SALMON RIVER SUTURE ZONE OF THE NORTHWESTERN U.S. CORDILLERA AND THE CENOZOIC PENGLAI SUTURE, TAIWAN

The Salmon River suture zone in western Idaho records subduction driven accretion of volcanic arc terranes within the Blue Mountains Province (BMP) to western Laurentia. Magmatism, metamorphism, and deformation have been the focus of many studies in the BMP; however, the timing of initial docking of the BMP has been contentious. Syntectonic magmatism suggests that docking of far-traveled island arc terranes began in the latest Jurassic (~158 Ma). Metamorphism in the Salmon River suture zone to the east records evidence that accretion of terranes to Laurentia continued well into the Cretaceous (141 to 112 Ma). Recently, the initial docking of island arcs within the BMP was suggested to have occurred prior to 160 Ma in the Early Jurassic, based on a subregional unconformity. To test these alternative interpretations, we use a modern analogue in Taiwan, where active accretion of the Cenozoic Luzon arc to Eurasia is occurring, to explore the tectonic architecture and geochronologic trends across arc-continent collision zones and address: (1) when does “docking” of a terrane occur?, (2) how long does terrane accretion last?, and (3) how are docking/suturing processes recorded in the rock record?

Oblique docking of the Luzon arc began at ~5 Ma and only the northernmost reaches of the arc are effectively sutured to the Eurasian continent today. To the south along the eastern coast of Taiwan, suturing is ongoing, with some Luzon arc still offshore at the southern tip of Taiwan. South of Taiwan, submarine parts of the Luzon arc within the Philippine Sea are located >100 km from structural elements of SE Eurasia. Assuming modern plate motion rates, the entire Luzon arc will not fully make contact with Eurasia for at least 1 m.y. Given the duration of ~4 m.y. from initial contact to suturing, oblique docking of this single island arc will have taken >10 m.y. Within the BMP, two island arcs ultimately docked, complicating and prolonging the accretion process. Detrital zircon data from Taiwan show that intra-collisional strata contain significant continental sediments. In contrast, sediments from the Philippines (part of the still undocked Luzon arc to the south) show little continental sediment influx, similar to ~160 Ma strata of the BMP, suggesting that parts of the BMP composite terrane were largely undocked and offshore at 160 Ma.