SEARCHING FOR CONTINENTAL SIGNALS AT THE TRENCH

Intraoceanic arcs have been proposed in many ancient collisional systems, but these interpretations are often controversial. Ideally, integrated data from the trench to the backarc would help us understand the system as a whole. However, preservation and exposure are imperfect, especially in collisional systems where later thrusting can overprint original contacts. Even when the whole system seems to be present, there are frequently debates about whether these components represent one subduction zone system or multiple coeval systems in different tectonic settings. The interpretation of a subduction zone as intraoceanic, marginal, or continental has significant implications for tectonic reconstructions and understanding the evolution of collisional systems. The subduction complex, built by material accreted and deformed at the trench, is the only component that is tied to a particular trench. When other parts of the forearc, arc, or backarc are missing or controversial, the siliciclastic material incorporated into the subduction complex can provide clues to the upper plate composition. Common tools to determine the provenance of subduction complexes include detrital zircon geochronology, sandstone petrography, mudrock geochemistry, and volcanic geochemistry. An intraoceanic arc setting should have no continental material in the trench and subduction complex. How do we prove a negative? Transporting sediment to the trench requires pathways. A lack of continental material could indicate an intraoceanic arc far from a continent or a lack of sediment pathways from the backarc so that young volcanic material dominates. We explore tectonic interpretations for multiple preserved subduction complexes, including new results from our work on the Early-Middle Jurassic Rattlesnake Creek terrane in the Klamath Mountains. Based on data from different subduction systems, we discuss best practices and caveats for data collection and tectonic interpretations.