LOST IN TRANSLATION: TRACING ISLAND-ARC GEOLOGIC RECORDS FROM MODERN ACTIVITY TO ANCIENT ACCRETED TERRANES

Records of ancient arc activity preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion, and to understand how continental crust is formed, recycled, and maintained through time. However, interpreting tectonic and sedimentary records from ancient terranes after arc-continent collision is complicated by preferential preservation of evidence for some processes and loss of evidence for others. Composition of accreted arc terranes differs as a function of arc-continent collision geometry. ‘Forward-facing’ collision can accrete an oceanic arc onto either a passive or an active continental margin, with the arc facing the continent and colliding trench- and forearc-side first. In ‘backward-facing’ collision, involving two subduction zones with similar polarity, the arc collides backarc-first with an active continental margin. The preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case. Preservation of arc terranes likely is biased towards those in a state of tectonic accretion for tens of millions of years before collision. The prevalence of tectonic erosion in modern oceanic subduction zones implies that valuable records of arc processes are commonly destroyed before collision with a continent. Arcs are most likely to undergo tectonic accretion shortly before forward-facing collision with a continent, and thus most forearc and accretionary-prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity, when sediment flux to the trench was greatest and tectonic accretion prevailed. Collision geometry and tectonic erosion vs. accretion are important controls on preservation of material from the trench, forearc, arc massif, intra-arc basins, and backarc basins, and thus on how well an ancient arc terrane preserves evidence for certain tectonic processes.