EVOLUTION OF MAGMATISM ALONG A FLAT-SLAB SUBDUCTION-TRANSFORM TRANSITION, WRANGELL ARC, ALASKA-YUKON (Invited Presentation)

The Wrangell arc in eastern Alaska and Yukon provides an archetypical environ in which to examine relations between magmatism, deformation, and plate interactions within a continental arc-transform junction. New geochronologic data from bedrock and modern river sediments demonstrate that the Wrangell arc was continuously active from ca. 30 Ma to present. New geochemical data from bedrock and modern river sediments reveal basalt through high-Si rhyolite compositions and have adakitic, calc-alkaline, and transitional-tholeiitic geochemistry, indicating magmatism was shaped chiefly by slab-edge melting due to mantle upwelling, mantle wedge-melting leading to calc-alkaline magmatism, and eruptions due to intra-arc extension. Palinspastic reconstruction of the arc along regional strike-slip faults allows reconstruction of the spatial migration of arc magmatism. Intra-arc strike-slip faults were active conduits for magmatism and magmatism was persistently focused within or adjacent to a remnant suture zone, indicating upper plate crustal heterogeneities influenced arc magmatism. Magmatism initiated ca. 30–17 Ma along both sides of the Totschunda fault along the north flank of the Wrangell-St. Elias Mountains in Alaska and progressed southeastward ca. 17–10 Ma along the Duke River fault in the Yukon. This spatial-temporal evolution is attributable to dextral strike-slip and a change in the subducting slab geometry (slab curling/steepening). Magmatism progressed generally westward outboard of the Totschunda and Duke River faults ca. 13–6 Ma along the south flank of the Wrangell-St. Elias Mountains and then northwestward ca. 6 Ma to present in the western Wrangell Mountains. The 13 Ma to present spatial-temporal evolution is consistent with dextral slip and changes in plate boundary conditions, including an increase in plate convergence rate and angle ca. 6 Ma. The arc length varied from ~110 to ~190 km, substantially shorter than most modern arcs owing to a relatively short slab subducting along the lateral edge of the Yakutat flat slab. Results from the Wrangell arc indicate that arc-transform junctions maybe identified in the geologic record by the presence of calc-alkaline to transitional geochemical signatures, complex spatial-temporal patterns, short arc lengths, and voluminous magmatism.