INVESTIGATING THE LOST ARC: GEOLOGICAL CONSTRAINTS ON ~25 MILLION YEARS OF MAGMATISM ALONG AN ARC-TRANSFORM JUNCTION, WRANGELL VOLCANIC BELT, ALASKA

The Aleutian Arc has been a fundamental site for increasing our understanding of continental crust growth and mega-thrust earthquakes. To the east magmatism along a classic arc-transform junction, the Wrangell Volcanic Belt (WVB) has received less attention. Arc-transform junctions as seen in the WVB are some of the most volcanically active provinces on earth. The geochemical evolution and temporal-spatial eruptive history of these composite arc-leaky transform volcanic belts are controlled by both upper plate and lower plate processes. The well preserved and still active WVB provides a simple analog to not only better our understanding of arc-transform volcanic belts, but to also increase our utilization of these volcanic belts to decipher the geological record of convergent margins.

Building on existing geochronology and geochemistry data sets, key questions we seek to address are: 1) Is magmatism along the edge of a flat slab continuous or punctuated? 2) Does the relative position of the intraplate arc vary due to northward translation along the Denali fault system? 3) Did magmatism shift diachronously through time in response to varying plate boundary conditions? 4) Are regional strike-slip faults conduits for magmatism, 5) Does the contribution of slab edge melt vary with time? To achieve these goals we tested a new ⁴⁰Ar/³⁹Ar geochronology technique on detrital lithic grains from modern river sediments to better constrain the timing of volcanism. ⁴⁰Ar/³⁹Ar analyses of detrital volcanic lithic grains and U-Pb analyses of detrital zircons from a Chisana River sample closely overlap. New bedrock (⁴⁰Ar/³⁹Ar whole rock) and detrital (⁴⁰Ar/³⁹Ar whole rock and U-Pb zircon) geochronology and bedrock geochemical constraints indicate initiation of a >~50 km swath of WVB arc magmatism by ~24 Ma. Slab edge melt was occurring at this time, which we link to the flat slab subduction of the Yakutat microplate.

Existing tectonic models invoke systematic northwestward progression of arc magmatism. Our new age data indicate a more complex history, including a southward shift followed by a northwestward shift. The relative location of the Wrangell arc was likely mainly controlled by both northwestward movement of the leading front of the Yakutat slab and northwestward translation of the upper plate along the Denali fault system.