EVIDENCE FOR CONTINUOUS SUBDUCTION THROUGHOUT THE JURASSIC ALONG THE OUTER MARGIN OF THE WRANGELLIA-PENINSULAR TERRANES, NORTHERN NORTH AMERICA CORDILLERA (Invited Presentation)

The Mesozoic North American Cordillera margin contains numerous segments of arc volcanism and associated fore- and back-arc basins, but collision and strike-slip processes have disrupted many of the sections. The two outermost (ocean-facing in modern geography) subduction complexes, the Franciscan and Chugach accretionary complexes, show no record of collisional events and thus contain the best-preserved examples of processes in long-lived subduction zones, with both containing evidence for subduction erosion.

We review unpublished and published geochronologic data from the Talkeetna and Chitina arcs and associated Chugach accretionary complex of southern Alaska and show that these results document continuous subduction in the Jurassic. The oldest direct record of subduction in the accretionary complex comes from a U-Pb titanite isochron and Ar-Ar ages from the blueschist facies rocks from three locales 100s of kms apart, ranging in age from 204–185 Ma. The main phase of Jurassic accretion is documented in detrital zircon MDAs from the accretionary package outboard of the blueschist, the Potter Creek assemblage near Anchorage, which contains basalt, mudstone, and lithic- and feldspar rich sandstone, with MDAs from 169–156 Ma. LA-ICPMS ages from locales along the Kenai Peninsula record MDAs that overlap continuously between ~177–150 Ma. The Talkeetna arc began at 213 Ma (CA-TIMS age on pluton closest to accretionary complex) and shifted inward (away from the subduction complex) at ~ 180 Ma; we interpret this as a time of local subduction erosion and partial exhumation of blueschist-facies parts of the accretionary complex. U-Pb zircon ages from plutonic rocks show continuous igneous activity from 202–164 Ma, with a final phase from 157–152 Ma. The Late Jurassic Chitina arc develops along strike in eastern Alaska as the Talkeetna arc wanes.

Recently published data from the Bonanza arc, built on the southernmost extent of Wrangellia, also document continuous arc development from ~205–165 Ma. The absence of an accretionary complex here has long been noted, and we interpret the high-angle fault that truncates the arc is the southernmost extent of the Border Ranges fault system. Dextral displacement along this fault in the Cretaceous-Eocene resulted in subsequent translation of the accretionary complex.