The Gulf of Alaska is one of the largest accretionary complexes on Earth. In this study, we examined the earliest phase of accretion in the Mesozoic McHugh Complex and Valdez Groups, exposed in SE Alaska. The oldest preserved fragment, the Mesomélange assemblage, is Jurassic (ca. 160–140 Ma) and consists of an ~3-km-thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts. Heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the Wrangellia and the Talkeetna oceanic arc. A tectonic erosion event affected the forearc just prior to ca. 120 Ma and was likely caused by seamount collision, ridge subduction, or both. This was followed at 105 Ma by mass wasting of sandstone and conglomerates, preserved as the Graywacke-Conglomerate assemblage (ca. 105–83 Ma). Heavy minerals indicate continued flux from arc sources, but with significant changes suggesting a larger, more diverse catchment area. Erosion of deeper crustal sources provided high-Mg diopside and garnets to the trench. Faster sediment fl ux was caused by rock uplift triggered by final accretion of the Wrangellia-Peninsula terrane to North America. The start of large-scale accretion in Alaska roughly coincided with the initiation of Shimanto Complex accretion in Japan and can be understood as primarily linked to sediment supply driven by plate-margin tectonics rather than climatically induced erosion onshore.
This work was funded by National Science Foundation grants EAR-0809608 to Amato and EAR-0809609 to Pavlis. Clift and Wares thank Nilanjan Chatterjee from the Massachusetts Institute of Technology for his help with the electron probe analyses and John Still from University of Aberdeen for help with the scanning electron microscope and advice on the interpretation of mineral data. Jeremy Preston provided software for calculation of mineral formulas from probe data from http://www.gabbrosoft.org/. Marc Robertson provided important assistance at an early stage in this project. Joe Hecker (New Mexico State University) helped with some of the mineral separations. The paper benefited from helpful reviews from Peter Cawood, David
Scholl, and an anonymous reviewer.