SEDIMENTARY RECORD OF PALEOGENE BASIN DEVELOPMENT, DEFORMATION, AND EXHUMATION DURING SPREADING RIDGE SUBDUCTION IN SOUTHERN ALASKA

Paleogene strata exposed in the Talkeetna Mountains, Matanuska Valley, and northern Chugach Mountains provide insight into sedimentation, volcanism, and tectonics during spreading ridge subduction. Depositional environments: Sedimentologic data document a two-sided basin with paleovalleys incised upon arc plutons, marine forearc basin strata, and subduction complex rocks. This paleodrainage system evolved after uplift of the Cretaceous forearc basin. Over 3 km of conglomerate, sandstone, mudstone, and coal were deposited in alluvial fan, channel, floodplain, lacustrine, and mire sub-environments. Sediment accumulation took place ca. 60-53 Ma based on geochronologic data from interbedded tuffs and lavas. Terrestrial fossils (wood, leaves, seeds, gastropods, bivalves, fish, insects) document aquatic and forested ecosystems. Leaf-character derived mean annual temperature and precipitation estimates (11–15°C; 125 cm) indicate warm temperate conditions during deposition, consistent with modest rates of chemical weathering and sedimentation (~500m/Ma). Detrital ages document erosion of Jurassic-Paleocene arc rocks to the north and Cretaceous metasedimentary rocks to the south. Volcanism: Sparse lavas in forearc strata (60-56 Ma) range from basaltic andesite to andesite with arc-like geochemical characteristics; these represent melting of arc-like mantle beneath the forearc, consistent with high heat flow through a slab window. Younger volcanic and intrusive rocks in the forearc region have more primitive/depleted compositions, consistent with tapping juvenile mantle that was emplaced through a slab window. The oldest depleted lavas unconformably overlying the 60-56 Ma strata are ca. 50-48 Ma. Tectonics: Deposition was coeval with exhumation of subduction complex rocks along the Border Ranges fault (BRF). Nonmarine strata with ca. 53 Ma tuffs unconformably overlie subduction complex phyllite with ca. 57 Ma ⁴⁰Ar-³⁹Ar white mica cooling ages. Strike-slip along the BRF was limited given that strata with similar lithologies, tuff ages, and detrital ages straddle the fault.

Collectively, our data document the surface response to a ridge subduction event in south-central Alaska, providing a comprehensive record of the upper plate response to slab window events along a convergent margin.