Upper plate proxies for flat-slab subduction processes in southern Alaska

Abstract

The timing of initiation of flat-slab subduction beneath southern Alaska and the upper plate record of this process are not well understood. We explore the record of flat-slab subduction in southern Alaska by integrating stratigraphic, provenance, geochronologic, and thermochronologic data from the region directly above and around the perimeter of ongoing flat-slab subduction. These datasets document a change from regional Paleocene–Oligocene subduction-related magmatism and basin development to an absence of magmatism and initiation of rock exhumation that continues to today. We infer that initiation of flat-slab subduction prompted crustal shortening, exhumation, inversion of sedimentary basins, and cessation of magmatism above and around the area of ongoing flat-slab subduction. Surface uplift and erosion above the flat slab resulted in deposition of thick, clastic wedges in sedimentary basins located along the western and northern perimeters of the flat-slab region. Along the eastern perimeter, northwestward-propagating Oligocene–Quaternary slab-edge volcanism and transtensional basin development along dextral strike-slip faults record progressive northwestward insertion of a shallow slab against the curved continental margin of eastern Alaska. Collectively, these geologic data indicate that flat-slab subduction was shaping southern Alaska by late Eocene–early Oligocene time, much earlier than previous models infer. Upper plate processes related to subduction of a flat slab in Alaska are similar to those documented in other modern flat-slab regions. These processes include: 1) shortening and exhumation of the upper plate several hundred kilometers inboard from the plate margin, 2) cessation of subduction-related magmatism within ten million years of the onset of shallow subduction, 3) shoaling or inversion of sedimentary basins above the flat-slab region, 4) deformation and/or erosion of the accretionary prism during flat-slab subduction, and 5) in some settings, reestablishment of sedimentation in the accretionary prism after the shallow slab has migrated laterally. Unlike other flat-slab margins, strike-slip-related volcanism and basin development characterizes one edge of the flat-slab region in Alaska, a consequence of oblique insertion of the flat slab into the corner of a curved continental margin.