Miocene‐Recent sediment flux in the south‐central Alaskan fore‐arc basin governed by flat‐slab subduction

Abstract

The Cook Inlet in south‐central Alaska contains the early Oligocene to Recent stratigraphic record of a fore‐arc basin adjacent to a shallowly subducting oceanic plateau. Our new measured stratigraphic sections and detrital zircon U‐Pb geochronology and Hf isotopes from Neogene strata and modern rivers illustrate the effects of flat‐slab subduction on the depositional environments, provenance, and subsidence in fore‐arc sedimentary systems. During the middle Miocene, fluvial systems emerged from the eastern, western, and northern margins of the basin. The axis of maximum subsidence was near the center of the basin, suggesting equal contributions from subsidence drivers on both margins. By the late Miocene, the axis of maximum subsidence had shifted westward and fluvial systems originating on the eastern margin of the basin above the flat‐slab traversed the entire width of the basin. These mud‐dominated systems reflect increased sediment flux from recycling of accretionary prism strata. Fluvial systems with headwaters above the flat‐slab region continued to cross the basin during Pliocene time, but a change to sandstone‐dominated strata with abundant volcanogenic grains signals a reactivation of the volcanic arc. The axis of maximum basin subsidence during late Miocene to Pliocene time is parallel to the strike of the subducting slab. Our data suggest that the character and strike‐orientation of the down‐going slab may provide a fundamental control on the nature of depositional systems, location of dominant provenance regions, and areas of maximum subsidence in fore‐arc basins.

Partitioning Pervasive Detrital Geochronologic Age Distributions in the Southern Alaskan Forearc

“…Shallow subduction of the Yakutat microplate continued during the Miocene. Previous qualitative assessment of detrital zircon U-Pb distributions and εHf (t) values in Lower-Middle Miocene strata suggest that sediment was derived from all margins of the basin, including the central and eastern Alaska Range, western Talkeetna Mountains, and northern Chugach Mountains (Finzel and Enkelmann, 2017). Some of those regions, including the central Alaska Range, western Alaska Range, and eastern Alaska Range produce detrital zircon and apatite fission track ages that reflect late Oligocene to early Miocene (∼30-18 Ma) initial and widespread exhumation (Lease et al, 2016;Enkelmann et al, 2019).…”

“…The Yakutat microplate is an ∼11-30 km thick, wedge-shaped oceanic plateau that is subducting at a dip angle of 11-16 • , decreasing from west to east, to near the modern coastline (Ferris et al, 2003;Eberhart-Phillips et al, 2006;Worthington et al, 2008Worthington et al, , 2012Christeson et al, 2010;Bauer et al, 2014). Yakutat shallow-subduction-related processes observed in the upper plate in Alaska include changes in (1) the style and location of volcanic arc magmatism, (2) sedimentary basin subsidence and inversion patterns, and (3) sediment sources as a result of accelerated surface uplift above the subducted flat slab (e.g., Enkelmann et al, 2008Enkelmann et al, , 2019Finzel et al, 2011Finzel et al, , 2015Finzel et al, , 2016Trop et al, 2012;Arkle et al, 2013;Finzel and Enkelmann, 2017). Shallow subduction still characterizes the present-day margin of southern Alaska.…”

“…The upper Miocene to Pliocene Sterling Formation is the only Cenozoic formation in the basin that is thickest near the eastern margin, where it is ∼3,300 m thick (Calderwood and Fackler, 1972;Kirschner and Lyon, 1973). Detrital zircon U-Pb and εHf (t) signatures in these strata record the shrinking of basin catchments and a shift toward more local sediment sources in the adjacent arc rocks in response to insertion of the shallow Yakutat slab (Finzel et al, 2016;Finzel and Enkelmann, 2017).…”

“…Five samples from rivers draining the southern flank of the central Alaska Range have n total = 487 and n < 250 Ma = 463 (Lease et al, 2016). The eastern Alaska Range is the least well-characterized with only two samples that produce with n total = 186 and n < 250 Ma = 184 (Lease et al, 2016;Finzel and Enkelmann, 2017). To the south, the detrital signature of Permian-Cretaceous accretionary prism strata in the northern Chugach Mountains is represented by three samples with n total = 969 and n < 250 Ma = 914 (Finzel and Enkelmann, 2017).…”

“…The eastern Alaska Range is the least well-characterized with only two samples that produce with n total = 186 and n < 250 Ma = 184 (Lease et al, 2016;Finzel and Enkelmann, 2017). To the south, the detrital signature of Permian-Cretaceous accretionary prism strata in the northern Chugach Mountains is represented by three samples with n total = 969 and n < 250 Ma = 914 (Finzel and Enkelmann, 2017). No modern river data was available for the Yukon Tanana Uplands located northeast of the eastern Alaska Range (Figure 1), but that region was previously identified as an important sediment source area during the early Cenozoic based on Paleozoic and Precambrian ages present in the basin strata (Finzel et al, 2016).…”