Integrated provenance record of a forearc basin modified by slab‐window magmatism: detrital‐ zircon geochronology and sandstone compositions of the Paleogene Arkose Ridge Formation, south‐central Alaska

Abstract

This study presents an integrated provenance record for ancient forearc strata in southern Alaska. Paleocene–Eocene sedimentary and volcanic strata >2000 m thick in the southern Talkeetna Mountains record nonmarine sediment accumulation in a remnant forearc basin. In these strata, igneous detritus dominates conglomerate and sandstone detrital modes, including plutonic and volcanic clasts, plagioclase feldspar, and monocrystalline quartz. Volcanic detritus is more abundant and increases upsection in eastern sandstone and conglomerate. U‐Pb ages of >1600 detrital zircons from 19 sandstone samples document three main populations: 60–48 Ma (late Paleocene–Eocene; 14% of all grains), 85–60 Ma (late Cretaceous–early Paleocene; 64%) and 200–100 Ma (Jurassic–Early Cretaceous; 11%). Eastern sections exhibit the broadest distribution of detrital ages, including a principal population of late Paleocene–Eocene ages. In contrast, central and western sections yield mainly late Cretaceous–early Paleocene detrital ages. Collectively, our results permit reconstruction of individual fluvial drainages oriented transverse to a dissected arc. Specifically, new data suggest: (1) Detritus was eroded from volcanic‐plutonic sources exposed along the arcward margin of the sampled forearc basin fill, primarily Jurassic–Paleocene magmatic‐arc plutons and spatially limited late Paleocene–Eocene volcanic centers; (2) Eastern deposystems received higher proportions of juvenile volcanic detritus through time from late Paleocene–Eocene volcanic centers, consistent with emplacement of a slab window beneath the northeastern part of the basin during spreading‐ridge subduction; (3) Western deposystems transported volcanic‐plutonic detritus from Jurassic–Paleocene remnant arc plutons and local eruptive centers that flanked the northwestern part of the basin; (4) Diagnostic evidence of sediment derivation from accretionary‐prism strata exposed trenchward of the basin fill is lacking. Our results provide geologic evidence for latest Cretaceous–early Paleocene exhumation of arc plutons and marine forearc strata followed by nonmarine sediment accumulation and slab‐window magmatism. This inferred history supports models that invoke spreading‐ridge subduction beneath southern Alaska during Paleogene time, providing a framework for understanding a mature continental‐arc/forearc‐basin system modified by ridge subduction. Conventional provenance models predict reduced input of volcanic detritus to forearc basins during progressive exhumation of the volcanic edifice and increasing exposure of subvolcanic plutons. In contrast, our results show that forearc basins influenced by ridge subduction may record localized increases in juvenile volcanic detritus during late‐stage evolution in response to accumulation of volcanic sequences formed from slab‐window eruptive centers.

Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska

“…56-54 Ma; Haeussler et al, 2003). In the Matanuska basin, a potential sedimentary source with a major population of Paleocene-Eocene detrital zircons is the older forearc strata of the Paleocene-Eocene Arkose Ridge Forma tion (22% of the total distribution; Kortyna et al, 2013;Figs. 2 and 8).…”

“…8). Paleocurrent indicators and provenance data from Matanuska basin strata support sediment flux away from the Talkeetna Mountains and toward Cook Inlet during Cenozoic time (Trop et al, 2003;Kortyna et al, 2013;Sunderlin et al, 2014).…”

“…57-54 Ma ( Fig. 3; Bradley et al, 2003;Haeussler et al, 2003;Cole et al, 2006;Kortyna et al, 2013). Spreading-ridge subduction causes progressively younger oceanic crust to be consumed, producing a shallow slab that can extend both several hundreds of kilometers or more away from the spreading ridge and inboard from the subduction zone beneath the overriding plate (Cloos, 1993).…”

“…Following ridge subduction, nonmarine depositional systems were quickly reestablished, reflecting rapid subsidence that frequently occurs after the ridge passes. Provenance studies of forearc strata in the Matanuska Valley that were deposited contemporaneously with ridge subduction have sand-sized and conglomeratic modes dominated by igneous detritus and record exhumation of the adjacent arc rocks, as well as erosion of local volcanic centers attributed to slab-window magmatism (Kortyna et al, 2013). Strata adjacent to the accretionary prism locally contain detrital modes and ages that indicate erosion of the accretionary prism as well (Little, 1988;Trop et al, 2012).…”

Andean Forearc Dynamics, As Recorded By Detrital Zircon From the Eocene Talara Basin, Northwest Peru

“…For instance, the Cretaceous-Paleogene forearc of California, one of the best preserved and most studied basins in the world (Ingersoll 1982;Dickinson 1995), preserves provenance patterns related to low-angle subduction that broadly affected western North America during the Laramide orogeny (Sharman et al 2014). Provenance analysis of the Paleocene-Eocene forearc of southern Alaska reveals a unique magmatic history associated with subduction of an oceanic ridge and migration of the slab window (Kortyna et al 2013). In the Cenozoic Moquegua forearc in southern Peru, a major late Eocene DZ shift marks onset of uplift of the Central Andes associated with flat-slab subduction and strong intraplate coupling, rather than tectonic compression or magmatic underplating (Decou et al 2013).…”