Rock uplift at the transition from flat-slab to normal subduction: The Kenai Mountains, Southeast Alaska

Abstract

The process of flat-slab subduction results in complex deformation of overlying forearcs, yet how this deformation decays with distance away from the zone of underthrusting is not well understood. In south central Alaska, flat-slab subduction of the Yakutat microplate drives shortening and rock uplift in a broad coastal orogenic belt. Defined limits of the zone of underthrusting allow testing how orogenesis responds to the transition from flat slab to normal subduction. To better understand forearc deformation across this transition, apatite (U-Th)/He low temperature thermochronometry is used to quantify the exhumation history of the Kenai Mountains that are within this transition zone. Measured ages in the northern Kenai Mountains vary from 10-20 Ma and merge with the exhumation pattern in the Chugach Mountains to the northeast, where high exhumation occurs due to flat-slab-related deformation. In the southern Kenai Mountains, however, ages increase to 30-50 Ma across a transition near Seward, Alaska, above the zone from flat-slab to normal subduction. These ages are relatively old in comparison to ages determined in other studies in southern Alaska and suggest minimal exhumation. Furthermore, transitions in topographic expression of the coastal orogen also occur at the margin of Yakutat underthrusting. These observations suggest that either deformation associated with flat-slab subduction requires tens of kilometers to decay with distance away from the zone of underthrusting, or that orogenesis in the Kenai Mountains is driven by a distinct tectonic cause. A potential driver of deformation is underplating of thick sediments, specifically the Surveyor Submarine Fan, along the Aleutian Megathrust, analogous to the tectonic mechanism responsible for the emergence of the Kodiak Island forearc. If correct, this may represent a recent tectonic transition in the region, given the minimal exhumation of the rugged Kenai Mountains despite the presence of an erosion-conducive glacial climate.National Science Foundation EAR-1123688/112364

Structural Heterogeneity in Source Zones of the 2018 Anchorage Intraslab Earthquake and the 1964 Alaska Megathrust Earthquake

“…In addition, the large coseismic slip area between the Kenai Peninsula and the Montague Island shows a localized low‐Vs and high‐ν anomaly in the overriding plate. Focused and rapid exhumation has taken place around the Montague Island as revealed by young apatite (U‐Th)/He ages of <5 Ma, especially in the southwestern island (Arkle et al, ; Ferguson et al, ; Valentino et al, ). Haeussler et al () suggested that several splay faults there have accommodated the exhumation and rooted into the megathrust where there is probably underplating.…”

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

“…Axial fluvial systems with an eastern provenance also migrated to the area adjacent to the southern Kenai Peninsula (CB‐23) during middle Miocene time. Apatite thermochronologic ages <20 Ma in the northern Kenai Peninsula and Talkeetna Mountains indicate exhumation of the eastern region during the Miocene [ Little and Naeser , 1991; Hoffman and Armstrong , ; Arkle et al ., ; Valentino et al ., ]. The eastern provenance region sits above the shallow part of the Yakutat slab (Figure a) and exhumation may have resulted from insertion of the slab [ Finzel et al ., ; Arkle et al ., ; Valentino et al ., ].…”

“…Furthermore, previous studies have demonstrated that very little sand‐sized sediment is recycled from the Paleozoic and Mesozoic strata in the northern region [ Finzel et al ., ; Lease et al ., ]. In contrast, exhumation of the eastern region, especially the western Chugach Mountains located above the flat‐slab, continued into late Miocene time [ Buscher et al ., ; Arkle et al ., ; Valentino et al ., ]. Therefore, uplift and erosion of the eastern region resulted in the dominance of an eastern provenance for almost the entire fore‐arc basin during late Miocene time, with an additional small contribution from plutonic rocks in the northern region.…”

“…Widespread exhumation continued during Pliocene time in the Chugach Mountains and northern Kenai Peninsula [ Arkle et al ., ; Valentino et al ., ]. The Sterling Formation rests unconformably on the Beluga Formation in the northwest and northeast parts of the basin [ LePain et al ., ] above the subducting flat‐slab.…”

“…Peninsula [Arkle et al, 2013;Valentino et al, 2016]. The Sterling Formation rests unconformably on the Beluga Formation in the northwest and northeast parts of the basin above the subducting flat-slab.…”