Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

Abstract

The western Chugach Mountains and Prince William Sound are located in a syntaxial bend, which lies above fl at-slab subduction of the Yakutat microplate and inboard of the Yakutat collision zone of southern Alaska. The syntaxis is characterized by arcuate fault systems and steep, high topography, which suggest focused uplift and exhumation of the accretionary prism. We examined the exhumation history with low-temperature thermochronometry of 42 samples collected across the region. These new apatite (U-Th)/He, apatite fi ssion-track, zircon (U-Th)/He, and zircon fi ssion-track ages, combined with ages from surrounding regions, show a bull'seye pattern, with the youngest ages focused on the western Chugach syntaxis. The ages have ranges of ca. 10-4 Ma, ca. 35-11 Ma, ca. 33-25 Ma, and ca. 44-27 Ma, respectively. The youngest ages are located on the south (windward) side of the Chugach Mountains and just north of the Contact fault. Sequentially higher closure temperature systems are nested across Prince William Sound in the south, the Chugach Mountains, and the Talkeetna Mountains to the north. Computed exhumation rates typically are 0.2 mm/yr across Prince William Sound, increase abruptly to ~0.7 mm/yr across and adjacent to the Contact fault system, and decrease to ~0.4 mm/yr north of the core of the Chugach Mountains. The abrupt age and exhumation rate changes centered on the Contact fault system suggest that it may be a critical structural system for facilitating rock uplift. Our data are most consistent with Yakutat fl at-slab subduction starting in the Oligocene, and since then ~11 km of rock uplift north of the Contact fault and ~4-5 km of rock uplift in Prince William Sound to the south. These data are consistent with a deformation model where the western Chugach core has approached long-term exhumational steady state, though exhumation rates have probably increased in the last ~5 m.y. We interpret that rock uplift in the overriding wedge has been driven dominantly by underplating, with long-term vertical displacement concentrated at the southern edge of the Chugach Mountains and centered on the Contact fault system. Though our data do not unequivocally differentiate between Pliocene tectonic-or climate-related causes for increased exhumation in the last ~5 m.y., we interpret the increased rates to be due to increased infl ux of underplated sediments that are derived from erosion in the Saint Elias orogen collision zone.

Landslides and Megathrust Splay Faults Captured by the Late Holocene Sediment Record of Eastern Prince William Sound, Alaska

“…The 5-10 m offset across the NGU near the Smith Islands does suggest a larger average fault displacement per earthquake than on the CS4 profile to the east. This increase in uplift to the west is consistent with exhumation patterns recorded from thermochronology data that average over thousands of earthquake cycles (Arkle et al, 2013; et al, 1994) with earthquake hypocenters of M w > 2 (from Doser et al, 2008) and interpreted faults that splay from the megathrust. The ellipse represents our interpreted zone of duplexing.…”

“…5). In addition, the lack of upperplate reflectors is consistent with few mapped thrust faults in this part of PWS and slow rates of long-term exhumation compared with other portions of PWS (Arkle et al, 2013;Haeussler et al, 2015).…”

“…Although the entire PWS region was uplifted in 1964, geodetic data show that central PWS has been gradually subsiding since the earthquake, consistent with a locked megathrust interface (Freymueller et al, 2008). The relatively moderate interseismic subsidence contrasts with the estimated millennial-scale exhumation rates of greater than 1 mm=yr recorded with thermochronology methods for rocks in the hanging wall of the Patton Bay megathrust splay fault (Arkle et al, 2013;Ferguson et al, 2015;Haeussler et al, 2015). Hence, coseismic uplift rates have outpaced interseismic subsidence rates in the hanging wall of these splay faults for PWS throughout the Quaternary record.…”

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

“…We show that 1964 uplift near the Junken Trough was likely similar to the uplift observed near the north end of Montague Island and that the uplift rate has remained relatively fixed through Holocene earthquake cycles. Using a >10 Ma time scale, apatite-fission track closure rates from Arkle et al [2013] show a similar pattern where they suggest one third of the exhumation rate from rocks along northern Montague Island when compared to uplifts recorded near Cape Cleare. These studies suggest that southern Montague Island and Cape Cleare Bank have remained the focus of coseismic uplift during the Quaternary.…”

Initiation of the Wrangell Arc: A Record of Tectonic Changes in an Arc-Transform Junction Revealed by New Geochemistry and Geochronology of the ~29–18 Ma Sonya Creek Volcanic Field, Alaska

“…Recent thermochronology research in the Chugach-St. Elias Range to the south of the SCVF reveals a record of uplift and exhumation as a result of the progressive subduction, collision, and underthrusting of the Yakutat microplate beneath North America (Berger et al, 2008a(Berger et al, , 2008bMcAleer et al, 2009;Enkelmann et al, 2008Enkelmann et al, , 2010Arkle et al, 2013;Grabowski et al, 2013;Falkowski et al, 2014Falkowski et al, , 2016Falkowski and Enkelmann, 2016;Enkelmann et al, 2017;Dunn et al;. After the subduction of a spreading ridge in Eocene time, the eastern Chugach-western St. Elias range experienced relative quiescence until the onset of Yakutat subduction no earlier than 35 Ma (Enkelmann et al, 2010;Falkowski et al, 2014Falkowski et al, , 2016.…”