Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska

Abstract

Megathrust splay fault systems in accretionary prisms have been identifi ed as conduits for long-term plate motion and signifi cant coseismic slip during subduction earthquakes. These fault systems are important because of their role in generating tsunamis, but rarely are emergent above sea level where their long-term (million year) history can be studied. We present 32 apatite (U-Th)/He (AHe) and 27 apatite fi ssion-track (AFT) ages from rocks along an emergent megathrust splay fault system in the Prince William Sound region of Alaska above the shallowly subducting Yakutat microplate. The data show focused exhumation along the Patton Bay megathrust splay fault system since 3-2 Ma. Most AHe ages are younger than 5 Ma; some are as young as 1.1 Ma. AHe ages are youngest at the southwest end of Montague Island, where maximum fault displacement occurred on the Hanning Bay and Patton Bay faults and the highest shoreline uplift occurred during the 1964 earthquake. AFT ages range from ca. 20 to 5 Ma. Age changes across the Montague Strait fault, north of Montague Island, suggest that this fault may be a major structural boundary that acts as backstop to deformation and may be the westward mechanical continuation of the Bagley fault system backstop in the Saint Elias orogen. The regional pattern of ages and corresponding cooling and exhumation rates indicate that the Montague and Hinchinbrook Island splay faults, though separated by only a few kilometers, accommodate kilometer-scale exhumation above a shallowly subducting plate at million year time scales. This long-term pattern of exhumation also refl ects short-term seismogenic uplift patterns formed during the 1964 earthquake. The increase in rock uplift and exhumation rate ca. 3-2 Ma is coincident with increased glacial erosion that, in combination with the fault-bounded, narrow width of the islands, has limited topographic development. Increased exhumation starting ca. 3-2 Ma is interpreted to be due to rock uplift caused by increased underplating of sediments derived from the Saint Elias orogen, which was being rapidly eroded at that time.

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

“…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.…”

“…Open circles in (a) and (c) represent island-based seismic stations used for refraction models (Brocher et al, 1994). Ferguson et al, 2015;Haeussler et al, 2015). To the west of the Smith Islands, there is no evidence from seismic profiling or seafloor bathymetry for along-strike uplift (Liberty, 2013).…”

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.…”

Tsunamigenic Splay Faults Imply a Long‐Term Asperity in Southern Prince William Sound, Alaska

“…Surface ruptures during subduction zone earthquakes can highlight patterns of coseismic motion (e.g., Fujiwara et al, 2011;Henstock et al, 2006), paleoseismic and geodetic observations can provide estimates of recurrence intervals and patterns of uplift/subsidence (e.g., Atwater & Hemphill-Haley, 1997;Cisternas et al, 2005;Saillard et al, 2017;Shennan et al, 2014;Sieh et al, 2008), and thermochronology measurements can provide regional uplift rates over thousands of earthquake cycles (e.g., Enkelmann et al, 2015;Ferguson et al, 2015;Haeussler et al, 2015). Surface ruptures during subduction zone earthquakes can highlight patterns of coseismic motion (e.g., Fujiwara et al, 2011;Henstock et al, 2006), paleoseismic and geodetic observations can provide estimates of recurrence intervals and patterns of uplift/subsidence (e.g., Atwater & Hemphill-Haley, 1997;Cisternas et al, 2005;Saillard et al, 2017;Shennan et al, 2014;Sieh et al, 2008), and thermochronology measurements can provide regional uplift rates over thousands of earthquake cycles (e.g., Enkelmann et al, 2015;Ferguson et al, 2015;Haeussler et al, 2015).…”

“…Thrust faults that splay from a megathrust within subduction zone accretionary wedges can pose major seismic and tsunami hazards, yet little is known about the spatial and temporal controls on this family of faults. Surface ruptures during subduction zone earthquakes can highlight patterns of coseismic motion (e.g., Fujiwara et al, 2011;Henstock et al, 2006), paleoseismic and geodetic observations can provide estimates of recurrence intervals and patterns of uplift/subsidence (e.g., Atwater & Hemphill-Haley, 1997;Cisternas et al, 2005;Saillard et al, 2017;Shennan et al, 2014;Sieh et al, 2008), and thermochronology measurements can provide regional uplift rates over thousands of earthquake cycles (e.g., Enkelmann et al, 2015;Ferguson et al, 2015;Haeussler et al, 2015). However, detailed slip partitioning and uplift patterns over multiple earthquake cycles remains unknown.…”