ALBERT

Description: Megathrust splay fault systems in accretionary prisms have been identified as conduits for long-term plate motion and significant 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 fission-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 reflects 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.

Description: The western Chugach Mountains and Prince William Sound are located in a syntaxial bend, which lies above flat-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 fission-track, zircon (U-Th)/He, and zircon fission-track ages, combined with ages from surrounding regions, show a bull’s-eye 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 flat-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 influx of underplated sediments that are derived from erosion in the Saint Elias orogen collision zone.

Description: The Cenozoic Susitna basin lies within an enigmatic lowland surrounded by the Central Alaska Range, Western Alaska Range (including the Tordrillo Mountains), and Talkeetna Mountains in south-central Alaska. Some previous interpretations show normal faults as the defining structures of the basin (e.g., Kirschner, 1994). However, analysis of new and existing geophysical data shows predominantly (Late Oligocene to present) thrust and reverse fault geometries in the region, as previously proposed by Hackett (1978). A key example is the Beluga Mountain fault where a 50-mGal gravity gradient, caused by the density transition from the igneous bedrock of Beluga Mountain to the 〉4-km-thick Cenozoic sedimentary section of Susitna basin, spans a horizontal distance of ~40 km and straddles the topographic front. The location and shape of the gravity gradient preclude a normal fault geometry; instead, it is best explained by a southwest-dipping thrust fault, with its leading edge located several kilometers to the northeast of the mountain front, concealed beneath the shallow glacial and fluvial cover deposits. Similar contractional fault relationships are observed for other basin-bounding and regional faults as well. Contractional structures are consistent with a regional shortening strain field inferred from differential offsets on the Denali and Castle Mountain right-lateral strike-slip fault systems.