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  • 1
    Publication Date: 2005-08-01
    Description: High-resolution aeromagnetic surveys over forearc basins can detect faults and folds in weakly magnetized sediments, thus providing geologic constraints on tectonic evolution and improved understanding of seismic hazards in convergent-margin settings. Puget Sound, Washington, and Cook Inlet, Alaska, provide two case histories. In each lowland region, shallow-source magnetic anomalies are related to active folds and/or faults. Mapping these structures is critical for understanding seismic hazards that face the urban regions of Seattle, Washington, and Anchorage, Alaska. Similarities in aeromagnetic anomaly patterns and magnetic stratigraphy between the two regions suggest that we can expect the aeromagnetic method to yield useful structural information that may contribute to earth-hazard and energy resource investigations in other forearc basins. ©2005 The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.〈br /〉〈br /〉〈a href="http://doi.org/10.1186/BF03351857" target="_blank"〉〈img src="http://bib.telegrafenberg.de/typo3temp/pics/f2f773b55e.png" border="0"〉〈/a〉
    Print ISSN: 1343-8832
    Electronic ISSN: 1880-5981
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2014-03-08
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
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  • 3
    Publication Date: 2013-01-23
    Description: We describe the modern distribution of salt-marsh and tidal-flat foraminifera from Sitkinak Island (Trinity Islands) and Simeonof Island (Shumagin Islands), Alaska, to begin development of a dataset for later use in reconstructing relative sea-level changes caused by great earthquakes along the Alaska-Aleutian subduction zone. Dead foraminifera were enumerated from a total of 58 surface-sediment samples collected along three intertidal transects around a coastal lagoon on Sitkinak Island and two intertidal transects on Simeonof Island. Two distinctive assemblages of salt-marsh foraminifera were recognized on Sitkinak Island. Miliammina fusca dominated low-marsh settings and Balticammina pseudomacrescens dominated the high marsh. These two species make up 〉98% of individuals. On Simeonof Island, 93% of individuals in high-marsh settings above mean high water were B. pseudomacrescens . The tidal flat on Simeonof Island was dominated by Cibicides lobatulus (60% of individuals), but the lower limit of this species is subtidal and was not sampled. These results indicate that uplift or subsidence caused by repeated earthquakes along the Alaska-Aleutian subduction zone could be reconstructed in coastal sediments using alternating assemblages of near monospecific B. pseudomacrescens and low-marsh or tidal-flat foraminifera.
    Print ISSN: 0096-1191
    Topics: Geosciences
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  • 4
    Publication Date: 2013-05-03
    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.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
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  • 5
    Publication Date: 2018-05-23
    Description: The Queen Charlotte Fault defines the Pacific–North America transform plate boundary in western Canada and southeastern Alaska for c. 900 km. The entire length of the fault is submerged along a continental margin dominated by Quaternary glacial processes, yet the geomorphology along the margin has never been systematically examined due to the absence of high-resolution seafloor mapping data. Hence the geological processes that influence the distribution, character and timing of mass transport events and their associated hazards remain poorly understood. Here we develop a classification of the first-order shape of the continental shelf, slope and rise to examine potential relationships between form and process dominance. We found that the margin can be split into six geomorphic groups that vary smoothly from north to south between two basic end-members. The northernmost group (west of Chichagof Island, Alaska) is characterized by concave-upwards slope profiles, gentle slope gradients (〈6°) and relatively low along-strike variance, all features characteristic of sediment-dominated siliciclastic margins. Dendritic submarine canyon/channel networks and retrogressive failure complexes along relatively gentle slope gradients are observed throughout the region, suggesting that high rates of Quaternary sediment delivery and accumulation played a fundamental part in mass transport processes. Individual failures range in area from 0.02 to 70 km 2 and display scarp heights between 10 and 250 m. Transpression along the Queen Charlotte Fault increases southwards and the slope physiography is thus progressively more influenced by regional-scale tectonic deformation. The southernmost group (west of Haida Gwaii, British Columbia) defines the tectonically dominated end-member: the continental slope is characterized by steep gradients (〉20°) along the flanks of broad, margin-parallel ridges and valleys. Mass transport features in the tectonically dominated areas are mostly observed along steep escarpments and the larger slides (up to 10 km 2 ) appear to be failures of consolidated material along the flanks of tectonic features. Overall, these observations highlight the role of first-order margin physiography on the distribution and type of submarine landslides expected to occur in particular morphological settings. The sediment-dominated end-member allows for the accumulation of under-consolidated Quaternary sediments and shows larger, more frequent slides; the rugged physiography of the tectonically dominated end-member leads to sediment bypass and the collapse of uplifted tectonic features. The maximum and average dimensions of slides are an order of magnitude smaller than those of slides observed along other (passive) glaciated margins. We propose that the general patterns observed in slide distribution are caused by the interplay between tectonic activity (long- and short-term) and sediment delivery. The recurrence (〈100 years) of M 〉 7 earthquakes along the Queen Charlotte Fault may generate small, but frequent, failures of under-consolidated Quaternary sediments within the sediment-dominated regions. By contrast, the tectonically dominated regions are characterized by the bypass of Quaternary sediments to the continental rise and the less frequent collapse of steep, uplifted and consolidated sediments.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 6
    Publication Date: 2014-03-01
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
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  • 7
    Publication Date: 2015-10-02
    Description: We present new marine seismic-reflection profiles and bathymetric maps to characterize Holocene depositional patterns, submarine landslides, and active faults beneath eastern and central Prince William Sound (PWS), Alaska, which is the eastern rupture patch of the 1964 M w  9.2 earthquake. We show evidence that submarine landslides, many of which are likely earthquake triggered, repeatedly released along the southern margin of Orca Bay in eastern PWS. We document motion on reverse faults during the 1964 Great Alaska earthquake and estimate late Holocene slip rates for these growth faults, which splay from the subduction zone megathrust. Regional bathymetric lineations help define the faults that extend 40–70 km in length, some of which show slip rates as great as 3.75 mm/yr. We infer that faults mapped below eastern PWS connect to faults mapped beneath central PWS and possibly onto the Alaska mainland via an en echelon style of faulting. Moderate ( M w 〉4) upper-plate earthquakes since 1964 give rise to the possibility that these faults may rupture independently to potentially generate M w  7–8 earthquakes, and that these earthquakes could damage local infrastructure from ground shaking. Submarine landslides, regardless of the source of initiation, could generate local tsunamis to produce large run-ups along nearby shorelines. In a more general sense, the PWS area shows that faults that splay from the underlying plate boundary present proximal, perhaps independent seismic sources within the accretionary prism, creating a broad zone of potential surface rupture that can extend inland 150 km or more from subduction zone trenches.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 8
    Publication Date: 2015-05-01
    Description: The Queen Charlotte fault (QCF) is a dextral transform system located offshore of southeastern Alaska and western Canada, accommodating ~4.4 cm/yr of relative motion between the Pacific and North American plates. Oblique convergence along the fault increases southward, and how this convergence is accommodated is still debated. Using seismic reflection data, we interpret offshore basement structure, faulting, and stratigraphy to provide a geological context for two recent earthquakes, an M w  7.5 strike-slip event near Craig, Alaska, and an M w  7.8 thrust event near Haida Gwaii, Canada. We map downwarped Pacific oceanic crust near 54° N, between the two rupture zones. Observed downwarping decreases north and south of 54° N, parallel to the strike of the QCF. Bending of the Pacific plate here may have initiated with increased convergence rates due to a plate motion change at ~6 Ma. Tectonic reconstruction implies convergence-driven Pacific plate flexure, beginning at 6 Ma south of a 10° bend the QCF (which is currently at 53.2° N) and lasting until the plate translated past the bend by ~2 Ma. Normal-faulted approximately late Miocene sediment above the deep flexural depression at 54° N, topped by relatively undeformed Pleistocene and younger sediment, supports this model. Aftershocks of the Haida Gwaii event indicate a normal-faulting stress regime, suggesting present-day plate flexure and underthrusting, which is also consistent with reconstruction of past conditions. We thus favor a Pacific plate underthrusting model to initiate flexure and accommodation space for sediment loading. In addition, mapped structures indicate two possible fault segment boundaries along the QCF at 53.2° N and at 56° N.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 9
    Publication Date: 2015-05-01
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 10
    Publication Date: 2015-05-01
    Description: The 28 October 2012 M w  7.8 Haida Gwaii earthquake was a megathrust earthquake along the very obliquely convergent Queen Charlotte margin of British Columbia, Canada. Coseismic deformation is not well constrained by geodesy, with only six Global Positioning System (GPS) sites and two tide gauge stations within 250 km of the rupture area. To better constrain vertical coseismic deformation, we measured the upper growth limits of two sessile intertidal organisms, which are controlled by physical conditions, relative to sea level at 25 sites 5 months after the earthquake. We measured the positions of rockweed ( Fucus distichus , 617 observations) and the common acorn barnacle ( Balanus balanoides , 686 observations). The study focused on the western side of the islands where rupture models indicated that the greatest amount of vertical displacement, but we also investigated sites well away from the inferred rupture area to provide a control on the upper limit of the organisms unaffected by vertical displacement. We also made 322 measurements of sea level to relate the growth limits to a tidal datum using the TPXO7.2 tidal model, rather than ellipsoid heights determined by GPS. Three methods of examining the data all indicate 0.4–0.6 m subsidence along the western coast of Moresby Island as a result of the 28 October 2012 Haida Gwaii earthquake. Our data are, within the errors, consistent with data from two campaign GPS sites along the west coast of Haida Gwaii and with rupture models that indicate megathrust rupture offshore, but not beneath, the islands.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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