Alaska's "Sleeping Lady"

Boulder, Colo., USA – Every year countless Alaskans and visitors gaze across Cook Inlet from Anchorage at the "Sleeping Lady" silhouette of Mount Susitna. This scenic mountain and the more rounded Beluga Mountain to the northwest rise up steeply from the adjacent lowlands of the Susitna basin where the Yentna and Susitna rivers meander on their way to the Inlet.

The geologic story to explain this low-lying basin in the midst of an otherwise mountainous terrain (the high Talkeetna Mountains to the north and portions of the Western Alaska Range to the west and south) has been difficult to tell. Conventional wisdom assumes a northeast-dipping fault (a "normal" fault) between the Mount Susitna/Beluga Mountain front and the Susitna basin.

This study by R.W. Saltus and colleagues of gravity, seismic, and magnetic data overturns this conventional interpretation and shows that the bounding fault instead dips to the southwest (beneath the mountains); it is a thrust fault. The existence of the Beluga Mountain thrust fault was previously postulated by an Alaskan geophysicist named Steve Hackett in the late 1970s. This work by Saltus and colleagues proves that he was correct.

This changes the current understanding of this structure, and of the recent geologic evolution of the mountains and adjacent basin that are such familiar parts of the local landscape.

FEATURED ARTICLE
Late Oligocene to present contractional structure in and around the Susitna basin, Alaska -- Geophysical evidence and geological implications
R.W. Saltus et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80305, USA. Themed issue: Geologic Evolution of the Alaska Range and Environs. This article is OPEN ACCESS online at http://geosphere.gsapubs.org/content/early/2016/08/11/GES01279.1.abstract.

All GEOSPHERE articles are available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosiciety.org.

All recently published articles are highlighted below.

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Magmatic history and crustal genesis of western South America: Constraints from U-Pb ages and Hf isotopes of detrital zircons in modern river
Martin Pepper et al., Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/07/22/GES01315.1.abstract.

This paper utilizes ages and Hf isotope signatures of detrital zircons collected from modern rivers to reconstruct the magmatic history and crustal evolution of western South America. The primary data consist of 5,524 new U-Pb ages and 1,199 new Hf isotope determinations from 59 samples that were collected from both the eastern and western flanks of the Andes along the length of western South America. These data, together with previously published geochronologic and isotopic information, indicate that (1) western South America has age maxima at 2.2-1.8 Ga, 1.6-0.9 Ga, 700-400 Ma, and 360-200 Ma, which is similar to most other continents and presumably records processes of crustal generation/preservation related to the supercontinent cycle, (2) <200 Ma magmatism in western South America has age maxima at approximately 183, 166, 149, 125, 110, 88, 65, 35, 21, and 4 Ma (with significant north-south and east-west variations), yielding an average cyclicity of ~33 m.y.; (3) no correlation exists between this <200 Ma magmatic history and the velocity of convergence between central South America and oceanic plates, the age of downgoing oceanic plates, or the absolute motion of South America; (4) Hf isotopes record reworking of older crustal materials during most time periods, with incorporation of juvenile crust at ~1.6-1.0 Ga, 500-300 Ma, and ~175-35 Ma.

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Growth and maturation of a mid- to shallow-crustal intrusive complex, North Cascades, Washington
Erin K. Shea et al., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA and Department of Geological Sciences, University of Alaska, Anchorage, Alaska 99775, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/07/22/GES01290.1.abstract.

Intrusive igneous rocks represent a series of complex magmatic processes, including the generation and movement of magma in Earth's crust. However, much remains unknown regarding the how quickly these bodies are assembled, which has important implications for understanding the existence and longevity of magma chambers in volcanic arcs. Our study sought to better understand the rate at which magma is intruded in the crust. We conducted a detailed geochronologic and isotopic study of the Black Peak intrusive complex, a ca. 90 Ma intrusion in the North Cascades of Washington. Our data suggests that it is plausible that the Black Peak intrusive complex existed in a partially molten ("mushy") state for at least part of its 4.5 My lifetime.

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Shallow geophysical imaging of the Olympia anomaly: An enigmatic structure in the southern Puget Lowland, Washington State
Jack K. Odum et al., Geologic Hazards Science Center, U.S. Geological Survey, PO Box 25046, MS 966, Denver, Colorado 80225, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/07/22/GES01248.1.abstract.

The near linear and overlapping regional gravity and magnetic anomaly traces (potential field anomalies) define the projected surface location of a narrow, approximately 80 km-long, northwest-trending feature, (the Olympia Structure), in the southern Puget lowlands of Washington state. Typically such linear potential field anomalies suggest tectonic deformation involving the folding and or faulting and displacement (uplift) of basement rock. Northeast of the Olympia Structure trace, basement volcanic rocks lie beneath 4 to 6 km of Tertiary and Quaternary strata in the Tacoma basin, whereas these same basement rocks are exposed at the surface in the mountains of the Black Hills southwest of the structure. Numerous marine seismic-reflection profiles have been acquired near the surface trace location of the Olympia structure as defined by potential field anomalies; however, its tectonic character remains enigmatic, in part because inlets of southern Puget Sound are too shallow for the collection of deep-penetration marine seismic profiles across the geophysical anomalies. To supplement existing shallow marine data near the structure, we acquired 14.6 km of land-based seismic-reflection data along a profile that extends from basement rock exposed in the Black Hills northward across the projected surface location of the Olympia structure. Overall, the seismic-reflection profile, reanalysis of the marine seismic profiles, and constructed magnetic boundary analysis models presented in this study image a relatively gentle northeast dipping basement surface with only minor shallow faulting at the projected surface location of the Olympia structure. We suggest that the mapped trace of the Olympic structure along the northern flank of the Black Hills, at least within the study area, may be constrained more by a combination of relatively small amounts of faulted and juxtaposed normal and reversely magnetized basement volcanic rock units along with small variations in the thickness of overlying Tertiary and Quaternary strata, rather than being solely aligned along a major fault offset of basement bedrock.

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Detrital zircon U‑Pb geochronology and Hf isotope geochemistry of the Yukon-Tanana terrane, Coast Mountains, southeast Alaska
Mark E. Pecha et al., Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/07/22/GES01303.1.abstract.

This study utilizes U-Pb ages and Hf geochemical information from detrital zircons to characterize the main assemblages of the Yukon-Tanana terrane within and adjacent to the Coast Mountains of southeast Alaska. The U-Pb geochronologic and Hf isotopic data provide a detailed record of Ordovician through Carboniferous magmatism in the terrane and provide critical insight into its tectonic evolution. The data also shed light upon potential connections with other assemblages (i.e. Alexander terrane) preserved in the North American Cordillera. Finally, the comparisons illustrated in this study demonstrate the power of coupled U-Pb/Hf data for tectonic analysis and suggest that such data from other Cordilleran terranes will be helpful in reconstructing their origins and displacement histories.

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New constraints on the magma distribution and composition beneath Volcán Uturuncu and the southern Bolivian Altiplano from magnetotelluric data
Matthew J. Comeau et al., Department of Physics, University of Alberta, Edmonton T6E 2E1, Canada. Themed issue: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes This article is online at http://geosphere.gsapubs.org/content/early/2016/08/11/GES01277.1.abstract.

Volcan Uturuncu is located in the Bolivian Andes in an area where many large eruptions have occurred in the last 10 million years. While not active today, the volcano is at the centre of a region where the surface is moving upwards at 10-20 mm per year. This paper uses geophysical exploration with low frequency radio waves to look for magma in the crust beneath the volcano. These data have detected (1) a zone of molten rock at a depth of 15-20 km below sea level that extends over a horizontal distance of hundreds of kilometers and (2) a shallower magma body close to sea level. While these bodies contain large amounts of molten rock, ongoing research is needed to determine the chances of future eruptions.

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U-Pb isotopic ages of euhedral zircons in the Rhaetian of British Columbia: Implications for Cordilleran tectonics during the Late Triassic
M.L. Golding et al., Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2020-2207 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada and Natural Resources Canada/Geological Survey of Canada Pacific, 1500-605 Robson Street, Vancouver, British Columbia V6B 5J3, Canada. This article is online at http://geosphere.gsapubs.org/content/early/2016/08/11/GES01324.1.abstract.

Radiometric dating of detrital zircon grains from the Triassic sedimentary rocks of British Columbia helps to constrain our understanding of the formation of the Canadian Cordillera. These grains were found in rocks that were deposited on what used to be the margin of the North American continent. However, the age of these grains suggests that they were derived from volcanic activity on one of the island arcs that lay to the west of the continent during the Triassic. This implies close proximity between this island arc and the North American continent at this time, which is contrary to previous ideas of where and when the arc collided with the continent. The collision must have taken place at the latitude of B.C. before the end of the Triassic, and not during the Jurassic far to the south, as has previously been suggested.

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Magmatic versus phreatomagmatic fragmentation: Absence of evidence is not evidence of absence
James D.L. White, Geology Department, University of Otago, PO Box 56, Dunedin 9054, New Zealand; and Greg A. Valentine, Department of Geology and Center for Geohazards Studies, University at Buffalo, 411 Cooke Hall, Buffalo, New York 14260, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/08/11/GES01337.1.abstract.

Explosive volcanic eruptions can be “magmatic” with volcanic gases, “phreatomagmatic” with magma-water interactions producing vapor explosions, or some combination of the two. Many papers have inferred that magma-water interaction leaves a simple signature, and when no such signature is recognized have inferred that an eruption was driven only by “magmatic” processes. We argue that this approach is inappropriate. Even things already known about phreatomagmatism are insufficiently recognized, but more importantly it is also a fact that much is NOT known about both phreatomagmatic and magmatic processes. Our knowledge of both will increase, but even so we must resist use of the logically fallacious approach, termed argument from ignorance, in which conclusions are based on an absence of evidence. Instead we should build cases based on positive evidence, and accept that with present knowledge we may not be able to distinguish in all cases phreatomagmatic versus magmatic eruptions from their deposits.

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17 August 2016
GSA Release No. 16-47

Kea Giles, Managing Editor,
GSA Communications
+1-303-357-1057
kgiles@geosociety.org

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CC by Sanchom
Mount Susitna (the "Sleeping Lady") as viewed from the Tony Knowles Trail in Anchorage, Alaska, looking across Cook Inlet. Photo taken 23 June 2008 by Sanchom (via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:MtSusitna.jpg. CC: BY-SA.