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Gravity and magnetic character of south-central Alaska: Constraints on geologic and tectonic interpretations, and implications for mineral exploration

By
Jonathan M.G. Glen
Jonathan M.G. Glen
1
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
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Jeanine Schmidt
Jeanine Schmidt
2
U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA
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Robert Morin
Robert Morin
3
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
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Published:
January 01, 2007

Recent gravity and aeromagnetic investigations of the Talkeetna Mountains of south-central Alaska (61.5–63.75°N, 145–151°W) were undertaken to study the region's framework geophysics and to reinterpret crustal structures and composition. Aeromagnetic data for this study were compiled from 13 available regional- and local-scale surveys. Over 400 new gravity stations were collected along 12 profiles in the study area and combined with 3286 existing regional data.

These data are brought together here with current stratigraphic, lithogeochemical, structural, isotopic, and paleontologic findings to bear on the tectonics and metallogeny of south-central Alaska, and in particular to: (1) help understand the regional tectonic character of south-central Alaska, especially related to the development of the southern Alaska orocline; (2) to determine the structural relationships between tectonostratigraphic terranes (including Kahiltna, Wrangellia, and Peninsular terranes, as well as smaller terranes such as Susitna, Broad Pass, and Maclaren); (3) to understand the character of major faults; and (4) to develop a geophysically based regional mineral assessment for the Talkeetna Mountains and surrounding region that identifies the locations, size, and depth of buried sources of important potential mineral targets such as ultramafic units associated with feeder zones to the Triassic Nikolai Greenstone flood basalt.

Within the Talkeetna Mountains region, we interpret four regional-scale domains, with internally consistent geophysical character, that mostly correspond with previously defined tectonostratigraphic terranes. These include a Wrangellia domain, a restricted Peninsular domain, and two domains within the Mesozoic overlap assemblage north of Wrangellia. At the broadest scale, potential field data suggest that a large block of the Talkeetna Mountains consists of relatively dense magnetic crust, likely of oceanic-crustal composition (corresponding with Wrangellia and Peninsular terranes) that may also have been underplated by mafic material during early to middle Tertiary volcanism. At the NW edge of this block lies a prominent gravity and magnetic gradient (∼3.25 mGal/km, ∼100 nT/km) that forms a NE-trending first-order crustal discontinuity between dense late Paleozoic to Cretaceous Wrangellia crust and low-density Jurassic to Cretaceous flysch to the northwest.

Potential field models indicate this crustal break is a deep (>10 km), steeply dipping structure—not a shallow-dipping thrust as previously thought. Confined to a narrow zone of a few tens of kilometers situated along this boundary is a belt of Nikolai Greenstone and related rocks that carry a distinct geophysical signature that allows for assessing their size and distribution.

A zone of transitional crust, located between Wrangellia and North America, is cut by several northeast-trending structures that may have been reactivated in Tertiary time, perhaps accommodating combined thrust and dextral strike-slip motion due to the westward escape of crust during and since Tertiary oroclinal bending.

Due to the geophysical contrast of continental North American crust and Wrangellia's oceanic crust, slivers of Wrangellia's margin can be seen offset, along the Talkeetna fault zone, from interior Wrangellia, perhaps allowing an estimate of the magnitude of strike-slip displacement.

The Mesozoic flysch northwest of this crustal break is distributed over two distinct geophysical domains that we interpret as a southeast domain underlain by transitional crust and a northwest domain rooted by continental crust. We infer this to reflect two different depositional basins, consistent with recent sediment provenance studies that demonstrate that two distinct subbasins, one to the northwest and one to the southeast, received their sediments from continental North America's Mesozoic margin and from Wrangellia, respectively.

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GSA Special Papers

Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska

Kenneth D. Ridgway
Kenneth D. Ridgway
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Jeffrey M. Trop
Jeffrey M. Trop
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Jonathan M.G. Glen
Jonathan M.G. Glen
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J. Michael O'Neill
J. Michael O'Neill
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Geological Society of America
Volume
431
ISBN print:
9780813724317
Publication date:
January 01, 2007

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