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Crustal structure of Wrangellia and adjacent terranes inferred from geophysical studies along a transect through the northern Talkeetna Mountains

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, Anchorage, Alaska, USA
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Louise Pellerin
Louise Pellerin
3
Green Engineering, Inc., Anchorage, Alaska, USA
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Darcy K. McPhee
Darcy K. McPhee
4
U.S. Geological Survey, Menlo Park, California, USA
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J. Michael O'Neill
J. Michael O'Neill
5
U.S. Geological Survey, Denver, Colorado, USA
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Published:
January 01, 2007

Recent investigations of the Talkeetna Mountains in south-central Alaska were undertaken to study the region's framework geophysics and to reinterpret structures and crustal composition. Potential field (gravity and magnetic) and magnetotelluric (MT) data were collected along northwest-trending profiles as part of the U.S. Geological Survey's Talkeetna Mountains transect project. The Talkeetna Mountains transect area comprises eight 1:63,360 quadrangles (∼9500 km2) in the Healy and Talkeetna Mountains 1° × 3° sheets that span four major lithostratigraphic terranes (Glen et al., this volume) including the Wrangellia and Peninsular terranes and two Mesozoic overlap assemblages inboard (northwest) of Wrangellia. These data were used here to develop 2½-dimensional models for the three profiles.

Modeling results reveal prominent gravity, magnetic, and MT gradients (∼3.25 mGal/km, ∼100nT/km, ∼300 ohm-m/km) corresponding to the Talkeetna Suture Zone—a first-order crustal discontinuity in the deep crust that juxtaposes rocks with strongly contrasting rock properties. This discontinuity corresponds with the suture between relatively dense magnetic crust of Wrangellia (likely of oceanic composition) and relatively less dense transitional crust underlying Jurassic to Cretaceous flysch basins developed between Wrangellia and North America. Some area of the oceanic crust beneath Wrangellia may also have been underplated by mafic material during early to mid-Tertiary volcanism.

The prominent crustal break underlies the Fog Lakes basin approximately where the Talkeetna thrust fault was previously mapped as a surface feature. Potential field and MT models, however, indicate that the Talkeetna Suture Zone crustal break along the transect is a deep (2–8 km), steeply west-dipping structure—not a shallow east-dipping Alpine nappe-like thrust. Indeed, most of the crustal breaks in the area appear to be steep in the geophysical data, which is consistent with regional geologic mapping that indicates that most of the faults are steep normal, reverse, strike-slip, or obliqueslip faults. Mapping further indicates that many of these features, which likely formed during Jurassic and Cretaceous time, such as the Talkeetna Suture Zone have reactivated in Tertiary time (O'Neill et al., 2005).

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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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