2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 143-7
Presentation Time: 10:30 AM


STONE, David, Geophysical Institute, University of Alaska, 903 Koyukuk Drive, P O Box 757320, Fairbanks, AK 99775, TOMSICH, Carla Susanne, Geology and Geophysics, University of Alaska, PO Box 755780, Fairbanks, AK 99775-5780 and WALLACE, Wesley K., Geology and Geophysics, University of Alaska Fairbanks, 900 Yukon Drive, Fairbanks, AK 99775

The visible parts of the Yakutat Terrane are bounded by the active Fairweather-Queen Charlotte Islands right-lateral strike-slip fault, the Chugach-St.Elias thrust fault and the Transition Fault. The Pacific Plate is moving in a northwesterly direction (more or less parallel to the Fairweather-Queen Charlotte Islands fault) at about 50mm/year with respect to continental North America. GPS measurements show that near coastal stations that have the highest velocities move in roughly the same direction and at the same speed as the Pacific Plate.

From the Alaska Peninsula to the east the subducting Pacific plate, represented by the deep earthquake zone curves northwards near Cook Inlet, diverging from the megathrust and crossing the Denali Fault under Mt. McKinley. A large gap in surface volcanism between Hayes Glacier volcano and Jumbo Dome, the northernmost arc volcanism, may be related to a thickened slab under the Alaska Range.

The Wrangell Mountains include a group of very large Quaternary volcanoes whose chemistry is characteristic of a subduction zone/arc setting. There are very few deep earthquakes spatially associated with this volcanic assemblage and the volcanoes are clumped together rather than spaced out along a subducting slab.

The very wide gap between the megathrust and the volcano line is highlighted by a high density of earthquakes at depths around 50km and shallower, strongly indicating Flat-Slab subduction. The Flat-Slab earthquakes end abruptly along a northwest-southeast line more or less parallel to the overall motion of the Pacific Plate and the Yakutat Terrane.

Current subsurface models of the subducting Yakutat Terrane are largely based on tomographic modeling and place the western part of the slab under the seismically active Flat-Slab area but it may extend eastwards under much of the Copper River Basin, currently a large seismically dead zone. These models are being actively pursued.

To allow speculation on these observations we present a layered map showing topography, earthquakes color coded for depth, GPS measurements color coded for velocities, the distribution of faults, the distribution of volcanoes and models for the subsurface extent of the Yakutat Terrane. All of these features can be switched on and off to allow clearer views of their complicated interactions with each other.