AmericanChemicalSociety.com

Subduction of the Yakutat microplate in southern Alaska, either directly or indirectly, has had a more profound effect on deformation in southern and central Alaska than any other geologic event in the late Tertiary.  Subduction underplating and related tectonics are probably responsible for deformation leading to hydrocarbon traps in the Cook Inlet and perhaps in the North Slope area.

The Prince William Sound and western Chugach Mountains (PWS-CM) form an ~100x100 km syntaxial region.  The PWS-CM syntaxial region lies ~20-30 km above the interface of the subducting Yakutat microplate and the North American plate and is located west of complicated strike-slip deformation in the St. Elias area.  The Prince William Sound and western Chugach Mountains are located where strain might be focused due to subduction above the shallowly dipping, relatively buoyant Yakutat microplate. 

Our principal hypothesis for this work is that the PWS-CM syntaxial region is the location of rapid material removal by exhumation/erosion to balance out the accretionary flux brought in with shallow subduction of the Yakutat microplate.  To test this hypothesis, we are using low-temperature thermochronometry and field study of key areas of the Prince William Sound and western Chugach Mountains. We are attacking the main syntaxial region where we believe the effect of focused erosion might be the greatest and low-temperature thermochronometry ages will be much younger than around the periphery areas.

Work to date includes three seasons of field work to sample most of the critical areas of the PWS-CM region. Bedrock samples of granite, felsic dikes, and sandstone were collected via boat access in the fiords of the Prince William Sound.  These samples are used to evaluate the exhumation history in a N-S- and plate-transport direction to test the hypothesis of younging of ages toward the syntaxial core.  Glacial outwash samples from toes of major glaciers draining the north and south sides (leeward and windward sides) were also collected; these will be used to evaluate whether the mountains that are crossed by the glaciers contain different populations of ages – the glaciers essentially act as conveyor belts that sample much of the elevation range of the glaciated mountains.  We also sampled across major faults and collected one elevation transect to evaluate exhumation rate in the southern Chugach. All of these samples have been processed to extract the minerals apatite and zircon for thermochronology work.  Additional samples were collected via helicopter in the difficult to access middle of the western Chugach in summer 2010.  These samples are now being processed and should yield valuable constraints on location of focused uplift/exhumation in the critical core of the syntaxial region.

To date, bedrock (U-Th)/He ages from sea level are much younger than around the periphery of the syntaxial region.  Single-grain ages as young as 2 Ma have been determined for our northernmost processed sandstone samples.  Overall, (U-Th)/He ages tend to decrease northward and into the syntaxial region, with ages as > 10 Ma in our southern samples.  These results are starting to bear out our initial hypothesis that exhumation is focused in the syntaxial region.  Apatite fission track ages show a similar trend, but with overall higher ages. Modern glacial outwash zircon fission-track ages from three major glaciers on the north side of the range to give age peaks of 36-42 Ma and 50-60 Ma on two of the samples; these age peaks correspond to major heating events associated with near trench plutonism.  The third sample gives similar peaks, but also contains a younger robust peak of ~3 Ma; this young age probably reflects young volcanism from the Wrangell Mountains, but we cannot yet rule out the possibility of these young ages being related to very young and rapid exhumation.  Future U-Pb ages on these samples will confirm or refute young volcanic origin.  On the south side of the range, glacial outwash samples from two major glaciers give much younger overall peak ages of about 20, 29, 45 Ma.  The youngest age peak probably corresponds to initial Yakutat exhumation, but overall the younger ages on south (windward) side indicate that higher magnitude and higher rate rock uplift has occurred there.

One surprising result of our work thus far is that one sample collected on Montague Island in the southernmost part of the PWS field area gives our youngest (U-Th)/He and fission-track ages to date (~1 and 3 Ma).  This indicates that rock uplift, though rapid in the syntaxial region farther north, may be even more concentrated in the southernmost areas.  Montague Island is an area that underwent several meters of uplift associated with south-vergent thrust faulting during the M9.2 Alaska earthquake in 1964.  This region needs much more field analysis and sample collecting to test if extremely focused rock uplift occurs here over longer time scales, and will be a prime target in the next field season.

Thus far, this work has helped fund research for three graduate students and four undergraduates and has led to the publication of nine abstracts presentations at national and international meetings.  One major paper is currently being written for submission this winter.