Focused exhumation in the southern Alaska syntaxis: New insights from apatite and zircon thermochronology
California State University, Fullerton
ix, 122 p., Illus. [Thesis accompanied by CD-ROM, IBM compatible]
The western Chugach Mountains are located in a key position that may link active margin-normal deformation, related to Yakutat microplate subduction, to the southern Alaska syntaxial core. This syntaxis is characterized by arcuate fault systems and the steepest topography of any accretionary prism in the world, whereupon mountain building may be focused and maintained by erosional output. The exhumation history and mass flux balances of the accretionary prism are addressed with apatite fission-track (AFT) and apatite (AHe) and zircon (ZHe) (U-Th)/He ages from a sea level transect in the Prince William Sound and western Chugach Mountains. New AHe, AFT, and ZHe ages generally decrease from south to north into the western Chugach syntaxis and range from â€šÃ Âº10 to 4 Ma, â€šÃ Âº35 to 11 Ma and from â€šÃ Âº33 Ma to 25 Ma, respectively. The youngest ages are in the core of the range, shifted slightly to the windward and seaward flank of the mountains. These data combined with thermochronometer ages from the surrounding regions show a decrease in age from all directions into the core of the range to form a bull's eye pattern of localized exhumation in the western Chugach syntaxis. Sequentially higher closure temperature systems are nested across the entire length of the Prince William Sound in the south to the Talkeetna Mountains to the north. Abrupt age changes across the range bounding Contact and Border Ranges fault zones indicate these are critical structures for facilitating Yakutat microplate driven strain. Between these structures, AHe ages in the syntaxis are nearly time-invariant, at about 5-6 Ma, and suggest nearly uniform rock uplift of the core of the range. Based on these data a new model of tectonic deformation is proposed that identifies underplating as the principal mechanism driving deformation in the western Chugach Mountains and Prince William Sound. Vertical strain may be enhanced by the east-to-west constriction of crust in the Border Ranges structural bend, which extended to million year time scales, has produced rapid exhumation and focused rock uplift above the locked section in the western Chugach Mountains. This temporal pattern and spatial concentration shows high-coupling between tectonic and erosional flux, which suggests the western Chugach Mountains have reached a dynamic exhumational steady state. Analysis of thermochronometer ages along the southern Alaska margin additionally suggests that arcuate structural boundaries, such as the Border Ranges and Contact fault zones, may control the locus of exhumation. Thus, fundamental control on the distribution of strain in the upper crust may be the arcuate structural boundaries, which produce positive feedbacks between the windward polarity of exhumation but not the orogen-scale distribution. Whereas glacial erosion imposes limits to mean topographic height and increases total relief tectonics are ultimately driving the highly dynamic and coupled system. The focused rock uplift between the Contact and Border Ranges fault systems is thus, interpreted as a result of the combination of underplating above the megathrust that is localized and controlled by the western Chugach syntaxial bend.
Minerals Data and Information Rescue in Alaska (MDIRA)