HR: 0800h
AN: T41C-1231 INVITED     [Abstracts]
TI: Crustal structure of transpressional margins based on along strike variations in kinematics and exposed level in strike-slip systems of southern Alaskan
AU: * Pavlis, T L
EM: tpavlis@uno.edu
AF: University of New Orleans, Dept. of Geology and Geophysics, New Orleans, LA 70148 United States
AB: Strike-slip systems dominate the Cenozoic tectonics of the northern North American Cordillera, and variable levels of exposure together with lateral kinematic variations make this region a premier area for studies of strike-slip processes. Neogene interactions between the Yakutat microplate and North America provide one of the most extreme examples on earth of an ongoing oblique-collision where the obliquity of convergence varies rapidly along strike. Here the St. Elias orogen occupies the transition from the Queen Charlottes transform to the Aleutian subduction zone and the microplate caught up in the transition shows clear spatial variations related to this variation in kinematics, but that process has clearly varied in time as the microplate has been carried into the strike-slip/subduction transition. Three important results from recent work on this orogenic system include: 1) evidence that slip partitioning recognized in the southern, highly oblique, segment of the orogen carries through to the core of the orogen where convergence angles are high; 2) strike-slip is transferred into a an oroclinal bend with complex structural overprinting including superimposed fold systems in unmetamorphosed sedimentary rock; and 3) development of a predominantly "one sided" orogen in which contraction is focused toward the outboard (oceanic) side of the orogen. The last appears to have been a long-term effect because exhumation rates from thermochronology and distribution of metamorphic assemblages suggest exhumation was focused on the rocks outboard of a strike-slip "backstop". This exhumation pattern is almost certainly the result of intense erosion on the oceanic side of the orogen driven by variable orographic precipitation, much of which is converted to glacial ice that profoundly influences erosion within the orogen. The St. Elias orogen also exposes older, deeply exhumed strike-slip systems that are surprising similar to the neotectonic system and provide insights into deep crustal processes. The Eocene Hanagita fault system, which represents a dextral reactivation of the Mesozoic arc-forearc boundary, shows mid to upper crustal level exposures of structures very similar to the St. Elias orogen-a dominantly one-sided, slip-partitioned "positive flower structure" along the eastern limb of the Alaskan orocline that transfers into a zone of complex overprinting in the "core" of the orocline. Finally, constraints on deep-crustal processes are provided in a spectacular down-plunge exposure of an Eocene forearc strike-slip shear zone that shows a low-angle mid-crustal detachment between uniformly deformed high-grade gneiss at depth and overlying amphibolite facies schist deformed by distinct ductile shear zones.
DE: 8015 Local crustal structure
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting