Underplating below the Western Chugach Mountains in the Southern Alaska Block Syntaxial Core Constrained by Low-Temperature Thermochronology

Flat-slab subduction of the Yakutat microplate beneath south-central Alaska has played an instrumental role in the deformation of the overlying Southern Alaska Block (SAB). Terranes of the SAB are bound by arcuate fault systems that delineate regions of transpression and compression. North-verging accretion of these terranes form a prominent orogenic chain along Alaska’s southern coast, comprised of the Chugach, St. Elias Mountains, and adjacent coastal regions of the Prince William Sound (PWS). Results from several low-temperature thermochronology studies show that focused exhumation is constrained to a few main regions in southern Alaska - these are mainly in the St. Elias region of southeast Alaska where collisional processes cause rapid rock uplift and in the western Chugach Mountains where underplating along the megathrust is interpreted to cause localized rock uplift. However, the localized rock uplift in the western Chugach is inferred from PWS sea level thermochronology age trends that are projected north into the high elevation, glaciated regions of the western Chugach Mountain syntaxial core where age constraints were previously lacking.

Six new apatite (U-Th)/He (AHe) ages were determined from samples in the rugged core of the sytaxial region to evaluate spatial patterns of rock uplift. Ages along a NNW transect from Harvard Glacier to Mt. Sergeant Robinson range from 6.6 ± 0.9 to 10.8 ± 1.6 Ma. Ages along an ENE transect from Inner Lake George to west of Mt. Marcus Baker range from 4.7 ± 0.9 to 7.6 ± 1.4 Ma. However, when these ages are corrected for topographic effects and projected to sea level for comparison with previous sea level ages, their sea level age range is 2 to 6 Ma, with average age 4.1 Ma. The topographically-corrected sea level ages generally decrease into the core of the range where elevations are greatest. Inferred exhumation rates range from 0.3 to 0.5 km/Ma across the syntaxial core; these exhumation rates are the same as rates determined for PWS fiord samples to the south and suggest that late Miocene to recent average exhumation rates have been spatially constant across the region between the terrane-bounding Contact and Border Ranges fault systems. The consistency of the sea level projected ages and inferred exhumation rates across the syntaxial core supports a kinematic pattern of uniform rock uplift caused predominantly by underplating rather than crustal shortening between the terrane-bounding faults.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California