UPPER CRUSTAL COOLING OF THE SOUTHERN FOOTHILLS OF THE WESTERN AND CENTRAL ALASKA RANGE AND LOW-TEMPERATURE DETRITAL THERMOCHRONOLOGY FROM THE ADJACENT SUSITNA BASIN, SOUTH-CENTRAL ALASKA

The Susitna basin (SB) is an actively subsiding coal-bearing basin situated above the shallowly-subducting Yakutat microplate. The SB is thought by some to be a northern extension of the Cook Inlet forearc basin (CIFB), an important commercial producer of microbial gas. The onset of nonmarine deposition in both basins was roughly synchronous during Paleogene time, but the SB is structurally separated from the CIFB by a major dextral reverse fault (Castle Mountain fault) and the basins exhibit disparate stratigraphic sequences and subsidence histories. To better understand the history of exhumation and sedimentation in this region, we compare results from 23 apatite fission-track (AFT) and 18 apatite (U-Th)/He (AHe) analyses of bedrock samples from the southern foothills of the western and central Alaska Range and basement highs within the SB to 8 paired detrital AFT (DAFT) and detrital zircon (DZ) analyses from 6 measured stratigraphic sections of SB Cenozoic strata.

Preliminary interpretation of the new bedrock cooling data from exhumed Late Cretaceous and Paleocene intrusive arc rocks and Cretaceous metasedimentary strata indicate major late Paleocene-late Eocene cooling and contemporaneous clastic deposition in the SB, suggesting relative uplift and denudation of the basin margin during that period. AFT ages range from 75.1-24.7 Ma, but are concentrated between ~56 and 40 Ma. AHe ages range from 63.3-7.6 Ma, and most commonly between ~53 and 39 Ma. AFT and AHe ages tend to be older nearer the interior of the basin, younger at the SW margin, and become progressively younger toward the NW margin, suggesting that cooling is in part structurally controlled and that the basin margin migrated northwestward over time. Paired DZ and DAFT ages from poorly dated Oligocene/Miocene or younger strata record mostly late Paleocene-Eocene cooling with principal detrital age populations from ~57.9-37.5 Ma for all but the northwestern-most sample locations. These data suggest that the upper crustal section denuded above the subducting Yakutat microplate since latest Oligocene time was relatively thin. The dominant cooling event in the region beginning in latest Paleocene time instead appears to overlap with, and closely follow late Paleocene-early Eocene subduction of the Kula-Farallon or Kula-Resurrection ridge system.