NEW STRATIGRAPHIC AND DETRITAL GEOCHRONOLOGIC CONSTRAINTS ON DEFORMATION, DEPOSITION, AND DEXTRAL DISPLACEMENT ALONG THE CASTLE MOUNTAIN FAULT, SOUTH-CENTRAL ALASKA

Resolving the complex deformation history of the >200 km long Castle Mountain fault in south-central Alaska requires detailed studies of potentially offset geologic features. Lithofacies mapping, measured sections, and geochronologic data document a unique stratigraphic succession exposed on opposite sides of the fault. At Puddingstone Hill (south side of fault) and Billy Mountain (north side of fault), an angular unconformity separates sedimentary strata (Eocene Wishbone Fm.) from overlying volcaniclastic strata (Eocene, unnamed). Elsewhere, the sedimentary strata are overlain by Eocene lavas or Oligocene conglomerate. The sedimentary strata consist of massive to imbricated, pebble-cobble conglomerate with volcanic-plutonic clasts and minor cross-stratified sandstone. Paleocurrent indicators are southwest- to southeast-directed. Volcaniclastic strata consist of tuff-breccia, tuff, volcaniclastic sandstone, and minor agglomerate and lava. Detrital zircon U-Pb ages from both units reveal two major populations: Late Cretaceous-Paleocene (46% of all grains) and Jurassic-Early Cretaceous (30%). The sedimentary units share similar detrital age spectra (two samples; n=185), including several shared peaks (54, 60, 63, 103, 350 Ma). Age spectra from the sedimentary strata are more similar to each other than with coeval strata exposed elsewhere along the fault. Volcaniclastic strata (two samples, n=54) share major age peaks (59 and 170 Ma) but exhibit variations in Mesozoic and Precambrian peaks. These differences are difficult to assess given the small number of grains analyzed (n=11 to 43). The datasets indicate: (1) Both units were deposited during Eocene time. (2) Detritus was eroded mainly from the Paleogene Caribou Creek volcanic field and Jurassic-Cretaceous plutons. (3) Sedimentary strata were deposited in braided rivers that drained the same source and were deformed prior to volcaniclastic deposition. (4) Volcaniclastic strata were deposited in paleovalleys by pyroclastic eruptions, lahars, and streamflow. (5) More speculatively, the succession was offset by up to ~43 km of Eocene right-lateral displacement, with most displacement occurring during unconformity development, consistent with past estimates of several tens of kilometers of Cenozoic right-lateral displacement.