EARLY TERTIARY EXHUMATION OF THE FLANK OF A FOREARC BASIN, SOUTHWEST TALKEETNA MOUNTAINS, ALASKA

New detrital and thermochronologic data from rocks near Hatcher Pass, southwest Talkeetna Mountains, Alaska, record earliest Paleocene erosional and structural exhumation on the flank of the active Cook Inlet forearc basin. Cretaceous plutons shed sediments to the south, forming the Paleocene Arkose Ridge Formation. A Paleocene(?)-Eocene detachment fault juxtaposed 60 Ma metamorphic rocks with the base of the Arkose Ridge Formation.

U-Pb (SHRIMP) zircon ages of the Cretaceous plutons, more diverse than previously documented, are 90.3 ± 0.3 (previously considered a Jurassic unit), 79.1 ± 1.0, 76.1 ± 0.9, 75.8 ± 0.7, 72.5 ± 0.4, 71.9 ± 0.3, 70.5 ± 0.2, and 67.3 ± 0.2 Ma. The cooling of these plutons occurred between 72 and 66 Ma (zircon fission track (FT) closure ~225°C). ⁴⁰Ar/³⁹Ar analyses of hornblende, white mica, and biotite fall into this range (Harlan et al., 2003). New apatite FT data collected on a W-E transect reveal sequential exhumation of fault blocks at 62.8 ± 2.9, 54 ± 2.5, 52.6 ± 2.8, and 44.4 ± 2.2 Ma.

Plutonic clasts accumulated in the Paleocene Arkose Ridge Formation to the south. Detrital zircon (DZ) ages from the formation reflect this provenance: a new sample yielded one grain at 61 Ma, a dominant peak at 76, and minor peaks at 70, 80, 88, and 92 Ma. The oldest zircon is 181 Ma. Our apatite FT ages range from 35.1 to 50.9 Ma.

Greenschist facies rocks now sit structurally between the plutonic rocks and the Arkose Ridge Formation. They are separated from plutonic rocks by the vertical Hatcher Pass fault and from the sedimentary rocks by a detachment fault. Ar cooling ages (Harlan et al., 2003) and new zircon FT ages for these rocks are concordant at ~60 Ma, synchronous with deposition of the Arkose Ridge Formation. A cooling age of ~46 Ma came from one apatite FT sample. The metamorphic protolith (previously considered Jurassic) was deposited at or after 75 Ma based on new DZ data. The probability curve has a major peak from 76 to 102 Ma, minor peaks at 186, 197, 213, 303, 346, 1828 and two discordant grains ~2700 Ma. This is similar to DZ populations in the Valdez Group. The short period of time between deposition, metamorphism, and exhumation are consistent with metamorphism in a subduction zone setting. Ductile and brittle structures in the metamorphic rocks are consistent with exhumation in a transtensional setting.