This study was done under the Research Experience for Undergraduates program at the Geophysical Institute. We investigated the thermal history of the Okpilak batholith, northeastern Brooks Range using the technique of 40Ar/39Ar thermochronology of potassium feldspars. The Okpilak batholith of the northeastern Brooks Range presents the unusual case of intimate involvement of crystalline rocks in thrusting near the foreland basin of a fold-and-thrust belt. Various dating techniques have provided an outline of the batholith's uncommon history; U-Pb zircon dating has yielded an initial crystallization age of 380 +/- 10 Ma, K-Ar and 40Ar/39Ar dating of recrystallized biotite indicates metamorphism and cooling at ~59, and apatite fission track analyses have produced ages of 31 - 43 Ma for subsequent structural uplift and unroofing. In this study, we have employed the multidomain theory of argon retention in K-feldspar to provide additional information regarding the history of the batholith.
Samples from four locations in the batholith yielded age spectra indicative of the multidomain behavior of feldspar. This has allowed us to model the cooling history of the batholith spanning a temperature range from ~450deg.C to 220deg.C and a time from ~130 Ma to ~30 Ma. When coupled with apatite and biotite age data, a consistent history emerges. The batholith underwent slow cooling (or uplift) from about 130 Ma to about 45 Ma. At this time the pluton underwent rapid uplift and cooling from a temperature of about 250deg.C to less than 100 deg.C in about 10 Ma. Thus the rocks were uplifted from a depth of > 10 km (assuming a 25deg.C/km geothermal gradient) in a relatively short period of time. These new data rule out a model of gradual uplift from > 10 km depth between 59 and 43 Ma as had been proposed.
Funded by NSF Grant.
Scientific personnel:
Paul Layer
and Julie Paegle
(University of Utah summer intern)