Nucl. Tracks Radiat. Meas., Vol. 17,No. 3, pp. 367-371, 1990 Int. J. Radiat. AppL Instrum., Part D Printedin Great Britain 0735-245X/90 $3.00 + .00 © 1990 Pergamon Press plc APATITE FISSION TRACK EVIDENCE FOR PALEOCENE AND OLIGOCENE UPLIFT EVENTS IN THE NORTHEASTERN BROOKS RANGE, ALASKA PAULB. O'SULLIVAN*~" and JOH~ DECK~R:~ *Department of Geology, University of Alaska, Fairbanks, AK 99775, U.S.A.; ?Department of Geology, LaTrobe University, Bundoora, Victoria 3083, Australia; and :~ARCO Alaska, Inc., Box 100360, Anchorage, AK 99510, U.S.A. (Received 5 September 1988; in revised form 21 August 1989) Abstract--Apatite fissiontrack analysis data from outcrop samples of elastic sedimentary rocks collected from two major areas in the northeastern Brooks Range (NEBR) in the Arctic National Wildlife Refuge of Alaska (ANWR) constrain the timing of uplifts within an advancing deformation front. Eighteen samples from a Permian to Early Cretaceous section at Bathtub Ridge have a range in apparent apatite fission track ages from 49 + 10 to 74 + 15 Ma (2a errors) with a mean value of 62 Ma. Within two sigma errors, all 18 samplesdefine an apparent age of 58-60 Ma. Track lengths are long (mean length 14-15/~m) with tight distributions (standard deviation < 1.50/zm). These results indicate the section has undergone rapid uplift of several km from temperatures above ~ 125°C at about 62 Ma, and then more protracted cooling from ~ 50°C to the surface. This 62 Ma event is also recorded by re-set biotite K-At and zircon U-Pb ages (59--60 Ma) from the Devonian Okpilak Batholith 40 km to the north. Seven samples from the Arctic Creek area located 75 km to the northwest all possess apparent apatite fission track ages ranging from 31 + 7 to 40 + 15 Ma (2~ errors) with a mean value 37 Ma. Mean track lengths are all long (> 14/~m) with standard deviations between 1.3 and 2.1 #m. These results indicate that previously existingfission tracks were annealed prior to rapid uplift of several kilometers and erosion of the section subsequent to 37 Ma. These apatite fission track data show conclusively that the events responsible for building the NEBR decrease in age from south to north. When deformation was ending throughout most of the Brooks Range during Earliest Tertiary, Bathtub Ridge was being rapidly uplifted. During the Oligocene, the deformation front had progressed northward resulting in the rapid uplift of the Arctic Creek exposures. INTRODUCTION GEOLOGICAL SE'ITING THE COASTAL plain of the Arctic National Wildlife Refuge (ANWR) in northeastern Alaska (Fig. l) has recently received a great deal of attention because of regional petroleum prospects. Sedimentary rocks likely to occur beneath the ANWR coastal plain are exposed along the mountain front of the northeastern Brooks Range (NEBR). As part of an ongoing study of the geological development of the NEBR by the Alaskan Division of Geological Surveys and the University of Alaska-Fairbanks, an apatite fission track analysis study was conducted on outcrop samples. The purpose of these analyses is to expand the present understanding of the thermal evolution, and the uplift and burial history of sediments exposed in the NEBR, all of which bear heavily on the petroleum potential beneath the ANWR coastal plain. Results from 25 analyses of samples collected from two important areas, Bathtub Ridge and Arctic Creek (Fig. l) suggest that the development of the mountain front present in the NEBR by imbrication of thrust sheets was advancing northward from Paleocene to Oligocene times. The coastal plain or North Slope of Alaska is a combined passive continental margin-foreland basin bounded on the north by the Arctic Ocean and on the south by the Brooks Range, a Late Jurassic to Tertiary fold and thrust belt. The stratigraphy of the Brooks Range and the North Slope may be divided into three unconformity-bounded strati- graphic sequences (Lerand, 1973; Mull, 1982; Bird and Molenaar, 1987). The Proterozoic to Mississippian Franklinian sequence documents a complex and poorly understood history culminating in a Devonian orogenic event. The Mississippian to Early Cretaceous Eliesmerian sequence was deposited in a south facing passive margin. The Early Creta- ceous and younger Brookian sequence consists of south-derived elastic deposits eroded from the Brooks Range orogen. The stratigraphy of these sequences has been discussed in detail by many authors (e.g. Brosge and Tailleur, 1970; Detterman et al., 1975; Palmer et al., 1979). Major tectonic elements of the NEBR are domi- nated by the Brooks Range fold-thrust belt which 367