Deformational history of the central Brooks Range, Alaska: Results from fission‐track and ⁴⁰Ar/³⁹Ar analyses

Abstract

The Brooks Range is the northernmost orogenic belt in Alaska. From south to north it consists of a thin belt of oceanic basalt and chert, followed by two belts of high‐pressure/low‐temperature metamorphic rocks (the schist and central belts), a basement‐cored anticlinorium (the Doonerak Window), a fold and thrust belt, and a foreland basin (the Colville Basin). We have used apatite and zircon fission‐track (FT) and 40Ar/39Ar white mica analyses of a N‐S transect of the central Brooks Range to study the cooling history. These data are used with a balanced cross section to constrain the timing of deformational events in northern Alaska. The oldest cooling ages from this study were clusters of ∼185 Ma and ∼135 Ma zircon FT ages from the fold and thrust belt. Based on the similarity of the 185 Ma ages with the age of crystallization of the western Brooks Range ophiolites [Wirth et al., 1993], we suggest that the Brooks Range orogeny in northern Alaska had begun prior to this time. Cooling rates in the fold and thrust belt were very slow from 185 to 135 Ma, suggesting little or no deformation was occurring. Apatite and zircon FT analyses from the fold and thrust belt indicate that a substantial amount of cooling (caused by uplift and erosion) occurred from 135 to 95 Ma. We suggest that an increase in the rate of cooling at ∼135 Ma was caused by contractional deformation associated with renewed subduction with the opening of the Canada Basin. The 40Ar/39Ar white mica cooling ages of 130 to 120 Ma from the schist belt rocks of the southern Brooks Range indicate that peak metamorphism must have occurred prior to 130 Ma. An 40Ar/39Ar muscovite age of 113 Ma from the southernmost Brooks Range is interpreted to be the age of movement on normal faults juxtaposing lower‐grade phyllite belt rocks against schist belt rocks. At the same time, rapid deposition of Brooks Range‐derived sedimentary sequences began to the north, in the Colville foreland basin, and to the south, in the Yukon‐Koyukuk basin. Unstrained high‐level granitic plutons, which intruded the Yukon‐Koyukuk basin and Ruby terrane from 107 to 95 Ma, indicate the end of collision with the Yukon‐Koyukuk arc. By 95 Ma, cooling rates in the Brooks Range had slowed and little or no deformation was occurring. At 60 Ma, a major episode of rapid cooling occurred throughout northern Alaska, when the active plate margin in Alaska was more than 500 km to the south. In the Colville Basin, cooling was caused by thrust faulting and folding, and hence erosion, within the Albian‐age and younger sedimentary rocks. Within the interior of the Brooks Range, a large‐scale basement‐involved anticlinorium (the Doonerak Window) became active at this time. At 25 Ma, another episode of rapid cooling occurred within the Doonerak Window region. Both of these events may be related to shallow subduction of the Kula plate from the southern Alaskan plate margin.

Seismic profiling constraints on the evolution of the central Brooks Range, Arctic Alaska

“…9. Duplexing of Doonerak rocks (previously autochthonous) occurred beneath the EMA (60 and 25 Ma; O'Sullivan et al, 1993;Blythe et al, 1996;O'Sullivan et al, this volume, Chapter 11).…”

“…The Skajit allochthon contains units that are age equivalent to the Schist belt (Palmer et al, 1984), but the nature of the subsurface contact between the two is poorly known. As indicated from published cross sections, most researchers interpret uplift of the Schist belt along the southern margin of the Skajit allochthon in the central part of the range and overthrusting of at least portions of the Skajit allochthon at depth (Oldow et al, 1987;Gottschalk, 1990;Grantz et al, 1991;Till, 1992;Moore et al, 1994;Blythe et al, 1996). Similarly, we interpret the Skajit allochthon to overlie rocks of the Schist belt and be overthrust by rocks of the Schist belt in the hanging wall of the Minnie Creek thrust.…”

“…3. Uplift timing estimates from apatite fission-track dating have determined that the rocks of the Doonerak antiform have undergone episodes of cooling/uplift at ~60 Ma and 25 Ma (Blythe et al, 1996). An additional episode of uplift has been recognized in the Colville basin and northeastern Brooks Range at 45 Ma (O'Sullivan et al, 1993).…”

Origin of the Lisburne Hills–Herald Arch structural belt: Stratigraphic, structural, and fission-track evidence from the Cape Lisburne area, northwestern Alaska

“…Comparison of such data from the two belts would reveal similarities or differences in geologic history, style of deformation, magnitude of shortening, and time of shortening. Although there has been much recent research on these topics in the Brooks Range (e.g., Blythe et al, 1996;Christiansen and Snee, 1994;Cole et al, 1997;Fuis et al, 1997;Little et al, 1994;Moore et al, 1997;Mull et al, 1987aMull et al, , 1987bMull et al, , 1997Oldow and Avé Lallemant, 1998;O'Sullivan, 1996;O'Sullivan et al, 1993O'Sullivan et al, , 1997Till and Snee, 1995;Wallace, 1993;Wallace and Hanks, 1990;Wallace et al, 1997), the Lisburne Peninsula has received scant attention. Geologic information on the Lisburne Peninsula is limited to the framework studies of Collier (1906) and Martin (1970), work by Campbell (1961Campbell ( , 1967 in the southern Lisburne Peninsula, a few stratigraphic studies that resulted from regional coal-or petroleum-related investigations (Armstrong et al, 1971;Blome et al, 1988;Moore et al, 1984;Murchey et al, 1988;Tailleur, 1965), and very limited unpublished work by oil companies.…”

Producing Light Oil from a Frozen Reservoir: Reservoir and Fluid Characterization of Umiat Field, National Petroleum Reserve, Alaska

“…1: Nunn et al, 1987;Coakley and Watts, 1991;Moore et al, 1994). The earliest sedimentary evidence for initiation of the Brookian orogeny is Hauterivian in age (Bird and Molenaar, 1992), but the Albian-Cenomanian (112-93.5 Ma; Walker and Geissman, 2009) Torok and Nanushuk formations were deposited during a period of extension and exhumation on the south side of the Brooks Range that started ~113 Ma (Blythe et al, 1996;O'Sullivan et al, 1997). Cooling and exhumation slowed by ~95 Ma (O'Sullivan et al, 1997) and by the late Albian, the high volume of sediment represented by the Torok and Nanushuk formations filled much of the Colville Basin and overtopped the Barrow Arch (Molenaar, 1985;LePain et al, 2009).…”