Introduction to the Brooks Range
Cold, Dry, Barren, and Often Dark
The Brooks Range is many things: the tail end of the extensive North American Cordillera; entirely north of the Arctic Circle; crossed by a single gravel road; the site of one of the largest zinc mines in the world; virtually without trees or any vegetation taller than a few centimeters; and relatively dry. Visitors might expect a frozen, ice-covered wasteland; in fact, the Brooks Range today is home to only a few small glaciers. During the last glacial maximum, large valley glaciers existed and did much work carving the landscape; however, the Brooks Range was too arid to become a center of glaciation and too high to be overrun by giant continental glaciers creeping into Alaska from Canadian centers. Unlike southeastern Alaska, the area remains too arid to support a large population of glaciers.
Today, the Brooks Range is dramatic for its steep terrain, relatively high elevations, isolation, and barrenness. Most of the east-west trending Range lies within Alaska, the state infamous (amongst geologists) for being made up of myriad tectonic terranes. In contrast, the Brooks Range has a relatively simple architecture, compared with some of the geology of British Columbia and southeastern Alaska. At least, we think so. Much work remains to be done in the Brooks Range. Paradoxically, just to the north of the Range, under the North Slope, lies the largest oil field in North America. The North Slope has been subjected to intense exploration via geophysical and other direct and indirect methods, making the subsurface geology here some of the best-explored in the world. However, the Brooks Range itself lacks significant oil reserves and therefore geological exploration has been somewhat limited. The lack of roads (except the Dalton Highway which transects the Brooks Range north-south) limits seismic surveys and access for geologic mapping. The short summer season also limits the amount of ground covered by mapping campaigns. However, the beauty of the jagged, treeless range undoubtedly draws many workers (and a few tourists) to the region. A timeline of Alaska and Brooks Range geological exploration is presented here if you are curious.
Much of what we know about the Brooks Range has come from the work of geologists. The Range is something of a geological playland due to the excellent exposure and classic structure, and in spite of the short field seasons, extra-large mosquitoes, and difficulty of access. Great sections of Paleozoic passive-margin sediments are exposed as entire mountainsides. Sitting on a hillside, enjoying lunch, a geologist can gaze across to mountainsides opposite and pick out thrust faults as they branch and rejoin, ramp and flat. Much of the Range is made up of unmetamorphosed sedimentary rocks, allowing paleontologists to pick out fossils and constrain the ages of rock units. This is key to detailed structural mapping, since displacement due to thrust faulting often means rock beds are not laterally continuous over more than a few kilometers.
The deep architecture of the Brooks Range long remained a mystery, until ambitious seismic surveying in 1990 by the USGS and Rice University. The survey results, combined with geological data, allow some interesting interpretations of thearchitecture at the heart of the Range. Still, this survey line makes only one pass across the middle of the Range, along the Dalton Highway. Nowhere else can trucks carrying the necessary equipment make a traverse of the Range. Other types of geophysical surveys have been carried out, but they are either very local (often used for targeted mineral exploration) or very broad and low-resolution (e.g., aeromagnetic surveys). In this respect, the Brooks Range suffers from being a "merely average" mountain range. It is not the biggest (e.g., Himalaya) or oldest or most unique geologically. Still, every mountain range holds its secrets, and the Brooks Range is no exception: a mysterious origin of the basement rocks (currently debated); huge rotations of the entire range; anomalous flanking double basins; giant ophiolite thrust sheets; metamorphic rocks brought up from great depth; and a surprising lack of volcanism. The Range continues to attract geoscientists to work on unique problems related to orogenesis. Welcome to the Brooks Range!
The Brooks Range during winter
A word on Nomenclature
One of the greatest difficulties in understanding geology is grasping all the names! This problem is compounded when different authors (or the same author!) use various names for the same rocks, or the same terranes, or the same plates. To lessen this problem, this site will mostly stick to the terrane names used by Jones et al., 1987 / Moore et al., 1994. If you are familiar with another system of names for "subterranes" of the Brooks Range, refer to the table below to orient yourself.