Although the Brooks Range is covered by large-scale, low-resolution gravity and magnetic maps, the most valuable geophysical information from the area derives from regional seismic surveys. The most recent (and most ambitious) seismic survey, dubbed the Trans-Alaska Crustal Transect (TACT), extends from the Gulf of Alaska to the North Slope. Across the Brooks Range, the TACT follows the course of the Dalton Highway (because vibroseis helicopters have not yet been brought into common use!). During May through August 1990, a 700 channel seismograph system was deployed repeatedly along a 315 km profile (see figure at left). The system simultaneously measured both reflection and refraction data.

The sequence of thrust sheets in the Brooks Range is well-defined by the seismic results (see figure, below). The resulting seismic sections show a strongly reflective crust (0-50 km depth) in the northern two-thirds of the Brooks Range, with zones of reflections at 2-3 s and 4-6 s. The upper zone is thought to be the thrust fault zone at the base of the Endicott Mountain Subterrane, while the lower zone is thought to be the basal decollement which all the Brooks Range thrusts root into. In the central part of the Range, the seismic results show an 80-km-long series of imbricated, south-dipping thrust sheets. Further south, the upper crust becomes nonreflective as the transect crosses into an area underlain by metamorphic rocks. This observation provides evidence that metamorphic rocks exposed at the surface extend to depth. A deeper reflection zone at 5-6 s is interpreted as an older decollement separating Precambrian basement from overlying metamorphosed Paleozoic rocks.

Shortening estimates derived from balanced cross sections constrained by the seismic data range from 400-600 km since Jurassic time. Crustal thickness as determined by the TACT survey varies from 35 km in the hinterland to ~50 km beneath the core of the Range. In this sense, the Brooks Range is a typical orogen; the thickened crustal welt provides buoyancy which maintains the elevation of the Range.

The figure to the left (from Wissinger et al., 1998) shows one of three plausible reconstructions based on seismic data, kinematics and geology.

The top part of the figure shows seismic reflectors and an interpretation of the current subsurface structure of the Brooks Range. Note that reflections die out at the base of the crust, the depth of which varies from ~35 km to nearly 60 km under Atigun Pass in the central Brooks Range. South of the surface expression of the Minnie Creek Thrust, crustal reflectors are few, indicating metamorphic rocks. A few tens of kilometers north, the strong reflections of relatively unmetamorphosed sedimentary rocks show up clearly.

The lower part of the figure is a balanced cross section (A-A') and the restored rocks. The restoration sections (six in all) are drawn separately because the rocks of the Brooks Range originally spanned over 1000 km. The rocks did not originally overlie one another; this is a product of crustal shortening during orogeny. The ratio of cross section to restored sections gives some idea of the huge amount of shortening which must have been accommodated by the upper crust.

The top figure is from Levander et al., 1994; the bottom figure is from Wissinger et al., 1998.