EPISODIC, MULTI-MECHANISTIC EXHUMATION OF THE BROOKS RANGE OROGEN AS REVEALED BY THERMOCHRONOLOGY

In this contribution we integrate 40Ar/39Ar and fission-track thermochronology with other geologic observations to delineate the timing, rates, and processes of exhumation of the Brooks Range (northern Alaska) orogen. The Brooks Range orogen formed as a result of collision between an island arc and a passive margin. The orogen comprises a metamorphic core (up to amphibolite facies), a N-directed fold-thrust belt, and a foreland basin to the north. The south side of the orogen is bound by extensional faults and a hinterland sedimentary basin. Our interpreted orogenic history includes Early Cretaceous N-directed thrusting, Albian S-directed backfolding, Albian-Cenomanian extension, renewed Tertiary N-directed thrusting.

Perhaps the most significant exhumation event occurred in the Albian–Cenomanian as revealed by 40Ar/39Ar Hbl and mica cooling ages. The 40Ar/39Ar ages indicate an increase in cooling/exhumation rates to the south suggesting that the range-bounding extensional faults played a role in the exhumation. Significant volumes of Albian-Cenomanian clastics were deposited in basins to the north and south indicating that major surface uplift accompanied extension. The data suggest that extension immediately followed contraction and was short-lived, lasting only a few million years. Furthermore, the combination of extensional and erosional unroofing during this event removed nearly half of the overlying material in the southern part of the range. Fission-track data suggest much less cooling during this event in the central part of the range and very little cooling at the range front.

Diffusion-domain modeling of K-feldspar from the deepest levels of the orogen and existing fission-track data from shallower levels both suggest limited cooling/exhumation during the Late Cretaceous and renewed rapid cooling during the Paleocene-Eocene. In contrast to the Albian-Cenomanian event, the amounts of exhumation in different parts of the orogen are broadly similar. Furthermore, the Paleocene-Eocene event is recorded is shortening-related structures in the foreland-basin. These features reflect the rebuilding of the orogen during renewed shortening.

Overall, the data indicate that the Brooks Range serves as an example of a collisional orogen that was exhumed episodically by multiple processes.