SLAB DETACHMENT—AN EXPLANATION FOR THE MID-CRETACEOUS EVOLUTION OF THE BROOKIAN OROGENY, NORTHERN ALASKA

WARTES, Marwan A., Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709, [email protected]

The Brooks Range Orogen records the Late Jurassic to Early Cretaceous collision of the Ellesmerian passive margin with an island arc to the south. This collision resulted in partial underthrusting and metamorphism of the leading edge of the continental Arctic Alaska Terrane and large scale north-vergent imbrication of the distal Ellesmerian margin. Following this major episode of contraction and crustal thickening, the Brooks Range underwent significant exhumation and erosion at ~ 100 Ma. This rapid Albian uplift event is well documented by a plethora of regionally consistent cooling ages throughout the Brooks Range. Available constraints suggest this phase of exhumation was short-lived and accommodated principally by footwall uplift associated with down to the south extension in the southern Brooks Range.

Although late orogenic exhumation is common in many collisional orogens, the underlying cause remains poorly understood. I would like to explore a tectonic interpretation, briefly suggested by previous workers (e.g. Vogl et al., 2002, Tectonics), that mid-Cretaceous unroofing and rapid denudation of the Brooks Range reflects detachment of the subducting slab. The removal of this dense load and its associated "slab-pull" forces would affect a major change in regional stress patterns. For example, the buoyant continental lithosphere, partially subducted during collision, will isostatically rise, driving the exhumation of high pressure metamorphic rocks and accelerated surface uplift.

Slab detachment is known to occur along several modern collisional plate boundaries and is often temporally and spatially linked with potassic magmatism in the hinterland. Although magmatism south of the Brooks Range spans the Cretaceous, a unique belt of potassic to ultrapotassic igneous rocks in the Koyukuk Terrane coincide with the proposed timing for slab detachment. Furthermore, the isotopic composition of these rocks suggests a contribution from sub-continental lithosphere, consistent with partial melting and upwelling of mantle material exposed via tearing of the downgoing slab. If the hypothesis presented here is supported by new data, the Brooks Range would mark one of the few documented pre-Cenozoic examples of a process which may be an inevitable consequence of attempted continental subduction.