Mechanisms and timing of exhumation of collision‐related metamorphic rocks, southern Brooks Range, Alaska: Insights from 40Ar/39Ar thermochronology

Abstract

New 40Ar/39Ar thermochronologic data document the spatial distribution of cooling ages and rates across a metamorphic culmination in the south central Brooks Range, providing important constraints on the timing and processes responsible for exhumation of deep‐seated metamorphic rocks. The data indicate widespread episodic exhumation‐related cooling, with short‐lived events in the mid‐Albian to early Cenomanian and Paleocene to Eocene. Hornblende ages indicate that cooling from the metamorphic peak began at 105–103 Ma across the epidote‐amphibolite facies core of the culmination. Mica ages, however, increase southward from ∼90 Ma to ∼100 Ma, indicating that average cooling rates increased southward. Thermal modeling suggests that Albian cooling is the result of a short‐lived exhumation event that ended by the early Cenomanian, and that exhumation rates were between ∼1 and 8 mm/yr, with higher rates toward the south. The southward increase in cooling/exhumation rates is attributed to a southward increase in the component of tectonic exhumation by normal faulting on the south flank of the range. Rapid erosion, which is recorded by thick molasse deposits in flanking basins that are coeval with rapid cooling, may have been aided by surface uplift in the footwall of these faults. The northward younging of mica ages may not be explained by a simple core complex model involving metamorphic rocks being progressively dragged out from beneath an extensional fault. The overall spatial distribution of metamorphic zones and cooling ages/rates is best explained by a two‐step process: (1) a brief episode of extensional faulting producing a regional northward tilt with superimposed doming during the Albian‐Cenomanian and (2) relative uplift of the core during renewed Tertiary contraction, which enhanced the domal geometry. Diffusion‐domain modeling of K‐feldspar data indicates a period of little or no cooling that spanned much of the Late Cretaceous followed by rapid increase in cooling rates in the Paleocene to Eocene. Increased Tertiary cooling rates are attributed to renewed contraction leading to surface uplift and erosion.