TIMING OF MESOZOIC EXHUMATION OF THE METAMORPHIC CORE OF THE SOUTHERN BROOKS RANGE: NEW CONSTRAINTS FROM LOW TEMPERATURE THERMOCHRONOLOGY

The tectonic process responsible for the Mesozoic exhumation of the metamorphic core of the Brooks Range (BR) remains unresolved and has been a point of controversy for over three decades. Previous workers have proposed that the uplift of blueschist to greenschist facies rocks is related to Jura-Cretaceous Brookian contraction and thrusting (Till and Snee, 1995; Till, 2016), while others have countered that exhumation mainly occurred during a post-contractional mid-Cretaceous thermal and extensional event (Miller and Hudson, 1991). To evaluate these two endmember tectonic models, this study integrates new zircon (U-Th)/He data with existing data from prior studies (Bigot-Buschendorf, 2015) to delineate thermal histories from distinct structural domains across the orogen. Northerly samples derived from unmetamorphosed to low-greenschist facies rocks of the Endicott Mountains Allochthon, Doonerak Fenster, and the northern Central Belt yield Paleocene and Eocene-Oligocene cooling ages, which are related to renewed contractional deformation in the Cenozoic (O’Sullivan et al., 1997a, 1997b). In comparison, southerly samples from middle-upper greenschist facies rocks from the normal fault-bound Schist and Phyllite belts are characterized by mid-late Cretaceous cooling ages (~105-75 Ma). Shared thermal histories across the Schist Belt-Phyllite Belt contact, which is a normal fault that omits structural and metamorphic section, indicate that cooling occurred syn to post-slip on normal faults. The youngest metamorphic fabric (S_d) also cuts across the normal fault system. New (U-Th)/He data provide a minimum age for both normal faulting and the youngest fabric (S_d). The timing of the observed mid-late Cretaceous cooling event is contemporaneous with exhumation and greenschist-amphibolite facies metamorphism documented in the Arrigetch-Igikpak thermal high of the central BR (Toro et al., 2002; Vogl et al., 2002), regional magmatism that is syn-extensional (Akinin et al., 2020), formation of the Yukon-Koyukuk successor basin (Patton et al., 1994, 2009; O’Brien et al., 2018), and opening of the Amerasia Basin (Miller et al., 2017). Collectively, results support models for the Mesozoic exhumation of the metamorphic core of the BR driven by heating and extensional doming of mid-lower crustal rocks.