THE BROOKS RANGE, ALASKA, HAS A COMPLICATED HISTORY AS AN OROGEN: TRIBUTE TO THE CONTRIBUTIONS OF GIL MULL (Invited Presentation)

I met Gil in 1987 on a long working field trip across the Brooks Range organized by ARCO geologists, long after he had acquired his vast expertise in North Slope geology and his encyclopedic knowledge of Brookian stratigraphy, structures and their fossil age constraints. What he taught us on that field trip represented the starting point for our attempts to untangle the difficult, hard-to-date structural relations in the southern metamorphic core of the range. Gil led us through the allochthons of the western Brooks Range and showed us evidence for Brookian thrust faulting into earliest Cretaceous, the age of the syn-tectonic deep marine Okpik Fm. He explained that stratigraphic relations along the northern side of the thrust belt constrained thrusting to have ended by Fortress Mt. time in the early Albian. He was puzzled by the nature of younger events and called upon several km’s of Aptian-Albian vertical uplift to deliver huge volumes of clastics to the North Slope at this time. He was with us in the Cosmos Hills as we chased the fault contacts between the mafic and ultramafic rocks of the highest Brooks Range allochthons and the deepest levels of the orogen in the Schist belt. The “root zone” for allochthons in the northern Brooks Range, an idea elegantly illustrated in Roeder and Mull (1978), became post-thrusting, down-to-the south normal faults, helping explain the marine clastics deposited in basins south of the Brooks Range. We now know that the coeval North Slope clastic sequences came from Russia (Moore et al., 2015; Lease et al., 2022) and that blueschist minerals in their heavy mineral suites (Till, 1992) are now mysterious in origin, and hard to link to Brookian events. We are free to mostly uplift the Brooks Range in the Cenozoic, compatible with low-T thermochron data sets. We don’t yet understand many aspects of the Brooks Range orogen, such as why the basal thrust of the Brooks Range fold-thrust belt dips consistently north instead of south, revealing the vast “metamorphic core” of the range. We are just beginning to understand the age and origin of rock units in the core of the Brooks Range but know little about their structural relations and the role they play in Brookian orogenesis. In addition, there is still little agreement as to how much deformation in the metamorphic core is thrust vs. extension related.