UPDATE ON THE DOONERAK FENSTER PROBLEM

Gil Mull (1987) described the Doonerak fenster as “the most significant locality for understanding the regional structure and tectonic history of the Central Brooks Range”. Doonerak is remarkable in that it exposes distinctive Lower Mississippian rocks of the Ellesmerian sequence unconformably above Early Paleozoic arc volcanic rocks in a relationship similar to that found in the subsurface of the North Slope and the NE Brooks Range. Furthermore, this stratigraphic succession is different from that directly north of Doonerak in the Endicott Mountains allochthon (EMA), where there is a great thickness of upper Devonian clastic rocks (Endicott Group) below the Ellesmerian units. Gil and others proposed a structural model where the core of Doonerak is part of the Brooks Range “autochthon” and the entire EMA was displaced northward on the Amawk thrust over Doonerak. More recent apatite fission track work revealed that episodic denudation of the Doonerak antiform occurred after 60 Ma, suggesting that it experience late-stage thrusting and uplift. This is confirmed by new U-Th/He thermochronology. Paleothermometry using Raman spectroscopy of carbonaceous matter shows an abrupt drop in maximum paleotemperature at the north flank of the antiform relative to the core and the south flank. Some more recent observations call the classic structural model into question: 1) the TACT deep crustal seismic line shows imbricated south-dipping reflectors down to 25 km suggesting that Doonerak is not an autochthon; 2) outcrop-scale kinematic indicators on the Ellesmerian strata directly below the Amawk thrust west of Bombardment Creek show clear top-to-the-south sense of shear (opposite to that predicted by the classic model); and 3) detrital zircon ages from slates and phyllites south of Doonerak, which had been correlated to the Hunt Fork shale of the Endicott Group, are more akin to the Trembley Creek Phyllite of the Hammond Terrane and other units of the southern flank of the Brooks Range. This shows that the Endicott Allochthon is not rooted on the southern side of Doonerak as some structural models show. Alternative models where the Endicott Group is an inverted Devonian basin, as originally suggested by Kelly and Brosges (1996), and the Amawk structure is a backthrust, as proposed by Dutro et al. (1976), deserve another look.