North-vergent thrust fault between Baltica and Laurentian affinity rocks in the frontal part of Romanzof orogen, NE Brooks Range, Alaska

One of the most striking features of Arctic physiography is the long, linear Canadian Arctic margin of the Arctic Basin, which extends from the Lincoln Sea north of Greenland to the eastern Beaufort Sea and projects into northeastern Alaska. Among other ideas, this margin has been proposed to have developed by sinistral transform faulting in the Middle Devonian as a result of tectonic escape of terranes from the Caledonites (the so-called "Northwest Passage"). The faults on which the transform motion might have occurred, however, have not been recognized along the northern margin of North America. One candidate for such a fault is exposed at the southern boundary of the Sadlerochit Mountains province in the Tertiary frontal part of the NE Brooks Range. In the Plunge Creek area of the Arctic National Wildlife Refuge, a contact between rocks of Laurentian affinity and Baltica affinity is exposed on the back limb of a basement-involved map-scale thrust block formed by Brookian thrusting. The regional unconformity at the base of the Mississippian to Triassic Ellesmerian Sequence provides a near flat-lying datum that overlaps the contact between the pre-Mississippian tectonic units and demonstrates that it was not reactivated by Brookian thrusting. The Sadlerochit Mountains succession to the north of the contact consists of a Neoproterozoic and lower Paleozoic carbonate sequence that rests on metaclastic rocks that yield Grenville-Sveconorwegian (0.95-1.2 Ga) and other Mesoproterozoic detrital zircon U-Pb age populations similar to those reported from the northern parts of Baltica and eastern Greenland. In contrast, the Romanzof Mountains succession to the south consists of Neoproterozoic deep-marine clastic rocks (Neruokpuk Quartzite) and overlying lower Paleozoic chert and argillite. Detrital zircon U-Pb age populations from the Neruokpuk are very similar to those from Laurentian-derived clastic rocks in the Canadian margin of North America. Field relations show that the Neroukpuk Quartzite lies in thrust contact on Devonian lithic clastic rocks. The Devonian rocks are calcareous and, in addition to Caledonian (c. 420 Ma) populations, yield detrital zircon age populations similar to those in the Sadlerochit Mountains metaclastic rocks indicating its provenance is mainly in the Sadlerochit Mountains sequence. The Devonian unit also contains lenses of black chert, interpreted to be olistoliths, that can only be derived from the Romanzof Mountains succession. Folds in the metaclastic rocks display north vergence with little plunge and moderate south-dipping semipenetrative axial planar foliation. Blocks of older rocks of Baltica affinity occur locally below the thrust, thereby indicating the presence of a footwall syncline with rocks of the Sadlerochit Mountains succession forming the foot wall beneath the Devonian unit. These relations indicate that the contact at the south margin of the Sadlerochit Mountains succession is a north-directed thrust fault rather than a sinistral transform fault. Although this fault does not confirm the existence of a Northwest Passage event, it provides the first strong evidence that the Romanzof orogen is north-vergent thrust belt-foreland basin system that placed the Romanzof Mountains succession onto the Sadlerochit Mountains succession early in Devonian time.