Abstract The east-central Alaska region extends west from the Canadian border to the southeast edge of the Koyukuk basin, and north from the Yukon-Tanana upland to the southeast flank of the Brooks Range (Fig. 1). The region encompasses all or parts of 13 1:250,000 quadrangles and covers an area of about 140,000 km 2 . The physiography of the region (Wahrhaftig, this volume) is extremely diverse (Fig. 1). Steep, mountainous areas having high to moderate relief include parts of the Ogilvie Mountains, White Mountains, Yukon-Tanana upland, Ray Mountains, and southeastern Brooks Range. Areas of dissected plateaus and rolling hills with moderate to low relief include the Porcupine plateau and the Kokrines-Hodzana upland. Lowlands characterize the Yukon Flats basin and extend upstream into other parts of the Yukon River and Porcupine River drainage basins. Bedrock exposures are severely limited in most of the east-central Alaska region. Mountainous areas are locally glaciated, but except in the most rugged parts of the Ogilvie Mountains and parts of the Brooks Range, bedrock is extensively mantled by surficial cover and the colluvial products of alpine weathering processes. Stream cuts commonly offer the best exposures in generally forested or tundra-covered areas of moderate relief. Lowland areas have few pre-Cenozoic bedrock exposures because of extensive surficial cover and tundra. The pioneering reconnaissance studies by J. B. Mertie and his predecessors were carried out in the east-central Alaska region prior to 1940. A relatively small group of geologists conducted the basic geologic mapping during the 1960s and 1970s that defined the

ABSTRACT Structural analysis of Neoproterozoic to lower Paleozoic rocks near Old Crow, in North Yukon, show that they were affected by widespread, but distributed sinistral shear zone deformation. This tectonic event occurred under brittle-ductile conditions, in the early Paleozoic, prior to intrusion of Late Devonian granitoids of the Old Crow plutonic suite (368–375 Ma). Although outcrops are scattered, the shear zone deformation can be inferred to extend over a broad ~W–E corridor, ~10–20 km-wide and ~145 km long, from eastern Alaska into northern Yukon. The sinistral Porcupine shear zone is interpreted to represent a major, early Paleozoic crustal structure along which elements of NE Laurentian and Caledonian affinities in the Arctic Alaska terrane were transferred across the Arctic region during the Paleozoic. Our observations do not support major Paleogene dextral strike-slip deformation along the Porcupine River near Old Crow.

Abstract Ordovician rocks, found in northern, east-central, interior and southern Alaska, formed in a variety of depositional and palaeogeographic settings. Shallow- and deep-water strata deposited along the northwestern Laurentian margin occur in east-central Alaska (Yukon River area) and probably correlative rocks crop out to the north in the Porcupine River area. Ordovician strata elsewhere in Alaska are parts of continental or island arc fragments that, as indicated by faunal and detrital zircon data, have been variously displaced. In northern Alaska, Ordovician rocks are included in the Arctic Alaska–Chukotka Microplate (AACM), a composite tectonic entity with a complex history. Some Ordovician strata in the AACM (parts of the North Slope subterrane) represent displaced fragments of the northern Laurentian margin. Coeval strata in southwestern parts of the AACM (York and Seward terranes, Hammond subterrane) share distinctive lithologic and biotic features with Ordovician rocks in interior Alaska (Farewell and related terranes). Ordovician strata in southeastern Alaska (Alexander terrane) also likely compose a composite crustal fragment that accumulated in a complex arc system. Shared features between many of these units suggest similar origins as part of one or more crustal fragments situated in the palaeo-Arctic between Laurentia, Baltica and Siberia during early Paleozoic time.