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.

ABSTRACT The passive margins of Laurentia that formed during Neoproterozoic–Cambrian breakup of the supercontinent Rodinia record subsequent histories of contraction and translation. This contribution focuses on the northern margin of Laurentia, where recent geologic and geochronologic data have provided new insight into the evolution of northern North America. The Laurentian margin in East and North-East Greenland records synorogenic sedimentation and deformation associated with the Caledonian orogeny—the Silurian to Devonian continent-continent collision between Baltica and Laurentia that followed closure of the northern tract of the Iapetus Ocean. The timing of ultrahigh-pressure metamorphism and simultaneous sinistral and dextral strike-slip faulting in North-East Greenland indicates that the Himalayan-style orogen persisted through the Devonian. In contrast, the Franklinian margin further west records sinistral strike-slip translation of allochthonous crustal blocks and arc fragments starting in the Ordovician–Silurian and culminating with the Devonian–Carboniferous Ellesmerian orogeny, the origin of which remains enigmatic. We suggest that Ellesmerian deformation was related to widespread transpression associated with northward motion of Laurentia during Acadian and Neo-Acadian deformation along the Appalachian margin rather than orthogonal ocean basin closure and microcontinent-continent collision. The Pearya terrane and North Slope subterrane of the Arctic Alaska terrane, separated from the Franklinian passive margin by the Petersen Bay fault and Porcupine shear zone, respectively, best preserve the Paleozoic translational and transpressional history of the northern Laurentian margin. These two major structures record a complex history of terrane accretion and translation that defines the Canadian Arctic transform system, which truncated the Caledonian suture to the east and ultimately propagated early Paleozoic subduction to the Cordilleran margin of western Laurentia.