NEOPROTEROZOIC-CAMBRIAN TECTONIC EVOLUTION OF THE NORTHERN ROCKY MOUNTAINS: THE DYNAMICS OF RIFTING ALONG WESTERN LAURENTIAN MARGIN

The Neoproterozoic to Cambrian evolution of the western Laurentian margin has long served as a type example for passive margin development. It is an ingrained paradigm that the western Cordilleran margin was generally a low-relief passive miogeocline, initiated by Neoproterozoic rifting and characterized by repetitive marine transgression throughout the early to mid-Paleozoic. Thick Neoproterozoic to Cambrian successions in southern Idaho and Utah exemplify the classic rift-to-drift transition characteristic of a passive margin assemblage. However, there are significant complexities in Neoproterozoic to Cambrian strata of the northern Rocky Mountains, and apparent episodicity in rift development. The protracted nature of passive margin subsidence is problematic, with the time frame from initial rifting (ca. 780 Ma) to thermally driven subsidence (ca. 530 Ma) too long for a monogenetic system (ca. ~250 m.y. vs. <100 m.y. in modern systems). North of the Snake River Plain, miogeoclinal siliciclastic Neoproterozoic and Cambrian strata are limited, thick Cambrian carbonate is lacking and there is evidence for an irregular topographic surface characterized by local (fault-bounded?) basins, subtle highlands, and lateral variations in stratigraphic thickness. These features, coupled with the presence of episodic Neoproterozoic to Cambrian alkalic magmatism, suggest a more complex paleogeographic setting. The profound differences in Neoproterozoic-Cambrian sediment thickness, stratigraphic age range, facies distributions and subsidence patterns in the northern Rocky Mountains requires significant along-strike (NW-SE) differences in the geometry and kinematics of extension. Various models for the kinematics and geometry of continental extension have been proposed, and will be evaluated in the context of geological constraints.