WHERE ARE HOFFMAN’S RIFTED MARGINS OF RODINIA?

The breakup of the Neoproterozoic supercontinent Rodinia has long been portrayed as the tectonic context for the initiation of Snowball Earth and the origin of animals. Hoffman’s (1991) standard model of Rodinia is based on tectonostratigraphic evidence for Mesoproterozoic orogenesis followed by Neoproterozoic rifting and passive margin development around a Laurentian core. Here we report age constraints on contractional structures in the Wernecke Mountain of Yukon, and Death Valley, CA that challenge this framework. Km-scale folds in Yukon (Eisbacher, 1981; Thorkelson, 2000) are developed in the >811 Ma Basinal Assemblage of the Little Dal Gp. and are sealed by the 665-635 Ma Mt. Profeit dolostone. In Death Valley, foliation and folding are developed in the Beck Spring dolomite and unit KP1 and are sealed by the Virgin Spring limestone and the 717-665 Ma Surprise diamictite. Micropaleontology, geochemistry, and geochronology, indicate that the Beck Spring dolomite and KP1 are correlative with the ca. 770-740 Ma Chuar Group exposed in the Grand Canyon, AZ. Traditionally, it is taken for granted that these strata were accommodated by extension related to the rifting of Rodinia. We propose that the contractional structures, basin formation, and the appearance of ca. 800-740 Ma detrital zircons in these strata are the product of ca. 740 Ma flexure and obduction of an unknown arc and continental fragment on the western margin of Laurentia. These putative foreland deposits are unconformably overlain by Neoproterozoic diamictites that were deposited in narrow grabens, which formed through post-orogenic collapse and failed rifting. The development of an open passive margin on the western margin of Laurentia did not occur until the latest Ediacaran, when the unknown continental fragment rifted away. Thus, we suggest the tectonic context for Neoproterozoic environmental change, including Snowball Earth, is not merely the rifting of a single supercontinent Rodinia over ~500 Myrs, but instead multiple tectonic cycles punctuated by large igneous events. Indeed, as Paul Hoffman argued throughout his career, Precambrian margins record rich tectonic and environmental histories comparable to Phanerozoic margins, which have an average tenure of ~130 Myrs and terminate with arc obduction and continental collision.