TECTONO-STRATIGRAPHIC FRAMEWORK OF NEOPROTEROZOIC TO EARLY PALEOZOIC STRATA, WESTERN US: PROTRACTED RIFTING AND EVOLUTION OF THE WESTERN LAURENTIA MARGIN

Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to early Paleozoic sedimentary and volcanic rocks in the western US record a dynamically evolving landscape along the western (present day geographic coordinates) Laurentian margin that included: (1) initial development of intracratonic basins and deposition of siliciclastic strata from ~770–740 Ma; (2) early-phase rifting, volcanism, and deposition of diamictite-bearing strata along a N-S basin system from ~720–660 Ma; (3) early-phase regional subsidence and deposition of mature siliciclastic strata from ~660–580 Ma; (4) final-phase rifting and transition to drift, volcanism, and deposition of variably immature siliciclastic strata from ~570–520 Ma; and (5) regional subsidence along a passive margin and deposition of Middle Cambrian to Devonian carbonate-rich strata. Detrital zircon patterns record changing sources, with sediments mostly derived from basement along the evolving basin margin during early- and final-phase rifting, and mostly from distal eastern Laurentian basement and recycled sedimentary rocks during early-phase subsidence. Overlying carbonate-rich strata record episodic influx of siliciclastic sediment derived from cratonic arches exposed during regressions. Basalt flows and trachyte to rhyolite clasts with ages of 710-670 Ma have geochemical signatures consistent with rifting, as do alkali basalt flows with ages of ~550 Ma. Stratigraphic thickness-age relations are consistent with two phases of rifting: at ~700–670 Ma with an estimated 25–40% extension of initially thick lithosphere that was weakened by igneous activity; and at 580–530 Ma with an estimated 20–35% regional extension, renewed igneous activity, thermal thinning of mantle lithosphere, and localized extension culminating in final separation along the continental margin. Available stratigraphic, geochronologic, and paleomagnetic data are consistent with linkage of western Laurentia to Australia-East Antarctica within Rodinia, followed by multi-phase rifting and transition to change to drift during Cambrian time.