THE NEOPROTEROZOIC RIFTING OF WESTERN LAURENTIA: A SINGLE EVENT OR TWO RIFTING PROCESSES?

Neoproterozoic rifting of western Laurentia is an important geologic event in the evolution of North America and provides a primary control on later deformation geometry. Evidence of the earliest stages of this rift may be found from the Northern Canadian Cordillera to SW USA, where geochronologic constraints of the rift are scarce. Rift related volcanism is dated from around 800 Ma for the earlier stages and as young as 569 Ma in the Hamil-Gog Group, Canada; while other volcanic units are stratigraphically constrained as reflecting volcanism during early Cambrian time (e.g. Cerro Rajon Formation, Mexico). Considered together, these stratigraphic and volcanic sequences may reflect a rift event spanning more than 200 My—with time gaps and variations in geochemical and petrogenetic characteristics. The earliest stages show LIPs with pillow lavas and felsic magmatism, suggesting an evolution of the rift, while the later stages constraint magmatism to basaltic volcanism in a continental environment with occasional shallow water interaction. The later rifting event exhibits volcanic flows ranging from 2 to 15 m in thickness in NW Mexico and north Utah. These field relations combined with geochemical results suggest a low degree of melting which is consistent with dominant alkaline volcanism that evolves to a slightly higher rates of melting in the eastern Nevada and southern Utah localities. Both effusive and pyroclastic products have low SiO₂%, high TiO₂%, anorogenic and intraplate volcanism characteristics, and an absence of differentiated products except for occasional intermediate volcanic clasts in the Browns Hole Formation of northern Utah. These characteristics suggest a restart of the western Laurentia rifting process in later stages which can be interpreted as an independent second rifting event. Together with the tectonic and stratigraphic framework for the Neoproterozoic successions, this second rifting event was probably aborted. With further work, understanding the western Laurentia rifting process age and petrogenesis could help us better constrain portions of proto-Pacific evolution and early western Laurentian margin geometry.