Precambrian Tectonic Discontinuities in Western Laurentia: Broadband Seismological Perspectives on the Snowbird and Great Falls Tectonic Zones

摘要

The Snowbird Tectonic Zone (STZ) in southwestern Canada and the Great Falls Tectonic Zone (GFTZ) in northern Montana are two structural lineaments with major implications for the formation and evolution of the western North American craton. In this study, we examine the origins of these two proposed Proterozoic sutures using broadband seismic data from Alberta and USArray stations. We find substantial spatial variations in crustal structure and thickness across both the STZ and GFTZ based on an integrated analysis of P-to-S receiver functions, shear velocities, and potential field measurements. The STZ marks the transition from deep (NW, similar to 44km) to shallow (SE, similar to 39km) Moho in central Alberta. This steep Moho relief, in conjunction with elevated Vp/Vs ratios, provide compelling evidence for Proterozoic subduction and magmatism. In northern Montana, the Moho is depressed by similar to 6km within the fold-and-thrust belts of the Trans-Montana Orogen, where high (1.84) Vp/Vs ratios spread along the collisional suture of the Dillion Shear Zone and terminates sharply along the reworked margin of the Wyoming craton. These observations, coupled with widespread lower crustal conversions, are concordant with a Paleoproterozoic orogeny resulting from collision(s) between Archean cratons. On the other hand, a flat Moho under major northeast striking potential field lineaments in the northernmost Montana favors an origin as a ductile shear zone, rather than suture, in the orogenic hinterland. We find evidence for crustal underplating beneath the GFTZ, though this anomalous layer is smaller than previous estimates and appears to be confined to the Trans-Montana Orogen.