Crustal low‐velocity zones (LVZs) have been reported in active orogens such as the Himalayas and the Andes but rarely in stable cratonic regions. In this study, we provide compelling evidence for a significant midcrustal LVZ beneath eastern‐central Alberta, an integral part of the Precambrian Canadian Shield covered by thick Phanerozoic sedimentary deposits. This 200 km wide, over 10 km thick midcrustal LVZ is well resolved by shear velocity inversions using P‐to‐S receiver functions from more than 4600 earthquakes. It is generally overlain by a high‐velocity upper crust in the depth range of 8–15 km, especially in western‐central Alberta, which coincides with the previously documented Winagami reflection sequence. We interpret the LVZ to be of granitic composition, potentially in connection with the crystallization of partially molten crust during the Paleoproterozoic eon. In addition to the Precambrian tectonic history of western Laurentia, which featured plate convergence conducive to crustal melting, our crustal model is further supported by (1) a moderate spatial correlation between the LVZ and heat flow, and (2) shear velocities consistent with that of granite. The well preserved Winagami reflection sequence and the LVZ are potential evidence of distinct episodes of magmatism and crust modification in the Precambrian basement of the Western Canada Sedimentary Basin. The existence of a broad crustal LVZ suggests extensive subduction, orogenesis, and crustal melting during the Precambrian assembly of the North American craton.

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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,~44 km) to shallow (SE,~39 km) 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~6 km 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

The arrival time and amplitude of long-period underside reflection from mantle seismic discontinuities (SS precursors) have made major contributions to the understanding of mantle composition and dynamics. In this study, we introduce a nonlinear waveform inversion technique to simultaneously constrain shear velocities and discontinuity depths beneath the northwestern Pacific subduction system. Based exclusively on a large SS precursor waveform dataset, we are able to clearly delineate the morphology of the descending Pacific plate, which flattens at the base of the upper mantle and extends westward by at least ~1500 km toward northern-central China.Our grid search over a range of angles indicates a maximum correlation between shear velocity and transition zone thickness at ~30 degrees, consistent with the reported average slab dip beneath the study region. The strongly positive correlation suggests predominantly thermal, rather than compositional, variations along the descending Pacific plate. Our joint depth-velocity solutions also suggest a 5-10 km depression of the 410 km discontinuity and an average decrease of 1.2% in upper mantle shear velocity beneath the intraplate volcanic fields in northeastern China. This anomaly, which reaches the middle of the upper mantle transition zone beneath the Changbai hotspot, initiates at a significantly shallower (~320 km) depth beneath the Wudalianchi regions. The high amplitude reflection at depths greater than 410 km suggests a water-poor melt layer in possible association with 1) decompression melting from passive upwelling and/or 2) active upwelling through a slab window.