Continental Tectonics Inferred From High‐Resolution Imaging of the Mantle Beneath the United States, Through the Combination of USArray Data Types

Abstract

Introduction Tectonic SettingThe United States portion of the long-lived part of the North American continent consists of Precambrian cratons, including Archean cratons such as the Wyoming craton, and the Proterozoic Interior Platform (Bleeker, 2003;Hoffman, 1988). Since cratonic accretion, the Cenozoic-Mesozoic Rocky Mountain Cordillera and Paleozoic Appalachian Mountains formed on the west and east sides of the craton (Figure 1), respectively. Extending further out from the relatively stable core of the continent and to the east of the Appalachian range, the Atlantic Coastal Plain province in the east is a Paleozoic passive plate margin (Bally et al., 1989). The western edge of North America has a more complex tectonic history. Its seismically and tectonically active continental margin, Mesozoic-Cenozoic orogenies, and arc volcanism are controlled by the interaction between oceanic Pacific, Kula and Farallon plates and the North American plate (Atwater, 1989). Currently, the Juan de Fuca plate, a remnant of the Farallon plate, is subducting beneath North America at the Cascadia subduction zone (Atwater, 1970), causing arc volcanism, earthquakes, and episodic tremor and slip (Rogers & Dragert, 2003). The further inland location of the Laramide orogeny and eastward migration of magmatism during the Mesozoic can be explained by the contemporaneous flattening of the Farallon slab, which was likely caused by the increase in subduction rate and slab buoyancy (Engebretson et al., 1984(Engebretson et al., , 1985Molnar & Atwater, 1978). The Tertiary extensional system of the Basin and Range is likely due to the steepening of the Farallon Plate (Coney & Reynolds, 1977;Davis, 1980). Compared to tectonically active western North America, there is minimal tectonic activity and topographic variation in the central and eastern United States. However, variations in crustal and mantle structure do exist and have been the

Upper Mantle Structure Beneath the Contiguous US Resolved With Array Observations of SKS Multipathing and Slowness Vector Perturbations

“…Then, we present our observations at selected depth slices highlighting possible mantle heterogeneity. Following this, our observations are compared to synthetically generated data using three upper mantle tomography models (Bedle et al, 2021;Fichtner et al, 2018;Schmandt & Lin, 2014). This allows There is a clear frequency dependence of slowness vector deviation magnitudes.…”

“…We interpret this as the boundaries of the C4 and C2 heterogeneities having a large velocity contrast and also transitioning over a longer distance. These anomalies have been imaged as fast anomalies between the 410 and 660 km deep transition zone in several tomography models (Bedle et al., 2021; Porritt et al., 2014; Schmandt & Lin, 2014; Sigloch et al., 2008; Wang et al., 2019). We interpret this as possible lithosphere which has descended into the mid mantle and is sufficiently old that it has been heated by the mantle and has weaker thermal gradients at its boundaries.…”

“…We choose the models from Fichtner et al (2018), Schmandt and Lin (2014) and Bedle et al (2021) because they use different approaches and data. These velocity models are for the upper to mid mantle only (depths of up to 500-1,200 km) and do not contain lower mantle structure.…”

“…Seismic phenomena analyzed on a continental scale have improved our understanding of whole‐Earth dynamics. Such studies have analyzed reflectors in the mid mantle (e.g., Bentham et al., 2017; Deuss, 2009; Deuss et al., 2006; Waszek et al., 2018), converted phases (Abt et al., 2010; Jenkins et al., 2017), and small‐scale heterogeneity from scattering (Hedlin & Shearer, 2000; Ma & Thomas, 2020; Waszek et al., 2015), from waveform complexity (Thorne et al., 2020, 2021), and through seismic tomography (Bedle et al., 2021; Fichtner et al., 2018; Schmandt & Lin, 2014; Sigloch et al., 2008). Using slowness vector measurements (backazimuth and horizontal slowness) to quantify diffraction and observe multipathing caused by mantle heterogeneity boundaries has yet to be performed on a continental scale.…”

Big Data Seismology

“…As described in the Introduction, the USArray has allowed 3D variations in seismic wavespeed and anisotropy to be mapped at high‐resolution in the crust and upper mantle at continental scale, leading to an improved understanding of the relationship between processes in the mantle and tectonic features at the surface. Much of the literature is cited in the most recent continental‐scale imaging studies (Bedle et al., 2021; Porritt et al., 2021). The deployment of similar high‐density arrays in Asia have enabled detailed mapping of the seismic structure of the upper mantle in Eastern Asia (Tao et al., 2018), Tibet (Xiao et al., 2020), the Alps (Paffrath et al., 2021), and the Iberian peninsula (Chevrot et al., 2014).…”