P Wave Teleseismic Traveltime Tomography of the North American Midcontinent

Abstract

The remains of the 1.1‐Ga Midcontinent Rift (MCR) lie in the middle of the tectonically stable portion of North America. Previous and ongoing studies have imaged strong heterogeneity associated with the MCR in the crust but have not imaged such within the mantle. It is unclear whether this is due to the absence of rift‐related mantle structures or these studies had insufficient resolution to image them. To address this issue, we measured 46,374 teleseismic P wave delay times from seismograms recorded by the USArray Transportable Array, Superior Province Rifting EarthScope Experiment, and surrounding permanent stations. We included these and 54,866 delay times from prior studies in our tomographic inversion. We find that high‐velocity anomalies are widespread in our study area, but there are also prominent low‐velocity anomalies. Two of these are coincident with high‐Bouguer gravity anomalies associated with the MCR in Iowa and the Minnesota/Wisconsin border at 50‐ to 150‐km depth. Extensive resolution testing shows that these anomalies could be the result of downward vertical smearing of relatively low velocities from rift‐related material that “underplated" the crust, although we cannot exclude that the subcrustal mantle lithosphere beneath the MCR is anomalously enriched, hydrated, or warm. Other anomalies occur at syntaxes of the Penokean Orogen. One with the Superior Province and Marshfield Terrane in southern Minnesota and another with the Yavapai and Mazatzal Terranes, both at 100‐ to 250‐km depth. In the midmantle, we image two linear high‐velocity anomalies, interpreted as subducted fragments of the Farallon and Kula plates.

The Upper Mantle Structure of Northwestern Canada From Teleseismic Body Wave Tomography

“…The network-averaged standard deviations are 0.03 and 0.09 s for the P and S wave delay time data sets, respectively ( Figure 2). Similar delay-time errors are found in other studies (Bollmann et al, 2019;Boyce et al, 2016). Figure 2 shows the average relative station delay times for both P and S wave data sets.…”

“…We use VanDecar's inversion procedure to resolve the three-dimensional isotropic velocity structure underlying northwestern Canada (VanDecar et al, 1995). This tomographic software has been applied to a variety of tectonic settings (e.g., subduction zones: Bostock & VanDecar, 1995;cratonic regions: Bastow et al, 2013;Bollmann et al, 2019;Boyce et al, 2016;Estève et al, 2019;Frederiksen et al, 2007;Mercier et al, 2009;Villemaire et al, 2012;and rift settings: Bastow et al, 2008;White-Gaynor et al, 2019). Relative delay times are inverted simultaneously for slowness perturbations, station static terms, and source terms.…”

Body Wave Speed Structure of Eastern North America

“…Another likely alternative is suggested by Bollmann et al. (2019) which imaged a similar low wave speed structure using teleseismic P waves, named S1 in (Bollmann et al., 2019). In conjunction with a conductivity study (B. Yang et al., 2015), Bollmann et al.…”

“…Another likely alternative is suggested by Bollmann et al (2019) which imaged a similar low wave speed structure using teleseismic P waves, named S1 in (Bollmann et al, 2019). In conjunction with a conductivity study (B. Yang et al, 2015), Bollmann et al (2019) suggests this anomaly is likely the result of a fossilized slab crust or melt infiltration originating from a subduction plate.…”

“…thought to be the MLD. Even further west, a slow wave speed anomaly near the mid-continent rift was interpreted to be due to a shallow mantle, volcanic intrusion (Bollmann et al, 2019), but not deeper structure. Finally, the mantle transition zone beneath North America contains a very strong, positive anomaly (Golos et al, 2018) interpreted as the Farallon slab (Schmandt & Lin, 2014) or possibly an older Grenville slab (Biryol et al, 2016).…”