Joint Local and Teleseismic Tomography in the Central United States: Exploring the Mantle Below the Upper Mississippi Embayment and the Illinois Basin

Abstract

Three-dimensional, high-resolution crustal and upper mantle P and S wave velocity (Vp and Vs) models are presented for the central United States using body wave tomography. The study utilizes local and teleseismic data recorded by the Northern Embayment Lithospheric Experiment stations, the CERI New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. Vp and Vs solutions are very similar and are well resolved in the depth range 40 to 400 km. Two anomalously slow regions are present below the Illinois Basin, forming a northwest dipping low-velocity zone (LVZ) extending from~200 to 400 km. Maximum anomaly magnitudes in the LVZ reach about −4% and −5% for Vp and Vs, respectively. The LVZ appears to connect to a well-documented LVZ located below the northern Mississippi Embayment. The Illinois Basin velocity anomalies cannot be explained by elevated temperature alone and require compositional heterogeneity involving elevated orthopyroxene content in addition to an increase in iron and water content. The need for additional orthopyroxene suggests that the LVZ is being produced by metasomatism of mantle rocks by hydrous, silica-rich fluids ascending from a slab fragment trapped in or near the transition zone. This supports previous interpretations for the existence of the LVZ below the Embayment. We suggest that the LVZs below the Mississippi Embayment and the Illinois Basin are linked to the presence of a large igneous province, which convection models suggest is currently located below the central United States. Plain Language Summary We present three-dimensional, high-resolution velocity models from the Earth's surface to 410 km depth under the central United States. Our study utilizes seismic tomographya technique that images the velocity structure of the Earth using seismic waves that traveled from earthquake sources to seismic stations. Travel-time data recorded by several seismic arrays (dense deployments of seismic stations) were acquired to increase the resolution of our model. In the resulting model, compressional wave and shear wave velocity anomalies are similar and are well-resolved in the depth range 40 to 400 km. An anomalously slow region is present below the Illinois Basin that dips to the northwest and extends to 400 km. The low velocity cannot be explained by elevated temperature alone and requires an increase in iron and water content as well as high amounts of orthopyroxene. Elevated orthopyroxene suggests that the mantle rocks have been chemically altered due to fluids coming from a trapped slab fragment. We suggest that the low-velocity zone below the Illinois Basin is associated with the presence of a slab fragment containing a thick oceanic plateau. According to several studies, the plateau was subducted millions of years ago and is presently located underneath the central United States.

Shear Velocity Structure Beneath the Central United States From the Inversion of Rayleigh Wave Phase Velocities

“…The lithospheric foundering might have caused heating and chemical differentiation, which explains the generation of silica-rich magmas and thus the observed low velocity. Low velocities at 60-150 km (labeled O) below the Mississippi Embayment with high velocities above (labeled I in the map view) agree with previous mantle velocity studies (Geng et al, 2020;, and are similar to velocity anomalies below the North China Craton (Santosh et al, 2010;Tian & Zhao, 2011). Two slow anomalous regions in the uppermost mantle are present below the WVSZ and the SGSZ (labeled Q and R in cross-sections DD' and EE', respectively), which are also present in the study by Geng et al (2020).…”

“…Another prominent low Vs anomaly is present on the Arkansas-Oklahoma border (labeled M in the 20 km slice of the map view). This low Vs area corresponds to the Arkoma Basin and is also imaged in the Geng et al (2020) study. A prominent low-velocity region (labeled N in the map view) is present in Alabama and middle Tennessee, which agrees with Geng et al (2020).…”

“…This low Vs area corresponds to the Arkoma Basin and is also imaged in the Geng et al (2020) study. A prominent low-velocity region (labeled N in the map view) is present in Alabama and middle Tennessee, which agrees with Geng et al (2020). Biryol et al (2016) hypothesized that this area is underlain by thinned lithosphere.…”

“…The low Vs zone detected in the mantle below the Reelfoot Rift is the upper portion of the LVZs imaged in and Geng et al (2020). According to the analyses presented in these previous studies, the low velocities are produced by silica-rich fluids ascending from a trapped portion of the Farallon slab.…”

“…The highest resolution image of the LVZ below the NMSZ is provided by , which shows the LVZ dips to the southwest and extends to a depth of at least 250 km. Another two low-velocity anomalies at depths ∼200-400 km below the Illinois Basin are imaged by Geng et al (2020). The slow velocities form a northwest dipping zone and indicate the presence of a slab fragment below the central US region.…”