Structure of the upper mantle in the north-western and central United States from USArray S-receiver functions

Abstract

Abstract. We used more than 40 000 S-receiver functions recorded by the USArray project to study the structure of the upper mantle between the Moho and the 410 km discontinuity from the Phanerozoic western United States to the cratonic central US. In the western United States we observed the lithosphere-asthenosphere boundary (LAB), and in the cratonic United States we observed both the midlithospheric discontinuity (MLD) and the LAB of the craton. In the northern and southern United States the western LAB almost reaches the mid-continental rift system. In between these two regions the cratonic MLD is surprisingly plunging towards the west from the Rocky Mountain Front to about 200 km depth near the Sevier thrust belt. We interpret these complex structures of the seismic discontinuities in the mantle lithosphere as an indication of interfingering of the colliding Farallon and Laurentia plates. Unfiltered S-receiver function data reveal that the LAB and MLD are not single discontinuities but consist of many small-scale laminated discontinuities, which only appear as single discontinuities after longer period filtering. We also observe the Lehmann discontinuity below the LAB and a velocity reduction about 30 km above the 410 km discontinuity.

Seismic velocity structure of the crust and shallow mantle of the Central and Eastern United States by seismic surface wave imaging

“…This is consistent with independent seismic velocity images [e.g., Van der Lee and Frederiksen , ; Eaton and Frederiksen , ; Schmandt and Lin , ]. The pattern of absolute v s versus depth at representative locations in these provinces (Figure ) suggests that the thickness of Proterozoic lithosphere is generally ∼150–200 km, as also suggested by receiver‐function imaging of the lithosphere‐asthenosphere boundary (LAB) [ Kind et al , ]. Kaban et al [] present a numerical model for North America whereby basal shear by the convecting upper mantle limits lithospheric thickness to ∼200 km.…”

Insights Into Layering in the Cratonic Lithosphere Beneath Western Australia

“…To the best of our knowledge, the deepest MLD was reported to be at 200 km beneath station FRB near the core of the North American Craton, where the LAB is as deep as 240 km beneath the FRB station (Calò et al, ). This MLD depth is much deeper than the average depth of approximately 100 km (e.g., Abt et al, ; Kind et al, ; Miller & Eaton, ; Rader et al, ; Rychert & Shearer, ; Wirth & Long, ; Yuan & Romanowicz, ). The MLD tends to be a little deeper (~140 km) beneath the Slave Craton (e.g., Bostock, ; Snyder et al, ).…”

The meaning of midlithospheric discontinuities: A case study in the northern U.S. craton

“…The presence of a clear NVG at lithosphere‐asthenosphere transition depths west of the Sevier Thrust Belt and the typical, although not universal, absence of a NVG at lithosphere‐asthenosphere transition depths to the east of the Sevier Thrust Belt are consistent with a number of prior analyses of Sp phases that overlap with our study region [e.g., Abt et al ., ; Levander and Miller , ; Kumar et al ., ; Kind et al ., ; Lekić and Fischer , ; Hopper et al ., ]. However, other studies overlapping with our study region have argued for a more widespread NVG at tomographically defined LAB depths [e.g., Hansen et al ., ; Foster et al ., ; Kind et al ., ].…”

“…Within the overall high‐velocity cratonic mantle, seismic discontinuities or vertically localized seismic velocity gradients have been found by a range of seismic studies. In particular, converted wave studies have imaged negative velocity gradients (often termed midlithospheric discontinuities, or MLDs) on a widespread basis, typically at depths of 60–150 km [e.g., Zurek and Dueker , ; Wittlinger and Farra , ; Savage and Silver , ; Chen et al ., ; Hansen et al ., 2013, ; Rychert and Shearer , ; Abt et al ., ; Ford et al ., ; Geissler et al ., ; Miller and Eaton , ; Kind et al ., ; Kumar et al ., ; Wölbern et al ., ; Hansen et al ., ; Bodin et al ., ; Cooper and Miller , ; Foster et al ., ; Hopper et al ., ; Lekić and Fischer , ; Porritt et al ., ], and discontinuities at comparable depths have also been characterized as anisotropic boundaries [ Bostock , ; Levin and Park , ; Saul et al ., ; Mercier et al ., ; Sodoudi et al ., ; Wirth and Long , ]. Evidence for a midlithospheric low‐velocity zone is also found in diverse studies based on reflected waves [ Revenaugh and Sipkin , ], surface waves [e.g., Weeraratne et al ., ; Chen et al ., ; Romanowicz , ; Yuan and Romanowicz , ; Yuan et al ., ], long‐range seismic profiles [ Thybo and Perchuć , ; Nielsen et al ., ; Thybo , ] and regional P waveforms [ Chu et al ., ].…”

“…The absence of clear converted phases (at the frequencies used in these studies) is consistent with a LAB velocity gradient that is distributed over 50–70 km or more and that can be explained by a purely thermal boundary [e.g., Abt et al ., ; Ford et al ., ]. However, other studies do report converted phases from LAB depths beneath portions of the North American craton [ Miller and Eaton , ; Hansen et al ., ; Foster et al ., ; Porritt et al ., ; Kind et al ., ].…”