Mantle transition zone discontinuities beneath the contiguous United States

Abstract

Using over 310,000 high‐quality radial receiver functions recorded by the USArray and other seismic stations in the contiguous United States, the depths of the 410 km and 660 km discontinuities (d410 and d660) are mapped in over 1,000 consecutive overlapping circles with a radius of 1°. The average mantle transition zone (MTZ) thickness for both the western and central/eastern U.S. is within 3 km from the global average of 250 km, suggesting an overall normal MTZ temperature beneath both areas. The Pacific Coast Ranges and the southern Basin and Range Province are underlain by a depressed d410, indicating higher‐than‐normal temperature in the upper MTZ. The proposed Yellowstone and Raton hot spots are not associated with clear undulations of the MTZ discontinuities, but d410 beneath another proposed hot spot, Bermuda, is depressed significantly and d660 has a normal depth. Low‐temperature regions are found in the upper MTZ associated with the subducted Juan de Fuca slab beneath the northern Rocky Mountains and in two circular areas beneath the northern Basin and Range Province and the southern Colorado Plateau. Part of the Great Plains is characterized by a depressed d660. This observation, when combined with results from seismic tomography, suggests the existence of a cold region in the lower MTZ, probably associated with subducted Farallon slab segments.

Imaging Upper‐Mantle Structure Under USArray Using Long‐Period Reflection Seismology

“…For the complete model, the median 410 and 660 depths ( d 410 and d 660 ) are 417 and 661 km, respectively, with a median transition‐zone thickness of 245 km. These values are very close to the average discontinuity depths estimated from global SS precursors (e.g., 418 and 660 km from Flanagan & Shearer, ) and the USArray receiver function analysis of Gao and Liu (), who obtained average depths across the United States of 416 and 667 km. Wang and Pavlis () obtained slightly different USArray receiver‐function average depths of 415 and 660 km; they attributed their shallower d 660 to their use of the AK135 reference 1‐D model rather than iasp91 (their application of timing corrections based on the 3‐D P wave model of Burdick et al, , may also have been a contributing factor).…”

“…For comparison to the maps presented here, Figure S4 plots discontinuity topography results from the USArray receiver‐function study of Gao and Liu (). In general, the depth variations agree reasonably well for the 410‐ and 660‐km topography, but their transition zone thickness map shows much less variation than our results.…”

The upper-mantle transition zone beneath the Ibero-Maghrebian region as seen by teleseismic Pds phases

“…Nevertheless, the values in Fig. S5a and b before correction are very low compared to other studies (Chevrot et al, 1999;Gao and Liu, 2014;Tauzin and Ricard, 2014). These poor correlation values probably indicate that depth changes are not controlled primarily by temperature changes and/or that the velocity structure in the upper-mantle and the TZ are anti-correlated with anomalies of the same order but opposite sign.…”

“…Nevertheless, the mantle structure leads to large positive correlation of the observed depth values (and relative travel times), e.g., +0.94 in Chevrot et al (1999), +0.84 in Gao and Liu (2014), and +0.6 in Tauzin and Ricard (2014). Ideally, a negative or near zero correlation value is expected after correction, but in practice it is observed that velocity corrections reduce the correlation to smaller positive values (e.g., Gao and Liu, 2014;Tauzin, 2009). This residual positive correlation is partly due to the limitations of the velocity models to reproduce the real structure of the Earth, due to the assumption that depth changes are controlled only by temperature changes, and to a lesser extent due to measurement errors.…”

“…This is expected in studies covering smaller areas. For example, Tauzin and Ricard (2014) studied an area beneath west USA which is contained in the study area of Gao and Liu (2014) and found a smaller correlation value than in Gao and Liu (2014).…”