Brandon Schmandt

Dehydration melting at the top of the lower mantle

Cycling water through the transition zone The water cycle involves more than just the water that circulates between the atmosphere, oceans, and surface waters. It extends deep into Earths interior as the oceanic crust subducts, or slides, under adjoining plates of crust and sinks into the mantle, carrying water with it. Schmandt et al. combined seismological observations beneath North America with geodynamical modeling and high-pressure and -temperature melting experiments. They conclude that the mantle transition zone—410 to 660 km below Earths surface—acts as a large reservoir of water. Science, this issue p. 1265 Downwelling of hydrous minerals may cause partial melting of Earth’s lower mantle. The high water storage capacity of minerals in Earth’s mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.

P and S wave tomography of the mantle beneath the United States

Mantle seismic structure beneath the United States spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic P and S wave traveltime tomography including USArray data up to May 2014. To mitigate artifacts from crustal structure 5–40 s, Rayleigh wave phase velocities were used to create a 3-D starting model. Major features of the final P and S models include two distinct low-velocity anomalies at depths of ~60–300 km beneath the central and northern Appalachians and passive margin. The central Appalachian low-velocity anomaly coincides with Eocene basaltic magmatism, and the northern anomaly is located along the Cretaceous track of the Great Meteor hot spot. At depths of ~300–700 km beneath the central and eastern U.S. large high-velocity anomalies are inferred to be remnants of the Farallon slab that subducted prior to ~40 Ma during the Laramide orogeny.

Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

The Yellowstone magmatic system from the mantle plume to the upper crust

Yellowstones missing magmatic link Yellowstone is an extensively studied “supervolcano” that has a large supply of heat coming from a pool of magma near the surface and the mantle below. A link between these two features has long been suspected. Huang et al. imaged the lower crust using seismic tomography (see the Perspective by Shapiro and Koulakov). Their findings provide an estimate of the total amount of molten rock beneath Yellowstone and help to explain the large amount of volcanic gases escaping from the region. Science, this issue p. 773; see also p. 758 The Yellowstone supervolcano has a large magma body between the mantle hot spot and the upper crustal magmatic reservoir. [Also see Perspective by Shapiro and Koulakov] The Yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world. An understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk. Using a joint local and teleseismic earthquake P-wave seismic inversion, we revealed a basaltic lower-crustal magma body that provides a magmatic link between the Yellowstone mantle plume and the previously imaged upper-crustal magma reservoir. This lower-crustal magma body has a volume of 46,000 cubic kilometers, ~4.5 times that of the upper-crustal magma reservoir, and contains a melt fraction of ~2%. These estimates are critical to understanding the evolution of bimodal basaltic-rhyolitic volcanism, explaining the magnitude of CO2 discharge, and constraining dynamic models of the magmatic system for volcanic hazard assessment.

Seismically imaged relict slab from the 55 Ma Siletzia accretion to the northwest United States

Teleseismic tomography images a large high-velocity “curtain” extending vertically beneath the ca. 50 Ma Challis magmatic trend to maximum depths of 230–600 km. We interpret this structure as subducted Farallon ocean lithosphere that stalled with the ca. 55 Ma accretion of the Siletzia microplate to North America within the Columbia Embayment, and we consider the regional tectonic implications. The abrupt switch ca. 53 Ma from Laramide thrusting and magmatic quiescence to extension and vigorous magmatism in the northwestern United States is evidence for foundering of the fl at-subducting Farallon slab. To account for the imaged curtain, foundering apparently occurred by rollback after Siletzia accretion terminated subduction within the Columbia Embayment. The magnitude of the high seismic velocity curtain is approximately that expected for 20–50 m.y. old ocean lithosphere that stalled in the upper mantle ca. 50 Ma. We suggest that the stalled slab became nearly neutrally buoyant as a result of removal of its basaltic crust, the melting of which likely contributed to the short-lived vigor of Challis magmatism. After Siletzia accretion, normal dip Cascadia subduction initiated west of Siletzia, evidenced by arc volcanism in Oregon and Washington beginning ca. 45–40 Ma, while continued quiescence of the Sierra Nevada arc suggests persistence of flsubduction to the south. Kinematically, this requires a tear in the subducted Farallon slab near the latitude of southern Oregon. We propose that north to south propagation of this torn slab edge propagated the ignimbrite fl are-up across what now is the northern Basin and Range and ended the Laramide orogeny.

Seismic heterogeneity and small‐scale convection in the southern California upper mantle

We invert traveltime residuals of teleseismic P and S phases for 3-D perturbations in VP, VS, and VP/VS structures in the southern California upper mantle. The tomographic inversion uses frequency-dependent 3-D sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals. In addition to separate VP and VS tomography, we jointly invert the P and S data sets for VP/VS perturbations by imposing a smoothness constraint on the δlnVS/δlnVP field. The regional upper mantle is very heterogeneous with the greatest amplitude of velocity and VP/VS perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern California. Our imaging improves the resolution of the geometry and amplitude of these features. In addition to the major structures imaged and discussed in previous papers, we find a large high-velocity anomaly at depths between ∼340 and 500 km beneath the Borderland, Transverse Ranges, and northern Peninsular Ranges. This anomaly is separate from the well-known uppermost mantle Transverse Ranges anomaly, which extends no deeper than ∼175–200 km. The strongest low-velocity, high VP/VS anomaly is found beneath and west of the Salton Trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning. The magnitudes of VP, VS, and VP/VS perturbations and knowledge of regional 1-D velocity and attenuation lead us to conclude that the asthenosphere contains up to ∼1% partial melt extending to depths as great as 150–200 km.

Joint inversion of Rayleigh wave phase velocity and ellipticity using USArray: Constraining velocity and density structure in the upper crust

Rayleigh wave ellipticity, or H/V ratio, observed on the surface is particularly sensitive to shallow earth structure. In this study, we jointly invert measurements of Rayleigh wave H/V ratio and phase velocity between 24–100 and 8–100 sec period, respectively, for crust and upper mantle structure beneath more than 1000 USArray stations covering the western United States. Upper crustal structure, in particular, is better constrained by the joint inversion compared to inversions based on phase velocities alone. In addition to imaging Vs structure, we show that the joint inversion can be used to constrain Vp/Vs and density in the upper crust. New images of uppermost crustal structure (<3 km depth) are in excellent agreement with known surface features, with pronounced low Vs, low density, and high Vp/Vs anomalies imaged in the locations of several major sedimentary basins including the Williston, Powder River, Green River, Denver, and San Juan basins. These results demonstrate not only the consistency of broadband H/V ratios and phase velocity measurements, but also that their complementary sensitivities have the potential to resolve density and Vp/Vs variations.

Shrinking of the Colorado Plateau via lithospheric mantle erosion: Evidence from Nd and Sr isotopes and geochronology of Neogene basalts

Geochronologic data from the southern margins of the Colorado Plateau (western United States) show an inboard radial migration of Neogene basaltic magmatism. Nd and Sr isotopic data show that as basaltic volcanism migrates inboard it also becomes increasingly more asthenospheric. Strongly asthenospheric alkali basalt (e Nd > 4) appeared on the western plateau margin ca. 5 Ma, on the southeastern margin at 7 Ma, and is lacking from the plateau’s other margins. Tomographic data suggest that low-velocity mantle underlies almost all recent (younger than 1 Ma) basaltic volcanism in a ring around much of the Colorado Plateau at a depth of 80 km. The combined isotopic and tomographic data indicate that the low-velocity mantle is asthenosphere along the western and southeastern margins of the plateau, but modifi ed lithosphere around the remaining margins. Temporal and spatial patterns suggest a process by which upwelling asthenosphere is progressively infi ltrating and replacing lithospheric mantle, especially where Proterozoic boundaries exist. This model explains (1) the dramatic velocity contrast seen well inboard of the physiographic edge of the plateau, (2) the inboard sweep of Neogene magmatism, and (3) isotopic evidence that much (but not all) of the low-velocity mantle is asthenospheric. These data support models that ongoing uplift of the edges of the Colorado Plateau is driven by mantle processes.

Fossil slabs attached to unsubducted fragments of the Farallon plate

As the Pacific–Farallon spreading center approached North America, the Farallon plate fragmented into a number of small plates. Some of the microplate fragments ceased subducting before the spreading center reached the trench. Most tectonic models have assumed that the subducting oceanic slab detached from these microplates close to the trench, but recent seismic tomography studies have revealed a high-velocity anomaly beneath Baja California that appears to be a fossil slab still attached to the Guadalupe and Magdalena microplates. Here, using surface wave tomography, we establish the lateral extent of this fossil slab and show that it is correlated with the distribution of high-Mg andesites thought to derive from partial melting of the subducted oceanic crust. We also reinterpret the high seismic velocity anomaly beneath the southern central valley of California as another fossil slab extending to a depth of 200 km or more that is attached to the former Monterey microplate. The existence of these fossil slabs may force a reexamination of models of the tectonic evolution of western North America over the last 30 My.

Upper mantle P velocity structure beneath the MidwesternUnited States derived from triplicated waveforms

Upper mantle seismic velocity structures in both vertical and horizontal directions are key to understanding the structure and mechanics of tectonic plates. Recent deployment of the USArray Transportable Array (TA) in the Midwestern United States provides an extraordinary regional earthquake data set to investigate such velocity structure beneath the stable North American craton. In this paper, we choose an M_w5.1 Canadian earthquake in the Quebec area, which is recorded by about 400 TA stations, to examine the P wave structures between the depths of 150 km to 800 km. Three smaller Midwestern earthquakes at closer distance to the TA are used to investigate vertical and horizontal variations in P velocity between depths of 40 km to 150 km. We use a grid-search approach to find the best 1-D model, starting with the previously developed S25 regional model. The results support the existence of an 8° discontinuity in P arrivals caused by a negative velocity gradient in the lithosphere between depths of 40 km to 120 km followed by a small (∼1%) jump and then a positive gradient down to 165 km. The P velocity then decreases by 2% from 165 km to 200 km, and we define this zone as the regional lithosphere-asthenosphere boundary (LAB). Beneath northern profiles, waves reflected from the 410 discontinuity (410) are delayed by up to 1 s relative to those turning just below the 410, which we explain by an anomaly just above the discontinuity with P velocity reduced by ∼3%. The 660 discontinuity (660) appears to be composed of two smaller velocity steps with a separation of 16 km. The inferred low-velocity anomaly above 410 may indicate high water concentrations in the transition zone, and the complexity of the 660 may be related to Farallon slab segments that have yet to sink into the deep mantle.