Hersh Joseph Gilbert

Active foundering of a continental arc root beneath the southern Sierra Nevada in California

Seismic data provide images of crust–mantle interactions during ongoing removal of the dense batholithic root beneath the southern Sierra Nevada mountains in California. The removal appears to have initiated between 10 and 3u2009Myr ago with a Rayleigh–Taylor-type instability, but with a pronounced asymmetric flow into a mantle downwelling (drip) beneath the adjacent Great Valley. A nearly horizontal shear zone accommodated the detachment of the ultramafic root from its granitoid batholith. With continuing flow into the mantle drip, viscous drag at the base of the remaining ∼35-km-thick crust has thickened the crust by ∼7u2009km in a narrow welt beneath the western flank of the range. Adjacent to the welt and at the top of the drip, a V-shaped cone of crust is being dragged down tens of kilometres into the core of the mantle drip, causing the disappearance of the Moho in the seismic images. Viscous coupling between the crust and mantle is therefore apparently driving present-day surface subsidence.

Receiver functions in the western United States, with implications for upper mantle structure and dynamics

[1]xa0Investigations into mechanisms driving surface tectonics commonly search for mantle sources, but few observations constrain flow in the upper mantle and transition zone. Here variations in the upper mantle discontinuities at 410 km and 660 km below the western United States are revealed through mapping depths of compressional-to-shear wave conversions recorded by broadband seismometers. The resulting image exhibits 20 and 30 km of topography on the 410- and 660-km discontinuities, similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone. The pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the Rocky Mountains, Colorado Plateau, or Basin and Range Province, providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region, at least at scale lengths of a few hundred kilometers. Furthermore, we find no clear correlation between the depths of the 410- and 660-km discontinuities. Undulations on the surfaces of both discontinuities appear to be spaced at distances of ∼800 km. If the topography were due only to lateral temperature differences, such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone. Alternatively, the topography may reflect lateral variations in composition. Variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity, but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth.

Images of crustal variations in the intermountain west

[1]xa0We develop a map of crustal thickness variations across the Great Basin, Colorado Plateau, Rocky Mountain, and Great Plains Provinces of the western United States using common conversion point stacking of teleseismic receiver functions. Below the Rocky Mountains and High Plains in Colorado we find the thickest crust in the region at 45–50 km thick. Beneath the Basin and Range, thinner, between 30 and 40 km, crust is found. Thin, 30 km thick, crust is present in the northern portion of Nevada and Utah despite elevations similar to those farther south. Crustal thickness across the Colorado Plateau can be characterized as a broad transitional region between the thin crust of Basin and Range to the thicker crust of the Rocky Mountains. The impedance contrast across the Mohorovicic discontinuity decreases below the Colorado Plateau, as converted arrivals recorded in this region appear weak compared to surrounding areas. Variations in VP/VS across the region indicate higher values along the western boundary of the Basin and Range, in the Rocky Mountains, and in the western Great Plains. We are not able to characterize VP/VS in the Colorado Plateau. We find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole. Differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western United States.

Single-chamber silicic magma system inferred from shear wave discontinuities of the crust and uppermost mantle, Coso geothermal area, California

[1]xa0Analysis of seismograms from teleseismic rays traversing the Coso geothermal area near Ridgecrest, California, suggests the geothermal system lies over a single shallow magma reservoir (∼5 km below the surface) that also plays a crucial role in the local change in deformation style from areas to the north and west. The character of the magma reservoir and the absence of a lower crustal magma reservoir is inferred from three crustal P-to-S conversions observed using receiver function analysis: (1) A high-amplitude, shallow, negative arrival, Ps-P time of 0.7–0.9 s (3–5 km below sea level (bsl)), (2) a moderate amplitude, positive conversion, Ps-P time of 2.1–2.5 s (14–17 km bsl), and (3) the Moho conversion, Ps-P time of 4.0–4.2 s (30–32 km bsl). Observations of Moho converted arrivals indicate that the interface is mostly flat and uncomplicated throughout the study area, while the midcrustal conversion is laterally variable in amplitude and depth. The absence of the large negative amplitude conversion on waveforms recorded at stations outside the geothermal area strongly suggests that the feature lies only underneath the modern geothermal area. In addition, rays sampling the shallow converter also contain later arrivals with retrograde moveout consistent with an origin as reverberations above the conversion. Receiver functions calculated from synthetic data using a single isotropic layer over a half-space indicates that the shear velocity decreases by 30% across the interface (VS1 = 2.6 km/s; VS2 = 1.8 km/s; layer one thickness 4.9 km), further supporting the presence of shallow magma.

Upper mantle discontinuity structure in the region of the Tonga Subduction Zone

We study the mantle structure below the southwest Pacific in order to examine the geometry of the Tonga slab at depth and its interaction with the 410- and 660-km discontinuities (hereafter called the 410 and the 660). We utilize data from stations of both the Lau Basin Ocean Bottom Seismogram experiment and island stations of the Southwest Pacific Seismic Experiment. The tectonic complexity of this region, containing both the Tonga subduction zone and the associated Lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions. We produce a high-resolution image of the upper mantle near the Tonga subduction zone to show the interaction between the discontinuities and the subducting slab. Our results show the 410 uplifted by 30 km near the Tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions.

Crustal images from San Juan, Argentina, obtained using high frequency local event receiver functions

[1]xa0Images of the crust and upper mantle obtained using teleseismic receiver functions exhibit a weak P-S conversion from the Moho in Western Argentina, where a segment of the subducting Nazca plate flattens out beneath the overriding South American plate. To better estimate depth to the Moho and search for mid-crustal impedance contrasts, we calculate high frequency receiver functions using 37 intermediate-depth local earthquakes. Radial receiver functions from a station located near San Juan, Argentina, show a strong signal from the Moho at 7 seconds corresponding to a depth near 50 km, and conversions from interfaces within the crust at depths of ∼20 and 35 km. The higher frequency results obtained using local events have twice the vertical resolution of the teleseismic results and the combined data sets indicate a gradational increase in S velocity in the lower crust that cannot be detected in the lower frequency teleseismic data alone.

The basement revealed: Tectonic insight from a digital elevation model of the Great Unconformity, USA cratonic platform

Across much of North America, the contact between Precambrian basement and Paleozoic strata is the Great Unconformity, a surface that represents a >0.4 b.y.-long hiatus. A digital elevation model (DEM) of this surface visually highlights regional-scale variability in the character of basement topography across the United States cratonic platform. Specifically, it delineates Phanerozoic tectonic domains, each characterized by a distinct structural wavelength (horizontal distance between adjacent highs) and/or structural amplitude (vertical distance between adjacent lows and highs). The largest domain, the Midcontinent domain, includes long-wavelength epeirogenic basins and domes, as well as fault-controlled steps. The pronounced change in land-surface elevation at the Rocky Mountain Front coincides with the western edge of the Midcontinent domain on the basement DEM. In the Rocky Mountain and Colorado Plateau domains, west of the Rocky Mountain Front, structural wavelength is significantly shorter and structural amplitude significantly higher than in the Midcontinent domain. The Bordering Basins domain outlines the southern and eastern edges of the Midcontinent domain. As emphasized by the basement DEM, several kilometers of structural relief occur across the boundary between these two domains, even though this boundary does not stand out on ground-surface topography. A plot of epicenters on the basement DEM supports models associating intraplate seismicity with the Midcontinent domain edge. Notably, certain changes in crustal thickness also coincide with distinct changes in basement depth.

Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

NSF [EAR-0738935, EAR-0739001, EAR-1565475]; Colorado College Patricia Buster Scholarship Fund; National Science Foundation through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation [EAR-1261681]