The mean composition of ocean ridge basalts Gale, Allison; Dalton, Colleen A.; Langmuir, Charles H. ...
Geochemistry, geophysics, geosystems : G3,
03/2013, Volume:
14, Issue:
3
Journal Article
Peer reviewed
Open access
The mean composition of mid‐ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data. A global ...catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. “ALL MORB” is the total composition of the crust apart from back‐arc basins, N‐MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D‐MORB the depleted end‐member. ALL MORB and N‐MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back‐arc spreading centers requires higher extents of melting and greater concentrations of fluid‐mobile elements, reflecting the influence of water on back‐arc petrogenesis. The average data permit a re‐evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ± 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of all morb and N‐MORB provide constraints on the hypothesis that Earth has a non‐chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.
Key points
We present a carefully compiled MORB database, unparalleled in size and coverageWe rigorously establish the mean composition of MORB
The temperature and composition of Earth's mantle control fundamental planetary properties, including the vigor of mantle convection and the depths of the ocean basins. Seismic wave velocities, ocean ...ridge depths, and the composition of mid-ocean ridge basalts can all be used to determine variations in mantle temperature and composition, yet are typically considered in isolation. We show that correlations among these three data sets are consistent with 250°C variation extending to depths >400 kilometers and are inconsistent with variations in mantle composition at constant temperature. Anomalously hot ridge segments are located near hot spots, confirming a deep mantle-plume origin for hot spot volcanism. Chemical heterogeneity may contribute to scatter about the global trend. The coherent temperature signal provides a thermal calibration scale for interpreting seismic velocities located distant from ridges.
Love Wave Tomography of the United States Hariharan, Anant; Dalton, Colleen A.
Geophysical research letters,
28 December 2022, Volume:
49, Issue:
24
Journal Article
Peer reviewed
Open access
Love wave phase velocity maps provide essential constraints on radial anisotropy and deformation in the crust and upper mantle. However, the phenomenon of overtone interference causes scatter and ...systematic bias in the velocity measurements and impedes efforts to image small‐scale anisotropic variations. We develop an approach for identifying Love wave measurements that are biased by overtone interference, demonstrate its efficacy with EarthScope USArray data, and determine the first earthquake‐derived Love wave phase velocity maps for the entire conterminous U.S. in the period range 35–75 s. We show that radial anisotropy in parts of the crust and most of the lithospheric mantle is necessary to reconcile these maps with Rayleigh wave phase velocities. Our results convey the impact and geographic variability of overtone interference, offer an easy‐to‐implement method to ameliorate this impact, and present high‐resolution constraints on radial anisotropy beneath North America.
Plain Language Summary
Measurements of Love wave phase speeds at different frequencies provide unique information that can help understand how the Earth's interior is deforming. These measurements are difficult to make because other seismic waves interfere with the one we are attempting to measure, resulting in low‐quality and biased measurements. Here, we calculate the times at which the different waves arrive and use these times to identify and remove measurements impacted by interference. The resulting cleaned data set increases the precision of derived images of the Earth's interior and reduces bias in these images. Our images suggest shear waves polarized in vertical and horizontal directions are required to have different speeds.
Key Points
We describe an approach to eliminate overtone interference and improve the quality of Love wave measurements
We present the first earthquake‐derived Love wave phase velocity maps of the conterminous U.S., at periods up to and including 75 s
Our maps require radial anisotropy in many regions of the crust and upper mantle beneath the U.S.
The rate of ocean‐crust production exerts control over sea level, mantle heat loss, and climate. Different strategies to account for incomplete seafloor preservation have led to differing conclusions ...about how much production rates have changed since the Cretaceous, if at all. We construct a new global synthesis of crust production along 18 mid‐ocean ridges for the past 19 Myr at high temporal resolution. We find that the global production rate during 6–5 Ma was only 69%–75% of the 16–15 Ma interval. The reduction in crust production is mostly due to slower seafloor spreading along almost all ridge systems. While the total ridge length has varied little since 19 Ma, some fast‐spreading ridges have grown shorter and slow‐spreading ridges grown longer, amplifying the spreading‐rate changes. Our production curves represent a new data set for investigating the forces driving plate motions and the role of tectonic degassing on climate.
Plain Language Summary
The question of whether the speeds of tectonic plates vary over time is controversial but has big‐picture implications for our understanding of the forces inside the Earth that drive the plates, the role of volcanoes in controlling climate change over millions of years, and the rise and fall of sea level. At mid‐ocean ridges, two plates move apart, and the volcanic rocks that comprise the ocean crust are created. Magnetic minerals in the rocks record their age of formation and therefore the relative speeds of the diverging plates. However, this record is incomplete because seafloor is destroyed at subduction zones. We used the preserved seafloor magnetic record to calculate diverging plate speeds over the past 19 million years. We find that the relative plate speed at almost all divergent plate boundaries has slowed down, with a major inflection point at 15–16 Myr. As a result, the rate at which new ocean crust is created also slowed down, by roughly 35%. We speculate that there is not a single explanation for the nearly global slowdown in plate speeds but rather several unrelated tectonic events, such as the emergence of the Andes.
Key Points
A new global synthesis of seafloor‐spreading rates at high temporal resolution is presented
Since 15 Ma, spreading rate decreased along 15 of 18 major ridge systems, with a total global reduction of 37%
High‐resolution reconstructions, new data for eastern Pacific, and astronomical ages allow more detail than previous studies
The resolution of and level of agreement between different attenuation models have historically been limited by complexities associated with extracting attenuation from seismic-wave amplitudes, which ...are also affected by the source, the receiver, and propagation through velocity heterogeneities. For intermediate- and long-period Rayleigh waves, removing the amplitude signal due to focusing and defocusing effects is the greatest challenge. In this paper, three independent data sets of fundamental-mode Rayleigh wave amplitude are analyzed to investigate how three factors contribute to discrepancies between the attenuation models: uncertainties in the amplitude measurements themselves, variable path coverage, and the treatment of focusing effects. Regionalized pure-path and fully two-dimensional attenuation models are derived and compared. The approach for determining attenuation models from real data is guided by an analysis of amplitudes measured from synthetic spectral-element waveforms, for which the input Earth model is perfectly known. The results show that differences in the amplitude measurements introduce only very minor differences between the attenuation models; path coverage and the removal of focusing effects are more important. The pure-path attenuation values exhibit a clear dependence on tectonic region at shorter periods that disappears at long periods, in agreement with pure-path phase-velocity results obtained by inverting Rayleigh wave phase delays. The 2-D attenuation maps are highly correlated with each other to spherical-harmonic degree 16 and can resolve smaller features than the previous generation of global attenuation models. Anomalously low attenuation is nearly perfectly associated with continental cratons. Variations in lithospheric thickness are determined by forward modeling the global attenuation variations as a thermal boundary layer of variable thickness. Temperature profiles that satisfy the attenuation values systematically overpredict and underpredict Rayleigh wave phase velocity in cratons at short and long periods, respectively. Introducing a low-velocity layer at depths 60–80 km and a high-velocity layer that begins at 200 km can resolve the discrepancy. The former is consistent with receiver-function detections of a mid-lithospheric discontinuity, and the latter may correspond to the Lehmann discontinuity.
•Three independent sets of Rayleigh wave amplitude yield highly similar attenuation models.•Short-wavelength attenuation structure can be reliably recovered.•Areas of low attenuation are nearly perfectly associated with continental cratons.•Frequency-dependent attenuation values are modeled as thermal boundary layers.•Geotherms that satisfy attenuation and xenolith constraints do not satisfy phase-velocity observations.
SUMMARY
Seismic tomography is a cornerstone of geophysics and has led to a number of important discoveries about the interior of the Earth. However, seismic tomography remains plagued by the large ...number of unknown parameters in most tomographic applications. This leads to the inverse problem being underdetermined and requiring significant non-geologically motivated smoothing in order to achieve unique answers. Although this solution is acceptable when using tomography as an explorative tool in discovery mode, it presents a significant problem to use of tomography in distinguishing between acceptable geological models or in estimating geologically relevant parameters since typically none of the geological models considered are fit by the tomographic results, even when uncertainties are accounted for. To address this challenge, when seismic tomography is to be used for geological model selection or parameter estimation purposes, we advocate that the tomography can be explicitly parametrized in terms of the geological models being tested instead of using more mathematically convenient formulations like voxels, splines or spherical harmonics. Our proposition has a number of technical difficulties associated with it, with some of the most important ones being the move from a linear to a non-linear inverse problem, the need to choose a geological parametrization that fits each specific problem and is commensurate with the expected data quality and structure, and the need to use a supporting framework to identify which model is preferred by the tomographic data. In this contribution, we introduce geological parametrization of tomography with a few simple synthetic examples applied to imaging sedimentary basins and subduction zones, and one real-world example of inferring basin and crustal properties across the continental United States. We explain the challenges in moving towards more realistic examples, and discuss the main technical difficulties and how they may be overcome. Although it may take a number of years for the scientific program suggested here to reach maturity, it is necessary to take steps in this direction if seismic tomography is to develop from a tool for discovering plausible structures to one in which distinct scientific inferences can be made regarding the presence or absence of structures and their physical characteristics.
While variations in crustal structure beneath the Denali fault in Alaska are well‐documented, the existence of fault‐correlated structures throughout the entire thickness of the continental ...lithosphere is not. A new model of shear‐wave velocity structure obtained through joint inversion of surface wave and converted body wave data shows a northward increase in lithospheric thickness and velocity occurring across the Denali fault system. In northern Alaska, a dramatic increase in lithospheric thickness at the southern margin of the Arctic‐Alaska terrane lies in the vicinity of the Kobuk fault system. These correlations support the view that transpressive deformation tends to localize at the margins of thicker, higher‐strength lithosphere.
Plain Language Summary
Major faults in Earth’s tectonic plates, such as Alaska's Denali fault, begin as brittle fractures of the shallow crust and progress to include ductile shearing of the deeper crust. Changes in tectonic plate thickness or internal temperature have been observed beneath some faults, consistent with feedback between the strength of the deeper plate and the location of the fault. Using global earthquake signals, we built a new 3‐D model of seismic velocity structure throughout Alaska, extending from the surface to hundreds of kilometers in depth. According to the model, the Denali fault coincides with a northward thickening and an apparent abrupt temperature increase within Alaska's plate. Further north, at another boundary between geological provinces near the Kobuk fault, we also find an abrupt thickening of the plate. These relationships are consistent with faults being located near the edges of thicker, stronger regions of the tectonic plates.
Key Points
A new model of seismic shear‐wave velocity beneath Alaska was obtained by jointly inverting surface wave and Sp body wave data
The Denali fault lies just south of the margin of thicker and higher velocity upper plate lithosphere, as does the Kobuk fault
These results support the view that gradients in lithospheric strength can be key to the localization of major continental strike‐slip faults
SUMMARY
Measurements of fundamental-mode (FM) surface waves along the minor arc are impacted by overtone interference. This interference is primarily due to major-arc overtones for Rayleigh waves and ...minor-arc overtones for Love waves. In both cases, interference contaminates measurements of phase and amplitude and can introduce bias in seismic images. Here, we use synthetic seismograms computed via normal mode summation to probe how interference can vary as a function of the surface wave group velocities, source mechanism and depth, which control the relative excitation of the FM and overtones, and period. By comparing seismograms that include all overtones to those that include only the FM, we can quantify the interference, i.e., how the presence of the overtones perturbs the FM phase and amplitude. We compare the strength of this interference to calculations of excitation of the overtones and FM. We show that these calculations explain well the varying strength of interference for different source mechanisms and depths. Notably, these calculations illuminate source depths where Love wave overtone excitation is quite low and therefore interference is unusually weak, and depths where Rayleigh wave FM excitation is low and therefore interference is unusually strong. Our analysis also reinforces the dependence of the interference on the FM and overtone group velocities. For Love waves, this results in weak minor-arc overtone interference at long periods and, for continental paths, short periods. For Rayleigh waves, the differing overtone and FM group velocities and the relative excitation of the overtones and FM explain rapid variations in the strength of Rayleigh wave major-arc overtone interference as a function of epicentral distance. We then show that real data are affected by the relative excitation of the FM and overtones. We find that errors in Rayleigh wave phase velocities determined at the EarthScope USArray stations are larger when the ratios of overtone to FM excitation are larger. We also find a dependence of phase velocity error on excitation ratio for Love waves, and we identify the presence of major-arc overtone interference in Love wave measurements. Our results highlight opportunities for more nuanced quality control of surface wave measurements. The relative excitation ratio of the overtones and FM may be a better criterion for event selection than source depth, allowing deeper events that well excite FMs to be included and shallower events with large overtone excitation to be excluded. This would allow the collection of more accurate measurements that will increase the precision of seismic images.
How seismic waves lose energy Dalton, Colleen A.
Science (American Association for the Advancement of Science),
12/2017, Volume:
358, Issue:
6370
Journal Article
Peer reviewed
Energy loss varies with frequency in the lithosphere but not in the asthenosphere
Ocean basins record the life history of a tectonic plate—its creation at a mid-ocean ridge, its thickening over time, ...and its consumption at a subduction zone. The movement of tectonic plates is possible because the lithosphere, Earth's stiff outermost shell, slides on top of a weak asthenosphere. Despite its fundamental role in facilitating plate tectonics, the nature of the lithosphere-asthenosphere boundary is poorly understood. The asthenosphere is on average warmer than the lithosphere, but the temperature contrast alone may not provide the necessary viscosity reduction. Previous work has also proposed a dehydrated lithosphere and damp asthenosphere (
1
), and a solid lithosphere and partially molten asthenosphere (
2
). On page 1593 of this issue, Takeuchi
et al.
(
3
) present an analysis of aftershocks of the 2011 Tohoku earthquake and show how the attenuation of seismic waves has a different frequency response in the lithosphere versus the asthenosphere.
Summary
We present a methodology for 1-D imaging of upper-mantle structure using a Bayesian approach that incorporates a novel combination of seismic data types and an adaptive parametrization based ...on piecewise discontinuous splines. Our inversion algorithm lays the groundwork for improved seismic velocity models of the lithosphere and asthenosphere by harnessing the recent expansion of large seismic arrays and computational power alongside sophisticated data analysis. Careful processing of P- and S-wave arrivals isolates converted phases generated at velocity gradients between the mid-crust and 300 km depth. This data is allied with ambient noise and earthquake Rayleigh wave phase velocities to obtain detailed V S and V P velocity models. Synthetic tests demonstrate that converted phases are necessary to accurately constrain velocity gradients, and S–p phases are particularly important for resolving mantle structure, while surface waves are necessary for capturing absolute velocities. We apply the method to several stations in the northwest and north-central United States, finding that the imaged structure improves upon existing models by sharpening the vertical resolution of absolute velocity profiles, offering robust uncertainty estimates, and revealing mid-lithospheric velocity gradients indicative of thermochemical cratonic layering. This flexible method holds promise for increasingly detailed understanding of the upper mantle.
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