Robert A. Dunn

Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30'N

The seismic structure of the crust and shallow mantle beneath the East Pacific Rise near 9°3O′N is imaged by inverting P wave travel time data. Our tomographic results constrain for the first time the three-dimensional structure of the lower crust in this region and allow us to compare it to shallow crustal and mantle structure. The seismic structure is characterized by a low-velocity volume (LVV) that extends from 1.2 km depth below the seafloor into the mantle. The cross-axis width of the LVV is narrow in the crust (5–7 km) and broad in the mantle (∼18 km). Although the width of the top of the LVV is similar to previous estimates, its narrow shape at lower crustal depths and its significant widening in the mantle are previously unknown features of the rise velocity structure. In the rise-parallel direction the LVV varies in magnitude such that the lowest velocities are located between two minor rise axis discontinuities near 9°28′N and 9°35′N. From the seismic results we estimate the thermal structure and melt distribution beneath the rise. The thermal structure suggests that heat removal is relatively efficient throughout the crust yet inefficient at Moho and mantle depths. Estimates of the melt distribution indicate that magma accumulates at two levels in the magmatic system. One is at the top of the magmatic system and is capped by the shallow melt lens detected by seismic reflection surveys; the other is within the Moho transition zone and topmost portion of the mantle. The highest melt fractions occur within the upper reservoir, whereas the lower reservoir contains a lower melt fraction distributed over a broader area. By volume, however, there may be up to 40% more melt in the lower reservoir than in the upper reservoir. Along-axis variations in crustal melt content are similar to those in the mantle, supporting the hypothesis that the mantle, midway between the 9°28′N and 9°35′N devais, is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south. We see no evidence (from seismic anisotropy) for diapiric mantle flow, suggesting that solid-state flow and melt migration are decoupled in the shallow mantle. Our results are not compatible with models that require a large, segment-scale redistribution of melt within the crust. Instead, our results imply that crustal magma chambers are replenished at closely spaced intervals along the rise.

The Predictive Accuracy of Shoreline Change Rate Methods and Alongshore Beach Variation on Maui, Hawaii

Abstract Beach erosion has direct consequences for Hawaiis tourist-based economy, which depends on the attraction of beautiful sandy beaches. Within the last century, however, beaches on Oahu and Maui have been narrowed or completely lost, threatening tourism and construction development. In order for the counties and state of Hawaii to implement coastal regulations to prevent infrastructure damage, it is necessary to find a statistically valid methodology that accurately delineates annual erosion hazard rates specific to Hawaii. We compare the following erosion rate methods: end point rate (EPR), average of rates (AOR), minimum description length (MDL), jackknifing (JK), ordinary least squares (OLS), reweighted least squares (RLS), weighted least squares (WLS), reweighted weighted least squares (RWLS), least absolute deviation (LAD), and weighted least absolute deviation (WLAD). To evaluate these statistical methods, this study determines the predictive accuracy of various calculated erosion rates, including the effects of a priori knowledge of storms, using (1) temporally truncated data to forecast and hindcast known shorelines and (2) synthetic beach time series that contain noise. This study also introduces binning of adjacent transects to identify segments of a beach that have erosion rates that are indistinguishable. If major uncertainties of the shoreline methodology and storm shorelines are known, WLS, RWLS, and WLAD better reflect the data; if storm shorelines are not known, RWLS and WLAD are preferred. If both uncertainties and storm shorelines are not known, RLS and LAD are preferred; if storm shorelines are known, OLS, RLS, JK, and LAD are recommended. MDL and AOR produce the most variable results. Hindcasting results show that early twentieth century topographic surveys are valuable in change rate analyses. Binning adjacent transects improves the signal-to-noise ratio by increasing the number of data points.

Skew of mantle upwelling beneath the East Pacific Rise governs segmentation

Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10° anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.

Seismological evidence for three-dimensional melt migration beneath the East Pacific rise

The extent to which crustal processes along mid-ocean ridges are controlled by either the pattern of mantle upwelling or the mode of magma injection into the crust is not known. Models of mantle upwelling vary from two-dimensional, passive flow to three-dimensional, diapiric flow. Similarly, beneath a ridge segment bounded by tectonic offsets, crustal magma chambers may be replenished continuously along the ridge or at a central injection zone from which magma migrates towards the segments ends. Here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle beneath the East Pacific Rise. The anisotropy is consistent with two-dimensional mantle flow diverging from the rise, whereas the anomalous isotropic structure requires a three-dimensional but continuous distribution of melt near the crust–mantle interface. Our results indicate that crustal magma chambers are replenished at closely spaced intervals along-axis and that crustal systems inherit characteristics of scale from melt transport processes originating in the mantle.

Contrasting crustal production and rapid mantle transitions beneath back-arc ridges

The opening of back-arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading. During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate). Current models predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In contrast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5 km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9 km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge.

Evaluating hot spot–ridge interaction in the Atlantic from regional‐scale seismic observations

We probe variations in mantle temperature, composition, and fabric along hot spot–influenced sections of the Mid-Atlantic Ridge (MAR), using surface waves from nearby ridge earthquakes recorded on broadband island-based seismic stations. We invert frequency-dependent phase delays from these events to estimate one-dimensional mean shear velocity and radial shear anisotropy profiles in the upper 200 km of the mantle within two seafloor age intervals: 5–10 Ma and 15–20 Ma. Mean shear velocity profiles correlate with apparent hot spot flux: lithosphere formed near the low-flux Ascension hot spot is characterized by high mantle velocities, while the MAR near the higher-flux Azores hot spot has lower velocities. The impact of the high-flux Iceland hot spot on mantle velocities along the nearby MAR is strongly asymmetric: the lithospheric velocities near the Kolbeinsey ridge are moderately slow, while velocities near the Reykjanes ridge estimated in previous studies are much slower. Within each region the increase in shear velocity with age is consistent with a half-space cooling model, and the velocity variations observed between Ascension, the Azores, and Kolbeinsey are consistent with approximately ±75° potential-temperature variation among these sites. In comparison, the Reykjanes lithosphere is too slow to result purely from half-space cooling of a high-temperature mantle source. We speculate that the anomalously low shear velocities within the lithosphere produced at the Reykjanes ridge result from high asthenospheric temperatures of +50–75 K combined with ∼12% (by volume) gabbro retained in the mantle due to the imbalance between high hot spot–influenced melt production and relatively inefficient melt extraction along the slow spreading Reykjanes. Radial shear anisotropy in the upper 150 km also indicates an apparent hot spot influence: mantle fabric near Ascension is quite weak, consistent with previous models of anisotropy produced by corner flow during slow seafloor spreading. The fabric near the Azores and the Kolbeinsey ridge is stronger, suggesting that the hot spot increases mantle deformation beyond that produced by slow seafloor spreading in these regions.

Building Domain-Specific Environments for Computational Science: a Case Study in Seismic Tomography

We report on our experiences in building a computational environment for tomographic image analysis for marine seismologists studying the structure and evolution of mid- ocean ridge volcanism. The computational environment is determined by an evolving set of requirements for this problem domain and includes needs for high performance parallel computing, large data analysis, model visualiza tion, and computation interaction and control. Although these needs are not unique in scientific computing, the integration of techniques for seismic tomography with tools for parallel computing and data analysis into a com putational environment was (and continues to be) an interesting, important learning experience for researchers in both disciplines. For the geologists, the use of the environment led to fundamental geologic discoveries on the East Pacific Rise, the improvement of parallel ray-trac ing algorithms, and a better regard for the use of compu tational steering in aiding model convergence. The com puter scientists received valuable feedback on the use of programming, analysis, and visualization tools in the en vironment. In particular, the tools for parallel program data query (DAQV) and visualization programming (Viz) were demonstrated to be highly adaptable to the problem do main. We discuss the requirements and the components of the environment in detail. Both accomplishments and limitations of our work are presented.

Seismic evidence of effects of water on melt transport in the Lau back-arc mantle

Processes of melt generation and transport beneath back-arc spreading centres are controlled by two endmember mechanisms: decompression melting similar to that at mid-ocean ridges and flux melting resembling that beneath arcs. The Lau Basin, with an abundance of spreading ridges at different distances from the subduction zone, provides an opportunity to distinguish the effects of these two different melting processes on magma production and crust formation. Here we present constraints on the three-dimensional distribution of partial melt inferred from seismic velocities obtained from Rayleigh wave tomography using land and ocean-bottom seismographs. Low seismic velocities beneath the Central Lau Spreading Centre and the northern Eastern Lau Spreading Centre extend deeper and westwards into the back-arc, suggesting that these spreading centres are fed by melting along upwelling zones from the west, and helping to explain geochemical differences with the Valu Fa Ridge to the south, which has no distinct deep low-seismic-velocity anomalies. A region of low S-wave velocity, interpreted as resulting from high melt content, is imaged in the mantle wedge beneath the Central Lau Spreading Centre and the northeastern Lau Basin, even where no active spreading centre currently exists. This low-seismic-velocity anomaly becomes weaker with distance southward along the Eastern Lau Spreading Centre and the Valu Fa Ridge, in contrast to the inferred increase in magmatic productivity. We propose that the anomaly variations result from changes in the efficiency of melt extraction, with the decrease in melt to the south correlating with increased fractional melting and higher water content in the magma. Water released from the slab may greatly reduce the melt viscosity or increase grain size, or both, thereby facilitating melt transport.

Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system

Hydrothermal circulation at mid-ocean ridge volcanic segments extracts heat from crustal magma bodies. However, the heat source driving hydrothermal circulation in ultramafic outcrops, where mantle rocks are exhumed in low-magma-supply environments, has remained enigmatic. Here we use a three-dimensional P -wave velocity model derived from active-source wide-angle refraction-reflection ocean bottom seismometer data and pre-stack depth-migrated images derived from multichannel seismic reflection data to investigate the internal structure of the Rainbow ultramafic massif, which is located in a non-transform discontinuity of the Mid-Atlantic Ridge. Seismic imaging reveals that the ultramafic rocks composing the Rainbow massif have been intruded by a large number of magmatic sills, distributed throughout the massif at depths of ∼2–10 km. These sills, which appear to be at varying stages of crystallization, can supply the heat needed to drive high-temperature hydrothermal circulation, and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting. Our results demonstrate that high-temperature hydrothermal systems can be driven by heat from deep-sourced magma even in exhumed ultramafic lithosphere with very low magma supply.

Crust and Lithospheric Structure – Seismic Structure of Mid-Ocean Ridges

Along mid-ocean ridges oceanic plates separate, inducing mantle upwelling. The upwelling mantle undergoes pressure-release partial melting because the temperature of the solidus decreases with decreasing pressure. The newly formed melt, being less viscous and less dense than the surrounding solid, segregates from the residual mantle matrix and buoyantly rises toward the surface where it forms new oceanic crust. Seismological studies have been designed to determine the pattern of mantle upwelling and melting, the pattern of melt migration to the base of the crust, the crustal magmatic system, the final destination of the melt recorded by crustal thickness and structure, and the evolution of the lithosphere as the plates separate. Key influences on mid-ocean ridge structure and crustal formation are spreading rate, mantle temperature, and mantle composition.