Sea Beam Data Research Articles

Seismicity Associated with the May 2010 Eruption of South Sarigan Seamount, Northern Mariana Islands

On 29 May 2010, at approximately 11:48 UTC, an undersea volcano south of Sarigan in the Northern Mariana Islands (Figs. 1 and 2) erupted sending a cloud of volcanic ash and water vapor to 40,000 feet (12 km; Washington Volcanic Ash Advisory Center). Bathymetric data (Stern and Smoot, 1998; Embley et al. , 2007) indicate an undersea vent exists at approximately 16.582° N and 145.821° E or about 12 km southwest of Sarigan and about 30 km northeast of Anatahan (Fig. 3). The vent depth is estimated to be 184 m below sea level. Based on physical observations of water discoloration (Juan Camacho, personal comm., 2010) and satellite imagery, the eruption plume is attributed to this undersea seamount. Figure 1. Map showing locations of islands and seismic stations in the Northern Mariana Island (NMI) chain. The names of active volcanoes are underlined. The location of the South Sarigan seamount is marked with an X. Inset shows the location of the NMI relative to Japan, Asia, the Philippine Islands, and New Guinea. Figure 2. Map showing the location of undersea vent that was the site of the 29 May 2010 eruption relative to the islands of Anatahan and Sarigan. Triangles, locations of seismic stations SARN, ANNE, and ANA2. Figure 3. Bathymetry of South Sarigan seamount and vicinity. Note data gap over part of summit. Individual minimum sounding are noted for some peaks. Contour interval is 200 m with 60 m grid‐cell size. Multibeam bathymetry data sets compiled in the area of Sarigan include: National Oceanographic and Atmospheric Administration (NOAA) Coral Reef Ecosystem Division EM300 and Reson 8101 surveys (AHI‐07‐02, AHI‐07‐03, HI‐07‐02, HI‐07‐03); NOAA Vents Program EM300 (TN153 and TN167); SeaBeam 2100 data (MGLN02MV, Onnuri); Japan Agency for Marine‐Earth Science and Technology (JAMSTEC) Seabat data (NT0518); National Geophysical Data Center (NGDC) Seabeam data (1 line north of data gap). Figure and caption courtesy of Robert Embley, …

Generalized Whittle–Matérn random field as a model of correlated fluctuations

This paper considers a generalization of the Gaussian random field with covariance function of the Whittle–Matérn family. Such a random field can be obtained as the solution to the fractional stochastic differential equation with two fractional orders. Asymptotic properties of the covariance functions belonging to this generalized Whittle–Matérn family are studied, which are used to deduce the sample path properties of the random field. The Whittle–Matérn field has been widely used in modeling geostatistical data such as sea beam data, wind speed, field temperature and soil data. In this paper we show that the generalized Whittle–Matérn field provides a more flexible model for wind speed data.

Tectonic/volcanic segmentation and controls on hydrothermal venting along Earth's fastest seafloor spreading system, EPR 27°–32°S

We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL‐120 side‐scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading (∼150 km/Myr) part of the global mid‐ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large‐scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side‐scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in‐phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL‐120 instrument package, as well as CTDO tow‐yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific‐Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high‐resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

Constraining Navigation by Matching Swath Bathymetry and Gravity Measurements at Ship Track Crossovers

A method which utilizes the lateral offset information obtained by comparing swath bathymetric data at track crossover points as a further constraint on the navigation is presented. The method, based on generalized least squares inversion theory, derives a new navigational solution that minimizes the overall misfit between the pairs of topography at crossovers while trying to remain smooth and close to the starting model. To achieve a high numerical efficiency during inversions of large matrices, we employed sparse matrix algorithms. The inversion scheme was applied to a set of Sea Beam data collected over the East Pacific Rise near 9° 30' N in early 1988 at the time when the Global Positioning System had limited coverage. The starting model was constructed by taking evenly spaced samples of positions along the tracklines. For each one of the 361 crossovers, we gridded the bathymetric data around the crossover point compared the gridded maps, and calculated the offset and uncertainty associated with this estimation. A suite of inversion solutions were obtained depending on the choice of three free parameters (that is, the a priori model variance, the correlation interval of a priori model, and the trade-off coefficient between fitting the data and remaining close to the a priori model). The best solution was chosen as one that minimizes both the Sea Beam topography and free-air gravity anomaly differences at crossovers. The improvement was significant; the initial rms mismatch between the tracks and free-air gravity anomalies at crossovers was reduced from 610m to 75m and from 2.5mGal to 1.9mGal, respectively.

Objective classification of oceanic ridge-crest terrains using two-dimensional spectral models of bathymetry: Application to the Juan de Fuca Ridge

An important application of detailed bathymetric mapping is the interpretation of geological processes based on the nature of the fine-scale morphology of the seafloor. This interpretation is usually accomplished through qualitative analysis of contour maps. In this paper, an objective classification technique, based on a two-dimensional spectral model of bathymetry developed by Fox and Hayes (1985) is applied to detailed Sea Beam data from the Juan de Fuca Ridge. Parameters of the model can be directly related to seafloor properties corresponding to 1) isotropic (non-directionally dependent) roughness; 2) anisotropic (directionally dependent) roughness; 3) orientation of the anisotropic component; and 4) spectral rool-off ( ∼ fractal dimension), by fitting the model surface to two-dimensional amplitude spectra of bathymetry determined on a regular grid over the study area. A test area was selected which encompasses the southern volcanic rift zone of Axial Volcano and the northern terminus of the Vance Segment. Parameters of the model clearly define the contrast between the constructional volcanic terrain (rough, isotropic, with high fractal dimension) and the tectonic extensional terrain (smoother, anisotropic, with low fractal dimension). An agglomerative, hierarchical cluster analysis is applied to the data, independent of spatial information, to delineate groups of spectra with similar characteristics. Distinct, mappable regions, corresponding to volcanic and tectonic provinces, are objectively determined. Also, coherent sub-regions of consistent spectral properties occur within the larger volcanic/tectonic divisions. The classification is extended to the Juan de Fuca Ridge system from 44°30′ N to 47°20′ N through combining these results with an a priori technique (K-means clustering). Broad-scale physiographic regions of the Juan de Fuca Ridge are delineated by the technique, which may aid geologists in the interpretation of crustal accretion processes at seafloor spreading centers.

Segmentation and morphotectonic variations of the Central Indian Ridge (21°10′S–22°25′S)

Bathymetric, magnetic, and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the Central Indian Ridge (CIR) between 21°10′S and 22°25′S. Volcanoes, abyssal hills, and steep slopes, corresponding to fault scarps and to volcanic constructional slopes, have been obtained by image processing methods applied to Sea Beam data. In the survey area, segmentation of the CIR is similar to that of a slow spreading ridge. The plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions. Hourglass‐shaped valleys are bounded by second and third‐order discontinuities located at local along‐axis depth maxima (400–700 m depth variations) and marked by oblique extensional basins and en échelon jogs, respectively. Large‐throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places. Along the axial valley, the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs. The morphology and structure of the rift valley reflect the three‐dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center. Abyssal hills are bounded by both inward and outward facing faults in ∼1.5 Ma crust, indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20–40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills. Reconstructions of the plate boundary geometry through time, using abyssal hills as isochrons, provide evidence to show that the nontransform discontinuity, located at 21°47′S, with an offset as small as 2–3 km, is associated with V‐shaped off‐axis traces in the form of alignments of disrupted abyssal hill lineations. These reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery. Three‐dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0.780 and 0.984 Ma and suggests postaccretionary crustal block rotations within the discordant zones.

Structural analysis of the segmentation of the Central Indian Ridge between 20�30?S and 25�30?S (Rodriguez Triple Junction)

The morphological characteristics of the segmentation of the Central Indian Ridge (CIR) from the Indian Ocean Triple Junction (25°30′S) to the Egeria Transform Fault system (20°30′S) are analyzed. The compilation of Sea Beam data from R/VSonne cruises SO43 and SO52, and R/VCharcot cruises Rodriguez 1 and 2 provides an almost continuous bathymetric coverage of a 450-km-long section of the ridge axis. The bathymetric data are combined with a GLORIA side-scan sonar swath to visualize the fabric of the ridge and complement the coverage in some areas. This section of the CIR has a full spreading rate of about 50 mm yr−1, increasing slightly from north to south. The morphology of the CIR is generally similar to that of a slow-spreading center, despite an intermediate spreading rate at these latitudes. The axis is marked by an axial valley 5–35 km wide and 500–1800 m deep, sometimes exhibiting a 100–600 m-high neovolcanic ridge. It is offset by only one 40km offset transform fault (at 22°40′S), and by nine second-order discontinuities, with offsets varying from 4 to 21 km, separating segments 28 to 85 km long. The bathymetry analysis and an empirical orthogonal function analysis performed on across-axis profiles reveal morphologic variations in the axis and the second-order discontinuities. The ridge axis deepens and the relief across the axial valley increases from north to south. The discontinuities observed south of 22°S all have morphologies similar to those of the slow-spreading Mid-Atlantic Ridge. North of 22°S, two discontinuities have map geometries that have not been observed previously on slow-spreading ridges. The axial valleys overlap, and their tips curve toward the adjacent segment. The overlap distance is 2 to 4 times greater than the offset. Based on these characteristics, these discontinuities resemble overlapping spreading centers (OSCs) described on the fast-spreading EPR. The evolution of one such discontinuity appears to decapitate a nearby segment, as observed for the evolution of some OSCs on the EPR. These morphological variations of the CIR axis may be explained by an increase in the crustal thickness in the north of the study area relative to the Triple Junction area. Variations in crustal thickness could be related to a broad bathymetric anomaly centered at 19°S, 65°E, which probably reflects the effect of the nearby Reunion hotspot, or an anomaly in the composition of the mantle beneath the ridge near 19°S. Other explanations for the morphological variations include the termination of the CIR at the Rodriguez Triple Junction or the kinematic evolution of the triple junction and its resultant lengthening of the CIR. These latter effects are more likely to account for the axial morphology near the Triple Junction than for the long-wavelength morphological variation.

Geometric corrections on sidescan sonar images based on bathymetry. Application with SeaMARC II and Sea Beam data

Geometric corrections on sidescan sonar images based on bathymetry. Application with SeaMARC II and Sea Beam data

Geometric corrections on sidescan sonar images based on bathymetry. Application with SeaMARC II and Sea Beam data

Acoustic backscatter images of the seafloor obtained with sidescan sonar systems are displayed most often using a flat bottom assumption. Whenever this assumption is not valid, pixels are mapped incorrectly in the image frame, yielding distorted representations of the seafloor. Here, such distortions are corrected by using an appropriate representation of the relief, as measured by the sonar that collected the acoustic backscatter information. In addition, all spatial filtering operations required in the pixel relocation process take the sonar geometry into account. Examples of the process are provided by data collected in the Northeastern Pacific over Fieberling Guyot with the SeaMARC II bathymetric sidescan sonar system and the Sea Beam multibeam echo-sounder. The nearly complete (90%) Sea Beam bathymetry coverage of the Guyot serves as a reference to quantify the distortions found in the backscatter images and to evaluate the accuracy of the corrections performed with SeaMARC II bathymetry. As a byproduct, the processed SeaMARC II bathymetry and the Sea Beam bathymetry adapted to the SeaMARC II sonar geometry exhibit a 35m mean-square difference over the entire area surveyed.

Ocean crust formation processes at very slow spreading centers: A model for the Mohns Ridge, near 72°N, based on magnetic, gravity, and seismic data

The Mohns Ridge, in the Norwegian Greenland Sea, is one of the slowest spreading centers of the mid‐ocean ridge system (8 mm/yr half rate). Sea Beam data acquired with R/V Jean Charcot near 72°N show that its rift valley floor is characterized by en échelon volcanic ridges, oriented obliquely relatively to the average strike of the ridge axis. These ridges are regularly spaced along the axis, about every 40 km, and are separated by nontransform discontinuities. Sharp positive magnetic anomalies, centered over the topographic highs, suggest that they are eruptive centers, considered as the surficial expression of active spreading cells. Over the rift valley, Bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free‐air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface. This body, if it exists, probably corresponds to the zone of low viscosity that can be inferred from the model of Chen and Morgan (1990b), which predicts the existence of a decoupling region, between the upper crust and the asthenophere below. Its width varies rapidly along‐strike, from less than about 5 km to more than 15 km. In plan view, it has a pinch and swell form, which defines a series of spreading cells, the center of one cell being where the Bouguer anomaly is widest. Short wavelength (less than 10 to 20 km) along‐strike variations, such as Bouguer anomaly lows centered on the topographic highs, reflect local effects associated with the presence of the eruptive centers. Seismic tomography data from a 20×10 km active oblique volcanic ridge near 72°22′N tend to indicate that the links between the main, low‐velocity body at depth, and the magma injections centers which lie within the rift valley inner floor are probably complex.

Causes and consequences of variations in faulting style at the Mid‐Atlantic Ridge

Both volcanism and faulting contribute to the rugged topography that is created at the Mid‐Atlantic Ridge (MAR) and preserved off‐axis in Atlantic abyssal hill terrain. Distinguishing volcanic from fault‐generated topography is essential to understanding the variations in these processes and how these variations are affected by the three‐dimensional pattern of mantle upwelling, ridge segmentation, and offsets. Here we describe a new quantitative method for identifying fault‐generated topography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcanic features, whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes. The combination of curvature and slope information allows inward and outward facing fault faces to be mapped. We apply the method to Sea Beam data collected along the MAR between 28° and 29°30′N. The fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework: long, linear, small‐throw faults occur toward segment centers, while shorter, larger‐throw, curved faults occur toward ends; these variations reflect those of active faults within the axial valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the MAR. In the first model only one fault is active at a time on each side of the rift valley. Each fault grows while migrating away from the volcanic center due to dike injection; extension across the fault causes a flexural rotation of nearby inactive faults. The amount of stress necessary to displace the fault increases as the fault grows. When reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley. This model yields a realistic terracelike morphology from the rift valley floor into the rift mountains; the relief is caused by the net rotation accumulated in the lithosphere from the active faults (e.g., 10° reached 20 km from the active fault). Fault spacing is controlled by lithospheric thickness, fault angle, and the ratio of amagmatic to magmatic extension. We hypothesize that this mechanism may be dominant toward ridge segment offsets. An alternative model considers multiple active faults; each fault relieves stresses as it grows and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of necking instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints, causing the homogeneous abyssal hill fabric in these regions.

Windows-based high-precision EMG analysis tool

The South China Sea is a mid-late Tertiary marginal basin. The magnetic anomaly lineations in the eastern part of the basin trend approximately east-west [1,2], suggesting a north-south direction of spreading. In the spring of 1985, two cruises on the French research vessel “Jean Charcot” provided Sea Beam coverage, seismic reflection, magnetic and gravity profiles. The Sea Beam data exhibit two major structural trends: scarps striking N50°E± 15°, interpreted as normal faults, and scarps striking N140°E± 15°, interpreted as fracture zones. This fabric implies a northwest-southeast direction of spreading, up to about 100 km north and south of the inferred spreading axis [3]. Dense Sea Beam coverage of a roughly 1° square area northwest of the Scarborough Seamounts chain shows that the emplacement of these seamounts was, at least in its initial stage, controlled by faulting in two orthogonal directions, N50°E and N140°E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50°E, which are disrupted by transform zones striking N140°E. This detailed study indicates that the fracture zones may be closely spaced (less than 20–30 km) east of Macclesfield Bank. Furthermore, magnetic anomalies identified as 6 and 6a (20 Myr) along two north-south profiles located at both edges to the north of this detailed study area may be correlated with the N80°E trend characteristic of such anomalies in the eastern part of the South China Sea. The east-west trend of magnetic anomalies 6 and 6a, south of Scarborough seamount chain, recognised by Taylor and Hayes (1983) [1] is incompatible with the trend of the fault scarps observed on Sea Beam data. We infer that progressive, right-lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east-west average trend of some of the magnetic anomalies, and of the Scarborough seamount chain, which represents the location of the relict spreading axis, in spite of a N50°E spreading direction. The whole central part of the basin east of 115°E may have formed in this way, since N130–140°E striking fracture zones are observed on the Sea Beam swaths. This model may correspond to the second of two successive spreading phases, with N-S and NW-SE directions of extension respectively, as was presented by the authors in a previous paper (Pautot et al., 1986 [3]). Alternatively, we may assume that the direction of extension did not change radically during the opening history [3], implying that oblique spreading occurred along N80°E trending ridge segments, creating the N80°E trending scarps and magnetic anomalies observed to the north and to the south of the study area. Such a direction of extension is compatible with mid-late Tertiary left-lateral movements along large N130–150°E strike-slip faults, such as the Red River and Wang Chao faults in South China and Sundaland.

Pre-spread rifting and heterogeneous volcanism in the southern Havre Trough back-arc basin

The Havre Trough is an active back-arc basin associated with Pacific—Australian plate convergence. Two segments within the southern Havre Trough (SHT), surveyed with gloria, Sea beam and SYS09 swath data, now provide the first detailed data to reliably document the structure and morphology of this complex back-arc region. At the southern segment (35°40′–37°S), along a strike length of some 120 km, the SHT structure comprises a series of segmented, en echelon, generally flat floored, sediment filled, axial rift grabens, flanked by ridge and basin rift block topography. Prominent fault escarpments, with relief of 500–1000 m and a marked zig-zag arrangement, mark the outer margins of the rift system. Within the rift grabens extensional tectonism is pervasive. Extensive shallow seismicity within the SHT can only partly be associated with the rift graben system at present. Youthful, constructional volcanic terrains within the rift grabens are not common, and invariably have a restricted areal extent and a relief < 400 m. Neovolcanism is fault controlled, being apparently restricted to the rift escarpments. Lavas within the rifts display a marked source heterogeneity as evinced by the eruption of back-arc basin and island arc basalts on the western and eastern escarpments, respectively, even though separated by as little as 14 km. An identical rift morphology has been identified at the northern (33°–34°S) site. Data from both sites show individual rifts are 15–35 km in length, 6–10 km wide, and have axial depths of 3100–4000 m. Apart from at least one 15–20 km wide, cross trough ridge, available data indicate the rift graben system is contiguous between 33°S and 37°S. This rift graben morphology, by analogy with other known rifting back-arc basins, is considered here not to be consistent with true oceanic back-arc spreading. Rather it is argued that the SHT, possibly indeed all of the Havre Trough, is undergoing back-arc rifting prior to true spreading. Further, the rifting tectonic fabric within the SHT, and in the onshore Taupo to the south, extension is consistently dextrally oblique relative to the bounding Colville and Kermadec Ridges. The degree of obliquity increases from south to north. If accepted that the SHT is rifting rather than spreading, magnetic anomaly data within the trough do not record the accretion of new oceanic crust, and consequently can no longer be used to establish the “spreading” history of the SHT. The SHT magnetic anomalies are here interpreted as “pseudo-linear” magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older, low magnetisation arc basement rift blocks flanking the axial rift. Proxy data from onshore North Island, New Zealand, and offshore SHT suggest that the rate of extension and age of initiation of back-arc rifting are, between 35° and 37°S, respectively, 15–20 mm yr −1 and 5 Ma.

Oblique rifting in a slow-spreading ridge

IN oceanic rifts, the spreading direction is usually nearly perpendicular to the ridge axis, but at the Reykjanes1 and Mohns2–4 ridges this direction is highly oblique (30° to 40° to the axis). These ridges possess axial valleys containing oblique raised features (highs), corresponding to second- or third-order ridge axis discontinuities5. At the Reykjanes ridge, Searle and Laughton1 observed small highs which they interpreted as volcanic domes. Here we present a fault map of the Mohns ridge obtained from image processing of Seabeam data. We argue that the oblique highs in this ridge are of tectonic origin. The axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading. We describe laboratory experiments on brittle–ductile models which support this interpretation, in which stretching at 30° to a deformable band reproduces the en echelon horst-and-graben pattern of the Mohns ridge.

Morphology and tectonics of the Mid‐Atlantic Ridge (25°–27°30′S) from Sea Beam and magnetic data

High‐resolution Sea Beam bathymetry and sea surface magnetic data have been collected between 25°S and 27°30′S on the Mid‐Atlantic Ridge. Analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three‐dimensional inversion of sea surface magnetic data are presented. This 227‐km‐long portion of the Mid‐Atlantic Ridge is offset 42 km by the right‐lateral Rio Grande Transform at 25°40′S and 9.5 km by the left‐lateral nontransform Moore Discontinuity at 26°30′S. A third nontransform discontinuity, the Midway Discontinuity, exists near 26°12′S and is defined by a small displacement of the axial magnetization distribution and the disruption in the along‐strike continuity of ridge‐parallel terrain elements. Although the detailed survey presented in this paper extends out to only 5‐ to 7‐m.y.‐old crust, a regional compilation of magnetic data from this area by Cande et al. (1988) indicates that the relative positions and dimensions of the spreading cells have characterized this part of the Mid‐Atlantic Ridge for at least the past 55 m.y. The offsets dimensions and geometries, however, have changed markedly because of prolonged differential asymmetric spreading between segments. Between and within the accretionary units that are spreading at uniform rates (35 mm/yr total opening rate), the morphology of the ridge axis varies dramatically, from a well‐defined rift valley (20–25 km wide, 1–1.5 km deep) to an axial swell that rises 1500 m above the adjacent valley floor. The axial magnetic structure is also highly variable along‐strike. The inversion solution shows a series of generally linear, short‐wavelength (&lt;10 km) magnetization highs within the central Brunhes anomaly over each of the four segments in the survey area. These anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world's mid‐ocean spreading centers. The distinct axial magnetization highs can extend out to 2‐m.y.‐old crust at ridge‐discontinuity intersections, suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks, e.g., highly fractionated and strongly magnetized basalts enriched in iron and titanium. The magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area. Both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed. In the case of an axial swell centered at 26°S, attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and/or a zone of substantial alteration. Taken together, the bathymetric data and magnetic modeling document strong along‐axis gradients in crustal and upper mantle properties.

Landward vergence and oblique structural trends in the Oregon margin accretionary prism: Implications and effect on fluid flow

The central Oregon margin spans a regional transition in accretionary structures from seaward-verging in the south to landward-verging in the north. New multichannel seismic (MCS) data image both landward- and seaward-vergent provinces along the central and northern Oregon margin. Landward-vergence is characterized by a deep décollement, with deformation distributed across a broad lower continental slope in a coherent structural style. In the landward-vergent area, virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet, forming fault-bend folds and a distinctive ridge/trough morphology. This style of landward-vergence is not explained by existing models. In contrast, seaward-vergence correlates with a shallower décollement, approximately 1.4 km above the oceanic crust, and a more intense style of deformation within a narrower slope. Initial thickening of the trench sediments occurs across a well-developed protothrust zone. The frontal thrust forms a ramp-anticline that is cut by a prominent backthrust. Previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high-amplitude, reversed-polarity reflections suggestive of enhanced porosity along the faults. Potential fluid sources and migration paths are strongly influenced by changes in the level of the décollement and vergence along the margin.Abrupt changes in structural style occur both along strike and updip, and are bounded by two sets of oblique-slip faults. Three NW-striking left-lateral faults are imaged in both MCS and SeaBeam data. Plunging anticlines developed along the NW-striking faults are venting fluids and were previously interpreted as mud volcanoes. The deformation front is locally disrupted where these faults intersect the prism, but they appear to have limited influence on the structural evolution of the prism. In contrast, the NE-striking right-lateral faults are confined to the deforming sediments of the upper plate. These faults interact with the thrusts within the prism, forming a rhomboidal pattern of three-dimensional deformation.

Bispectral analysis of ocean volume reverberation.

The univariate probability density function (pdf) of the volume reverberation process is typically modeled as symmetric and non-Gaussian, with a variable kurtosis parameter. However, very little is known about the joint distribution of volume reverberation. In this study, the bispectral analysis of actual ocean volume reverberation data is performed in an effort to gain more insight into the structure of the joint probability density function of the volume reverberation process. The test vehicle is the Hinich test, a statistical method based on the sample bispectrum, which can be used to quantify the degree of non-Gaussianity and asymmetry in the joint pdf of a random process. Simulated data records, distributed χ2 with decreasing numbers of degrees freedom, are first examined to provide a baseline measure of test statistic behavior as a function of pdf asymmetry. The test is then applied to actual volume reverberation data records obtained using seabeam. Although the empirically determined univariate density function for each seabeam data record exhibits a very high degree of symmetry, the Hinich test statistic indicates the presence of a large degree of asymmetry in the joint pdf of the volume reverberation process.

A global and regional stochastic analysis of near‐ridge Abyssal Hill morphology

This paper presents the results of a global and regional stochastic analysis of near‐ridge abyssal hill morphology. The analysis includes the use of Sea Beam data for the estimation of stochastic parameters up to order 4. These parameters provide important quantitative physical information regarding abyssal hills, including their rms height, azimuthal orientation, characteristic width, aspect ratio, Hausdorff dimension, skewness, tilt, and peakiness. The global data set consists of 64 Sea Beam swaths near the Rivera, Cocos, and Nazca spreading sections of the East Pacific Rise, the Mid‐Atlantic Ridge, and the Central Indian Ridge. In one form of analysis, the parameters are averaged among spreading rate bins. Each of the spreading rate subsets can be identified as unique from the others in at least one aspect The slowest spreading rate subset (Mid‐Atlantic data) exhibit the largest scales (rms height and characteristic width and length) of abyssal hills. These parameters generally decrease as spreading rate increases up to the fast spreading rate data (Pacific‐Cocos) but increase going from fast to very fast (Pacific‐Nazca) spreading rate data. This indicates some complexity in the relationship between spreading rate and abyssal hill morphology. The plan view aspect ratio is nearly twice as large for the fast spreading rate data than for any of the other subsets and is smallest for the very fast spreading rate data. The fractal dimension is nearly identical for all spreading rate subsets. The vertical skewness is positive for the slow and medium spreading rate data, indicating larger peaks than troughs, and negative for the fast spreading rate data, indicating larger troughs than peaks. The kurtosis, or peakiness is everywhere larger than the Gaussian value of 3 and tends to be larger in the Atlantic than the Pacific. The tilting parameter provides substantial evidence indicating steeper inward facing slopes in the medium and fast spreading rate data, but only marginal evidence for it in the slow spreading rate data. From an analysis of correlations among parameters it is found that subsets sometimes behave differently from the entire data set. In particular, while over the global data set the characteristic width exhibits a well‐resolved positive trend when plotted versus rms height, these parameters exhibit a more gradual positive trend in the Mid‐Atlantic data and a negative trend in the Pacific‐Cocos data. In addition, the plan view aspect ratio, while generally uncorrelated with rms height for the global data set, is positively correlated with rms height for the Pacific‐Cocos data set. These results emphasize a strong uniqueness of the Pacific‐Cocos data relative to the rest of the data global set The Pacific‐Cocos data consist of 27 swaths concentrated between the Siquieros and Orozco fracture zones. These data provide very good abyssal hill coverage of this well‐mapped and well‐studied region and form the basis of a regional analysis of the correlation between ridge morphology and stochastic abyssal hill parameters. In this analysis, it is found that abyssal hill parameters are correlated with the depth of the adjacent ridge axis, indicating that the relative abundance in magma supply, which likely controls the ridge axis depth, may also be an important influence on the formation of abyssal hills.

Modeling of sea beam acoustic backscatter from ridge–crest terrains.

An approach based on a Kirchhoff approximation model [D. Jackson etal., J. Acoust. Soc. Am. 79, 1410–1422 (1986)] conveniently relates to a power law the ocean bottom roughness spectrum based on two parameters α and β, i.e., W(k)=βk−γ, where γ=2(α+1). Initial studies [R. Dziak etal., Eos 70, 71 (1989, 1990)] revealed that α and β, solved by multiregression analysis using sea beam data from the Juan de Fuca Ridge, compared favorably with the spectral models derived from similar tectonic provinces by Fox etal. [Rev. Geophys. 23 (1985)]. In comparison to a similar method applied to a heavily sedimented area by de Moustier etal. [J. Acoust. Soc. Am. 89, 1962–1963 (1991)], a lack of volume reverberation in hard-rock areas aids in deriving solutions for the model since less bottom parameters are involved. On the other hand, the fluctuation of the echo levels between the pings from the rough and hard bottom made it difficult to fit to a realistic bottom roughness model. The statistics of the peak echo amplitudes from the sea beam system and a method adopted to reduce their standard deviation are described.

Description and classification of seafloor terrains using two-dimensional spectra models of bathymetry.

A two-dimensional spectral model of bathymetry developed by Fox and Hayes [Rev. Geophys. 23(1), 1–48 (1985)] is applied to gridded sea beam data from the central Juan de Fuca Ridge in the northeast Pacific Ocean. Parameters of the model corresponding to (1) isotropic (nondirectionally dependent) roughness, (2) anisotropic (directionally dependent) roughness, (3) orientation of the anisotropic component, and (4) spectral rolloff (∼fractal dimension) are derived by fitting the model surface to two-dimensional amplitude spectra of bathymetry determined on a regular grid over the study area. Each term of the model is individually mapped and compared to ground truth data from SeaMARC-I sidescan sonar imagery. Parameters of the model clearly define the contrast between constructional volcanic terrain (rough, isotropic, with high fractal dimension) and tectonic extensional terrain (smoother, anisotropic, with low fractal dimension). An agglomerative, hierarchical cluster analysis is applied to the data, independent of spatial information, to delineate groups of spectra with similar characteristics. Distinct, mappable regions, corresponding to volcanic and tectonic provinces, are objectively determined. Also, coherent subregions of consistent spectral properties occur within the larger volcanic/tectonic divisions. The classification of large data sets, such as the entire Juan de Fuca Ridge system, can best be accomplished through the use of apriori techniques, such as k-means clustering, which requires the input of predefined characteristics for bathymetric provinces.