Gravity Lineations Research Articles

Sub‐Lithospheric Small‐Scale Convection Tomographically Imaged Beneath the Pacific Plate

AbstractSmall‐scale convection beneath the oceanic plates has been invoked to explain off‐axis nonplume volcanism, departure from simple seafloor depth‐age relationships, and intraplate gravity lineations. We deployed 30 broadband ocean bottom seismometer stations on ∼40 Ma Pacific seafloor in a region notable for gravity anomalies, measured by satellite altimetry, elongated parallel to plate motion. P‐wave teleseismic tomography reveals alternating upper mantle velocity anomalies on the order of ±2%, aligned with the gravity lineations. These features, which correspond to ∼300°–500°K lateral temperature contrast, and possible hydrous or carbonatitic partial melt, are—surprisingly—strongest between 150 and 260 km depth, indicating rapid vertical motions through a low‐viscosity asthenospheric channel. Coherence and admittance analysis of gravity and topography using new multibeam bathymetry soundings substantiates the presence of mantle density variations, and forward modeling predicts gravity anomalies that qualitatively match observed lineations. This study provides observational support for small‐scale convective rolls beneath the oceanic plates.

Upper-plate structure and tsunamigenic faults near the Kodiak Islands, Alaska, USA

AbstractThe Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modeling and geophysical data analysis. Using seismic and bathymetric data, we characterize a regionally extensive seafloor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km-long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but we suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper-plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short wave travel paths and little tsunami warning time for nearby communities.

Radial gravity anomalies associated with the ejecta of the Orientale basin

GRAIL gravity data shows small-scale gravity anomalies radiating out from the Orientale basin between distances of ∼550 km (average distance to the Cordillera ring) and ∼1000 km. These radial gravity lineations are sometimes associated with secondary crater chains or catenae, of which the specific details of formation and their relation to the basin ejecta remain unclear. However, many of the radial gravity anomalies have no clear topographic signature at all. Typical anomalies have an average width of about 20 km, with a magnitude of ± 20 mGal, and a length of 50–200 km. Here we use gravity inversions, hydrocode modeling, and observations to investigate the radial gravity anomalies in more detail. Density inversion models show that the gravity can be matched by solutions ranging from broad low-amplitude anomalies with density contrasts of ± 20 kg/m3 extending to depths of tens of kilometers, to shallow high-amplitude anomalies with density contrasts of about ± 200–400 kg/m3 confined to the top ∼1–2 km of the crust. Hydrocode models of the low-velocity secondary impacts of basin ejecta show that the impacting material does not disperse – as occurs for hypervelocity impacts – but instead remains as a thin layer within the secondary craters. This remaining secondary projectile material derived from Orientale ejecta can explain the linear gravity anomalies associated with secondary crater chains and catenae. However, this does not explain the majority of linear gravity features that are not associated with secondary crater chains. Therefore, we conclude that the majority of radial gravity anomalies represent the structure of the ejecta deposit. This density variability within the ejecta of basins likely contributes substantially to the density variability of the shallow upper crust as a whole.

Wavelet-based directional analysis of the gravity field: evidence for large-scale undulations

SUMMARY In the eighties, the analysis of satellite altimetry data leads to the major discovery of gravity lineations in the oceans, with wavelengths between 200 and 1400 km. While the existence of the 200 km scale undulations is widely accepted, undulations at scales larger than 400 km are still a matter of debate. In this paper, we revisit the topic of the large-scale geoid undulations over the oceans in the light of the satellite gravity data provided by the GRACE mission, considerably more precise than the altimetry data at wavelengths larger than 400 km. First, we develop a dedicated method of directional Poisson wavelet analysis on the sphere with significance testing, in order to detect and characterize directional structures in geophysical data on the sphere at different spatial scales. This method is particularly well suited for potential field analysis. We validate it on a series of synthetic tests, and then apply it to analyze recent gravity models, as well as a bathymetry data set independent from gravity. Our analysis confirms the existence of gravity undulations at large scale in the oceans, with characteristic scales between 600 and 2000 km. Their direction correlates well with present-day plate motion over the Pacific ocean, where they are particularly clear, and associated with a conjugate direction at 1500 km scale. A major finding is that the 2000 km scale geoid undulations dominate and had never been so clearly observed previously. This is due to the great precision of GRACE data at those wavelengths. Given the large scale of these undulations, they are most likely related to mantle processes. Taking into account observations and models from other geophysical information, as seismological tomography, convection and geochemical models and electrical conductivity in the mantle, we conceive that all these inputs indicate a directional fabric of the mantle flows at depth, reflecting how the history of subduction influences the organization of lower mantle upwellings.

Detailed tectonic evolution of the Reykjanes Ridge during the past 15 Ma

We present a new detailed tectonic model of the Reykjanes Ridge which examines the rift propagation hypothesis for the V‐shaped ridges and its asymmetric lithospheric accretion. Four major southward rift propagations extend through our entire survey area and several additional small scale rift propagations are observed, including northward propagators. If plume pulses only drive southward propagators, then two different driving mechanisms for propagators must exist. There is a major difference in the crustal accretion asymmetry between the area immediately off the Iceland shelf and that farther south, both in rift propagation pattern and free air gravity lineations. Furthermore, we identify two small offset features coined ponsu‐transforms, from which rift propagation is both initiated and stopped. The pattern of the V‐shaped ridges on the Reykjanes Ridge is not symmetric about the Reykjanes Ridge and the V‐shaped ridges are not linear continuous features. Our rift propagation model produces excellent fits to magnetic data and provides a self‐consistent model for the evolution of the Reykjanes Ridge during the past 15 Ma.

Influence of the Iceland mantle plume on oceanic crust generation in the North Atlantic

When a mantle plume with elevated temperature underlies an oceanic spreading centre it affects the generation of oceanic crust by creating thicker crust. We map the variation in crustal thickness and seismic velocity along three long-offset seismic profiles acquired over oceanic crust generated shortly after continental breakup in the North Atlantic: a 212-kmlong flowline from the Faroes rifted continental margin across crust of 51–42 Ma age, where oceanic spreading developed close to the inferred centre of the Iceland mantle plume; a 256 km flowline extending from the Hatton rifted continental margin across crust of 52–40 Ma age, about 800 km south of the presumed centre of the mantle plume; and a 99 km strike line over oceanic crust formed at 43 Ma in the Iceland Basin off the Hatton continental margin. The crustal velocity structure along each profile is constrained by multichannel seismic reflection data, which is used primarily to map the sediments, and by densely spaced ocean-bottom seismometers, which recorded wide-angle reflections and refractions to offsets of more than 100 km. Over 56 000 crustal diving wave and Moho wide-angle reflection arrivals were used in joint crustal refraction and reflection tomographic inversions. Quantitative error analysis shows that the seismic velocity of the crust is mostly constrained to within 0.1 km s−1 and the depth of the Moho to within ±250 m. We interpret the crustal thickness and velocity changes along the profiles as caused primarily by changes in the mantle temperature at the time of crustal formation. If all the oceanic crustal thickness variations are ascribed to mantle temperature changes, we infer that as mature seafloor spreading developed following continental breakup, the mantle cooled by ca. 75 ◦C over a 10 Myr period, although it still remained hotter than the global average of normal oceanic crust. The crust formed close to Iceland is at all times thicker than that formed further away, which we interpret as reflecting higher temperatures close to the centre of the thermal anomaly created by the mantle plume. Currently at the Reykjanes Ridge, south of Iceland, we interpret thicker than normal oceanic crust as being caused by the presence of hotter mantle, modulated by thickness variations of 1.5–2.0 km which are attributed to temporal variations in the mantle plume temperature of about 25 ◦C on a 3–6 Myr timescale. A 1.5 km increase in thickness of oceanic crust generated between 48 and 45 Ma on the Faroes line is similar in magnitude and duration to those occurring on the present day Reykjanes Ridge, which we suggest is due to a temperature pulse of ∼25 ◦C. Gravity lineations in the northern North Atlantic suggest that the oceanic crust has exhibited small thickness fluctuations of similar size throughout its history, interpreted as due to small fluctuations in the temperature of the Iceland mantle plume.

Using Ambient Seismic Noise to Determine Short-Period Phase Velocities and Shallow Shear Velocities in Young Oceanic Lithosphere

Using 10 broadband ocean bottom seismometers from the 11-month-long deployment of the Gravity Lineations Intraplate Melting Petrologic and Seismologic Expedition (GLIMPSE) passive seismic experiment located in the south central Pacific, we have estimated the seismic impulse responses from ambient seismic noise for 45 station-to-station paths. The raw impulse responses show moveout with station-to-station distance, and there is a trend of decreasing signal-to-noise ratios as the station-to-station distance increases. The decrease in signal-to-noise ratio is expected as a smaller range of azimuths of propagating surface waves will contribute constructively to the cross-correlated signal with increasing distance, although scattering may also play a role in the coherence of seismic noise at periods less than 16 sec. From these station-to-station paths, we determined group velocities for the fundamental mode Rayleigh waves of a 2–16-sec period and the second mode Rayleigh wave of a 3.5–7-sec period. We calculate phase velocities for the fundamental mode and second mode Rayleigh wave over the same period range as the group velocities by applying a time variable filter to the noise correlation function and carefully unwrapping the phase spectrum of the resulting filtered impulse responses. Within this period range, there is a transition from waves at short periods, whose energy is mostly in the water column, to waves sensitive to crustal and upper mantle structure. The phase velocities for the second mode, which have peak sensitivity in the lower crust and shallow mantle, show evidence for azimuthal anisotropy. The average phase velocities of the station-to-station paths in the east–west direction are 2% faster than the north–south paths at the 4–7-sec period, consistent with the fast directions determined from SKS wave splitting measurements of N100°E. By incorporating the short-period fundamental and higher mode phase velocities from ambient seismic noise with longer period (16–100 sec) fundamental mode Rayleigh-wave phase velocities determined from teleseismic events, we inverted for the average crustal and upper mantle shear velocity structure and water column depth and velocity. The predicted phase velocities are extremely sensitive to the water column compressional velocity. We determined the average water column velocity to be 1466±3 m/sec, in contrast to the average of 1500 m/sec estimated from shipboard measurements weighted according to the Rayleigh-wave sensitivity kernel. The difference may be due to the dispersive effects of scattering by bathymetry or by the thin variable thickness sediment layer. The inversion also produces a Vp/Vs ratio of 1.88 for the crust. This method can provide useful information about the shallow seismic structure of the oceanic crust and uppermost mantle and is an important complement to longer period studies.

Crustal structure beneath the gravity lineations in the Gravity Lineations, Intraplate Melting, Petrologic and Seismic Expedition (GLIMPSE) study area from seismic refraction data

The active source seismic component of the Gravity Lineations, Intraplate Melting, Petrologic and Seismic Expedition (GLIMPSE) experiment investigated the velocity structure and crustal thickness of the Pacific plate along a ∼450 km line paralleling the southern East Pacific Rise where prominent intraplate ridges lie within lows of the lineated free‐air gravity pattern. Compressional wave traveltime data collected by 10 ocean bottom seismometers indicate that the median crustal thickness is ∼6.2 km and increases to ∼8.2 km beneath the Sojourn Ridge. The conspicuous absence of crustal thinning beneath both the Sojourn and Hotu‐Matua ridge systems is strong support against a boudinage‐style model for the formation of these volcanic complexes. Our results suggest the ridge loads are partially supported by both plate flexure and underplating, with an additional mantle component inferred from the analysis of gravity data, Rayleigh wave tomography, and teleseismic body wave delays. Small‐scale convection could produce the observed crustal thickness pattern but fails to explain the geochemical and formation age trends along the ridges. We prefer a model where the channelized flow of low‐density material in the upper mantle, possibly undergoing small‐scale convection, is advected toward the ridge crest. Lows in the gravity field reflect the density contrast within the asthenosphere, and decompression melting amid upwelling limbs of the convective cells sustains the development of the associated ridge systems.

Rayleigh wave tomography beneath intraplate volcanic ridges in the South Pacific

We test models for the origin of intraplate volcanic ridges and gravity lineations on young seafloor west of the East Pacific Rise using Rayleigh wave dispersion measured in the Gravity Lineations and Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) seismic experiment. The excellent azimuthal distribution of teleseismic sources recorded over a 12‐month period provides resolution of phase velocities at periods up to 100 s. The average phase velocities for the study area reveal a pronounced low‐velocity zone reaching a minimum shear velocity of ∼3.95 km/s. The negative velocity gradient defining the base of the lithosphere, observed at 40 ± 15 km, abruptly reverses at 70 km depth. The underlying positive gradient changes slope at ∼125 km. We attribute these changes in gradient to the onset of incipient partial melting of upwelling mantle in the presence of water at 125 km, followed by increased melt production at 70 km that leads to dehydration of the residual matrix and migration of melt to the surface spreading center. Rayleigh wave tomography shows that there are anomalously low shear velocities extending to at least 50 km depth beneath the Sojourn Ridge and the Hotu Matua volcanic complex, with relatively high velocities between these volcanic chains. These observations are not consistent with passive models for the origin of the volcanic ridges involving lithospheric extension or thermoelastic cracking. Dynamic models invoking flow in the asthenosphere in the form of small‐scale convection or viscous fingering instabilities may explain the observed pattern of seismic velocity anomalies.

P and S wave delays beneath intraplate volcanic ridges and gravity lineations near the East Pacific Rise

The Gravity Lineations Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) explored the origin of two intraplate, non–hot spot ridge systems associated with gravity lineations in the south central Pacific. Using the 11 month ocean bottom seismometer (OBS) deployment of the GLIMPSE experiment, we determined the average P and S station delays for the region. Across the array, station delays varied by 1.22 s for P waves and 2.13 s for S waves. The presence of ultraslow shear velocity (<400 m/s) sediment layers of variable thickness throughout the region introduced S delays, determined from P‐to‐S‐converted phases generated at the crust‐sediment boundary, of 0.08 to 0.43 s. Consequently, we corrected the S delays for these sediments to examine seismic anomalies due to crustal and mantle structure. We used the sediment delay times and the relative amplitudes of the converted phases to estimate the average regional shear velocity and thickness of the sediment layer beneath the stations. The range of sediment thickness in the region is 10–70 m, and the average shear velocity in the sediments is 140 m/s. Both the average P and sediment‐corrected S delays show a positive correlation with seafloor age, with faster arrivals on older seafloor where the seismically fast lithosphere has thickened away from the spreading axis. The delay times also correlate with the residual mantle Bouguer gravity anomaly (rMBA), with slow arrivals coinciding with rMBA lows and the associated ridge systems. The delays are much too large to be caused by variations in crustal thickness and must represent anomalies within the mantle. These observations qualitatively support the either the channelized asthenospheric return flow model or the small‐scale convection hypothesis for the origin of the gravity lineations. These models predict both positively correlated density and velocity anomalies caused by thermal anomalies and the pressure‐release melting that migrates to the surface to form the volcanic ridges above the upwelling limbs of convection cells or channels.

Distribution of recent volcanism and the morphology of seamounts and ridges in the GLIMPSE study area: Implications for the lithospheric cracking hypothesis for the origin of intraplate, non–hot spot volcanic chains

Lithospheric cracking by remotely applied stresses or thermoelastic stresses has been suggested to be the mechanism responsible for the formation of intraplate volcanic ridges in the Pacific that clearly do not form above fixed hot spots. As part of the Gravity Lineations Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) project designed to investigate the origin of these features, we have mapped two volcanic chains that are actively forming to the west of the East Pacific Rise using multibeam echo sounding and side‐scan sonar. Side‐scan sonar reveals the distribution of rough seafloor corresponding to recent, unsedimented lava flows. In the Hotu Matua volcanic complex, recent flows and volcanic edifices are distributed over a region 450 km long and up to 65 km wide, with an apparent, irregular age progression from older flows in the west to younger in the east. The 550‐km‐long Southern Cross Seamount/Sojourn Ridge/Brown Ridge chain appears to have been recently active only at its eastern end near the East Pacific Rise. A third region of recent flows is found 120 km north of Southern Cross Seamount in seafloor approximately 9 Myr old. No indication of lithospheric extension in the form of faulting or graben formation paralleling the trend of the volcanic chains is found in the vicinity of recent flows or anywhere else in the study area. Thermoelastic cracking could be a factor in the formation of a few small, very narrow volcanic ridges, but most of the volcanic activity is broadly distributed in wide swaths with no indication of formation along narrow cracks. The Sojourn and Brown chains appear to begin as distributed zones of small seamounts that later develop into segmented ridges, perhaps under the influence of membrane stresses from self‐loading. We suggest that the linear volcanic chains are created by moving melting anomalies in the asthenosphere and that lithospheric cracking plays at most a secondary role.

Analysis of gravity and topography in the GLIMPSE study region: Isostatic compensation and uplift of the Sojourn and Hotu Matua Ridge systems

The Gravity Lineations Intraplate Melting Petrologic and Seismic Expedition (GLIMPSE) Experiment investigated the formation of a series of non–hot spot, intraplate volcanic ridges in the South Pacific and their relationship to cross‐grain gravity lineaments detected by satellite altimetry. Using shipboard gravity measurements and a simple model of surface loading of a thin elastic plate, we estimate effective elastic thicknesses ranging from ∼2 km beneath the Sojourn Ridge to a maximum of 10 km beneath the Southern Cross Seamount. These elastic thicknesses are lower than predicted for the 3–9 Ma seafloor on which the volcanoes lie, perhaps due to reheating and thinning of the plate during emplacement. Anomalously low apparent densities estimated for the Matua and Southern Cross seamounts of 2050 and 2250 kg m−3, respectively, probably are artifacts caused by the assumption of only surface loading, ignoring the presence of subsurface loading in the form of underplated crust and/or low‐density mantle. Using satellite free‐air gravity and shipboard bathymetry, we calculate the age‐detrended, residual mantle Bouguer anomaly (rMBA). The rMBA corrects the free‐air anomaly for the direct effects of topography, including the thickening of the crust beneath the seamounts and volcanic ridges due to surface loading of the volcanic edifices. There are broad, negative rMBA anomalies along the Sojourn and Brown ridges and the Hotu Matua seamount chain that extend nearly to the East Pacific Rise. These negative rMBA anomalies connect to negative free‐air anomalies in the western part of the study area that have been recognized previously as the beginnings of the cross‐grain gravity lineaments. Subtracting the topographic effects of surface loading by the ridges and seamounts from the observed topography reveals that the ridges are built on broad bands of anomalously elevated seafloor. This swell topography and the negative rMBA anomalies contradict the predictions of lithospheric cracking models for the origin of gravity lineaments and associated volcanic ridges, favoring models with a dynamic mantle component such as small‐scale convection or channelized asthenospheric return flow.

Influence of rheology on realignment of mantle convective structure with plate motion after a plate reorganization

Small‐scale convection (SSC) rolls below the oceanic lithosphere have the tendency to align with the large‐scale shearing direction and thus with the plate motion direction relative to the deep mantle. Understanding the timescales of and processes responsible for realignment would contribute significantly to our understanding of the unresolved phenomena in the Pacific such as gravity lineations, small‐scale seismic velocity variations, and intraplate volcanism that cannot be explained by hot spots. In this study we examine the evolution of those convection rolls when this relative plate motion direction is suddenly changed, as suggested by the kink in the Hawaii‐Emperor seamount chain. Using three‐dimensional numerical flow models, we investigate the realignment of SSC rolls after a change in plate motion direction. From the nature of the SSC, it is expected that rheological parameters dominate the characteristics of this realignment. Our results show that this is indeed the case. We find that (1) using constraints from onset timing of SSC, realignment of rolls can occur as fast as within 20 Ma, but might also take much longer, dependent on the rheology; (2) the realignment period is strongly correlated to the sum of large‐scale shear stress induced by plate motion and small‐scale shear stress from the SSC itself; (3) in a mantle deforming by dislocation creep, realignment occurs faster than by diffusion creep, because dislocation creep SSC is more vigorous; and (4) activation energy has little influence on the realignment time. Possible evidence for the realignment period might come from precise age determination of intraplate volcanism or azimuthal seismic anisotropy.

Temporal variations of the segmentation of slow to intermediate spreading mid‐ocean ridges 1. Synoptic observations based on satellite altimetry data

The high‐resolution geoid and gravity maps derived from ERS‐1 and Geosat satellite geodetic missions reveal a set of small‐scale lineations on the flanks of slow to intermediate spreading mid‐ocean ridges. Assuming that these lineations reflect the variations in crustal structure induced by mid‐ocean ridge axial discontinuities, we use them to investigate how the discontinuities, and the segments they bound, appear, migrate, and disappear. We provide a synoptic description of the main characteristics of the crustal structure variations, as well as their evolution in time, over the flanks of the Mid‐Atlantic, Indian, and Pacific‐Antarctic Ridges. The second‐order segment length does not appear to vary with the spreading rate for the slow to intermediate spreading ridges investigated here. The amplitude of the gravity signal associated with off‐axis discontinuity traces increases with the obliquity of the ridge to spreading and decreases with spreading rate and with the proximity of a ridge section to a hot spot. The patterns of the gravity lineations appear to be very homogeneous over 500‐ to 1000‐km‐large corridors bounded by large fracture zones. Far from hot spots, corridors are characterized either by segments bounded by discontinuities migrating back and forth along the axis, implying a lifetime of 10–30 Myr for the segments, or by segments and discontinuities very stable in space and time, surviving for 40–50 Myr. Closer to hot spots, the segmentation is affected in two ways. First, segments tend to migrate along axis away from hot spots, or toward cold spots. Second, asymmetric spreading tends to keep sections of ridges closer to hot spots than normal spreading would. These observations support the hypothesis that ridge segmentation and its evolution are controlled by mantle dynamics. Our analysis provides observational constraints for further models of crustal production along ridges, which are presented in the companion paper by Rabinowicz and Briais [2002].

Antarctic Tectonics: Constraints From an ERS-1 Satellite Marine Gravity Field

A high-resolution gravity field of poorly charted and ice-covered ocean near West Antarctica, from the Ross Sea east to the Weddell Sea, has been derived with the use of satellite altimetry, including ERS-1 geodetic phase, wave-form data. This gravity field reveals regional tectonic fabric, such as gravity lineations, which are the expression of fracture zones left by early (65 to 83 million years ago) Pacific-Antarctic sea-floor spreading that separated the Campbell Plateau and New Zealand continent from West Antarctica. These lineations constrain plate motion history and confirm the hypothesis that Antarctica behaved as two distinct plates, separated from each other by an extensional Bellingshausen plate boundary active in the Amundsen Sea before about 61 million years ago.

Pacific Plate motion and undulations in geoid and bathymetry

Previous studies have shown that the Pacific geoid and gravity fields exhibit lineated anomalies, trending approximately in the direction of absolute plate motion over the underlying mantle. Because the undulations obliquely cross fracture zones they have often been attributed a convective origin. Recently, lithospheric boudinage caused by diffuse extension has been proposed as a possible mechanism. We have examined the undulations in the free-air anomalies, geoid and bathymetry over a portion of the Pacific Plate to determine quantitatively how the undulations are related to plate motion. We compare the observed data to an axisymmetric, sinusoidal undulation defined in an arbitrary frame of reference; in particular, we seek the north pole of this reference frame that maximizes the correlation between data and model. Poles that are close to the Pacific hotspot pole represent copolar undulations possibly related to plate motion. The distance between the best-fitting poles and the hotspot pole is determined as a function of undulation wavelength and reveals several minima (with distance < 10°) for discrete geoid wavebands centered on wavelengths of 160 km, 225 km, 287 km, 400 km, 660 km, 850 km, 1000 km and 1400 km. Bathymetry data have copolar bathymetric expressions as well, giving an implied admittance of 2–3 m/km. The most co-polar geoid/bathymetry undulations (with poles within 2–3° of the average Pacific Euler pole) have wavelengths of ∼ 280 km and ∼ 1050 km, respectively. The latter could have a convective origin or be related to the spacing of hotspot swells. The former may reflect lithospheric boudinage formed in response to diffuse extension, but could also have a dynamic origin since flexural dampening may only have attenuated the bathymetric amplitude by 50% or less. Radiometric dating of volcanic ridges found in the troughs of prominent gravity lineations gives ages that correlate well with documented changes in Pacific and Indo/Australian Plate motion, suggesting the ridges formed in response to intermittent plate boundary stresses and not as a direct consequence of small-scale convection or diffuse extension.

Short-wavelength gravity lineations and unusual flexure results at the Puka Puka volcanic ridge system

Short-wavelength gravity lineations aligned parallel to the direction of absolute motion of the Pacific plate, and a newly discovered series of linear, elongate volcanic ridges in the south-central pacific Ocean are just two of the many geophysical and geochemical anomalies that have been observed in this area. These Puka Puka volcanic ridges can be traced for 2600 km along the trough of a major gravity lineation and stretch from close to the East Pacific Rise, in the east, to the Tuamotu Islands in the west. The ridges were the focus of a recent cruise to this area that collected high quality gravity and multi-beam swath bathymetry data, in addition to dredge samples that are suitable for radiometric age dating. A complete 2-D lithospheric flexure analysis of these new data reveals that each of the volcanic ridges is associated with an unusually low effective elastic plate thickness. Previous workers showed this region to be characterised by lower than expected elastic thickness values, which were interpreted in terms of both regional and more localised thermal anomalies in the oceanic lithosphere. The new flexure results obtained for the volcanic ridges in this study confirm these low values. Lithospheric stretching and small-scale convection models that have been put forward to explain the origin of the lineations and volcanic ridges have been re-examined in light of these new results but neither is found to satisfactorily explain all of the observations.

Observations of ridge‐hotspot interactions in the Southern Ocean

The evolution of ridge‐hotspot systems is not well understood. In this investigation, satellite‐derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge‐hotspot interaction at four sparsely explored systems in the Southern Ocean. These systems illustrate three different stages of ridge‐hotspot interaction in which a migrating spreading center approaches a hotspot (Pacific‐Antarctic/Louisville), passes over or is captured by the hotspot (Mid‐Atlantic/Shona‐Discovery), and ultimately migrates away from the hotspot (Southeast Indian/Kerguelen). All of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading. Interestingly, these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge‐hotspot systems. A simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump. The presence of previously uncharted, obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered. The azimuths of the aseismic ridges cannot be explained by plate kinematics alone; they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure. The features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets.

Evidence for diffuse extension of the Pacific Plate from Pukapuka ridges and cross‐grain gravity lineations

Satellite altimeter measurements of marine gravity reveal 100 to 200‐km wavelength lineations over a wide area of the Pacific plate oriented roughly in the direction of absolute plate motion. At least three mechanisms have been proposed for their origin: small‐scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere; diffuse N‐S extension of the lithosphere resulting in lineated zones of extension (boudins); and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism. Recently, several chains of linear volcanic ridges have been found to be associated with the gravity lineations. Following ridgelike gravity signatures apparent in high‐resolution Geosat gravity measurements, we surveyed a series of volcanic ridges that extend northwest from the East Pacific Rise flank for 2600 km onto 40 Ma seafloor. Our survey data, as well as radiometric dates on samples we collected from the ridges, provide tight constraints on their origin: (1) Individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion. (2) The ridges formed along a band 50 to 70‐km‐wide in the trough of one of the more prominent gravity lineations. (3) Radiometric dates of the largest ridges show no hotspot age progression. Moreover, the directions predicted for minihotspot traces older than 24 Ma do not match observed directions of either the gravity lineations or the ridges. Based on this last observation, we reject the minihotspot model. The occurrence of the ridges in the trough of the gravity lineation is incompatible with the small‐scale convection model which would predict increased volcanism above the convective upwelling. We favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere. However, the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model. Perhaps the fault scarps are buried under more than 30 m of sediments and/or covered by the elongated ridges. Finally, we note that if ridge‐push force is much smaller than trench‐pull force, then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion.

Crenulated seafloor: Evidence for spreading-rate dependent structure of mantle upwelling and melting beneath a mid-oceanic spreading center

The availability of dense GEOSAT-GM satellite altimetry data south of 30°S [1] provides the first opportunity to compare at ∼ 25 km resolution seafloor generated at ridges spreading with a wide range of spreading rates. The axis of the slow-spreading Southwest Indian Ridge as well as the seafloor that it has generated has a distinctly crenulated appearance. This crenulated seafloor shows regularly spaced gravity lineations in the spreading direction, crenulations that record stationary centers of mantle upwelling over the life of the Southwest Indian Ridge. A similar pattern of crenulation is seen along sections of the slow-spreading northern Mid-Atlantic Ridge, but along the faster (intermediate) spreading southern Mid-Atlantic Ridge, while regularly spaced highs are apparent along sections of the spreading axis, stationary crenulation does not persist off the axis. This pattern implies that the faster spreading southern Mid-Atlantic Ridge has variable centers of time-dependent upwelling that migrate along the ridge. Crenulations are not visible near the axis of the fast-spreading Pacific-Antarctic Ridge. Crenulation amplitudes decrease strongly with increasing spreading rate. The absence of crenulation at high spreading rate could be due to a 3D/2D transition in flow structure or the effect of mechanisms to redistribute crust along axis. While crenulations are not ubiquitous, even at slowly spreading ridges, where they exist they provide a time record to observe the characteristics of mid-ocean ridge upwelling, melting and melt migration not only beneath active spreading centers but through a much longer spreading history.