Louisville Seamount Research Articles

Evidence for a contribution from two mantle plumes to island-arc lavas from northern Tonga

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Development of sediment drifts approaching an active plate margin under the SW Pacific Deep Western Boundary Current

The abyssal Pacific Ocean is fed by a 1000 km wide, deep western boundary current (DWBC) that flows northward along the continental margin, east of New Zealand. Between the passive margin of Chatham Rise and the subduction zone of Kermadec Trench, a distance of 1200 km, the DWBC has formed a suite of sediment drifts over a depth range of 2200–5700 m. Airgun and 3.5‐kHz profiles record a variety of drift types that reflect regional variations in bathymetry, sediment supply, and the tectonic/volcanic framework. On Chatham Rise the DWBC has deposited a sinuous, linear body along the south flank (3000 m), an extensive apronlike drift on the north flank (2200–4500 m), and a ridgelike drift about the rise base (4500–5200 m). The flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby Louisville Seamount Chain. Further downcurrent, the 250 km long Rekohu Drift (3600–4190 m) has developed northward to 39°S. South of this latitude, drifts comprise mainly reworked pelagic/hemipelagic material and sediment transported from distant southerly sources. In contrast, drifts north of 39°S have received a major injection of terrigenous sediment from Hikurangi Channel which runs 1400 km from New Zealand, eastward across the Hikurangi Plateau to disgorge on to the abyssal floor at the plateau edge. En route, turbidity current overspill from the channel has moved north under the influence of the shallow DWBC to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500–4200 m. Off Hikurangi Channel mouth, a large fan has accumulated. The DWBC has extended the fan into a drift running over 250 km along the base of Hikurangi Plateau (5150–5770 m) toward Kermadec Trench. Here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area. The seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the DWBC in the region with the late Oligocene opening of the Australian‐Antarctic seaway. Drift construction commenced during the Miocene but was punctuated in the late Miocene by another period of erosion that coincided with increased bottom water production in Antarctica. Deposition resumed in Plio‐Pleistocene times when large quantities of sediment from the rapidly rising landmass of New Zealand were injected into the boundary current. The modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of Hikurangi Plateau and widespread scour zones and sediment wave fields.

Hunting for azimuthal anisotropy beneath the Pacific Ocean region

Lateral variations in azimuthal anisotropy cause significant waveform anomalies in long‐period surface waves (ƒ < 15 mHz, T ≳ 70 s), as a result of coupling between fundamental branch Rayleigh and Love waves. These anomalies, termed “quasi‐Love” (QL) waves, have elliptical polarization, arrive slightly behind the Love wave but prior to the Rayleigh wave, and are observed on many propagation paths in the Pacific Ocean region. Our observations of quasi‐Love waves indicate the existence of strong lateral anisotropic gradients in the western Pacific Ocean, in particular, near Hawaii and seaward of the Tonga‐Kermadec, Kurile and Marianas‐Izu‐Bonin subduction zones. A lack of quasi‐Love generation beneath the Phillipine Sea suggests weak azimuthal anisotropy in the region. In the southwest Pacific Ocean, the long‐period quasi‐Love waveforms recorded at station SNZO (South Karori, New Zealand) can be fit well with a simple anisotropic Earth model with a 90° rotation in the orientation of the fast P velocity axis near the boundary between the Indo‐Australian and the Pacific plates. In this model, 6% P wave velocity anisotropy from the Moho to 210 km depth, with a NNE‐SSW fast direction (roughly parallel to the plate boundary), extends 500–1000 km east of the Tonga‐Kermadec trench, where it shifts to a WNW‐ESE orientation (parallel to fracture zones in the southernmost Pacific plate). This solution is nonunique, however, as a similar lateral variation of 2% S wave anisotropy at asthenospheric depths (100–300 km) generates similar waveform anomalies. We attribute the laterally varying anisotropy to either (1) flow variations in the asthenosphere, consistent with lateral shear detected in deep Tonga‐Kermadec seismicity, (2) variations in fossil spreading direction in the Cretaceous long normal polarity interval, (3) the disturbance of fossil anisotropy caused by the passage of the Louisville Ridge hotspot, and (4) lithospheric compression associated with continental collision along the Alpine Fault. The apparent location of the Love‐to‐Rayleigh “scatterer” favors options 1 and 3. Gradients in azimuthal anisotropy are inferred in the northwestern Pacific, associated with one or more of these mechanisms. A quasi‐Love wave observed on a path from Alaska to PPT (Papeete, Tahiti) is bandlimited to ƒ > 8 mHz, suggesting lateral variations with wavelength twice the spacing of fracture zones in the north central Pacific.

Large-scale deformation associated with ridge subduction

SUMMARY Continuum models are used to investigate the large-scale deformation associated with the subduction of aseismic ridges. Formulated in the horizontal plane using thin viscous sheet theory, these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction. Modelling was used to compare the Tonga arc and Louisville ridge collision with the New Hebrides arc and d’Entrecasteaux ridge collision, which have disparate arc-ridge intersection speeds but otherwise similar characteristics. Models of both systems indicate that diffuse deformation (low values of the effective stress-strain exponent n) are required to explain the observed deformation. Deformation is somewhat insensitive to the vertically integrated strength of the arc (inversely proportional to the Argand number Ar), but indicates that the arc lithosphere is not extremely weak (Ar < 100). Low values of both Ar and n suggest that the thermal structure is typical of ‘cold’ or ‘normal’ arcs and that deformation is dominated by flow in the lower crust and mantle. In addition, low values of n (approaching Newtonian flow) may indicate that specific deformation mechanisms dictate deformation of the arc lithosphere. Possible mechanisms include low-stress, grain-size dependent creep, pyroxenite-controlled rheology and mechanisms associated with water weakening. Changes in the boundary conditions greatly affect deformation within island arcs. High rates of arc-ridge intersection speed (Tonga-Louisville system) yield arc-parallel tension and crustal thickening in the wake of ridge subduction. In contrast, low rates of arc-ridge intersection speed (New Hebrides-d’Entrecasteaux system) yield compressional deformation directly arcward of the collision zone and transverse strike-slip faulting adjacent to the region of compressional deformation. Localized regions of extensional deformation along the frontal part of the arc adjacent to the collision zone may contribute to the formation of re-entrants.

A Mantle Plume Initiation Model for the Wrangellia Flood Basalt and Other Oceanic Plateaus

The vast Wrangellia terrane of Alaska and British Columbia is an accreted oceanic plateau with Triassic strata that contain a 3- to 6-kilometers thick flood basalt, bounded above and below by marine sedimentary rocks. This enormous outpouring of basalt was preceded by rapid uplift and was followed by gradual subsidence of the plateau. The uplift and basalt eruptions occurred in less than approximately 5 million years, and were not accompanied by significant extension or rifting of the lithosphere. This sequence of events is predicted by a mantle plume initiation, or plume head, model that has recently been developed to explain continental flood volcanism. Evidence suggests that other large oceanic basalt plateaus, such as the Ontong-Java, Kerguelen, and Caribbean, were formed as the initial outbursts of the Louisville Ridge, Kerguelen, and Galapagos hot spots, respectively. Such events may play an important role in the creation and development of both oceanic and continental crust.

Seismic strain rate and deep slab deformation in Tonga

Seismic strain rates estimated from volume‐averaged moment tensor data are combined with the strike, dip and extent of the subducting lithosphere obtained from residual sphere analysis and seismicity to map deformation in the deep Tonga slab. Tonga strain rate orientation tensors, constructed from the Harvard Centroid Moment‐tensor catalog, are consistent with slab deformation models in which the lithosphere encounters resistance to penetration at the 650‐km discontinuity. Those which sample lithospheric volumes from 18°S to 25°S at 500–700 km depth indicate that the axis of maximum shortening lies within 15° of the slab's downdip azimuth and plunge. The discrepancy between shortening and slab dip may be larger in northernmost Tonga (17°S–18°S) where slab morphology is less well constrained, but overall the slab is compressing roughly parallel to its trajectory. Tonga strain rate orientation tensors also indicate horizontal strain both parallel and perpendicular to the strike of the slab. The proportion of along‐arc strain rate decreases from northern to southern Tonga. The scalar rate of seismic strain peaks at certain latitudes, but its values averaged over northern and central Tonga and southern Tonga are roughly equal. Strain rate data may be related (nonuniquely) to constraints on slab structure and finite strain inferred from seismicity and travel time data. Assuming that aseismic and seismic flow have the same geometry, and that the total rate of strain is constant along the arc, the degree of cross‐strike advective thickening increases to the south, while along‐arc strain decreases. If along‐arc strain rate exclusively produces southward shear of the deeper lithosphere, and the amount of shear is estimated from the offset during subduction of the northern edge of the slab and the Louisville Ridge, thickening of at least 50% is required in central Tonga. This lower bound improves the resolution of minimum slab width provided by travel time observations. If 50% thickening occurs, an upper bound on the estimated rate of seismic moment release is large enough to account for scalar strain rate in the seismogenic core of the slab. Larger thickening factors or flow in the outer regions of the slab require aseismic deformation. Assuming that the observed travel time residuals are the result of a single, coherent slab, a synthesis of seismicity, strain rate and travel time data indicate a slab structure in northern Tonga which curves sharply to the west at the base of the upper mantle, but which strikes roughly to the north at shallower depths and in the lower mantle. Such a contortion in the deep lithosphere is consistent with the subduction zone configuration prior to 8 m.y. and the tectonic history of the region.

Mapping the Tonga Slab

We applied residual sphere analysis to 25 intermediate‐ and deep‐focus earthquakes in the Tonga subduction zone and mapped variations in slab strike and dip, depth extent and deep lateral advection at all latitudes along the arc. As in the northwest Pacific, travel time data from intermediate‐ and deep‐focus events are well modeled by simple thermal slab structures and a thermal coefficient of velocity (∂νp/∂T) of −0.5 ± 0.2 m/s/°K (i.e., peak velocity perturbations of about 3–7%). In all regions where the seismicity extends to depths of 600 km or more, the shortest thermal slab models that cannot be rejected at the 90% confidence level reach depths of 750 km (northern Tonga) to 850 km (southern Tonga). These models, however, require a value of ∂νp/∂T = −1.5 m/s/°K, 3 times the value suggested by laboratory studies. If the magnitude of ∂νp/∂T is constrained to be less than −1.0 m/s/°K, the minimum penetration depth varies from 800 km to 900 km. The travel time data are equally consistent with slab structures that deform at the base of the upper mantle, thickening by a factor of 3, and models that contain no advective thickening. Inferred slab structure varies considerably throughout the subduction zone, and the nearly ubiquitous match between the observed seismicity and detailed undulations in slab strike and dip is evidence that residual spheres do reflect near‐source heterogeneity. Travel times in southern Tonga (21.5°S–27°S) are consistent with, and in some cases require, slab models that bend to a steeper dip at depth. This phenomenon is also observed in parts of the northwest Pacific but is not required in central (19°S–21.5°S) and northern (13°S–19°S) Tonga. The residual spheres of both intermediate‐ and deep‐focus events suggest a sharp bend in slab strike at 25°S. This feature coincides with a kink in strike observed at all depths in the seismicity and occurs at the intersection of the trench and the Louisville Ridge. Northern Tonga is the locus of considerable complexity in deep slab structure. Seismicity, strain rate and travel time data are consistent with (although they do not uniquely require) a slab structure in northern Tonga which curves sharply to the west at the base of the upper mantle but which strikes to the north at shallower depths and in the lower mantle. The contorted segment would have begun subducting roughly 10 m.y. ago during a major plate reorganization. In northernmost Tonga, a decrease in the maximum depth of seismicity and inferred slab structure provides evidence for southward shear of deep material relative to the shallow slab. Compared to data from the northwest Pacific, where travel time observations also suggest slab penetration below the deepest earthquakes, Tonga seismicity and residual spheres show more complexity at depth and may indicate more deformation internal to the slab. These observations argue against convective models which are rigorously stratified at 650 km depth, but they are consistent with a discontinuity in mantle properties, such as an increase in viscosity or a small percent variation in chemistry, if it merely resists and deforms the descending slabs without acting as a complete barrier to flow.

Provenance mixing in an intraoceanic subduction zone: Tonga Trench-Louisville Ridge collision zone, southwest Pacific

Dredging on the lower slope of the Tonga forearc, at the intersection between the Louisville Ridge hotspot chain and the trench, yielded both Late Tertiary volcaniclastic sediments and Late Cretaceous slightly tuffaceous pelagic sediments. Petrography and phase chemistry of volcanic debris of Tertiary age samples indicates derivation from a low-K tholeiitic arc: volcanic glass has a low K 2O content and shows an Fe enrichment trend; plagioclase grains are high in An and low in Or components; pyroxene grains (calcic clinopyroxene, orthopyroxene and pigeonite) have low TiO 2 contents and show an Fe enrichment trend; alkali feldspar and biotite are absent. The composition of the Tertiary samples is similar to other Tonga forearc sediments and is consistent with their derivation from the adjacent magmatic arc. The character and composition of volcanogenic debris in Cretaceous age samples indicates derivation from an intraplate alkali igneous source: plagioclase compositions show an increasing Or component with increasing Ab; alkali feldspar is a rare additional component of these samples; calcic clinopyroxene has high TiO 2 contents and is titanaugite; amphibole is also rich in TiO 2 and is kaersutite; minor volcanic glass is rich in alkalis. Samples were dredged from the seaward slope of a small terrace on the lower trench slope. A planar reflector, located 2.6 km below the terrace is marked by an abrupt jump in seismic velocity and is interpreted as the top of an essentially undamaged Late Cretaceous guyot of the Louisville chain that was subducted at the Tonga trench about 0.5 Ma. This guyot is the likely source of the Late Cretaceous, intraplate igneous detritus collected at the dredge site. Some dredge samples yielded a mixed volcanic arc/intraplate provenance and/or Late Tertiary/Late Cretaceous ages. This probably represents the mixing of the Cretaceous seamount and Tertiary magmatic arc sources during post-collision slumping of the wedge of lower slope material as it re-establishes its critical taper.

Subduction of a Late Cretaceous Seamount of the Louisville Ridge at the Tonga Trench: A model of normal and accelerated tectonic erosion

The hotspot‐generated Louisville Ridge is a 4000‐km chain of seamounts (typically 2–2.5 km high and 10–40 km in diameter) and an underlying crustal swell (1.5 km high and 100+ km wide) trending NNW across the southwestern Pacific. The northwest end of the Ridge collides with the north trending Tonga Trench (26°S) which, just north of that point, is exceptionally deep (10.8 km) and lacks both a turbidite wedge and a bordering accretionary complex. The collision zone is moving rapidly southward. Multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2–3 km beneath the lower western trench slope, which is interpreted as the flat summit of a subducted guyot, Motuku, of the Louisville chain. Projected eastward, the summit plain passes 1–2 km above the trench floor. Dredging of the nearby inner trench wall recovered uppermost Cretaceous (Maestrichtian) oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted Louisville seamount The principal effects of hotspot‐ridge collision with a sediment‐starved trench are (1) the impacting seamounts are subducted rather than accreted, and (2) although some seamount rocks are temporarily accreted, the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km/m.y. Accelerated tectonic erosion is related to (1) fracturing, shearing and general weakening of arc substrate rocks as they are lifted by the swell, penetrated by impacting seamounts, and left to collapse as the ridge moves away, (2) a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell, (3) a possible elevation of the subduction decollement to account for the removal of as much as 30,000 km³ of material from a 400 km sector of the trench in 1 million years, and (4) a reduction in supply of arc‐derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge. "Normal" tectonic erosion in the Tonga Trench is apparently minor, and we conclude that the bulk of the ∼37,000 km³ of material which fills subducting grabens each million years is arc‐derived volcanic and pelagic sediment.

Late cretaceous pelagic sediments, volcanic ASH and biotas from near the Louisville hotspot, Pacific Plate, paleolatitude ∼42°S

Dredging on the deep inner slope of the Tonga Trench, immediately north of the intersection between the Louisville Ridge hotspot chain and the trench, recovered some Late Cretaceous (Maestrichtian) slightly tuffaceous pelagic sediments. They are inferred to have been scraped off a recently subducted Late Cretaceous guyot of the Louisville chain. In the vicinity of the Louisville hotspot (present location 50°26′S, 139°09′W; Late Cretaceous location ∼42°S, longitude unknown) Late Cretaceous rich diatom, radiolarian, silicoflagellate, foraminiferal and coccolith biotas, accumulated on the flanks of the guyot and are described in this paper. Rich sponge faunas are not described. ? Inoceramus prisms are present. Volcanic ash is of within-plate alkalic character. Isotope ratios in bulk carbonate δ 18O − 2.63 to + 0.85, δ 13C + 2.98 to 3.83) are normal for Pacific Maestrichtian sediments. The local CCD may have been shallower than the regional CCD, because of high organic productivity. In some samples Late Cretaceous materials have been mixed with Neogene materials. Mixing may have taken place on the flanks of the guyot during transit across the western Pacific, or on the trench slope during or after subduction and offscraping about 0.5 Ma.

Seismic stratigraphy of the central Tonga Ridge

Seismic stratigraphic analyses of single- and multichannel seismic reflection profiles from the central part of the Tonga arc system, SW Pacific, are used to interpret its geological evolution. On the central Tonga Ridge (a high-standing part of the frontal arc, as defined by Karig (1970)) and its flanks between 18°S and 22°S, we identify and map six seismic sequences overlying inferred lower(?)—middle Eocene volcanic (acoustic) basement. Available island sections and boreholes, along with seismic facies, suggest that the upper Eocene to upper Miocene stratigraphic succession on the central Tonga Ridge consists of volcaniclastic turbidites and hemipelagic sediments. In contrast, carbonate deposition predominates from the early Pliocene to the present. We interpret major hiatuses during the early Oligocene, early Miocene and early Pliocene. Based upon the available seismic data and previous results, we infer that the extreme eastern Tofua Trough, a partially filled basin that now separates the central Tonga Ridge from the Tofua Ridge (the modern volcanic arc), may be the site of latest Eocene or Oligocene extension which may be related to rifting and back-arc spreading in the South Fiji Basin to the west. Another late Miocene or early Pliocene phase of rifting deformed strata deposited in the Palaeogene basin, and led to creation of at least part of the modern Tofua Trough. Available evidence suggests that the Neogene extension is related to initiation of sea floor spreading in the Lau Basin. Widespread carbonate deposition beginning in the early Pliocene suggests a decrease of volcanic activity in the vicinity of the central Tonga Ridge. An apparent N-S migration of Miocene(?) and younger depocentres may be related to sequential uplift of parts of the central Tonga Ridge in response to progressive southward subduction of the Louisville Ridge at the Tonga Trench.

LARGE EARTHQUAKES IN THE TONGA REGION ASSOCIATED WITH SUBDUCTION OF THE LOUISVILLE RIDGE

Subduction of the Louisville Ridge influences both interplate and intraplate seismicity in the southern Tonga region (22°S–26°S). Five earthquakes with diverse mechanisms and seismic moments greater than 1 × 1027 dyn cm have occurred in this region since 1975. These include two outer rise, one underthrusting, and two intermediate depth events, all of which occurred in close proximity to the subducting ridge. In order to understand better the nature of these events and to determine the role played by the Louisville Ridge in the subduction process, detailed rupture models have been determined for each earthquake by body and surface wave analysis. Three events occurred at shallow depths near the oblique intersection of the Louisville Ridge and the Tonga are. These are the compressional (thrust) outer rise event of October 11, 1975 (Mw = 7.4); the tensional (normal) outer rise event of October 10, 1977 (Mw = 7.4); and the subsequent underthrusting event on December 19, 1982 (Mw = 7.5), the latter being the largest documented interplate event in Tonga. The intermediate depth events of June 22, 1977 (d = 70 km, Mw = 8.1) and April 13, 1980 (d = 160 km, Mw = 7.6) both have normal fault mechanisms; however; the 1977 event has a downdip tension axis, whereas the 1980 event has a downdip compression axis. The 1977 event is unusual due to its large size and the fact that most intermediate depth activity in Tonga involves downdip compression. The body wave radiation for the 1977 event indicates concentrated rupture in the depth range 70–90 km, near the downdip projection of the Louisville Ridge. It appears that the Louisville Ridge perturbs the regional stress regime at both intermediate and shallow depths in the slab. The ridge appears to be a buoyant feature that produces locally strong interplate coupling to the north of its junction with the Tonga trench, which accounts for the 1982 thrust event and the preceding 1975 compressional outer rise event located seaward of the thrust rupture zone. South of the ridge intersection the plate interface appears to be less strongly coupled: tensional stresses due to slab pull are communicated to shallow depths, resulting in the October 10, 1977 (Mw = 7.4), tensional outer rise event. The locally enhanced interplate coupling, along with a downdip stress barrier to the intraplate compressional stresses that arise from deep resistance to the slab subduction, are two effects of the ridge that appear to explain the accumulation of in‐plate tension in the region of the June 22, 1977, event, the April 13, 1980, downdip compressional event reflects a return to in‐plate compression following the great 1977 rupture which relaxed the tensional stress. The unusual stress regime of the subducting ridge and its oblique trajectory down the plate are thus responsible for the diversity of large earthquake occurrence in the southern Tonga arc.

Cooling of the oceanic lithosphere—evidence from geoid anomalies across the Udintsev and Eltanin fracture zones

The often step-like variation in the marine geoid across fracture zones is believed to reflect the change in age of the ocean floor, and hence in temperature structure of the plate across the fracture zone. The rate of change of the geoid with age has been estimated as a function of age from geoid offsets across fracture zones and used to constrain thermal models of lithospheric cooling. Results from three studies of the geoid offsets across the Mendocino fracture zone are consistent with a thermal plate cooling model having a plate thickness between 100 and 125 km. This is in contrast to results obtained from the combined measurements from fracture zones in the South Pacific, which are not consistent with either the plate model or boundary layer model of cooling. To examine this inconsistency we have measured geoid offsets on Seasat profiles across the Udintsev and Eltanin fracture zones in the South Pacific and tested the robustness of the measurement technique. We used a second-order polynomial that included a step function centered on the fracture zone to estimate the regional geoid and the geoid offset. The polynomials were fit to the geoid profiles using least-squares minimization criteria. Data within a certain distance of the fracture zone were excluded from the fitting procedure in order to minimize the effects of horizontal cooling and lithospheric deformation near the fracture zone. We find that the estimate of the geoid offset is very sensitive to the length of data excluded and that generally, an exclusion width of 100–200 km is needed to avoid anomalies (unrelated to the regional geoid offset) near the fracture zone. Exclusion widths of less than 100 km generally lead to unpredictable errors in the geoid offset estimates. Using exclusion widths of between 150 and 200 km on most profiles, we constructed geoid slope (geoid offset divided by the age offset) versus average age plots for each of the limbs of the two fracture zones. Similar trends were found on both the east and west limbs of the Eltanin and the east limb of the Udintsev. The trends were inconsistent with a single thermal plate thickness and instead showed an initial decrease in slope between 0 and 35 m.y., followed by an abrupt increase followed again by a decrease in slope. In contrast, the west branch of the Udintsev shows an increase in geoid slope values between 20 and 35 m.y., followed by a decrease. The difference in observed trends across the western limb of the Udintsev may be due to the effects of the Louisville Ridge (north of the fracture zone) on the regional geoid. The similarity in trends on the remaining three limbs argues against the effects of isolated thermal or bathymetric anomalies and appears instead, to reflect a general feature of the geoid-slope versus average age relationship across fracture zones. Although the thermal plate cooling model is successful in predicting both seafloor depths and heat flow values out to ages of at least 80 m.y. B.P., it cannot explain the observed geoid slope values for these two fracture zones. It is not clear at this point whether this is due to inadequacies in the cooling model or to peculiarities in fracture zone evolution, possibly caused by small-scale convection driven by the horizontal temperature gradients beneath fracture zones.

Origin of the Louisville Ridge and its relationship to the Eltanin Fracture Zone System

We have combined shipboard and Seasat altimeter derived data in an intergrated geological and geophysical study of the Louisville Ridge; a 3500‐km‐long seamount chain extending from the Tonga trench to the Eltanin Fracture Zone. A break in the smooth trend of the ridge at latitude 37.5°S has been recognized in both bathymetric and altimetric data. The 40Ar‐39Ar dating of rocks dredged either side of the break suggest that it is analogous to the bend in the Hawaiian‐Emperor seamount chain. Although the general trend of the ridge can be fit by small circles about Pacific absolute motion poles determined from other seamount chains, the new bathymetric and age data allow us to refine Pacific absolute motion poles. The continuity in smooth trend of the ridge and the Eltanin Fracture Zone suggests some relationship between them. However, a major offset developed on this transform between 60 and 80 Ma, prior to the oldest dated rocks from the ridge. Although magmatism was more or less continuous on the ridge during 28–60 Ma, it probably occurred on crust with little or no offset. Thus magmatism appears to have been little influenced by the developing fracture zone. By 28 Ma, the distance between the magmatic source and the fracture zone had decreased sufficiently for a portion of the ridge to have been emplaced on crust with an offset. After about 12 Ma, however, volcanic activity on the Louisville Ridge apparently waned, despite a possible influence on the magmatism of the fracture zone.

The application of inverse theory to seamount magnetism

The traditional least squares method for modeling seamount magnetism is often unsatisfactory because the models fail to reproduce the observations accurately. We describe an alternative approach permitting a more complex internal structure, guaranteed to generate an external field in close agreement with the observed anomaly. Potential field inverse problems like this one are fundamentally incapable of a unique solution, and some criterion is mandatory for picking a plausible representative from the infinite‐dimensional space of models all satisfying the data. Most of the candidates are unacceptable geologically because they contain huge magnetic intensities or rapid variations of magnetization on fine scales. To avoid such undesirable attributes, we construct the simplest type of model: the one closest to a uniform solution as measured by the norm in a specially chosen Hilbert space of magnetization functions found by a procedure called seminorm minimization. Because our solution is the most nearly uniform one we can say with certainty that any other magnetization satisfying the data must be at least as complex as ours. The theory accounts for the complicated shape of seamounts, representing the body by a covering of triangular facets. We show that the special choice of Hilbert space allows the necessary volume integrals to be reduced to surface integrals over the seamount surface, and we present numerical techniques for their evaluation. Exact agreement with the magnetic data cannot be expected because of the error of approximating the shape and because the measured fields contain noise of crustal, ionospheric, and magnetospheric origin. We examine the potential size of the various error terms and find that those caused by approximation of the shape are generally much smaller than the rest. The mean magnetization is a vector that can in principle be discovered from exact knowledge of the external field of the seamount; this vector is of primary importance for paleomagnetic work. We study the question of how large the uncertainty in the mean vector may be, based on actual noise, as opposed to exact, data; the uncertainty can be limited only by further assumptions about the internal magnetization. We choose to bound the rms intensity. In an application to a young seamount in the Louisville Ridge chain we find that remarkably little nonuniformity is required to obtain excellent agreement with the observed anomaly while the uniform magnetization gives a poor fit. The paleopole position of ordinary least squares solution lies over 30° away from the geographic north, but the pole derived from our seminorm minimizing model is very near the north pole as it should be. A calculation of the sensitivity of the mean magnetization vector to the location of the magnetic observations shows that the data on the perimeter of the survey were given the greatest weight and suggests that enlargement of the survey area might further improve the reliability of the results.

Anomalous high-frequency wave propagation from the Tonga--Kermadec seismic zone to New Zealand

Summary. Bulletins of the International Seismological Centre (ISC) show very large residuals, up to 15 s early, for arrivals from events in the TongaKermadec subduction zone to the New Zealand network of seismometers. The very early arrivals are confined to events south of about 22S, and shallower than about 350 km. The waveforms show two distinct phases: an early, emergent, first phase with energy in the high-frequency band 2-10 Hz, and a distinct second phase, containing lower frequency energy, arriving at about the time predicted by JB tables. The residuals are attributed to propagation through the cold, subducted lithosphere, which has a seismic velocity 5 per cent faster, on average, than normal. Ray tracing shows that the ray paths lie very close to the slab for events south of 22S, but pass well beneath the slab for events further north, corresponding to the change in residual pattern. This characteristic of the ray paths is due to the curved shape of the seismic zone, and in particular to the bend in the zone where the Louisville ridge intersects the trench at 25s. The residuals can only be explained if the high velocity anomaly extends to a depth of 450 km in the region of the gap in deep seismicity from 32 to 36s. The very high-frequency character of the first phase requires the path from the bottom of the slab to the stations to be of high Q, and to transmit 2-10 Hz energy with little attenuation. The absence of low-frequency energy in the first phase is due to the narrowness of the high-velocity slab, which transmits only short-wavelength waves. The second phase, which contains low frequencies, is identified as a P-wave travelling beneath the subducted slab in normal mantle. There is no need to invoke any special structures, such as low-velocity waveguides or reflectors, to explain any of the observations. The S-wave arrivals show similar effects.

On geoid heights and flexure of the lithosphere at seamounts

The sea surface height has now been mapped to an accuracy of better than ±1 m by using radar altimeters on board orbiting satellites. The major influence on the mean sea surface height is the marine geoid which is an equipotential surface. We have carried out preliminary studies of how oceanic volcanoes, which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges, contribute to the marine geoid. Simple one‐ and two‐dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum. Previous studies based on gravity and bathymetry data and uplift/subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness Te increase with the age of the lithosphere at the time of loading. The models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid‐ocean ridge crest will be associated with relatively low amplitude geoid anomalies (about 0.4–0.5 m/km of height), while seamounts formed on relatively old lithosphere, on ridge flanks, will be associated with much higher amplitude anomalies (1.4–1.5 m/km). Studies of the Seasat altimetric geoid prepared by NASA's Jet Propulsion Laboratory support these model predictions; geoid amplitudes are relatively low over the Mid‐Pacific Mountains and Line Islands, which formed on or near a mid‐ocean ridge crest, and relatively high over the Magellan Seamounts and Wake Guyots, which formed off ridge. Direct modeling of the altimetric geoid over these features is complicated, however, by the wide spacing of the satellite tracks (which can exceed 100 km) and poor bathymetric control beneath individual satellite tracks. In regions where multibeam bathymetric surveys are available, models can be constructed that fit the altimetric geoid to better than ±1 m. Studies of geoid anomalies over the Emperor seamount chain, for example, suggest that these seamounts formed on 20–30 Ma old oceanic lithosphere, while anomalies over the Louisville ridge and the Hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 Ma. This indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting. We examine the implications of these results to studies which attempt (1) to separate temporal variations of the sea surface due to oceanographic effects, (2) to predict seamounts directly from satellite‐derived gravity and geoid data, and (3) to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due to mantle convection.

Geoid heights over the Louisville Ridge (South Pacific)

Geoid heights over the Louisville Ridge seamount chain, deduced from the Seasat satellite altimeter data, have been analyzed and interpreted. Two uncharted seamounts belonging to the chain have been detected. The elastic thickness of the oceanic lithosphere has been determined along the chain. This was done rigorously using the three‐dimensional geoid and bathymetry maps and was also computed approximately using spectral methods on geoid and bathymetry profiles. A slightly increasing elastic thickness from the southeast to the northwest, in the range 15.5–22.5 km has been estimated.

The empirical age‐depth relation and depth anomalies in the Pacific Ocean Basin

This paper presents a new age‐depth relation and a comprehensive depth anomaly map for the Pacific Ocean. Age, depth, and sediment thickness were averaged for each 1°×1° area. Bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water. Seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep. Hot spots were treated as thermal anomalies, and data within 800 km of them were eliminated from further analysis. An age‐depth equation was determined by fitting a least squares line to the remaining data. The resulting depth versus age distribution confirms the linear dependence of depth on √age up to 80 Ma. All older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted. This “flattening” of the age‐depth distribution requires that a separate function be fit to data older than 80 Ma. As a result, data representing the deepest seafloor (older than 80 Ma) were selected for curve fitting, and the age‐depth distribution was approximated by an exponential function. It is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age‐depth data from the Pacific Ocean. A depth anomaly map prepared by comparing observed depth with the depth predicted by the new age‐depth formula shows several newly recognized features, including swells over the Louisville Ridge in the South Pacific and in the Gulf of Alaska Seamounts.

Seismic quiescence and asperities in the Tonga‐Kermadec Arc

Highly significant temporal and spatial seismicity rate changes were found along the TongasKermadec plate boundary. The standard deviate (z) test was used to compare systematically the rates in different periods and different volumes, and to test for uniqueness of anomalies found. The NOAA hypocenter data file showed a pronounced decrease in the number of small‐magnitude earthquakes reported after 1969 at the same time as such a reporting decline exists in the worldwide data. Therefore only earthquakes with mb≥4.9 were studied. A search for quiescence before recent mainshocks yielded two uniquely significant precursory anomalies, one missed mainshock and one false alarm. Quiescence started 63 months before the January 1976, M = 8.0, Kermadec; and 25 months before the June 1977, Tonga, M = 7.2, earthquakes. The December 1975, M = 7.5, shock was not preceded by a seismicity rate change. The seismic gaps in the Tonga‐Kermadec arc with sufficient seismicity for analysis (17°–22° and 31°–34°) show constant rates up to the present. Therefore, on the basis of the quiescence hypothesis, earthquakes are not forecast for these segments. The stress drops, Δσ, of 380 interface events were estimated using the mb/Ms method. The average Δσ was uniform along the island arc with the exception of two segments with average Δσ of less than half the normal values. These segments coincided with the two largest recent interface ruptures; however, in the northern Tonga the Δσ was low before the 1975 rupture, whereas in the Kermadec the 1976 aftershocks showed low Δσ. Asperities could not be identified on the basis of high stress drops. A systematic study of the seismicity rate as a function of space defined large variations along the arc. The areas of highest and lowest seismicity rate correlate with the northern end of the trench and with the Louisville ridge intersection, respectively. However, other large rate contrasts exists between arc segments without obvious tectonic differences. This implies that seismicity doughnut patterns do not reliably define precursors. One segment of outstandingly high seismicity rate (30°–31°S) is identified as a major asperity because it contains a large number of m&gt;6 events and a large number of recent mainshock‐aftershock sequences and because it stopped the great 1976 rupture. We propose that the only other segment of high seismicity rate (near 20.5 °S) also represents a major asperity.