Rift Tip Research Articles

GPS-Derived Strain Rates on an Active Ice Shelf Rift

Ice shelves are important components of the Antarctic ice sheet due to their ice-ocean-atmosphere interface and vulnerability to global increases (or decreases) in atmospheric and oceanic temperatures. The development of rifts, which are fractures that penetrate through the entire ice shelf thickness, precede large tabular iceberg detachment and can lead to ice shelf break-up. Changes in strain rates on an active propagating rift system on the Amery Ice Shelf, East Antarctica are determined using in-situ Global Positioning System (GPS) measurements. Results for the 2002/03 Antarctic summer period (Dec-Feb) confirm previous observations by [2] that rift propagation occurs in episodic bursts separated by several days. Transverse-to-flow (i.e. parallel-to-rift) strain rates exceed longitudinal-to-flow (i.e. normal-to-rift) rates by up to a factor of 5 and maximum principal strain rates around the rift tip vary from 12 to 21 [x 10-3/yr]. A rotation in the direction of the principal strain is evident around the rift tip, indicating a change in the mechanics of rift fracture. It is demonstrated that cumulative sum analysis [12], obtained by differencing a pair of residual baseline time series situated approximately normal and parallel to the rift, is an effective method to detect small baseline length changes.

Geochemistry, petrogenesis and tectonic setting of the Samothraki mafic suite, NE Greece: Trace-element, isotopic and zircon age constraints

The Samothraki mafic suite in the north-eastern Aegean Sea, Greece, is an ‘in situ’ magmatic complex comprising gabbros, sparse dykes and basalt flows and pillows cut by late dolerite dykes. We have determined the age of the complex by SHRIMP zircon geochronology of a gabbro as 159.9 ± 4.5 Ma (i.e. Oxfordian; early Late Jurassic), which precludes any correlation with the so-called Lesvos ophiolite further south (253.1 ± 5.6 Ma; Latest Permian). Six distinct, hitherto unrecognised, geochemical groups have been identified among the basalts and dolerites of the Samothraki mafic suite on the basis of trace-element and Nd–Sr isotopic characteristics. All groups show the presence of an enriched component in their source region, be that a long-term enriched mantle reservoir or an enriched melt. They also invariably show evidence of the addition of subduction-related hydrous fluids. Some groups testify to recent depletion events of long-term enriched mantle, some to the involvement of strongly depleted, recently refertilised mantle, and others to significant addition of melts either from subducted sediments or continental crust. None of the groups bear resemblance to mid-ocean ridge or back-arc basin basalts thus suggesting that the Samothraki mafic suite cannot represent mature back-arc basin crust. In view of recent advances in back-arc basin geology and the data collected here, the Samothraki mafic suite could represent a slab-edge segment of an evolved back-arc basin near the trench, a situation that satisfies the inflow of fertile mantle, the strong signature of sediment melts and the rare occurrence of boninite-like melts. Alternatively, it could represent the embryonic stages of back-arc basin formation either by a propagating rift tip into continental crust or by oblique subduction causing back-arc rifting of continental crust and creation of nascent transtensional basins. Restoration of block configuration in NE Greece before extensional collapse of the Hellenic hinterland and exhumation of the Rhodope Metamorphic Core Complex (mid-Eocene to mid-Miocene) results in a continuous ophiolite belt from Guevgueli to Samothraki, thus assigning the latter to the Innermost Hellenic Ophiolite Belt. We propose that the Samothraki mafic suite originated via rift propagation of the Sithonia ophiolite spreading ridge into the Chortiatis calc-alkaline arc (and its basement) which was partly assimilated. Our model does not favour southward intra-oceanic subduction of the Meliata–Maliac Ocean under the Vardar Ocean whereby Samothraki is located in the back-arc spreading centre. The autochthonous rift-related origin, the enriched geochemical characteristics and the lack of common features with known ophiolite types suggests calling the igneous complex of Samothraki Island a “mafic suite” rather than an “ophiolite”.

Tabular iceberg collisions within the coastal regime

AbstractDuring 2000–07, five giant icebergs (B15A, B15J, B15K, C16 and C25) adrift in the southwestern Ross Sea, Antarctica, were instrumented with global positioning system (GPS) receivers and other instruments to monitor their behavior in the near-coastal environment. The measurements show that collision processes can strongly influence iceberg behavior and delay their progress in drifting to the open ocean. Collisions appear to have been a dominant control on the movement of B15A, the largest of the icebergs, during the 4-year period it gyrated within the limited confines of Ross Island, the fixed Ross Ice Shelf and grounded C16. Iceberg interactions in the near-coastal regime are largely driven by ocean tidal effects which determine the magnitude of forces generated during collision and break-up events. Estimates of forces derived from the observed drift trajectories during the iceberg-collisioninduced calving of iceberg C19 from the Ross Ice Shelf, during the iceberg-induced break-off of the tip of the Drygalski Ice Tongue and the break-up of B15A provide a crude estimate of the stress scale involved in iceberg calving. Considering the total area the vertical face of new rifts created in the calving or break-up process, and not accounting for local stress amplification near rift tips, this estimated stress scale is 104 Pa.

Seismicity and deformation associated with ice-shelf rift propagation

Previous observations have shown that rift propagation on the Amery Ice Shelf (AIS), East Antarctica, is episodic, occurring in bursts of several hours with typical recurrence times of several weeks. Propagation events were deduced from seismic swarms (detected with seismometers) concurrent with rapid rift widening (detected with GPS receivers). In this study, we extend these results by deploying seismometers and GPS receivers in a dense network around the tip of a propagating rift on the AIS over three field seasons (2002/03, 2004/05 and 2005/06). The pattern of seismic event locations shows that icequakes cluster along the rift axis, extending several kilometers back from where the rift tip was visible in the field. Patterns of icequake event locations also appear aligned with the ice-shelf flow direction, along transverse-to-rift crevasses. However, we found some key differences in the seismicity between field seasons. Both the number of swarms and the number of events within each swarm decreased during the final field season. The timing of the slowdown closely corresponds to the rift tip entering a suture zone, formed where two ice streams merge upstream. Beneath the suture zone lies a thick band of marine ice. We propose two hypotheses for the observed slowdown: (1) defects within the ice in the suture zone cause a reduction in stress concentration ahead of the rift tip; (2) increased marine ice thickness in the rift path slows propagation. We show that the size–frequency distribution of icequakes approximately follows a power law, similar to the well-known Gutenberg–Richter law for earthquakes. However, large icequakes are not preceded by foreshocks nor are they followed by aftershocks. Thus rift-related seismicity differs from the classic foreshock and aftershock distribution that is characteristic of large earth quakes.

Topographic analysis of Devana Chasma, Venus: implications for rift system segmentation and propagation

Devana Chasma is a rift system on Venus formed in association with the Beta Regio and Phoebe Regio volcanic highlands, which are interpreted as mantle plumes. We present a new analysis of a 2500-km-long segment of Devana. Based on the rift topography, the horizontal extension across the rift boundary faults is 3–9 km. This is a lower bound on the total rift extension because the altimetry does not resolve the topographic relief across the numerous faults that are visible in radar images of the rift floor. The total extension across Devana is approximately 20 km, similar in magnitude to continental rift systems on Earth. Rift flank elevations are up to 3.1 km in the regions nearest the mantle plumes and decay strongly with increasing distance from the plumes, indicating a strong thermal component to the rift flank topography, unlike the situation usually reported for terrestrial rifts. As on Earth, there is also a flexural uplift component to the flank topography. Rift depths are up to 2.5 km below the surrounding plains, with considerable along-strike variability. There is a 600 km lateral offset along Devana Chasma near the mid-point between the two mantle plumes. Devana most likely formed as two distinct rifts due to the horizontal stresses created by outflow from the upwelling plumes. The offset zone formed as a result of the interaction between the two rift tips, which requires that upwelling at the two mantle plumes overlapped in time.

Dynamics of continental rift propagation: the end-member modes

An important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation. In regions of rift propagation, specifically transition zones from continental rifting to seafloor spreading, it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones. The focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere. More specifically, this study explores how modeled rift behavior depends on the value of rheological parameters of the crust. A three-dimensional finite element model is used to simulate lithosphere deformation in an extensional regime. The chosen approach emphasizes understanding the tectonic forces involved in rift propagation. Dependent on plate strength, two end-member modes are distinguished. The stalled rift phase is characterized by absence of rift propagation for a certain amount of time. Extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone, which requires a buildup of shear stresses near the rift tip and significant intra-plate deformation. This stage represents a situation in which a rift meets a locked zone. Localized deformation changes to distributed deformation in the locked zone, and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend. In the alternative rift propagation mode, rift propagation is a continuous process when the initial crust is weak. The extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses. Model predictions address aspects of previously unexplained rift evolution in the Laptev Sea, and its contrast with the tectonic evolution of, for example, the Gulf of Aden and Woodlark Basin.

Patterns of stress at midocean ridges and their offsets due to seafloor subsidence

The effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two-dimensional (2D) horizontal stresses arising from isostatically compensated vertical loads. The topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge-parallel tensile stresses. These may predominate over the ridge-perpendicular stresses and explain observations at midocean ridge offsets such as (1) oblique normal faulting at ridge-transform intersections trending up to 60° relative to the ridge axis, and (2) nontransform offsets consisting of structures oriented at 45° relative to the ridge trend. At midocean ridge overlaps, rotation of the ridge-parallel tensile stresses favours rift propagation at more than 45° relative to the ridge trend. It is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets, and as a result large overlaps may eventually start to rotate and evolve into a microplate.

Glass compositions, plume‐ridge interaction, and hydrous melting along the Galápagos Spreading Center, 90.5°W to 98°W

The Galápagos Spreading Center (GSC) between 90.5°W and 98°W manifests its interaction with the nearby Galápagos plume by way of variations in lava geochemistry, crustal thickness, and morphology along the ridge axis. Natural glasses from stations with ∼9 km average spacing were analyzed for major and minor elements, H2O, and CO2. Samples can be classified as enriched mid‐ocean ridge basalts (E‐MORB), transitional MORB (T‐MORB), or normal MORB (N‐MORB) on the basis of K/Ti ratios. E‐MORB dominate the GSC east of 92.6°W. T‐MORB are mainly found between 92.6°W and 95.5°W. West of the propagating rift tip at 95.5°W, N‐MORB dominate. High K/Ti E‐MORB also have higher H2O, Al2O3, and Na2O and lower FeO*, SiO2, and CaO/Al2O3 relative to N‐MORB at similar values of MgO, characteristics consistent with lower mean extents of partial melting relative to N‐MORB. We examine the melting process along this section of the GSC with a set of equations that simulate a deep zone of hydrous melting related to the depression of the mantle solidus by H2O. This model constrains the range of mantle source compositions, the depth of the additional hydrous melting zone, the melt productivity in the hydrous region, and the ratio of mantle flow rate through the hydrous zone relative to the anhydrous zone (Uw/U0) that can explain the measured crustal thickness as well as the fractionation‐corrected concentrations of K, Na2O, H2O, and Ti along the GSC. Far from the hot spot, the measured crustal thickness and N‐MORB compositions are explained by passive mantle upwelling (Uw/U0 = 1), mean melt fraction () ∼ 0.06, and a source with ∼35 ppm K, 130 ppm H2O, 2300 ppm Na2O, and 1050 ppm Ti. The transitional zone has a source enriched in K and could have a slight excess plume‐driven flow through the hydrous melting zone (Uw/U0 ≤ 1.5). The crustal thickness and glass compositions in the “enriched” region of the GSC nearest the hot spot are best explained by only a slight increase in the temperature of the mantle (<∼20°C), coupled with a mantle source moderately enriched (relative to N‐MORB source) and plume‐driven flow through the hydrous zone of Uw/U0 = 1.5–3.5.

Easter microplate dynamics

We use two‐dimensional elastic finite element analysis, supplemented by strength estimates, to investigate the driving mechanism of the Easter microplate. Modeled stresses are compared with the stress indicators compiled from earthquake focal mechanisms and structural observations. The objective is to constrain the tectonic forces that govern the Easter microplate rotation and to test the microplate driving hypothesis proposed by Schouten et al. [1993]. We infer that the mantle basal drag cannot drive the microplate rotation but opposes it, and that the asthenospheric viscosity is no more than about 1 × 1018 Pa s. At most, the basal drag comprises 20% of the force resisting microplate rotation. The outward pull of the main plates can drive the rotation by shear drag applied along the northern and southern boundaries of the microplate. However, we propose an additional driving force which arises from the strong variation of the ridge resistance force along the east and west rifts, so that the main driving torques come from the pull of the major plates acting across the narrowing and slowing rifts. This requires the strength to increase substantially toward the rift tips due to thickening of the brittle lithosphere as the spreading rate slows.

3‐D crustal velocity structure at the rift tip in the Western Woodlark Basin

We present a three‐dimensional (3‐D) crustal velocity model for part of the western Woodlark Basin in the SW Pacific, where rifting of a continental arc transitions to seafloor spreading. Velocities to ∼14 km depth are constrained by regularized inversion of first arrival traveltimes recorded at 14 ocean bottom instruments from a network of airgun profiles within an 80 km × 80 km area. The 3‐D velocity model images the axis and flanks of the rift basin adjacent to the westernmost spreading segment. Velocity contours that dip north at ∼10° parallel seismic reflectors from within a thick ophiolitic basement. High velocities (7.5 km/s) at the base of our model and preliminary analysis of earthquake P‐wave arrival times are consistent with an average crustal thickness of 15–20 km. Anomalously shallow upper mantle (8 km/s) velocities occur 6 km south of the spreading center at a depth of 3 km below the seafloor, and in the western part of the model, ∼30 km east of the D'Entrecasteaux Islands, at a depth of 6 km below the seafloor.

Crustal structure, accretionary processes and rift propagation: a gravity study of the intermediate-spreading Valu Fa Ridge, Lau Basin

The Valu Fa Ridge is an intermediate‐spreading (full rate of 60 mm yr−1) ridge located in the Lau Basin. In 1995 this ridge was surveyed using a multidisciplinary, geophysical approach to image crust and upper mantle structure, with the aim of investigating the processes of oceanic crustal accretion in a back‐arc tectonic environment. As part of this experiment a network of gravity profiles was acquired, together with seismic, magnetic, swath bathymetry and controlled‐source electromagnetic data. Presented in this paper are the results of forward modelling of a subset of the acquired gravity profiles, two oriented ridge‐perpendicular and one ridge‐parallel, using the preferred seismic models of Turner . (1999) as a basis of initial model construction. In addition, the gravity data set in its entirety has been used to calculate the mantle and residual mantle Bouguer anomalies with the aim of investigating variability in crustal structure, both density and layer thickness, and the nature of the underlying upper mantle. Of particular interest are the overlapping spreading centre between the Central and Northern Valu Fa Ridges, where seismic modelling implies a generally thickened crust and a magma chamber located beneath the overlap basin rather than separate chambers supplying each ridge, and the propagating rift tip and associated basin‐bounding pseudo‐fault. Modelling results suggest that the pre‐ and post‐rift crusts have different compositional origins, with lower densities required > 12 km off‐axis to fit the observed free‐air gravity anomaly. The locations of the transitions into regions of lower density correspond with those of Turner . (1999) derived from seismic modelling, which in turn correspond in location to the rift‐related pseudo‐fault identified by Wiedicke & Collier (1993). Calculation and interpretation of the mantle and residual mantle Bouguer anomalies also confirms the lower off‐axis densities and indicates a general increase in crustal thickness northwards towards the overlapping spreading centre between the Central and Northern Valu Fa Ridge segments. The presence of thicker crust, corresponding to a region of negative mantle and residual mantle Bouguer anomaly, implies that the region surrounding the overlapping spreading centre has been in the recent geological past, or is presently, the site of an increased magma supply. The residual mantle Bouguer anomaly also reveals features related directly to off‐axis lateral variation in density and layer thickness, associated with the southward propagation of the Valu Fa Ridge into the Havre Trough. Modelling of the north–south variation in crustal thickness along‐axis shows that the anomaly trend can be explained simply by thicker crust beneath each of the Valu Fa Ridge segments and the overlapping spreading centre between the Northern and Central Valu Fa Ridges. The observation of segment‐centred crustal thickening is consistent with models of ridge segmentation that suggest that melt influx is located towards segment centres and that segment length is controlled by the maximum lateral extent to which this melt can flow ( Macdonald . 1988 ; Tolstoy . 1993 ). However, the overlapping spreading centre remains anomalous from these models in that it also appears to be associated with thicker crust and an increased melt supply.

Comparisons of gravity anomalies at pseudofaults, fracture zones, and nontransform discontinuities from fast to slow spreading areas

Published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading. We investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments. We find that altimetry‐based gravity anomalies on the Mathematician, Bauer, Easter, Juan Fernandez, and northern Chile Ridge pseudofaults, located in 75–150 mm/yr (full rate) seafloor spreading environments, are similar in amplitude and form to Atlantic fracture zones with 20–30 mm/yr spreading rates. A 5–15 mGal positive mantle Bouguer anomaly is observed on the pseudofault bounding the eastern Juan Fernandez microplate, comparable to those at some similar age‐offset nontransform discontinuities in slow spreading environments. Our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust, the latter of which remains frozen at pseudofaults. We predict that any pseudofaults with age offsets more than ∼1 m.y. and not coincident with hotspot volcanism will be associated with thin (and possibly unusually dense) crust, even in superfast seafloor spreading environments.

Hydrothermal sedimentation adjacent to the propagating Valu Fa Ridge, Lau Basin, SW Pacific

The partition geochemistry, and hydrothermal metal accumulation rates (AR) during the Late Quaternary (<120 ka), have been investigated in eight cores adjacent to the Valu Fa Ridge (VFR) in the southern Lau Basin. The sediments consist mainly of foraminiferal nannofossiliferous ooze with ash and abundant amorphous Fe and Mn oxyhydroxides. Plume fall-out from active hydrothermal vents was collected in the immediate vicinity of hydrothermal outlets. It contains barite, sphalerite and feldspar. Solutions adjacent to known outlets exhibit a significant enrichment in Mn, Al, Pb, Ag, Co, and Zn; these elements may find their way into the sediments by precipitation and/or adsorption. Sea water at greater distances (>3 km) from hydrothermally active areas still shows an enrichment of Mn, Ag, Ba and Zn when compared to regional (500 m depth) Lau Basin sea water. The elements, Ni, Zn, Cu, Co and Mo, are associated with Mn oxides in the sediment and are thought to have been scavenged by hydrothermal oxides during the transport of hydrothermal oxides away from the spreading center. Only a low proportion of Fe is bound to the hydrothermal oxide phase in the sediments and this element is mainly associated with silicate phases. Principal components analysis (PCA) of geochemical data in the detrital-free, CaCO 3-free fraction of the sediments supports a geochemical “hydrothermal oxide” association between Mn, Ni, Mo, Cu, Zn, Pb, P, Be, Ba and La, with Fe associated with the silicate/weathered detrital fraction. Iron/manganese ratios in the oxide phase, range between 1.2 and 2.8. These values are below those found in sediments at mid-ocean ridges. Peaks in the downcore distribution in Mn appear to be contemporaneous in the cores studied and are probably the result of hydrothermal discharges on the VFR. Hemipelagic sedimentation rates throughout the area average 2.3 cm ka −1. Hydrothermal Mn AR range between 3 and 82 mg cm −2 ka −1 during the 0–30 ka B.P. period, and between 5 and 65 mg cm −2 ka −1 during the 30–120 ka B.P. period. Maximum Mn AR adjacent to the VFR are comparable to the highest values previously reported for Recent sediments proximal to the East Pacific Rise (EPR). During the entire period (0–120 ka B.P.), Mn AR are highest in core SO48/101 located about 90 km behind the propagating tip of the VFR. Recent hydrothermal input has also occurred about 33 km behind the propagator. This is in accord with the suggested location of fully evolved `steady state' magma chambers occurring some distance (tens of kilometers) behind propagating rift tips.

Forces driving ridge propagation

Recent multibeam bathymetric and geophysical data from the Southeast Indian Ridge (SEIR) reveal eight propagating ridges (PRs) moving down the regional bathymetry gradient toward the Australian‐Antarctic Discordance (AAD). These PRs are propagating at a nearly uniform rate of 40‐mm yr−1 despite dramatic variations in axial morphology and segment length, and local reversals of the regional bathymetric gradient. Because existing dynamic rift propagation models do not explain the continued, uniform propagation of the diverse SEIR PRs, we have developed a new model that includes variable crustal thickness and thermally‐driven, along‐axis asthenospheric flow, in addition to topographic gradients, as positive contributions to the forces driving rift propagation. Along‐axis asthenospheric flow and variations in crustal thickness along the SEIR appear to provide the first‐order driving force required for ridge propagation, even in the highly segmented, magmatically starved region in and near the AAD where previous models for ridge propagation fail. Further, our model development requires that the rift valley at the propagating tip is a consequence of rifting in the cooler thermal regime of preexisting lithospherc, rather than a consequence of viscous resistance to flow. This explanation of PR tip topography is consistent with gravity data from the SEIR that suggest that the rift tip and associated pseudofaults are at least partially compensated by crustal thinning.

Structure and morphology of submarine volcanism in the hotspot region around Réunion Island, western Indian Ocean

The form of the Deccan-Maldives-Mascarene-Réunion hotspot trace suggests that it has, at least in part, been strongly controlled by crustal structures, especially fracture zones. This makes it difficult to assess the present-day or past location of the hotspot, and thus complicates the interpretation of African plate motion reconstruction. We present here results of a cruise to the Réunion area of which the aims were: (a) to determine the extent of present-day volcanism associated with the Réunion hotspot in the region; and (b) to examine the rôle of pre-existing oceanic crustal structures in controlling the location of present-day volcanism. Additionally, we examined the morphology and geology of the important extinct spreading centre southwest of Réunion abandoned when spreading jumped to separate Seychelles from India during the Deccan flood basalt episode some 60–65 Ma ago. The extensive bathymetrie, seismic and geological investigation shows that significant present-day hotspot volcanism is confined to the Piton de la Fournaise edifice on Réunion Island itself. Apparently, the location of recent Réunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits. For plate motion reconstruction and plume flux calculation purposes, Piton de la Fournaise must be taken as the present location of the Réunion hotspot. Accretion at the extinct spreading centre progressively ceased at the time of anomaly A27 (63 Ma), and was associated with marked propagation of the rift tips.

Oxygen isotope variations in Lau Basin lavas

New laser-fluorination oxygen isotope data are presented for volcanic glasses and phenocrysts from Lau and North Fiji Basin lavas. The low oxygen blank of the technique allows accurate analysis of these often scarce igneous phases. δ 18O values of volcanic glass range from 5.53‰ to 6.06‰. Oxygen isotope ratios of ferromagnesian and plagioclase phenocrysts are lower and higher than their host glasses, respectively, and coexisting phases appear to represent oxygen isotope equilibrium at magmatic temperatures. Evolved lavas from the propagating and dying rift tips in the Central Lau Basin display significant 18O-enrichment with magmatic evolution due to FeTi-oxide crystallisation and require that precursor melts had relatively low δ 18O values immediately prior to oxide saturation. Development of these precursor melts from basaltic parents involved assimilation of hydrothermally altered sheeted dykes or gabbros in recently accreted oceanic crust accompanied by crystallisation of a plagioclase-rich assemblage. The stable isotope geochemistry and petrology of Central Lau Basin lavas constrain magmatic evolution to depths between 2.5 and 5 km beneath the seafloor. A dacite from Valu Fa Ridge is slightly 18O-enriched relative to Valu Fa basalts, consistent with crystallisation of a mineral assemblage similar to mid-ocean ridge basalts. 18O/ 16O ratios of Lau Basin basaltic glasses decrease with Mg Number; this observation could be reconciled with the assimilation and fractional crystallisation model proposed for initial differentiation of the evolved lavas. However, oxygen isotope data for Central Lau Basin basalts correlate positively with Ba/La while basaltic glasses from throughout the region display a negative correlation between δ 18O and Na 8.0 suggesting that recycling of subducted oxygen and/or mantle fertility may also exert an influence on 18O/ 16O of primary melts. Presently, the lack of quantitative control over oxygen isotope fractionation in basaltic systems prohibits resolution of these effects. Detailed investigation of less complex suites of lavas, employing the precision offered by laser fluorination, may provide new insights into the behaviour of oxygen at magmatic temperatures.

Cross-sectional areas of mid-ocean ridge axes bounding the Easter and Juan Fernandez microplates

We have calculated cross-sectional areas for the ridges bounding the Easter and Juan Fernandez microplates, 22°–28°S and 31°–35°S, obtaining accurate results where complete bathymetric data exist and estimates in other regions with partial bathymetric coverage and predicted bathymetry. We consider the reliability and usefulness of global predicted bathymetry in these calculations and the possible application of this dataset in other localities. The spreading rates on ridges bounding these microplates span the range from slow to superfast, allowing an investigation of ridge axis inflation over most of the rates active on Earth today. The across-axis areas of the Easter microplate ridge axes range from −29 km2 to 7 km2, while the Juan Fernandez ridge axis areas range from −27 km2 to 8 km2. Positive values correlate with regions usually interpreted as magmatically robust. Negative values arise from calculations in areas of propagating rift tips and deep grabens, such as Pito and Endeavor Deeps. Geochemical trends of Easter microplate axial basalts show decreasing MgO toward propagating rift tips and slight positive correlations between variables such as MgO vs. cross-sectional area, Na8.0 vs. axial depth, and Na8.0 vs. cross-sectional area. We document the decrease in the axial area approaching segment ends and propagating rift tips along both the West and East ridges of the microplates. On the Easter microplate both East and West ridge systems undergo large variations in spreading rate from >130 km Myr−1 to <50 km Myr−1. Inflation on these ridge segments is highly variable and only weakly correlated with spreading rate. On the Juan Fernandez microplate, West ridge spreading rates vary only between ∼115–140 km Myr−1 and are systematically faster than on the East ridge, where rates vary between ∼10–35 km Myr−1. Cross axis areas are systematically greater and significantly less variable on the faster spreading West ridge. Overall, compared to oceanic spreading centers bounding major plates with similar spreading rates, the axial areas are smaller on the microplate ridge systems, possibly because their rapidly changing configurations create a lag in the mantle response to the rigid plate boundary.

The East ridge system 28.5–32°S East Pacific rise: Implications for overlapping spreading center development

We report here on geophysical data from the East ridge and surrounding areas of the large-offset overlapping spreading centers (OSCs) that accommodate Pacific-Nazca opening between 28.5° and 32°S. The East ridge overlaps and is offset from the West ridge system by ∼ 120 km, forming the largest known pair of OSCs. In this area spreading rates reach the fastest currently active on Earth of ∼ 149 mm/yr. Although the East ridge is composed of 4 morphologically defined segments separated by 3 small OSCs, other geophysical characteristics imply 1 upwelling segment. All the active ridge segments in this area (including the propagating tips of the East and West ridges) form relative topographic highs with respect to the flanking sea floor; however, identified abandoned ridge tips form deeps. We interpret these data in terms of a model in which the propagating segment represents an overshoot of a surficial rupture of the brittle lithospheric layer, only partially coupled to the diverging flow of a more broadly distributed ductile deformation zone (DDZ), surrounding the steady-state ridges and crossing the offset between the OSCs. The topographic high of the propagating segment may be maintained primarily by along-axis melt migration from the stable spreading segments rather than by direct upwelling from beneath the ridge. The large overlapping ridges are inherently unstable and continued extension causes the overlapping axes to become offset from the stably spreading segments, cut off from the supply of melt, and replaced by a new set. The failed rift tips, for a period of time, overlie the broad DDZ and preferentially undergo continued extension and subsidence. The DDZ surrounding the ridge axes may be very broad in this area because of the very fast spreading rate, creating a very thin lithosphere susceptible to perturbation by relatively small mantle heterogeneities advected near the ridge axis, leading to the formation of the smaller OSCs observed.

Shallow dips of normal faults during rapid extension: Earthquakes in the Woodlark‐D'Entrecasteaux rift system, Papua New Guinea

Earthquakes in the Woodlark‐D'Entrecasteaux rift system reveal changes in fault geometry along strike, as extension progresses from continental to oceanic modes. In one segment, evidence is found for earthquakes on shallow‐dipping normal faults. To examine these trends, locations of 147 earthquakes are redetermined, and focal mechanisms for 21 events are derived from waveform inversion. Where seafloor spreading occurs, east of 152°E, most earthquakes illuminate a NE‐SW strike‐slip fault north of the oceanic rift system along the Woodlark Rise. Elsewhere, earthquakes occur along a well‐defined rift zone, and focal mechanisms reveal normal faulting with north‐south T axes and depths less than 10 km. Most normal‐faulting earthquakes show high‐angle fault planes (dips 40°–50°), but several are consistent with slip on moderately low‐angle planes dipping 15°–35°. These low‐angle nodal planes are only found in a region between 150.5°E and 152.5°E, where the transition occurs from seafloor spreading to continental rifting. A seismic reflection profile near 151°E reveals a major normal fault that dips to the north at 25°–35° just landward of the oceanic rift tip. That fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied. Hence normal faults dipping ∼30° are likely to be seismically active in at least one part of this rift system. In this region just landward from the rift tip the highest extension rates are expected (∼40 mm/yr) and strain appears localized on a single system of normal faults. Because shallow dips for normal faults are seen near the rift tip, but not elsewhere along strike, high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips. Perhaps high shear traction at the base of the upper crust, localized by nearby spreading, sufficiently alters the local stress regime.

A history of the Easter microplate, 5.25 Ma to present

GLORIA and other geophysical data are used to interpret the morphology and tectonic pattern of the Easter microplate. We model magnetic anomaly data and calculate spreading rates for use in Euler pole calculations. We calculate three‐plate closure and best fit poles for Nazca‐Pacific, Nazca‐Easter, and Pacific‐Easter plate motions and compare the results with earthquake focal mechanisms and seismicity. We use the three‐plate closure poles to rotate digital GLORIA and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate's East Rift first started to propagate northward at around 5.25 Ma. The microplate has evolved in two stages. The first is from 5.25 Ma to around 2.5 Ma, and involved continuous northward propagation of the East Rift, while the West Rift became segmented along new transform faults as it adjusted to keep the overall Nazca‐Pacific velocities constant. The transpressive northern boundary migrated with the East Rift tip, while the transtensional southern boundary remained more or less still. At some time after 2.5 Ma the East Rift ceased northward propagation and the Southwest Rift began to open up along the transtensional Pacific‐Easter transform. A convergent northern boundary formed involving southward thrusting of the Nazca plate over the microplate. No further Nazca plate lithosphere was transferred from north of the microplate to the microplate interior, and the Pito Deep region opened by pervasively rifting apart preexisting Nazca plate lithosphere.