Spreading Rift Research Articles

Magmatism at an ultra-slow spreading rift: high-resolution geomorphological studies of a Red Sea Rift segment in Hadarba Deep

The mid-ocean rift in the Red Sea is one of the youngest rifting systems on Earth. Only recently, state-of-the-art methods and modern deep-sea instruments have been used to explore this young and unique volcanic system. During the first autonomous underwater vehicle surveys of the Red Sea Rift in Spring 2022, we collected multibeam bathymetry, backscatter, sub-bottom profiler data, and water column data over a 9 km long ridge segment in the Hadarba Deep between 22.49°N and 22.56°N to investigate the volcano-tectonic processes of this ultra-slow spreading segment (12 mm/year spreading rate). The high-resolution hydroacoustic data was used to (1) delineate and quantify the geometry of tectonic structures and individual lava flows, (2) define lava flow morphology and eruption style, (3) estimate relative ages of flows and features, and (4) retrace the evolution of the volcanic activity. In addition, the geochemistry of several young lava flows provides information on the relation between the different magma that supply these eruptions. About 90 eruptive units with variable sedimentary cover have been identified within the 43 km2mapped region. The oldest lava flows are buried under 3 to 4.2 m of sediment, indicating ages of up to ~30 ka based on average sedimentation rate estimates (~14 cm/ka), while the youngest eruptions are covered by<10 cm of sediment, and are thus younger than 700 years. Three volcanic phases have been identified based on changes in flow morphology and distribution, and tectonic pattern. All three axial phases have an average eruptive frequency of ~100-250 years. The segment displays an overall low tectonic extension (<10% of the total extension) and low vertical offset. Our geomorphological maps, analyses, and statistics reveal a moderately faulted, ultra-slow spreading MOR segment in the Red Sea with a surprisingly large amount of magmatic extension, implying that the segment has been underlined by a large magma supply for at least 15 ka. All these observations provide valuable implications for the formation history of the Red Sea Rift and the formation of ultra-slow spreading crust.

Deep sourced magma and ore-metal mobility in the D. João de Castro submarine volcano (Azores): a mineral chemistry and melt inclusion study

The D. João de Castro is a submarine volcano with known hydrothermal activity located in the Terceira ultra-slow spreading rift within the Azores triple junction (ATJ). Several well-known mafic and ultramafic rock-hosted seafloor hydrothermal systems lay along the Mid-Atlantic Ridge, to the north and to the south of the Azores platform, yet little is known about seafloor hydrothermal activity, ore-metal availability, and magmatic–hydrothermal interactions within the ATJ. Here, we investigate multi-phase melt inclusions hosted in early formed phenocrysts (olivine, clinopyroxene and plagioclase), and metallic precipitates found in groundmass vesicles. Combining detailed petrographic observations with geochemical data and thermobarometry calculations, we assess P–T conditions of early formed phenocrysts, melt pathways towards surface, timing of sulfide saturation and composition of immiscible sulfide melts. Results show that D. João de Castro is characterized by a multi-level magmatic system where primary melt segregated from the upper mantle and moved up through the oceanic crust with little residence time. Sulfide saturation with the formation of immiscible magmatic sulfide liquid (Fe–Ni–Cu) occurred early in primitive magmas with clinopyroxene and olivine crystallization and continued during plagioclase crystallization. At shallower levels, the magmatic degassing of volatiles carrying base metals (Cu–Zn–Pb–Co) and Ba have contributed to the element budget of the D. João de Castro hydrothermal system. The study of multi-phase melt inclusions and vesicles at D. João de Castro submarine volcano contributes to the understanding of source to surface magmatic processes at the Terceira Rift and underline the importance of magmatic degassing into seafloor hydrothermal systems.

Thermal State of the Guaymas Basin Derived From Gas Hydrate Bottom Simulating Reflections and Heat Flow Measurements

AbstractSeafloor heat flow provides information about the thermal evolution of the lithosphere, the magnitude and timing of volcanic activity, and hydrothermal circulation patterns. In the central Gulf of California, the Guaymas Basin is part of a young marginal spreading rift system that experiences high sedimentation (1–5 km/Myr) and widespread magmatic intrusions in the axial troughs and the off‐axis regions. Heat flow variations record magmatic and sedimentary processes affecting the thermal evolution of the basin. Here, we present new seismic evidence of a widespread bottom‐simulating reflection (BSR) in the northwestern Guaymas Basin. Using the BSR depths and thermal conductivity measurements, we determine geothermal gradient and surface heat flow variations. The BSR‐derived heat flow values are less than the conductive lithospheric heat flow predictions for mid‐oceanic ridges. They suggest that high sedimentation (0.3–1 km/Myr) suppresses the lithospheric heat flow. In the central and southeastern regions of the basin, the BSR‐derived geothermal gradient increases as the intruded magmatic units reach shallower subsurface depths. Thermal modeling shows that recent (<5,000 years) igneous intrusions (<500 m below the seafloor) and associated fluid flow elevate the surface heat flow up to five times. BSR‐derived geothermal gradients correlate little with the depth of the shallowest magmatic emplacements to the north, where the intrusions have already cooled for some time, and the associated hydrothermal activity is about to shut down.

Geologic aspects (faults, landslides and volcanic dikes) of the Nateekin River shallow earthquake swarm of 15 June 2020 near Makushin Volcano, Unalaska Island, Alaska, U.S.A.-siftdesk

Starting 15 June 2020, a shallow earthquake swarm occurred in the Nateekin River valley just east of Makushin Volcano of Unalaska Island, Alaska, U.S.A. Focal mechanism determinations were made on the larger earthquake events. These correlated with four known fault groups originally recognized during 1980-85 field work in this region. These are (a) numerous N 54° W ± striking normal faults, (b) prominent N 74° W ± striking normal faults, (c) N 50° E ± strike-slip faults, and (d) a N 36° E ± striking reverse faults. These fault groups correlate with known dikes for the region, which suggest magmatic intrusions are associated with this earthquake swarm. These faults reflect a predictable fault structure for a regional maximum horizontal tectonic stress direction of N 54° W ± with a localized spreading rift zone associated with Makushin Volcano that is striking N 74° W ±. This rift zone has a reactivated landslide and numerous nearby small landslides caused by this June/July earthquake swarm. This swarm sheds insight into the faulting, volcanic and geothermal processes of the region. Key words: Shallow earthquake swarm in a volcanic rift zone; four different seismic focal (fault) mechanisms recognized; associated active faults and landslides observed; related Holocene dikes and volcanic activity; regional maximum horizontal tectonic stress and magmatic processes; extensive geothermal resources.

Strike-slip tectonics during rift linkage

The kinematics of rift segment linkage in magmatic rifts remain debated. Strain patterns from Afar provide tests of current models of how segmented rifts grow in areas of incipient oceanic spreading. Here, we present a combined analysis of seismicity, interferometric synthetic aperture radar (InSAR), and GPS–derived strain rate maps to reveal that the plate-boundary linkage between the Red Sea and Gulf of Aden rifts of Afar is accommodated primarily by distributed extensional faulting. Large rotations about vertical axes predicted by bookshelf faulting models are not detected. Additionally, models of stress changes and seismicity induced by recent dikes provide poor fits to the observed time-space patterns of strike-slip earthquakes. Instead, we explain these features as resulting from rift-perpendicular shearing at the tips of spreading rifts where extension terminates against less stretched lithosphere. Our results demonstrate that distributed extension drives rift-perpendicular shearing, achieving plate-boundary linkage during incipient seafloor spreading.

Correlated Changes Between Volcanic Structures and Magma Composition in the Faial Volcanic System, Azores

The interaction between magmatic and tectonic processes in ocean intraplate volcanism yields insights into the ascent and transport of magmas. Many oceanic intraplate studies lack a temporal component and do not consider changes in tectonic regime and/or magmatic processes during the evolution of magmatic systems. The eastern part of the Azores archipelago formed under the influence of both an intraplate melting anomaly and a system of ultraslow spreading rift axes. The majority of recent submarine and subaerial eruptions in the Azores occur along volcanic rift zones and thus melt transport and volcanic processes are largely controlled by tectonic processes, and are therefore ideally suited to study volcano-tectonic interactions. Here, we investigate how variable the magmatic and tectonic processes are in space and time and how they interact. We present new bathymetric, geophysical, geochemical and Sr-Nd-Pb isotope data from Faial Island and the surrounding seafloor, providing insights into the interaction between the asthenospheric melting anomaly and extensional lithospheric stresses. The bathymetry reveals large submarine volcanic rift zones on the western flank of Faial, including that of the 1957-1958 Capelinhos eruption. Based on absolute ages and seismic imagery, we develop a relative chronology of the magmatic evolution of the submarine rift zones. Their preferred WNW-ESE orientation implies that the stress field has not changed within the last ~1 Ma. We can however show that melt productivity progressively decreased with time. Compositionally different magma batches fed distinct volcanic rift zones and edifices, suggesting that changes in the melting regime occur on a small spatial scale and that the distribution of compositionally similar lavas is tectonically controlled. As melt supply decreases, the tectonic influence on volcanism increases with a stronger localisation of melts along tectonically controlled lineaments. The youngest mafic and intermediate melts (<10 ka) on Faial are exclusively erupted along single rift zones and cover a smaller area, whereas the older volcanism was more widespread.

MOLECULAR AND BIOLOGICAL CHARACTERISTICS, AND CULTIVATION OF AN ATTENUATED STRAIN 1974-VNIIVViM OF RIFT VALLEY FEVER VIRUS

In recent decades, a threat of spreading Rift Valley fever (RVF) to Asia and Europe raises serious concerns. Within many years, vaccines against RVF were developed in several ways including designing attenuated vaccine versions, inactivated or genetically engineered vaccines. A serious hazard of the RVF agent requires developing sufficiently immunogenic preparations providing both RVF specific prevention and safety precautions in production procedures. Therefore, this work was aimed at molecular and biological characterization of an attenuated RVF virus (RVFV) strain 1974VNIIVViM including search for high-tech cell culture systems for the virus growth and determination of its antigenic and/or immunogenic relationship with a virulent strain Entebbe. The results of the investigations showed that the virus accumulated in continuous cell cultures like saiga kidney (SK), Siberian mountain goat kidney (PSGK-60) and MDVK (calve kidney cell culture) at high titers with infectious (8.53±0.21 lg MICLD50/cm) and antigenic (1:64 to 1:128 in the passive hemagglutination test, PHT) activity levels. We determined optimal regime of the virus growth in BHK-21/13 (Syrian hamster kidney cell culture) using a roller culture method: the culture bottle rotation speed of 12 to 15 r.p.h.; the infection dosage of 0.01 to 0.001 MLD50/cell; the first 12 hours of the culture at pH = 6.4 to 6.8. Further, pH is maintained at 7.2 to 7.6 for 48 to 72 hours of culture at 37 С, which provides the raw virus production at high infectivity (8.5 to 8.9±0.2 lg MICLD50/cm) and antigenicity (1:128 to 1:256 in PHT and 1:625 to 1:3125 in solid phase ELISA) levels. Phylogenetic analysis of the nucleotide sequences of 3 sites in S and/or M segments showed that it was closely related to strains Smithburn and Entebbe allowing their grouping into a separate cluster. Comparative analysis of nucleotide sequences of genes encoding glycoproteins Gn and Gc, and the nucleoprotein N, shows that most of the substitutions in the sequences of the genes of the RVFV strain 1974-VNIIVViM are synonymous and do not alter the primary amino acid sequence, with insertion and deletion missing. In the nucleotide sequence encoding glycoprotein Gn of strain 1974-VNIIVViM we found 8 substitutions relative to a similar sequence of M segment of a virulent strain Entebbe, and 14 substitutions when compared with M segment of strain Smithburn. A lower (as compared with other attenuated strains) amount of significant amino acid substitutions in RVFV antigenic components was revealed; thus, strain 1974-VNIIVViM selection as a source of immunodominant proteins and/or raw virus for vaccine production seems to be well-grounded.

Multiple ways of producing intermediate and silicic rocks within Thingmúli and other Icelandic volcanoes

Major and trace element compositions of rocks and coexisting phenocrysts of the Thingmuli volcano suggest a revision of the existing models for the formation of intermediate and silicic melts in Iceland. The new data define two compositional tholeiitic trends with a significant gap between them. A high-iron trend (HFe) contains 6–14 wt% total FeO in silicic rocks with c. 1 wt% MgO, as well as sodic plagioclase and hedenbergite phenocrysts. A low-iron trend (LFe) contains 3–5 wt% FeO at c. 1 wt% MgO, which is typical of Iceland but higher than MORB compositions. The most evolved phenocrysts of the LFe trend do not reach iron-rich end members. The HFe trend is interpreted as a result of fractional crystallization; numerical modelling using the MELTS algorithm suggests that crystallization took place under redox conditions constrained to one-log unit below the fayalite-magnetite-quartz oxygen buffer (FMQ-1). The LFe trend is explained by a combination of mixing between rhyolite and ferrobasalt, assimilation of hydrated crust and fractional crystallization under higher redox conditions (FMQ). The two trends and the gap are best defined in a plot of Mg# versus SiO2 that is useful to unravel petrogenetic processes. For example, intermediate and silicic rocks of the Holocene volcanic systems of spreading rifts (e.g. Krafla), propagating rifts (e.g. Hekla) and off-rifts (Oraefajokull) also fall into high- and low-iron fields and outline a gap similar to Thingmuli. The identification of two compositional trends in erupted intermediate and silicic volcanic products shows that processes in the deep roots of single volcanic systems are highly diverse and likely controlled by local variations in the thermal gradients and the extend of hydrothermal alteration. Generalizations about the relationship between the compositions of intermediate and silicic rocks and plate tectonic setting, therefore, should be avoided.

The origin and geochemical evolution of the Woodlark Rift of Papua New Guinea

Geochemical, isotopic, and geochronologic data for exhumed rocks in the Woodlark Rift of Papua New Guinea (PNG) allow a tectonic link to be established with the Late Cretaceous Whitsunday Volcanic Province (WVP) of northeastern Australia. Most of the metamorphic rocks in the Woodlark Rift have Nd isotopic compositions (εNd=+1.7 to +6.2) similar to the Nd isotopic compositions of rocks in the WVP (εNd=+1.3 to +6.6; Ewart et al., 1992), and contain inherited zircons with 90 to 100Ma U–Pb ages that overlap the timing of magmatism in the WVP. None of the metamorphic rocks in the Woodlark Rift have the highly evolved Hf and Nd isotopic compositions expected of ancient continental crust. Magmas were erupted in the WVP during the middle Cretaceous as eastern Gondwana was rifted apart. The protoliths of felsic and intermediate metamorphic rocks in the Woodlark Rift are interpreted to be related to the magmatic products produced during this Cretaceous rifting event. Some mafic metamorphic rocks exposed in the western Woodlark Rift (eclogites and amphibolites) are not related to the WVP and instead could have originated as basaltic lavas crystallized from mantle melts at (U)HP depths in the Late Cenozoic, or as fragments of Mesozoic aged oceanic lithosphere.Isotopic and elemental comparisons between basement gneisses and Quaternary felsic volcanic rocks demonstrate that felsic lavas in the D'Entrecasteaux Islands did not form solely from partial melting of metamorphic rocks during exhumation. Instead, the isotopic compositions and geochemistry of Quaternary felsic volcanic rocks indicate a significant contribution from the partial melting of the mantle in this region. When combined with geophysical data for the western Woodlark Rift, this suggests that future seafloor spreading will commence south of Fergusson Island, and west of the present-day active seafloor spreading rift tip.

Evolution of the Malvinas Plate South of Africa

We confirm that a Malvinas Plate is required in the Agulhas Basin during the Late Cretaceous because: (1) oblique Mercator plots of marine gravity show that fracture zones generated on the Agulhas rift, as well as the Agulhas Fracture Zone, do not lie on small circles about the 33o-28y South America-Africa stage pole and were therefore not formed by South America-Africa spreading, (2) the 33o-28y South America-Africa stage rotation does not bring 33o magnetic anomalies on the Malvinas Plate into alignment with their conjugates on the African Plate, and (3) errors in the 33o-28y South America-Africa stage rotation cannot account for the misalignment. We present improved Malvinas-Africa finite rotations determined by interpreting magnetic anomaly data in light of fracture zones and extinct spreading rift segments (the Agulhas rift) that are clearly revealed in satellite-derived marine gravity fields covering the Agulhas Basin. The tectonic history of the Malvinas Plate is chronicled through gravity field reconstructions that use the improved Malvinas-Africa finite rotations and more recent South America-Africa and Antarctica-Africa finite rotations. Newly-mapped triple junction traces on the Antarctic, South American, Malvinas, and African Plates, combined with geometric and magnetic constraints observed in the reconstructions, enable us to investigate the locations of the elusive western and southern boundaries of the Malvinas Plate.

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Grabens have been mapped immediately adjacent to recently erupted submarine lava flows on the Cleft and CoAxial segments of the Juan de Fuca Ridge (JdFR). The grabens are 10–100 m wide and 5–15 m deep and are located uprift and/or downrift of the eruptive vents that fed the flows. We interpret that these structures formed (or were reactivated) directly over the dike that fed the eruptions as it was intruding toward the surface. These graben structures are primary conduits for diffuse hydrothermal venting on the seafloor during the cooling of newly intruded dikes. The axial summit “graben” or “caldera” on the East Pacific Rise (EPR) in some locations has similar dimensions to the grabens observed on the JdFR, and we interpret that it too may be a dike‐induced graben structure (although often buried by subsequent eruptions where it is narrowest). These grabens on the JdFR and the EPR are distinctly narrower and deeper than grabens that formed during well‐documented dike intrusions on slow spreading rifts on land (Iceland and Afar). Mechanical modeling suggests that narrow grabens would form when a dike is at shallow depth where it imposes a high stress perturbation on the ambient stress state to cause faulting. Therefore the narrow grabens on the JdFR and EPR imply that a relatively high level of horizontal compressive stress typically exists perpendicular to the ridges, and this must be overcome by high dike‐induced perturbations to cause faulting. This is probably because gradual plate spreading rarely gets enough time to lower the compressive stress significantly due to the high frequency of dike intrusion events relative to the plate spreading rate. The dikes that do intrude in this environment must have relatively high internal magma pressure. Therefore the size and character of dike‐induced grabens that form at the surface on intermediate to fast spreading ridges reflect the fact that volcanism dominates over tectonism in regulating the local stress state where a robust magma supply is available.

Geochemistry of basalts from the western Woodlark, Lau and Manus basins: implications for their petrogenesis and source rock compositions

Geochemical compositions are reported for back-arc and marginal basin basalts sampled during the 1990 cruise of R/V Akademik Mstislav Keldysh in the Lau Basin (North Lau, LN; King's Triple Junction, LK; Central Lau, LC; East Lau, LE), Manus Basin (Red Star region, MRS), and western Woodlark Basin (Dobu Seamount region, WD, continental rifting segment; East Basin region, WE, the boundary between continental and spreading segments; and Franklin Seamount region, WF, spreading rifting segment). Two main geochemical types of basalts have been distinguished within the Lau and Manus back-arc marginal basins: (i) depleted basalts comparable with N-MORB lavas (LC, LE, MRS); and (ii) enriched basalt lavas (LN, LK vesicular pillow basalts: LK, VPB) correspond to typical BABB. The sources of melts were depleted spinel or plagioclase lherzolite (N-MORB type) for LC, LE, MRS basalts; the N-MORB source was metasomatized by adding LILE and volatiles (‘arc-source’ components) for enriched lavas. Direct observations from Mir submersibles in the area of the King's triple junction (Lau Basin) established that LILE-depleted massive sheet-flow basalts overlie vesicular pillow basalts. The western Woodlark Basin is a tectonic transition zone between typical oceanic spreading in the central part of the basin and continental rifting in easternmost Papua-New Guinea. The WD basalts show strong LILE, LREE, P, and Zr enrichment and are spatially associated with alkaline rhyolites. The WE and WF lavas demonstrate moderate enrichment in these elements. The western Woodlark lavas formed from multiple sources, including N-MORB mantle, ‘arc-source’ components and enriched mantle like OIB.

Fine‐scale magnetic anomaly field over the southern Juan de Fuca Ridge: Axial magnetization low and implications for crustal structure

Sea surface, deep tow, and submersible magnetic surveys show that the spreading axis of the Cleft segment of the southern Juan de Fuca Ridge (SJDF) is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent Vance segment and most mid‐ocean ridge spreading centers. Forward iterative thickness modeling, based on the near‐bottom magnetic data, shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the Cleft axial valley. Thick crust is required to produce the magnetic anomaly high over the Vance axial valley. This magnetic modeling demonstrates that variations in the base of the magnetic source layer, which is assumed to be equivalent to the base of the extrusive lavas, can produce a significant proportion of the observed short‐wavelength magnetic anomaly signal. These magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2A thins beneath the axial valley of the Cleft and is thicker beneath the Vance axial valley. Recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading, which could produce significant magnetic contrasts. The results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration, first‐order change in upper crustal thickness is one of the most important variables to consider in the production of fine‐scale magnetic anomalies in young ocean crust.

GLORIA survey of the oceanic-continental transition of the Havre-Taupo back-arc basin

A GLORIA (Geological Long-Range Inclined Asdic) side-scan sonar survey, covering 23,000 km2, provides the first complete imagery of an active and contiguous, oceanic to continental back-arc system, namely, the Havre Trough to Taupo Volcanic Zone (TVZ) New Zealand. Havre Trough tectonism and volcanism relates to a series of laterally discontinuous, mutiple spreading rifts which terminate southward at the 3-km-deep Ngatoro Basin. A 45-km sinistral offset attributed toen echelon synthetic shearing separates the basin from the actively spreading TVZ. Sonographs reveal a youthful and complex volcanic seascape with 20 newly discovered seamounts, whereas flanking regions are mantled with a largely featureless mud blanket.

Eocene reorganization of the Pacific‐Farallon Spreading Center north of the Mendocino Fracture Zone

During a 1984 Sea Beam and magnetics survey in the North Pacific we collected data in the area north of the Mendocino fracture zone where a change in the direction of seafloor spreading occurred between magnetic anomalies 24 and 21 and also conducted detailed surveys of the bends in the Surveyor and Mendocino fracture zones associated with this change in plate motion. We have analyzed these data combined with all available magnetics and bathymetric data from previous cruises to determine whether the ridge reorientation and reorganization occurred by way of rift propagation or rift rotation through differential asymmetric spreading. The reorganization began just after anomaly 24 to the north of the Surveyor fracture zone but did not begin between the Mendocino and Surveyor fracture zones until the time of anomaly 22, a 2.5‐m.y. delay produced by the old Mendocino transform trend constraining the Farallon plate lithosphere between the Mendocino and Surveyor transforms to move in the old spreading direction until the fracturing through of a new Mendocino transform fault in the new orientation. Most of the reorientation of spreading rift segments occurs within 2–3 m.y.; the entire reorganization requires about 7 m.y. We find direct evidence for seven propagation episodes accounting for the primary reorientation of 75% of the length of the ridge system, and nowhere is the data more consistent with rift rotation than with rift propagation as the principal means of reorientation. We conclude that the primary mechanism by which the reorganization occurred was rift propagation. Assuming that rift propagation can account for all of the ridge reorganization, we present forward models of magnetic anomaly profiles which succeed in fitting the data well. We then present a model for the tectonic evolution of the reorganization which fits the data well using 11 episodes of both northward and southward propagation.

East African rift and northeast lineaments: continental spreading—transform system?

Seafloor spreading and transform faulting processes are also likely to be operative during continental rifting events. Continental lines of old weakness oriented at high angles to the direction of continental rifting may be reactivated by transform faulting. These older continental transform faults, which predate and accomodate the rifting, will continue to propagate as younger oceanic transform faults as the rift develops into seas and oceans. This model is applied to the East African Rift which is postulated to be a continental spreading rift that is accommodated by east-northeast continental transform lineaments that are reactivated older crustal defects of appropriate orientation. At least five continental transform lineaments can be tentatively identified by empirical best fits to oceanic transform directions of the South Atlantic Ocean and to various continental African northeast-trending structures: (1) Cape Town-Maputo (CT-LM); (2) Orange River-Beira (OR-B); (3) Luderitz-Lindi (L-Li); (4) Walvis Bay-Mombasa-Mogadishu (WB-M-Mo); and (5) Luanda-Afar (Lu-Af). As these postulated lineaments are perennial deep seated crustal defects they may also control the development of mineral deposits.

Rifting of old oceanic lithosphere

Geophysical data from five regions in the Pacific and Indian oceans reveal that long distance (&gt;400 km) spreading center jumps have occurred in the past. The present‐day seafloor morphology is used to develop a scenario for a spreading center jump. The major events are (1) thinning and weakening of the lithosphere at the future rifting site, (2) rifting of the weakened lithosphere (during rifting, the crack is filled from above by normal faulting and wedge subsidence; viscous upwelling fills the crack from below), (3) spreading at the rift site results in a ridge bounded by two troughs (spreading ceases at the dying spreading center, resulting in a deep central graben surrounded by flexural ridges; periods of slow spreading at both spreading centers produce rough topography), (4) ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site. The new spreading center develops into a normal spreading rift. The major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site. The proximal trough (nearer the new spreading ridge) and the distal trough (farther from the new ridge) are first‐order topographic features, 100–1000 km long and 300 km wide, resembling fracture zones with which they are often confused. They share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages, but unlike fracture zones they are generally parallel to magnetic lineations.

Emperor Fracture Zone Morphology by Multi-Beam Sonar

U.S. Naval Oceanographic Office surveys have updated bathymetry for 1300 km of the Emperor Fracture Zone (EFZ). Multi-beam swath mapping sonar provides up to total coverage on survey lines of various spacings. The feature's accurate position is obtained with LORAN-C data plus navigational satellite data. Data are presented at a 200 fm (366 m) contour interval on a Mercator projection. The EFZ apparently was a spreading rift along a paleo-plate boundary of a microplate some 70 m.y. ago. At present, it is a relict feature. The EFZ is a series of secondary and tertiary linear ridges and troughs. The overall trend is from NW to SE. There are three principal secondary segments: from 46°15'N, 172°50'E to 41°30'N, 178°E to 40°30'N; and 180° to 36°45'N, 175°30'W. Apparent pull-apart basins are found within each segment. The growth of listric normal faults may also explain the growth of the intrafracture zone basins. The most complex bathymetry is found where the Chinook Fracture Zone intersects the EFZ.

Rifting History of the Woodlark Basin in the Southwest Pacific

Marine geophysical evidence has been obtained for the rate and history of sea-floor spreading in the Woodlark Basin. The eastern part of this basin is presently separating from the Australian Plate at over 4 cm per yr in a northerly direction. The western part of the basin is not presently spreading. This spreading rift marks the southern boundary of the Solomons Plate which is bounded by subduction zones in the north and east (the New Britain and northern Solomons Trenches, respectively) and in the west by a combination strike-slip rifting (dip-slip) boundary in eastern Papua (New Guinea). A vector triangle solution near the Solomons Trench-Woodlark Rift triple point gives underthrusting of the Solomons Plate beneath the northern Solomon Trench in a northeasterly direction at about 11 cm per yr. The Woodlark Basin began opening as a sphenochasm, with a pole near the tip of eastern Papua about 20 m.y. B.P. This was caused by left-lateral shear in the region induced by a change in the relative motion pole of the Australia and Pacific Plates. The basin opened only a few degrees at this time, then stopped. Rifting in the entire basin resumed about 3 m.y. B.P., based on magnetic anomaly data. About 1 m.y. B. P., the spreading center in the western basin shifted to the Woodlark Rise.