Magmatic evolution of the Easter microplate-Crough Seamount region (South East Pacific)

The Easter microplate-Crough Seamount region located between 25° S–116° W and 25° S–122° W consists of a chain of seamounts forming isolated volcanoes and elongated (100–200 km in length) en echelon volcanic ridges oriented obliquely NE (N 065°), to the present day general spreading direction (N 100°) of the Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26°30 S–115° W was surveyed and sampled. The southern boundary including the Orongo fracture zone and other shallow ridges (< 2000 m high) bounding the Southwest Rift of the microplate consists of fault scarps where pillow lava, dolerite, and metabasalts are exposed. The degree of rock alternation inferred from palagonitization of glassy margins suggests that the volcanic ridges are as old as the shallow ridges bounding the Southwest Rift of the microplate. The volcanics found on the various structures west of the microplate consist of depleted (K/Ti < 0.1), transitional (K/Ti = 0.11–0.25) and enriched (K/Ti > 0.25) MORBs which are similar in composition to other more recent basalts from the Southwest and East Rifts spreading axes of the Easter microplate. Incompatible element ratios normalized to chondrite values [(Ce/Yb)_N = 1–2.5}, {(La/Sm)_N = 0.4–1.2} and {(Zr/Y)_N = 0.7–2.5} of the basalts are also similar to present day volcanism found in the Easter microplate. The volcanics from the Easter microplate-Crough region are unrelated to other known South Pacific intraplate magmatism (i.e. Society, Pitcairn, and Salas y Gomez Islands). Instead their range in incompatible element ratios is comparable to the submarine basalts from the recently investigated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO80, 1993) and centered at about 80 km west of Easter Island. The oblique ridges and their associated seamounts are likely to represent ancient leaky transform faults created during the initial stage of the Easter microplate formation ( 5 Ma). It appears that volcanic activity on seamounts overlying the oblique volcanic ridges has continued during their westward drift from the microplate as shown by the presence of relatively fresh lava observed on one of these structures, namely the first Oblique Volcanic Ridge near 25° S–118° W at about 160 km west of the Easter microplate West Rift. Based on a reconstruction of the Easter microplate, it is suggested that the Crough seamount (< 800 m depth) was formed by earlier (7–10 Ma) hotspot magmatic activity which also created Easter Island.     

A SeaMARC II survey of recent submarine volcanism near Easter Island

SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated 400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km² and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation. Two additional areas of high backscatter immediately north of Easter Island cover a combined area of over 300 km². The sidescan sonar records show that these features are clearly of volcanic origin and are not debris flows from the nearby island. The flows are nearly 300 m thick and are morphologically similar to subaerial pahoehoe lava shields. Their high backscatter indicates that they are also the products of relatively recent submarine volcanic activity. The presence of these large areas of recent volcanism in the vicinity of Easter Island has important implications for the various models that have been proposed to explain the origin of the Easter Seamount Chain. In addition, the similar ages of Easter Island and the Easter Microplate suggest that the presence of a hotspot near or beneath this fast-spreading portion of the East Pacific Rise about 4.5 m.y. ago may have initiated the large-scale rift propagation that created the microplate.     

High-resolution seafloor mapping using the DSL-120 sonar system: Quantitative assessment of sidescan and phase-bathymetry data from the Lucky Strike segment of the Mid-Atlantic Ridge

High-resolution, side-looking sonar data collected near the seafloor (100 m altitude) provide important structural and topographic information for defining the geological history and current tectonic framework of seafloor terrains. DSL-120 kHz sonar data collected in the rift valley of the Lucky Strike segment of the Mid-Atlantic Ridge near 37° N provide the ability to quantitatively assess the effective resolution limits of both the sidescan imagery and the computed phase-bathymetry of this sonar system. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are <1 m, statistical analysis of DSL-120 sonar data collected from the Lucky Strike segment indicates that the effective spatial resolution of features is 1–2 m for sidescan imagery and 4 m for phase-bathymetry in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparison of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10–30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is 20 times smaller than that of hull-mounted multibeam at these seafloor depths (2000 m). Comparison of DSL-120 bathymetry with itself on crossing lines indicates that residual depth values are ±20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better navigation and less complex seafloor terrain had residual depth values half those of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly beneath the tow vehicle and also towards the swath edges.Variations in sidescan amplitude observed across the rift valley and on Lucky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct changes in sonar backscatter amplitude were also observed between areas covered with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydrothermal vent chimneys or mounds taller than 2 m and greater than 9 m² in surface area, can easily be resolved and mapped using this system. These features at Lucky Strike have been confirmed visually using the submersible Alvin, the remotely operated vehicle Jason, and the towed optical/acoustic mapping system Argo II.     

Vesteris Seamount: An enigma in the Greenland Basin

Vesteris Seamount is a solitary submarine volcano located at 73°30 N, 9°10W in the Greenland Basin. Steeply rising from a base depth of 3100 m to a minimum depth of ~ 130 m and striking 030°/210°, the feature lies ~ 300 km east of the east Greenland margin on an otherwise nearly flat and featureless seafloor. The main body of the seamount appears to have been formed episodically, the last of which culminated about 110 000 years ago. Subsequent, lower intensity volcanic activity continued sporadically until about 25 000 years ago, as evidenced by ash layers found in cores near the base of the feature. The smoothed surfaces at the summit make it likely that the seamount actually broached the surface during the Weichselian glacial period, between 8000 and 13 000 years ago. Two multibeam bathymetric investigations aboardPFS Polarstern during ARKTIS II/4 (1984) and ARKTIS VII/1 (1990), combined with geologic sampling, single-channel seismic profiling and underwater television coverage, have resulted in a new interpretation of both the morphology and origins of the seamount. Data collected aboardPolarstern from ARKTIS II/4 (1984) have been previously reported by Hempelet al. (1991), however, when combined with the ARKTIS VII/1 (1990) data set, a more detailed interpretation of the morphology and structure was feasible. This included the elongated shape of the feature and showed the existence of several small volcanic cones on the seamount flanks.The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

A detailed magnetic study of the Reykjanes Ridge between 63°00′N and 63°40′N

Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00N and 63°40N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.     

Submersible observations of Equatorial Atlantic mantle: The St. Paul Fracture Zone region

The St. Paul F.Z. is a large structural domain made up of multiple transform faults interrupted by several Intra-Transform Ridge (ITR) spreading segments. Two regions were studied in details by submersible: (1) The ITR short (<20 km in length) segment near 0° 37N–25° 27W and 1° N–27° 42W and (2) The St. Peter and St. Paul's Rocks (SPPR) massif located at 29° 25W (¡3700 m depth). (1) The short ITR segments consist of a magma starved rift valley with recent volcanic activities at 4700 m depth. A geological profile made along the rift valley wall showed localized volcanics (basalts and dykes) which are believed to overlay and intrude the ultramafics. The geological setting and the high ultramafic/volcanic ratio suggest an extremely low magmatic supply and crustal-mantle uplift during lithospheric stretching and denudation. (2) The St. Peter and St. Paul's Rocks (SPPR) massif consists of a sigmoidal ridge within the active transform zone. The SPPR is divided into two different geological domains called the North and the South Ridges. The North Ridge consists of strongly tectonized fault scarps composed of banded and mylonitized peridotite, sporadic gabbros (3900–2500 m) and metabasalts (2700–1700 m). The South Ridge is less tectonized with undeformed, serpentinized spinel lherzolite (2000–1400 m) and basalts. Extensional motion and denudation accompanied by diapirism affected the South Ridge within a transform domain. Instead, the North Ridge was formed during an important strike-slip and faulting motion resulting in the uplifted portion of the St. Paul F.Z. transverse ridge. There is a regional compositional variation of the volcanics where E-MORBs and alkali basalts are produced on the SPPR massif and are comparable to the adjacent northern segments of the Mid-Atlantic Ridge. On the other hand, N and T- MORBs collected from the eastern part of the St. Paul F.Z. (25° 27W IRT) are similar to the volcanics from the southern segments of the MAR. The peridotites exposed in these provinces (SPPR and ITR) are similar in their REE and trace element distribution. Different degrees (3–15%) of partial melting of a mixed composite mantle consisting of spinel and amphibole bearing lherzolite veined with 5–40% clinopyroxenite gave rise to the observed MORBs and alkali basalts.     

Seamount gravity anomaly modelling with variably thick sediment cover

Inversion modelling of marine gravity anomalies to derive predicted seafloor topography has provided significant advance in delineating deep-ocean bathymetry where the seafloor both conforms to the half-space cooling model of seafloor spreading, and largely sediment-free. Similar modelling for elevated ridges and seamounts, that are formed by processes other than seafloor spreading and/or have proximal sediment sources (e.g., continental margins and volcanic arcs), have significantly higher errors when validated against modern shipborne echo-sounding data. A three-dimensional, five-layer gravity model is emulated for the cases of both synthetic and real seamounts, with varying degrees of sediment burial, to establish the sensitivity of variable sediment cover as a source of error. A simple `Gaussian' seamount with base radius of 30 km, 2000 m of relief, has a maximum 140–160 mGal anomaly, that decreases to 50 mGal with the addition of 1 km of sediment cover with simple `flood' geometry. Complete burial, with a typical sediment density of 2300 kg m^–3, results in a 120 mGal difference from a sediment-free seamount model. Increasing sediment density results in an exponential decay of the seamount anomaly. More complex synthetic geometries of varying basement relief and sediment thickness show that the anomaly amplitude remains significant, especially where the latter is >700–800 m thick. For the real case, seamounts of the Three Kings Ridge (northern New Zealand) imaged with seismic reflection data, with varying degrees of sediment cover of up to 1 km, when modelled both with and with-out the inclusion of a sediment layer, typically have rms differences of 30 mGal between observed and modelled gravity anomalies. Significantly, the rms errors are reduced by 50% with the inclusion of a sediment layer that corresponds to a reduction of predicted seafloor topography rms errors of 192–684 m to 78–360 m.     

Geology of an active hot spot: Teahitia-Mehetia region in the South Central Pacific

The Teahitia-Mehetia hot spot region located in the southeastern extension of the Society Islands chain, near 18° S–148° W consists of several active volcanoes. The distribution of recent volcanic activity correlates with seismic epicenters, and covers an area of more than 1000 km². Intermittent volcanic activity has given rise to large (>1000 m high) and small (<500 m high) edifices composed of various types of flows. Several recent volcanic events have produced a suite of alkalic rocks ranging from ankaramites, through alkali basalts to trachy-phonolites. The presence of altered MORB-like tholeiites on one small seamount suggests that a different mantle source material was involved in forming some of the crust in this hot spot region.     

Abundant seamounts of the Rano Rahi seamount field near the Southern East Pacific Rise, 15° S to 19° S

A widespread seamount province, the Rano Rahi Field, is located near the superfast spreading Southern East Pacific Rise (SEPR) between 15°–19° S. Particularly abundant volcanic edifices are found on Pacific Plate aged 0 to 6.5 Ma between 17°–19° S, an area greater than 100,000 km². The numbers of seamounts and their volume are several times greater than those of a comparablysurveyed area near the Northern East Pacific Rise (NEPR), 8°–17° N. Most of the Rano Rahi seamounts belong to chains, which vary in length from 25 km to >240 km and which are very nearly collinear with the Pacific absolute and relative plate motion directions. Bends of 10°–15° occur along a few of the chains, and some adjacent chains converge or diverge slightly. Many seamount chains have fluctuations in volume along their length, and statistical tests suggest that some adjacent chains trade-off in volume. Several seamount chains split into two lines of volcanoes approaching the axis. In general, seamount chains composed of individual circular volcanoes are found near the axis; the chains consist of variably-overlapping edifices in the central part of the survey; to the west, volcanic ridges predominate. Near the SEPR, the volume of nearaxis seamount edifices is generally reduced near areas of deflated cross-sectional area of the axial ridge. Fresh lava flows, as imaged by sidescan sonar and sampled by dredging, exist around some seamounts throughout the entire survey area, in sharp contrast to the absence of fresh flows beyond 30 km from the NEPR. Also, the increases in seamount abundance and volume extend to much greater crustal ages than near the NEPR. Seamount magnetization analysis is also consistent with this wider zone of seamount growth, and it demonstrates the asynchronous formation of most of the seamount chains and volcanic ridges. The variety of observations of the SEPR seamounts suggests that a number of factors and mechanisms might bring about their formation, including the mantle upwelling associated with superfast spreading, off-axis mantle heterogeneities, miniplumes and local upwelling, and the vulnerability of the lithosphere to penetration by volumes of magma. In particular, we note the association of extensive, recent volcanism with intermediate wavelength gravity lineaments lows on crust aged 6 Ma. This suggests that the lineaments and some of the seamounts share a common cause which may be related to ridge-perpendicular asthenospheric convection and/or some manner of extension in the lithosphere.     

Ridge–hotspot interaction: the Pacific–Antarctic Ridge and the foundation seamounts

The study of different magmatic provinces between the Resolution fracture zone (33°S–131°W) and the Pacific–Antarctic Ridge (PAR) axis (37°S–111°W) suggests that similar processes of interaction between hotspot and spreading axial magmatism occurred 20–25 Ma and 0–5 Ma ago. There is evidence of this process from the changes in composition observed in the lavas erupted near 400–300 km between the present day PAR axis (37°S–111′W) and the eastern tip of the Foundation Seamount (FS) hotspot near 36°20′S–114°W where the last alkali enriched volcanics [K/Ti>0.30, Zr/Y>6 and (Ce/Yb)_N>4] have erupted. This transitional province between the PAR and the FS consists of volcanic cones built on several volcanic ridges (<200 km in length) which have erupted less enriched volcanics such as E-[K/Ti=0.25–0.33, Zr/Y=5–6 and (Ce/Yb)_N=3–4] and T-[K/Ti=0.11–0.25, Zr/Y=2–4 and (Ce/Yb)_N=1–2] MORBs than those from the FS. It is also noticed that there is a general decrease in the degree of the basalt alkalinity (more T-MORBs) towards the PAR axis. The limit of the FS hotspot influence corresponds to the area where the VR intersect the PAR axis for a distance of about 100 km along its strike between 37°10′S and 38°20′S. Indeed, the lava erupted further to the north and to the south of these latitudes contains N-MORBs [K/Ti=0.05–0.11, Zr/Y<3 and (Ce/Yb)_N=<2]. Many Old Pacific Seamounts (OPS, >20 Ma) also built on volcanic ridges are identified west (>1200 km from the PAR axis) of a Failed Rift Propagator (FRP) forming the eastern boundary of the ancient Selkirk microplate. Some of these seamounts made of alkali basalts were built during the initiation of the FS hotspot 20–23 Ma ago. The interaction and the influence (thermal) of mantle plume magmatism with the ancient spreading ridge of the Farallon–Pacific plates was responsible for the eruptions of the T-MORBs and andesitic lavas. This situation is comparable to that presently observed on the PAR axis where silicic lavas are also erupted in association with T-MORBs.     

Pito Rift: How a large-offset rift propagates

The Pito Rift area is the site of actively deforming oceanic lithosphere that has been primarily under extension for at least the past million years, based on kinematic reconstructions. The major morphologic features, Pito Deep and Pito Seamount, are aligned toward the Euler pole for relative motion between the Easter and Nazca plates. SeaMARC II side-scan and bathymetry data indicate that there are two general modes of faulting currently active in the Pito Rift area. One is associated with incipient rifting of old (3 Ma) Nazca lithosphere by large NW-SE normal faults, and the other is associated with a broad area of right-lateral transform shear between the Nazca and Easter plates. This transform shear is distributed over a broad region because of the northward growth of the East Rift and parallel tectonic rifting within the Pito Rift area. The majority of the Pito Rift area is composed of preexisting blocks of Nazca plate that are back-tilted away from Pito Deep and strike perpendicular to present and previous relative plate motions. This observation suggests that block-faulting and back-tilting are the primary mechanisms responsible for the distributed lithospheric extension, in agreement with gravity and magnetic analyses (Martinez et al., this issue).The only recent volcanic flows observed in side-scan data are from the Pito Seamount area and to the outside of the outer pseudofault of the East Rift. The significance of the young flows near the outer pseudofault is not understood. We interpret the flows extending northwest from the Pito Seamount as representing a newly formed seafloor spreading axis within the Pito Rift area. Gravity and magnetic analyses (Martinez et al., this issue) together with SeaMARC II bathymetry and side-scan data support this interpretation.Based on the tectonic evolution of the Easter microplate, we propose an evolutionary model for the formation of the Pito Rift area, where new tectonic grabens form immediately west of the previous graben and with slightly more counterclockwise orientation. The duration and history of tectonic activity for each graben are not well constrained.     

Offset Spreading Centers near 12°53′ N on the East Pacific Rise: Submersible observations and composition of the volcanics

The Offset Spreading Center located between 12°52 and 12°54 N on the East Pacific Rise (Macdonald and Fox, 1983) has been studied in 1982 and 1984 with submersible Cyana and in 1983 with the deep towed vehicle Seamarc I. The two O.S.C. segments, about 1.5 km apart and 4 km in length, separated by a depression (about 100 m in depth) show different volcano-tectonic settings. The Western Spreading Center (WSC) segment is characterised mainly by recent volcanic constructional features, while the Eastern Spreading Center (ESC) is highly fissured and consists essentially of older pillow-lava terrain. The intervening depression located between the two segments is floored by small constructional mounds (<10 m in height) of pillow lava. The crust of both segments becomes older along strike towards their respective tips. However, the W.S.C. comprises generally younger flows than does the E.S.C. A small central volcano (80 m in height and 1 km in diameter) located near 12°51 N near the Southern tip of the W.S.C. contains a different type of volcanics than that found on both spreading centers. The volcanics collected along the O.S.C. ridges are depleted tholeiites, with low K₂O (<0.15%), Na₂O (<3%) and TiO₂ (<1.76%) contents, comparable to other MORB from the axial graben of the E.P.R. south of the area of overlap. Instead the specimen from the small volcano is enriched in K₂O (>0.2%), Na₂O (>3%) and TiO₂ (2%).Although there is a morphological overlap of the spreading centers in the study area there is no overlap in the present active axial volcanic zones. The bottom observations suggest that the Western spreading center is younger than the E.S.C. and thus that the W.S.C. could be propagating to the south.Contribution N^o 39 du Centre de Brest de L'IFREMER.     

ALVIN investigation of an active propagating rift system,Galapagos 95.5° W

ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.^–1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.     

Volcanic Morphology of the East Pacific Rise Crest 9°49′–52′: Implications for volcanic emplacement processes at fast-spreading mid-ocean ridges

Deep sea photographs were collected for several camera-tow transects along and across the axis at the East Pacific Rise crest between 9°49 and 9°52 N, covering terrain out to 2 km from the ridge axis. The objective of the surveys was to utilize fine-scale morphology and imagery of seafloor volcanic terrain to aid in interpreting eruptive history and lava emplacement processes along this fast-spreading mid-ocean ridge. The area surveyed corresponds to the region over which seismic layer 2A, believed to correspond to the extrusive oceanic layer, attains full thickness (Christeson et al., 1994a, b, 1996; Hooft et al., 1996; Carbotte et al., 1997). The photographic data are used to identify the different eruptive styles occurring along the ridge crest, map the distribution of the different morphologies, constrain the relative proportions of the three main morphologies and discuss the implications of these results. Morphologic distributions of lava for the area investigated are 66% lobate lava, 20% sheet lava, 10% pillow lava, and 4% transitional morphologies between the other three main types. There are variations in inferred relative lava ages among the different morphological types that do not conform to a simple increase in age versus distance relationship from the spreading axis, suggesting a model in which off-axis transport and volcanism contribute to the accumulation of the extrusive layer. Analysis of the data suggests this ridge crest has experienced three distinctly different types of volcanic emplacement processes: (1) axial summit eruptions within a 1 km wide zone centered on the axial summit collapse trough (ASCT); (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3) off-axis eruptions and local constructional volcanism at distances of 0.5-1.5 km from the axis. Major element analyses of basaltic glasses from lavas collected by Alvin, rock corer and dredging in this area indicate that the most recent magmatic event associated with the present ASCT erupted relatively homogeneous and mafic (>8.25 weight percent wt.% MgO) basalts compared to older, off-axis lavas which tend to be more chemically evolved (Perfit and Chadwick, 1998; Perfit and Fornari, unpublished data). The more primitive lavas have a more extensive distribution within and east of the ASCT. More evolved basalts (MgO <8.0wt.%) are concentrated in a broad area a few kilometers east of the axis, and in an oval-shaped area south of 9°50 N, west of the ASCT. Transitional and enriched (T- and E-) mid-ocean ridge basalts exist in relatively small areas (<1 km²) on the crestal plateau and correlate with scarps or fissures where pillow lavas were erupted. Mafic lavas in this area are primarily related to the youngest magmatic events. Geochemical analysis of samples collected at distances >500 m from the ASCT suggests that regions of off-axis volcanism may be sourced from older and cooler sections of the axial magma lens. Analysis of these data suggests that this portion of the EPR has not experienced large scale volcanic overprinting in the past 30 ka. The predominance of lobate flows (66%) throughout much of the crestal region, and subtle variations in sediment cover and apparent age between flows, suggest that eruptive volumes and effusion rates of individual eruptions have been similar over much of the last 30 ka and that most of the eruptions have been small, probably similar in volume to the 1991 EPR flow which had an estimated volume of 1×10⁶ m³ (Gregg et al., 1996).     

白垩纪以来太平洋上地幔组成和温度变化

The geological evolution of the Earth during the mid-Cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the Cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling(DSDP/ODP/IODP) sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-Cretaceous are broadly similar in average Na8, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge(the East Pacific Rise). The Ontong Java plateau(125–90 Ma) basalts have distinctly lower Na8 and143Nd/144 Nd, and higher La/Sm and 87Sr/86 Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the Cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-Cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate.     

Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3°25′S

A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25S, where the Pacific and Nazca plates are separating at 152 mm/yr. A single 90 km-long traverse of the rise crest extends near-bottom observations onto the rise flanks. A ridge at the spreading axis is defined by its steep regional slopes, probably caused by rapid crustal contraction as the spreading magma chamber freezes, rather than by outward-facing fault scarps. It can be divided into a marginal horst-and-graben zone with low (<50 m), symmetric fault blocks, and a 2 km-wide elongate axial shield volcano that is unfaulted except for a narrow crestal rift zone. This has a summit graben (10–35 m deep) probably formed by caldera collapse, and narrow pillow basalt walls built over vent fissures. Small, low (<50 m) volcanic peaks occur on the shield volcano and the horst-and-graben zone, and some may have been built away from the spreading axis. Major plate-building lava flows issue from the crestal rift zone and flow an average of 500 m down the sides of the volcano. The marginal horst-and-graben zone results from tensional faulting of a thin crust of lava, and evolves by progressive shearing on inclined fault planes. Crustal extension continues at least as far as 20 km from the axis, and the large, long fault blocks formed in thicker crust beyond the subaxial magma chamber develop into abyssal hills. Pelagic sedimentation, at a maximum rate of 22 m/10⁶ years, gradually infills open fissures and smooths the small-scale roughness of the fault blocks. Off-axis volcanism has also resulted in smoother crust, and built seamounts.Comparison of the EPR at 3°25S with the Famous Rift and Galapagos Rift reveals a similarity in the processes and small-scale landforms at fast, medium and slow-spreading ridges. There are significant differences in the medium-scale landforms, probably because plate-boundary volcanic and tectonic processes act on crust of very different strength, thickness, and age.Contribution of the Scripps Institution of Oceanography, new series.     

Seafloor electromagnetic induction studies in the Bay of Bengal

Seafloor magnetometer array experiments were conducted in the Bay of Bengal to delineate the subsurface conductivity structure in the close vicinity of the 85°E Ridge and Ninety East Ridge (NER), and also to study the upper mantle conductivity structure of the Bay of Bengal. The seafloor experiments were conducted in three phases. Array 1991 consisted of five seafloor stations across the 85°E Ridge along 14°N latitude with a land reference station at Selam (SLM). Array 1992 also consisted of five seafloor stations across 85°E Ridge along 12°N latitude. Here we used the data from Annamalainagar Magnetic Obervatory (ANN) as land reference data. Array 1995 consisted of four seafloor stations across the NER along 9°N latitude with land reference station at Tirunelveli (TIR). OBM-S4 magnetometers were used for seafloor measurements. The geomagnetic Depth Sounding (GDS) method was used to investigate the subsurface lateral conductivity contrasts. The vertical gradient sounding (VGS) method was used to deliniate the depth-resistivity structure of the oceanic crust and upper mantle. 1-D inversion of the VGS responses were conducted and obtained a 3-layer depth-resistivity model. The top layer has a resistivity of 150–500 m and a thickness of about 15–50 km. The second layer is highly resistive (2000–9000 m) followed by a very low resistive (0.1–50 m) layer at a depth of about 250–450 km. The 3-component magnetic field variations and the observed induction arrows indicated that the electromagnetic induction process in the Bay of Bengal is complex. We made an attempt to solve this problem numerically and followed two approaches, namely (1) thin-sheet modelling and (2) 3-D forward modelling. These model calculations jointly show that the observed induction arrows could be explained in terms of shallow subsurface features such as deep-sea fans of Bay of Bengal, the resistive 85°E Ridge and the sea water column above the seafloor stations. VGS and 3-D forward model responses agree fairly well and provided depth-resistivity profile as a resistive oceanic crust and upper mantle underlained by a very low resistive zone at a depth of about 250–400 km. This depth-range to the low resistive zone coincide with the seismic low velocity zone of the northeastern Indian Ocean derived from the seismic tomography. Thus we propose an electrical conductivity structure for the oceanic crust and upper mantle of the Bay of Bengal.     

Tectonics of the Nereus Deep,Red Sea: A deep-tow investigation of a site of initial rifting

The Nereus Deep (23°N) lies in the central portion of the Red Sea, in a region which marks a transition between the nearly continuous axial rift valley of the southern Red Sea and the northern Red Sea, where a well defined axial rift is absent. The deep-tow survey and associated heat flow measurements reported here show that the Nereus Deep is a short segment of axial rift, and it is the northernmost deep where petrology, heat flow, magnetics, and morphology all indicate classic seafloor spreading. Heat flow measured in the Nereus Deep is characterized by non-linear gradients and closely-spaced variability indicative of active hydrothermal circulation associated with seafloor spreading. The two axial highs which we have mapped in Nereus differ markedly in that the southernmost appears younger or at least has had a more recent phase of volcanism. The two axial highs are offset left laterally approximately 2 km. This small offset or bend in the axial course has been labelled the Nereus shear zone, and, despite its small extent, it mimics many of the major features of small offset, slow-slipping transform faults. This shear zone may result from shear stresses associated with misalignments in succeeding volcanic episodes. The Nereus Deep appears to represent one of the earliest phases of seafloor spreading. The Red Sea seems to be opening towards the north, and the Nereus Deep is near the tip of propagation, but it is clear from this study that rift propagation in a site of initial rifting differs greatly from that observed along a well developed, fast spreading center like the East Pacific Rise.     

Geophysical study of a seamount located on the continental margin of India

Bathymetric, magnetic, and gravity data obtained over a 2,464 m high seamount located in the Arabian Sea indicate that the seamount (inferred mean density=2.65 gm/cc) extends for 1 km beneath the seafloor and is locally isostatically compensated. With a reversely magnetized upper part and normally magnetized base, the seamount was probably formed by at least two volcanic episodes. The base was formed during the Late Paleocene (ca. 58 Ma) while the Indian Plate was moving over the Reunion hotspot. A renewed period of volcanism, contemporaneous with the major changes in direction of Indian Plate motion during the early Oligocene (ca. 36 Ma) probably formed the cap.     

Tectonics of a fast spreading center: A Deep-Tow and sea beam survey on the East Pacific rise at 19°30′ S

Sea Beam and Deep-Tow were used in a tectonic investigation of the fast-spreading (151 mm yr^-1) East Pacific Rise (EPR) at 19°30 S. Detailed surveys were conducted at the EPR axis and at the Brunhes/Matuyama magnetic reversal boundary, while four long traverses (the longest 96 km) surveyed the rise flanks. Faulting accounts for the vast majority of the relief. Both inward and outward facing fault scarps appear in almost equal numbers, and they form the horsts and grabens which compose the abyssal hills. This mechanism for abyssal hill formation differs from that observed at slow and intermediate spreading rates where abyssal hills are formed by back-tilted inward facing normal faults or by volcanic bow-forms. At 19°30 S, systematic back tilting of fault blocks is not observed, and volcanic constructional relief is a short wavelength signal (less than a few hundred meters) superimposed upon the dominant faulted structure (wavelength 2–8 km). Active faulting is confined to within approximately 5–8 km of the rise axis. In terms of frequency, more faulting occurs at fast spreading rates than at slow. The half extension rate due to faulting is 4.1 mm yr^-1 at 19°30 S versus 1.6 mm yr^-1 in the FAMOUS area on the Mid-Atlantic Ridge (MAR). Both spreading and horizontal extension are asymmetric at 19°30 S, and both are greater on the east flank of the rise axis. The fault density observed at 19°30 S is not constant, and zones with very high fault density follow zones with very little faulting. Three mechanisms are proposed which might account for these observations. In the first, faults are buried episodically by massive eruptions which flow more than 5–8 km from the spreading axis, beyond the outer boundary of the active fault zone. This is the least favored mechanism as there is no evidence that lavas which flow that far off axis are sufficiently thick to bury 50–150 m high fault scarps. In the second mechanism, the rate of faulting is reduced during major episodes of volcanism due to changes in the near axis thermal structure associated with swelling of the axial magma chamber. Thus the variation in fault spacing is caused by alternate episodes of faulting and volcanism. In the third mechanism, the rate of faulting may be constant (down to a time scale of decades), but the locus of faulting shifts relative to the axis. A master fault forms near the axis and takes up most of the strain release until the fault or fault set is transported into lithosphere which is sufficiently thick so that the faults become locked. At this point, the locus of faulting shifts to the thinnest, weakest lithosphere near the axis, and the cycle repeats.