Ridge Jumps Research Articles

Piston core record of Late Paleogene (31 Ma) to recent seafloor hydrothermal activity in the Southwest Pacific Basin

A large diameter piston core containing 8.35 m of metalliferous sediment has been recovered from a small abyssal valley in the remote Southwest Pacific Basin (31° 42.194′S, 143° 30° 331′W; 5082 m water depth), providing unique insight into hydrothermal activity and eolian sedimentation there since the early Oligocene. A combination of fish‐teeth Sr‐isotope stratigraphy and INAA geochemical data reveals an exponentially decreasing hydrothermal flux 31 Ma to the present. Although hydrothermal sedimentation related to seafloor spreading explains this trend, a complex history of late Eocene/early Oligocene ridge jumps, propagating rifts and plate tectonic reorganization of South Pacific seafloor could have also played a role. A possible hiatus in deposition, as recorded by changes in core composition just below 2 m depth, is beyond the resolution of the fish teeth Sr isotope dating method employed here; however, the timing of this interval may be coincident with extinction of the Pacific‐Farallon Ridge at ∼20 Ma. A low flux eolian component accumulating at this site shows an increase relative to the hydrothermal component above 2 m depth, consistent with dust‐generating continental sources far to the west (Australia/New Zealand). This is the first long‐term paleoceanographic record obtained from within the South Pacific “bare zone” (Rea et al., 2006), an anomalous region where Pacific seafloor has largely escaped sediment accumulation since the Late Cretaceous.

Chert in the Pacific: Biogenic silica and hydrothermal circulation

Data on sections drilled by DSDP and ODP in the Pacific have been compiled for 187 sites, 105 of which contain chert. The spatial pattern of siliceous deposits in the Pacific generally follows the pattern of biologic productivity. There is some evidence that enhanced vertical diffusivity in the ocean during the Eocene warm climate gave rise to siliceous deposits on and near bathymetric highs and basin margins. The most common occurrence of chert is in Eocene and older sections; however, cherts do occur in sections as young as 7 Ma in the sites investigated. Over half of the youngest chert occurrences in sedimentary sections are within 150 m of basement. Nearly all the chert occurrences on crust younger than 60 Ma are within 150 m of basement. The deficit in heat flow over ocean crust younger than 60 Ma is explained as the result of heat advection by hydrothermal waters circulating in the upper part of the ocean crust. These waters appear to dissolve biogenic silica and deposit it as chert in the lower parts of sedimentary sections. Diffusion of these waters into the lower sediment section is enhanced by small-scale faulting. This faulting may be associated with intra-plate stresses that develop during ridge jumps and reorientations of plate motion. The relatively warm bottom waters of the early Cenozoic may have also enhanced the dissolution of biogenic silica during hydrothermal circulation in the upper oceanic crust and basal sediment section.

Mid-ocean ridge jumps associated with hotspot magmatism

Hotspot–ridge interaction produces a wide range of phenomena including excess crustal thickness, geochemical anomalies, off-axis volcanic ridges and ridge relocations or jumps. Ridges are recorded to have jumped toward many hotspots including, Iceland, Discovery, Galápagos, Kerguelen and Tristan de Cuhna. The causes of ridge jumps likely involve a number of interacting processes related to hotspots. One such process is reheating of the lithosphere as magma penetrates it to feed near-axis volcanism. We study this effect by using the hybrid, finite-element code, FLAC, to simulate two-dimensional (2-D, cross-section) viscous mantle flow, elasto-plastic deformation of the lithosphere and heat transport in a ridge setting near an off-axis hotspot. Heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width. To determine the conditions necessary to initiate a ridge jump, we vary four parameters: hotspot magmatic heating rate, spreading rate, seafloor age at the location of the hotspot and ridge migration rate. Our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non-linearly with increasing spreading rate and seafloor age. Models predict that magmatic heating, itself, is most likely to cause jumps at slow spreading rates such as at the Mid-Atlantic Ridge on Iceland. In contrast, despite the higher magma flux at the Galápagos hotspot, magmatic heating alone is probably insufficient to induce a ridge jump at the present-day due to the intermediate ridge spreading rate of the Galápagos Spreading Center. The time required to achieve a ridge jump, for fixed or migrating ridges, is found to be on the order of 10 5–10 6 years. Simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux. Model results also suggest a mechanism for ridge reorganizations not related to hotspots such as ridge jumps in back-arc settings and ridge segment propagation along the Mid-Atlantic Ridge.

On the Forces Driving Plate Tectonics: Inferences from Absolute Plate Velocities and Intraplate Stress

Summary Absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries. Plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere. Intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress. The absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar. While the global pattern of absolute velocity is probably close to those predicted by such models, the absolute motions do not therefore provide a strong test of the driving mechanism. Comparison of predicted intraplate stress with the long-wavelength features of the global stress field, however, as determined by in situ measurements, earthquake mechanisms, and stressinduced geological structures, does prove to be a powerful test of possible driving forces. All absolute velocity models have several interesting properties. Lithosphere in the equatorial half of the Earth is moving significantly faster than lithosphere in the polar half. Some connection to the Earth's rotation is implied since this statement is demonstrably untrue for co-ordinate poles much different from the geographic pole. Subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate, confounding attempts to explain the dip angles of Benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate. Ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere; such rapid migration may be a necessary but is not a sufficient condition for ridge jumps. Force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere. From the rms absolute velocities of individual plates, there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere. The predicted intraplate deviatoric stress cannot match the well-determined stress fields in North America and Europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system. The mid-plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities.

Submarine lava flow emplacement and faulting in the axial valley of two morphologically distinct spreading segments of the Mariana back‐arc basin from Wadatsumi side‐scan sonar images

High‐resolution, deep‐tow side‐scan sonar data were collected over two distinct spreading segments in the central part of the Mariana back‐arc basin. These data allow mapping of small fissures and faults and the distinguishing of hummocky from smooth lava flows. Using these data, we observe spatial variations in seafloor deformation and volcanic activity within each segment, and also significant differences in the degree of tectonic deformation between the two segments. One segment, characterized by an hourglass shape suggestive of magmatic processes dominating over tectonic processes, is in fact currently dominated by intense deformation rather than volcanism. The other segment, which exhibits morphology (wide and deep linear axial valley) typical of magma‐starved segments, is subjected to very limited deformation and is covered by mostly unfaulted, recent flows. Each segment also displays along‐axis variations in the degree of tectonic deformation and in lava flows freshness. We observe a decrease of lava effusion rate from segment centers toward their ends. We also investigated the apparent asymmetry of the Mariana basin. On the southern segment of the study area, azimuths of tectonic structures are divided into two groups, one segment‐parallel and one ∼15° oblique to strike of the segment. These two trends of linear features developed synchronously with volcanic activity. Currently oblique structures seem to be the most active ones. The tectonic structures that are parallel to the overall valley trend are distributed over the entire valley, whereas the oblique structures are only located proximal to the eastern valley wall. They are likely related to changes in the local stress field related to the obliquely trending eastern axial valley wall. Asymmetric character, such as nonuniform spacing and throw of faults, was not observed. These observations suggest that the asymmetry of the basin is not due to asymmetric spreading, but rather to eastward ridge jumps of several kilometers. Such small‐amplitude ridge jumps likely occurred frequently during basin development because the asymmetric character of the basin is strongly pronounced and no abandoned rift valley is recognizable within the entire basin.

Early Central Atlantic Ocean seafloor spreading history

Twenty-three Mesozoic “Chrons” (specific time intervals) from M0 to M40, including several in the Jurassic Magnetic Quiet Zone (“Jurassic Quiet Zone”), as well as Cenozoic Chron C34, are identified and mapped between the Atlantis and Fifteen-Twenty fracture zones on the North American plate, and between the Atlantis and Kane fracture zones on the African plate. Asymmetric seafloor spreading is indicated by the distances spanned over Chron intervals for the western and eastern flanks of the Central Atlantic ocean basin: C34 to the Mid-Atlantic Ridge (84 Ma to 0 Ma), M0 to C34 (120.6 Ma to 84 Ma), and M25 to M0 (154 Ma to 120.6 Ma). Chron M40 (167.5 Ma) is mapped ∼65 km outboard of the S1 magnetic anomaly over the African flank, and its conjugate, the Blake Spur Magnetic Anomaly (“Blake Spur Anomaly”) over the North American flank. Another pair of conjugate anomalies, the S3 magnetic anomaly over the African flank, and the East Coast Magnetic Anomaly (“East Coast Anomaly”) over the North American flank, are respectively located ∼30 km and 180 km inboard of the S1-Blake Spur Magnetic Anomaly pair. Therefore, the ridge jump to the east between “Blake Spur” and “East Coast” anomalies at ∼170 Ma theorized by [Vogt and others in 1971][1] is supported by this study. Between the Atlantis and Kane fracture zones, the width of the African Jurassic Magnetic Quiet Zone is ∼70 km greater (22%) than the North American Jurassic Magnetic Quiet Zone. Correlatable anomalies exist over the African plate, suggesting a second ridge jump, to the west. Modeling results indicate that this jump occurred between 164 Ma and 159 Ma (Chrons M38 and M32). The ridge jumps can be related to plate interactions as North America separated from Gondwana. However, we note that the second ridge jump occurred approximately at the time suggested for the onset of seafloor spreading in the Gulf of Mexico. [1]: #ref-40

The rise and fall of axial highs during ridge jumps

We simulate jumps of ocean spreading centers with axial high topography using elastoplastic thin plate flexure models. Processes considered include ridge abandonment, the breaking of a stressed plate on the ridge flank, and renewed spreading at the site of this break. We compare model results to topography at the East Pacific Rise between 15°25′N and 16°N, where there is strong evidence of a recent ridge jump. At an apparently abandoned ridge, gravity data do not suggest buoyant support of topography. Model deflections during cooling and melt solidification stages of ridge abandonment are of small vertical amplitude because of plate strengthening, resulting in the preservation of a “frozen” fossil high. The present‐day high is bounded by slopes with up to a 40% grade, a scenario very difficult to achieve flexurally given generally accepted constraints on lithospheric strength. We model these slopes by assuming that the height at which magma is accreted increases rapidly after the ridge jumps. This increase is attributed to high overburden pressure on melt that resided in an initially deep magma chamber, followed by a rapid increase in temperature and melt supply to the region shortly after spreading began. The high is widest at the segment center, suggesting that magmatic activity began near the center of the segment, propagated south and then north. The mantle Bouguer anomaly exhibits a “bull's‐eye” pattern centered at the widest part of the high, but the depth of the axis is nearly constant along the length of the segment. We reconcile these observations by assigning different cross‐axis widths to a low‐density zone within the crust.

Seafloor spreading magnetic anomalies south off Sri Lanka

Results obtained from compilation and reinterpretation of about 21,200 line km of bathymetry, magnetic and satellite gravity data between 10°S to 10°N latitudes and 75 to 90°E longitudes south off Sri Lanka are presented here. Magnetic data and the synthetic seafloor spreading model reveal the presence of Mesozoic anomaly sequence M11 through M0 south of Sri Lanka. The oldest magnetic anomaly M11 (134 Ma) occurs between 110 and 140 km away from the Sri Lankan coast. The seafloor created during the Early Cretaceous is estimated to have evolved with variable half-spreading rates ranging from 5.5 to 1.53 cm/yr. The trends of the fracture zones inferred from the offsets in the magnetic anomalies have been constrained using the satellite gravity mosaic. The Cretaceous Magnetic Quiet Zone (CMQZ, 121–84 Ma) crust between the isochrons M0 and A34 has an unequal width and widens from about 170 km in the west (80°E longitude) to about 500 km towards east (85°E longitude). Plate reconstruction models for 160 (Fit), 134 (M11), 121 (M0) and 84 Ma (A34) are generated under the constraints of newly identified magnetic anomaly isochrons and fracture zones. The mismatch in the A34 reconstruction is attributed to the emplacement of the Ob, Lena and Marion Dufresne seamounts at the spreading ridge axis and/or the frequent ridge jumps in the Middle Cretaceous during the major plate reorganization.

Faulting and volcanism in the axial valley of the slow‐spreading center of the Mariana back arc basin from Wadatsumi side‐scan sonar images

We analyzed in detail the geology of the median valley floor of the Mariana Basin slow‐spreading ridge using sea surface geophysical data and a high‐resolution deep‐tow side‐scan sonar survey over one spreading segment. Analysis of surface magnetic data indicates highly asymmetric accretion, with the half‐spreading rate on the western side of the basin being two to three times larger than on the eastern side. Surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of ∼10 km of amplitude. Deep‐tow backscatter data indicate along‐axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end, respectively. This variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end. Concerning tectonic processes, we find a power law distribution of the fractures, with an exponent of 1.74. This suggests that within the inner valley floor, fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8% of the strain. According to our calculation based on a ratio of 0.02 to 0.03 between the vertical displacement and the length of faults, the amount of tectonic strain accommodated in the inner valley floor would consistently be ∼1.1–3.4%. Data also show two distinct sets of fractures. One trend is parallel to the rift direction at the segment center (∼N160°E) and perpendicular to the plate separation direction. Another set trends ∼17° oblique to this direction (∼N175°E) and is located over the eastern part of the valley, in the vicinity of a major bounding fault also trending ∼N175°E, that is, obliquely to the direction of plate motion. We modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three‐dimensional boundary element approach. We found that the orientation of the predicted fissuring near the oblique fault is locally rotated by ∼15° due to a flexure of the bending plate close to this fault.

The Agulhas Ridge, South Atlantic: The Peculiar Structure of a Fracture Zone

The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the stress field across the fracture zone. Moreover, passing the Agulhas Ridge’s location between 80 and 69 Ma, the Bouvet and Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridge’s structure. The low crustal velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South Atlantic. In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which distributes plume material southwards across the AAFZ, as a source for the magmatic material.

Geodynamic evolution of the Galápagos hot spot system (Central East Pacific) over the past 20 m.y.: Constraints from morphology, geochemistry, and magnetic anomalies

We report results of magnetic data from the Nazca Plate and of geochemical (major element and Sr‐Nd‐Pb‐isotope) analyses of rocks dredged from the Galápagos hot spot tracks (Cocos, Carnegie, Malpelo and Coiba Ridges and adjacent seamounts) in the Central East Pacific. Magnetic anomalies indicate that the Malpelo and Carnegie Ridges were once attached and that seafloor spreading separated the two ridges between 14.5 Ma and 9.5 Ma. The variations in Sr‐Nd‐Pb isotopic composition show that three of the mantle components currently observed at the Galápagos (Central, Southern, and Eastern) existed in the hot spot for at least 20 m.y., whereas the Northern Galápagos mantle component has been present for at least ∼15 Ma. Our data are consistent with the existence of a compositionally zoned/striped Galápagos plume since ∼20 Ma. Combined constraints from the morphology of the hot spot tracks, the magnetic record, and the isotope geochemistry of the rock samples provide new insights into the hot spot‐ridge geometry and interaction of the Galápagos hot spot with the Cocos‐Nazca spreading center (CNS) over the past 20 m.y. At 19.5 Ma a ridge jump moved the spreading axis to the northern edge of the hot spot. Between 19.5 and 14.5 Ma, the spreading axis was located above the center of the hot spot. At 14.5 Ma, a new ridge jump moved the spreading axis to the south, splitting the paleo‐Carnegie Ridge into the present Carnegie and Malpelo Ridges. The repeated ridge jumps reflect capture of the northwardly drifting spreading center by the Galápagos hot spot. At 11–12 Ma an offset of the spreading axis lay above the plume center. Spreading between the Carnegie and Malpelo Ridges continued until 9.5 Ma.

Seismic structure of the Mid‐Atlantic Ridge, 8–9°S

The Mid‐Atlantic Ridge at 8–9°S is characterized by a transition from axial valley to axial high and recent episodes of ridge jumping and ridge propagation. We present constraints on the structure of 0–4 Ma crust in this region on the basis of the analysis of wide‐angle seismic data from a grid of profiles across and parallel to the current and abandoned spreading centers. A 350–800 m thick oceanic layer 2A, interpreted as high‐porosity extrusive basalts, is underlain by a ∼2.0–2.5 km layer 2B with velocities which increase with age and decrease in the vicinity of the pseudofaults. Layer 3 velocities are uniform across the area except for a possible localized anomaly at the ridge axis. The crustal thickness varies from 6–7 km near the pseudofaults formed by ridge propagation to 9–10 km at the segment center of the recently (∼0.3 Ma) abandoned spreading center. Seismically determined crustal thickness and density variations and age‐related lithospheric cooling can plausibly account for all observed variations in gravity across the area, and there is no requirement for the thicker crust at the segment center to be underlain by hot mantle. The transition from axial valley to axial high occurs at a crustal thickness of ∼8 km.

Geochemical and Nd isotopic variations in sediments of the South China Sea: a response to Cenozoic tectonism in SE Asia

Secular variations in geochemistry and Nd isotopic data have been documented in sediment samples at ODP Site 1148 in the South China Sea. Major and trace elements show significant changes at ca. 29.5 Ma and 26–23 Ma, whereas ϵ Nd values show a single change at ca. 26–23 Ma. Increases in Al/Ti, Al/K, Rb/Sr, and La/Lu ratios and a decrease in the Th/La ratio of the sediments beginning at 29.5 Ma are consistent with more intense chemical weathering in the source region. The abrupt change in Nd isotopes and geochemistry at ca. 26–23 Ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments, implying a drastic change in sedimentary provenance and environment at the drill site. Comparison of the Nd isotopes of sediments from major rivers flowing into the South China Sea suggests that pre-27 Ma sediments were dominantly derived from a southwestern provenance (Indochina–Sunda Shelf and possibly northwestern Borneo), whereas post-23 Ma sediments were derived from a northern provenance (South China). This change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the South China Sea, associated with a southwestward expansion of the ocean basin crust and a global rise in sea level. Thus, the geochemical and Nd isotopic changes in the sediments at ODP Site 1148 are interpreted as a response to a major plate reorganization in SE Asia at ca. 25 Ma.

The West Philippine Basin: An Eocene to early Oligocene back arc basin opened between two opposed subduction zones

Based on geological and geophysical data collected from the West Philippine Basin and its boundaries, we propose a comprehensive Cenozoic history of the basin. Our model shows that it is a back arc basin that developed between two opposed subduction zones. Rifting started around 55 Ma and spreading ended at 33/30 Ma. The initial spreading axis was parallel to the paleo‐Philippine Arc but became inactive when a new spreading ridge propagated from the eastern part of the basin, reaching the former one at an R‐R‐R triple junction. Spreading occurred mainly from this second axis, with a quasi‐continuous counter‐clockwise rotation of the spreading direction. The Gagua and Palau‐Kyushu ridges acted as transform margins accommodating the opening. Arc volcanism occurred along the Palau‐Kyushu Ridge (eastern margin) during the whole opening of the basin, whereas the paleo‐Philippine Arc decreased its activity between 43 and 36 Ma. The western margin underwent a compressive event in late Eocene‐early Oligocene time, leading to the rising of the Gagua Ridge and to a short subduction episode along Eastern Luzon. In the western part of the basin, the spreading system was highly disorganized due to the presence of a mantle plume. Overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed. Shortly after the end of the spreading, a late stage of amagmatic extension occurred between 30 and 26 Ma in the central part of the basin, being responsible for the deep rift valley that cut across the older spreading fabric.

Ridge-plume interaction in the North Atlantic and its influence on continental breakup and seafloor spreading

Abstract Development of the rifted continental margins and subsequent seafloor spreading in the North Atlantic was dominated by interaction between the Iceland mantle plume and the continental and oceanic rifts. There is evidence that at the time of breakup a thin sheet of particularly hot asthenospheric mantle propagated beneath the lithosphere across a 2500 km diameter region. This event caused transient uplift, massive volcanism and intrusive magmatism, and a rapid transition from continental stretching to seafloor spreading. Subsequently, the initial plume instability developed to an axisymmetric shape, with the c. 100 km diameter central core of the Iceland plume generating 30–40 km thick crust along the Greenland-Iceland-Faroes Ridge. The surrounding 2000 km diameter region received the lateral outflow from the plume, causing regional elevation and the generation of thicker and shallower than normal oceanic crust. We document both long-term (10–20 Ma) and short-term (3–5 Ma) fluctuations in the temperature and/or flow rate of the mantle plume by their prominent effects on the oceanic crust formed south of Iceland. Lateral ridge jumps in the locus of rifting are frequent above the regions of hottest asthenospheric mantle, occurring in both the early history of seafloor spreading, when the mantle was particularly hot, and throughout the generation of the Greenland-Iceland-Faroes Ridge.

Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models

The oceanic Cocos Plate subducting beneath Costa Rica has a complex plate tectonic history resulting in segmentation. New lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the East Pacific Rise from lithosphere formed at the Cocos‐Nazca spreading center. They also define two early phase Cocos‐Nazca spreading regimes and a major propagator. In addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of Cocos Plate over the Galapagos hot spot. The subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab. Newly identified seafloor‐spreading anomalies show oceanic lithosphere formed during initial breakup of the Farallon Plate at 22.7 Ma and opening of the Cocos‐Nazca spreading center. A revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the Cocos‐Nazca spreading center. At 19.5 Ma a major ridge jump reshaped its geometry, and after ∼14.5 Ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere. A suspected cause of ridge jumps is an interaction of the Cocos‐Nazca spreading center with the Galapagos hot spot.

Crustal structure and tectonics of the Ninetyeast Ridge from seismic and gravity studies

Seismic reflection and refraction, gravity, and bathymetric data across and along the central part of the Ninetyeast Ridge were analyzed to determine the crustal structure of the ridge and to understand its tectonics. The ridge in the study area rises to ∼3.5 km above the seafloor of adjacent ocean basins. Basement is covered by sediments up to 0.8 km thick but exposed at some locations. Seismic refraction and gravity models of the ridge show that the crustal thickness is ∼22 km, greater than the average thickness (7 km) of oceanic crust estimated in the Central Indian and Wharton Basins. The ridge is compensated by flexure of layers 2A, 2B, and 3A to the extent of ∼2 km and by a 12 km thick deep crustal body (layer 3B) of underplated material. Upper crustal velocities (layer 2A) beneath the ridge are lower, up to 3.4 km s−1 than in adjacent basins because of weathering process and hydrothermal circulation of the waters in the tectonized blocks. While velocities in deep crustal body (layer 3B) are higher, up to 7.8 km s−1 and seem to be resulted from magmatic rocks accreted within the lithosphere. Steep down ward faulting of ∼2 km along the eastern flank of the Ninetyeast Ridge possibly originated because of compressional and extensional stresses applied along the 89°E Fracture Zone (FZ). The stresses are interpreted as resulting from multiple ridge jumps of Wharton spreading centers during the Late Cretaceous and middle Eocene. The velocity‐depth model suggests that the rocks of layers 2A and 2B and layer 3A are exposed on the eastern flank of the Ninetyeast Ridge and 90°E FZ, respectively. They are consistent with basalt, dolerite, and gabbro rocks, collected from the fault scarps. The central part of the ridge between 1°N and 11°S has uniformly deepened by ∼0.7 km and is bounded by major sublatitudinal deep‐seated fractures. We interpret that the deepened structural block and associated fractures resulted from the activity of the intense deformation of the central Indian Ocean.

Seafloor spreading on the Amsterdam‐St. Paul hotspot plateau

The Amsterdam‐St. Paul (ASP) platform on the intermediate rate Southeast Indian Ridge (SEIR) is the only oceanic hotspot plateau outside the Atlantic Ocean containing an active, mid‐ocean ridge spreading axis. Because the ASP hotspot is small and remotely located, it has been relatively unstudied, and the ridge axis location in many places near the ASP plateau was previously unknown or ambiguous. We mapped the SEIR out to 1 Ma crust (Jaramillo anomaly) both on and near the ASP platform. We located the spreading center to within a few kilometers, based on side‐scan sonar reflectivity. Recent off‐platform magnetic anomalies and lineated abyssal hill topography are consistent with a simple spreading history. Off‐platform full spreading rates increase from ∼63 km/Myr on segment H to the north of the platform to ∼65.5 km/Myr on segment K to the south. In contrast, inversions of seafloor magnetization based on uniform and variable thickness magnetic source layers reflect a complex on‐platform tectonic history with ridge jumps, off‐axis volcanism, and propagating rifts. On one section of the ASP plateau the spreading location has stabilized and is beginning to rift the plateau apart, generating symmetric magnetic anomalies and lineated topography for the last several hundred thousand years. The larger, more stable, spreading segments of the ASP platform are aligned with major volcanic edifices, suggesting that along‐axis magma flow away from plume‐fed centers is an important influence on spreading geometry. Many complex tectonic features observed on the ASP plateau, such as ridge jumps, en echelon, oblique spreading centers, and transforms oblique to the spreading direction, are comparable to features observed on Iceland. The similarities suggest that moderate crustal thickening at an intermediate rate spreading center may have similar effects to pronounced thickening at a slow rate spreading center.

Paleocene on-spreading-axis hotspot volcanism along the Ninetyeast Ridge: An interaction between the Kerguelen hotspot and the Wharton spreading center

Investigations of three plausible tectonic settings of the Kerguelen hotspot relative to the Wharton spreading center evoke the on-spreading-axis hotspot volcanism of Paleocene (60-54 Ma) age along the Ninetyeast Ridge. The hypothesis is consistent with magnetic lineations and abandoned spreading centers of the eastern Indian Ocean and seismic structure and radiometric dates of the Ninetyeast Ridge. Furthermore, it is supported by the occurrence of oceanic andesites at Deep Sea Drilling Project (DSDP) Site 214, isotopically heterogeneous basalts at Ocean Drilling Program (ODP) Site 757 of approximately the same age (59-58 Ma) at both sites. Intermix basalts generated by plume-mid-ocean ridge (MOR) interaction, exist between 11° and 17°S along the Ninetyeast Ridge. A comparison of age profile along the Ninetyeast Ridge between ODP Sites 758 (82 Ma) and 756 (43 Ma) with similarly aged oceanic crust in the Central Indian Basin and Wharton Basin reveals the existence of extra oceanic crust spanning 11° latitude beneath the Ninetyeast Ridge. The extra crust is attributed to the transfer of lithospheric blocks from the Antarctic plate to the Indian plate through a series of southward ridge jumps at about 65, 54 and 42 Ma. Emplacement of volcanic rocks on the extra crust resulted from rapid northward motion (absolute) of the Indian plate. The Ninetyeast Ridge was originated when the spreading centers of the Wharton Ridge were absolutely moving northward with respect to a relatively stationary Kerguelen hotspot with multiple southward ridge jumps. In the process, the spreading center coincided with the Kerguelen hotspot and took place on-spreading-axis volcanism along the Ninetyeast Ridge.

Torfajökull: a radiogenic end-member of the Iceland Pb-isotopic array

Torfajökull is the largest silicic center in Iceland and is located on the eastern branch of Iceland's neovolcanic zone. Torfajökull's location on the Iceland volcanic crust is unique because (1) it is situated behind the tip of a propagating ridge, and (2) several ridge jumps made it possible for lavas to extrude through relatively old basaltic crust (up to 10 Ma). Torfajökull is dominated by large amounts of silicic rocks, with less abundant basaltic and intermediate compositions. Several lavas show evidence of mixing of basaltic and rhyolitic melts. Varying degrees of assimilation of melts from hydrated and metamorphosed basaltic crust into primitive mantle-derived melts have been argued as the key mechanism in generating the variation of rock types and isotopic signatures found in the Icelandic lavas. Torfajökull, with its underlying old basaltic crust, offers the most favourable place to identify the contribution of crustal melts through radiogenic isotope analysis. Our data show that the Torfajökull lavas are among the most radiogenic Sr- and Pb-containing lavas in Iceland, but we find no correlation between major elements and the radiogenic isotope compositions. Basaltic, intermediate and rhyolitic lavas from the Torfajökull silicic center have 87Sr/ 86Sr- and 143Nd/ 144Nd-values of 0.70323–0.70342 and 0.512963–0.512999, respectively. Despite a large variation in SiO 2 concentrations (50%–74%), there are no significant differences in isotopic compositions between the different rock types. Torfajökull lavas plot towards the high 87Sr/ 86Sr end of the Icelandic Sr–Nd-isotope array. The Pb-isotope compositions are even more strikingly uniform ( 206Pb/ 204Pb 19.168–19.308, 207Pb/ 204Pb 15.513–15.587, 208Pb/ 204Pb 38.729–39.066) with no indication of radiogenic enrichment in the silicic lavas. The Pb-isotopic signature of the Torfajökull basaltic and silicic lavas clearly defines the most radiogenic end of the Iceland Pb-isotopic arrays. The radiogenic Sr- and Pb-isotope component of the Iceland isotopic array is well defined as a FoZo (focal zone) mantle component.