Non-transform Discontinuities Research Articles

Gravity Data Reveal New Evidence of an Axial Magma Chamber Beneath Segment 27 in the Southwest Indian Ridge

Hydrothermal systems are integral to mid-ocean ridge activity; they form massive seafloor sulfide (SMS) deposits rich in various metallic elements, which are potential mineral resources. Since 2007, many hydrothermal fields have been discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR). The Duanqiao hydrothermal field is located at segment 27’s axis between the Indomed and Gallieni transform faults; tomography models reveal an obvious low-velocity anomaly beneath it, indicating a possible axial magma chamber (AMC). However, confirmation of an AMC’s existence requires further study and evidence. In this study, we first calculated the gravity effect to identify the heterogeneous distribution of crustal density beneath segment 27 and the surrounding area. Next, we used the gravity-inversion method to obtain the crustal density structure beneath the study area. The results indicate that a thickened crust and low-density crustal materials exist beneath segment 27. The low-density anomaly in the lower crust beneath the Duanqiao hydrothermal field suggests the existence of an AMC covered with a cold and dense upper crust. The density results identify several faults, which provide potential channels for magma migration. In addition, the melt migrates westward and redistributes laterally toward the segment’s western end. However, when migrating toward the segment’s eastern end, the melt is affected by a rapid cooling mechanism. Therefore, the segment’s ends present different density features and morphologies of nontransform discontinuities (NTDs).

Mantle melting, lithospheric strength and transform fault stability: Insights from the North Atlantic

The offset ridge-transform structure of ocean basins is one of the most prominent expressions of plate tectonics. Yet why this configuration is favored over a continuous divergent boundary has remained unresolved. We examine this issue using mantle Bouguer anomalies (MBAs) from contrasting ridge systems in the North Atlantic. The Reykjanes Ridge north of the Bight transform fault has no transform offsets and is characterized by rapidly propagating melting centers along a linear axis and a continuous MBA low. To the south, the adjoining Mid-Atlantic Ridge has typical ridge segments each underlain by a discrete MBA “bulls-eye” low and offset by transform and non-transform discontinuities. We hypothesize that the pattern of mantle melting, as reflected in the MBAs, creates chemical and rheologic variations in the residual mantle that either favor or hinder transform fault formation. Within ridge segments, mantle melting efficiently extracts water producing dry and strong residual mantle. At segment ends, low extents of melting and inefficient melt extraction preserve damp and weak mantle. On the linear Reykjanes Ridge continuous melting and rapidly propagating melting centers create continuous strong mantle with possibly weak rheologic variations at high angles to the opening direction, not favoring transform faults. In contrast, stable segmented mantle melting on the Mid-Atlantic Ridge forms bands of strong and weak residual mantle aligned in the spreading direction, the latter creating favorable locations for shear deformation and transform faults. Our hypothesis also explains the lack of transform faults at Earth's endmember spreading rates. At ultra-slow ridges, overall melting is limited and irregular and melt extraction is inefficient. At ultra-fast ridges, mantle melting is pervasive and melt extraction is efficient. In both cases, organized spreading-parallel compositional rheological variations do not form and transform faults are not favored. Our model implies that beyond cooling and strengthening with age, the pattern of mantle melting shapes the rheological structure of oceanic lithosphere and the geometry of plate tectonics.

Rheological control on the segmentation of the mid-ocean ridges: Laboratory experiments with extension initially perpendicular to the axis

Mid-ocean ridges (MOR) axes are not straight, but segmented over scales of 10s to 100s of kilometers by several types of offsets including transform faults (TF), overlapping spreading centers (OSC) and non-transform, non-overlapping offsets (NTNOO). Variations in axial morphology and segmentation have been attributed to changes in magma supply, axial thermal structure (which depends on mantle temperature and spreading rate), and axial mechanical properties. To isolate the effect of each of these processes is difficult with field data alone. We therefore present a series of analogue experiments using colloidal silica dispersions as an Earth analogue. Diffusion of salt from saline solutions placed in contact with these fluids, causes formation of a skin, whose rheology evolves from viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults, and ridge segments. Lithospheric thickness is varied independently by changing the surface water layer salinity. Experimental results depend on the axial failure parameter ΠF, the ratio of a mechanical length scale (Zm) and the axial elastic thickness (Zaxis), which depends on mantle temperature and spreading velocity. Slow-spreading fault-dominated, and fast-spreading fluid intrusion-dominated, ridges on Earth and in the laboratory are separated by the same critical value ΠFc±0.024, suggesting that the axial failure mode governs ridge geometry. Here, we examine ridge axis segmentation. Measurements of >4000 experimental ridge segments and offsets yield an average segment length Lm that is quasi-constant at all spreading velocities. Scaled to the Earth, Lm∼55 km, in agreement with the natural data. Experiments with low ΠF show offset size varying as dl=csteLmZaxis regardless of offset type, a correlation well explained by fracture mechanics. Finally, as on Earth, experimental ridge segments are separated by transform and non-transform discontinuities, and their nature and occurrence vary with ΠF. NTNOOs develop when ΠF<ΠFc, while OSCs develop when ΠF>ΠFc. In contrast, TF may form at any ΠF, but the proportion of TFs relative to OSCs or NTNOOs decreases when ΠF/ΠFc>> 1 or << 1, in agreement with natural MOR.

Long‐Term Evolution of Nontransform Discontinuities at the Mid‐Atlantic Ridge, 24°N–27°30′N

AbstractWe studied long‐term evolution of nontransform discontinuities (NTDs) on the Mid‐Atlantic Ridge from 0‐ to ~20‐ to 25‐Ma crust using plate reconstructions of multibeam bathymetry, long‐range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly (RMBA), and gravity‐derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi‐stable. Fast, short‐term (&lt;2 Myr) propagation is driven by reduced magma supply (increased tectonic extension) in the propagating ridge tip when NTD ridge‐axis offsets are small (≲5 km). Propagation at larger offsets generally is slower and longer term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge‐axis offset, the evolution of NTDs appears to be controlled by three factors: (1) gross volume and distribution of magma supplied to ridge segments as controlled by 3‐D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma‐poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long‐wavelength (5‐10 Myr) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle.

Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow‐Spreading Southwest Indian Ridge (49.2° to 50.8°E)

AbstractBeneath ultraslow‐spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultraslow‐spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation.

Widespread tectonic extension at the Central Indian Ridge between 8°S and 18°S

The middle part of the Central Indian Ridge (MCIR) between 8°S and 18°S is representative of mid-ocean ridges in the Indian Ocean but has not previously been systematically surveyed. Here we present results from the first high-resolution mapping survey over both on- and off-axis sections of the MCIR including multibeam bathymetry, magnetics, hydrocasting, and seabed sampling. The 700-km-long MCIR consists of six first-order segments that are offset by >30km along well-developed transform faults. Three of the first-order segments are further divided into seven second-order segments with <30km offset along non-transform discontinuities. We have recognized for the first time 11 prominent ocean core complexes (OCCs). These occur at nearly all segment ends, corresponding to an occurrence every 60km of the surveyed ridge. Seafloor spreading model studies using magnetic reversals show that the MCIR is a slow-spreading ridge with average full opening rates ranging from 33.7 to 45.1mm/yr, increasing from north to south. The highly curved and intermittent axial ridge geometry, rugged flank fabric, variation in the depth and width of the middle valley, and the frequent occurrences of ocean core complexes and non-transform discontinuities demonstrate that asymmetric accretionary processes are dominant along the ridge. The spreading rate symmetry combined with morphotectonic features, reveal that the MCIR segments developed mainly via tectonic extension with little magmatism. Segments with asymmetric accretion controlled by tectonic extension makes up ~96% of the MCIR, whereas symmetric accretion controlled by robust magmatism make up <4%. Hydrothermal plumes with high methane concentrations occur frequently over the OCCs. This finding indicates that abundant OCCs exposed by detachment faults lead to extensive hydrothermal circulation at off-axis areas and that detachment faults are the primary fluid path for hydrothermal fluid circulation at off-axis regions. Serpentinization of mantle-derived rock at OCCs may be one of the major sources of heat and methane in off-axis areas.

Gravity anomalies over the Central Indian Ridge between 3∘S and 11∘S, Indian Ocean: Segmentation and crustal structure

High-resolution shipboard geophysical investigations along the Indian Ocean ridge system are sparse especially over the Carlsberg and Central Indian ridges. In the present study, the shipboard gravity and multibeam bathymetry data acquired over a 750 km long section of the Central Indian Ridge between 3 ∘S and 11 ∘S have been analysed to understand the crustal structure and the ridge segmentation pattern. The mantle Bouguer anomalies (MBA) and the residual mantle Bouguer anomalies (RMBA) computed in the study area have shown significant variations along the ridge segments that are separated by transform and non-transform discontinuities. The MBA lows observed over the linear ridge segments bounded by well-defined transform faults are attributed to the thickening of the crust at the middle portions of the ridge segments. The estimates of crustal thickness from the RMBA shows an average of 5.2 km thick crust in the axial part of the ridge segments. The MBA and relative RMBA highs along the two non-transform discontinuities suggests a thinner crust of up to 4.0 km. The most significant MBA and RMBA highs were observed over the Vema transform fault suggesting thin crust of 4 km in the deepest part of the transform fault where bathymetry is more than 6000 m. The identified megamullion structures have relative MBA highs suggesting thinner crust. Besides MBA lows along the ridge axis, significant off-axis MBA lows have been noticed, suggesting off-axis mantle upwelling zones indicative of thickening of the crust. The rift valley morphology varies from the typical V-shaped valley to the shallow valley floor with undulations on the inner valley floor. Segments with shallow rift valley floor have depicted well-defined circular MBA lows with persistent RMBA low, suggesting modulation of the valley floor morphology due to the variations in crustal thickness and the mantle temperature. These are supported by thicker crust and weaker lithospheric mantle.

Evidence of serpentinized mantle beneath a non-transform discontinuity at an ultra-slow spreading ridge from wide-angle ocean bottom seismometer data

Non-transform discontinuities (NTDs) are frequently found on ultra-slow spreading ridges. They have crustal structures that differ significantly from those of neighboring neo-volcanic ridges (NVRs). For an ultra-slow spreading ridge with weakened lithosphere and thinned crust, NTDs play an important role in understanding the mechanisms of ridge spreading, hydrothermal venting and mineral resource deposition. In this paper, we present an extensive ocean bottom seismometer (OBS) experiment that was carried out in 2010 at the Southwest Indian Ridge (SWIR) during R/V Dayang Yihao cruise DY115-21. The seismic source was a four-air-gun array with a total volume of or 98.32 L (6000 cubic inches) and a seismic trace spacing of ~250 m. A total of 10832 shots were fired and 52 survey lines (about 2650 km in length) were acquired. Thirty-eight OBSs, each containing a three-component geophone and a hydrophone, successfully recovered information from the crust and upper mantle in NVR and NTD settings. In this paper, we use two wide-angle seismic profiles and a ray-tracing method to constrain the velocity structure of the crust and upper mantle of the NTD between segments 28 and 29 of the SWIR. The processing methods include relocations of shot and OBS positions, time drift corrections and filtering. The widely used ray-theoretical travel-time inversion codes of Zelt and Smith (1992) were used for the modeling. The method includes a damped least square inversion of picked travel times, and we computed the statistical misfit, resolution and uncertainty of model parameters to ensure that the final velocity model had the best fit to our data. The main conclusions are as follows: (1) NTD has a thinner crust (3.2-4.5 km), with an oceanic layer 2 that is 2 km thick and an oceanic layer 3 that is very thin (<1 km) or absent. (2) Oceanic layer 2 has significant lateral velocity variation, indicating the presence of various faults. (3) The uppermost mantle has low velocities (7.2 km/s), indicative of large and deep faults or the presence of serpentinization. We suggest that the low velocity zone in the upmost mantle, together with the thinner crust (<5 km) of the NTD and the presence of faults, result from large-scale serpentinization of the uppermost mantle, providing a good example for investigations of the difference between the Moho discontinuity and a serpentinization front.

Along‐axis variation in crustal thickness at the ultraslow spreading Southwest Indian Ridge (50°E) from a wide‐angle seismic experiment

AbstractThe Southwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thin crust, and extensive outcrops of serpentinized peridotite. Previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the SWIR, from the Prince Edward (35°30′E) to the Gallieni (52°20′E) fracture zones. Here we present a new analysis of wide‐angle seismic data along the ridge 49°17′E–50°49′E. Our main conclusions are as follows: (1) we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient, underlain by a layer 3 with variable thickness and low velocity gradient; (2) the crustal thickness varies from ∼5 km beneath nontransform discontinuities (NTDs) up to ∼10 km beneath a segment center; (3) the melt supply is focused in segment centers despite a small NTD between adjacent segments; (4) the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust. Our observation of thick crust at an ultraslow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate, and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate, and that mantle temperature or chemistry (or both) must vary significantly along axis.

Geochemistry and petrogenesis of mafic-ultramafic rocks from the Central Indian Ridge, latitude 8°–17° S: denudation of mantle harzburgites and gabbroic rocks and compositional variation of basalts

This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7°45′–17°10′ S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12°45′ S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)N variations in MORB along the CIR (8°–21°S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.

Nature of crust in the central Red Sea

A transition between continental crust in the northern Red Sea and oceanic crust in the southern Red Sea coincides broadly with a southward increase in plate tectonic separation rate and with a decrease in upper mantle seismic velocity. We re-evaluate here the nature of crust in the intervening central Red Sea with the results of legacy seismic refraction experiments and recently released marine gravity anomalies derived from satellite altimeter measurements. In the refraction data, collected east of Thetis Deep, velocities of 6.6–6.9kms−1 of a deep refracting layer, which are similar to measured velocities of unaltered gabbro samples, extend outside the deep to 65km from the axis. The new version of the marine gravity field reveals trends crossing the central Red Sea. Whereas some of them connect with major lineaments in the surrounding African-Arabian shield, those around Thetis Deep die out towards the coastlines. They can be paired across the ridge and lie slightly oblique to plate motions, as is typical of oceanic fracture zones or non-transform discontinuities migrating away from hotspots. Taken together these observations support the view that an oceanic rather than extended continental crust underlies this part of the central Red Sea.The crestal mountains around the median valleys of slow-spreading ridges are typically 500–1000m lower at spreading discontinuities. Around Thetis Deep, the similar pattern in the gravity field to those of slow-spreading ridges suggests that the crestal mountains may variably block or impede flowage of evaporites towards the spreading centre, whereas the discontinuities may mark areas where flowage is unobstructed. Limited multibeam data collected in transits outside Thetis Deep show oblique fabrics as expected from these predicted movements.

Segmentation and morphology of the Central Indian Ridge between 3°S and 11°S, Indian Ocean

The segmentation pattern of 750km long Central Indian Ridge (CIR) between 3°S and 11°S latitudes in the Indian Ocean has been studied using multibeam bathymetry and magnetic data. Twelve ridge segments were identified that are separated by well defined transform faults and non-transform discontinuities. Magnetic model studies qualify the ridge as a slow spreading ridge with average full spreading rates varying from 26 to 38mm/yr. The disposition of the magnetic anomalies suggests that the plate opening direction has not changed during the last 0–4Ma period. Along axis variations in the magnetic anomalies, presence of axial volcanic ridges on the inner valley floor, variations in the depth and geometry of the rift valley, suggest distinct variati'ons in the accretionary processes along the ridge. Based on these characteristics and the segmentation pattern, we suggest that ten ridge segments are undergoing less magmatic phase of extension, while two segments have shown characteristics of magmatic accretion. The linear segments with narrow and shallow rift valley floor and near symmetric magnetic anomalies are identified as segments with magmatic accretion. The influence of diffuse plate boundary zone on the young seafloor fabric generated by the CIR has been examined. The seafloor topography different from the normal ridge parallel fabric observed at few places over the NE flank of the CIR is suggested to be the consequence of gradual and progressive influence of the distributed diffuse plate boundary (between the Indian–Capricorn plates), on the newly generated oceanic lithosphere. Further, we documented distinct ridge-transform intersection (RTI) highs, three of these RTI highs are identified as oceanic core complexes/megamullion structures. Megamullion structures are found to be associated with less magmatic sections. Increased seismicity has been observed at the less magmatic segment ends suggesting the predominance of tectonic extension prevalent at the sparsely magmatic sections.

Structural analysis of fault populations along the oblique, ultra-slow spreading Knipovich Ridge, North Atlantic Ocean, 74°30′N-77°50′N

The Knipovich Ridge (73°30′–78°40′N) is an extreme end-member of the mid-ocean ridge spreading system, both in terms of spreading rate (<1.5 cm/yr) and angle between the ridge and the spreading direction (40°–53°). Structural analysis of side-scan sonar images obtained along ∼400 km of the ridge axis reveals systematic relationships between fault population parameters, obliquity, and axial segmentation along the ridge. Fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems. For the ridge as a whole, faults in the axial region are short, straight, isolated (not linked into complex fault zones), and exhibit length-scaling relationships characteristic of young and active fault systems. Faults are generally oblique to both the ridge axis and the spreading direction, and orientations vary systematically with angle between ridge and spreading direction. Along-axis analysis reveals the influence of axial segmentation on fault population characteristics. Segment centers are dominated by faults perpendicular and sub-perpendicular to plate motion with longer characteristic length and generally lower fracture density. Conversely, segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density. We infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion, while faulting in segment ends is controlled by the mechanics of oblique rifting, non-transform discontinuities, and/or accommodation zones. The contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra-slow spreading rate of this spreading system.

Morphotectonics of the Carlsberg Ridge between 62°20′ and 66°20′E, northwest Indian Ocean

The segmentation pattern of 440 km long section of the Carlsberg Ridge between 62°20′E and 66°20′E, in the northwest Indian Ocean, has been studied using multibeam bathymetry and magnetic data. A well-defined transform fault is the only first order discontinuity documented in this section of the ridge. Four segments have been identified based on the topographic fabric, along-axis depth variations and the presence of off-axis traces of transform and non-transform discontinuities. These segments constitute four distinct domains representing magmatic and sparsely magmatic crustal accretion. Magnetic model studies show this section of the ridge is a slow spreading ridge with spreading half rates varying from 11 to 16 mm/yr. Along-axis variations in the magnetic anomalies and crustal magnetization, the presence of axial volcanic ridges on the inner valley floor, variations in the depth and geometry of the rift valley, and recovery of mantle derived peridotites from the inner valley floor suggest distinct variations in the accretionary processes along the ridge. It is suggested that the linear section in the east with ridge-parallel topographic fabric, continuous magnetic lineations, and the segment with a shallow valley floor with along-axis ridge propagation fabric, represent a magmatic phase of crustal accretion. The wide rift valley, recovery of mantle rocks, deformed topographic fabric, along-axis depth maxima and discontinuous magnetic lineations documented along the middle part of the ridge and the existence of short ridges east of the transform fault represent sparsely magmatic sections with dominance of tectonic extension.

Evolution of the Southwest Indian Ridge from 55°45′E to 62°E: Changes in plate‐boundary geometry since 26 Ma

From 55°45′E to 58°45′E and from 60°30′E to 62°00′E, the ultraslow‐spreading Southwest Indian Ridge (SWIR) consists of magmatic spreading segments separated by oblique amagmatic spreading segments, transform faults, and nontransform discontinuities. Off‐axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the SWIR. Individual magmatic segments show varying magnitudes and directions of asymmetric spreading, which requires that the shape of the plate boundary has changed significantly over time. In particular, since 26 Ma the Atlantis II transform fault grew by 90 km to reach 199 km, while a 45‐km‐long transform fault at 56°30′E shrank to become an 11 km offset nontransform discontinuity. Conversely, an oblique amagmatic segment at the center of a first‐order spreading segment shows little change in orientation with time. These changes are consistent with the clockwise rotation of two ∼450‐km‐wide first‐order spreading segments between the Gallieni and Melville transform faults (52–60°E) to become more orthogonal to spreading. We suggest that suborthogonal first‐order spreading segments reflect a stable configuration for mid‐ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge‐axis that can more easily accommodate seafloor spreading.

Numerical modelling of non-transform discontinuity geometry: Implications for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends

Ocean ridge discontinuities partition and offset spreading centres at a range of scales. Large scale discontinuities (10's–100's km) are synonymous with first-order transform faults, which have well defined linear fault zone valleys. In contrast, Non-Transform Discontinuities (NTDs) are diffuse, smaller scale offsets (0 to < 20 km), characterised by central basins or topographic highs. The geometry of NTD offsets can be categorised by the sense of offset, either right-stepping or left-stepping, and by the relative positions of the segment tips. The segment tip configurations include under-lapping, over-lapping or simple across-axis jumps or stepping in the ridge axis. In this study finite difference software is used to model segment geometry at a slow-spreading ridge under a normal tensile-stress within a homogeneous and isotropic medium. Along- and across-axis segment separations were varied incrementally for left- and right-stepping senses. The results show that the ratio of along-axis to across-axis segment tip separation is a dominant control of stress field rotation within an NTD. Features which most clearly show rotation within an NTD include basins and tectonically controlled constructional ridges. The obliquity of these features along with measurements of the surrounding fault fabrics are used as a way of observing and determining stress rotations within NTDs along the Central Indian Ridge (CIR). These rotations were used to obtain segment geometries from models where the central tensor showed an equivalent rotation. The results show that geometry has a profound effect on stress field rotation under which large- and small-scale volcano-tectonic fabrics form. In addition, a shortfall of the predicted model tip relative to interpreted positions, along with morphology and observation of the ridge fabrics at the terminations to some segments, suggests the existence of a zone, broadly analogous to the process zone observed in fracture mechanics, which we call a damage zone. Given the criteria for the promotion of hydrothermal circulation, this damage zone would have a greater potential for hosting hydrothermal activity.

Deformation of oceanic lithosphere near slow-spreading ridge discontinuities

Transform and non-transform discontinuities that offset slow spreading mid-ocean ridges involve complex thermal and mechanical interactions. The truncation of the ridge axis influences the dynamics of spreading and accretion over a certain distance from the segment-end. Likewise, the spreading system is expected to influence the lithospheric plate adjacent to the ridge-end opposite of the discontinuity. Tectonic effects of the truncated ridge are noticeable in for example the contrast between seafloor topography at inside corners and outside corners, along-axis variations in rift valley depth, style of crustal accretion, and ridge segment retreat and lengthening. Along such slow-spreading discontinuities and their fossil traces, oceanic core complexes or mega-mullion structures are rather common extensional tectonic features. In an attempt to understand deformation of oceanic lithosphere near ridge offsets, the evolution of discontinuities, and conditions that may favor oceanic core complex formation, a three-dimensional thermo-mechanical model has been developed. The numerical approach allows for a more complete assessment of lithosphere deformation and associated stress fields in inside corners than was possible in previous 3-D models. The initial suite of results reported here focuses on deformation when axial properties do not vary along-strike or with time, showing the extent to which plate boundary geometry alone can influence deformation. We find that non-transform discontinuities are represented by a wide, oblique deformation zone that tends to change orientation with time to become more parallel to the ridge segments. This contrasts with predicted deformation near transform discontinuities, where initial orientation is maintained in time. The boundary between the plates is found to be vertical in the center of the offset and curved at depth in the inside corners near the ridge–transform intersection. Ridge–normal tensile stresses concentrate in line with the ridge tip, extending onto the older plate across the discontinuity, and high stress amplitudes are absent in the inside corners during the magmatic accretionary phase simulated by our models. With the tested rheology and boundary conditions, inside corner formation of oceanic core complexes is predicted to be unlikely during magmatic spreading phases. Additional modeling studies are needed for a full understanding of extensional stress release in relatively young oceanic lithosphere.

Ridge segmentation and the magnetic structure of the Southwest Indian Ridge (at 50°30′E, 55°30′E and 66°20′E): Implications for magmatic processes at ultraslow‐spreading centers

The aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow‐spreading Southwest Indian Ridge. Contrary to faster spreading ridges, magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities. There is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities. We suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase, result in thinner extrusive lavas and thus control the along‐axis magnetization variations by thinning the magnetic source layer. A unique segment centered at 50°28′E shows a marked low magnetization anomaly at its center similarly to the segments of the slow‐spreading Mid‐Atlantic Ridge. We suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated. A higher rate of melt production to the west of Gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low‐magnetization lavas. To the east, magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas. Finally, we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley. Off‐axis, in thin crust areas, upper mantle rocks may become progressively more altered, as distance from the axis increases. The strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities. By contrast, in thicker crust areas, the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting, as on the Mid‐Atlantic Ridge, in slightly more positive magnetization values along the traces of axial discontinuities, regardless of polarity.

Spreading process of the northern Mariana Trough: Rifting‐spreading transition at 22°N

We have conducted a geophysical survey of the northern Mariana Trough from 19°N to 24°N. The trough evolves southward from incipient rifting to seafloor spreading within this region. This study aims to clarify the location and time of the rifting‐to‐spreading transition, which was controversial previously, and processes of seafloor spreading after the transition. The new data set includes swath bathymetry with side‐scan images and magnetic vector anomaly. The mantle Bouguer gravity anomaly (MBA) was calculated using the free‐air gravity anomaly from satellite altimetry. The rifting‐to‐spreading transition occurs at about 22°N, which is proved by seafloor‐spreading fabric in the bathymetry, clear magnetic lineations, and the bull's‐eye pattern in MBA. Four ridge segments separated by three nontransform discontinuities are recognized between 19°N and 22°N. The northernmost segment has relatively abundant magma supply compared with the other segments, which is estimated from a larger segment length, shallower axial depths with no rift valley, and lower MBA. The next segment to the south is, on the other hand, a magma‐starved segment with a prominent rift valley. Two anomalously deep grabens (called the Central Grabens) formed by amagmatic extension occur near the segment ends. The succession of magma‐rich, magma‐starved, and normal segments with increasing distance from the volcanic arc is the same as the observation in the Lau Basin reported by Martinez and Taylor [2002]. The magnetic anomaly revealed the detailed history of the spreading. The seafloor spreading between 19°N and 20°N began prior to 5 Ma, and that between 20°N and 21°30′N began at about 4 Ma. Spreading half‐rates in the western side of the spreading center were 2 to 3 cm/year before 2.58 Ma south of 21°30′N and during the Matuyama Chron north of 21°30′N, but an average during the Brunhes Chron is 1 cm/year or less. Orientations of the ridge axes, which range from −20° to 0° at present, have rotated about 20° clockwise since the start of the spreading. These changes in rate and direction might be associated with changes in the motion of the Philippine Sea plate. Spreading has been asymmetric in the northern Mariana Trough. The spreading rates of the western side of the spreading center have been significantly larger than the eastern counterpart in general. The asymmetry may have been caused by an interaction of mantle upwelling systems under the volcanic front and the backarc spreading center and would be a characteristic of backarc spreading.

A multibeam-sonar, magnetic and geochemical flowline survey at 14°14′S on the southern East Pacific Rise: insights into the fourth dimension of ridge crest segmentation

A detailed bathymetric and magnetic survey of the eastern flank of the East Pacific Rise at 14°14′S covering seafloor ages of 0–10 Ma has been carried out and used, along with a flowline profile on the conjugate western ridge flank, to reveal the spreading history and the temporal ridge crest segmentation. Additional information from basaltic lavas is included to study the relationship between physical and magmatic segment boundaries. The sequence of magnetic reversals indicates a total spreading rate of 150 mm/yr since 10 Ma. Symmetric spreading, however, occurred only since 2.8 Ma. Between 7 and 2.8 Ma spreading was asymmetric, with a higher spreading rate toward the east. Migration events of at least five overlapping spreading centres (OSC) left discordant zones on the Nazca plate consisting of hummocky basins and motley texture of curved lineations striking a few degrees oblique to the strike of the ridge crest. Four of the OSCs were right-stepping and migrated northward and one was left-stepping and migrated southward. By transferring Pacific lithosphere to the Nazca plate, these migration events may account for most of the asymmetric accretion observed. The basaltic samples from the eastern flank have been analysed and back tracked to the position of eruption on the ridge crest. In terms of their geochemical signature (Mg# 0.41–0.68) the samples reveal that the magmatic segment boundary between the Garrett transform and 14°30′S has remained stationary over the last 10 Myr and therefore provide no evidence for a link between magmatic and physical segmentation. We therefore propose that migrating non-transform ridge axis discontinuities are governed by propagating giant cracks; as a crack front advances a melt reservoir is tapped and magma rises passively into the crack and erupts subsequently on the seafloor. Some of the OSCs seem to have originated close to transform faults and therefore argue that far-field stresses, perhaps caused by the evolution of the Bauer microplate, rather than mantle upwelling create non-transform ridge axis discontinuities.