Non-transform Discontinuity Research Articles

Seismic observation of an active detachment faulting system beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge

Hydrothermal processes in detachment settings at slow and ultraslow spreading ridges differ greatly from those at melt-rich faster spreading ridges. Active detachment faulting provides the possibility for off-axis high-temperature hydrothermal vents located far away from the heat source beneath the axial volcanic ridge. Seismic data from the slow spreading Mid-Atlantic ridge revealed that hydrothermal fluids may exploit detachment faults to extract heat from a melt zone near the crust-mantle interface. However, knowledge of the subsurface structure and the kinematic processes of detachment faults, and their interaction with hydrothermal fields at the slowest spreading ridges is still insufficient. Here, we report new insights on the seismicity beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge from three ocean bottom seismometer monitoring experiments. The seismicity outlines the subsurface geometry of a detachment faulting system (DF1 and DF2). The strongly flexed DF1 (45°) is a mature detachment fault with a domed-shaped OCC where ultramafic rocks are exposed on the seafloor. An active young DF2 with intense earthquake activity along the steep subsurface (67°) suggests the initiation phase of rotation. The diversity of hydrothermal activities in the Longqi detachment-hydrothermal systems is closely related to the evolution of detachment faults. Additionally, both the non-transform discontinuity on the western margin of the Longqi-1 field and local faults facilitate hydrothermal circulation. Our study provides baseline observations for a Longqi-type of hydrothermal circulation at the inside corner associated with detachment faulting and non-transform offsets.

Quantifying the influence of magmatism and tectonism on ultraslow-spreading-ridge hydrothermal activity: Evidence from the Southwest Indian Ridge

Hydrothermal activity in mid-ocean ridges (MORs) is an important intermediary for the mass and heat exchange between the ocean and lithosphere. The development of hydrothermal activity on MORs is primarily controlled by coupled magmatic and tectonic activities. In ultraslow-spreading ridges, deep-dipping low-angle normal faults with large offsets, typically detachment faults in the inside corners of ridge offsets, favor the formation of tectonic-related hydrothermal activities, whereas volcanic-related hydrothermal fields are typically developed in neovolcanic zones in this category of the ridge system. However, whether tectonic or magmatic activity is dominant and to what extent they control the formation of hydrothermal activities on ultraslow-spreading ridges remain unclear. Segments in the west and east of the Gallieni transform fault (TF) located in the ultraslow-spreading Southwest Indian Ridge (SWIR), namely, western area (WA) and eastern area (EA), exhibit distinct magma-supply conditions that provide favorable conditions for examining the influence of magmatic and tectonic activities. We generated prediction models for these areas using the spatial analysis of the water depth, minor faults, large faults, ridge axis, nontransform discontinuity (NTD) inside corners, TF inside corners, Bouguer gravity anomaly, magnetic anomalies, and seismic activities. By employing the weights of evidence method, we reported that the formation of seafloor hydrothermal systems in SWIR was primarily correlated to the NTD inside corner, ridge axis, and minor fault (i.e., contrast values (C) of 4.186, 3.727, and 3.482 in WA and 4.278, 3.769, and 3.135 in EA). Furthermore, EA was significantly affected by the TF inside corner (C = 3.501), whereas WA was influenced by large faults (C = 4.062). Our results demonstrated that tectonism was the primary controlling factor in the development of hydrothermal activities in the study area, and the contribution of magmatism was secondary, even in WA, which has a relatively robust magma supply. We delimited prominent prospecting areas at each side based on posterior probability. Our results provided insights into the formation mechanisms of hydrothermal activities and support prospecting in MORs.

Shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

SUMMARY Shear wave velocities provide an important constraint on crustal lithology. Limited crustal shear wave data are available from the ultraslow spreading mid-ocean ridges. We combine observations of both compressional (P) and shear (S) waves in ocean–bottom seismometer data from the Southwest Indian Ridge to determine crustal P-wave velocity (Vp), S-wave velocity (Vs), Vp/Vs and Poisson's ratio variations along the ridge at 49°17′E–50°49′E. Similar layered crustal structures were revealed beneath both the magmatically robust segment centres (Vp/Vs of 1.76–1.94, Poisson's ratio of 0.26–0.32) and the non-transform discontinuity (NTD) between them (Vp/Vs of 1.76–2.03 and Poisson's ratio of 0.26–0.34). Because laboratory measurements show an overlap in Poisson's ratio between mafic igneous and ultramafic rocks, particularly at Vp values typical of oceanic Layer 3, it can be difficult to distinguish crustal composition using this parameter only. However, our observed Vp gradients of 0.1 ± 0.1 s−1 suggest that in this area, oceanic Layer 3 consists primarily of mafic igneous rocks both at segment centres and at the NTD. Oceanic crustal Layers 2A and 2B above are likely also to consist of mafic igneous rocks, with some evidence for increased fracturing at the NTD.

Seafloor Hydrothermal Activity around a Large Non-Transform Discontinuity along Ultraslow-Spreading Southwest Indian Ridge (48.1–48.7° E)

Non-transform discontinuity (NTD) is one category of tectonic units along slow- and ultraslow-spreading ridges. Some NTD-related hydrothermal fields that may reflect different driving mechanisms have been documented along slow-spreading ridges, but the discrete survey strategy makes it hard to evaluate the incidence of hydrothermal activity. On ultraslow-spreading ridges, fewer NTD-related hydrothermal activities were reported. Factors contributing to the occurrence of hydrothermal activities at NTDs and whether they could be potential targets for hydrothermal exploration are poorly known. Combining turbidity and oxidation reduction potential (ORP) sensors with a near-bottom camera, Chinese Dayang cruises from 2014 to 2018 have conducted systematic towed surveys for hydrothermal activity around a large NTD along the ultraslow-spreading Southwest Indian Ridge (SWIR, 48.1–48.7° E). Five new potential hydrothermal anomaly sites (2 inferred and 3 suspected) of high or low temperature and the previously inferred Sudi hydrothermal field occurred in diverse morphotectonic settings along a 78 km long ridge axis. The calculated vent frequency (Fs, sites/100 km) was ~7.7 over the entire study area, higher than the modified value (Fs ≈ 6.5) between 48 and 52° E of SWIR. Even only for the 54 km long large NTD, three hydrothermal anomaly sites yielded an Fs of ~5.6, which is higher than that of most ridge sections and is comparable to some fast-spreading ridges with high-resolution surveys. This indicates that NTDs along ultraslow-spreading ridges could be promising areas in fertilizing hydrothermal activities. Moreover, the deeply penetrating faults on the rift valley walls and strain-focused areas may contribute to the formation of NTD-related hydrothermal circulations. We suggest that NTDs along ultraslow-spreading ridges may be potential targets for further exploration of hydrothermal activities and seafloor sulfide deposits.

Magmatic and tectonic segmentation of the intermediate-spreading Costa Rica Rift—a fine balance between magma supply rate, faulting and hydrothermal circulation

SUMMARY 3-D tomographic modelling of wide-angle seismic data, recorded at the intermediate-spreading Costa Rica Rift, has revealed a P-wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non-transform discontinuity at the ridge axis. This low velocity zone displays a maximum velocity anomaly relative to the ‘background’ ridge axis crustal structure of ∼0.5 km s−1, has lateral dimensions of ∼10 × 5 km, and extends to depths ≥2.5 km below the seabed, placing it within layer 2 of the oceanic crust. We interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre, that results in higher upper crustal bulk porosity and permeability. Evidence for ongoing magmatic accretion at the Costa Rica Rift ridge axis takes the form of an axial magma lens beneath the western ridge segment, and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system. We propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate. These fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present. The Costa Rica Rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading. The characteristics inherited from each spreading mode have been preserved in the crustal morphology off-axis for the past 7 Myr. Using potential field data, we contextualize our seismic observations of the axial ridge structure at the whole segment scale, and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off-axis may also be apparent along-axis, and that the current larger-scale magma supply system at the Costa Rica Rift may be relatively weak. Based on all available geophysical observations, we suggest a model for the inter-relationships between magmatism, faulting and fluid circulation at the Costa Rica Rift across a range of scales, which may also be influenced by large lithosphere scale structural and/or thermal heterogeneity.

Mafic-hosted seafloor sulfide mineralization at the margin of a non-transform discontinuity on the southern mid-Atlantic ridge

Mineralogical, geochemical and S-isotopic studies were carried out on seafloor massive sulfides (SMS) and hydrothermally altered rocks from the mafic-hosted TaiJi field (including TaiJi-1 and TaiJi-2 sites), which is located on the margin of a nontransform discontinuity (NTD) on the Southern Mid-Atlantic Ridge (SMAR). The main hydrothermal precipitates of TaiJi-1 were massive sulfides, while TaiJi-2 produced a large amount of semi-massive sulfides. Significant rock alteration is an important feature of this field, and large amounts of clay minerals (chlorite) occurred in the semi-massive sulfides. Geochemically, notable negative correlations were identified between rock forming elements (Mg) and major hydrothermal metal elements such as Zn + Cu and Co in semisulfides. Such mineralogical and geochemical characteristics, together with low Co/Ni ratio (similar to the surrounding rocks) of the semi-massive sulfides and the unique REE features of some altered rocks, tell us that the prominent mineralization mechanism for these semi-massive sulfides was probably related to rock mineralization. Thus, TaiJi-2 appears to differ significantly from the TaiJi-1 in terms of sulfide mineralization mechanism. Moreover, the TaiJi sulfides are remarkable for large δ34S variation (7.2–15.3%). We suggest that seawater corrosion after sulfides precipitation were responsible for 34S enrichment in sulfides. Furthermore, our work sheds some light on the comparison between TaiJi and other NTD-related fields along the Mid-Atlantic Ridge. We argue that rock mineralization may represent an important mineralization type in NTDs.

Tectonic environments and local geologic controls of potential hydrothermal fields along the Southern Mid-Atlantic Ridge (12–14°S)

Abstract Systematic hydrothermal exploration and multi-beam bathymetry mapping have been conducted along a 220-km-long section of the Southern Mid-Atlantic Ridge (SMAR) from 12°S (Bode Verde Fracture Zone) to 14°S (Cardno Fracture Zone), and previously reported deposits (Tao et al., 2011) are now being thoroughly investigated. Here, we present the characterization of three possible hydrothermal fields, a complete bathymetry data set of the ridge segment, gravity data, and the petrologic characteristics of collected rock samples. The magmatism characteristics, evolution of the ridge segment, and the local geological controls of the possible hydrothermal fields are then discussed. The studied segment can be divided into two segments by a Non-Transform Discontinuity (NTD). Our morphotectonic analysis shows significant along-axis heterogeneity in the surveyed segments: three distinctive cross-axis grabens were identified in the northern segment, and two were identified in the southern segment. Moreover, based on the gravity data (a relatively low spherical Bouguer anomaly) and petrologic data (low Mg# values and relatively low FeO and relatively high Al2O3 and CaO contents compared to nearby seafloor samples), a volcanic feature, the ZouYu seamount, on this segment is considered to be associated with strong magmatic activity, and the magmatic activity of the inside corner at the southern end of the segment has increased and decreased. The three possible hydrothermal fields occur in different local geological settings: a shallow magmatic seamount (ZouYu), an NTD (TaiJi), and an inside-corner high (CaiFan). These potential hydrothermal fields are significantly different from other fields in similar tectonic settings in terms of local geologic controls and products. The ZouYu field is primarily related to a newly formed cone, resulting in the production of sulfides, and differs from other fields on shallow magmatic seamounts. The TaiJi field is largely controlled by the tectonic evolution of the NTD and is based on mafic rocks. The inside corner containing the CaiFan field is a magmatic seamount rather than an Ocean Core Complex, making it strikingly different from other inside corner-related fields.

Local Seismicity of the Rainbow Massif on the Mid‐Atlantic Ridge

AbstractThe Rainbow massif, an oceanic core complex located in a nontransform discontinuity on the Mid‐Atlantic Ridge (36°N), is notable for hosting high‐temperature hydrothermal discharge through ultramafic rocks. Here we report results from a 9 month microearthquake survey conducted with a network of 13 ocean bottom seismometers deployed on and around the Rainbow massif as part of the MARINER experiment in 2013–2014. High rates (~300 per day) of low‐magnitude (average ML ~ 0.5) microearthquakes were detected beneath the massif. The hypocenters do not cluster along deeply penetrating fault surfaces and do not exhibit mainshock/aftershock sequences, supporting the hypothesis that the faulting associated with the exhumation of the massif is currently inactive. Instead, the hypocenters demarcate a diffuse zone of continuous, low‐magnitude deformation at relatively shallow (< ~3 km) depths beneath the massif, sandwiched in between the seafloor and seismic reflectors interpreted to be magmatic sills driving hydrothermal convection. Most of the seismicity is located in regions where seismic refraction data indicate serpentinized ultramafic host rock, and although the seismic network we deployed was not capable of constraining the focal mechanism of most events, our analysis suggests that serpentinization may play an important role in microearthquake generation at the Rainbow massif.

Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system

Hydrothermal circulation at mid-ocean ridge volcanic segments extracts heat from crustal magma bodies. However, the heat source driving hydrothermal circulation in ultramafic outcrops, where mantle rocks are exhumed in low-magma-supply environments, has remained enigmatic. Here we use a three-dimensional P -wave velocity model derived from active-source wide-angle refraction-reflection ocean bottom seismometer data and pre-stack depth-migrated images derived from multichannel seismic reflection data to investigate the internal structure of the Rainbow ultramafic massif, which is located in a non-transform discontinuity of the Mid-Atlantic Ridge. Seismic imaging reveals that the ultramafic rocks composing the Rainbow massif have been intruded by a large number of magmatic sills, distributed throughout the massif at depths of ∼2–10 km. These sills, which appear to be at varying stages of crystallization, can supply the heat needed to drive high-temperature hydrothermal circulation, and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting. Our results demonstrate that high-temperature hydrothermal systems can be driven by heat from deep-sourced magma even in exhumed ultramafic lithosphere with very low magma supply.

Segment‐scale variations in seafloor volcanic and tectonic processes from multibeam sonar imaging, M id‐ A tlantic R idge Rainbow region (35°45′–36°35′N)

Along-axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid-ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow-spreading Mid-Atlantic Ridge (35o45'-36o35'N) centered on the Rainbow non-transform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multi-directional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone-dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif. This article is protected by copyright. All rights reserved.

Heterogeneous and asymmetric crustal accretion: New constraints from multibeam bathymetry and potential field data from the Rainbow area of the Mid‐Atlantic Ridge (36°15'N)

AbstractAt slow‐spreading mid‐ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma‐poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multibeam bathymetry and potential field data from the Rainbow area of the Mid‐Atlantic Ridge (35°40'N–36°40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T, and the fraction of magmatic accretion, M. Estimated T values range from 0.2–0.4 on ridge segments to 0.6‐0.8 at the Rainbow nontransform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high‐temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep‐penetrating second‐generation faults.

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.

Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an ∼ 18 km 2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a ∼ 24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at ∼ 3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.

The rate of oceanic detachment faulting at Atlantis Bank, SW Indian Ridge

The rates of slip on oceanic detachment faults and how those rates compare to sea-floor spreading rates constitute fundamental data required to constrain how oceanic core-complexes form and their role during crustal accretion. We combine sea-surface magnetic data, with the magnetic polarity of shallow-core samples and Pb/U SHRIMP ages of igneous zircon to determine the time-averaged half-spreading rate during oceanic detachment faulting at Atlantis Bank, 100 km south of the ultraslow-spreading Southwest Indian Ridge (SWIR). The Pb/U zircon ages correlate well with the magnetic ages and so highlight that magmatic accretion and faulting were coeval for over 2 Myr, creating and exposing a >1.5-km-thick layer of gabbro for >35 km parallel-to-spreading. We use bivariate linear regression of distance–age data and forward modeling of magnetic anomaly data to calculate a half-spreading rate during detachment faulting of 14.1+1.8/−1.5 km/Myr (95% confidence limits). When integrated with regional constraints on spreading history, we note that detachment faulting coincided with a short-lived regional increase in the full-spreading rate along the SWIR and, for the ridge segment containing Atlantis Bank, spreading was highly asymmetric with ~80% of plate-motion accommodated by detachment faulting. Consequently, the detachment fault effectively formed the plate-boundary at the surface in this spreading segment. Highly asymmetric spreading was confined to the spreading segment containing Atlantis Bank and to the duration of detachment faulting. So the ridge segment containing Atlantis Bank migrated northward relative to its symmetrically spreading eastern neighbour, such that the intervening non-transform discontinuity shortened. We suggest that the highly asymmetric spreading may be a characteristic feature of oceanic detachment faulting, an inference supported by more poorly constrained half-spreading rates determined at several other oceanic core-complexes.

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.

The Rainbow Hydrothermal Plume, 36°15′N, MAR

The Rainbow hydrothermal plume was discovered during a recent geophysical survey along 200 km of the Mid‐Atlantic Ridge (MAR), SW of the Azores Triple Junction, in which at least seven new sites of hydrothermal activity were identified. Here, we present the first hydrographic study of the Rainbow plume, 36°15′N, the strongest of the features located during that survey. The plume is detectable from real‐time in situ nephelometer anomalies and extends 10–15km, W‐E, in a non‐transform discontinuity (NTD) between two adjacent ridge‐segments. Maximum anomalies in the Rainbow plume indicate particle enrichments at least as large as those seen directly above the Trans Atlantic Geotraverse (TAG) vent‐field (26°N, MAR). Analysis of hydrographic data indicates a vent source at Rainbow with a thermal output of up to 98 MW, representing 140% of the thermal output previously attributed to TAG. Both lines of evidence indicate the Rainbow plume to be the strongest such feature yet found on the MAR.

Segmentation and morphotectonic variations of the Central Indian Ridge (21°10′S–22°25′S)

Bathymetric, magnetic, and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the Central Indian Ridge (CIR) between 21°10′S and 22°25′S. Volcanoes, abyssal hills, and steep slopes, corresponding to fault scarps and to volcanic constructional slopes, have been obtained by image processing methods applied to Sea Beam data. In the survey area, segmentation of the CIR is similar to that of a slow spreading ridge. The plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions. Hourglass‐shaped valleys are bounded by second and third‐order discontinuities located at local along‐axis depth maxima (400–700 m depth variations) and marked by oblique extensional basins and en échelon jogs, respectively. Large‐throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places. Along the axial valley, the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs. The morphology and structure of the rift valley reflect the three‐dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center. Abyssal hills are bounded by both inward and outward facing faults in ∼1.5 Ma crust, indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20–40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills. Reconstructions of the plate boundary geometry through time, using abyssal hills as isochrons, provide evidence to show that the nontransform discontinuity, located at 21°47′S, with an offset as small as 2–3 km, is associated with V‐shaped off‐axis traces in the form of alignments of disrupted abyssal hill lineations. These reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery. Three‐dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0.780 and 0.984 Ma and suggests postaccretionary crustal block rotations within the discordant zones.

Morphology and tectonics of the Mid‐Atlantic Ridge (25°–27°30′S) from Sea Beam and magnetic data

High‐resolution Sea Beam bathymetry and sea surface magnetic data have been collected between 25°S and 27°30′S on the Mid‐Atlantic Ridge. Analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three‐dimensional inversion of sea surface magnetic data are presented. This 227‐km‐long portion of the Mid‐Atlantic Ridge is offset 42 km by the right‐lateral Rio Grande Transform at 25°40′S and 9.5 km by the left‐lateral nontransform Moore Discontinuity at 26°30′S. A third nontransform discontinuity, the Midway Discontinuity, exists near 26°12′S and is defined by a small displacement of the axial magnetization distribution and the disruption in the along‐strike continuity of ridge‐parallel terrain elements. Although the detailed survey presented in this paper extends out to only 5‐ to 7‐m.y.‐old crust, a regional compilation of magnetic data from this area by Cande et al. (1988) indicates that the relative positions and dimensions of the spreading cells have characterized this part of the Mid‐Atlantic Ridge for at least the past 55 m.y. The offsets dimensions and geometries, however, have changed markedly because of prolonged differential asymmetric spreading between segments. Between and within the accretionary units that are spreading at uniform rates (35 mm/yr total opening rate), the morphology of the ridge axis varies dramatically, from a well‐defined rift valley (20–25 km wide, 1–1.5 km deep) to an axial swell that rises 1500 m above the adjacent valley floor. The axial magnetic structure is also highly variable along‐strike. The inversion solution shows a series of generally linear, short‐wavelength (&lt;10 km) magnetization highs within the central Brunhes anomaly over each of the four segments in the survey area. These anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world's mid‐ocean spreading centers. The distinct axial magnetization highs can extend out to 2‐m.y.‐old crust at ridge‐discontinuity intersections, suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks, e.g., highly fractionated and strongly magnetized basalts enriched in iron and titanium. The magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area. Both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed. In the case of an axial swell centered at 26°S, attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and/or a zone of substantial alteration. Taken together, the bathymetric data and magnetic modeling document strong along‐axis gradients in crustal and upper mantle properties.

Three‐dimensional analysis of gravity anomalies in the Mark Area, Mid‐Atlantlc Ridge 23°N

We have carried out a three‐dimensional analysis of the gravity field over the Mid‐Atlantic Ridge at Kane (MARK) area, a well‐studied portion of the Mid‐Atlantic Ridge south of the Kane fracture zone between 23°40′N and 22°30′N. The Mid‐Atlantic Ridge in this area is composed of two distinct spreading segments separated by a small nontransform discontinuity at 23°15′N. The mantle Bouguer anomalies in the MARK area are dominated by V‐shaped bands of anomalous mass and small‐amplitude gravity lows and highs located both on and off‐axis, which indicate that the anomaly distribution is more complex than the simple bulls eye pattern described along some other parts of the Mid‐Atlantic Ridge. Within the Kane fracture zone, and in the ridge‐transform intersection area, mantle Bouguer anomalies do not accurately reflect the extent of crustal thinning that has been documented seismically. This is attributed to lower average crustal and mantle densities, due to fracturing and alteration, within the fracture zone and the inability of gravity data to resolve the abrupt change to thinner fracture zone crust which is confined to a narrow zone along the fracture zone trough and beneath the nodal basin at the ridge‐transform intersection. Outside of the Kane fracture zone the magnitude of the crustal thickness variations predicted solely from the residual gravity anomalies compare reasonably well with the crustal structure derived from seismic refraction studies. The largest residual gravity anomalies, implying significant reductions in crustal thickness, are observed over the small nontransform offset at 23° 15′N. The anomalous mass associated with this feature forms a v‐shape that can be traced well off‐axis suggesting that these discontinuities are fundamental ridge segmentation boundaries that can persist for millions of years. The distinctive V‐shaped pattern of gravity anomalies in the MARK area reflects systematic crustal thickness variations related to the northward migration of this nontransform offset over the past 3–4 m.y.