Oblique Spreading Research Articles

Morphology and tectonics of the Romanche Transform and its environs

We describe the results of GLORIA surveys of the Romanche Transform and neighbouring areas of the Equatorial Atlantic Ocean. These surveys have imaged the whole of the Romanche Transform Fault, the eastern end of the Saint Paul Transform, the spreading centre joining them, and the northern part of the spreading centre between Romanche and Chain transforms. They have also imaged representative parts of the off-axis seafloor structure in these areas, and have facilitated preparation of a new bathymetric map, which is presented here. Contrary to previous descriptions of the area, we have found that there is ‘normal’ spreading fabric (volcano-tectonic lineaments trending NNW-SSE orthogonal to the spreading direction) between these major equatorial fracture zones. This implies that normal seafloor spreading processes have operated there. The Romanche Transform Domain, within which normal spreading fabric is either absent or heavily modified, and transform-related structures predominate, is about 100 km wide. There are multiple transform faults within Saint Paul Fracture Zone, of which we have imaged the active traces of the two southernmost ones. These transforms are joined by short but otherwise normal spreading segments. The Romanche Transform is shorter than previously thought, with a length of 840 km. This is equivalent to an age offset of about 50 Ma. The transform has a complex system of short normal and oblique spreading centres at its eastern intersection, forming an overall oblique section of ridge which may have recently cut off the corner of an earlier, simple, orthogonal ridge-transform intersection. The recent complex system appears to have developed at about 1.5 Ma. An oblique valley is mapped south of the western end of Romanche Transform, and is thought to be the fracture zone trace of an old transform offset between Romanche and Chain transforms. This transform disappeared at about chron 8 (26.9 Ma), as its trace is not seen between 20° W and the spreading centre.

Extensional transform zones and oblique spreading centers

Extensional transform zones (ETZs) are plate boundary segments of order 100 km long that strike at angles between 15° and 45° to the extension direction. They are characterized by neovolcanic/tectonic zones comprising overlapping en echelon volcanic systems and/or faults that trend 30°–75° to the extension direction, sometimes accompanied by a Riedel shear. Below these surficial en echelon structures the deformation is aseismic and ductile, and the plate boundary is probably continuous. ETZs occur in fast and slow spreading and rifting environments and may persist in a stable configuration for several million years. ETZs link oblique spreading segments to transform faults in the Manus and probably the Lau backarc basins. The Reykjanes Peninsula and Tjornes Fracture Zone in Iceland and the Mak'Arrasou in Afar are ETZs that link subaerial to submarine spreading or rifting segments. The Brawley and Cerro‐Prieto seismic zones appear to be ETZs in the Imperial and Mexicali valleys that link the San Andreas, Imperial, and Cerro‐Prieto transform faults. Experimental and analytical models of transtensional deformation in brittle‐ductile systems match many of the observed characteristics of ETZs and oblique spreading centers, including variably sigmoidal to straight en echelon faults that are not parallel to the extension direction. The contrasting fault patterns reflect the rheology of the models and lithosphere: they are more sigmoidal when the strain in the lower ductile layer is more focused, causing the axial faults to curve as they propagate toward parallelism with the less ductile rift margins. The angle (Ø) between the faults and the extension direction decreases with the angle (α) between the strike of the zone and the extension direction. ETZs occur in the range 15° ≤ α ≤ 45°, whereas oblique spreading centers have 45° < α < 90° and transform relay zones have 0° < α < 15°. Oblique fast spreading segments exhibit ridge‐parallel faults and volcanic systems (Ø = α), presumably reflecting locally rotated stress fields, whereas at oblique slow spreading centers, Ø is closer to orthogonal (α < Ø < 90°).

Early and late Pan-African orogenies in the Aïr assembly of terranes (Tuareg shield, Niger)

Our study of the (60,000 km 2) Aïr massif, which comprises the southeastern extension of the Tuareg shield in the Niger Republic, has identified three major displaced terranes: Aouzegueur, Barghot and Assodé. These terranes were all affected differentially by the Pan-African orogeny during an early major phase (750–660 Ma) and a late “follow-up” phase (650–580 Ma). The greenschist facies Aouzegueur terrane comprises an ophiolite and is intruded by a late-kinematic medium-K calc-alkaline tonalite-trondhjemite-granodiorite (TTG) suite (∼730 Ma). The amphibolite facies Barghot terrane, made up of a basement and a supracrustal sequence, is intruded by late- to post-kinematic high-K calc-alkaline granitoids (715–665 Ma). Both terranes form a NNE to E-verging belt thrust (680–670 Ma) upon an unknown eastern rigid continent and are covered unconformably by an early Pan-African molasse. In contrast, the amphibolite facies Assodé terrane, composed of a basement and several supracrustal sequences, collided with the eastern rigid block without being thrust upon it. Tibetan-type crustal thickening occurred and HT-LP metamorphism proceeded to regional anatexis enhanced by widespread emplacement of anatectic granites of lower crustal origin (∼6770 Ma). This thermal event is attributed to the delamination of the continental lithospheric mantle and consequent rise of the asthenosphere. The late Pan-African phase affecting mainly the Assodé terrane is marked by the intrusion of high-K calc-alkaline N-S elongated batholiths (645–580 Ma)) and by right-lateral displacement along the N-S Raghane mega-shear zone, located on the edge of the eastern rigid continent. This movement along the northern extent of the rigid continent in Eastern Hoggar (8°30′E) created the Tiririne intracontinental linear fold belt made up of early Pan-African molasse. In the model proposed, we estimate that the Assodé terrane, driven by an oblique spreading ridge, moved to the north over a distance of ∼1000 km druing 60 Ma along the lithospheric Raghane shear zone. The latter was the subsequent locus of the Silurian-Devonian alkaline ring-complexes. The calc-alkaline granitoids are regarded as subduction-related mantle products contaminated by lower and medium-level crust. The model and mechanisms proposed can be applied to the entire Tuareg shield.

Windows-based high-precision EMG analysis tool

The South China Sea is a mid-late Tertiary marginal basin. The magnetic anomaly lineations in the eastern part of the basin trend approximately east-west [1,2], suggesting a north-south direction of spreading. In the spring of 1985, two cruises on the French research vessel “Jean Charcot” provided Sea Beam coverage, seismic reflection, magnetic and gravity profiles. The Sea Beam data exhibit two major structural trends: scarps striking N50°E± 15°, interpreted as normal faults, and scarps striking N140°E± 15°, interpreted as fracture zones. This fabric implies a northwest-southeast direction of spreading, up to about 100 km north and south of the inferred spreading axis [3]. Dense Sea Beam coverage of a roughly 1° square area northwest of the Scarborough Seamounts chain shows that the emplacement of these seamounts was, at least in its initial stage, controlled by faulting in two orthogonal directions, N50°E and N140°E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50°E, which are disrupted by transform zones striking N140°E. This detailed study indicates that the fracture zones may be closely spaced (less than 20–30 km) east of Macclesfield Bank. Furthermore, magnetic anomalies identified as 6 and 6a (20 Myr) along two north-south profiles located at both edges to the north of this detailed study area may be correlated with the N80°E trend characteristic of such anomalies in the eastern part of the South China Sea. The east-west trend of magnetic anomalies 6 and 6a, south of Scarborough seamount chain, recognised by Taylor and Hayes (1983) [1] is incompatible with the trend of the fault scarps observed on Sea Beam data. We infer that progressive, right-lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east-west average trend of some of the magnetic anomalies, and of the Scarborough seamount chain, which represents the location of the relict spreading axis, in spite of a N50°E spreading direction. The whole central part of the basin east of 115°E may have formed in this way, since N130–140°E striking fracture zones are observed on the Sea Beam swaths. This model may correspond to the second of two successive spreading phases, with N-S and NW-SE directions of extension respectively, as was presented by the authors in a previous paper (Pautot et al., 1986 [3]). Alternatively, we may assume that the direction of extension did not change radically during the opening history [3], implying that oblique spreading occurred along N80°E trending ridge segments, creating the N80°E trending scarps and magnetic anomalies observed to the north and to the south of the study area. Such a direction of extension is compatible with mid-late Tertiary left-lateral movements along large N130–150°E strike-slip faults, such as the Red River and Wang Chao faults in South China and Sundaland.

Segmentation, volcanism and deformation of oblique spreading centres: A quantitative study of the Reykjanes Ridge

New multi-beam swath bathymetry charts and high-resolution side-scan sonar imagery provide detailed evidence for volcanic and magmatic segmentation along the Reykjanes Ridge. By combining these data sets, we have quantified some relationships between different orders of segmentation, various characteristics of volcanism, and changes in tectonism along the ridge axis. Three orders of segmentation along the Reykjanes Ridge have been recognised: (1) A long-wavelength depth variation, or swell, that is related to the shoaling of the ridge towards Iceland; (2) Intermediate-wavelength rises of the order of 40–120 km; and (3) A short-wavelength, volcanic segmentation of the order of 5–30 km. The long-wavelength depth variation, which is probably controlled by the temperature of the underlying asthenosphere, has a maximum positive deviation from a predicted depth curve that forms a break in slope at 59–60°N. This feature migrated southward along the ridge at between 5 and 10 cm/yr at least four times in the past 10 Ma. The intermediate-wavelength rises, which are also temporally and spatially unstable, are probably related to local Rayleigh-Taylor style asthenospheric instabilities. The short-wavelength segments have an evolutionary life-cycle, starting with eruption via fissure volcanoes, and progressing by increasing magma flux to the formation of conical, and then shield-like, volcanoes. These segments are tectonically dismembered by extensional faulting, following a rapid decrease in magma supply. We propose a model for the construction of the volcanic layer of the Reykjanes Ridge which involves a cyclicity of magmatic and tectonic activity.

En echelon axial volcanic ridges at the Reykjanes Ridge: a life cycle of volcanism and tectonics

New deep-towed sidescan records and multibeam bathymetric data along the Reykjanes Ridge allow us to assess the varying thermal effect of the Icelandic hotspot. The increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest. Numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay, and a “cycle” of alternating construction and dismemberment is proposed. Unfaulted, short, narrow, lensoid ridges, occasionally in small groups are interpreted as immature and developing axial volcanic ridges. Broad convex-flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained. Narrow, “bow”-form ridges, occasionally with rectilinear flanks, represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation. Isolated polygonal blocks, generally seen off-axis represent the most advanced stage of dismantling of the surface crustal accretionary features. While this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge, it is recognised that this needs modelling. Although the oblique spreading of the Reykjanes Ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small-scale accretionary processes, we propose that the major processes identified in this study are directly applicable to all slow-spreading sections of mid-ocean ridges.

Energy dissipation rates at ridge and transform fault segments—A model of oblique spreading—

We present an energy dissipation model that can easily be applied to oblique transform faults as well as to oblique spreading ridges. By using a simple model of a rising incompressible Newtonian fluid in a vertical conduit of the ridge‐transform fault system, the energy dissipation rates of the ridge and transform fault segments are calculated as a function of the angles between the strikes of segments and the spreading directions, and the spreading rates. The stability is estimated for about 400 present‐day segments from the energy dissipation rates of ridge and transform fault segments. It is concluded that most of the present‐day configurations of the ridge‐transform fault system are nearly stable, even though the strikes of ridge and transform fault segments are oblique to the spreading directions. Several exceptions seem to be associated with extraordinary phenomena, such as the occurrence of an extremely large earthquake and overlapping spreading centers.

Constraints of Sea Beam data on crustal fabrics and seafloor spreading in the South China Sea

The South China Sea is a mid-late Tertiary marginal basin. The magnetic anomaly lineations in the eastern part of the basin trend approximately east-west [1,2], suggesting a north-south direction of spreading. In the spring of 1985, two cruises on the French research vessel “Jean Charcot” provided Sea Beam coverage, seismic reflection, magnetic and gravity profiles. The Sea Beam data exhibit two major structural trends: scarps striking N50°E± 15°, interpreted as normal faults, and scarps striking N140°E± 15°, interpreted as fracture zones. This fabric implies a northwest-southeast direction of spreading, up to about 100 km north and south of the inferred spreading axis [3]. Dense Sea Beam coverage of a roughly 1° square area northwest of the Scarborough Seamounts chain shows that the emplacement of these seamounts was, at least in its initial stage, controlled by faulting in two orthogonal directions, N50°E and N140°E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50°E, which are disrupted by transform zones striking N140°E. This detailed study indicates that the fracture zones may be closely spaced (less than 20–30 km) east of Macclesfield Bank. Furthermore, magnetic anomalies identified as 6 and 6a (20 Myr) along two north-south profiles located at both edges to the north of this detailed study area may be correlated with the N80°E trend characteristic of such anomalies in the eastern part of the South China Sea. The east-west trend of magnetic anomalies 6 and 6a, south of Scarborough seamount chain, recognised by Taylor and Hayes (1983) [1] is incompatible with the trend of the fault scarps observed on Sea Beam data. We infer that progressive, right-lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east-west average trend of some of the magnetic anomalies, and of the Scarborough seamount chain, which represents the location of the relict spreading axis, in spite of a N50°E spreading direction. The whole central part of the basin east of 115°E may have formed in this way, since N130–140°E striking fracture zones are observed on the Sea Beam swaths. This model may correspond to the second of two successive spreading phases, with N-S and NW-SE directions of extension respectively, as was presented by the authors in a previous paper (Pautot et al., 1986 [3]). Alternatively, we may assume that the direction of extension did not change radically during the opening history [3], implying that oblique spreading occurred along N80°E trending ridge segments, creating the N80°E trending scarps and magnetic anomalies observed to the north and to the south of the study area. Such a direction of extension is compatible with mid-late Tertiary left-lateral movements along large N130–150°E strike-slip faults, such as the Red River and Wang Chao faults in South China and Sundaland.

Structure and development of the Midocean Ridge Plate Boundary in the Gulf of Aden: Evidence from GLORIA Side Scan Sonar

A GLORIA side scan sonar survey along the crest of the midocean ridge in the Gulf of Aden has provided information on the small‐scale morphology of its spreading centers and fracture zones. Spreading center offsets increase in length to the east. This is thought to result from changes in the behavior of the mantle with increasing distance from the Afar hot spot. A 300‐km‐long spreading center in the western Gulf of Aden is oriented obliquely to the spreading direction. Nested within the obliquely oriented median valley is an en echelon arrangement of a number of smaller, elongated basins, each oriented approximately normal to the spreading direction. Close to the axis of spreading, volcanic and closely spaced tectonic structures run approximately normal to the spreading direction. Higher on the walls of the median valley, more‐widely‐spaced normal faults have formed subparallel to the valley. In the western Gulf of Aden there are no indications of sonar targets that might represent transform faults parallel to the NNE spreading direction. In the east, spreading center offsets of 15–25 km are made at fracture zones which are best described as narrow zones of highly oblique spreading. Offsets of greater than 25 km are achieved on single transform faults, and the associated tectonic fabric has a spreading direction trend. The boundaries between crusts of different ages along inactive transform fault extensions lie along the bases of fracture zone scarps. These scarps are situated in the older crust and face the younger. This is the opposite of the situation predicted on the basis of simple plate cooling models. At some places in the Gulf of Aden the fracture zone topography is very subdued, and this is attributed to the failure of the two slabs of lithosphere at the end of the transform fault to couple completely before the younger lithosphere on the nontransform side of the spreading center completes its ascent of the median valley wall. There is evidence of a change of spreading direction 3.5 m.y. ago, from about 040° to 022°. It is argued that the small‐scale morphology of the transform fault/spreading center intersections can be understood in terms of the transfer of shear fracture on a transform fault to tensile fracture on a spreading center propagating away from the intersection. It is concluded that the transform fault must be weaker than the spreading center.

Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5� S: Upwelling centers and variations in crustal thickness

To decipher the distribution of mass anomalies near the earth's surface and their relation to the major tectonic elements of a spreading plate boundary, we have analyzed shipboard gravity data in the vicinity of the southern Mid-Atlantic Ridge at 31–34.5° S. The area of study covers six ridge segments, two major transforms, the Cox and Meteor, and three small offsets or discordant zones. One of these small offsets is an elongate, deep basin at 33.5° S that strikes at about 45° to the adjoining ridge axes. By subtracting from the free-air anomaly the three-dimensional (3-D) effects of the seafloor topography and Moho relief, assuming constant densities of the crust and mantle and constant crustal thickness, we generate the mantle Bouguer anomaly. The mantle Bouguer anomaly is caused by variations in crustal thickness and the temperature and density structure of the mantle. By subtracting from the mantle Bouguer anomaly the effects of the density variations due to the 3-D thermal structure predicted by a simple model of passive flow in the mantle, we calculate the residual gravity anomalies. We interpret residual gravity anomalies in terms of anomalous crustal thickness variations and/or mantle thermal structures that are not considered in the forward model. As inferred from the residual map, the deep, major fracture zone valleys and the median, rift valleys are not isostatically compensated by thin crust. Thin crust may be associated with the broad, inactive segment of the Meteor fracture zone but is not clearly detected in the narrow, active transform zone. On the other hand, the presence of high residual anomalies along the relict trace of the oblique offset at 33.5° S suggests that thin crust may have been generated at an oblique spreading center which has experienced a restricted magma supply. The two smaller offsets at 31.3° S and 32.5° S also show residual anomalies suggesting thin crust but the anomalies are less pronounced than that at the 33.5° S oblique offset. There is a distinct, circular-shaped mantle Bouguer low centered on the shallowest portion of the ridge segment at about 33° S, which may represent upwelling in the form of a mantle plume beneath this ridge, or the progressive, along-axis crustal thinning caused by a centered, localized magma supply zone. Both mantle Bouguer and residual anomalies show a distinct, local low to the west of the ridge south of the 33.5° S oblique offset and relatively high values at and to the east of this ridge segment. We interpret this pattern as an indication that the upwelling center in the mantle for this ridge is off-axis to the west of the ridge.

GLORIA investigations of oceanic fracture zones: comparative study of the transform fault zone

The tectonic patterns of some twenty spreading centre offsets have been determined using the long range sidescan sonar GLORIA and are reviewed in this paper. Offsets of less than about 20 km fail to produce transform faults but are accommodated by short sections of oblique spreading (with overlapping spreading centres in the fast spreading case). At slow slip rates such offsets can be associated with topographic fracture zones that are indistinguishable from those produced at true transform faults. Such features may account for a significant proportion of spreading centre offsets on slowly spreading mid-ocean ridges. Offsets larger than about 30 km produce a true 'transform fault'. This comprises a 1 to 5 km-wide band of individual faults that are parallel or subparallel to the spreading direction. The band corresponds to the 'Transform Fault Zone' (TFZ) of continental wrench faulting. It includes the 'Principal Transform Displacement Zone' (PTDZ) and secondary structures such as Riedel shears. A narrow TFZ is normally associated with a prominent PTDZ, which appears to be either a narrow furrow or a single scarp; on the other hand wider zones often fail to show a clear PTDZ on GLORIA. Both types are common on the slowly spreading Mid-Atlantic Ridge, and may even occur at different points along the same large offset transform (such as Romanche), although the narrow type appears to occur preferentially where the local Ridge axis is oblique to the spreading direction. Narrow TFZs and prominent PTDZs appear to be more common on the fast spreading East Pacific Rise. It is suggested that broader TFZs and less prominent PTDZs may form in response to the complicating effects of small components of compression across the transform fault, perhaps as a result of departure of the transform fault trace from a true small circle. Such compression would be more easily supported in the thicker, stronger lithosphere of slowly spreading ridges, but could be relieved by tension due to the ridge-push force where the ridge is oblique to the spreading direction.

The northern Red Sea and the Dead Sea rift

The results of geophysical measurements at sea, when combined with mesostructural measurements on land, demonstrate systematic extensional features in the region of the northern Red Sea and the Dead Sea rift. Rifts and diapirs were encountered in the northern Red Sea, showing an arcuate trend. Whereas the faulted continental margins at that region are oriented NW-SE, the rifts and the diapirs changed their trend northwards from NNW-SSE to N-S. Systems of rifts and diapirs are considered to be extensional features, therefore the tectonic extensional axis probably changed its orientation accordingly. A series of rifts and diapirs was found also in the Gulf of Elat, trending approximately parallel to the coastlines, and is interpreted to be the northern extension of the Red Sea rift and diapir system. Mesostructural measurements that were carried out in the western flank of the Dead Sea rift indicated further evidence for structural extension, trending E-W to ENE-WSW, in addition to measurements of left-lateral offset of approximately 10 km. All these data, when combined with the geophysical measurements of the characteristics of the earth crust underneath the Dead Sea rift, suggest a tectonic regime of oblique extension in the region of the Red Sea and the Dead Sea rift. This tectonic regime indicates that the Red Sea spreading center extends northwards along the Dead Sea rift, and the clockwise change of the axial trend imposed a sinistral offset to the tectonic extension. Thus it is suggested that the Dead Sea rift is an incipient oblique spreading center.

The study of magnetic anomalies over Davis Strait

Magnetic data collected during several cruises in Davis Strait show north-south lineations over most of the area. The central positive magnetic zone coincides well with a major gravity low. These results suggest that the magnetic anomalies were produced by seafloor spreading and that the gravity low marks an extinct spreading center in Davis Strait. Comparison between the geomagnetic polarity time-scale and dated anomalies in Labrador Sea suggests identification of anomalies 13–27 (37 to 62 MY). In southern Davis Strait (63°N), the spreading rates are 8.4 mm/y. east of the spreading center, and 2.7 mm/y to the west. In northern Davis Strait (66°N), the spreading rates are 4.5 mm/y east of the spreading center, and 2.1 mm/y to the west. Seafloor spreading in Davis Strait was there-fore very asymmetrical, with decreasing rates from south to north. Spreading is interpreted as having begun at 61–63 M and ending at 37 MY. Three fracture zones have been delineated, suggesting oblique spreading about the ridge axis in the Davis Strait. A distinct northeast-southwest anomaly zone is interpreted as the, expression of faults caused by volcanically-induced vertical crustal movements, and not by spreading-related horizontal crustal movement. Faulting along this northeast-southwest anomaly zone is inferred to have occurred after the spreading and before 30 MY. The magnetic anomalies off Cape Dyer are proposed to be different from the anomalies striking in northeast-southwest direction. The Cape Dyer anomaly is interpreted as being formed at 58 MY while the northeast-southwest anomaly zone formed more recently.

Tectonic and magmatic ridges in the eltanin fault system, South Pacific

A Seabeam reconnaissance of the 400 km-long fast-slipping (88 mm yr-1) Heezen transform fault zone and the 55 km-long spreading center that links it to Tharp transform defined and bathymetrically described several types of ridges built by tectonic uplift and volcanic construction. Most prominent is an asymmetric transverse ridge, at which abyssal hills adjacent to the fault zone have been raised 2–3 km above normal rise-flank depths. Topographic and petrologic evidence suggests that this uplift, which has produced a 5400 m scarp from the crest of the ridge to the floor of a 10 km-wide transform valley, is caused by rapid serpentinization of upper mantle which has been exposed to hydrothermal circulation by fault-zone fracturing of an unusually thin crust. Transverse ridges have been thought atypical of fast-slipping transforms. One class of volcanic ridge more common at these sites is the overshot ridge, formed by prolongation of spreading-center rift zones obliquely across the transform. Overshot ridges are well developed at Heezen transform, especially at the eastern end where an eruptive rift zone extending 60 km from the southern tip of the East Pacific Rise has built a transform-parallel ridge that fills the eastern transform valley. Obliteration of fault-zone structure by ridges overshooting from the spreading center intersections means that the topography of the aseismic fracture zones is not just inherited from that of the active transform fault zone. The latter has several en echelon and overlapping fault traces, linked by short oblique spreading axes that generally form pull-apart basins rather than volcanic ridges. Interpretation of the origin and pattern of the fault zone's tectonic and volcanic relief requires refinement of the plate geography and history of this part of the Pacific-Antarctic boundary, using new Seabeam and magnetic traverses to supplement and adjust the existing geophysical data base.

The role of possible transverse faults in the development of the Sabzevar ophiolites, North-East Iran, with special reference to magma chamber tectonics

In the Sabzevar ophiolites, North-East Iran, structural studies have shown that accretion have rather occurred along an oblique spreading axis. A discontinuous ridge may have developed either at a ridge-transform intersection with a small offset or in a pull-apart basin linked to a wrench fault subparallel to the mountain belt. Special attention was paid to the origin of the setting of the layered cumulates which probably were formed in several disrupted magma chambers. The layering, of variable dip ahd strike, results from crystallization taking place either along inclined planes linked to fault activities or near temporary subhorizontal floor regulated by gravitational forces. Tectonics in accretion zones also control the shape and size of the chamber, the evolution of the more or less asymmetric channel, the setting of gabbroic dykes and flow-layered secant cumulates.

Asymmetric geophysical signatures in the Greenland-Norwegian and Southern Labrador Seas and the Eurasia Basin

A thorough examination of geophysical data from the Greenland-Norwegian Sea, Eurasia Basin and southern Labrador Sea shows significant asymmetry of several parameters (basement topography adjusted for sediment loading, free-air gravity anomaly, spreading half-rate and seismicity) with respect to crustal age: 1. (1) Average zero-age depth (0–57 m.y. B.P.), depth of highest rift mountain summits, and depth to magnetic basement (10–30 km from axis of Mohns and Knipovich ridges) is less on the North American plate flanks. The zero-age depth asymmetry is 400–500 m for the Eurasia Basin (0–57 m.y. B.P.) and for Mohns Ridge (57-22 m.y. B.P.), and 150–200 m for younger Mohns Ridge crust (22-0 m.y. B.P.) and for the extinct Aegir Ridge (57-27 m.y. B.P.). There is little or no asymmetry in the Labrador Sea except near the extinct rift valley, where the east flank is 150–300 m shallower. Magnetic depth-to-source computations provide an independent confirmation of basement asymmetry: The belts 10–30 km from the axis of Mohns and Knipovich ridges are 100–150 m shallower on the west flank of these ridges. The shallower ridge flank is topographically rougher, so that average rift mountain summits are 300 m shallower on the west flanks of the Mohns-Knipovich ridges, a larger asymmetry than for average zero-age depth. The amount of topographic asymmetry is greatest near the Mohns-Knipovich bend. Asymmetry appears to be greatest for ridges oriented normal to the spreading direction, and less for oblique spreading. 2. (2) Free-air gravity anomaly asymmetries of +5 to +20 mGal ( + sign indicates west flank is more positive) are associated with topographic asymmetry at least within 10–15 m.y. of the axis of Mohns and Knipovich ridges. Gravity is reduced on the older flanks west of the extinct Mid-Labrador Ridge and east of Mohns Ridge; asymmetric crustal layer thicknesses or densities provide one possible explanation, although deep-seated sources (e.g., mantle convection), unrelated to the crust, cannot be excluded. 3. (3) Spreading half-rate was about 5–15% lower on the North American plate flanks of Mohns Ridge (57-35 m.y.) and in the Eurasia Basin (0–57 m.y.); thus the fast-spreading flank tends to produce deeper, smoother crust. However, topographic asymmetry cannot relate only to spreading-rate asymmetry, since for the young Mohns Ridge crust ( <9 m.y. B.P.) faster spreading and higher topography are both associated with the west flank. 4. (4) Mid-plate seismicity is higher on the Eurasia (eastern) flank of Mohns and Knipovich ridge, but this effect may be unrelated to the other three. The fluid-dynamical model of Stein et al. correctly explains the sense of spreading-rate asymmetry (the North American plate, moving faster over mantle, is growing more slowly). However, the other asymmetries and their causal relationships remain theoretically unexplained.

Eocene to recent development of the South-west Indian Ridge, a consequence of the evolution of the Indian Ocean Triple Junction

Summary. The South-west Indian Ridge, the contact between the African and Antarctic plates, lies between the Bouvet Triple Junction in the South Atlantic and the Indian Ocean Triple Junction about 2100 km east of Madagascar. From the vicinity of Prince Edward Island at 40° E it trends north-easterly and it is segmented by a suite of deep north—south gashes terminating on the north-east with two spectacular meridional fracture zones, the ‘Atlantis II’ and the ‘Melville’, at 57° 30′E and 60° 30′E respectively. From there northeast to the Indian Ocean Triple Junction at 25° 30′S, 70° 00′E the ridge trends N75° E; it is characterized by a triangle of rough topography with the triple junction at the eastern apex. From all available data an instantaneous pole of relative motion for Africa/Antarctica was computed; it lies at 8.4° N, 42.4° W, with a rate of 0.15° Myr−1. Since the marked change in the direction and rate of spreading in the Madagascar, Crozet and Central Indian Basins that occurred in the Eocene (44 Ma, Anomaly 19), the poles of relative motion for the African, Indian and Antarctic plates have changed very little. We fixed the Africa/Antarctica and Africa/India poles and computed that for India/Antarctica. We justified this pole by comparisons of predicted isochrons with observed magnetic lineations and determined the tectonic history of the triple junction. Since the Eocene (44 Ma, Anomaly 19), this junction has moved as rapidly east-wards with respect to Africa as Antarctica has moved south. The resultant geometry and slow spreading account for the triangle of rough topography produced by the South-west Indian Ridge east of the Melville Fracture Zone. The triple junction evolved as a stable ridge-ridge-ridge type with the South-east Indian Ridge remaining approximately constant in length. It was not resolved whether this constancy in length is maintained by frequent ridge jumps or by oblique spreading on the South-west and Central Indian Ridges near the triple junction.

Tectonic pattern of the Azores spreading centre and triple junction

The major tectonic elements of the Azores triple junction have been mapped using long-range side-scan sonar. The data enable the Mid-Atlantic Ridge axis to be located with a precision of a few kilometres. Major faults and other tectonic and volcanic elements of the ridge maintain their regional trend of 010° to 020° past the triple junction area. There is no oblique spreading, and only minor transform offsets of the Mid-Atlantic Ridge occur here. The main effect of the triple junction or Azores hot spot is to diminish the amplitude of the median valley to 200 m or less. There is no axial high: a topographic high seen on several profiles is located to the east of the Mid-Atlantic Ridge spreading axis and does not appear to have any fundamental significance. The third arm of the triple junction includes the Azores srreading centre which appears to have developed as a series of en echelon rifted basins (the Terceira Rift) extending from Formigas Trough at 36.8°N, 24.5°W to a point near 39.3°N, 28.8°W. There are indications that recent activity in the spreading centre may be concentrated in a series of ridges which flank the older rifted basins. Until recently the northwest end of the Terceira Rift was connected to the Mid-Atlantic Ridge axis either directly at an RRR junction, or via a transform fault. The triple junction has probably moved south during the last 6 Ma to a positin on the Mid-Atlantic Ridge near 38.7°N. Initiation of the Azores spreading centre may have occurred during the 36 Ma B.P. rearrangement of poles, with an RFF triple junction north from the East Azores fracture zone to the North Azores fracture zone and transferring a wedge of European plate to the African plate. The tectonic elements revealed by this study are in good agreement with inferred earthquake mechanisms and with the RM2 plate tectonic model of Minster and Jordan, but east-west motion between North America and Africa does not seem to be compatible with the other motions at the triple junction unless it is of very recent (2>3 Ma) origin.

New geophysical evidence for sea-floor spreading in central Baffin Bay

Geophysical data collected during a detailed survey in Baffin Bay have shown that lineated magnetic anomalies trending north-northwest occupy the deep central region. These anomalies exhibit maximum amplitudes of about 300 nT and can be modelled by a 1-km thick magnetic source layer divided into blocks of normal and reversed polarity. The magnetizations required are comparable with those of oceanic basalts. A striking feature of the gravity field is a 20 mGal gravity low, about 20 km wide, which runs through the centre of the bay with approximately the same trend as the magnetic lineations. The gravity low is associated with a change in crustal structure measured from seismic refraction data and sometimes with a deepening of the sediment-basement interface, reminiscent of a median valley. These results suggest that the magnetic anomalies were produced by sea-floor spreading and that the gravity low marks an extinct spreading centre in Baffin Bay. Comparisons of the magnetic anomaly profiles with a model profile computed for magnetic anomalies 13–24 (38 to 60 Ma), show good correlation between the observed and computed anomalies in the time period represented by anomalies 13–21, with slow spreading rates of about 0.3–0.4 cm yr−1 perpendicular to the spreading axis. These results are in reasonable agreement with magnetic anomaly identifications and spreading rates deduced from geophysical data in the Labrador Sea. The direction of plate motion in Baffin Bay is not well defined from the data, but the Labrador Sea data require plate motions at a highly oblique angle to the spreading centre in the bay. Peculiarities of the postulated spreading centre, including the change in crustal structure beneath the gravity low along its strike from south to north, and the decrease in coherence and amplitude of the magnetic anomalies immediately north of the survey area, may be the result of these very low spreading rates, oblique spreading and changes in spreading direction, or the proximity of this area to the junction with a possible major transform fault through the Nares Strait.

The oblique intersection of the Mid-Atlantic Ridge with Charlie-Gibbs transform fault

The junction angle between the western Charlie-Gibbs transform fault and the spreading axis of the Mid-Atlantic Ridge diverges by 40° from the orthogonal intersection assumed in many studies of plate boundaries. This has been established by a surface-ship reconnaissance and by mapping fault trends in a transponder-navigated deep-tow survey of the fracture valley 25 km from the intersection. One set of normal faults trends 325–330°, parallel to the obliquely spreading ridge axis, and another set trends 275°, parellel to the direction of relative plate motion. Although the near-bottom survey was in the theoretically inactive part of the fracture zone, beyond the transform fault section, there is evidence for recent motion on faults that cut the thick sediment fill of the fracture valley. Oblique spreading of a ridge axis near a transform fault may result from distortion of the regional stress field by a strike-slip couple. Tension parallel to the long axis of the strike-slip strain ellipse, which is responsible for oblique normal faulting in transform valleys, causes oblique dike injection and oblique faulting in the axial rift valley. These effects extend further from transfrom fault intersections on slow-spreading ridges than on fast-spreading rises.