Ridge-transform Intersection Research Articles

Deep‐tow studies of the Vema Fracture Zone: 1. Tectonics of a major slow slipping transform fault and its intersection with the Mid‐Atlantic Ridge

The Vema transform fault, which slips at a rate of 24 mm/yr, displaces the Mid‐Atlantic Ridge (MAR) 320 km in a left‐lateral sense. High‐resolution deep‐tow studies of the Vema ridge‐transform intersection (RTI) and the eastern 130 km of the active transform fault reveal a complex pattern of dip‐slip and strike‐slip faults which evolve in time and space. At the intersection, both the neovolcanic zone and the west wall of the MAR rift valley curve counterclockwise toward the transform fault along trends approximately 30° oblique to the regional north‐south trend of the spreading axis. The curving of extensional structures in the rift valley, such as normal faults and the axial zone of dike injection, appears to be related to transmission of transform related shear stresses into the spreading center domain. Intermittent locking of the American and African lithospheric plates across the RTI causes shear stresses to penetrate up to 4 km into the MAR axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block‐faulted west wall of the rift valley where the lithosphere is thicker. The degree of shear coupling across the RTI may vary with time due to changes in the thickness of the lithosphere along the axis (0–10 km), the strength of a “mantle weld” at depth, and the presence or absence of an axial magma chamber, so that extensional structures at the RTI may be either spreading center parallel when coupling is weak or oblique when coupling is strong. Oblique extension across the RTI in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin. The easternmost 20 km of the active transform fault zone near the RTI displays a braided network of three to nine tectonically active grabens and V‐shaped furrows in a zone 2–4 km wide, interpreted to consist of interwoven Riedel shears, P shears, and oblique normal faults. Clay cake deformation experiments and deep‐tow observations suggest that P shears and R shears, which are 10°–20° oblique to the transform slip direction, develop during the initial stages of transform faulting near the RTI as the newly accreted lithosphere accelerates to full plate velocity. Some of the R shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the RTI. Subsequent to this merging of the two fault types, some of the R shears develop a significant component of dip slip, while other R shears merge with P shears creating a complex anastomosing fault pattern up to 4 km wide. A continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the RTI. Twenty kilometers west of the RTI the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays. This narrow groove cuts through thinly sedimented basalt 20–40 km west of the RTI and continues as a narrow furrow (less than 100 m wide) through up to 1.5 km of layered turbidite fill most of the way to the western RTI. Such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old, thick, and rigid and has completed its acceleration to full plate velocity. The transform fault zone is closely associated with a partially buried median ridge and widens to 1–2 km where it transects exposed portions of the ridge. The transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro, metagabbro, and basalt.

Structural and volcanic expression of a fast slipping Ridge‐Transform‐Ridge‐Plate Boundary: Sea MARC I and photographic surveys at the Clipperton Transform Fault

A high‐resolution near‐bottom survey has been conducted of the Clipperton transform fault and adjoining segments of the East Pacific Rise (EPR), using the Sea MARC I side‐looking sonar system and the Lamont‐Doherty Geological Observatory Olympus‐based camera system. The transform fault zone (TFZ) is a narrow, well‐defined belt of transform‐parallel lineaments, which varies along strike from a single, sharp‐edged notch to a complex band of subparallel lineaments up to 1 km wide. The TFZ is set within a 5‐km‐wide band of unusually fine‐grained side scan texture, which could indicate nonbasaltic seafloor and/or pervasively sheared and mass‐wasted basaltic crust The fine‐grained swath is surrounded by constructional volcanic terrain with no hint of strike‐slip motion; this observation puts an upper limit of 5 km on the extent of lateral migration of the TFZ in the last 1.5 m.y. Both ridge transform intersections (RTIs) are dominated by bathymetric highs located on the old plate opposite the spreading center. A mantling of fresh‐looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism; thermal expansion may also play a part. The EPR south of Clipperton has recently experienced extrusion of high effusion rate basalts, burial of faults and fissures by lava flows, and development of vigorous hydrothermal circulation. On the EPR north of Clipperton, the axial zone of faults and fissures tapers toward the transform fault; this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault.

Are oceanic fracture zones locked and strong or weak?: New evidence for volcanic activity and weakness

Traditionally, oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity. This notion has recently been questioned on the basis of evidence showing that initial depth offsets (scarp height) across some Pacific fracture zones are preserved even after ∼100 m.y. of differential subsidence. This evidence suggests that some parts of oceanic fracture zones are locked shortly) after they drift away from ridge-transform intersections. Because the fracture zones seem to be strong welds, differential cooling stresses cause flexure in the lithosphere on both sides of the fracture. In this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones (locked and strong vs. slipped and weak) and present new evidence indicating that parts of some fracture zones are volcanically active. Volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits. This and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior. More data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones.

Anomalous seismic crustal structure of oceanic fracture zones

Summary. The seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the North Atlantic. The crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3. Anomalous crust is present in both large- and small-offset fracture zones. Since they are among the most common tectonic features in the ocean basins, and are particularly closely spaced on slow-spreading ridges, fracture zones represent a major source of seismic crustal heterogeneity. We interpret the anomalous crust as a thin, intensely fractured, faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that, in places, are extensively serpentinized. The unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes; firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section; and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point, so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness, culminating in the very thin crust of the fracture zones at either end of the ridge segment.

A tectonic model for ridge-transform-ridge plate boundaries: Implications for the structure of oceanic lithosphere

The first-order geologic and morphologic relationships at, along and proximal to ridge-transform-ridge plate boundaries are used to construct an empirical and speculative tectonic model. The distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere (slowly-slipping ridge-transfrom-ridge plate boundaries) to extremely high rates of strain associated with very thin edges of lithosphere (fast-slipping ridge-transform-ridge plate boundaries). The geometry of a ridge-transform intersection necessitates the juxtaposition of a relatively cold, thick edge of lithosphere against the truncated end of an accreting plate boundary. The cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time. The shallow level manifestation of this cold edge effect is a thinner oceanic crust. At depth, the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere. The ridge-transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis, at some distance from the ridge-transform intersection, to an oblique angle near the boundary. The brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique, dip-slip faults. The model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge. At very slow rates of accretion ( < 4 cm/yr full rate) large offset transforms (> 100 km) juxtapose thick (30–50 km) edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere. In this environment, a relatively strong mantle weld will form at the ridge-transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike-slip tectonism to a relatively narrow and temporally stable principal transform displacement zone. In contrast, at very fast rates of accretion large offset transforms ( >100 km) place relatively thin edges of lithosphere ( <15 km) against axes of accretion and consequently the changes on accretionary processes will not be as profound: the crust will not thin dramatically; the mantle weld will be relatively weak; and the upper mantle will not be as heterogeneous. In addition, the thin edges of lithosphere and the higher strain rates associated with fast-slipping transforms create an environment that makes it relatively easy for complex and temporally unstable geometries to develop within the principal transform displacement zone.

Multichannel seismic evidence for anomalously thin crust at Blake Spur fracture zone

A two-ship, wide-aperture, multichannel seismic reflection profile obtained across a small western North Atlantic fracture zone provides convincing new evidence for unusually thin crust at oceanic fracture zones. Reflections from the crust-mantle boundary, typically observed at about 2 s below the basement reflection, rise to only 0.8 s beneath the fracture zone trough. This corresponds to crustal thicknesses of perhaps as little as 2 to 2.4 km in the fracture zone, less than half the thickness of normal oceanic crust. This is the first time that Moho reflections have been observed across an oceanic fracture zone and the first convincing evidence that anomalously thin crust may be associated even with fracture zones of relatively small offset. The existence of thin crust at both large- and small-offset fracture zones suggests that the magnitude of the thermal edge effect at ridge-transform intersections is not the only factor responsible for the accretion of unusually thin crust in fracture zones.

The geology of the Oceanographer Transform: The ridge-transform intersection

Seven dives in the submersible ALVIN and four deep-towed (ANGUS) camera lowerings have been made at the eastern ridge-transform intersection of the Oceanographer Transform with the axis of the Mid-Atlantic Ridge. These data constrain our understanding of the processes that create and shape the distinctive morphology that is characteristic of slowly-slipping ridge-transform-ridge plate boundaries. Although the geological relationships observed in the rift valley floor in the study area are similar to those reported for the FAMOUS area, we observe a distinct change in the character of the rift valley floor with increasing proximity to the transform. Over a distance of approximately ten kilometers the volcanic constructional terrain becomes increasingly more disrupted by faulting and degraded by mass wasting. Moreover, proximal to the transform boundary, faults with orientations oblique to the trend of the rift valley are recognized. The morphology of the eastern rift valley wall is characterized by inward-facing scarps that are ridge-axis parallel, but the western rift valley wall, adjacent to the active transform zone, is characterized by a complex fault pattern defined by faults exhibiting a wide range of orientations. However, even for transform parallel faults no evidence for strike-slip displacement is observed throughout the study area and evidence for normal (dip-slip) displacement is ubiquitous. Basalts, semi-consolidated sediments (chalks, debris slide deposits) and serpentinized ultramafic rocks are recovered from localities within or proximal to the rift valley. The axis of accretion-principal transform displacement zone intersection is not clearly established, but appears to be located along the E-W trending, southern flank of the deep nodal basin that defines the intersection of the transform valley with the rift floor.

Petrologic consequences of rift propagation on oceanic spreading ridges

The production of anomalously differentiated lava compositions at several mid-ocean spreading centers can be attributed to magmatic processes associated with propagating rifts. The degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers. Low supply rates and moderate cooling rates allow advanced degrees of closed-system fractionation to occur. High supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma. Propagating rift tips are a special class of ridge-transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge. This balance is affected by the spreading rate, the propagation rate of the rift, the length of the bounding transform and proximity to hotspots. Maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance. Rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion. Geochemical anomalies may persist, though changing in degree and extent, for several million years on ridge segments that stop propagating. The concept of “magnetic telechemistry” is generally supported by our study, but in the vicinity of hotspots, magnetic anomaly amplitude may be controlled more by bathymetric and/or thickened magnetic layer effects than by geochemistry.

Tectonics of ridge-transform intersections at the Kane fracture zone

The Kane Transform offsets spreading-center segments of the Mid-Atlantic Ridge by about 150 km at 24° N latitude. In terms of its first-order morphological, geological, and geophysical characteristics it appears to be typical of long-offset (>100 km), slow-slipping (2 cm yr-1) ridge-ridge transform faults. High-resolution geological observations were made from deep-towed ANGUS photographs and the manned submersible ALVIN at the ridge-transform intersections and indicate similar relationships in these two regions. These data indicate that over a distance of about 20 km as the spreading axes approach the fracture zone, the two flanks of each ridge axis behave in very different ways. Along the flanks that intersect the active transform zone the rift valley floor deepens and the surface expression of volcanism becomes increasingly narrow and eventually absent at the intersection where only a sediment-covered ‘nodal basin’ exists. The adjacent median valley walls have structural trends that are oblique to both the ridge and the transform and have as much as 4 km of relief. These are tectonically active regions that have only a thin (<200 m), highly fractured, and discontinuous carapace of volcanic rocks overlying a variably deformed and metamorphosed assemblage of gabbroic rocks. Overprinting relationships reveal a complex history of crustal extension and rapid vertical uplift. In contrast, the opposing flanks of the ridge axes, that intersect the non-transform zones appear to be similar in many respects to those examined elsewhere along slow-spreading ridges. In general, a near-axial horst and graben terrain floored by relatively young volcanics passes laterally into median valley walls with a simple block-faulted character where only volcanic rocks have been found. Along strike toward the fracture zone, the youngest volcanics form linear constructional volcanic ridges that transect the entire width of the fracture zone valley. These volcanics are continuous with the older-looking, slightly faulted volcanic terrain that floors the non-transform fracture zone valleys. These observations document the asymmetric nature of seafloor spreading near ridge-transform intersections. An important implication is that the crust and lithosphere across different portions of the fracture zone will have different geological characteristics. Across the active transform zone two lithosphere plate edges formed at ridge-transform corners are faulted against one another. In the non-transform zones a relatively younger section of lithosphere that formed at a ridge-non-transform corner is welded to an older, deformed section that initially formed at a ridge-transform corner.

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.

Ultramafic intrusions in the Lewis Hills Massif, Bay of Islands Ophiolite Complex, Newfoundland: Implications for igneous processes at oceanic fracture zones

The Coastal Complex of western Newfoundland has been interpreted as oceanic crust and upper mantle which formed in an oceanic fracture zone. Within the Coastal Complex, a suite of peridotite bodies and associated basaltic dikes occur. The least-deformed peridotites in these bodies have cumulate textures and have apparently been separated from any gabbroic cumulates which formed in association with them. Intrusive relationships with surrounding metamorphic and cumulate rocks are well preserved and suggest that the peridotite bodies are crystal mush intrusions derived from mobilized ultramafic cumulates which were originally deposited within relatively small isolated magma chambers. These processes are believed to be characteristic of ridge-transform intersections or “leaky” transform regions. Two petrologically distinct types of peridotite have been identified. One type has lithologies and mineral chemistry similar to those of many oceanic plutonic rocks, and these lithologies may be interpreted as differentiates produced in the evolution of typical Mid-Ocean Ridge Basalts (MORBs). The second type of peridotite, although lithologically similar to the first, has relatively high CaO/Al 2 O 3 ratios and relatively low TiO 2 and Na 2 O. This second type of peridotite cannot be simply related to the genesis of typical MORBs. Chilled margins and fine-grained dikes along the margins of some of these peridotite bodies are high-magnesium basalts.

Evolution of abyssal lavas along propagating segments of the Galapagos spreading center

The unusual petrological diversity of abyssal lavas erupted along some segments of the Galapagos spreading center is a direct consequence of the propagation (elongation) of these segments into older oceanic crust. With increasing distance behind propagating rift tips, relatively unfractionated MORB erupted close to the tips are joined first by FeTi basalts (bimodal assemblage) and then by a wide range of basaltic and siliceous lavas. Further behind propagating rift tips, this broad range diminishes again, approaching the narrow compositional range of adjacent normal ridge segments. These compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre-existing plate. We envisage this evolution as proceeding from small, isolated, ephemeral magma chambers through increasing numbers of larger, increasingly interconnected chambers to the steady-state buffered system of a normal ridge. Throughout this evolution, magma supply rates gradually increase and cooling rates of crustal magma bodies decrease. High degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved. At other propagating and non-propagating ridge-transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas. These effects may be evaluated provided a number of tectonic variables including transform length, spreading and propagation rates are taken into account.

A simple thermal-mechanical model for mid-ocean ridge topographic variation

Summary. A simple dynamic model, based on the geometry of mantle divergence and thermal parameters controlling equilibrium size of the axial magma chamber, explains the variation in topography along mid-ocean ridges. Among morphological characters accounted for are: (1) the change from axial-valley to axial-high type ridge crests with increasing spreading rate, (2) the localized occurrence of deeps at ridge-transform intersections, and (3) the correlation of average transform spacing with spreading rate. The model also yields an explanation for anomalous ridge topography associated with oceanic hot spots. Incorporation of smaller-scale bathymetric and ophiolite data into this scheme permits construction of a comprehensive model of ocean crust accretion.

Membrane stresses near mid-ocean ridge-transform intersections

An anisotropic two-dimensional finite element model was used to determine constraints on relative material properties and the horizontal stress field of mid-ocean ridges near transform faults given the regional stress field. A ratio of 10 3 in the viscous equivalent of Young's modulus between the ridge crest and the oceanic plate is indicated. The horizontal stress field suggests that the concentration of near ridge-transform intersection seamount chains and the occurrence of ridge jumps are a result of regional stresses. Since dike intrusions are perpendicular to local deviatoric tension, ridges are shown to become oblique such that the connecting transform fault is shortened. Excessively oblique ridges, however, tend to produce intrusions that result in the cancellation of that obliqueness. Thermal variations related to hot spots are unlikely to cause asymmetric spreading and short ridge segments are unstable to preemption by longer adjoining segments.