Ridge-transform Intersection Research Articles

Gabbroic intrusions and magmatic metasomatism in harzburgites from the Garrett transform fault: implications for the nature of the mantle–crust transition at fast-spreading ridges

Mafic and ultramafic rocks sampled in the Garrett transform fault at 13°28′S on the East Pacific Rise (EPR) provide insight on magmatic processes occurring under a fast-spreading ridge system. Serpentinized harzburgite from Garrett have modal, mineral and bulk chemical compositions consistent with being mantle residue of a high degree of partial melting. Along with other EPR localities (Terevaka transform fault and Hess Deep), these harzburgites are among the most residual and depleted in magmatophile elements of the entire mid-ocean ridge system. Geothermometric calculations using olivine-spinel pairs indicate a mean temperature of 759 ± 25 °C for Garrett residual harzburgite similar to the average of 755 °C for tectonite peridotites from slow-spreading ridges. Results of this study show that mid-ocean ridge peridotites are subject to both fractional melting and metasomatic processes. Evidence for mantle metasomatism is ubiquitous in harzburgite and is likely widespread in the entire Garrett peridotite massif. Magma-harzburgite interactions are very well preserved as pyroxenite lenses, plagioclase dunite pockets or dunitic wall rock to intrusive gabbros. Abundant gabbroic rocks are found as intrusive pockets and dikes in harzburgite and have been injected in the following sequence: olivine-gabbro, gabbro, gabbronorite, and ferrogabbro. The wide variety of magmas that crystallized into gabbros contrast sharply with present-day intratransform basalts, which have a highly primitive composition. Ferrogabbro dikes have been intruded at the ridge-transform intersection and as they represent the last event of a succession of gabbros intrusive into the peridotite, they likely constrain the origin of the entire peridotite massif to the same location. In peridotite massifs from Pacific transform faults (Garrett and Terevaka), primitive to fractionated basaltic magmas have flowed and crystallized variable amounts of dunite (±plagioclase) and minor pyroxenite, followed by a succession of cumulate gabbroic dikes which have extensively intruded and modified the host harzburgitic rocks. The lithosphere and style of magmatic activity within a fast-slipping transform fault (outcrops of ultramafic massif, discontinuous gabbro pockets intrusive in peridotite, magnesian and phyric basalts) are more analogous to slow-spreading Mid-Atlantic Ridge type than the East Pacific Rise.

Origin of extensional core complexes: Evidence from the Mid‐Atlantic Ridge at Atlantis Fracture Zone

The contrast in geologic structure observed on opposing flanks of the Mid‐Atlantic Ridge, where it is offset by the Atlantis transform fault, illustrates how significant differences in crustal structure can result from tectonic processes that operate near the ends of slow spreading segments. New geophysical and geological data provide information on the nature of large massifs that punctuate the strips of crust formed at the inside corner of ridge‐transform intersections (RTI), as well as of the low‐relief volcanic morphology that typifies the outside corners. The geological relations mapped at the inside corner of the eastern Atlantis RTI are strikingly similar to those seen in the Basin and Range where metamorphic core complexes are unroofed through asymmetric detachment faulting. The core of the eastern RTI massif exposes deep‐seated rocks beneath a shallow‐dipping, corrugated surface which is interpreted as a fault surface. On the median valley side of the massif, this seafloor detachment is overlain by upper crustal blocks bounded by steeper fault scarps. The western side of the 15‐km‐wide massif is characterized by en echelon faults which face away from the ridge axis. Similar features are mapped at two fossil massifs that are interpreted to have formed at the inside comers of each RTI and to have rafted off‐axis as plate spreading proceeded. Analysis of new and preexisting shipboard gravity data indicates that high‐density material is not continuously emplaced at the inside corner. Rather, peaks in the gravity anomaly map are patchily distributed along the transform valley walls. The gravity highs associated with the three massifs (oceanic core complexes) in this area are not centered with respect to their morphology but are located toward their spreading axis and transform sides. Gravity modeling suggests that the western boundary of a high‐density wedge at the eastern RTI massif is steeply dipping, whereas the eastern boundary may dip about 15° toward the median valley. In contrast to the inside corners of the RTIs in our study area, the outside corner seafloor is characterized by volcanic constructions similar to those found on either side of the spreading axis at the center of the segments and inferred to be typical basaltic upper crust. Kinematic analysis at the Mid‐Atlantic Ridge‐Atlantis Transform RTI suggests that the formation of seafloor detachments may occur when the rate of extension not accommodated by magmatic input exceeds about 4 mm/yr. Isolated volcanic ridges that extend into the fracture zone domain, curving as they approach the fault trace, mark times of abundant magma supply at the segment ends. The apparent interplay between magmatic and tectonic strain accommodation at a mid‐ocean ridge, as well as the overall structure of oceanic core complexes, may provide important kinematic constraints on core complex formation and the development of shallow‐dipping detachment faults.

Computer‐aided interpretation of side‐looking sonar images from the eastern intersection of the Mid‐Atlantic Ridge with the Kane Transform

Approximately 600 km2 of Deep‐Tow side‐looking sonar data were collected at the eastern intersection of the Mid‐Atlantic Ridge with the Kane Transform. The merged digital image mosaic provides a synoptic view of the oceanic crust exposed across major escarpments at this ridge‐transform intersection (RTI). We characterized this large acoustic data set using textural attributes extracted from the backscatter image by means of a gray level co occurrence matrix method and Fourier fractal analysis. False‐color texture attribute maps created from these methods aid in the interpretation and discrimination of surficial deposits and bedrock units on the seafloor. Principal components analysis performed on the textural attributes reduced the dimensionality of the textural feature vector and optimized imago texture discrimination. The textural attributes were calibrated to ground truth geologic data and extrapolated to regions with no ground data. The resultant texture and classification maps are consistent with the results of submersible studies but reveal more details of the seafloor geology than the conventional visual interpretation of the backscatter image and ground geologic observations. The inferred distribution of gabbros, basalts, surficial sediments, and rubble deposits shown in the classification map provides new constraints on seafloor geology at the eastern Kane RTI. Over 60% of the southern transform valley wall is covered by pelagic sediments and talus or rubble deposits, with intermittent exposures of coarse‐grained gabbroic rocks and basaltic volcanic rocks. The western rift valley wall is classified as mainly massive outcrops of gabbroic rocks. The contact between the gabbroic outcrops on the western rift valley wall and the pillow basalt terrane on the rift valley floor can be traced for several kilometers along the base of the median valley wall. This new perspective supports the interpretation of the “inside‐corner massif” as an “oceanic core complex” which is being degraded by mass wasting along tectonically active escarpments of the bounding median valley and transform valley walls.

A submersible study of the western intersection of the Mid-Atlantic ridge and Kane fracture zone (WMARK)

In 1994, a joint Japanese-American dive program utilizing the worlds deepest diving active research submersible (SHINKAI 6500) was carried out at the western ridge-transform intersection (RTI) of the Mid-Atlantic Ridge and Kane transform in the central North Atlantic Ocean. A total of 15 dives were completed along with surface-ship geophysical mapping of bathymetry, magnetic and gravity fields. Dives at the RTI traced the neovolcanic zone up to, and for a short distance (2.5 km) along, the Kane transform. At the RTI, the active trace of the transform is marked by a narrow valley (<50 m wide) that separates the recent lavas of the neovolcanic zone from the south wall of the transform. The south wall of the transform at the western RTI consists of a diabase section near its base between 5000 and 4600 m depth overlain by basaltic lavas, with no evidence of gabbro or deeper crustal rocks. The south wall is undergoing normal faulting with considerable strike-slip component. The lavas of the neovolcanic zone at the RTI are highly magnetized (17 A m−1) compared to the lavas of the south wall (4 A m−1), consistent with their age difference. The trace of the active transform changes eastwards into a prominent median ridge, which is composed of heavily sedimented and highly serpentinized peridotites. Submersible observations made from SHINKAI find that the western RTI of the Kane transform has a very different seafloor morphology and lithology compared to the eastern RTI. Large rounded massifs exposing lower crustal rocks are found on the inside corner of the eastern RTI whereas volcanic ridge and valley terrain with hooked ridges are found on the outside corner of the eastern RTI. The western RTI is much less asymmetric with both inside and outside corner crust showing a preponderance of volcanic terrain. The dominance of low-angle detachment faulting at the eastern RTI has resulted in a seafloor morphology and architecture that is diagnostic of the process whereas crust formed at the WMARK RTI must clearly be operating under a different set of conditions that suppresses the initiation of such faulting.

Transform tectonics and thermal rejuvenation on the Côte d’Ivoire-Ghana margin, west Africa

Formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break-up, including transpressional crustal thickening, and thermal rejuvenation and igneous underplating during passage of a spreading ridge. ODP drill holes on the Côte d’Ivoire-Ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin. Apatite fission-track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra-continental wrench zone that predated margin formation. Rapid cooling of &gt;120°C at 120–115 Ma corresponds to erosion of 3.5–5 km along the conjugate Brazilian margin, reflecting c. 1 km of tectonically driven uplift, subaerial erosion, and isostatic uplift due to unloading. Following rift initiation at 120 Ma (Aptian), an oceanic spreading axis passed adjacent to this part of the margin at 90 Ma (Cenomanian). Maximum uplift during the ridge-transform intersection was 390 m, considerably less than the 2000+ m predicted by heat conduction models in local isostatic equilibrium. The modern ridge is partially the product of thicker crust (22 km) underlying the ridge than the adjacent Deep Ivorian Basin (19 km), and partially related to flexural unloading of the transform ridge between the end of intra-continental wrenching and ridge-transform intersection. Flexural coupling between the continental and oceanic plates since ridge-transform intersection has caused a progressive depression of the offshore margin, estimated at about 650 m in the study area.

Velocity structure from forward modeling of the eastern ridge‐transform intersection area of the Clipperton Fracture Zone, East Pacific Rise

In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge‐transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two‐dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low‐velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to “fill in” the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma‐starved, displays the crustal thickness and apparent signal attenuation characteristic of a plentiful, but perhaps episodic, magma supply.

Variation in lithospheric stress along ridge‐transform plate boundaries

Three‐dimensional numerical models of asthenospheric flow and deformation in the oceanic lithosphere predict variability in the stress field that reflects the geometry of the ridge‐transform boundary. A series of 3‐D Boundary Element calculations shows how spreading rate, transform offset and segment length each influence the flow and stress fields that develop during plate‐driven asthenospheric flow beneath a ridge‐transform boundary. The predicted patterns of stress‐supported seafloor relief generally follow those observed: median valleys are predicted at slow‐spreading ridges vs no rift valley or, in some cases, a small axial high at fast spreading ridges; nodal deeps occur at ridge‐transform intersections for offsets greater than 25 km; longer segments have more along‐axis deepening than short segments which do not shoal much in their center. The predicted amplitude of the stress supported topography is up to 20% (across axis) and 40% (along axis) of that observed for an assumed asthenospheric viscosity of 5×1019 Pa s and a weak lithosphere (local compensation).

Tectonic reconstruction of the Clipperton and Siqueiros Fracture Zones: Evidence and consequences of plate motion change for the last 3 Myr

Bathymetry, side‐looking sonar and magnetics data from the northern East Pacific Rise have been analyzed to determine the tectonic history of the Clipperton Fracture Zone (CFZ) and the Siqueiros Fracture Zone (SFZ) over the last 2–3 m.y. Results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at ∼2.5 Ma (1–2°), ∼1.5 Ma (1–2°), and ∼0.5 Ma (4–5°). Along the right‐stepping Clipperton Transform, the most recent change in spreading direction has resulted in fracture zone normal compression, which we propose has created a median ridge and transform‐parallel troughs along the active transform fault. Volcanic intersection highs located near the ridge‐transform intersections (RTI) of the CFZ are interpreted to be the result of fracture‐zone‐normal extension that has created pathways for magma emplacement into/onto the older lithosphère across from the RTI. Along the left‐stepping SFZ, we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges. Tectonic reconstructions indicate a single Euler pole is unable to describe the motion of the Pacific‐Cocos plate boundary between the CFZ and SFZ during periods of spreading direction change since ∼3 Ma. Transtensional transforms will adjust to a new spreading direction almost immediately, while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform.

Stratigraphic numerical modelling of a carbonate platform on the Romanche transverse ridge, equatorial Atlantic

The Romanche transverse ridge (equatorial Atlantic) is located in the northern flank of the fracture zone, opposite the eastern ridge-transform intersection (RTI). It constitutes a major positive topographic anomaly that reaches a height of over 4 km above the level predicted by the thermal subsidence curve. A series of E–W aligned peaks, located on the crest of the transverse ridge, were at sea level during early and middle Miocene times; they are presently capped by a ~300 m thick, shallow-water carbonate platform that grew on a subsiding oceanic crust basement surface flattened by erosion at sea level. These structures are now about 1 km below sea level. High resolution seismic reflection surveys and multibeam morphobathymetry as well as study of samples recovered from the carbonate platform allowed a reconstruction of the paleoenvironment and of the vertical movements of the peaks starting from the lower Miocene. Ages derived from microfossils suggest that the base of the carbonate platform dates from 17–25 m.y. ago and the sinking of the platform started between 18 and 13 m.y. ago. These data were included in a numerical simulation model that takes into account thermal and tectonic subsidence, growth potential of the carbonates, subaerial and submarine erosion rates and Mio-Pliocene absolute sea level fluctuations. The results outline the subsidence history of the Romanche transverse ridge and suggest post-Miocene subsidence faster than that predicted by the thermal cooling model.

Earthquake activity near Ascension Island, South Atlantic Ocean, as seen by a combined seismic/hydrophone array

Earthquake activity in the vicinity of Ascension Island is studied using data from a recently installed, very broadband seismographic station and a network of ocean hydrophones deployed around the island. The seismic station ASCN recorded 121 local earthquakes over a 188-day period. Distance from ASCN is computed for 41 of these events, most of which occurred at approximately 100 km—the distance to the Mid-Atlantic Ridge. A local magnitude scale is calibrated and applied to the same 41 events with mahnitudes ranging from 0.4 to 4.2. Six earthquakes were located using data from the hydrophones and ASCN. Two of the earthquakes occur on the Ascension Fracture Zone 35 km northwest of the island and the other four near the ridge-transform intersection. The locations have errors less than 10 km and frequently less than 3 km. There is no direct evidence for seismicity connected with geothermal activity on Ascension Island during the observation period.

Diffuse impact of the Mid‐Atlantic Ridge with the Romanche transform: an ultracold ridge‐transform intersection

The Romanche is a long offset (∼950 km), slow slip (∼1.7 cm/yr) transform; thus a hot ridge axis should meet a ∼50‐m.y.‐old, thick and cold lithosphere at the ridge‐transform intersection (RTI). A strong thermal/topographic “transform cold edge effect” is therefore predicted. A morphobathymetric, seismic reflection and petrologic study of the eastern Romanche RTI shows that as the Mid‐Atlantic Ridge approaches the transform, a well‐formed axial rift valley disappears about 80 km from the RTI and is substituted by short en echelon, poorly developed axial ridge segments; they too disappear about 30 km from the edge of the transform valley. The predicted gradual deepening of the ridge axis toward the transform was not observed. An active nodal deep and an “inside corner high” are also absent. These observations, and the distribution of earthquake epicenters, suggest a poorly developed, diffuse RTI. An inactive rift valley ∼80 km west of the present RTI suggests ridge jumping within the last ∼4 m.y. The present poorly developed RTI may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform. We infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle‐derived serpentinized peridotites outcrop ubiquitously on the seafloor starting ∼30 km from the edge of the transform valley. The unusually deep (∼4 km below sea level) axial ridge segments, the lack of crust, and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region. Absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column. Mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform. These results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold RTIs and that ultracold RTIs are magma starved, short lived, and unstable in time and space.

The Southeast Indian Ridge between 127° and 132°40′E: contrasts in segmentation characteristics and implications for crustal accretion

Abstract The Australian-Antarctic Discordance is an anomalously deep section of the Southeast Indian Ridge which overlies a colder than normal region of the upper mantle. The Southeast Indian Ridge exhibits large contrasts in its geophysical and geochemical characteristics across the eastern boundary of the Discordance. We present new geophysical data collected along the Southeast Indian Ridge between 127°00′E and 132°40′E which define the segmentation characteristics of this portion of the Indo-Australian-Antarctic plate boundary. The Southeast Indian Ridge within our survey area can be broken into three first-order segments bounded by one transform fault and two propagating rifts. The transform fault, located at the west end of the study area, forms the eastern boundary of the Australian-Antarctic Discordance. The morphology of the spreading axis is that of an axial high akin to that found along the East Pacific Rise. The non-transform discontinuities found along the axis of accretion are also similar to those encountered at fast-spreading centres. These characteristics are in marked contrast with those of the spreading axis within the Australian-Antarctic Discordance, which resembles a slow-spreading centre despite its intermediate spreading rate ( c. 74 mm a −1 ). We suggest that the geophysical and geochemical transitions that occur along the Southeast Indian Ridge near the eastern boundary of the Australian-Antarctic Discordance may be ascribed to varying mantle temperature and melt production rate along the spreading centre at constant spreading rate. The two propagating rifts, located near 127°45′E and 131°E, are actively migrating toward the Discordance down the regional, along-axis gradient in bathymetry at the rates of 43 and 46 mm a −1 , respectively. Propagating rifts play a dominant role in the reorganization of the spreading centre east of the Australian-Antarctic Discordance. Satellite gravity data reveal a number of V-shaped, west-pointing structures on the flanks of the Southeast Indian Ridge. These structures appear to have been spawned near the George V ridge-transform intersection at 139°E. Four of these features display a clear association with active propagating rifts. Some of the remaining features may be due to the migration of melting anomalies entrained by asthenospheric flow along the axis of the Southeast Indian Ridge.

Construction of the oceanic lithosphere by magmatic intrusions: Petrological evidence from plutonic rocks formed along the fast-spreading East Pacific Rise

We report textural and compositional characteristics of plutonic rocks found in four sites exposed 5000 m below sea level along the southern East Pacific Rise system. Studied outcrops of plutonic rocks are located along escarpments in two transform faults (Garrett and Terevaka) and in two deep regions exposed by the action of propagating rifts (Pito Deep and Hess Deep). The transform faults have a large variety of heterogeneous plutonic rocks with mineral compositional trends extending from relatively primitive values in the olivine-bearing gabbros to very fractionated values in the ferrogabbros. An abundance of ferrogabbros in these transform faults suggests small and ephemeral magma chambers in which liquid and crystal mush evolved by crystal fractionation in small pockets and intrusive sills. Conversely, Pito Deep and Hess Deep have mainly homogeneous cumulates without the fractionated end members; instead, mineral compositions range to highly primitive values. The ubiquity of homogeneous olivine-bearing gabbros at Pito Deep and Hess Deep suggests a more robust magma chamber with frequent injection and mixing, involving larger volumes of crystal-liquid mush, which buffers compositions. We infer that the lithosphere created near ridge-transform intersections of fast-spreading centers is controlled by a colder mantle regime and will be lithologically and compositionally different from lithosphere created near hotter segment centers and away from such discontinuities. Our observations show that magmatic intrusion is a fundamental process in the construction of the lower oceanic crust.

Morphology and crustal structure of a small transform fault along the Mid-Atlantic Ridge: The Atlantis Fracture Zone

The Atlantis Fracture Zone (30° N) is one of the smallest transform faults along the Mid-Atlantic Ridge with a spatial offset of 70 km and an age offset of ~ 6 Ma. The morphology of the Atlantis Fracture Zone is typical of that of slow-slipping transforms. The transform valley is 15–20 km wide and 2–4 km deep. The locus of strike-slip deformation is confined to a narrow band a few kilometers wide. Terrain created at the outside corners of the transform is characterized by ridges which curve toward the ridge-transform intersections and depressions which resemble nodal basins. Hooked ridges are not observed on the transform side of the ridge-transform intersections. Results of the three-dimensional inversion of the surface magnetic field over our survey area suggest that accretionary processes are sufficiently organized within 3–4 km of the transform fault to produce lineated magnetic anomalies. The magnetization solution further documents a 15-km, westward relocation of the axis of accretion immediately south of the transform about 0.25 Ma ago. The Atlantis Transform is associated with a band of high mantle Bouguer anomalies, suggesting the presence of high densities in the crust and/or mantle along the transform, or anomalously thin crust beneath the transform. Assuming that all the mantle Bouguer anomalies are due to crustal thickness variations, we calculate that the crust may be 2–3 km thinner than a reference 6-km thickness beneath the transform valley, and 2–3 km thicker beneath the mid-points of the spreading segments which bound the transform. Our results indicate that crustal thinning is not uniform along the strike of the fracture zone. Based on studies of the state of compensation of the transform, we conclude that the depth anomaly associated with the fracture zone valley is not compensated everywhere by thin crust. Instead, the regional relationship between bathymetry and gravity is best explained by compensation with an elastic plate with an effective thickness of ~ 4 km or greater. However, the remaining isostatic anomalies indicate that there are large variations away from this simple model which are likely due to variations in crustal thickness and density near the transform.

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.

The origin of bathymetric highs at ridge-transform intersections: A multi-disciplinary case study at the Clipperton Fracture Zone

Bathmetric highs on the old crust proximal to ridge-transform intersections (RTIs), termed “intersection highs”, are common but poorly understood features at offsets of fast to intermediate rate spreading centers. We have combined new reflection seismic, photographic, and geochemical data with previously published Seabeam, SeaMARC I, and SeaMARC II data to address the nature of the intersection highs at the Clipperton Fracture Zone. The Clipperton Intersection Highs are both topped by a carapace of young lavas at least 100 m thick. These lavas, which were erupted on the intersection highs, are chemically similar to their adjacent ridge segments and different from the surrounding older crust. At least some of the erupted magma traveled directly from the adjacent ridge at a shallow crustal level. Ridge-related magma covers and intrudes at least the upper 500 m of the transform tectonized crust at the RTI. We suspect that additional magma enters the intersection highs from directly below, without passing through the ridge. The young oceanic crust near the western Clipperton RTI is not thin by regional comparison. The 1.4 m.y. old crust near the eastern Clipperton RTI thickens approaching the transform offset. If the thermal effects of the proximal ridge were negligible, the eastern intersection high crust would appear to be in isostatic equilibrium. We believe that thermal effects are significant, and that the intersection high region stands anomalously shallow for its crustal thickness. This is attributable to increased temperature in the mantle below the ridge-proximal crust. Although ridge magma is injected into the proximal old crust, plate boundary reorganization is not taking place. Intersection high formation has been an ongoing process at both of the Clipperton RTIs for at least the past 1 m.y., during which time the plate boundary configuration has not changed appreciably. We envision a constant interplay between the intruding ridge magma and the disrupting transform fault motion. In addition, we envision a nearly constant input of magma from below the high, as an extension of the magma supply to the ridge from the mantle. Because the proximal ridge profoundly affects the juxtaposed crust at the RTI, sea floor fabric along the aseismic extensions of this fast-slipping transform fault is primarily a record of processes at work at the RTI rather than a record of transform tectonism.

Oceanic crust thickens approaching the Clipperton Fracture Zone

A multi-channel seismic reflection image shows the reflection Moho dipping toward the Clipperton Fracture Zone in crust 1.4 my old. This seismic line crosses the fracture zone at its eastern intersection with the East Pacific Rise. The seismic observations are made in travel time, not depth. To establish constraints on crustal structure despite the absence of direct velocity determinations in this region, the possible effects of temperature, tectonism, and anomalous lithospheric structure have been considered. Conductive, advective, and frictional heating of the old crust proximal to the ridge-transform intersection can explain <20% of the observed travel-time increase. Heating has a negligible effect on crustal seismic velocity beyond ~10 km from the ridge tip. The transform tectonized zone extends only 6 km from the ridge tip. Serpentinization is unlikely to have thickened the seafloor-to-reflection Moho section in this case. It is concluded that, contrary to conventional wisdom, the 1.4 my old Cocos Plate crust thickens approaching the eastern Clipperton Ridge-Transform Intersection. Increase in thickness must be at least 0.9 km between 22 and 3 km from the fracture zone.

Intratransform origins of mid-ocean ridge microplates

Research Article| November 01, 1994 Intratransform origins of mid-ocean ridge microplates Robert T. Bird; Robert T. Bird 1Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882 Search for other works by this author on: GSW Google Scholar David F. Naar David F. Naar 2Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 Search for other works by this author on: GSW Google Scholar Author and Article Information Robert T. Bird 1Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882 David F. Naar 2Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (11): 987–990. https://doi.org/10.1130/0091-7613(1994)022<0987:IOOMOR>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert T. Bird, David F. Naar; Intratransform origins of mid-ocean ridge microplates. Geology 1994;; 22 (11): 987–990. doi: https://doi.org/10.1130/0091-7613(1994)022<0987:IOOMOR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present evidence that the Easter and Juan Fernandez microplates originated by rift propagation from within transform fault zones at locations far from ridge-transform intersections. An intratransform spreading center may have served as the focal point for each new propagation. Both events appear coincident with a circum-Pacific plate reorganization dated at 5.6 Ma, which may have been related to a change in Pacific absolute plate motion and a counterclockwise rotation of Pacific-Nazca relative plate motion. This intratransform model may be applicable to the generation of other microplates and may thus serve as a general model for microplate origins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A view of the lower crustal component of hydrothermal systems at the Mid‐Atlantic Ridge

An extensive suite of hydrothermally altered gabbros was recovered by Alvin and dredging from the Mid‐Atlantic Ridge, in the vicinity of the eastern ridge‐transform intersection (RTI) of the Kane Fracture Zone (MARK), where tectonic extension has provided a window into the lower crustal component of hydrothermal cells. Four alteration types are distinguished on the basis of metamorphic assemblage, mineral composition, and deformation textures. A conceptual model is presented that places temperature ‐ fluid/rock ratio relationships, in conjunction with styles of deformation, into a spatial and temporal framework. Exsolution of late stage magmatic fluids in the vicinity of the magma‐hydrothermal interface at temperatures &gt;700°C marks the onset of fluid‐rock interaction in the MARK gabbros (Kelley et al., this issue), with subsequent alteration involving seawater‐derived fluids being strongly influenced by deformation mechanisms. In zones of ductile shear, hydration was initiated at temperatures between 550° and 700°C and moderate fluid/rock ratios. Elsewhere in the lower crust, the onset of seawater penetration occurred at temperatures of 400°–550°C and was facilitated by brittle fracturing. Early vein networks indicate very low initial fluid/rock ratios and are interpreted as being related to the solidification of plutons. Later fractures indicate higher water/rock ratios and may be related to the downward propagation of shallow fault systems. Throughout the crust, fluid‐rock interaction ceased at temperatures 180°–300°C. Subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the RTI massif produced moderate fluid/rock ratios within localized zones. Hydrothermal alteration in the crustal section exposed at MARK was initiated at lower temperatures, and water/rock interaction proceeded to lower temperatures than many other plutonic suites. These differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust.

Vema Fracture Zone (central Atlantic): Tectonic and magmatic evolution of the median ridge and the eastern ridge‐Transform intersection domain

The eastern Vema fracture zone intersection with the Mid‐Atlantic Ridge (MAR) axis was surveyed with the French submersible Nautile during the Vemanaute cruise in 1989. At this ridge‐transform intersection (RTI), an elongated, E‐W ridge, more than 50 km long, is present in the transform valley. This median ridge rises up to 1000 m above the surrounding seafloor. The crest of the median ridge is lower and presents an arcuate shape at the tip of the MAR axis. Dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation. This detrital cover consists of polymictic sedimentary breccias, sandstones and siltstones, composed of basaltic, doleritic, and gabbroic clasts, with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle. Most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation. Disaggregation, mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls. This event could be related to the uplift of the southern wall of the fracture zone (the “transverse ridge”) which took place probably between 10 and 3 Ma ago. Since this uplift episode, the transverse ridge (which is now undergoing subsidence) and the detritus covered transform valley floor, separated by the transform fault zone, have migrated westward and eastward respectively. Vertical tectonics of the median ridge at the approach of the RTI can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors. A possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western RTI, is more than 1 km out of isostatic equilibrium. Recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the MAR axis have been recorded on the southern flank of the median ridge. Several stages in the very recent tectonic‐volcanic history of the eastern RTI, that is, roughly during the last 300,000 years, can thus be defined. The lower elevation and narrow, arcuate shape of the median ridge east of 40°57′W are inferred to have resulted from tectonic extension during the creation of the nodal basin.