Valley Walls Research Articles

Evolution of undercut slopes on abandoned incised meanders in the Eastern Highland Rim of Tennessee, USA

Field surveys, location-for-time reasoning, and computer modeling were used to study the evolution of slopes on valley walls of abandoned bedrock meanders on the Eastern Highland Rim, Tennessee. Hillslopes on the undercut slopes of cutoff incised meanders were ordered as to relative age by the height of their meander floors above the modern stream level. The assumption is that the undercut slope is actively eroded by the stream until abandonment of the meander, at which time the slope begins to evolve to a different form. More-advanced stages of evolution occur on walls of higher meanders that were abandoned earlier. The most rapid change in this initial form is the elimination of a free face, which occurs soon after the meander is abandoned. In addition, the hillslopes associated with even the lowest (youngest) cutoff meanders show somewhat gentler overall gradients than the actively undercut slopes. Hillslopes associated with meanders 3 to about 20 m above modern stream level maintain straight segments with angles showing only a slight decrease from the 36–38° associated with the lowest cutoffs; overall angles decrease, however, as the straight segment becomes shorter. The oldest slopes, those on cutoffs 30 m or greater above modern stream level, have developed into convex–concave slopes with maximum slopes of 15°. A hillslope evolution model based on previously published algorithms was used to simulate the transition of actively undercut hillslopes into hillslopes on abandoned meanders. Hillslope modeling is particularly useful in this setting. If the valley incision rate is known, an age can be estimated for the cutoff and hence for the hillslope. Alternatively, if hillslope process rates are known, a model age obtained for the hillslope can be used to estimate an incision rate. Even where both incision rates and hillslope process rates are poorly constrained, as in the present setting, modeling allows assumptions about specific rates to be evaluated by determining their implications for other rates. For example, for three cutoff meanders along one stream, best-fit criteria were used to select process rates for the model. Model ages of hillslopes were then obtained and compared with those calculated from a valley-incision rate measured elsewhere in the same physiographic province. For two of the hillslopes, model ages were found to be much younger than those calculated from the incision rate. In order to make the two ages agree, unreasonably low process rates had to be used in the model, thus implying that the incision rate probably underestimates the actual incision rate in this valley. Experimentation with heights of initial profiles, again using best-fit criteria, suggests that since abandonment of the highest cutoff, the plateau has been downwasting at a rate about one-fourth that of the valley incision rate, a finding in agreement with published rates of chemical denudation in the area.

Seismic structure across the rift valley of the Mid‐Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges

The results from a 53‐km‐long, wide‐angle seismic profile across the rift valley of the Mid‐Atlantic Ridge south of the Kane transform (near 23°20′N, MARK area) provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid‐ ocean ridges. Anomalously low middle and lower‐crustal P wave velocities beneath the neovolcanic Snake Pit ridge are consistent with elevated axial temperatures and with the presence of 4±1% partial melt evenly distributed within the lower crust in preferentially oriented, elongated thin films. If the melt inclusions have larger aspect ratios, melt fractions can be up to 17±3%. This and other geological observations suggest that the study area is presently in a magmatically active period. The igneous crust is anomalously thin beneath both flanks of the median valley (≤2.3–2.5 km). Thus the mantle rocks observed along the western rift valley wall at Pink Hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting. The crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous, with igneous crustal thickness variations of ≥2.2 km over horizontal distances of ∼5 km. This heterogeneity indicates that the magma supply in the area has fluctuated during the last ∼2 m.y. Thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data.

Dendrochronology and lichenometry: colonization, growth rates and dating of geomorphological events on the east side of the North Patagonian Icefield, Chile

This paper highlights the importance for dating accuracy of initial studies of delay before colonization for both trees and lichens and tree age below core height, particularly in recently deglaciated terrain where colonization and growth rates may vary widely due to differences in micro-environment. It demonstrates, for the first time, how dendrochronology and lichenometry can be used together in an assessment of each other's colonization and growth rates, and then cross-correlated to provide a supportive dating framework. The method described for estimating tree age below core height is also new. The results show that on the east side of the North Patagonian Icefield in the Arco and Colonia valleys, Nothofagus age below a core height of 112 cm can vary from 5 to 41 years and delay before colonization may range from a maximum of 22 years near water to a minimum of 93 years on the exposed flanks of the Arenales and Colonia Glaciers. Tree age plus colonization delay supplied a maximum growth rate of 4.7 mm/year for the lichen Placopsis perrugosa and lichen colonization is estimated to take from 2.5 to approximately 13 years. A minimum lichenometric date of 1883 was estimated for an ice-formed trimline at the junction of the Arenales and Colonia glaciers and a maximum dendrochronological date of 1881 for a water-formed trimline in the Arco valley. Tree and lichen ages around the valley suggest that a glacial outburst drained the 1881 high level lake releasing approximately 265 million cubic metres of water. Repeated flooding, with a minimum of 38 high lake levels, is suggested by horizontal sediment lines on the Arco valley walls and moraine flanks. Dating confirmed diminishing flood levels with a last minor flood in 1963. The wider significance of the work is that it should produce more accurate dating of recent glacier fluctuations around the North Patagonia Icefield, an area where dated reference surfaces are extremely scarce.

Record of seamount production and off‐axis evolution in the western North Atlantic Ocean, 25°25′–27°10′N

Using multibeam bathymetry, we identified 86 axial and 1290 off‐axis seamounts (near‐circular volcanoes with heights ≥70 m) in an area of 75,000 km2 on the western flank of the Mid‐Atlantic Ridge (MAR), 25°25′N to 27°10′N, extending ∼400 km from the inner rift valley floor to ∼29 Ma crust. Our study shows that seamounts are a common morphological feature of the North Atlantic seafloor. Seamount‐producing volcanism occurs primarily on the inner rift valley floor, and few, if any, seamounts are formed on the rift valley walls or the ridge flank. The high abundance of off‐axis seamounts is consistent with 1–3 km wide sections of oceanic crust being transferred intact from the axial valley to the ridge flank on crust >4 Ma. Significant changes in seamount abundances, sizes, and shapes are attributed to the effects of faulting between ∼0.6 and 2 m.y. off axis in the lower rift valley walls. Few seamounts are completely destroyed by (inward facing) faults, and population abundances are similar to those on axis. However, faulting reduces the characteristic height of the seamount population significantly. In the upper portions of the rift valley, on 2–4 Ma crust, crustal aging processes (sedimentation and mass wasting), together with additional outward facing faults, destroy and degrade a significant number of seamounts. Beyond the crest of the rift mountains (>4 Ma crust) faulting is no longer active, and changes in the off‐axis seamount population reflect crustal aging processes as well as temporal changes in seamount production that occurred at the ridge axis. Estimates of population density for off‐axis seamounts show a positive correlation to crustal thickness inferred from analysis of gravity data, suggesting that increased seamount production accompanies increased magma input at the ridge axis. We find no systematic variations in seamount population density along isochron within individual ridge segments. Possible explanations are that along‐axis production of seamounts is uniform or that seamount production is enhanced in some regions (e.g., segment centers), but many seamounts do not meet our counting criteria because they are masked by younger volcanic eruptions and low‐relief flows.

Heavy metal trends in floodplain sediments and valley fill, River Lahn, Germany

Sampling along the River Lahn, a major tributary to the River Rhine that flows through an agrarian region of west-central Germany, reveals that heavy metal pollution occurs outside major industrial regions and mined landscapes. Along a 60-km reach of the river between Marburg and Wetzlar, mean concentrations of Cu, Pb and Zn at depths of 5 and 15 cm in four floodplain transects were greater than background levels. Concentrations declined sharply between 15 and 25 cm. Laterally, concentrations peaked at the bank top and in the near-channel zone of the floodplain. Beyond the near-channel zone (generally up to 100 m from the channel) concentrations were much less and fairly uniform to the valley wall. The clustering of metals close to the channel suggests that they are a fluvial rather than an eolian deposit. During floods of a moderate magnitude, sediments and attached metals are deposited soon after the river overtops its banks. Although the overall store of metals in the valley is less than that along the Rhine, near-channel metal concentrations are similar to those along rivers draining major industrial and mined areas.

Accretionary processes in the axial valley of the Mid-Atlantic Ridge 27 degrees N - 30 degrees N from TOBI side-scan sonar images

We analyse TOBI side-scan sonar images collected during Charles Darwin cruise CD76 in the axial valley of the Mid-Atlantic Ridge (MAR) between 27° N and 30° N (Atlantis Transform Fault). Mosaics of the two side-scan sonar swaths provide a continuous image of the axial valley and the inner valley walls along more than six second-order segments of the MAR. Tectonic and volcanic analyses reveal a high-degree intra-segment and inter-segment variability. We distinguish three types of volcanic morphologies: hummocky volcanoes or volcanic ridges, smooth, flat-topped volcanoes, and lava flows. We observe that the variations in the tectonics from one segment to another are associated with variations in the distribution of the volcanic morphologies. Some segments have more smooth volcanoes near their ends and in the discontinuities than near their mid-point, and large, hummocky axial volcanic ridges. Their tectonic deformation is usually limited to the edges of the axial valley near the inner valley walls. Other segments have smooth volcanoes distributed along their length, small axial volcanic ridges, and their axial valley floor is affected by numerous faults and fissures. We propose a model of volcano-tectonic cycles in which smooth volcanoes and lava flows are built during phases of high magmatic flux. Hummocky volcanic ridges are constructed more progressively, by extraction of magma from pockets located preferentially beneath the centre of the segments, during phases of low magma input. These cycles might result from pulses in melt migration from the mantle. Melt arrival would lead to the rapid emplacement of smooth-textured volcanic terrains, and would leave magma pockets, mostly beneath the centre of the segments where most melt is produced. During the end of the volcanic cycle magma would be extracted from these reservoirs through dikes with a low magma pressure, building hummocky volcanic ridges at low effusion rates. In extreme cases, this volcanic phase would be followed by amagmatic extension until a new magma pulse arrives from the mantle.

Submersible observations of Equatorial Atlantic mantle: The St. Paul Fracture Zone region

The St. Paul F.Z. is a large structural domain made up of multiple transform faults interrupted by several Intra-Transform Ridge (ITR) spreading segments. Two regions were studied in details by submersible: (1) The ITR short (<20 km in length) segment near 0° 37′N–25° 27′W and 1° N–27° 42′W and (2) The St. Peter and St. Paul's Rocks (SPPR) massif located at 29° 25′W (i3700 m depth). (1) The short ITR segments consist of a magma starved rift valley with recent volcanic activities at 4700 m depth. A geological profile made along the rift valley wall showed localized volcanics (basalts and dykes) which are believed to overlay and intrude the ultramafics. The geological setting and the high ultramafic/volcanic ratio suggest an extremely low magmatic supply and crustal-mantle uplift during lithospheric stretching and denudation. (2) The St. Peter and St. Paul's Rocks (SPPR) massif consists of a sigmoidal ridge within the active transform zone. The SPPR is divided into two different geological domains called the North and the South Ridges. The North Ridge consists of strongly tectonized fault scarps composed of banded and mylonitized peridotite, sporadic gabbros (3900–2500 m) and metabasalts (2700–1700 m). The South Ridge is less tectonized with undeformed, serpentinized spinel lherzolite (2000–1400 m) and basalts. Extensional motion and denudation accompanied by diapirism affected the South Ridge within a transform domain. Instead, the North Ridge was formed during an important strike-slip and faulting motion resulting in the uplifted portion of the St. Paul F.Z. transverse ridge. There is a regional compositional variation of the volcanics where E-MORBs and alkali basalts are produced on the SPPR massif and are comparable to the adjacent northern segments of the Mid-Atlantic Ridge. On the other hand, N and T- MORBs collected from the eastern part of the St. Paul F.Z. (25° 27′ W IRT) are similar to the volcanics from the southern segments of the MAR. The peridotites exposed in these provinces (SPPR and ITR) are similar in their REE and trace element distribution. Different degrees (3–15%) of partial melting of a mixed composite mantle consisting of spinel and amphibole bearing lherzolite veined with 5–40% clinopyroxenite gave rise to the observed MORBs and alkali basalts.

Petaloid Organs Preserved in an Arctic Plant Macrofossil Assemblage from Full-Glacial Sediments in Southeastern Minnesota

A small suite of plant macrofossils indicates that southeastern Minnesota supported subarctic to arctic vegetation 18,700 yr B.P. Fossil tepals of Polygonum viviparum are exceptionally well preserved; they occur with more fragmentary remains of Dryas integrifolia, Vaccinium uliginosum var. alpinum, and probable species of arctic Salix, S. cf. herbacea, and S. cf. arctica. The pollen spectrum from this site is dominated by Picea, Pinus, and Cyperaceae, which are typical of midwestern full-glacial sequences. Tundra-like conditions with permafrost were present in southeastern Minnesota during full-glacial time. Local environments 18,700 yr B.P. reconstructed from both physical and paleobotanical evidence include wind-swept ridge tops with thin loess, outcrops of dolostone and sandstone on valley walls, colluvial slopes, sandy to gravelly floodplains, shallow floodplain pools, wet meadows, and peaty turfs, all in a treeless or nearly treeless environment.

Depositional features and transportation mechanism of valley-filling Iwasegawa and Kaida debris avalanches, Japan

The depositional features of two valley-filling debris avalanche deposits were studied to reveal their transportation and depositional mechanisms. The valley-filling Iwasegawa debris avalanche deposit (ca. 0.1 km3) is distributed along the valleys at the southeastern foot of Tashirodake Volcano, northern Honshu, Japan. Debris-avalanche blocks range in size from 0.3 km3) is distributed along the valleys at the eastern-southeastern foot of Ontake Volcano, central Japan. Debris-avalanche blocks range in size from <25 m proximally to <7 m in the medial zone. Debris-avalanche matrix percentages increase from 50–70% in the proximal zone to 80% in the distal zone. The debris-avalanche matrix is more abundant (80–90%) at the bottom part of the deposit. Deformation structures observed in the debris-avalanche blocks include elongation, folding, conjugate reverse faults, and numerous minor faults in unconsolidated materials. Lithic components within the debris-avalanche matrix tend to have a higher percentage of plucked clasts from the adjacent underlying formations. A Bingham "plug flow" model is consistent with the transportation and depositional mechanisms of the valley-filling debris avalanches. In the plug of the debris avalanche, fragile blocks were transported without major rupturing due to relatively small shear stresses in regions of small strain rate. The debris-avalanche matrix was mainly produced by shearing at the bottom and margins of the avalanche. Valley-filling debris avalanches tend to have smaller debris-avalanche blocks and larger amounts of debris-avalanche matrix than do unconfined debris avalanches. These differences may be due to disaggregation of debris-avalanche blocks by shearing against valley walls and interaction between debris-avalanche blocks and valley walls. Oriented wood logs and branches, reverse grading of clasts at the base, and a higher proportion of incorporated clasts at the base are interpreted to result from shearing along the bottom and valley walls.

Gap Winds and Wakes: SAR Observations and Numerical Simulations

The nature of terrain-induced gap winds and wakes in the atmosphere is examined using surface wind data from synthetic aperture radar (SAR) and the shallow water equations. The shallow water model is used to predict the types of wake–jet wind patterns that might occur behind an idealized pair of bell-shaped hills with a gap between them. A regime diagram is constructed based on the width of the gap and the upstream Froude number. Specific predictions of the model are found to compare moderately well with SAR data from four examples of airflow near Unimak Island in the Aleutian Chain. The model predicts the observed wakes and jets, including jets that exceed the upstream speed. Theoretical analysis considers the relative importance of rising terrain and narrowing valley walls in the acceleration of gap winds. Wind speeds in the wake region are controlled by the Bernoulli function and regional pressure. Gap winds therefore are streams of air that have avoided Bernoulli loss over the terrain by passing through gaps. The speed of gap winds can exceed the upstream speed only in ridgelike situations when the regional leeside pressure is lower than the upstream pressure.

Architecture and sequence stratigraphy of a late Neogene incised valley at the shelf margin, southern Celtic Sea

ABSTRACT Valleys on the outer Celtic Sea shelf were cut and filled during the late Pliocene/early Pleistocene. The Kaiser valley is one of several valleys forming an anastomosed network. The main valley, directed 20° (Azimuth true N), is 50 m deep and > 10 km wide. It is connected to the parallel Dompaire and Parsons valleys by several 120-140° directed incisions of lesser width and depth. Analyzed by mean of very high-resolution seismic data, the Kaiser valley is interpreted as containing a compound fill consisting of eight erosionally based depositional sequences. A typical sequence comprises two facies: (1) fluvial channels at the base, which represent lowstand to early transgressive deposits; and (2) onlapping transgressive bay-fill deposits that are locally interbedded at the top with isolated small channels attributed to flood tidal deltas. The erosional bases of the fluvial facies correspond to sequence boundaries. These are interpreted to result from relative sea-level falls. Successive fluvial and bay-fill facies are separated by flat erosional surfaces of high acoustic amplitude, which extend laterally across the entire composite valley, locally beveling sequence boundaries and creating terraces on the valley walls. These flat facies contacts are interpreted as bay ravinement surfaces produced by waves in an estuarine setting. The larger-scale stacking pattern of the depositional sequences defines a progradational-retrogradational trend, in which the lowest sequence is mainly constituted by fluvial channel deposits, whereas upper sequences display mostly bay-fill facies. The sequences are related to fifth-order glacioeustatic fluctuations, whereas their progradational-retrogradational trend reflects fourth-order eustatic variations and/or rapid tectonic tilting of the area, as indicated by the presence of two incision orientations. The preservation of the system took place during a third-order sea-level rise, and was favored by subsidence of the margin, leading to its present occurrence down to -240 m below present sea level.

The Enigma of the Giant Forest Hog, Hylochoerus meinertzhageni (Mammalia: Suidae), in Tanzania Reviewed

ABSTRACT The giant forest hog, Hylochoerus meinertzhageni, is distributed across the West African forest belt and into eastern Africa. It is known from Uganda and is reasonably common in suitable habitat in Kenya. Evidence suggests that a taxon described from a photograph as Hylochoerus schulzi Zukowsky 1921 from the Mutjek Mountains in northern Tanzania along the western Rift Valley wall is in fact a bushpig, Potamochoerus larvatus. Previously published records of the giant forest hog in Tanzania are discussed and shown to be non-definitive; the need for a specimen or unambiguous photographic record of H. meinertzhageni from Tanzania still exists.

Abstract: Characterizing a Morrow Sandstone Reservoir Through Stratigraphic Interpretation of 3-D Seismic Data, Sorrento Field, Colorado&amp;nbsp;

Sorrento field, southeastern Colorado, contains an estimated 21 million bbls of original oil in place in the Pennsylvanian Morrow sandstone. The reservoir consists of a coinplex of stacked channel sequences within an incised valley. Interpretation of 3-D compressional seismic data has outlined the configuration of the incised valley and provided essential information about the characteristics of the rocks that fill it. This seismic interpretation provides stratigraphic details on the compartmentalization of the four flow units which explain the multiple fluid contacts and variable performance of wells in this field. reservoir is encased in marine mudstone; normal compressional seismic data are not capable of imaging reservoir rock because the acoustic impedance is not sufficient to distinguish the two lithologies. Rather, seismic modeling shows that the compressional seismic character of the Morrow interval is the result primarily of high-velocjty/hjgh-density nonreservoir rocks. The distribution of these nonreservoir rocks is critical in delineating the reservoir because the channel sandstone is bordered by reflective rocks such as floodplain deposits or high-density facies which lie outside of the valley wall. Nonreservoir rocks that are barriers to flow within the reservoir, such as carbonate-cemented zones, define the compartmentalization. The petroleum industry has been challenged to characterize Morrow reservoirs because the sandstone

Seafloor geomagnetic vector anomaly of the intersection of the Mid‐Atlantic Ridge and the Kane Transform Fault: Implications for magnetization of the oceanic crust

We investigated the fine‐scale magnetic structure of the oceanic crust and upper mantle around the western intersection of the Mid‐Alantic Ridge and the Kane Transform Fault, using the geomagnetic vector field measured at the seafloor. The strikes of magnetic boundaries were found to reflect of the local tectonics represented by fault strikes and dips. The ridge‐transform intersection is characterized by magnetic boundary strikes rotated eastward. The strike directions reflect the direction of recent magmatic activity. These indicate that a few traces of faults extending obliquely from the ridge axis to the transform fault are associated with neovolcanism. Magnetic boundary strikes in the lower part of the transform valley wall and near a linear trace of the transform fault indicate deformation or tilting of the crust associated with transform faulting. It is also indicated that the crustal block around the median ridge is tilted by uplift, accompanied by injection or emplacement and serpentinization of peridotites. The nodal basin consists of extrusive basalts in the E–W direction and N–S trending magnetic strikes. Submersible observation of in situ magnetization of oceanic crust shows that extrusive basalts are typically strongly magnetized. The most recent neovolcanic ridge has a mean magnetization of 50 A/m. The mean magnetization of the ridge axis is 20 A/m. The magnetization of the 3 Ma ridge flank in the inside corner high is about 10 A/m. The gabbroic layer in the transform valley has a magnetization less than 2 A/m. The serpentinized peridotites in the median ridge have magnetization less than 1 A/m. This magnetization records a reversal in polarity, which is important because the serpentinized peridotites contribute to the marine geomagnetic anomaly pattern.

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.

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.

FAMOUS and AMAR segments on the Mid-Atlantic Ridge: ubiquitous hydrothermal Mn, CH 4, δ3He signals along the rift valley walls and rift offsets

Dynamic hydrocast experiments enabled Mn (TDM), CH 4 concentrations and δ 3He ratio to be recorded through vertical cross-sections of hydrothermal plumes along the FAMOUS segment and the southern part of the AMAR segment on the Mid-Atlantic Ridge between 36°N and 37°N. Mn, CH 4 and δ 3He figures all along both segments are well above the seawater background in the open ocean: they are interpreted to be the result of time-integrated hydrothermal discharges dispersed and mixed in a closed basin delineated by the rift valley and the segment ends. Hydrothermal activity along the FAMOUS and AMAR segments appears to be similar. A comparison of the residence times of the three tracers from the dispersed, time-integrated signals is proposed. Although the background values in these closed basin are high, some proximal and (or) large hydrothermal inputs, overprinted on the general time-integrated plume, can be detected (i.e. the Rainbow site south of AMAR). Based on the depth and the location of plumes, hydrothermal activity is not, by far, limited to the neo-volcanic inner floor of the valley and should involve the walls and complex offsets of the rift valley. Considering the Mn and CH 4 concentrations in these plumes, two types of ocean–mantle interaction may be represented: hot, focused discharges on ultramafic exposures (Rainbow site) and low-temperature diffuse serpentinisation.

Seismic structure and crustal magmatism at the Mid‐Atlantic Ridge, 35°N

We have determined the three‐dimensional P wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the Mid‐Atlantic Ridge (MAR) near 35°N. The best fitting one‐dimensional model is ∼0.8 km/s lower in velocity at a given depth than for fast spreading ridges, while the isotropic three‐dimensional velocity structure shows a strong lateral heterogeneity (up to 1.4 km/s peak‐to‐peak variation at 1 km depth) in both the along‐ and cross‐axis directions. An anomalous region, with velocities as much as 0.8 km/s lower than average and 3–5 km in lateral extent, is located beneath a near‐axis seamount. We interpret this feature as a region of near‐solidus temperatures with possibly a small degree of partial melt. From the variation of travel time residuals with azimuth we determine a best fitting P wave anisotropy model that has 4% anisotropy from 500 m to 1 km depth, 2% anisotropy in the depth range 1–1.5 km, and no anisotropy at greater depth. This finding is consistent with vertical cracks aligned parallel to the axial valley. We find no evidence for the shallow, high‐velocity, ridge‐parallel feature commonly observed at the axis of fast spreading ridges and attributed to high‐velocity dikes beneath a thin extrusive layer. The absence of this anomaly at the MAR implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25,000 years. An unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes, which may account for the generally lower velocities beneath the MAR compared with the East Pacific Rise, and a lateral variation in upper crustal lithology.

The Reykjanes Ridge: structure and tectonics of a hot-spot-influenced, slow-spreading ridge, from multibeam bathymetry, gravity and magnetic investigations

We report a comprehensive morphological, gravity and magnetic survey of the oblique- and slow-spreading Reykjanes Ridge near the Iceland mantle plume. The survey extends from 57.9°N to 62.1°N and from the spreading axis to between 30 km (3 Ma) and 100 km (10 Ma) off-axis; it includes 100 km of one arm of a diachronous `V-shaped' or `chevron' ridge. Observed isochrons are extremely linear and 28° oblique to the spreading normal with no significant offsets. Along-axis there are ubiquitous, en-echelon axial volcanic ridges (AVRs), sub-normal to the spreading direction, with average spacing of 14 km and overlap of about one third of their lengths. Relict AVRs occur off-axis, but are most obvious where there has been least axial faulting, suggesting that elsewhere they are rapidly eroded tectonically. AVRs maintain similar plan views but have reduced heights nearer Iceland. They are flanked by normal faults sub-parallel to the ridge axis, the innermost of which occur slightly closer to the axis towards Iceland, suggesting a gradual reduction of the effective lithospheric thickness there. Generally, the amplitude of faulting decreases towards Iceland. We interpret this pattern of AVRs and faults as the response of the lithosphere to oblique spreading, as suggested by theory and physical modelling. An axial, 10–15 km wide zone of high acoustic backscatter marks the most recent volcanic activity. The zone's width is independent of the presence of a median valley, so axial volcanism is not primarily delimited by median valley walls, but is probably controlled by the lateral distance that the oblique AVRs can propagate into off-axis lithosphere. The mantle Bouguer anomaly (MBA) exhibits little mid- to short-wavelength variation above a few milliGals, and along-axis variations are small compared with other parts of the Mid-Atlantic Ridge. Nevertheless, there are small axial deeps and MBA highs spaced some 130 km along-axis that may represent subdued third-order segment boundaries. They lack coherent off-axis traces and cannot be linked to Oligocene fracture zones on the ridge flanks. The surveyed chevron ridge is morphologically discontinuous, comprising several parallel bands of closely spaced, elevated blocks. These reflect the surrounding tectonic fabric but have higher fault scarps. There is no evidence for off-axis volcanism or greater abundance of seamounts on the chevron. Free-air gravity over it is greater than expected from the observed bathymetry, suggesting compensation via regional rather than pointwise isostasy. Most of the observed variation along the ridge can be ascribed to varying distance from the mantle plume, reflecting changes in mantle temperature and consequently in crustal thickness and lithospheric strength. However, a second-order variation is superimposed. In particular, between 59°30′N and 61°30′N there is a minimum of large-scale faulting and crustal magnetisation, maximum density of seamounts, and maximum axial free-air gravity high. To the north the scale of faulting increases slightly, seamounts are less common, and there is a relative axial free-air low. We interpret the 59°30′N to 61°30′N region as where the latest chevron ridge intersects the Reykjanes Ridge axis, and suggest that the morphological changes that culminate there reflect a local temperature high associated with a transient pulse of high plume output at its apex.

Intense CH 4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20′N fracture zone and the Mid-Atlantic Ridge

As part of the FARA French-US Program designed to study the Mid- Atlantic Ridge (MAR) between 15°N and the Azores, twenty-three dives with the submersible Nautile were conducted during the French-US Faranaut 15N cruise on the eastern and western parts of the 15°20′N Fracture Zone/Ridge axis intersection. South of the eastern ridge-transform fault intersection, nine Nautile dives were made within the rift valley and along the western rift valley wall. CH 4 concentrations in the bottom waters reach 53.2 nmol/kg along faulted zones on top and on the east flank of the ultramafic inner corner high (15°05′N, 44°59′W) where serpentinized rocks outcrop. No 3He anomaly is associated with methane, ruling out any primary mantle component. Fourteen dives were also made in the rift valley to the north, close to the western intersection of the 15°20′N Transform. High CH 4 anomalies (up to 22 nmol/kg) are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop. Seven vertical hydrocasts carried out in the axial valley (4500 m deep) show an intense CH 4 anomaly, with a maximum (35.8 nmol/kg) at 3200 m depth. This CH 4 concentration is among the highest found along the Mid-Atlantic Ridge (Charlou and Donval, 1993; Charlou et al., 1997). CH 4 concentrations of 9.9–14.9 nmol/kg are also present on the western wall along the 3200 m isobath. The high CH 4 concentrations correspond to only weak 3He anomalies. This CH 4-rich plume is also associated with active fault zones that expose peridotite rocks. CH 4 output from ultramafic outcrops on the western and eastern intersections of the 15°20′N Fracture Zone with the MAR is believed to reflect ongoing serpentinization. These results associated with many other CH 4 anomalies measured in the water column above ultramafic outcrops found between 12°N and the Azores most likely reflect serpentinization processes extending along the whole slow spreading Mid-Atlantic ridge.