Crustal structure of Atlantic fracture zones--II. The Vema fracture zone and transverse ridge

Potts, C. G.; White, R. S.; Louden, K. E.

doi:10.1111/j.1365-246x.1986.tb03840.x

Cited by 39 publications

(11 citation statements)

References 33 publications

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“…The RMBA pattern indicates a decrease of crustal thickness moving from the centre of the EMARS towards the Vema eastern ridge-transform intersection (RTI), and a minimal crustal thickness along the southern edge of the transform valley ( Fig. 4), consistent with seismic refraction experiments 20,21 . RMBA profiles along a flow line from the centre of the EMARS show (1) a long-range steady increase of crustal thickness from ,20-Myrold crust to the present (note that this pattern of steady crustal thickening would be enhanced if we were to take into account the observed variations of spreading rate in correcting for the thermal component of gravity); (2) 3-4-Myr-long oscillations of crustal thickness, superimposed on the steady thickening.…”

Section: Variations Of Gravity and Crustal Thicknesssupporting

confidence: 69%

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere

Bonatti

Ligi²,

Brunelli

et al. 2003

Nature

111

110

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A 20-Myr record of creation of oceanic lithosphere is exposed along a segment of the central Mid-Atlantic Ridge on an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of approximately 3-4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the mantle is upwelling at an average rate of approximately 25 mm x yr(-1), but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north-south mantle flow.

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Section: Variations Of Gravity and Crustal Thicknesssupporting

confidence: 69%

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere

Bonatti

Ligi²,

Brunelli

et al. 2003

Nature

111

110

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“…The width of the anomalous crust is similar to that inferred from the gradual thinning observed on multi-channel reflection profiles across the Blake-Spur Fracture Zone (Mutter et aZ. 1984), and from wide-angle seismic lines across the Vema fracture zone (Whitmarsh & Calvert 1986, Potts, White & Louden 1986. Careful design of the experimental geometry could produce much more refined images of appropriate structures, though the accuracy could not approach that which could be obtained from a more detailed but more expensive 3-D seismic survey.…”

Section: E L a Y T I M E A N A L Y S I S A N D T O M O G R A P H I supporting

confidence: 74%

Crustal structure of Atlantic Fracture Zones - III. The Tydeman fracture zone

Potts¹,

Calvert²,

White³

1986

Geophysical Journal International

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An extensive seismic survey was performed in March 1982 on two different parts of the Tydeman fracture zone. This survey consisted of two separate experiments: a reversed seismic refraction experiment, centred on 36"N, 26"W, using ocean-bottom seismometers and free-floating sonobuoys, which was shot with explosives; and a two-ship multichannel experiment of the expanding spread type, also shot with explosives at approximately 36"N, 23'30'W. These experiments are the first, and t o date only, wide-angle seismic surveys of fracture zone crust older than 25 Myr.Slope-intercept and tau-p inversion of the OBS/sonobuoy travel time data indicates low crustal and mantle seismic velocities within the fracture zone. These results are confirmed and refined by a more detailed analysis using raytracing and synthetic seismogram modelling for laterally varying structures, which implies velocities for the upper mantle beneath the fracture zone of 7.3-7.5kms-'. Similar analysis of a line perpendicular t o the fracture zone reveals the transition from normal mantle velocities to the low values found beneath the fracture zone, and suggests that the width of the anomalous region is approximately 12 km. The geometry of the OBS experiment permitted a delay time analysis and a further simple tomographic inversion of the traveltime data, which produced an approximate image of the low velocity mantle region associated with the fracture zone. The width of the region of anomalous crust is the same as the width of the sediment filled trough, which marks the fracture zone location at the line intersection.Analysis of the two-ship seismic data from a site some 240 km t o the east, also by synthetic seismogram modelling, gave similar results to the OBS/sonobuoy experiment with a mantle velocity of 7.2 km s-l detected beneath the crust, which possessed lower than normal seismic velocities. The close seismogram spacing of less than 0.1 km allowed us t o transform the dataset into the tau-p domain and enabled us to detect arrivals obscured in the X -T domain by the reflected water wave.Both experiments indicate low crustal seismic velocities within the fracture zone together with unusually low velocities of 7.2-7.5 km s-' in the upper mantle. Comparison with other fracture zone surveys reveals that the seismic velocity in the upper mantle beneath fracture zones decreases with increasing age. We attribute this to serpentinization of mantle peridotite brought about by hydrothermal circulation to the base of the fractured and faulted crust beneath the fracture zone.The Tydeinan fracture z m e 91 1 anomaly 30/31 by Collette et 01. (1984), using the time-scales of La Brecque et al. (1977)). At the OBS and sonobuoy experimental site the identification of anomaly 24, on the north side of the fracture zone, gives an age between 53 and 56 Ma.There is no evidence of present day large-scale natural seismic activity along the Tydeman fracture zone itself (Anderson 1985). The location of the fracture zone from our bathymetric contour map (Fig. 1) is in close ...

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“…However, the gravity data for the AFFZ do not support this but show that the flanking ridges actually represent substantial mass excesses and are not supported locally by underlying mass deficiencies. The main mechanism responsible for raising the ridges involves flexure of the lithosphere and regional isostatic compensation, as also suggested for the Vema fracture zone by Potts et al [1986]. However, the slight crustal thickening beneath the ridges in the AFFZ gravity models (Figure 11) may be indicative of a degree of serpentinization there.…”

Section: Origin Of the Transverse Ridgesmentioning

confidence: 64%

Structure of the Alula‐Fartak Fracture Zone, Gulf of Aden

Tamsett¹,

Searle²

1990

J. Geophys. Res.

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The Alula‐Fartak transform fault is associated with a broad, steep‐sided valley running NNE‐SSW across the mouth of the Gulf of Aden. A discussion of the tectonics of the fracture zone is based principally on GLORIA side scan sonar and gravity data. The transform valley is 180 km long and 25 km across, has 3.5 km of relief, and is bounded by transverse ridges. The northern two‐thirds is highly asymmetrical in cross section. The eastern wall dips 38° and appears to be a single scarp exposing a section of oceanic crust 3100 m thick. The western wall is much less steep and accommodates several normal faults oriented at about 15° to its general trend. A very fine, long sonar target running along part of the valley delineates the main active strike‐slip fault. The sonar data also reveal many fine faultlike targets between the main walls, running approximately at right angles to the trend of the valley, which may be anti‐Riedel shears. A 50‐km‐long, 7.5‐km‐wide, and 1.5‐km‐high median ridge runs along the southern third of the transform valley. Gravity data show that the valley is underlain by a significant mass deficiency thought to be a large serpentinite intrusion. The median ridge is suggested to be the surface manifestation of a large serpentinite diapir. The ridges flanking the transform valley are above a level compatible with isostatic equilibrium and are interpreted as flexural features raised in response to the upward loads imposed on the lithosphere by the fracture zone valley and its underlying mass deficiency. Very shallow crust is located close to the ridge‐transform intersections. This is considered to result from static flexure induced by the negative load of the transform valley superimposed on dynamic compensation of the spreading center valley. Evidence of a c. 3.5 m.y. B.P. change in spreading direction suggests the transform has experienced a recent component of opening, accounting for several aspects of its structure.

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Crustal structure of Atlantic fracture zones--II. The Vema fracture zone and transverse ridge

Cited by 39 publications

References 33 publications

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere

Crustal structure of Atlantic Fracture Zones - III. The Tydeman fracture zone

Structure of the Alula‐Fartak Fracture Zone, Gulf of Aden

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