1,721,051 research outputs found
Extension across the Indian-Arabian plate boundary: the Murray Ridge
No full textSeismic reflection profiles from the Murray Ridge in the Gulf of Oman, northwest Indian Ocean, show a significant component of extension across the predominantly strike-slip Indian–Arabian plate boundary. The Murray Ridge lies along the northern section of the plate boundary, where its trend becomes more easterly and thus allows a component of extension. The Dalrymple Trough is a 25 km wide, steep-sided half graben, bounded by large faults with components of both strike-slip and normal motion. The throw at the seabed of the main fault on the southeastern side of the half grabenr eaches 1800 m. The northwest side of the trough is delineated by a series of smaller antithetic normal faults. Wide-angle seismic, gravity and magnetic models show that the Murray Ridge and Dalrymple Trough are underlain by a crystalline crust upto 17 km thick, which may be continental in origin. Any crustal thinning due to extension is limited, and no new crust has been formed. We favour a plate model in which the Indian–Arabian plate boundary was initially located further west than the Owen Fracture Zone, possibly along the Oman continental margin, and suggest that during the Oligocene–Early Miocene Indian Ocean plate reorganization, the plate boundary moved to the site of the present Owen Fracture Zone and that motion further west ceased. At this time, deformation began along the Murray Ridge, with both the uplift of basement highs, and subsidence in the trough stilting the lowest sedimentary unit. Qalhat Seamount was formed at this time. Subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produce the present basement topography. The normal faulting was accompanied by hanging-wall subsidence, footwall uplift, and erosion. Flat-lying recent sediments show that the major vertical movements have ceased, although continuing earthquakes show that some faulting is still active along the plate boundary
Effects of tectonism and magmatism on crack structure in oceanic crust: a seismic anisotropy study
No full textWe analyzed 25,675 traveltime residuals from a three-dimensional seismic tomographic inversion to investigate crack-induced seismic anisotropy in the upper oceanic crust. The study covered two regions with contrasting levels of magmatic activity on the western limb of the 9°N overlapping spreading center on the East Pacific Rise. The level of anisotropy gradually decreases with depth in the magmatically and hydrothermally active ridge region. In contrast, we observed a highly variable anisotropic structure in the magmatically and hydrothermally less active tip region at the end of the dying ridge segment: a weakly anisotropic layer beneath strongly anisotropic extrusive volcanic rocks is likely to be the result of relatively shallow cracks closed by hydrothermal precipitation. Strongly anisotropic dikes with inferred narrow and water-saturated cracks provide important along-axis pathways for the circulation of hydrothermal fluids beneath the shallow cracks in the less magmatically active regions. Furthermore, a significant clockwise rotation (20°–30°) of fast directions occurs in both regions with increasing depth. Such a rotation provides evidence that the geometry of the underlying crack structure of the western limb is significantly different from that defined by the bathymetric ridge crest
Melt generation at very slow-spreading oceanic ridges: constraints from geochemical and geophysical data
No full textWe show that there is a strong and consistent correlation between geochemical and geophysical estimates of the amount of melt generated in the mantle beneath oceanic ridges. This correlation holds across all spreading rates and on scales down to the size of individual ridge segments. There is an abrupt decrease in the amount of melt generated at full spreading rates below ~20 mm/a. Our observations are consistent with the conclusion that <10% of the melt is frozen in the mantle before it reaches the crust and that serpentine probably represents only a small percentage of the material above the Moho. The melt is well mixed on a ridge segment scale, probably in high level magma chambers, but the melts remain distinct between segments. The rare earth element concentrations of basalts from very slow-spreading ridges are higher than those from normal oceanic ridges, which is directly indicative of reduced mantle melting, and they show characteristic light rare earth element enrichment, interpreted as caused by a deep tail of small percentage wet melting. The decrease in melt production at rates below ~20 mm/a points to the importance of conductive cooling inhibiting melting of the upwelling mantle at very slow-spreading centres
Crustal structure of the Southwest Indian Ridge at 66E: seismic constraints
No full textThe Southwest Indian Ridge represents a slow-spreading end-member of the global mid-ocean\ud
ridge system, and as such its structure places important constraints on models of melt supply\ud
and delivery from the mantle at ridges. We present results from a wide-angle seismic experiment\ud
conducted at the ridge axis at 66◦E, in a region that has comprehensive swath bathymetric,\ud
gravity and magnetic data coverage and where the full spreading rate is ∼12 mm yr−1.\ud
Based on these data, the experiment traversed four spreading segments. Crustal thickness and\ud
velocity structure were determined along three intersecting profiles each ∼100 km long using\ud
shots from a 10-gun, 71 L tuned airgun array towed at 15 m depth and fired at 40 s intervals,\ud
recorded on three ocean bottom hydrophones on each profile. OBH data show high-amplitude\ud
arrivals from oceanic Layer 2, lower-amplitude arrivals from Layer 3 and wide-angle reflections\ud
from the Moho. Forward modelling and inversion of traveltime picks from these data\ud
show that the crust consists of a 1.5–2.5-km-thick Layer 2 with a high velocity gradient and a\ud
0.5–3.0-km-thick Layer 3 with a low velocity gradient, and a crustal thickness of 2.2–5.4 km.\ud
Additional constraints on the models come from 2-D modelling of gravity data along the profiles,\ud
corrected for 3-D effects of off-line bathymetry. Along-axis, the thickness of Layer 2\ud
varies little, but Layer 3 is thick at segment centres and very thin at segment boundaries. Along\ud
a flowline profile, crustal thickness varies by up to 75 per cent from its minimum value in\ud
∼3 Myr. The reduced crustal thickness is consistent with observations from very slow-spreading\ud
ridge axes elsewhere and may be explained by conductive cooling of the upwelling mantle.\ud
The large along-axis variations in Layer 3 thickness indicate that magmatic accretion is focused\ud
at segment centres and melt is delivered to segment ends perhaps only by lateral dyke propagation.\ud
Flowline variations in crustal thickness may result from episodicity of melt supply on\ud
timescales of ∼3 Myr and by tectonic extension during amagmatic periods. Velocities at the\ud
top of Layer 2 are poorly correlated with crustal age based on magnetic anomalies, suggesting\ud
also that episodicity is decoupled between adjacent segments
Anomalous melt production after continental breakup in the southern Iberia Abyssal Plain
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Low degree melting under the Southwest Indian Ridge: the roles of mantle temperature, conductive cooling and wet melting
No full textBoth low mantle temperatures and conductive cooling have been suggested as the cause of the atypically thin oceanic crust and the incompatible element enrichment characteristic of very slow-spreading ridges. Here we present a model of melting under the Southwest Indian Ridge, which takes into account mantle temperature, conductive cooling, source composition and wet melting. The model parameters are constrained by oceanic crustal thickness, lava chemistry and isotopic composition and water content. The results suggest that conductive cooling to a depth of around 20 km, expected in areas with a full spreading rate of 15 mm/yr, is necessary to generate the Southwest Indian Ridge lava chemistry, but not that from faster spreading rate ridges at 23°N on the Mid Atlantic Ridge or 45°N on the Juan de Fuca Ridge. The mantle potential temperatures of ~1280°C, estimated for the Southwest Indian Ridge lavas are close to the global average of the upper mantle. Mantle water contents of 150-300 ppm can explain the observed melt water contents and allow sufficient melting at depth to explain the observed heavy rare earth element depletions in the melts
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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