On: “Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment” by G. M. Kent, I. I. Kim, A. J. Harding, R. S. Detrick, and J. A. Orcutt (May‐June 1996 GEOPHYSICS, 61, 821–834)

Geophysics ◽  
1998 ◽  
Vol 63 (1) ◽  
pp. 316-318
Author(s):  
A. J. Calvert
Keyword(s):  
Sea Floor ◽  
Counter Example ◽  
Scattered Energy ◽  
Mid Ocean Ridge ◽  
Out Of Plane ◽  
History Of ◽  
The Common ◽  
Velocity Filtering ◽  
Ocean Ridge ◽  
Water Depths

In their paper, Kent et al. (1996) present an excellent case history of the use of dip moveout (DMO) and velocity‐filtering in the common midpoint (CMP) domain for the suppression of out‐of‐plane arrivals scattered from a deep sea‐floor. However, they imply that as a result of a “small offset approximation” the use of DMO in this way is limited to surveys recorded in water depths of at least a few kilometers with conventional streamer offsets. This is incorrect. I argue here that the application of DMO will reduce to water velocity the stacking velocity of arrivals scattered from the upper surface of the seafloor without any restriction on water depth. Furthermore, I argue that this use of DMO is simply an example of the equivalence between 2-D and 3-D DMO for marine surveys where all source‐receiver azimuths are equal, and that no “small offset approximation” is required. I first present a counter‐example to the claim of Kent et al. (1996) that seafloor scattering cannot be suppressed using DMO in shallow water, and then consider in more detail their argument for the application of DMO to out‐of‐plane scattering. In the discussion that follows, I only consider DMO in the context of a constant velocity medium.

Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment

Geophysics ◽  
1996 ◽  
Vol 61 (3) ◽  
pp. 821-834 ◽  
Author(s):  
Graham M. Kent ◽  
Isaac I. Kim ◽  
Alistair J. Harding ◽  
Robert S. Detrick ◽  
John A. Orcutt
Keyword(s):  
Deep Water ◽  
East Pacific Rise ◽  
Processing Scheme ◽  
Sea Floor ◽  
East Pacific ◽  
Scattered Energy ◽  
Mid Ocean Ridge ◽  
Out Of Plane ◽  
Dip Angle ◽  
Ocean Ridge

Multichannel seismic (MCS) images are often contaminated with in‐ and out‐of‐plane scattering from the sea floor. This problem is especially acute in the mid‐ocean ridge environment where sea‐floor roughness is pronounced. Energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as Moho, and scattering off of elongated sea‐floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections. Moreover, stacking at normal subsurface velocities may enhance these water‐borne events, whose stacking velocity depends on azimuth and generally increases with time, making them indistinguishable from subsurface arrivals. To suppress scattered energy in deep water settings, we propose a processing scheme that invokes the application of dip moveout (DMO) to deliberately increase the differential moveout between sea‐floor‐scattered and subsurface events, thereby facilitating the removal of unwanted energy in the stacked section. After application of DMO, all sea‐floor scatterers stack at the water velocity, while subsurface reflections like Moho still stack at their original velocity. The application of DMO in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used. Unlike previous approaches to suppress scattered energy, dip filtering is applied in the common‐midpoint (CMP) domain after DMO. Moveover, our DMO‐based approach suppresses out‐of‐plane scattering, and therefore is not limited to removal of in‐plane scattering as is the case with shot and receiver dip filtering techniques. The success of our DMO‐based suppression scheme is limited to deep water (a few kilometers of water depth for conventional offsets), where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle. The application of DMO‐based suppression to synthetics and MCS data collected along the southern East Pacific Rise demonstrates the effectiveness of our approach. Cleaner images of primary reflectors such as Moho are produced, even though present shot coverage along the East Pacific Rise is unduly sparse, resulting in a limited effective spatial bandwidth.

Reply by the authors to the discussion by A. J. Calvert

Geophysics ◽  
1998 ◽  
Vol 63 (1) ◽  
pp. 318-318
Author(s):  
A. J. Calvert
Keyword(s):  
Sea Floor ◽  
Scattered Energy ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

The authors of “Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment” chose not to reply to the discussion by.

A near-ridge origin for seamounts at the southern terminus of the Pratt-Welker Seamount Chain, northeast Pacific Ocean

1999 ◽  
Vol 36 (6) ◽  
pp. 1021-1031 ◽  
Author(s):  
Brian Cousens ◽  
Jarda Dostal ◽  
T S Hamilton
Keyword(s):  
Gulf Of Alaska ◽  
Trace Element Composition ◽  
Primitive Mantle ◽  
Sea Floor ◽  
Northeast Pacific ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Juan De Fuca ◽  
Seamount Chain ◽  
Ocean Ridge

Three seamounts close to the south end of the Pratt-Welker Seamount Chain, Gulf of Alaska, have been sampled to test whether or not mantle plume-related volcanism extends south of Bowie Seamount. Lavas recovered from Oshawa, Drifters, and Graham seamounts are weathered, Mn-encrusted pillow lavas and sheet-flow fragments, commonly with glassy rims. The glasses and holocrystalline rocks are tholeiitic basalts, with light rare earth element depleted to flat primitive mantle normalized incompatible element patterns and radiogenic isotope compositions within the ranges of mid-ocean ridge and near-ridge seamount basalts from the Explorer and northern Juan de Fuca ridges. Chemically, the seamount lavas strongly resemble older, "shield-phase" tholeiitic rocks dredged from the flanks of southern Pratt-Welker seamounts, but are distinct from the younger alkaline intraplate lavas that cap Pratt-Welker edifices. The weathered, encrusted basalts were most likely erupted in a near-ridge environment, adjacent to Explorer Ridge, between 11 and 14 Ma. No evidence of plume-related activity is found in this area. Compared with northeast Pacific mid-ocean ridge and alkaline intraplate basalts, Graham seamount lavas have anomalously high 206Pb/204Pb, which does not appear to be a function of sea-floor alteration, magma contamination, or mixing between previously identified mantle components. All near-ridge seamounts in the northeast Pacific exhibit isotopic heterogeneity that does not correlate with major or trace element composition, suggesting that the mantle sources of all near-ridge seamounts have been variably depleted by prior, but recent melting events.

scholarly journals Rift- and subduction-related crustal sequences in the Jinshajiang ophiolitic mélange, SW China: Insights into the eastern Paleo-Tethys

Lithosphere ◽  
2019 ◽  
Vol 11 (6) ◽  
pp. 821-833
Author(s):  
Wen-Jun Hu ◽  
Hong Zhong ◽  
Wei-Guang Zhu ◽  
Zhong-Jie Bai
Keyword(s):  
Mantle Metasomatism ◽  
Spreading Ridge ◽  
Crustal Assimilation ◽  
Ophiolitic Mélange ◽  
Mid Ocean Ridge ◽  
History Of ◽  
T Values ◽  
Ocean Ridge ◽  
Subducting Slab ◽  
Important Branch

Abstract The Paleozoic Jinshajiang ophiolitic mélange in southwest China marks an important branch ocean (i.e., the Jinshajiang Ocean) of the Paleo-Tethys. Basic-intermediate rocks are widespread features in the mélange; their formation age is well known, but the petrogenesis has not been well studied, which means that the evolutionary history of the Jinshajiang Ocean is not well constrained. To understand the nature of the mélange and the ocean, we present a set of elemental and isotopic data from two typical crustal sequences in two areas of the Jinshajiang ophiolitic mélange, Zhiyong and Baimaxueshan. The basalts in the ca. 343 Ma Zhiyong crustal sequence show mid-ocean-ridge basalt–like geochemical compositions with Nb/La ratios of 0.98–1.15 and εNd(t) values of +6.5 to +7.7, indicating that the basalts formed in the spreading ridge of the ocean. In contrast, the 283 Ma Baimaxueshan crustal sequence consists of gabbros and basaltic-andesitic lavas, which have an arc affinity with Nb/La ratios of 0.54–0.67 and εNd(t) values of +5.1 to +6.5. The geochemical differences were not caused by crustal assimilation but reflect mantle metasomatism by fluids dehydrated from the subducting slab. Therefore, we propose that the Zhiyong and Baimaxueshan crustal sequences formed in seafloor spreading and subduction settings, which were related to the opening and closure of the ocean, respectively.

Melvicalathis, a new brachiopod genus (Terebratulida: Chlidonophoridae) fromdeep sea volcanic substrates, and the biogeographic significance of the mid-oceanridge system

Zootaxa ◽  
2008 ◽  
Vol 1866 (1) ◽  
pp. 136 ◽  
Author(s):  
DAPHNE E. LEE ◽  
MURRAY R. GREGORY ◽  
CARSTEN LÜTER ◽  
OLGA N. ZEZINA ◽  
JEFFREY H. ROBINSON ◽  
...  
Keyword(s):  
Deep Sea ◽  
New Genus ◽  
Significant Component ◽  
Ocean Ridges ◽  
Southeast Indian Ridge ◽  
Mid Ocean Ridge ◽  
History Of ◽  
Cryptic Habitats ◽  
Ocean Ridge ◽  
Ridge System

Brachiopods form a small but significant component of the deep-sea benthos in all oceans. Almost half of the 40 brachiopod species so far described from depths greater than 2000 m are small, short-looped terebratulides assigned to two superfamilies, Terebratuloidea and Cancellothyridoidea. In this study we describe Melvicalathis, a new genus of cancellothyridoid brachiopod (Family Chlidonophoridae; Subfamily Eucalathinae) from ocean ridge localities in the south and southeast Pacific Ocean, and cryptic habitats within lava caves in glassy basalt dredged from the Southeast Indian Ridge, Indian Ocean. These small, punctate, strongly-ribbed, highly spiculate brachiopods occur at depths between 2009 m and 4900 m, and appear to be primary colonisers on the inhospitable volcanic rock substrate. The ecology and life-history of Melvicalathis and related deep-sea brachiopods are discussed. Brachiopods are rarely reported from the much-studied but localised hydrothermal vent faunas of the mid ocean ridge systems. They are, however, widespread members of a poorly known deep-sea benthos of attached, suspension-feeding epibionts that live along the rarely sampled basalt substrates associated with mid-ocean ridge systems. We suggest that these basalt rocks of the mid-ocean ridge system act as deep-sea “superhighways” for certain groups of deep-sea animals, including brachiopods, along which they may migrate and disperse. Although the mid-ocean ridges form the most extensive, continuous, essentially uniform habitat on Earth, their biogeographic significance may not have been fully appreciated.

Porphyry intrusions and albitites in the Bardoc–Kalgoorlie area, Western Australia, and their role in Archean epigenetic gold mineralization

1992 ◽  
Vol 29 (8) ◽  
pp. 1609-1622 ◽  
Author(s):  
W. K. Witt
Keyword(s):  
Western Australia ◽  
Gold Mineralization ◽  
Shear Zones ◽  
Rock Composition ◽  
Gold Mines ◽  
Mid Ocean Ridge ◽  
History Of ◽  
Compatible Elements ◽  
Ocean Ridge ◽  
Textural Evidence

Minor intrusions in the Menzies – Kambalda greenstone belt of the Archean Eastern Goldfields Province, Western Australia, range from quartz–feldspar porphyry to plagioclase–hornblende porphyry. The porphyries display enrichment of mobile and incompatible elements (K to Zr) and depletion of relatively compatible elements, with negative Nb, P, and Ti anomalies, on mid-ocean-ridge basalt-normalized spidergrams. The composition and timing of emplacement of the porphyries are consistent with a genetic relationship with spatially related granitoids. Porphyries occur in 30% of gold mines in the Menzies–Kambalda belt. The association appears to be largely structural, since both the intrusions and the mineralizing fluids exploit zones of weaknesses, such as lithological contacts and shear zones. Porphyries have been modified to varying degrees by hydrothermal alteration, especially pervasive albitization. Textural evidence indicates that secondary albite and associated sodic amphibole formed late in the deformation history of the greenstones and were broadly contemporaneous with secondary phyllosilicate, carbonate and sulphide minerals related to gold mineralization. Recent studies in the Alleghany district of California suggest the initial rock composition may critically influence the nature of alteration associated with gold mineralization. Therefore, albitization of porphyries may be caused by the same hydrothermal fluids that deposit gold and produce potassic alteration in mafic rocks.

scholarly journals Steadying Mid-Ocean Ridge Spreading Rates

Eos ◽  
2020 ◽  
Vol 101 ◽  
Author(s):  
Kate Wheeling
Keyword(s):  
Magnetic Field ◽  
Magnetic Anomaly ◽  
Data Set ◽  
Mid Ocean Ridge ◽  
History Of ◽  
Spreading Rates ◽  
Ocean Ridge

Researchers used an up-to-date global magnetic anomaly data set to track the history of magnetic field reversals and obtain more accurate estimates of tectonic spreading rates.

Movements of thick evaporites on the flanks of a mid-ocean ridge: the central Red Sea Miocene evaporites

2020 ◽  
Author(s):  
Neil Mitchell ◽  
Wen Shi ◽  
Ay Izzeldin ◽  
Ian Stewart
Keyword(s):  
Red Sea ◽  
North Atlantic ◽  
Ocean Basin ◽  
Vertical Movements ◽  
Oceanic Spreading ◽  
Mid Ocean Ridge ◽  
History Of ◽  
Global Sea Level ◽  
Ocean Ridge ◽  
Thermal Subsidence

<p>Thick evaporites ("salt") were deposited in the South and North Atlantic, and Gulf of Mexico basins, in some parts deposited onto the flanks of nascent oceanic spreading centres.  Unfortunately, knowledge of the history of evaporite movements is complicated in such places by their inaccessibility and subsequent diapirism.  This is less of a problem in the Red Sea, a young rift basin that is transitioning to an ocean basin and where the evaporites are less affected by diapirism.  In this study, we explore the vertical movements of the evaporite surface imaged with deep seismic profiling.  The evaporites have moved towards the spreading axis of the basin during and after their deposition, which ended at the 5.3 Ma Miocene-Pliocene boundary.  We quantify the evaporite surface deflation needed to balance the volume of evaporites overflowing oceanic crust of 5.3 Ma age, thermal subsidence of the lithosphere and loss of halite through pore water diffusion, allowing for isostatic effects.  The reconstructed evaporite surface lies within the range of estimated global sea level towards the end of the Miocene.  Therefore, the evaporites appear to have filled the basin almost completely at the end of the Miocene.  Effects of shunting by terrigenous sediments and carbonates near the coast and contributions of hydrothermal salt are too small to be resolved by this reconstruction.</p>

A new Tertiary sill complex of mid-ocean ridge basalt type NNE of the Shetland Isles: a preliminary report

1986 ◽  
Vol 77 (3) ◽  
pp. 223-230 ◽  
Author(s):  
F. G. F. Gibb ◽  
R. Kanaris-Sotiriou ◽  
R. Neves
Keyword(s):  
North Atlantic ◽  
Ridge Basalt ◽  
The North ◽  
Tectonic History ◽  
Mid Ocean Ridge ◽  
Intrusive Rocks ◽  
History Of ◽  
Geochemical Analyses ◽  
Ocean Ridge ◽  
Basalt Type

ABSTRACTBasic intrusive rocks recently encountered in wells N and NNE of the Shetland Isles are probably parts of a single large sill complex which extends for over 130 km along the edge of the Faeroe-Shetland Trough. The sills intrude thick Mesozoic sediments which almost certainly overlie continental crust but the complex also appears to underlie, and extend beyond the SE edge of, the Faeroes basaltic lava plateau. Petrographic and geochemical analyses of drill core samples recovered from some of these sills reveal that they are of mid-ocean ridge basalt (MORB) type; an observation which provides evidence regarding the plate tectonic history of this area of the North Atlantic and has major implications for the nature of the continental/oceanic crust transition.

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