scholarly journals Porosity of very young oceanic crust from sea floor gravity measurements

1998 ◽  
Vol 25 (11) ◽  
pp. 1959-1962 ◽  
Author(s):  
Matthew J. Pruis ◽  
H. Paul Johnson
Keyword(s):  
Oceanic Crust ◽  
Sea Floor ◽  
Gravity Measurements

Early Paleozoic Volcanism and Metamorphism of the Moretons Harbour–Twillingate Area, Newfoundland

1973 ◽  
Vol 10 (9) ◽  
pp. 1363-1379 ◽  
Author(s):  
D. F. Strong ◽  
J. G. Payne
Keyword(s):  
Oceanic Crust ◽  
Volcanic Rocks ◽  
Metal Sulfides ◽  
Early Paleozoic ◽  
Extensive Area ◽  
Sea Floor ◽  
Pillow Lavas ◽  
Intense Deformation ◽  
Sea Floor Spreading ◽  
Harbour Area

In the Moretons Harbour area, at the eastern end of the Lushs Bight terrane of central Newfoundland, the volcanic rocks of the "Lushs Bight Supergroup" are divided into two new groups, viz, the Moretons Harbour Group and the Chanceport Group. The former is separable into four formations, consisting primarily of variable proportions of basaltic pillow lavas and volcanoclastic sediments, with a composite thickness in excess of 6 km, or around 8 km including an extensive area of 'sheeted' diabase dikes. These formations are steeply dipping and face southwest; they are separated by the Chanceport fault from the Chanceport Group to the south. The latter consists of interbedded basaltic pillow lavas with graywackes and banded red and green cherts, all facing north and steeply dipping to overturned, with a composite thickness of approximately 3 km.The Moretons Harbour Group has been intruded by the Twillingate trondhjemitic granite–granodiorite pluton and abundant basic dikes intrude the granite, indicating that the mafic and felsic magmatism were coeval. Both have undergone intense deformation and the volcanics show a change from greenschist to amphibolite facies mineralogy within a distance of 2 km on approaching the pluton, a result of buttressing by the pluton during deformation, and not an intrusive effect.Base metal sulfides are common throughout the area, but the main occurrences of Cu, As, Sb, and Au are concentrated in the Little Harbour Formation, a 2600 m thick sequence of volcanoclastic rocks within the Moretons Harbour Group.The great thickness of volcanic rocks is interpreted as having formed in an island arc environment, although it is possible that the lowermost parts of the sequence represent oceanic crust. It is unlikely that the sheeted diabases of the Moretons Harbour area were produced by sea-floor spreading.

Accretionary processes and stratigraphic reconstruction of Neoproterozoic oceanic crust in North Wales, UK

2020 ◽  
Author(s):  
Niall Groome ◽  
David Buchs ◽  
Åke Fagereng ◽  
Margaret Wood ◽  
Stewart Campbell ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Regional Scale ◽  
Ocean Floor ◽  
Sea Floor ◽  
Structural Controls ◽  
Small Series ◽  
North Wales ◽  
Intraplate Magmatism ◽  
Detrital Zircon Ages ◽  
Ocean Ridge

<p>Extending across Anglesey and Llŷn Peninsula in North Wales, UK, the Mona Complex is a collection of Neoproterozoic-Cambrian units formed through the collision of the Iapetus oceanic plate with the Avalonian microcontinent [1].  One of these units, the Gwna Complex, represents accreted ocean floor material that is largely characterised as a regional-scale tectonic mélange.  Detrital zircon ages in terrigenous sediments suggest that subduction occurred around 600-540 Ma [2].  Accreted sequences of volcanics, pelagic sea floor sediments and turbidites can be used to reconstruct the history, stratigraphy and origin of the ancient ocean floor, whilst the presence of these different lithologies also have major influences on structural controls of accretion.</p><p>In Newborough, Anglesey, sub-greenschist (T < 300°C) Gwna Complex material has been accreted in the form of imbricated semi-coherent lenticular slices 5 – 200 m thick with a subvertical orientation.  Large volumes of terrigenous sediment (turbidite-derived muds and fine sands) are present elsewhere in the Gwna Complex, acting as the mélange matrix, incorporating blocks of stronger, more brittle surrounding units.  In Newborough, however, the Gwna Complex has experienced comparatively little terrigenous input, localising mélange formation to metre-scale layers towards the upper unit interfaces.  This leads to the semi-coherent preservation of ocean floor stratigraphy.  Highly foliated hyaloclastite layers within thick volcanic sequences were exploited as weak horizons during accretion, allowing relatively thick, coherent volcanic sequences to be preserved.  Hyaloclastites typically make up to basal unit of lenticular slices.</p><p>Lenticular units record a stratigraphy consisting of relatively undeformed pillow basalts with intermittent hyaloclastite horizons, grading upwards into peperites and then carbonates as sea floor sedimentation becomes more prominent. Overlying layers of pelagic cherts and terrigenous turbiditic sediment are typically more dismembered and mélange formation is localised within turbiditic sediment, and rarely within clast-poor hyaloclastites.  The geochemistry of pillow basalts and associated volcanics from throughout the Gwna Complex is similar, albeit not identical, to typical modern MORB.  This suggests that the volcanics originated from a mid-ocean ridge source, with overlying sediments accumulating on the sea floor representing different stages in the life cycle of the oceanic crust leading up to subduction and accretion.  A small series of accreted sills and related amygdalar hyaloclastites that occur in Newborough show a distinct OIB signature and are likely related to a later episode of minor intraplate magmatism.</p><p>References:</p><p>[1] Horák J et al. (1996) J Geol Soc London 153: 91-99</p><p>[2] Asanuma H et al. (2017) Tectonophysics 706-707: 164-195</p>

Pumpellyite from the oceanic crust, DSDP/ODP Hole 504B

1999 ◽  
Vol 63 (6) ◽  
pp. 891-900 ◽  
Author(s):  
H. Ishizuka
Keyword(s):  
Costa Rica ◽  
Oceanic Crust ◽  
Hydrothermal Alteration ◽  
Fluid Phase ◽  
Plagioclase Phenocryst ◽  
Chemical Compositions ◽  
Oceanic Ridge ◽  
Sea Floor ◽  
Fine Grained ◽  
Compositional Variations

AbstractPumpellyite has been found in doleritic basalt of a sheeted dyke complex drilled from 2072.1 m below sea floor in DSDP/ODP Hole 504B, south of the Costa Rica Rift, eastern Pacific. It occurs as fine-grained crystal aggregates accompanied by albite, chlorite and chalcopyrite, which partially replace a plagioclase phenocryst (An85–88) that is also associated with primary magnetite. Chemical compositions of the pumpellyite vary antithetically in relation to Fe* and Al as well as Fe* and Mg, indicating the dominant substitution of Fe3+ by Al with the minor substitution of Fe2+ by Mg. Such compositional variations overlap with those of prehnite-pumpellyite facies rocks dredged from other oceanic ridges and intra-oceanic arcs, and those of similar facies rocks from ophiolites, but are aluminous compared with those of zeolite facies metabasites in ophiolites. These observations suggest that the breakdown of the plagioclase phenocryst and magnetite in the presence of a Cu- and S-bearing fluid phase led to the formation of pumpellyite + albite + chlorite + chalcopyrite during oceanic ridge hydrothermal alteration.

CORRELATING SEA‐SURFACE AND AERIAL GRAVITY MEASUREMENTS

Geophysics ◽  
1966 ◽  
Vol 31 (1) ◽  
pp. 264-266 ◽  
Author(s):  
Stephen Thyssen‐Bornemisza
Keyword(s):  
Ground Surface ◽  
Sea Surface ◽  
Surface Gravity ◽  
Sea Floor ◽  
Gravity Measurements ◽  
Vertical Gradients

When sea‐surface gravity observations were supplemented by corresponding values from the airborne meter, average vertical gradients of gravity could be computed. In a borehole these gradients are observable by moving the borehole gravity meter up and down to another level (Thyssen‐Bornemisza, 1963, 1964, 1965a), but measurements taken on two horizontal profiles separated by the constant vertical interval h could furnish only relative gradient values or variations in the profile direction. Of course, gravity profiles on the ground surface or the sea floor can be likewise supplemented by aerial observations.

Thermal aspects of sea-floor spreading and the nature of the oceanic crust

1973 ◽  
Vol 18 (1-2) ◽  
pp. 1-17 ◽  
Author(s):  
Y. Bottinga ◽  
C.J. Allegre
Keyword(s):  
Oceanic Crust ◽  
Sea Floor ◽  
Sea Floor Spreading

Density and porosity of the upper oceanic crust from seafloor gravity measurements

2000 ◽  
Vol 27 (7) ◽  
pp. 1053-1056 ◽  
Author(s):  
H. Paul Johnson ◽  
M. J. Pruis ◽  
D. Van Patten ◽  
M. A. Tivey
Keyword(s):  
Oceanic Crust ◽  
Gravity Measurements

scholarly journals Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka

2011 ◽  
Vol 3 (1) ◽  
pp. 453-476
Author(s):  
G. Delisle
Keyword(s):  
Heat Flow ◽  
Oceanic Crust ◽  
High Heat ◽  
Great Depth ◽  
Alternative Interpretation ◽  
High Heat Flow ◽  
Flow Measurements ◽  
Sea Floor ◽  
Erosion Rates ◽  
Bathymetric Data

Abstract. Heat flow measurements were carried out in 2009 offshore Kamchatka during the German-Russian joint-expedition KALMAR. An area with elevated heat flow in oceanic crust of Cretaceous age – detected ~30 years ago in the course of several Russian heat flow surveys – was revisited. One previous interpretation postulated anomalous lithospheric conditions or a connection between a postulated mantle plume at great depth (> 200 km) as the source for the observed high heat flow. However, the positive heat flow anomaly – as our bathymetric data show – is closely associated with the fragmentation of the western flank of the Meiji Seamount into a horst and graben structure, initiated during descend of the oceanic crust into the subduction zone offshore Kamchatka. This paper offers an alternative interpretation, which connects high heat flow primarily with natural convection of fluids in the fragmented rock mass and, as a potential additional factor, high rates of erosion, for which evidence is available from our collected bathymetric image. Given high erosion rates, warm rock material at depth rises to nearer the sea floor, where it cools and causes temporary elevated heat flow.

Sign in / Sign up

Export Citation Format

Share Document