Mid-Atlantic Ridge at 37° N: Geophysical anomalies in the area of Leg 37 drilling

1977 ◽  
Vol 14 (4) ◽  
pp. 664-673 ◽  
Author(s):  
D. I. Ross ◽  
R. K. H. Falconer
Keyword(s):  
Magnetic Anomaly ◽  
Fracture Zone ◽  
Spreading Rate ◽  
Geophysical Data ◽  
Fracture Zones ◽  
Ridge Crest ◽  
Layer 2 ◽  
Mid Atlantic Ridge ◽  
Drilling Site ◽  
Geophysical Anomalies

Geophysical data collected as part of Leg 37 are compiled with more recent data collected for new IPOD drilling site surveys. Bathymetric and magnetic maps covering the area of sites 332–335 are presented. On the basis of magnetic anomaly correlations it is suggested that site 334 is on normal crust between fracture zones A and B and not closer than 15 km to either fracture zone. Magnetic anomaly inversion is applied to a composite profile, extending from the ridge crest out to beyond anomaly 5. It shows a definite change in spreading rate at 4.7 ± 0.5 Ma. Average rates for the periods 0–4.7 Ma and 4.7–10 Ma are 10.2 ± 0.9 mm/yr and 14.0 ± 1.9 mm/yr respectively. The inversion results are consistent with a simple magnetic source layer 2 km thick.

Magnetic anomaly patterns on Mid-Atlantic Ridge crest at 26°N

1977 ◽  
Vol 82 (2) ◽  
pp. 231-238 ◽  
Author(s):  
Bonnie A. McGregor ◽  
C. G. A. Harrison ◽  
J. William Lavelle ◽  
Peter A. Rona
Keyword(s):  
Magnetic Anomaly ◽  
Ridge Crest ◽  
Mid Atlantic Ridge

Metamorphism in the Mid-Atlantic Ridge near 24° and 30° N

1971 ◽  
Vol 268 (1192) ◽  
pp. 589-603 ◽  
Keyword(s):  
Geothermal Gradient ◽  
Greenschist Facies ◽  
Amphibolite Facies ◽  
Fracture Zones ◽  
Ridge Crest ◽  
Transitional State ◽  
Group I ◽  
Ocean Floor Spreading ◽  
Mid Atlantic Ridge ◽  
Group Ii

Metabasites (metabasalts and metagabbros) occur abundantly in association with serpentinites in transverse fracture zones and on walls of the median valley. These metabasites were formed by burial metamorphism probably in deeper parts of the crust and the upper mantle beneath the Ridge crest, and were brought up to the surface of the crust probably by serpentinites rising along fracture zones and by normal faulting along the median valley. The metabasalts are in the zeolite and greenschist facies and a transitional state from the greenschist to the amphibolite facies, whereas metagabbros tend to have been recrystallized at higher temperatures, being in the greenschist and amphibolite facies. Compositionally the metabasites are divided into two groups, I and II. Group I comprises those which retain the approximate composition of the original rocks except for water content, whereas group II comprises those which underwent marked chemical migration, as regards sodium in zeolite-facies rocks and calcium and silicon in greenschist-facies rocks. In rocks of group I, calcic igneous plagioclase remains unaltered, and albite and epidote did not form. This fact, along with the absence of epidote-amphibolite facies rocks, would be due to the low rockpressure during metamorphism. In some rocks of group II, albite and epidote occur. Burial metamorphism takes place probably mainly beneath the Ridge crest where the geothermal gradient is great. The resultant metamorphic rocks are probably of the low-pressure type, and move laterally by ocean-floor spreading to form the main part of the oceanic crust. Contact metamorphic gneisses, probably derived from gabbros, have been found. Some metagabbros were subjected to cataclasis by fault movements along fracture zones and the median valley.

scholarly journals Oceanic fracture zones do not provide deep sections in the crust

1976 ◽  
Vol 13 (9) ◽  
pp. 1223-1235 ◽  
Author(s):  
J. Francheteau ◽  
P. Choukroune ◽  
R. Hekinian ◽  
X. Le Pichon ◽  
H. D. Needham
Keyword(s):  
Oceanic Crust ◽  
Fracture Zone ◽  
Fault System ◽  
Published Data ◽  
Transform Domain ◽  
Average Thickness ◽  
Fracture Zones ◽  
Transform Fault ◽  
The Third ◽  
Layer 2

Data from rock-dredging have often been used to infer that oceanic fracture zones provide a 'window' into layers of the oceanic crust lying at a depth below the surface that is approximately equivalent to the vertical offset of the fracture zone, and thus permit the reconstruction of a crustal stratigraphy for the whole of acoustic layer 2 (commonly considered to have an average thickness of ~2 km) and, in some interpretations, for the upper part of layer 3. Alternatively, it has been suggested that fracture zones are preferential sites of serpentinite mega-dykes differing in composition from layer 3 but containing inclusions of the third layer. The published data indicate that basalts and basaltic rubble are abundant in fracture zones and, on analysis, do not justify the assumptions that have been made. The structure of fracture zones limits the possible extent of crustal sections exposed on their walls. Moreover, it is suggested that rocks of different layers of the lithosphere can be emplaced in the transform domain due to the dynamic of the transform fault system, itself.

Fracture zones on the Mid-Atlantic Ridge between 43° N and 44° N

1970 ◽  
Vol 7 (5) ◽  
pp. 1352-1355 ◽  
Author(s):  
M. J. Keen
Keyword(s):  
Fracture Zone ◽  
Seismic Reflection ◽  
Seismic Reflection Profile ◽  
Fracture Zones ◽  
Sea Floor ◽  
Eastern Flank ◽  
Reflection Profile ◽  
Mid Atlantic Ridge ◽  
Sea Floor Spreading

A seismic reflection profile across the fracture zones on the Mid-Atlantic Ridge between 43° N and 44° N shows that the thickness of sediment increases markedly south of the fracture zone at 43° 05′ N on the eastern flank of the ridge. The thickness there is in accord with observations made west of the ridge in the region of the survey at 45° N. This suggests that the effects of rates of sea-floor spreading and sedimentation have been similar on this eastern part of the ridge to those west of the ridge farther north.

Imaging of fracture zones in the Finnsjön area, central Sweden, using the seismic reflection method

Geophysics ◽  
1995 ◽  
Vol 60 (1) ◽  
pp. 66-75 ◽  
Author(s):  
Christopher Juhlin
Keyword(s):  
Fracture Zone ◽  
Seismic Profile ◽  
Decay Rates ◽  
Damping Factor ◽  
Fracture Zones ◽  
Borehole Data ◽  
Seismic Reflection Method ◽  
Adequate Quality ◽  
Initial Processing ◽  
Geophone Spacing

In 1987 the Swedish Nuclear Fuel and Waste Management Co. (SKB) funded the shooting of a 1.7-km long, high‐resolution seismic profile over the Finnsjön study site using a 60‐channel acquisition system with a shotpoint and geophone spacing of 10 m. The site is located about 140 km north of Stockholm and the host rocks are mainly granodioritic. The main objective of the profile was to image a known fracture zone with high hydraulic conductivity dipping gently to the west at depths of 100 to 400 m. The initial processing of the data failed to image this fracture zone. However, a steeply dipping reflector was imaged indicating the field data were of adequate quality and that the problem lay in the processing. These data have now been reprocessed and a clear image of the gently dipping zone has been obtained. In addition, several other reflectors were imaged in the reprocessed section, both gently and steeply dipping ones. Correlations with borehole data indicate that the origin of these reflections are also fracture zones. The improvement over the previous processing is caused mainly by (1) refraction statics, (2) choice of frequency band, (3) F-K filtering, and (4) velocity analyses. In addition to reprocessing the data, some further analyses were done including simulation of acquisition using only the near‐offset channels (channels 1–30) and the far‐offset channels (channels 31–60), and determining the damping factor Q in the upper few hundred meters based upon the amplitude decay of the first arrivals. The data acquisition simulation shows the far‐offset contribution to be significant even for shallow reflectors in this area, contrary to what may be expected. A Q value of 10, determined from observed amplitude decay rates, agrees well with theoretical ones assuming plane wave propagation in an attenuating medium.

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