Rifting origin for the vema fracture in the North Atlantic

doi:10.1016/S0012-821X(68)80055-X

Earth and Planetary Science Letters

Volume 5, 1968, Pages 296-300

https://doi.org/10.1016/S0012-821X(68)80055-X Get rights and content

The mid-Atlantic ridge crest is offset more than 300 km along the E-W trending Vema Fracture valley. The trend of the extremeties of this valley outside the region of recent earthquakes, as well as that of apparently inactive transverse valleys to the south, is somewhat south of east. These observations, and the fact that the valleys to the south apparently do not extend across the ridge crest, strongly suggest that a new pattern of ridge growth and sea-floor spreading, represented by the Vema Fracture valley, has been superimposed on an older one. A geometrical model is proposed for the recent development of the Vema Fracture valley, which is shown to be consistent with other geological and geophysical observations in this region.

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Cited by (31)

Does the Mid-Atlantic Ridge affect the distribution of abyssal benthic crustaceans across the Atlantic Ocean?
2018, Deep-Sea Research Part II: Topical Studies in Oceanography
Citation Excerpt :
Our approach highlights widely distributed deep-sea species within Munnopsidae and three more families from one trans-Atlantic expedition (see Riehl et al., this issue for Macrostylidae, Brix et al. (this issue) for Desmosomatidae and Nannoniscidae). The VFZ offsets the crest of the MAR by 320 km (Van Andel et al., 1968, 1971; van Andel, 1969). Between 41°10 ´ and 44°30`W there is a narrow west-east trending trough at approximately 5000 m depth (Van Andel et al., 1971).
A trans-Atlantic transect along the Vema Fracture Zone was sampled during the Vema-TRANSIT expedition in 2014/15. The aim of the cruise was to investigate whether the Mid-Atlantic Ridge (MAR) isolates the abyssal fauna of the western and eastern abyssal basins.
Based on two genetic datasets of Macrostylidae and Desmosomatidae/Nannoniscidae studied by Riehl et al. and Brix et al. in this issue we found that most of the therein-delimitated species were found at only one side of the MAR. We analysed those species of Macrostylidae and Desmosomatidae that were sampled across the MAR and complemented these with one species of a third family: Munnopsidae. With these datasets we were further able to consider the effect of different niche adaptations: Macrostylidae are infaunal (burrowing), Munnopsidae are considered epifaunal with pronounced swimming capabilities and Desmosomatidae and Nannoniscidae are partly able to swim, but are not as well adapted to swimming as Munnopsidae. We concluded that the MAR seems to be a dispersal barrier for the non-swimming Macrostylidae as well as weakly-swimming Desmosomatidae and Nannoniscidae. However, four species of Macrostylidae and Desmosomatidae did cross the MAR, but evidence for regular unrestricted gene flow is still lacking. For the swimming Munnopsidae we were able to detect persistent gene flow across the MAR.
Timing of transverse ridge uplift along the Vema transform (Central Atlantic)
2017, Marine Geology
Citation Excerpt :
It follows that the VTR may have risen close to 10 Ma along the entire length of the transform offset. This could be explained as the effect of a major change of the stress regime, which occurred about 10 Ma at the VTR (van Andel et al., 1969; Fox et al., 1969; Bonatti et al., 2003). Comparison of the Sr isotopic ages of the pelagic limestones encrusting the basement rocks from the lower part of the Vema Lithospheric Section with crustal ages estimated from magnetic anomalies shows limestone ages either close to, or younger than 10–12 Ma even at sites with crustal ages ≫ 10 Ma (Fig. 3).
Transverse ridges are large topographic anomalies running adjacent to slow-slip oceanic transforms. They form due to different processes, including thermal stresses, hydration-dehydration of peridotites, non-linear viscoelastic rheology of the oceanic crust and vertical tectonic motions of lithospheric slivers induced by changes in ridge/transform geometry, causing transpression and/or transtension along the transform boundary. A prominent transverse ridge on the southern side of the Vema transform (Central Atlantic) rose probably between 12 and 10 Ma along the entire length (≈ 320 km) of the transform, exposing a relatively undisturbed section of oceanic lithosphere. We used pelagic limestones encrusting serpentinized peridotites sampled from the lower slopes of the uplifted lithospheric section to date this uplift and define mechanisms of its emplacement. Ages were obtained both by micropaleontology (foraminifera and nannofossils) and by ⁸⁷Sr/⁸⁶Sr isotope ratios. No ages older than ≈ 12 Ma were obtained, even in samples recovered at sites with crustal ages (determined by magnetic anomalies) well over 12 Ma; on the other side, ages as young as 5.6–8.3 Ma were found in clusters of samples collected from the eastern part of the transverse ridge, probably due to mass-wasting episodes that rejuvenated the substratum. These results support the hypothesis that the Vema Transverse Ridge rose between 12 and 10 Ma due to flexural uplift related to transtension along the transform, in line with a general model whereby transverse ridges rise during discrete events as a consequence of changes in ridge-transform geometry.
Paleontological evidence for early exposure of deep oceanic crust on the Vema Fracture Zone southern wall (Atlantic Ocean, 10°45'N)
1992, Marine Geology
We present the results of a paleontological study of samples collected during the Vemanaute cruise on the Vema Fracture Zone southern wall where an almost complete section of oceanic crust is exposed. The three studied samples are poorly lithified yellowish calcareous sediments lying horizontally over gabbros. They contain abundant, generally well-preserved, and relatively diverse tropical late Neogene calcareous nannofloras and planktonic foraminiferal faunas, and yield a few poorly preserved radiolarians. The estimated ages are 8.8–10 Ma; 5.6–6.4 Ma (late Miocene); 4.5–4.2 Ma (early Pliocene), derived from deepsea correlations between microfossil zonations and magnetic stratigraphy. Our results are compared to theoretical ages proposed for the crust in the surveyed area. This leads us to discuss the timing and location of mechanisms responsible for the exposure of the observed section and allows us to emphasize that early morphologies and structures may be preserved during the transform walls migration.
Thermal implications for the evolution of the spitsbergen transform fault
1982, Tectonophysics
Heat flow taken between Svalbard and Greenland reveal three thermal provinces:
- 1.
  (1) the Molloy Ridge within the Spitsbergen Transform,
- 2.
  (2) the Yermak Plateau
- 3.
  (3) the northeastern margin of Svalbard (Nordaustlandet).
The Molloy Ridge is a short spreading segment and the average heat flow is much above the Sclater et al. (1971), cooling curve but agrees with values from the Norwegian-Greenland Sea. An additional zone of intrusion identified by heat flow lies to the northwest of the Molloy Ridge. It straddles both the visible fracture zone and part of the Yermak Plateau. A thermal boundary lies between the warm western segment of the Yermak Plateau and the shelf off Nordaustlandet. If the thermal subsidence of the western Yermak Plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m.y. (approximately 20 m.y. younger than the northeastern segment of the plateau). Plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7. We postulate that the original transform is associated with the Hornsund Fault zone. In response to deviatoric stress across the oblique ridge-transform system, the Nansen Ridge propagated southwestward aborting the old transform trace, and shifted to its present position.
It is suggested that this propagation and migration of the ridge-transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the Western Yermak Plateau. The propagation phenomenon coincides with large-scale Tertiary volcanic activity on Svalbard.
Readjustment and migration of the oblique transform is still taking place. As the transform-ridge system is liberated from continental constraints, the migration rate will diminish as orthogonality is approached.
Evidence for upper cretaceous transform fault metamorphism in West Cyprus
1981, Earth and Planetary Science Letters
Metamorphic rocks of low-pressure/medium-temperature facies occur in West Cyprus as blocks and slivers with mafic and ultramafic screens in high-angle, serpentinite-filled fault zones. A satisfactory explanation for the origin of the metamorphic rocks has previously remained a subject of controversy. The evidence presented here, based on a study of their bulk chemistry, mineralogy and⁴⁰Ar/³⁹Ar geochronology, indicates they were produced by greenschist to amphibolite facies dynamothermal metamorphism of alkalic and tholeiitic mafic rocks and associated sediments at between 83 and 90 m.y. Their field relations show similarities with present-day oceanic fracture zones suggesting that metamorphism occurred within strike-slip faults, some of which were probably extensions of the Arakapas transform. We propose that hot crust generated at an oceanic spreading centre provided the heat for metamorphism when juxtaposed against older, cooler rocks during ridge-ridge transform movements. In addition, shear heating may have been facilitated by the strike-slip faulting and contributed to the total heat available. These interpretations are compatible with many aspects of the broader regional geology of Cyprus, provide new constraints on the early evolution of the Troodos Complex and form the basis of a model for transform fault metamorphism.
The petrochemistry of ophiolite gabbroic complexes: A key for the classification of ophiolites into low-Ti and high-Ti types
1981, Earth and Planetary Science Letters
Ophiolites have been divided into two groups: high-Ti and low-Ti types. These can be discriminated by studying the fractionation trends of both gabbroic complexes (this work) and lavas and dykes [16], particularly in the TiO₂/M.I. diagram. The first type typically shows MORB-like magmas whereas in the second the magma types have a spectrum of composition from mid-ocean ridge basalts to island arc tholeiites and boninite-like magmas often occur.
High-Ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back-arc basin crusts as well as oceanic crust generated during the intermediate and late-stage opening of intraoceanic back-arc basins.
Parental magmas and fractionation processes of low-Ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back-arc basins.

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Rifting origin for the vema fracture in the North Atlantic

The Vema fracture zone in the equatorial Atlantie

J. Geophys. Res.

The intersection between the Mid-Atlantic ridge and the Vema fracture zone in the North Atlantic

J. Mar. Res.

The structure and development of rifted rises

J. Mar. Res.

A new class of faults and their bearing on continental drift

Nature

Mechanism of earthquakes and nature of faulting on mid-oceanic ridges

J. Geophys. Res

Magnetic anomalies over Oceanic ridges

Nature