Transform faults and longitudinal flow below the Midoceanic Ridge

Abstract

Transform faults offset the pipe‐shaped region of partial melting and magma generation below the Midoceanic ridge. Hence if there is flow along the pipe, it will be blocked or at least impeded at major transform faults. There is evidence on the Reykjanes ridge that minor transform faults, with offsets of a few tens of kilometers, may be converted to oblique spreading axes by asthenosphere flow. Quantitative estimates of the extent of blocking are derived from a Parker‐Oldenburg law of plate thickness increasing as the square root of crustal age. There are several kinds of evidence that the subcrustal partial melts generally moving away from long‐wavelength topographic and gravity highs on the Midoceanic ridge (hot spots) are partly blocked at transform faults: (1) elevations of particular isochrons, including the present spreading axis, frequently jump discontinuously across major transform faults (the best examples of this are in the northeast Atlantic); (2) morphology and seismicity change abruptly across the Blanco fracture zone, which separates a ridge influenced by a hot spot (the Juan de Fuca ridge) from one not so influenced (the Gorda ridge); (3) a zone of relatively high magnetic amplitudes is associated with the hot spot influenced Juan de Fuca and central Galapagos ridges (this zone may delineate how much crust was produced by the Fe/Ti‐rich melts of hot spot origin (whether due to distinctive source composition or subsequent fractionation); on both ridges the high‐amplitude magnetic zones are terminated on both ends at transform faults, again suggesting blockage of the hot spot melts); (4) prominent ridges, such as the Mendocino and Charlie Gibbs ridges, may form on the ‘upstream’ side of some fracture zones at certain times during their evolution (such fracture ridges seem to be constructional volcanic piles formed continuously at or near the fracture‐ridge crest junction; they are here interpreted as being due to excess basalt melts, produced from the partial melts ponded on the upstream side of the transform fault; fracture ridges are proposed to record the past intensity of pipe flow and hence hot spot activity); and (5) recent studies by Solomon (1973) show a region of high S wave attenuation and low Q at the southern end of the Reykjanes ridge, below the prominent fracture ridge just north of the Charlie Gibbs fracture zone. Taken together, the various observations support the existence of pipelike flow at shallow depths (up to a few tens of kilometers) below the Midoceanic ridge. The interaction between mantle hot spots and nearby plate boundaries discussed in this paper may produce features, such as the Ninetyeast and Broken ridges, that exhibit a strong plate tectonic fabric (structures parallel or perpendicular to transform faults) but nevertheless attest to special convective processes in the mantle.

The origin of the asymmetry in the Iceland hotspot along the Mid-Atlantic Ridge from continental breakup to present-day

“…In these cases, plume influence to the AR is inhibited not only by plate shear, but also by the large difference in lithospheric thickness across the transform between the RR and AR and the relatively thick thermal lithosphere of the JMMC, which has long been predicted to inhibit mantle flow between ridges (e.g. Vogt and Johnson, 1975). These results strongly support Hypothesis 2, that variations in lithospheric thickness can enhance the asymmetry of plume influence.…”

Rare-Earth, Se, Cr, Fe, Co, and Na Abundances in DSDP Leg 38 Basement Basalts: Some Additional Evidence on the Evolution of the Thulean Volcanic Province

“…These data further suggest that no light RE-enriched mantle component from the Iceland or Jan Mayen mantle plume reached this region of the Iceland-Jan Mayen Ridge at the time of deposition of the Site 348 basalts. The apparent lack of northward Iceland mantle plume flow beneath the ridge has been interpreted to reflect: (1) repeated jumping of the ridge axis, which would tend to prevent the establishment of a relatively steady-state shallow mantle plume flow pattern for the region; and/or (2) damming of shallow mantle plume flow by a ridge offset, such is the present case with the Tjörnes Fracture Zone (Schilling, 1973;Vogt and Johnson, 1975).…”

Three‐dimensional structure of asthenospheric flow beneath the Southeast Indian Ridge

“…Although focused on a much more localized scale than that which we address, Vogt and Johnson [1975] [Klein and Langrnuir, 1987] of adiabatic decompressional melts. In regions that contain significant along-axis flow, our models predict that warmer upper mantle can be advected into the low-viscosity region beneath the ridge axis (Figures 10 and 14).…”