Bathymetric segmentation and faulting on the Mid-Atlantic Ridge, 24°00'N to 24°40'N

Allerton, S; Searle, R C; Murton, BJ

Abstract

Two segments in the MARNOK area (Mid-Atlantic Ridge North of Kane) have been investigated with an integrated program of TOBI high-resolution sidescan sonar, deep tow photoanalysis, and dredging. The petrological effect of the transform is minor compared to the manifestation of the second order segmentation. The southern, narrowgate-type segment has a poorly developed neovolcanic zone (NVZ) which tapers towards the bathymetric centre where continuous faulting causes rapid dismemberment. Flat-topped seamounts are preferentially located at the ends of the segment, particularly near the non-transform offset (NTO). There is a striking variation in the degree of fractionation along the length of the narrowgate segment. Primitive lavas are sampled at the centre, while more fractionated basalts, showing a greater range of parental compositions form discrete volcanic edifaces at the ends. In contrast, the northern segment has a wider inner valley and syn- magmatic faults extend up to 15km along the crest of the robust neovolcanic ridge (NVR). The along-segment trend of increasing MgO towards the bathymetric crest of the NVZ is noisier, but similar to that of the narrowgate segment, being less well constrained because of low sampling density. There is no variation in the bulk degree of melting along the segments as determined from the major element geochemistry. Hence melt migration is favoured over mantle upwelling as the main cause of the crustal thickness variations which define second order segmentation. Radiogenic isotopes show only very small variations with no systematic pattern emerging within or between the ridge segments. The isotopic irregularities are not related to the variations in incompatible elements implying that the latter result from the action of dynamic melting processes rather than from long-lived source heterogeneity. A model is developed which relates the volcanology, tectonic style and the geochemical trends to the episodicity of magma supply and eruption.

Citation

Allerton, S., Searle, R. C., & Murton, B. (1996). Bathymetric segmentation and faulting on the Mid-Atlantic Ridge, 24°00'N to 24°40'N. In C. MacLeod, P. Tyler, & C. Walker (Eds.), Tectonic, Magmatic, Hydrothermal and Biological Segmentation of Mid-Ocean Ridges (49-60). The Geological Society

• Publication Date: 1996
• Publisher: The Geological Society
• Pages: 49-60
• Series Number: Special Publication 118
• Book Title: Tectonic, Magmatic, Hydrothermal and Biological Segmentation of Mid-Ocean Ridges