Detailed volcanic geology of the MARNOK area, Mid-Atlantic Ridge north of Kane transform

Abstract

We present the results of a detailed study of the volcanic geology of the median valley floor of the Mid-Atlantic Ridge in two adjacent but contrasting spreading segments immediately north of the Kane transform fault, based on a high-resolution deep-towed sidescan sonar (TOBI) survey, near-bottom photographic traverses, and geochemical analyses from 28 precisely sited sets of dredge samples. This has allowed us to assess the relative importance of small (intersegment) and large (transform) boundaries on magmatic processes at slow spreading ridges.We find that the petrological effect of the transform (so-called 'transform fault effect') is minor compared to the manifestation of the second-order segmentation. The southern, 'narrowgate'-type segment has a poorly developed neovolcanic zone, which tapers towards its 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. There is a striking variation in the degree of fractionation along the length of this segment: primitive lavas were sampled at the centre, while more fractionated basalts, showing a greater range of parental compositions, form discrete volcanic edifices 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 its robust axial volcanic ridge. The along-segment trend of increasing MgO towards the bathymetric crest of the axial volcanic ridge is similar to, but less well-defined than, that for the southern segment.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 focused 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 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.

Detailed bathymetry and magnetic anomaly in the Central Ryukyu Arc, Japan: implications for a westward shift of the volcanic front after approximately 2.1 Ma

“…The largest one with a crater, at 27°22′ N, 127°39′ E, is approximately 500 m high (Figure 2b). Just west of these volcanoes, hummocky terrain (H1 in Figure 2a) similar to those often found in the axial areas of mid-ocean ridges (e.g., Lawson et al 1996) is observed. Many small conical structures less than 1 km in diameter are also observed in this area (Figure 2b).…”

Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

“…2-2). The geometry is of a draped survey performed by the RSS Charles Darwin towing the TOBI (Towed Ocean Bottom Instrument) system across the rift valley of the MidAtlantic Ridge north of the Kane transform in 1991 [Lawson et al, 1996]. A magnetic reversal pattern, in accordance with appropriate spreading rates is imposed on a 500 m thick layer, as is an area of strong magnetization at the neovolcanic axis (central anomaly magnetic high-CAMH).…”

“…Figure 2-3: Deep-tow magnetic profile across the MAR (24°20'N) [Lawson et at., 1996]: Panels arranged as in Figure 2-2. The neovolcanic zone coincides with the magnetic high in both solutions.…”

“…Seafloor observations [e.g., Ballard and Van Andel, 1977;Chadwick et al, 1995;Lawson et al, 1996] and kinematic models [Cann, 1974;Dewey and Kidd, 1977;Kidd, 1977;Palmason, 1980;Hooft et al, 1996] have established that the bulk of extrusive crust is built by dike injection and lava extrusion within a neovolcanic zone at the spreading axis, be it within a summit trough (EPR) or along a median valley ridge (MAR). However, the lateral deposition of lava flows produces a zone of volcanic accretion that is wider than the off-axis extent of dike emplacement.…”

“…The survey included several near-bottom side-scan and magnetometer transects across the median valley using the Towed Ocean Bottom Instrument (TOBI) [Allerton et al, 1995] (Figure 3-1). Numerous dredges within the axial valley and ridge-transform intersection (RTI), and two Wide-Angle Seafloor Photography (WASP) [Kleinrock, 1992] camera tows across the valley floor were also performed [Lawson et al, 1996]. The near-bottom profiles lie within the MARNOK area and provide an excellent opportunity to study the fine-scale magnetic structure of ocean crust in relation to seafloor morphology and volcanism, as well as its correlation with sea surface measurements.…”