Anatomy of an Axial Volcanic Ridge: The Mid-Atlantic Ridge at 45°N
Abstract
Study of a single axial volcanic ridge in the Mid-Atlantic Ridge median valley at 45°N has enabled us to construct a detailed volcano-stratigraphic model and thrown new light on the structure and development of AVRs. Data sets include 50 m resolution multibeam bathymetry, comprehensive 3 m resolution deep-towed sidescan sonar, a grid of twenty-two 1.4 km-spaced lines of deep-towed magnetic field measurements, continuous video observations and 270 rock samples from eleven ROV dives, and two approximately 8 km2 areas of very-high-resolution bathymetry and magnetics. A continuous topographic ridge extends ~35 km along the segment, and strikes 010°, ~5-10° CCW of the regional ridge trend. The northernmost 10 km appears older, as attested by lower topographic relief, acoustic backscatter and crustal magnetisation and greater degree of faulting. The rest, which we infer to be most recently constructed, is 25 km long, ~ 4 km wide and ~500 m high. It has a sharp crest, and lateral spurs trending NE that we attribute to tectonic control from the right-stepping MAR axis. The recent AVR is covered by approximately 3000 small (<450 m diameter, 200 m high) circular volcanoes ranging from steep-sided (45°) cones to more rounded domes. They tend to align in rows parallel to the AVR axis, to its NE-trending spurs, or, on its lower flanks, sub-normal to the AVR trend. These latter lineaments, which are spaced 1-2 km apart, comprise short (1-2 km) rows of single cones. We infer that their emplacement is controlled by down-flank magma transport. The AVR itself contains only one volcano >450 m diameter, though about ten, all flat-topped and up to 1.2 km diameter, occur elsewhere on the median valley floor. The high-resolution surveys show all cones >70 m high suffered significant flank collapse, often with near-vertical collapse scars. The active AVR is partly flanked by hummocky volcanic terrain similar to the AVR but of lower acoustic backscatter, which we infer to be older, and partly by flat-lying, mostly sediment-covered low-relief lavas. These typically have low relief lobate surfaces, often with collapse structures and occasional lava tubes. Most appear acoustically dark (implying significant sediment cover and age), though an extensive area off the NE flank is bright and apparently fairly young. The active AVR has open fissures, mostly along its crest, but few other faults are observed directly. Steep, outward-facing slopes draped by elongate pillows may be small normal faults covered by lava. The surrounding median valley floor is heavily fissured. Normal faults cut it and the older part of the AVR, with significant outward facing faults along the NW flank of the old AVR. High crustal magnetization is confined to the active AVR and is highest towards its southern tip. The high-resolution maps show magnetisation highs along the AVR flanks and spurs as well as its crest. This may indicate younger volcanism or thicker accumulations in these areas. Magnetic palaeointensity measurements on a few samples are consistent with ages of up to ~13 ka for samples from the active AVR, and ~15 ka for one sample from the median valley floor.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMOS21B..02A
- Keywords:
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- 3035 MARINE GEOLOGY AND GEOPHYSICS / Midocean ridge processes;
- 3045 MARINE GEOLOGY AND GEOPHYSICS / Seafloor morphology;
- geology;
- and geophysics;
- 8416 VOLCANOLOGY / Mid-oceanic ridge processes;
- 9325 GEOGRAPHIC LOCATION / Atlantic Ocean