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- Title
The Mid‐Atlantic Ridge Near 13°20′N: High‐Resolution Magnetic and Bathymetry Imaging.
- Authors
Searle, R. C.; MacLeod, C. J.; Peirce, C.; Reston, T. J.
- Abstract
We describe detailed magnetic and bathymetric studies around 13°N on the Mid‐Atlantic Ridge, a site of extensive detachment faulting. Inversion of closely spaced sea surface magnetic anomalies reveals a disorganized pattern of magnetization, with anomalies younger than anomaly 2 being poorly delineated. The Brunhes anomaly width is highly variable but averages ~60% of that predicted for the regional spreading rate. It is often split, both along and across axis, by apparently reversely magnetized crust. Gaps in the Brunhes anomaly match gaps in the neovolcanic zone inferred from acoustic backscatter. A strong negative magnetization is associated with the oceanic core complex (OCC) at 13°20′N (OCC1320) and is inferred to arise from exhumed old, reversely magnetized lithosphere. The inferred position of the magmatic axis implies ~30% asymmetry of crustal accretion post‐anomaly‐2. Higher spatial resolution magnetic anomalies near the seafloor, measured by autonomous underwater vehicle, are qualitatively similar to earlier deep‐towed data but differ somewhat from the sea surface magnetics. We interpret this mismatch as reflecting the differing sensitivities of the two observing geometries and the existence of a highly heterogeneous topography and magnetization. This suggests that a strongly three‐dimensional structure exists, more compatible with a geodynamic model where neighboring OCCs are not connected but evolve independently. A modeled near‐seafloor profile through OCC1320 shows low positive magnetization below the smooth dome. A second profile, running E‐W between two OCCs, shows high magnetization coinciding with a large seamount, reflecting recent off‐axis volcanism. Measured microbathymetry reveals extensive small volcanic cones on this seamount and confirms previous interpretations of OCC morphology. Plain Language Summary: Slow spreading ridges form new crust both by magma accreting onto the trailing edges of tectonic plates, or by deeper, ductile mantle being pulled up ("exhumed") and becoming rigid, in "oceanic core complexes" (OCCs): a process of "asymmetric accretion." As part of a larger study of OCCs, we made detailed measurements of seafloor topography and variations in the magnetic field at the sea surface and, while other operations were underway, at the seafloor, using a robotic submarine. Measuring topography allows us to map volcanoes and fractures, while processing and modeling magnetic field variations yield the seafloor spreading history by mapping well‐dated reversals of the Earth's field. Rocks magnetized with the current north pointing polarity during the past 800,000 years have a complex pattern and form a narrower band than expected, with significant gaps and asymmetry. The western plate has been accreting almost twice as fast as the eastern plate. The topography and magnetic pattern suggest a complex interplay of accretion by volcanism and exhumation, with volcanic activity jumping from place to place, with crustal stretching along large "detachment faults" (which cut through the entire lithosphere and form OCCs) and many smaller fractures. Thus, crustal development here is thoroughly three‐dimensional. Key Points: Crustal magnetization is heterogeneous and three‐dimensional, with a narrow and discontinuous Brunhes anomalyNormal magnetization broadly matches the side scan sonar‐derived neovolcanic zone, consistent with independent core complex evolutionPost‐anomaly‐2 crustal accretion has been highly asymmetric, faster to the west (core complex side)
- Subjects
MID-Atlantic Ridge; BATHYMETRY; GEOLOGIC faults; MAGNETIC anomalies; MAGNETIC anisotropy
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2019, Vol 20, Issue 1, p295
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2018GC007940