HR: 08:00h
AN: T51F-01 INVITED    [Abstracts]
TI: Development of Oceanic Core Complexes on the Mid-Atlantic Ridge at 13-14N: Deep-Towed Geophysical Measurements and Detailed Seafloor Sampling
AU: * Searle, R
EM: r.c.searle@durham.ac.uk
AU: MacLeod, C
EM: macleod@cardiff.ac.uk
AU: Murton, B
EM: bjm@noc.soton.ac.uk
AU: Mallows, C
EM: Christopher.Mallows@durham.ac.uk
AU: Casey, J
EM: jfcasey@uh.edu
AU: Achenbach, K
EM: KAY@uwyo.edu
AU: Unsworth, S
EM: samwise@noc.soton.ac.uk
AU: Harris, M
EM: mh9@noc.soton.ac.uk
AB: The first scientific cruise of research vessel James Cook in March-April 2007 targeted the Mid-Atlantic Ridge at 13-14°N, to investigate details of lithospheric generation and development in a low-magmatic setting. Overall objectives were to 1) investigate the 3D pattern of mantle upwelling and melt focusing; 2) study how plate accretion and separation mechanisms differ between magma-rich and magma-poor areas; and 3) test mechanisms of detachment faulting and extensional strain localisation in the lower crust and upper mantle. Smith et al. (Nature 2006) had shown this to be an area of widespread detachment faulting and formation of oceanic core complexes (OCC), and published bathymetry showed an extensive area of blocky rather than lineated topography, which elsewhere has correlated with areas of low effusive magmatism. We conducted a TOBI deep-towed geophysical survey over a 70 km length of ridge extending to magnetic chron C2n (1.9 Ma) on each flank. This included sidescan sonar and high resolution bathymetry and magnetic measurements on 13 E-W tracks spaced 3 - 6 km apart. The area includes 1 active, 1 dying, and 1 defunct OCC and borders well-lineated, apparently magmatically robust seafloor to the north. The geophysical survey was complimented by recovery of 7 oriented and 18 unoriented core and 29 dredge samples, including some from a probable OCC south of the TOBI survey. Deep-towed sidescan, bathymetry and video show the OCCs typically comprise a steeply outward tilted volcanic ridge marking the breakaway (as suggested by Smith et al., 2006); a high, rugged central massif that is complexly deformed as a result of uplift and bending, and may be separated from the breakaway ridge by what we interpret as a late outward dipping normal fault; and a smooth, corrugated surface that generally dips c. 20° towards the ridge axis at the termination but gradually rotates to horizontal or gently outward dipping near its junction with the central massif. Older OCCs have flatter central areas than the youngest OCC, perhaps as a result of unbending late in their development. They also have steep, ridge-dipping normal faults at or near their terminations that may play a role in ending the detachment faulting. The axial neovolcanic zone is absent or poorly developed opposite the active and dying OCCs, and appears to be propagating towards them, thus playing an important part in terminating OCC activity. We recovered peridotite from the central parts of the three northern OCCs, the median valley walls near 13°00'N, and the neovolcanic ridge near the OCC at 13°20'N. Basalt was recovered from the breakaway ridges and often the central massifs. Most of the corrugated surfaces were extensively covered in sediment and basaltic rubble which prevented us obtaining drill samples, though dredging recovered peridotite, fault rocks and basalt rubble. The toes of the 13°30'N and 13°20'N OCCs showed evidence of extensive and some recent hydrothermal activity. Basalt and fresh glass were recovered from axial volcanic ridges. We recovered very little gabbro. A reasonably coherent magnetic central anomaly is seen in the sea-surface field, but the higher-resolution deep- towed magnetic data reveal a pattern of crustal magnetisation that does not obviously match a simple reversal history, even in areas away from obvious OCCs. We are modelling these results in terms of asymmetric spreading, ridge jumping and tectonic rotations varying in time and space.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3075 Submarine tectonics and volcanism
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting