Journal of Conference Abstracts

High-resolution TOBI Study of the Broken Spur Segment, Mid-Atlantic Ridge, 29°N

R. C. Searle Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

R.C.Searle@durham.ac.uk

N. C. Mitchell Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

J. Escartín Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

A. P. Slootweg Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

S. Russell Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

P. A. Cowie Department of Geology and Geophysics, University of Edinburgh, West Mains Road,

Edinburgh, EH9 3JW, UK

S. Allerton Department of Geology and Geophysics, University of Edinburgh, West Mains Road,

Edinburgh, EH9 3JW, UK

C. J. MacLeod Department of Earth Sciences, University of Wales College of Cardiff, P.O. Box 914,

Cardiff, CF1 3YE, UK

T. Tanaka Department of Earth Sciences, Faculty of Science, Chiba University,

1-33 Yayoi-cho Inage-ku Chiba, JAPAN 263

C. Flewellen Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK

I. Rouse Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK

The survey

In March-April 1996, on RRS Charles Darwin cruise CD99, we used an upgraded version of the Southampton Oceanography Centre's deep-tow vehicle TOBI (Towed Ocean Bottom Instrument) to conduct a comprehensive survey of the Broken Spur spreading segment (28°45' - 29°15'N) on the slow spreading Mid-Atlantic Ridge. The survey covered approximately the southern two-thirds of the segment, including all of the second-order non-transform offset at its southern end, and extended approximately 35 km off-axis (~2.8 Ma) to both east and west. Track lines were east-west and spaced 2 km apart, yielding near 100% side-scan sonar coverage in each of two scan-directions (north and south). We are thus able to examine the tectonic and magmatic development from segment centre to segment end and to assess any E-W asymmetry in these processes, e.g. from inside-corner to outside-corner.

Instrumentation

TOBI carried the following sensors: a 30 kHz side-scan sonar with 3 km range and ~6 m resolution; 7 kHz sub-bottom profiler; CTD; three-component fluxgate magnetometer with gyro-compass and pitch-and-roll meters for attitude determination; and phase-difference swath bathymetry system. The magnetometer and phase bathymetry were newly developed for this study. The magnetometer worked extremely well, and produced excellent three-component data after corrections for the vehicle's attitude and its own permanent field and heading-dependent induced magnetic field. The bathymetry system, still under development at the time of the cruise, produced phase data, albeit somewhat noisy, to ranges of about 1.5 km. We anticipate being able to recover high-resolution bathymetry from these phase data after further processing. In addition to TOBI, we ran a Simrad EM12 hull-mounted multibeam echosounder, surface-towed proton precession magnetometer, shipboard three-component magnetometer (Isezaki, 1986), and LaCoste-Romberg gravitymeter.

Preliminary conclusions

The following are preliminary conclusions based on an initial examination of the partially, shipboard processed data, especially the TOBI side-scan, Simrad bathymetry and magnetics. In particular, the sidescan mosaics have provided very high resolution images of seafloor faults, including clear details of their evolution, cross-cutting relationships, and mass-wasting. They also allow us to readily distinguish fault scarps from steeply dipping volcanic terrain.

We confirm the suggestion of Shaw (1992) that the segment centre is characterised by relatively symmetric, closely-spaced, small-throw, predominantly inward-facing normal faults, whereas the segment end is asymmetric and characterised on the inside corner by relatively widely-spaced, large throw normal faults. Faults may extend for substantial proportions of the segment length, but often show evidence of growth from linkage of shorter, initially independent faults. We find the degree of asymmetry to be extreme at the segment end, with apparent down-flexing of the eastern plate toward the first major normal fault on the west, and relatively minor, mainly inward-facing faulting on the down-flexed plate on the outside corner. The inside corner faulting is of an entirely different order from that elsewhere in the segment.

Outward facing faults appear rare: even where there are steep bathymetric slopes, the side-scan usually fails to show typical fault morphology, and where there are outward facing faults they are usually relatively short.

On the inside corner side, faults tend to terminate by bending into the direction of offset forming a hooked shape. Some of the faults from mid-segment terminate well before the non-transform offset (NTO) by hooking round to join adjacent ones, thus increasing the fault spacing as the segment end is approached. The inside corner high appears to be bounded by an extreme version of one of these hooked faults, although extensive mass-wasting here tends to obscure the actual fault plane.

The NTO is marked by regional-scale swinging of fault azimuths from near N-S to NE-SW, and development of some large, linear NE-SW faults in the offset region, very similar to those seen at Kurchatov Fracture Zone (Searle and Laughton, 1977). No significant spreading-parallel faults occur there, despite some steep E-W slopes in the bathymetry.

We confirm the observation (Sempéré et al., 1993; Pariso et al., 1995) that the current axial volcanic ridge is situated highly asymmetrically and obliquely with respect to both the median valley floor and all the magnetic reversal boundaries in the area. It lies close to the western wall of the median valley, and converges obliquely to intersect the wall just north of the segment centre. It is also close to the western side of the Brunhes normal magnetisation block. The 'neovolcanic zone' as defined by high acoustic backscatter has a gradational boundary to the east, but terminates abruptly against the first valley wall scarp to the west.

The tectonic structures also bear witness to a history of substantial segment evolution. If we take the southern limit of near N-S, ridge-parallel faulting, as the northern edge of the NTO, we see this has migrated south, north, and now south again during the 2.8 Ma represented by our survey. This is supported by evidence of NE-SW NTO faults cross-cutting inferred older, N-S segment-centre faults and volcanic terrain, and vice versa. It also appears that the southern edges of the two major segment-end fault blocks identified by Shaw (1992) represent the present and a past position of the segment boundary; we have also identified a third such feature, and find they occur at roughly 0.5 Ma intervals.

References

Isezaki, N., Geophysics 51, 1992-1998 (1986).

Pariso, J.E., Sempéré, J.-C. & Rommevaux, C., J. Geophys. Res. 100, 17,787-17,794 (1995).

Searle, R.C. & Laughton, A.S., J. Geophys. Res. 82, 5313-5328 (1977).

Sempéré, J.-C., Lin, J., Brown, H.S., Schouten, H. & Purdy, G.M., Mar. Geophys. Res. 15, 153-200 (1993).

Shaw, P., Nature 358, 490-493 (1992).