Abstract. Magnetic field measurements made by submersible define the cross-sectional geometry of a magnetic polarity reversal boundary and the vertical variation of crustal magnetization in upper oceanic crust. Measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas, and steeper dip in the lower extrusive lavas. This geometry is a consequence of the emplacement of extrusive lava at a midocean ridge. Reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava sequence to the overlying marine magnetic anomaly signal. From the forward modeling, the highly magnetized extrusive lavas contribute the majority (50-75%) of the observed sea surface magnetic anomaly, consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies.

Mantle-derived serpentinized peridotites crop out in a belt approximately 2 km wide and 20 km long along the western median valley wall of the Mid-Atlantic Ridge just south of the Kane Transform in the MARK area. Serpentinites extend southward from extensive exposures of gabbroic rocks near the Kane Transform. The belt crops out along approximately half the length of a well-defined ridge segment parallel to a prominent neovolcanic ridge. It terminates in a segment boundary zone to the south marked by a bathymetric depression whose trace extends obliquely off axis northwestward into older crust where serpentinites have also been dredged. The seΦentinites are considered to have been exposed by extreme lithospheric extension along a major crustal detachment, as suggested for mafic plutonic rocks that crop out as an oceanic "core complex" to the north. Gently dipping metamoΦhic fabrics and fault surfaces in the seΦentinites suggest a similar structural history for these two adjacent areas. Consistently oriented high-and low-temperature fabrics along the length of the belt of seΦentinized peridotites do not support diapiric uplift as a mechanism for the exposure of these exotic rocks. Gabbroic to diabasic intrusions and overlying basaltic lavas suggest that the seΦentinites were exposed by uplift and faulting of upper mantle material that did not develop an extensive overlying magmatic crust as it rose beneath the ridge axis and spread laterally. These types of exposures of hydrated upper mantle material appear to be common elements of oceanic crust formed at slow-spreading ridges with low magma budgets.

Extensive exposures of gabbroic rocks, inferred to represent mid-crustal intervals of the oceanic crust, crop out on the western median valley wall of the spreading segment of the Mid-Atlantic Ridge immediately south of the Kane Transform. Surface exposures of the gabbroic rocks have been mapped by numerous submersible dives, deeply towed camera transects, and sidescan sonar surveys, making this region one of the most intensively studied areas of the mid-ocean ridge system. Recent drilling in this tectonic window at Sites 921-924 provides a new perspective on the architecture and mode of accretion of mid-crustal rocks of the oceanic crust.Surface studies show that the gabbroic rocks crop out in an area that extends at least 15 km along the ridge axis and 8-10 km normal to the ridge axis. The median valley wall of the Mid-Atlantic Ridge here is essentially a dip-slope on a major crustal detachment fault. This low-angle normal fault zone has been cut by numerous steeply dipping normal faults that impart a stairstep morphology to the terrain. Basaltic pillow lavas crop out at the top of the median valley wall and probably represent upper crustal rocks that formed structurally above the gabbroic units. To the east, the median valley floor of the Mid-Atlantic Ridge is dominated by relatively young pillow lavas and an extensive neovolcanic ridge. These lavas overlie the detachment footwall, and are probably the surface expression of dikes and plutonic bodies that cut the detachment at depth.Gabbroic and related rocks recovered from Sites 921 to 924 indicate that the mid-crustal terrain exposed in this tectonic window was assembled as a collage of relatively small (less than about 1 km across) plutons. Individual plutons have been variably deformed and metamorphosed, and subsequently intruded by later plutons and dikes. Thus, whereas surface exposures show evidence of major faulting over the past 0.5 m.y., subsurface data from Ocean Drilling Program cores indicate a more continuous style of synkinematic intrusion and crustal stretching. These processes may be broadly coeval expressions of slow seafloor spreading in this environment. Exposures of similar crustal levels in the adjacent Kane Transform suggest a similar pattern of accretion. This type of spreading may be common at ridge-transform intersections and other places on slow-spreading ridges with very low or episodic magma budgets. 1 Karson, J. A., Cannat, M., Miller, D.J., and Elthon, D. (Eds.), 1997. Proc. ODP, Sci. Results, 153: College Station, TX (Ocean Drilling Program). department

Although the general concept of linear magnetic anomalies generated by seafloor spreading processes is well established, the details of the source distribution responsible for these anomalies remain uncertain. We summarize here magnetic properties from variably altered and deformed olivine gabbro, gabbro, and less abundant troctolite, gabbronorite, and oxide gabbros sampled at four sites drilled on the western median valley wall of the Mid-Atlantic Ridge south of the Kane Fracture Zone. The overall mean natural remanent magnetization (NRM) intensity (1.54 ± 2.6 A/m) and Koenigsberger ratio (8.05 ± 15.2) for these samples suggest that lower crustal gabbros are a significant contributor to marine magnetic anomalies. However, dual magnetic polarities were recorded at all four sites, with apparent polarity reversals sometimes occurring over spatial scales of tens of centimeters. Detailed demagnetization and rock magnetic studies of one such interval suggest that the complex remanence, dual polarities, and the occurrence of spurious well-defined magnetization components are related to production of magnetite during high-temperature alteration and/or cooling in periods of opposite polarity. These complexities, if generally applicable to oceanic gabbros, may reduce the integrated contribution from the gabbroic layer to marine magnetic anomalies.

Drilling at Ocean Drilling Program Sites 920 to 924 recovered core with a diverse set of pervasive structural elements. Site 920 recovered predominantly peridotitic rocks that display an early crystal-plastic fabric overprinted by at least five generations of veins. Sites 921 to 924 recovered gabbros that contain magmatic and metamorphic foliations and lineations developed to varying intensities throughout. Brittle features in the gabbro core include Cataclastic zones, faults, and several generations of veins. The characteristic magnetization direction was used to estimate the in situ orientation of structural features within the core. Although significant uncertainty is associated with the unknown effects of anisotropy and tectonic rotations on the remanent declinations, the corrected attitudes of the dominant foliations at Site 920 dip gently east-northeast, parallel to other observations of seafloor structures in the area. Other vein generations and structural features in the rocks do not have a consistent orientation with respect to each other or a consistent variation with core depth. Sites 921-924 were drilled into a section of mostly gabbroic rocks that typically have complicated magnetic properties, with several remanence components identifiable during demagnetization. Reorientation of the gabbro cores is less certain because of the complexity of the remanent magnetization components, however, many structures in the gabbro from Hole 923A also seem to have gentle dips to the northeast after such a reorientation.