Data report: reorientation of structural features at Sites 920 to 924 using remanent magnetization and magnetic characteristics

Abstract

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.

Serpentinization of mantle‐derived peridotites at mid‐ocean ridges: Mesh texture development in the context of tectonic exhumation

“…They are white on the core cut surfaces (Figure a), commonly sigmoidal in shape, and filled with chrysotile fibers [ Dilek et al ., ; Andreani et al ., ] that grow perpendicular to the vein contacts (Figure c). The reorientation of these V2 veins based on core demagnetization data [ Hurst et al ., ] shows that they have a statistically consistent geometry, with a 30° average dip throughout the 200 m deep Hole 920D. Hole 920B, drilled only 20 m away, shows a similar pattern but is only 100 m deep.…”

“…However, these pieces can rotate about the vertical axis during recovery. A reorientation of these long pieces of core at ODP site 920 was proposed based on paleomagnetic data [ Cannat et al ., ; Hurst et al ., ]. It has significant uncertainties in terms of absolute reorientation in the geographic frame [ Hurst et al ., ], but provides robust constraints on the reorientation of long pieces of core relative to one another.…”

“…Hurst et al . [] further proposed that if the magnetic declination was 0° when the magnetization was acquired, V2 veins in the cored section can be reoriented to a dominant north to northwest strike and an eastward 30° dip (Figure d).…”

“…Serpentine ribbons (fast axis of lizardite subperpendicular to axis of wave plate) and the corresponding “blue” microfractures are parallel to V2 veins (fast axis of chrysotile subparallel to axis of wave plate). (d) Reoriented azimuths and dips of V2 veins in ODP hole 920D based on paleomagnetic data [ Hurst et al ., ]. The proposed absolute reorientation has significant uncertainties [ Hurst et al ., ].…”

“…[]), from the Southwest Indian Ridge at 62°E–65°E [ Sauter et al ., ] and a set of drilled samples from ODP Site 920 at 23°N on the Mid‐Atlantic Ridge (ODP Leg 153 [ Cannat et al ., ]), and from ODP Site 1274 at 15°N also on the MAR (ODP Leg 209 [ Kelemen et al ., ]). Some of these drilled samples have been partially oriented using paleomagnetic measurements [ Hurst et al ., ]. Previous sample studies of serpentinization at slow ridges have focused on mineralogical and geochemical aspects to unravel successive phases of serpentinization and veining [ Dilek et al ., ; Andreani et al ., ; Früh‐Green et al ., ; Bach et al ., ], to constrain element mobility and water‐rock exchanges during serpentinization [ Paulick et al ., ; Boschi et al ., ] and to study the magnetic properties of variably serpentinized peridotites [ Oufi et al ., ; Bach et al ., ].…”