Paleomagnetic studies of the geometry and kinematics of faulting at the slow-spreading mid-Atlantic ridge (MARK area).

摘要

The Mid-Atlantic Ridge (MAR) south of the Kane Transform (the MARK area) is divided into three segments with distinct morphologies, a northern asymmetric segment abuts the Kane Transform, a central symmetric segment and a southern asymmetric segment. The evolution of slow-spreading ridges and the particular problem of quantifying fault-block rotations (commonly predicted to be associated with mechanical extension) is addressed by integrating paleomagnetic, structural and morphologic data. This approach is applied to the northern and southern segments where mechanical extension is believed to play a dominant role in seafloor spreading.;The evolution of the crustal section exposed along the eastern median-valley margin (in the SMARK area) was studied during manned submersible dives. The first paleomagnetic results from oriented dike samples collected on the MAR were combined with photogeology and Sea Beam bathymetry to conclude that recent spreading in this segment has been predominantly mechanical.;Magnetic susceptibility and remanence measurements on serpentinites from the northern MARK area reveal a strong magnetic anisotropy characterized by an oblate ellipsoid. The close correspondence between these ellipsoids and the preferred orientation of magnetite grain-clusters in serpentine veins suggests the magnetic "fabric" reflects the orientation of these veins. Corrected remanent inclinations were used to determine the applied field directions at the time of acquisition and thereby orient the cores and all structural features within them. These results indicate that there has been no rotation of the cored section since the acquisition of remanence.;The detailed study of the asymmetric segments of the MARK area suggest a general model for asymmetric ridge segment evolution in which crustal accretion results from the complex interplay of diffuse volcanism and laterally discontinuous faulting within the median valley. The lack of a discrete sheeted dike complex and the widely distributed sites of volcanism within the median valley indicate that these ridges cannot be characterized by models involving uniform magmatism. Paleomagnetic data indicates that the median valley is characterized, structurally, by small discontinuous faults that can juxtapose rotated and non-rotated blocks. However, the large scale separation of plates is dominated by non-rotational faulting, perhaps along a simple planar fault zone.