Tectonic Evolution of the Atlantis II Fracture Zone

Abstract

SeaBeam echo sounding, seismic reflection, magnetics, and gravity profiles were run along closely spaced tracks (5 km) parallel to the Atlantis II Fracture Zone on the Southwest Indian Ridge, giving 80% bathymetric coverage of a 30-× 170-nmi strip centered over the fracture zone. The southern and northern rift valleys of the ridge were clearly defined and offset north-south by 199 km. The rift valleys are typical of those found elsewhere on the Southwest Indian Ridge, with relief of more than 2200 m and widths from 22 to 38 km. The ridge-transform intersections are marked by deep nodal basins lying on the transform side of the neovolcanic zone that defines the present-day spreading axis. The walls of the transform generally are steep (25°-40°), although locally, they can be more subdued. The deepest point in the transform is 6480 m in the southern nodal basin, and the shallowest is an uplifted wave-cut terrace that exposes plutonic rocks from the deepest layer of the ocean crust at 700 m. The transform valley is bisected by a 1.5-km-high median tectonic ridge that extends from the northern ridge-transform intersection to the midpoint of the active transform. The seismic survey showed that the floor of the transform contains up to 0.5 km of sediment. Piston-coring at two locations on the transform floor recovered more than 1 m of sand and gravel, which appears to be turbidites shed from the walls of the fracture zone. Extensive dredging showed that more than two-thirds of the crust exposed in the transform valley and its walls were plutonic rocks, principally gabbros and residual mantle peridotites. In contrast, based on dredging and seafloor morphology, only relatively undisrupted pillow basalt flows have been exposed on crust of the same age spreading away from the transform. Magnetic anomalies are well defined out to 11 m.y. over the flanking transverse ridges and transform valley, even where layer 2 appears to be absent. The total opening rate is 1.6 cm/yr, but the arrangement of the anomalies indicates that the spreading for each ridge is asymmetric, with the ridge flanks facing the transform spreading at a rate of 1.0 cm/yr. Such an asymmetric spreading pattern requires that both the northern and southern ridges migrate away from each other at 0.2 cm/yr, thus lengthening the transform at 0.4 cm/yr for the last 11 m.y. To the north, the fracture zone valley is oriented differently from the present-day transform, indicating a paleospreading direction change at 17 m.y. from N10°E to due north-south. This change placed the transform into extension for the 11-m.y. period required for simple orthogonal ridge-transform geometry to be reestablished and produced a large transtensional basin within the transform valley. This basin was split by continued transform slip after 11 m.y., with the larger half moving to the north with the African Plate.

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

“…The spacing of the gravity spikes along IC crust near the MAR axis ranges from 10 to 50 km, (Figure 8), similar to the spacing of fossil topographic highs on the transverse ridges bounding the Atlantis II Transform [Dick et al, 1991] and other MAR transverse ridges [Bonatti and Honnorez, 1976;Bonatti et al, 1983;Pockalny et al, 1988]. If, as proposed here, the gravity spikes are created by faults, the spacing implies that the formation of individual low-angle faults is a transient process that occurs episodically (every -2 m.y.).…”

“…We examine digital multibeam bathymetric and residual gravity data collected over four areas of the slow-spreading MAR Morris and Detrick, 1991; (Figure 1), two areas of the fast-spreading EPR between 7°-9"30'S [Wang and Cochran, 1993] and 8050 '-9°50*N [Madsen et al, 1990], and the very slow-spreading Southwest Indian Ridge (SWIR) near the Atlantis II Transform [Dick et al, 1991; Dick, unpublished data, 1990]. These data sets encompass a total of 8 transforms and 14 nontransform offsets with age offsets from 0.1 to 23 Ma and half-spreading rates from 0.8 to 7.7 cm/yr.…”

“…Significant differences in lithology between IC and OC crust have also been found in dredged rock samples. Upper mantle and lower crustal rocks are frequently dredged at IC crust, suggesting that low-angle faults may locally expose the lower crust and upper mantle there Dick, 1983, 1984;Dick et al, 1991;Mdvel et al, 1991;. In contrast, mantle and lower crustal rocks were rarely found at OC crust or at segment centers .…”

“…This will result in the pinning of the exposed fault surface away from ridge offsets (shaded regions in Figure 15a). Minor transversal faults (thin lines in Figure 15a) could contribute to accommodate some along-axis variations in the strain distribution of low-angle faults [Dick et al, 1991] as earlier studies indicated [Karson, 1991;.The spacing of the gravity spikes along IC crust near the MAR axis ranges from 10 to 50 km, (Figure 8), similar to the spacing of fossil topographic highs on the transverse ridges bounding the Atlantis II Transform [Dick et al, 1991] and other MAR transverse ridges [Bonatti and Honnorez, 1976;Bonatti et al, 1983; Pockalny et al, 1988]. If, as proposed here, the gravity spikes are created by faults, the spacing implies that the formation of individual low-angle faults is a transient process that occurs episodically (every -2 m.y.).…”

The oceanic crust in 3D: Paleomagnetic reconstruction in the Troodos ophiolite gabbro

“…Geophysical surveys, ocean drilling holes, and submersible means provide indications for the present-day lower oceanic crust deformation. For instance, magnetic investigations [2, 5, 6] and structural studies [7,8] of the gabbroic section have shown that the lower crust is occasionally suffer significant rigid body rotation and is mostly decoupled from the upper crust. Yet, these views are either limited in their spatial coverage or provide indirect evidence for deformation.…”