North Atlantic crustal structure
Springer Online Journal Archives 1860-2000
Abstract The Kane Fracture Zone probably is better covered by geophysical survey data, acquired both by design and incidentally, than any other fracture zone in the North Atlantic Ocean. We have used this data to map the basement morphology of the fracture zone and the adjacent crust for nearly 5700 km, from near Cape Hatteras to the middle of the Mesozoic magnetic anomalies west of Cap Blanc, northwest Africa. We use the trends of the Kane transform valley and its inactive fracture valley to determine the record of plate-motion changes, and we interpret the basement structural data to examine how the Kane transform evolved in response to changes in plate motion. Prior to about 133 Ma the Kane was a small-offset transform and its fracture valley is structurally expressed only as a shallow ( 〈 0.5 km) trough. In younger crust, the offset may have increased to as much as 190 km (present offset 150 km) and the fracture valley typically is up to 1.2 km deep. This part of the fracture valley records significant changes in direction of relative plate motion (5°–30°) near 102 Ma, 92 Ma, 59 Ma, 22 Ma, and 17 Ma. Each change corresponds to a major reorganization of plate boundaries in areas around the Atlantic, and the fracture-zone orientation appears to be a sensitive recorder of these events. The Kane transform has exhibited characteristic responses to changes in relative plate motion. Counterclockwise plate-motion changes put the left-lateral transform offset into extension, and the response was for ridge tips at the ridge-transform intersections to propagate across the transform valley and against the truncating lithosphere. Heating of this lithosphere appears to have produced uplift and formation of a well developed transverse ridge that bounds the inactive fracture valley on its older side. The propagating ridge tips also rotated toward the transform fault in response to the local stress field, forming prominent hooked ridges that now extend into or across the inactive fracture valley. Clockwise (compressional) changes in relative plate motion produced none of these features, and the resulting fracture valleys typically have a wide-V shape. The Kane transform experienced severe adaptions to the changes in relative plate motion at about 102 Ma (compressional shift) and 92 Ma (extensional shift), and new transform faults were formed in crust outside the contemporary transform valley. Subsequently, the transform offset has been smaller and the rates of change in plate motion have been more gradual, so transform-fault adjustment has been contained within the transform valley. The fracture-valley structure formed during extensional and compressional changes in relative plate motion can be decidedly asymmetrical in conjugate limbs of the fracture zone. This asymmetry appears to be related to the ‘absolute’ motion of the plate boundary with respect to the asthenosphere.
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