Changes in the axial configuration of the East Pacific Rise near 6°S during the past 2 m.y.

Abstract

The axis of the East Pacific Rise (EPR) in the area of detailed survey, 5°−7°S, is generally marked by a topographic block about 300 m high and 15–20 km wide. This axial block trends 14° through much of the survey area, but at 5.8°S it changes to a regional trend of 22° which continues to the north. A 200-m-high step (downdropped to the west) lies subparallel to and 20 km west of that part of the axial block which trends 14° and dies out to the north where the axial trend changes. Large fault blocks 1000 m high and 6–10 km wide occur 175 km west of the axis in the survey area, and a topographic depression of similar dimensions occurs 80 km east of the rise axis. Identification of magnetic anomalies here on the magnetic equator is complicated by the electrojet effect which produces a large and irregular diurnal variation in the earth's magnetic field. The few identifications possible indicate a whole spreading rate of 153 mm/yr for the EPR at 6° S. Mapped anomalies reveal a trend of 42° for the Gilsa anomaly, a 20° deviation from the present regional axis trend, and show the present topographic axis to be 10 km west of a central position between the flanking Brunhes/Matuyama anomalies. At about 6.5°S the rise axis is offset 55 km right laterally along a fracture zone which is poorly defined bathymetrically. A 1000-γ magnetic anomaly trends 108° along the fracture zone and coincides with its active portion. Both the bathymetric and the magnetic expressions of the fracture zone terminate a few tens of kilometers outside the offset axes, suggesting a change in the process that formed the fracture zone a few hundred thousand years ago. These data, combined with the areal distribution of earthquake epicenters, suggest that (1) during the past 1.7 m.y. the trend of the spreading axis has changed from highly oblique to more orthogonal to the spreading direction, (2) during the past 0.69 m.y. the spreading axis has shifted 10 km to the west, and (3) at present, that portion of the rise axis trending 14° is unstable and is in the process of reorienting to the regional trend of 22°. The westward shift of the rise axis may have occasioned the change in the morphologic and magnetic signatures of the 6.5°S fracture zone.