Large-scale oblique features in an active transform fault,the Wilkes fracture zone near 9°S on the East Pacific Rise |
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| Authors: | Charles L Kureth Jr David K Rea |
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| Institution: | (1) Department of Geological Sciences, The University of Michigan, 48109 Ann Arbor, Michigan, U.S.A.;(2) Oceanography Program, Department of Atmospheric and Oceanic Science, The University of Michigan, 48109 Ann Arbor, Michigan, U.S.A.;(3) Present address: the Traverse Group Inc., 2480 Gale Road, 48105 Ann Arbor, Michigan, USA |
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| Abstract: | The Wilkes fracture zone offsets the East Pacific Rise about 200 km right-laterally near 9°S. The bathymetric expression of the fracture zone ranges from a simple slope or step along its inactive extension to a 100 km wide zone of oblique structural features in the active portion. A low ridge 200 to 300 m high, 5 to 15 km wide and 185 km long is the dominant oblique structure; it trends 23° north of the main transform trend. A high-amplitude magnetic anomaly trends 097° along the southern part of the active portion and apparently marks the main transform direction. The structurally simple, inactive portions of the Wilkes fracture zone trend 105°. Plots of epicenter locations reveal two groupings of earthquakes, one along an 082° trend in the central part of the fracture zone, and a cluster near the southwestern fracture zone — spreading center intersection.Taken together the data suggest that some event, other than a shift in the Nazca-Pacific pole of rotation, occurred 0.9 m.y. ago to change the Wilkes fracture zone from a simple fault to a complex zone of shearing. Since that time the long oblique ridge, probably the surface expression of a Riedel shear, was formed. At present the entire 200 km long, 100 km wide region between the offset axes is seismically active, but transform motion may be largely confined to the southern margin of the active zone, coincident with the high-amplitude magnetic anomaly there. |
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