MAGNETIC AND TECTONIC STUDIES OF THE DUELING PROPAGATING SPREADING CENTERS AT 20-DEGREES-40'S ON THE EAST PACIFIC RISE - EVIDENCE FOR CRUSTAL ROTATIONS

We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20-de grees-40'S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/V Moana Wave during which conti nuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly-2-a ged crust. A three-dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distributio n was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the, recent history of the discontinuity. T he results indicate consistent asymmetric spreading faster to the east , discontinuous high magnetizations in the discordant zone associated with the discontinuity, and Southward migration of the feature at a ra te of 90-100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma ) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas o f oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonunifo rm magnetization unit vector. Results from this second model support t he presence of highly rotated pre-Brunhes Nazca crust within Brunhes P acific crust which has been deformed by bookshelf faulting. This indic ates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary bet ween rigid plate tectonics to the north and deformation within the Naz ca plate between the discontinuity and the Easter microplate to the so uth. The detailed history of the discontinuity involves dueling propag ation with a great deal of variation in the amount of overlap of the t wo ridges as well as inward and outward cutting and abandonment of the tips of both ridges.