Bathymetric, magnetic and side scan sonar data from a June 1987 SeaMARC II cruise aboard the R/V Moana Wave have been combined with additional data in an area 140 km by 154 km centered on the overlapping spreading center pair (OSC) located at 11°45′N on the East Pacific Rise (EPR). The purpose of this study is to correlate and interpret these data sets in order to determine the recent (<1 m.y.) history of this OSC. The data sets include gridded bathymetry and magnetic field anomalies, a side scan sonar mosaic, and a fault map. The bathymetry and anomaly grids were used as inputs in a three‐dimensional Fourier inversion resulting in a crustal rock magnetization distribution solution. The most prominent feature of all five maps is a roughly V‐shaped anomalous zone centered on the OSC. The zone is characterized by a positive depth anomaly, a negative magnetic field anomaly within the central (positive) anomaly and a zone of high magnetization in the inversion solution. A hummock y texture and sparse, curved faults mark the zone in the side scan mosaic and fault map. The shape of this zone indicates a short period of northward migration of the discontinuity 0.72–0.6 Ma at approximately 75 mm/yr followed by slow (less than 30 mm/yr) and then fast (140–200 mm/yr) southward migration for the past 0.6 m.y.. The anomalous zone is not well defined beyond the edge of the central anomaly which is straight and continuous, indicating that if this discontinuity of the EPR existed 0.72 Ma, its offset was negligible. The present‐day axis has an orientation of 349°, the edge of the central anomaly has an orientation of 353°, and Jaramillo‐age crust to the east of the EPR has an orientation of 353°–355°. This variation in magnetic linealion directions suggests a 4° to 6°‐counter‐clockwise change in the direction of spreading in the past 1 m.y. which seems to have been accommodated by asymmetric spreading. Variations in mean fault trend with age are consistent with a counter‐clockwise change in the direction of spreading for the same time period.
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°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 continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly‐2‐aged crust. A three‐dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The 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 rate 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 of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre‐Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges.
Abstract. Six Deep-Tow magnetic profiles across the axis of the East Pacific Rise [EPR] in two small areas between 19~ ' and 20~ were collected during the 1983 Protea 1 cruise of the R/V Melville. These near-bottom profiles are of extremely high resolution allowing the interpretation of very short wavelength features. We have inverted the magnetic field data to determine the rock magnetization distribution near the axis of this ultrafast speading center (162 mm yr t). The solutions reveal large amplitude (up to 35 A m -l) short wavelength (I-3 km) variations in magnetization. Specifically all crossings show a narrow (0.5 to 1.5 km) low in magnetization superimposed on a broader (2.5 to 4 km) high directly over the ridge axis. Four profiles in the northern area (19~ ' to 19'~33'S) also show symmetrical near-axis (within 4 km) lows which are remarkably continuous along strike. Explanations for the short-wavelength variations are discussed which fall into the following categories: (1) variations in the thickness of the magnetized layer, (2) variations in rock chemistry (e.g. alteration due to hydrothermal activity), and (3) paleofield intensity variations. None of the mechanisms discussed alone adequately explain the observed phenomena in the study area or on a world-wide scale. Further sampling and high resolution surveying will be required in order to accurately determine the relative importance of the mechanisms discussed.
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