Magnetic Anomalies and Plate Tectonics of the Central East Pacific

In the late Oligocene, the Pacific-Farallon spreading center collided with the subduction zone bordering North America. This event triggered a major plate reorganization that in the East Pacific eventually led to the break-up of the Farallon plate into the Cocos and Nazca plates. The opening of this new oceanic spreading center perpendicular to the existing Pacific spreading is unique and had far-reaching consequences for the active continental margins of Central- and South America. Since most of the original fissure where the Farallon plate split into the Cocos plate and the Nazca plate has already been subducted beneath Central- and South America together with much of the oceanic crust that was formed during the early phase of Cocos-Nazca spreading, the full reconstruction of plate kinematics in the area back to the time of the opening required studying the oceanic crust conjugate to the Farallon remains offshore Costa Rica and Ecuador in the Central Pacific. This conjugate piece of crust is located around 120°W, just south of the Equator. For the plate tectonic reconstruction a thorough identification of seafloor spreading anomalies from the Early Miocene was required. The problem is that magnetic anomalies in the region have low amplitudes because seafloor spreading anomalies in this part of the Pacific strike more or less north-south and thus almost parallel to the present geomagnetic field. In the near-horizontal geomagnetic field at the equator, these 2D source bodies generate only very small magnetic anomalies in the total field. To make things worse, the Equatorial Electrojet causes strong geomagnetic variations in the vicinity of the geomagnetic dip equator with amplitudes similar to those of the crustal field magnetic anomalies measured at the sea surface. This challenge can be overcome by the use of total field gradiometers and vector magne¬tometers. It was possible to identify seafloor spreading anomalies between Chrons 7 and 5A for a large area in the Central Pacific. In combination with picks of the same anomalies from the Cocos- and Nazca plates it was possible to trace back the history of the Farallon break-up in a three-plate reconstruction. The plate motion of the Pacific plate in the hotspot reference frame provides the absolute position of the initial triple junction and the strike direction of the newly formed Cocos-Nazca spreading center, revealing its close relation to the Galapagos hotspot. Multibeam bathymetry data from the research area highlight the details of an in¬creased magmatic activity near the newly formed triple junction and the reorganization of the seafloor spreading that followed the break-up and which finally resulted in a major ridge jump at the East Pacific Rise during Chron 6.