Relative Motion History of the Pacific-Nazca (Farallon) Plates since 30 Million Years Ago

Relative plate motion history since 30 Ma between the Pacific and the southern portion of the Nazca (Farallon) plates is examined. The history is constrained by available seafloor magnetic anomaly data and a two-minute grid of predicted bathymetry derived from satellite altimetry and shipboard sensors. These data are used to create a new plate motion reconstruction based on new magnetic anomaly identifications and finite poles of motion. The new identified magnetic isochrons and tectonic reconstruction provides greater resolution to the tectonic history between chrons 7y (24.73 Ma) and 3 (4.18 Ma) than previous interpretations. Shipboard magnetics and aeromagnetic data from over 250 expeditions were plotted and used to extrapolate magnetic anomalies picked from 2D magnetic modeling from selected cruises. Magnetic anomalies were further constrained by tectonic features evident in the predicted bathymetry. Previously published magnetic anomaly locations consistent with this work were used where interpretation could not be constrained by 2D modeling and map extrapolation. Point locations for anomalies were used as input for calculation of finite poles of motion for chrons 10y, 7y, 6c, 5d, 5b, 5aa, 5o, 4a and 3a. An iterative process of anomaly mapping, pole calculation and anomaly point rotations was used to refine the finite poles of motion. Eleven stage poles were calculated from the nine finite poles from this study and two published instantaneous Euler vectors. Tectonic reconstructions indicate a history dominated by two major southward ridge propagation events, the first starting by 28 Ma and completed by 18 Ma. The second event initiated in association with breakup of the Farallon plate around 24 Ma and ceased by about 11 Ma. Lithosphere was transferred from Nazca to Pacific during the first event and in the opposite sense during the second. Development of the Mendoza microplate east of the later propagator occurred at about 20 Ma and this dual spreading process appears to have lasted until about 15 Ma