T43D-2709: Spreading behaviour of the Pacific-Farallon ridge system between 83 and 28 Ma
Authors: Christopher J Rowan, David B Rowley
Author Institutions: Department of the Geophysical, University of Chicago, Chicago, IL, USA
At 83 Ma, the roughly N-S oriented Pacific-Farallon ridge extended more than 10,000 km from 51 ° N to 43 ° S. Despite substantial shortening of the ridge system since ~55 Ma, this ridge and its remnants (e.g., the East Pacific Rise/EPR) have produced as much as 45% of all the reconstructable oceanic lithosphere created in the Late Cretaceous and Cenozoic. Accurately reconstructing the past spreading history of the Pacific-Farallon ridge is therefore of paramount importance for determining possible variations in global spreading rates over geological time, which are the basis of suggested interactions between mantle dynamics, surface tectonics, sea-level rise and climate in the past 100 Myr. However, attempts to accurately determine Pacific-Farallon spreading face the twin challenges of extensive subduction of Farallon crust – which precludes reconstruction by fitting conjugate magnetic anomaly and fracture zone traces – and the well-established asymmetric spreading behaviour of the EPR and its ancestor ridges for at least the past 51 Myr. We present improved rotation poles for the Pacific-Farallon spreading system between geomagnetic chrons 34y (83 Ma) and 10y (28.28 Ma), complete with uncertainties that allow easier combination into global plate circuits. These poles are derived by combining magnetic anomaly and fracture zone data from both the northern and southern Pacific plate, maximising the data distribution along the original ridge length to average out local variations in spreading behaviour. We have calculated best fit ‘half’-stage poles for Pacific-Farallon spreading between nine Pacific plate magnetic anomalies (34y, 33y, 29o, 24.3o, 20o, 18.2o, 17.1y, 13y and 10y). For poles younger than chron 24.3o, full stage poles have been calculated by using anomaly picks from yet-to-be subducted Farallon/Nazca crust in the south Pacific to determine spreading asymmetry. Characterisation of the variation in spreading asymmetry in the past 50 Ma also allows bounds to be placed on the asymmetry between 50 and 83 Ma and full stage poles to be calculated for this period. Our new poles plot in a similar region to previously published Pacific-Farallon rotations, but synthetic flowlines produce a better match to Pacific fracture zone trends, with significantly less latitudinal drift. With the exception of the 18o-20o stage pole, the six stage poles between chrons 33y and 13y (74-33 Ma) all cluster tightly at 60-75 ° E, 60-68 ° N, which is consistent with the relatively constant orientation of the major Pacific fracture zones. This period of stability spans several episodes of Farallon plate fragmentation, as segments of the Pacific-Farallon ridge were subducted beneath the Americas. Therefore, these events do not appear to have particularly affected the position of the instantaneous rotation pole; in contrast, Farallon plate fragmentation events do appear to correlate to increases in spreading rate on the surviving ridge. Net Nazca:Pacific spreading asymmetry over the past 50 Myr is 56-58:42-44%, but over periods of less than 15 Myr there is considerably more variability. There are also quasi-periodic long-term variations in spreading asymmetry, with higher asymmetry between 15-25 Ma and 35-45 Ma, and more symmetric spreading in the past 10 Ma, between 25-35 Ma and possibly at around 50 Ma.