Timing of Eocene True Polar Wander Inferred from Pacific Plate Marine Magnetic Anomalies

Abstract:

Recent evidence strongly suggests that the entire solid Earth re-oriented relative to its spin axis (i.e., true polar wander, TPW) by ~7° during Early to Middle Eocene time. Specifically, spin axis locations estimated from paleomagnetic analysis of Pacific-plate seafloor magnetic anomalies C32 to C24r show that Pacific hotspots remained approximately stationary relative to the spin axis from 72 Ma until at least 53.9 Ma. Paleomagnetic analysis, combined with spin axis locations from paleo-equatorial sediment bands, shows that Pacific hotspots were again stationary from 46.3 Ma to 12 Ma. We interpret this shift to have been a result of TPW.

This shift occurred between 53.9 Ma and 46.3 Ma, a time interval which coincides with global tectonic re-organizations. This interval is also important for paleoclimate as it shortly follows the Paleocene-Eocene Thermal Maximum (at ~56 Ma) and includes the Early Eocene Climatic Optimum (EECO, ~53.2 to 49.1 Ma [Westerhold et al. 2018]) and the ~49.1-Ma onset of Cenozoic cooling marking the transition from greenhouse to icehouse Earth.

To further narrow the timing of this global re-orientation, we conduct a paleomagnetic analysis of the skewness (asymmetry of shape) of Pacific plate marine magnetic anomaly 21n, which spans ~46.3 to 47.9 Ma. Anomaly 21n is a readily distinguished seafloor magnetic anomaly on seafloor just older than chron C20r, which defines the old end of the 46.3 Ma to 12 Ma TPW stillstand. We use both shipboard total-intensity and vector aeromagnetic tracklines in our analysis to obtain a well-constrained paleomagnetic pole.

When reconstructed into the Pacific hotspot reference frame, our estimated paleomagnetic pole lies close to the 46.3 to 12-Ma TPW stillstand location, indicating that true polar wander was completed by 47.9 Ma, if not earlier. Thus the ~7° shift occurred in, at most, 6.0 Ma (between the young end of chron 24r and the old end of chron 21n) and was completed near or before the beginning of Middle Eocene time. Within resolution, the episode of TPW and the EECO are coeval. Both also mainly occurred before the 47.8 Ma-old bend in the Hawaiian-Emperor Chain (Gaastra and Gordon, this session), but may overlap in age with the early part of the bend.