NEW PALEOLATITUDE DATA FROM MIDWAY ATOLL: EVIDENCE FOR LONG-TERM STABILITY OF EARTH'S SPIN AXIS

Mantle-circulation models demonstrate the potential for long-term stability of the Earth's spin axis in the presence of the necessary core-mantle heat flux to generate plumes. Polar wander, the rotation of the entire solid Earth, is a geophysically plausible process having important implications for our understanding of the history of the mantle and surface processes. To gauge polar wander, one needs paleomagnetic data from geographically widespread positions; an associated problem in evaluating polar wander has been the difficulty in obtaining robust paleomagnetic data from the region represented by the Pacific Ocean basin.

On the basis of interpretations of the skewness of marine magnetic anomalies from a small portion of the Pacific plate (between the Galapagos and Clarion fracture zones), Horner-Johnson and Gordon (2010) call for significant polar wander in a fixed hotspot reference frame (called “true polar wander”, or TPW) at ~32 Ma. Here we test this interpretation using new paleolatitude data collected from Midway Atoll, one of the oldest outcrops of the Hawaiian-Emperor (H-E) seamount chain, with an age of 27.7 Ma (Dalrymple et al., 1977). Midway Atoll was drilled in 1965 (Ladd, 1967); two basement sections composed of basalts and soil horizons were recovered and these are the focus of our study.

The Horner-Johnson and Gordon (2010) interpretation predicts a paleolatitude of 12.7° N for the formation of Midway Atoll. Our preliminary paleomagnetic analyses indicate a paleolatitude of 18.7° N. The latter is consistent with the present-day latitude of the H-E hotspot and suggests little (or no) cumulative polar wander since 27 Ma. Although we feel that it is unlikely that the ~5° of TPW reported by Horner-Johnson and Gordon (2010) occurred during the nominal ~4 million year window between 32 Ma and the formation of Midway Atoll, the rate in this case would have been ~1.2°/Myr, which disagrees with the polar wander rate modeled at 30 Ma of 0.15°/Myr (Schaber et al., 2009). We suggest instead that the marine magnetic anomaly skewness data reflect oceanic crustal formation processes rather than purely paleolatitude. Overall, our results are consistent with mantle-circulation models indicating that considerable core-mantle boundary heating can occur when polar wander is weak (Schaber et al., 2009).