A Late Cretaceous-Eocene Geomagnetic Polarity Time Scale (MQSD20) that steadies spreading rates on multiple mid-ocean ridge flanks

Abstract

Magnetic anomalies over mid-ocean ridge flanks record the history of geomagnetic field reversals, and the width of magnetized crustal blocks can be combined with absolute dates to generate a Geomagnetic Polarity Timescale (GPTS). We update here the current GPTS for the Late Cretaceous-Eocene (chrons C33-C13,~84-33 Ma) by extending to several spreading centers the analysis that originally assumed smoothly varying spreading rates in the South Atlantic. We assembled magnetic anomaly tracks from the southern Pacific (23 ship tracks), the northern Pacific (35), the southern Atlantic (45), and the Indian Ocean (51). Tracks were projected onto plate tectonic flow lines, and distances to magnetic polarity block boundaries were estimated by fitting measured magnetic anomalies with a Monte Carlo algorithm that iteratively changed block model distances and anomaly skewness angles. Distance data from each track were then assembled in summary sets of block model distances over 13 ridge flank regions. We obtained a final MQSD20 GPTS with another Monte Carlo algorithm that iteratively perturbed ages of polarity chron boundaries to minimize the variability of spreading rates over all ridge flanks and fit an up-to-date set of radioisotopic dates. The MQSD20 GPTS highlights a major plate motion change at 50-45 Ma, when spreading rates decreased in the Indian Ocean as India collided with Eurasia while spreading rates increased in the South Atlantic and Northern Pacific and the Hawaii-Emperor seamount chain changed its orientation. Plain Language SummaryAs the Earth's magnetic field reversed its polarity during geological time, seafloor spreading created a series of magnetized blocks on mid-ocean ridge flanks that give rise to magnetic anomalies and field highs and lows measured by survey ships. These reversal records are combined with age ties from radioisotopic dating to construct a geomagnetic polarity timescale (GPTS) that lists the ages of magnetic field reversals. Our study updates the GPTS in the Late Cretaceous-Eocene (~84-33 Ma) by minimizing the variation of spreading rates in the southern Atlantic, Indian, and southern and northern Pacific Oceans using an up-to-date set of 154 ship tracks. By providing independent age information, the new GPTS will aid the developing discipline of astrochronology, which is based on the correlation of sediment cycles with astronomical cycles in the Earth's orbit and spin axis orientation. The new GPTS also refines the global history of ocean spreading and highlights a major change at 50-45 Ma. At that time, seafloor spreading in the Indian Ocean slowed down as India collided with Eurasia while spreading became faster in the northern Pacific, coinciding with a bend in the orientation of the Hawaii-Emperor seamount chain.