Browsing by Author "Woodworth, Daniel"

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    High-Resolution Pacific Apparent Polar Wander Since the Paleocene: Evidence for Two Episodes of True Polar Wander and Two True Polar Stillstands
    (2022-08-11) Woodworth, Daniel; Gordon, Richard G; de Hoop, Maarten
    Paleomagnetic poles from the continents have long provided evidence for apparent polar wander (APW), the motion over geologic time of the spin axis relative to a continent or tectonic plate. In contrast, the APW paths of oceanic plates are much less developed, mainly because oceanic plates lack subaerially exposed surface, making conventional paleomagnetic approaches difficult or impossible. Herein two alternative approaches, skewness analysis of marine magnetic anomalies and paleo-spin axis estimation from the distribution of paleo-equatorial sediment bands, are developed and applied. With these approaches we determined a high-resolution APW path for the Pacific Plate from 56 Ma to 12 Ma. Combined with existing Pacific poles for 72 Ma to 58 Ma, our Pacific APW allows study of finer detail than is possible using conventional methods. We identify two tracks in the Pacific APW path, suggesting northward plate motion relative to the spin axis 72 Ma to 56 Ma and 46 Ma to 12 Ma. Surprisingly, there is no net apparent polar wander between 12 Ma and the present and during the gap between these tracks southward motion from 56 Ma to 46 Ma is indicated. When reconstructed into the reference frame of the Pacific hotspots, which has been used to approximate an absolute reference frame, the two Pacific APW tracks correspond to two stillstands in the motion of the paleo-spin axis at locations significantly different both from one another and the present spin axis. These locations are separated by spin axis motion from 56 to 46 Ma and 12 Ma to the present. We interpret these stillstands in paleo-spin axis motion relative to the Pacific hotspots as true polar stillstands and the intervals separating them, corresponding to anomalous Pacific APW, as episodes of true polar wander of 7° and 3°, respectively.
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    Pacific Plate Apparent Polar Wander, Hot Spot Fixity, and True Polar Wander During the Formation of the Hawaiian Island and Seamount Chain From an Analysis of the Skewness of Magnetic Anomaly 20r (44ﾠMa)
    (Wiley, 2018) Zheng, Lin; Gordon, Richard G.; Woodworth, Daniel
    While it is well documented that the Hawaiian hot spot has shifted southward relative to the spin axis since the formation of some of the Emperor seamounts, the paleolatitude of the hot spot during the formation of the Hawaiian chain is poorly known. To better determine the latter, here we estimate the location of the 44 Ma Pacific plate paleomagnetic pole by investigating the skewness (asymmetry) of 14 airplane and 19 ship‐board crossings of magnetic anomaly 20r between the Murray and Marquesas fracture zones on the Pacific plate. The new 44 Ma paleomagnetic pole (78.0°N, 26.0°E, A95_1 = 5.4° at 101°, A95_2 = 2.0°) differs by ≈4° from its position expected if the Pacific hot spots have been fixed relative to the spin axis. This shift is independently recorded by the chron 12r (32 Ma) Pacific plate skewness paleomagnetic pole and is also confirmed by paleomagnetic poles reconstructed from the continents, indicating that global hot spots have moved in unison with respect to the spin axis, probably due to true polar wander, which may continue today as recorded by optical astronomy and geodetic very long baseline interferometry. An analysis of spreading rates recorded in the magnetic profiles indicates that spreading rates doubled between ≈50 and ≈42 Ma (confirming prior results), as expected if the bend in the Hawaiian‐Emperor chain records a change in Pacific plate motion relative to the deep mantle.
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    Paleolatitude of the Hawaiian Hot Spot Since 48 Ma: Evidence for a Mid‐Cenozoic True Polar Stillstand Followed by Late Cenozoic True Polar Wander Coincident With Northern Hemisphere Glaciation
    (Wiley, 2018) Woodworth, Daniel; Gordon, Richard G.
    Paleospin axis locations since 48 Ma inferred from the distribution of equatorial sediment accumulation rates on the Pacific plate, together with paleomagnetic poles from magnetic anomaly skewness, indicate that the Hawaiian hot spot was nearly fixed in latitude from 48 to 12 Ma, but ≈3° north of its current latitude. From 48 to 12 Ma in the Pacific hot spot reference frame, which we take to be equivalent to the global hot spot reference frame, the spin axis was located near 87°N, 164°E, recording a stillstand in true polar wander. Global hot spots shifted coherently relative to the spin axis since ≈12 Ma, consistent with an episode of true polar wander, which may continue today. The motion of the spin axis away from the Hawaiian hot spot and toward Greenland since ≈12 Ma coincided with, and may have contributed to, the onset of northern hemisphere glaciation.