Browsing by Author "Zheng, Lin"
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Item Absolute plate velocities from seismic anisotropy: Importance of correlated errors(American Geophysical Union, 2014) Zheng, Lin; Gordon, Richard G.; Kreemer, CornéThe errors in plate motion azimuths inferred from shear wave splitting beneath any one tectonic plate are shown to be correlated with the errors of other azimuths from the same plate. To account for these correlations, we adopt a two-tier analysis: First, find the pole of rotation and confidence limits for each plate individually. Second, solve for the best fit to these poles while constraining relative plate angular velocities to consistency with the MORVEL relative plate angular velocities. Our preferred set of angular velocities, SKS-MORVEL, is determined from the poles from eight plates weighted proportionally to the root-mean-square velocity of each plate. SKS-MORVEL indicates that eight plates (Amur, Antarctica, Caribbean, Eurasia, Lwandle, Somalia, Sundaland, and Yangtze) have angular velocities that differ insignificantly from zero. The net rotation of the lithosphere is 0.25 ± 0.11° Ma−1 (95% confidence limits) right handed about 57.1°S, 68.6°E. The within-plate dispersion of seismic anisotropy for oceanic lithosphere (σ = 19.2°) differs insignificantly from that for continental lithosphere (σ = 21.6°). The between-plate dispersion, however, is significantly smaller for oceanic lithosphere (σ = 7.4°) than for continental lithosphere (σ = 14.7°). Two of the slowest-moving plates, Antarctica (vRMS = 4 mm a−1, σ = 29°) and Eurasia (vRMS = 3 mm a−1, σ = 33°), have two of the largest within-plate dispersions, which may indicate that a plate must move faster than ≈ 5 mm a−1 to result in seismic anisotropy useful for estimating plate motion. The tendency of observed azimuths on the Arabia plate to be counterclockwise of plate motion may provide information about the direction and amplitude of superposed asthenospheric flow or about anisotropy in the lithospheric mantle.Item Current absolute plate motion from seismic anisotropy and hotspot tracks; bounds on the latitudinal shift of the Hawaiian hotspot during formation of the Hawaiian island and seamount chain(2013-12-06) Zheng, Lin; Gordon, Richard G.; Lenardic, Adrian; Symes, William W.; Sawyer, Dale S.Hotspots, the volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere, can serve as a reference frame to track the motion of the plates relative to the mantle beneath the asthenosphere. The application of the hotspot reference frame, however, is limited by the relative hotspot motion ranging from a few mm a-1 [Morgan, 1971; Duncan, 1981; Muller et al., 1993; Koivisto et al. 2013] to 80 mm a-1 [Raymond et al., 2000]. This dissertation aims to evaluate the hotspot reference frame. In the first part, I estimate the plate motion relative to the sub-asthenospheric mantle from seismic anisotropy data [Kreemer, 2009]. Prior studies based on seismic anisotropy assume that errors in the azimuths inferred from shear-wave splitting are uncorrelated. In this dissertation, I show that the residuals of azimuths inferred from shear-wave splitting beneath any one tectonic plate are strongly correlated with other residuals from the same plate. I account for these correlations in an inversion for absolute plate angular velocity by adopting a two-tier analysis of plate absolute velocities. First I find the pole of rotation and confidence limits for each plate individually. Then I perform a global inversion in which each plate is represented not by multiple individual estimates of the orientation of seismic anisotropy but by a single best-fitting pole and confidence limits. In the second part, I estimate plate motion from two hotspot data-sets: T57, consisting of 57 trends of hotspot tracks distributed on ten major plates [Morgan and Phipps Morgan, 2007]; and HS3, consisting of 11 trends and 2 propagation rates of hotspot tracks distributed on four major plates [Gripp and Gordon, 2002]. I test the assumption of uncorrelated errors in hotspot trend, following the same approach as previously used for seismic anisotropy data. I estimate the best-fitting angular velocities after removing the correlated errors within plates in T57 hotspot data-set. Volcanic age dates used in HS3 tend to be younger than true ages due to the bias inherent in K-Ar measurements. Therefore, the volcanic propagation rates tend to be too high. I use the difference between an astrogeochronologic-based, and a K-Ar-based, geomagnetic reversal time scale as a proxy to recalibrate the K-Ar age dates, which reduces the volcanic propagation rates of the Hawaii hotspot track and the Society hotspot track by 8% and 4% respectively. The global set of best-fitting angular velocities estimated from seismic anisotropy data in the first part, are compared with those estimated from hotspot data-sets in the second part. In the third part, I test the hotspot mobility from the magnetic anomaly data due to seafloor spreading. I estimate the location of the Pacific paleomagnetic pole of anomaly 20r (44 Ma B.P.) by analyzing the skewness of marine magnetic anomalies. Then I reconstruct this paleomagnetic pole in the Pacific hotspot reference frame [Andrew et al., 2006; Koivisto et al., 2013], and compare it with prior paleomagnetic pole at the similar age in the Indo-Atlantic hotspot reference frame [Besse and Courtillot, 2002].Item Unknown 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, DanielWhile 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.