Browsing by Author "Gordon, Richard G"
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Item 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, MaartenPaleomagnetic 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.Item Hotspot Motion during the Cenozoic and True Polar Wander across the Cretaceous-Paleogene Boundary(2022-12-01) Gaastra, Kevin; Gordon, Richard GThe motion of Earth’s tectonic plates relative to sites of mid-plate or excessive plate boundary volcanism produce age-progressive chains of volcanoes. These hotspot volcanoes are thought to be caused by plumes of hot mantle material rising in the solid state from near the core-mantle boundary. These mantle plumes and the hotspot tracks they produce are one of the only records of the motion of Earth’s surface relative to its interior. Therefore, understanding how these mantle plumes move relative to each other and the deep mantle is paramount to understanding the nature of plate tectonics. Here I present methods of analysis of volcano locations and age dates and apply them to three prominent Pacific plate hotspot tracks (Hawaii, Louisville, and Rurutu). I find that motion between these hotspots is insignificant for the last 80 million years. Therefore the mantle plumes underlying these Pacific plate hotspots may be more stable in a convecting mantle than previously inferred. Rates of hotspot motion become more uncertain further back in Earth’s history. Here I combine a Monte Carlo inversion method with objectively assigned uncertainties of the trends of the young portions of global hotspot chains to place bounds on neotectonic (i.e., the past 5-10 million years) rates and directions of hotspot motion. I find that a non-zero but slow plate-motion perpendicular rate of merely 2–4 mm/yr is indicated when considering most or all global hotspots. Though the trends of the Marquesas and Comores hotspots are distinct outliers which may indicate more recent rapid motion. The Earth’s magnetic field also provides a reference for the absolute motion of Earth’s tectonic plates. As over long time scales (>100,000 years) the Earth’s magnetic pole averages to the location of its axis of rotation. By analyzing the phase of marine magnetic anomaly C27r in the Pacific Ocean Basin I estimate the location of the Earth’s axis of rotation during the formation of this seafloor (63 million years ago). I use this to test hypotheses of true polar wander, a re-orientation of Earth’s crust and mantle relative its axis of rotation across the Cretaceous-Paleogene boundary.Item New Evidence that the Emperor Chain Records Motion of the Pacific Plate Relative to the Deep Mantle(2015-10-02) Seidman, Lily E; Gordon, Richard GA key question for Pacific and circum-Pacific tectonics is whether the Emperor seamount chain records the northward motion of the Pacific plate relative to the deep mantle. To investigate this question, we determine a new Pacific plate paleomagnetic pole for ≈60 Ma BP from the analysis of the skewness of marine magnetic anomaly 26r recording Pacific-Farallon motion in low paleolatitudes. We further update a previously published Pacific plate pole for ≈65 Ma from the analysis of anomalies 27r to 31 by incorporating a larger correction for anomalous skewness. These two poles, along with prior poles for 58 Ma and 72 Ma allow us to test how much, if any, the Hawaiian hotspot moved relative to the spin axis for ≈14 Ma of the ≈30 Ma during which the Emperor chain was formed. We find that the Hawaiian hotspot moved insignificantly northward (0.04 ±0.15° Ma-1 (4± 17 mm a-1)) from 72 Ma to 58 Ma while the Pacific plate moved significantly northward (0.38 ±0.15° Ma-1 (42 ± 17 mm a-1)). We further compare the apparent polar wander of the Pacific hotspots with that of the Indo-Atlantic hotspots over the past 65 Ma. The two paths are offset by several degrees, which may indicate ~10 mm a-1 of motion between Pacific and Indo-Atlantic hotspots. The two paths indicate a jump of ≈8° in the position of the spin axis relative to global hotspots between ≈60 Ma and ≈45 Ma presumably due to true polar wander.Item Oblique Seafloor Spreading, the Malpelo Plate Hypothesis, and Cocos-Nazca-Pacific Plate Motion Circuit(2018-11-16) Zhang, Tuo; Gordon, Richard GWe show that oblique seafloor spreading occurs in several regions where obliquity, α, was not recognized before. These include the slow spreading centers of the Red Sea (α≈20°), intermediate spreading centers of the Cocos-Nazca plate boundary between 91°W and 94°W (α≈9°), and superfast spreading centers of the East Pacific Rise at the Nazca-Pacific plate boundary between 29°S and 32°S (α≈10°) and perhaps between ≈16°S and ≈22°S (α≈4°). Thus, oblique spreading occurs across slow, intermediate, and superfast spreading centers, but not across fast spreading centers. Across slow and intermediate spreading centers, obliquity tends to decrease with increasing spreading rate, while across fast and superfast spreading centers it tends to increase with increasing spreading rate. Oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these. We show along a segment of the Cocos-Pacific plate boundary that inferred magma overpressure is only one-fourth as large as remote tectonic stress, consistent with a prior inference from other observations. The highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume, but not all ridge segments near plumes spread obliquely. For one set of estimates of plume fluxes, the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate. Using global multi-resolution topography, we estimate new transform-fault azimuths along the eastern Cocos-Nazca plate boundary and show that the direction of relative plate motion is 3.3° ±1.8° (95% confidence limits) clockwise of prior estimates. The new direction of Cocos-Nazca plate motion is, moreover, 4.9° ±2.7° (95% confidence limits) clockwise of the azimuth of the Panama transform fault. We infer that the plate east of the Panama transform fault is not the Nazca plate, but instead is a microplate that we term the Malpelo plate. With the improved transform fault data, the non-closure of the Nazca-Cocos-Pacific plate-motion circuit is reduced from 15.0 mm a–1 ±3.8 mm a–1 to 11.6 mm a–1 ±3.8 mm a–1 (95% confidence limits). Last, we examine the closure of the Cocos-Nazca-Pacific plate motion circuit, which were previously shown to fail closure by a linear velocity of 11.6 mm a–1 ±3.8 mm a–1. We tested eliminating the spreading rates along the Cocos-Pacific plate boundary north of the Orozco transform fault. The non-closure velocity is reduced to 9.9 mm a–1 ±3.8 mm a–1 (95% confidence limits). By further replacing the spreading rates with those from NUVEL plate motion model [DeMets et al., 1990] and keeping the new transform fault azimuth estimates, the non-closure velocity is reduced to 8.2 mm a–1 ±3.8 mm a–1 (95% confidence limits), which indicates significant non-closure. Finally, we tested further by eliminating both the spreading rates and transform faults along the traditionally defined Cocos-Nazca plate boundary east of the Galapagos transform fault, the non-closure velocity is reduced to 4.2 mm a–1 ±3.8 mm a–1 (95% confidence limits), which is insignificant. This result indicates that the plate boundary east of the Galapagos Transform Fault does not record motion between the Cocos and Nazca plate. Instead it records motion between a previously unrecognized plate and either the Cocos or Nazca plate.Item The Absolute Motion of Trenches and Age of the Subducting Slab(2014-12-05) Mathews, David Christopher; Gordon, Richard G; Sawyer, Dale S; Niu, FenglinPrevious work proposes that many trenches advance, but inferred motions vary considerably between different estimates of plate motion relative to hotspots. We remedy this by using recent estimates of absolute plate motions inferred from seismic anisotropy, incorporating the MORVEL relative plate angular velocities, and fully propagating the uncertainties. Nearly half the trench velocities differ significantly from prior results, with the greatest differences at the Kermadec, New Hebrides, and Marianas trenches. Trench velocity ranges from retreat of 126 ± 20 mm a-1 to advance of 52 ± 14 mm a-1 with a median of 9 mm a-1 of retreat. Out of 57 locations, trench advance is significant at only five locations (along the Hikurangi, Marianas, and Izu-Bonin trenches), retreat is significant at 23 locations, and trench motion differs insignificantly from zero at 29 locations. Trench advance increases with age and absolute velocity of subducting lithosphere and with angle of subduction.Item Toward a Definitive Estimate of Geologically Current Absolute Plate Motion: Correlation of Hotspot Trends, Realistic Trend Uncertainties, How Fast Hotspots Move, and Combination of Hotspot Trends and Orientations of Seismic Anisotropy(2019-04-18) Wang, Chengzu; Gordon, Richard GHotspots are widely used to construct a reference frame to track the motion of plates relative to lower mantle. How fast hotspot move is crucial in constructing such reference frame. In addition, hotspot motion itself bears some implication about mantle dynamics. While previous studies show different opinion about hotspot motion. Slow motion is supported by many studies, such as Morgan [1981, 1983] and Duncan [1981], which estimated that individual hotspots move relative to a mean hotspot reference frame by 3-5 mm a-1. In contrast, more recently, it has been popular to assume that hotspot in the Pacific basin move relative to those in the Indo-Atlantic at rates of 10-30 mm a-1 or more [Norton, 1995; Raymond et al., 2000; Tarduno et al., 2003]. And fast motion motivates the usage of a moving hotspot reference (e.g. Doubrovine et al., [2012]) with modeled hotspot motion. This dissertation aims to evaluation hotspot motion and hotspot reference frame using an updated global hotspot data set (HS4, revised from by Morgan and Phipps Morgan [2007]). In the first part, I fit hotspot trends from HS4 and calculate hotspot motion from data misfit. It shows that the angular misfit of fitting observed hotspot trends is dominated by differences in trends between plates rather than difference within plates, which indicate misfit of fitting hotspot trends in the same plate are correlated with each other. And the estimated plate-mean hotspot motion in the direction perpendicular to plate motion differ significantly from zero for hotspot beneath only 3 of 10 plates and then only by 0.3 to 1.4 mm a-1. In the second part, we objectively estimate uncertainty in hotspot trend using hotspot track width and length for hotspot in HS4 data set. It shows that uncertainties in trends are underestimated by most previous studies [e.g. Morgan and Phipps Morgan, 2007]. Our newly estimated uncertainties in trends are consistent with the misfits observed in the last part. We update plate-mean hotspot motion using the new uncertainties in trends, 54 of 56 hotspot motion differ insignificantly from zero and none of 10 plates differ significantly from zero. In the third part, we test hotspot fixity by fitting 41 hotspot trends (T41, a subset of HS4) using both fixed hotspot and moving hotspot reference frame. The modeled hotspot motions used in moving hotspot reference frame are calculated from Doubrovine et al. [2012]. The results show that more hotspots and more plates are fitted better in fixed hotspot reference frame than moving hotspot reference frame. Assuming modeled hotspot motion directions are correct, we calculate an average hotspot motion for T41 is 1.5 mm a-1, with lower bound zero and upper bound as 4.6 mm a-1 for magnitude (95% confidence limit). Thus, fixed hotspot reference frame is superior than moving hotspot for T41, and possibly for all available hotspot data set. In the fourth part, we calculate a new of absolute plate motion using both hotspot trend and orientation of seismic anisotropy (HS4-SKS-MORVEL). We first calculate a new set of absolute plate motion using only hotspot trends data. (HS4-MORVEL). HS4-MORVEL is superior than previous plate motions using hotspots. First, it considers that hotspots in the same plate are correlated with each other by using a two-tier analysis. Second, it uses newly estimate hotspot trend uncertainties. Then we combine HS4-MORVEL and SKS-MORVEL [Zheng et al., 2014] to get HS4-SKS-MORVEL, which has better resolution than both sets.Item Transform Faults, Fracture Zones, and the Kinematics of Horizontal Thermal Contraction of Oceanic Lithosphere(2015-12-04) Mishra, Jay Kumar; Gordon, Richard G; Lenardic, Adrian; Sawyer, Dale S; Akin, John EPlate rigidity is a key assumption of the plate tectonics theory. The assumption allows us to study plate motion on Earth with great simplicity. However, recent developments in understanding plate cooling have put the assumption of plate rigidity to test. Kumar and Gordon [2009] advance the theory of horizontal thermal contraction of oceanic lithosphere as the ultimate test of plate rigidity and predict that relative intraplate velocities due to horizontal thermal contraction within the oceanic lithosphere of a tectonic plate can be as high as 3 – 10 mm a-1. Contractional strains have been predicted to vary inversely with age and hence most of the strains related with horizontal thermal contraction should be accumulated and dissipated by young oceanic lithosphere. Transform faults provide the necessary free boundaries across which most of the contractional stresses can be dissipated by ridge-parallel contraction of the oceanic lithosphere. The strike of the transform faults, earlier predicted to be parallel to the relative plate motion direction, thus, should be biased as a result of transform fault perpendicular contraction of oceanic lithosphere. I first improve the global transform fault dataset used in MORVEL [DeMets et al., 2010] and then calculate residuals between the observed transform fault azimuths and those predicted for the rigid oceanic plates. I find that on an average, for the six plate pairs with both left-lateral and right-lateral slipping transform faults, azimuths are biased by about 0.75°±0.36° clockwise for left-lateral slipping and by −0.73°±0.22° (= ±1 standard error) counter-clockwise for the right-lateral slipping. I, then investigate if this observed bias can be caused by horizontal thermal contraction. For the six selected pairs with both right-lateral and left-lateral slipping transform faults, the mean bias is predicted to be only 0.38° for only right-lateral slipping faults and −0.46° for only left-slipping transform faults. Thus the magnitudes of bias predicted by horizontal thermal contraction are ≈60% as large as the observed residual between the observed strikes of transform faults and the strikes observed for the assumption of plate rigidity. Thus we cannot exclude some normal faulting in transform fault valleys. A global analysis of plate motion based on the transform fault azimuths corrected for the predicted bias due to horizontal thermal contraction shows that the hypothesis of no horizontal thermal contraction can be rejected with at least 40% of the contractional strains causing the rotation of strikes of transform faults and the remaining being accommodated by normal faulting in transform fault valleys. Thus we conclude that plates are not rigid. Based on the predictions of horizontal thermal contraction, I further test its effect on intraplate velocities and build a 2D kinematic model of the Pacific lithosphere between the Rivera and the Heezen fracture zones to calculate the predicted intraplate relative velocity field due to horizontal thermal contraction. For the kinematic assumption that balances net contraction and extension across the fracture zones, young oceanic lithosphere along the Rivera fracture zone is predicted to have a contractional velocity of 2.6 mm a-1 in the South-Southeast direction. This is big enough to account for the misfit of 5±3 mm a-1 in the PA-NA-NB-AN plate circuit and not big enough to explain the misfit of 14±5 mm a-1 for the CO-PA-NZ plate circuit.