R-3 Repository :: Browsing by Author "Gordon, Richard G."

Browsing by Author "Gordon, Richard G."

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Absolute plate velocities from seismic anisotropy: Importance of correlated errors
(American Geophysical Union, 2014) Zheng, Lin; Gordon, Richard G.; Kreemer, Corné
The 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.
Bounds on geologically current rates of motion of groups of hot spots
(Wiley, 2017) Wang, Chengzu; Gordon, Richard G.; Zhang, Tuo
It is widely believed that groups of hot spots in different regions of the world are in relative motion at rates of 10 to 30 mm a−1 or more. Here we present a new method for analyzing geologically current motion between groups of hot spots beneath different plates. In an inversion of 56 globally distributed, equally weighted trends of hot spot tracks, the dispersion is dominated by differences in trend between different plates rather than differences within plates. Nonetheless the rate of hot spot motion perpendicular to the direction of absolute plate motion, vperp, differs significantly from zero for only 3 of 10 plates and then by merely 0.3 to 1.4 mm a−1. The global mean upper bound on |vperp| is 3.2 ± 2.7 mm a−1. Therefore, hot spots move slowly and can be used to define a global reference frame for plate motions.
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].
Deformation of Indian Ocean lithosphere: Evidence for a highly nonlinear rheological law
(Wiley, 2015) Gordon, Richard G.; Houseman, Gregory A.
The width of diffuse oceanic plate boundaries is determined by the rheology of oceanic lithosphere. Here we apply thin viscous sheet models, which have been successfully applied to deformation in several continental deforming zones, to investigate the deformation of oceanic lithosphere in the diffuse oceanic plate boundaries between the India, Capricorn, and Australia Plates. We apply kinematic boundary conditions based on the current motion between these plates. We neglect buoyancy forces due to plate thinning or thickening and assume that the thin viscous sheet has the same depth-integrated nonlinear viscosity coefficient everywhere. Our initial models have only one adjustable parameter, n, the power-law exponent, with n = 1, 3, 10, 30, and 100. The predicted width of the deforming zone decreases with increasing n, with n ≥ 30 explaining the observations. This n value is higher than has been estimated for continental lithosphere and suggests that more of the strength of oceanic lithosphere lies in layers deforming by faulting or by dislocation glide than for continental lithosphere. To obtain a stress field that better fits the distribution and type of earthquake focal mechanisms in the diffuse oceanic plate boundary, we add a second adjustable parameter, representing the effect of slab pull stretching the oceanic plate near the Sumatra Trench. We show that an average velocity increment on this boundary segment of 5 mm a−1 (relative to the average velocity of the India and Australia Plates) fits the observed distribution of fault types better than velocities of 3.3 mm a−1 or 10 mm a−1.
Diffuse oceanic plate boundaries: Kinematic observations and rheological constraints
(2006) Mutnuri, Kartik; Gordon, Richard G.
We use a thin viscous sheet model to interpret the observed shortening along a seismic reflection profile collected on the Phedre cruise (on the deforming oceanic lithosphere in the central Indian Ocean). We determine the 95% confidence region of the 1/e fall-off distance of strain rate to be between 7.1° and 20.4°, the vertically averaged power-law exponent to be between 1.6 and 18.5, and the ratio of the strength of the upper lithosphere to the lower lithosphere to be between 1 and 5.5. We calculate a simplified strength envelope of a two layered oceanic lithosphere based on the lower lithosphere deforming in accordance with the Dorn equation. This model estimates a range of between 0.2 and 20 TN m-1 for the strength of the oceanic lithosphere. The ratio of the strength of the upper lithosphere with respect to the lower lithosphere ranges between 8.6 and 13.98.
Horizontal thermal contractional strain of oceanic lithosphere: The ultimate limit to the rigid plate hypothesis
(2007) Kumar, Ravi Ranjan; Gordon, Richard G.
Depth-averaged horizontal strain rates in oceanic lithosphere due to thermal contraction are determined. Calculated strain rates range from ≈10 -2 Myr-1 (near the mid-ocean ridge) to ≈10 -5 Myr-1 (for the oldest oceanic lithosphere). The average thermal contractional strain rate in oceanic lithosphere is ≈10 -4 Myr-1. Newly created lithosphere is displaced toward old ocean basins at a rate that is 1.35% of the half-rate of seafloor spreading, giving displacement rates of 0.1 to 1.1 km Myr-1. The bias in plate displacement rates estimated from marine magnetic anomalies, expressed as a percentage of the full spreading rate, is 0.60% or 0.85% depending on the age of the magnetic isochron used to estimate current plate velocity. The displacement rate due to thermal contraction parallel to a mid-ocean ridge could be as large as ≈10 mm/yr. Strain rates due to thermo-elastic stresses are an order of magnitude smaller than the strain rate calculated when these stresses are neglected.
Kinematics and visualization of strain in three dimensions
(2000) Koomen, Amy Corinne; Gordon, Richard G.
A new computer program for strain visualization has been written. Given an upper triangular deformation tensor, which corresponds to homogeneous deformations where no shearing occurs in the z-direction (assuming a right-handed reference system), the program calculates and displays the strain ellipsoid. In addition, it can show the orientations and magnitudes of the finite strain axes, paths of particle motion, instantaneous strain axes, vorticity vector, and flow apophyses. The strain visualization program has been applied to the wrench-dominated transconvergence of the Pacific-Sierra Nevada plate boundary. I find that, within the uncertainties, the two end-members of accommodation of fault-normal contraction can produce finite strain axes matching the orientation of the mean fold-axis azimuth of the California Coast Range. Furthermore, I find that modeling the regional geology as though it were experiencing coaxial deformation is not an invalid oversimplification.
The Malpelo Plate Hypothesis and implications for nonclosure of the Cocos-Nazca-Pacific plate motion circuit
(Wiley, 2017) Zhang, Tuo; Gordon, Richard G.; Mishra, Jay K.; Wang, Chengzu
Using global multiresolution topography, we estimate new transform-fault azimuths along the 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 nonclosure 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). The nonclosure seems too large to be due entirely to horizontal thermal contraction of oceanic lithosphere and suggests that one or more additional plate boundaries remain to be discovered.
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.
Pacific plate deformation from horizontal thermal contraction
(Geological Society of America, 2014) Kreemer, Corné; Gordon, Richard G.
The central approximation of plate tectonics is that the plates are rigid, which gives the theory its rigor and predictive power. Space geodetic measurements are consistent with the rigidity of stable plate interiors, but some failures of plate-circuit closure, in particular of oceanic plates, indicate that plates may be measurably non-rigid. We explore the hypothesis that horizontal thermal contraction causes deformation of oceanic plates. Here we show significant expected displacement fields due to thermal contraction for the Pacific plate based on a previously proposed relationship between sea-floor age and strain rate and on two end-member assumptions on how strain compatibility is enforced. The predicted maximum 2.2 mm/yr southeastward motion of the north-eastern part of the plate relative to the Pacific-Antarctic Rise may contribute to a large part of the non-closure of the Pacific-North America plate motion circuit. Our predicted displacement rates cannot (yet) be confirmed by current space-geodetic data and will require seafloor geodesy with 1 mm/yr accuracy. The spatial distribution of predicted moment rate agrees reasonably well with that of intraplate earthquakes epicenters, similar to what is observed for plate boundary zones. Our results suggest that plate-scale horizontal thermal contraction is significant and that it may be partly released seismically.
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.
Receiver Function Analysis and Acoustic Waveform Modeling for Imaging Earth’s Crust: New Techniques and Their Applications
(2013-09-16) Liu, Huafeng; Niu, Fenglin; Gordon, Richard G.; Zelt, Colin A.; Symes, William W.
The crust is the outer-most layer of the earth with thickness up to 80 km. Massive seismic waveform data have enabled imaging fine crustal structures with the aid of new imaging techniques. In this thesis, I develop seismic imaging techniques to take full
Revised estimates and uncertainties of motion between hotspots: Implications for the global plate circuit
(2005) Andrews, David Lanning; Gordon, Richard G.
New quantitative methods of determining plate motions with respect to the hotspots, and their associated uncertainties are presented. These methods are then applied to Pacific hotspot data and combined with the global plate circuit to make estimates of the Cenozoic motion between Pacific and Indo-Atlantic hotspots. It is found that Pacific and Indo-Atlantic hotspots have little to no resolvable motion, about 3 +/- 6 mm yr-1, since 48 million years before present (Ma), but do show large motions, about 50 +/- 15 mm yr-1, between 48 and 68 Ma. This implies that a global hotspot reference frame for plate motions is attainable, at least since 48 Ma. It is also demonstrated, with the aid of paleomagnetic data that the observed large scale motions prior to 48 Ma are likely, at least in part, a result of unresolved links in the global plate circuit across Antarctica before 48 Ma.
Russian Oil Futures
(James A. Baker III Institute for Public Policy, 2004) Gordon, Richard G.; James A. Baker III Institute for Public Policy
Sedimentological Investigations of Paleo-Ice Sheet Dynamics in West Antarctica
(2013-09-16) Kirshner, Alexandra; Anderson, John B.; Dasgupta, Rajdeep; Hight, Christopher; Gordon, Richard G.
Modern Pine Island and Thwaites Glaciers, which both drain into Pine Island Bay, are some of the fastest moving portions of the cryosphere and may be the most unstable ice streams in Antarctica. I examined over 133 cores to conduct a detailed sedimentological facies analysis. These data, augmented by new radiocarbon and 210Pb dates, and bathymetric data, are used to reconstruct the post-LGM deglacial history of PIB and gain a better understanding of the causes of ice sheet retreat. My results record a clear retreat stratigraphy in PIB composed of, from top to base; terrigenous sandy silt (plumite), pebbly sandy mud (ice-proximal glacimarine), and till. Initial retreat from the outer-continental shelf began shortly after the LGM and before 16.4 k cal yr BP, in response to rising sea level. Bedforms in outer PIB document episodic retreat in the form of back-stepping grounding zone wedges and are associated with proximal glacimarine sediments. A sub-ice shelf facies is observed in central PIB that spans ∼12.3–10.6 k cal yr BP. Widespread impingement of warm water onto the continental shelf caused an abrupt change from sub-ice shelf sedimentation to distal glacimarine sedimentation dominated by dispersal of terrigenous silt between 7.8 and 7.0 k cal yr BP. The uppermost sediments in Pine Island Bay were hydrodynamically sorted by meltwater plumes. Inner Pine Island Bay contains several large basins that are linked by channels. The most recent release of sediment coincides with rapid retreat of the grounding line, and has an order of magnitude greater flux relative to the entire unit, indicating episodic sedimentation. This is the first identification of a meltwater-derived deposit in Antarctica and demonstrates that punctuated meltwater-intensive glacial retreat occurred at least three times throughout the Holocene in this region. Quartz sand grains were used to conduct an analysis of mode of transport for sediments in the Antarctic Peninsula region from the Eocene to present to record the onset of glaciation. Glacial transport imparts a unique suite of microtextures on quartz grains from high shear-stresses. Eocene samples are free of glacial influence. Late Eocene samples show the inception of glacially derived high-stress microtextures, marking the onset of alpine glaciation. Oligocene grains are similar to the late Eocene samples. Middle Miocene microtextures are characteristic of transport from far-field large ice sheets, originating from ice rafting from the West Antarctic Ice Sheet. The Pliocene and Pleistocene samples indicate the existence of the northern Antarctic Peninsula Ice Sheet at this time, consistent with other proxies.
Seismic imaging with traveltime and waveform inversion: Naga Thrust and Fold Belt, India
(2009) Jaiswal, Priyan; Zelt, Colin A.; Sawyer, Dale S.; Gordon, Richard G.; Hirasaki, George J.
I demonstrate the utility of traveltime and waveform inversion in depth imaging of seismic data with the help of two 2-D multichannel seismic lines, Reg-07 and PO-03, shot perpendicular to the trend of the Naga Thrust and Fold Belt (NTFB), India. I use both lines to demonstrate that a) a velocity model from traveltime inversion is suitable for pre-stack depth migration (PSDM) and can effectively serve as a starting model for frequency-domain full-waveform inversion; b) both PSDM and waveform inversion yield interpretable depth images but a combination of the two best describes the geology; and c) there still exists untested and unexploited hydrocarbon potential in the NTFB. Using the Reg-07 data I establish a working methodology for combining traveltime inversion with waveform inversion and PSDM. The results are validated using a nearby well. Using the PO-03 data I first demonstrate the advantages of combining traveltime inversion with PSDM for depth imaging; the combined method is referred to as unified imaging. Unified Imaging simultaneously yields a velocity model and a depth image that are consistent with each other. The velocity model from unified imaging is validated using an in-line well. The depth image from unified imaging reveals the presence of a triangle zone along PO-03 that was previously unknown and could be promising for exploration. Waveform inversion using the velocity model from unified imaging reveals the presence of a complex conjugate fault system in the supra-thrust along PO-03 which was also previously unknown and could also be promising for exploration. Traveltime inversion estimates a velocity model that is representative of the large scale features of the subsurface. This makes traveltime inversion a necessary first step regardless of the final choice of imaging--PSDM or waveform inversion. While PSDM uses the traveltime model as a whole for estimating a reflectivity image, waveform inversion enhances the resolution of a part of the traveltime model to yield a detailed acoustic property map. Even though the data and model requirements for PSDM are less stringent than for waveform inversion, waveform inversion appears to be resolving structural features that are imaged inadequately by PSDM.
Strength envelopes for deforming oceanic lithosphere with some implications for the history of lithospheric deformation of the equatorial Indian Ocean
(2009) Ghosh, Jaydip; Gordon, Richard G.
Strength envelopes for oceanic lithosphere inferred from laboratory experiments indicate that the oceanic lithosphere in the central Indian basin is several times stronger (≈27 TN m -1 ) than the force available to drive deformation (9±2 TN m -1 [95% confidence limits]). Thus the strength of oceanic lithosphere is over-estimated. Either pore pressure is much higher than hydrostatic, or the effective coefficient of friction is much lower than found in laboratory experiments, or both. For models in which the pore pressure is nominally fixed at zero, a coefficient of friction (0.11±0.04 [95% confidence limits]) is needed to explain the discrepancy. Although very low, this coefficient of friction is significantly larger than the coefficient of friction inferred for the interplate thrust at most subduction zones. For models resulting in acceptable strengths from various combinations of coefficient of friction and pore pressure, the ratio of the force per unit length supported by the brittle and semi-brittle components of the lithosphere to that supported by the lower lithosphere deforming by creeping flow at a reference strain rate of 10 -16 s -1 lies between limits of 3.5 and 12, which indicates that 78% to 92% of the strength of oceanic lithosphere lies in its brittle and semi-brittle portions. It follows that the onset of lithospheric folding at ≈8 Ma ago in the equatorial Indian Ocean was the result of an increase of driving force of no more than ≈28%, but a much greater increase in driving force at or before 15 to 20 Ma ago cannot be excluded.
Tests of fixity of the Indo-Atlantic hot spots relative to Pacific hot spots
(John Wiley & Sons, Inc., 2014) Koivisto, Emilia A.; Andrews, David L.; Gordon, Richard G.
Rates of inter-hot spot motion have been debated for decades. Herein we present updated predictions for the tracks of the Tristan da Cunha, Réunion, and Iceland hot spots assuming them to be fixed relative to Pacific hot spots. Uncertainties in Pacific hot spot rotations, which include uncertainties in the current locations of hot spots of 100–200 km, are combined with uncertainties in relative plate motions accumulated through the plate circuit to obtain the final uncertainty in the predicted positions (including uncertainties of 150–200 km in the current locations of the Indo-Atlantic hot spots). Improvements to reconstruction methods, to relative plate reconstructions, to age dates along the tracks, and to the geomagnetic reversal timescale lead to significant changes from prior results. When compared with the observed tracks, the predicted tracks indicate nominal rates of motion of only 2–6 mm a−1 of these Indo-Atlantic hot spots relative to Pacific hot spots over the past 48 Ma. Within the uncertainties, the rates range from no motion to rates as high as 8–13 mm a−1. For reconstructions prior to 48 Ma B.P., however, the apparent rates of inter-hot spot motion are much larger, 46–55 ± 20 mm a−1, if the motion occurred entirely between 68 Ma B.P. and 48 Ma B.P. Either hot spots moved rapidly before 48 Ma B.P., and slowed drastically at ≈ 48 Ma B.P., or global plate circuits through Antarctica become less reliable as one goes increasingly further into the past. Most paleomagnetic data favor the latter explanation.
Tests of inter-hotspot motion and of hotspot motion relative to the spin axis
(2010) Koivisto, Emilia Anna-Liisa; Gordon, Richard G.
First, an updated Pacific paleomagnetic skewness pole for chron 32 (72 Ma) is presented. The updated paleomagnetic pole corrects for the spreading-rate dependence of anomalous skewness, a correction which hasn't been applied to Pacific skewness poles before. The presence of anomalous skewness is one of the main factors limiting the accuracy of paleomagnetic poles determined from the skewness data. Thus, successfully correcting for the anomalous skewness, as was done in this study, significantly improves the reliability of the skewness poles. The earlier assertions that the Hawaiian hotspot has shifted southward relative to the spin axis by 13° since ≈72 Ma are also confirmed. Second, updated reconstructions of the Pacific plate relative to the hotspots for the past 68 million years are presented, with the uncertainties in the reconstructions. Plate-circuit reconstructions are used to predict the tracks of some major Indo-Atlantic hotspots (Tristan da Cunha, Reunion and Iceland) from the Pacific-hotspot plate motion and the rates of relative motion between the Pacific and Indo-Atlantic hotspots are estimated. Within the uncertainties, motion between the hotspots is found insignificant for the past 48 million years. For earlier times, a systematic error in the plate circuit used to make the predictions is inferred and which may be due to unmodeled motion between East and West Antarctica. If the observed discrepancy can be shown to correspond to an error in the plate circuit, the southward motion of the Hawaiian hotspot of 13° since ≈72 Ma can likely be attributed to true polar wander. Building on the above-mentioned work, finally, for the first time, a globally self-consistent model of plate motions relative to the global hotspots for the past 48 million years is presented, and the implications of this model to the question of relative hotspot motion discussed. The provided globally self-consistent set of reconstructions can be used as a fixed frame of reference for absolute plate motions, and true polar wander, for the past 48 million years.
The motion between Nubia and Somalia from magnetic anomaly and fracture zone crossings flanking the Southwest Indian Ridge
(2000) Lemaux, James Wilmer, II; Gordon, Richard G.
Previous reconstructions for anomaly 5 and older anomalies flanking the Southwest Indian Ridge (SWIR) have indicated no statistically significant evidence for motion between the Nubian and Somalian plates. Recently, an analysis of current plate motion across the SWIR indicates significantly different Nubia-Antarctica and Somalia-Antarctica angular velocities. Herein the motion across the SWIR is examined since chron 5 (11 Ma) and chron 6 (20 Ma). I identify 238 Anomaly 5 crossings, 140 Anomaly 6 crossings, and many fracture zone crossings. The new results show that the Nubia-Antarctica and Somali a-Antarctica rotations differ significantly. The results have several implications. (1) Earlier motion differs significantly from that since 3.2 Ma, implying that Nubia-Somalia motion began before 3.2 Ma. (2) Poles of rotation for motion since chron 5 and since chron 6 poles differ significantly from those after 3.2 Ma suggesting a component of right-lateral shearing has occurred along the East African rift since 11 and 20 Ma. (3) Data suggest that the boundary between Nubia and Somalia is narrow where it intersects the SWIR near the Andrew Bain fracture zone complex.

Browsing by Author "Gordon, Richard G."

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