Hotspot Fixity Research Articles

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 (44 Ma)

AbstractWhile 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.

Uncertainties in plate reconstructions relative to the hotspots; Pacific-hotspot rotations and uncertainties for the past 68 million years

SUMMARY A method previously used for estimating the uncertainty in rotation of a plate relative to the hotspots is shown to be insufficient because of the neglect of the uncertainty in one of the three degrees of freedom of a rotation. Two new methods for estimating best-fitting rotations and associated uncertainties for the rotation of a plate relative to the hotspots are presented. Both methods require a priori estimates of the uncertainties in the locations of individual hotspots and their ancient tracks for a particular age. The use of a priori uncertainties permits the hypothesis of hotspot fixity to be tested. The first method, the two-hotspot method, leads to simple geometrical interpretations of best-fitting rotations and of the eigenvectors and eigenvalues of the appropriate covariance matrix. This simplicity comes at the cost of using only two hotspots and their associated tracks with assumed equal circular uncertainties. The second method, the N-hotspot method, allows use of an arbitrary number of hotspots (≥2), with unequal elliptical uncertainties. This generality and power comes at the cost of losing the simple geometrical interpretation of the first method. Elliptical uncertainties are generally more appropriate because hotspot tracks are less well known along-strike than across-strike because of gaps and uncertainties in ages along the tracks. When both methods are applied to the estimate of the Pacific-hotspot rotation since the age of the Hawaiian‐Emperor elbow, using the tracks of both the Hawaiian and Louisville hotspots, they produce similar rotations and similar uncertainties. The N-hotspot method is further applied to estimate Pacific-hotspot motion at six additional ages from 10.9 Ma to 67.7 Ma, corresponding to the ages of key magnetic anomalies used in global plate reconstructions. The goodness of the resultant fit shows that the assigned uncertainties and assumption of hotspot fixity are mutually consistent. The pre-elbow (i.e. pre-47.9 Ma or Emperor) pole of rotation is shown to differ significantly from the post-elbow (i.e. post-47.9 Ma or Hawaiian) pole of rotation. Moreover, rotation over the past 47.9 Ma is shown to have occurred in at least three statistically distinct stages, from 47.9 to 20.1 Ma, from 20.1 Ma to ≈6 Ma, and from ≈6 Ma to the present. The change in pole position at 20.1 Ma is accompanied by a highly significant 50 per cent increase in the rate of rotation of the Pacific plate relative to the hotspots. These encouraging results suggest that the new methods can be used to improve and to quantify plate motions relative to the hotspots for a wide variety of applications.

Moving hotspots or reorganized plates?

Hotspot fixity and no major plate reorganization in the past ∼40 m.y. are two propositions important for studying absolute plate motion and mantle dynamics. Reexamining the hotspot data with the NUVEL-1A relative plate motion model, we find that these two propositions cannot be simultaneously tenable: either hotspots have been moving relative to each other, or a major reorganization of plate motion occurred in the past ∼40 m.y. Statistical compatibility tests show that hotspot rate data are incompatible with trend data, implying that hotspots have moved. Furthermore, hotspot rates are consistently lower than those predicted by best-fitting absolute plate motion models. This may be explained by hotspots moving systematically opposite to the plate motion. If so, the moving hotspots still provide a useful reference frame for defining absolute plate motion. However, all hotspot trend data, with ages as old as 40 Ma, are statistically compatible, indicating no major reorganization of plate motion in the past ∼40 m.y. In this case, relative plate motion models based on young marine magnetic anomalies may be used to infer older (<40 Ma) plate kinematics, which lack direct constraints.

Motion of the Easter hot spot relative to Hawaii and Louisville hot spots

Hot spots have been widely used as a fixed reference frame for plate motions; however, it should be expected that the underlying plumes are advected in a dynamic mantle. Here a simple numerical model of hot spot motion due to large‐scale mantle flow is applied to the Easter, Hawaii and Louisville hot spots. Computations are performed for a range of different buoyant plume rising speeds, ages and locations, mantle density and viscosity structures, etc. It is shown that a westward motion of the Easter hot spot at several cm per year relative to Hawaii and Louisville hot spots is a robust result for a large range of model parameters. In order to obtain an appropriate boundary condition for mantle flow near the Easter hot spot and to find out on which plate to expect the Easter hot spot track for a given time, published isochrons were rotated to the past ridge locations in a reference frame that takes the computed motion of Hawaii and Louisville hot spots into account. Calculations that include flow in the mantle and therefore relative movement of hot spots yield a better explanation of the observed age distribution along the Sala y Gomez ridge than assuming hot spot fixity; at the same time they predict a hot spot track on the Nazca plate that is roughly similar to the shape of the Nazca and Sala y Gomez ridges. Furthermore, the Easter hot spot was in an intraplate location on the Nazca plate during the past 43 Ma, somewhat closer to the ridge between ∼43 and 26 Ma than between ∼26 Ma and now. In contrast, if hot spots are assumed fixed, a location close to the ridge and rather on the Pacific plate between ∼43 and 26 Ma is predicted. Based on morphology and gravity signature it is suggested that the Nazca ridge and the eastern Tuamotu Island Plateau were created by plume material that erupted at the East Pacific Rise while the Easter plume was located sufficiently close to enable such an interaction, whereas the Sala y Gomez ridge was formed above the Easter plume in an intraplate setting. This implies that, if hot spots have indeed moved as computed here, during formation of the Nazca ridge (between ∼43 and 26 Ma) plume material erupted at the ridge several 100 km away from the plume, whereas no volcanism occurred directly above the plume. It also implies that any seamounts on the Pacific plate east of the Tuamotu Plateau are unrelated to the Easter plume.

Is plate tectonis withstanding the test of time?

Since the theory of plate tectonics was first proposed thirty years ago, some problems have arisen in its practical application. These call into question its fundamental assumptions of horizontal plate motion, hotspot fixity, true polar wander, Panthalassa, and the Earth’s constant size while leaving seafloor spreading and subduction intact. A rapidity expanding earth solves these problems and privides an alternative viewpoint worth reconsidering.

A geometric technique for relocating hotspots and refining absolute plate motions

An age-independent, geometric relationship is presented that links hotspots to the seamounts which they produce, and so permits the use of undated seamounts to refine the motion of tectonic plates. This technique has the potential to rigorously assess hotspot fixity and to locate extinct hotspots. The present application of this method points to a recent change in Pacific plate motion, and suggests a relocation of the Louisville hotspot to the Hollister ridge, south of the Eltanin fracture zone.

Large-scale motion between Pacific and Atlantic hotspots

STUDIES of true polar wander (TPW), the rotation of the solid Earth with respect to the spin axis1, have suggested that there has been 10–15° of relative motion over the past 130 Myr (refs 2–4). In such studies, the orientation of the spin axis is recovered from continental palaeomagnetic poles (corrected for relative plate motions), and compared with a deep-mantle reference frame defined by hotspot locations. But deducing relative plate motions becomes increasingly difficult for older (Mesozoic) time periods, hindering tests of TPW on timescales comparable to those of large-scale mantle convection; moreover, the assumption of hotspot fixity is controversial5,6. We examine here a more direct approach7,8, using palaeolatitudes derived from Pacific guyots. Contrary to predictions from TPW models, these data suggest only minor latitudinal shifts of Pacific hotspots during the Cretaceous period. Instead of TPW, relative motion between the Atlantic and Pacific hotspot groups9 is required at a velocity of approximately 30 mm yr−1, more than 50% larger than previously proposed5.