Plumes, Motion Between Hot Spots, and True Polar Wander I
Presiding: R G Gordon, Rice University; R J O'Connell, Harvard University; V Courtillot, Institut de Physique du Globe de Paris
GP21A-01 08:30h
Pacific Plate Apparent Polar Wander and Latitudinal Shift of the Hawaiian Hotspot: A Brief Review and Future Prospects
Except for young volcanic islands, the Pacific plate is largely lacking outcrop amenable to traditional paleomagnetic analysis. As a result, a wide variety of alternative methods have been developed for estimating pole positions including analysis of the magnetic anomalies over seamounts ("seamount poles"), shape analysis of magnetic anomalies due to seafloor spreading ("skewness poles"), analysis of the amplitudes of magnetic anomalies due to seafloor spreading, analysis of azimuthally unoriented piston cores and deep-sea drilling cores in both sedimentary and igneous rocks, and analysis of equatorial sediment facies. In the 1970s, studies of seamount poles, of skewness, and of equatorial sediment facies all provided strong evidence for northward motion of the Pacific plate, in general agreement with the northward motion of the Pacific plate predicted if it is assumed that the hotspots are fixed relative to the spin axis. Beginning in about 1980, however, strong evidence was obtained that the northward motion of the Pacific plate relative to the spin axis was less than its northward motion relative to the Hawaiian and other hotspots, implying a southward drift of the Hawaiian hotspot. Results from DSDP Leg 55 first demonstrated southward motion of the Hawaiian hotspot [Kono 1980, Jackson, Koizumi et al. 1980]. Morgan [1981] showed that paleomagnetic poles and hotspot tracks from the Indian and Atlantic Oceans (and bordering continents) predicted this southward motion if the Pacific hotspots are fixed relative to those in the Indo-Atlantic, while both move together relative to the spin axis. This coherent motion of global hotspots can be interpreted as the result of true polar wander. Gordon & Cape [1981] and Gordon [1982] analyzed equatorial sediment facies, published piston core paleomagnetic data, and skewness data. They showed that the data were mutually consistent and indicated southward motion of the Hawaiian hotspot relative to the spin axis consistent with Morgan's predictions. Since then, the apparent polar wander of the Pacific plate has been elaborated in considerably greater detail. Here I concentrate on the results from skewness. Because the ages of the magnetic anomalies analyzed are unambiguous and automatically known to the nearest polarity chron, the ages of skewness poles are precisely known. The geometry of spreading on the Pacific plate during Cenozoic and Late Cretaceous time enables the estimation of poles with very compact confidence regions. Poles have been determined for many different anomalies and give a detailed, but far from complete, view of Pacific plate apparent polar wander since 83 Ma. In contrast, results from igneous rocks obtained by deep sea drilling are far sparser, less accurate, and give only paleolatitudes and not unique poles. The data are nevertheless useful, especially for time intervals lacking magnetic reversals (i.e., the Cretaceous Normal Polarity Superchron) and to provide an independent test of the results from skewness and other approaches. These independent tests reveal a high level of consistency with skewness results, strongly supporting the usefulness and reliability of skewness analysis.
GP21A-02 08:45h
Theoretical Constraints on True Polar Wander
For the present geologic epoch, long term true polar wander (TPW) is relatively small, but simple theoretical considerations suggest that it could have been larger in other epochs and could have been responsible for 90 degree inertial interchange events. In this work, we use scaling arguments to assess the qualitative behavior of TPW and a simple Maxwell model to analytically describe how changes in mass anomalies translate into TPW. Unlike previous work, our goal is to derive simple analytical estimates of TPW based on the characteristic amplitudes and timescales for changes in the moment of inertia. We find estimates for both the amplitude and speed of TPW as a function of various Earth properties. Our analysis shows that there are four main factors that strongly influence how large the maximum TPW can be: the (geological) timescale of the forcing τforce, the viscosity structure of the mantle which yields a weighted average viscosity η, the characteristic amplitude of the non-hydrostatic changes in moment of inertia ΔCdyn, and the added moment of inertia due to the equatorial bulge (C-A). The maximum TPW speed is not sensitive to the timescale of forcing although the total TPW reorientation angle and the TPW acceleration do depend on this timescale. The maximum TPW speed is (7 degrees per Million years) * (ΔCdyn/0.003(C-A)) * (1022 Pa*s/η). For a model where τforce = 108 years, an η of 1022 Pa*s (higher than standard upper mantle estimates), and ΔCdyn/(C-A) = 0.003, we obtain a maximum TPW angle of 88 degrees. With the same values except τforce = 107 years, the maximum TPW angle is 23 degrees. The same analysis provides the dependence of maximum TPW angle on the 4 factors. TPW is shown to act as a low pass filter: rapid changes in the moment of inertia produce smaller and delayed TPW. Thus, TPW may have been an important contributor to plate motions over relatively long timescales but not over shorter timescales. Our simple approach allows us to assess whether multiple TPW events are possible but the major uncertainty continues to be the mantle viscosity structure.
GP21A-03 INVITED 09:00h
Hotspot motion, paleomagnetic tests of plate circuits,and coherency of the Pacific plate
Paleomagnetic analyses conducted during Ocean Drilling Program (ODP) Leg 197, studies of plate circuits and geodynamic modeling results have all pointed toward motion of the Hawaiian hotspot during formation of the Emperor Seamounts. Here we examine plate circuits that transfer data to the Pacific basin through West Antarctica (e.g. Cande et al., 1995) and through the Lord Howe Rise (Steinberger et al., 2004). We find that both circuits pass paleomagnetic consistency tests and yield hotspot motion rates that are compatible with ODP Leg 197 data based on rigorous rock magnetic and paleomagnetic investigations. An important part of this evaluation is the recognition that prior Pacific apparent polar wander paths (APWPs) fail internal consistency tests, and that the physical processes derived from these paths, including rates of true polar wander, are untenable. The failure of prior Pacific APWPs can be traced to an overreliance on remote sensing data (modeling of seamount anomalies and marine magnetic anomaly skewness) and an underappreciation of the inherent limitations of such data. Differences between paleomagnetic confidence intervals and those typically portrayed for remote sensing data will be briefly reviewed. Notwithstanding the elegance of some technical analyses, remote sensing data are clearly past their prime in terms of answering topical questions regarding hotspots and mantle plumes. Similarities between the Hawaiian-Emperor and Louisville tracks of the Pacific plate suggest that the hotspot motion defined by the ODP Leg 197 paleomagnetic data may reflect basin-wide flow. New age data, however, also point toward intra-basin hotspot motion and/or lithospheric control on hotspot tracks. We also test coherency of the Pacific plate since the Late Cretaceous to evaluate the degree to which intra-plate deformation could have affected hotspot tracks.
GP21A-04 INVITED 09:20h
Mantle Dynamics and the Long-Term Rotational Stability of the Earth
The long term (10-100 Ma) rotational stability of a dynamic, evolving Earth is a classic problem in geophysics framed by a series of seminal studies (e.g., Gold, 1955; Goldreich and Toomre, 1969). Gold (1955), for example, considered the stability of a hydrostatic planet subject to an imperfectly compensated (internal or external) load. In this case, the hydrostatic bulge provides no long-term rotational stability and the reorientation of the pole, or so-called true polar wander (TPW), would be governed solely by the location of the load. In particular, a mass excess of any size (indeed, as small as Gold's beetle) would drive a TPW that would eventually reorient the load to the equator. Gold's (1955) arguments were extended by Goldreich and Toomre (1969) who demonstrated that a group of anomalous masses moving randomly on the surface (the classic set of scurrying beetles) could drive rapid (relative to the speed of the masses) reorientation of the rotation pole. This inherent instability of the rotation axis appears to be at odds with observational evidence for a relatively stable rotation axis over the last 200 Ma. Previous studies have explained this stability through some combination of a high viscosity (sluggish) lower mantle and/or a relatively fortuitous distribution of mantle heterogeneity. In this talk we present a new set of predictions of long term TPW based on a large suite of three-dimensional convection simulations. These simulations, which are constrained by recent estimates of the radial profile of mantle viscosity and initiated using seismically-inferred mantle heterogeneity, yield a suite of simple conditions governing rotational stability in the post-Jurassic Earth.
GP21A-05 INVITED 09:40h
Ediacaran Rapid True Polar Wander: Constraints on a Possible Driving Mechanism From the Paleomagnetism and Geochronology of the Sept-Îles Intrusive Suite, Quebéc, Canada
The 80 km-diameter Sept-Îles intrusive suite and associated regional dikes are interpreted as the eroded remnant of a plume that was emplaced beneath NE North America in mid-Ediacaran time(1,2). We report here new paleomagnetic, zircon U/Pb, apatite (U-Th-Sm)/He, and conodont alteration results from rocks associated with the Sept-Îles intrusive suite which help constrain the Laurentian apparent polar wander path (APWP) during the newly-defined Ediacaran Period. Our paleomagnetic results are in general agreement with those of an earlier study by Tanczyk et al.(3), which identified a two-polarity, low-latitude component (A), here dated as young as ~562 Ma, overprinted by a more steeply-inclined two polarity magnetization (B), here constrained to be not older than ~561 Ma by U-Pb crystallisation ages on samples that contain only the B component and cross-cut those with the A component. Low conodont alteration index in an early Whiterockian (early Middle Ordovician) fauna extracted from flat, overlying limestone, and early Cretaceous helium closure ages in igneous apatites from the middle part of the intrusion, indicate that neither component of magnetization may be attributable to post-Cambrian thermal remagnetization. Older, robust Ediacaran paleomagnetic studies imply that Laurentia occupied high paleolatitudes for most of the early part of Geon 5. Thus, our results suggest that the emplacement of the Sept-Îles plume was coincident with rapid movement of Laurentia. Perhaps the intrusion triggered a pair of rapid TPW bursts, first moving the site towards the equator and then with subsequent thermal relaxation moving it back to higher latitude. If the Earth's principal and intermediate moments of inertia were of similar magnitude, relatively small perturbations in the Earth's mass distribution could have initiated inertial interchange TPW events. This is consistent with the hypothesis of Evans(4) that multiple bursts of true polar wander dominate the APWP signals for all continents during Ediacaran and Early Cambrian time. 1.M. D. Higgins, Lithos (in press). 2. M. D. Higgins, O. van Breemen, Journal of Geology 106, 421 (1998). 3. E. I. Tanczyk, P. Lapointe, W. A. Morris, P. W. Schmidt, Can. J. Earth Sci. 24, 1431 (1987). 4. D. A. Evans, Earth and Planetary Science Letters 157, 1 (1998).