A paleomagnetic analysis of Cambrian true polar wander

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Abstract

The latest Neoproterozoic through Cambrian is one of the most remarkable intervals in geologic time. Tectonically, the period from 580 to 490 Ma marks a time of rapid plate reorganization following the final stages of supercontinental breakup and Gondwana assembly. The apparent speed at which this reorganization occurred led some to propose a link between tectonic events, biologic changes and climatic changes. One of the more intriguing proposals is that the tectonic changes were triggered by an episode of inertial interchange true polar wander (IITPW) which resulted in a rapid (6°/m.y.) shift of the spin axis relative to the geographic reference frame. IITPW is a special case of true polar wander (TPW) that makes specific demands on the length of apparent polar wander paths (APWPs) recording the motion. Specifically, each path must allow for ∼90° of synchronous motion during the interval from 523 to 508 Ma. A review of paleomagnetic data for Laurentia, Baltica, Siberia and Gondwana indicates that none of the APWPs approaches the necessary length, each path is of a different length and the apparent motions are non-synchronous. Collectively, these observations negate the premise of a Cambrian IITPW event. Since the IITPW hypothesis was proposed as an alternative to rapid plate motion of Laurentia and Gondwana during the Neoproterozoic–Cambrian interval, any alternative model must account for this rapid motion. I suggest that a reasonable explanation for `anomalously' high rates of plate motion for some continents, possibly on the order of 20–40 cm yr−1, is enhanced plate motion driven by lower-mantle thermal anomalies and possibly true polar wander. In fact, the enhanced plate motions driven by these lower-mantle sources may provide a dynamic feedback triggering true polar wander.

Introduction

The Vendian–Cambrian boundary represents one of the most puzzling and intriguing transitions in earth history. It marks the first time that all major phyla are well represented in the fossil record 1, 2, 3, major transitions in seawater chemistry 4, 5, plate reorganization, the breakup of the vestiges of the Rodinia supercontinent 6, 7 and a possible change from a severe icehouse climate to a greenhouse climate [5]. All of these changes may have taken place over a relatively short interval of geologic time, giving rise to speculation about cause and effect among the observed changes. For example, did the breakup of the remnants of the Rodinia supercontinent lead to an icehouse climate (snowball earth) followed by a rise in δ13C and 87Sr/86Sr isotopic values 4, 5? Did this climatic change in turn trigger the rise of the metazoans? Was the rise of the metazoans stimulated by the changes in the oceanic environment due to a rapid redistribution of landmasses 5, 8? Indeed, this period in earth history may represent more than a simple time marker; it may represent a fundamental shift in the modus operandi of the earth's climatological and geodynamic systems.

Several authors 6, 9, 10 noted the rapid migration of continents at the close of the Precambrian. These authors remarked that the calculated minimum velocities suggested that large continents were able to move at much higher velocities than was typical of Phanerozoic time but provided no geodynamic explanation for this rapid motion. Gurnis and Torsvik [11] demonstrated that upper-mantle convection was not sufficient to drive large plates at velocities in excess of 25 cm yr−1 due to drag forces at the base of a thickened lithosphere. They did show, through the use of finite-element models, that such velocities could be attained provided a continent with a thick lithospheric root was pushed away from a region of elevated deep-mantle temperatures (plumes) or pulled toward a deep-mantle cold spot (Fig. 1a).

An alternative explanation [8] for this rapid continental motion is that it resulted from an episode of inertial interchange true polar wander (IITPW) during the period from Early Cambrian (late Tommotian ∼523 Ma) to early-Middle Cambrian (Amgan ∼508 Ma). True polar wander (TPW) is the migration of the net lithospheric and mantle reference frame relative to the spin axis (Fig. 1b). IITPW is a special case of TPW wherein the Earth's intermediate inertial axis and maximum inertial axis interchange (i.e. the maximum axis becomes the intermediate axis and vice versa, Fig. 1c). IITPW requires a 90° shift in the geographic reference frame relative to the spin axis in a relatively short amount of geologic time [8]. The exact time it takes for these axes to interchange depends on the material properties of the interior of the earth; however, time periods as short as 10–15 m.y. have been proposed 8, 12, 13. The IITPW hypothesis was expanded upon by Evans [14] who proposed that TPW is a geodynamic legacy of the preceding supercontinent of Rodinia.

Paleomagnetic studies are typically used to determine the relative motion of continents with respect to the spin axis of the earth. Paleomagnetists represent this motion through the use of apparent polar wander paths (APWPs). These paths represent the motion of the continent through time in terms of latitudinal drift and rotation. Because of the assumed symmetry of the geomagnetic field, longitudinal motion is not quantifiable from paleomagnetic studies alone. However, should the motion of the entire lithosphere take place in unison and at rates exceeding typical plate velocities, then the amount of APW measured by paleomagnetic studies will reflect the amount of true polar wander (APW = TPW) and the relative longitudinal positions of landmasses can be determined [8]. Thus, in order to demonstrate an episode of rapid TPW several requirements must be met. One requirement is that APWPs from all continents show nearly the same lengths and shapes for a given time interval. This is theoretically possible to demonstrate with a complete paleomagnetic database. The second requirement is that all plates show more or less the same amount of APW for the interval of proposed TPW. Since we have no paleomagnetic record from the oceanic plates that must have existed in pre-Mesozoic times, any analysis of TPW prior to the Mesozoic will be incomplete.

Section snippets

Cambrian IITPW episode

Kirschvink et al. [8] examined selected paleomagnetic data available from Baltica, Laurentia, Gondwana and Siberia for the interval from latest Vendian through Ordovician time. They concluded that selected paleomagnetic data were compatible with the hypothesis of an episode of inertial interchange true polar wander beginning during the Early Cambrian (late Tommotian) until early-Middle Cambrian time (Amgan; 523–508 Ma using the revised Cambrian time scale discussed below). Both [8] and [15]

Cambrian time scale

The hypothesis of IITPW was forwarded in part to explain the rapid radiation that occurred during the Tommotian–Toyonian stages of the Early Cambrian [8]. A revised Cambrian time scale 16, 17 places additional constraints on the IITPW hypothesis. Fig. 2 shows the revised Cambrian time scale along with the radiometric ages used to constrain the time scale 16, 17, 18. Specifically, the new time scale shifts the end of the Lower Cambrian from ∼518 Ma to ∼510 Ma 16, 18. Furthermore, the Upper

Baltica

Support for an episode of IITPW from the Baltica database was permissible because there are only two paleomagnetic results from that continent that bracket the proposed interval. Kirschvink et al. [8] used paleomagnetic results from the Fen Central Complex (FCC) at 583 ± 15 Ma [19] and the Lower Ordovician (Arenig–Llanvirn ∼475–470 Ma) Swedish limestones [20] for the analysis.

Reconstructions using the IITPW model resulted in an overlap between Baltica and Gondwana using the south-pole option

Discussion

Inertial interchange true polar wander was introduced by Kirschvink et al. [8] to explain the apparently rapid continental motion that occurred near the end of the Neoproterozoic. The IITPW hypothesis makes specific predictions about both the magnitude and duration of APW. Specifically, IITPW requires 90° of APW in about 15 m.y. for each continent. This translates to a rate of APW of 66 cm yr−1. As previously noted, this 90° of APW represents an ideal value since any plate motion may be added

Conclusions

Kirschvink et al. [8] suggested that the rapid continental motion that occurred at the dawn of the Phanerozoic was triggered by a pulse of inertial interchange true polar wander. A careful analysis of the original paleomagnetic data used in that study, new paleomagnetic data from Laurentia and Baltica, a re-evaluation of the Siberian and Gondwana paleomagnetic database and a revised Cambrian time scale indicate that the IITPW hypothesis is not supported by the available data. Rejection of the

Acknowledgements

This research was supported in part by NSF grant EAR98-05306. The author wishes to thank Prodip Dutta and Chad Pullen for comments on an early draft of this paper and Paul Hoffman and Joe Hodych for their reviews of the manuscript. I also wish to thank Dave Evans, who disagrees with many of the conclusions in this paper, for a thorough critique of the data and interpretations that resulted in an improved version of the manuscript. [RV]

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