Plate tectonics plays a crucial role in shaping Earth's geography, impacting the evolution of life and climate. To really understand the long-term evolution of Earth’s systems, we need to quantify the past motions of the tectonic plates. Plate reconstructions have been crucial in figuring out an enormous array of Earth processes.
Conventionally, past plate motions are inferred from physical characteristics of the sea floor, specifically its magnetic anomalies and fracture zones. Together, those features have allowed the construction of global plate reconstructions back to the Cretaceous (~130 Ma). But the inherent nature of plate tectonics masks its own origin story: oceanic lithosphere records are progressively destroyed by subduction, so they cannot be used in deeper time. Before 130 Ma, plate motions can only be quantified through the study of paleomagnetism.
Over long time scales (~105-106 years), the geomagnetic field can be approximated by a geocentric axial dipole (GAD), where the vertical field component is specifically linked to latitude and the horizontal component consistently points north. This means that if a rock can record the direction of the paleomagnetic field during its formation and the GAD hypothesis remains valid (at least back to ~540 Ma), we can establish its original paleolatitude and azimuthal orientation! However, analytical limitations have so-far prevented us from using this tool to its full potential. For example, owing to the axial symmetry of the Earth’s magnetic field, the determination of paleolongitude from paleomagnetic data –although theoretically possible– cannot be constrained. Paleolongitude has thus remained the greatest uncertainty in pre-Cretaceous plate reconstructions (top panel figure).
Paleomagnetic records are collected from individual rock samples and subsequently grouped to develop global-scale paths called apparent polar wander (APW) paths. These APWPs represent the time-dependent position of Earth's spin axis relative to a given block of lithosphere or continent.
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| Hypothetical APWP track (filled dots). Top panel: Conventional paleomagnetic reconstruction where paleolongitude remains unconstrained. Bottom Panel: the APWP segment traces a small circle, the centre of which represents the Euler vector. Rotations about Euler poles can completely define plate motion so PEP analysis yields east-west motion. |
Euler’s theorem states that any displacement across the surface of a sphere can be represented by a rotation about an axis (i.e. Euler vector). Consequently, if a continent rotates about a fixed axis, the corresponding paleomagnetic poles (i.e. APWP segment) will trace an arc which can be defined by a small circle, the centre of which represents the Euler vector. The inversion of paleomagnetic data to retrieve Euler vectors – or paleomagnetic Euler pole (PEP) analysis – is of particular interest because it offers the possibility to recover full kinematic descriptions of past plate motion. Because an Euler vector can fully express the kinematics of a continent, if paleomagnetic data can be used to compute Euler vectors describing a given plate’s history of motion, its paleolongitude is determinable!
This exciting method could overcome the paleolongitudinal indetermination of paleomagnetism, but despite being first conceptually introduced more than half a century ago has seen limited application. This appears to be due, at least in part, to the fact that paleomagnetic data is inherently noisy, with noise coming from both intrinsic (geomagnetic secular variation) and extrinsic (e.g. measurement errors, erroneous age assignments in rocks, inclination shallowing, etc.) uncertainties. For instance, current APWPs describe plate motion in 10 Ma steps, yielding a crude description of plate latitudinal and azimuthal motion.
Recent advances in APWPs construction methods have demonstrated that through new methodologies and computational methods, it is possible to generate APW paths with unprecedented spatial and temporal resolution (~1Ma). These new methods may offer new insights into Earth's deep time evolution. Great things are on the horizon!
Leandro Gallo is a Maria Skłodowska-Curie postdoctoral fellow at the Center for Planetary Habitability (PHAB), a Center of Excellence funded by the Norwegian Research Council and hosted at the University of Oslo (UiO), Norway. His research focus is reconstructing the long-term changes in the ancient spatial configuration of continents (paleogeography). A major focus of this research is on paleomagnetic data synthesis tools, combining data-analysis, data-science and statistics to constrain polar wander through deep time.
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