Analysis of an Updated Paleointensity Database (Q(PI)-PINT) for 65-200 Ma: Implications for the Long-Term History of Dipole Moment Through the Mesozoic

The global paleointensity database for 65–200 Ma was analyzed using a modified suite of paleointensity quality criteria (QPI) such that the likely reliability of measurements is assessed objectively and as consistently as possible across the diverse data set. This interval was chosen because of dramatic extremes of geomagnetic polarity reversal frequency ranging from greater than 10 reversals per million years in the Jurassic hyperactivity period (155–171 Ma) to effectively zero during the Cretaceous Normal Superchron (CNS; 84–126 Ma). Various attempts to establish a relationship between the strength of Earth\u27s magnetic field and the reversal frequency have been made by previous studies, but no consensus has yet been reached primarily because of large uncertainties in paleointensity estimates and sensitivity of these estimates to data selection approaches. It is critical to overcome this problem because the evolution of the dipole moment is a first order constraint on the behavior of the geodynamo. Here we show that conventional statistical tests and Bayesian changepoint modeling consistently indicate the strongest median/average virtual dipole moment during the CNS. In addition, the CNS and Jurassic hyperactivity period are characterized by the highest and lowest percentage of virtual dipole moments exceeding the overall median for the 65‐ to 200‐Ma interval, respectively. These observations suggest that the superchron dynamo was able to generate stronger fields than the dynamo operating in the frequently reversing regime. While the precise mechanism remains unclear, our results are compatible with the hypothesis that field strength and reversal rate variation are controlled by changes in core‐mantle boundary thermochemical conditions

Phanerozoic polar wander, palaeogeography and dynamics

A significant number of new palaeomagnetic poles have become available since the last time a compilation was made (assembled in 2005, published in 2008) to indicate to us that a new and significantly expanded set of tables with palaeomagnetic results would be valuable, with results coming from the Gondwana cratonic elements, Laurentia, Baltica/Europe, and Siberia. Following the Silurian Caledonian Orogeny, Laurentia's and Baltica's Apparent Polar Wander Paths (APWPs) can be merged into a Laurussia path, followed in turn by a merger of the Laurussia and Siberia data from latest Permian time onward into a Laurasian combined path. Meanwhile, after about 320. Ma, Gondwana's and Laurussia/Laurasia's path can be combined into what comes steadily closer to the ideal of a Global Apparent Polar Wander Path (GAPWaP) for late Palaeozoic and younger times. Tests for True Polar Wander (TPW) episodes are now feasible since Pangaea fusion and we identify four important episodes of Mesozoic TPW between 250 and 100. Ma. TPW rates are in the order of 0.45-0.8°/M.y. but cumulative TPW is nearly zero since the Late Carboniferous. With the exception of a few intervals where data are truly scarce (e.g., 390-340. Ma), the palaeomagnetic database is robust and allows us to make a series of new palaeogeographic reconstructions from the Late Cambrian to the Palaeogene. © 2012 Elsevier B.V

A Precambrian microcontinent in the Indian Ocean

The Laccadive–Chagos Ridge and Southern Mascarene Plateau in the north-central and western Indian Ocean, respectively, are thought to be volcanic chains formed above the Réunion mantle plume1 over the past 65.5 million years2,3. Here we use U–Pb dating to analyse the ages of zircon xenocrysts found within young lavas on the island of Mauritius, part of the Southern Mascarene Plateau. We find that the zircons are either Palaeoproterozoic (more than 1,971 million years old) or Neoproterozoic (between 660 and 840 million years old). We propose that the zircons were assimilated from ancient fragments of continental lithosphere beneath Mauritius, and were brought to the surface by plume-related lavas. We use gravity data inversion to map crustal thickness and find that Mauritius forms part of a contiguous block of anomalously thick crust that extends in an arc northwards to the Seychelles. Using plate tectonic reconstructions, we show that Mauritius and the adjacent Mascarene Plateau may overlie a Precambrian microcontinent that we call Mauritia. On the basis of reinterpretation of marine geophysical data4, we propose that Mauritia was separated from Madagascar and fragmented into a ribbon-like configuration by a series of mid-ocean ridge jumps during the opening of the Mascarene ocean basin between 83.5 and 61 million years ago. We suggest that the plume-related magmatic deposits have since covered Mauritia and potentially other continental fragments