A paleomagnetic study of ca. 580 Ma volcanics near Grand Bank, Avalon zone of Newfoundland, and implications for true polar wander in the ediacaran

Farrell, Sarah (2019) A paleomagnetic study of ca. 580 Ma volcanics near Grand Bank, Avalon zone of Newfoundland, and implications for true polar wander in the ediacaran. Masters thesis, Memorial University of Newfoundland.

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Abstract

Paleomagnetic studies suggest that Laurentia moved from the equator to the pole and then back again within ~60 Ma during the Ediacaran. Hypotheses to explain the large 90⁰ apparent oscillations include unreliable paleomagnetic data and/or unreliable dating and complex magnetic field behaviour. Inertial interchange true polar wander (IITPW) has also been suggested. Since IITPW would be a global event, it would require the large oscillations suggested by the paleomagnetic record of Laurentia to have affected all the continents in the Ediacaran. Such oscillations have been found in selected paleomagnetic data from Baltica and West Africa but not Australia. Avalonia paleomagnetism also does not appear to show the oscillations expected from the IITPW hypothesis. The present study provides new paleomagnetic data for the Marystown Group volcanics, dated at 580±3 Ma, collected near Grand Bank in the Avalon Zone of Newfoundland. The volcanics were studied with alternating field and thermal demagnetization which showed that remanence is carried mostly by magnetite rather than hematite. Seven sites provide stable remanence directions with mean tilt-corrected declination and inclination of D=287⁰ and I=58⁰ (α95 = 13.7⁰). A positive conglomerate test, using rhyolitic crystal lithic tuff clasts from an agglomerate within the section of flows, shows that the magnetite-bearing clasts carry primary remanence. Magnetic polarity reversals are also present within the section (six normal flows and one reversed flow). The corresponding paleolatitude is 39⁰+17/-13⁰ at 580±3 Ma. This agrees with the paleolatitude of 35⁰+11/-10⁰ at 580±10 Ma previously found for three basalt flows and five red bed sites in the Marystown Group (omitting the likely Devonian Famine Back Cove basalts). The Marystown Group results, along with other stable paleomagnetic data from Avalonia, suggest that Avalonia remained at mid to low latitudes during the mid-Ediacaran. The declination of Avalonia suggests that it remained at 225⁰ from 606 Ma to 592 Ma and then underwent a steady change towards 330⁰ at 550 Ma. Although IITPW cannot be disproved for the mid-Ediacaran, it is more likely that complex magnetic field behaviour is the reason multiple large oscillations can be seen in the Laurentia, Baltica, and West African Craton paleomagnetic data.

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