PASS · Paleozoic Seafloor Spreading

Paleozoic Seafloor Spreading

Seafloor spreading forms two-thirds of the surface of our planet and involves accretion of magmatic rocks at spreading ridges in the oceans. However, plate divergence in slow-spreading oceans may also be taken up by extensional detachment faulting. This project attempted to extend the record of this newly-recognised detachment-mode of seafloor spreading back to the Paleozoic, by investigating a potential detachment system preserved in the Ordovician Thetford Mines Ophiolite of Canada, a slice of oceanic lithosphere emplaced onto the North American continent during the closure of the Iapetus Ocean. The intention was to perform a field-based structural and paleomagnetic investigation to test the hypothesis that this ophiolite contains the oldest known analogue for modern oceanic detachment systems. The objectives were to: (i) systematically back-strip successive tectonic events that have affected the ophiolite, in order to recover primary seafloor relationships; (ii) determine whether relative tectonic rotation has occurred across the detachment fault, which is a defining feature of examples in the modern oceans; and (iii) quantify the role of large- and small-scale faulting in accommodating displacement within the detachment system. The project was also designed to provide extensive training in field-based tectonic applications of paleomagnetism, directly complementing the Fellow’s existing expertise and experience in laboratory techniques, thereby allowing her to conduct future investigations in complex tectonic environments. Following fieldwork in Canada and laboratory analyses of the samples collected from the Thetford Mines ophiolite, it became apparent that its magnetic record has been completely remagnetized during the Acadian orogeny. However, the analyses yielded exciting new results on how the magnetic fabric record in ophiolite units that formed by different magmatic processes have become obliterated during emplacement and post-emplacement tectonic deformation. These analyses form the bulk of the data ready for publication to date. However, as remagnetization made it impractical to meet the training objectives relating to net tectonic rotation analyses (which were central to the training needs of the Fellow), this technique was then applied to the early rotation history of the Oman ophiolite, where we could guarantee that original magnetizations are preserved. This additional component to the project allowed the full training objectives to be delivered by: (i) performing detailed sampling through extrusive sequences of the northern half of the ophiolite, extending through a rocks believed to have experienced rotation during seafloor spreading and seafloor volcanism; (ii) using paleomagnetic data from these samples to test the hypothesis that intraoceanic rotation started during the phase of crustal construction, during spreading (as in the Thetford Mines example); and (iii) undertaking net tectonic rotation analysis of these data to determine the evolution of rotational strain during the early history of crustal accretion.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз