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Paleozoic Seafloor Spreading

Periodic Reporting for period 1 - PASS (Paleozoic Seafloor Spreading)

Reporting period: 2015-09-04 to 2017-09-03

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.
The project proceeded via four work packages: database compilation; field sampling; laboratory analyses; and consolidation and dissemination. After the initial literature review, samples were collected from the Thetford Mines ophiolite. Magnetic anisotropy (fabric) data were acquired on these samples, and they were then demagnetized using standard paleomagnetic techniques. The main results from the magnetic fabric analysis are being turned into a paper (to be submitted in November 2017 to Solid Earth, the open access journal of the European Geosciences Union), and have been disseminated at national and international meetings. However, paleomagnetic data from the ophiolite showed that it has experienced complete remagnetization, so the remanence data could not be used to test the oceanic core complex hypothesis for this ophiolite, leaving a gap in the skills set the Fellow needed to acquire to enhance her professional maturity. To overcome this, an additional component of research analysing early seafloor spreading rotations in the unremagnetized Oman ophiolite was performed. This allowed the training objectives relating to tectonic analysis of magnetic remanence data to be met, and have yielded results that overturn the accepted view that crustal rotation was active during the eruption of the lava sequences. The data do not support the results from the less systematic analyses in this region published by other workers, with different parts of the lava sequence recording identical directions of magnetisation when appropriate structural corrections are applied. The data from this additional component are currently being analysed and will be written up for publication in a high-impact paper.
The research conducted in the Thetford Mines ophiolite represents the first magnetic fabric (anisotropy) study to quantify complete tectonic obliteration of original magmatic fabrics in an ophiolite. There have been many publications that have documented and described the evolution of magnetic fabrics in deformed sedimentary rock successions, but very few have addressed deformation-related fabrics in igneous successions and none in an important ophiolite. Results show that magnetic fabrics in all components of the main Thetford Mines ophiolite (serpentinized mantle rocks, gabbros, dykes and lavas), that formed by distinctly different magmatic processes, share the same fabric that reflects structurally-controlled crystallographic preferred orientations of iron-rich phases. Fabrics have consistent NW-SE oriented minimum anisotropy axes and steeply plunging NE maximum axes, and most likely developed during the Acadian orogeny in response to regional NW-SE shortening. This suggests complete obliteration of earlier magmatic fabrics during orogenesis. In contrast, gabbros in the south of the ophiolite retain a magnetic fabric parallel to compositional layering, suggesting that primary fabrics may survive locally in low strain zones. Importantly, consideration of anisotropy data from individual overprinted sites could lead to erroneous interpretations, as the tectonic fabric has anisotropy axes aligned coincidentally with magmatic morphologies such as dike margins in some cases. This emphasizes the need for caution during analysis of magnetic fabric data in complexly deformed terranes.

The additional component of training in the Oman ophiolite has yielded data that are still being analysed by the Fellow, but preliminary interpretations indicate that: (i) remanence directions within different levels of the northern massifs of the ophiolite have been highly rotated in a clockwise sense since formation of the ophiolite in the Cretaceous, consistent with previous studies; but (ii) that this rotation did not start during magmatic construction of the crust (as previously suggested), as different levels in the lava sequences record identical rotations when analysed using a net tectonic rotation approach. This is in contrast to previously published results from these rocks. These results therefore overturn c. 20 years of acceptance of these earlier interpretations, and remove the need for progressive rotation of the ophiolite during seafloor spreading, simplifying the reconstruction of the regional evolution of the world’s largest ophiolite.

There are no socio-economic impacts or wider societal implications of the project.
Summary of the magnetic fabric data from the Thetford Mines ophiolite.