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A Dual Rift Model for the Opening of the NE Atlantic

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Existing models for the opening of the NE-SW oriented NE Atlantic invoke a near instantaneous rotation in extension direction, from E-W to NW-SE associated with eventual break-up during the Early Eocene. The implication of this model is that NW-SE extension is accommodated on margin-parallel normal faults and margin-oblique (NW-SE) strike-slip “transfer zone” faults, acting contemporaneously, and resulting in basin segmentation along the margin. Recent studies of fault kinematics in the Faroe Islands and Faroe-Shetland Basin, indicate that the extension vector rotation was prolonged (>10 My) and rift-oblique structures accommodate minor extension as relatively small dykes and normal faults (sub-basin-scale lateral continuity), rather than basin-scale strike-slip systems. Tectonostratigraphic mapping in the Faroe Shetland basin indicates a younging-direction for margin-oblique structures throughout the Palaeogene, from the NE to the SW along the margin, related to propagation of the newly forming Atlantic. Here we use high-resolution structural mapping and paleostress analysis from the Faroe Islands and Kangerlussuaq region of East Greenland, and compare this with an analogous study of ancillary deformation surrounding the tips of two NE-SW trending rift-fault segments in the Krafla Fissure Swarm, NE Iceland, to constrain the relative timing, distribution, and kinematics, of propagating rift systems. In the Faroes, extension vector rotation involves: (1) NE-SW extension accommodated by NW-SE normal faults and dykes, which are cut by (2) ENE -WSW and ESE -WNW striking dykes and strike-slip faults accommodating N-S extension, which in turn are cut by (3) NE-SW striking (margin-parallel) oblique-slip faults. Structural mapping of fault sets in Krafla reveals multiple fault sets associated with the main rift faults: (1) ESE -WNW striking mode I fractures, (2) NNW -SSE striking normal faults and mixed-mode fractures, and (3) NE-SW trending normal faults and mode I fractures, which form the final hard linkage between the bounding fault segments. Comparison between the margin datasets, and the Krafla system shows that relative timings and relative kinematics are the same for both systems. We find no evidence for rift segmentation across transfer zone faults in either setting. The distribution and scale of rift-oblique structures in Krafla indicates they are laterally discontinuous, rather than forming a rift-scale structure. Comparison between the Krafla dataset, and numerical models for ancilliary fault development ahead of propagating tips, shows a positive correlation, suggesting the evolution of structural sets in the Krafla system reflects an evolving local stress field ahead of the main bounding faults. We infer therefore that “transfer” structures interpreted along the Atlantic margins, instead represent ancilliary faults and dykes associated with tip-zone stress perturbations in the overlap zone of two active rift systems: in this instance, a dual rift system involving the NE-propagating Reykjanes ridge from the SW, and a SW-propagating Aegir ridge from the NE.

References Ellis D, Stoker MS. 2014. The Faroe–Shetland Basin: a regional perspective from the Paleocene to the present day and its relationship to the opening of the North Atlantic Ocean. Geological Society, London, Special Publications 397. Moy DJ, Imber J. 2009. A critical analysis of the structure and tectonic significance of rift-oblique lineaments (‘transfer zones’) in the Mesozoic–Cenozoic succession of the Faeroe–Shetland Basin, NE Atlantic margin. Journal of the Geological Society, London, 166, 1-14 Walker RJ, Holdsworth RE, Imber J, Ellis D. 2011. Onshore evidence for progressive changes in rifting directions during continental break-up in the NE Atlantic: Journal of the Geological Society, v. 168, p. 27-48.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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