NJG94-4-01
2014
Some remarks on the earthquakes of Fennoscandia: A conceptual seismological model drawn from the perspective of hyperextension
94
4
pp. 233–262
978-82-92-39492-2

To a first order, patterns of Fennoscandia’s seismicity reflect the benchmark domain boundaries of its Mesozoic rifted margin. Three distinct belts of earthquakes strike sub-parallel to the generalized line of breakup. The outermost seismic belt (SB1) marks the Taper Break (TB), or the zone of flexural coupling/decoupling between the distal (seaward) and proximal/necking (landward) domains. A coastal belt (SB2) follows the Innermost Limit of Extension, defined as the onset of 39 km-thick crystalline continental crust. An interior belt (SB3) follows the Hinterland Break in Slope, or the landward limit of the Scandinavian rifted margin. Between each belt, large portions of the necking, proximal, and hinterland domains are seismically quiescent. Evaluation of the ’Cumulative Seismic Moment’ (CSMw) per unit area indicates that the release of seismic energy is asymmetric. Although some of Fennoscandia’s largest seismic events occur in the dominantly Proterozoic to Archean lithosphere of the eastern craton, 80% of Fennoscandian CSMw maps to the domain boundaries of the western rifted margin. CSMw energy tends to be highest at the TB and decreases systematically towards the continental interior.

As proposed by many previous studies, a first-order spatial correlation between Scandinavia’s offshore earthquake belt and voluminous, geologically rapid, sedimentary loading during the Neogene period is evident. However, the presence of thinned, faulted crystalline basement is also a very important factor behind Scandinavia’s offshore seismicity. Where the Neogene deposits are at their thickest the underlying crust is dominantly oceanic; CSMw is lower per unit area there than where lesser Plio–Pleistocene loading impacts continental crust that was fully prepared by Mesozoic necking and hyperextension. CSMw data suggest that the ‘strength’ of the TB and the continental margin’s distal domain is significantly less than that of relatively young (ca. 54 Ma) oceanic lithosphere. Our data imply that ridge push does not contribute significantly to Fennoscandia’s seismicity. Rather, we find that thin-plate bending stresses stemming from offshore depositional loading conspire with unbuttressed Gravitational Potential Energy (GPE), onshore erosion, and post-glacial isostatic rebound to generate Fennoscandia’s earthquakes.

We present a conceptual seismological model for Fennoscadia that is consistent with modern hypotheses of extended margin evolution, including post-breakup reactivation by footwall uplift in regions adjacent to sharp crustal taper. Illustrated by simple concepts of elastic thin-plate theory, the model honors our conclusion that Fennoscandian seismicity is principally the product of locally derived stress fields and that far field stress from the oceanic domain is unlikely to penetrate deeply into a hyperextended continental margin. It predicts the locations of the observed seismic belts and seismic gaps with the mathematics of thin-plate bending. It describes how the outer seismic belt will remain localized in the vicinity of the TB in response to the permanently contrasting material properties and flexural rigidities of the distal and necking/proximal domains. It suggests that overprinting stress may sweep across the proximal domain and magma-stiffened portions of the distal domains as post-thinning cooling progressively increases both the effective lithospheric rigidity and its associated flexural wavelength. Our model is built from concepts and data presented by previous authors in many earlier studies, and is at least partly applicable to other post-breakup rifted margins.

T.F. Redfield, Geological Survey of Norway, 7491 Trondheim, Norway. P.T. Osmundsen, Geological Survey of Norway, 7491 Trondheim, Norway. Department of Arctic Geology, University Center in Svalbard, 9171 Longyearbyen, Norway

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