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Fast ductile rock deformation accommodating dyke emplacement in the middle crust

Hans Jørgen Kjøll1 , Olivier Galland1, Thomas Scheiber2

  • Affiliations: 1Njord Center, Department of Geosciences, University of Oslo, Oslo, Norway; 2Department of Civil Engeneering and Environmental Sciences, Western Norway University of Applied Sciences, Sogndal, Norway 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 195

  • Programme No: 1.8.26

  • Theme 1 > Session 8

Abstract

Growth of magmatic sheet intrusions is generally assumed to occur through initial tensile fracturing and elastic bending of the host rock. In addition, most models assume that rates of dyke emplacement are too fast to be accommodated by ductile flow of the host rock, even at mid-crustal levels. There is, however, increasing field evidence that suggests that inelastic processes of the host rock play an important role in emplacement dynamics. We have studied a mafic dyke complex emplaced in the middle crust of an Ediacaran-aged magma-rich rifted margin in Northern Sweden. The host rocks in the field area are composed primarily of dolostone intercalated with silicic beds. The peak ambient temperature during dyke emplacement was approximately 700°C. Field observations show that the host rock beds are tightly folded (up to 30% shortening) close to the contacts of the surrounding dykes, and strain decreases rapidly with increasing distance from the dykes. The axial planes of the folds are sub-parallel to the dyke contacts themselves, suggesting that the folds formed as a consequence of dyke emplacement and inflation. Estimates of dykes cooling times suggest that the magma reached its solidus temperature within one year. Our results suggest that the strain rate accommodating dyke emplacement was very rapid (on the order of 10-7 s-1) compared to typical tectonic strain rate estimates (ca 10-12 - 10-15s-1). Our data show that inelastic deformation is an important aspect during dyke emplacement in the middle crust, and that fast strain rates are possible.