Anna Ksienzyk

Post-Caledonian fault activity in W-Norway: onshore and offshore don’t match – or do they?

by Ksienzyk, A.K., Wemmer, K., Rotevatn, A., Fazlikhani, H., Jacobs, J., Fossen, H.

K-Ar or Ar-Ar analyses of fault gouge fine fractions are becoming well-established methods to investigate the timing of fault activity. They rely on the synkinematic growth of illite, a K-bearing clay mineral, to date periods of fault movement. These methods come with a number of challenges, including contamination with host rock illite/muscovite and other K-bearing minerals, potential Ar-loss from the very fine-grained illite and/or convoluted histories of polyphase faulting. However, our understanding of fault systems and the behaviour of illite in fault gouges, while still far from complete, has improved to allow increasingly complex structural histories to be disentangled.
Isotopic dating of fault rocks can provide valuable information on the timing of fault activity, particularly in areas where stratigraphic evidence of faulting is absent or not preserved. In Norway, a number of recent studies (Davids et al., 2013; Torgersen et al., 2014; 2015; Ksienzyk et al., in press; Viola et al., in press) gradually reveal a distinct pattern of onshore fault activity: (1) The oldest illite ages associated with post-Caledonian fault movement are Middle Devonian-Carboniferous, depending on the study area. (2) An early Permian period of fault activity is recognised over large parts of onshore Norway and correlates well with rifting in the northern North Sea and Oslo Graben. (3) In the Mesozoic, onshore and offshore tectonics appear to be diachronous. Offshore, the Early Triassic-Middle Jurassic is generally considered to be an inter-rift period of tectonic quiescence. Onshore, on the other hand, illite ages document significant Late Triassic-Early Jurassic fault activity in southern and western Norway. Additionally, a younger period of onshore fault reactivation in the Mid-Cretaceous post-dates Late Jurassic-Early Cretaceous rifting in the northern North Sea.

Fault reactivation during Late-Jurassic-Early Cretaceous North Sea rifting appears to have migrated outwards from the rift centre. Faults in the Viking Graben were active during the early stages of this rift phase, in the Middle-Late Jurassic, while faults on the Horda Platform towards the east were reactivated later, in the Late Jurassic-Early Cretaceous (Bell et al., 2014). The Mid-Cretaceous reactivation of onshore faults, even further to the east, might thus be a continuation of this trend. The Late Triassic-Early Jurassic inter-rift fault activity observed onshore, on the other hand, cannot easily be linked to offshore rifting. However, recent studies as part of the MultiRift project have revealed significant inter-rift fault movement also in the northern North Sea, offshore western Norway (Refvem, 2016; Deng et al., in review). A closer look at offshore fault activity through time might thus reveal that onshore and offshore tectonics are not diachronous after all.

References:
Bell, R.E., Jackson, C.A.-L., Whipp, P.S. & Clements, B. (2014) Strain migration during multiphase extension: observations from the northern North Sea. Tectonics 33, 1936-1963.
Davids, C., Wemmer, K., Zwingmann, H., Kohlmann, F., Jacobs, J. & Bergh, S.G. (2013) K–Ar illite and apatite fission track constraints on brittle faulting and the evolution of the northern Norwegian passive margin. Tectonophysics 608, 196-211.
Deng, C., Fossen, H., Gawthorpe, R.L., Rotevatn, A., Jackson, C.A.-L. & Fazli Khani, H. (in review) Structural evolution during multiphase rifting: Oseberg area, northern North Sea.
Ksienzyk, A.K., Wemmer, K., Jacobs, J., Fossen, H., Schomberg, A.C., Süssenberger, A., Lünsdorf, N.K. & Bastesen, E. (in press) Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K-Ar illite fault gouge dating. Norwegian Journal of Geology.
Refvem, M.A. (2016) Spatial and temporal evolution of fault arrays on the northren Utsira High, northern North Sea. MSc thesis, University of Bergen, 75 pp.
Torgersen, E., Viola, G., Zwingmann, H. & Harris, C. (2014) Structural and temporal evolution of a reactivated brittle–ductile fault – Part II: Timing of fault initiation and reactivation by K–Ar dating of synkinematic illite/muscovite. Earth and Planetary Science Letters 407, 221-233.
Torgersen, E., Viola, G., Zwingmann, H. & Henderson, I.H.C. (2015) Inclined K–Ar illite age spectra in brittle fault gouges: effects of fault reativation and wall-rock contamination. Terra Nova 27, 106-113.
Viola, G., Scheiber, T., Fredin, O., Zwingmann, H., Margreth, A. & Knies, J. (in press) Deconvoluting complex structural histories archived in brittle fault zones. Nature Communications.