Jean-Baptiste P. Koehl1,2, Steffen G. Bergh1,2 ...

Mid/Late Devonian-early Carboniferous collapse basins in the Barents Sea and Svalbard Jean-Baptiste P. Koehl1,2, Steffen G. Bergh1,2, Tormod Henningsen1, Jan-Inge Faleide2,3, Aleksandra A. Smyrak4 4 4 Sikora , Snorre Olaussen , Erik P. Johannessen . 1 Department

of Geology , University of Tromsø, N-9037 Tromsø, Norway. 2 Research Centre for Arctic Petroleum Exploration (ARCEx), University of Tromsø, N-9037 Tromsø, Norway. 3 Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, NO0316 Oslo, Norway. 4 The University Centre in Svalbard (UNIS), Pb 156, NO-9171 Longyearbyen, Norway. * Email corresponding author: [email protected]

Take-home message Mid/Late Devonian and lower Carboniferous sedimentary strata in the Barents Sea and Svalbard were deposited during widespread extension related to the collapse of the Caledonides. Introduction The Caledonian Orogeny was followed by a phase of late/post extensional collapse that resulted in the formation of thick, Middle Devonian sedimentary basins in western Norway and Spitsbergen in Svalbard (Figure 1). However, in North Norway and the Barents Sea, this collapse episode is yet to be demonstrated and, in Spitsbergen, the setting in which early Carboniferous sedimentary rocks of the Billefjorden Group were deposited is partly debated.

Results (1) In the Barents Sea, we uncovered the existence of a kmthick shear zone off the coasts of NW Finnmark, the SørøyaIngøya shear zone, which displays a NE-plunging, spoonshape geometry in map-view (Figure 1). This spoon-shaped shear zone accommodated the deposition of potential Devonian growth strata, potentially similar to those in Middle Devonian collapse basins in western Norway (Figure 2). These sediments deposited on the flanks of NE-SW trending basement ridges exhumed as core complexes along bowed portions of the Sørøya-Ingøya shear zone (Figure 1 & 2). In addition, major fault complexes, e.g. the Måsøy and Troms-Finnmark fault complexes (Figure 1), merge and sole into the Sørøya-Ingøya shear zone at depth (Figure 2a). We propose that these major fault complexes initiated as extensional brittle splays during inversion of the shear zone.

Figure 1: (a) Location of the study area in the Barents Sea; (b) Structural map with a time-surface of the upper boundary of the Sørøya-Ingøya shear zone (SISZ) showing a potential NE-SW trending core complex (dashed red line). Structural elements are from Indrevær et al. (2013) and Koehl et al. (2018). Location of (b) is shown in (a) as a black frame. Abbreviations: Fpe = Finnmark Platform east; FPw = Finnmark Platform west; GL = Gjesvær Low; HfB = Hammerfest Basin; Ma = Magerøya; MFC = Måsøy Fault Complex; NBSW = Nordkapp Basin SW; NFC = Nysleppen Fault Complex; Rf = Rolvsøya fault; SISZ = Sørøya-Ingøya shear zone; sNB = southwesternmost Nordkapp basin; TFFC = Troms-Finnmark Fault Complex.

Results (2) In Svalbard, the Late Devonian-early Carboniferous EllesmerianInuitian Orogeny and Tertiary transpressional deformation related to the opening of the NE Atlantic Ocean partly deformed lower Carboniferous strata. However, in central Spitsbergen, newly exposed lower Carboniferous rocks show extensional growth strata along widespread normal faults. Among these, WNW-ESE trending faults appear not to propagate into overlying, upper Carboniferous sedimentary strata (Figure 3). This suggests widespread extension in the early Carboniferous and gradual localization of extension-related normal displacement along fewer, major normal faults in the late Carboniferous, e.g. the Billefjorden Fault Zone, which is comparable to what is observed in the Barents Sea, e.g. the Måsøy Fault Complex (Figure 2).

Conclusion (1) In the SW Barents Sea, the Sørøya-Ingøya shear zone was inverted during the collapse of the Caledonides and localized the deposition of Devonian-lower Carboniferous sedimentary rocks and formation of major, extensional splay-faults, which contributed to exhume NE-SW trending basement ridges as core complexes during a phase of widespread extension. (2) Analogously, in Spitsbergen, NW-SE trending faults accommodated the deposition of lower Carboniferous growth strata of the Billefjorden Group, which deposited during a phase of widespread extension. These faults die out in lower/mid Carboniferous sedimentary strata.

Acknowledgements The present study is part of the ARCEx project (Research Centre for Arctic Petroleum Exploration) which is funded by the Research Council of Norway (grant number 228107) together with 10 academic and 9 industry partners. We would like to express our gratitude to all the persons and the partner institutions that are involved in this project.

Figure 2: (a) Seismic section across the Finnmark Platform west and the Gjesvær Low showing the Sørøya-Ingøya shear zone (dashed yellow) and potential Devonian, syn-tectonic, sedimentary growth strata (dotted white) along potentially inverted, SE-dipping Caledonian thrusts (dotted yellow). Location shown as a red line in Figure 1; (b) uninterpreted and (c) interpreted zoom in Devonian growth strata and SE-dipping Caledonian thrusts. Abbreviations as in Figure 1.

References

Figure 3: Outcrop photograph of a syn-tectonic growth strata (dashed green) in lower Carboniferous, coal-bearing sedimentary rocks of the Billefjorden Group along a NWSE trending fault (red line) in Odellfjellet, central Spitsbergen.

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