Background: The present contribution reexamines the geometry of a segment of a presumably long-lived fault in Svalbard, the Balliolbreen Fault segment of the Billefjorden Fault Zone, along which presumably two basement terranes of Svalbard accreted in the early-mid Paleozoic after thousands of kilometers strike-slip displacement.
Methods: We performed structural fieldwork to Billefjorden in central Spitsbergen and interpreted satellite images.
Results: Field observations demonstrate that the Balliolbreen Fault formed as a top-west thrust fault in the early Cenozoic and that weak sedimentary units such as shales of the Lower Devonian Wood Bay Formation and coals of the uppermost Devonian-Mississippian Billefjorden Group partitioned deformation, resulting in significant contrast in deformation intensity between stratigraphic units. For example, tight early Cenozoic folds are localized in shales of the Wood Bay Formation and contemporaneous top-west brittle-ductile thrusts within coals of the Billefjorden Group, whereas Pennsylvanian deposits of the Hultberget (and/or Ebbadalen?) Formation are simply folded into gentle open folds. Rheological contrasts also resulted in the development of décollements locally, e.g., between tightly folded strata of the Wood Bay Formation and Billefjorden Group and flat-lying, brecciated limestone-dominated strata of the Wordiekammen Formation. Despite the limited quality and continuity of outcrops in the area, the eastward-thickening character (i.e., away from the fault) of Pennsylvanian deposits of the Hultberget, Ebbadalen, and Minkinfjellet formations suggests that the fault did not act as a normal fault in Pennsylvanian times.
Conclusions: The study suggests that strain partitioning of early Cenozoic Eurekan contraction alone may explain the deformation patterns in Paleozoic rock units in central Spitsbergen, i.e., that Late Devonian Svalbardian contraction is not required, and that a major segment of the Billefjorden Fault Zone formed in the early Cenozoic. The present work illustrates the crucial need for interdisciplinary approaches and composite educational backgrounds in science.
Keywords: Billefjorden Fault Zone; Carboniferous; Cenozoic; Devonian; Eurekan; Permian; Svalbard; Svalbardian Orogeny; décollement; fault; thrust.
The present study reexamines the geometry, kinematics, and tectonic history of a major fault zone in the Norwegian Arctic, the Balliolbreen Fault segment of the Billefjorden Fault Zone. Thus far, the latter fault was believed to represent a hundreds to thousands of kilometers long discontinuity in the crust, possibly linking with major faults in northern Scotland (e.g., Great Glen Fault). In addition, it was believed to have accommodated hundreds to thousands of kilometers of lateral displacement between 500 Ma and 400 Ma based on similar rock types in northern Scotland and northern Spitsbergen and between northeastern Svalbard and northern Greenland, thus suggesting discrete origins for the crustal blocks constituting the Svalbard Archipelago. Our results and recent studies show that the Balliolbreen Fault formed c. 65–30 million years ago, i.e., much later than previously proposed, that it dips gently to the east (instead of steeply), and that it accommodated a limited amount of vertical (instead of large lateral) tectonic movements. The fault likely formed between weak Lower Devonian (c. 420-390 million years old) rock units, which consist of fine-grained rocks easily deformed into folds during tectonic events, and strong Pennsylvanian-Permian (c. 330-290 million years old) rock units, which consist of coarse-grained rocks that do not deform easily and, instead, rupture once enough tectonic movement has occurred. The study has implications for how geoscientists map major faults during fieldwork and how they interpret structural datasets and extensively eroded outcrops transects. It also suggests that the Svalbardian Orogeny, a major episode of tectonic contraction, which presumably occurred c. 380-370 million years ago in Arctic regions, did not occur. The study calls for more interdisciplinary approaches when performing scientific research, especially when mapping major fault zones during geological fieldwork. It also highlights the need of more composite educational backgrounds for researchers.
Copyright: © 2024 Koehl JBP et al.