The northern Bergen Are Shear Zone an oblique-lateral ramp In the Devonian extensional detachment system of western Norway OLE PETTER WENNBERG, ALAN GEOFFREY MILNES & INGER WINSVOLD Wennberg, O. P., Milnes, A. G. & Winsvold, I. The northe Bergen Are Shear Zone - an oblique-lateral ramp in the Devonian extensional detaehment system of weste Norway. Norsk ologisk Tidsskrt, Vol. 78, pp. 169-184. Oslo 1998. ISSN 0029-196X. ISSN 0029-196X. Structural investigations in the northe part of the Bergen area (weste Norway) have been earried out on the major, 2-3 km-thiek, late/post-Caledonian, extensional Bergen Are Shear Zone. This shear zone dips steeply SW with a ehange in strike from 115-120° in the northeast to 150-160° in the southwest, over a distance of 50 km. The footwall eonsists of the autochthonous Western Gneiss Complex and the lowermost Caledonian Nappes. The hanging wall eonsists of strueturally higher Caledonian Nappes uneonformably overlain by eoarse Devonian eonglomerates. The orientation of the mylonitic foliation within the shear zone, in addition to several types of shear-sense indieators, predominantly indieate a dextral shear component. The related stretehing lineation pl unges about 20° W to NW, implying a significant eomponent of normal slip, as indieated by the regional geology. The minimum displacement on the Bergen Are Shear Zone is approximately 16 km, with a normal dip slip eomponent of 8 km, and a dextral strike slip eomponent of 14 . The Bergen Are Shear Zone post-dates all Caledonian eontraetional deformation in its vieinity. We suggest that the Bergen Are Shear Zone represents a braneh of the Nordord-Sogn Detaehment Zone, the main Devonian detaehment, developed as an oblique-lateral ramp. O. P. Wennberg,• A. G. Mi/nes & l Winsvold, Geologisk Institutt, Universitet i Bergen, A/legt 41, 5007 Bergen, Norway. • Z & S Geologi ajs. Sverdrupsgate 23, N-4007 Stavanger, Norway. Introduction The Bergen Are Shear Zone (BASZ, Fig. l) has been suggested to represent the southward continuation of the well-known large-scale (displacement �50 km), Devo- nian, extensional detachment system in western Norway - the Nordord-Sogn Detachment Zone (Milnes et al. 1988; Seranne et al. 1991; Fossen 1992; Wennberg & Milnes 1994). The Nordord-Sogn Detachment Zone (NSDZ, Fig. l) and BASZ cut through, on a regional scale, the Caledonian Nappe pile, which was formed during the final collisional event of the Caledonian Orogeny (the Scandian phase), which ended in early Devonian times (ca. 395 Ma; Milnes et al. 1997). The Fig. l. Tectonie overview map of SW Norway. Abbreviations: BASZ = Bergen Are Shear Zone; NSDZ =Nordord-Sogn Detaehment Zone; HFSD = Hardangerord Shear Zone; LGF =Lærdal-Gjende Fault; WGC =Weste Gneiss Complex. Modified from Sigmond et al. (1984), Milnes et al. (1997) and Wennberg (1996). Devonian sediments l i i C aledonian allochthon :::::::::J Autochthon/Parautochthon � Late-/post-Caledonian shear zone
The northern Bergen Are Shear Zone an oblique-lateral ramp In the Devonian extensional detachment system of western Norway
Jun 23, 2023
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