Geometry and kinematics of extensional deformation along the northern edge of the Rombak Window, Nordland, North Norway.

Abstract

Deformation along the northern edge of the Rombak Window documents an unusual extensional event during the evolution of a colliosional orogen. The extension occurred along a series of ductile deformation zones that strike slightly east of north and dip moderately west; both mesoscopic and microscopic kinematic indicators demonstrate normal motion on these zones. Two adjacent zones are major features, extending at least 6 km along strike (representing a structural depth of 1.1 - 1.6 km) without changing orientation. The remainder are fairly superficial features, dying out at dephts of 100 to 200 m; they are thought to represent local adjustments to offset on the major zones. Fault zones with normal offset along strike at the southern edge of the Rombak Window are thought to be part of the same feature, although they formed at shallower dephts. A prominent N-S-trending structural lineament and isolated patches of Dividal Group sedimentary rocks in the central part of the window may represent the trace of this extensional feature. The extension was preceded by emplacement of the Bjørnfjell Thrust Complex, containing granitic basement and autochthonous sedimentary rocks of Baltoscandian affinity. Both of these events was followed by emplacement of higher nappe sheets containing a variety of rock types, and generally considered to be exotic with respect to the Baltic craton.Three types of models which could create the required extension of the subducting craton, preceded and followed by large-scale compressional deformation, are being considered: (1) Extension of the upper surface of the Baltic craton during flexing prior to subduction. (2) Extension due to the flexing of a crystalline thrust sheet as it moved over a ramp in an underlying detachment. (3) Extension due to an unrecognized deformational event during the 80 Ma age bracket allowed by the broadest interpretation of the age constraints on Caledonien deformation in this area.