KEISERHJELMEN DETACHMENT FAULT ROCK DEVELOPMENT, NW SPITSBERGEN

The Keiserhjelmen Detachment (KD) is an up to several hundred meter thick, ductile to brittle, top-to-North, fault zone that separates lower plate Caledonian migmatitic rocks from overlying Silurian-Devonian sediments as part of a metamorphic core complex in NW Spitsbergen. Exposed in the Woodfjorden and Raudfjorden areas, the KD is folded into an anticlinorium – synclinorium pair with transport parallel axes. Associated fault rocks include mylonites, ultramylonites, phyllonites, marble tectonites, foliated proto to ultra-cataclasites, breccias, and pseudotachylites with polished slip surfaces. A ductile to brittle transformation occurs with structural ascent. Fault zone textures include composite surfaces, asymmetric and rotated porphyroclasts, mineral elongation, asymmetric boudins, intrafolial folds, slickensides and striae. In thin section quartz deformational textures vary from equigranular and polygonal to seriate and interlobate, and from internally strain free to extreme undulose extension. Quartz ribbons with internal oblique fabrics are common. Feldspar porphyroclasts show an array of ductile to brittle features, also consistent with significant spatial and temporal variation in PT conditions and recovery processes. In quartz, and to a lesser degree feldspar, fluid inclusion planes occur at a high angle to the fault zone fabric and slip direction, and can reach a density of >10 per mm. Syn-kinematic granitic veins and sheets may be related to larger footwall intrusive bodies dated at 420-390 Ma. Micas are seriate with order of magnitude grain size reduction within shear bands. Extensive sericitization of feldspars and cordierite, and growth of chlorite and sericite in shear bands, porphyroclast shadows, and micro-veins suggests significant fluid migration produced hydrolytic weakening and aided retrogression. Silicate and carbonate, foliated, proto to ultra-cataclasites are common in upper reaches, and are associated with significant opaque enrichment and evidence for pressure solution. In conclusion, KD fault rocks are consistent with local Caledonide deformation concluding with development of a long-lived crustal-scale fault zone with orogen parallel transport, that formed during lower crust exhumation, retrogression and fluid flow.