Core complex fault rocks of the Silurian to Devonian Keisarhjelmen detachment in NW Spitsbergen

Abstract

ABSTRACT A Silurian–Devonian metamorphic core complex has recently been recognized in northwest Spitsbergen, on the northwest corner of the Barents Shelf at the junction between the Atlantic and Arctic oceans. The associated Keisarhjelmen detachment, a major, ductile-brittle fault zone, is 200–500 m thick and has a map trace >150 km. A top-to-the-north transport direction is parallel to the axis of a large-scale, shallowly north-plunging, detachment corrugation. This detachment zone separates overlying faulted Silurian–Devonian aged cover strata from underlying migmatitic rocks in the core. The detachment shows a diverse array of fault and metamorphic rocks with structural ascent, ranging from sheared migmatite, mylonite, ultramylonite, foliated cataclasite, pseudotachylite, and breccia. Footwall post-kinematic granitic intrusions occurred shortly prior to, and likely during, deposition of the older cover strata. Variably deformed, syn-kinematic granitic sheets and veins within the detachment zone are considered coeval. Thin sections show significant grain size reduction, porphyroclasts, and well-developed composite fault surfaces. Relict garnet sigma porphyroclasts associated with chlorite and sericite indicate retrogression. Feldspar porphyroclasts show significant sericite alteration, undulose extinction and limited recrystallization low in the detachment, and brittle deformation throughout. Quartz deformation textures and grain size vary considerably within and between samples. Deformation during retrogression continued into the brittle realm with the development of thick foliated cataclasites, fault breccias, and local pseudotachylites concentrated at the top of the detachment. Biotite in particular shows grain size reduction, concentration along C-surfaces, and shredding and redistribution, suggesting it played a significant role in both ductile and brittle faulting. Veins, micro-veins, and fluid inclusion planes are ubiquitous throughout the detachment, indicating substantial fault-related fluid flow. Given existing geochronologic and P-T (pressure-temperature) data from the basement rocks of the area, the kinematics, retrogression, and ductile-brittle transition are consistent with exhumation of a core complex developing by orogen-parallel extension associated with transtension during the Late Silurian and Early to Middle Devonian in northwest Spitsbergen. Remaining questions include how this core complex connects with coeval plate-scale strike-slip faults in Svalbard, and its relationship to mainland Norwegian core complexes and Devonian basins to the south.