THE BYGDIN CONGLOMERATE, NORWEGIAN CALEDONIDES: QUARTZ CRYSTAL FABRICS AND THEIR IMPLICATIONS FOR 3D STRAIN AND TECTONIC SETTING

In this theme session honoring the career of Arthur Sylvester we present previously unpublished quartz crystal fabric data from the deformed Bygdin conglomerate in the Norwegian Caledonides. The Bygdin area is internationally renowned as a classic location for 3D strain analysis (Hossack 1968) using plastically deformed quartz-rich pebbles which range in shape from oblate to prolate. Quartz c-axis fabrics forming apparent single girdles perpendicular to the gently SE plunging mineral lineation in the pebbles have been described by Strand (1944) and indicate that the lineation marks the maximum principal stretch direction. However, the plotting of these historical fabric data in geographic coordinates makes kinematic interpretation of the fabrics difficult.

Optical analysis of thin sections cut perpendicular to foliation and parallel to lineation reveals that the quartz-rich pebbles have recrystallized by subgrain rotation (SGR) with some grain boundary bulging (BLG). A top to the NW shear sense is ubiquitously indicated by microstructures such as alignment of elongate dynamically recrystallized quartz grains oblique to foliation, shear bands and rare sigma shaped fibrous wings on rigid feldspar clasts. Universal stage-based optical microscopy yields asymmetric cross-girdle quartz c-axis fabrics in which both internal and external fabric asymmetry indicate a top to the NW shear sense in all pebbles. Quartz c- and a-axis fabrics derived from EBSD analyses confirm this shear sense. Numerical and experimental studies (Lister & Hobbs 1980; Heilbronner & Tullis 2006) indicate that cross (and single) girdle fabrics should form under approximate plane strain conditions. The recording of cross-girdle fabrics in all pebble shapes suggests that the 3D (oblate–prolate) shapes of these pebbles are not indicators of 3D plastic strain.

Deformation temperatures indicated by opening angles in optically measured c-axis fabrics (Faleiros et al. 2016 linear thermometer) range from 440-525 °C but the quartz SGR and BLG microstructures suggest these may be overestimates. The ubiquitous top to the NW shear sense indicates that the Bygdin conglomerate has been deformed (under approximate plane strain conditions) during Mode I phase extension (Fossen 1992) that post-dates Caledonian top to the SE thrusting.