Structure and petrofabrics of quartzite and elongate pebbles at Sandviksfjell, Bergen, Norway

Thrust slices of Precambrian gneiss and its thin cover of Lower Paleozoic (?) metasedimentary rocks were imbricated during the Caledonian orogeny and are now exposed at Sandviksfjellet on the northeast side of Bergen. Locally one of the main thrust faults between metasedimentary rocks and the overlying crystalline nappe is folded into a 50 m-high Z-fold whose axis is parallel to the main lineation and to the overall southeastward Caledonide transport direction. Pebbles in the hinges and mid-limb of the Z-fold are greatly elongated into prolate ellipsoids up to 1.25 m long. External to the fold, quartzite pebbles are oblate spheroids. Preferred orientation of c axes of quartz in both pebbles and associated meta-arkose is surprisingly weak and not clearly related to the mesoscopic fabric. Partitioning of strain along the thrust faults and compositional heterogeneity of the metasedimentary rocks are regarded as the main reasons why the quartz c axis subfabrics are so weak. Most of the final shape of the greatly elongated prolate pebbles in the Z-fold is postulated to result from an extremely inhomogeneous strain distribution, however, little of the sequence of events resulting in pebble deforrnation is preserved in the quartz subfabric. In fact, considerable pebble strain may have been accomplished by pressure solution without intracrystalline plastic flow. Two deforrnation models are presented to explain the relations between the folded thrust and the prolate pebbles: one synkinematic and the other late- or post-kinematic. Synkinematic forrnation of the large Z-fold and elongation of the pebbles involved alternative extension and constriction of the metasedimentary sequence beneath the
main nappe when it overrode a structural asperity. In the alternate two-stage model, emplacement of younger, higher nappes deformed the older, main thrust and part of its footwall of metasedimentary rocks. After the Ulriken Gneiss Nappe was emplaced above the metasedimentary rocks, with consequently increased load pressure and increased ductility in the lower plate, weak nearly orthorhombic preferred orientations of quartz c axes were imposed in all of the quartzose rocks, and pebbles were flattened even more.