STRAIN, DEFORMATION MECHANISMS AND KINEMATICS RECORDED IN A QUARTZ PEBBLE CONGLOMERATE

Finite strain and microfabrics were analyzed in a Neoproterozoic quartz pebble conglomerate deformed in the footwall of a detachment fault during the development in Miocene time of the Northern Snake Range metamorphic core complex (east-central Nevada, USA). The motivation was to seek evidence for inhomogeneity of strain and fabric on the scale of individual pebbles and to consider the results in the context of previous work on the strain and kinematics of this well-studied core complex. Pebble strain was analyzed using the Rf/φ method, which produced results that plot along plane strain (k = 1) on a Flinn diagram. Strain ratios (X:Y:Z) in the two samples are 15.1:4.0:1 and 9.2:2.9:1. The samples analyzed were not in place, but can be confidently oriented by the uniformity in orientation of foliation and lineation in the area where they were collected. The plane of flattening (XY) is parallel to foliation and dips gently to the east. The X direction is parallel to the mineral lineation and is down dip. X-ray computed tomography (XRCT) detected small particles, composed of individual grains or clusters of several grains, that possess a shape fabric reflecting the orientation (but not magnitude) of the pebble strain. The identity of these phases is uncertain, but possibilities include metal oxides and garnet. Strong evidence of non-coaxiality is indicated by the presence of shear bands and a fabric of recrystallized grains oblique to the cleavage and plane of flattening. Electron back-scattered diffraction (EBSD) analysis of quartz c-axis fabrics show a strong preferred orientation (LPO), with one or two maxima near Z in a partial girdle in the YZ plane. This is consistent with strain kinematics between pure shear and simple shear. These results are interpreted to reflect a deformation history recording slight non-coaxial shear imposed on earlier coaxial shear. Overall, deformation is homogeneous on the scale of the samples with subtle differences in LPO among individual quartz grains.