LOW TO NO STRAIN IN ROCKS ADJACENT TO THE MELONES FAULT ZONE: IMPLICATIONS FOR STRAIN ANALYSIS TECHNIQUES AND THE NEVADAN OROGENY

This research involves evaluating new methods of measuring strain with implications to the styles of deformation in arcs, how deformation is localized, and differentiating between tectonic versus pre-lithification strain. We compare novel methods of 3D strain analysis on nine metavolcanic and metasedimentary samples collected along an 85 m thick, steeply dipping, continuous measured section of the Late Jurassic Mariposa Formation within the Sierra Nevada foothills. This section is notable as it has undergone rigid body rotation, but the nature and magnitude of internal strain is unknown. A weak bedding parallel fabric is preserved in slates; sandy to cobble conglomerates (matrix and clast-supported) preserve a very weak fabric that may be depositional. Three perpendicular faces were cut on oriented samples and axial ratios of coarse sand to pebbles on each face were measured using two applications: Ellipsefit and StraboTools. Edge Fabric Ellipse (EFE) results from both methods show low values of fabric magnitude, ε = (0.09-0.48) with largely flattening strains, v = -0.4 to 0.76. The digitization method of Ellipsefit also yields low strain magnitudes ranging from 0.10 to 0.68, and flattening strains of -0.6 to 0.7. Comparison with published results of undeformed sandstones yields nearly identical magnitudes and shapes. Results for EFE were relatively close to each other with higher deviation between values for lower ε. EFE results are based on gradients of image lightness which may be caused by many factors, including fractures and weathering patterns, so caution must be used when interpreting the results. Nevertheless, StraboTools provides a rapid, quantitative, and reproducible measurement of rock fabric strength that is similar to Ellipsefit results. The results from digitized sections also yield very low strains; only two samples yield NW-trending oblated ellipsoids consistent with regional cleavages noted elsewhere in the field area. Although the sample traverse was within 640 m of the regional Melones Fault Zone, there is scant evidence of internal strain within the measured section. Rather, the region has undergone rigid body deformation as well as tilting presumably during the Late Jurassic Nevadan orogeny but is not internally strained.