STRAIN IMPLICATIONS FOR THE FORMATION OF FLUID ESCAPE STRUCTURES DURING SYNEXTENSIONAL DEPOSITION OF MIOCENE VOLCANICLASTIC SEDIMENTARY ROCKS IN THE VOLCANIC HILLS, SOUTHWEST NEVADA

The Eastern California Shear Zone (ECSZ) and the Central Walker Lane (CWL) constitute a tectonic boundary zone that accommodates differential displacement between the Sierra Nevada and the central Great Basin. The ECSZ and CWL are misaligned and, prior to 4 Ma and after ~13 Ma, displacement transfer between the two fault systems was accommodated along the Silver Peak-Lone Mountain (SPLM) extensional complex. The SPLM is comprised of a low-angle, northwest-dipping detachment fault separating metamorphic tectonites of the lower plate from metasedimentary Paleozoic rocks and Mesozoic plutons overlain by Cenozoic volcanic and sedimentary rocks of the upper-plate. During displacement on the SPLM detachment, the upper-plate was partially disarticulated with the formation of fault-bounded synextensional basins. In the Volcanic Hills of northern Fish Lake Valley, Nevada, the upper-plate is well preserved and is composed of synextensional sedimentary and volcanic rocks. The sedimentary rocks are dated between 13 and 11 Ma and are overlain unconformably by tuff ranging in age from 6.0 to 5.9 Ma. In this area, the sedimentary and volcanic succession exhibits abrupt changes in thickness from 1100 m to 300 m due to differential subsidence on basin-bounding faults. Where exposed in the Volcanic Hills, the lowermost member of the synextensional succession is composed of interbedded sandstone and conglomerate, containing Cenozoic volcanic and Paleozoic detritus, interbedded with ash-flow tuff. Parts of this lithologic succession show evidence of rapid sedimentation with the formation of fluid escape structures in the form of sub-vertical sandstone and conglomerate dikes. The sedimentary dikes range in thickness from 0.4 cm to 2 cm and can be traced along strike for up to 3 m. The sedimentary dikes are found in two orientations; N40°E ± 25° and N15°W ± 30° and are mutually cross-cutting at the outcrop; often forming a herringbone pattern. The development of two sets of dilatational fluid escape structures with mutually cross-cutting relations suggests that they formed during flattening strain associated with displacement on basin bounding faults. These results are consistent with fault-slip inversion studies showing finite flattening during displacement on faults in the Volcanic Hills.