Strain Implications for the Formation of Clastic Dikes During Synextensional Deposition of Miocene Volcanoclastic Rocks in the Volcanic Hills, Southwest Nevada




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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 upperplate 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 Ma and 11 Ma and are overlain unconformably by tuff ranging in age from 6.0 Ma 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, the Silver Peak formation, is composed of interbedded sandstone and conglomerate, containing Cenozoic volcanic and Paleozoic detritus, interbedded with ashflow tuff. Parts of this lithologic succession show evidence of rapid sedimentation with the formation of fluid escape structures in the form of subvertical sandstone and conglomerate dikes. The clastic dikes range in thickness from 0.4 cm to 2 cm and can be traced along strike for up to 3 m. The clastic dikes are primarily found in two orientations; N40°E ± 25° and N15°W ± 30° and are mutually cross-cutting at the outcrop; often forming a rhombohedral pattern. The development of two sets of dilational clastic dikes with mutually crosscutting relations suggests that they formed during flattening strain associated with displacement on basin bounding faults. These results are consistent with faultslip inversion studies showing finite flattening during displacement on faults in the Volcanic Hills.



Geology, Structural, Clastic dikes, Faults (Geology), Fish Lake (Nev.)