Paleomagnetic Evaluation of Crustal-Scale Block Rotations in the Mina Deflection of the Central Walker Lane

Crustal deformation resulting from relative Pacific-North America plate motion is broadly distributed on faults across the western U.S.. Geodetic observations show that some 25 percent of transform plate motion is currently accommodated by faults east of the Sierra Nevada, from the eastern California shear zone to the central Walker Lane (CWL). The northwest trending faults of the CWL are joined to the Furnace Creek - Owens Valley fault system by the Mina deflection, a belt of east-northeast striking faults that serve as a displacement transfer system linking the two fault zones. The Furnace Creek - Owens Valley fault system has a cumulative right-slip of 40 - 50 km, and this displacement is transferred in an extensional right-step to the curved high-angle faults of the Mina deflection. The high-angle fault geometry and basin depths preclude more than 10 km of right-lateral displacement accommodated along structures in the Mina deflection and thus a slip deficit of 30 to 40 km exists. We are attempting to resolve the amount of permanent Neogene strain in the Mina deflection and determine if the geodetically observed strain accumulation and local heterogeneities are consistent with the permanent Neogene strain, in particular, vertical axis rotation of blocks in the Mina deflection area. To in part quantify the magnitude and potential heterogeneities of crustal block rotation and to more clearly define block boundaries, paleomagnetic data have been obtained from numerous stratigraphic sections of upper Tertiary volcanic rocks in a 300 sqkm area centered on the Candeleria Hills, NV. Eight to ten oriented samples from 260 sites distributed across multiple tectonic blocks have been fully demagnetized with all sites yielding interpretable results (23 sites (two sections) in mid Miocene andesite flows, 102 sites (10 sections) in upper Miocene basalt flows, and 135 sites (27 sections) in lower Miocene, regionally extensive ash flow tuffs). Although some anomalous directions were observed, the stratigraphically corrected (typically less than 15° dip) data from the volcanic rocks are internally consistent, well-grouped at the site level, and discordant clockwise to late Cenozoic expected field direction (358\deg, 58\deg). For example, a Miocene basalt flow section in the central Candelaria Hills yields a group mean (D = 16.5, I = 50.0, a95 = 1.8, k = 497.3) that provides an inferred rotation of +18.5\deg +/- 6.9\deg compared to a late Cenozoic expected field. Two sites in the Candelaria Junction regional ash flow tuff located in separate structural blocks yield similar inclinations, yet statistically distinct declinations (Site1, 255.8, -48.6, 4.4; Site2, 296.4, -41.0, 4.6), providing evidence for a modest clockwise vertical axis rotation between structural blocks. Overall, we interpret most data obtained to reflect at most moderate clockwise vertical axis rotation among structural blocks in the Mina deflection region. The absence of large magnitude rotation of any individual block and, thus between blocks, places limits on the amount of cumulative slip on fault networks in the Mina deflection. The area may be accommodating strain by net northwest extension on a curved fault array with very low rates of vertical axis rotation and minor slip on individual faults. To compare the geodetic observations with long-term strain, we are developing a tectonic model, using existing fault geometries and paleomagnetic data for the CWL that links vertical axis rotations of the fragmented upper crust to the driving mechanisms associated with slip transfer across the region.