Paleomagnetic Data from Upper Tertiary Volcanic Rocks in the Central Walker Lane: Crustal-Scale Block Rotation and Transitional Field Directions

Late Cenozoic deformation is broadly distributed across the western plate boundary of North America from the San Andreas Fault system eastward into the Basin and Range Province. Comparison of plate motion and geodetic measurements indicates that 75 percent of the strain between the Pacific and North America plates is accommodated along the San Andreas with the residual strain partitioned inboard along the eastern California shear zone (ECSZ). Displacement is partitioned northward from the ECSZ to a narrow zone of deformation in the southern Walker Lane along the Owens Valley and Furnace Creek fault systems. Recent GPS data from sites in the Central Walker Lane indicate differential motion among tectonic blocks, which is interpreted to be a result of strain partitioning (Oldow et al., 2001). To in part quantify the magnitude of heterogeneous tectonic block rotation in the Neogene and to more clearly define block boundaries in the central Walker Lane, reconnaissance paleomagnetic data have been obtained from upper Tertiary volcanic rocks in Candeleria Hills, Nevada. Eight to ten oriented samples from 47 sites have been demagnetized with all sites yielding interpretable results: 14 sites in mid Miocene andesite flows (Ta), 17 sites in upper Miocene basalt flows (Tb), and 16 sites in lower Miocene (?) rhyolite ash flow tuffs (Taf) (one site per eruptive flow unit). After removal of a recent viscous overprint (by 10-25 mT and 100° to 300° C) most samples yield one well-defined magnetization well-grouped at the site level. After correcting for the modest dip of the flows; Ta yield a normal polarity magnetization direction (D = 14.4° , I = 69.1° , α ₉₅ = 5.53° , κ = 101.2, 17N); Tb1 yield a dual polarity magnetization (D = 34.0° , I = 45.0° , α ₉₅ = 8.49° , κ = 43.5, N = 3R5N); Tb2 yield a reversed polarity magnetization (D = 204.1° , I = -56.0° , α ₉₅ = 7.53° , κ = 47.6, R9); Taf yield anomalous northeast-directed, moderate negative inclination magnetizations (12 sites) and northwest-directed, moderate to steep negative inclination magnetizations (4 sites). The data from sites in Ta and Tb are interpreted to reflect moderate clockwise vertical axis rotation of parts of the Candeleria Hills structural block and are consistent with tectonic block rotation accommodating crustal strain in the Central Walker Lane since the Neogene. We tentatively interpret the anomalous directions from Taf to reflect either a high amplitude field excursion or the transitional part of a field reversal.