Abstract Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques. In the absence of valid strain markers, it can be difficult to identify the presence of an early tectonic fabric development and resulting layer parallel shortening (LPS). In order to identify early LPS, we carried out anisotropy of magnetic susceptibility (AMS) analyses on Mississippian limestones from the Sawtooth Range of Montana. The Sawtooth Range is an arcuate zone of north-trending, closely spaced, west-dipping, imbricate thrust sheets that place Mississippian Madison Group carbonates above Cretaceous shales and sandstones. This structural regime is part of the cordilleran mountain belt of North America, which resulted from accretion of allochthonous terrains to the western edge of the North American continent. Although the region has a general east–west increase in thrust displacement and related brittle deformation, a similar trend in penetrative deformation or the distribution of tectonic fabrics is not observed in the field or in the AMS results. The range of magnetic fabrics identified in each thrust sheet ranges from bedding controlled depositional fabrics to tectonic fabrics at a high angle to bedding.

Paleomagnetic data from three regionally extensive Oligocene ignimbrite sheets, two sequences of Miocene andesite flows, and ten sequences of Upper Miocene to Pliocene basaltic andesite flows in the Candelaria Hills and adjacent areas, west-central Nevada, provide further evidence that, since the late Miocene, and possibly between latest Miocene and earliest Pliocene time, the broad region that initially facilitated Neogene displacement transfer between the Furnace Creek and central Walker Lane fault systems experienced some 20° to 30° of clockwise vertical-axis rotation. The observed sense and magnitude of rotation are similar to those previously inferred from paleo-magnetic data from different parts of the Silver Peak Range to the south. We propose that clockwise rotation within the transfer zone formed in response to horizontal components of simple and pure shear distributed between early-formed, northwest-striking right-lateral structures that initiated in mid- to late Miocene time. Notably, the spatial distribution of the early-formed transfer zone is larger and centered south of the presently active stepover, which initiated in the late Pliocene and is characterized by a trans-tensional deformation field and slip on east-northeast–oriented left-oblique structures that define the Mina deflection. The sense and magnitude of rotation during this phase of deformation, which we infer to be of pre–latest Pliocene age, are inconsistent with the geodetically determined regional velocity field and seismologically determined strain field for this area. As a consequence, the longer-term kinematic evolution of the stepover system, and the adjoining parts of the Furnace Creek and Walker Lane fault systems, cannot be considered as a steady-state process through the Neogene.

Paleomagnetic and geochronologic data from mafic intrusive rocks, inferred to contain magnetizations of early Late Cretaceous age, and upper Tertiary volcanic rocks, all part of the upper plate of the Silver Peak extensional complex in the southern Silver Peak Range, add to the growing body of results suggesting that Neogene displacement transfer within the central Walker Lane involved components of modest magnitude crustal tilting and, at least locally, rotation of structural blocks. Mesozoic intrusions and upper Tertiary volcanic rocks yield paleomagnetic data that are discordant to expected field directions. The data from 49 accepted sites in mafic dikes that cut granitic rocks, 4 sites in a single Oligocene(?) ash flow tuff, 20 sites in mid-Miocene andesite flows, and 28 sites in upper Miocene to lower Pliocene pyroclastic rocks may imply a systematic progression in the magnitude of vertical axis rotation and tilting with age. At a minimum, the data are consistent with at least some 20° of clockwise rotation of upper-plate rocks in this part of the Silver Peak Range and demonstrate a greater regional extent to the area affected by clockwise rotation during Neogene displacement transfer. Eight new 40 Ar/ 39 Ar age determinations from the mafic dikes and adjacent host rocks, all somewhat disturbed age spectra, imply that these rocks cooled below ∼300 °C during the Late Cretaceous between about 90 and 80 Ma. Four mafic dike groundmass concentrates yield integrated apparent ages between 86.31 Ma ± 0.12 Ma and 80.80 Ma ± 0.11 Ma, and four age spectra from biotite from the host granite yield integrated values between 93.6 ± 0.9 Ma and 78.6 Ma ± 0.2 Ma. The mafic dikes yield in situ exclusively normal polarity results consistent with an early Late Cretaceous age of magnetization acquisition, with an overall group mean (D = 25.1°, I = 55.4°, α 95 = 3.4°) that is discordant to an early Late Cretaceous expected field (D = 337°, I = 66°). Ten of 20 sites from steeply dipping mid-Miocene andesite flows and 21 of 28 sites in gently tilted upper Miocene ash flow tuffs yield overall stratigraphically corrected group means (D = 24.4°, I = 36.7°, α 95 = 7.1°) and (D = 16.5°, I = 53.5°, α 95 = 7.6°, respectively) that are discordant in a clockwise sense to the Miocene expected direction (D = 358°, I = 55°). The paleomagnetic data support a history of tilting and vertical axis rotation of the southern Silver Peak Range, most of which occurred coincidently with latest Miocene and Pliocene exhumation of the lower-plate rocks in the extensional complex. In addition, it is possible that the paleomagnetic data from Mesozoic intrusions record an additional, modest phase of deformation that predated development of the extensional complex. The observations are consistent with a tectonic model where deformation of upper-plate rocks in this area involved a small component of west- to southwest-side-down tilting, likely related to range-scale folding during the late Miocene and Pliocene, accompanied by modest clockwise vertical axis rotation.