The Salmon River suture zone, western Idaho, is a fundamental lithospheric boundary between the North American craton and the accreted terranes of the Cordilleran margin. The initial juxtaposition along this north-south-oriented structure occurred during Early Cretaceous time. This zone was potentially reactivated twice by subsequent tectonism, once during Cretaceous time and once during Miocene time. The Late Cretaceous western Idaho shear zone formed along the Salmon River suture zone, as denoted by a sharp gradient in the isotopic signature of the granitoids that intruded the lithospheric boundary zone. The reconstructed Late Cretaceous orientation of the western Idaho shear zone contains subvertical fabrics (lineation, foliation). The same boundary also acted as a locus for subsequent Miocene Basin and Range extensional deformation. Domino-style normal faulting and deep (2100 m) basin formation accommodated the motion between the extending accreted terranes to the west and the unextended Idaho batholith to the east. Whereas either the mantle boundary or a crustal-scale structuring controls the regional extent of the extensionally reactivated zone, locally crustal basement faults and lithological contacts control the orientation and precise location of faults that accommodate reactivation. The multiple reactivation of the Salmon River suture zone is critical for several reasons. The Early Cretaceous suture zone apparently created a fundamental lithospheric flaw, which was reactivated after terrane accretion. Whether this zone was a fracture or a shear zone, the fabric in the mantle lithosphere was apparently not ‘healed’ during orogenesis. Thus, juxtaposition of mantle lithosphere, which is inferred to occur by faulting in the uppermost mantle, acts as a weakness during later tectonism. Second, the paucity of strike-slip plate boundaries in the geological record makes sense in the context of reactivation. The vertical, lithospheric-scale nature of these structures makes them particularly susceptible to lithospheric-scale reactivation during both transcurrent and/or extensional deformation. These reactivations both overprint the earlier deformation and modify the original geometry. Steeply dipping fabrics, rather than vertical fabrics, may be the general signature of major, ancient strike-slip faults.

Three well‐characterized glass‐ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations (pTRMs) in the temperature interval 400°–350°C for varying lengths of time (10 min to 5 days) and in magnetic fields from 0.5 to 5 Oe (0.05–0.5 mT). The thermal demagnetization behavior of the pTRMs changes with domain state in the sense that with decreasing coercivity (increasing grain size), demagnetization curves become less steep and may not completely demagnetize until heated to the Curie temperature. Numerical calculations based on Néel's single domain (SD) theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures. We observe that experimental results do not match any of the theoretical predictions. There is no discernible field dependence of demagnetization curves for the SD and pseudo‐single domain samples and even for the “soft” multidomain (MD) sample in the field range investigated. An important consequence for thermally derived paleointensity data is that they may not be entirely reliable, especially if the sample remanence is due to MD grains.

S U M M A R YMost rocks contain both ferromagnetic and paramagnetic minerals that contribute to their bulk magnetic susceptibility and the anisotropy of magnetic susceptibility. Anisotropy of magnetic susceptibility techniques typically measure the net susceptibility and are not able to separate paramagnetic and ferromagnetic contributions. Since different minerals may form at various times and/or under different conditions, examination of their individual contributions provides unique information related to the rock's formation and evolution. By subjecting a sample to high magnetic fields, the ferromagnetic minerals become saturated and the contribution of the paramagnetic minerals can be evaluated (the slope of the line at high field values, on a field vs magnetization plot). Using this approach, we developed a new technique that separates the ferromagnetic and paramagnetic components of standard 1 inch cylindrical samples using a Vibrating Sample Magnetometer. This separation is tested by artificially combining separate samples with known paramagnetic-only and ferromagnetic-only behaviour. By comparing the high-field results of a combined paramagnetic and ferromagnetic signal to the classic low field alternating current susceptibility of the paramagnetic-only signal, we demonstrate that the high field anisotropy is the result solely of the paramagnetic fabric even when the low field anisotropy of magnetic susceptibility is dominated by the ferromagnetic minerals. A ferromagnetic-only fabric is calculated for the combined paramagnetic and ferromagnetic rock, by tensor subtraction of the high field (paramagnetic-only) and low field (paramagnetic plus ferromagnetic) measurements on the same sample. Application of this technique to natural samples of combined paramagnetic and ferromagnetic behaviour is discussed.

We present an integrated study of the postcollisional (post-Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4-123.8 Ma crystallization ages and exhibit a north-northeast-younging trend of the magmatism. The 40 Ar/ 39 Ar analyses on biotite and hornblende indicate very rapid (<1 m.y.) cooling below biotite closure temperature (~350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ~200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic-Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.

Results of paleomagnetic and rock magnetic measurements are presented from gabbroic samples recovered during Ocean Drilling Program Leg 147 at the Hess Deep. Paleomagnetic measurements indicate that samples acquired up to two small components of secondary remanent magnetization. Stable magnetic inclinations determined after alternating-field and thermal demagnetization reveal a mean stable magnetic direction (38°) that is significantly steeper than that predicted for this equatorial site (<5°). Thus, it is likely that remanent magnetization was acquired before tectonic uplift. The mean intensity of natural remanent magnetization for the recovered gabbros is 2.3 A/m, and the Koenigsberger ratio indicates that the in situ magnetization is dominated by remanent, rather than induced, magnetization. Measurements of hysteresis loop parameters indicate that the effective magnetic grain size of the gabbro samples falls within the pseudo-single domain region. Although recovered from a fast-spreading ridge, the paleomagnetic and rock magnetic properties of the gabbros from Hole 894G are very similar to those of gabbros recovered from slow-spreading ridges.