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.

The effect of hydrothermal alteration on the thermoremanent magnetization (TRM) of synthetic titanomagnetite (TM40: Fe2.6Ti0.404) has been studied, to simulate the alteration that occurs in the oceanic crust. Pseudo-single-domain titanomagnetite grains, similar in size to those often found in oceanic basalts, were dispersed in a permeable but rigid glass matrix. This resulted in a TRM in the sample which was subsequently oxidized in acidic solutions while a magnetic field (0.1 mT) was applied perpendicular to the TRM direction. The experiments were conducted in a non-magnetic stainless steel pressure vessel at 150øC in solutions of acidity varying from pH=2 to pH=7. In addition to being time and temperature dependent, the acquired chemical remanent magnetization (CRM) was also found to be very pH dependent. The degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions. Long-term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating. It was found that during the alteration of TM40 to titanomaghemite the CRM is along the a•nbie. nt field direction from the onset, and not partly or wholly along the TRM direction as has been found in previous air oxidation experiments. This has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples. 19,54519,546 KELSO ET AL.: CRM IN T1TANOMAGNETITE DUE TO HYDROTttERMAL ALTERATION (i.e., the CRM direction) being perpendicular to the original TRM of the sample. They found that CRM and TRM were parallel up to z = 0.7, where z, the oxidation parameter, is the proportion of ferrous iron converted to ferric iron, which varies from 0 to 1.

Serpentinized oceanic peridotites collected during Ocean Drilling Program Leg 147 at Site 895 in the Hess Deep were studied magnetically to further our understanding of the magnetization of the oceanic lithosphere. The majority of the samples are dunites and harzburgites, with varying degrees of serpentinization. Rock magnetic studies suggest that the dominant magnetic mineral is relatively pure pseudo-single-domain magnetite. Optical observations reveal that the magnetite has a secondary origin related to the serpentinization process. The dunites and the harzburgites have median natural remanent magnetization values of 3.2 and 1.2 A/m and magnetic susceptibility values of 0.053 and 0.023 (S1 volume units), respectively. The NRM is the dominant component of magnetization and has a magnitude similar to values observed for oceanic basalts of Layer 2a. Similarly serpentinized peridotites residing within the oceanic lithosphere are likely contributors to magnetic anomalies. Thermal and alternating-field demagnetizations usually yield a stable remanent magnetization direction that often has an inclination much greater than expected for the time-averaged geomagnetic field (>5°) at Site 895. This disparity suggests that the samples acquired their magnetization and were subsequently reoriented. Hole 895E samples have relatively coherent inclinations (30°) that become shallower in the bottom 30 m of the hole. Inclinations in Hole 895D vary from +60° to -60°, with no systematic trends evident with depth in the hole, suggesting differential vertical rotation of blocks on a scale of a few meters. Experimental evidence suggests that room-temperature magnetic viscosity does not significantly effect the magnetization intensity or direction of these samples.

Rock magnetic and petrolopic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudo-single-domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to thermal demagnetization, with nearly 50% of the magnetization remaining after 400øC demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long-wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous reinanent magnetization along the present-day field is likely to dominate. The magnetic susceptibilities of the samples are only capable of producing a magnetization of less than 1 A/m in the 0.05 mT present-day field of Central Australia. Susceptibility is nearly constant with temperature to within 30øC of the Curie temperature where it decreases rapidly, i.e., there is no significant Hopkinson peak. The granulite samples from this study have magnetizations, both reinanent and induced components, that are large enough to account for most long-wavelength magnetic anomalies if they are juxtaposed with relatively nonmagnetic rocks, similar to the high-grade rocks in the Canadian Shield. INTRODUCI•ON Background InformationThere has been an increasing interest in long-wavelength magnetic anomalies in recent years, in great part due to the magnetic data collected by the POGO and Mapsat satellites. These data sets have provided many insights into the different sources contributing to the earth's magnetic field, from core processes to external fields. Of direct interest to this paper is the component of the magnetic field due to the continental lithosphere. Attempts to identify and model the lithospheric components of magnetization (for reviews, see Mayhew et al. [1985]; and Mayhew and LaBrecque [1987]) have resulted in the recognition of widespread long-wavelength (>100 kin) magnetic anomalies over both the continents and the oceanic basins. Modeling the shape and amplitude of these anomalies suggests that their source resides in the lithosphere. The few magnetic studies of mantle material [Wasilewski et al., 1979; Wasilewski and Mayhew, 1982, 1992] have found that the continental mantle is relatively nonmagnetic, as is argued from thermodynamic considerations by Frost and Shire [1986]. Thus most modelers have considered the Moho to be the lower limit for the source of the observed anomalies. Inverse modeling of the anomalies typically shows that contrasts in mag...