B. Benford

Fabric development in the mantle section of a paleotransform fault and its effect on ophiolite obduction, New Caledonia

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. New, detailed field measurements document the rotation of foliation, lineation, and pyroxenite dikes across a 50-km-wide region. Deformation intensity recorded by folding and boudinage of dikes increases toward a central, 3-km-wide mylonitic zone. We used several additional methods to characterize fabric patterns across the shear zone. The shape-preferred orientation of orthopyroxene grains, computed from outcrop tracings, closely parallels field fabrics, with increased alignment and flattening near the center of the shear zone. The lattice-preferred orientations of olivine are consistent with high-temperature fabrics; the a axes within the mylonitic core were used to constrain the orientation of shear zone boundaries. Seismic anisotropy calculations, based on the lattice-preferred orientation of olivine, indicate 5%–11% shear-wave anisotropies, with increased values in the center of the shear zone. The magnetic silicate fabric in the rocks, determined from anisotropy of magnetic susceptibility techniques, broadly matches field fabrics but provides less consistent information across the shear zone than other fabric methods. This suite of field and laboratory data provides a unique and detailed view of strain and fabric patterns across a shear zone in oceanic mantle lithosphere. Because the primary mantle fabrics seem to be related to the present distribution of ophiolitic rocks in New Caledonia, we propose that ophiolite obduction and Neogene extension may have been controlled by preexisting fabrics and structures in the oceanic lithosphere.

Mesozoic magmatism and deformation in the northern Owyhee Mountains, Idaho: Implications for along-zone variations for the western Idaho shear zone

The northern Owyhee Mountains of southwestern Idaho contain granitoid rocks that are the same age as the Cretaceous western border zone of the Idaho batholith to the north of the Snake River Plain. They contain a well-developed and consistently oriented 020° foliation, zircon yielding U-Pb dates of ca. 160–48 Ma, and initial 87Sr/86Sr isotopic compositions that show a steep west-to-east transition in values from 0.704 to 0.708 over a distance of ∼30 km. The rocks of the northern Owyhee Mountains are interpreted to be the southward continuation (Owyhee segment) of the western Idaho shear zone. Similar to a well-studied section of the western Idaho shear zone by McCall (McCall segment), the Owyhee segment displays steep foliation and lineation orientations, deformation of 98–90 Ma plutons, steep Sr isotopic gradients, and syntectonic tonalite intrusions. However, the Owyhee segment has three major differences from the McCall segment: (1) significantly less well-developed solid-state strain fabric foliations; (2) trend of 020° rather than 000°; and (3) a wider transition zone in initial Sr ratios from 0.704 to 0.708. We present a simple tectonic model to explain these differences, assuming a 20° along-zone difference in the initial orientation of the western margin of the Laurentia, a rigid-body collision, homogeneous material behavior, and transpressional kinematics. For the Owyhee segment, the model predicts a lower oblique-convergence angle, less convergent displacement, more dextral transcurrent displacement, and an overall lower finite strain relative to the McCall segment.

Lithospheric Control on the Initiation of the Yellowstone Hotspot: Chronic Reactivation of Lithospheric Scars

The Yellowstone hotspot is generally interpreted to have resulted from a mantle plume that initiated beneath the Idaho—Oregon—Nevada region at ~18 Ma. We explore an alternative model in which the initiation of Yellowstone magmatism is a result of lithospheric-scale processes—transtensional reactivation of the western Idaho shear zone—rather than a mantle plume. This model is based on both spatial and temporal correlations with deformation in the U. S. West. The first-order observation is that hotspot-related magmatism occurs as a linear N-S feature, which exploits the mantle portion of the inactive western Idaho shear zone and a deep crustal feature along the future Northern Nevada rift. The location for initiation of the McDermitt caldera, the interpreted initiation site of the Yellowstone hotspot, is spatially coincident with the southernmost known extent of the western Idaho shear zone in the lithospheric mantle. In contrast, the feeder zones of the voluminous magmatism of the Columbia River basalts intruded farther north, facilitated by the better-developed fabrics of the northern segment of the western Idaho shear zone. The timing of initial Yellowstone magmatism and Columbia River basalts is coincident with the coupling of western California and the North American plate to the Pacific plate. The kinematics of this coupling suggest oblique divergence and transtensional deformation. Previous numerical modeling of vertical fabrics in the lithospheric mantle show preferential reactivation by transtensional deformation. Thus, the mantle lithosphere—rather than deeper portions of the mantle—appears to exert a fundamental control on the initiation of the Yellowstone hotspot.