Mantle strength of the San Andreas fault system and the role of mantle-crust feedbacks

In lithospheric-scale strike-slip fault zones, upper crustal strength is well constrained from borehole observations and fault rock deformation experiments, but mantle strength is less well known. Using peridotite xenoliths, we show that the upper mantle below the San Andreas fault system (California, USA) is dry and its maximum resolved shear stress (5–9 MPa) is similar to the shear strength of the upper, seismogenic portion of the fault. These results do not fit with any existing lithospheric strength profile. We propose the “lithospheric feedback” model in which the upper crust and lithospheric mantle act together as an integrated system. Mantle flow controls displacement and loads the upper crust. In contrast, the upper crust controls the stress magnitude in the integrated system. Crustal rupture transiently increases strain rate in the upper mantle below the strike-slip fault, leading to viscous strain localization. The lithospheric feedback model suggests that lithospheric strength is a dynamic property— varying in space and time—in actively deforming regions

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

Rock magnetic stratigraphy of a mafic layered sill: a key to the Karoo volcanics plumbing system

The Insizwa sill is an 1 km-thick subhorizontal layered mafic intrusion and part of the Karoo Large Igneous Province in South Africa. This well-exposed intrusion consists of several superimposed petrologically and geochemically distinct units. Magnetic methods were used to study the intrusion in order to constrain the physical processes active in these types of bodies during crystallization. Rock magnetism studies indicate that within different petrologic units bulk susceptibility is controlled by primary magnetite (with minor pyrrhotite) and/or paramagnetic minerals (olivine, pyroxene). New magnetic data based on 659 specimens obtained from 3 vertical borehole cores, each spaced 5 km apart, confirm the prominent vertical zonation in low field magnetic susceptibility (Klf), degree of anisotropy (Pj) and orientation of the anisotropy of magnetic susceptibility (AMS) axes