Clyde J. Northrup

Taking mylonites' temperatures

We measured titanium-in-quartz (TitaniQ) temperatures by ion microprobe for three deformed rocks having different quartz microstructure to assess the possibility of constrain- ing the temperature of mylonitization directly from dynamically recrystallized quartz grains. Calibration via ion microprobe indicates analytical precisions and accuracies of ~±2 °C and ±10 °C, respectively (2σ). High- versus low-temperature mylonites yield high versus low Ti concentrations and temperatures that are consistent with other fi eld studies of mylonites that have similar microstructures; these observations imply that TitaniQ accurately measures dynamic recrystallization temperatures. Variations in temperature in a single domain exceed analytical errors, implying that one microstructure in a rock can refl ect different tempera- tures and, presumably, strain rates. Combined with paleopiezometric and phase-equilibrium estimates of differential stress and water fugacity, strain rates may be estimated.

Timing of plutonism and deformation in the White Mountains of eastern California

New mapping and U-Pb zircon geochronology help establish the timing of contractional deformation and magmatism in the White Mountains of California. In the Redding Canyon area of the west-central White Mountains, Mesozoic deformation characterized by east-directed movement along reverse faults as well as recumbent folding was followed by development of upright folds with axes that plunge moderately to the north. This later folding event produced penetrative, vertical, north-striking axial-planar cleavage that is present along much of the western flank of the range. Deformed units include folded and/or boudinaged diorite dikes (ca. 165 Ma; U-Pb zircon) that contain the later penetrative cleavage. The cleavage is clearly cut by the Redding Canyon pluton (ca. 164 Ma; U-Pb zircon), demonstrating that at least some of the intense deformation preserved in the area is Middle Jurassic and correlative with the East Sierran thrust system identified elsewhere in California. Dates for the Beer Creek pluton (ca. 179 Ma; U-Pb zircon) and the Sage Hen Flat pluton (ca. 175 Ma; U-Pb zircon), which cut deformation in their wall rocks, suggest that East Sierran thrust deformation did not propagate as far eastward as these plutons at the present level of exposure. The new dates also cast doubt on the presence of any Late Jurassic–Early Cretaceous plutonism in the White Mountains. Throughout the east-central Sierra Nevada and White Mountains, high-precision U-Pb zircon geochronology is resolving significant plutonism into two short-lived events that occurred at ca. 180–165 Ma and 102–86 Ma.

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.

U-Pb geochronology and geochemistry of intrusive rocks from the Cougar Creek Complex, Wallowa arc terrane, Blue Mountains Province, Oregon-Idaho

The Cougar Creek Complex is a compositionally diverse intrusive suite constructed of variably deformed and nondeformed dikes and plutons interpreted as the roots of the Wallowa arc terrane in the Blue Mountains Province, Oregon-Idaho. New high-precision U-Pb zircon ages, geochemical data, and structural analysis define two compositionally and temporally distinct cycles of magmatism, and two episodes of contractional deformation in the Cougar Creek Complex. From Middle Permian to Early Triassic time (265.4 ± 0.2 Ma to 248.8 ± 0.1 Ma), the Wallowa arc was dominated by silicic arc magmatism. During the Early to Middle Triassic (248.8 ± 0.1 Ma to 229.4 ± 0.1 Ma), an apparent hiatus in intrusive activity suggests a cessation or lull in arc magmatism, or a shift of the arc axis. This lull in volcanic activity coincides with D1 deformation, uplift, and erosion of the Wallowa arc. In the Late Triassic (229.4 ± 0.1 Ma to 229.1 ± 0.5 Ma), voluminous mafic to intermediate magmatism, exhibiting trace-element characteristics of a subduction-modified, strongly depleted mantle source, was initiated, and this represents a change in “normal” arc magmatism. We interpret the sequence of D1 deformation, an apparent gap in igneous activity, exhumation of the Wallowa arc, and a switch from “normal” silicic arc magmatism to depleted, mid-ocean-ridge basalt–like mafic volcanism as a response to spreading-ridge subduction and subsequent slab window magmatism beneath the overriding Wallowa arc. Late Triassic D2 synmagmatic left-lateral mylonitic shear zones in the Cougar Creek Complex illustrate strain partitioning in an intrusive, transpressional arc axis environment undergoing sinistral-oblique subduction. Synchronous U-Pb crystallization ages, 40Ar/39Ar cooling ages, the abundance of plutonic clasts within conglomerate units, and unimodal Late Triassic detrital zircon ages for volcanic sandstone units suggest uplift and erosion of the Wallowa arc during the Late Triassic; this is inferred to be related to the upwelling of buoyant asthenosphere through an opening slab window beneath the Wallowa arc. We propose that Late Triassic mafic igneous rocks from the Wallowa terrane and Wrangellia resulted from the subduction of a spreading ridge and slab window magmatism beneath a late Paleozoic arc, thus linking the Late Triassic geologic evolution of two fundamental components of the Cordilleran orogen.

Isotopic Compositions of Intrusive Rocks from the Wallowa and Olds Ferry Arc Terranes of Northeastern Oregon and Western Idaho: Implications for Cordilleran Evolution, Lithospheric Structure, and Miocene Magmatism

Late Paleozoic and Mesozoic intrusive rocks from the Wallowa and Olds Ferry arc terranes of the Blue Mountains Province, Oregon-Idaho, provide constraints on the paleogeographic and tectonic setting of magmatism preserved in both arcs. Sr, Nd, and Pb isotopic data show that the Wallowa terrane represents an isotopically depleted, juvenile intra-oceanic island arc. By contrast, isotopic data for intrusive rocks of the Olds Ferry arc are more isotopically enriched, and thereby establish a clear distinction between the two arcs. This distinction strengthens paleogeographic interpretation of the Olds Ferry terrane as a fringing continental arc, and it provides a basis for correlation to other inboard Cordilleran arc terranes including Quesnellia and Stikinia. The Wallowa terrane is by contrast more similar geologically and isotopically to the outboard Insular terranes. These isotopic data also constrain interpretations of regional lithospheric architecture. Isotopic profiles generated orthogonal to the inferred Wallowa–Olds Ferry terrane boundary and the western Idaho shear zone show abrupt increases in initial 87 Sr/ 86 Sr that mark the transitions between three geochemically distinct lithospheric columns. West-to-east spatial variability in the isotopic compositions of Neogene volcanic rocks is explained by the partial melting of these three geochemically distinct mantle reservoirs coupled to their respective crustal columns since the early Mesozoic, rather than alternative models of lithosphere-scale decollement offset during Sevier shortening. The inherited arc-related mantle of the Olds Ferry arc may also have played a primary role in the petrogenesis of distinctive Neogene low-K, high-alumina olivine tholeiites of the High Lava Plains.