Search

The abrupt boundary between accreted terranes and cratonic North America is well exposed along the Salmon River suture zone in western Idaho and eastern Oregon. To constrain the post-suturing deformation of this boundary, we assess the cooling history using zircon and apatite (U-Th)/He thermochronology. Pre-Miocene granitic rocks, along a regional transect, were sampled from accreted terranes of the Blue Mountains Province to cratonic North America (Idaho batholith). Each sample was taken from a known structural position relative to a paleotopographic surface represented by the basal unit of the Miocene Columbia River basalts. An isopach map constructed for the Imnaha Basalt, the basal member of the Columbia River Basalt Group (CRBG), confirms the presence of a Miocene paleocanyon parallel to the northern part of Hells Canyon. The (U-Th)/He zircon dates indicate mostly Cretaceous cooling below 200°C, with the ages getting generally younger from west to east. The (U-Th)/He apatite dates indicate Late Cretaceous-Paleogene cooling, which post-dates tectonism associated with the western Idaho shear zone (WISZ). However, (U-Th)/He apatite dates younger than the Imnaha Basalt, with one date of 3.4 ± 0.6 Ma, occur at the bottom of Hells Canyon. These young (U-Th)/He apatite dates occur along the trend of the Miocene paleocanyon, and cannot be attributed to local exhumation related to faults. We propose that burial ofMesozoic basement rocks by the Columbia River basalts occurred regionally. However, the only samples currently exposed at the Earth's surface that were thermally reset by this burial were at the bottom of the Miocene paleocanyon. If so, exhumation of these samples must have occurred by river incision in the last 4 million years. Thus, the low-temperature thermochronology data record a combination of Late Cretaceous- Paleogene cooling after deformation along the WISZ that structurally overprinted the suture zone and Neogene cooling associated with rapid river incision. [ABSTRACT FROM AUTHOR]

The physical process of rhyolite segregation from crystal mushes remains elusive as microstructural evidence of conventional segregation mechanisms is not available. This study provides direct fabric evidence for deformation‐assisted segregation of eruptible rhyolite in the Chilean Andean arc. The shallow (<7 km), 6.4–6.2 Ma Huemul pluton comprises domains of quartz monzonite, granite, and high‐silica granite. Compositional modeling shows that rhyolitic melt (high‐silica granite) was extracted from a granitic parent, leaving behind silicic cumulates (quartz monzonite). To understand mechanisms of rhyolite segregation, we investigate magmatic fabrics in the pluton. Anisotropy of Magnetic Susceptibility analyses reveal oblate magnetic fabrics and NNW‐striking, subvertical magnetic foliations throughout Huemul. Within the high‐silica granite, magnetic lineations are subvertical and parallel to elongate miarolitic cavities. Magnetic lineations in the quartz monzonite plunge moderately to the NNW, away from the high‐silica granite. In the quartz monzonite, the Shape‐Preferred Orientation of early feldspars is parallel to the magnetic lineation and developed while suspended in melt. Estimations of early feldspar clustering and crystallinity yield ~38% of interstitial volume loss in the quartz monzonite and no volume loss in the granite. These fabric data suggest ENE tectonic shortening coeval with rhyolite extraction. We explain these observations with a model of tectonic filter pressing in which shortening is accommodated by interstitial melt flow at slow (10−5 km3/yr) rates, segregating moderate volumes of rhyolite in Myr time scales. These interactions link plutonism, tectonic deformation, and upward mobility of eruptible rhyolite in tectonically active margins. Key Points: Granitic fabrics in a silicic pluton show rhyolite segregation, accumulation, and extraction coeval with tectonic shorteningWe interpret that a portion of tectonic shortening is accommodated by displaced interstitial melt, that is, tectonic filter pressingTectonic filter pressing may be the key connecting segregation of rhyolite in volcanic and tectonic time scales in active arcs [ABSTRACT FROM AUTHOR]

The Idaho batholith and spatially overlapping Challis intrusive province in the North American Cordillera have a history of magmatism spanning some 55 Myr. New isotopic data from the ∼98 Ma to 54 Ma Idaho batholith and ∼51 Ma to 43 Ma Challis intrusions, coupled with recent geochronological work, provide insights into the evolution of magmatism in the Idaho segment of the Cordillera. Nd and Hf isotopes show clear shifts towards more evolved compositions through the batholith's history and Pb isotopes define distinct fields correlative with the different age and compositionally defined suites of the batholith, whereas the Sr isotopic compositions of the various suites largely overlap. The subsequent Challis magmatism shows the full range of isotopic compositions seen in the batholith. These data suggest that the early suites of metaluminous magmatism (98–87 Ma) represent crust–mantle hybrids. Subsequent voluminous Atlanta peraluminous suite magmatism (83–67 Ma) results primarily from melting of different crustal components. This can be attributed to crustal thickening, resulting from either subduction processes or an outboard terrane collision. A later, smaller crustal melting episode, in the northern Idaho batholith, resulted in the Bitterroot peraluminous suite (66–54 Ma) and tapped different crustal sources. Subsequent Challis magmatism was derived from both crust and mantle sources and corresponds to extensional collapse of the over-thickened crust. [ABSTRACT FROM PUBLISHER]

• Different generations of magmatic to subsolidus titanite can be distinguished. • Recrystallized and neoblastic titanite can be used to date shear zone initiation. • The western Idaho shear zone initiated between 98 and 96 Ma. We present an integrative petrochronological approach to dating the initiation of shear zone deformation in granitic rocks, using the mineral titanite (CaTiSiO 5). This method is suited to granitoid-hosted shear zones in continental arc settings, where the interplay between tectonics, magmatism, and deformation is actively debated. Microstructural observations including crystallographic misorientations, fabric context, and backscattered electron images were used to identify relict magmatic, partially to fully recrystallized, and neoblastic titanite crystals. Principal component analysis of trace element compositional variance in titanite was used to further distinguish and quantify the crystal chemical response to deformation. High-precision isotope dilution U-Pb geochronological measurements on relict magmatic, partially to fully recrystallized, and neoblastic titanite crystals was used to constrain the timing of shear zone initiation. For a sample of a porphyritic orthogneiss from the western Idaho shear zone of the northern U.S. Cordillera, U-Pb zircon geochronology dates emplacement of this unit to between ca. 105 and 103 Ma, whereas the age of partially recrystallized and neoblastic titanite grains indicate that the western Idaho shear zone initiated between ca. 98 and 96 Ma. The >5 Ma lag between pluton emplacement and the onset of deformation indicates that mid-Cretaceous deformation in the western Idaho shear zone is temporally distinct from Late Jurassic–Early Cretaceous accretionary tectonics. Our integrated petrochronological investigation documents that: 1) deformation induces textural, chemical, and isotopic changes in titanite; 2) the geochemical properties of primary magmatic titanite collapse into a discernible trend of subsolidus syn-deformational titanite compositions; and 3) the onset of deformation and progression of strain accumulation are preserved in the U-Pb ages of these titanite crystals. [ABSTRACT FROM AUTHOR]

Abstract: We documented polygonal fracture networks in the Tuolumne Intrusive Suite, Sierra Nevada Batholith by collecting data on the size, orientation, and length of fractures and fracture-bound polygons in nearly isotropic granitic rocks. Polygonal fracture networks are constrained to sheets of rock sub-parallel to the underlying rock, with the sheets bound above and below by fractures (i.e., free surfaces). Data indicate that most geometric characteristics of the polygonal fracture networks are scale-invariant; large (m-scale) networks exhibit the same geometry as small (cm-scale) networks. A linear correlation exists between sheet thickness and average polygon size. Polygons within the networks are anisotropic, with the degree of anisotropy correlated with the degree of sheet curvature. The preferred orientation of the polygons correlates with the inclination of the sheet, with long axes of polygons parallel to sheet strike. Based on geometry, we suggest that the polygonal fracture networks in granitic rocks observed in this study formed from temperature fluctuations that caused thermal expansion and contraction in the rock. Repeated cycles of expansion/contraction led to the initiation and propagation of fractures within the network. We propose that the observed fracture patterns are not a product of extrinsic conditions but rather a product of interactions between fractures, implying that the polygonal fracture networks are self-organizing systems. [Copyright &y& Elsevier]

The Johnson Granite Porphyry and the Silver Pass Porphyry are two late-stage porphyritic intrusions in the Cretaceous Sierra Nevada Batholith, California. Both intrusions are characterized by fine-grained textures, miarolitic cavities, pegmatite veins, and abundant xenoliths. Microstructures in the Johnson Granite Porphyry record magmatic flow. Magnetic fabrics in the Johnson Granite Porphyry, determined by anisotropy of magnetic susceptibility, show that the poles to magnetic foliations define a NNE-SSW girdle and magnetic lineations plunge shallowly to moderately to the WNW and ESE. A gravity survey and three-dimensional inversion indicate that the Johnson Granite Porphyry is shallow and tabular, extending to 1.5 km depth beneath the southwestern margin. A second gravity anomaly was delineated southeast of the Johnson Granite Porphyry, suggesting the intrusion continues to depth. The Silver Pass Porphyry is deformed by the dextral Rosy Finch shear zone and records a gradient of high- to low-temperature, solid-state fabrics. Field and magnetic foliations in the Silver Pass Porphyry are subvertical and lineations are shallowly plunging. We propose that regional shear zones controlled the emplacement of these intrusions. The Johnson Granite Porphyry was emplaced in a three-dimensional tension gash associated with the termination of the Bench Canyon shear zone. The Rosy Finch shear zone facilitated the emplacement of the Silver Pass Porphyry, probably as a tension fracture associated with the shear zone. The field observations and magnetic fabrics suggest that the Johnson Granite Porphyry may record an extrusive event, suggesting that volcanism may be related to deformation along active shear zones. [ABSTRACT FROM AUTHOR]