New in situ laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe U–Pb geochronology of zircons from the Idaho batholith and spatially overlapping Challis intrusions reveals a series of discrete magmatic belts of different ages and compositions. Following the accretion of the Blue Mountains province to North America along the Salmon River suture zone, two compositionally diverse belts of metaluminous plutons formed both adjacent to the suture and well inboard of it. These were constructed from ~100 to 85 Ma and were followed by a voluminous pulse of peraluminous magmatism, forming the bulk of the Atlanta lobe and largest fraction of the batholith between ~80 and 67 Ma. Around 70 Ma, a later and more spatially restricted suite of metaluminous plutons formed around the Bitterroot lobe of the batholith. This was followed by another pulse of voluminous peraluminous magmatism in the Bitterroot lobe, lasting from ~66 to 54 Ma. The changes from low volume metaluminous to high volume peraluminous magmatism may reflect a combination of changes in the angle and segmentation of the subducting Farallon plate and over thickening of the continental lithosphere. All of these features were then cut by plutons and dikes associated with the Challis volcanic field, lasting from ~51 to 43 Ma. Inherited components are pervasive in zircons from most phases of the batholith. While Precambrian components are very common, zircons also often contain cores or mantles that are 5–20 million years older than their rims. This suggests that the early phases of the batholith were repeatedly cannibalized by subsequent magmas. This also implies that the older suites may have been originally more aerially extensive than their currently exposed forms.

Migrating magmatism in the northern US Cordillera: in situ U–Pb geochronology of the Idaho batholith

Regional tectonic features of the western Canadian Cordillera can be interpreted in terms of Middle Jurassic accretion of a single composite superterrane (Stikinia + Wrangellia + Alexander) to ancestral North America. Closure of the intervening Cache Creek-Bridge River ocean moved the continental edge to a new position west of the accreted superterrane. The Coast Belt is primarily a succession of superimposed Middle Jurassic to early Tertiary magmatic arcs related to prolonged subduction of Pacific Ocean lithosphere beneath the new North American margin. Discrete magmatic pulses, separated by lulls or periods of relative quiescence, successively overprinted the new margin. The Insular and Intermontane superterranes, previously viewed as widely separate entities prior to mid-Cretaceous time, were already amalgamated before an initial Middle Jurassic overprint produced an ancestral Coast Belt. Late Jurassic to Early Cretaceous rift-related(?) marine basins, mid to Late Cretaceous compressional structures, and early Tertiary extensional features coincident with the Coast Belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary, subduction-related Andean magmatic arc.

A Middle Jurassic to Early Tertiary Andean‐Sierran Arc Model for the Coast Belt of British Columbia

Obduction, backfolding and piggyback thrusting in the metamorphic hinterland of the southeastern Canadian Cordillera

Upper Jurassic and Lower Cretaceous basinal strata are preserved in a discontinuous belt along the inboard margin of the Alexander-Wrangellia-Peninsular terrane (AWP) in Alaska and western Canada, on the outboard margin of terranes in the Canadian Cordillera accreted to North America prior to Late Jurassic time, and along the Cordilleran margin from southern Oregon to southern California. Nearly all of the basinal assemblages contain turbiditic strata deposited between Oxfordian and Albian time. Arc-type volcanic rocks and abundant volcanic detritus in many of the assemblages suggest deposition within or adjacent to a coeval arc complex. On the basis of the general similarities between the basinal sequences, we propose that they record involvement of the AWP in the Late Jurassic-Early Cretaceous evolution of the Cordilleran margin. A geologically reasonable scenario for the accretion of the AWP includes (1) Middle Jurassic accretion to the Cordilleran margin, in particular the Stikine and Yukon-Tanana terranes, in a dextral transpressional regime, (2) Late Jurassic-Early Cretaceous overall northward translation of the AWP and evolution of a series of transtensional basins within a complex dextral strike-slip system along the Cordilleran margin, and (3) mid-Cretaceous structural imbrication of the AWP and inboard terranes that either terminated or resulted in a change in the character of deposition in the marginal basins. Mid-Cretaceous deformation along the inboard margin of the AWP was broadly synchronous with contractional deformation throughout the Cordillera and most likely due to changes in subduction zone parameters along the Cordilleran margin, outboard of the AWP, rather than collision of the AWP.

/pdf/upper-jurassic-lower-cretaceous-basinal-strata-along-the-1obd9kgh4z.pdf

Upper Jurassic-Lower Cretaceous basinal strata along the Cordilleran Margin: Implications for the accretionary history of the Alexander-Wrangellia-Peninsular Terrane

Using paleomagnetically derived estimates of latitudinal displacement, within the northern Cordillera we identify two major tectonic domains that were active during Late Cretaceous through Eocene time (roughly 83 to 45 Ma). At 100 to 90 Ma, the Interior Domain (comprising much of interior British Columbia, central Yukon, and eastern and central Alaska) was situated in the latitudes of Oregon and northern California, and the Coast Domain (southeastern Alaska, much of the Coast Ranges and islands of British Columbia, and the Cascade Mountains of Washington) was in latitudes similar to those of northern Mexico. Subsequently, both domains moved northward, reaching their present positions before 45 Ma. Within each domain, displacement estimates from northern locations are greater than those from southern locations, as if the domains had been elongated in an orogen-parallel sense. Evidently, during the latest Cretaceous through Eocene, the Cordillera was a huge dextral shear system, which (83 to 65 Ma) carried the Coast Domain about 2000 km northward until it combined with the Interior Domain and then (65 to 45 Ma) drove the combined domains an additional 1000 to 2000 km north northwestward, spreading them out along the continental margin. Following previous workers, we ascribe these motions to the rapid oblique subduction of the oceanic Kula Plate beneath the continental western margin of the North American Plate, and derive a model based, in part, on comparisons with the oblique subduction of the Nazca Plate beneath the central Andes. Between the Interior Domain and the craton, geologic evidence in the north has been interpreted as indicating displacements comparable in magnitude to paleomagnetic estimates; in the south, the record has been obscured by Eocene extension. Between the Interior and Coast Domains, geological relationships are complex, and most current geologically based interpretations propose that post-mid-Cretaceous relative motions between them did not exceed a few hundred kilometers, whereas paleomagnetic evidence indicates 2000 km. Our reading of the geologic evidence is that it neither requires nor precludes the very large displacements inferred from paleomagnetic observations. Hence we argue that although the paleomagnetically derived displacements far exceed those proposed by geologic interpretations, they are not inconsistent with the geologic evidence itself.

Large (1000 to 4000 km) northward movements of tectonic domains in the northern Cordillera, 83 to 45 Ma

K–Ar dates and U–Pb zircon dates define three periods of igneous activity in the southern Kootenay Arc: (1) emplacement of late-synkinematic to post-kinematic granodioritic plutons in mid-Jurassic time (170–165 Ma) accompanying amphibolite-facies regional metamorphism; (2) emplacement of post-kinematic granitic plutons in mid-Cretaceous time (~100 Ma); and (3) emplacement of small bodies of syenite in Eocene time (~50 Ma) in the western part of the area. Micas from mid-Jurassic plutons that yield the oldest K–Ar dates (158–166 Ma) also yield plateau-shaped 40Ar/39Ar age spectra. Age spectra for biotites younger than these but older than 125 Ma reflect thermal overprinting.In southeastern British Columbia, the Kootenay Arc marks the transition from the North American rocks of the Cordilleran miogeocline to the tectonic collage of allochthonous terranes that have been accreted to it.Deformation, metamorphism, and plutonism recorded in rocks of the southern Kootenay Arc commenced in mid-Jurassic time as a co...

Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern British Columbia. Part I: Jurassic to mid-Cretaceous

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Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern British Columbia. Part I: Jurassic to mid-Cretaceous