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U-Pb analyses of 101 detrital zircon grains from Paleozoic and Triassic clastic strata of the Alexander terrane indicate that most detritus in the terrane was derived from intraterrane igneous rocks, although the occurrence of 1.0-3.0 Ga grains in a Lower Devonian sandstone indicates proximity to a continental region during the Silurian-Devonian Klakas orogeny. Because these Precambrian grains are not the same age as grains that were accumulating along the western margin of North America, it appears unlikely that the terrane was in proximity to the Cordilleran margin during Early Devonian time. Continental regions that could have shed grains of the appropriate ages, and that also record Silurian-Devonian orogenic activity, include the paleo-Pacific margin of Australia and the Scandinavian portion of Baltica. Existing paleomagnetic data are consistent with either paleoposition, whereas previous biogeographic studies suggest closer ties with Baltica.

New analyses for U-Pb ages and εHft values, along with previously published U-Pb ages, from Mississippian-Permian sandstones in synorogenic clastic wedges of the Ouachita foreland and nearby intracratonic basins support new interpretations of provenance and sediment dispersal along the southern Midcontinent of North America. Recently published U-Pb and Hf data from the Marathon foreland confirm a provenance in the accreted Coahuila terrane, which has distinctive Amazonia/Gondwana characteristics. Data from Pennsylvanian-Permian sandstones in the Fort Worth basin, along the southern arm of the Ouachita thrust belt, are nearly identical to those from the Marathon foreland, strongly indicating the same or a similar provenance. The accreted Sabine terrane, which is documented by geophysical data, is in close proximity to the Coahuila terrane, suggesting the two are parts of an originally larger Gondwanan terrane. The available data suggest that the Sabine terrane is a Gondwanan terrane that was the provenance of the detritus in the Fort Worth basin. Detrital-zircon data from Permian sandstones in the intracratonic Anadarko basin are very similar to those from the Fort Worth basin and Marathon foreland, indicating sediment dispersal from the Coahuila and/or Sabine terranes within the Ouachita orogen cratonward from the immediate forelands onto the southern craton. Similar, previously published data from the Permian basin suggest widespread distribution from the Ouachita orogen. In contrast to the other basins along the Ouachita-Marathon foreland, the Mississippian-Pennsylvanian sandstones in the Arkoma basin contain a more diverse distribution of detrital-zircon ages, indicating mixed dispersal pathways of sediment from multiple provenances. Some of the Arkoma sandstones have U-Pb age distributions like those of the Fort Worth and Marathon forelands. In contrast, other sandstones, especially those with paleocurrent and paleogeographic indicators of southward progradation of depositional systems onto the northern distal shelf of the Arkoma basin, have U-Pb age distributions and εHft values like those of the "Appalachian signature." The combined data suggest a mixture of detritus from the proximal Sabine terrane/Ouachita orogenic belt with detritus routed through the Appalachian basin via the southern Illinois basin to the distal Arkoma basin. The Arkoma basin evidently marks the southwestern extent of Appalachian-derived detritus along the Ouachita-Marathon foreland and the transition southwestward to overfilled basins that spread detritus onto the southern craton from the Ouachita-Marathon orogen, including accreted Gondwanan terranes. [ABSTRACT FROM AUTHOR]

Paleomagnetic data from 29 sites within the Lower Devonian Karheen Formation pass fold and reversal tests, and define two polarity zones that are correlative between two separate stratigraphic sections. We interpret this magnetization to be Early Devonian and thus propose a paleopole for the Alexander terrane that places the terrane at about 14 [degrees] (north or south). Due to uncertainties of the Early Devonian parts of the North American and Gondwana apparent polar wander paths, several interpretations of the Early Devonian position of the Alexander terrane relative to either North America (Laurentia) or Gondwana are permissible. We use a combined data set of paleomagnetic, detrital zircon, and paleontological observations to narrow the paleogeographic options. Eastern Australia and the Scandinavian part of Baltica lie within the paleolatitude bands, and both are possible sources of Precambrian detrital zircon grains found in the Karheen Formation; however, paleontological data from the underlying Heceta Formation are only consistent with a position near northern Laurentia-Baltica. Therefore, the combined data sets indicate an Early Devonian position near Baltica.

The determination of the ages of 176 abraded single detrital zircon grains in the Cambrian to Triassic miogeoclinal and eugeoclinal strata in Nevada reveals that the grains might have originated from the crystalline provinces of central and western North America. The ages of the zircon grains are determined by U-Pb analyses of the samples. The detrital zircon grains probably originated from cratonal rocks, the Peace River arch area and the basement rocks of southwestern US.

A long-standing debate over the provenance of lower Paleozoic sedimentary rocks in the Roberts Mountains allochthon, Nevada, is addressed through abraded single-grain U-Pb detrital zircon analysis of the Harmony and Valmy Formations. Twenty-seven zircon grains from mature quartz arenite of the Middle Ordovician Valmy Formation yielded concordant to a few percent discordant ages of [approximately] 1050 Ma (n = 2), 1830-1845 Ma (n = 4), 1910-1960 Ma (n = 5), 2270-2340 Ma (n = 2), 2650-2750 Ma (n = 8), and [approximately] 2900 Ma (n = 1), and moderately discordant ages of [approximately] 2070 Ma (n = 1) and [approximately] 3240 Ma (n = 1). These ages are consistent with ultimate derivation from the north (Slave craton and Medicine Hat province and central/northern Alberta) and suggest a link between the Valmy Formation and the coeval Eureka Quartzite on the adjacent shelf and miogeoclinal prism. Twenty-seven analyses of zircon from immature quartzo-feldspathic wacke of the Upper Cambrian Harmony Formation include concordant analyses in the range of 695-710 Ma (n = 9), 1015-1225 Ma (n = 12), [approximately] 1330 Ma (n = 2), [approximately] 1745 Ma (n = 1), [approximately] 1915 Ma (n = 1), and 2570 Ma (n = 1). The [approximately] 700 Ma and [approximately] 1100 Ma ages (which represent >80% of the grains) have no known western North American basement source of reasonable proximity and age, suggesting derivation from an unknown western or southern source, or perhaps from rocks that presently underlie the Roberts Mountains allochthon. A growing body of U-Pb isotopic evidence indicates that 1000-1270 Ma and 670-750 Ma zircon ages are relatively common in offshelf continental margin strata. The latter set of ages is also represented by orthogneiss that intrudes or is overlain by continental margin strata, necessitating the widespread exposure of felsic plutonic rocks of these ages along the Cordilleran margin during early Paleozoic time. Subsequently, such rocks may have been overlain by younger continental margin strata, structurally overlain by allochthonous units, or tectonically removed from the region by younger rifting events. The presence of widespread mid- and late Proterozoic felsic magmatic rocks along the Cordilleran margin is not easily accommodated by popular pre-570 Ma plate reconstructions that place Nevada and the central Cordillera opposite the Transantarctic Mountains or eastern Australia.

Gold-bearing quartz vein systems in the Juneau gold belt formed within a 160-km-long by 5- to 8-km-wide zone along the western margin of the Coast Mountains, Alaska. Vein systems are spatially associated with shear zones adjacent to terrane-bounding, mid-Cretaceous thrust faults. Analysis of vein orientations and sense of shear data define a stress configuration with greatest and least principal axes oriented subhorizontally with northeast-southwest trends and subvertically, respectively. This local stress configuration is compatible with the far-field plate configuration during Eocene time. Isotopic ages of vein formation indicate that fluid cycling occurred between 56.5 and is greater than or equal to 52.8 Ma, and are consistent with a genetic link between veining and a change in plate motion in early Eocene time. Veining was also synchronous with the latter stages of rapid exhumation and voluminous plutonism immediately inboard of the gold belt. We propose a model in which interacting tectonic events facilitated fault-valve action and vein development along now-exhumed shear zones.

Regional tectonic analysis suggests that the entire Cordilleran continental margin, from Nevada to the Yukon, underwent contractional orogeny involving emplacement of deep basinal, submarine-fan, and mafic volcanic strata over autochthonous continental margin strata during Late Devonian to Mississippian time (Antler orogeny). Displaced parautochthonous strata are present from Nevada to east-central British Columbia, and perhaps within the Yukon-Tanana terrane of Yukon and eastern Alaska. Manifestations of the orogenic event in the northern Cordillera are preserved primarily by normal faults and related coarse clastic strata of the Selwyn basin. We propose that these features formed due to flexural extension during foreland deformation, rather than mid-Paleozoic rifting or transtension.

Basinal layers from the Upper Jurassic to the Lower Cretaceous are preserved in an interrupted array along the periphery of the Alexander-Wrangellia-Peninsular (AWP) terrane. Turbiditic strata deposited from the Oxfordian to the Albian time were found in nearly all the sedimentary structures. The presence of igneous rocks and volcanic ash in the assemblages is suggestive of deposition at or near to coeval arcs. A geologic scenario explaining the evolution of the assemblages is presented.

The stratigraphy of the Coast Mountains batholith between Cape Fanshaw and Taku Inlet, Southern Alaska is comprised of Jura-Cretaceous rocks of the Gravina belt, pre-Permian to Triassic rocks of theTaku terrane, and mid-Proterozoic to upper paleozoic continental margin assemblages. These assemblages may be part of the Yukon-Tanana terrane, which lies underneath most of Yukon and eastern Alaksa. Gravina belt, Taku and Yukon-Tanana terrane strata lie side-by-side along a mid-Cretaceous thrust fault system comprising the Alexander-Wrangellia terrane inboard margin.

Lower Paleozoic eugeoclinal rocks lie between autochthonous miogeoclinal strata and outboard volcanic arc terranes in the Kootenay Arc in northeastern Washington and southeastern Britsh Columbia. The eugeoclinal strata, divided into the Covada Group and the Bradeen Hill assemblage, records deposition and tectonic processes on the oceanic side of the lower Paleozoic continental margin. The data shows deposition of continental sediment, intermittent volcanism, local faulting, and starved basin conditions, implying tectonic synchroneity along the outer continental margin during the early Paleozoic.

Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390,660 and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910,1770, and 2450 Ma and highly radiogenic [.sup.87 Sr]/[.sup.86 Sr] ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin.

The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Continental Granodiorite over metasedimentary rocks of Proterozoic and early Paleozoic age. This fault is structurally beneath the San Pedro detachment fault and associated ductile deformational fabrics, which formed during early Oligocene to early Miocene time. A syntectonic leucogranite sill within the Little Rincon shear zone yields a U-Pb concordia-intercept age of 66 [+ or -]10 Ma for zircon and a concordant age of 51 [+ or -]2 Ma for fractions composed of monazite and xenotime. This demonstrates that compressional deformation in the Catalina and Rincon mountains is generally coeval with Laramide thrust faults that extend at least from southeastern California to southeastern Arizona. A peraluminous granite pluton that truncates the shear zone but displays extension-related fabrics yields a lower-intercept age of 24 [+ or -]12 Ma for zircon and an age of 30 [+ or -]6 Ma for monazite. This indicates that some peraluminous plutons in the region were emplaced during regional crustal extension.

The Duncan Canal shear zone, central southeastern Alaska, disrupts and penetratively deforms Devonian, Pennsylvanian, and Upper Triassic strata of the Alexander terrane. The deformation is interpreted to have occurred within a regime of right-lateral shear during Early or Middle Jurassic time. Clastic and volcanic rocks correlated with the Upper Jurassic and Lower Cretaceous strata of the Gravina belt lack the fabrics characteristic of the deformed Alexander terrane strata and are inferred to unconformably overlie the shear zone. Deformational fabrics of mid-Cretaceous age observed in the Gravina rocks locally truncate and deform the shear-zone strata. Dextral strike-slip faults of Late Cretaceous and (or) Tertiary age that disrupt and modify fabrics of the shear zone are interpreted to accommodate some of the displacement observed on the Clarence Strait fault. The Duncan Canal shear zone extends southward to the Mosman Inlet region on Etolin Island where it projects beneath Jura-Cretaceous Gravina strata. In Duncan Canal and Mosman Inlet, the shear zone evolved within the Alexander terrane and does not coincide with any recognized terrane boundaries. Since the Duncan Canal shear zone is restricted to the currently exposed eastern margin of the Alexander terrane, the shear zone is interpreted to reflect deformation along the eastern margin of the Alexander terrane.

Detrital zircon (DZ) U‐Pb laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) has revolutionised the way geologists approach many Earth science questions. Although recent research has focused on rapid sample throughput, acquisition rates are limited to 100–300 analyses h−1. We present a method to acquire zircon U‐Pb dates at rates of 120, 300, 600 and 1200 analyses h−1 (30, 12, 6 and 3 s per analysis) by multi‐collector LA‐ICP‐MS. We demonstrate the efficacy of this method by analysing twelve zircon reference materials with dates from ~ 3465 to ~ 28 Ma. Mean offset from high‐precision dates increases with faster rates from 0.9% to 1.1%; mean random 1s uncertainty increases from 0.6% to 1.3%. We tested this new method on a sandstone sample previously characterised by large‐n DZ geochronology. Quantitative comparison shows increased correspondence among age distributions comprising > 300 dates. This new method holds promise for DZ geochronology because (a) it requires no major changes to hardware, but rather modifications to software; (b) it yields robust age distributions well‐suited for quantitative analysis and maximum depositional age calculations; (c) there is only a minor sacrifice of accuracy and measurement uncertainty; and (d) there is less burden to researchers in terms of time investment and analytical cost. [ABSTRACT FROM AUTHOR]