scholarly journals Early Paleozoic post-breakup magmatism along the Cordilleran margin of western North America: New zircon U-Pb age and whole-rock Nd- and Hf-isotope and lithogeochemical results from the Kechika group, Yukon, Canada

Geosphere ◽  
2019 ◽  
Vol 15 (4) ◽  
pp. 1262-1290 ◽  
Author(s):  
Roderick W. Campbell ◽  
Luke P. Beranek ◽  
Stephen J. Piercey ◽  
Richard Friedman
Keyword(s):  
North America ◽  
Continental Shelf ◽  
Mantle Source ◽  
Tectonic Setting ◽  
Plate Boundary ◽  
Hf Isotope ◽  
Passive Margin ◽  
Western North America ◽  
South Central ◽  
Cordilleran Margin

AbstractPost-breakup magmatic rocks are recognized features of modern and ancient passive margin successions around the globe, but their timing and significance to non-plume-related rift evolution is generally uncertain. Along the Cordilleran margin of western North America, several competing rift models have been proposed to explain the origins of post-breakup igneous rocks that crop out from Yukon to Nevada. New zircon U-Pb age and whole-rock geochemical studies were conducted on the lower Paleozoic Kechika group, south-central Yukon, to test these rift models and constrain the timing, mantle source, and tectonic setting of post-breakup magmatism in the Canadian Cordillera. The Kechika group contains vent-proximal facies and sediment-sill complexes within the Cassiar platform, a linear paleogeographic high that developed outboard of continental shelf and trough basins. Chemical abrasion (CA-TIMS) U-Pb dates indicate that Kechika group mafic rocks were generated during the late Cambrian (488–483 Ma) and Early Ordovician (473 Ma). Whole-rock trace-element and Nd- and Hf-isotope results are consistent with the low-degree partial melting of an enriched lithospheric mantle source during margin-scale extension. Equivalent continental shelf and trough rocks along western North America are spatially associated with transfer-transform zones and faults that were episodically reactivated during Cordilleran rift evolution. Post-breakup rocks emplaced along the magma-poor North Atlantic margins, including those near the Orphan Knoll and Galicia Bank continental ribbons, are proposed modern analogues for the Kechika group. This scenario calls for the release of in-plane tensile stresses and off-axis, post-breakup magmatism along the nascent plate boundary prior to the onset of seafloor spreading.

scholarly journals Supplemental material: Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America

2013 ◽  
Author(s):  
George Gehrels ◽  
Mark Pecha
Keyword(s):  
North America ◽  
Detrital Zircon ◽  
Isotope Geochemistry ◽  
Hf Isotope ◽  
Passive Margin ◽  
Western North America ◽  
File Size ◽  
Image File ◽  
Supplemental File

Geosphere, February 2014, v. 10, p. 49-65, doi:10.1130/GES00889.1, Supplemental File 2 - CL image file (238 pages). File size is ~23 MB.

scholarly journals Supplemental material: Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America

2013 ◽  
Author(s):  
George Gehrels ◽  
Mark Pecha
Keyword(s):  
British Columbia ◽  
North America ◽  
Detrital Zircon ◽  
Southern California ◽  
Isotope Geochemistry ◽  
Hf Isotope ◽  
Passive Margin ◽  
Western North America ◽  
Standard Data ◽  
Data Table

Geosphere, February 2014, v. 10, p. 49-65, doi:10.1130/GES00889.1, Supplemental Tables - Zipped file containing 13 Excel table files. Table 1: Alaska U-Pb data. Table 2: Northern British Columbia U-Pb data. Table 3: Southern British Columbia U-Pb data. Table 4: Nevada-Utah U-Pb data. Table 5: Southern California U-Pb data. Table 6: Sonora U-Pb data. Table 7: Hf standard data. Table 8: Alaska Hf data. Table 9: Northern British Columbia Hf data. Table 10: Southern British Columbia Hf data. Table 11: Nevada-Utah Hf data. Table 12: Southern California Hf data. Table 13: Sonora Hf data.

scholarly journals Supplemental material: Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America

2020 ◽  
Author(s):  
George Gehrels ◽  
Mark Pecha
Keyword(s):  
North America ◽  
Detrital Zircon ◽  
Analytical Methods ◽  
Isotope Geochemistry ◽  
Hf Isotope ◽  
Passive Margin ◽  
Western North America ◽  
File Size ◽  
Supplemental File

Geosphere, February 2014, v. 10, p. 49-65, doi:10.1130/GES00889.1, Supplemental File 1 - Analytical methods file (32 pages, 20 figures). File size is ~6.7 MB.

Non-lithostatic pressure in North American core complexes

2020 ◽  
Author(s):  
Andrew Zuza ◽  
Drew Levy ◽  
Christopher Henry ◽  
Sean Long ◽  
Seth Dee
Keyword(s):  
North America ◽  
North American ◽  
Tectonic Setting ◽  
Shear Zones ◽  
Passive Margin ◽  
Lithostatic Pressure ◽  
Western North America ◽  
Geothermal Gradients ◽  
Deep Burial ◽  
Heterogeneous Rock

<p>The core complexes of western North America are generally thought to exhume deeply buried rocks (as much as 30 km) from the Cordilleran infrastructure, from beneath an inferred orogenic plateau to the surface today. However, how deep these rocks were buried has been intensely debated over the past three decades, especially for the Ruby Mountain-East Humboldt Range (RER) and northern Snake Range core complexes, eastern Nevada: published thermobarometry calculations, including robust modern techniques, suggest deep burial to 2-3x stratigraphic depths (as much as 30 km), whereas generations of field studies support burial only to roughly stratigraphic depths (~12-15 km). This has led to fierce debate that either field geologists are missing major structures or geobarometric estimates may neglect important considerations, such as reaction overstepping. Here we propose that a model of non-lithostatic conditions can resolve both field and petrologic datasets, and therefore the North American core complexes represent an example of tectonic overpressure. Western North America is covered by a remarkably well-characterized ~12-15-km-thick passive margin sequence that allows for careful structural reconstructions. Our observations focus on the RER geology, including new detailed geologic mapping (1:24,000 scale), structural traverses, thermochronology, and peak temperature (Tp) estimates. In particular, peak P-T conditions that suggest deep burial require (1) relatively low geothermal gradients of ≤20°C/km and (2) enigmatic structures that are not observed and would be atypical of other Cordilleran fold-thrust belts or even other analogous intra-plateau thrust systems. Instead, our Tp compilation (e.g., Raman spectroscopy of carbonaceous material, Conodont color alteration index, thermochronology) across continuous stratigraphy suggests high geothermal gradients (≥40°C/km) that are consistent with the region being extensively intruded and mineralized—i.e., the region underwent major Jurassic, Cretaceous, and Eocene intrusive episodes and hosts an Eocene(?) world-class Carlin-type gold deposit—and matches thermal gradients observed in other eastern Nevada studies and analogous orogens. Systematic mapping does not reveal any structural break across a section of Neoproterozoic to undeformed Permian passive margin strata that was supposedly deeply buried beneath an additional entire stratigraphic section. The approach of using a Tp traverse to test deep burial models allows for self-consistent evaluation of the data. That is, interpretations are based on a trend of temperature variations deduced from numerous measurements rather than relying on a single (or few) pressure data point(s). Our observations suggest that non-lithostatic pressure may have affected Cordilleran core complexes. We explore how the local rheologically heterogeneous rock types and specific tectonic setting may have created conditions favorable for tectonic overpressure in North American core complexes. For example, paleo-stress estimates from across several shear zones demonstrate significant strength variations that may have facilitated mean stress (pressure) perturbations.</p>

Late Cambrian ptychaspidid trilobites from western Utah: implications for trilobite systematics and biostratigraphy

2005 ◽  
Vol 142 (4) ◽  
pp. 377-398 ◽  
Author(s):  
J. M. ADRAIN ◽  
S. R. WESTROP
Keyword(s):  
North America ◽  
Distinct Species ◽  
Western North America ◽  
Closely Related Species ◽  
Widespread Species ◽  
Late Cambrian ◽  
South Central ◽  
New Information ◽  
The Family ◽  
Notch Peak

The Notch Peak Formation (Late Cambrian, Sunwaptan) of western Utah yields diverse silicified trilobite faunas that provide new information on the anatomy of many taxa. The family Ptychaspididae Raymond, 1924, is represented by species of Keithiella Rasetti, 1944; Idiomesus Raymond, 1924; Euptychaspis Ulrich in Bridge, 1931; and Macronoda Lochman, 1964. At least four species are new, of which E. lawsonensis and M. notchpeakensis are named formally. Much previous work on Late Cambrian trilobites has emphasized biostratigraphic utility and the recognition of geographically widespread species. Data from new silicified collections indicate that this approach is difficult to justify because many putative ‘index species’ actually represent a plexus of closely related species whose biostratigraphic significance has yet to be determined. One such plexus is represented by E. kirki Kobayashi, 1935, whose previously reported occurrences in Texas, Oklahoma, Utah, Nevada and northern Canada record at least four distinct species. Similarly, Macronoda can now be shown to consist of at least five late Sunwaptan species in south-central and western North America.

Synopsis of late Palaeozoic and Mesozoic terrane accretion within the Cordillera of western North America

1990 ◽  
Vol 331 (1620) ◽  
pp. 479-486 ◽  
Keyword(s):  
North America ◽  
Western North America ◽  
Island Arcs ◽  
Multidisciplinary Research ◽  
The West ◽  
Marginal Sea ◽  
Faunal Diversity ◽  
Cordilleran Margin ◽  
Terrane Accretion ◽  
Faunal Studies

Establishing the paleogeographic origin of most of the terranes within the Cordillera remains an ellusive goal; despite more than 10 years of multidisciplinary research, the home port of any major terrane has not been identified unequivocally. Even most continental fragments that show affinities to North America cannot be repositioned confidently along the Cordilleran margin, and some continental fragments (e.g. Chulita) probably are not North American in origin. Cordilleran oceanic terranes, including island arcs, seamounts, off-ridge islands, and scraps of ocean basins, are especially difficult to reposition because Panthalassa has been destroyed. Faunal studies with emphasis on palaeobiogeographic affinities are the most useful, particularly when coupled with analyses of faunal diversity and endemism. Such studies suggest that some terranes previously thought to have formed near the Cordillerran margin were situated thousands of kilometres to the west, and were separated from the continent by broad ocean basins, rather than by a narrow marginal sea.

Phylogeography of the Habronattus amicus species complex (Araneae: Salticidae) of western North America, with evidence for localized asymmetrical mitochondrial introgression

Zootaxa ◽  
2009 ◽  
Vol 2307 (1) ◽  
pp. 39-60 ◽  
Author(s):  
MARSHAL HEDIN ◽  
MICHAEL C. LOWDER
Keyword(s):  
North America ◽  
Species Complex ◽  
Mitochondrial Gene ◽  
Microhabitat Use ◽  
Nominal Species ◽  
Western North America ◽  
Ecological Data ◽  
Dune Systems ◽  
South Central ◽  
Mitochondrial Introgression

The frequency and significance of hybridization and gene introgression in spiders (Araneae) is unclear, largely because few studies have combined multiple lines of evidence in the study of systems where introgression is likely. Here we consider multiple types of data to gauge the prevalence and importance of mitochondrial introgression for three species of the Habronattus amicus species complex distributed in western North America. A comprehensive phylogeographic sample recovers some geographically-coherent mitochondrial clades, but conspicuously fails to resolve nominal species as monophyletic. This pattern of discordance between molecules and morphological taxonomy is studied in more detail for two species, H. amicus (Peckham and Peckham) and H. ustulatus (Griswold), found in syntopy on three dune systems in south-central Oregon. Here, multivariate morphological analyses clearly distinguish these two species, but also reveal a narrow window of size overlap between small H. amicus females and large H. ustulatus males. A quantitative assessment of microhabitat use shows that these species occupy different microhabitats at two of three dune systems, but that this difference breaks down at a single site (Summer Lake). Consistent with the size overlap and ecological data, mitochondrial data suggest interspecific gene flow at Summer Lake, which is apparently asymmetric from H. amicus into H. ustulatus. The introgression dynamics revealed in south-central Oregon impact our interpretation of mitochondrial gene trees for the entire species complex, and provide important insight into how hybridization may be influencing evolution in this diverse spider genus.

Icacinaceae in the early middle Paleocene Raton Formation, Colorado

2018 ◽  
Vol 55 (2) ◽  
pp. 75-86 ◽  
Author(s):  
Keith Berry
Keyword(s):  
North America ◽  
Tropical Rainforest ◽  
Tropical Rainforests ◽  
Early Eocene ◽  
Western North America ◽  
South Central ◽  
Late Paleocene ◽  
Single Locality

Icacinicaryites corruga is reported from the upper coal zone of the Raton Formation in south-central Colorado. Prior to this report, this endocarp imprint was known from only a single locality near Pagosa Springs in southwestern Colorado (Animas Formation). This demonstrates that Icacinaceae, which characterized the late Paleocene – early Eocene tropical rainforests of western North America, already were present in Colorado’s early middle Paleocene tropical rainforest. This determination agrees with the results of a reevaluation of the stratigraphic distribution of previous records of Icacinaceae from the Paleocene of western North America.

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