Cordilleran Orogen Research Articles

Detrital zircon geochronology of Neoproterozoic to Permian miogeoclinal strata in British Columbia and Alberta

U-Pb ages have been determined on 250 detrital zircon grains from Neoproterozoic through Permian miogeoclinal strata in British Columbia and Alberta. Most of the grains in these strata are >1.75 Ga and are interpreted to have been derived from nearby basement provinces (although most grains were probably cycled though one or more sedimentary units prior to final deposition). Important exceptions are Ordovician sandstones that contain grains derived from the Peace River arch, and upper Paleozoic strata with detrital zircons derived from the Franklinian orogen, Salmon River arch (northwestern U.S.A.), and (or) Grenville orogen. These provenance changes resulted in average detrital zircon ages that become progressively younger with time, and may also be reflected by previously reported shifts in the Nd isotopic signature of miogeoclinal strata. In addition to the grains that have identifiable sources, grains of ~1030, ~1053, 1750-1774, and 2344-2464 Ma are common in our samples, but igneous rocks of these ages have not been recognized in the western Canadian Shield. We speculate that unrecognized plutons of these ages may be present beneath strata of the western Canada sedimentary basin. Collectively, our data provide a record of the ages of detrital zircons that accumulated along the Canadian Cordilleran margin during much of Paleozoic time. Comparisons between this reference and the ages of detrital zircons in strata of potentially displaced outboard terranes may help reconstruct the paleogeography and accretionary history of the Cordilleran orogen.

Metamorphic CO 2 degassing from orogenic belts

Kerrick and Caldeira (1993, 1994a) concluded that metamorphic CO 2 degassing in collisional orogens, and especially the Himalayan orogenic belt, could have been an important factor in enhancing paleoatmospheric CO 2 levels and contributing to early Cenozoic global greenhouse warming [Kerrick, D.M., Caldeira, K., 1993. Paleoatmospheric consequences of CO 2 released during early Cenozoic regional metamorphism in the Tethyan orogen. In: Touret, J.L.R., Thompson, A.B. (Guest-Eds.), Fluid–Rock Interaction in the Deeper Continental Lithosphere. Chem. Geol. 108, 201–230.] [Kerrick, D.M., Caldeira, K., 1994a. Metamorphic CO 2 degassing and early Cenozoic paleoclimate. GSA (Geol. Soc. Am.) Today 4, 57–65.]. However, our revised CO 2 mass loss computations for regional metamorphism in the Himalaya–Karakoram belt incorporating recent geochronologic data and revised estimates of the proportion of carbonate source rocks indicate that metamorphic CO 2 degassing from this orogen cannot explain Early Eocene warmth. Widespread pluton-induced hydrothermal flow occurred during the Eocene in the Cordilleran belt of western North America. Synmetamorphic intrusions, which are common in metamorphic belts, may cause significant regional fluid flow. To obtain a representative CO 2 flux from such environments, we computed a CO 2 flux of 1.5×10 12 mol km −2 Ma −1 from petrologic and geochemical studies of the Paleozoic plutonic–metamorphic belt in New England (northeastern United States). For the 2×10 6 km 2 area of Eocene metamorphism in the North American Cordillera, the CO 2 fluxes derived from the New England metamorphic belt yield an area-integrated flux of ∼3×10 18 mol Ma −1. If a significant fraction of this CO 2 entered the atmosphere, this degassing flux would alone account for Eocene greenhouse global warming. For the Ominica belt within the Cordilleran orogen, a volumetric estimate of the mass of carbonate veins indicates that the consumption of CO 2 by precipitation of carbonate veins may not significantly decrease the amount of CO 2 in fluids that convect to near-surface crustal levels. Compared to other Eocene metamorphic belts, the widespread hydrothermal activity in the North American Cordillera may have been the largest, and most climatically significant, source of metamorphic CO 2 to the Eocene atmosphere. CO 2 degassing by active metamorphism is most significant in extensional regimes of high heat flow. Extensional tectonism and hydrothermal activity in metamorphic belts may have substantially contributed to atmospheric CO 2 content throughout the Phanerozoic. Examples include the Mesozoic circum-Pacific metamorphic belt, and Oligocene–Miocene regional metamorphism in the Himalayan orogen.

Tectonic setting of synorogenic gold deposits of the Pacific Rim

More than 420 million oz of gold were concentrated in circum-Pacific synorogenic quartz lodes mainly during two periods of continental growth, one along the Gondwanan margin in the Palaeozoic and the other in the northern Pacific basin between 170 and 50 Ma. These ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary-rock dominant terranes. The auriferous veins contain only a few percent sulphide minerals, have gold:silver ratios typically greater than 1:1, show a distinct association with medium grade metamorphic rocks, and may be associated with large-scale fault zones. Ore fluids are consistently of low salinity and are CO 2-rich. In the early and middle Palaeozoic in the southern Pacific basin, a single immense turbidite sequence was added to the eastern margin of Gondwanaland. Deformation of these rocks in southeastern Australia was accompanied by deposition of at least 80 million oz of gold in the Victorian sector of the Lachlan fold belt mainly during the Middle and Late Devonian. Lesser Devonian gold accumulations characterized the more northerly parts of the Gondwanan margin within the Hodgkinson–Broken River and Thomson fold belts. Additional lodes were emplaced in this flyschoid sequence in Devonian or earlier Palaeozoic times in what is now the Buller terrane, Westland, New Zealand. Minor post-Devonian growth of Gondwanaland included terrane collision and formation of gold-bearing veins in the Permian in Australia's New England fold belt and in the Jurassic-Early Cretaceous in New Zealand's Otago schists. Collision and accretion of dozens of terranes for a 100-m.y.-long period against the western margin of North America and eastern margin of Eurasia led to widespread, latest Jurassic to Eocene gold veining in the northern Pacific basin. In the former location, Late Jurassic and Early Cretaceous veins and related placer deposits along the western margin of the Sierra Nevada batholith have yielded more than 100 million oz of gold. Additional significant ore-forming events during the development of North America's Cordilleran orogen included those in the Klamath Mountains region, California in the Late Jurassic and Early Cretaceous; the Klondike district, Yukon by the Early Cretaceous; the Nome and Fairbanks districts, Alaska, and the Bridge River district, British Columbia in the middle Cretaceous; and the Juneau gold belt, Alaska in the Eocene. Gold-bearing veins deposited during the Late Jurassic and Early Cretaceous terrane collision that formed the present-day Russian Far East have been the source for more than 130 million oz of placer gold. The abundance of gold-bearing quartz-carbonate veins throughout the Gondwanan, North American and Eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth. Such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata. The common temporal association between gold veining and magmatism around the Pacific Rim reflects these thermal episodes. Melting of the lower thickened crust during arc formation, slab rollback and extensional tectonism, and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore-forming processes.

Strontium and neodynium isotope ratios in the Fraser River, British Columbia: a riverine transect across the Cordilleran orogen

The Fraser is the largest undammed river to traverse the North American Cordillera. Its source is in Precambrian/Palaeozoic sedimentary rocks of the Rocky Mountains, at the margin of the North American Craton, from where it flows west across accreted ensimatic terranes, to the young magmatic arc of the Coast Range. 87 Sr 86 Sr and 143 Nd 144 Nd in suspended matter from the Fraser and its tributaries outline a mixing curve between two end members, (a) LILE-rich cratonic material with 87 Sr 86 Sr ∼ 0.77 and ϵ Nd (0) ∼ −23 and an Archaean crustal residence age and (b) material from mantle-derived igneous rocks with 87 Sr 86 Sr ∼ 0.704 and ϵ Nd (0) ∼ +4 and a crustal residence age of ∼ 0.6 Ga. In the eastern, ensialic region, dissolved Sr is less radiogenic than Sr in suspended matter, because of the contribution of dissolved Sr from Rb-poor carbonate rocks. As the Fraser flows west over younger terranes, this relationship inverts, with dissolved Sr more radiogenic. Inversion reflects a difference in the transport behaviour of dissolved and suspended material. Dissolved Sr from upstream, with a radiogenic signature, remains in solution, whereas the suspended load from upstream tends to fall out where the current decreases, to be replaced by local, less radiogenic material. For riverborne fluxes entering the ocean at active margins, compositions of conservative dissolved elements, such as Sr, represent both the interior craton and the outer orogen, while elements carried mainly in suspended form, such as Nd, are biased towards the outer orogen.

Reply [to “Comment on ‘The Butte Valley and Layton Well thrusts of eastern California: Distribution and regional significance’”

TectonicsVolume 16, Issue 1 p. 184-185 CommentariesFree Access Reply [to “Comment on ‘The Butte Valley and Layton Well thrusts of eastern California: Distribution and regional significance’”] Chester T. Wrucke, Chester T. WruckeSearch for more papers by this authorCalvin H. Stevens, Calvin H. StevensSearch for more papers by this authorJoseph L. Wooden, Joseph L. WoodenSearch for more papers by this author Chester T. Wrucke, Chester T. WruckeSearch for more papers by this authorCalvin H. Stevens, Calvin H. StevensSearch for more papers by this authorJoseph L. Wooden, Joseph L. WoodenSearch for more papers by this author First published: 01 February 1997 https://doi.org/10.1029/96TC02357AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Busby-Spera, C., J. Mattinson, N. Riggs, E. Schermer, The Triassic-Jurassic magmatic arc in the Mojave-Sonoran Deserts and in the Sierran-Klamath region: Similarities and differences in palcogeographic evolution, Paleozoic and Early Mesozoic Paleogeographic Relation; Sierra Nevada, Klamath Mountains and related terranes, Geol. Soc. of Am., Spec. Pap., 255 D. Harwood, M. Miller, 325– 338, 1990. Davis, G. A., andB. C. Burchfiel, Comment on "The Butte Valley and Layton Well Thrusts of eastern California: Distribution and regional significance" by Chester T. Wrucke, Calvin H. Stevens, and Joseph L. Wooden,Tectonics 16(1), 1997. Dunne, G. C., J. D. Walker, Age of Jurassic volcanism and tectonism, southern Owens Valley region, east-central California, Geol. Soc. Am. Bull., 105, 1223– 1230, 1993. Grose, L. T., Structure and petrology of the northeast part of the Soda Mountains, San Bernardino County, California, Geol. Soc. Am. Bull., 70, 1509– 1548, 1959. Jennings, C. W., J. L. Burnett, andB. W. Troxel, Trona sheet, Geologic map of California, scale1∶250,000,Calif. Div. of Mines and Geol.,Sacramento,1962. Lipman, P. W., The roots of ash-flow calderas in western North America: Windows into the tops of granitic batholiths, J. Geophys. Res., 89, 8801– 8841, 1984. Lipman, P. W., M. A. Dungan, L. L. Brown, A. Deino, Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs, Geol. Soc. Am. Bull., 108, 1039– 1055, 1996. Saleeby, J. B., C. Busby-Spera, Early Mesozoic tectonic evolution of the western U.S. Cordillera, The Cordilleran Orogen, Contertninous U.S., The Geology of North America, G-3 B. C. Burchfiel, P. W. Lipman, M. L. Zoback, 107– 168, Geol. Soc. of Am., Boulder, Colo., 1992. Schermer, E. R., Cathy Busby, Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences, Geol. Soc. Am. Bull., 106, 767– 790, 1994. Smith, R. L, Ash-flow magmatism, Ash-flow Tuffs, Geol. Soc. Am. Spec. Pap., 180 C. E. Chapin, W. E. Elston, 5– 27, 1979. Wrucke, C. T., C. H. Stevens, J. L. Wooden, The Butte Valley and Layton Well Thrusts of eastern California: Distribution and regional significance, Tectonics, 14, 1165– 1171, 1995. Volume16, Issue1February 1997Pages 184-185 ReferencesRelatedInformation

Pressure – temperature and tectonic evolution of Triassic lawsonite – aragonite blueschists from Pinchi Lake, British Columbia

A blueschist and eclogite terrane is associated with one of the largest faults in the Canadian Cordilleran Orogen, the Pinchi fault. Blueschists (in situ) and retrogressed eclogite blocks occur along the Pinchi fault zone near 54°30'N and 124°W. Critical blueschist facies mineral assemblages include lawsonite–glaucophane, jadeite–lawsonite–glaucophane–quartz, and aragonite. White mica 40Ar/39Ar spectra on blueschist and eclogite yield ages in the range 221.8 ± 1.9 to 223.5 ± 1.7 Ma, establishing a direct link between the blueschists and eclogites. Preservation of aragonite sets rigid constraints on the pressure–temperature–fluid–time conditions of unroofing. K–Ar dates indicate that this is some of the oldest documented metamorphic aragonite. Comparison with computed petrogenetic grids suggests that metamorphic temperatures were in the range 200–300 °C, with pressures greater than 8–10 kbar (1 kbar = 100 MPa). Unroofing likely occurred during collision of the Cache Creek terrane with Quesnellia in the Late Triassic to Middle Jurassic. The fault was initiated as a plate boundary and was active as late as Eocene time as a strike-slip zone. The Pinchi blueschist terrane is similar to others in the North American Cordillera and highlights a tectonic regime of repeated blueschist metamorphism and rapid unroofing along many parts of the western margin of North America in the early Mesozoic.

Long-term sediment accumulation in the Middle Jurassic–early Eocene Cordilleran retroarc foreland-basin system

Research Article| July 01, 1996 Long-term sediment accumulation in the Middle Jurassic–early Eocene Cordilleran retroarc foreland-basin system P. G. DeCelles; P. G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar B. S. Currie B. S. Currie 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Geology (1996) 24 (7): 591–594. https://doi.org/10.1130/0091-7613(1996)024<0591:LTSAIT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation P. G. DeCelles, B. S. Currie; Long-term sediment accumulation in the Middle Jurassic–early Eocene Cordilleran retroarc foreland-basin system. Geology 1996;; 24 (7): 591–594. doi: https://doi.org/10.1130/0091-7613(1996)024<0591:LTSAIT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The late Middle Jurassic–early Eocene (∼ 120 m.y.) sediment-accumulation history of the Cordilleran foreland basin in northern Utah exhibits a sigmoidal pattern on a rate vs. time plot, with moderate rates of accumulation during late Middle Jurassic, very low net rates during Late Jurassic–earliest Cretaceous, increasingly rapid rates during Early-middle Cretaceous, and low rates during Late Cretaceous–early Eocene time. This pattern is consistent with deposition in a prograding foreland-basin system that comprised integrated back-bulge, forebulge, foredeep, and wedge-top depozones. The upper Middle Jurassic represents the back-bulge depozone; the Upper Jurassic was deposited on the eastern flank of a flexural forebulge; the basal Cretaceous unconformity is the result of eastward migration of the forebulge; the thick, Lower-middle Cretaceous succession represents the foredeep depozone; and the Upper Cretaceous–early Eocene embodies the syndepositionally deformed wedge-top depozone. Previous models that explain Middle-Late Jurassic stratigraphic patterns in terms of foredeep subsidence (alone) and a Late Jurassic hiatus in crustal shortening in the Cordilleran orogen are shown to be neither necessary nor supported by evidence from the Cordilleran hinterland. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU’s FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC01448, Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States, Christian J. Bjerrum and Rebecca J. Dorsey). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $15.00. Payment must accompany order.A thick succession of Jurassic nonmarine and marine sedimentary rocks is exposed in a large area from northern Arizona to eastern Idaho and western Wyoming. These sediments accumulated in the Utah‐Idaho trough, a deep elongate cratonal basin whose origin has recently been debated. Detailed stratigraphic analysis, subsidence analysis, and first‐order flexural modeling of these deposits (this study) provide new insights into the timing and mechanisms of subsidence in the Utah‐Idaho trough. Lower and Middle Jurassic strata are divided into six unconformity‐bounded sequences. In contrast to the overall uniform thickness of Lower Jurassic sequences (1 and 2), Middle Jurassic strata (sequences 3 through 6) consist of distinctly westward thickening sedimentary packages in which basal shallow marine deposits onlap eastward onto bounding unconformities. Basal strata of sequences 3 through 6 pass upward into widespread progradational continental deposits that are truncated progressively toward the east (cratonward) by the next unconformity. Decompacted total subsidence curves show that the rate of subsidence in most sections increased sharply at the end of sequence 2 time (J‐2 unconformity; ∼170 Ma). This is interpreted to record the onset of Middle Jurassic deposition in the distal part of a retroarc foreland basin. The unconformities and distinctive stratal geometries may have formed in response to forebulge migration caused by episodic thrusting in the Cordilleran orogen to the west. First‐order flexural modeling was carried out to test the hypothesis of flexural subsidence in the Utah‐Idaho trough. Trial‐and‐error comparisons produce a close match between decompacted stratigraphic profiles and model deflection profiles. The best fit is obtained using an infinite elastic plate (D = 1 × 1024 Nm), a moderate load topography, elevated base level, and an overfilled basin. Using recently published tectonic reconstructions for Nevada and Utah, we find close spatial agreement between a large Middle Jurassic fold‐thrust belt and the supracrustal load inferred from model simulations. Our integrated basin analysis thus supports the interpretation of some previous studies that the Middle Jurassic Utah‐Idaho trough was a retroarc foreland basin that formed east of a belt of regional contractile deformation and crustal thickening in western and central Nevada and southeastern California. Late Jurassic extension and normal faulting in northeastern Nevada may have been related to gravitational collapse of overthickened crust in the Cordilleran orogen. This coincides with a period of slowed subsidence in the Utah‐Idaho trough that began at about 157 Ma.

K-Ar and U-Pb zircon chronology of Late Cretaceous and Tertiary magmatism in central Chihuahua State, Mexico

Magmatism in central Chihuahua occurred from at least 68 to 27.5 Ma, a duration that can be separated into two periods by an order of magnitude increase in the intensity of magmatism starting at 46 Ma. Activity prior to 46 Ma was Volumetrically minor and possibly intermittent. The oldest rocks (68 Ma) are a localized, very thick, intermediate-composition sequence of lava flows, debris flows, and related sediments. From 62 until at least 52 Ma, numerous small intrusives and felsic tuffs were emplaced locally. Between 46 and 27.5 Ma, the area was inundated with a quasi-steady supply of volcanic products. The post-46 Ma activity evolved through four distinct stages that produced two sequences of felsic ash-flow tuffs separated by a sequence of massive coarsely porphyritic lavas and tuffs of intermediate to felsic composition. These three calc-alkaline stages were gradually succeeded at about 31 Ma by mildly alkaline basaltic andesite lavas and rhyolitic tuffs with peralkaline characteristics. All volcanic activity terminated abruptly at 27.5 Ma. The most significant events in the evolution of this magmatism are its onset, its termination, and the profound increase in activity beginning at 46 Ma. In its timing and subdued intensity, the pre-46 Ma magmatism fits a setting within the interior of the Laramide Cordilleran orogen. A convergent magmatic are had advanced eastward to central Chihuahua by at least 68 Ma. The oldest rocks are tilted, but volcanic rocks 46 Ma and younger were not deformed. The abrupt increase in magmatism at 46 Ma may be a manifestation of diminished compressional stress related to a decrease in the rate of plate convergence along western North America. The regional stress field remained in compression until about 31 Ma, which corresponds approximately to the time of transition from calc-alkaline to mildly alkaline mafic and peralkaline felsic volcanism. Magmatism ended at 27.5 Ma, prior to extensional faulting in the area, but generally coincident with the end of plate convergence along part of the continental margin of western Mexico.

The Across-Strike Anatomy of the Cordilleran Orogen at 40 degrees N Latitude and Implications for the Mesozoic Paleogeography of the Western United States

The Across-Strike Anatomy of the Cordilleran Orogen at 40 degrees N Latitude and Implications for the Mesozoic Paleogeography of the Western United States

Distribution and characteristics of metamorphic belts in the south‐eastern Alaska part of the North American Cordillera

The Cordilleran orogen in south‐eastern Alaska includes 14 distinct metamorphic belts that make up three major metamorphic complexes, from east to west: the Coast plutonic–metamorphic complex in the Coast Mountains; the Glacier Bay–Chichagof plutonic–metamorphic complex in the central part of the Alexander Archipelago; and the Chugach plutonic–metamorphic complex in the northern outer islands. Each of these complexes is related to a major subduction event. The metamorphic history of the Coast plutonic–metamorphic complex is lengthy and is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Stikine terrane to the east. The metamorphic history of the Glacier Bay–Chichagof plutonic–metamorphic complex is relatively simple and is related to the roots of a Late Jurassic to late Early Cretaceous island arc. The metamorphic history of the Chugach plutonic–metamorphic complex is complicated and developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes.The Coast plutonic–metamorphic complex records both dynamothermal and regional contact metamorphic events related to widespread plutonism within several juxtaposed terranes. Widespread moderate‐P/T dynamothermal metamorphism affected most of this complex during the early Late Cretaceous, and local high‐P/T metamorphism affected some parts during the middle Late Cretaceous. These events were contemporaneous with low‐ to moderate‐P, high‐T metamorphism elsewhere in the complex. Finally, widespread high‐P–T conditions affected most of the western part of the complex in a culminating late Late Cretaceous event. The eastern part of the complex contains an older, pre‐Late Triassic metamorphic belt that has been locally overprinted by a widespread middle Tertiary thermal event.The Glacier Bay–Chichagof plutonic–metamorphic complex records dominantly regional contact‐metamorphic events that affected rocks of the Alexander and Wrangellia terranes. Widespread low‐P, high‐T assemblages occur adjacent to regionally extensive foliated granitic, dioritic and gabbroic rocks. Two closely related plutonic events are recognized, one of Late Jurassic age and another of late Early and early Late Cretaceous age; the associated metamorphic events are indistinguishable. A small Late Devonian or Early Mississippian dynamothermal belt occurs just north‐east of the complex. Two older low‐grade regional metamorphic belts on strike with the complex to the south are related to a Cambrian to Ordovician orogeny and to a widespread Middle Silurian to Early Devonian orogeny.The Chugach plutonic–metamorphic complex records a widespread late Late Cretaceous low‐ to medium/high‐P, moderate‐ T metamorphic event and a local transitional or superposed early Tertiary low‐P, high‐T regional metamorphic event associated with mesozonal granitic intrusions that affected regionally deformed and metamorphosed rocks of the Chugach terrane. The Chugach complex also includes a post‐Late Triassic to pre‐Late Jurassic belt with uncertain relations to the younger belts.

Mid‐Cretaceous extensional fragmentation of a Jurassic‐Early Cretaceous Compressional Orogen, Alaska

Several key geologic relations of interior and northern Alaska suggest that the hinterland of the Brooks Range thrust belt, which formed part of a major Middle Jurassic to earliest Cretaceous compressional orogen, underwent significant regional extension in the mid‐Cretaceous. These geologic relations include (1) simultaneous development of depositional basins in both foreland and hinterland of the Brooks Range fold and thrust belt, (2) regional normal faults along the “root zone” for Brooks Range allochthons, (3) anomalously thin (10–20 km) crust in the hinterland of the Brooks Range developed as a consequence of extension, (4) missing source regions for arc‐derived clastic rocks of the foreland, perhaps removed by rifting, (5) preservation of high pressure/low temperature metamorphic rocks in the hinterland, possibly resulting from extensional denudation and rapid uplift, (6) omission rather than duplication of structural section along many regional terrane boundaries in the hinterland, (7) shallow dipping metamorphic foliations, stretching lineations, and high‐temperature cooling histories consistent with normal sense displacement of overlying allochthonous rocks, (8) widespread crustal anatexis in parts of the hinterland, and (9) emplacement of rift‐related alkaline and associated igneous rocks in the Yukon‐Koyukuk basin. Consideration of the amount of crustal thinning, tectonic denudation, and throw on normal faults implied by the above relations indicates that mid‐Cretaceous events could have been responsible for at least 100% stretching (ß=2) across most of interior Alaska. Evaluation of timing constraints suggests that extensional tectonism in interior Alaska spanned an interval from about 130 to 90 Ma. Review of known geologic relations indicates that extension postdated an important episode of crustal shortening in the Brooks Range related to closure of an ocean basin (now represented by the Angayucham terrane) during the approach and collision of a Jurassic and older arc system with the continental margin. Extension in interior Alaska was coeval with the inception of rifting and with rift‐related magmatism in the Canadian Arctic and likely occurred during the opening of the Canada Basin. This important period of extensional deformation fragmented the various components of an earlier Jurassic to earliest Cretaceous compressional orogen in Alaska, broadening the cross‐sectional width of the Cordilleran orogen at this latitude and drastically modifiying paleogeography with the formation of large marine basins in the Cretaceous. As such, this episode of extension has important implications for our understanding of the paleogeography and tectonic setting of earlier crustal shortening and our understanding of the tectonic and thermal controls on subsequent Cretaceous sedimentation across Alaska. This extension also plays a critical role in larger‐scale plate tectonic problems such as the pre‐Cretaceous reconstruction of the Arctic and opening of the Canada Basin.

Detrital zircon geochronology of metasedimentary rocks in the southern Omineca Belt, Canadian Cordillera

We address two problems of Cordilleran geology in this study using U–Pb dating of single detrital zircon grains from metasedimentary rocks: the provenance of the Windermere Supergroup, and the age and correlation of metasedimentary rocks within the Shuswap Complex that are at high metamorphic grade. Because some of these rocks are clearly of North American affinity, the ages of zircons provide indirect constraints on the age and distribution of continental basement from which the zircons were derived.A consistent pattern emerges from ages of about 50 grains from six rocks. Nearly all samples analyzed (48–53°N) are characterized by a bimodal distribution of zircon ages of 1.65–2.16 Ga and &gt; 2.5 Ga, with a distinct lack of ages between 2.1 and 2.5 Ga. Exceptions to this pattern are young zircons from two samples, from Valhalla and Grand Forks – Kettle complexes of southeastern British Columbia, that have grains 1435 ± 35 and 650 ± 15 Ma, respectively. These younger grains are inferred to have been derived from magmatic rocks, and they have no obvious source in either the Canadian Shield or the Alberta subsurface basement to the east. The Early Proterozoic and Archean ages of detrital zircons resemble those of dated basement rocks beneath the Alberta Basin as well as basement exposed within the Cordilleran hinterland (gneisses of Thor–Odin, Frenchman Cap, and Malton regions). However, 2.1–2.4 Ga rocks that are extensive in the subsurface of northern Alberta are not represented in the inventory of detrital zircon ages presented in this paper.This pattern suggests that much of the Cordilleran basement between these latitudes is underlain by Archean crust of the Hearne–Wyoming provinces that may be mantled to the west by an orogenic–magmatic belt of Early Proterozoic (1.7–1.9 Ga) age which may largely have been parallel to the present Cordilleran orogen.

Permian-Triassic plutonism and tectonics, Death Valley region, California and Nevada

Research Article| June 01, 1991 Permian-Triassic plutonism and tectonics, Death Valley region, California and Nevada J. Kent Snow; J. Kent Snow 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar Yemane Asmerom; Yemane Asmerom 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar Daniel R. Lux Daniel R. Lux 2Department of Geological Sciences, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar Author and Article Information J. Kent Snow 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Yemane Asmerom 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Daniel R. Lux 2Department of Geological Sciences, University of Maine, Orono, Maine 04469 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (6): 629–632. https://doi.org/10.1130/0091-7613(1991)019<0629:PTPATD>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. Kent Snow, Yemane Asmerom, Daniel R. Lux; Permian-Triassic plutonism and tectonics, Death Valley region, California and Nevada. Geology 1991;; 19 (6): 629–632. doi: https://doi.org/10.1130/0091-7613(1991)019<0629:PTPATD>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Significant contractional structures that deform Permian rocks but predate an Early Triassic overlap sequence are recognized within the Cordilleran orogen, western United States. Thrusting in the Death Valley region of the orogen, however, has been regarded as Middle Triassic or younger and thus kinematically distinct. We present new isotopic age limits on two posttectonic stocks that intrude major structures of the Death Valley thrust belt. The stocks are no younger than Middle Triassic, but are likely Late Permian in age, consistent with stratigraphic and structural data suggesting that thrusting predates the overlap sequence. We hypothesize that Permian shortening may have affected more than 700 km of the Cordilleran orogen at the same time arc activity began within cratonic North America but prior to Early Triassic emplacement of the structurally higher Sonomian arc terrane. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Origin of deep crystal reflections: seismic profiling across high-grade metamorphic terranes in Canada

In an attempt to better understand the origin of deep crustal reflections LITHOPROBE has sponsored or co-sponsored Seismic reflection surveys across tracts of high-grade metamorphic rock in the Archean Superior craton, the Proterozoic Grenville orogen and the Phanerozoic Cordilleran orogen. Common to these three diverse terranes are near-surface zones of prominent Seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology. At some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks (stratified assemblages, sills, etc.), whereas in other regions it was generated by extreme attenuation, stretching and ductile flow of weakly layered or irregularly organized rocks. It seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust, with reflections originating at lithological boundaries and zones of mylonite.

Uniqueness of geological correlations: An example from the Death Valley extended terrain

The northern Death Valley-Furnace Creek fault zone is a major northwest-trending fault system in the Death Valley region, but the magnitude of offset and its significance to extensional tectonism in the region are controversial. Stratigraphic data have provided first-order constraints on offset across the fault zone but offer limited resolution. Previous correlations of Mesozoic thrust faults across the fault zone have relied on regional stratigraphic trends to fingerprint structurally similar thrust plates. We show that a northeast-trending Mesozoic backfold within the predominantly east-directed fold and thrust belt was probably continuous from the southern Panamint Range, California, to the Belted Range, Nevada. The relative position of this antithetic structure allows uniquely compatible correlations of three Mesozoic structures between two range blocks across the northern Death Valley-Furnace Creek fault zone independent of stratigraphic arguments. The offset structures have geometric properties of size, vergence, order, and spacing that indicate a probability less than 10 -4 of the Mesozoic orogen identically repeating this structural suite in two unrelated places. Reconstruction of these correlative structures indicates 68 ± 4 km N48°W ± 6°W of apparent dextral offset between the Cottonwood and Funeral Mountains, in general agreement with previous estimates based on stratigraphic observations but of substantially higher precision. This reconstruction, and the restoration of a 30-km separation between the Funeral and Grapevine Mountains indicated by offset segments of the backfold, suggest a simple pre-extensional geometry for the Mesozoic orogen in the Death Valley region consistent with observations from unextended blocks and analogous to that of the Cordilleran orogen at the latitude of Calgary, Alberta.

The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

Metamorphic core complexes form as the result of major continental extension, when the middle and lower continental crust is dragged out from beneath the fracturing, extending upper crust. Movement zones capable of producing such effects evolve in space as well as with time. Deforming rocks in the footwall are uplifted through a progression of different metamorphic and deformational environments, producing a characteristic sequence of (overprinted) meso- and microstructures. The movement zone is folded as the result of the bowing upwards of the lower crust to form a broad basement culmination, as the result of isostatic rebound due to tectonic denudation, but most likely also as the result of local isostatic adjustments due to granite intrusion in the middle crust. A succession of splays branch off from the master detachment fault at depth, excising substantial portions of the lower portions of the upper plate as successive detachment faults eat upwards through it. At the same time, detachment faults incise into progressively deeper levels of the lower plate, although the amount of incision is limited, because the locus of movement remains at approximately the same level in the lower plate. The detachment faults presently observed in the metamorphic core complexes are relatively young features, formed late in the geological evolution of these bodies, and are only the last in a succession of low-angle normal faults that sliced through the upper crust at the upward terminations of major, shallow-dipping, ductile shear zones in the extending Cordilleran orogen. Excisement of listric fault bottoms can explain some of the enigmatic domino-like fault blocks, and other structural relations observed in these terranes. Evidence in support of this model is illustrated from detachment terranes in the northern Colorado River region of southern Nevada, southeastern California and western Arizona, where multiple generations of detachment faults have produced remarkable excisement and incisement geometries.

Ophiolitic thrust nappes in western Nevada: implications for the Cordilleran Orogen

New field and geophysical evidence from Jurassic mafic and ultramafic rocks in western Nevada indicate the presence of two major ophiolite complexes tectonically overlying the rocks of the North American continental margin. Coupled with the existence of Jurassic and older island arc-marginal basin complexes and overthrust relations in the adjacent Sierran-Klamath region, these ophiolitic thrust nappes in Nevada support collision models for the middle Jurassic deformation. The geometry of the observed structures and their relative age relationships suggest that this deformation might have resulted from the collision of the North American continent with a west-dipping subduction zone which resulted in ophiolite emplacement and continental underthrusting. Continued westward motion of North America beneath the allochthonous oceanic terranes during the late Mesozoic developed extensive thrust and possible strike-slip faulting in the western US Cordillera and caused migration of deformation towards the east. This eastward propagation of deformation is interpreted to have resulted in the development of the Sevier foreland thrust belt of latest Jurassic to latest Cretaceous age. These collisional processes and the geometry of the proposed collision orogen are reminiscent of the alpine-type collision belts.

Cordilleran Orogenic Belt of Northern Chihuahua, Mexico: ABSTRACT

Cordilleran Orogenic Belt of Northern Chihuahua, Mexico: ABSTRACT

A historical perspective of exploration geochemistry in Canada — The first 30 years

The history of geochemical exploration in Canada during the period from its initiation in 1938 by Dr. Hans Lundberg to 1968 is outlined. During this 30-year period, methods based on rock, soil, drainage sediment, and vegetation analyses in terranes as varied as those in the Appalachian Orogen, the Canadian Shield and Cordilleran Orogen have been applied with considerable success. Geochemical methods have led directly to or assisted in the discovery of some 18 significant metal deposits of a varied nature in Canada during this period and since then of another 23 deposits. The discoveries include: porphyry copper and molybdenum deposits, lead-zinc-silver deposits, zinc deposits, a rare earth deposit, uranium deposits and precious metal deposits.