Western Laurentia Research Articles

Between the supercontinents: Mesoproterozoic Deer Trail Group, an intermediate age unit between the Mesoproterozoic Belt–Purcell Supergroup and the Neoproterozoic Windermere Supergroup in northeastern Washington, USA

Mesoproterozoic and Neoproterozoic basins in western North America record the evolving position of the Laurentian craton within two supercontinents during their growth and dismemberment: Columbia (Nuna) and Rodinia. The western-most exposures of the Columbia rift-related Belt–Purcell Supergroup are preserved in northeastern Washington, structurally overlain by the Deer Trail Group and depositionally overlying the Neoproterozoic Windermere Supergroup. It has been disputed whether the Deer Trail Group is correlative with the Belt–Purcell Supergroup, or younger. To help resolve the uncertain correlation of these units and their bearing on supercontinent evolution, we characterized the detrital zircon age populations of units from the Deer Trail Group, the Windermere Supergroup, and the Belt–Purcell Supergroup in northeastern Washington. These data show that the western part of the Columbia supercontinent (now located in Australia and eastern Antarctica) remained attached to western Laurentia and continued to supply 1600–1500 Ma detrital zircon grains to the Belt–Purcell Supergroup until after ca. 1391 Ma. The Deer Trail Group is younger than the Belt–Purcell strata, with the basal unit younger than ca. 1362 Ma and a middle unit younger than ca. 1300 Ma. The Deer Trail Group has a pre-Grenville-age provenance from the southwestern USA and possibly east Antarctica. The Buffalo Hump Formation is younger than the Deer Trail Group, with Grenville-age (ca. 1112 Ma) detrital zircon grains and a detrital zircon signature like that of the overlying Neoproterozoic Windermere Supergroup. We interpret the Deer Trail Group to have been deposited during the rift-demise of supercontinent Columbia and before the Grenville-age assembly of the supercontinent Rodinia.

Petrochronology of the Terre Adélie Craton (East Antarctica) evidences a long-lasting Proterozoic (1.7–1.5 Ga) tectono-metamorphic evolution — Insights for the connections with the Gawler Craton and Laurentia

The Terre Adélie Craton displays superimposed strain fields related to the Neoarchean (2.6–2.4 Ga, M1) and Paleo-Mesoproterozoic (1.7–1.5 Ga, M2) metamorphic events. M1 is a regional granulite facies event, constrained by P-T modelling at ~0.8–1.0 GPa – 800–850 °C, followed by a decompressional retrogression in the upper amphibolite facies at ~0.6 GPa – 750 °C. M2 Stage 1 P-T peak is constrained at 0.6–0.7 GPa – 670–700 °C, followed by a steep P-T path down to 0.3 GPa – 550 °C. Retrogression after M2 PT peak occurred in a context of dextral shearing along the Mertz Shear Zone along with thrust motions within the eastern Terre Adélie Craton. In this paper, we present a series of 63 new 40Ar/39Ar ages of biotite and amphibole pairs in mafic rocks from a complete traverse of the Terre Adélie Craton. 40Ar/39Ar dating constrains M2 amphibolite facies metamorphism at a regional scale between 1700 and 1650 Ma, during stage 1 peak metamorphism. During retrogression, lower amphibolite facies recrystallization mainly occurred along vertical shear zones and mafic dykes between 1650 and 1600 Ma (Stage 2), followed by amphibolite to greenschist facies metamorphism until after 1500 Ma (Stage 3). At the scale of the Mawson continent, this event is related to the growth of an active margin above an oblique subduction zone. The supra-subduction model best explains opening of Dumont D'Urville and Hunter basins at 1.71 Ga followed by their rapid closure and metamorphism at 1.70 Ga. In this context, episodic shear zone reactivation and magmatic dyke emplacement led to a partial reequilibration of the 40Ar/39Ar system until <1500 Ma. This latter phase of mafic magmatism largely coincides with a hot spot event at the scale of the Gawler Craton and western Laurentia paleocontinent.

Orogenic link ∼41°N–46°N: Collisional mountain building and basin closure in the Cordillera of western North America

AbstractPolyphase structural mapping and mineral age dating across the Salmon River suture zone in west-central Idaho (Riggins region; ∼45°30′N, ∼117°W–116°W) support a late Mesozoic history of penetrative deformation, dynamothermal metamorphism, and intermittent magmatism in response to right-oblique oceanic-continental plate convergence (Farallon–North America). High-strain linear-planar tectonite fabrics are recorded along an unbroken ∼48 km west-to-east transect extending from the Snake River (Wallowa intra-oceanic arc terrane; eastern Blue Mountains Province) over the northern Seven Devils Mountains into the lower Salmon River Canyon (ancestral North America; western Laurentia). Given the temporally overlapping nature (ca. 145–90 Ma) of east-west contraction in the Sevier fold-and-thrust belt (northern Utah–southeast Idaho–southwest Montana segment), we propose that long-term terrane accretion and margin-parallel northward translation in the Cordilleran hinterland (∼41°N–46°N latitude; modern coordinates) drove mid- to upper-crustal shortening &amp;gt;250 km eastward into the foreland region (∼115°W–113°W). During accretion and translation, the progressive transfer of arc assemblages from subducting (Farallon) to structurally overriding (North American) plates was accommodated by displacement along a shallow westward-dipping basal décollement system underlying the Cordilleran orogen. In this context, large-magnitude horizontal shortening of passive continental margin strata was balanced by the addition of buoyant oceanic crust—late Paleozoic to Mesozoic Blue Mountains Province—to the leading edge of western Laurentia. Consistent with orogenic float modeling (mass conservation, balance, and displacement compatibility), diffuse dextral-transpressional deformation across the accretionary boundary (Salmon River suture: Cordilleran hinterland) was kinematically linked to eastward-propagating structures on the continental interior (Sevier thrust belt; Cordilleran foreland). As an alternative to noncollisional convergent margin orogenesis, we propose a collision-related tectonic origin and contractional evolution for central portions of the Sevier belt. Our timing of terrane accretion supports correlation of the Wallowa terrane with Wrangellia (composite arc/plateau assemblage) and implies diachronous south-to-north suturing and basin closure between Idaho and Alaska.

The Mesoproterozoic Baoban Complex, South China: A missing fragment of western Laurentian lithosphere

Abstract New age data for the Baoban Complex, South China establishes that it lay outboard of western Laurentia in the early Mesoproterozoic but was not part of the Cathaysia Block, with which it is traditional linked, until the mid-Paleozoic. Our geochronology data for detrital zircon and authigenic monazite grains from metasedimentary rocks indicate accumulation between ca. 1.55 Ga and 1.45 Ga for the Gezhencun succession of the Baoban Complex and ca. 1.45 Ga and 1.30 Ga for the Ewenling succession. The former unit is dominated by detrital zircon populations between 1900 Ma and 1500 Ma with two peaks at 1780 Ma and 1580 Ma. The Ewenling succession has detrital zircon peaks at 1720 Ma and 1450 Ma. Newly discovered gneissic granites were emplaced at 1550 Ma and intruded by 1450 Ma leucogranite dykes that are coeval with 1460–1430 Ma bimodal magmatism. The whole Baoban Complex was metamorphosed over the range of 1.3–0.9 Ga based on ages of authigenic zircon and monazite. Depositional ages of metasedimentary rocks are coeval with successions of the Belt-Purcell Basin, western Laurentia. Detrital zircon from the two regions have similar age populations and Lu-Hf compositions, and display a synchronous provenance shift at ca. 1.45 Ga. Basement lithologies on Hainan Island range in age from ca. 1.55–1.43 Ga and underwent metamorphism during 1.3–0.9 Ga. This is younger than basement rocks on the mainland of the Cathaysia Block in South China, suggesting the two regions are spatially unrelated at this time and hence the Mesoproterozoic record of the island cannot constrain the location of the Cathaysia Block in the Nuna and Rodinia supercontinents.

Geochronological and palaeomagnetic investigation of the Madiyi Formation, lower Banxi Group, South China: Implications for Rodinia reconstruction

New geochronological and palaeomagnetic data are presented for the Madiyi Formation of the lower Banxi Group in the South China Block (SCB). Zircon U–Pb dating of two tuff beds within the sampled section yielded consistent ages of 801.9 ± 6.3 Ma and 804.6 ± 9.6 Ma. A total of 137 palaeomagnetic samples were collected and thermally stepwise demagnetized. Three components were isolated. Component A was identified in 30 samples below 300 °C. It has a mean direction resembling the recent geomagnetic field, and is interpreted of recent viscous remanent magnetization. Component B, typically isolated below ~660 °C, yields a pole in situ close to the Cretaceous poles of the SCB and is interpreted as a Cretaceous overprint. Component C was identified in 90 samples, determined in the high-temperature steps between 660 and 690 °C. It yielded a mean direction of Ds = 293.1°, Is = 69.9° (k = 22.3, α95 = 3.2°) after tilt correction, corresponding to a palaeopole of 35.3°N, 67.9°E (dp = 5.5°, dm = 4.7°) and palaeolatitude of 53.8 +4.9/−4.7°N for the study area. Inclination correction using an empirical flattening factor f = 0.6 yielded a corrected mean direction of Ds* = 293.0°, Is* = 77.3° (k = 51.4, α95 = 2.1°), corresponding to a palaeopole of 34.3°N, 82.4°E (dp = 3.9°, dm = 3.7°) and palaeolatitude of 65.7 +3.7/−3.6°. Component C records five polarity zones and passes a reversal test. The pole diverges from all younger poles of the SCB, and is interpreted to be of primary remanent magnetization. Our results reveal a high-latitude position of the SCB at ~800 Ma. Comparing with other two poles from the Xiaofeng dykes (~821 Ma) and Yanbian dykes (~824 Ma) reveals no significant latitudinal difference for the SCB between ~820 and 800 Ma. However, coeval poles from the global palaeomagnetic database, along with geological evidence, indicate that East Svalbard, Australia and Laurentia were located around the equator at ~800 Ma. The striking difference in palaeolatitude suggests that the SCB was unlikely to be located between southeastern Australia and western Laurentia in the Rodinia supercontinent at ~800 Ma.

Onset of the Late Paleozoic Glacioeustatic Signal: A Stratigraphic Record from the Paleotropical, Oil-Shale-Bearing Big Snowy Trough of Central Montana, U.S.A.

AbstractIn the Big Snowy Mountains of central Montana, USA, late Visean to Bashkirian strata preserve a nearly complete, but poorly documented, paleotropical stratigraphic succession that straddles the range of current estimates of the onset of the Late Paleozoic Ice Age (LPIA). Sedimentologic and stratigraphic investigation of the Otter (late Visean to Serpukhovian) and Heath (Serpukhovian) formations, with secondary focus on the overlying Tyler (late Serpukhovian to Bashkirian) and Alaska Bench (Bashkirian) formations, facilitated an appraisal of paleotropical environmental change preserved in this succession. Three facies associations reminiscent of environments currently forming in Shark Bay, Australia, were identified in the Otter Formation: shallow semi-restricted littoral platform, intertidal platform, and supratidal plain. Five facies associations broadly comparable to modern environments present in the Sunda Shelf and southern coast of the Persian Gulf were identified in the Heath Formation: offshore outer ramp, mid- to outer ramp, inner ramp, coastal plain, and sabkha. Facies associations preserved in the Heath Formation are here explained in the context of a protected, homoclinal carbonate ramp situated in a partially silled epicontinental embayment. A shift from low-magnitude relative sea-level oscillations preserved in the Otter Formation to a cyclothemic stratigraphic pattern entailing ≥ 6 fourth-order, high-frequency and high-magnitude relative sea-level fluctuations in the Heath Formation is here interpreted to record the main eustatic signal of the LPIA in central Montana. Current published biostratigraphic constraints for the observed stratigraphy estimate the main eustatic signal of the LPIA to have occurred approximately between 331 (base Serpukhovian) and 327 Ma in central Montana. A distinct upward transition from coal and paleosol-bearing depositional sequences in the lower Heath to evaporite and limestone-bearing depositional sequences in the upper Heath preserves a broad humid to arid paleoclimate shift during deposition of this unit, which influenced hydrographic circulation patterns and the resultant distribution of anoxic environments in the Big Snowy Trough during this time interval. Improved depositional and sequence stratigraphic models of the Heath Formation proposed in this study permit new insight into the theoretical distribution of, and water depth necessary to preserve, black, organic-rich claystone and shale in partially silled intracratonic basins, in addition to new temporal constraints on LPIA onset in paleotropical western Laurentia.

Study and Use of Upper Cambrian to Lower Ordovician conodonts in central, southern, and western Laurentia, 1933–2018

The first descriptions of Laurentian Lower Ordovician conodonts were in the 1930s in the central USA. Authors boiled shales to free the conodont elements. The use of acetic acid began in 1940 at the University of Missouri. The first study of Cambrian conodonts was in 1959; part of the material was from the western USA and part from northern Europe. In the mid 1960s, research began to be based on measured sections and systematically collected samples; many new taxa still were being described. Histological studies resulted in recognising three major groups of Cambrian conodonts, protoconodonts, paraconodonts, and euconodonts. The latter group was the most successful. Preliminary biozonal frameworks were established in the early 1970s and were tied into biozonations based on other fossil groups. Conodont studies became more widespread geographically during the 1980s, biozonations were refined, and conodont zones began to be correlated globally based on many cosmopolitan taxa. In the 1980s, geochemists found that conodonts were useful source materials for chemostratigraphy. The Pander Society was founded in 1967. Regular meetings of the society have been held since 1968 in the USA and internationally, resulting in many important proceedings volumes. The International Working Group on the Cambrian–Ordovician Boundary decided to use conodonts as the primary fossil group to define that boundary, thus stimulating intensive study of conodonts of that age. Definition of that boundary was achieved in 2001, although problems remain because of redeposited conodonts at the stratotype section. A Working Group will recommend a GSSP for the base of the highest stage of the Cambrian, Stage 10. The lowest occurrence of the euconodont Eoconodontus notchpeakensis (Miller Journal of Paleontology, 43(2), 413–439 1969) is being considered to define the base of that stage. The species occurs in depositional environments from nearshore sandstones and dolostones to deep-ocean radiolarian cherts, indicating that conodonts had become adapted to many depositional environments during the late Cambrian. The alternative fossil for marking the base of Stage 10 is the lowest occurrence of an agnostoid arthropod, Lotagnostus americanus (Salter 1860), which has a controversial taxonomy.

Geochemical, isotopic, and U–Pb zircon study of the central and southern portions of the 780 Ma Gunbarrel Large Igneous Province in western Laurentia

Spanning 2500 km along the western margin of North America are 780 Ma dykes, sills, and minor volcanic packages of the Gunbarrel Large Igneous Province. This study focuses on southern (northwestern United States) and central (northern British Columbia) Gunbarrel intrusions and metavolcanics rocks of the Irene and Huckleberry formation (Washington State). Southern Gunbarrel U–Pb ages range from 780 to 769 Ma and new U–Pb zircon dates for the Turah and Rogers Pass sills are 778.6 ± 0.7 and 778.7 ± 0.9 Ma, respectively. Southern Gunbarrel intrusions are medium- to coarse-grained diabases that are moderately evolved basaltic, continental tholeiites. Intrusions display negative Nb–Ta and positive Pb anomalies in normalized multielement plots, and εNd780 values vary from +3.6 to +1.5. The Irene and Huckleberry volcanic rocks are E-MORB in composition with higher εNd780 (+5 to +6) and likely represent partial melts of a mantle plume responsible for the Gunbarrel event. Assuming an Irene and Huckleberry parental magma, mixing models indicate that the southern Gunbarrel magmas were crustally contaminated, but local host rocks are not appropriate crustal contaminants. The modeling points to average upper crust as the crustal contaminant, with an εNd780 of approximately –2. This crustal contaminant likely resides on the craton impinged upon by the mantle plume. The remarkable geochemical homogeneity of Gunbarrel intrusions from the Yukon to Wyoming is best explained if primary, plume-derived E-MORB magmas were contaminated in large magma reservoirs near the plume centre and were then injected laterally into the crust 100s to 1000s of kilometres from the reservoir.

Paleomagnetism of the Chuar Group and evaluation of the late Tonian Laurentian apparent polar wander path with implications for the makeup and breakup of Rodinia

AbstractPaleogeographic models commonly assume that the supercontinent Rodinia was long-lived, with a static geometry involving Mesoproterozoic links that developed during assembly and persisted until Neoproterozoic rifting. However, Rodinian paleogeography and dynamics of continental separation around its centerpiece, Laurentia, remain poorly constrained. On the western Laurentian margin, geological and geochronological data suggest that breakup did not occur until after 720 Ma. Thus, late Tonian (ca. 780–720 Ma) paleomagnetic data are critical for reconstructing paleogeography prior to dispersal and assessing the proposed stasis of Rodinia. Here, we report new paleomagnetic data from the late Tonian Chuar Group in the Grand Canyon, Arizona. We combined this new data set with reanalyzed existing data to obtain a new paleopole preserved in hematite, the reliability of which is supported by six of the seven (Q1–Q6) Van der Voo reliability quality criteria. In addition, we identified pervasive mid- to high-temperature overprints. This new paleomagnetic pole was incorporated with recent high-precision geochronological data and existing paleomagnetic data to present a new late Tonian Laurentian apparent polar wander path (APWP). Having examined the paleomagnetic data of other cratons, global reconstructions for 775 Ma, 751 Ma, and 716 Ma are presented. These reconstructions are consistent with Australia located near the present southern margin of Laurentia. However, a stringent analysis of the global data set does not support a good match between any major craton and the rifted conjugate margin to western Laurentia. Breakup on the western Laurentian margin may have involved rifting of a continental fragment or a craton with uncertainties in its late Tonian geochronologic and paleomagnetic constraints. Our revised Laurentian APWP will allow for more robust tests of paleogeography and evaluation of the proposed supercontinent Rodinia.

Early Cambrian alkaline volcanism on the southern margin of Laurentia: evidence in the volcaniclastic units from the Puerto Blanco Formation in the Caborca block, NW Mexico

ABSTRACTVolcaniclastic units are exposed at the base of the Puerto Blanco Formation in the Caborca region, northwestern Mexico. The lower unit reveals the presence of Early Cambrian mafic volcanism in this region. It consists of a volcano-sedimentary sequence represented by tuffaceous conglomerates, agglomerates, lapillistones, tuffs, and altered mafic volcanic flows. Petrographic analysis classified the volcanic clasts as albite-sphene-calcite-actinolite granofels, with a moderate to intense hydrothermal alteration, precisely characterized by EPMA analysis. Albite-actinolite geothermometry indicates temperatures from 400 to 500°C, suggesting metamorphic conditions in the upper temperature greenschist facies. Geochemistry analysis shows a high TiO2 basic–ultrabasic volcanism that originated the volcanic clasts. Rock protoliths were studied using immobile trace elements, which classified them as OIB-type alkaline basalts with the characteristic spider hump-shaped pattern, situated in an anorogenic intracontinental tectonic setting with enriched mantle signatures. 40Ar/39Ar geochronology shows metamorphic ages of 52.58 ± 2.0 and 91.67 ± 0.55 Ma, consistent with the emplacement of Laramidic granitoids identified in the region. Possible correlations of this alkaline volcanism include the Southern Oklahoma Aulacogen and the late stages of the rifting of north western Laurentia represented in western United States.

Rifting of western Laurentia at 1.38 Ga: The Hart River sills of Yukon, Canada

The Hart River sills are a set of mafic to intermediate intrusions that occur in northern Yukon, Canada. The largest sills are over 500 m thick and over 200 km long. New U-Pb dates of 1382.15 ± 0.39 Ma and 1382.14 ± 0.36 Ma were obtained via chemical abrasion thermal ionization mass spectrometry on zircon. Whole rock initial neodymium isotopic compositions of the Hart River sills are juvenile and have εNdi from +1.5 to +4.0. The primary mineralogy of the Hart River sills is predominated by clinopyroxene and plagioclase. Geochemical modeling indicates that the Hart River sills lie on a common liquid line of descent defined by a fractionating assemblage of plagioclase, clinopyroxene and minor olivine. The Hart River sills have rare earth element and high field strength abundances similar to normal mid-ocean ridge basalts (N-MORB) but are enriched in large ion lithophile elements. The Sm/Yb and Dy/Zr ratios indicate >8% partial melting of spinel-bearing mantle. During the emplacement of the Hart River sills, western Laurentia was juxtaposed with Australia and eastern Antarctica within the supercontinent Columbia. The degree of partial melting, similarity to N-MORB, and juvenile isotopic signature are consistent with an episode of rifting at 1.38 Ga. Coeval magmatism and intracontinental rift basins farther south on Laurentia provide additional evidence for rifting of supercontinent Columbia at 1.38 Ga.

Cambrian oncolites from San José de Gracia, Sonora, Mexico

Recently discovered oncolites from a Lower Cambrian locality in San José de Gracia (SJG), Sonora (Mexico), illustrate the wide geographic distribution that they had in this part of Laurentia some 515–510 Ma ago. They are exceptionally large (~44 mm) in comparison to other marine forms of the North American Cambrian Carbonate Bank, but similar in their overall morphology and depositional environment. Their association with abundant biomorphs, especially indigenous filamentous structures with the curvature and spatial arrangement that fit the minimum criteria for biogenicity, supports their biological origin. Key morphological features include laminated growth in at least a portion of the oncolite, subspherical shape with internal porosity, and a nucleated habit on skeletal debris (mollusk shells) and mud concretions. The similarity of SJG oncolites with others from Cambrian outcrops separated today by hundreds of kilometers, suggests that massive microbially-driven, shallow marine ecosystems existed in western Laurentia during this time. This speaks of the impact that microbes had in shaping marine sediments, and the global primary productivity and carbon cycle during early stages of the Phanerozoic.

Geochemistry and U-Pb geochronology of 1590 and 1550 Ma mafic dyke swarms of western Laurentia: Mantle plume magmatism shared with Australia

Three new UPb ID-TIMS isotopic ages confirm previous evidence for two geochemically distinct, early Mesoproterozoic pulses of mafic magmatism along the western margin of Laurentia. The first is a UPb baddeleyite age of 1590 ± 5 Ma from a dyke swarm in the Tobacco Root Mountains of western Laurentia. It is the first evidence for magmatism of this age in west-central Laurentia. The second is a UPb baddeleyite age of 1592.4 ± 2.5 from the Western Channel Diabase, 2000 km to the north in NW Laurentia, which supports two previous UPb baddeleyite ages of ca. 1590 Ma for these intrusions. The third is a UPb baddeleyite age of 1551 ± 5 Ma, also from the Tobacco Root Mountains, and provides evidence for a distinct younger pulse of mafic magmatism.We propose that the ca. 1590 Ma mafic intrusions in northwestern and west-central Laurentia represent components of a large igneous province (LIP), which we name the “ca. 1590 Ma Mammoth-Western Channel LIP.”This 1590 Ma LIP is geochemically similar to the contemporaneous volcanic rocks in the Gawler Craton and Curnamona Province of Australia. Furthermore, the 1599 Ma Wernecke Breccias near the Western Channel Diabase are geologically similar to the 1590 Ma Olympic Dam Breccias on the South Australian craton. We propose that a mantle plume at ca. 1590 Ma centered between the Laurentian and Southern Australian cratons, located by converging dyke swarms, fed the intrusions on Laurentia plus the Gawler Range Volcanics, the Hiltaba Suite granites and the Ninnerie Supersuite on the South Australian craton. Additionally, magmatic underplating from the plume set up the hydrothermal system responsible for the formation of the Wernecke and Olympic Dam Breccias. The younger 1551 Ma magmatism in the Tobacco Root Mountains, which shows less contamination of a metasomatized subcontinental lithospheric mantle than the older 1590 Ma pulse, may represent an early stage of rifting that pre-dates the ca. 1470 Ma Belt-Purcell Basin extension of western Laurentia.Felsic magmatism, hypothesized to have accompanied the 1590–1550 Ma LIP magmatism, could be a potential source for detrital zircon, thereby reducing the requirement for a non-Laurentian source for detrital zircon ages within the 1610–1490 Ma “North American Magmatic Gap.” Additionally, the 1590 Ma and 1550 Ma ages on western Laurentia dykes provide tighter constraints for previous paleomagnetic studies.

Tectonic Analysis of the Pennsylvanian Ely‐Bird Spring Basin: Late Paleozoic Tectonism on the Southwestern Laurentia Margin and the Distal Limit of the Ancestral Rocky Mountains

AbstractDuring Pennsylvanian to Early Permian time, much of southwestern Laurentia experienced broad deformation far from the nearest plate boundaries. The bulk of this deformation is known as the Ancestral Rocky Mountains orogeny, which consists of several basement uplifts and proximal basins north and northwest of the Ouachita‐Marathon suture. The Ely‐Bird Spring basin formed during this time, located between the classic Ancestral Rocky Mountains and the complex and poorly constrained western Laurentian margin. In this study, we used tectonic subsidence curve analyses to evaluate the tectonic style of basin subsidence in this basin and several northern Ancestral Rocky Mountains basins. The Ely‐Bird Spring and Wood River basins have convex upward and stair‐stepped tectonic subsidence curves similar to curves from migrating foreland basins. These curves, combined with sedimentological and structural data, are consistent with these basins forming due to loading from the west and northwest as part of the complex western Laurentian plate boundary, not due to classic Ancestral Rocky Mountains tectonics. The other northern Ancestral Rocky Mountains basins have much steeper and more linear subsidence curves, similar to foreland basins with fixed loads. However, the Oquirrh basin, in particular, displays a large amount of subsidence far from hypothesized tectonic drivers of deformation. A combination of transmitted stress from both the western Laurentia margin and traditional Ancestral Rocky Mountains tectonism explains the anomalously large subsidence in the Oquirrh basin. Therefore, transmitted stresses from more than one side of western and southern Laurentia are required to explain the observed pattern of deformation.

Latest Neoproterozoic to Cambrian detrital zircon facies of western Laurentia

Abstract Late Neoproterozoic to Cambrian sandstone units are common in western Laurentia and record initial transgression of the craton after the formation of the western passive margin during the latest Neoproterozoic to earliest Cambrian. Detrital zircon measurements from 42 latest Neoproterozoic to Cambrian basal Sauk sequences and five older Neoproterozoic sandstone samples from a region extending from the Mexico–United States border to central British Columbia, Canada, are combined with previous results to characterize sediment source areas and dispersal systems. Detrital zircon populations in Neoproterozoic and Cambrian sedimentary rocks are divided into six facies based on a statistical comparison using multidimensional scaling. Detrital zircon facies are found in unique geographical regions reflecting proximity to the major tectonic provinces of Laurentia. Samples from northern regions are dominated by Archean and Paleoproterozoic zircons derived from Archean tectonic provinces and the orogenic belts that record the assembly of the Laurentian craton. More southerly sample locations show an increase in detrital zircons derived from younger Paleoproterozoic orogenic belts and early Mesoproterozoic intrusive suites. Detrital zircons from Grenville-aged sources are common in the south. The Transcontinental Arch, a feature interpreted to have controlled large-scale sediment dispersal patterns in the mid- to late Cambrian, likely played a major role in isolating the southern and northern signatures. Our data set can be used to test tectonic models for the Cordilleran orogen that invoke Jurassic or Cretaceous collision of a ribbon continent as the driving mechanism for orogenesis. Cambrian rocks of the Cassiar-Antler platform juxtaposed with North America during the hypothetical ribbon continent collision show the same geographic distribution of detrital zircon facies as similar-aged rocks from autochthonous and parautochthonous locations on the Laurentian margin. The concordance of detrital zircon facies across the proposed suture is a negative result for models that predict large dextral displacements, on the order of 2000 km, across the suture.

Permian-Triassic Tethyan realm reorganization: Implications for the outward Pangea margin

We present a new conceptual model to explain the first order Permian-Triassic evolution of the whole > 30 000 km long Pangea margin facing the Panthalassa ocean. Compilation of available geological, geochemical, geochronogical and paleomagnetic data all along this system allowed us to distinguish three part of the margin: western Laurentia, western Gondwana and eastern Gondwana. These segments record distinct tectonic and magmatic events, which all occur synchronously along the whole margin and correlate well with the main geodynamic events of this period, i.e. subduction of the Paleotethys mid-ocean ridge at 310-280 Ma, opening of the Neotethys at 280-260 Ma, counterclockwise rotation of Pangea at 260-230 Ma and closure of the Paleotethys at 230-220 Ma. Between 260 and 230 Ma, the reorganization of the Tethyan realm triggered the up to 35° rotation of Pangea around an Euler pole located in northernmost South America. This implied both an increase and a decrease of the convergence rate between the margin and the Panthalassa ocean, north and south of the Euler pole, respectively. Thus, the Permian-Triassic Pangean margin was marked: in western Laurentia by marginal sea closure, in western Gondwana by widespread bimodal magmatic and volcanic activity, in eastern Gondwana by transpressive orogenic phase. Therefore, we propose that the Permian-Triassic evolution of the outward margin of Pangea was controlled by the Tethyan realm reorganization.

Proterozoic crustal evolution of central East Antarctica: Age and isotopic evidence from glacial igneous clasts, and links with Australia and Laurentia

Rock clasts entrained in glacial deposits sourced from the continental interior of Antarctica provide an innovative means to determine the age and composition of ice-covered crust. Zircon U-Pb ages from a suite of granitoid clasts collected in glacial catchments draining central East Antarctica through the Transantarctic Mountains show that crust in this region was formed by a series of magmatic events at ∼2.01, 1.88–1.85, ∼1.79, ∼1.57, 1.50–1.41, and 1.20–1.06Ga. The dominant granitoid populations are ca. 1.85, 1.45 and 1.20–1.06Ga. None of these igneous ages are known from limited outcrop in the region. In addition to defining a previously unrecognized geologic history, zircon O and Hf isotopic compositions from this suite have: (1) mantle-like δ18O signatures (4.0–4.5‰) and near-chondritic Hf-isotope compositions (εHf ∼ +1.5) for granitoids of ∼2.0Ga age; (2) mostly crustal δ18O (6.0–8.5‰) and variable Hf-isotope compositions (εHf=−6 to +5) in rocks with ages of ∼1.88–1.85, ∼1.79 and ∼1.57Ga, in which the ∼1.88–1.79Ga granitoids require involvement of older crust; (3) mostly juvenile isotopic signatures with low, mantle-like δ18O (∼4–5‰) and radiogenic Hf-isotope signatures (εHf=+6 to +10) in rocks of 1.50–1.41Ga age, with some showing crustal sources or evidence of alteration; and (4) mixed crustal and mantle δ18O signatures (6.0–7.5‰) and radiogenic Hf isotopes (εHf=+3 to +4) in rocks of ∼1.2Ga age. Together, these age and isotopic data indicate the presence in cratonic East Antarctica of a large, composite igneous province that formed through a punctuated sequence of relatively juvenile Proterozoic magmatic events. Further, they provide direct support for geological correlation of crust in East Antarctica with both the Gawler Craton of present-day Australia and Proterozoic provinces in western Laurentia. Prominent clast ages of ∼2.0, 1.85, 1.57 and 1.45Ga, together with sediment source linkages, provide evidence for the temporal and spatial association of these cratonic elements in the Columbia supercontinent. Abundant ∼1.2–1.1Ga igneous and metamorphic clasts may sample crust underlying the Gamburtsev Subglacial Mountains, indicating the presence of a Mesoproterozoic orogenic belt in the interior of East Antarctica that formed during final assembly of Rodinia.

Northern Laurentian provenance for Famennian clastics of the Jasper Basin (Alberta, Canada): A Sm-Nd and U-Pb detrital zircon study

Within the carbonate-dominated Devonian succession of the Western Canada Sedimentary Basin, sandstones of the Famennian Sassenach Formation represent a sedimentological and isotopic anomaly inferred to be related to western sources uplifted during Antler tectonism. Previously reported evidence for a changing tectonic regime along western Laurentia during deposition of the Sassenach renders this unit an important potential link to shifts in source areas associated with this tectonism. We report the first set of U-Pb detrital zircon ages and new Sm-Nd whole rock data from the Sassenach Formation of the Jasper Basin in the Alberta Rocky Mountain fold-and-thrust belt. The 147 Sm/ 144 Nd ratios (0.109–0.122) and e Nd values (–9.6 to –10.9) at the time of deposition are within a narrow range that overlaps with published data, confirming a post-Ordovician shift to more positive e Nd values along the northern and western Laurentian margin coinciding with the introduction of more juvenile sources associated with Innuitian and Grenville sources. A previously recognized subtle negative shift in e Nd values in the Sassenach relative to under­lying Frasnian deposits is also confirmed, which suggests a minor contribution from more mature sources, opposite the overall post-Ordovician positive trend. Zircon ages show a wide spread (378–3506 Ma), with three broad age groups separated by a paucity of ages in the 700–900 Ma and 2100–2500 Ma ranges: (1) a narrow Paleozoic to Neoproterozoic group (378–700 Ma), (2) a broad Proterozoic group of scattered ages (900–2100 Ma), and (3) a narrow Archean group (2500–2900 Ma). Paleozoic to Neoproterozoic detrital zircon ages are absent from the western Laurentian record, and thus link the Sasse­nach Formation to the Arctic realm where terranes with zircons of these ages are well known. Discordance modeling suggests a consistent Pb loss in the Sassenach zircon data set (ca. 390 Ma) that coincides with the first orogenic pulse in Innuitian–Ellesmerian tectonics in the paleo-Arctic realm. Together these data suggest that the bulk of terrigenous sediment in the Jasper Basin was recycled from Ellesmerian foreland strata and subsequently transported southward along the margins of western Laurentia. Sediment transport may have been facilitated by southward-directed, shore-parallel shallow-marine currents initiated by Northern Hemisphere wind patterns.

New U-Pb constraints on the age of the Little Dal Basalts and Gunbarrel-related volcanism in Rodinia

Diabase exposed amongst outcrops of the Little Dal Basalts (LDB) between the Coates Lake Group and overlying the Little Dal Group in the Mackenzie Mountains of northern Canada has a geochemical signature that is characteristic of the LDB and associated Tsezotene sills. Marginally concordant magmatic zircons from a sample of the diabase yielded CA-ID-TIMS weighted mean 207Pb/206Pb and 206Pb/238U dates of 778.4±1.8Ma (2σ) and 775.10±0.54Ma (2σ), respectively. These dates provide the first direct constraint on the age of the LDB and confirm a temporal relationship to the widespread Gunbarrel magmatic event that affected western North America during the protracted breakup of the supercontinent Rodinia. Hafnium isotopes were measured on three grains and calculated values of εHf at 775Ma are 9.5±0.4, 9.7±0.2 and 9.3±0.2 (2σ), consistent with a mantle plume genesis.Gunbarrel magmatism was not temporally related to the final fragmentation of Rodinia and the establishment of the west Laurentian continental margin (rift-drift transition), which did not occur until after 720Ma. However, the age and stratigraphic position of the LDB suggest the Gunbarrel event may have thermally weakened the lithosphere leading to a period of intracontinental rifting and the deposition of the Coates Lake Group rift-sediments prior to 732Ma.The sample of diabase also yielded a single 207Pb/206Pb 1565.7±1.5Ma xenocrystic zircon with a model Th/U ratio of 0.47 indicative of a crustal origin. Although there is limited evidence for felsic magmatism, metamorphism, and orogenesis of this age in western Laurentia, a range of felsic volcanic rocks and/or intrusive suites of this age do occur in southern, central and eastern Australia, supporting existing reconstructions of the Nuna/Columbia supercontinent that place the northwest margin of Laurentia adjacent to Australia.

A Pangean rim of fire: Reviewing the Triassic of western Laurentia

A synthesis of U-Pb detrital zircon data from the Triassic of western Laurentia is placed in the context of paleogeographic models for Pangea. We find that an emerging body of evidence supports the hypothesis that a Triassic subduction zone and continental magmatic arc system fringed western Laurentia starting in the southwestern United States, continuing northward along the Cordillera, including arc rocks of the Quesnel and Yukon-Tanana terranes, and extending further into the Arctic region. In the context of this geodynamic setting, the western interior basins of North America would have formed, probably by subduction dynamics, as a collage of backarc and retroarc foreland basins. The convergent tectonic model for western Laurentia is consistent with paleogeographic reconstructions that show a subduction zone encircling Pangea, called the Pangean rim of fire.