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Ian W D Dalziel - One of the best experts on this subject based on the ideXlab platform.
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Laurentia‐Kalahari Collision and the Assembly of Rodinia
The Journal of Geology, 2020Co-Authors: Ian W D Dalziel, Sharon Mosher, Lisa M GahaganAbstract:Abstract The Llano Orogenic Belt along the present southern margin of Laurentia, regarded as continuation of the Grenvillian Orogen along the eastern Laurentian margin and exposed in basement uplifts in central and western Texas, records an ∼300‐m.yr. history of orogenesis culminating in arc‐continent and continent‐continent collision between ∼1150 and 1120 Ma and continuing until ∼980 Ma. The shape of the orogen and kinematics of the contractional deformation along the belt, together with the high‐P metamorphic conditions attained, indicate that a previously unidentified craton served as an indentor. It is paleomagnetically acceptable for the Kalahari Craton of southern Africa to have been opposed to this margin and within ∼1500 km of present‐day central Texas at ∼1100 Ma. Moreover, the Kalahari Craton is the correct size, and the structural and metamorphic evolution of the 1200–950 Ma Namaqua‐Natal Orogenic Belt that wraps around its present southern margin is compatible with that craton having been the...
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coats land crustal block east antarctica a tectonic tracer for Laurentia
Geology, 2011Co-Authors: S L Loewy, Ian W D Dalziel, Sergei Pisarevsky, James N Connelly, Jennifer Tait, Richard E Hanson, D BullenAbstract:Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1112 ± 4 Ma, are identical in age to both the Umkondo large igneous province (LIP) of the Kalahari craton (southern Africa) and the early Keweenawan LIP of Laurentia (North America). Although marine and satellite data demonstrate that Coats Land was close to Kalahari within the Gondwana supercontinent, the Coats Land rocks yield Pb isotope compositions strikingly distinct from those of the Umkondo province, yet indistinguishable from rocks of the Keweenawan province. The anorogenic Red Bluff granitic suite, along the present-day southern Laurentian margin in the Franklin Mountains (Texas, USA), is of comparable age, general rock type, and Pb isotope composition to rocks of Coats Land and may provide a piercing point for a Coats Land–Laurentia link. Paleomagnetic poles permit the Coats Land block to be close to this part of Laurentia ca. 1100 Ma and allow juxtaposition of Kalahari and southern Laurentia ca. 1000 Ma. The Coats Land crustal block may therefore be a critical tectonic tracer for placing Laurentia within late Mesoproterozoic and Neoproterozoic paleogeographic reconstructions. If this hypothesis is correct, Laurentia collided with the Kalahari craton along Antarctica's Maud orogen, which would represent a continuation of the ca. 1000 Ma Grenville orogen of eastern and southern Laurentia.
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eastern Laurentia in rodinia constraints from whole rock pb and u pb geochronology
Tectonophysics, 2003Co-Authors: Staci L Loewy, Ian W D Dalziel, James N Connelly, Charles F. GowerAbstract:Abstract Whole-rock Pb isotopic signatures and U/Pb geochronology refute a Rodinian correlation of northeastern Laurentia and proto-Andean Amazonia. According to this previously proposed model, the Labrador–Scotland–Greenland Promontory (LSGP) of northeastern Laurentia collided with the proto-Andean margin of Amazonia, at the Arica Embayment, during the Grenville/Sunsas Orogeny (ca. 1.0 Ga). Links between the two margins were based upon the correlation of the LSGP with Arequipa-Antofalla Basement (AAB), a Proterozoic block along the proto-Andean margin of Amazonia adjacent to the Arica Embayment. Specifically, similarities in 1.8–1.0 Ga basement rocks in both regions suggested that the AAB was originally a piece of the LSGP. Furthermore, similarities in unique, post-collisional, but pre-rift, glacial sedimentary sequences also supported a link between the AAB and LSGP. Tests of these apparent similarities fail to support correlation of the AAB and the LSGP and, thus, eliminate a direct link between northeastern Laurentia and southwestern Amazonia in Rodinia. However, Pb isotopic compositions and U/Pb geochronology provide the basis for two new correlations, namely, (1) the ca. 1.3–1.0 Ga basement in the central and southern Appalachians may be an allochthonous block that was transferred to Laurentia from Amazonia at ca. 1.0 Ga, and (2) an allochthonous AAB may be a piece of the Kalahari Craton that was transferred to Amazonia at ca. 1.0 Ga. Based on these new correlations and a previously proposed Grenvillian connection between southern Laurentia (Llano) and Kalahari, we propose that Amazonia may have collided with a contiguous southeastern Laurentia/Kalahari margin at ca. 1.0 Ga.
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Laurentia kalahari collision and the assembly of rodinia
The Journal of Geology, 2000Co-Authors: Ian W D Dalziel, Sharon Mosher, Lisa M GahaganAbstract:Abstract The Llano Orogenic Belt along the present southern margin of Laurentia, regarded as continuation of the Grenvillian Orogen along the eastern Laurentian margin and exposed in basement uplifts in central and western Texas, records an ∼300‐m.yr. history of orogenesis culminating in arc‐continent and continent‐continent collision between ∼1150 and 1120 Ma and continuing until ∼980 Ma. The shape of the orogen and kinematics of the contractional deformation along the belt, together with the high‐P metamorphic conditions attained, indicate that a previously unidentified craton served as an indentor. It is paleomagnetically acceptable for the Kalahari Craton of southern Africa to have been opposed to this margin and within ∼1500 km of present‐day central Texas at ∼1100 Ma. Moreover, the Kalahari Craton is the correct size, and the structural and metamorphic evolution of the 1200–950 Ma Namaqua‐Natal Orogenic Belt that wraps around its present southern margin is compatible with that craton having been the...
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paleozoic Laurentia gondwana interaction and the origin of the appalachian andean mountain system
Geological Society of America Bulletin, 1994Co-Authors: Ian W D Dalziel, Luis Hugo Dalla Salda, Lisa M GahaganAbstract:Laurentia, the rift-bounded Precambrian nucleus of North America, may have broken out from a Neoproterozoic supercontinent between East and West Gondwana. Several lines of evidence suggest that the Appalachian margin of Laurentia subsequently collided with the proto-Andean margin of the amalgamated Gondwana supercontinent in different relative positions during early and mid-Paleozoic time, in route to final docking against northwest Africa to complete the assembly of Pangea. Hence the Appalachian and Andean orogens may have originated as a single mountain system. The overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the Paleozoic Era. Laurentia-Gondwana collisions may help to explain contemporaneous unconformities in the Paleozoic sedimentary cover of the Laurentian, Gondwanan, and Baltic cratons.
Robert Lopez - One of the best experts on this subject based on the ideXlab platform.
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late ordovician early silurian continental collisional orogeny in southern mexico and its bearing on gondwana Laurentia connections
Geology, 1999Co-Authors: Fernando Ortegagutierrez, Mariano Eliasherrera, Margarita Reyessalas, Consuelo Maciasromo, Robert LopezAbstract:New zircon and monazite U-Pb data, tectonic mapping, and petrologic studies in key units of the Acatlan Complex show a previously undocumented phase of continental collision orogeny of Late Ordovician–Early Silurian age in southern Mexico. The event involved the partial eclogitization of oceanic lithosphere and continental crust, which traveled westward more than 200 km over siliciclastic metasedimentary rocks of the trench-forearc of an opposing continental margin. The overriding eastern margin was the Oaxaquia microplate attached to Gondwana, and the western overridden margin is considered to have been the eastern margin of Laurentia. This event, which we name the Acatecan orogeny, was roughly synchronous with the possible closure of Iapetus along the Appalachian margin, which involved, according to current models, either the docking of peri-Gondwanan terranes such as Avalonia and Carolina or the direct collision between Gondwana and Laurentia. The permanence of Oaxaquia in northwestern Gondwana until the end of the Silurian, as suggested by Tremadocian to Silurian marine faunas in the cover of Oaxaquia, is more consistent with the direct collision of Gondwana and Laurentia at the end of the Ordovician, forming the Acatlan Complex between.
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Late Ordovician–Early Silurian continental collisional orogeny in southern Mexico and its bearing on Gondwana-Laurentia connections
Geology, 1999Co-Authors: Fernando Ortega-gutiérrez, Mariano Elías-herrera, Margarita Reyes-salas, Consuelo Macías-romo, Robert LopezAbstract:New zircon and monazite U-Pb data, tectonic mapping, and petrologic studies in key units of the Acatlan Complex show a previously undocumented phase of continental collision orogeny of Late Ordovician–Early Silurian age in southern Mexico. The event involved the partial eclogitization of oceanic lithosphere and continental crust, which traveled westward more than 200 km over siliciclastic metasedimentary rocks of the trench-forearc of an opposing continental margin. The overriding eastern margin was the Oaxaquia microplate attached to Gondwana, and the western overridden margin is considered to have been the eastern margin of Laurentia. This event, which we name the Acatecan orogeny, was roughly synchronous with the possible closure of Iapetus along the Appalachian margin, which involved, according to current models, either the docking of peri-Gondwanan terranes such as Avalonia and Carolina or the direct collision between Gondwana and Laurentia. The permanence of Oaxaquia in northwestern Gondwana until the end of the Silurian, as suggested by Tremadocian to Silurian marine faunas in the cover of Oaxaquia, is more consistent with the direct collision of Gondwana and Laurentia at the end of the Ordovician, forming the Acatlan Complex between.
Chris M Hall - One of the best experts on this subject based on the ideXlab platform.
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ediacaran paleogeography of Laurentia paleomagnetism and 40ar 39ar geochronology of the 583ma baie des moutons syenite quebec
Precambrian Research, 2011Co-Authors: P. J.a. Mccausland, Fatim Hankard, Chris M HallAbstract:Abstract Laurentia has unclear paleogeographic relations during the Precambrian–Cambrian transition. Published paleomagnetic results from the late Neoproterozoic (Ediacaran period) imply that Laurentia may have lain at high southerly latitudes during 590–570 Ma, but conflict with other results from 615 Ma and from 565 to 550 Ma which place Laurentia at low paleolatitudes. Paleomagnetic results from other continents over the same Ediacaran period are either sparse or equally ambiguous, giving rise to proposals of unusual geodynamic events such as rapid plate motions and/or large-scale true polar wander or unusual geomagnetic field geometry. To address this problem for Laurentia, we have obtained paleomagnetic and 40Ar–39Ar geochronologic results from the mid-Ediacaran Baie des Moutons (Mutton Bay) syenite, exposed in a failed Iapetan rift along the north shore of the Gulf of St. Lawrence in eastern Quebec. Early and late intrusive units of the Mutton Bay syenite bear hornblende and biotite grains which provide overlapping 40Ar–39Ar plateau ages, indicating that the intrusion cooled rapidly upon emplacement at 583.4 ± 2.0 Ma. Of the 44 sites sampled in the Baie des Moutons syenite, 8 yielded consistent characteristic paleomagnetic ChRM A directions that were easterly and steep (D = 98.6° I = 78.0°; α95 = 6.5°, k = 71.7), retained by PSD magnetite. Six sites from associated feldspar porphyry dykes and late intrusive units gave shallow southeasterly or northwesterly ChRM B directions (D = 163.1°, I = 6.0°; α95 = 21.7°, k = 10.5). Other sites fail to carry a detectable or stable ancient remanence, mainly due to the dominant presence of a viscous remanence carried by MD magnetite. Baked contact tests for both ChRMs A and B at several sites proved inconclusive due to the lack of stable remanence in the host syenite in each case. The eight sites bearing ChRM A directions yield a paleopole at 42.6°N, 332.7E (dp = 11.7°, dm = 12.4°) which is close to three 590–570 Ma previously published paleopoles that also place Laurentia at high paleolatitudes. The six sites with ChRM B directions give a paleopole at location −34.2°N, 321.5E (dp = 10.9°, dm = 21.8°), in good agreement with a published paleopole from the 565 Ma Sept Iles intrusion. The results from this study are enigmatic, providing support for both high and low paleolatitude interpretations of Laurentia's location during the mid-Ediacaran. It is nevertheless possible to propose relative Ediacaran paleogeographies that are consistent with Ediacaran paleopoles and with later Cambro-Ordovician relationships between Laurentia and other circum-Iapetus continents. The candidate paleogeographies do require, however, some contribution from rapid plate motion or true polar wander and also indicate that the opening of the Iapetus Ocean began well before 550 Ma rifting along eastern Laurentia.
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circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from Laurentia
Precambrian Research, 2007Co-Authors: P. J.a. Mccausland, Chris M HallAbstract:Abstract The paleogeography of the Precambrian–Cambrian transition is still poorly known, but is fundamental for understanding Late Neoproterozoic climate extremes and the spatial associations of the rapidly evolving Ediacaran and Cambrian fauna. This period has also been proposed to host unusual geodynamic events such as rapid plate motions and/or the bulk tumbling of the Earth with respect to its spin axis, called true polar wander (TPW). New paleomagnetic and 40Ar–39Ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western Quebec. The Mont Rigaud and the Chatham-Grenville stocks are related to a failed rift arm of the Iapetus Ocean. Both intrusions bear hornblendes which provide overlapping 40Ar–39Ar plateau ages of 533.2 ± 1.1 Ma (Mont Rigaud) and 531.4 ± 3.4 Ma (Chatham-Grenville), corresponding to Early Cambrian (Nemakit-Daldynian) time. Characteristic paleomagnetic (reversed) directions from both intrusions are easterly and shallow (D = 89.3°, I = 30.1°; α95 = 6.2, k = 38.8; N = 15 sites), and the Mont Rigaud intrusion also carries antipodal normal-polarity directions. The combined result yields a north paleopole at location 11.9°S 184.6°E; dp = 3.8° dm = 6.9°, placing the Montreal region at 16 ± 4°S paleolatitude in the Early Cambrian, in good agreement with Late Ediacaran and Middle to Late Cambrian paleomagnetic results from Laurentia. Laurentia likely resided at low southerly paleolatitudes throughout the Cambrian, ruling out a major inertial-interchange TPW event for Early to Middle Cambrian time. A comparison of Late Ediacaran to Middle Cambrian paleomagnetic results from Laurentia, Baltica, Siberia and elements of West Gondwana implies that by Early Cambrian time a large paleolatitudinal gap existed between Laurentia and its presumed conjugate rift margins, thus placing a wide Iapetus Ocean between them. The position of Baltica within the circum-Iapetus paleogeography is contentious, due to differing interpretations of the few paleomagnetic results. During Ediacaran time, Laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 Ma to the south polar region at 590–575 Ma, and then rapidly back to low latitude by 565–550 Ma. The large and perhaps rapid apparent polar wander of Laurentia and other continents in the Ediacaran remains enigmatic and controversial.
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Circum-Iapetus paleogeography of the Precambrian–Cambrian transition with a new paleomagnetic constraint from Laurentia
Precambrian Research, 2007Co-Authors: Phil J.a. Mccausland, Chris M HallAbstract:Abstract The paleogeography of the Precambrian–Cambrian transition is still poorly known, but is fundamental for understanding Late Neoproterozoic climate extremes and the spatial associations of the rapidly evolving Ediacaran and Cambrian fauna. This period has also been proposed to host unusual geodynamic events such as rapid plate motions and/or the bulk tumbling of the Earth with respect to its spin axis, called true polar wander (TPW). New paleomagnetic and 40Ar–39Ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western Quebec. The Mont Rigaud and the Chatham-Grenville stocks are related to a failed rift arm of the Iapetus Ocean. Both intrusions bear hornblendes which provide overlapping 40Ar–39Ar plateau ages of 533.2 ± 1.1 Ma (Mont Rigaud) and 531.4 ± 3.4 Ma (Chatham-Grenville), corresponding to Early Cambrian (Nemakit-Daldynian) time. Characteristic paleomagnetic (reversed) directions from both intrusions are easterly and shallow (D = 89.3°, I = 30.1°; α95 = 6.2, k = 38.8; N = 15 sites), and the Mont Rigaud intrusion also carries antipodal normal-polarity directions. The combined result yields a north paleopole at location 11.9°S 184.6°E; dp = 3.8° dm = 6.9°, placing the Montreal region at 16 ± 4°S paleolatitude in the Early Cambrian, in good agreement with Late Ediacaran and Middle to Late Cambrian paleomagnetic results from Laurentia. Laurentia likely resided at low southerly paleolatitudes throughout the Cambrian, ruling out a major inertial-interchange TPW event for Early to Middle Cambrian time. A comparison of Late Ediacaran to Middle Cambrian paleomagnetic results from Laurentia, Baltica, Siberia and elements of West Gondwana implies that by Early Cambrian time a large paleolatitudinal gap existed between Laurentia and its presumed conjugate rift margins, thus placing a wide Iapetus Ocean between them. The position of Baltica within the circum-Iapetus paleogeography is contentious, due to differing interpretations of the few paleomagnetic results. During Ediacaran time, Laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 Ma to the south polar region at 590–575 Ma, and then rapidly back to low latitude by 565–550 Ma. The large and perhaps rapid apparent polar wander of Laurentia and other continents in the Ediacaran remains enigmatic and controversial.
David A D Evans - One of the best experts on this subject based on the ideXlab platform.
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paleomagnetic evidence for a large rotation of the yukon block relative to Laurentia implications for a low latitude sturtian glaciation and the breakup of rodinia
Geological Society of America Bulletin, 2017Co-Authors: Athena Eyster, David A D Evans, R R Fu, Justin V Strauss, B P Weiss, Charlie F Roots, Galen P Halverson, Francis A MacdonaldAbstract:Understanding the tectonic history of the supercontinent Rodinia is crucial for testing proposed links among Neoproterozoic tectonics, supercontinent cycles, climate, and biogeochemistry. The Neoproterozoic Mount Harper volcanics of the Ogilvie Mountains, Yukon, Canada, interfinger with Sturtian-age (ca. 717−660 Ma) glacial deposits that were deposited in narrow, fault-bounded basins related to the breakup of Rodinia. Here, we present new paleomagnetic data from the Mount Harper volcanics and isolate four paleomagnetic directions: a low-temperature direction recording the present geomagnetic field, a mid-temperature direction consistent with a Cretaceous overprint, and two high-temperature directions, one of which is carried by hematite and likely represents a chemical overprint, and the other of which is carried by magnetite and likely is a primary direction. This primary pole passes the fold and conglomerate tests and includes a reversal but is 50° away from the coeval 721−712 Ma Laurentian Franklin large igneous province pole. This difference can be reconciled using a 50° counterclockwise rotation of the Yukon block relative to Laurentia. The prerotation reconstruction of the Yukon block relative to Laurentia aligns Neoproterozoic fault orientations and facies belts between the Wernecke and Mackenzie Mountains, rectifies paleoflow measurements in Mesoproterozoic and Paleoproterozoic strata, and realigns the orientation of the ca. 1260 Ma Bear River dikes with the Mackenzie dike swarm of northern Canada. This reconstruction also facilitates future studies that relate Neoproterozoic sedimentary and structural patterns to the fragmentation of Rodinia. Finally, this low-latitude pole supports the snowball Earth interpretation of the ca. 717 Ma Sturtian glacial deposits.
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wyoming on the run toward final paleoproterozoic assembly of Laurentia
Geology, 2016Co-Authors: Taylor M Kilian, David A D Evans, Wouter Bleeker, Kevin R Chamberlain, Brian CousensAbstract:Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America’s assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
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Wyoming on the run—Toward final Paleoproterozoic assembly of Laurentia
Geology, 2016Co-Authors: Taylor M Kilian, David A D Evans, Wouter Bleeker, Kevin R Chamberlain, Brian CousensAbstract:Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America’s assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
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paleomagnetism of mesoproterozoic margins of the anabar shield a hypothesized billion year partnership of siberia and northern Laurentia
Precambrian Research, 2016Co-Authors: David A D Evans, Roman Veselovsky, Peter Yu Petrov, A V Shatsillo, V E PavlovAbstract:Abstract Siberia and Laurentia have been suggested as near neighbors in Proterozoic supercontinents Nuna and Rodinia, but paleomagnetic evidence has been sparse and ambiguous. Here we present four new paleomagnetic poles from undeformed Paleo-Mesoproterozoic (lower Riphean) sedimentary rocks and mafic intrusions of the northwestern Anabar uplift in northern Siberia. Combining these results with other Proterozoic data from Siberia and Laurentia, we propose a tight juxtaposition of those two blocks (Euler parameters 77°, 098°, 137° for Anabar to North America) spanning the interval 1.7–0.7 Ga, constituting a long-lived connection that outlasted both the Nuna and Rodinia supercontinental assemblages.
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sutton hotspot resolving ediacaran cambrian tectonics and true polar wander for Laurentia
American Journal of Science, 2011Co-Authors: Ross N Mitchell, David A D Evans, Taylor M Kilian, Timothy D Raub, Wouter Bleeker, Adam C MaloofAbstract:Hotspot tracks represent plate motions relative to mantle sources, and paleomagnetic data from magmatic units along those tracks can quantify motions of those mantle anomalies relative to the Earth's magnetic field and rotational axis. The Ediacaran Period is notable for rapid and large paleomagnetic apparent polar wander (APW) for many continents. Whereas magmatic units attributed to the "Sutton" mantle plume suggest a practically stationary hotspot track, paleolatitudes of Laurentia for that interval vary dramatically; geologic and paleomagnetic data are at odds unless true polar wander (TPW) is invoked to explain a majority of APW. Here we test the plume-TPW hypothesis by generating the predicted Sutton hotspot track for a station- ary plume under a moving plate along the Laurentian margin during the interval from 615 to 530 Ma. Our model is the first to provide a kinematic framework for the extensive large igneous province associated with opening the Iapetus Ocean.
P. J.a. Mccausland - One of the best experts on this subject based on the ideXlab platform.
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precisely locating the ordovician equator in Laurentia
Geology, 2013Co-Authors: David A T Harper, P. J.a. Mccausland, Robin L M Cocks, Christian M O Rasmussen, Peter M SheehanAbstract:The Late Ordovician equatorial zone, like the zone today, had few hurricane-grade storms within 10o of the equator, as emphasized by the preservation of massive-bedded Thalassinoides ichnofacies in a trans-Laurentian belt more than 6000 km long, from the southwestern United States to North Greenland. That belt also includes nonamalgamated shell beds dominated by the brachiopod Proconchidium , which would not have been preserved after hurricane-grade storms. The belt lacks such storm-related sedimentary features as rip-up clasts, hummocky cross-stratification, or large channels. In contrast, other contemporaneous Laurentian Thalassinoides facies and shell beds on either side of the belt have been disturbed by severe storms below fair-weather wave base. The position of the biofacies-defined equatorial belt coincides with the Late Ordovician equator deduced from paleomagnetic data from Laurentia, thus providing both a high-precision equatorial location and an independent test of the geocentric axial dipole hypothesis for that time.
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ediacaran paleogeography of Laurentia paleomagnetism and 40ar 39ar geochronology of the 583ma baie des moutons syenite quebec
Precambrian Research, 2011Co-Authors: P. J.a. Mccausland, Fatim Hankard, Chris M HallAbstract:Abstract Laurentia has unclear paleogeographic relations during the Precambrian–Cambrian transition. Published paleomagnetic results from the late Neoproterozoic (Ediacaran period) imply that Laurentia may have lain at high southerly latitudes during 590–570 Ma, but conflict with other results from 615 Ma and from 565 to 550 Ma which place Laurentia at low paleolatitudes. Paleomagnetic results from other continents over the same Ediacaran period are either sparse or equally ambiguous, giving rise to proposals of unusual geodynamic events such as rapid plate motions and/or large-scale true polar wander or unusual geomagnetic field geometry. To address this problem for Laurentia, we have obtained paleomagnetic and 40Ar–39Ar geochronologic results from the mid-Ediacaran Baie des Moutons (Mutton Bay) syenite, exposed in a failed Iapetan rift along the north shore of the Gulf of St. Lawrence in eastern Quebec. Early and late intrusive units of the Mutton Bay syenite bear hornblende and biotite grains which provide overlapping 40Ar–39Ar plateau ages, indicating that the intrusion cooled rapidly upon emplacement at 583.4 ± 2.0 Ma. Of the 44 sites sampled in the Baie des Moutons syenite, 8 yielded consistent characteristic paleomagnetic ChRM A directions that were easterly and steep (D = 98.6° I = 78.0°; α95 = 6.5°, k = 71.7), retained by PSD magnetite. Six sites from associated feldspar porphyry dykes and late intrusive units gave shallow southeasterly or northwesterly ChRM B directions (D = 163.1°, I = 6.0°; α95 = 21.7°, k = 10.5). Other sites fail to carry a detectable or stable ancient remanence, mainly due to the dominant presence of a viscous remanence carried by MD magnetite. Baked contact tests for both ChRMs A and B at several sites proved inconclusive due to the lack of stable remanence in the host syenite in each case. The eight sites bearing ChRM A directions yield a paleopole at 42.6°N, 332.7E (dp = 11.7°, dm = 12.4°) which is close to three 590–570 Ma previously published paleopoles that also place Laurentia at high paleolatitudes. The six sites with ChRM B directions give a paleopole at location −34.2°N, 321.5E (dp = 10.9°, dm = 21.8°), in good agreement with a published paleopole from the 565 Ma Sept Iles intrusion. The results from this study are enigmatic, providing support for both high and low paleolatitude interpretations of Laurentia's location during the mid-Ediacaran. It is nevertheless possible to propose relative Ediacaran paleogeographies that are consistent with Ediacaran paleopoles and with later Cambro-Ordovician relationships between Laurentia and other circum-Iapetus continents. The candidate paleogeographies do require, however, some contribution from rapid plate motion or true polar wander and also indicate that the opening of the Iapetus Ocean began well before 550 Ma rifting along eastern Laurentia.
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circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from Laurentia
Precambrian Research, 2007Co-Authors: P. J.a. Mccausland, Chris M HallAbstract:Abstract The paleogeography of the Precambrian–Cambrian transition is still poorly known, but is fundamental for understanding Late Neoproterozoic climate extremes and the spatial associations of the rapidly evolving Ediacaran and Cambrian fauna. This period has also been proposed to host unusual geodynamic events such as rapid plate motions and/or the bulk tumbling of the Earth with respect to its spin axis, called true polar wander (TPW). New paleomagnetic and 40Ar–39Ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western Quebec. The Mont Rigaud and the Chatham-Grenville stocks are related to a failed rift arm of the Iapetus Ocean. Both intrusions bear hornblendes which provide overlapping 40Ar–39Ar plateau ages of 533.2 ± 1.1 Ma (Mont Rigaud) and 531.4 ± 3.4 Ma (Chatham-Grenville), corresponding to Early Cambrian (Nemakit-Daldynian) time. Characteristic paleomagnetic (reversed) directions from both intrusions are easterly and shallow (D = 89.3°, I = 30.1°; α95 = 6.2, k = 38.8; N = 15 sites), and the Mont Rigaud intrusion also carries antipodal normal-polarity directions. The combined result yields a north paleopole at location 11.9°S 184.6°E; dp = 3.8° dm = 6.9°, placing the Montreal region at 16 ± 4°S paleolatitude in the Early Cambrian, in good agreement with Late Ediacaran and Middle to Late Cambrian paleomagnetic results from Laurentia. Laurentia likely resided at low southerly paleolatitudes throughout the Cambrian, ruling out a major inertial-interchange TPW event for Early to Middle Cambrian time. A comparison of Late Ediacaran to Middle Cambrian paleomagnetic results from Laurentia, Baltica, Siberia and elements of West Gondwana implies that by Early Cambrian time a large paleolatitudinal gap existed between Laurentia and its presumed conjugate rift margins, thus placing a wide Iapetus Ocean between them. The position of Baltica within the circum-Iapetus paleogeography is contentious, due to differing interpretations of the few paleomagnetic results. During Ediacaran time, Laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 Ma to the south polar region at 590–575 Ma, and then rapidly back to low latitude by 565–550 Ma. The large and perhaps rapid apparent polar wander of Laurentia and other continents in the Ediacaran remains enigmatic and controversial.
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Opening Iapetus: Constraints from the Laurentian margin in Newfoundland
Bulletin of the Geological Society of America, 2001Co-Authors: Peter A Cawood, P. J.a. Mccausland, G. R. DunningAbstract:Late Neoproterozoic to Early Cambrian geologic, geochronologic, and paleomagnetic data from along the Iapetus margin of Laurentia may be reconciled,within a multistage rift history that involved an initial separation of Laurentia from the west Gondwana cratons ca. 570 Ma, followed by rifting of a further block or blocks from Laurentia ca 540-535 Ma into an already open Iapetus Ocean to establish the main passive-margin sequence in the Appalachians. Paleomagnetic data suggest that Laurentia rifted from Amazonia-Rio de la Plata cratons and began its northward movement ca. 570 Ma to produce a wide Iapetus Ocean by 550 Ma. Geologic data from the Newfoundland segment of the Laurentian margin provide evidence for a rift-drift transition ca. 540-535 Ma, as constrained by the youngest rift-related magmatism at 550.5 (+3)/(-2) Ma (U/Pb zircon) for the Skinner Cove Formation and 555 (+3)/(-5) Ma for the Lady Slipper pluton, and a late Early Cambrian age of ca, 525-520 Ma for the oldest drift-related sedimentation. Rifting between the Laurentia and the west Gondwana cratons was probably distributed among multiple rift systems that fostered the production of a number of terranes (such as the Argentine Precordillera, Oaxacan) as well as the Iapetus Ocean. Development of Laurentian-derived Iapetan terranes during the final breakout of Laurentia from Rodinia may have been facilitated by preexisting 760-700 Ma rift weaknesses and apparently rapidly changing plate vectors during latest Neoproterozoic time.