Search

Paleomagnetism of the North American Midcontinent Rift provides a robust paleogeographic record of Laurentia (cratonic North America) from ca. 1110 to 1070 Ma, revealing rapid equatorward motion of the continent throughout rift magmatism. Existing age and paleomagnetic constraints on the youngest rift volcanic and sedimentary rocks have been interpreted to record a slowdown of this motion as rifting waned. We present new paleomagnetic and geochronologic data from the ca. 1090-1083 Ma "late-stage" rift volcanic rocks exposed as the Lake Shore Traps (Michigan), the Schroeder-Lutsen basalts (Minnesota), and the Michipicoten Island Formation (Ontario). The paleomagnetic data allow for the development of paleomagnetic poles for the Schroeder-Lutsen basalts (187.8°E, 27.1°N; A95 = 3.0°, N = 50) and the Michipicoten Island Formation (174.7°E, 17.0°N; A95 = 4.4°, N = 23). Temporal constraints on late-stage paleomagnetic poles are provided by high-precision, 206Pb-238U zircon dates from a Lake Shore Traps andesite (1085.57 ± 0.25 Ma; 2σ internal errors), a Michipicoten Island Formation tuff (1084.35 ± 0.20 Ma) and rhyolite (1083.52 ± 0.23 Ma), and a Silver Bay aplitic dike from the Beaver Bay Complex (1091.61 ± 0.14 Ma), which is overlain by the Schroeder-Lutsen basalt flows. These Michipicoten Island Formation dates are the youngest yet obtained from Midcontinent Rift volcanic rocks and indicate that rift magmatism was active for at least 25 m.y. The addition of these late-stage paleomagnetic poles to the Laurentian apparent polar wander path suggests that rapid motion of Laurentia continued throughout the entirety of rift volcanism.

A mid-Permian (Guadalupian epoch) extinction event at approximately 260 Ma has been mooted for two decades. This is based primarily on invertebrate biostratigraphy of Guadalupian-Lopingian marine carbonate platforms in southern China, which are temporally constrained by correlation to the associated Emeishan Large Igneous Province (LIP). Despite attempts to identify a similar biodiversity crisis in the terrestrial realm, the low resolution of mid-Permian tetrapod biostratigraphy and a lack of robust geochronological constraints have until now hampered both the correlation and quantification of terrestrial extinctions. Here we present an extensive compilation of tetrapod-stratigraphic data analysed by the constrained optimization (CONOP) algorithm that reveals a significant extinction event among tetrapods within the lower Beaufort Group of the Karoo Basin, South Africa, in the latest Capitanian. Our fossil dataset reveals a 74-80% loss of generic richness between the upper Tapinocephalus Assemblage Zone (AZ) and the mid-Pristerognathus AZ that is temporally constrained by a U-Pb zircon date (CA-TIMS method) of 260.259 ± 0.081 Ma from a tuff near the top of the Tapinocephalus AZ. This strengthens the biochronology of the Permian Beaufort Group and supports the existence of a mid-Permian mass extinction event on land near the end of the Guadalupian. Our results permit a temporal association between the extinction of dinocephalian therapsids and the LIP volcanism at Emeishan, as well as the marine end-Guadalupian extinctions.

Three Orosirian basins and associated foreland thrust-fold belts are preserved on the margins of the Slave craton. All three are related to orogenic belts where oceans opened and later closed, uniting new crustal partners. The Great Slave basin differs from the Kilhigok and Coronation basins in ways that have defied explanation. It lacks a passive margin sequence and hosts two discrete igneous suites, separated by large-scale thrusting, that occurred well after the adjacent paleocean had closed. Here we report U–Pb zircon geochronology by chemical abrasion isotope dilution thermal ionization mass spectrometry for a member of each suite to constrain the age and origin of postcollisional thrusting. A widespread pulse of mainly phreatic alkaline volcanism, coeval with renewed foredeep flexure, occurred at 1889.0 ± 0.7 Ma (2σ internal error). A quartz-monzodiorite body, one of a belt-parallel chain of laccoliths that postdate thrusting, was emplaced at 1866.9 ± 0.9 Ma. These ages bracket renewed foredeep sedimentation and thrusting that telescoped major facies zones and was rooted within the basin. The older age is 70 and 30−60 Myr younger than collision in the Thelon and Taltson orogens, respectively. We attribute postcollisional thrusting and foredeep subsidence to "eduction"—the upward and outward ejection of partly subducted crust—and postulate that the top of the ejected wedge was a normal-sense detachment fault projecting beneath the Nonacho basin. We infer that eduction was triggered by slab failure, producing alkaline volcanism, and ended with delamination and laccolith emplacement. Eduction was facilitated by tradewind-driven erosion. Delamination was enabled by crustal transfer to the educted wedge, reducing footwall buoyancy. Slab failure and/or delamination removed the passive margin. [ABSTRACT FROM AUTHOR]

The 1.9 Ga Coronation 'geosyncline' to the west of Slave craton was among the first Precambrian continental margins to be identified, but its duration as a passive margin has long been uncertain. We report a new U-Pb (isotope dilution--thermal ionization mass spectrometry (ID-TIMS)) [sup.207]pb/[sup.206]Pb date of 2014.32 ± 0.89 Ma for zircons from a felsic pyroclastic rock at the top of the Vaillant basalt, which underlies the passive margin sequence (Epworth Group) at the allochthonous continental slope. A sandstone tongue within the basalt yields Paleoproterozoic (mostly synvolcanic) and Mesoarchean detrital zircon dates, of which the latter are compatible with derivation from the Slave craton. In contrast, detrital zircon grains from the Zephyr arkose in the accreted Hottah terrane have Paleoproterozoic and Neoarchean dates. The latter cluster tightly at 2576 Ma, indistinguishable from igneous zircon dates reported here from the Badlands granite, which is faulted against the Vaillant basalt and underlying Drill arkose. We interpret these data to indicate that Badlands granite belongs to the hanging wall of the collisional geosuture between Hottah terrane and the Slave margin, represented by the Drill-Vaillant rift assemblage. If 2014.32 ± 0.89 Ma dates the rift-to-drift transition and 1882.50 ± 0.95 Ma (revised from 1882 ± 4 Ma) the arrival of the passive margin at the trench bordering the Hottah terrane, the duration of the Coronation passive margin was ~132 million years, close to the mean age of extinct Phanerozoic passive margins of ~134 million years (see Bradley 2008). Le << geeosynclinal >> Coronation, 1,9 Ga, a l'ouest du craton des Esclaves a ete parmi les premieres bordures continentales precambriennes a etre identifieees, mais le temps qu'il a servi de bordure passive demeure incertain depuis longtemps. Nous signalons une nouvelle datation U-Pb (ID-TIMS) [sup.207]Pb/[sup.206]Pb de 2014,32 [+ or -] 0,89 Ma sur des zircons provenant d'une roche pyroclastique felsique au sommet du basalte Vaillant, lequel est sous la sequence de bordure passive (Groupe d'Epworth) a la pente continentale allochtone. Une langue de gres dans le basalte a donne, pour des zircons detritiques, des ages Paleoproterozolque (surtout synvolcanique) et Mesoarcheeen; ce qui est, pour le Mesoarcheen, compatible avec une provenance du craton des Esclaves. Par contre, des grains de zircons detritiques provenant de l'arkose Zephyr dans le terrane accrete de Hottah ont donne des dates de Paleoproterozolque et de Neoarcheen. Ces dernieres dates sont regroupees de maniere serreea 2576 Ma et ne peuvent etre distinguees des dates de zircons ignes rapportees ici et provenant du granite Badlands, lequel est juxtapose; au basalte Vaillant et a l'arkose Drill par une faille. Selon nous, ces donnees indiquent que le granite Badlands appartient a l'eponte superieure de la geosuture de collision entre le terrane de Hottah et la bordure des Esclaves, representee par l'assemblage de distension Drill-Vaillant. Si 2014,32 [+ or -] 0,89 Ma constitue la date de la transition de distension a derive et si 1882,50 ± 0,95 Ma (revise de 1882 ± 4 Ma) represente l'arriveee de la bordure passive a la fosse en bordure du terrane de Hottah, alors la bordure passive Coronation aurait dure ~132 Ma, ce qui est pres de l'age moyen des bordures passives phanerozoiques disparues, soit ~134 Ma (voir Bradley 2008). [Traduit par la Redaction], Introduction The first pre-Mesozoic succession to be identified as a former continental terrace was the Neoproterozoic-Cambrian Adelaide 'geosyncline' of South Australia (Sprigg 1952). In the then prevailing view that ocean [...]

The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversification was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tommotian) records of [[delta].sup.13] [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] variability from Siberia, Mongolia, and China onto a Moroccan U/Pb--[[delta].sup.13][MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] age model constrained by five U/Pb ages from interbedded volcanic ashes. The [[delta].sup.13] [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] correlations ignore fossil tie points, so we assume synchroneity in [[delta].sup.13]C trends rather than synchroneity in first appearances of animal taxa. We present new [[delta].sup.13][C.sub.org], [sup.87]Sr/sup.86]Sr, uranium, and vanadium data from the same carbonate samples that define the Moroccan [[delta].sup.13] [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] curve. The result is a new absolute time line for first appearances of skeletal animals and for changes in the carbon, strontium, and redox chemistry of the ocean during the Nemakit-Daldynian and Tommotian ages at the beginning of the Cambrian. The time fine suggests that the diversification of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil first appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540-538 Ma), a larger pulse in the mid- to late Nemakit-Daldynian (ca. 534-530 Ma), and a moderate pulse in the Tommotian (ca. 524-522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydrothermal flux of Sr. doi: 10.1130/B30346.1

The record of metamorphism, deformation, and melting in the North Cascades continental arc provides insights into the timing and mechanisms of extensional un-roofing that followed crustal thickening. The Skagit Gneiss (North Cascades) is composed of variably deformed 90-45 Ma tonalitic to granitic intrusive rocks. These lithologies and metasedimentary rocks are migmatitic. Zircon and monazite from different textural varieties of migmatite from three outcrops along an east-west transect were analyzed using the isotope dilution-thermal ionization mass spectrometry (ID-TIMS) technique. The data reveal two main migmatization pulses: (1) 68-63 Ma and (2) 53-47 Ma. In the westernmost locality, leucosome zircon yields group 1 dates, and mesosome (sillimanite-garnet-biotite gneiss) zircon is as young as 61 Ma. Leucosome zircon in the easternmost outcrop yields only group 2 dates, and biotite gneiss contains 47 Ma zircon. The third outcrop has leucosome dates from both groups: 65 Ma and 53 Ma. Monazite from leucosomes and mesosomes likely records the timing of prograde metamorphism (ca. 69 Ma) and later synkinematic [+ or -] fluid-mediated growth (49-46 Ma). These results indicate that partial melting occurred during 71-61 Ma metamorphism and lasted through 46 Ma deformation associated with exhumation. The results and field observations suggest the presence of ductilely deforming crust from 68 to 46 Ma. The similarity between the youngest U-Pb dates in the migmatites and cooling and basin-filling ages suggests a link between ductile and brittle processes over a range of structural levels in the arc, from a zone of crustal melting, flow, and migmatite crystallization to Earth's surface. doi: 10.1130/B30060.1

The Nemakit-Daldynian--Tommotian (ND-T) boundary marks the first appearance of metazoan reefs and calcite biomineralizers and is associated with the largest [delta][sup.13]C shift during the Phanerozoic Eon. Biological transitions in Earth history are often accompanied by excursions in the carbon isotopic composition ([delta][sup.13]C) of the ocean, where [delta][sup.13]C variability is interpreted to reflect changes in the global carbon cycle. The duration and thus rate of these [delta][sup.13]C anomalies are rarely known, making it difficult to constrain their possible causes and their relationship, if any, to biologic transitions. We report sedimentological and [delta][sup.13]C data from a new 2.5-km-thick section that spans the early Cambrian evolutionary 'explosion' in the Moroccan Anti-Atlas Mountains. Three new zircon [sup.206]Pb-[sup.238]U ages from tufts within the stratigraphy constrain the timing of the ND-T boundary to 524.84 [+ or -] 0.09 Ma. Two of the tufts exactly bracket the ND-T transition and constrain the duration of the -8[per thousand] [delta][sup.13]C shift to 506 [+ or -] 126 k.y. With a simple box model, we explore a range of geochemical processes that could account for such a rapid ND-T [delta][sup.13]C shift, and conclude that metamorphic and/or volcanic fluxes of carbon may have been sustained at levels 4-16 times higher than today for millions of years. doi: 10.1130/G30726.1

The Wopmay orogen is a Paleoproterozoic orogenic belt formed in part by the accretion of Hottah terrane, an east-facing continental magmatic arc, to the western margin of the Archean Slave craton at ca. 1.88 Ga. The arc-continent collision was responsible for the Calderian orogeny. Just prior to the collision, arc volcanism of the Hottah terrane bad migrated trenchward and changed in composition from an aphyric calc-alkaline to a bimodal tholeiitic suite. The change in magmatism, along with subsidence and consequent high-temperature--low-pressure metamorphism of rocks on the upper plate, is attributed to extension and asthenospheric upwelling during rollback of the lower plate. The upwelling led to regional heating of the crust, melting, and generation of the metaluminous to peraluminous Hepburn intrusive suite within the arc just prior to accretion of Hottah terrane onto the Slave craton. During the collision, a 200-km-long, orogen-parallel swarm of mafic sills was intruded into the lower plate and trench-fill sediments at the passive-margin shelf-slope break. The mafic magmatism may have been induced by slab breakoff. The Calderian orogeny provides an excellent example of the magmatic history of an arc-continent collision. The Calderian orogeny was short-lived. Within a few million years, an east-dipping subduction zone formed outboard of the accreted terrane, creating volcanic rocks that were erupted atop the eroded Hottah-Slave collision zone. The short duration of the Calderian orogeny seems typical of arc continent collisions, perhaps because the attenuated arc crust, coupled with the extended crust of the lower plate, never generates severely overthickened crust. Rise of asthenospheric mantle due to slab breakoff generates rapid uplift, erosion, and collapse of the mountains built during collision. doi: 10.1130/B26521.1