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Paleogeographic 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. [ABSTRACT FROM AUTHOR]

Lonar Crater, India, is one of the youngest and best preserved impact structures on Earth. The 1.88-km-diameter simple crater formed entirely within the Deccan traps, making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the Moon. In this study, we present a meter-scale--resolution digital elevation model, geological map of Lonar Crater and the surrounding area, and radiocarbon ages for histosols beneath the distal ejecta. Impact-related deformation of the target rock consists of upturned basalt flows in the upper crater walls and recumbent folding around rim concentric, subhorizontal, non-cylindrical fold axes at the crater rim. The rim-fold hinge is preserved around 10%-15% of the crater. Although tearing in the rim-fold is inferred from field and paleomagnetic observations, no tear faults are identified, indicating that large displacements in the crater walls are not characteristic of small craters in basalt. One significant normal fault structure is observed in the crater wall that offsets slightly older layer-parallel slip faults. There is little fluvial erosion of the continuous ejecta blanket. Portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules, in particular in the eastern and western rim, as well as in the depression north of the crater known as Little Lonar. The emplacement of the continuous ejecta blanket can be likened to a radial ground-hugging debris flow, based on the preserved thickness distribution of the ejecta, the efficient exchange of clasts between the ejecta flow and the underlying histosol, and the lack of sorting and stratification in the bulk of the ejecta. The ejecta profile is thickened at the distal edge and similar to fluidized ejecta structures observed on Mars. [ABSTRACT FROM AUTHOR]

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 Sturtianage (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 lowtemperature 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. [ABSTRACT FROM AUTHOR]

We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5-6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50° shifts in paleomagnetic direction are coincident with equally abrupt shifts in δ13C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fields, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent's changing position relative to Earth's spin axis. [ABSTRACT FROM AUTHOR]