Worldwide database for magnetostratigraphy available

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95549/1/eost11864.pd

Archean geodynamics : Ephemeral supercontinents or long-lived supercratons

Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they were pieces of some ancestral landmass(es). The idea that such an ancient continental assembly represents an Archean supercontinent has been proposed but remains to be justified. Starkly contrasting geological records between different clans of cratons have inspired an alternative hypothesis where cratons were clustered in multiple, separate "supercratons." A new ca. 2.62 Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible with either two successive but ephemeral supercontinents or two long-lived supercratons across the Archean-Proterozoic transition. Neither interpretation supports the existence of a single, long-lived supercontinent, suggesting that Archean geodynamics were fundamentally different from subsequent times (Proterozoic to present), which were influenced largely by supercontinent cycles.Peer reviewe

Paleomagnetism of Cryogenian Kitoi mafic dykes in South Siberia: Implications for Neoproterozoic paleogeography

We present a new paleomagnetic pole of 1.1°N, 22.4°E, A95 = 7.4° from the 760 Ma gabbro-dolerite Kitoi dykes located in the southern part of the Siberian Craton. The pole is supported by contact tests and suggests closer position of Siberia relative to Laurentia at 760 Ma than in Mesoproterozoic. We propose that this closer configuration was achieved by dextral transpressive motion of Siberia relative to Laurentia between 780 and 760 Ma. This motion was probably initiated at the first stage of the Rodinia breakup and is coeval with the 780 Ma Gunbarrel magmatic event of the western Canadian shield

The magnificent seven : A proposal for modest revision of the Van der Voo (1990) quality index

Thirty years ago, Rob Van der Voo proposed an elegant and simple system for evaluating the quality of paleomagnetic data. As a second-year Ph.D. student, the lead author remembers Rob waxing philosophical about the need to have an appropriate, but not overly rigid evaluation system. The end result was a 7-point system that assigned a (1) or (0) for any paleomagnetic result based on objective criteria. The goal was never to reject or blindly accept any particular result, but merely to indicate the degree of quality for any paleomagnetic pole. At the time, the global paleomagnetic database was burgeoning and it was deemed useful to rank older paleo magnetic results with the newer data being developed in modern laboratories. Van der Voo's, 1990 paper launched a silent revolution in paleomagnetism. Researchers began to evaluate their data against those seven criteria with the anticipation that reviewers would be similarly critical. Today, paleomagnetism is a mature science. Our methods, analyses, and results are more sophisticated than they were 30 years ago. Therefore, we feel it is appropriate to revisit the Van der Voo (1990) criteria in light of those developments. We hope to honor the intention of the original paper by keeping the criteria simple and easy to evaluate while also acknowledging the advances in science. This paper aims to update the criteria and modernize the process. We base our changes on advances in paleomagnetism and geochronology with a faithful adherence to the simplicity of the original publication. We offer the "Reliability" or "R" index as the next generation of the Van der Voo "Quality" or "Q" index. The new R-criteria evaluate seven different information items for each paleomagnetic pole including age, statistical requirements, identification of magnetic carriers, field tests, structural integrity, presence of reversals and an evaluation for possible remagnetization.Peer reviewe

Linking collisional and accretionary orogens during Rodinia assembly and breakup: Implications for models of supercontinent cycles

Periodic assembly and dispersal of continental fragments has been a characteristic of the solid Earth for much of its history. Geodynamic drivers of this cyclic activity are inferred to be either top-down processes related to near surface lithospheric stresses at plate boundaries or bottom-up processes related to mantle convection and, in particular, mantle plumes, or some combination of the two. Analysis of the geological history of Rodinian crustal blocks suggests that internal rifting and breakup of the supercontinent were linked to the initiation of subduction and development of accretionary orogens around its periphery. Thus, breakup was a top-down instigated process. The locus of convergence was initially around north-eastern and northern Laurentia in the early Neoproterozoic before extending to outboard of Amazonia and Africa, including Avalonia–Cadomia, and arcs outboard of Siberia and eastern to northern Baltica in the mid-Neoproterozoic (~760 Ma). The duration of subduction around the periphery of Rodinia coincides with the interval of lithospheric extension within the supercontinent, including the opening of the proto-Pacific at ca. 760 Ma and the commencement of rifting in east Laurentia. Final development of passive margin successions around Laurentia, Baltica and Siberia was not completed until the late Neoproterozoic to early Paleozoic (ca. 570–530 Ma), which corresponds with the termination of convergent plate interactions that gave rise to Gondwana and the consequent relocation of subduction zones to the periphery of this supercontinent. The temporal link between external subduction and internal extension suggests that breakup was initiated by a top-down process driven by accretionary tectonics along the periphery of the supercontinent. Plume-related magmatism may be present at specific times and in specific places during breakup but is not the prime driving force. Comparison of the Rodinia record of continental assembly and dispersal with that for Nuna, Gondwana and Pangea suggests grouping into two supercycles in which Nuna and Gondwana underwent only partial or no break-up phase prior to their incorporation into Rodinia and Pangea respectively. It was only after this final phase of assembly that the supercontinents then underwent full dispersal

Palaeoproterozoic to Eoarchaean crustal growth in southern Siberia: a Nd-isotope synthesis

Nd-isotope analyses from 114 rock samples are reported from the southern part of the Siberian craton to establish a first-order crustal formation scheme for the region. The Nd-isotopedata show considerable variability within and among different cratonic units. In many cases this variability reflects differing degrees of mixing between juvenile and older (up to Eoarchaean) crustal components. The fragments of Palaeoproterozoic juvenile crust within the studied segment of the Siberian craton margin have Nd-model ages of 2.0-2.3 Ga. Voluminous Palaeoproterozoicgranites ( 1.85 Ga) were intruded into cratonic fragments and suture zones. These granites mark the stabilization of the southern Siberian craton. The complexity in the Nd data indicatea long history of crustal development, extending from the Eoarchaean to the Palaeoproterozoiceras, which is interpreted to reflect the amalgamation of distinct Archaean crustal fragments, with differing histories, during Palaeoproterozoic accretion at 1.9-2.0 Ga and subsequent cratonic stabilization at 1.85 Ga. Such a model temporally coincides with important orogenic events on nearly every continent and suggests that the Siberian craton participated in the formation of a Palaeoproterozoic supercontinent at around 1.9 Ga

Integrated stratigraphic correlation of Upper Devonian platform-to-basin carbonate sequences, Lennard Shelf, Canning Basin, Western Australia: advances in carbonate margin-to-slope sequence stratigraphy and stacking patterns

High-resolution, time-significant correlations are integral to meaningful stratigraphic frameworks in depositional systems, but may be difficult to achieve using traditional sequence stratigraphic or biostratigraphic approaches alone, particularly in geologically complex settings. In steep, reefal carbonate margin-to-slope systems, such correlations are essential to unravel shelf-to-basin transitions, characterize strike variability, and develop predictive sequence stratigraphic models – concepts which are currently poorly understood in these heterogeneous settings. The Canning Basin Chronostratigraphy Project (CBCP) integrates multiple independent datasets (including biostratigraphy, magnetostratigraphy, stable isotope chemostratigraphy, and sequence stratigraphy) extracted from Upper Devonian (Frasnian and Famennian) reefal platform exposures along the Lennard Shelf, Canning Basin, Western Australia. These were used to generate a well-constrained stratigraphic framework and shelf-to-basin composite reconstruction of the carbonate system. The resultant integrated framework allows for unprecedented analysis of carbonate margin-to-slope heterogeneity, depositional architecture, and sequence stratigraphy along the Lennard Shelf. Systems tract architecture, facies partitioning, and stacking patterns of margin to lower-slope environments were assessed for six composite-scale sequences that form part of a transgressive-to-regressive supersequence and span the Frasnian-Famennian (F-F) biotic crisis. Variations are apparent in margin styles, foreslope facies proportions, dominant resedimentation processes, downslope contributing sediment factories, and vertical rock successions, related to hierarchical accommodation signals and ecological changes associated with F-F boundary. We present these results in the form of carbonate margin-to-basin sequence stratigraphic models and associations that link seismic-scale architecture to fine-scale facies heterogeneity. These models provide a predictive foundation for characterization of steep-sided flanks of reefal carbonate platform systems that is useful for both industry and academia. This study emphasizes the utility of an integrated stratigraphic approach and the insights gained from better-constrained facies and stratal architecture analysis; insights that were not achievable with traditional sequence stratigraphic or biostratigraphic techniques alone

Unraveling the geometry of the New England oroclines (eastern Australia): Constraints from magnetic fabrics

The southern New England Orogen (NEO) in eastern Australia is characterized by tight curvatures(oroclines), but the exact geometry of the oroclines and their kinematic evolution are controversial. Here we present new data on the anisotropy ofmagnetic susceptibility (AMS), which provide a petrofabric proxy for the finite strain associated with the oroclines.We focus on a series of preoroclinal Devonian-Carboniferous fore-arc basin rocks, which are aligned parallel to the oroclinal structure, and by examining structural domains, we test whether or not the magnetic fabric is consistent with the strain axes. AMS data show a first-order consistency with the shape of the oroclines, characterized, in most of structural domains, by subparallelism between magnetic lineations, “structural axis” and bedding. With the exception of the Gresford and west Hastings domains, our results are relatively consistent with the existence of the Manning and Nambucca (Hastings) Oroclines. Reconstruction of magnetic lineations to a prerotation (i.e., pre–late Carboniferous) stage, considering available paleomagnetic results, yields a consistent and rather rectilinear NE-SW predeformation fore-arc basin. This supports the validity of AMS as a strain proxy in complex orogens, such as the NEO. In the Hastings Block, magnetic lineations are suborthogonal to bedding, possibly indicating a different deformational historywith respect to the rest of the NEO