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Now showing items 1 - 16 of 3823

  • Constraining Absolute Plate Motions Since the Triassic

    Tetley, Michael G.   Williams, Simon E.   Gurnis, Michael   Flament, Nicolas   Muller, R. Dietmar  

    The absolute motion of tectonic plates since Pangea can be derived from observations of hotspot trails, paleomagnetism, or seismic tomography. However, fitting observations is typically carried out in isolation without consideration for the fit to unused data or whether the resulting plate motions are geodynamically plausible. Through the joint evaluation of global hotspot track observations (for times <80Ma), first-order estimates of net lithospheric rotation (NLR), and parameter estimation for paleo-trench migration (TM), we present a suite of geodynamically consistent, data-optimized global absolute reference frames from 220Ma to the present. Each absolute plate motion (APM) model was evaluated against six published APM models, together incorporating the full range of primary data constraints. Model performance for published and new models was quantified through a standard statistical analyses using three key diagnostic global metrics: root-mean square plate velocities, NLR characteristics, and TM behavior. Additionally, models were assessed for consistency with published global paleomagnetic data and for ages <80Ma for predicted relative hotspot motion, track geometry, and time dependence. Optimized APM models demonstrated significantly improved global fit with geological and geophysical observations while performing consistently with geodynamic constraints. Critically, APM models derived by limiting average rates of NLR to similar to 0.05 degrees/Myr and absolute TM velocities to similar to 27-mm/year fit geological observations including hotspot tracks. This suggests that this range of NLR and TM estimates may be appropriate for Earth over the last 220Myr, providing a key step toward the practical integration of numerical geodynamics into plate tectonic reconstructions.
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  • Rift and plate boundary evolution across two supercontinent cycles

    Merdith, Andrew S.   Williams, Simon E.   Brune, Sascha   Collins, Alan S.   Mueller, R. Dietmar  

    The extent of continental rifts and subduction zones through deep geological time provides insights into the mechanisms behind supercontinent cycles and the long term evolution of the mantle. However, previous compilations have stopped short of mapping the locations of rifts and subduction zones continuously since the Neoproterozoic and within a self-consistent plate kinematic framework. Using recently published plate models with continuously closing boundaries for the Neoproterozoic and Phanerozoic, we estimate how rift and peri-continental subduction length vary from 1 Ga to present and test hypotheses pertaining to the supercontinent cycle and supercontinent breakup. We extract measures of continental perimeter-to-area ratio as a proxy for the existence of a supercontinent, where during times of supercontinent existence the perimeter-to-area ratio should be low, and during assembly and dispersal it should be high. The amalgamation of Gondwana is clearly represented by changes in the length of peri-continental subduction and the breakup of Rodinia and Pangea by changes in rift lengths. The assembly of Pangea is not clearly defined using plate boundary lengths, likely because its formation resulted from the collision of only two large continents. Instead the assembly of Gondwana (ca. 520 Ma) marks the most prominent change in arc length and perimeter-to-area ratio during the last billion years suggesting that Gondwana during the Early Palaeozoic could explicitly be considered part of a Phanerozoic supercontinent. Consequently, the traditional understanding of the supercontinent cycle, in terms of supercontinent existence for short periods of time before dispersal and re-accretion, may be inadequate to fully describe the cycle. Instead, either a two-stage supercontinent cycle could be a more appropriate concept, or alternatively the time period of 1 to 0 Ga has to be considered as being dominated by supercontinent existence, with brief periods of dispersal and amalgamation.
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  • Rift and plate boundary evolution across two supercontinent cycles

    Merdith, Andrew S.   Williams, Simon E.   Brune, Sascha   Collins, Alan S.   Müller, R. Dietmar  

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  • Kinematic constraints on the Rodinia to Gondwana transition

    Merdith, Andrew S.   Williams, Simon E.   Mueller, R. Dietmar   Collins, Alan S.  

    Earth's plate tectonic history during the breakup of the supercontinent Pangea is well constrained from the seafloor spreading record, but evolving plate configurations during older supercontinent cycles are much less well understood. A relative paucity of available palaeomagnetic and geological data for deep time reconstructions necessitates innovative approaches to help discriminate between competing plate configurations. More difficult is tracing the journeys of individual continents during the amalgamation and breakup of supercontinents. Typically, deep-time reconstructions are built using absolute motions defined by palaeomagnetic data, and do not consider the kinematics of relative motions between plates, even for occasions where they are thought to be 'plate-pairs', either rifting apart leading to the formation of conjugate passive margins separated by a new ocean basin, or brought together by collision and orogenesis. Here, we use open-source software tools (GPlates/pyGPlates) to assess quantitative plate kinematics inherent within alternative reconstructions, such as rates of relative plate motion. We analyse the Rodinia-Gondwana transition during the Neoproterozoic, investigating the proposed Australia-Laurentia configurations during Rodinia, and the motion of India colliding with Gondwana. We find that earlier rifting times provide more optimal kinematic results. The AUSWUS and AUSMEX configurations with rifting at 800 Ma are the most kinematically supported configurations for Australia and Laurentia (average rates of 57 and 64 mm/a respectively), and angular rotation of similar to 1.4 degrees/Ma, compared to a SWEAT configuration (average spreading rate similar to 76 mm/a) and Missing-Link configuration (similar to 90 mm/a). Later rifting, at, or after, 725 Ma necessitates unreasonably high spreading rates of >130 mm/a for AUSWUS and AUSMEX and similar to 150 mm/a for SWEAT and Missing-Link. Using motion paths and convergence rates, we create a kinematically reasonable (convergence below 70 mm/a) tectonic model that is built upon a front-on collision of India with Gondwana, while also incorporating sinistral strike-slip motion against Australia and East Antarctica. We use this simple tectonic model to refine a global model for the breakup of western Rodinia and the transition to eastern Gondwana. (C) 2017 Elsevier B.V. All rights reserved.
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  • Oblique rifting: the rule, not the exception

    Brune, Sascha   Williams, Simon E.   Mueller, R. Dietmar  

    Movements of tectonic plates often induce oblique deformation at divergent plate boundaries. This is in striking contrast with traditional conceptual models of rifting and rifted margin formation, which often assume 2-D deformation where the rift velocity is oriented perpendicular to the plate boundary. Here we quantify the validity of this assumption by analysing the kinematics of major continent-scale rift systems in a global plate tectonic reconstruction from the onset of Pangea breakup until the present day. We evaluate rift obliquity by joint examination of relative extension velocity and local rift trend using the script-based plate reconstruction software pyGPlates. Our results show that the global mean rift obliquity since 230 Ma amounts to 34 degrees with a standard deviation of 24 degrees, using the convention that the angle of obliquity is spanned by extension direction and rift trend normal. We find that more than similar to 70 % of all rift segments exceeded an obliquity of 20 degrees demonstrating that oblique rifting should be considered the rule, not the exception. In many cases, rift obliquity and extension velocity increase during rift evolution (e.g. Australia-Antarctica, Gulf of California, South Atlantic, India-Antarctica), which suggests an underlying geodynamic correlation via obliquity-dependent rift strength. Oblique rifting produces 3-D stress and strain fields that cannot be accounted for in simplified 2-D plane strain analysis. We therefore highlight the importance of 3-D approaches in modelling, surveying, and interpretation of most rift segments on Earth where oblique rifting is the dominant mode of deformation.
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  • East African topography and volcanism explained by a single,migrating plume

    Hassan, Rakib   Williams, Simon E.   Gurnis, Michael   Mueller, Dietmar  

    Anomalous topographic swells and Cenozoic volcanism in east Africa have been associated with mantle plumes. Several models involving one or more fixed plumes beneath the northeastward migrating African plate have been suggested to explain the space-time distribution of magmatism in east Africa. We devise paleogeographically constrained global models of mantle convection and, based on the evolution of flow in the deepest lower mantle, show that the Afar plume migrated southward throughout its lifetime. The models suggest that the mobile Afar plume provides a dynamically consistent explanation for the spatial extent of the southward propagation of the east African rift system (EARS), which is difficult to explain by the northeastward migration of Africa over one or more fixed plumes alone, over the last approximate to 45 Myr. We further show that the age-progression of volcanism associated with the southward propagation of EARS is consistent with the apparent surface hotspot motion that results from southward motion of the modelled Afar plume beneath the northeastward migrating African plate. The models suggest that the Afar plume became weaker as it migrated southwards, consistent with trends observed in the geochemical record.
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  • Global patterns of Earth's dynamic topography since the Jurassic

    Rubey, Michael   Brune, Sascha   Heine, Christian   Davies, David Rhodri   Williams, Simon E.   Müller, Ralph Dietmar  

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  • Abrupt plate accelerations shape rifted continental margins

    Williams, Simon E.   Butterworth, Nathaniel P.   Mueller, R. Dietmar  

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution(1,2), yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength-velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas(3) and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.
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  • A reconstruction of the Eurekan Orogeny incorporating deformation constraints

    Gion, Austin M.   Williams, Simon E.   Mueller, R. Dietmar  

    The Eurekan Orogeny records Paleogene convergence between Greenland and the Canadian Arctic. The complexity of the region, well represented by the disputed magnitude of Cenozoic sinistral displacement of Greenland relative to Ellesmere Island, stems from the simultaneous evolution of multiple tectonic regimes, as well as overprinting of later tectonic activity. Presented here is a plate model of regional crustal deformation constructed with the interactive GPlates software that enables an evaluation of previous reconstructions of the Eurekan Orogeny. This model is built upon a synthesis of published geological and geophysical data and their interpretations. It incorporates two phases of deformation from similar to 63 to 35 Ma. Phase 1, similar to 63 to similar to 55 Ma, involves similar to 85 km of Paleocene extension between Ellesmere and Devon Island, extension of similar to 20 km between Axel Heiberg and Ellesmere Island, and similar to 85 km of sinistral strike slip along the Nares Strait/Judge Daly Fault System, matching a range of 50-100 km indicated by the offset of marker beds, facies contacts, and platform margins between the conjugate Greenland and Ellesmere Island margins. Phase 2, similar to 55 to 35 Ma, results in total east-west compression of similar to 30 km and similar to 200 km of north-south compression across Ellesmere Island. This model confirms, for the first time, that key observations from subregions deformed by the Eurekan Orogeny are compatible on a broad scale. We have also identified potential problem areas in southwestern Ellesmere Island that are less compatible with a best fit model, given current constraints. This deforming plate model offers a platform and base model for future research.
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  • Alignment between seafloor spreading directions and absolute plate motions through time

    Williams, Simon E.   Flament, Nicolas   Müller, R. Dietmar  

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  • Ocean Basin Evolution and Global-Scale Plate Reorganization Events Since Pangea Breakup

    Mueller, R. Dietmar   Seton, Maria   Zahirovic, Sabin   Williams, Simon E.   Matthews, Kara J.   Wright, Nicky M.   Shephard, Grace E.   Maloney, Kayla T.   Barnett-Moore, Nicholas   Hosseinpour, Maral   Bower, Dan J.   Cannon, John  

    We present a revised global plate motion model with continuously closing plate boundaries ranging from the Triassic at 230 Ma to the present day, assess differences among alternative absolute plate motion models, and review global tectonic events. Relatively high mean absolute plate motion rates of approximately 9-10 cm yr(-1) between 140 and 120 Ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time. An event at similar to 100 Ma is most clearly expressed in the Indian Ocean and may reflect the initiation of Andean-style subduction along southern continental Eurasia, whereas an acceleration at similar to 80 Ma of mean rates from 6 to 8 cm yr-1 reflects the initial northward acceleration of India and simultaneous speedups of plates in the Pacific. An event at similar to 50 Ma expressed in relative, and some absolute, plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4-5 cm yr(-1) indicates that an increase in collisional forces (such as the India-Eurasia collision) and ridge subduction events in the Pacific (such as the Izanagi-Pacific Ridge) play a significant role in modulating plate velocities.
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  • Potential links between continental rifting,CO2 degassing and climate change through time

    Brune, Sascha   Williams, Simon E.   Mueller, R. Dietmar  

    The concentration of CO2 in the atmosphere is a key influence on Earth's climate. Today, significant quantities of CO2 are emitted at continental rifts, suggesting that the spatial and temporal extent of rift systems may have influenced deep carbon fluxes and thus climate change throughout geological time. Here we test this hypothesis by conducting a worldwide census of continental rift lengths over the last 200 million years. We estimate tectonic CO2 release rates through time and show that along the extensive Mesozoic and Cenozoic rift systems, rift-related CO2 degassing rates reached more than 300% of present-day values. Using a numerical carbon cycle model, we find that two prominent periods of enhanced rifting 160 to 100 million years ago and after 55 million years ago coincided with greenhouse climate episodes, during which atmospheric CO2 concentrations were more than three times higher than today. We therefore propose that continental fragmentation and long-term climate change could plausibly be linked via massive CO2 degassing in rift systems.
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  • Full-fit, palinspastic reconstruction of the conjugate Australian-Antarctic margins

    Williams, Simon E.   Whittaker, Joanne M.   Mueller, R. Dietmar  

    Despite decades of study the prerift configuration and early rifting history between Australia and Antarctica is not well established. The plate boundary system during the Cretaceous includes the evolving Kerguelen-Broken Ridge Large Igneous Province in the west as well as the conjugate passive and transform margin segments of the Australian and Antarctic continents. Previous rigid plate reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history. We investigate a range of scenarios for the early rifting history of these plates by developing a deforming plate model for this conjugate margin pair. Potential field data are used to define the boundaries of stretched continental crust on a regional scale. Integrating crustal thickness along tectonic flow lines provides an estimate of the prerift location of the continental plate boundary. We then use the prerift plate boundary positions, along with additional constraints from geological structures and large igneous provinces within the same Australian and Antarctic plate system, to compute "full-fit" poles of rotation for Australia relative to Antarctica. Our preferred model implies that the Leeuwin and Vincennes Fracture Zones are conjugate features within Gondwana, but that the direction of initial opening between Australia and Antarctica does not follow the orientation of these features; rather, the geometry of these features is likely related to the earlier rifting of India away from Australia-Antarctica. Previous full-fit reconstructions, based on qualitative estimates of continental margin overlaps, generally yield a tighter fit than our preferred reconstruction based on palinspastic margin restoration.
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  • Full-fit, palinspastic reconstruction of the conjugate Australian-Antarctic margins

    Williams, Simon E.   Whittaker, Joanne M.   Müller, R. Dietmar  

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  • A rapid burst in hotspot motion through the interaction of tectonics and deep mantle flow

    Mueller, R. Dietmar   Gurnis, Michael   Williams, Simon E.   Flament, Nicolas  

    Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth's mantle(1). Seismic imaging reveals that these plumes can be of deep origin(2)-probably rooted on thermochemical structures in the lower mantle(3-6). Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally(7,8), the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian-Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian-Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian-Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.
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  • Revision of Paleogene plate motions in the Pacific and implications for the Hawaiian-Emperor bend

    Wright, Nicky M.   Mueller, R. Dietmar   Seton, Maria   Williams, Simon E.  

    Understanding the relative motion between the Pacific plate and its neighboring plates in the Paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the Pacific Ocean basin, the history of circum-Pacific subduction, and the cause of the Hawaiian-Emperor bend (HEB). We quantitatively model the Farallon/Vancouver-Pacific-Antarctic seafloor spreading history from 67 to 33 Ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications. We find a well-constrained increase from 75 +/- 5 mm/yr to 101 +/- 5 mm/yr in Pacific-Farallon full spreading rates between 57.6 Ma and 55.9 Ma, followed by a stepwise increase to 182 +/- 2 mm/yr from 49.7 to 40.1 Ma. The increases in Pacific-Farallon spreading rates are not accompanied by any statistically significant change in spreading direction. The 57.6-55.9 Ma surge of Pacific-Farallon spreading reflects an eastward acceleration in Farallon plate motion, as it precedes west Pacific subduction initiation and is not associated with any significant change in Pacific-Antarctic spreading. We interpret the increase in Pacific-Farallon spreading rates after ca. 50 Ma as a consequence of further acceleration in Farallon plate motion. We find no indication of a major change in Pacific plate absolute motion at this time. Our model suggests that changes in relative motion direction between the Pacific and Farallon and Pacific and Antarctic plates were insignificant around the formation time of the HEB (ca. 47.5 Ma), and the bend is largely a consequence of Hawaiian hotspot motion, which ceased rapid motion after 47 Ma.
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