Alaska is made up of a mosaic of terranes that have enigmatic origins. Several plate restorations for the assembly of Alaska have been proposed, but their validity remains debated, partly due to the removal of vast volumes of oceanic plate material via subduction at the accretionary margins. The position, depth and volume of this subducted lithosphere, recognised as seismically fast anomalies in tomographic images, can be used to track the locations of subduction plate boundaries of the past, thus serving as an important constraint for plate restorations of convergent margins. Existing plate tectonic reconstructions can be assessed and developed further by integrating seismic tomographic models of the mantle with geological and palaeomagnetic bedrock datasets, a procedure which we term “tomotectonic analysis”.Previous tomotectonic studies (e.g., Sigloch & Mihalynuk, 2017, GSA Bulletin) have highlighted various discrepancies between the most generally accepted tectonic reconstruction models of the western coast of North America and tomographic observations of slabs in the mantle. For example, the kinematic reconstruction of Laurentia, constrained by the opening of the Atlantic Ocean, places the Cordilleran margin thousands of kilometres east of the tomographically imaged Angayucham and Mezcalera slabs in the mantle during the Early to Late Jurassic. This suggests that there was extensive westward subduction beneath the Insular and Intermontane superterranes that involved multiple plates, rather than a single subduction zone. Though a recent plate reconstruction that employed tomotectonic methods (Clennett et al., 2020, G-Cubed) provided a coherent explanation of bedrock, plate kinematic and mantle observations for the Cordilleran margin, application of this model to Alaska and the Arctic was hindered by low tomographic resolution beneath that region and requires further investigation. In particular, restoration of the Arctic Alaska terrane is complicated further by its possible relationship with the proposed Arctic Alaska-Chukotka microcontinent and its involvement in the accretionary development of the Siberian peninsula and the opening of the Canada Basin, for which several working hypotheses continue to be debated.In this study we consider the application of tomotectonic analysis to Mesozoic reconstructions of the western Arctic and central Alaska. We will compare and contrast these tectonic reconstructions with respect to the distribution of slabs in the deep mantle based on observations from the latest seismic tomographic models, such as DETOX-P1, P2 and P3 (Hosseini et al., 2020, GJI). We will also highlight the limitations of current tomographic models and the need for targeted seismic investigations with greater resolution of the underlying mantle. This discussion provides the motivation and rationale for a new seismic tomographic model of the mantle beneath North America currently being produced by the authors using a more complete USArray dataset.

The Western Gneiss Region (WGR) of Norway, part of the Caledonian Orogenic Belt, is one of the largest and best studied examples of exhumed ultra-high pressure (UHP) continental terrains in the world. This makes it an ideal candidate for studying the poorly understood processes that facilitate and control the exhumation of UHP continental material. Although the WGR is often considered the type example of the eduction model of UHP exhumation (Andersen et al., 1991), validation of exhumation models requires robust estimates of pressure and temperature across the full range of retrograde conditions which follow peak metamorphism. However, such constraints are often difficult to obtain as there is commonly overprinting of early-stage exhumation records during later stages of exhumation. UHP assemblages in the WGR are primarily preserved within numerous mafic eclogite enclaves, making them ideal candidates for studying processes and conditions that occur during exhumation from UHP conditions. In this study, we present detailed Electron Probe Micro-Analyses (EPMA) combined with Scanning Electron and Optical Microscopy characterization from a suite of mafic eclogite samples from the Stadlandet Peninsula of Western Norway. Our analyses focus on diopside–plagioclase (± amphibole) symplectite, which form from breakdown of omphacite during exhumation. Spatial variations in the compositions of minerals within these symplectites reflect a detailed record of P-T conditions during exhumation (Boland & van Roermund, 1983; Joanny et al., 1991; Waters, 2002). We used a novel technique of high resolution, low voltage EPMA, combined with secondary fluorescence corrections, which permits the analysis of individual symplectite lamellae with widths down to 1μm. Retrograde P-T pathways were then constructed from these data using the hornblende-plagioclase thermometer and clinopyroxene-plagioclase-hornblende barometer (Waters, 2002). P-T estimates from the symplectites fall in the range 470-720°C and 3-16 kbar. Combining the P-T arrays with existing peak P-T estimates indicates a two-stage exhumation path, with a steep initial isothermal decompression from depth followed by a more gentle cooling trajectory at lower pressures. The inflection in the exhumation path is estimated to be around 10-15 kbar at 650-700°C. The path shape is usually interpreted to record an initial rapid buoyancy driven exhumation from UHP to the base of the crust or lithosphere, followed by a second stage of slow exhumation to crustal depths. This confirmation of two-stage exhumation paths helps to constrain models of exhumation for the WGR, which in turn provides insights into how UHP terrains exhume globally. References: Andersen, T. B., Jamtveit, B., Dewey, J. F. & Swensson E. (1991). Subduction and Eduction of Continental Crust: Major Mechanisms during Continent-Continent Collision and Orogenic Extensional Collapse, a Model Based on the South Norwegian Caledonides. Terra Nova, 3(3), 303–10 Boland, J., & van Roermund, H. (1983). Mechanisms of exsolution in omphacites from high temperature, type B, eclogites. Physics and Chemistry of Minerals, 9(1), 30–37. Joanny, V., van Roermund, H. & Lardeaux, J. M. (1991). The clinopyroxene/plagioclase symplectite in retrograde eclogites. Geologische Rundschau, 80(2), 303–320 Waters, D. J. (2002). Clinopyroxene-amphibole-plagioclase symplectites in Norwegian eclogites. Mineralogical Society, Winter Conference, Derby.

The India-Asia collision is one of the most globally significant tectonic events of the Cenozoic era. It is widely cited as providing a unique natural laboratory for studying collisional tectonics, offering invaluable insights of processes associated with continental collision across a multitude of scales. Yet despite its importance, significant debate continues to surround the validity of three mutually exclusive models to explain the India-Asia collision. These include: (1) the single subduction model; (2) the double subduction model; and (3) the Greater India Basin hypothesis. In our recent review (Parsons et al. 2020, Earth-Science Reviews) we demonstrated that available constraints from the Himalayan orogen and Tibetan plateau, including tomographic analysis of subducted slabs beneath these regions, are unable to robustly define the relative likeliness of each model. In this contribution, we expand upon the work of Parsons et al. (2020), with geological, geophysical, and plate kinematic constraints from the southern Eurasian margin between Myanmar and Sulawesi. Our analysis focuses on the interpretation of subducted oceanic lithosphere beneath Myanmar to Sulawesi and includes a cross sectional area-based restoration of actively subducting India-Australia plate oceanic lithosphere. Our results provide a new restoration for the southern Eurasian margin and the India-Australia plate boundary (the Wharton ridge) during the India-Asia collision. Our integration of plate kinematic constraints with tomographic interpretation of subducted slabs suggests that the plate boundary between the Indian continent and southern Tibet migrated ~1000-2000 km northwards during collision. This includes ~1000 km lateral migration of subducted Indian plate oceanic lithosphere, now imaged beneath northern India and southern Tibet. Our reconstruction proposes that northward migration of the India-Tibet suture and subducted Indian plate oceanic lithosphere initiated at ~43 Ma and reflects a major plate network reorganisation event. At this time, “hard collision” of the Indian continent with southern Eurasia occurred synchronously with (1) reduction in Indian plate velocity; (2) cessation of the Wharton ridge and coupling of the Indian and Australian plates; (3) subduction initiation of Australian plate oceanic lithosphere beneath southeast Eurasia, and onset of northeast migration of the Australian continent; (4) accelerated ocean spreading between Australia and Antarctica; and (5) southwest ridge jump of the Central India spreading ridge. Buoyancy of the Indian continent kept it afloat, whilst oceanic lithosphere to the east continued to drive wholesale motion of the coupled India-Australia plate. This forced the Indian continent to migrate northwards, dragging the subducted Indian plate oceanic slab with it, which effectively unzipped the coupled India-Australia plate along the extinct Wharton ridge, during subduction. Our findings are most consistent with models (2) and (3), which are characterised by two collisions, the latter of which occurred between India and the Eurasian margin at ~45-40 Ma. More generally, our study demonstrates how changes in the balance of forces within a plate network, caused by events such as continental collision, can lead to significant plate network reorganisations. Such events can have dramatic effects on the position and geometry of subducted slabs and should be considered when interpreting plate restorations from deep-mantle structure.

The India-Asia collision is one of the most well-studied orogenic events on Earth; it recorded the terminal stages of the central Tethys ocean basins and offers invaluable insight into the geological processes associated with continental collision. In this study, we integrate bedrock datasets, observations of subducted slabs in the mantle, and plate kinematic constraints, to constrain models for the India-Asia collision and the central Tethys oceans. Previously proposed models for the India-Asia collision differ in terms of subduction zone configurations and paleogeographic reconstructions of Greater India, which represents to northern passive margin of India prior to collision. Five distinct subduction zone configurations have been proposed previously, which differ in the number of active trenches (one or two trenches) in the central Neotethys Ocean and differ in the respective timing, duration, location and migration of those trenches. Three distinct paleogeographic reconstructions of Greater India have been proposed previously, which differ in size and structure. Here, we consider the validity of these subduction zone configurations and Greater India reconstructions with respect to the bedrock record, plate kinematics and the deep mantle structure of subducted slabs beneath the Indian hemisphere. Following the assumption that slabs sink vertically through the mantle, the positions and geometries of subducted slabs determined from seismic tomography constrain the locations and kinematics of paleo-subduction zones. Integrating this with bedrock constraints allows us to constrain post-Triassic subduction zone configurations for the central Tethys oceans. Our analysis demonstrates that the Neotethys Ocean was consumed by at least two subduction zones since the Jurassic. At the onset of the India-Asia collision at 59±1 Ma, one subduction zone was active along the southern Asian continental margin at ~20°N. At that time, a second may have been active at subequatorial latitudes, but support for this from a bedrock perspective is lacking. This subduction zone configuration allows for three reconstructions for Greater India: The (1) minimum-area; (2) enlarged-area; and (3) Greater India Basin reconstructions. We integrate these reconstructions and subduction zone configurations in a plate kinematic framework to test their validity for the India-Asia collision. Our findings show that no single model is entirely satisfactory and each invokes assumptions that challenge accepted concepts. These include our understanding of suture zones, subduction-erosion processes, and the limits of continental subduction. We explore these challenges and their implications for our understanding of the India-Asia collision and continental collisions in general.