4D Tectonic Reconstruction
Creators: name: O'Kane, Tomas
Publisher: name: The Australian National University
Year: 2013
Subjects: Tectonics
Reconstruction
Plate Modelling
Subduction
Earthquakes

description: The study of plate tectonics spans the entire field of Earth Sciences. Plate tectonic reconstructions, created within the paradigm of the plate tectonic theory, result from the synthesis of extensive geological, geophysical, geochemical, geochronological and seismological data. The images produced through such reconstruction have numerous practical applications. Considerable amounts of data are captured in each snapshot of time. When such data is assimilated, geologic trends and patterns of motion can be revealed that would otherwise not be apparent. Such uses are growing, for the scope and potential of tectonic reconstructions has exploded in line with the growth in computer processing power. Initially, reconstructions involved the simple Euler rotation of continents on a 2D surface. However, now we can view and analyse huge datasets in the realm of subduction and mantle convection models, erosion simulations, or rifting algorithms, for example. Yet for all the high-resolution models, the complex data processing and integrated simulations, limited attention has been given to two crucial components of plate tectonics: crustal deformation and the geometrical evolution of subducted lithosphere. In this thesis we analyse two tectonic systems, and using a new approach incorporate deformation of both the crust and subducted lithosphere into our modelling. The results offer a different outcome to previous studies. In a study of Northern Melanesia, it is found that constraints provided by the extent of subducting lithosphere and crustal deformation require substantial revisions of kinematic histories. We also show that the 3D geometry of the Solomon Sea slab has 4D implications for the tectonic evolution of the region as a whole. Specifically, we made meaningful discoveries about the tectonic evolution of the region by detailing the geometrical evolution of the subducted lithosphere. In this instance, we showed that the Solomon Sea slab, currently subducting at the New Britain Trench, can be described in terms of ailerons – tabular segments suspended from the overlying lithosphere, with each segment potentiallybounded by a vertical slab tear. Such segmentation could result from slab tears, formed during rollback of this strongly curved subduction zone. However, this geometry required the isolation (and eventual drop-off) of a large slab segment. Evidence for this previously undocumented tectonic scenario was found through visualisation of a detached slab segment, located in the upper mantle beneath a geochemically distinct arc. We tentatively drew a link between the fluids and magmas emanating from the detached slab and high-K, volatile-rich magmas that characterise the overlying plate. We also illustrate that the subducted lithosphere beneath PNG has undergone a more complex evolution than previously thought. We provide further evidence that this subducted lithosphere is doubly-dipping, with both north- and south-dipping limbs. Analysis of the spatial distribution seismic activity, and focal mechanisms, also suggests the presence of a slab tear, located beneath the Huon Peninsula. This slab tear detaches the doubly-dipping slab segment from the part that is actively subducting beneath the New Britain Arc. We employed a similar methodology in our analysis of the evolution of the Andean subduction system of western South America. Through modelling of earthquake hypocentre distribution and “slab refloating”, we found that the margins of flat slab segments may, in fact, be vertical slab tears. Tectonic reconstruction reveals a spatial, and possibly a causal, relationship between these tears and subducted oceanic fracture zones. We also examine the evolution of the Andean orogen, illustrating that prior to crustal shortening and oroclinal rotation the western margin of South America was essentially a linear subduction zone. This outcome has many applications in the way we view the subduction history of the system as a whole. The research presented in this thesis demonstrates that if sufficient attention is given to the analysis of large-scale deformation of the continental and subducted lithosphere, alternative theories as to the tectonic history of the region may be uncovered. The methodology described here could be applied in alternative tectonic settings, potentially with similarly important results.