N. Butterworth

Abrupt plate accelerations shape rifted continental margins

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, 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 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.

Cenozoic uplift of south Western Australia as constrained by river profiles

Abstract The relative tectonic quiescence of the Australian continent during the Cenozoic makes it an excellent natural laboratory to study recent large-scale variations in surface topography, and processes that influence changes in its elevation. Embedded within this topography is a fluvial network that is sensitive to variations in horizontal and vertical motions. The notion that a river acts as a ‘tape recorder’ for vertical perturbations suggests that changes in spatial and temporal characteristics of surface uplift can be deduced through the analysis of longitudinal river profiles. We analyse 20 longitudinal river profiles around the Australian continent. Concave upward profiles in northeast Australia indicate an absence of recent surface uplift. In contrast, the major knickzones within longitudinal profiles of rivers in southwest Australia suggest recent surface uplift. Given the lack of recent large-scale tectonic activity in that region, this uplift requires an explanation. Applying an inverse algorithm to river profiles of south Western Australia reveals that this surface uplift started in the Eocene and culminated in the mid-late Neogene. The surface uplift rates deduced from this river profile analysis generally agree with independent geological observations including preserved shallow-marine sediment outcrops across the Eucla Basin and south Western Australia. We show that the interplay between global sea level and long-wavelength dynamic topography associated with south Western Australias plate motion path over the remnants of an ancient Pacific slab is a plausible mechanism driving this surface uplift.

Tectonic environments of South American porphyry copper magmatism through time revealed by spatiotemporal data mining

Porphyry ore deposits are known to be associated with arc magmatism on the overriding plate at subduction zones. While general mechanisms for driving magmatism are well established, specific subduction-related parameters linking episodes of ore deposit formation to specific tectonic environments have only been qualitatively inferred and have not been formally tested. We develop a four-dimensional approach to reconstruct age-dated ore deposits, with the aim of isolating the tectono-magmatic parameters leading to the formation of copper deposits during subduction. We use a plate tectonic model with continuously closing plate boundaries, combined with reconstructions of the spatio-temporal distribution of the ocean floor, including subducted portions of the Nazca/Farallon plates. The models compute convergence rates and directions, as well as the age of the downgoing plate through time. To identify and quantify tectonic parameters that are robust predictors of Andean porphyry copper magmatism and ore deposit formation we test two alternative supervised machine learning methods; the ‘random forest’ (RF) ensemble and ‘support vector machines’ (SVM). We find that a combination of rapid convergence rates (~100 km/Myr), subduction obliquity of ~15°, a subducting plate age between ~25–70 Myr old, and a location far from the subducting trench boundary (>2000 km), represent favorable conditions for porphyry magmatism and related ore deposits to occur. These parameters are linked to the availability of oceanic sediments, the changing small-scale convection around the subduction zone, and the availability of the partial melt in the mantle wedge. When coupled, these parameters could influence the genesis and exhumation of porphyry copper deposits.

Identifying tectonic niche environments of South American porphyry magmatism through geological time: a spatio-temporal data mining approach

Porphyry ore deposits are well known to be associated with arc magmatism related to subduction on the overriding plate. Furthermore, the regional mechanisms for magmatism and the resulting formations of porphyry deposits are well established. Specific parameters leading to these events have been inferred, but not formally tested. We aim to identify the specific set of tectono-magmatic parameters that result in a subducting slab producing particular types of magmatism on the overriding plate, and their link to the formation of ore deposits. We use a four-dimensional approach to reconstruct age-dated magmatism back through space and time to isolate the tectono-magmagic parameters leading to the formation of a metalliferous deposit during subduction. By utilising machine learning techniques we identify and quantify geodynamic parameters that are robust predictors of back-arc magmatism and porphyry formation. The ‘random-forest’ ensemble and ‘support vector machines’ learning classification methods are employed to prioritise parameters that are considered influential in the development of magmatism and the subsequent metallogenesis of porphyry ore deposits. We find that a combination of convergence rates and directions, seafloor age, subduction obliquity, and the distance to a trench edge help predict whether magmatism and related ore deposits occur.