Philip J. Heron

Influences on the positioning of mantle plumes following supercontinent formation

Several mantle convection studies analyzing the effects of supercontinent formation and dispersal show that the genesis of subcontinental plumes results from the formation of subduction zones at the edges of the supercontinent rather than from the effect of continental thermal insulation or thermochemical piles. However, the influence of subduction zone location on the position of subcontinental plumes has received little attention. This study analyzes 2-D and 3-D numerical models of supercontinent formation (in an isochemical mantle) to assess the role of subduction and mantle viscosity contrast in the generation of subcontinental mantle plumes. We find that once a critical supercontinent width is reached, plumes do not form under the center of a supercontinent. In studies featuring a low viscosity lower mantle, the surface positions of the initial plumes (arriving within 90 Myr of supercontinent assembly) become locked beneath the continent at a distance 2000–3000 km from the continental margin. However, the broad downwellings in simulations that feature a high-viscosity lower mantle trigger plumes at a greater distance from the continental margin subduction. For all mantle viscosity profiles, subcontinental plumes show dependence on the location of supercontinent margin subduction. As theories differ on the role of core-mantle boundary chemical piles in plume formation, it is significant that our isochemical models show that the formation of subduction zones at the margins of a supercontinent has a profound effect on subcontinental mantle dynamics. Our results may help to explain what determined the eruption sites of past (and future) large igneous provinces.

The impact of Rayleigh number on assessing the significance of supercontinent insulation

Several processes unfold during the supercontinent cycle, more than one of which might result in an elevation in subcontinental mantle temperatures, thus multiple interpretations of the concept of continental insulation exist. Although a consensus seems to have formed that subcontinental mantle upwellings appear below large continents extensively ringed by subduction zones, there are differing views on what role continental insulation plays in the production of elevated mantle temperatures. Here we investigate how the heating mode of the mantle can change the influence of the “thermal blanket” effect. We present 2‒D and 3‒D Cartesian geometry mantle convection simulations with thermally and mechanically distinct oceanic and continental plates. The evolution of mantle thermal structure is examined after continental accretion at subduction zones (e.g., the formation of Pangea) for a variety of different mantle‒heating modes. Our results show that in low‒Rayleigh number models the impact of the role of continental insulation on subcontinental temperatures increases, when compared to models with higher convective vigor. Broad, hot upper mantle features generated in low‒Rayleigh number models (due, in part, to the thermal blanket effect) are absent at higher Rayleigh numbers. We find that subcontinental heating in a high‒Rayleigh number flow occurs almost entirely as a consequence of the influence of subduction initiation at the continental margin, rather than the influence of continental insulation. In our models featuring Earth‒like convective vigor, we find that it is difficult to obtain subcontinental temperatures in significant excess of suboceanic temperatures over timescales relevant to supercontinent aggregation.

Inherited structure and coupled crust‐mantle lithosphere evolution: Numerical models of Central Australia

Continents have a rich tectonic history that have left lasting crustal impressions. In analyzing Central Australian intraplate orogenesis, complex continental features make it difficult to identify the controls of inherited structure. Here the tectonics of two types of inherited structures (e.g., a thermally enhanced or a rheologically strengthened region) are compared in numerical simulations of continental compression with and without “glacial buzzsaw” erosion. We find that although both inherited structures produce deformation in the upper crust that is confined to areas where material contrasts, patterns of deformation in the deep lithosphere differ significantly. Furthermore, our models infer that glacial buzzsaw erosion has little impact at depth. This tectonic isolation of the mantle lithosphere from glacial processes may further assist in the identification of a controlling inherited structure in intraplate orogenesis. Our models are interpreted in the context of Central Australian tectonics (specifically the Petermann and Alice Springs orogenies).

Identifying mantle lithosphere inheritance in controlling intraplate orogenesis

Crustal inheritance is often considered important in the tectonic evolution of the Wilson Cycle. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological make-up. Recently, increased resolution in lithosphere imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a ‘jelly sandwich’ rheology, common in stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in generating deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics.

Exploring the theory of plate tectonics : the role of mantle lithosphere structure.

Abstract This review of the role of the mantle lithosphere in plate tectonic processes collates a wide range of recent studies from seismology and numerical modelling. A continually growing catalogue of deep geophysical imaging has illuminated the mantle lithosphere and generated new interpretations of how the lithosphere evolves. We review current ideas about the role of continental mantle lithosphere in plate tectonic processes. Evidence seems to be growing that scarring in the continental mantle lithosphere is ubiquitous, which implies a reassessment of the widely held view that it is the inheritance of crustal structure only (rather than the lithosphere as a whole) that is most important in the conventional theory of plate tectonics (e.g. the Wilson cycle). Recent studies have interpreted mantle lithosphere heterogeneities to be pre-existing structures and, as such, linked to the Wilson cycle and inheritance. We consider the current fundamental questions in the role of the mantle lithosphere in causing tectonic deformation, reviewing recent results and highlighting the potential of the deep lithosphere in infiltrating every aspect of plate tectonics processes.