Slabs in the lower mantle and their modulation of plume formation

Numerical mantle convection models indicate that subducting slabs can reach the core-mantle boundary (CMB) for a wide range of assumed material properties and plate tectonic histories. An increase in lower mantle viscosity, a phase transition at 660 km depth, depth-dependent thermal expansivity, and depth-dependent thermal diffusivity do not preclude model slabs from reaching the CMB. We find that ancient slabs could be associated with lateral temperature anomalies ~500°C cooler than ambient mantle. Plausible increases of thermal conductivity with depth will not cause slabs to diffuse away. Regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the CMB, even for radially simple mantle structures. The observation from tomography showing only a few continuous slab-like features from the surface to the CMB may be a result of complex plate kinematics, not mantle layering. There are important consequences of deeply penetrating slabs. Our models show that plumes preferentially develop on the edge of slabs. In areas on the CMB free of slabs, plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin. However, in areas beneath slabs, the basal thermal boundary layer would be thicker and plume formation infrequent. Beneath slabs, a substantial amount of hot mantle can be trapped over long periods of time, leading to “mega-plume” formation. We predict that patches of low seismic velocity may be found beneath large-scale high seismic velocity structures at the core-mantle boundary. We find that the location, buoyancy, and geochemistry of mega-plumes will differ from those plumes forming at the edge of slabs. Various geophysical and geochemical implications of this finding are discussed

Asymmetric continental deformation during South Atlantic rifting along southern Brazil and Namibia

Plate restoration of South America and Africa to their pre-breakup position faces the problem of gaps and overlaps between the continents, an issue commonly solved with implementing intra-plate deformation zones within South America. One of these zones is often positioned at the latitude of SE/S Brazil. However, geological evidence for the existence of a distinct zone in this region is lacking, which is why it remains controversial and is not included in all modeling studies. In order to solve this problem we present a study of multiple geological aspects of both parts of the margin, SE/S Brazil and its conjugate part NW Namibia at the time of continental breakup. Our study highlights pronounced differences between these regions with respect to Paraná-Etendeka lava distribution, magmatic dyke emplacement, basement reactivation, and fault patterns. In Namibia, faults and dykes reactivated the rift-parallel Neoproterozoic basement structure, whereas such reactivation was scarce in SE/S Brazil. Instead, most dykes, accompanied by small-scale grabens, are oriented margin-perpendicular along the margin from northern Uruguay to São Paulo. We propose that these differences are rooted in large-scale plate movement and suggest a clockwise rotation of southern South America away from a stable northern South America and Africa, in a similar way as proposed by others for a Patagonian continental section just prior to South Atlantic rifting. This rotation would produce margin-parallel extension in SE/S Brazil forming margin-perpendicular pathways for lava extrusion and leading to the asymmetric distribution of the Paraná-Etendeka lavas. NW Namibia instead remained relatively stable and was only influenced by extension due to rifting, hot spot activity, and mantle upwelling. Our study argues for significant margin-parallel extension in SE/S Brazil, however not confined to a single distinct deformation zone, but distributed across ~ 1000 km along the margin

Dynamics of pyroclastic density currents : : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University, Palmerston North, New Zealand

Listed in 2016 Dean's List of Exceptional ThesesPyroclastic density currents (PDCs) are the most dangerous mass flows on Earth. Yet they remain poorly understood because internal measurements and observations are hitherto non-existent. In this thesis, the first measurements and views into experimental large-scale PDCs synthesized by “column collapse” provide insights into the internal structure, transport and emplacement dynamics of dense PDCs or pyroclastic flows. While from an outside point of view, PDCs resemble dilute gravity currents, the internal flow structure shows longitudinal and vertical complexities that greatly influence the PDCs‟ propagation and emplacement dynamics. Internal velocity and concentration profiles from direct observations provide the evidence of an unforeseen intermediate zone that plays an important role into the transfer of mass from the ash-cloud to the underflow. The intermediate zone is a “dense suspension” where particle cluster in bands to form mesoscale structures. These reduce particle drag and yield an extreme sedimentation rate of particles onto the newly-formed underflow. These findings call into question the existing paradigm of a continuous vertical concentration profile to explain the formation of massive layers and an underflow from ash-clouds. Instead, a sharp concentration jump occurs between the intermediate zone, with concentrations of the order of few volume percent, and the underflow, with concentrations of c.45%. PDCs were found to be composed of 4 main zones identified as the underflow, and the ash-cloud head, body and wake. Following the evolution of the PDC structure over time allows the formation of a complex ignimbrite deposit sequence to be uncovered, reproducing experimentally the “standard ignimbrite sequence” reported from field studies. Experiments revealed that each flow zone deposited the particulate load under contrasting emplacement timescales (spanning up to 5 orders of magnitude), which are primarily controlled by the concentration of the zone. The ash-cloud head is the most dynamic zone of the PDC, where proximally mass is intensively transferred downward and feeds the underflow front, while at all times, the finest particles are entrained upward and feed the wake through detachment of large Kelvin-Helmholtz instabilities. Subsequently, kinematic coupling between the moving underflow and overriding ash-cloud leads to a forced-supercriticality, preferentially affecting the head. The wide range of particle sizes and densities yield a spectrum of gas-transport behaviours ranging from a poorly coupled and rapid-sedimenting mesoscale regime up to a homogeneously coupled long-lived suspending regime. Internal velocity and concentration profiles illuminate the role of boundary velocity, which yields forced-acceleration of the ash-cloud. Kinematic coupling of the ash-cloud with the underflow induces a velocity at the lower flow boundary, while shear stress at the ash-cloud/underflow wanes and results in the shrinking of the maximum velocity and concentration heights. Therefore, the ash-cloud can reach high velocities and multiply its destruction potential. The experimental work presented in this thesis provides the first datasets of the internal physical properties of PDCs, which can be used to test the validity of current numerical models and highlight their limitations. This thesis also presents the study of a small hydrothermal blast that occurred at Mt. Tongariro, New Zealand, on the 6th of August 2012. The study of the blast is subdivided into two phases: the PDC phase and the ballistic phase. The detailed study of the PDC along the main propagation axis highlighted the role of the longitudinal zoning of the current, which was reflected in the complex tripartite deposit architecture. The study of the blast-derived ballistic crater field revealed a zone of high crater density that was related to the focus of ballistic trajectories around the main explosion direction. Simple inverse ballistic modelling provided evidence for a shallow blast (c. 5° above horizontal) from Te Maari. Furthermore, a comparison of ballistic block lithologies confirmed the origin of the elongated succession of craters or fissures formed by successive blasting during the eruption

Subsidence Mechanisms of Sedimentary Basins Developed over Accretionary Crust

This thesis uses forward modelling to investigate the formation of intercratonic basins upon accretionary crust. It began from the hypothesis that accretionary crust forms with a near normal thickness crust, but a thin lithosphere inherited from the terranes that compose it. After the accretion process has ceased the lithosphere stabilises and begins to cool, causing it to grow thicker and this in turn drives subsidence of the accretionary crust. A 1-D finite difference computer code was developed to model conductive heat flow through a column of cooling lithosphere and asthenosphere. To test the hypothesis, the subsidence produced by the modelling of this process was compared to the observed subsidence from backstripping numerous basins situated on accretionary crust The model produced a good fit to the subsidence in a detailed case study of two of the Palaeozoic basins in North Africa. The study was then extended to test the applicability of to accretionary crust globally. It found that while using measured values of the crust and lithospheric thickness for each region the model produced subsidence curves that matched the observed subsidence in each basin. It makes a more coherent argument for the formation of these basins that is able to explain a wider variety of features than other proposed subsidence mechanisms such as slow stretching or dynamic topography. These results suggest that such subsidence is an inherent property of accretionary crust which could influence the evolution of the continental crust over long time periods. The model was used to investigate the subsidence of the West Siberian Basin and found the subsidence patterns to be consistent with the decay of a plume head which thinned the lithosphere. This subsidence patterns indicate the plume material thinned the lithosphere over an area of 2.5 million km2 resulting in uplift before it cooled and subsided

Localized precipitation and runoff on Mars

We use the Mars Regional Atmospheric Modeling System (MRAMS) to simulate lake storms on Mars, finding that intense localized precipitation will occur for lake size >=10^3 km^2. Mars has a low-density atmosphere, so deep convection can be triggered by small amounts of latent heat release. In our reference simulation, the buoyant plume lifts vapor above condensation level, forming a 20km-high optically-thick cloud. Ice grains grow to 200 microns radius and fall near (or in) the lake at mean rates up to 1.5 mm/hr water equivalent (maximum rates up to 6 mm/hr water equivalent). Because atmospheric temperatures outside the surface layer are always well below 273K, supersaturation and condensation begin at low altitudes above lakes on Mars. In contrast to Earth lake-effect storms, lake storms on Mars involve continuous precipitation, and their vertical velocities and plume heights exceed those of tropical thunderstorms on Earth. Convection does not reach above the planetary boundary layer for lakes O(10^2) mbar. Instead, vapor is advected downwind with little cloud formation. Precipitation occurs as snow, and the daytime radiative forcing at the land surface due to plume vapor and storm clouds is too small to melt snow directly (<+10 W/m^2). However, if orbital conditions are favorable, then the snow may be seasonally unstable to melting and produce runoff to form channels. We calculate the probability of melting by running thermal models over all possible orbital conditions and weighting their outcomes by probabilities given by Laskar et al., 2004. We determine that for an equatorial vapor source, sunlight 15% fainter than at present, and snowpack with albedo 0.28 (0.35), melting may occur with 4%(0.1%) probability. This rises to 56%(12%) if the ancient greenhouse effect was modestly (6K) greater than today.Comment: Submitted to JGR Planet

Tertiary-Quaternary subduction processes and related magmatism in the Alpine-Mediterranean region

During Tertiary to Quaternary times, convergence between Eurasia and Africa resulted in a variety of collisional orogens and different styles of subduction in the Alpine-Mediterranean region. Characteristic features of this area include arcuate orogenic belts and extensional basins, both of which can be explained by roll-back of subducted slabs and retreating subduction zones. After cessation of active subduction, slab detachment and post-collisional gravitational collapse of the overthickened lithosphere took place. This complex tectonic history was accompanied by the generation of a wide variety of magmas. Most of these magmas (e.g. low-K tholeiitic, calc-alkaline, shoshonitic and ultrapotassic types) have trace element and isotopic fingerprints that are commonly interpreted to reflect enrichment of their source regions by subduction-related fluids. Thus, they can be considered as ‘subduction-related’ magmas irrespective of their geodynamic relationships. Intraplate alkali basalts are also found in the region generally postdated the ‘subduction-related’ volcanism. These mantle-derived magmas have not been, or only slightly, influenced by subduction-related enrichment. This paper summarises the geodynamic setting of the Tertiary-Quaternary “subduction-related” magmatism in the different segments of the Alpine-Mediterranean region (Betic-Alboran-Rif province, Central Mediterranean, the Alps, Carpathian-Pannonian region, Dinarides and Hellenides, Aegean and Western Anatolia), and discusses the main characteristics and compositional variation of the magmatic rocks. Radiogenic and stable isotope data indicate the importance of continental crustal material in the genesis of these magmas. Interaction with crustal material probably occurred both in the upper mantle during subduction (‘source contamination’) and in the continental crust during ascent of mantle-derived magmas (either by mixing with crustal melts or by crustal contamination). The 87Sr/86Sr and 206Pb/204Pb isotope ratios indicate that an enriched mantle component, akin to the source of intraplate alkali mafic magmas along the Alpine foreland, played a key role in the petrogenesis of the ‘subduction-related’ magmas of the Alpine-Mediterranean region. This enriched mantle component could be related to mantle plumes or to long-term pollution (deflection of the central Atlantic plume and recycling of crustal material during subduction) of the shallow mantle beneath Europe since the late Mesozoic. In the first case, subduction processes could have had an influence in generating asthenospheric flow by deflecting nearby mantle plumes due to slab roll-back or slab break-off. In the second case, the variation in the chemical composition of the volcanic rocks in the Mediterranean region can be explained by “statistical sampling” of the strongly inhomogeneous mantle followed by variable degrees of crustal contamination

High-resolution geologic mapping of seafloor structures and identification of structural systematics

Two comprehensive geologic mapping projects, which were conducted in the eastern Manus Basin, Papua New Guinea, are the core of this dissertation. They provide new perspectives on the local geologic framework and distribution of hydrothermal discharge sites at felsic-hosted hydrothermal systems in an opening back-arc basin. Both mapped areas are interpreted as present-day analogs to volcanogenic massive sulfide (VMS) deposits preserved in the geologic record on land. Our results advance the knowledge of submarine volcanic eruption styles and related eruption products, the interplay of back-arc volcanism with the formation of VMS ore deposits and finally the spatial distribution and influence of hydrothermal activity at the resultant seafloor morphologies

Causes and Consequences of Diachronous V-Shaped Ridges in the North Atlantic Ocean

In the North Atlantic Ocean, the geometry of diachronous V-shaped features that straddle the Reykjanes Ridge is often attributed to thermal pulses which advect away from the center of the Iceland plume. Recently, two alternative hypotheses have been proposed: rift propagation and buoyant mantle upwelling. Here, we evaluate these different proposals using basin-wide geophysical and geochemical observations. The centerpiece of our analysis is a pair of seismic reflection profiles oriented parallel to flowlines that span the North Atlantic Ocean. V-shaped ridges and troughs are mapped on both Neogene and Paleogene oceanic crust, enabling a detailed chronology of activity to be established for the last 50 million years. Estimates of the cumulative horizontal displacement across normal faults help to discriminate between brittle and magmatic modes of plate separation, suggesting that crustal architecture is sensitive to the changing planform of the plume. Water-loaded residual depth measurements are used to estimate crustal thickness and to infer mantle potential temperature which varies by 25◦C on timescales of 3–8 Ma. This variation is consistent with the range of temperatures inferred from geochemical modeling of dredged basaltic rocks along the ridge axis itself, from changes in Neogene deep-water circulation, and from the regional record of episodic Cenozoic magmatism. We conclude that radial propagation of transient thermal anomalies within an asthenospheric channel that is 150 50 km thick best accounts for the available geophysical and geochemical observations

Site evaluation for laser satellite-tracking stations

Twenty-six locations for potential laser satellite-tracking stations, four of them actually already occupied in this role, are reviewed in terms of their known local and regional geology and geophysics. The sites are also considered briefly in terms of weather and operational factors. Fifteen of the sites qualify as suitable for a stable station whose motions are likely to reflect only gross plate motion. The others, including two of the present laser station sites (Arequipa and Athens), fail to qualify unless extra monitoring schemes can be included, such as precise geodetic surveying of ground deformation

The preserved plume of the Caribbean Large Igneous Plateau revealed by 3D data-integrative models

Remnants of the Caribbean Large Igneous Plateau (C-LIP) are found as thicker than normal oceanic crust in the Caribbean Sea that formed during rapid pulses of magmatic activity at similar to 91-88 and similar to 76 Ma. Strong geochemical evidence supports the hypothesis that the C-LIP formed due to melting of the plume head of the Galapagos hotspot, which interacted with the Farallon (Proto-Caribbean) plate in the eastern Pacific. Considering plate tectonics theory, it is expected that the lithospheric portion of the plume-related material migrated within the Proto-Caribbean plate in a north-north-eastward direction, developing the present-day Caribbean plate. In this research, we used 3D lithospheric-scale, data-integrative models of the current Caribbean plate setting to reveal, for the first time, the presence of positive density anomalies in the uppermost lithospheric mantle. These models are based on the integration of up-to-date geophysical datasets from the Earth's surface down to 200 km depth, which are validated using high-resolution free-air gravity measurements. Based on the gravity residuals (modelled minus observed gravity), we derive density heterogeneities both in the crystalline crust and the uppermost oceanic mantle (<50 km). Our results reveal the presence of two positive mantle density anomalies beneath the Colombian and the Venezuelan basins, interpreted as the preserved fossil plume conduits associated with the C-LIP formation. Such mantle bodies have never been identified before, but a positive density trend is also indicated by S-wave tomography, at least down to 75 km depth. The interpreted plume conduits spatially correlate with the thinner crustal regions present in both basins; therefore, we propose a modification to the commonly accepted tectonic model of the Caribbean, suggesting that the thinner domains correspond to the centres of uplift due to the inflow of the hot, buoyant plume head. Finally, using six different kinematic models, we test the hypothesis that the C-LIP originated above the Galapagos hotspot; however, misfits of up to similar to 3000 km are found between the present-day hotspot location and the mantle anomalies, reconstructed back to 90 Ma. Therefore, we shed light on possible sources of error responsible for this offset and discuss two possible interpretations: (1) the Galapagos hotspot migrated (similar to 1200-3000 km) westward while the Caribbean plate moved to the north, or (2) the C-LIP was formed by a different plume, which - if considered fixed - would be nowadays located below the South American continent