ALBERT

Description: OS34A-05: Hydrothermal convection is an important process that occurs in the oceanic lithosphere as well as within continents where the geothermal gradient is high enough to drive fluid flow. This process efficiently mines heat from the lithosphere, sustains life in the otherwise bleak settings at oceanic depths and is associated with mineral deposits. Although recent focus on hydrothermal systems has greatly improved our understanding on how they work, the detailed effects of topography on these systems has largely been ignored. While the qualitative effects of topography on hydrothermal flow are largely known (e.g. Ingebritsen 2006), we here present results from systematic numerical modeling on the importance of topography for both, subaerial and submarine hydrothermal convection. The model is based on a 2-D Finite Element Method (FEM) solver for fully compressible, single-phase, porous media fluid flow and is used to simulate hydrothermal convection in a number of synthetic studies as well as for two case studies for the Lucky Strike vent field (submarine) and the Amiata volcano (subaerial). The results of synthetic studies using sinusoidal topography variations show that topography indeed has a profound effect on the distribution and flow field of the convection cells. In the submarine case, fluid venting occurs at the topographic highs while the recharge zones are restricted to the lows. For the subaerial scenarios, the opposite occurs where groundwater flow focuses venting at flank regions and the recharge zones are situated at the highs. For example, in the submarine case, ~90% of the hydrothermal fluids vent at upper 50% of topographic highs if the number of topographic highs equals the number of plumes in a flat-top reference simulation. The results show that the focusing effect into topographic highs (submarine) and lows (subaerial) is highly dependent on the wavelength and amplitude of topography, i.e. wavelengths that are too high or low result in venting at flanks or even topographic lows (submarine case). Amplitude also has a first-order effect of focusing the vent sites on topographic highs and lows. Another observation is that the wavelength of the topography affects the number of plumes generated in the model. These findings are confirmed in two case studies for the submarine Lucky Strike hydrothermal field on the Mid-Atlantic Ridge and the subaerial geothermal field of Amiata, Italy. In both case studies the predicted vent locations fit well with the observed ones.

Description: We present quantitative modeling results for the effects of surface relief on hydrothermal convection at ocean-spreading centers investigating how vent site locations and subsurface flow patterns are affected by bathymetry induced sub-seafloor pressure variations. The model is based on a 2-D FEM solver for fluid flow in porous media and is used to simulate hydrothermal convection systematically in 375 synthetic studies. The results of these studies show that bathymetric relief has a profound effect on hydrothermal flow: bathymetric highs induce subsurface pressure variations that can deviate upwelling zones and favor venting at structural highs. The deviation angle from vertical upwelling can be expressed by a single linear dependence relating deviation angle to bathymetric slope and depth of the heat source. These findings are confirmed in two case studies for the East Pacific Rise at 9°30′N and Lucky Strike hydrothermal fields. In both cases, it is possible to predict the observed vent field locations only if bathymetry is taken into account. Our results thereby show that bathymetric relief should be considered in simulations of submarine hydrothermal systems and plays a key role especially in focusing venting of across axis hydrothermal flow onto the ridge axis of fast spreading ridges.

Description: Mid-ocean ridges and volcanic passive continental margins are prime regions to explore active and extinct hydrothermal systems. In both settings, a large number of hydrothermal vents have already been discovered by direct observations and/or geophysical surveys. The growing interest in these systems results from their relevance for different fields of marine sciences. For example, commercially interesting ore deposits form as a byproduct of hydrothermal venting at the seafloor, unique ecosystems evolve around submarine vent sites, and hydrothermal systems driven by sill intrusions into organic sediments are related to hydrocarbon maturation and even venting of greenhouse gases into the atmosphere. Numerical simulations of hydrothermal fluid flow can help to gain a quantitative understanding of the subsurface physicochemical processes that control these systems. This thesis contributes to a better understanding of hydrothermalism in oceanic and continental settings by presenting a newly developed hydrothermal flow model and two case studies of hydrothermal flow at mid-ocean ridges and volcanic passive margins. To explore the effects of bathymetric relief on hydrothermal fluid flow in submarine settings, a systematic study has been carried out using 375 simulations. These simulations show that temperature-induced pressure variations in the subsurface result in the deviation of hydrothermal plumes towards bathymetric highs in submarine settings. The plume deviation from its origin is directly related to the surface slope and depth of the heat source. A case study for the fast-spreading East Pacific Rise at 9° 30’N shows that bathymetric effects help to focus venting directly onto the ridge axis – only if bathymetry is taken into account can across axis fluid flow be reconciled with exclusive on-axis venting. A second case study for the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge shows that also here venting is likely to occur at local bathymetric highs. The effects of hydrothermal convection triggered by sill intrusions in continental settings have been explored in a case-study for the Gjallar Ridge area on the Norwegian margin. This area is affected by a swarm of sill intrusions originated from North-Atlantic continental break-up during the Paleocene-Eocene transition as well as pre-break-up faults resulting from extensional tectonics. The structures are interpreted using 3D multichannel seismic data in combination with a structural and thermal reconstruction of the margin using TECMOD software. The reconstructed temperature is used as initial condition for sediments prior to sill injection and the detailed thermal history of sediments is modeled by a 2D fluid flow simulation. The simulation results show that high-temperature venting (〉200°C) occurs less than 1000 years following sill emplacement. The faults play strong roles for transferring the fluids to far-off regions. As a result of circulating hot fluids, the maturity of sedimentary rocks is greatly enhanced, especially where the hot fluids are trapped below impermeable sills during their ascent, thereby suggesting potential zones for future hydrocarbon explorations. Furthermore, solution strategies for modeling hydrothermal fluid flow by finite element, finite volume and semi-Lagrangian methods are explained in particular in order to find out how the temperature equation is solved. Different schemes of fully-implicit, Crank-Nicolson and exponential for temperature diffusion and finite volume and semi-Lagrangian for temperature advection are evaluated. The results suggest that the most accurate method for solving temperature diffusion is Crank-Nicolson. However, other methods such as fully implicit and exponential are still valid. The mass conserving finite volume method is the most accurate method for solving temperature advection; however, limited time-stepping is its major drawback and thus semi-Lagrangian method is usually preferred. Therefore, the definition of optimum method is linked to the accuracy of interest and complexity of the media.