Subsurface structure and hydrothermal fluid circulation at the Mohns mid-ocean ridge: a multi-geophysical investigation
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Despite the small size of submarine hydrothermal vents exposed on seafloor, the extent and geological significance of hydrothermal systems span over distance of kilometers both laterally and in depth. However, due to remoteness and very limited accessibility of mid-ocean ridges, our understanding of sub-seafloor configuration of these systems is still poorly constrained and lacking empirical observations, especially for the Arctic Mid-Ocean Ridge System. This doctoral work is a natural extension and contribution to the global effort of developing knowledge about hydrothermal fluid circulation at mid-ocean ridges. It presents new geophysical data collected at the Mohns Ridge: magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data transect in the vicinity of an active Loki’s Castle vent field; near-seafloor magnetic and high-resolution bathymetric surveys from two axial volcanic ridges, and active and inactive/extinct vent fields located within the limits of the median valley. Comprehensive investigation of these data along with legacy seismic profiles, regional magnetic data, rock samples, and other available geological and geophysical information provides valuable and currently most accurate constraints on subsurface characterization of the Mohns Ridge and its hydrothermal fluid circulation system. The exceptional quality of the CSEM and MT data yields a multitude of important observations that allow us to delineate lithospheric and crustal structure of the ultraslow-spreading Mohns Ridge. These data present a solid framework for answering a range of fundamental questions such as: what controls the deep structure of the ultralsow-spreading ridges, what are the crustal thickness and melt distribution beneath the ridge, or what drives the crustal formation process. Integration of seismic, magnetic, and bathymetric data helps to better constrain the crustal architecture and analyze it with respect to fluid circulation. Based on this analysis and using electric conductivity contrasts in the subsurface as a proxy for fluid saturation of crustal rocks, we outline the extent and patterns of the fluid flow along the transect. In this context, we define major elements of the Loki’s Castle hydrothermal system and use forward magnetic modeling to test possible geological scenarios. D-surveys from the median rift valley provide additional insights into interplay between hydrothermal circulation, volcanism, and tectonic processes along the ridge. A significant applied value of this work concerns marine-mineral exploration. We map areal extent and infer subsurface configuration of several seafloor massive sulfide deposits associated with hydrothermal venting, including localization and characterization of two new vent sites. Through this multi-geophysical and multi-scale investigation, we acknowledge complexity of the process of hydrothermal circulation in the mid-ocean ridge environment; we connect vent-scale seafloor observations to crustal- and lithosphere-scale subsurface data; describe each case study to the best of our knowledge; and finally, integrate this information to reveal general tendencies and advance our understanding of the studied phenomenon.