A wide-angle seismic study of the interplay between oceanic detachment faulting and hydrothermal systems

Abstract

Seafloor spreading at slow rates can be accommodated by large-offset oceanic detachment faults, that exhume lower crustal and mantle rocks in footwall domes termed oceanic core complexes (OCCs). At detachment faults, seafloor massive sulphide (SMS) deposits—a direct result of black smoker venting—have been found at both hanging wall and the footwall. SMS deposits at mid-ocean-ridge hydrothermal systems are of interest not only for their potential as future mineral supply but also for their implications to global heat balance. Yet, slow-spreading ridges are found to have highly variable melt supply and lack stable shallow magma chambers to maintain high-temperature fluid flow. Important aspects of the tectonic and magmatic processes that dominate these hydrothermal systems—particularly the heat source driving the fluid circulation—are not clearly understood. To ascertain lithospheric composition of detachments and key drivers of long-lived hydrothermal activity at OCCs, this thesis presents P-wave velocity structures in a fine scale of two OCCs that host large SMS accumulation. We conducted travel-time tomography and full waveform inversion (FWI) of wide-angle seismic data, at the Trans-Atlantic Geotraverse (TAG) and the Semenov hydrothermal fields on the slow-spreading Mid-Atlantic Ridge.

TAG is a mafic-hosted hydrothermal system located at the hanging wall of a detachment. The tomographic velocities derived in this study, along two ~10 km profiles across the TAG detachment, reveal the presence of gabbro in the footwall that is exhumed by a young detachment within an immature OCC. Application of downward continuation to the data improves the visibility of seismic refraction arrivals within the shallowest crust and shows a velocity reaching 6.5 km/s at 500 m depth. In turn, the resulting velocity models illuminate the along-ridge-axis variation in exhumation of lower crustal rocks and confirm the detachments are complex structures. On the other hand, The Semenov hydrothermal field is hosted at a mature and corrugated OCC associated with an inactive detachment. Three-dimensional travel-time tomography reveals a highly serpentinised footwall with P-wave velocities < 6 km/s within the upper 1.5 km lithosphere. Additionally, two-dimensional FWI reveal that shallow mafic intrusion is likely driving the focused, long-lasting hydrothermal flow at the footwall, leading to enhanced serpentinisation that in turn, formed the large SMS deposits at ~5 km west of the neo-volcanic ridge axis. This study offers new insight into the interplay of magmatic processes, hydrothermal systems and detachment faulting at the inception and dying stage during the OCC evolution.

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Identifiers

ORCID for Szu-Ying Lai: orcid.org/0000-0003-4711-514X

ORCID for Tim Minshull: orcid.org/0000-0002-8202-1379

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