Preliminary Investigation of Apparent Hydrothermalism Associated with a Long-Lived Detachment Fault at Kane Megmullion


DOEI Project Funded: 2007


Within the past decade it has been discovered that very long-lived (~1-2 m.y.) normal faults can form at slow-spreading mid-ocean ridges. These 'detachment' faults slip at low angles (20°-30°), and the rollover of the fault footwalls forms corrugated, domed edifices termed megamullions. Most studies of these features have been structural, but recent discoveries at LostCity and TAG show that major detachments can be intimately linked with hydrothermal flow. The former is low-temperature, serpentinite-hosted, and precipitates carbonates and brucite, while the latter is high-temperature, basalt-hosted, and precipitates sulfides and anhydrite. We believe that we may have discovered a third type of association, namely probably low-temperature hydrothermal precipitation of Fe- Mn-carbonate associated with a dominantly ultramafic substrate. During a recent cruise (KN180-2), we discovered evidence that there may have been significant flow of hydrothermal fluids along the detachment fault that formed Kane megamullion. The evidence for hydrothermal fluids is primarily in the form of cones of basaltic debris that are highly cemented by Fe-Mn oxyhydroxide-carbonate deposits. The cones are scattered across the megamullion footwall, which is dominantly serpentinized peridotite. This is the first reported instance of apparent, Fe-Mn hydrothermal deposits from a dominantly ultramafic-hosted system. We hypothesize that the cones were formed where hydrothermal fluids flowed up the shear zone of the detachment fault, percolated through the basaltic hanging wall, and caused cementation of basalt debris. These cemented zones were then rafted with the footwall as it was exhumed, while mass wasting transported the surrounding, un-cemented debris down the fault slope. Carbonate sediments on the megamullion surface also exhibit unusual cementation and coloration patterns, and this may reflect further effects of hydrothermal alteration.

Here we propose to conduct mineralogical, chemical, and stable isotope (ä18ï and ä13C) analyses of authigenic and sedimentary samples from the cones and surrounding sediment to test our hypothesis. This will establish whether they are associated with hydrothermal fluid flow, and will document the nature (e.g., T, pH, Eh) of the discharging fluids. In addition, the form and distribution of the cones will be mapped from KN180-2 Jason and ABE data and integrated with the analytic results to document the character and a real distribution of fluid flow from the detachment fault and megamullion surface. This research will have wide-ranging implications for understanding the geometry and chemistry of fluid flow associated with major normal fault systems.

Significance and Relation to Other Work

The proposed work relates directly to the DOEI theme of fluid flow in geologic systems. It provides a unique opportunity potentially to document that fluid flow from major normal faults in ocean crust is a significant process, as well as to document the chemical character and spatial scale of venting in such systems. This will have wide-ranging implications for the role of fluids in altering and weakening rocks within fault shear zones, in creating possible overpressure that facilitates fault slip, and thus in explaining why detachment faults are abnormally long-lived and also appear to slip at anomalously low angles. In addition, it will provide the first data on Fe-Mn deposits from a dominantly ultramafic-hosted hydrothermal system.