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| Titel |
Hydrothermal circulation in fault slots with topography |
| VerfasserIn |
Sofya Titarenko, Andrew McCaig |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095478
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| Publikation (Nr.) |
 EGU/EGU2014-10934.pdf |
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| Zusammenfassung |
| There are numerous cases where the circulation of hydrothermal fluid is likely to be confined
within a permeable fault slot. Examples are (1) the Lost City Hydrothermal Field (LCHF) at
30 N in the Atlantic, which is likely to be controlled by large E-W faults related to
the Atlantis transform fault and mass wasting on the southern wall of the Atlantis
Massif, and (2) large normal faults bounding the Hess Deep rift in the East Pacific,
which contain intense hydrothermal metamorphic assemblages in lower crustal
gabbros formed at 200-350 °C. This type of circulation could occur anywhere
where steep faults cut the oceanic crust, including large near-axis normal faults,
transform faults and faults at subduction bend zones, and could be the major way in
which the upper mantle and lower crust are hydrated. It is therefore important to
constrain the controls on temperature conditions of alteration and hence mineral
assemblages.
Previous 2-D modelling of the LCHF shows that seafloor topography and permeability
structure combine together to localise the field near the highest point of the Atlantis Massif.
Our new models are 3-D, based on a 10km cube with seafloor topography of ~ 2km affecting
both the fault slot and impermeable wall rocks. We have used Comsol multiphysics in this
modelling, with a constant basal heatflow corresponding to the near conductive thermal
gradient measured in IODP Hole 1309D, 5km north of the LCHF, and a constant temperature
seafloor boundary condition. The wall rocks of the slot have a permeability of 10-17 m2
while permeability in the slot is varied between 10-14 and 10-15 m2. Initial conditions
are a conductive thermal structure corresponding to the basal heatflow at steady
state.
Generic models not based on any particular known topography quickly stabilise a
hydrothermal system in the fault slot with a single upflow zone close to the model edge with
highest topography. In models with a depth of circulation in the fault slot of about
6 km, after an initial period of higher temperature venting which removes heat
from the initial condition, venting temperature is approximately 200 °C with a
permeability of 3x10-15 m2. This falls to about 170 °C with a permeability of
5x10-15 m2. Temperatures can be reduced by restricting the depth of hydrothermal
circulation.
These temperatures correspond to prehnite-chlorite assemblages seen in fault rocks at
Hess Deep, but are higher than those observed at the LCHF. Work is continuing to vary
permeability, fault slot geometry and topography to better match the conditions in the Atlantis
Massif, and to model the effects of dyke intrusion into the fault zone as observed at Hess
Deep. |
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