Hydrothermal circulation and subsidence in ocean basins : evidence from the Southeast Indian Ridge. Implications on the permeability structure within fault zones.
Abstract
Recent heat flow data collected with 18 m long probes along a 14 Ma old isochron exhibit high variability, low heat flow values, but rather linear temperature profiles. This linearity may reflect conductive heat transfer from the sediment surface to the highly permeable, igneous basement. However, near troughs, where heat flow values as low as 2 to 12 mWm-2 are measured, linear temperature profiles are observed together with large variations in conductivity with depth. Here, we suggest that heat transfer near troughs is likely to be mainly controlled by downward advection of seawater. To explain the linearity, we propose that the depth of penetration (H) of seawater along faults is much greater than the probe length (d). Hence, the convex curvature of the temperature profile cannot be resolved along the probe length, resulting in an apparent linear temperature profile. Assuming that seawater penetrates down to the base of the faults, we predict (by solving the 1D equation of heat transfer with vertical advection of water near troughs) that temperatures as low as 100°C to 130°C can be found at the base of the crust (H ~3 to 7 km). Such low temperatures can be achieved with average, vertical flow velocities of 10-10 ms-1 and formation average permeabilities as low as 1015 to 10-17 m2, consistently with what is known on the physical properties of sediments and of oceanic crust. In presence of convenient fault spacing, steady state circulation of water within the highly permeable, damaged zone that probably exist along faults can maintain (by lateral, conductive cooling) low, but non-zero, temperatures at the base of the crust. Hence, while the crust is instantaneously ``pre-subsided'' at the ridge axis, the sub-crustal lithosphere contracts slowly, at a rate that is proportional to the difference between the mantle temperature and the average temperature at the base of the ``wet'' domain. This process significantly affected subsidence rates during the [0-25 Ma] period, during which off-axis seawater circulation is known to have been active. The first-order consistency of our model with the available data supports the model hypothesis, sugesting that highly permeable zones are likely to exist along sediment-depleted, submarine faults, allowing steady-state water circulation within fault zones.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.T53C1440G
- Keywords:
-
- 3614 Mid-oceanic ridge processes (1032;
- 8416);
- 8010 Fractures and faults;
- 8045 Role of fluids;
- 8130 Heat generation and transport;
- 8135 Hydrothermal systems (0450;
- 1034;
- 3017;
- 3616;
- 4832;
- 8424)