Numerical simulations of coupled heat and fluid flow were used to study the importance of various structural as well as temporal parameters in a vertical cross-section through the upper crust at a continental rift zone such as, for instance, the Rheingraben. the following are investigated in detail: (1) the effects of permeability variation, anisotropy, vertical layering, and channeling by fault zones, (2) the temporal evolution of the involved processes in response to temporal and lateral variations of the basal boundary conditions, (3) the relative importance of buoyancy-induced free and topography-controlled forced convection, and (4) alternative driving forces connected with the release of heat and fluid by the cooling and the dehydration of a lower-crustal plutonic intrusion. The main results are:
* The transition from a conductive to an advective thermal regime in the earth's upper crust occurs within one permeability decade.
* A laterally variable, sinusoidal heat flow density anomaly of 20 mW m-2 at 15 km depth over a period of 6 Ma (which corresponds to the heat release of a cooling pluton at depth), combined with permeabilities greater than 5·10-18 m2, is sufficient to drive a thermally relevant free convection system in the upper crust. For permeabilities in excess of 3·10-17 m2 the thermally induced convection is sufficiently strong to support an advection-dominated regime.
* Permeabilities such as required for advection dominated regimes are reported in experimental studies for averaging intervals of 1-10 km.
* Even a very modest amount of anisotropy has a great influence on the temperature and flow field geometry.
* For a horizontally layered sedimentary cover the results suggest that the type of heat transfer or redistribution in the sedimentary top layer does not influence the onset of thermally relevant convection in the crystalline basement. However, there is definitely fluid flowing across the sediment-basement interface. This is supported by geochemical and isotope data.
* Fault zones, modeled as localized high-permeability zones, drastically modify the temperature and flow fields, when introduced into the previous model geometries. Depending on the ambient permeability they either support a pervasive recharge of the model from the fault or a closed circulation cell within the fault itself.
* Evidence from oxygen isotope studies of hydrothermally altered rocks indicates that meteoric water must have percolated down to depths of 7-12 km and deeper (Hoefs, 1989). In all of the above models great amounts of meteoric water infiltrate the upper crust to great depths by virtue of thermally initiated free convection systems. On the continents, far away from subduction zones, we cannot think of any other physically plausible process capable of accomplishing this.
Hoefs, J., Niedersächsische. Akad. Geowiss. Veröfftl., 1, 43-48, (1989).