Relationships of Heat Transfer and Decompression Processes in Fault Zones

Sergey O. Balyshev Institute of the Earth's Crust RAS, Lermontov str. 128, 664033, Irkutsk, Russia

The processes of heat transfer were modelled. The model systems consist of: 1) consolidated pluton, which cooled down from the temperature 800 oC, and schematically represented as a sphere 5 km in diameter; 2) monotonous series about 5 km in thickness, overlapping the pluton; 3) tectonic zone with hydrothermal fluid in it. The convectice movement of fluid assumed in the thick series is not contrary to the facts conserning natural hydrothermal systems. To estimate the medium filtration competence near fault zones the experimental investigation of rock permeability after decompression (pressure release) was carried out.

Pressure gradient geosystems were considered in the experiments. Amphibolitic granodiorites and rapakivi-granites were used as subjects of the investigation. Experiments were performed at high T, P and symultaneous effect of aqueous fluid. Various regimes of dissipation (from gradual bringing of the system to standard conditions to shock decompression) were simulated. Permeability coefficients were determined for samples before and after experiments; thin sections were examined.

Microscopic studies of rocks subjected to shock decompression reveal the following variations. Clevage cracks are clearly manifested in biotite and small grains of the ore mineral. Biotite scales are split and become swelled. Rather wide unmineralized joints (to 0.02 mm) are developed almost in every grain of plagioclase (to 6-7 in a grain). Large joints are often accompanied by smaller feathering ones. Amphibole grains are cut with coarse subparallel joints with noticeable chloritization. Not open joints but zones (to 0.012 mm wide) are often observed where amphibole is deformed. Many large joints cut all minerals of thin sections. From studied physical properties, permeability of rocks is most regularly and unambiguously dependent on the regime of pressure release. When the system is most rapidly brought to standard conditions permeability of granodiorites increases from 7.97×10-3 mD in the reference experiment to 113.83×10-3 mD in samples obtained at shock decompression (the time of pressure release is 2 seconds). For rapakivi-granites it is 0.992 mD and 1.978 mD, correspondingly (mean values of a set of experiments are used). Permeability uncrease for granodiorites is accompanied by the decreased coefficient of open porosity from 4.57 % for primary rocks and to 2.33 % for samples after shock decompression.

The obtained experimental results were used in order to evaluate of fluid rates in models. The development of several model systems with increasing filtration rates has been considered during 10 thousand years. The plane, where the heat processes were calculated, was the longitudinal section directly along the fault zone, just the part of the Earth crust after shock decompression.

In the case with low permeabilities and correspondingly low filtration rates the gradual heat spreading from sources take place. As a result, a new quasistationary temperatures distribution is formed. With increasing of medium permeability and filtration rate qualitative changes in geothermal field appear in the form of local temperature maxima and minima. The splitting of the temperature field in discret thermal anomalies occurs. In the model systems, where the filtration rate is about 0.3 km/century or more, the conductive heat transfer can not reach essential values and it is of secondary importance; the convection predominats. Begining from some threshold values of the filtration rates, all model systems develope by the same way. But the systems, where fluid rates are more, evolve faster to the end structure of the thermal field.

Summing up the obtained results, it's quite reasonable to pay attention to structural self-organization of pore and crack media in rocks, which is possibleif the rate of pressure release is rapid enough (more than determined critical value). The connected system of pores and cracks is favorable for intens ascending convective heat transfer, that is reflected in structurallization of temperature fields in modelling.

Supported by the Russian Foundation for Fundamental researches (grant 96-05-64770).