GEOTHERMAL SYSTEMS IN ICELAND - STRUCTURE AND CONCEPTUAL MODELS .2. LOW-TEMPERATURE AREAS

A review and assessment of data pertaining to the origin and nature of low-temperature geothermal activity in Iceland are presented. This ac tivity is widely distributed in Quaternary and Tertiary formations on the American plate in western Iceland west of the active belts of volc anism and rifting but it is very sparse on the European plate east of these belts. Low-temperature systems occur in a few places within the active volcanic belts. Temperatures range from just above ambient to a little over 150 degrees C. Generally speaking, reservoir temperatures decrease with increasing distance from the active volcanic belts. The distribution of the low-temperature areas can be correlated to a larg e extent with active tectonism. In Iceland the European plate is tecto nically stable but in the American plate the shear stress field is com plicated, leading to complex fracturing and faulting of the crust at p resent. No single generalized conceptual model describes the basic fea tures of all low-temperature areas in Iceland. Low-temperature geother mal activity is considered to develop by one of the following four pro cesses, or any combination of them: (1) deep Row of groundwater from h ighland to lowland areas through permeable structures driven by the hy draulic gradient; (2) convection in young fractures formed by tectonic movements in old and relatively impermeable bedrock; (3) drift of hig h-temperature geothermal systems out of the active volcanic belts in c onjunction with their cooling and extinction of the magma heat source; and (4) magma intrusion into Quaternary or Tertiary formations adjace nt to the active volcanic belts. Formation of permeable fractures by r ecent earth movements is probably the most common process responsible for the development of low-temperature activity through convection in these fractures. Convection in low-temperature systems with temperatur es above some 60 degrees C is probably mostly driven by pressure diffe rences created by a relatively light hot water column within the syste m and a denser cold water column outside it. in systems of lower tempe rature the convection is driven by hydrostatic head in the recharge ar eas. The source of the low-temperature waters is largely meteoric. How ever, in some coastal areas a significant seawater-groundwater compone nt is present, up to 10%. Waters not containing a seawater component a re low in dissolved solids, or in the range 150-500 ppm. The reason is the low content of anions, particularly Cl, in the basaltic rock form ing soluble salts with the major aqueous cations. Geothermal waters fr om the low-temperature areas in Iceland typically possess lower delta D-values (more negative) than the local precipitation. This difference is variable; most often it lies in the range of 10-30 parts per thous and delta D, but it may be as large as 70 parts per thousand. This dif ference has been considered to indicate that the recharge areas to the low-temperature areas lie inland on higher ground, the distance being as much as 150 km. The interpretation favoured here is that at least some of the low-temperature waters contain a component of ''ice-age wa ter'', i.e. water that is older than about 10,000 years. The ''ice-age water'' is depleted in deuterium relative to today's precipitation. W hen ''ice-age water'' is present in the geothermal water, deuterium ca nnot be used as a tracer to locate the recharge areas to the geotherma l areas and in this way to deduce about regional groundwater Row.