Page 47 - Geothermal Energy Systems Exploration, Development, and Utilization

1.3 Conceptual Models of Geothermal Reservoirs 23
mid-oceanic ridges and back arc extensional systems, immerged or emerged, and
to less extent pulls part systems developed along strike-slip faults. The geological
setting of such rift zones is then the most favorable context because of the high
mantle heat flow (Figure 1.2), the shallow depth of the mantle crust boundary,
and the periodic magmatic activity (emplacement of stocks, sills, and dykes)
and volcanic flow of hot mafic lavas. Moreover, convection of heat is enhanced
by fluid – rock interaction – intense fracturing related to extensional tectonics
favoring exchange between fluids of superficial and deep origin in the vicinity of
magma chambers. Numerical modeling of rifting processes illustrates the shift of
the isotherms toward surface depending on rifting velocities, presence of strain
softening, and time (Huismans and Beaumont, (2002), Figure 1.12).
Iceland is the best case history for illustrating this first-order parameter for
location of high geothermal gradients (Figure 1.13). A large active volcanic zone,
corresponding to the mid-oceanic ridge, is running SW–NE, and displays various
heat sources (dikes and magma chambers). Seawater, meteoric water, and volcanic
fluids are mixed in pressurized water-dominated reservoirs, often associated with
young tectonic fractures, carrying heat from several kilometers depth toward
the surface (Fl´ ovenz and Saemundsson, 1993; Arn´ orsson, 1995). The regional
◦
temperature gradient varies from 50 to 150 Ckm −1 and the highest values are
found close to the volcanic rift zone.
Active margins related to subduction are sites of intense convection with respect to
magmatic activity. A computed model at the scale of the lithosphere (Figure 1.6)
shows that the subduction of cold lithosphere is accompanied by a raise of hot
lithosphere just above the main plate boundary. Thus, large crustal zones have
◦
temperatures greater than 300 C at very shallow depth and undergo melting con-
ditions at few kilometers depth. Generated calc-alkaline magmatism is responsible
for the intrusion of voluminous granitic suites at shallow depth and related vol-
canism of intermediate to felsitic composition. This convective phenomenon at the
scale of the lithosphere is also responsible of the concentration of U, K, and Th
radioelements in the upper crust, which will contribute to the thermal budget of
the continents over a long period.
Active margin settings are zones with almost infinite source of fluids from
meteoric origin, as generated high relief is bordered by oceanic areas, and from
deep source, in relation to magmatic and metamorphic processes.
Presently, New Zealand and Philippines are the zones where the exploita-
tion of geothermal energy is the most advanced within these subduction-related
contexts.
Collision zones and convergent plate boundaries may also be sites of high geothermal
gradients. Collision is responsible for the development of large thrust systems that
lead to a crustal thickening of several tens of kilometers. Zones situated at mid-
crustal depth within the underthrust slab will then be buried and will undergo
an immediate increase in pressure and a progressive increase in temperature.
As discussed previously (Figure 1.6), the equilibrium thermal field is reached
several ten million years after the thrusting event has ceased. This evolution is