Valiya M. Hamza

HEAT FLOW MAP OF SOUTH AMERICA

The results of geothermal investigations carried out in South America have been compiled with the purpose of preparing regional maps of terrestrial heat flow. The compilation revealed that 655 heat flow values had been determined, giving an overall data density of 37106 km2 and a representative mean heat flow of 63+-36 mW/m2. The quality of the data set is variable, depending on the nature of the primary geothermal data, and the geographic distribution of the data set is also non-uniform. In spite of such difficulties a careful analysis of the data set, following suitable priority schemes, has allowed not only the determination of reliable mean heat flow values for a large number of major geological structures in South America, but also the preparation of mosaics of regional heat flow variations. Heat flow is extremely variable in the Cordilleran regions, with the eastern and southern parts having relatively high values compared to the western and northern parts. The general trend of increasing heat flow from the western coastal regions towards inland areas is interrupted by a N-S trending low heat flow belt in the Pre-Cordilleran basins. In the eastern part of the continent heat flow is low to normal (<75 mW/m2) but there are indications that in the Patagonian Platform it is higher than in the Brazilian Platform. There are, however, several isolated localities of high heat flow in the northeastern and south-central parts of Brazil. The Mesozoic rift basins (Potiguar, Reconcavo and Taubate) are also characterized by relatively high values. In order to examine the tectonic significance of variations in the regional geothermal regime, heat flow maps have been prepared using manual and automatic contouring methods. The comparative study of automatic contour maps generated by means of a variety of data interpolation and gridding schemes has led to the identification of some geothermal features that are believed to be related to tectonic processes affecting the South American continent. Prominent among these are E-W trending belts of low heat flow in northern Peru and in central Chile (extending into the Sierras Pampeans in Argentina), as well as high heat flow belts in northern Chile (extending into the Altiplano in Bolivia) and southern Chile (extending into western Argentina). The low heat flow belts coincide approximately with zones of sub-horizontal subduction, while the high heat flow belts are situated in regions of high-angle subduction. Some of these features correlate well with the results of studies on anelastic attenuation, electrical resistivity distribution and some patterns of global seismic tomography. On the other hand, many of these features are not evident in the recent spherical harmonic analysis of global heat flow, which suggests that the use of empirical predictors based on a heat flow-age relation in devising global heat flow maps should be restricted to tectonically stable areas.

Geothermal gradient and heat flow in the state of Rio de Janeiro

Results of geothermal studies carried out at 72 localities have been used in evaluation of temperature gradient and heat flow values of the upper crust in the state of Rio de Janeiro. The investigations included temperature logs in boreholes and wells, calculation of geothermal gradients, measurements of thermal conductivity and determination of heat flow density. In addition, estimates of temperature gradients and heat flow were also made for areas of thermo-mineral springs, based on the so-called geochemical methods. Analysis of these data sets, after incorporation of appropriate corrections (for the perturbing effects of drilling operations, topography and climate changes) has allowed for the first time a better understanding of the regional distribution of thermal gradients and heat flow within the study area. The results obtained indicate that geothermal gradient values are in the ranges of 14 to 26oC/km in Precambrian metamorphic terrain and 19 to 33oC/km in areas of Phanerozoic sedimentary basins. Most of the rock formations are characterized by thermal conductivity values varying from 2.2 to 3.6 Wm-1K-1. Consequently regionally averaged mean heat flow values are found to fall in the interval of 40 to 70 mW/m2. Computer generated contour maps reveal that geothermal gradients and heat flow are systematically high in the western compared to the eastern parts of the state of Rio de Janeiro. There are indications that this geothermal anomaly is probably associated with the belt of Tertiary alkaline intrusives, between Itatiaia and Cabo Frio. Residual heat of large scale magma intrusions in the later part of the Tertiary period may be one of the possible mechanisms responsible for this thermal anomaly.

Tectonic leakage of fault bounded aquifers subject to non-isothermal recharge: a mechanism generating thermal precursors to seismic events

Abstract The thermal consequences of flow transients generated by deformation induced leakage at fault zones intersecting confined aquifers have been examined using a simple analytical model. The recharge of the aquifer is assumed to take place by vertical infiltration through the confining layers. The energy equation relevant in this case has been derived by evaluating enthalpy fluxes associated with lateral and vertical components of fluid flows as well as that due to volume changes in the permeable layer, generated by deformation induced alterations in the hydraulic head. Results of numerical simulations indicate that thermal transients arise during the stress build-up period when the infiltrating fluids mix with those in the aquifer, but relatively rapid return to equilibrium conditions occur as soon as the leakage is cut-off. The model is thus capable of reproducing the characteristic features of thermal precursors found in observational records. According to model results, the growth of thermal perturbation as well as the time for attaining steady-state conditions is proportional to the recharge rate. The magnitude of the perturbation is related to the thermal gradients in the confining layers but permeability contrast between the confining layer and the aquifer also plays a significant role. The model results also indicate that precursory signals are likely to be significant only in areas close to the fault plane where tectonic leakage has a direct influence on the hydraulic head of the aquifer. If the fault plane is sensitive to deformation the tectonic leakage rate may undergo substantial changes in response to changing stress patterns and this in turn may easily lead to abrupt and substantial changes in the local thermal regime. Some of the transient thermal anomalies observed in records of temperatures in boreholes in tectonically active areas have characteristics similar to those predicted by the ‘deformation induced leakage’ model. Examples are presented illustrating model fits to thermal transients, identified as precursors to seismic events, in Firjusa (Turkmenistan), Didiweli (Georgia) and Izu peninsula (Japan). The possibility of obtaining complementary information on processes responsible for fluid flow during pre-seismic periods is pointed out. It appears that the occurrence of thermal anomalies associated with small-scale tectonic deformation may not be uncommon. Its detection depends on availability of suitable boreholes and of the facilities for monitoring of temperatures using high precision sensors over periods comparable to local recurrence periods of earthquakes.

Geothermal investigations in an area of induced seismic activity, northern São Paulo State, Brazil

Abstract Geothermal investigations were carried out in Nuporanga, state of Sao Paulo (Brazil), where occurrence of seismic activity has been found to be closely related to opening of groundwater wells. Results of macroseismic studies show that seismic activity had its beginning in May 1977, soon after completion of drilling of the COLABA well and most of the initial seismic events are located close to it. Geothermal investigations were initiated in September 1977 on the assumption that fluid movements associated with seismic activities are capable of producing short-term time-dependent changes in the local thermal regime and in the hope that identification of such time-dependent changes would contribute to a better understanding of the nature of local seismicity. Results of thermal logs in the COLABA well reveal the existence of an unusual thermal regime with a constant temperature zone (CTZ) down to 175 m followed by a zone in which temperature rises rapidly (TGZ), in the interval of 180–204 m. Repeated thermal logs carried out over a period of three months reveal temperature drops of up to 0.8°C taking place in the TGZ immediately after periods of intense microseismic activity. Temperature measurements of pumped water also show changes of lesser magnitude, occurring in the CTZ, closely related to the frequency of seismic events. Substantial temperature changes related to periods of seismic activity were also observed in two nearby wells in Nuporanga. The available geothermal and macroseismic data have been used in the development of a simple model of the process that have triggered seismicity in Nuporanga. According to this model the COLABA well acts as a natural siphon drawing water from a perched aquifer at depths of less than 40 m and injecting it to a fault zone at about 175 m. The model allows a coherent explanation for the observed correlation between seismicity and absence of pumping. During periods in which pumping is suspended the static level of water in the well is high and the pressure exerted by the water column induces a reduction in frictional resistance at the fault zone. During pumping the water level falls to its dynamic level and consequently the pressure at the fault zone is also lower. Such pressure changes are apparently sufficient to trigger microseismic activity in Nuporanga. The observed temperature drops immediately after tremors could be attributed to the cooling effect associated with the penetration of relatively cold water from upper levels into newly opened fractures. Also the small but significant rise in temperatures during seismically quiescent periods can be considered as a result of warming up of stationary fluid bodies within fracture zones. Measures taken on the basis of this model has been successful in “switching off” seismicity on at least two occasions. In view of this success we conclude that geothermal investigations can be of considerable use in understanding the nature of fluid movements associated with near-surface earthquakes.