HEAT FLOW AND TEMPERATURE GRADIENTS IN CHILE# MIGUELMu~oz I ANDVALIYA HAMZA2 S u m m a r y : Conventional heat-flow measurements in Chile carried out by other workers are summarized, Between latitudes 26 - 29 ° S heat flow is consistently low (< 42 m Wm -2) excepting a site in the Andes slope (75:3 mWm-2) In Central Chile (33 °S) near Santiago, a value in the Andes (60.7 m W m -2) is lower than the value in the Santiago basin (78.7 mWm-2) Heat flow through the sea bottom around the Chile Ridge (about 44 - 48 ° S; 75 - 80 ° W) ranges between 25 and 414 mWm-2; heat-flow estimates based upon the location in depth of the phase of gas hydrates have also been carried out in this area In Tierra de/Fuego the only heat-flow value is 96.3 m Wm -2 The present heat-now studies in Chile not allow any conclusions to be drawn on the genered heat-flow distribution and its description within the frame of new global tectonics Only some preliminary modal results comparing heat-flow measurements in the area of the Chile Ridge to thermal effects produced by a ridge-trench collision may presently be partially adopted A general discussion regarding the results from global seismic tomography, maximum depth of seismic coupling and thermal processes in Chile is also presented The silica geotemperature in the Santiago basin resulting from 257 groundwater analyses is 77.4 +_10.4 °C; the equivalent heat flow is 92.5 ± 16.6 m W m -2 which is in agreement with the conventional heat-flow value in this area Geochemical thermometry indicates fluid temperature at depth higher than 200 °C in some of the 33 hot-spring areas evaluated using SiO2 , Na-K-Ca and 1Va-Li geothermometers The evalutation of fluid rock equilibrium and CO2 - fugacities by means of relative Na, K, Mg and Ca contents of thermal waters indicates that only in El 7htio and Puchuldiza in Northern Chile have fluids attained partial equilibrium with both K-Na and K-Mg mineral systems Other geothermal areas in the north, and many hot springs in Central Chile, correspond to/mmature waters which are generally unsuitable for the evaluation of K/Na and K/Mg equilibrium temperatures In Central Chile the evaluation of some hot-spring waters in partial equilihrium condition indicate deep temperatures between 80 °C and 245 °C In the area of El Tatio the combined heat flow (conductive and convective) yields a value of 1465 m Wm -2 with fluid circulating within I km of an underlying magmatic intrusion at - k m depth The water catchment area may be situated 20 krn to the east of the geothermal area, with the underground fluid moving at a rate of about 1.3 k m y -1 Temperature logs in wells for oil prospection show that temperatures are affected by drilling disturbances Some preliminary BHT estimates of gradients yield between 26.3°C km -1 and 72.4 °C km-1 Thermal conductivity and diffusivity data from these wells are also shown # Presented at the International Meeting on Terrestrial Heat Flow and the Structure of Lithosphere, Bechyn~ Castle, Czech Republic, September - 7, 1991 Address: Departamento de Geoffsica, Universidad de Chile, Casilla 2777, Santiago, Chile Address: Institute Astron6mico e Geoffsico, Universidade de S~o Paulo, Caixa Postal 9638, 01050 S~o Paulo, SP, Brasil Studiagooph, etgeod.37 (1993) 15 M, Mtr~oz and V Hamza INTRODUCTION In this paper several results involving fundamental and applied problems of heat flow and thermal processes in Chile are presented Many of these results were presented at the International Meeting on Geothermics and Geothermal Energy held in Guaruj~ (Sao Paulo) in 1986 [1] A more detailed account concerning heat-flow studies, carried out by other workers, is given here The results concerning geochemical thermometry are discussed in more detail in the present paper, including new outputs from the evaluation of fluid-rock equilibrium and CO L fugacities using Na-K-Mg-Ca geoindicators Also, some new geothermal data from wells for oil prospection are shown; particularly, thermal conductivity data of deep boreholes in continental Chile were obtained for the first time after the work by Uyeda et al [2] Some of the ongoing work concerning radiogenic heat production in Central Chile will not be presented here; a summary of preliminary results can be found in [3] Also the effects of climatic changes on geothermal data are not discussed - a general approach to this problem in the southern hemisphere is summarized in [4] 10o 60I ° | -20 o x ~7;3 33,5 SAINTACLARAIO.O EL SALVaO,OR~ EUSA311~ BOOUERONG'HAf ~ ? 30O • #JVALLENAR • 4L9)21.8 (L:'9.3o~LA AFRICANA78.7 • ~ DiSPUTAOAeO.7 -4O ° ~OIm -SO o ISLATIERRAOELFUEGO9G.3 Fig Heat flow in mWm -2 Values in the Pacific Ocean are from von Herzen [5], values in brackets from Diment et al [6] Other values are from Uyeda et [2] Heat-flow values in the Chile Ridge are presented in Figs and and in Table 316 Studia geoph, et geod 37 (1993) Heat Flow and Temperature Gradients in Chile The perspective of heat-flow studies in Chile is one which points to the integration of fundamental and applied (energy) problems This has been tried from the very beginning of heatflow research conducted during the last decade In this way this perspective can be set in agreement with the statement by Keflis-Borok [7]: "The integral approach to other studies of the lithosphere, besides it dynamics on short time scales, deserves attention It is a major and, I believe, the major workhorse in the present perestroika of the solid-Earth sciences: their integration and globalization, integration of basic and applied problems, change of theoretical base, etc." As will be shown in this paper no conclusions about the general heat-flow distribution in Chile can be drawn from the present state of studies Only some remarks concerning small areas and localized thermal processes can be put forward HEAT-FLOW MEASUREMENTS BY CONVENTIONAL METHOD The first heat-flow density determinations in continental Chile were communicated by Diment et al [6]; these were the only heat-flow values known in South America till 1969 Also, two values in the Pacific Ocean near the coast of northern Chile were obtained previously by yon Herzen [5] Other values of temperature gradients and conventional heat-flow density were published later by Uyeda and Watanabe [8], Uyeda et al [2], Herron et al [9] and Cande et al [10] Two heat-flow values in the Pacific Ocean (Fig - 37.3 and 33-5 mWrn-2) near the coast of northern Chile are from [5] The lowest heat flows from the south-eastern Pacific may be due to the small amount of heat generated by radioactivity in the crust, with little or no heat coming from the mantle; no correction has been applied to the heatflow measurements from these sites, and irregular sea-bottom topography may have no effect in these cases [5] Between latitudes 26°S and 29°S (Fig 1) heat flow is consistently low (< 42 mWm -2) excepting the area of El Salvador, in the middle slope of the Andes, with a heat-flow value of 75 mWm -2 Among the heat-flow measurements in continental Chile, a topography correction was felt to be needed only for this site [2] - the uncorrected value is 59.9 mWrn-2 The site of E1 Salvador is near the southern limit of the active volcanic chain of northern Chile Low heat-flow values between latitudes 28 °S and 33 °S are encountered in a region where no active volcanism is observed Also, it is worth noticing that no Andean thermal springs are found in Chile in zones with no active volcanos The La Africana Mine in the Santiago basin is characterized by a high heat flow (79 m W m -2) - estimates based upon geochemical thermometry corroborate this high value At the same latitude (nearly 33 °S) the measurement in the La Disputada Mine in the western slope of the Andes yielded a "normal" value of 61 mWm -2 The only measurement in Tierra del Fuego - carried out in the area of an oil field - yielded a high value of 96.3 rnWm-2 As stated by Uyeda et al [2], the foregoing beat-flow data from Chile are neither sufficient to draw any solid conclusions on the heat-flow distribution of the South American continent, nor they allow any definitive description of the distribution of heat flow in active arcs areas Also, preliminary models used to study thermal processes s~dia g~o~ et g~d 37 (~993) 17 M, M t ~ o z and V Hamza in subduction zones and to understand the volcanism in Chile - proposed by Honda and Uyeda [11], and based on the simple fluid dynamical model of McKenzie [12] - show only small differences in the thermal structure of the region with volcanic activity when compared with the region where volcanic activity is not observed This suggests that more sophisticated models and additional heat-flow data are needed to explain the volcanism in Chile Data are too scarce as to establish the trend of heat flow from the trench-arc zone to the volcanic line and to the back arc region The contribution from crustal radiogenic sources to heat flow in back arc regions may be sustantial if the back arc region is continental [ 11] A further preliminary model for the thermal structure of the Central Andes, proposed by Honda [13], shows a strong cooling of the upper continental mantle due to interaction with the subsiding oceanic plate; nevertheless, the model parameters assumed by Honda [13] not correspond to the Chile subduction zone Other studies on the thermal state of western South America are contained in [14,15] A general vertical distribution of temperature in different regions of South America as well as the evolution of the thermal regime of the Archean crust are described in [16] In the area of the Chile Ridge (44 °S - 48 °S; 75 °W - 79 °W) - including a ridgetrench collision zone - heat flow is very variable with values between 25 and 414 mWm-2 In this case a mean heat-flow value cannot be estimated-complex processes envolving hydrothermal circulation and sedimentation in tectonic areas such as the Chile Ridge may be linked to anomalous dispersion in heat-flow values [17,18] Values in the area of the Chile Ridge are shown in Figs and and in Table Cande et al [10] have found that the value of 212 mWm-2 on the seawardmost end of line A-A' ,,,o lo9 -4Ei° ~ e Fig 318 c' PE~l.s Heat flow [mWm-2] in the Chile Ridge from Herron et al [9] and Cande et al [10] Heat-flow values in cross-sections A-A' to D-D' are presented in Fig and Table Studia geoph, et geodo37 (1993) Heat Flow and Temperature Gradients in Chile Table Heat flow in the area of the Chile Ridge [10] Latitude (S) Longitude (W) Water Depth [m] Heat flow [mWrn-2] 47°23.6' 47022.8 ' 47019.4 ' 47019.0 ' 45054.4 ' 45055.0 ' 45054.5 ' 45°54.2' 45054.2 ' 45°54.1 ' 47044"0' 47°44"0' 47044"0' 47044"5, 47°44"1 ' 45003"9, 45004"3' 45003"8' 45004.6 ' 76°24.7 ' 76022.2' 76010.5 ' 76008.2 ' 75044.9 ' 75042.4' 75052.9 ' 75052.0' 75051.9' 75051.6 ' 75051"7' 75047"9' 75047"8' 75°45"8' 75042'4 ' 75047"2' 75049"8' 75°53"1 ' 75055.8 ' 3656 3579 3510 3403 1701 1626 3153 2775 2625 2441 1879 1710 1110 1260 842 2017 2122 2358 3225 129 116 148 112 116 162 90 343 316 194 79 I10 92 116 132 72 80 62 132 (Figs and 3) is anomalously high by roughly 100 m W m -2 when compared with the theoretical Parsons and Sclater [19] value, and considering the effects of rapid sedimentation indicated by the model of Langseth et al [20]; they conclude that this high value m a y be due to the effects of the flow of hot water from deeper levels along thrust faults near the foot of the landward slope In the case of line C-C', the predicted value of heat flow, corrected for sedimentation, is about 112 m W m -2, which is only slightly less than the average of observed values See also the caption to Fig for further comments on these heat-flow sites Also, in this area the heat-flow estimates based upon the location in depth of the phase of gas hydrates in marine sediments [10] are found to be in good correlation with conventional measurements The boundary of gas hydrates was determined by means of seismic reflection - this method of estimating heat flow is described in [21] Cande et al [10] have also compared the heat-flow measurements and estimates to the numerical model of DeLong et al [22] which considers the thermal effects of a ridge-trench collision and assumes that heat is transferred conductively The observed heat-flow values compare favourably with the predicted values at a point near (5 km) the trench axis in the case of line B-B'; along lines A-A' and D-D' the measured heat flow is higher than predicted values - Cande et al [10] suspect that this difference may be due to the simplicity of the model describing the complex structure of the forearc region The measurements along C-C, are not included in this comparison as they were made above recently deformed trench sediments and not record the effects of ridge collision Studia geoph, et geod 37 (1993) !9 M Muffoz and V Hamza GEOCHEMICAL THERMOMETRY OF HOT SPRINGS AND F L U I D - R O C K EQUILIBRIUM Fluid temperature at depth has been estimated in 33 areas with hot springs activity [ 1,23] The geothermometers used were SiO2 [24], Na-K-Ca [25], Na-Li and Li [26] The complex processes controlling fluid-rock equilibrium, and the evolution of fluid chemical composition along their circulation path in the earth, require a comparative study of temperature estimates provided by different geothermometers Temperature estimates are shown in Table 2, and in a more detailed analysis in Table and for E1 Tatio - the most studied of the geothermal areas in Chile HEAT FLOW (mW/rn2) A K c c' -0 -1 so -2 -2 -3 OSR -4 -4 -5 "~ B B' ° -6 D D' b32 ~ -0 ,oI/4 o ~ ~km ~4 ~ !o _ -4 -5 Fig Heat flow in the cross-sections of Fig (from [10]) The value of 212 mWm -2 [9] on line A-A' corresponds to the m y old oceanic crust being subducted here; measurements along A-A' reflect the condition of the trench's landward slope about m y, before the collision The heat-flow measurements along B-B' reflect the effect of ridge subduction; sites with heat-flow values over 300 mWm -2 on the lower slope above the oceanic crust are roughly 0.5 m y old Sites along C-C' on the crust m, y old are above recently deformed sediments and not reflect the effects of the ridge-trench collision Measurements along D-D' reflect the effect of the ridge-trench collision that ocurred about m y ago 320 Studia geoph, et geod 37 (1993) Heat Flow and Temperature Gradients in Chile Table Hot springs and geothermal areas in Chile Ts: Temperature of spring discharges T(SiO2), T(Na-Li), T(Na-K-Ca): Temperatures estimated by chemical geothermometers TMg(Na-K-Ca): T(Na-K-Ca), Mg corrected Temperatures in *C Locality Latitude Ref TS T(SiO2) T(Na-Li) T(Na-K-Ca) TMg(Na-K-Ca) Longitude Bafios Juras¢ 18°12'S 69°32~/ Untupujo 18013'S 69"17'W Suriri 18055'S 68059'W Chinchillani 19"08'S 68°55'W Puchuldiza (1) 19"23'S 68°58'W Chusmiza 19041'S 69012'W Pampa L'Lrima 19°51'S 68o56'W Mamifia 20°15'S 69°10'W E1 "ratio(2) 22°20'S 68o01'W Socos 30043'S 71°35'W Colina 33010'S 70°38'W Apoquindo 33025'S 70°25~/ Bafios de Colina 33048'S 70000'W Bafios Morales 33°50'S 700035V Cauquenes 34°16'S 70°35'W Vegas del Flaco 34°57'S 70o28'W San Pedro 35008'S 70027'W fl = 1/3 fl = 4/3 a 66 105 218 131 63* 129orn.a a 15 13 183 151 81 n.a a 82 164 229 234* a 29 116 176 193 79* n.a a 85 200 237 205* 245 196 a 46 101 58 133 76* n.a a 69 179 264 195 147 48 b 41-52 92 112 111 53* a,c,d,e 78 171 262 207* b 26 64 11 74 29* f 30 58 189 112 38* 112 or n.a f 22 67 101 26* n.a g 50 - 257 170 208 77 f 22 69 75 126 116 126 f 48 90 134 226* 121 226 f 77 145 150 205* 193 194 f 34 69 235 234* 313 209 233* 197 (see Table 3) to be continued Studia geoph,et geod, 37 (1993) 321 M Muftoz and V Hamza Table continuation LocMi~ Latitude Ref TS T(SiO2) T(Na-Li) Longitude Bafios de Azufre 35°16'S 70o38'W Panim~ivida 35045'S 71°25~V Campanario(3) 35°56'S 70033'W Catillo 36°16'S 71°34'W ChillOn 36°57'S 71033'W Pemehue 38°03'S 71o44'W Tolguaca 38°14'S 71o44'W Manzanar 38°27'S 71043'W Rfo Blanco 38°35'S 71o42'W AguasdelaVaca 38°37'S 71o37'W Minettie 39°19'S 71o44'W San Luis 39o2rs 71o33,w Palgufn 39%3'S 71 °47'W Llif6n 40°12'S 72°17a~V AguasC~ientes 40°37'S 72°23"~V Puyehue 40°39'S 72o21'W T(Na-K-Ca) TMg(Na-K-Ca) ]]= 1/3 /l= 4/3 f 39 150 141 193 92* n.a f 32 77 124 98 38* 98 or n.a f 32 120 90(259) 172 172 153 f 35 97 185 103 64* 102or n.a f 89 204 242 191 71 n.a f 37 128 148 152 126 152 f 90 145 266 150 93* 81 f 48 110 196 - f 90 173 - 192 46* n.a f 60 141 280 212 125 173 f 36-46 110 124 138 100 f 36-46 100 158 140 73* f 44-46 102 157 129 70* 94 or n.a f 17 81 287 165 36* n.a f 50-75 131 164 154 109 147 f 50-60 124 156 142 108 112 to be continued 322 Studiageoph, et geod 37 (1993) Heat b'low and Temperature Gradients in Chile Table Ref.: continuation Chemical composition taken from: a - Lahsen [27]; b - Merino [28]; c - Cusicanqui et al [29]; d - Lahsen and Trujillo [30]; e - Ellis [31] as reported in Giggenbach [32]; f - De Grys [33]; g - Benado and Maffn [34] Notes: (1) (2) (*) temperatures are the mean calculated temperatures of three springs temperatures are the mean calculated temperatures of the thirty-three springs sampled (see Table 3) T(Na-Li) has been calculated for C1- < 0.2 M and (CI- > 0.3 M) [26] as C1- in Campanario is 0.29 M indicates the temperature to be chosen according to the rule by Fournier and Truesdell (-) lack of data (3) [25] The Mg correction of the calculated T(Na-K-Ca) was carried out by referring to the graphs of Fournier and Potter [35] n.a an Mg correction according to [35] was not applied; in some particular cases an Mg corrected temperature is given for the 13= 113 branch of the Na-K-Ca temperature [25] An Mg correction to T(Na-K-Ca) was not applied (n.a.) if T(Na-K-Ca) was below 70 *(2, or if R = {Mg/(/vlg + Ca + K)} x 100 (in equivalent units of concentration) was less than 0.5, or if the ATMg correction was negative Neither the SiO2 nor the Na-K-Ca geothermometers are reliable for acid sulfate-rich waters that contain little chloride [25] This is the case of the Chillfm thermal waters (pH: 2.4 - 5-87; SO4/C1:300-500 - [36]) - a much lower SO4/CL ratio of 4.274 is given by De Grys [33]; a Na-Li temperature estimate for these fluids is 242 °C Following the classification of White et [37], the characteristics of this system seem to be those of a dry vapor system These systems may provide a mechanism to separate volatile Fig from other volatile elements; Hg deposits could then be encountered in their surroundings and - this being more speculative - porphyritic copper below their phreatic level [37] Rio Blanco (Table 2) is one of the moderate acid sulfate waters (pH: 4.33; SO4/C1: 5.0); no data on the Li content are available to calculate a T(Na-Li) estimate For Baftos Jurase, Chinchillani and Chusmiza, the SO4/C1 ratio ranges between 4.3 and 5.0 (pH is about 7-5); the content of these waters corresponds approximately to the acid HCO3-SO4 type waters of White et al [38] Also, the composition of the Tolguaca springs is similar to those of acid sulfate waters, but their pH = 6.43 and the HCO3/C1 ratio is lower than that of acid HCO3-SO4 type waters The analyses and classification of waters of central Chile were reported by De Grys [33] A magnesium correction to the Na-K-Ca geothermometer [35] has also been carried out, and the TMg(Na-K-Ca) estimates are shown in Table Magnesium corrections are important in the cases of Suriri, San Pedro, Campanario, Aguas de la Vaca and Puyehue Studia geoph, et geod 37 (1993) 323 M Mufloz and V Hamza Table El Tatio Analysis of 33 hot springs Number of sampled springs: 33 Mean surface temperature: 78 °C Standard deviation: 13 °C mean T(SiO2): mean T(Na-K-Ca): mean T(Na-Li): st dev: st dev: st dev: 171 °C 207 *C 262 °C 170C 20°C 9°C ATMg: Mg correction to T(Na-K-Ca) - Temperature va,lues in *C • Number of springs with available Mg analysis: 12 Spring T(Na-K-Ca) R% ATMg 65 80 109 149 181 186 202 218 227 238 244 339 173 175 202 196 211 208 218 229 227 230 231 229 3.8 0,6 0.3 0.8 3.3 0.5 1-7 0-8 0.1 0.2 0-1 0-2 6.2 7.2 n.a 1-1 11.7 8.8 0.6 0.3 n.a n.a n.a n.a = { Mg/(Mg + Ca + K)} x 100, using equivalent units of concentration; R% ,aTMg to be subtracted from the calculated T(Na-K-Ca): ATMg = - 1.03 + 59.971 log R + 145.05 (log R)2 - 36711 (log R)21T- 1.67 xl07 log R/T the calculated Na-K-Ca temperature in K; T an Mg correction to T(Na-K-Ca) according to [35] was not applied n.a For many hot waters a Mg correction is not applied according to [35] - see last note in Table The cases of Pampa Lirima and Bafios de Colina will be considered later The results of geochemical thermometry applied to 33 hot spring-waters of the E1 Tatio geothermal area are shown in Table The tectonic features and location of springs and wells of El Tatio are shown in Fig As can be seen in Table 3, the Mg correction to T(Na-K-Ca) is generally not relevant in E1 Tatio The highest temperature estimate is given by the Na-Li geothermometer (262 °C) showing the smallest standard deviation (9 °C) The temperature estimates from well discharges are given in Table 324 Studia geoph, et geod 37 (1993) M Mu~oz and K Hamza (h < H) and themml conductivity K, and provided the magma supply can maintain the isothermal condition (Tin) during the initial stages, an equilibrium state will be established in the overlying rocks at a characteristic time "r(H) [46]; the hydrothermal convection that should occur through the fractured zone of thickness h will set up an effective conductivity NK (N > 1) As shown in [46], the mean combined heat flow Qc from the system is equal to K TmI[H- h(N- 1)IN] The expression in brackets can be called the "effective depth" of magma; it is the depth that would be implied by heat-flow observations if convection were absent [46] The difference between depth H and the "effective depth" of magma is the thickness that is made "transparent" by hydrothennal convection Considering the temperature and volumetric flow characteristics of E1 Tatio - and following a procedure like the one described in [47] - a value of 2.03 x 10s W is reached for the heat discharge by convection [1] Taking into account the structure of the El Tatio graben the combined heat flow is estimated to be Qc = 1465 mWm-2 An "effective depth" of magma of 1.1 km has been obtained, indicating that hydrothermal convection evolves between the surface and within 1-1 km of the magmatic intrusion if circulation is vigorous (large N) If N is moderate circulation will develop even nearer to the magma intrusion Magnetotellurie soundings [48] in the north of Chile indicate a highly anomalous zone of 0.4 ohm m of electrical resistivity at - km depth in the area of El Tatio ~ig 9) This zone may correspond to granite at a temperature of about 800 °C enriched by a high electrical conductivity component (H20), as can be inferred from [49] Assuming the value of 1.1 km for the magma "effective depth" in El Tatio, fluid circulation is evolving to a depth of - km, with magma becoming "transparent" through this thickness by means of hydrothermal convection [ 1] The isotopic analyses of spring, well and meteoric water samples from El Tatio deuterium/hydrogen and ISo/160 ratios - carried out by Giggenbach [32] and NE SW SAG W-Cordillera (:HI TUR TAT Altlplano LAC POR SON ESC - ~ E-Cordillera MAR ~ ~r~ -~e=_ '~ 0,7 " - 1)0 ~ AZU i';;' " , ,, ! l ~ - 5Okra Fig Electrical structure of the Andean crust in northern Chile and southern Bolivia as inferred from magnetotelluric studies [48] Resistivity values in ohm rn The anomalous zone in the area of El Tatio (TAT) may correspond to a rnagmatic intrusion at - km depth Hydrothermal convection evolves within I km of the intrusion upper contact 334 Studia geoph, et geod 37 (1993) Heat Flow and Temperature Gradients in Chile re-examined by Youngman [41], indicate that the composition of the groundwater (the cold unminemlized fluid) is - 9.5%o81sO, - 54~o b'D which is essentially the composition of the only meteoric sample, snow (other waters considered by Giggenbach [32] as meteoric were cold spring and drainage waters) Then the El Tatio groundwater is of meteoric origin With a deuterium content of waters of about -78%0 the recharge catchment area is found to lie some 12 - 16 km to the east of the geothermal area [32] Preliminary data for the tritium content of undiluted E1 Tatio waters yields a value of 3.2 T.U (1 T.U = 10-8 T/H) [29] Upon this result an age of about 15 - 17 years is suggested by Cusicanqui [29] for the E1 Tatio waters on discharge Taking into account the former results on the depth of fluid circulation ( - kin) the actual distance travelled by the water may be estimated to be 20 kin, at a rate of aboutt 1.3 km/year TEMPERATURE OF WATER AND HEAT FLOW IN THE SANTIAGO BASIN (RESULTS FROM GEOCHEMICAL THERMOMETRY) Estimates of water temperature and heat flow based upon geochemical methods is particularly significant in areas with no recent magmatic activity In some cases sedimentary basins may be considered as potential areas of low-enthalpy geothermal / J • ~" ~'~'~, G ,Yl/, Yit // () i Ii i ~I ~,.k.~ ~al / ~ r~ a "°~ _i ''-'~ L ~ ~ ',,.// / v t • I "h Fig lO The Santiago basin (~om [50]) Water analyses of the enclosed area (broken line) were considered to estimate the temperature at depth and equivalent heat flow (see Table 5) The zone of water discharge along the northwest basin boundary is characterized by the larger relative number of heat anomalies Also shown is La Africana Mine with a conventional heat-flow value of 78.7 mW'2; in this zone the anomalous mean fluid temperature (silica) at depth and mean surface equivalent heat flow are, respectively, 94 oC and 117 roW"2 Stadia geoph, et geod 37 (1993) 335 M Muffoz and V Hamza Table Temperature at depth T(SiO2) and equivalent heat flow (Q equiv.) in the Santiago basin Number of samples T(SiO2) [°C] Q equiv [mWm-2] Q conventional [mWm-2] All data Anomalies only Anomalies excluded La Africana Mine Area 257 54 203 77-4 + 10-4 92.1 :t: 6-7 73.5 + 7.1 92.5 + 16-6 114-1 + 18-7 86.8 + 16.0 All data Anomalies only Anomalies excluded 56 14 42 75.6 + 12.4 94.4 + 5-5 69-4:1:5.9 89.9 + 16-3 117.4 5:19.0 80.8 5:15.4 78-7 Q equiv.: calculated after [24]: T(SiO2) = mQ equiv + b; for the Santiago basin the following values have been adopted [1]: m = 683 + 67 [°C row-l]; b = 14.2 + 5-1 [°C] Q conventional : value taken from [2] systems - some insights on the existence of such systems in relation to tectonics in Chile have been presented in [51] In the Santiago basin (Fig 10) estimates from the S i t geothermometer [24] have been carried out which indicate anomalous high water temperature and high heat flow compatible with the only conventional heat-flow measurement in the area [1] The larger relative number of anomalies is encountered in the northwest basin boundary - the zone of water discharge The pattern of heat-flow anomalies was found to be in agreement with studies concerning this kind of areas (e.g., Smith and Chapman [52], Woodbury and Smith [53]) which indicate that deviations result in negative anomalies of heat flow in the recharge area and positive anomalies in the discharge area, with an intermediate zone characterized by a base heat flow A further discussion of the hydrological characteristics of the Santiago basin can be found in [1] The results for the Santiago basin are shown in Table 5, The possibility of using this basin as a low-enthalpy geothermal system - with industrial applications, heating, greenhouses - has to be studied Recently, a preliminary project for the assessment of low and middle-enthalpy geothermal fields in Chile - including the case of the Santiago basin - has been proposed to the Chilean government It is woth noting that the application of the Na-K-Ca geothermometer of Fournier and Truesdell [25] to well waters of the Santiago basin gives irregular results in respect to the selection of the 13- value to estimate temperatures The 13- value is assumed to be 1/3 or 4/3 [25] but - as these authors have observed - in some places the net rock-water reaction may be such that 13 is very different from the former values Also, the evaluation of the fluid-rock equilibrium by means of the K-Na and K-Mg subsystems [39] indicates that the Santiago basin waters are immature and that many waters plot 336 Studia geoph, etgeod 37 (1993) Heath'love and Temperature Gradients in Chile near the Mg comer of the equilibrium diagram If K/Mg temperatures were still valid, deeper equilibrium temperatures in the Santiago basin may be of about 80 °C TEMPERATURE GRADIENTS IN BOREHOLES; THERMAL CONDUCTIVITY OF SAMPLES Geothermal gradient data are shown in Table and in Fig 11 (the temperature axis does not start at T= °C) The general form of T-logs show that temperatures are affected by drilling disturbances The least affected part is at the bottom of the hole, but even here corrections are necessary There is no reliable information about the duration of the last mud circulation, and the time elapsed between the end of mud circulation and the time of the bottom-bole temperature (BHT) measurement Some preliminary and uncorrected estimates of gradients (G) are shown in Table 6, using the relation G = (TnHr- To)IZnHT); the value - that must be corrected - To = 10 °C was assumed Gradient values (CVL) by least-squares fit to the T-log data along one depth interval are also given in Table Presently, it is not possible to ascertain the propriety of these interval-gradient values - temperatures may be strongly affected by mud circulation A change from high to very low values of temperature gradients in one borehole has been observed in other areas, and may be due to terrain effects (e.g [54]) Thermal conductivity of samples from the Pehu6n-1 borehole (depth interval between 475 and 1932 m) is given in Table and in Table - the geological formation and rock type have been considered For other boreholes (Lebu-2, Dolores-1 and Curanilahue-2), thermal conductivities and diffusivities are shown in Table The needle-probe method Table Geothermal gradients in boreholes G: calculated by considering the bottom borehole temperature (BHT) CVL: gradient values by least-squares fit to temperature data along one depth interval (see Fig 7) Borehole Latitude Longitude Pehu~n-1 37°39'08"S 73°32'44"W Lebu-2 37°38'25"S 73037'59"W Dolores-1 19041'S 69055'W (approx.) Curanilahue-2 37"25'S 73030'W Cholchol-1 38035'S 72o50'W (approx.) Studiageoph, etgeod.37 (1993) Depth interval [m] G [°C/KIn] - 1900 300 - 1800 - 1300 200- 1150 - 400 27.2 - 1550 325 - 1500 - 315 26.3 CVL [°C/Km] 6.0 72.4 7.9 50.0 9-0 38.8 37 M Mt~oz and V Hamza Table Borehole Pehu6n-1 Thermal conductivity (K) by considering geological formations of the site Geological formation Depth interval [m] K [W/m°C] Millongue 475.0 - 478.0 633-0 - 639.0 1.7 1-5 Trihueco 873.5 1039.01051.01072.01098.0- 877.5 1045.0 1057.0 1078.0 1104.0 2,1 1.8 3.2 1,7 2.6 Boca Lebu 1180.0 - 1183.0 1.9 Curanilahue 1456.0- 1460.0 1501,0- 1505,0 1.7 2-2 Quiriquina 1542-0- 1545.0 1704.0- 1708.0 1859-0- 1863-5 1930.0-1932.5 2,6 1.6 2.8 4-1 Thermal conductivity Pehudn-l: 2.3 ± 0.7 [W/m°C] was used for conductivity measurements The equipment used is a half-space line source device built at the Institute de Pesquisas Tecnol6gieas (Sao Paulo) This equipment is calibrated using fused silica discs which are internationally accepted as primary standards; in addition, reek samples of known thermal conductivity are also used as substandards, Table Rock Bomhol¢ Pehu6n-1 Thermal conductivity of rocks Number of samples K [W/m°C] Conglomerate Sandstone Motasandstone Lutit¢ (argillite) Basalt 338 3-4 ± 0-9 2.2 ± 0.6 2-2 1-7 + 0.1 2.0 ± 0,8 Studia geoph, et geod 37 (1993) Heat Flow and Temperature CTadients in Chile T("C) Z o 4O,.,,~,, ?,.?,.,~,,.,~, sF ~, ~.,.sP.,.~P o~,.,.~.,.~ ~ ~ -~ ,~ ~ ~ ! _~ 200; O T("C) ~ 7o • 'V"I 200, S*'~, PEHUEN-t \ 8001ooo-, t• LEBU-2 400600, 800- *,f 12001400 '"'% , I000 16(X) ~ |" t 1800, 1200- • • 2000- t400i TPC) Z(m) zLm) , p, ,.~0 ,., ,~ ~., ,~, ,., o ' " T(°C) ¥ ~ :~ ~,, ,4,e,,.~, ~, ~ V' 5~ CURANILAHUE- e~l • • J • •dP ~ I II • e e i'°Z(m) ° l • , Zlm) ::21 ~o T(oc) ,4 ~p ? ep 2e , , 12, , , I es 2p ~ 3F # CHOLCHOL-I IOO- ==*% • I 2oo 3oo Z(m) Fig Ii T-logs in boreholes.Location of boreholes is given in Table Lcbu-2: the temperature value atZ= 1300 m (T= 104 °C) is not shown 0N.B.: The temperature axis does not startat °C) Studia geoph, et good 37 (1993) 339 M Muffoz and V Hamza Table Thermal conductivity (K) and thermal diffusivity 0¢) in samples of boreholes Lebu-2, Dolores-4 and Curanilahue-2 Borehole Depth interval [m] K [W/m'C] ~¢ [10-6m2/s] Lcbu-2 157.0- 161.0 348-0 - 353.0 446.0 - 448.5 664-0 - 669.0 818.0 - 822-0 909.0 - 915.0 1345.0- 1351.0 1539.0- 1543.5 1583.0- 1586.0 2.67 2.96 2.13 1.30 1.78 1.93 1-92 2.84 3.91 1.08 0.91 1.30 1-47 1-33 1-43 1.14 1.09 1.19 Dolores-1 198.5 - 200.5 374.0 - 380.0 453.0 - 458-5 582.0 - 585.0 763.0 - 767.5 1452-0- 1457.5 1687.0- 1693.0 1798-0- 1802-0 1952.0- 1955.0 2344.0 - 2346.0 2.14 2.04 1-63 1-26 1.33 2.45 1.75 1-42 2.28 2.20 1.25 1.45 1.27 1.25 1.43 1-20 1.45 1-42 1.22 1.41 Curanilahue-2 1518.0- 1521.0 1536.0- 1541.0 1558.0- 1563.0 2.68 4.07 2-32 0.95 1.15 Lebu-2 2.38 + 0.74 1-22 + 0.17 Dolores-1 1.85 + 0.41 1-34 + 0.10 Curanilahue-2 3.02 5- 0.75 1-05 + 0-10 340 Studia geoph, et geod 37 (1993) Heat b'low and Temperature Gradients in Chile CONCLUSIONS AND DISCUSSION The heat flow measured in Chile is extremely variable with generally low values at latitudes 26.5 °S- 29 °S and higher values in the central region (33 °S) The latitude range characterized by low heat flow in part correlates with that where active volcanism is not observed Higher values at latitude 33 °S coincide with the re-initiation of volcanic activity A high value of heat flow is observed at Tierra del Fuego in the jigsaw mosaic of southern Chile Also, geothermal BHT gradients in boreholes are very variable with normal or higher than normal values as compared to the standard value for the Earth The highest gradients are encountered in Chile's extreme north (Dolores) and in the central-south region (Lebu) - both sites are situated in the coastal range in areas characterized inland by strong geothermal and volcanic activity But near the Lebu borehole other BHT data ~ehu~n, Curanilahue, Cholchol) indicate nearly standard gradient values BHT gradients are preliminary as it has not been possible to correct temperature measurements for mud circulation The former results of heat flow in Chile not allow for any conclusions on the general beat-flow distribution and its description in the frame of new global tectonics The present state of research does not show a simply connected relation of heat flow with other geophysical processes This can be more clearly seen by considering the maximum depth of the seismic coupling along Chile and its relation to the heat flow and the geothermal structure The maximum depth of seismic coupling is defined by Tichelaar and Ruff [55] as the depth of large magnitude (Mw>6) underthrusting earthquakes with foei at the downdip edge of the coupled plate interface, Their results, concerning the Chilean subduction zone [55] indicate that the coupling extends deeper (down to 48 - 53 kin) between latitudes 28 °S and 32 °S as compared to the coupling in the regions immediately north and south (36 - 41 km and 44 - 49 km, respectively) The largest depth range of seismic coupling is then encountered in the region with no active volcanoes and where heat flow at two sites - Boquer6n Chanar (iron mine) and Vallenar (Cachiyuyo area, copper mine) - is extremely low In the calculations by Tichelaar and Ruff [55] a constant P-wave velocity of 6.7 ms-1 between the seismic source and the free surface along the Chilean subduction zone was assumed - as they pointed out this can be a handicap for the correctness of their conclusions Tichelaar and Ruff [55] discuss various processes that may control the maximum depth of the seismic coupling It seems to them that temperature effects associated with volcanism are not sufficient to explain the maximum depth of the seismically coupled zone - the large 1967 M w = 7.1 Tocopilla earthquake occurred at a depth of 45 - 48 km and Andean volcanism is still active [55] The possibility of this control effect was suggested by the thermal modelling carried out by Van den Beukel and Wortel [56,57] To these authors, reduced strength, due to thermally activated creep, seems the most likely explanation of the change from a seismically coupled zone at shallow depths to one uncoupled at larger depths But a caveat in the correlation between volcanism and maximum depth of seismic coupling [55] cannot arise from the results by Van den Beukel and Wortel [57]: they have inferred that the volcanic line and the boundary of the asthenospheric wedge approximately coincide and that the asthenospheric wedge does not extend significantly Studia geoph, et geod 37 (1993) 34 M Mu~oz and V Hamza into the arc-trench region The 1967 Tocopilla earthquake used in [55] to counteract the correlation between volcanism and maximum depth of seismic coupling occurred in the coastal range, 100 krn west of the volcanic zone, far away from the part of the arc-trench transition zone where - as proposed in [57] - equilibrium exists between cooling (caused by the subsiding oceanic lithosphere) and heating (caused by the nearly continental asthenospheric wedge) With respect to this, magnetotelluric and geomagnetic deep soundings show distinctly different electrical conductivity structures from the coastal range to the Andes at latitudes 22 ° - 230 S in northern Chile [48,58] A controversial point in the thermo-mechanical modelling by Van den Beukel and Wortel [56,57] is the requirement of an average heat flow in the central part of the arc-trench regions falling between 30 and 35 mWm-2 This average value is suggested to the authors by the published data on heat flow from northeast Honshu 0apan), Cascades (USA), Perd and northern Chile, as well as from heat-flow values for other subduction zones that have been active for more than 50 My (Kurile Islands, Central America) Northern Chile's low heat-flow values considered in [56,57] are from sites between latitudes 26°32 ' S and 28o05 ' S - in a region where active volcanism is almost absent - and from one site in the central Chile Andean region with a normal value of heat flow Heat flow in the Santiago basin and preliminary uncorrected temperature gradients and conductivity measurements in boreholes in the coastal range are instead suggesting a normal or a high heat flow in other segments of the arc-trench zone Also low heat-flow values in Perd taken by Van den Beukel and Wortel [56,57] are in regions with no active volcanism inland Heatflow measurements in western South America are too scarce for proposing an equivalence with heat flow in other subduetion zones Even if the Kurile Islands, Japan, Pent, Chile and M6xico off Oaxaca can be considered, due to the dominant stress regime in the present phase of subduction, as Chilean-type subduction zones [59], there are other features which indicate the diversity of these regions It is not clear to what extent the statement by Smirnov and Sugrobov [60] concerning the paradoxical relationship from the viewpoint of radiogenic nature of heat flow - between high heat flow and thin ernst in continent-ocean transition zones - made by these authors on the basis of studies in the northwestern Pacific - can be applied to South American subduction zones The distribution of radiogenic elements should be studied in detail for proposing more solid hypotheses concerning the similarity and disparity between these regions and for attempting to establish the relative importance of radiogenesis and zonal melting in volcanism and in the thermal evolution of the crust, as follows more specifically from part of the geophysical and geological literature (e.g [61,62]) Also, the results of seismic tomography - which in part concern the thermal state of the Earth - are not conclusive as regards the degree of similarity between the northwestern and southeastern Pacific transition zones through different depth ranges in the upper and lower mantle [63-67] Seismic imaging of these zones points out some features in common but also shows disparities which should be important in establishing the temperature structure and rbeological processes Presently, many more constraints on upper mantle boundaries can be applied in the case of subduction zones in the northwestern Pacific where seismic coverage is more dense; preliminary results concerning temperature perturbations to the 660-km discontinuity within subduction plates in the northwestern Pacific argue against 342 SOadiageoph, et geod 37 (1993) Heat Flow and Temperature Gradients in Chile models of clean plate slice through this discontinuity, and favor relatively models that include a gravitationally trapped megalith hypothesis and models of subducting plates encountering a viscosity contrast [68] Besides the global complexity shown by seismic tomography, local effects having consequences on heat flow and volcanism in Chile may be considered An increasing degree of coupling at the plate interface thrust between latitudes 37 ° S and 46°S has been estimated by Spence [69] Variations of coupling fully coincide with maximum values of the degree of partial melting of mantle peridotite in volcanic products as inferred from Sr/Ca-Ba/Ca systematics [70] The maximum values of the partial melting index are related by Onuma and L6per-Escobar [70] to the reduced strength of coupling between two neighboring segments of the fractured subducting oceanic lithosphere, through which deep mantle material could ascend into the upper layers Also, the distinctly different uplift and erosional histories in western South America- as summarized in [14] - should be considered in establishing an appropriate description of heat flow and thermal processes in this region The complexity of the tectonic structure along the Peni-Chile arc-trench zone is also apparent in the variegated picture of seismic velocities - also including low-velocity zones in different crustal depth ranges and important variations between the coastal and Andean ranges [71-77] The intricate characteristics of the Chilean subduction zone indicate that solving for the major features of the associated heat flow makes unavoidable the measurement of temperature, conductivity and radiogenic heat generation through several deep drillings in different tectonic areas A more dense coverage of measurements carried out in the ocean in the area of the Chile Ridge [9,10] has shown the arduous problems that may occur in interpreting heat flow on the basis of some hypothetical models This scenario may be reproduced in some areas of continental Chile as more abundant data become available In respect to geothermal areas and hot springs in Chile, somewhat more definitive conclusions regarding temperatures at depth and fluid-rock equilibrium may be advanced It has been shown that a large number of hot-spring areas are characterized as immature systems where partial fulfillment of fluid-rock equilibrium has been not attained In northern Chile, the geothermal areas of E1 Tatio and Puchuldiza are instead mature, high-enthalpy systems that may be regarded as potential areas for energy production Particularly, full equilibrium temperatures up to 285 °C were obtained for the E1 Tatio fluids; the recharge area is about 12 - 16 km to the east of the geothermal area and fluid circulation evolves to a depth of - kin, and within km of a magmatic intrusion In central Chile only seven of twenty hot-spring areas evaluated by the Na-K-Mg geoindicators satisfy partial fulfillment of fluid-rock equilibrium; temperatures at depth are generally much lower than in systems in northern Chile which satisfy the same condition ~1Tatio, Puchuldiza) San Pedro shows the highest degree of partial equilibrium, with equilibrium temperatures between 195 °C and 245 °C Chill(m and Rio Blanco are acid sulfate waters characterized by high temperatures at depth (170°C- 240 °C) Hot-spring areas in central Chile are generally associated to active volcanoes, and no extended area showing surface thermal activity is encountered in this region In northern Chile, besides the extrusive type of magmatic activity, it is possible Studia geoph, etgeod 37 (1993) 343 M M ~ o z and V Hamza to distinguish a developed intrusive type - like at E1 Tatio - leading to the genesis of larger geothermal areas with fluids circulating over longer horizontal paths in the crust The thermal structure of the Santiago basin is still not well determined Conventional measurements yield a high value of heat flow which is in agreement with estimates from the SiO2 geothermometer Silica estimates indicate a temperature at depth of about 77 °C and up to 94 °C in the zone of water discharge Evaluation of fluid-rock equilibrium by means of the Na-K-Mg geoindicators shows that waters are immature; if K/Mg temperatures were still valid, deeper equilibrium temperatures could be about 80 °C On the basis of these results, further study of this area regarding low-enthalpy geothermal applications can be proposed Acknowledgements: This work was supported by Departamento T6cnico de Investigaci6n (Universidad de Chile) with the collaboration of the Instituto de Pesquisas Tecnol6gicas (Sao Paulo, Brasil) and ENAP (Chilean Oil Concern) We especially wish to thank E d u a r d o G o n z ~ile z (ENAP) for data concerning borehole temperatures Received: Revised: 22 1992 20 1992 References [1] M M u f l o z : Flujo de calor en Chile 6nfasis en las ~reas de El Tatio y de la cuenca de Santiago In: V.M Hamza, A Frangipani, A.E Beck and F.B Ribeiro (Editors), Proc Int Meeting on Geotherrnics and Geothermal Energy, Guaruj~i (Sao Paulo), 1986 Spec Issue Rev Brasfleira Geoffsica, (1987), 153-164 [2] S U y e d a , T W a t a n a b e , E K a u s e l , M K u b o , Y Y a s h i r o : Report of heatflow measurements in Chile Bull Earthq Res Inst., Tokyo, 53 (1978), 131-163 [3] M M u i l o z : Radiogenic heat producltion and erosion process in Central Chile In: Abstracts Int Meeting on Terrestrial Heat Flow and the Structure of the Lithosphere, Bechyn~ 1991, p.60 [4] V M H a m z a , F B R i b e i r 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