XXIX IAH CONGRESS 1999, 6-10 September, Bratislava, Slovak Republic.
Geothermal potential in Western Anatolia and its relationship between electrically conductive zones ILYAS CAGLAR Istanbul Technical University, Faculty of Mines, Geophysics Dept., Istanbul-Türkiye. Email:
[email protected] Abstract
High electrical conductivity and high heat flow coexist in much of the western Anatolia, a region tectonically active. I first reviewed the several crustal conductive zones lied beneath the six magnetotelluric lines in northwest Turkey. The temperature calculations are made for western Anatolia, using the depth values to the conductive zones beneath the magnetotelluric lines. The results reflect that northern Aegean has high Earth’s inner temperature.
Aegean Sea
In Fig. 7, the layered models one-dimensional found from the inversion of magnetotelluric data, are placed beneath of each site to illustrate conducting zones. All sites are dominated by a middle, very conductive (0.8-10 ohm-m) structure with total conductance 3000Si, which is almost overlain by a more resistive cover. A conductive zone lies in depth between ( 6.5 and 12km. Here, the upper conductive zone is considerable shallow than that of previous four lines. The more complex structure with high resistivity may be occured in lower crust. Its upper boundary is more shallower around Biga Peninsula than in southern region. Another two conductive zones with resistivities Introduction Turkey (Fig. 1 and 2) is located within an important geothermal area, and has an estimated 31,500 0.1-0.3 ohm-m and 0.1-1 ohm-m are found at much deeper region (at least deeper than 21.5km). MWt geothermal heat potential (Mertoglu et al., 1995). The Biga peninsula, Gediz graben (between Manisa and Salihli), B. Menderes graben (between Germencik and Denizli), Dikili, Izmir, Bursa ρ (ohm-m) and Eskisehir that are well known as geothermal sites are located in western Anatolia (Fig. 3a). Fig. 6. One-dimensional geoelectric models of the conductive 5 MT sites on the line L-4: 10 Black Sea zone-I Trache (a) Tarakli, (b) Pasalar, (c) Seyfiler, (d) Türkmenköy, NAFZ Black Sea 4 (e) Muttalip, (f) Çiftlik. Caspian 10 Sea Anatolia Turkey CHIN A
Syria Iraq
Iran
Egypt
Over the line L-6 in the southwestern Anatolia (Fig. 8), three conductive zones generally observed. The known geothermal sites around this line is not so much, but high heat flow values are estimated.
INDI A
Saudi Arabia
Arabian Sea
Figure 2. Anatolia and surroundigs.
Indian Ocean
200
30o
36 o
Heat Flow (mWm-2 )
L-1 Alacoglu Tekirdag S Lüleburgaz Marmara Yesilsirt Sea Dügünbasi Karahidir Kofcaz Kavak Cesme Saricali Hanoglu
0 10 20 30 40
km
Conductive zones 0
Sedimentary units
40
0
km
W
Ihsaniye Kaymakdere
20 km
L-3
E
Safranbolu
20 40
0
20 km
km
NGFZ
S Tarakli L-4 Çiftlik MuttalipKadilar Seyfiler Gölpazari Türkmenköy 0.1
10
0.06
20
r VAL= r CL LC
km
r CL 0.01
0.065 r LC
20 km
0
0.01
5 km
A c i p a y a m Tavas
Menderes massif
60
0
Sedimentary Upper crust Lower crust units
1-10 ohm-m
km
10
20 km
Highly conducting zones and its connection with the inner geothermal state of the crust
Temperature and the circulation of subsurface hydrothermal fluids, both of which are characteristic features of geothermal systems, are capable of generating high electrical conductivity within the crustal rocks, that can be measured by magnetotelluric method. Resistivity of these rocks that can be observed in geothermal areas is lower than in surrounding rocks. The use of magnetotelluric method in detecting conductive zones is based on the fact that the resistivity of thermal ground water in the rocks decreases significantly at high temperatures and that geothermal activity can produce conductive alteration minerals. The area of line L-4 (Fig. 5) is geothermally more active when compared with northern areas, and high heat flow values up to 130 mWm are found here. The conductive zone is also shallower along the line L-4, indicating the effect of a high heat flow value. Fig. 6 represents the summary of the reasonable one-dimensional geoelectric models for all sounding sites produced from the inversion of magnetotelluric data. Two conductive zones could be described; first conducting layer at a depth of 15.8±6.2 km and second conducting layer in the lower crust at a depth 60.0±15.0 km.
10-30 ohm-m
Figure 8. Crustal geoelectric structure along the line L-6 (re-interpreted from Ilkisik, 1987)
An approch to the inner thermal state beneath western Anatolia
The temperature T(d), in the crust at depth d, could be calculated using the equation given by Lachenbruch (1971). The temperature calculations are made for western Anatolia to approach the inner thermal state of the earth. For this purpose, the depth values to the conductive zones beneath the magnetotelluric lines are used as d parameter in eq. (4). For the calculations we used K=2.5 W/mK, a representative value for the upper and lower crust (Majorowicz et al., 1993). The calculated temperatures are grouped into two regions. From Fig. 9, it is evidence that the temperature beneath Thrace is low. The northwest Anatolia generally has high 600 values. In the northern Aegean, temperatures are highest for upper and lower crust. It can be associated with 500 active tectonics and high geothermal activity. 400
Acknowledgments: 300
200
1 2 3 4 5
a 5 6 7 810
L-2 Safranbolu NW SE Ihsaniye Ismetpasa Kaymakdere Cakraz 20
10
K a y a b a þ i Dimril ArmutluTefenni
80
N
Figure 5 Simplified geoelectric structures along the lines L-2, L-3 and L-4 showing highly conducting zones.
NW
40
From the magnetotelluric survey carried out over the line L-1 ( Fig. 4) the crustal thickness in mid of the basin was proposed as about 28-32 km. The two region, Thrace and Aegean, differ in heat flow, the Aegean region having a value generally higher than that of the Thrace. The grabens (i.e. Gediz and Büyük Menderes) are geothermally active regions, showing high heat flow values.
50
Akifköy Elmali Fethiye Y.Köy
20
Figure 3 (a) Heat flow distribution in western Anatolia and major fault zones. The heat values are symbolized by rectangulars of different patterns Ilkisik (1995). (b) Tectonic map of western Anatolia. The lines with black triangles show Mesozoic sutures and the lines with open triangles indicate major intra-continental thrusts. The magnetotelluric lines are indicated by thick dashed lines. (Geology is simplified from Akyüz and Okay, 1996)
Figure 4. Conductive zones in the geoelectric structure of the line L-1 (in Thrace basin).
SE 0
Antalya
28o
L-6 Finike
b Mediterranean Sea
Mediterranean Sea
28o
40 o
a k a r y azone S Biga P. Ta v s a n l i Kirsehir zone L-5 a v Afyon o n z o n e massif onre Menderes Bzo G e d i z Lycian n. . B.Menderes g Aegean Sea
30o
L-3
L-4
Eskisehir
Dikili
L-2
Istanbul zone ANKARA
Gönen
36o
B L A C K S E A
Trache basin L-1
Ýstanbul Tuzla Akyazi
32o
2
2 3 4 5 6 7 810
This study is partly supported by a Türkiye Bilimsel ve Teknik Arastirma Kurumu (TÜBITAK) grant YDABÇAG-22.
References
Akyüz, S. and Okay, A. 1996: Int. Geology Rev. 38, 400-418. Depth ( km ) Garcia, M. M. 1992: Surv. Geophysc.13, 600 409-434. Ilkisik, O.M., 1981: I.Ü. Yerbilimleri Derg. 2, 307-319. Ilkisik, M., 1987:Tübitak Int. Report, 82 500 pp. Ilkisik, M. 1990: Doga Tr. J. of Eng. and Env. Sci.14, 598-617. 400 Ilkisik, O.M. 1995:Tectonophysics 244, 175-184. Lachenbruch, A. H. 1971: J. Geophys. 300 Res. 76, 3842-3851. 1 Lewis, T. J., Bentkowski, W. H., Davis, 2 E. E., Hyndman, R. D., Souther, J. G. and 3 200 Wright, J. A. 1988: J. Geophys. Res. 93, b 4 15207-15225. 2 Majorowicz, J. A., Gough, D. I. and Lewis 5 6 7 810 2 3 4 5 6 7 810 T. J. 1993:Earth and Planet. Sci. Lett.115, Depth ( km ) 57-64. Mertoglu, O., Mertoglu, F.M. and Basarir, N. 1995: Geothermal Res. Conc.17, 19Fig. 9. Calculated temperature within the earth as 22. function of depth. (a) Thrace basin and northwest Anatolia. (1) L-2 and L-3; For upper crust: (2) L-4 and (5) L-1; for lower crust: (3) L-4 and (5) L-1. (b) Northwestern Aegean and Southwestern Anatolia. For upper crust: (1) L-5 and (4) L-6; for lower crust: (2) L5 and (3) L-6.
Temperature ( oC )
Trache basin
28o
Temperature ( o C )
36o
32o
28 o
