Temperature-depth curves and heat flow in central part of Anatolia, Turkey

Accepted Manuscript Temperature-depth curves and heat flow in central part of Anatolia, Turkey

Elif Balkan Pazvantoğlu, Kamil Erkan PII: DOI: Reference:

S0040-1951(19)30064-2 https://doi.org/10.1016/j.tecto.2019.02.019 TECTO 128054

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

3 September 2018 22 February 2019 27 February 2019

Please cite this article as: E.B. Pazvantoğlu and K. Erkan, Temperature-depth curves and heat flow in central part of Anatolia, Turkey, Tectonophysics, https://doi.org/10.1016/ j.tecto.2019.02.019

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ACCEPTED MANUSCRIPT TEMPERATURE-DEPTH CURVES AND HEAT FLOW IN CENTRAL PART OF ANATOLIA, TURKEY Elif BALKAN PAZVANTOĞLU1, Kamil ERKAN2 1 2

Geophysical Engineering, Dokuz Eylül University, Izmir, Turkey, Environmental Engineering, Marmara University, Istanbul, Turkey

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ABSTRACT The objective of this study is to determine the crustal thermal regime of Central Anatolia

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Province and its relationship with regional tectonics. Investigations include new temperaturedepth measurements in wells, calculations of geothermal gradients from new and previously

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measured temperature logs, and determinations of heat flow with terrain corrections if necessary. The results provide us with a better understanding of the regional distribution of

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subsurface temperatures and heat flow within the study area. Although Central Anatolia Province is bordered with tectonically active areas, moderate heat flow values (64±16 mWm2

) are generally observed within the region in accordance with the relatively less tectonic

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activity. Two thermally anomalous zones are indicated in the vicinity of Kırşehir and Kızılcahamam with heat flow values of higher than 100 mWm-2. Anomalous heat flow in

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these areas can best be explained by high radioactivity of basement rocks although this is not

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directly confirmed for the Kızılcahamam anomaly. The high elevation and recent volcanic activity in the region indicate that thermal activity may be present at lithospheric levels but have not yet reached up to crustal levels. Also, no surface heat flow anomaly due to fault

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activity was observed at the section of the North Anatolia fault within the study area.

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Keywords; heat flow, geothermal gradient, central Anatolia

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ACCEPTED MANUSCRIPT 1. Introduction Central Anatolia (CA) is situated in the central part of the Anatolian microcontinent which is dominated by extension in the west, collision in the east and transcurrent motion along its northern and southeastern boundaries (Whitney et al., 2007). In previous studies, CA has been associated with young volcanism, rapid surface uplift (~1 km mean elevation), and complex tectonic deformation over the last 10 Ma (Cosentino et al., 2012; Yildirim et al. 2011;

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Schildgen et al. 2012; Aktuğ et al., 2013; Uluocak et al., 2016) (Fig. 1a). The CA province is also mentioned with prevalent geothermal systems based on a limited

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number of high heat flow values (Tezcan and Turgay, 1991), shallow Curie point depth

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investigations (Ateş et al, 2005) and other geophysical and geochemical observations (İlkışık et al., 1997; Kıyak et al., 2015; Yurteri and Şimşek, 2017). Determinations of regional heat

measurements

in

boreholes

with

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flow by conventional methods require high-resolution temperatures-versus-depth (T-D) thermal

equilibrium,

and

thermal

conductivity

determinations on related rocks. The only published conventional heat flow study for the

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study area, which is based on geothermal gradients from non-equilibrium bottom hole temperature data and a constant thermal conductivity assumption, indicates moderate to high

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heat flow values (70-100 mWm-2) (Tezcan and Turgay, 1991) for CA. Tezcan (1995) proposed that high heat flow areas within the region are generally connected with the

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metamorphic massifs. In addition, relatively shallow Curie point depths (12-14 km) were calculated using aeromagnetic anomalies in the region with shallowest values being associated with the CA Volcanic structures (Ateş et al., 2005). Kızılcahamam geothermal area

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located in NW CA was delineated by above 90 mWm-2 and associated with the Tertiary volcanism (Tezcan, 1995). Akın and Çiftçi (2011) estimated heat production rate between

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0.62-5.68 μWm-3 for the Kırşehir massif from airborne radioactivity mapping studies, and suggest that 8-38% of the mean heat flow of 72 mWm-2 originates from the radiogenic heat production.

Although CA has been characterized by significant neotectonic features, including volcanic structures, major fault zones, and widespread geothermal activity, the crustal thermal conditions have not been investigated using conventional methods. The study area geographically covers the CA region, the western Black Sea region and the Mediterranean region of Turkey. We analyzed T-D data from 157 shallow boreholes (83 points from the Central Anatolia region, 18 points from the western Black Sea region, and 56 points from the 2

ACCEPTED MANUSCRIPT Mediterranean region). Thermal conductivities were determined from measurements of outcrops of related rocks or assigned from literature values based on the lithologic information. Finally, we report a total of 60 geothermal gradients and 52 heat flow determinations for the region. We interpret the distribution of new heat flow data with general tectonics of the regions and previous geophysical investigations. 2. Data collection

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The dataset used in this study consists of new measurements and previous measurements which were not analyzed and published before. Part of the temperature-depth (T-D) data used

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in this study was collected along with rock thermal conductivity measurements in CA as a part of a government-funded project in Turkey (İlkışık et al., 1997). Data collection was

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carried out by the General Directorate of Mineral Research and Exploration (MTA) of Turkey. An Amerada portable logging tool was used in T-D measurements recorded for each

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meter of depth below the water table. Thermal conductivity measurements were done on the rock samples collected from surface outcrops in the vicinity of each borehole using QTM-500

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device in the laboratory of MTA. Generally, more than one type of lithology was sampled for each site, resulting in a larger data set of thermal conductivity measurements. They reported the raw T-D logs and thermal conductivities without doing further analyses. A statistical

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analysis of rock thermal conductivity dataset with respect to lithologic types in both western

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and central Anatolia has recently been made by Balkan et al. (2017). The second data set consists of new T-D measurements obtained between 2013 and 2017 in CA. We used a custom-designed thermistor probe four-wire measurement portable tool in

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the acquisition of the data with the 1-5m sampling interval. All T-D measurements were done

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below the water table.

In total, we studied T-D measurements from 157 boreholes to obtain geothermal gradients and heat flow values. All the T-D logs were recorded in thermally equilibrated boreholes that were drilled for fresh water supply (but not producing) or groundwater monitoring. The wells were partly provided by the State Hydrological Works (DSI) regional directorates, and partly by local drilling companies. Location, depth, static water level, lithologic etc., information are obtained from the personnel of the state offices or from the drillers.

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ACCEPTED MANUSCRIPT All T-D data analyzed together after correcting for effects of the groundwater flow, topography. Thermal conductivities required for heat flow determinations are obtained from Balkan et al (2017) and Erkan (2015) if there are no available measurements. The distribution of the locations of the entire dataset is shown in Fig.1b.

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(a)

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(b)

Figure 1.a) Simplified tectonic map of Turkey including major structures (from Şengor et al., 1985; Barka and Reilinger, 1997; Kiratzi and Louvari, 2001; Bozkurt 2001; Bozkurt and Sozbilir, 2004; Koralay et al., 2011) b) Data locations with the corresponding quality classes (see the text). Geographical borders of the Black Sea, Central Anatolia, and Mediterranean regions are indicated with black lines. Red stars symbols show the location of the hot springs. Acronyms for administrative provinces are as follows: ADA: Adana; AKS; Aksaray; ANK: Ankara; ANT: Antalya; BAR: Bartın; BOL: Bolu; BRD: Burdur; CNK: Çankırı; ESK: Eskişehir; HAT: Hatay; ISP: Isparta; KRB: Karabük; KAS: Kastamonu; KIR: Kırıkkale; KIR: Kırşehir; KON: Konya; NIG: Niğde; SIN: Sinop; ZON; Zonguldak.

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ACCEPTED MANUSCRIPT 3. Data Analysis Recorded T-D measurements may include some perturbations resulting from hydrogeological effects, climatic changes and topographic contrasts around mountainous terrains. To calculate reliable heat flow values, the effects of these factors must be eliminated. Climate changes are one of the problems which can cause a significant error on temperature measurements and must be removed from the data to obtain reliable heat flow

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values. The paleoclimatic correction on regional heat flow calculations usually considers the warming of the ground since the beginning of the last interglacial period (c.a. 15000 Ka).

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These corrections have significant magnitudes at high latitudes (up to 20 mW/m-2) but

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diminish toward the low latitudes (Majorowicz and Wybraniec, 2011). Majorowicz and Wybraniec (2011) calculate the paleoclimate effect on the heat flow values for Anatolia to be

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0–4mWm-2, and well below the error bounds of the present dataset. A second correction for the climate deals with global warming which considers the 20th-century warming in the ground. Based on instrumental SAT (surface annual temperature) records from 52 stations in

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Turkey, Tayanç et al (2009) found no significant change in temperature trend in Turkey until 1993, and significant warming since then. The effect of these recent warming trends is easily

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observed in the gradients as departures from the linear gradients toward the high temperatures with increasing amplitude toward the surface (Pollack and Huang, 2000). We did not observe

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such curvature in the data taken before 2000. For the dataset measured after 2015, we observed a recent climate change effect in two stations (Akcavakif-1 and Cayobasi, see Table

these depths.

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2), in the first 50m of the boreholes. In these boreholes, the gradients were calculated below

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3.1 Data quality classifications The boreholes used in this study were opened for hydrological purposes thus a rigorous quality analysis was applied in order to eliminate hydrological effects from the dataset measurements. We applied the method of Erkan (2015) for quality classification (Table 1). According to this, T-D curves must be linear with depth as long as the thermal conductivity of related geological section is constant in a well. Class A and B data correspond to the solution of 1-D heat transfer along a borehole (Jaeger, 1965). This kind of data consists of a linearly increasing temperature with depth and should extrapolate to the mean annual ground surface temperature (GST) at the measurement point. Groundwater movements and fluid flows in some sections of borehole results in a partly disturbed T-D curve. Such kinds of data are 5

ACCEPTED MANUSCRIPT classified as class C. If water movement affects the large part of the T-D curve, or the borehole is too shallow (<50m), it is rated as class D. Table 1 Definitions of the data quality classes (Erkan, 2015) Class A B C

Relative error in Geothermal gradient 5% 10% 25% not suitable for heat flow determination not suitable for heat flow determination

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D G X

Criteria Greater than 100m conductive (linear) T –D section Greater than 50m conductive (linear) T –D section Disturbed T –D curve due to intra-borehole fluid activity Intermittent conductive sections Intense intra-borehole fluid activity; conductive section too shallow Dominated regional geothermal activity on T-D curve Dominated groundwater activity on T-D curve

If the T-D curves are completely under the influence of the groundwater movement, they

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are not used for heat flow determination and rated class X. T–D curves for these wells generally show isothermal behavior, indicating fast vertical groundwater flow. Other types of

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hydrologically active sites are found near geothermal systems. These sites show the effect of local geothermal activity, which shows distinctly higher temperatures. These types of data are

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rated class G and are also not suitable for conductive heat flow determinations (Erkan, 2015). The resulting data classes are shown in Fig.1b by various types of symbols. According to

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this, 91 sites were found to be under the groundwater effect (Class X in Fig.1b), and 6 sites were found to be under geothermal activity (G). The remaining 60 sites were found to be suitable for conductive thermal modeling in varying degrees from Class A to Class D. Among

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the thermally conductive boreholes, Class A and B holes are the most reliable sites where

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entire of the T-D data show conductive (linear) behavior. Class C holes show intra-borehole fluid flow (IBF) activity in some sections. Class D holes are the least reliable sites, either too

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shallow or highly disturbed by the IBF activity. 3.2 Topographic correction

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Topographic differences alter the subsurface temperature distribution under mountainous terrains. It is crucial to be aware of the topography effect when studying in mountainous regions (Beardsmore and Cull, 2001). Lees (1910) suggested a correction in two dimensions to eliminate the distortion in the geothermal field beneath idealized mountain ranges (Beardsmore and Cull, 2001). Lees’ (1910) correction method is applied on the 7 T-D data where there are steep topographic changes near the measurement points the corrected geothermal gradients (cG) are listed in Table 2.

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ACCEPTED MANUSCRIPT 4. Results 4.1. Temperature-depth curves Classes A/B/C T-D data falling in the same or adjacent provinces in Fig.1b are clustered and plotted in several panels in Fig.2. Elevations of the boreholes are given in Table 1. Analysis of nearby boreholes is especially useful for comparison of the extrapolated surface temperatures for the reasons as follows. Extrapolation of the T-D profile to the surface

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represents the ground surface temperature (GST) at the location of the borehole. GST can be correlated with the mean annual surface temperature (MAST) at low elevations where the

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ground and the surface are fully coupled (Pollack and Huang, 2000). For nearby holes, MAST

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changes in accordance with the adiabatic lapse rate (~ 5 °Ckm-1). At high elevations, GST and MAST are decoupled but show the same trends in long time periods (i.e. decades) (Gonzalez-

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Rouco et al, 2003). As a result, the elevation of the borehole can be used as a reference for the expected ground surface temperatures in the vicinity of each borehole site, and pose further evidence for the conductive nature of the heat transfer along the borehole (e.g., hydrologically

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active boreholes do not show these relationships).

Boreholes measured in Kastamonu (KAS), Karabük (KRB), Bartın (BAR), and Sinop

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(SIN) are shown in Fig. 2a. These are the northernmost boreholes and located in the western

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Black Sea region. Extrapolated surface temperatures generally agree with elevations of the boreholes. There is an exception for the boreholes named Sinop and Gobu. They have the highest surface temperatures although they are located at the lowest elevations. In Kastamonu,

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T-D curves are generally suitable for conductive geothermal gradients calculations. Downflow of groundwater disturbs both Godel and Kale wells. In Kale, probably the borehole

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has been cooling down to the depth of 130m. Below this depth to the bottom of the well, temperatures show conductive behavior. T-D curve in K.orencik is an almost straight line for the entire length of the curve. A strong upflow affects the first 85m of Alatarla but the rest of the curve shows conductive behavior. In A.sehiroren the interval of 162-202 m is appropriate for the geothermal gradient calculation as the interval 80m-140m shows isothermal behavior. Sallar and Gobu wells show conductive regimes for their entire depths. For Sinop, the first 35m of the borehole seems to be affected by lateral movements of the groundwater but the rest of the curve is conductive.

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ACCEPTED MANUSCRIPT T-D curves of Bolu, Ankara, and Çankırı are given in Fig. 2b. The holes are generally at elevations around 1000m. Kuscular is an exception located at an elevation of 1912m and also shows a relatively lower surface temperature. Yunluyayla is rated as B class since the conductive regime is dominant along the borehole. For Hasanaslan the abrupt change in the gradient at 94m can be explained by the thermal conductivity contrast. A strong IBF affects the deeper part of the Gunbasi, therefore, the geothermal gradient was calculated from the first 138m. The entire hole in Kuscular shows a conductive thermal regime. A weak upflow

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(21-38m) in Eldivan and downflow (25-76m) in Akcavakif-1 are inferred.

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In Fig. 2c, Karaarkac, and Karacaoren are located in Kırşehir. Both of them are under the effect of downflow so the geothermal gradients are calculated using the bottom hole

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temperature and the projected surface temperature. T-D curves for Kırıkkale are also given in Fig. 2c and their general character show conductive behavior. Their projected surface

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temperatures are also in accordance with the mean annual surface temperature (MAST) of the region.

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The five T-D curves of Konya are all rated as C class (Fig. 2d). In this region, high-quality temperature logs were difficult to obtain. The geology of this area mainly consists of

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carbonates and, as a result of groundwater activity, only a few measurements show linear character. The upper parts of Karaagac and Saricalar curves are disturbed by upflow so the

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geothermal gradients at these sites are calculated using the bottom hole temperatures and the projected surface temperatures. The lateral water movement is dominant at the first 50 m of Yamac. Multiple groundwater movements disrupt the curves of Sazlipinar and Argithani. The

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only hole in Aksaray, Belisirma, is an exception in this group which penetrates into an ignimbrite lithology for the entire of its depth but shows a similar characteristic of the

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boreholes in Konya.

In Fig. 2e, T-D curves located in Eskişehir are shown. Depths of holes are generally between 100-200m. Sivrihisar and Aydinli are likely to be affected by multiple upflow zones along the holes. A conductive section is apparent below 180m for Mesudiye. The geothermal gradient is calculated below 50m in Cukurhisar due to the downflow at shallower depths. Both lateral and upflow of groundwater at the first 50m of the Kayakent curve is inferred thus the geothermal gradient is evaluated below this level. Groundwater activity has a strong effect on Guneli so the geothermal gradient is calculated using the projected surface temperature and the bottom-hole temperature. 8

ACCEPTED MANUSCRIPT T-D curves in Fig.2f belong mostly to the Mediterranean region of Turkey. For holes in Burdur, projected surface temperatures are relatively lower than those in Adana and Hatay, in accordance with the elevation differences of these provinces. T-D curves for Cendik, Elmacik, and Dedeler are entirely linear.

In Cekmece, it is possible to calculate the conductive

geothermal gradient between 13-111m but below this level, IBF seems to be dominant. A strong downflow is inferred for T.sokmen so the gradient is calculated by using the bottom

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hole temperature and the surface temperature.

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ACCEPTED MANUSCRIPT

b

c

d

f

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e

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D

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a

Figure 2.Temperature–depth (T–D) curves for classes A/B/C data for a) KAS-Kastamonu, KRB-Karabük, BARBartın, and SİN-Sinop b) BOL-Bolu, ANK-Ankara, and CNK-Çankırı c) KIR-Kırşehir and KRK-Kırıkkale d) KON-Konya and AKS-Aksaray e) ESK-Eskişehir f)BRD-Burdur, ADA-Adana, and HAT-Hatay. The location of the provinces can be found in Fig. 1b.

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ACCEPTED MANUSCRIPT 4.2 Heat flow A list of classes A/B/C/D boreholes (total of 60 points), calculated geothermal gradients, and heat flow determinations are given in Table 2. Errors for gradients are calculated using the method of the Chapra and Canale (2010) (see the Supplementary data file). Generally, for class D data, gradients are evaluated by drawing a line between the bottom of the hole and the ground surface (Table 2). A histogram of classes A/B/C (total of 40 points) geothermal

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gradients are given in Fig. 3a. Most of the data lie between the 10-50 °Ckm-1 and the mean geothermal gradient is calculated as 39±17 °Ckm-1 for the study area.

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Thermal conductivity values are assigned according to the lithologic information for the

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depths interval where the geothermal gradient is calculated. Available thermal conductivity measurements of surface outcrops were made on wet conditions. When thermal conductivity

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measurements are not available, literature values from Erkan (2015) and Balkan et al (2017) are used.

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The calculated heat flow values for the study area are given in Table 2, and their statistical distribution is shown in Fig. 3b. The heat flow values could not be calculated for 8 sites whose lithological descriptions are not available. The rest of the heat flow data consists of 34

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values are shown in Fig. 4.

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points for classes A/B/C, and 18 points for class D, regional distribution of the heat flow

The study area covers three regions of the Anatolia; Central Anatolia, the western Black Sea, and Mediterranean regions. Average geothermal gradients and heat values are calculated

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as 45±18 ᵒCkm-1 and 64±16 mWm-2 for Central Anatolia region, 35±12 ᵒCkm-1 and 73±19 mWm-2 for western Black sea region, 24±8 ᵒCkm-1 and 34±12 mWm-2 for Mediterranean

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region respectively.

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ACCEPTED MANUSCRIPT Table 2 Class A/B/C/D-type data used in this study, along with gradients (G), corrected gradients (cG) after topographic correction, thermal conductivities (λ), heat flow (Q) values, and their respective errors. For sites where thermal conductivities cannot be quantified, only gradients are listed. Literature thermal conductivities are indicated by (L) next to the value and are obtained from Erkan (2015) for Q.Alluvium and from Balkan et al. (2017) for the other rocks types. See the caption of Fig. 1 for administrative province names.

(15.3) (49.0) 26.8 44.0 23.1 25.0 22.5 39.8 (60.7) (67.1) 62.0 (66.7) (13.6) 37.0 23.5 (17.6) 40.8 36.7 19.0 74.1 36.5 17.5 39.7 (18.7) 20.0 35.2 (16.0) 34.8 (43.4) 76.1 (11.2) (23.1) 66.7 30.2 (40.5) 27.1 32.9 54.7 35.3 33.3 20.7 (63.8) (17.6) 84.6 (11.3) 18.6 27.8 60.0 (60.1) 22.2 31.4 (29.8) 50.0 28.9 23.2 36.6 (26.2) 61.1 (12.2) 64.8

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cG σG (ᵒCkm-1)

Q σQ (mWm-2) (32) (74) 40 18 78 21

Lithology

0.3 0.1 0.3 0.3 0.3 0.1 0.3 0.2 0.3 0.3 0.1 0.3

53 75 72 (230) (121) 42 (87) (37) 56 35 (13) 61

21 10 30

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Limestone with clay Schist Marl Granite Andesite Ignimbrite Marl Limestone with clay Q.Alluvium Q. Alluvium Tuff Q.Alluvium

0.3 0.3 0.2

28 133 93

13 29 30

Q.Alluvium Marl Limestone

0.3 0.2 0.3 0.3

59 (58) 30 81

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Q.Alluvium Limestone Q.Alluvium Flysch

2.8 3.0 19.0 1.5(L) 0.9

0.2 0.2 0.3 0.1

99 (132) 114 (10)

32

6.7 7.6

1.8(L) 2.2 3.4(L)

0.3 0.1 0.3

120 66 (138)

32 20

Marl Flysich Granite

1.5(L) 1.5(L) 3.0(L) 3.2(L) 3.0(L) 3.4(L) 1.5(L) 8.6 1.6(L) 12.9 2.2(L) 4.7 1.3(L) 7.0 3.0(L) 15.0 2.0(L) 1.0(L) 2.2 2.6 7.9 1.5(L) 2.1 12.5 1.0(L) 7.2 5.8 9.2 1.5(L) 1.3(L) 15.3 1.3 2.7(L) 55.0 5.5 1.8(L)

0.3 0.3 0.3 0.3 0.5 0.3 0.3 0.1 0.3 0.3 0.5 0.3 0.2 0.2 0.3 0.1 0.2

49 82 106 105 62 (217) (26) 135 (28) 23 83 120 (60) 58 47 (62) 50

22 37 37 36 17

Q. Alluvium Q.Alluvium Limestone Limestone/Flysich Conglamerate Granite Q.Alluvium Tuff Gravel with clay Silt Conglamerate C.stone with conglomerate Claystone Limestone Q.Alluvium Limestone with clay Claystone

0.3 0.3 0.1 0.3 0.3

55 (33) 79 (33) 99

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6.7 4.4 5.8 6.3 2.3 9.9

λ σλ (Wm-1K-1) 2.1(L) 0.5 1.5(L) 0.3 1.5(L) 0.3 1.8(L) 0.3

2.1(L) 3.3 1.8(L) 3.4(L) 1.8(L) 52 13.0 0.8 1.3(L) 2.72 3.7 1.5(L) 5.9 1.5(L) 0.8 4.1 1.5(L) 43.5 2.2 4.7 1.5(L) 3,7 1.8(L) 34.5 8.6 2.7 0.9 9.9 1.5(L) 3.1 2.0 1.5(L) 40.6 4.1 2.0(L)

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G

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D D C B C C B C D D C D D B C D B A C A C A C D B A D C D C D D B C D B C C C C B D D B D C C C D B C D C C C C D C D B

Interval (m) 70-188 10-20 0-110 60-85 84-110 72-92 162-202 0-140 20-54 0-180 76-184 104-110 124-146 65-105 0-68 0-108 55-93 13-111 24-82 0-112 52-80 42-208 0-55 54-70 14-74 20-120 138-188 0-94 0-76 0-138 0-180 46-98 46-94 0-184 0-42 36-84 0-158 0-192 0-136 0-94 186-258 0-58 0-136 20-98 12-92 55-98 180-190 170-190 0-138 56-74 0-70 0-84 76-113 38-76 80-107 0-186 0-122 64-100 0-98 10-74

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Class

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Elev. (m) 1006 1000 873 840 627 1102 1168 879 895 873 1206 1006 1171 850 65 1375 775 309 908 1128 811 197 951 1490 1004 222 1003 1020 1300 893 809 302 1000 1006 871 940 1381 1255 1177 1182 1013 986 1043 1912 1793 79 925 748 1196 1302 1005 1192 1007 74 1002 90 1102 1050 922 1419

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KON BOL KRK CNK KAS KON KAS ESK KIR ANK AKS KON KON KRK ADA NIG KRK HAT BRD CNK ESK ADA CNK KON BRD BAR KON KAS KIR ANK ESK ZON ANK ESK KIR KAS KAS KON KIR KIR ESK KIR BRD ANK ISP ANT ESK KRK KIR KRB KON KON KON SIN ESK HAT KON KON KON BOL

Depth (m) 188 20 115 95 110 130 202 140 54 185 102 110 146 110 68 108 93 188 82 112 80 242 55 70 74 128 188 94 76 202 180 98 290 184 42 84 158 192 136 110 264 58 136 98 96 98 190 190 138 74 70 84 164 76 108 186 122 100 98 96

D

Prov.

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Long. (ᵒN) 32.75 32.04 33.67 33.60 34.02 31.70 34.42 31.70 33.72 32.09 34.29 33.12 32.04 33.63 35.58 34.89 33.66 36.20 30.16 32.90 30.30 35.57 33.50 32.28 30.09 32.29 32.91 33.72 34.29 32.71 30.95 31.97 33.05 31.71 33.73 33.58 33.41 32.40 34.63 34.28 31.80 33.79 30.12 32.86 31.12 30.33 30.93 33.54 34.22 32.75 32.62 31.81 33.20 35.12 31.45 36.33 31.89 33.75 31.89 32.39

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Abditolu Adakoy Akcakavak Akcavakif-1 Alatarla Argithani Asehiroren Aydinli Ballica Basri Belisirma B.aslama Buyukoba Cagildas Catalpinar Cavdarli Cayobasi Cekmece Cendik Cerkes Cukurhisar Dedeler Eldivan Elmaagac Elmacik Gobu Gocu Godel Gollu Gunbasi Guneli Gurbuzler Hasanaslan Hkhisar Jandarma K.orencik Kale Karaagac Karaarkac Karacaoren Kayakent Kirlangic Kozluca Kuscular Kuyucak Mavikent Mesudiye Nitrosan O.Sanayi Sallar Saricalar Sarikoy Sazlipinar Sinop Sivrihisar T.Sokmen Tasagil Yamac Yavasli Yunluyayla

Lat. (ᵒE) 37.74 40.80 39.91 40.69 41.49 38.30 41.37 39.06 39.92 39.62 38.26 37.67 38.54 39.85 36.83 38.10 40.25 36.20 37.66 40.81 39.82 37.07 40.54 37.13 37.47 41.53 37.87 41.27 39.45 40.22 39.73 41.14 40.12 39.47 39.91 41.49 41.15 37.68 39.25 39.23 39.32 39.88 37.49 40.52 37.94 36.32 39.51 39.85 39.13 40.96 38.10 37.80 37.69 42.02 39.44 36.25 37.39 37.91 38.75 40.64

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Site name

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33.5

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Sand-Gravel Q.Alluvium Q.Alluvium L sandstone

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Q. Alluvium Claystone with sand Marl Limestone with clay Andesite

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Figure 3. Histograms of the a) geothermal gradient and b) heat flow using class A/B/C data

Figure 4. Regional distribution of heat flow data and their associated error values. Heat flow values in square brackets are estimated without formal error (for class D-type data).

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ACCEPTED MANUSCRIPT 5. Discussion Heat flow determinations have important implications in terms of the neotectonics of the study region. A map of the heat flow distribution in tectonic provinces of CA is shown in Fig. 5. Also shown in Fig. 5 is the crustal dilatation/compression map based on GPS studies of Aktuğ et al (2013). According to this, CA region is a relatively undeformed tectonic unit bounded by the tectonically active western Anatolia extensional province in the west, North Anatolian transform zone in the north, and the Taurus Mountains thrust zone in the south Excluding the two anomalous zones

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(Barka and Reilinger, 1997; Kahle et al., 2000).

(discussed below), we observe moderate heat flow values (60-65 mWm-2) within the CA

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block. This is in contrast to high heat flow values observed in western Anatolia (Erkan, 2015),

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which is under crustal extension due to the subduction of the Aegean arc. As Barka and Reilinger (1997) pointed out, unlike western Anatolia, no arc-parallel extension zone exists in

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CA due to subduction of the Cyprus arc. This is also in agreement with moderate heat flow values observed in CA. On the other hand, considering the recent volcanic activity and high elevation in the CA block, elevated temperatures may exist at the deeper levels of the

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lithosphere, as inferred by seismic tomography studies (Biryol et al., 2011) and thermomechanical modeling (Menant et al. 2016). However, our study suggests that even if

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these elevated temperatures are present in the mantle lithosphere they may have not reached the crustal levels yet. Such transient thermal regimes have been observed in the Colorado

2009) of North America.

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Plateau (Morgan and Swanberg, 1985) and the Sierra Nevada regions (Erkan and Blackwell,

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Fig. 6 shows an N-S profile in the study region showing most of the tectonic features with observed heat flow values (green line). Also shown in Fig. 6 are the depths of seismicity and a

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seismic tomographic model of the lithospheric structure (Biryol et al, 2011). The depth of seismicity is almost uniform (~20 km) from North to South except the southern end of the profile above the subducting Eastern Cyprus slab, where relatively lower heat flow values are also observed. Another interesting result we observe in Fig. 6 is the lack of a heat flow anomaly associated with the North Anatolia Fault zone. That is, although high heat flow values are observed south of the fault zone, no such values are observed at the north of the fault zone, suggesting no heat flow anomaly due to the fault activity.

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Figure 5. Distribution of reliable heat flow values (Classes A/B/C) with respect to the tectonic divisions of central Anatolia (Barka and Reilinger, 1997). See text for the discussion of the tectonic divisions. Regions with anomalously high heat flow (> 100 mWm-2) are highlighted by ellipses. The rectangular dashed area outlines the boundary of granitic basement rocks of Kırşehir Massif. The background color map indicates the crustal dilatation/compression based on GPS measurements (orange-dilatation; bluecompression; yellow-no significant deformation; Aktuğ et al, 2007).

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Figure 6. A N-S profile of the study region showing the important tectonic features and observed heat flow values (green line) with error bars, the trace of the profile (A-A’) can be found in Fig.5. Blue dashed line shows the heat flow values excluding two high anomalies. Also shown are the depth of seismicity (from KOERI) and seismic tomographic interpretations from Biryol et al (2011). Heat flow and depth of seismicity values are obtained within a distance of ±50 km of the profile. CACC-G shows the approximate location of the granitic basement of the Central Anatolia Crystalline complex. CAV: Central Anatolian Volcanoes. NAF: North Anatolia Fault.

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Within CA region, there are two zones (Kırşehir and Kızılcahamam anomalies) where high heat flow values (>100 mW/m2) are observed (Fig. 5 and Fig. 6, ellipses in green). These thermally anomalous zones were also observed in the previous study of Tezcan and Turgay

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(1991) using bottom hole temperatures from deep wells. From the lack of present-day deformation (Fig. 5), neither of them can be explained by tectonism. The Kırşehir anomaly is

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located within the Central Anatolian Crystalline complex where granitic monzonite rocks are dominant (Okay, 2008). High radioactive contents were reported for the region in previous studies (Güncüoğlu, 1986; Tureli, 1993; İlbeyli et al., 2004; Köksal et al., 2004; Köksal and Güncüoğlu, 2007; Köksal et al., 2012;2013) suggesting that high heat flow in this zone may be related to the high radioactivity of the basement units. Using the reported values, we obtained a range of heat production values 2-11 µWm-3 with the mean value of 4.6 µWm-3 for the region (see the supplementary file). These heat production values are within the range of 0.5-10 mW/m3 given by a global compilation of granitic rocks (Artemieva et al. 2017). Besides, radioactive heat production of >10 µWm-3 for granites are reported in various studies (Iyer et al., 1984; Vila et al., 2010). 16

ACCEPTED MANUSCRIPT The relationship between surface high heat flow and heat generation in the granitic basement of Kırşehir massif (see Fig 5) is explained by a 1-D thermal model with assumptions of steady-state thermal conduction and constant heat generating crustal layers (Fig 7). The heat production distribution is applied as a step function and the depth of the radiogenic layer is assumed to be 10 km (Blackwell, 1971). Thermal conductivity of upper (0-15) and lower (15-35) crust are assigned as 3.0Wm-1K-1 and 2.5 Wm-1K-1 respectively (Kukkonen et al., 1999). Thermal conductivity is corrected for temperature and pressure as

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suggested by Kukkonen et al. (1999). Three different crustal temperatures are investigated describing the thermal condition of the area. The Reference Model (Fig. 7a) represents the

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regions outside of the granitic basement where average surface heat flow is reported to be 64

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mWm-2 in the present study. The blue temperature-depth curve in Fig. 7b represents the reference model, which reaches to 670 ᵒC at the base of the crust. The reference model gives a

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mantle heat flow of 40 mWm-2. This value for the mantle heat flow is imposed for modeling the region of granitic basement. Model A and Model B (Fig 7a) represent two different radiogenic heat production models for the region with granitic basement, and are constructed

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to give the anomalous heat flow observed in Kırşehir massif (105 mWm-2). According to this, the Kırşehir anomaly can be explained by either a 5 km thick layer of very highly radioactive

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intrusion (Model A) or a 10 km layer of high radioactive intrusion (Model B). Intermediate

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models are also possible to explain the observed heat flow anomaly.

Figure 7. a) Three different simplified conceptual models of the crust with the radiogenic heat production (A) and the thermal conductivity (λ) of the different layers (for details see supplementary material). The thickness of the crustal layers is obtained from (Ateş et al., 2012). b) Resulting T-D curves for the three different model scenarios.

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ACCEPTED MANUSCRIPT The Kızılcahamam anomaly is located just south of North Anatolia fault, and almost entirely covered with volcanic rocks. Bilim (2011) investigated the magnetic field on this zone and estimated shallow Curie point depths (~15 km at the center of the anomalous zone). As the volcanic rocks in this area are relatively old (Miocene age), and cannot play a role for the elevated temperatures, we propose that a high radioactive basement unit may also be the origin of the anomaly here.

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Heat flow determinations reported in this study are in broad agreement with the heat flow map of Tezcan and Turgay (1991) even though two studies use entirely different datasets.

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Especially, the anomalies observed around Kızılcahamam and Kırşehir are indicated by both

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studies. In addition, moderate heat flow in the western Black Sea region, and low heat flow values in the Mediterranean region are also in accordance with both studies. However, the

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present study provides more detailed information about the distribution of the regional heat flow, particularly within the CA region, around the thermal anomaly zones, and along the

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North Anatolia fault zone. 6. Conclusions

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This study reports a comprehensive investigation of the regional heat flow and geothermal

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gradients with their errors in the CA using conventional measurement and processing techniques. We analyzed a total of 157 borehole sites in the region. Out of 157 sites, 60 sites were used for conductive geothermal gradient calculations, and 52 sites were used for regional heat flow determinations. Excluding the high anomalies in Kırşehir and

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Kızılcahamam, the average heat flow is calculated to be 64±16 mWm-2 in CA high platform

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based on class A/B/C type data. We observed heat flow anomalies (>100 mWm-2) in Kırşehir and Kızılcahamama areas. A 1-D thermal model supports that Kırşehir anomaly can be related to highly granitic rocks in this area The study area includes part of the North Anatolia fault zone; the heat flow data in this part of the fault zone indicate no thermal anomaly associated with the fault activity.

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ACCEPTED MANUSCRIPT Acknowledgments The authors would like to express their thanks to O M İlkışık, who provided the field data conducted in 1996-1999. The authors are grateful to B O Akkoyunlu for valuable assistance during the field studies between 2013 and 2017. We also thank the anonymous reviewers for their helpful comments on the manuscript. Many thanks go to John Sand and Chris Hare for

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language editing. This study was partly supported by TUBITAK Project No 113R017.

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ACCEPTED MANUSCRIPT Yurteri, C., Simsek, S., 2017. Hydrogeological and hydrochemical studies of the Kaman-

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ACCEPTED MANUSCRIPT Highlights Regional heat flow distribution of central Anatolia was obtained. Geothermal gradients were calculated. Moderate heat flow values are in accordance with the tectonic state of central Anatolia.

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Radiogenic heat production in Kırşehir was the main reason for the high surface heat flow.

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