Heat flow, heat production, thermal structure and its tectonic implication of the southern Tan-Lu Fault Zone, East–Central China

Geothermics 82 (2019) 254–266

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Heat flow, heat production, thermal structure and its tectonic implication of the southern Tan-Lu Fault Zone, East–Central China

T

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Yibo Wanga,b,c,d, Shengbiao Hua,b,c, Zhuting Wanga,b,c, Guangzheng Jianga,c, Di Hua,b,c, , Kesong Zhange, Peng Gaoa,b,c, Jie Hua,b,c, Tao Zhangd a

Institute of Geology and Geophysics Chinese Academy of Sciences, State Key Laboratory of Lithospheric Evolution, Beijing, China University of Chinese Academy of Sciences, College of Earth and Planetary Sciences, Beijing, China c Chinese Academy of Sciences, Institutions of Earth Science, Beijing, China d Shandong University of Science and Technology, College of Earth Science and Engineering, Qingdao, China e Institute of Anhui Bureau of Geological Exploration, First Hydrology and Engineering Geological Exploration, Bengbu, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Heat flow Thermal structure Tan-Lu Fault Zone Focal depth Heat production

We present new heat flow and heat production data to constrain the lithospheric thermal structure along the southern Tan-Lu Fault Zone (STLFZ), a continuous active deep crustal fault zone in East Asia, and use these thermal data to infer how the thermal structure of the STLFZ influences regional focal depths. We report five new high-quality heat flow measurements derived from temperature–depth profiles at five well sites and detailed thermal conductivity measurements of 13 outcrop samples and 115 dry core samples from five wells along the STLFZ. We analyze these new data in combination with published data to derive a thermal profile along the STLFZ and map the 350 °C isotherm along the STLFZ, which correlates well with the maximum depth of recorded seismicity. The results indicate that: (1) the thermal gradient along the STLFZ increase from 22.2 °C km–1 in the south to 34.8 °C km–1 in the north; (2) heat flow values are in the 44.0–75.4 mW m–2 range along the STLFZ, with a mean value of 59.9 ± 10.5 mW m–2; (3) the highest heat flow value is located in Lu-Zong Basin, with extremely high radioactive heat production of 15–20 μW m–3 in the upper crust that may be related to local ore bodies; and (4) the bottom boundary of the shallow-earthquake seismic zone in the STLFZ coincides with the 350 °C isotherm.

1. Introduction The tectonics and evolution of the Tan-Lu Fault Zone (TLFZ), a major seismogenic fault zone in East Asia, continue to be controversial due to conflicting interpretations of the deep structures and mechanisms controlling TLFZ earthquakes (Li et al., 2017; Yang et al., 2017), with few geothermal studies conducted in the region to characterize its thermal structure and evolution. The thermal structure of the lithosphere, as well as the vertical variations in crustal and mantle heat flow, which are calculated from the high-quality surface heat flow and heat production, provide key constraints on the evolution of cratons and deep dynamic processes and obtaining the deep temperature field of Earth’s crust (Furlong and Chapman, 2013; He et al., 2008). Reliable heat flow values and heat production of the TLFZ are therefore of paramount importance in reconstructing its thermal state.

Geothermal studies have been conducted in the TLFZ since the 1970s and the first heat flow measurements were primarily acquired in Shandong Province, along the central section of the TLFZ, and published by the Geothermal Research Division, Chinese Academy of Sciences (1979). Further heat flow measurements have since been collected (Wang and Huang, 1988, 1990; Hu et al., 2000), with 1230 measurements compiled by Jiang et al. (2016a), which include 473 heat flow measurements in the NCC and 231 in the YC, with mean values of 62.0 and 61.8 mW m–2, respectively (Figs. 1 and 2). However, almost 90% of the geothermal well sites are distributed in sedimentary basins, with a limited focus on the geothermal structure of the fault zone, such that these measurements primarily capture the local thermal history, as opposed to the thermal evolution of the TLFZ. The southern TLFZ (STLFZ) is at the boundary of two plates, and previous studies of TLFZ in this zone have found that: (1) the

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Corresponding author at: No. 19, Beitucheng West Road, Chaoyang District, Beijing, 100029, China. E-mail addresses: [email protected] (Y. Wang), [email protected] (S. Hu), [email protected] (Z. Wang), [email protected] (G. Jiang), [email protected] (D. Hu), [email protected] (K. Zhang), [email protected] (P. Gao), [email protected] (J. Hu), [email protected] (T. Zhang). https://doi.org/10.1016/j.geothermics.2019.06.007 Received 10 March 2019; Received in revised form 9 June 2019; Accepted 14 June 2019 0375-6505/ © 2019 Published by Elsevier Ltd.

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Fig. 1. A: Schematic geological map in East China (modified from Grimmer et al. (2002)). B: Structural map around the southern Tan-Lu Fault Zone. C: Geological boundary and sampling position for DC01.

and YC, and is broadly divided into two areas by the TLFZ (Fig. 1). The western area covers the southern North China Basin and southeastern margin of the NCC, and the eastern area covers the lower YC. The east–west-trending Dabie–Sulu Orogenic Belt lies along the boundary for approximately 2000 km, and is truncated by the TLFZ to the east (Fig. 1; Yin and Nie, 1993; Zhang et al., 2009; Wan, 2011; Guo et al., 2012). The southern North China Basin is a superimposed basin comprising many small- to medium-sized Mesozoic–Cenozoic basins that formed and evolved on a basement block that extended approximately east–west (Jia et al., 2004; Huang et al., 2005; Jin and Song, 2005; Shi et al., 2011). The TLFZ gradually became the dominant factor in the middle–later stages of basin development, whereas the Dabie Orogenic Belt was the dominant factor in the early stage (Chen et al., 2004). YC subducted beneath the NCC during the Late Triassic, forming the Sulu–Dabie Orogen (Yin and Nie, 1993; Zhang et al., 2009; Guo et al., 2012). And basin development has primarily occurred since the Late Cretaceous in the lower Yangtze region (Li et al., 2011; Zhang et al., 2014). The Tan-Lu Fault has undergone three primary episodes of activity since the Mesozoic: (1) strike-slip movement from the Late Jurassic to Early Cretaceous; (2) extensional tectonics from the Late Cretaceous to Paleogene; and (3) compressional tectonics since the Neogene (Xu, 1992; Yin and Nie, 1993; Zhu et al., 2004, 2016).

occurrence of deep subduction and exhumation on both sides of the fault zone (Guo et al., 2012); (2) the earliest occurrence of strike-slip movement along the TLFZ, along with the ongoing fault slip (Zhu et al., 2009); (3) a series of deep magmatic events during the Early Cretaceous due to large-scale strike-slip faulting (Zhu et al., 2016); (4) the east–west to northeast–southwest change in the maximum principal stress direction of the STLFZ (Yang et al., 2017); and (5) the predominant earthquake distribution in the vicinity of the fault zone, with reduced seismicity in the STLFZ compared with the northern TLFZ (Wei et al., 1993). Numerous deep seismic sounding surveys have been conducted in the STLFZ and adjacent areas since the 1980s (Fig. 1B; Zheng, 1989; Wang et al., 2000; Liu et al., 2003; Bai and Wang, 2006; Xu et al., 2014). These seismic studies have revealed the characteristics of seismic wave group and basic crustal structure of the STLFZ, which are vital in constraining its thermal structure and deep dynamics (He, 2015). Here we present new local steady-state continuous temperature logs and thermal physical parameters along the STLFZ and report on its deep thermal structure. Local thermal anomalies are inferred to confirm the absence of high fault-centered frictional heat flow. Finally, we demonstrate that the observed focal depths are constrained by the thermal structure of the surrounding region.

2. Geological setting 3. Data and methods The TLFZ is well known for its deep-seated faults, which have had a complicated formation and evolutionary history (Xu, 1992). It plays an important role in controlling the regional structure, mineralization, magmatism, seismicity, and volcanism of East Asia (Liu et al., 2001; Mackey, 2003; Mao et al., 2008; Zhu et al., 2009). The TLFZ also crosses several major tectonic units, including the Yangtze Craton (YC), Dabie Orogenic Belt, North China Craton (NCC), Sulu Orogenic Belt, and Central Asian Orogenic Belt (Fig. 1). The study region is located in the wedge-shaped body near the paleo-plate boundary between the NCC

3.1. Borehole temperatures Heat is the fundamental driving force for plate tectonics and deep dynamic processes, and it also influences the shallow response to tectonic events. Present-day geothermal research in the massif is based on thermal–physical parameters, such as the formation temperature, thermal conductivity, and radiogenic heat production, to determine the present-day geothermal gradient, heat flow, and temperature 255

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Fig. 2. Histogram of heat-flow values in the North China Craton and Yangtze Craton. Table 1 Primary lithologies and temperature gradients of the analyzed boreholes. Borehole No.

Locality

Latitude (°N)

Longitude (°E)

Tb

Depth interval (m)

Main Rock Type

TG (SD) (°C m−1)

Mean TG (SD) (°C m−1)

DC01

Qianshan

30°40′7″

116°29′6″

> 2 years

Hefei Dingyuan

31°43′14″ 32°30′29″

117°15′43″ 117°31′42″

> 2 years 161 days

DC04 LZSD

Wuhe Lujiang

33°09′25″ 30°58'47"

117°50′4″ 117°28'10"

278 days 8 days

Andesite gneissic rocks Siltstone Mudstone gypsum Siltstone Tuff, trachyandensite Syenite, monzonite

20.2(0.6) 22.6(0.5) 28.5 (0.5) 37.6 (1.3) 13.9 (1.2) 34.8 (1.6) 23.6 (0.9) 31.0 (0.9)

22.2(0.6)

DC02 DC03

0-290 290-1300 0-1329 0-380 380-528 0-1448 0-1650 1650-3000

30.3 (5.5)

27.4 (1.9)

Tb Shut-in time.

3.1.1. Temperature logging High-quality heat flow and crustal heat production measurements are essential prerequisites for mapping the regional lithospheric properties and thermal structure of a given area (Morgan and Gosnold, 1989). Borehole SCTs and thermal conductivity measurements on drill cores in a core depository are two ideal approaches for determining heat flow (Blackwell and Steele, 1989). However, drilling processes can cause strong temperature disturbances, with major effects including drilling fluid circulation and frictional heat generation. Adequate time must therefore be provided between the cessation of drilling and temperature logging to guarantee that the temperature data represent the

distribution at different depths and its controlling factors. Temperature is the key parameter in geothermal research. Commonly used formation temperature data consist of a number of different measurements, including systematically continuous temperature (SCT) measurements, formation test oil temperatures, and bottom hole temperatures (BHTs), with SCT data being the most reliable measurements (Beardsmore and Cull, 2001). SCT refers to the temperature measurement that is taken after thermal equilibrium has been reached between the borehole and formation.

256

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data exhibit two different kinds of vertical gradient variations: (1) the DC01 and DC02 TG values are approximately constant with increasing depth; (2) the TG values of the deep LZSD section and the two wells in the north show a gradual decrease in TG with depth.

true formation temperatures. The time required to recover 90% and 99% of the formation temperature is approximately 0.5–1.5 times and 10–20 times the drilling time, respectively (Yu, 1991). The shut-in time (time between the temperature logging and cessation of drilling) are listed in Table 1, with each temperature log representative of a steadystate to quasi-steady-state temperature measurement curve. The well temperature data used in this study were derived from the SCT measurements. High-quality continuous temperature logs were obtained from four deep geothermal wells (DC01–DC04 from north to south in Fig. 1B and Table 1) and one pilot hole of the Chinese Continental Scientific Drilling Project in Lu-Zong Basin (LZSD). The borehole temperature data were measured using a continuous logging system with a 42.9-mm-long platinum thermal resistance sensor (Robertson Geologging Ltd; UK) and a 5000-m-long cable (Downhole Surveys; Australia). The sensitivity and accuracy of the temperature measurements are ± 0.01 °C and ± 0.1 °C, respectively. The probe was calibrated once every two years at the Beijing Institute of Metrology, as well as after each temperature measurement, using a Hg thermometer to test and record the sensitivity and accuracy of the probe, in order to obtain a unified calibration. The depth data and corresponding temperature data were acquired using the Matrix Logging System (Mount Sopris Instruments; USA). The response time of the system due to the sensor assembly is approximately 2 s, with the downhole rate set to approximately 6 m min–1 (He et al., 2008). The well locations are distributed along the STLFZ and its adjacent area (within 50 km) for this geothermal study. Borehole temperatures were measured at 0.1 m intervals, with sampling conducted at 2 m intervals. The temperature–depth (T–Z) profiles are shown in Fig. 3. The thermal state of the uppermost continental crust is determined by measuring either the heat flow from Earth’s deep interior or ground surface temperature fluctuations (Huang et al., 2000). The temperature increases linearly with depth below 1000 m, resembling a type of conductive curve (Hu and Xiong, 1994; Pasquale et al., 2014). The LZSD curve provides an accurate temperature profile near the STLFZ to > 3000 m depth. Although most of the temperature curves are influenced by a disturbed section to varying degrees, each of the affected curves extends through the disturbed section, which allows the upper section to be used in conjunction with the lower section for temperature gradient and heat flow determinations.

3.2. Thermal conductivity 3.2.1. Sample measurements We collected 28 DC02 core samples and 23 DC04 core samples from 570 and 580 m depth, respectively, to the bottom of each borehole. We obtained another 28 core samples from the entire DC03 section. We acquired only six core samples from the deep LZSD section, and 30 core samples from the 80–300 m DC05 depth section, approximately 20 km from LZSD, which we assume to be representative of the upper LZSD section. These 115 core samples, along with 13 outcrop samples that captured the main rock types observed in DC01, were used for the thermal conductivity measurements. The sampling position of each core is marked at the corresponding depth in Fig. 3. Optical scanning technology (Popov et al., 1999, 2016; Lippmann and Rauen GbR; Germany) was employed to measure the thermal conductivity (TC), with a ± 3% measurement accuracy and 0.20–25 W m–1 K–1 measurement range. The instrument scans the samples using a centralized, moving, and continuous heat source, and the TC is calculated as a function of the temperature difference before and after exposing a given sample to the moving heat source and a calibration to standard samples with known TC values (Popov et al., 2016). The samples varied by < 1 mm along their planar–cylindrical surfaces and approximately 5 cm along their lengths. At least three tests were first conducted on each standard sample to calibrate the system error. The method employing a half-space line apparatus (Ou et al., 2004; Yao et al., 2005) and the method employing optical scanning technology, as employed by He et al. (2008) to analyze the same orthogneiss core samples of the Chinese Continental Scientific Drilling project, yielded TC values of 2.91, 2.96, and 2.95 W m–1 K–1, respectively, confirming the reliability of the instrument and method. The TC measurements are shown in Table 2. The DC02 and DC04 cores consisted primarily of a single lithology (i.e., sandstone), with approximately constant TC values of 1.6–2.3 and 1.3–2.6 W m–1 K–1, respectively. However, there were large fluctuations in the DC03 TC values that spanned the 1.2–5.5 W m–1 K–1 range. The DC01 core was dominated by gneiss, with TC values of 2.4–3.4 W m–1 K–1.

3.1.2. Temperature gradient The temperature gradients (TG) were calculated via a linear leastsquares regression method (Powell et al., 1988) at 10 m depth intervals (20 m intervals for LZSD) along the five temperature logs. The temperature data above the liquid surface were removed during the TG calculations due to the sensitivity differences in the surface air temperature variation between air and water (Wen and Ding, 2004). The TG variations with depth are shown in Fig. 3 and Table 1. The TG values varied from 14.6 to 29.8 °C km–1 in the DC01 borehole, with a mean TG value of 22.2 ± 0.6 °C km–1. A relatively stable TG was also obtained in the DC02 borehole, with a mean TG value of 28.5 ± 0.5 °C km–1. However, the TG values ranged from 9.2 to 50.6 °C km–1 in the DC03 borehole, with a mean of 30.3 ± 5.5 °C km–1. The TG curve can be divided into two sections: a high TG (37.6 ± 1.3 °C km–1) section from the surface to 380 m depth and a low TG (13.9 ± 1.2 °C km–1) section to the bottom of each borehole. A greater range of fluctuations is present in the DC04 borehole (35.0–93.2 °C km–1, with a mean of 34.8 ± 1.6 °C km–1), with the largest TG occurring at approximately 1150 m depth. The LZSD temperature log can be divided into an upper (200–1650 m) section that is characterized by relatively low TG values (15.2–31.8 °C km–1), with a mean value of 23.6 ± 0.9 °C km–1, and a lower (1650–3000 m) section that is characterized by relatively high TG values (14.9–36.1 °C km–1), with a mean value of 31.0 ± 0.9 °C km–1. The mean TG values for the five boreholes exhibit an increasing trend from south to north near the STLFZ, with minimum and maximum values of 22.2 and 34.8 °C km–1, respectively. The TG

3.2.2. Thermal conductivity estimation The LZSD TC measurements are based on the root-mean-square method proposed by Roy et al. (1981) due to the difficulty in obtaining the LZSD core samples (only six core samples were obtained). This model has been generally applied to rock samples that possess a random distribution and orientation of different minerals. The TC values can therefore be calculated using the known TC values of the various rock components using the following equation:

λb =

n

∑i =1 ∅i

λi

(1)

where ∅i is the volume fraction of the i th mineral, and λi and λb denote the TC of the i th mineral and mean TC, respectively (W m–1 K–1). The volume fraction of each mineral in the rock samples was calculated via CIPW (Verma et al., 2003; Pruseth, 2009), with the major and trace element compositions measured following Zhang et al. (2017). The mineral TC values were obtained from previous studies, with 0.6 and 0.023 W m–1 K–1 used as the TCs of water and air, respectively (Birch and Clark, 1940; Beck et al., 1956; Horai, 1971). The calculated mineral volume fractions and TC values of the three syenite and eight monzonite LZSD core samples (Zhang et al., 2017) are listed in Table 3. 257

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Fig. 3. Lithological columns, temperature logs, and vertical trend in temperature gradient for the five well sites.

m–1 K–1) at room temperature T0 (°C), and Tm and λm are calibration coefficients, where Tm = 1473 °C and λm = 1.05 W m–1 K–1. The pressure-corrected TC, λ (P ) (W m–1 K–1), is given by:

3.2.3. Thermal conductivity correction The primary factors affecting TC are pressure, temperature, porosity, and water saturation (Pribnow et al., 2013). High-temperature and high-pressure tests of rock thermal properties have shown that TC is very sensitive to pressure at low pressures (P ≤ 50 MPa), and that the TC increases rapidly with increasing pressure (Seipold and Huenges, 1998; Sun, 2017). TC is a function of temperature, with TC decreasing as the temperature increases. The water-saturation correction is generally applied to sedimentary rocks, whereas metamorphic and igneous rocks do not require a full-saturation correction due to their lower porosities (Rhee, 1975). The measured porosities and published values are shown in Table 4. We performed temperature, pressure, and porosity corrections on the core samples. When it comes to correction model, we take into account the main lithology and porosity of the core samples, following Eq. (2) (Sekiguchi, 1984), 3 (Sun, 2017), and 1 (Roy et al., 1981) for temperature, pressure, and porosity corrections, respectively. Our results show maximum temperature, pressure, and porosity corrections of 8%, 6%, and 13% for the deep DC04 samples, 1298 m DC04 sample, and 416 m DC03 sample, respectively. The temperature-corrected TC, λ (T ) (W m–1 K–1), is given by:

TT 1 1 ⎞ + λm λ (T ) = ⎛ 0 m ⎞ (λ 0 − λm ) ⎛ − Tm ⎠ ⎝T ⎝ Tm − T0 ⎠ ⎜

⎟

⎜

λ (P ) = 0.0005P + λ 0

(3)

where P denotes the in situ formation pressure (MPa). The pressure and saturation corrections increase the TC, whereas the temperature correction generally decreases the TC, with the three corrections offsetting each other to a certain extent. For example, the DC02 porosity correction is generally greater than the pressure correction above 900 m depth, whereas both corrections are approximately equivalent below 1000 m depth. Based on Eqs. (1)–(3), the corrected TC values are generally larger than the measured values. 3.3. Radiogenic heat production We measured the concentrations of radioactive heat-producing elements, including uranium (U), thorium (Th), and potassium (K), in 85 core samples (the same samples used in the TC calculations and plotted in Fig. 3) from DC02, DC03, LZSD, and DC04 for our heat production calculations. The U and Th concentrations were obtained by inductively coupled plasma mass spectrometry, and the K concentrations were determined by atomic absorption spectroscopy. The core sample densities were also measured in the laboratory by the

⎟

(2)

where T is the in situ formation temperature (°C), λ 0 is the TC (W 258

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Table 2 Thermal conductivity and heat production measurements.

*

Borehole No.

Depth

Rock Type

TC (W m−1 K−1)

A (μW m−3)

Borehole No.

Depth

Rock Type

TC (W m−1 K−1)

A (μW m−3)

DC01 DC01 DC01 DC01 DC01 DC01 DC01 DC01 DC01 DC01 DC01 DC01 DC01 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 DC02 LZSD LZSD LZSD LZSD LZSD LZSD DC03 DC03

* * * * * * * * * * * * * 574 584 607 658 688 697 819 843 881 887 903 909 922 971 1056 1092 1118 1325 1328 1393 1407 1417 1440 1447 1457 1473 1484 1503 1762 1892 2330 2488 2683 2871 71 87

gneiss gneiss gneiss gneiss gneiss gneiss gneiss gneiss gneiss gneiss gneiss gneiss gneiss siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone syenite monzonite monzonite monzonite monzonite monzonite mudstone mudstone

2.444 3.093 2.504 2.235 2.882 2.671 3.354 2.520 2.754 2.752 2.860 2.419 2.406 1.733 1.887 1.767 2.058 1.759 2.268 2.333 1.651 1.777 2.081 1.99 1.581 1.969 1.730 2.022 1.616 2.130 1.690 1.929 1.619 1.637 1.572 1.797 1.647 1.776 1.811 1.802 1.898 2.661 2.918 2.669 1.970 2.360 2.296 1.202 2.612

– – – – – – – – – – – – – 0.75 0.71 1.77 0.56 – 1.41 0.63 0.77 2.39 2.31 2.80 0.77 1.12 0.75 1.40 1.96 1.63 0.60 1.13 0.74 0.82 0.80 0.79 1.46 1.08 – 1.48 0.69 94.44 27.53 11.88 4.90 4.87 7.86 2.68 2.57

DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC03 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04 DC04

99 138 151 180 213 245 254 261 263 281 284 297 307 322 336 348 368 375 384 395 401 416 418 430 470 510 586 608 612 623 752 770 786 793 843 848 853 857 866 945 1176 1185 1193 1203 1223 1261 1271 1276 1298

mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone gypsum gypsum gypsum gypsum gypsum gypsum gypsum gypsum gypsum gypsum siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone siltstone

1.156 1.424 1.177 1.458 1.384 1.359 1.518 1.500 1.661 1.425 2.129 1.605 1.604 2.814 1.762 1.421 3.789 5.007 5.258 5.513 5.220 5.000 4.905 5.127 4.814 3.323 1.765 1.316 2.649 1.738 1.656 1.607 1.397 1.289 1.491 2.172 1.201 1.403 1.353 1.590 2.233 1.919 2.179 1.870 1.970 2.211 2.292 1.719 2.209

2.91 2.52 3.27 3.09 1.87 3.02 1.93 2.53 2.73 1.71 1.54 0.92 3.41 1.24 3.14 3.12 2.41 3.67 3.59 1.24 0.50 0.26 0.43 0.17 0.53 0.32 1.99 1.71 0.95 2.28 1.61 2.34 2.44 1.61 1.86 1.82 1.59 1.88 1.69 2.71 2.47 2.71 4.21 2.33 3.20 5.07 4.22 6.82 1.73

Outcropping rocks.

m–3 range with a mean of 2.50 ± 0.82 μW m–3 for the DC03 mudstone (Table 5). Gypsum generally has low U, Th, and K concentrations, resulting in low heat production. The common accessory minerals in the rock samples are apatite, zircon, titanite, and epidote, where the large differences in heat production, and U and Th concentrations, may primarily be attributed to the differences in their radiogenic mineral compositions (Ray et al., 2015; Podugu et al., 2017). The U/K, Th/K, and Th/U ratios range from 0.43 to 4.40, 0.54–3.68, and 0.54–3.68, respectively, for the DC02 siltstone, 0.41–3.21, 0.41–3.21, and 0.14–2.43, respectively, for the DC04 siltstone, and 2.68–32.39, 0.53–9.67, and 0.16–2.01, respectively, for the DC03 mudstone (Table 5). The mean Th/U ratio is high for DC02 (2.36) relative to that for DC04 (0.31), despite their similar lithologies and mean siltstone Th/U ratio of approximately 2.2–3.1 (Wollenberg and Smith, 1987; Haenel et al., 1988). This large difference may imply different sediment sources between DC02 and DC04 since the two well sites are separated by about 200 km, or different evolutionary processes in the later period, with dominant

Archimedean method. The heat production values were calculated via the empirical formula proposed by Rybach (1976):

A = 10−5ρ (9.25CU + 2.56CTh + 3.48CK )

(4) –3

–3

where A is the heat production (μW m ), ρ is the density (kg m ), CU and CTh are the U and Th concentrations (ppm), respectively, and CK is the K concentration (%). The results are listed in Table 2 and summarized in Table 5. The heat production values vary as a function of well site, depth, and lithology along the STLFZ. The DC02 heat production is in the 0.56–1.11 μW m–3 range, with a mean value of 0.77 μW m–3. The DC04 siltstone and DC03 mudstone exhibit large heat production fluctuations, as well as high U and Th concentrations, compared with the DC02 siltstone and DC03 gypsum. Heat production is in the 0.56–2.80 μW m–3 range, with a mean of 1.20 ± 0.62 (1 SD) μW m–3 for the DC02 siltstone, in the 0.95–6.82 μW m–3 range with a mean of 2.58 ± 1.35 μW m–3 for the DC04 siltstone, and in the 0.92–3.67 μW 259

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Table 3 Volume fractions (%) of the mineral compositions in the LZSD samples, and estimated thermal conductivity and heat production values. Sample ID

LZSD1-B1

LZSD1-B2

LZSD1-B3

LZSD1-B4

LZSD1-B7

LZSD1-B8

LZSD1-B9

LZSD1-B14

LZSD1-B15

LZSD1-B16

LZSD1-B17

Sampling Depth Quartz (7.69) Plagioclase (2.32) Orthoclase (2.32) Corundum (3.56) Diopside (5.57) Hypersthene (4.19) Olivine (5.02) Ilmenite (2.30) Magnetite (5.10) Hematite (11.28) Apatite (1.38) Zircon (4.54) TC (W m−1 K−1) U (ppm) Th (ppm) K2O (%) A (μW m−3)

1679 2.55 55.67 34.01 0 1.82 2.18 0 0.77 1.71 0.63 0.54 0.13 2.58 68.7 196 5.29 32.20

1711 1.17 52.2 37.16 0.21 0 5.45 0 0.84 2.19 0 0.63 0.16 2.50 56 168 5.73 27.09

1846 1.23 49.07 40.82 0 3.66 1.28 0 0.81 2.12 0.33 0.53 0.14 2.55 46 118 6.48 20.89

2000 0.68 54.9 32.85 0 4.67 2.8 0 0.92 2.32 0 0.8 0.05 2.56 32.8 102 5.12 16.36

2080 0 55.34 34.29 0 4.26 1.42 0.39 0.8 2.52 0.17 0.77 0.05 2.23 19.3 59.6 5.34 7.85

2165 0.47 55.01 35.4 0 3.39 1.88 0 0.8 2.28 0 0.68 0.09 2.23 27.4 98.8 5.58 11.83

2255 2.58 53.09 34.52 0 2.94 2.83 0 0.87 2.31 0 0.68 0.17 2.33 54.3 148 5.38 21.24

2789 1.77 51.58 39.63 0 2.51 2.48 0 0.59 0.99 0 0.39 0.05 2.27 27.3 56.3 6.32 9.58

2798 2.67 51.41 39.61 0 2.27 1.77 0 0.59 1.26 0 0.37 0.04 2.29 31.9 56.1 6.33 10.67

2980 1.3 50.5 40.73 0 3.17 2.16 0 0.61 1.08 0 0.39 0.05 2.26 71.2 56.8 6.5 19.06

2985 1.23 50.71 40.59 0 2.76 2.95 0 0.58 0.74 0 0.38 0.05 2.26 80.7 59.8 6.29 21.25

low quality due to the lack of in situ core samples, whereas the heat flow values decrease with depth in the lower syenite and monzonite sections, from approximately 1500 m depth to the bottom of the borehole. The heat flow for the 1590–1720, 1860–1980, 1980–2160, 2200–2480, and 2620–2850 m depth sections are 74.1 ± 1.8, 87.2 ± 2.0, 76.2 ± 1.2, 71.8 ± 2.1, and 66.7 ± 1.0 mW m–2, respectively. The vertical heat flow values generally decrease with depth due to the contribution of radiogenic heat production in the rocks (Haenel et al., 1988; He et al., 2008).

oxidation processes making U soluble in water, whereas Th remains in the weathered product due to its insolubility (Dickin and Muller, 2005). 3.4. Heat flow determination The heat flow values of the five boreholes were calculated via multiplication of the TG values from the least-squares method with the TC values at different depth intervals. The corrected core TC values were used for the heat flow determinations, with the heat flow calculated from sections with stable TG and TC measurements. The results are shown in Table 6 and Fig. 3. The stable TG and approximately constant TC yields a high-quality DC01 heat flow value of 59.9 ± 1.7 mW m–2. The lack of segmentation in the core sampling highlights two sections with slightly different heat flow values: an upper (580–640 m) section, with a mean heat flow of 57.1 ± 2.4 mW m–2, and a lower (820–1240 m) section, with a mean heat flow of 55.5 ± 1.4 mW m–2. The small difference in heat flow confirms relatively stable heat flow throughout the entire borehole. Stable heat flow is also calculated throughout the DC04 borehole, with the upper (810–900 m) section yielding a mean heat flow of 66.4 ± 2.7 mW m–2 and the lower (1190–1310 m) section yielding a mean heat flow of 61.1 ± 2.1 mW m–2. The difference between these two depth sections may be due to fluid convection within borehole at 1150 m. The boreholes with obvious lithological changes, such as the gypsum and mudstone units in DC03, and the trachyandesite–monzonite interface in LZSD, require greater attention to determine their respective heat flow values. The DC03 borehole is divided into three sections for the heat flow determination: a 150–200 m section with a mean heat flow of 59.3 ± 1.9 mW m–2, a 300–330 m section with a mean heat flow of 58.9 ± 3.1 mW m–2, and a 380–520 m section with a mean heat flow of 65.8 ± 3.8 mW m–2. The vertical heat flow variations span the 66.7–87.2 mW m–2 range in the LZSD borehole, and exhibit a slight decrease with depth. The heat flow in the upper (200–1495 m) section of the LZSD borehole is 80.7 mW m–2 and is of

4. Discussion We have presented the first systematic study of heat flow along the STLFZ, and provide a new TC and heat production database for the primary rock types found in the study area. The data sets highlight a variable thermal background, with the calculated isotherm used to constrain the focal depth distribution. The prominent features are discussed below.

4.1. Influence of the vertically varying temperature gradients There are many reasons for the observed changes in TG with depth, but these changes can generally be divided into two categories: (1) the redistribution of heat from Earth’s interior to the surface, including the thermal refraction due to the differences in the local heat flow and thermal properties of the rocks (such as TC and heat production), climate change, groundwater activity, topographic relief, tectonic movements, faults, and other parameters; and (2) transient or periodic changes in the near-surface temperature, such as surface temperature changes and mud circulation effects during drilling. Here we discuss the thermal properties of the rocks, climate change, and groundwater activity in detail to better understand how they may shape the observed TG changes.

Table 4 Thermal conductivity corrected porosities in the DC02, DC03, and DC04 boreholes. Borehole No.

DC02 DC03 DC04

Depth (m)

500-1500 70-500 700-1300

Rock Type

Siltstone Mudstone, gypsum Sandstone

Porosity Range (%)

1.73-5.81 3.07-14.37 4.60-9.50

Na Number of measurements. 260

Porosity (%)

Source References a

Mean (SD)

N

4.47 (1.68) 10 6.88 (1.59)

8 7

This study (Chen, J. P., 2004) This study

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Table 5 U, Th, K, heat production (HP), and Th/U, Th/K, and U/K ratios along the STLFZ. Rock Type

Na

Siltstone (DC02) Siltstone (DC04) Mudstone

28

Gypsum

6

23 22

U (μg/g) Range Mean ± SD

T (μg/g) Range Mean ± SD

K (%) Range Mean ± SD

A (μW m−3) Range Mean ± SD

Th/U Range Mean ± SD

Th/K (10−4) Range Mean ± SD

U/K (10−4) Range Mean ± SD

0.71-9.12 3.26 ± 2.64 2.18-25.3 9.22 ± 5.20 2.9-15 9.41 ± 3.63 0.32-1.42 0.92 ± 0.46

1.06-6.47 3.43 ± 1.49 1.14-5.3 2.09 ± 1.12 1.72-11.8 3.69 ± 2.24 0.60-3.00 1.87 ± 0.92

1.27-2.11 1.68 ± 0.19 1.65-6.73 3.07 ± 1.15 0.32-4.1 2.12 ± 1.12 0.08-0.78 0.30 ± 0.25

0.56-2.8 1.20 ± 0.62 0.95-6.82 2.58 ± 1.35 0.92-3.67 2.50 ± 0.82 0.17-0.53 0.37 ± 0.14

0.54-3.68 2.36 ± 1.90 0.14-2.43 0.31 ± 0.47 0.16-2.01 0.49 ± 0.44 0.58-5.74 2.57 ± 1.81

0.54-3.68 2.04 ± 0.87 0.41-3.21 0.75 ± 0.60 0.53-9.67 2.67 ± 2.53 1.95-26.67 10.78 ± 9.07

0.43-4.4 1.94 ± 1.52 1.32-4.11 2.88 ± 0.72 2.68-32.39 6.29 ± 7.09 1.75-4.80 3.80 ± 1.12

Na Number of rock samples.

temperature measurements therefore reflect a longer surface temperature history. The temperature change related to the last ice age would have penetrated the Earth to a depth of ˜2 km (Majorowicz et al., 2012), whereas the depth influenced by surface temperature and annual variations in formation depth is generally several tens of meters throughout most of China (Yu, 1991). However, in the present study we calculate the heat flow (except DC04) for a stable TG section below 500 m depth, where climate change has minimal impact on heat flow and lithospheric thermal structure.

4.1.1. Thermal properties of the rocks The product of TC and TG yield a certain value at a given borehole depth, with the TG and rock TC at different depths showing a relationship of mutual thermal growth and decline. LZSD and DC03 exhibit obvious segmentation, as shown in Fig. 3. DC03 has two TG segments defined by a lower gypsum lithology and an upper mudstone lithology. The corresponding TG values are 37.6 ± 1.3 and 13.9 ± 1.2 °C km–1, respectively. LZSD can be divided into three sections: an upper (20–1495 m) section of Zhuanqiao Formation volcanic rocks, which are characterized by relatively low TG values (16.7–36.4 °C km–1) with a mean value of 24.5 ± 0.7 °C km–1, a lower (1650–3000 m) section of crystalline rocks that possess high TG values (25.0–41.7 °C km–1) with a mean value of 30.9 ± 0.9 °C km–1, and a contact segment (1495–1650 m) that has similar TG values to the upper section. The TG changes in the upper and lower sections are primarily due to the TC differences between the volcanic and crystalline rocks. For the depth section where the lithology is relatively uniform, the TG values decrease systematically with increasing depth due to the presence of radioactive elements in Earth’s crust, with a more pronounced decrease in the presence of a high heat-producing radioactive element layer. Our analysis of the LZSD radioactive elements shows that the heat production from the lower section (1600–3000 m) is as high as 20 μW m–3. This extremely high heat production leads to a decrease of 7 °C km–1 from 1900 m depth to the bottom of the borehole.

4.1.3. Groundwater activity Groundwater activity is widespread throughout the shallow portion of Earth’s crust. The vertical flow of groundwater can significantly affect the thermal state of the shallow crust, with the direction and velocity of groundwater flow evaluated using drilling temperature data. The temperature profile is generally linear or piecewise linear throughout the well for the case of heat conduction only (excluding the upper vadose zone (i.e., above the water table)). However, the temperature profile deviates from linearity when the borehole encounters a water-conducting fracture or high permeability layer, with a local temperature rise (upflow) or fall (downflow) observed. The temperature curve in the regional groundwater recharge zone is “concave” when the permeability of the entire well is high and relatively uniform, whereas the groundwater discharge area exhibits a “convex” temperature curve, with the shape and magnitude of the concave–convex curvature dependent on the rate of vertical groundwater flow. A generalized pattern of the effects of groundwater activity (recharge, runoff, and discharge) on the shallow temperature field is shown in Fig. 4, assuming a uniform formation medium and background heat flow. Some temperature sections within 1000 m of the groundwater profiles used in

4.1.2. Climate change Long-term climate change will affect the subsurface temperature field. However, any surface temperature changes will propagate very slowly through the subsurface due to the very low thermal diffusion coefficient of the rock (approximately 1 × 10–6 m2 s–1). Deeper Table 6 Heat flow determinations for DC01, DC02, DC03, DC04, and LZSD. Borehole No.

DC01 DC02 DC03

DC04 LZSD

Depth interval (m)

200-1400 580-640 820-1240 150-220 300-330 380-520 810-900 1190-1310 80-1495 1590-1720 1860-1980 1980-2160 2200-2480 2620-2850

TC (W m−1 K−1)

TG (SD) (°C m−1)

22.58 30.21 28.59 41.34 37.07 13.89 41.83 29.73 23.59 29.17 32.71 32.71 31.40 28.95

(0.55) (1.65) (0.29) (1.60) (2.38) (1.20) (1.46) (1.24) (0.93) (1.11) (0.96) (0.30) (0.43) (0.67)

Na Number of measurements. * Data from DC05. 261

Heat flow (SD) (mW m−2) a

Mean (SD)

N

2.65 1.89 1.94 1.43 1.59 4.74 1.59 2.05 3.42 2.54 2.67 2.33 2.29 2.30

13 4 11 4 4 7 6 9 30 2 3 3 3 4

(0.31) (0.15) (0.24) (0.04) (0.14) (0.72) (0.30) (0.20) (0.64) (0.06) (0.21) (0.19) (0.35) (0.04)

*

59.90 57.12 55.51 59.32 58.89 65.83 66.38 61.09 80.68 74.07 87.19 76.22 71.83 66.69

(1.73) (2.42) (1.35) (1.94) (3.14) (3.83) (2.71) (2.09) (3.59) (1.80) (1.98) (1.18) (2.07) (1.04)

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Fig. 4. Generalized model of groundwater effects on the shallow temperature field.

was an important reason for the large differences in the heat flow values between the Huainan and Huaibei coalfields. The distribution of heat flow values across the STLFZ and its neighboring areas point to high heat flow values in Lujiang to the north and low values to the south. This zonal control of heat flow may be explained by the heterogeneity of the STLFZ, which is characterized by differences in magmatism, seismicity, mineralization, mantle properties, and neotectonic processes (Mackey, 2003; Mao et al., 2008; Zhu et al., 2009; Xiao et al., 2013; Wei et al., 2014; Liu et al., 2015). Nevertheless, He et al. (2008) suggested that the occurrence of high heat flow was primarily controlled by deep dynamic mechanisms, with higher mantle heat flow resulting from postcollision delamination and magmatic processes and higher crustal heat flow because of the over-thickened lithosophere being the primary reason for the high heat flue at the Chinese Continental Scientific Drilling site. We observe two heat flow anomalies in the study area based on the distribution of heat flue values (i.e., Lu-Zong Basin and Jiashan-Laian Massif), which are represented by LZSD and DC04, respectively. The mean heat flow is 56 mW m–2 when these two heat flow anomalies are excluded, which implies that the thermal anomalies are likely due to the superposition of later tectono-thermal events. However, if we employ the heat production measurements and U, Th, and K2O concentrations from Zhang et al. (2017), the LZSD heat production (1600–3000 m section) calculated using Eq. (4) is as high as 20 μW m–3 (Tables 2 and 3). It can also be inferred that the thermal anomaly in LuZong Basin may be closely related to ore bodies with high radioactive heat generation. The heat flow anomalies on the northern side (near DC04) are primarily concentrated in the Jiashan–Laian area. A large number of basaltic lavas were erupted during the Paleocene–Pliocene (Zhi and Zhang, 1994), with these eruptive events typically affected by the latest tectono-thermal events. Whether the seismogenic fault is in a high stress state is a scientific problem that puzzles seismologists (Chen, 2014): low-intensity seismogenic faults are generally unlikely to accumulate high energy, making it impossible for large earthquakes; high-intensity faults slip

this study appear to exhibit convex T–Z profiles, with the temperature suddenly rising or falling over a given depth range. The DC02 T–Z profile may therefore be heavily influenced by groundwater at 450–480 m depth, which can be attributed to upward flow, with a similar trend observed in the DC03 T–Z profile, with transient perturbations at 330–350 m depth. The disturbance of the middle–lower depth section of the DC04 T–Z profile indicates the influence of varying degrees of upward flow, possibly due to DC04 being located at the intersection of two buried fractures (Reiter, 2001; Verdoya et al., 2007).

4.2. The relatively high heat flow in LZSD and DC04 A major scientific question is whether the STLFZ is a thermal anomaly belt. Previous geothermal studies focused primarily on the Huainan–Huaibei Coalfield and Subei Oilfield (Wang, 1989; Peng et al., 2015), with only four centrally distributed heat flow values reported near the STLFZ (Deng and Wang, 1982). We obtained a mean heat flow of 61.4 mW m–2 after combining our five new heat flow values to these previous values, which indicate heat flow in the STLFZ near the mean value of 61.5 mW m–2 in continental China (Jiang et al., 2016a). The STLFZ heat flow is similar to those in the two adjacent cratons, the NCC and YC, which have heat flow values of 62.0 and 61.8 mW m–2, respectively. The mean heat flow values of the southern North China Basin to the west and Cenozoic extensional Subei Basin to the east are 54 and 72 mW m–2, respectively (Wang, 1989). The continental heat flow decreases with the age of the last tectono-thermal event experienced by the geological terrane (Icaro and Pollack, 1980; Pollack et al., 1993), with the Subei Basin yielding a high background heat flow value. Numerical simulations of the NCC deep processes have indicated that dehydration of the Pacific Plate yielded a smaller mantle viscosity coefficient on the eastern side of the NCC than on the western side, indicating stronger mantle convection in the east (He, 2014; 2015). Jiang et al. (2016b) considered the influence of the TLFZ as a key reason for the high heat flow values on the Jiaodong Peninsula and Peng et al. (2015) suggested that the distance from the plate boundary 262

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during repeated seismic cycles, which will cause obvious "heat flow anomaly" due to friction. However, the geothermal study of the San Andreas fault does not have the high heat flow anomaly, so Lachenbruch and Sass (1988) proposed the concept of “heat flow paradox”, which triggered a series of debates about the strong/weak nature of faults (Lockner et al., 2011; Scholz, 2000; Zoback, 2000). Therefore, the observed regional variations in heat flow were probably not controlled by the inhomogeneity of the TLFZ, but may rather be similar to the San Andreas Fault (Lachenbruch and Sass, 1988). That is, the TLFZ only exists as a paleo-plate boundary, such that the plausible control on the heat flow distribution may only be a heat flow paradox.

Table 7 to examine the relationship between the extent of seismicity and the deep thermal regime. The temperature at a given depth for conductive heat flow in a three-layered crust with a constant surface heat flow Qs , surface temperature T0 , thermal conductivity K , and radiogenic heat production A (see Table 7 for the values) is defined as:

T (z ) = T0 +

Qs A 2 z− z K 2K

(5)

A three-layer crustal structure is observed at each well site, consisting of an upper, middle, and lower crust, based on the deep seismic sounding studies. It is well known that TC is a function of temperature. Here we employ a correction to reduce the effect of radiative heat transfer (Modest, 1993) at depth (Kukkonen et al., 1997):

4.3. Coupling with focal depth Stick-slip has been cited as the most important mechanism for inducing shallow earthquakes (Brace and Byerlee, 1966; Kaproth and Marone, 2013), and is favored in the presence of “brittle” minerals, such as feldspar and quartz, under high effective stress conditions perpendicular to the fault and at low temperatures (< 350 °C) (Byerlee and Brace, 1968; Sibson, 1977; Stesky, 1978). The base of the seismogenic zone extends along the northeast–southwest-oriented TLFZ (Wei et al., 1993). A number of extensive deep seismic sounding surveys have been conducted in the STLFZ and its adjacent areas since the 1970s, with basic wave group features obtained from the wide-angle reflection–refraction sections and crustal structure profiles (Zheng, 1989; Wang et al., 2000; Li et al., 2002; Liu et al., 2003; Dong et al., 2009; Bai et al., 2016). We constructed a twodimensional profile of the STLFZ lithospheric structure using these seismic results, which is shown in Fig. 5b, with most of the earthquake foci located above the upper or middle–lower crust where there are dramatic structural changes. Approximately 90% of the earthquakes occur above 19 and 13 km depths to the southwest and northeast of DC02, respectively (Fig. 5b). Based on the stable boundary of common minerals from Hyndman and Wang (1993); Wang et al. (1995) determined that the rock deformation and metamorphic temperature and pressure in the STLFZ are 350–500 °C and 4 kbar, respectively. The deep temperature was calculated for each of the well sites in Fig. 5b and

1 K (T ) = Kref ⎡ + c (T + 273.15)3⎤ ⎦ ⎣ 1 + bT

(6)

where Kref is the thermal conductivity at room temperature, and b and c are experimental constants. The results from Roy and Rao (1999); Correia and Aafanda (2002), and Schatz and Simmons (1972) were used for the upper crust, middle–lower crust, and mantle lithosphere to determine the best-fitting parameter values of b1 = 0.0015 K−1 and b2 = 0.0001 K−1. A value of c = 1 × 10−10 Wm−1 K−4 was applied when the temperature exceeded 800 °C. The calculated 350 °C depth at each heat flow site is plotted in Fig. 5b. Both the 350 °C depths and crustal extent of seismicity shallow by approximately 6 km from northeast to southwest. The mean depth of the 350 °C isotherm is 18.6 km in the southwest, whereas is it 12.6 km in the northeast. Since heat flow consists primarily of two components, crustal and mantle heat flow, mantle heat flow can be obtained via the “layer-by-layer deduction” method based on the three-layer crustal model plotted in Fig. 5b. Here Qc is the sum of the heat flow generated by the radioactive elements (QA ) in Earth’s crust, where Qc = ∑ QA , and the mantle heat flow is Qm = Qs − ∑ QA . The calculated mantle heat flow and the ratio of mantle to surface heat flow are shown in Fig. 5a and Table 7. Segmentation of the seismogenic zone along the fault zone

Fig. 5. (a) Along-strike section of the STLFZ showing thermal observations: surface heat-flow (Qs ), mantle heat-flow (Qm ), and Qm/ Qs . (b) M > 1 seismicity for the 1965–2017 period, which extends to the estimated depth of the 350 °C isotherm based on heat-flow observations, with the mean isotherm depth indicated by the dashed blue line (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 263

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Table 7 Lithological and thermal properties for the well sites in Dabie Orogenic Belt, North China Craton, and Yangtze Craton. a

Tectonic Unit

crust structure

lithology

Vp

Dabie

Upper Middle Lower Upper Middle Lower Upper Middle Lower

Felsic rocks, Eclogite d Gneiss Granite d Granulite d Basement f Gneiss, Amphibolite f Granulite f Basement e Gneiss e Granulite e

5.6-6.2 6.2-6.4 6.6-7.0 6.0-6.2 6.2-6.4 6.4-7.0 6.0-6.2 6.2-6.5 6.5-7.3

YZ

NC

TC (W m−1 K−1) d d d f f f e e e

2.9 2.8 2.5 3c 2.8 2.6 3c 2.8 2.6

b b b

c c

c c

A (μW m−3)

Well Site

Qs (mW m−2)

Depths to 350 ℃

Qm (mW m−2)

Qm/Qs

1.2 b 0.9 b 0.3 b 2.16 f 1.26 f 0.25 f 1.26 e 0.86 e 0.31 e

DC01 AQ01 AQ02 LZ01 LZSD

59.73 50.7* 52.8* 75.4* 75.20

15.5 17.5 19.8 16.0 16.2

29.96 21.83 19.05 30.48 30.28

1.00 0.70 0.60 0.68 0.67

LZ02 DC02 DC03 DC04

44.0* 56.32 61.35 63.74

29.3 15.7 12.2 13.0

15.95 29.47 32.94 34.05

0.57 1.10 1.16 1.15

* From Deng and Wang (1982). a P-wave velocity (km s–1). b From He et al. (2009). c From Zang et al. (2002). d From Zhang et al. (2004). e From Chi and Yan (1998). f From Wang (1989).

corresponds to the deep temperature interface, with no obvious connection to surface heat flow. For example, the change in surface heat flow occurs at LZSD, as opposed to DC02. Therefore, the relationship between surface heat flow and active fault slip is not straightforward, even though spatial correlations are observed. The “brittle” to “ductile” transition along the base of seismogenic zone is controlled primarily by ambient thermal conditions, such that higher heat flow along the STLFZ to the northeast of LZSD may not reflect the deep thermal state. This decoupling between heat flow and the deep thermal state may be due to the previously mentioned extremely high radioactive heat generation in the upper crust of the Lu-Zong Basin. Seismogenic areas should therefore be constrained by deep rheological structures, with deep heat flow anomalies due to heterogeneous mantle activity likely playing an important role.

Acknowledgements

5. Conclusions

References

We determined the crustal thermal conditions along the STLFZ using new high-quality heat flow, radioactive heat production, and TC measurements of the main rock formations in the studied boreholes. We infer the following conclusions on the thermal structure of the STLFZ:

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This research was supported by grants from the Anhui Fund Project (Basic Measurement of Heat Flow in Anhui Province, No. 2014-g-11), the Jiangsu Fund Project (Investigation of Geothermal Field and Special Study on Crustal Thermal Structure in Subei Basin, 2017ZX05008-004) and a National Youth Science Fund Project (Methods Study Based on Petrophysics Experiments for Accurate Recognition of LithofaciesDiagenetic Facies with Well Logs in Tuffaceous Sandstone, 41602135). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.geothermics.2019.06. 007.

1 The mean TG exhibits an increasing trend from south to north near the STLFZ, with minimum and maximum observed values of 22.23 and 34.80 m km−1, respectively. Heat flow in the core of the massifs along the STLFZ ranges from 44.0 to 75.4 mW m−2, with a mean value of 59.9 ± 10.5 mW m−2. The highest heat flow is observed in Lu-Zong Basin. 2 The heat flow and radiogenic heat production measurements in this study have an important implication for the deep thermal structure of the lithosphere. The 350 °C depth at DC02 is consistent with a depth transition along the STLFZ, where the 350 °C isotherm thins from 18.6 km to the southwest to 12.6 km to the northeast, correlating with the maximum depth of recorded seismicity across the region. 3 These heat flow and heat production analyses along the STLFZ likely validate the absence of a fault-centered heat flow anomaly.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 264

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