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Cenozoic thermal history of the Bohai Bay Basin: constraints from heat flow and coupled basin–mountain modeling
Physics and Chemistry of the Earth 28 (2003) 421–429 www.elsevier.com/locate/pce
Cenozoic thermal history of the Bohai Bay Basin: constraints from heat flow and coupled basin–mountain modeling Lijuan He *, Jiyang Wang Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China Received 12 March 2002; received in revised form 18 September 2002; accepted 4 April 2003
Abstract Heat flow measurements show a moderate thermal background (61 mW/m2 ) in the Bohai Bay Basin, which although experienced multi-phase rifting in the Cenozoic era. In contrast, its surrounding mountain areas are characterized by low heat flow. Constraint by heat flow measurements, the thermal evolution of the Bohai Bay Basin during the Cenozoic era was performed by a numerical basin–mountain model. The model incorporates differential lithosphere stretching and shortening by finite-element method in the Lagrangian frame. The predicted heat flow in the center of the three depressions of the Bohai Bay Basin is calculated to have varied between 51 and 63 mW/m2 through the Cenozoic evolution, indicating a rather smooth variation of basin thermal state, and a cooling trend from the Oligocene to present-day. Model results also suggest that the Taihang Mountains probably uplifted in the Quaternary, which resulted in low heat flow in the mountain area. Both heat flow constraints and modeling imply that a new phase of rifting in the Pliocene existed in the Huanghua and Bozhong Depressions, which was suggested by tectonic subsidence analysis. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Bohai bay basin; Heat flow; Coupled basin–mountain model; Thermal history
1. Introduction Heat flow, which reflects the present-day thermal state of the lithosphere, is the result of multi-phase evolution of the lithosphere. Therefore, heat flow is a key constraint on lithosphere thermal modeling based on extensional models. At present, there are many extensional models describing the evolution of lithosphere, including kinematic models and dynamic models, which were summarized in the literatures (e.g. Ruppel, 1995; Fernandez and Ranalli, 1998). These models assumed that the areas outside are non-deformed lithosphere. However, crustal shortening and extension, two of the most important tectonic processes reshaping continental lithosphere, link closely in many regions. In eastern China, many extensional basins are coupled with mountains (Liu et al., 2000). There are three typical couples: Songliao rift basin and Daxingan Mountains in the northern part, the Bohai Bay rift basin and the Taihang Mountains in the middle, and the Jianghan rift * Corresponding author. Tel.: +86-10-62007813; fax: +86-1062010846. E-mail address: [email protected] (L. He).
basin and the Xuefeng Mountains in the southern area. We chose the Bohai Bay Basin for modeling in this paper not only because it is the largest Cenozoic rift basin in eastern China, but also due to its special thermal regime compared to typical modern rift basins with high heat flow. Study on its thermal evolution and coupling with the Taihang Mountains may shed some light on understanding the dynamics of basin–mountain systems in eastern China. In this paper, a coupled basin–mountain model is used, which incorporates differential lithosphere stretching and shortening similarly as in the model of Negredo et al. (1999), but based on a finite element algorithm by the Lagrangian method. We modeled the Cenozoic thermal history of the Bohai Bay Basin based on a crosssection passing through the Taihang Mountains, Jizhong Depression, Huanghua Depression, and Bozhong Depression (Figs. 1 and 2). Through modeling, we want to understand the possible time of the uplift of the Taihang Mountains and its influence on the thermal history of the Jizhong Depression, and the possibility of a new phase of extension existing in the Huanghua and Bozhong depressions, and its influence on the present-day heat flow regime.
1474-7065/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1474-7065(03)00062-7
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Fig. 1. Map of Bohai Bay Basin (modified from Ye et al., 1985). Note the basin, comprising of Jizhong, Huanghua, Bozhong, Linqing, Jiyang and Liaohe depressions, is surrounded by five mountain ranges: the Yan, Luxi, Taihang, Liaodong and Jiaodong Mountains. The profile (PA–PB, PC–PD) used for modeling (Fig. 2) is shown as a full line.
2. Tectonic setting The Bohai Bay Basin (Fig. 1), a Cenozoic basin, is surrounded by five mountain ranges: the Yan, Luxi, Taihang, Liaodong and Jiaodong Mountains. It is comprised of six major depressions, which are the Jizhong, Huanghua, Bozhong, Linqing, Jiyang, and Liaohe depressions. Tertiary strata rest unconformably on
a variety of older pre-rift strata and are covered conformably or unconformably by Quaternary sediments. The succession is typically 4000–7000 m thick. Lithologies are dominated by sandstone and mudstone. The Cenozoic succession has been divided into six formations: the Kongdian, Shahejie, Dongying, Guangtao, Minghuazhen, and Pingyuan formation, which resulted by two phases of rifting (Allen et al., 1997). The earlier, Paleocene-early-Eocene phase resulted in the deposition of the Kongdian Formation and the fourth (lowest) member of the Shahejie Formation in the west and south of the present basin. The second phase started at about 43–45 Ma (middle Eocene), beginning with the deposition of the third member of the Shahejie Formation. In part, these sediments were deposited in the same half grabens as the Kongdian Formation, but the Bozhong Depression in the central part of the basin originated at this time, and became the major depocentre. It was generally believed that regional extension ended at the end of the Oligocene, and the basin as a whole began to subside in a post-rift phase of thermal subsidence that has lasted until the present day (Allen et al., 1997). The latest study of the tectonic subsidence curves both from bore holes and seismic profiles in the Bozhong Depression (e.g. Hu et al., 2001a,b) and the Huanghua Depression (e.g. Zhou and Cong, 1999) indicate an intense subsidence in the Pliocene. Whether this suggests a new phase of tectonic activity will require further study. From a geographical point of view, the formation of the Jizhong Depression was coupled with the uplifting of the Taihang Mountains. Unfortunately, the timing of the Taihang uplift is still debated. Three opinions exist (Wang, 1998). The first believes that the Taihang Mountains formed in the Mesozoic era (e.g. 120 Ma).
Fig. 2. Strata of the profile (PA–PB, PC–PD) (see Fig. 1 for position) which passes through three depressions of Jizhong, Huanghua and Bozhong.
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The second suggests that its kilometer of uplift was completed during late Cretaceous and Cenozoic (Wang, 1998; Xu et al., 2001). The third proposed that the Taihang uplifted during the Quaternary (Wu et al., 1999).
3. Heat flow Our understanding of the heat flow pattern of the Bohai Bay Basin is developing as the increasing of heat flow measurements. Heat flow investigation in the Bohai Bay Basin began in the early 1980Õs, and the first 16 heat flow measurements in the basin ranged between 46 and 106 mW/m2 (Xie et al., 1980; Zhang et al., 1982). Five measurements in the Huanghua Depression averaged 83 mW/m2 and two in the Bozhong Depression averaged 85 mW/m2 (Table 1). The high heat flow in these depressions was explained as the result of a new phase of rifting in the Quaternary (Ye et al., 1985). However, the two heat flow values in the Bozhong Depression, recalculated by Chen et al. (1984) with the temperature data in the same boreholes but new thermal gradients in different depths and new thermal conductivities, turned out to be only 62 mW/m2 on average. These two values were later compiled into the heat flow database of China (2nd and 3rd edition) (Wang and Huang, 1990; Hu et al., 2001a). Wang and Wang (1986) published 45 heat flow data distributed in the Liaohe Depression, ranging between 44 and 94 mW/m2 with a mean of 66 mW/m2 . In 1988, based on 26 new heat flow measurements with coordinates and 139 results without coordinates, Chen (1988) pointed out that the Bohai Bay Basin had a moderate heat flow background. In his results, the mean heat flow is 56 ± 13 mW/m2 in the Jizhong Depression, 66 ± 10 mW/m2 in the Jiyang Depression, 546 mW/m2 in the Huanghua Depression, and 62 mW/m2 in the Bozhong Depression (Table 1). In recent years, some new investigations of heat flow have been made (e.g. Wang et al., 2000; Hu et al., 2001b;
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Wang et al., 2002). Unfortunately, because some of these data are from an unpublished report and some of them did not give coordinates (Wang et al., 2000; Wang et al., 2002), they were not included in the new compilation of heat flow by Hu et al. (2000, 2001a). Considering these results may aid in understanding the heat flow pattern of this basin, we list them in Table 1. Wang et al. (2002) had made 76 heat flow measurements with an average of 66 mW/m2 in the Bohai offshore (usually called Bohai Basin), based on large number of borehole temperatures and new thermal conductivity measurements. Because they did not give the sites of these boreholes, we cannot distinguish them into different depressions, and therefore did not list them in Table 1. Based on the heat flow investigation, it can be concluded that the Bohai Bay Basin is characterized by moderate heat flow (61 mW/m2 ), which is similiar to the average value of 61 ± 16 mW/m2 in the continental area of China (Hu et al., 2000). However, the heat flow in the Bohai Bay Basin is significantly lower than that (79 mW/m2 ) in the Yinggehai Basin (He et al., 2002), or that (77 mW/m2 ) in the South China Sea Basin (He et al., 2001), the Cenozoic rift basin in eastern China also likely experienced multiple phases of extension. Compared to the basin area, only a few heat flow measurements have been made in the surrounding mountain areas. Sixteen heat flow measurements show a rather low heat flow (48 ± 15 mW/m2 ) (Chen, 1988). There are only two heat flow measurements distributed in the Taihang Mountains, which are 47 and 26 mW/m2 respectively.
4. Crustal structure Fig. 3 illustrates the crustal thickness variations in the Bohai Bay Basin and its surrounding areas. The depth to Moho is roughly 10 km shallower in the center of basin than beneath the surrounding mountaineous areas. The shallowest moho at about 30 km, is beneath the
Table 1 Heat flow statistics of each depression in the Bohai Bay Basin Jizhong D.
Jiyang D.
Xie et al. (1980); Zhang et al. (1982)
Huanghua D.
Bozhong D.
78–106(5) 83
72–99(2) 85
Wang and Wang (1986) Chen et al. (1984); Chen (1988) Wang et al. (2000)
Liaohe D.
44–94(45) 66 ± 10 (68) 56 ± 13
(24) 66 ± 10
(7) 54 ± 6
Hu et al. (2001a,b) Table entries are heat flow range with data number and heat flow average in mW m2 .
57–67(2) 62 46–70(12) 56 ± 7 53–69(5) 61 ± 7
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Fig. 3. Map of depth to Moho (after Feng et al., 1989). The dash line shows the coastline, and the darkest line illustrates the profile (PA–PB, PC–PD) for modeling.
Bozhong and Liaohe Depressions. It rises gradually toward the surrounding areas, with greatest depth to the Moho, 42 km, beneath the Yan, Taihang and Liaodong Mountains.
5. Numerical modeling 5.1. A coupled basin–mountain model A kinematic approach based on a finite element algorithm in the Lagrangian system is presented, which incorporates differential lithosphere stretching and shortening. The algorithm assumes pure-shear deformation and local isostasy for the lithosphere. The mode of deformation is controlled by different stretching factors (Fig. 4). In the stretching area (I), the stretching factors (b) in different units are greater than 1, for the shortening area (II), they are less than 1, and in the undeforming area (III), they equal 1. The algorithm is based on the resolution of the 2-D heat conduction equation:
Fig. 4. Coupled basin–mountain model. I: stretching area with b > 1; II: shortening area with b < 1; III: undeforming area with b ¼ 1.
qc
oT ¼ kr2 T þ A ot
ð1Þ
where q is density, c is specific heat, T is temperature, k is thermal conductivity, and A is radiogenic heat production. The thermal parameters used in our model are listed in Table 2. For the shortening area, the lower boundary, keeping temperature fixed at 1330 °C, moves down with time according to volume conservation in the shortening process. For the stretching area, the lower boundary keeps a temperature of 1330 °C at certain depth (e.g. 125 km), as defined by McKenzie (1978), by using preserved grids (He et al., 2001). The upper boundary keeps the fixed temperature of 0 °C, and sedimentation or erosion are neglected. Zero horizontal heat flow is assumed at the lateral boundaries. For the first phase, the initial temperature is assumed stable. The initial lithosphere is stretched or shortened from time t0 to t1 (e.g. t0 ¼ 57 Ma, t1 ¼ 45 Ma in this case), and then undergoes thermal relax until time t2 (e.g. t2 ¼ 43 Ma). The transient thermal field of the deforming lithosphere is calculated to predict the thermal history, and the upper boundary subsides (to form basin) or uplifts (to form mountain) with time according to isostatic compensation and the thermal evolution of lithosphere. For the second phase, the lithosphere was stretched or shortened again at time t2 with the structure and temperature at that time. The transient thermal field was calculated once more. In this process, our model assumed that the stretching (or shortening) rate declined linearly during the stretching (or shortening) period (He et al., 2001). The stretching factors of each phase in different units of
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Table 2 Thermal parameters used in model Parameters
Upper and medium crust (23 km)
Lower crust (12 km)
Upper mantle
Heat production (lW m3 ) (Chi and Yan, 1998) Thermal conductivity (W m1 K1 ) (Govers and Wortel, 1993) Density (kg/m3 ) (Ma et al., 1991) Specific heat (J kg1 K1 ) (Liu and Furlong, 1993)
1.1 2.56 2700 1000
0.3 2.6 2900 1000
0 3.2 3300 1000
the model profile are obtained by matching the calculated subsidence (or uplift) to the observed values using a trial-and-error approach. Our model is therefore based on rifting history of a basin, and the geophysical data, such as heat flow and Moho depth, provide additional constraints. 5.2. Modeling results The cross-section extends from the Taihang Mountains, through the Jizhong Depression, Canxian Uplift, and Huanghua Depression, to the Bozhong Depression (Fig. 1). According to the tectonic history of this area (Allen et al., 1997), two stages of rifting were considered in the modeling, which are 57–45 Ma and 43–23 Ma, respectively. Two stages of thermal relaxation are from 45 to 43 Ma and from 23 to 0 Ma, respectively. 5.2.1. Influence of Taihang Mountains uplift To understand the influence of the Taihang Mountains uplift of thermal history of basin, we modeled the thermal history of the Jizhong Depression and the Taihang Mountains (PA–PB, Fig. 1) in the following four cases based on different tectonic histories suggested by different authors. Case 1: Only two stages of rifting were simulated in the modeling without considering rising of the Taihang Mountains. In this case, the basin–mountain model equals the basin–extension model (He et al., 2001). Case 2: The Taihang Mountains is assumed to rise in the Mesozoic (120 Ma) with duration of 20 m.y.s. Case 3: The Taihang Mountains was uplifting during the period of the first rifting of Jizhong Depression (57–45 Ma) (Wang, 1998; Xu et al., 2001). Case 4: The Taihang Mountains rose in Quaternary (2.4–0 Ma) (Wu et al., 1999). Unlike the thermal history of the basin illustrated by McKenzie (1978), the heat flow of the mountain region decreased rapidly during uplift period and increased gradually in the post-uplift period. Modeling results indicate that the younger the mountain, the lower the
heat flow in the mountain area (Fig. 5(a)). In the former three cases, the present-day heat flow predicted in the Taihang Mountains show about 52 mW/m2 . In case 4, however, the heat flow is as low as 42 mW/m2 , closer to the observed heat flow. From this we might infer that the Taihang Mountains were uplifted relatively recently. The evolution curves of heat flow show differences in different cases (Fig. 5b), indicating the influence of mountain uplift is strong in such an area. If the Taihang Mountains rose in the Mesozoic (case 2), the basin shoulder would be cooled in the early Cenozoic, but heated later compared with the results of case 1 (Fig. 5(b)). If the Taihang Mountains uplifted in the early Cenozoic (case 3), it would be cooled during all the Cenozoic evolution, but this cooling would weaken in the late Cenozoic, and the predicted present-day heat flow should be similar to that modeled in case 1. In case 4, the Taihang Mountains rose in the Quaternary, and its rising cooled the basin shoulder continuously (Fig. 4(a)). In the center of the Jizhong depression, the differences in the heat flow evolution curves are almost not detectable (Fig. 5(c)), indicating that the influence of mountain uplift extends to a finite range. In such a kinematic model with local Airy isostasy, the thermal history of inner basin and its present-day heat flow background is mainly controlled by the multi-phase extension it experienced. 5.2.2. Influence of a new phase of extension At present, researchers dispute whether there is new tectonic activity from the Oligocene to the present in the Huanghua and Bozhong Depressions, which are also called Bohai Basin by some authors (e.g. Hu et al., 2001b). Obverse proofs come from the tectonic subsidence analysis from both boreholes and seismic profiles, which are usually used to constrain the modeled evolution of the basin through time (Negredo et al., 1999). Results of tectonic subsidence analysis from both boreholes and seismic profiles indicate that there is a new rapid subsidence stage from 12 Ma to now (Hu et al., 2001b). Intense subsidence in the Pliocene can be seen in the offshore basins of China, such as the South China Sea and the Yinggehai basin, where much higher present-day heat flow values (79 mW/m2 ) are observed. The relatively low heat flow (63 mW/m2 ) in the
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Fig. 5. Modeling results in four cases. (a) Present-day heat flow predicted by model in four cases. (b) Heat flow evolution curves predicted by model in four cases in the basin shoulder; (c) Heat flow evolution curves predicted by model in four cases in the Jizhong Depression.
Huanghua and Bozhong Depressions do not seem to support the opinion of new tectonic activity. Two models were used to simulate the thermal histories of the basin. Model-A assumed no new rifting, and Model-B assumed a new rifting in the Pliocene (12– 2 Ma). The evolution curves of tectonic subsidence predicted by the two models are not-surprisingly different (Fig. 6), and the second is closer to the observed values. The present-day heat flow trend predicted by model-A (50–55 mW/m2 ) is lower than that by model-B (50–58 mW/m2 ) (Fig. 7). The later is closer to the measured values (Table 1). The predicted depths to Moho (30–32 km) by these two models are similar (Fig. 8) to observed geophysical data (Fig. 2). Based upon the modeling results and constraints from geophysical data, a new phase of extension possi-
bly occurred in the Pliocene. This also indicates that even if a new phase of rifting did exist in the Pliocene, it could not have led to high heat flow in this area as suggested by Ye et al. (1985). Therefore, the moderate heat flow in this area does not conflict with the idea that a new phase of rifting may exist in the Pliocene. 5.2.3. Thermal history The heat flow evolution predicted by model-B is shown in Fig. 9. The highest paleo-heat flow occurred in different time at different depressions. The highest paleo-heat flow in the Jizhong Depression (58 mW/m2 ) resulted from the first-phase of extension. The later extension was too weak to produce a higher heat flow. Generally, the heat flow shows a decreasing trend from Eocene to present in the Jizhong Depression
Fig. 6. Evolution curves of tectonic subsidence of four points (A–D) in these three depressions. The left panel shows the results of model-A, and the right panel shows the results of model-B. For comparison, the observed tectonic subsidence from backstripping is shown in the figures as different symbols for different points.
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Fig. 7. Predicted heat flow by two models. The dash line shows the heat flow predicted by model-A, in which no new rifting assumed in the Pliocene. The solid line shows the results of model-B, in which a new rifting was assumed in the Pliocene.
Fig. 8. Predicted Moho depth by two models. The dash line illustrates the result of model-A and the solid line of model-B. The differences of results between these two models are slight.
Fig. 9. Evolution curves of basement heat flow for the Jizhong, Huanghua and Bozhong Depressions predicted by model-B. Unlike Jizhong Depression, which experienced three phases of extension starting from 57 Ma, the Huanghua and Bozhong Depressions experienced two phases of extension starting from 43 Ma. The episodes of extension are clearly reflected in the evolution of basement heat flow in these depressions.
(Fig. 9). In the Huanghua and Bozhong depressions, the highest paleo-heat flow occurred in the Eocene. The second heat flow peak resulting from the latest rifting is smaller than the first, showing a cooling trend of heat flow.
The heat flow in these depressions varied between 51 and 63 mW/m2 through the Cenozoic evolution. It indicates a rather smooth variation of basin thermal state, and a cooling trend from the Oligocene to the presentday. The present-day heat flow is about 60 mW/m2 ,
Table 3 Comparison of stretching factors between Bohai Bay Basin and Yinggehai Basin
Bohai Bay Basin (this study)
Yinggehai Basin (He et al., 2002)
Point Point Point Point
A B C D
1st Phase
2nd Phase
3rd Phase
57–45 Ma
43–23 Ma
12–2 Ma
1.37 1.02 1 1
1.05 1.25 1.41 1.53
1.02 1.08 1.11 1.12
50–45 Ma
28–22 Ma
5.2–1.9 Ma
1.4
1.7
2.4
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which is just normal, not as high as the Yinggehai Basin (79 mW/m2 ), a similar Cenozoic basin in the eastern China. The great difference of present-day heat flow between these two basins can be explained through comparing of their extension characteristic. Although both of these two basins experienced three phases of extension during the Cenozoic evolution, the stretching strength, time, and duration are significantly different (Table 3). The largest stretching factor in the Jizhong Depression (point A) is 1.37, which is in the first phase of extension (57–45 Ma), and it is 1.53 in the Bozhong Depression (point D) happened during 43–23 Ma. In contrast, the strongest extension with stretching factor of 2.4 in the Yinggehai Basin occurred in the third phase (5.2–1.9 Ma), which is much stronger and newer than that in the Bohai Bay Basin. Therefore, the Yinggehai Basin, has a rather high present-day heat flow, and different thermal history, which shows a warming trend during the Cenozoic evolution.
6. Conclusions Based on modeling results, following conclusions can be made: (1) The heat flow in these three depressions varied between 51 and 63 mW/m2 through the Cenozoic evolution, indicating a rather smooth variation of basin thermal state, and a cooling trend from Oligocene to now. (2) The heat flow is higher in the western area of the basin in the early Cenozoic, but higher in the eastern area of the basin in middle and late Cenozoic. (3) The coupling of basin and mountain might influence the thermal history of the basin shoulder. (4) The present-day moderate heat flow does not conflict with the opinion that a new phase of rifting might have existed in the Huanghua and Bozhong Depressions in the Pliocene.
Acknowledgements This work was supported by National Key Basic R&D Program 973 (G1999043302), NSFC (Grant no. 40274021) and Chinese Academy of Sciences (Grant no. KZCX1-07). We thank two anonymous reviewers for constructive reviews.
References Allen, M.B., Macdonald, D.I.M., Zhao, X., Vincent, S.J., BrouetMenzies, C., 1997. Early Cenozoic two-phase extension and the late
Cenozoic thermal subsidence and inversion of the Bohai Basin, northern China. Mar. Petrol. Geol. 14, 951–972. Chen, M. (Ed.), 1988. Geothermics of North China. Science Press, Beijing, p. 218 (in Chinese with English abstract). Chen, M., Huang, G., Wang, J.A., Deng, X., Wang, J., 1984. A preliminary research on the geothermal characteristics in the Bohai Sea. Sci. Geol. Sin. 4, 392–401 (in Chinese). Chi, Q., Yan, M., 1998. Radioactive elements of rocks in North China platform and the thermal structure and temperature distribution of the modern continental lithosphere. Acta Geophys. Sin. 41, 38–48 (in Chinese with English abstract). Feng, R., Huang, G., Zheng, S., Wang, J., Yan, H., Zhang, R., 1989. Crustal structure and earthquake activities in North China. Acta Geol. Sin. 63, 111–124 (in Chinese with English abstract). Fernandez, M., Ranalli, G., 1998. The role of rheology in extensional basin formation modelling. Tectonophysics 282, 129– 145. Govers, R., Wortel, M.J.R., 1993. Initiation of asymmetric extension in continental lithosphere. Tectonophyscics 223, 75–96. He, L., Wang, K., Xiong, L., Wang, J., 2001. Heat flow and the thermal history of the South China Sea. Phys. Earth Planet. Int. 126, 211–220. He, L., Xiong, L., Wang, J., 2002. Heat flow and thermal modeling of the Yinggehai Basin, South China Sea. Tectonophysics 351, 245–253. Hu, S., He, L., Wang, J., 2000. Heat flow in the continental area of China: a new set. Earth Planet. Sci. Lett. 179, 407–419. Hu, S., He, L., Wang, J., 2001a. Compilation of heat flow data in the China continental area (third ed.). Chin. J. Geophy. 44 (5), 611– 626. Hu, S., OÕSullivan, B.P.B., Raza, A., Kohn, B.P., 2001b. Thermal history and tectonic subsidence of the Bohai Basin, Northern China: a Cenozoic rifted and local pill-apart basin. Phys. Earth Planet. Int. 126, 221–235. Liu, M., Furlong, K.P., 1993. Crustal shortening and Eocene extension in the southeastern Canadian Cordillera: some thermal and rheological considerations. Tectonics 12, 776–786. Liu, H., Liang, H., Li, X., Yin, J., Zhu, D., Liu, L., 2000. The coupling mechanism of mesozoic–cenozoic rift basins and extensional mountain system in eastern China. Earth Sci. Front. 7, 477–486 (in Chinese with English abstract). Ma, X., Liu, C., Liu, G., 1991. Xiangshui (Jiangsu province) to Mandal (Nei Monggol) Geoscience Transect. Acta Geophys. Sin. 5, 199–215 (in Chinese with English abstract). McKenzie, D.P., 1978. Some remarks on the development of sedimentary basins. Earth Planet. Sci. Lett. 40, 25–32. Negredo, A., Fernandez, M., Torne, M., Doglioni, C., 1999. Numerical modeling of simultaneouse extension and compression: The Valencia through (western Mediterranean). Tectonics 18, 361– 374. Ruppel, C., 1995. Extensional processes in continental lithosphere. J. Geophys. Res. 100, 24187–24215. Wang, Y., 1998. Evolution Processes and Dynamics of the Orogenic Belts and Basins in North China Since the Mesozoic. Geological Press, Beijing, p. 92 (in Chinese with English abstract). Wang, J., Huang, S., 1990. Compilation of heat flow data in the China continental area (second ed.). Seismol. Geol. 12, 351–366 (in both Chinese and English). Wang, J., Wang, J.A., 1986. Heat flow measurement in the Liaohe Basin, North China. Chin. Sci. Bull. 31, 686–689. Wang, Y., Hu, S., Wang, J., Wu, T., Wang, Y., Feng, D., 2000. Heat flow in the eastern subdepression of the Liaohe Basin. J. Graduate School, Acad. Sin. 17, 62–68. Wang, L., Liu, S., Xiao, W., Li, C., Li, H., Guo, X., Liu, B., Luo, Y., Cai, D., 2002. Heat flow pattern of the Bohai Basin. Chin. Sci. Bull. 47, 151–155 (in both Chinese and English).
L. He, J. Wang / Physics and Chemistry of the Earth 28 (2003) 421–429 Wu, C., Zhang, X., Ma, Y., 1999. The Taihang and Yan Mountains rose mainly in Quaternary. North China Earthquake Sci. 17 (3), 1– 7 (in Chinese with English abstract). Xie, Z., Wu, Q., Zhang, R., Xie, Y., 1980. A preliminary analysis of some heat flow values in the Bohai Sea and adjacent region. Seismol. Geol. 2, 57–63. Xu, J., Gao, Z., Sun, J., Song, C., 2001. A preliminary study of the coupling relationship between basin and mountain in the extensional environments––a case study of the Bohai Bay Basin and Taihang Mountain. Acta Geol. Sin. 75, 165–174.
429
Ye, H., Shedlock, K.M., Hellinger, S.J., Sclater, J.G., 1985. The North China Basin: an example of a Cenozoic rifted intraplate basin. Tectonics 4, 153–169. Zhang, R., Xie, Z., Wu, J., Xie, Y., 1982. The distribution of heat flow values in Tangshan and its surroundings. Seismol. Geol. 4, 57– 67. Zhou, H., Cong, L., 1999. Polyphase extension and its impact on hydrocabon accumulation in fault basin-case analysis of polyphase rifting in Nanpu Depression. Earth Sci.-J. Chin. Univ. Geos. 24, 625–629 (in Chinese with English abstract).
Cenozoic thermal history of the Bohai Bay Basin: constraints from heat flow and coupled basin–mountain modeling Lijuan He *, Jiyang Wang Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China Received 12 March 2002; received in revised form 18 September 2002; accepted 4 April 2003
Abstract Heat flow measurements show a moderate thermal background (61 mW/m2 ) in the Bohai Bay Basin, which although experienced multi-phase rifting in the Cenozoic era. In contrast, its surrounding mountain areas are characterized by low heat flow. Constraint by heat flow measurements, the thermal evolution of the Bohai Bay Basin during the Cenozoic era was performed by a numerical basin–mountain model. The model incorporates differential lithosphere stretching and shortening by finite-element method in the Lagrangian frame. The predicted heat flow in the center of the three depressions of the Bohai Bay Basin is calculated to have varied between 51 and 63 mW/m2 through the Cenozoic evolution, indicating a rather smooth variation of basin thermal state, and a cooling trend from the Oligocene to present-day. Model results also suggest that the Taihang Mountains probably uplifted in the Quaternary, which resulted in low heat flow in the mountain area. Both heat flow constraints and modeling imply that a new phase of rifting in the Pliocene existed in the Huanghua and Bozhong Depressions, which was suggested by tectonic subsidence analysis. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Bohai bay basin; Heat flow; Coupled basin–mountain model; Thermal history
1. Introduction Heat flow, which reflects the present-day thermal state of the lithosphere, is the result of multi-phase evolution of the lithosphere. Therefore, heat flow is a key constraint on lithosphere thermal modeling based on extensional models. At present, there are many extensional models describing the evolution of lithosphere, including kinematic models and dynamic models, which were summarized in the literatures (e.g. Ruppel, 1995; Fernandez and Ranalli, 1998). These models assumed that the areas outside are non-deformed lithosphere. However, crustal shortening and extension, two of the most important tectonic processes reshaping continental lithosphere, link closely in many regions. In eastern China, many extensional basins are coupled with mountains (Liu et al., 2000). There are three typical couples: Songliao rift basin and Daxingan Mountains in the northern part, the Bohai Bay rift basin and the Taihang Mountains in the middle, and the Jianghan rift * Corresponding author. Tel.: +86-10-62007813; fax: +86-1062010846. E-mail address: [email protected] (L. He).
basin and the Xuefeng Mountains in the southern area. We chose the Bohai Bay Basin for modeling in this paper not only because it is the largest Cenozoic rift basin in eastern China, but also due to its special thermal regime compared to typical modern rift basins with high heat flow. Study on its thermal evolution and coupling with the Taihang Mountains may shed some light on understanding the dynamics of basin–mountain systems in eastern China. In this paper, a coupled basin–mountain model is used, which incorporates differential lithosphere stretching and shortening similarly as in the model of Negredo et al. (1999), but based on a finite element algorithm by the Lagrangian method. We modeled the Cenozoic thermal history of the Bohai Bay Basin based on a crosssection passing through the Taihang Mountains, Jizhong Depression, Huanghua Depression, and Bozhong Depression (Figs. 1 and 2). Through modeling, we want to understand the possible time of the uplift of the Taihang Mountains and its influence on the thermal history of the Jizhong Depression, and the possibility of a new phase of extension existing in the Huanghua and Bozhong depressions, and its influence on the present-day heat flow regime.
1474-7065/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1474-7065(03)00062-7
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Fig. 1. Map of Bohai Bay Basin (modified from Ye et al., 1985). Note the basin, comprising of Jizhong, Huanghua, Bozhong, Linqing, Jiyang and Liaohe depressions, is surrounded by five mountain ranges: the Yan, Luxi, Taihang, Liaodong and Jiaodong Mountains. The profile (PA–PB, PC–PD) used for modeling (Fig. 2) is shown as a full line.
2. Tectonic setting The Bohai Bay Basin (Fig. 1), a Cenozoic basin, is surrounded by five mountain ranges: the Yan, Luxi, Taihang, Liaodong and Jiaodong Mountains. It is comprised of six major depressions, which are the Jizhong, Huanghua, Bozhong, Linqing, Jiyang, and Liaohe depressions. Tertiary strata rest unconformably on
a variety of older pre-rift strata and are covered conformably or unconformably by Quaternary sediments. The succession is typically 4000–7000 m thick. Lithologies are dominated by sandstone and mudstone. The Cenozoic succession has been divided into six formations: the Kongdian, Shahejie, Dongying, Guangtao, Minghuazhen, and Pingyuan formation, which resulted by two phases of rifting (Allen et al., 1997). The earlier, Paleocene-early-Eocene phase resulted in the deposition of the Kongdian Formation and the fourth (lowest) member of the Shahejie Formation in the west and south of the present basin. The second phase started at about 43–45 Ma (middle Eocene), beginning with the deposition of the third member of the Shahejie Formation. In part, these sediments were deposited in the same half grabens as the Kongdian Formation, but the Bozhong Depression in the central part of the basin originated at this time, and became the major depocentre. It was generally believed that regional extension ended at the end of the Oligocene, and the basin as a whole began to subside in a post-rift phase of thermal subsidence that has lasted until the present day (Allen et al., 1997). The latest study of the tectonic subsidence curves both from bore holes and seismic profiles in the Bozhong Depression (e.g. Hu et al., 2001a,b) and the Huanghua Depression (e.g. Zhou and Cong, 1999) indicate an intense subsidence in the Pliocene. Whether this suggests a new phase of tectonic activity will require further study. From a geographical point of view, the formation of the Jizhong Depression was coupled with the uplifting of the Taihang Mountains. Unfortunately, the timing of the Taihang uplift is still debated. Three opinions exist (Wang, 1998). The first believes that the Taihang Mountains formed in the Mesozoic era (e.g. 120 Ma).
Fig. 2. Strata of the profile (PA–PB, PC–PD) (see Fig. 1 for position) which passes through three depressions of Jizhong, Huanghua and Bozhong.
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The second suggests that its kilometer of uplift was completed during late Cretaceous and Cenozoic (Wang, 1998; Xu et al., 2001). The third proposed that the Taihang uplifted during the Quaternary (Wu et al., 1999).
3. Heat flow Our understanding of the heat flow pattern of the Bohai Bay Basin is developing as the increasing of heat flow measurements. Heat flow investigation in the Bohai Bay Basin began in the early 1980Õs, and the first 16 heat flow measurements in the basin ranged between 46 and 106 mW/m2 (Xie et al., 1980; Zhang et al., 1982). Five measurements in the Huanghua Depression averaged 83 mW/m2 and two in the Bozhong Depression averaged 85 mW/m2 (Table 1). The high heat flow in these depressions was explained as the result of a new phase of rifting in the Quaternary (Ye et al., 1985). However, the two heat flow values in the Bozhong Depression, recalculated by Chen et al. (1984) with the temperature data in the same boreholes but new thermal gradients in different depths and new thermal conductivities, turned out to be only 62 mW/m2 on average. These two values were later compiled into the heat flow database of China (2nd and 3rd edition) (Wang and Huang, 1990; Hu et al., 2001a). Wang and Wang (1986) published 45 heat flow data distributed in the Liaohe Depression, ranging between 44 and 94 mW/m2 with a mean of 66 mW/m2 . In 1988, based on 26 new heat flow measurements with coordinates and 139 results without coordinates, Chen (1988) pointed out that the Bohai Bay Basin had a moderate heat flow background. In his results, the mean heat flow is 56 ± 13 mW/m2 in the Jizhong Depression, 66 ± 10 mW/m2 in the Jiyang Depression, 546 mW/m2 in the Huanghua Depression, and 62 mW/m2 in the Bozhong Depression (Table 1). In recent years, some new investigations of heat flow have been made (e.g. Wang et al., 2000; Hu et al., 2001b;
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Wang et al., 2002). Unfortunately, because some of these data are from an unpublished report and some of them did not give coordinates (Wang et al., 2000; Wang et al., 2002), they were not included in the new compilation of heat flow by Hu et al. (2000, 2001a). Considering these results may aid in understanding the heat flow pattern of this basin, we list them in Table 1. Wang et al. (2002) had made 76 heat flow measurements with an average of 66 mW/m2 in the Bohai offshore (usually called Bohai Basin), based on large number of borehole temperatures and new thermal conductivity measurements. Because they did not give the sites of these boreholes, we cannot distinguish them into different depressions, and therefore did not list them in Table 1. Based on the heat flow investigation, it can be concluded that the Bohai Bay Basin is characterized by moderate heat flow (61 mW/m2 ), which is similiar to the average value of 61 ± 16 mW/m2 in the continental area of China (Hu et al., 2000). However, the heat flow in the Bohai Bay Basin is significantly lower than that (79 mW/m2 ) in the Yinggehai Basin (He et al., 2002), or that (77 mW/m2 ) in the South China Sea Basin (He et al., 2001), the Cenozoic rift basin in eastern China also likely experienced multiple phases of extension. Compared to the basin area, only a few heat flow measurements have been made in the surrounding mountain areas. Sixteen heat flow measurements show a rather low heat flow (48 ± 15 mW/m2 ) (Chen, 1988). There are only two heat flow measurements distributed in the Taihang Mountains, which are 47 and 26 mW/m2 respectively.
4. Crustal structure Fig. 3 illustrates the crustal thickness variations in the Bohai Bay Basin and its surrounding areas. The depth to Moho is roughly 10 km shallower in the center of basin than beneath the surrounding mountaineous areas. The shallowest moho at about 30 km, is beneath the
Table 1 Heat flow statistics of each depression in the Bohai Bay Basin Jizhong D.
Jiyang D.
Xie et al. (1980); Zhang et al. (1982)
Huanghua D.
Bozhong D.
78–106(5) 83
72–99(2) 85
Wang and Wang (1986) Chen et al. (1984); Chen (1988) Wang et al. (2000)
Liaohe D.
44–94(45) 66 ± 10 (68) 56 ± 13
(24) 66 ± 10
(7) 54 ± 6
Hu et al. (2001a,b) Table entries are heat flow range with data number and heat flow average in mW m2 .
57–67(2) 62 46–70(12) 56 ± 7 53–69(5) 61 ± 7
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Fig. 3. Map of depth to Moho (after Feng et al., 1989). The dash line shows the coastline, and the darkest line illustrates the profile (PA–PB, PC–PD) for modeling.
Bozhong and Liaohe Depressions. It rises gradually toward the surrounding areas, with greatest depth to the Moho, 42 km, beneath the Yan, Taihang and Liaodong Mountains.
5. Numerical modeling 5.1. A coupled basin–mountain model A kinematic approach based on a finite element algorithm in the Lagrangian system is presented, which incorporates differential lithosphere stretching and shortening. The algorithm assumes pure-shear deformation and local isostasy for the lithosphere. The mode of deformation is controlled by different stretching factors (Fig. 4). In the stretching area (I), the stretching factors (b) in different units are greater than 1, for the shortening area (II), they are less than 1, and in the undeforming area (III), they equal 1. The algorithm is based on the resolution of the 2-D heat conduction equation:
Fig. 4. Coupled basin–mountain model. I: stretching area with b > 1; II: shortening area with b < 1; III: undeforming area with b ¼ 1.
qc
oT ¼ kr2 T þ A ot
ð1Þ
where q is density, c is specific heat, T is temperature, k is thermal conductivity, and A is radiogenic heat production. The thermal parameters used in our model are listed in Table 2. For the shortening area, the lower boundary, keeping temperature fixed at 1330 °C, moves down with time according to volume conservation in the shortening process. For the stretching area, the lower boundary keeps a temperature of 1330 °C at certain depth (e.g. 125 km), as defined by McKenzie (1978), by using preserved grids (He et al., 2001). The upper boundary keeps the fixed temperature of 0 °C, and sedimentation or erosion are neglected. Zero horizontal heat flow is assumed at the lateral boundaries. For the first phase, the initial temperature is assumed stable. The initial lithosphere is stretched or shortened from time t0 to t1 (e.g. t0 ¼ 57 Ma, t1 ¼ 45 Ma in this case), and then undergoes thermal relax until time t2 (e.g. t2 ¼ 43 Ma). The transient thermal field of the deforming lithosphere is calculated to predict the thermal history, and the upper boundary subsides (to form basin) or uplifts (to form mountain) with time according to isostatic compensation and the thermal evolution of lithosphere. For the second phase, the lithosphere was stretched or shortened again at time t2 with the structure and temperature at that time. The transient thermal field was calculated once more. In this process, our model assumed that the stretching (or shortening) rate declined linearly during the stretching (or shortening) period (He et al., 2001). The stretching factors of each phase in different units of
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Table 2 Thermal parameters used in model Parameters
Upper and medium crust (23 km)
Lower crust (12 km)
Upper mantle
Heat production (lW m3 ) (Chi and Yan, 1998) Thermal conductivity (W m1 K1 ) (Govers and Wortel, 1993) Density (kg/m3 ) (Ma et al., 1991) Specific heat (J kg1 K1 ) (Liu and Furlong, 1993)
1.1 2.56 2700 1000
0.3 2.6 2900 1000
0 3.2 3300 1000
the model profile are obtained by matching the calculated subsidence (or uplift) to the observed values using a trial-and-error approach. Our model is therefore based on rifting history of a basin, and the geophysical data, such as heat flow and Moho depth, provide additional constraints. 5.2. Modeling results The cross-section extends from the Taihang Mountains, through the Jizhong Depression, Canxian Uplift, and Huanghua Depression, to the Bozhong Depression (Fig. 1). According to the tectonic history of this area (Allen et al., 1997), two stages of rifting were considered in the modeling, which are 57–45 Ma and 43–23 Ma, respectively. Two stages of thermal relaxation are from 45 to 43 Ma and from 23 to 0 Ma, respectively. 5.2.1. Influence of Taihang Mountains uplift To understand the influence of the Taihang Mountains uplift of thermal history of basin, we modeled the thermal history of the Jizhong Depression and the Taihang Mountains (PA–PB, Fig. 1) in the following four cases based on different tectonic histories suggested by different authors. Case 1: Only two stages of rifting were simulated in the modeling without considering rising of the Taihang Mountains. In this case, the basin–mountain model equals the basin–extension model (He et al., 2001). Case 2: The Taihang Mountains is assumed to rise in the Mesozoic (120 Ma) with duration of 20 m.y.s. Case 3: The Taihang Mountains was uplifting during the period of the first rifting of Jizhong Depression (57–45 Ma) (Wang, 1998; Xu et al., 2001). Case 4: The Taihang Mountains rose in Quaternary (2.4–0 Ma) (Wu et al., 1999). Unlike the thermal history of the basin illustrated by McKenzie (1978), the heat flow of the mountain region decreased rapidly during uplift period and increased gradually in the post-uplift period. Modeling results indicate that the younger the mountain, the lower the
heat flow in the mountain area (Fig. 5(a)). In the former three cases, the present-day heat flow predicted in the Taihang Mountains show about 52 mW/m2 . In case 4, however, the heat flow is as low as 42 mW/m2 , closer to the observed heat flow. From this we might infer that the Taihang Mountains were uplifted relatively recently. The evolution curves of heat flow show differences in different cases (Fig. 5b), indicating the influence of mountain uplift is strong in such an area. If the Taihang Mountains rose in the Mesozoic (case 2), the basin shoulder would be cooled in the early Cenozoic, but heated later compared with the results of case 1 (Fig. 5(b)). If the Taihang Mountains uplifted in the early Cenozoic (case 3), it would be cooled during all the Cenozoic evolution, but this cooling would weaken in the late Cenozoic, and the predicted present-day heat flow should be similar to that modeled in case 1. In case 4, the Taihang Mountains rose in the Quaternary, and its rising cooled the basin shoulder continuously (Fig. 4(a)). In the center of the Jizhong depression, the differences in the heat flow evolution curves are almost not detectable (Fig. 5(c)), indicating that the influence of mountain uplift extends to a finite range. In such a kinematic model with local Airy isostasy, the thermal history of inner basin and its present-day heat flow background is mainly controlled by the multi-phase extension it experienced. 5.2.2. Influence of a new phase of extension At present, researchers dispute whether there is new tectonic activity from the Oligocene to the present in the Huanghua and Bozhong Depressions, which are also called Bohai Basin by some authors (e.g. Hu et al., 2001b). Obverse proofs come from the tectonic subsidence analysis from both boreholes and seismic profiles, which are usually used to constrain the modeled evolution of the basin through time (Negredo et al., 1999). Results of tectonic subsidence analysis from both boreholes and seismic profiles indicate that there is a new rapid subsidence stage from 12 Ma to now (Hu et al., 2001b). Intense subsidence in the Pliocene can be seen in the offshore basins of China, such as the South China Sea and the Yinggehai basin, where much higher present-day heat flow values (79 mW/m2 ) are observed. The relatively low heat flow (63 mW/m2 ) in the
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Fig. 5. Modeling results in four cases. (a) Present-day heat flow predicted by model in four cases. (b) Heat flow evolution curves predicted by model in four cases in the basin shoulder; (c) Heat flow evolution curves predicted by model in four cases in the Jizhong Depression.
Huanghua and Bozhong Depressions do not seem to support the opinion of new tectonic activity. Two models were used to simulate the thermal histories of the basin. Model-A assumed no new rifting, and Model-B assumed a new rifting in the Pliocene (12– 2 Ma). The evolution curves of tectonic subsidence predicted by the two models are not-surprisingly different (Fig. 6), and the second is closer to the observed values. The present-day heat flow trend predicted by model-A (50–55 mW/m2 ) is lower than that by model-B (50–58 mW/m2 ) (Fig. 7). The later is closer to the measured values (Table 1). The predicted depths to Moho (30–32 km) by these two models are similar (Fig. 8) to observed geophysical data (Fig. 2). Based upon the modeling results and constraints from geophysical data, a new phase of extension possi-
bly occurred in the Pliocene. This also indicates that even if a new phase of rifting did exist in the Pliocene, it could not have led to high heat flow in this area as suggested by Ye et al. (1985). Therefore, the moderate heat flow in this area does not conflict with the idea that a new phase of rifting may exist in the Pliocene. 5.2.3. Thermal history The heat flow evolution predicted by model-B is shown in Fig. 9. The highest paleo-heat flow occurred in different time at different depressions. The highest paleo-heat flow in the Jizhong Depression (58 mW/m2 ) resulted from the first-phase of extension. The later extension was too weak to produce a higher heat flow. Generally, the heat flow shows a decreasing trend from Eocene to present in the Jizhong Depression
Fig. 6. Evolution curves of tectonic subsidence of four points (A–D) in these three depressions. The left panel shows the results of model-A, and the right panel shows the results of model-B. For comparison, the observed tectonic subsidence from backstripping is shown in the figures as different symbols for different points.
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Fig. 7. Predicted heat flow by two models. The dash line shows the heat flow predicted by model-A, in which no new rifting assumed in the Pliocene. The solid line shows the results of model-B, in which a new rifting was assumed in the Pliocene.
Fig. 8. Predicted Moho depth by two models. The dash line illustrates the result of model-A and the solid line of model-B. The differences of results between these two models are slight.
Fig. 9. Evolution curves of basement heat flow for the Jizhong, Huanghua and Bozhong Depressions predicted by model-B. Unlike Jizhong Depression, which experienced three phases of extension starting from 57 Ma, the Huanghua and Bozhong Depressions experienced two phases of extension starting from 43 Ma. The episodes of extension are clearly reflected in the evolution of basement heat flow in these depressions.
(Fig. 9). In the Huanghua and Bozhong depressions, the highest paleo-heat flow occurred in the Eocene. The second heat flow peak resulting from the latest rifting is smaller than the first, showing a cooling trend of heat flow.
The heat flow in these depressions varied between 51 and 63 mW/m2 through the Cenozoic evolution. It indicates a rather smooth variation of basin thermal state, and a cooling trend from the Oligocene to the presentday. The present-day heat flow is about 60 mW/m2 ,
Table 3 Comparison of stretching factors between Bohai Bay Basin and Yinggehai Basin
Bohai Bay Basin (this study)
Yinggehai Basin (He et al., 2002)
Point Point Point Point
A B C D
1st Phase
2nd Phase
3rd Phase
57–45 Ma
43–23 Ma
12–2 Ma
1.37 1.02 1 1
1.05 1.25 1.41 1.53
1.02 1.08 1.11 1.12
50–45 Ma
28–22 Ma
5.2–1.9 Ma
1.4
1.7
2.4
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which is just normal, not as high as the Yinggehai Basin (79 mW/m2 ), a similar Cenozoic basin in the eastern China. The great difference of present-day heat flow between these two basins can be explained through comparing of their extension characteristic. Although both of these two basins experienced three phases of extension during the Cenozoic evolution, the stretching strength, time, and duration are significantly different (Table 3). The largest stretching factor in the Jizhong Depression (point A) is 1.37, which is in the first phase of extension (57–45 Ma), and it is 1.53 in the Bozhong Depression (point D) happened during 43–23 Ma. In contrast, the strongest extension with stretching factor of 2.4 in the Yinggehai Basin occurred in the third phase (5.2–1.9 Ma), which is much stronger and newer than that in the Bohai Bay Basin. Therefore, the Yinggehai Basin, has a rather high present-day heat flow, and different thermal history, which shows a warming trend during the Cenozoic evolution.
6. Conclusions Based on modeling results, following conclusions can be made: (1) The heat flow in these three depressions varied between 51 and 63 mW/m2 through the Cenozoic evolution, indicating a rather smooth variation of basin thermal state, and a cooling trend from Oligocene to now. (2) The heat flow is higher in the western area of the basin in the early Cenozoic, but higher in the eastern area of the basin in middle and late Cenozoic. (3) The coupling of basin and mountain might influence the thermal history of the basin shoulder. (4) The present-day moderate heat flow does not conflict with the opinion that a new phase of rifting might have existed in the Huanghua and Bozhong Depressions in the Pliocene.
Acknowledgements This work was supported by National Key Basic R&D Program 973 (G1999043302), NSFC (Grant no. 40274021) and Chinese Academy of Sciences (Grant no. KZCX1-07). We thank two anonymous reviewers for constructive reviews.
References Allen, M.B., Macdonald, D.I.M., Zhao, X., Vincent, S.J., BrouetMenzies, C., 1997. Early Cenozoic two-phase extension and the late
Cenozoic thermal subsidence and inversion of the Bohai Basin, northern China. Mar. Petrol. Geol. 14, 951–972. Chen, M. (Ed.), 1988. Geothermics of North China. Science Press, Beijing, p. 218 (in Chinese with English abstract). Chen, M., Huang, G., Wang, J.A., Deng, X., Wang, J., 1984. A preliminary research on the geothermal characteristics in the Bohai Sea. Sci. Geol. Sin. 4, 392–401 (in Chinese). Chi, Q., Yan, M., 1998. Radioactive elements of rocks in North China platform and the thermal structure and temperature distribution of the modern continental lithosphere. Acta Geophys. Sin. 41, 38–48 (in Chinese with English abstract). Feng, R., Huang, G., Zheng, S., Wang, J., Yan, H., Zhang, R., 1989. Crustal structure and earthquake activities in North China. Acta Geol. Sin. 63, 111–124 (in Chinese with English abstract). Fernandez, M., Ranalli, G., 1998. The role of rheology in extensional basin formation modelling. Tectonophysics 282, 129– 145. Govers, R., Wortel, M.J.R., 1993. Initiation of asymmetric extension in continental lithosphere. Tectonophyscics 223, 75–96. He, L., Wang, K., Xiong, L., Wang, J., 2001. Heat flow and the thermal history of the South China Sea. Phys. Earth Planet. Int. 126, 211–220. He, L., Xiong, L., Wang, J., 2002. Heat flow and thermal modeling of the Yinggehai Basin, South China Sea. Tectonophysics 351, 245–253. Hu, S., He, L., Wang, J., 2000. Heat flow in the continental area of China: a new set. Earth Planet. Sci. Lett. 179, 407–419. Hu, S., He, L., Wang, J., 2001a. Compilation of heat flow data in the China continental area (third ed.). Chin. J. Geophy. 44 (5), 611– 626. Hu, S., OÕSullivan, B.P.B., Raza, A., Kohn, B.P., 2001b. Thermal history and tectonic subsidence of the Bohai Basin, Northern China: a Cenozoic rifted and local pill-apart basin. Phys. Earth Planet. Int. 126, 221–235. Liu, M., Furlong, K.P., 1993. Crustal shortening and Eocene extension in the southeastern Canadian Cordillera: some thermal and rheological considerations. Tectonics 12, 776–786. Liu, H., Liang, H., Li, X., Yin, J., Zhu, D., Liu, L., 2000. The coupling mechanism of mesozoic–cenozoic rift basins and extensional mountain system in eastern China. Earth Sci. Front. 7, 477–486 (in Chinese with English abstract). Ma, X., Liu, C., Liu, G., 1991. Xiangshui (Jiangsu province) to Mandal (Nei Monggol) Geoscience Transect. Acta Geophys. Sin. 5, 199–215 (in Chinese with English abstract). McKenzie, D.P., 1978. Some remarks on the development of sedimentary basins. Earth Planet. Sci. Lett. 40, 25–32. Negredo, A., Fernandez, M., Torne, M., Doglioni, C., 1999. Numerical modeling of simultaneouse extension and compression: The Valencia through (western Mediterranean). Tectonics 18, 361– 374. Ruppel, C., 1995. Extensional processes in continental lithosphere. J. Geophys. Res. 100, 24187–24215. Wang, Y., 1998. Evolution Processes and Dynamics of the Orogenic Belts and Basins in North China Since the Mesozoic. Geological Press, Beijing, p. 92 (in Chinese with English abstract). Wang, J., Huang, S., 1990. Compilation of heat flow data in the China continental area (second ed.). Seismol. Geol. 12, 351–366 (in both Chinese and English). Wang, J., Wang, J.A., 1986. Heat flow measurement in the Liaohe Basin, North China. Chin. Sci. Bull. 31, 686–689. Wang, Y., Hu, S., Wang, J., Wu, T., Wang, Y., Feng, D., 2000. Heat flow in the eastern subdepression of the Liaohe Basin. J. Graduate School, Acad. Sin. 17, 62–68. Wang, L., Liu, S., Xiao, W., Li, C., Li, H., Guo, X., Liu, B., Luo, Y., Cai, D., 2002. Heat flow pattern of the Bohai Basin. Chin. Sci. Bull. 47, 151–155 (in both Chinese and English).
L. He, J. Wang / Physics and Chemistry of the Earth 28 (2003) 421–429 Wu, C., Zhang, X., Ma, Y., 1999. The Taihang and Yan Mountains rose mainly in Quaternary. North China Earthquake Sci. 17 (3), 1– 7 (in Chinese with English abstract). Xie, Z., Wu, Q., Zhang, R., Xie, Y., 1980. A preliminary analysis of some heat flow values in the Bohai Sea and adjacent region. Seismol. Geol. 2, 57–63. Xu, J., Gao, Z., Sun, J., Song, C., 2001. A preliminary study of the coupling relationship between basin and mountain in the extensional environments––a case study of the Bohai Bay Basin and Taihang Mountain. Acta Geol. Sin. 75, 165–174.
429
Ye, H., Shedlock, K.M., Hellinger, S.J., Sclater, J.G., 1985. The North China Basin: an example of a Cenozoic rifted intraplate basin. Tectonics 4, 153–169. Zhang, R., Xie, Z., Wu, J., Xie, Y., 1982. The distribution of heat flow values in Tangshan and its surroundings. Seismol. Geol. 4, 57– 67. Zhou, H., Cong, L., 1999. Polyphase extension and its impact on hydrocabon accumulation in fault basin-case analysis of polyphase rifting in Nanpu Depression. Earth Sci.-J. Chin. Univ. Geos. 24, 625–629 (in Chinese with English abstract).