Distribution of the crustal magnetic anomaly and geological structure in Xinjiang, China

Journal of Asian Earth Sciences 77 (2013) 12–20

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Distribution of the crustal magnetic anomaly and geological structure in Xinjiang, China Guoming Gao ⇑, Guofa Kang, Chunhua Bai, Guangquan Li Department of Geophysics, Yunnan University, 2 North Green Lake Rd., Kunming, Yunnan 650091, PR China

a r t i c l e

i n f o

Article history: Received 22 March 2013 Received in revised form 30 July 2013 Accepted 9 August 2013 Available online 22 August 2013 Keywords: Geomagnetic field model Crustal magnetic anomaly Curie surface Mafic dykes Xinjiang

a b s t r a c t Based on the high-order crustal magnetic field model NGDC-720-V3, we investigate the distribution of crustal magnetic anomaly, the decay characteristics of the anomaly, and the relationship between the magnetic anomaly and geological structure in Xinjiang, China. Topography of the magnetic layer basement is studied through Curie isothermal surface using the power spectrum method. It is found that south Tarim Basin, Junggar Basin, and Turpan–Hami Basin have strong positive magnetic anomaly, whereas west Kunlun Mountain, Altun Mountain, Tianshan Mountain, and Altai Mountain have weak or negative anomaly. The magnetic anomaly well reflects the regional tectonic structure, i.e., three alternating mountains intervened by two basins. The magnetic anomaly on the ground surface in Tarim Basin is well corresponding to the mafic dykes. The decay of the magnetic anomaly with altitude indicates that Xinjiang is a large massif composed of several magnetic blocks with different sizes in different directions. The Curie surface presents a feature of being shallow under mountains whereas being deep under basins, roughly having an anti-mirror correspondence with the Moho depth. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Crustal magnetic field arises from magnetic rocks in the crust and upper mantle. Because rocks change in magnetism with tectonic evolution, they contain information on crustal magnetic material distribution and record of tectonic evolution (Xu, 2009; Hemant and Maus, 2005). Xinjiang is located on the north side of Tibetan Plateau, lying among several paleocontinents such as Indian Platform, Arabian–Nubian Shield, Russian Platform, and Siberian Platform. Under their mutual actions, tectonics in this region is relatively active and mineral resources are rich. Study of the spatial distribution of crustal magnetic field in Xinjiang and analysis of the relationship between the magnetic anomaly and the geological structure can provide insight on the origin of the crustal magnetic anomaly. Since 1950s, magnetic anomaly and geological structure in Xinjiang have been investigated by aeromagnetic survey and satellite magnetic survey. Deng et al. (1992) utilized the data from aeromagnetic survey to make a preliminary geological interpretation of magnetic anomaly in Xinjiang. Zhang and Zhang (2007) investigated the geological structure and tectonic evolution of Altun Mountain fault zone based on aeromagnetic survey data. Achache et al. (1987), Arkani et al. (1988), Zhang (2002), and Kang et al. (2010) studied satellite magnetic anomaly over Xinjiang and concluded that positive ⇑ Corresponding author. Tel.: +86 15198892593. E-mail address: [email protected] (G. Gao). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.08.014

magnetic anomaly covers most of Xinjiang, with the focus located in South Tarim Basin. However, there are limitations in the study of crustal magnetic anomaly using aeromagnetic survey or satellite data alone (Zhou et al., 2002; Xiong, 2009). Aeromagnetic anomaly cannot well reflect the deep magnetic source; while satellite magnetic anomaly just reflects the distribution feature of macro-scale basement anomaly. In June of 2009, the National Geophysical Data Center (NGDC) of the United States established a geomagnetic field model NGDC-720-V3 (see http://geomag.org/models/index.html) by combining data from satellite, ground, oceanic, and aeromagnetic surveys. In that model, the order of spherical harmonic functions is up to 740 and the resolvable spatial wavelength is as fine as 55 km. Thus the model enables us to study the crustal magnetic anomaly in Xinjiang accurately (Kang et al., 2011). Curie isothermal surface is a thermal boundary for crustal rock minerals to transform from ferromagnetism to paramagnetism. It is the lower boundary of magnetic layer. Researches on Curie isothermal surface can help understand deep structure of crust and have important application in assessment/exploration of hydrocarbon resource. Curie isothermal surface can be inverted for from crustal magnetic anomaly. For instance, the distribution of the Curie isothermal surface in Tarim Basin was inverted for Lu and Yang (1996), Li and Xiao (1999). In this paper, we shall according to the NGDC-720-V3 model, calculate the spatial distribution of crustal magnetic anomaly in Xinjiang, study how the magnetic anomaly decays with altitude, invert for the Curie isothermal surface using the power spectrum

G. Gao et al. / Journal of Asian Earth Sciences 77 (2013) 12–20

analysis, and finally discuss the relationship between the magnetic anomaly and geological structure in this region. 2. Geotectonic background The current crustal structure in Xinjiang was finalized in Cenozoic Era. Due to the collision between Indian-Australian Plate and Euroasian Plate, the major tectonics of Xinjiang is featured with basins and mountains. There are three great mountains in Xinjiang, i.e., Altai Mountain in the north, Tianshan Mountain in the center, and West Kunlun Mountain plus Altun Mountain in the south. Between these three mountains lie Junggar Basin and Tarim Basin. The landform framework is three alternating mountains intervened by two basins (Fig. 1). Tianshan Mountain, Kunlun Mountain, and Altun Mountain are orogenic belts that formed in Cenozoic Era (Zhu et al., 2011), which indicates intense tectonic activity (Li and Xiao, 1999; Li et al., 2006). Now we briefly introduce the geotectonic history of Tarim Basin and Junggar Basin. Recent paleogeographic models (Li et al., 1996; Li and Powell, 2001; Metcalfe, 2009; Turner, 2010) have inferred that the Tarim Block is a craton and was originally connected to the Kimberley region of northwest Australia, forming part of East Gondwana. Correlations of the stratigraphy in the two areas revealed that both have a crystalline basement overlain by remarkably similar Neoproterozoic strata (Brookfield, 1994; Corkeron et al., 1996; Li et al., 1996; Grey and Corkeron, 1998) and occupied similar paleo latitudes during the Middle to Late Neoproterozoic (Chen et al., 2004; Huang et al., 2005; Zhan et al., 2007). Later on, the cratonic, Precambrian basement of Tarim Basin was overlain by a thick (3–16 km) sedimentary succession (Bally et al., 1986) which records the progressive tectonic and paleogeographic evolution of the basin and the adjacent region since the Late Neoproterozoic (Carroll et al., 1995; Carroll et al., 2001). In south and central Tarim Basin is a large-scale marine-continental sedimentary rock from Paleozoic to Cenozoic (Xu et al., 2005), while in north Tarim Basin, the Precambrian metamorphosed basement widely crops out (Xu et al., 2013). Permian molasses and continental volcanic rocks (including A-type rapakivi granites) are exposed at the northern margin of Tarim Basin (Yang et al.,

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2007b; Pirajno et al., 2008; Tian et al., 2010). These volcanic rocks are associated with a mantle plume beneath the Tarim Block (Zhang et al., 2010a, b). Junggar Basin is located in northern Xinjiang, separated from Tarim Basin by Tianshan Mountain. Geologically, this basin is not a craton (Zhao and Cawood, 2012), but a junction between Siberia Plate and Kazakhstan Plate, and is part of the Central Asian Orogenic Belt (Windley et al., 2007; Kröner et al., 2007; Pirajno et al., 2009; Rojas-Agramonte et al., 2011). Unlike Tarim Block, the ancient Junggar Block had formed by the superposition of island arcs in the Ordovician, the Silurian, the Devonian, and the Carboniferous (Li and Xiao, 1999). Thus, a lot of calc-alkali igneous rock and its clasolite are present in Junggar Block. A variety of granitic rocks, including M-, A- and I-types, have been found around Junggar Basin (Xiao et al., 1992). Previous research suggested that most of the granitic rocks intruded between 320 and 350 Ma (Zhou, 1989) with a few at 400 Ma (Hu et al., 1997). In late Carboniferous, the Junggar block was transformed into a residual sea basin with the closing of Kangur Basin (Li and Xiao, 1999). Thus Junggar Basin has sediments of marine-terrigenous facies mixed with volcanic rock (associated with island arcs). In the subsurface, the basin is filled with thick continental sedimentary rocks not older than early Permian (XBGMR, 1993) and surrounded by a number of Paleozoic ophiolite belts. The surrounding of the thick, young continental sediments by the Paleozoic ophiolite belts (a section of oceanic crust) strongly indicates that Junggar Basin was a sea basin which later on was uplifted above sea level to become part of the continent). 3. Calculation of the crustal magnetic anomaly In the spherical harmonic series of geomagnetic field, terms with harmonic degrees n P 16 serve as the crustal magnetic field (Maus et al., 2007; Purucker, 2007; Hemant et al., 2007). The spherical harmonic series of vertical component DZ of the crustal magnetic anomaly can be expressed as follows: N X n anþ2 X m m ðn þ 1Þ ðg m n cos mk þ hn sin mkÞP n ðcos hÞ r n¼16m¼0

DZ ¼ 

ð1Þ

Fig. 1. Topography and tectonics of Xinjiang and its neighboring area. Topographical data courtesy from http://www1.gsi.go.jp/geowww/globalmap-gsi/gtopo30.html. Note: The study region (red in the small map) is encircled by blue line. Location of geological structure from Xiao et al. (2010), Liu (2006), Song (2005), Zhang (1982). A—Junggar basin; B—Turpan–Hami Basin; C—Tarim Basin; D—Yili basin; WJ—West Junggar; EJ—East Junggar. 1—Irtysh Fault; 2—Darbut Fault; 3—Karamaili Fault; 4—North Tianshan Mountains Piedmont Fault; 5—North Margin of Central Tianshan Mountains Fault; 6—Ebinur Lake Fault; 7—South Margin of Central Tianshan Mountains Fault; 8—Korla Fault; 9—Arakan Fault; 10—Tumxuk Deep Fault; 11—Kegang-Tacang Fault; 12—Karakax Fault; 13—Great North Minfeng Fault; 14—Luobuzhuang Fault; 15—Altun Fault; 16— Kangxiwar Fault.

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Fig. 2. Crustal magnetic anomaly from (a) NGDC-720 model and (b) aeromagnetic anomaly in Xinjiang, as well as regional faults.

where k and h are longitude and latitude, respectively, a is the Earth radius (6371.2 km), Pm n ðcos hÞ is the Schmidt quasi-normalized assom ciated Legendre function of degree n and orderm, g m n and hn are spherical harmonic coefficients, and N is a truncation level. In this paper, N is set to degree 720, and the grid is 0.1°  0.1°. Substituting the NGDC-720-V3 spherical harmonic coefficients into Eq. (1) yields the crustal magnetic anomaly on the earth surface for Xinjiang. The spatial distribution is shown in Fig. 2a. Based on the 1:5,000,000 aeromagnetic anomaly map of China and its neighboring seas compiled by China Aero Geophysical Survey and Remote Sensing Center for Land and Resources in 2004, we downward-continued the data to the Earth surface, to obtain the aeromagnetic anomaly distribution in Xinjiang. For the convenience of comparing the modeled magnetic anomaly with the aeromagnetic anomaly, we plot the latter in Fig. 2b. 4. Relation between the crustal magnetic anomaly and geological structure

Mountain Fault and Arakan Fault (Fig. 2a). One positive anomaly zone is distributed in South Tarim Basin, and the other is from Junggar Basin to Turpan–Hami Basin. The negative anomaly zone is distributed in the west and middle of Tianshan Mountain and North Tarim Basin. The south Tarim positive magnetic anomaly zone is distributed in Tarim Basin from 40°N–41°N line southward to the Karghilik– Hotan–Qakilik line, with an extension direction approximately in the east–west or north–east. This zone is broad in the west and narrow in the east, like a triangle. Its northern boundary is Arakan Fault which divides Tarim Basin magnetic anomaly into south and north parts. The north part is negative while the south part is positive. The southwest boundary is Kegang-Tacang Fault which separates the strong positive magnetic anomaly in south Tarim Basin from the weak magnetic anomaly in west Kunlun Mountain. The southeast boundary is Great North Minfeng Fault which separates the positive anomaly in south Tarim Basin from the weak anomaly in Altun Mountain. This positive anomaly zone has small negative anomaly appearing in Guma, Hotan and Tazhong–Qakilik. The other positive anomaly zone, namely, the Junggar–Turpan– Hami positive anomaly zone, lies on the north side of North Tianshan Mountain, comprising magnetic anomaly of different patterns (Fig. 2a). From central Junggar Basin to its northwest margin is one annular positive anomaly with anomaly intensity of 100–200 nT. From the south margin of Junggar Basin to Turpan–Hami Basin is a positive anomaly band approximately in the east–west direction with intensity at 80–310 nT. In Yumin–Fuhai–Ertan is another annular positive anomaly. Altai Mountain has a negative magnetic anomaly, with the strongest anomaly focus being 310 nT at (92.2°E, 42.2°N). Please note that the positive magnetic anomalies in Junggar Basin and Turpan–Hami Basin are separated by the negative magnetic anomaly along Bogda Mountain. The negative magnetic anomaly zone is distributed widely in the west and center of Tianshan Mountain and in the north of Tarim Basin, extending in the north–west direction as a whole. The strongest negative anomaly focus is 307 nT at (80.3°E, 40.6°N). 4.2. Crustal magnetic anomaly and aeromagnetic anomaly Comparison of the aeromagnetic anomaly distribution (Fig. 2b) with the crustal magnetic anomalies calculated from NGDC-720V3 model (Fig. 2a) reveals only a minor discrepancy and a consistent trend generally. The crustal magnetic anomaly calculated from the model presents clearer magnetic anomaly bands and better relationship between the magnetic anomaly and the faults. In contrast, the aeromagnetic survey cannot be extrapolated to the altitude of satellite due to the limitation from the continuation technique. Hence, it provides a limited help for studying deep basement distribution. Fortunately, a high-order geomagnetic field model constructed using satellite, ground, oceanic, and aeromagnetic survey data can help us obtain magnetic anomalies in all horizons from the ground surface to the satellite altitude.

4.1. Basic feature of the crustal magnetic anomaly 4.3. Decay characteristics of the crustal magnetic anomaly As shown in Fig. 2a, the crustal magnetic anomaly distribution is complex, i.e., having positive anomaly zones, negative anomaly zones, dramatic gradient zones, and banded anomaly zones with alternating positive and negative values. Anomaly intensity varies significantly, from 307 to 465 nT. Magnetic anomaly diverges in the west of Xinjiang and converges in the east. It extends primarily in the north–west direction in north Xinjiang, but in the east–west or north–east direction in south Xinjiang. The two groups in the west merge into approximately the east–west direction in the east, forming a flared planar shape. In terms of polarity, the magnetic anomaly divides into two positive zones and one negative zone, demarcated roughly by north Margin of Central Tianshan

Analyzing the distribution of magnetic anomaly at different altitudes above ground can help understand the decay characteristics of the anomaly. We calculate the distribution of magnetic anomaly on four horizons at altitude 20 km, 100 km, 200 km and 400 km, as shown in Fig. 3. The result indicates that local strong magnetic anomalies decay rapidly with altitude, i.e., high-frequency components diminish and the distribution patterns become simpler with the increase of altitude. At altitude 400 km, the magnetic anomaly pattern is simple, with a low intensity. Except a negative anomaly over Yili Basin and Altai Mountain in north Xinjiang, other areas have positive

G. Gao et al. / Journal of Asian Earth Sciences 77 (2013) 12–20

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Fig. 3. Distribution of the crustal magnetic anomaly at altitude (a) 20 km; (b) 100 km; (c) 200 km; and (d) 400 km.

anomaly. The strongest positive anomaly is located in Tarim Basin, with a focus (81.3°E, 38.2°N) of 8 nT; this anomaly covers the whole Tarim Basin, extending to Turpan–Hami Basin and Junggar Basin. Because high-frequency components decay, the major anomaly at altitude 400 km come from long wavelength of the crust, representing macroscale crustal basement. At altitude 200 km, a negative anomaly zone begins to appear in north Tarim Basin and Tianshan Mountain, extending to Urumqi. At altitude 100 km, small positive anomaly emerges in the central and south of Tarim Basin and spanning Junggar–Turpan–Hami Basin, while the contour of Yili wedge-shaped positive magnetic field begins to appear. At altitude 20 km, the outline of the magnetic anomaly becomes visible. It is worth noting that the negative magnetic anomaly over Bogda Mountains only appears at altitude 20 km. 4.4. Relation between the magnetic anomaly and geological structure Comparing Fig. 1 with 2, we find that the spatial distribution of magnetic anomaly in Xinjiang basically agrees with the geological structure, well reflecting the tectonic feature, i.e., alternating mountain ranges intervened by basins. In general, except north Tarim Basin, there are distinct boundaries between the basin’s strong positive anomaly and negative magnetic anomaly over the mountains. The southwest and southeast boundaries of Tarim Basin are bordered by weak magnetic anomalies over West Kunlun Mountain and Altun Mountain, respectively. Margin of Junggar Basin is mountain-based negative anomaly over east and west Junggar Mountain and Tianshan Mountain. The quasi-triangular negative magnetic anomaly is intervened by the positive magnetic anomaly over Junggar Basin. Negative magnetic anomaly over Tianshan Mountain is intervened by the positive magnetic anomaly over Turpan–Hami Basin and Yili Basin. Crustal magnetic anomaly is caused by magnetic bodies in the crust, such that the spatial distribution of the magnetic anomaly reflects the distribution of magnetic bodies in the consolidated crust. The magnetic anomaly distribution from the ground to the

satellite altitude indicates that the entire Xinjiang is a sophisticated massif composed by magnetic bodies with different sizes, different directions, and different depths. The deepest magnetic body is a positive magnetic basement covering Tarim Basin and Junggar–Turpan–Hami Basin. This basement is overlaid by the negative magnetic massif covering north Tarim Basin and Tianshan Mountains. Yili Basin is a wedge-shaped positive magnetic massif bordered by Ebinur Lake Fault and south Margin of Central Tianshan Mountains Fault. Tarim Basin may be seen as a double-layer structure (Li and Xiao, 1999). The lower layer is basin wide, whereas the upper basement is divided into South Tarim positive magnetic massif and North Tarim negative magnetic massif. The south Tarim magnetic massif is roughly surrounded by Arakan Fault, Kegang–Tacang Fault and Great North Minfeng Fault, resulting in a triangle wide in the west and narrow in the east. The Junggar–Turpan–Hami Basin basement is bordered by Darbut Fault, Karamaili Fault and north Margin of Central Tianshan Mountains Fault, presenting a fan shape. The Bogda Mountain negative magnetic anomaly splits Junggar–Turpan–Hami Basin positive anomaly. The distribution of intensity of the crustal magnetic anomaly in Xinjiang is well corresponding with the faults. This is because on the two sides of a fault are mountain and basin, which have negative and positive magnetic anomalies, respectively. Thus, the fault should present a weak intensity in magnetic anomaly. Magnetic anomaly on both sides of Arakan Fault has very different features, with negative anomaly on the north side while positive anomaly on the south side. This fact leads to a conclusion that the fault divides the whole Tarim Basin into two tectonic parts: to the north are non-magnetic or weak magnetic rocks, while to the south are partially magnetic cover and magnetic crystalline basement. Tumxuk Deep Fault divides the south Tarim positive anomaly zone into one part approximately in east–west strike and the other part in north–east strike. The distinct anomalous feature exists in magnetic anomaly on both sides of the faults. Such good correlation between magnetic anomaly and fault zone indicates that major faults

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in Xinjiang acts as the boundary among geologic bodies of different natures. Magnetic anomaly distribution in south Tianshan Mountain is distinctly different from that in north Tianshan Mountain. South Tianshan Mountain and north Tarim Basin have negative magnetic anomaly with blurred mountain-basin boundary. Studies on deep structure of lithosphere in Tianshan Mountain and north margin of Tarim Basin (Zhao et al., 2004; Xu et al., 2001) revealed that during subduction to the South Tianshan Mountain orogenic belt, the substances in north margin of Tarim Basin were carried to the orogenic belt and reformed into the crust of south Tianshan Mountain. The current crust of south Tianshan Mountain was part of Tarim Block. South Tianshan Mountain has no clear mountain root, because the magnetic anomaly in south Tianshan Mountain and north Tarim Basin are both negative. This is consistent with the conclusion from seismic wave inversion (Tang et al., 2011; Li et al., 2007). The north of Central Tianshan Mountain Fault has strong positive magnetic anomaly, whereas to the south of the Fault is negative magnetic anomaly. When underthrusting toward Tianshan Mountain, magnetic substance of Junggar Basin may be blocked by Central Tianshan Mountain Fault, such that north Tianshan Mountain and Junggar Basin have the same positive magnetic anomaly. The distribution feature of the magnetic anomaly is consistent with different magnetic/density structure (Zhao et al., 2004), and different velocity structure (Xiong et al., 2011; Lei et al., 2013) between north Tianshan Mountain and south Tianshan Mountain. Now we discuss the correlation between the magnetic anomaly on the ground surface (Fig. 2a) and the mafic dykes in Tarim Basin as shown in Fig. 4 (Zhang, 1982; Li et al., 2009; Zhu et al., 2011). Geologically, in north Tarim Basin is a Proterozoic weakly or nonmagnetic basement lithofacies, which is very well corresponding to the negative magnetic anomaly in the north Tarim Basin in Fig. 2a. The central Tarim Basin is dominated by Paleoproterozoic magnetic granite, also very well corresponding to a clear positive magnetic anomaly in the central Tarim Basin. As shown in Fig. 2a, south Tarim Basin is characterized with a series of NE belts with alternating positive magnetic anomaly and negative anomaly. On the other hand, in Fig. 4, in the south Tarim Basin is the NE Hotan fold system composed of four magnetic basement lithofacies belts separated by three weakly magnetic basement lithofacies belts. Overall, the maganetic anomaly in Tarim Basin is well corresponding to the mafic dykes.

decreasing the accuracy of Curie surface inversion. For this reason, the impact of shallow magnetic bodies has to be filtered away from magnetic anomaly. Methods of suppressing the effect of shallow magnetic source include low-pass filter (Gasparini et al., 1981), upward continuation (Guan, 2005), multiscale 2D discrete wavelet transform analysis (Maurizio and Tatiana, 1998; Hou and Yang, 2011), and calculation of magnetic anomalies over long-to-medium wavelength bands using crustal magnetic field model (Kang et al., 2011; Kumar and Alice, 2009; Mita et al., 2009). In this paper, we select NGDC-720-V3 model of suitable order to eliminate influence from shallow magnetic bodies. According to Lowes-Mauersberger energy spectrum algorithm (Lowes, 1974), the geomagnetic field energy spectrum is

WðnÞ ¼ ðn þ 1Þ

n X m 2 2 ½ðg m n Þ þ ðhn Þ 

ð2Þ

m¼0

Variation of crustal magnetic field energy spectra in the NGDC720-V3 model with harmonic orders, as shown in Fig. 5, indicates that the crustal magnetic energy spectra is not homogeneous. Energy spectra at order 16–60 increases with the increase of harmonic order, called long wavelength zone. Energy spectra at order 61–220 is relatively stable, called medium wavelength zone. Energy spectra at order 221–720 gradually decreases, called short wavelength zone. Magnetic anomaly in long to medium wavelengths primarily reflects magnetic feature of deep and intermediate crustal lithosphere, and magnetic anomaly in short wavelength is primarily generated by shallow magnetic bodies (Kang et al., 2011). Fig. 6a and b are the modeled magnetic anomaly with order 16–220 and that with order 221–720. The latter is dominated by local feature reflecting the shallow magnetic sources. Therefore, the magnetic anomaly at order 16–220 is chosen and the power spectrum method (Spector and Grant, 1970) is employed in this paper to calculate Curie surface beneath Xinjiang. The calculation of Curie surface is conducted in the wavenumber domain. Assuming that a magnetic-source body extends infinitely in the horizontal direction and its depth is much smaller compared with the horizontal scale, depth to the top of the magnetic-source body, Zt, and depth to the center point Z0 are (Li, 2003; Li et al., 2009 )

Zt ¼

ln½AðjkjÞ  ln D jkj

ð3Þ

5. Curie isothermal surface Crustal magnetic anomaly is a combination of the effects from shallow and deep crustal magnetic sources. If the wavelength is close to each other, magnetic anomaly sources at different depths will mix and lead to aberration in deep magnetic body anomaly,

Fig. 4. Basement lithofacies beneath Tarim Basin, modified from Zhang (1982).

Fig. 5. Energy spectra of the NGDC-720-V3 model with the harmonic order.

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Fig. 6. The crustal magnetic anomaly at different wavelength bands: (a) n = 16– 220; and (b) n = 221–720.

Z0 ¼

ln½AðjkjÞ=jkj  ln E ; jkj

ð4Þ

respectively, where A(|k|) is the amplitude spectrum of magnetic anomaly, k is wavenumber, and D and E are constants. The lower boundary of the magnetic body, i.e., the depth of the Curie surface, Zb, is

Z b ¼ 2Z 0  Z t

ð5Þ

In the procedure of calculation, we firstly divide the study region into a grid of 37  23 squares. Each square has a size of 125 km  125 km, and two neighboring squares have an overlap of 50%. Secondly, we calculate the amplitude spectrum of magnetic

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anomaly and the ratio of amplitude spectrum to wavenumber for each square. Thirdly, we select the amplitude spectra in frequency interval 0.053–0.1 rad/km (the high-to-medium wavenumber domain) to fit Zt via (3), and then select the ratio of amplitude spectrum to wavenumber in frequency interval 0.001–0.05 rad/km (the low wavenumber domain) to fit Z0 via (4). The depth to Curie isothermal surface is got via (5). After getting the depths to Curie surface for all squares, we use a minimum curvature method to interpolate the depths of Curie surface for the whole study region (Fig. 7a). The histograms in Fig. 7a give a statistical distribution of the depths of Curie surface. The inversion result shows that the Curie surface in Xinjiang varies significantly in depth, ranging from 30 to 50 km with an average of 39 km. This result agrees well with the Curie surface depths from aeromagnetic survey (Li and Xu, 1999; Zhang, 2003; Wang et al., 1996). The most distinct feature of the Curie surface in Xinjiang is that beneath mountains is a shallow Curie surface while beneath basins is a deep Curie surface. This indicates that the basins are cold, rigid, and stable, while the mountains are orogenically active. In the south and east of Tarim Basin and Kuqa, in Yili Basin, in the south of Turpan–Hami Basin, in Turpan area, and in the central and west of Junggar Basin, Curie surface is as deep as 46 km. In West Kunlun Mountain, Altun Mountain, Tianshan Mountain, and Altai Mountain, Curie surface is generally shallow with depth less than 39 km. The Curie surface in Qarqan is only 30 km deep, being the shallowest in Xinjiang. In mountain-basin transitional zones, especially the transition zone between West Kunlun Mountain, South Tianshan Mountain and Tarim Basin, on the basin side is a deep Curie surface, while on the mountain side is a shallow Curie surface. In Junggar Basin, the Gurbantunggut Desert has a shallow Curie surface with depth at only 38 km,indicating that Junggar Basin is probably a relatively hot basin. Comparison of the Curie surface depth with the faults shows a good relationship between them. Across major abyssal faults, Curie surface depth varies significantly. For example, Curie surface is deep on the south side of Arakan Fault but shallow on the north side. Curie surface is deep on the west side of North Minfeng Fault but shallow on the east side. Curie surface is deep on the north side of South Tianshan Mountain Piedmont Fault but shallow on the south side. There exists a certain correlation between the crustal magnetic anomaly and the Bouguer gravity anomaly in Xinjiang. The striking feature of gravity anomaly (Peng, 2005) is similar to that of magnetic anomaly, i.e., both are parallel to major mountains. Gravity

Fig. 7. Depth of the Curie surface in Xinjiang: (a) from the model; and (b) from aeromagnetic anomaly.

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Fig. 8. Depth of the Moho in Xinjiang.

Table 1 Value of terrestrial heat flow in Xinjiang (Wang et al., 1996, 2000). Region

Terrestrial heat flow (mW/m2)

Tarim Basin Junggar Basin Tianshan Mountain Turpan–Hami Basin Qakilik–Qarqan region

42–56 41–61 70 44 73

anomaly appears low over mountains but high over basins. Magnetic anomaly is negative over mountains but positive over basins. The relationship between magnetic anomaly and gravity anomaly is indirectly reflected in Curie surface and Moho interface. As shown in Fig. 8, the Moho interface (Lin and Liu, 1995) and the Curie surface present an anti-mirror correspondence in Tianshan Mountain and its south. In Tianshan Mountain, Altun Mountain and West Kunlun Mountain, the Moho is deep, and the Curie surface there is shallow. In Tarim Basin, the situation is in reverse, also presenting an anti-mirror correspondence. The depths of the Curie surface and the Moho are basically the same in the central Junggar Basin. Therefore, in this region, the Moho could be considered as a magnetic boundary. Above is the magnetic lower crust, while below is the nonmagnetic upper mantle. In the east and west Junggar Basin and Gurbantunggut Desert, the depth of the Curie surface is less than that of the Moho. The Curie surface has a certain correlation with the terrestrial heat flow in Xinjiang. Table 1 lists terrestrial heat flows in some areas of Xinjiang. From Fig. 5 and Table 1, Curie surface is relatively shallow in areas with high terrestrial heat flow, but deep in areas with low terrestrial heat flow. For example, in Qakilik–Qarqan region and Tianshan Mountains, terrestrial heat flow value is high and Curie surface is shallow. In Junggar Basin, Tarim Basin and Turpan–Hami Basin, terrestrial heat flow value is low and Curie surface is deep. The fact of a high heat flow value in the center but a low flow value in the margins of Tarim Basin (Wang et al., 1996) is well corresponding to a shallow Curie surface in the center but a deep surface in the margins of the basin. Junggar Basin is featured with a low terrestrial heat flow in the center and a high value in the margins (Wang et al., 2000), corresponding to a deep Curie surface in the center and a shallow surface in the margins of the basin. The difference between Tarim Basin and Junggar Basin in Curie surface implies different thermal structures and thermal evolution histories.

compute the Curie surface. Firstly, we continuate the observed aeromagnetic anomaly upward to 20 km over the ground surface, by multiplying a decaying factor in the frequency domain (Li et al., 2009). This step aims to remove the effect from the shallow magnetic bodies (Hu et al., 2006). Secondly, the aeromagnetic anomaly is used to compute the Curie surface, with the same procedure as in Section 5. The Curie surfaces from the NGDC-720-V3 model and the aeromagnetic anmoly are plotted in Fig. 7a and b, respectively. Comparing Fig. 7a with b, we find that two approaches leads to roughly the same result. Nonetheless, Fig. 7a appears to be more consistent with the geological structure in Fig. 1. The magnetic anomaly is negative in north Tarim Basin and northeast Junggar Basin. The reason why north Tarim Basin has a negative magnetic anomaly is due to a Proterozoic weakly or nonmagnetic basement lithofacies; see Fig. 4. Unfortunately, the reason why northeast Junggar Basin has a negative anomaly is difficult to explain currently due to the lack of geological evidence there. The average Curie surface depth beneath north Tarim Basin is approximately 49 km, while that beneath northeast Junggar Basin is approximately 39 km. According to previous study of the geothermal field (Liu et al., 2003; Hu et al., 2001), the lithosphere in north Tarim Basin is cold, with a low terrestrial heat flow (45 mW/m2), whereas the terrestrial heat flow in northeast Junggar Basin is at 53 mW/m2. The reason why the two basins have different Curie depths is likely due to different terrestrial heat flows. 7. Conclusions (1) The distribution of crustal magnetic anomaly in Xinjiang agrees well with the geological structure, reflecting the tectonic framework in Xinjiang, i.e., alternating mountains intervened by basins. For instance, west Kunlun Mountain and Altun Mountain have negative magnetic anomaly, intervened by strong positive anomaly in South Tarim Basin. The distribution of positive and negative magnetic anomaly is consistent with the striking of major faults. The magnetic anomaly on the ground surface presents good correspondence with the mafic dykes in Tarim Basin. (2) The decay of the magnetic anomaly with altitude from the ground to altitude 400 km shows that Xinjiang is a sophisticated massif composed of magnetic massifs of different sizes in different directions at different depths. The deepest magnetic body is a positive magnetic basement covering Tarim Basin and Junggar–Turpan–Hami Basin. This basement is overlaid by negative magnetic massifs in North Tarim Basin and Tianshan Mountain. Tarim Basin may be a double layer structure, with the lower layer being basin-wide. (3) The depth of Curie isothermal surface reveals a distinct feature, i.e., a shallow surface in mountains but a deep surface in basins. This indicates that the basins are cold, rigid, and stable, while the mountains are orogenically active. Across the major abyssal faults, Curie surface varies significantly. The Moho interface and the Curie surface present an antimirror correspondence in Tianshan Mountain and its south., i.e., the Curie surface is shallow where the Moho interface is deep, and vice versa. Curie surface is shallow in areas with high terrestrial heat flow, and deep in regions with low terrestrial heat flow.

Acknowledgements 6. Discussion To validate the Curie isotherm surface inverted for from the NGDC-720-V3 Model, we use aeromagnetic anomaly in Fig. 2b to

This study was sponsored by the National Science Foundation of China under Grant 41264003, as well as the Foundation of Provincial Education Department of Yunnan under contract 2012Y488.

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