Geometry and Emplacement of the Late

中国科技论文在线 Journal of Asian Earth Sciences 79 (2014) 302–311 http://www.paper.edu.cn

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Journal of Asian Earth Sciences

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Geometry and emplacement of the Late Cretaceous maﬁc dyke swarms on the islands in Zhejiang Province, Southeast China: Insights from high-resolution satellite images ⇑ Ning-hua Chen a, Jin-jin Dong a, Jian-yu Chen a,b, , Chuan-wan Dong a, Zhong-yue Shen a

a Department of Earth Sciences, Zhejiang University, Hangzhou 310027, China b State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China

article info abstract

Article history: The geology of Zhejiang coastal area in Southeast China is characterized by numerous Late Mesozoic Received 24 April 2013 intrusive rocks and widespread mafic dyke swarms, which indicate continental lithospheric extensional Received in revised form 1 October 2013 events during the Jurassic and Cretaceous. This work is focused on using multisource high-resolution Accepted 1 October 2013 remote sensing images (Worldview2, Geoeye1 and Quickbird2) to identify the geometry, morphology Available online 9 October 2013 and location of previously undocumented and poorly understood dyke swarms exposed on the islands in Zhejiang Province. The geometry of each dyke is described by its strike, length and thickness. The spa- Keywords: tial distribution of the dyke density and crustal dilation are obtained based on the statistics of 774 Mafic dyke swarms extracted mafic dykes. Field surveys are performed in some islands in order to analyze the detailed geo- High-resolution remote sensing Geometry metric features and assess the interpretative accuracy. The spectral measurement and analysis of mafic Southeastern China dykes are performed as well for remote sensing imagery processing and lithological interpretation. The results show that the frequency distributions of the length and thickness of dykes follow power law curves. The maximum and mean dyke thicknesses are 11.2 m and 1.43 m, respectively. The crustal dilation of the islands ranges from 0.09% to 7.4%. From the north to the south islands, the dilation decreases gradually. The dyke frequency and density have the same distribution as the dilation. According to 40Ar–39Ar age (Zhongshanjie archipelago) and U–Pb zircon age (Sijiao Island) of the mafic dyke, the dyke swarms on the eastern Zhejiang islands emplaced at around 87–97 Ma. The cross-cutting relationships of dykes and host rocks show that the mafic dyke swarms have close spatial correlations with granite. The dykes stretch in various directions whereas the NE-trending dykes dominate. Our research reveals four intrusive events that imply the crustal extension and intermittent variation of the regional stress field in the coastal area of southeastern China in the Late Cretaceous. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction of mafic dyke swarms have been extensively studied through mul- tidisciplinary approaches. Mafic dyke swarms occur in a variety of tectonic settings such The geometric and kinematic features of dyke swarms are a as upwelling mantle plumes (e.g. Ernst and Baragar, 1992; Ernst good indicator of magma flow directions, location of magma and Buchan, 2003; Li et al., 2010; Kiselev et al., 2012), backarc source and palaeostress field (e.g. Ernst and Buchan, 2003; Klau- extension (e.g. Xiong et al., 2011), post-orogenic collapse (e.g. Iln- sen, 2006a, 2006b; Annen et al., 2001; Olsson et al., 2011; Kiselev icki, 2010; Liu et al., 2013), intra-continental rifting (e.g. Fahrig, et al., 2012). Multisource remote sensing provides an effective 1987; Smythe et al., 1995; Hou et al., 2005), and constitute a com- way to determine the distribution of dyke swarms on a large mon expression of crustal extension. They serve as major conduits scale (e.g. Babiker and Gudmundsson, 2004; Mège and Korme, for transferring magma to the upper crust, and contain ample 2004), and it is possible to measure the detailed geometric fea- information about the nature of the mantle source. The composi- tures of dyke swarms (including strike, dip, spacing and thick- tion, age, geometry, palaeomagnetism, flow pattern and tectonics ness) through high-resolution satellite imagery. The changes in dyke attitude and rock magnetic fabric can be used to study the propagation modes of dykes, which reveal the overall process of ⇑ the transfer of magma. Corresponding author at: State Key Laboratory of Satellite Ocean Environment Adjacent to the west of Pacific Ocean, the southeastern islands Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China. Tel.: +8657181963116. and coastal areas of China are characterized by numerous Late E-mail address: [email protected] (J.-y. Chen). Mesozoic intrusive rocks and widespread mafic dyke swarms,

转载中国科技论文在线 http://www.paper.edu.cn N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311 303 which indicate continental lithospheric extensional events during appropriate methods for remote sensing interpretation and assess the Jurassic and Cretaceous. Although many models have been pro- the accuracy of measurements, we first conduct field surveys and posed as geodynamical causes to explain this major extensional spectral measurements of dykes on some of the islands that allow event in the last two decades, debates continue (e.g. Martin human access. We used DGPS, compass, tape and laser rangefinder et al., 1994; Li, 2000 and references therein). As a key evidence for the measurements and location. Moreover, 3D point clouds of of extensional events, mafic dyke swarms have been studied in the dyke swarms on Miaozihu Island of Zhongshanjie archipelago lithogeochemistry (e.g. Li et al., 1995; Lapierre et al., 1997; Chen were obtained using high-resolution terrestrial laser scanner (Rie- and Jahn, 1998; Li and McCulloch, 1998; Li, 2000; Zhou and Li, gl-Vz1000), which offers 5 mm point spacing within 100 m dis- 2000; Ren et al., 2002) and palaeomagnetism (e.g. Gilder et al., tance. The geometry of each dyke is described by its strike, 1996; Liu and Morinaga, 1999; Pan et al., 2011) in parts of this re- length and thickness. The lithological spectral were measured gion. However, the distribution, geometry, and kinematics of the using an Analytical Spectrometer Devices (ASD) field-portable mafic dyke swarms largely remain unknown. spectrometer, which records 2151 channels within the 0.4- to Located in the southeast of China, Zhejiang Province has the 2.5-Am-wavelength region. Sample site locations are on Sijiao Is- most islands in China. In spite of the heavy vegetation cover, reefs land and Luhua Island (Fig. 1b). provide completely exposed dyke swarms on most of the islands. In this paper, we use multisource high-resolution remote sensing 3.2. Data and image processing images to identify the geometry, morphology and relative spatial location of previously undocumented and poorly understood dyke Medium resolution images, such as Landsat ETM, ASTER have swarms exposed on the islands where field surveys are difficult. been widely used to extract the length and strike of dyke swarms Based upon the field surveys of representative dyke swarms and in arid and semi-arid areas (e.g. Merged and Brkaiby, 1997; Bilotti the analysis of dyke geometry, we investigated the emplacements et al., 2000; Babiker and Gudmundsson, 2004; Mathieu et al., of the dyke swarms. We also discuss the geodynamic environment 2011). However, it is difficult to use these images for identifying of Late Cretaceous on the islands of the southeastern China. the dyke swarms, which are narrowly and shortly exposed on the island. Thus, we used multisource high-resolution satellite images 2. Geological background (Worldview2, Geoeye1 and Quickbird2) to extract the dyke geometric features (i.e., strike, length and thickness) on the islands Zhejiang Province is situated in the south of the Yangtze River in Zhejiang Province. Delta in the southeastern China and constitutes partially the South Because of its very high spatial (0.5 m) and spectral (8-bands) China Block (SCB), which includes the Proterozoic and Paleozoic resolution, Worldview2 imagery is primarily used to extract the terrain of the Yangtze Craton, and South China Fold Belt. The bound- geometric features of dyke swarms. Based on the characters of ary between the Yangtze Craton and South China Fold Belt is the the 8 bands and spectral characters of dykes, we select band 5, 3, sub-EW-trending Jiangshan-Shaoxing Fault (Fig. 1). The geology and 2 for generating false color image. The image is preprocessed of southeast China is characterized by extensive Mesozoic Yansha- for fusion, geometric correction and edge enhancement. All bands nian magmatism (Charvet et al., 1994; Ren et al., 2002; Shu et al., of imagery are geo-referenced to a Transverse Mercator projection 2009; Zhu et al., 2010). SCB has experienced complex and alternate with the root mean square error (RMSE) less than 1 pixel in order extension, compression and strike-slip activities affected by the to minimize the change in the spatial autocorrelation pattern. Geo- interaction of Paleoasian, Tethys and Pacific tectonic domain in eye1 imagery (0.5 m) and Quickbird2 (0.61 m) are used to identify the Mesozoic and Cenozoic eras (Shu et al., 2009). The widespread the dykes as well. Fig. 2 shows the false color and fusion imagery of NE-trending Mesozoic volcanic-intrusive complex area and tec- Wordview2 on Nandingxing Island of Shengsi archipelago. tonic zone formed the basic tectonic framework in Late Mesozoic southeast areas of China (Wang and Zhou, 2002; Hu et al., 2012). 4. Results The geological and geomorphologic characteristics on the islands of the southeastern China are similar to those of the adjacent main- 4.1. Lithology and spectral features of dykes land as the basement is comprised mainly of volcanic rocks and granite. In recent years, the igneous rock and tectonic evolution in According to the field surveys on some islands of Zhejiang Prov- the southeast of China have been studied extensively (e.g. Hsu ince, the mafic dyke swarms have close spatial correlations with et al.,1988,1990; Gilder et al., 1996; Chen and Jahn, 1998; Li, granite, which consists mainly of potash feldspar and quartz. Most 2000; Zhu et al., 2010). Some dyke swarms were found and studied of the dykes are fresh with dark and greenish color, and their com- in the fields of lithogeochemistry and isotopic geochronology in ponents are mainly diabase and proterobase. The dyke rocks are Zhejiang and Fujian coastal areas, as well as in Meizhou Island, Don- mostly coarse-grained with an ophitic texture in the middle of gji Island and Putuo Island (Dong et al., 2010). The age of the dyke dykes and fine-grained at their edges. However, a few dyke rocks swarms, which indicates the extensional events, is about 90– on some islands, especially on Luhua Island, are badly weathered 140 Ma. Based on this, many models have been proposed to explain and altered (Fig. 3a). Fig. 3b shows the spheroidal weathering of the geodynamic environment during the Jurassic and Cretaceous in the mafic dykes with yellow color. Moreover, thin section analyses this region (Lapierre et al., 1997; Liu and Morinaga, 1999; Li, 2000; indicate that the dykes have been badly chloritized (Fig. 3c). Some Zhou and Li, 2000). However, the work on the geometry and dyke rocks have an amygdaloidal structure filled with chlorite, emplacement of the mafic dyke swarms that reveal the geodynamic calcspar and quartz. Oriented mineral phenocrysts are sporadically environment is still very limited at present. scattered in some dyke rocks. Fig. 3d shows that the long axis of hornblende phenocrysts is along the NE direction, which is in 3. Methodology accordance with the dyke strike. The mafic dykes on Sijiao Island and Luhua Island have the sim- 3.1. Field survey and spectral measurement ilar spectral reflectivity curves which were from in situ measurements (Fig. 4). The reflectance of mafic dykes is from 5% to 12%. The mafic dyke swarms are well exposed in Zhejiang coastal Overall, the reflectivity value on Luhua Island is lower than that areas, especially on the islands (Fig. 1b). In order to choose on Sijiao Island. There is an obvious absorption at 2.3 lm in the 中国科技论文在线 http://www.paper.edu.cn 304 N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311

Fig. 1. (a) Tectonic sketch map of eastern Asia. NCB: North China Block, QDOB: Qinling-Dabie orogenic belt, SCB: South China Block. The red line indicates the regional fault zone. The black rectangle shows the location of ﬁgure b. (b) Simpliﬁed geological map of Zhejiang Province. HNI: Huaniao Island, LHI: Luhua Island, ZSA: Zhongshanjie archipelago, PTI: Putuo Island, JSI: Jiushan Island, DMI: Dameng Island, DT: Dongtou Island, JI: Nanji Island. The red and green dotted lines indicate the dykes of different intrusion. The blue and green color indicates volcanic rock and granite respectively. Rose diagrams show the dyke strikes on the islands.

composition. The maﬁc dyke is easier to be altered and weathered than the host rock so that many of them form slight topographical depressions.

4.2. Length and thickness of dykes and their relationships with orientation

As dykes intrude host rocks acting mechanically as a single fracture system, they rarely form simple, planar, parallel-sided walls, but segments linked by sudden sidesteps, horn structures and bro- ken bridges (Jolly and Sanderson, 1995). As branch and compound are the typical geometric features of dykes, it is difficult to distin- guish the number of dykes accurately. In order to facilitate statistical analysis, dykes that meet the following conditions are considered as a single dyke: (i) The spacing of dykes is larger than the thickness of adjacent dykes. (ii) No joint is found or the length of branched dyke segment is ten times longer than its thickness. Using these criteria, 774 dyke segments are identified on the islands in Zhejiang Province from high-resolution remote sensing images and field surveys. The geographical distribution and strike of dykes are shown in Fig. 1b and Table 1. Fig. 2. The false color and fusion imagery of Worldview2 showing the dykes on Because of the coverage of dense vegetation or dyke submerg- Nandingxing Island of Shengsi archipelago. The red arrows point to the mafic dykes. ing, the extension of observed dykes is quite small. The length of the measured dyke segments range from 3 to 325 m. The average spectral reflectivity curve of Luhua Island, which indicates more length is 40 m. The length distribution of dyke segments is reason- chloritization of dyke rocks. The host rock, which is granite, has a ably close to following a power law (Fig. 5). Actually, many dykes higher spectral reflectivity because it has more quartz in its intruded the host rock throughout the island and extended several 中国科技论文在线 http://www.paper.edu.cn N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311 305

Fig. 3. (a) Dyke swarms in eastern area of Luhua Island. The red arrows point to the weathered dykes. (b) The outcrop of the maﬁc dyke on Sijiao Island and the red arrows indicate spheroidal weathering. (c) Microscopic thin section showing chloritization of the maﬁc dyke, single polarized light. Chl: chlorite. (d) Oriented thin section of dyke showing long axis of mineral point to NE, cross polarized light. Hbl: hornblendes.

mean dyke thickness is 1.43 m. Only a few dykes whose thickness is less than 0.5 m are identiﬁed. This is partly because of the restriction of the image resolutions. Moreover, the thin dykes are often regarded as branches of thick dykes and ignored to count in the ﬁeld surveys. The thickness distribution of dyke segments follows a power law as well (Fig. 6). Power-law distribution is very common in natural, especially in volcano tectonics (Gudmundsson, 2011; Mohajeri and Gudmundsson, 2012; Gudmundsson and Mohajeri, 2013). The variation in length and thickness as functions of dyke strike as shown in Fig. 7, indicate that NE-trending dykes dominate. Although the rangeabilities of average values (length and thickness) are relatively low, the number, maximum length and thickness of NE-trending dykes are much larger than those of other direction dykes. Table 1 shows that the dyke thicknesses are very similar on the 12 main islands in Zhejiang Province.

4.3. Strike, shapes and cross-cutting relationship Fig. 4. The spectral curves of maﬁc dykes and granite on Sijiao Island and Luhua Island. The dyke swarms are widespread throughout most of the islands of Zhejiang Province from north to south, especially on Si- kilometers. Dyke thickness varies along the length and breadth of jiao Island (Figs. 1 and 8), and Zhongshanjie archipelago (Fig. 9). the dyke segments. The maximum dyke thickness is 11.2 m. The These dykes record both the source geochemistry and the tectonic

Table 1 Statistical parameters of distribution and density of dykes on the islands.

Islands Area (km2) Amount Dyke thickness(m) Dyke length(m) Density Min Max Mean Min Max Mean

Huaniao 3.67 31 0.72 10.85 2.09 11.83 121.17 34.97 5.91 104 Luhua 2.55 102 0.59 11.2 1.52 11.53 108.03 34.06 2.11 103 Sijiao 24.59 233 0.46 9.69 1.50 6.28 91.45 26 3.59 104 Huangxing 2.42 65 0.49 6.3 1.45 8.73 135.89 32.78 1.29 103 Miaozihu 2.61 92 0.61 2.95 1.36 17.53 91.43 37.95 1.87 103 Qingbang 1.41 131 0.48 3.51 1.25 9.1 229.05 46.32 5.27 103 Shizhushan 0.5 24 0.73 2.06 1.27 18.43 78.6 40.21 2.39 102 Xifushan 0.68 38 0.6 3.43 1.48 12.88 225.04 49.07 4.68 103 Jiushan 4.17 32 0.6 3.08 1.46 16.67 264.35 63.48 7.11 104 Damen 30.01 12 0.62 1.84 1.13 23.31 84.64 38.05 1.74 105 Dongtou 31.71 9 0.67 1.89 1.12 24.67 292.48 191.42 5.35 105 Nanji 7.8 5 0.2 1.1 0.8 8 33 12.3 8.76 106 中国科技论文在线 http://www.paper.edu.cn 306 N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311

Fig. 5. Length distribution of 774 dykes measured from satellite images and ﬁeld survey. (a) Histogram of dyke length. (b) Fitting curve showing a power-law distribution of dyke length and the estimated coefﬁcient of determination (R2) is about 0.92.

Fig. 6. Thickness distribution of 774 dykes measured from satellite images and field survey. (a) Histogram of dyke thickness. (b) Fitting curve showing a power-law distribution of dyke thickness and the estimated coefficient of determination (R2) is about 0.93. The first bin includes 0–1 m thick dykes because of the limit of resolution.

emplacements (Gautneb and Gudmundsson, 1992; Babiker and Gudmundsson, 2004). The rose diagram of dyke strike shows that NEE-trending (60–70°) dykes dominate on Luhua Island of Shengsi archipelago followed by the NE-trending (20–45°) and NW-trending (290–300°) dykes. The dykes on Sijiao Island, the largest island of Shengsi archipelago, have same dominant orientations as Luhua Island. Besides NEE and NW directions, a secondary EW-trending (85–95°) is also apparent. To the south, more dykes on Zhongshan- jie archipelago were detected with satellite images. In ﬁve islands, i.e. Huangxin, Miaozihu, Qingbang, Shizhushan and Xifushan, to- tally 350 dykes are found (Fig. 9). Different with Shengsi archipelago, the dominant orientation of the dykes on Zhongshanjie archipelago is NNE-trending (10–30°), and followed by NNW (330–350°) and EW-trending (85–105°). The main strike of dykes on Dameng, Dongtou and Nanji islands, which are almost the most south islands in Zhejiang Prov- ince, shows a dominated NW-trending. Most of the dykes in these islands are subvertical, except some inclined dykes on Nanji Island. Fig. 7. The number, mean thickness and length of dykes versus dykes strike. The The dykes on the islands of Zhejiang are often braided consist- vertical bars indicate the range of values. ing of anastomosing segments. Fig. 10 shows the dyke swarm at information at the time of emplacement. In general, dykes do not Liujingtan on Sijiao Island. It consists of more than 20 braided dyke appear to be affected by fractures or faults. Therefore, we can as- segments and its width exceeds 90 m. The dykes cross the bay and sume that the dyke trends are perpendicular to the direction of extend 1.2 km along the NE direction. Some of the dykes trace pre- the minimum compressive principal stress, r3, at the time of dyke existing fracture patterns and form zigzag shapes (Fig. 11a). 中国科技论文在线 http://www.paper.edu.cn N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311 307

Fig. 8. Geological map of Sijiao Island showing the geographical distribution and rose diagram of strikes of 233 dyke segments. The proﬁle perpendicular to the envelope of the swarm is marked with a gray line. The black lines represent the measured dyke width along the proﬁle and these data are used for the calculated dyke dilatation displayed in Table 2.

Fig. 9. Geological map of Zhongshanjie archipelago showing the geographical distribution and rose diagram of strikes of 350 dyke segments on the ﬁve islands.

The cross-cutting relationship of dyke swarms is an important indicate that the NE strike direction experienced two pulsatile geometric feature, which reflects the relative age of dykes. intrusive events. Fig. 11b and c shows that two intrusive events are identified on Lu- hua Island based on the field observation and satellite images. NWW-trending (290–300°) dykes are cut by NNE-trending (20– 5. Discussion 45°) dykes. This cross-cutting relationship is similar to that of Zhongshanjie archipelago. On the contrary, NE-trending dykes 5.1. Dyke distribution and crustal dilation are cut by NNW- and NWW-trending dykes on Sijiao Island. Only the number of NNW- and NWW-trending dykes is very small Fig. 12a and b shows the measured and interpreted geometric and the thickness is no more than 1 m (Fig. 11b). In addition, values (i.e. strike, thickness) for 22 points of the dykes on Sijiao Is- new NE-trending dykes are intruded within the older NE-trending land. The high correlation coefficients indicate a fairly positive cor- dykes along the NE-trending joints (Fig. 11d). These phenomena relation between the interpreted and measured values, and 中国科技论文在线 http://www.paper.edu.cn 308 N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311

Fig. 10. (a) Remote sensing image showing the dyke swarm at Liujingtan on Sijiao Island. The dykes extend about 1.2 km along NE–NW direction, cross the bay and expose on the cliff. (b) Remote sensing image showing the dyke swarm consists of more than 20 braided dyke segments. The red arrows point to the older NE-trending dyke and the green arrows point to the younger NW-trending dyke. (c) Remote sensing image showing the dykes exposed on the cliff. (d) Field photograph showing the dykes on the cliff. (e) Field photograph showing the dykes at Liujingtan.

Fig. 11. (a) Field photograph showing the dyke traced pre-existing fracture. (b) Field photograph showing two intrusive events on Luhua Island. (c) Geoeye1 image showing two intrusive events on Luhua Island. (d) Field photograph showing the new NE-trending dyke intruded within the old NE-trending dyke. The red arrows point to the older dykes and the green arrows point to the younger dykes.

X illustrating that it is an efficient and accurate replacement for tra- Density ¼ t l=A ditional field measurements using high-resolution satellite images. The spatial distribution of dykes is quantified by integrating both the data obtained from satellite images and the field surveys where t and l are the average thickness and the length of dyke seg- along the Zhejiang islands. The spatial density of dyke swarms is ments, and A is the area of the island. Table 1 shows that the dyke expressed as the percentage of the exposed dyke to the areas of swarms on Shengsi archipelago (Huaniao, Luhua, and Sijiao) and these islands. The density is given by: Zhongshanjie archipelago (Huangxin, Miaozihu, Qingbang, 中国科技论文在线 http://www.paper.edu.cn N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311 309

Fig. 12. Graphics showing Field-thickness/Images-thickness (a) and Field-strike/Image-strike (b) relationship. The black line represents the tendency line between data from ﬁeld measurements and data from image measurements.

Shizhushan, and Xifushan), which are located in the north of coastal the dykes, we conclude that the dykes intrude the granite in a pul- Zhejiang, are more frequently exposed than the southern islands. satile manner. Firstly, the dykes intruded along NWW direction. The crustal dilation due to dykes in the regional swarms varies This pulsation was weak and influenced only in the north of Zhe- widely (Gudmundsson, 1995). jiang islands. Secondly, a large-scale pulsation intruded along NE The total dilation is obtained by projecting the dykes on a line direction, which affected most of Zhejiang islands from Huaniao Is- that is drawn perpendicular to the envelope of the swarm and land (northernmost) to Nanji Island (southernmost). Thirdly, NE- dividing the cumulative dyke by the length of the line (Jolly and trending dykes intruded again along the joints within the dykes Sanderson, 1995). The percentage extension is given by: when the older NE-trending dykes cooled. This pulsation was weak X and only affected Sijiao Island. At last, another NNW-trending dyke Dilation ¼ t 100=L intruded at a small scale. The spatial distribution of dyke swarms shows that the main dyke swarm (NNE-NE–trending) is exposed where t is the thickness of dykes, and L is the length of the projected on the islands along 450-km-long coastal area with a gradual nar- section. rowing of the high-density dyke zone from the northern islands to Although the distributions of exposed dykes on the islands of the southern islands. The strike of the dyke swarm is consistent Zhejiang province are not uniform, the width of the swarm is com- with the regional fracture, which is generally regarded as compres- monly larger than that of the island. Therefore, the profiles we sional shear faults and varies along the coastal islands in a zigzag choose usually cross the entire island (Fig. 8). According to this shape. equation, the dilations of the 12 main islands are listed in Table 2, According to the spatial distribution of dyke swarms and their ranging from 0.09% to 7.4%. Shizhushan Island, which belongs to relationships with regional fractures, we estimate that the stress Zhongshanjie archipelago has, the most dilation (7.4%). The aver- field in the north of Zhejiang coastal area is likely weaker than that age dilation on Zhongshanjie archipelago is 4.7%, followed by Shen- of the southern Zhejiang coastal area, and controlled by basement gsi archipelago. From the northern to the southern islands, the fractures. Zhongshanjie archipelago that exposed much more dilation decreases gradually. The dyke frequency has the same dis- dykes is possibly close to the mafic magma, or is the main mag- tribution characteristics as the density and dilation. matic conduit of dyke intrusion. Few occurrences of conjugate dykes mean the lateral emplacement of mafic magma. The dyke 5.2. Events of dyke emplacements swarms of different orientations imply the intermittent variation of the regional stress field. The statistics of the isotope age data indicate that Cretaceous magmatism in SE China occurred in four major episodes during 6. Conclusions 136–146 Ma, 122–129 Ma, 101–109 Ma and 87–97 Ma (Li, 2000). Mafic dyke swarms intruded in granite and volcanic rocks follow- We compiled a new mafic dyke distribution map on the islands ing the pre-existing joints in each period of Late Mesozoic (Cao in Zhejiang Province, Southeast China. Combining with the field et al., 2009; Lou et al., 2011). The active peak of intrusion is at surveys and interpretations over satellite images on the geometric 110 Ma and 90 Ma. Some isotopic ages of granite and dyke swarms characters of the mafic dykes, we have found: on Zhejiang islands have been reported. For example, the A-type granite on Putuo and Taohua islands has U–Pb zircon ages of (1) Mafic dyke swarms are widely exposed on the islands of 93.6 Ma and 92.9 Ma, respectively (Qiu et al., 1999). The Zhongsha- Zhejiang Province. The length and thickness distributions jie archipelago dyke has 40Ar–39Ar age of 93.4 Ma (Dong et al., of dykes are close to following power-law curves. The max- 2010) and Sijiao islands dyke has U–Pb zircon ages of 93.9 Ma imum dyke thickness is 11.2 m, and the mean dyke thick- (Pan et al., 2011). Therefore, the intrusion of dyke swarms on Zhe- ness is 1.43 m. jiang islands corresponds to the fourth episode (i.e. 87–97 Ma). (2) These dykes swarms emplaced at around 87–97 Ma, and are According to the distribution of granite and dyke swarms, mafic strong evidence of crustal extension in the Late Cretaceous. dyke swarms have close spatial relationships with granite. Most of The crustal dilation of the islands ranges from 0.09% to the dykes emerged accompanying with granite and only a few in- 7.4%. From north to the south islands, the dilation decreases truded in volcanic rocks. Although the intrusion of dyke and gran- gradually. The dyke frequency and density have the same ite belongs to the same episode of magmatism, the dyke intrusions spatial patterns as the dilation. The cross-cutting relation- are later than that of granite. From the cross-cutting relationship of ships of dykes and host rocks show that the mafic dyke 中国科技论文在线 http://www.paper.edu.cn 310 N.-h. Chen et al. / Journal of Asian Earth Sciences 79 (2014) 302–311

Table 2 Percentage dilation and dyke frequency for individual traverses on the islands.

Location and traverse Traverse length (m) Number of dykes Total dyke thickness (m) Amount of dilation (%) Dyke frequency (km1) Huaniao 2020.6 14 59.5 2.9 6.9 Luhua 2132.6 35 96.2 4.5 16.4 Sijiao 10143.5 65 162.97 1.6 6.4 Huangxing 1557.8 28 72.8 4.7 17.9 Miaozihu 2,095.8 30 97.7 4.7 10.2 Qingbang 919.8 22 43.6 4.7 23.9 Shizhushan 360.3 15 26.8 7.4 41.7 Xifushan 789.8 10 22.1 2.8 12.7 Jiushan 1,934.9 14 27.2 1.4 7.2 Damen 3826.7 7 5.1 0.13 1.8 Dongtou 4470.1 9 13.6 0.3 2.0 Nanji 7051.7 6 6.4 0.09 0.9

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