Eruptive-Volcanic-basalt structures in the Truong SaSpratly Islands and adjacent areas from interpreting gravity and magnetic data

Vietnam Journal of Earth Sciences, 39(1), 1-13, DOI: 10.15625/0866-7187/39/1/9167 1 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences Eruptive-volcanic-basalt structures in the Truong Sa- Spratly Islands and adjacent areas from interpreting gravity and magnetic data Tran Tuan Dung*1, 2, Nguyen Quang Minh1 1Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology 2Graduate Universiy of Science and Technology, Vietnam Academy of Science and Technology Received 28 April 2016. Accepted 20 January 2017 ABSTRACT In the Spratly Islands (Truong Sa Islands) and adjacent areas, volcanic activities are quite strong after the sea- floor spreading in the Cenozoic Era. However, it is difficult to define their ranges and spatial locations. Based on the different characteristic between eruptive volcanic basalt and sedimentary rocks near the surface, it can be said that, the blocks which are higher density and magnetization than those surroundings could be identified as eruptive volcan- ic basalt. This paper presents the methods of reduction to the magnetic equator in low latitudes to bring out a better correlation between magnetic anomalies and their causing-sources; High-frequency filtering is to separate gravity and magnetic anomalies as well as information about the volcanic basalts in the upper part of the Earth's crust; 3D total gradient is to define the spatial location of high density and magnetic bodies. The potential structures of eruptive vol- canic basalt are predictively determined by multi-dimensional correlation analysis between high-frequency gravity and magnetic anomalies with weighted total gradient 3D. The results from the above-mentioned methods have shown that the distribution of the eruptive volcanic basalt mainly concentrates along the Spratly Island’s seafloor-spreading axis, transitional crust, Manila trench and some large fault zones. These results are improved by available seismic data in the study area. Keyword: Spratly Islands, Reduction to the equator, 3D total gradient, Eruptive volcanic basalt. ©2017 Vietnam Academy of Science and Technology 1. Introduction1 The East Vietnam Sea has been formed from the collision between the Indian and the Eurasian plates. The opening process of the East Vietnam Sea leads to sea-floor spreading to form the basin. Some of these basins were *Corresponding author, Email: trantuandung@yahoo.com opened by pull-apart mechanism resulting in lithospheric rift and separating of sub- continent as well. It is said that, the spreading process terminated in the Late Miocene after the collision among North Palawan formation and west Philippine Arc; Taiwan Island and Chinese mainland (Li S-L., et al., 2011). The initiate phase of the lithospheric rift is often Tran Tuan Dung and Nguyen Quang Minh/Vietnam Journal of Earth Sciences 39 (2017) 2 accompanied by magmatic activities (eruption and intrusion). Volcanic activities are quite strong after sea-floor spreading and the mag- matic eruption occurs not only in the Spratly Islands but also elsewhere in the Southeast Asia. The magmatic activity cut-through the ocean floor, sub-continent such as Paracel and Spratly (Hoang Sa and Truong Sa) Islands, Reed Bank is forming the basalt layers in the region. Many suggested that the opening pro- cess of the Spratly Islands is more complex than the existing document. Researching re- sults from the geothermal and gravity methods show that either the rate of subsidence ex- tremely slow or the intrusion rate of heat flow come up so high. The relationship of the time and space of magmatic field after rift has led to the question, that is, main motion for the magmatic field: the mantle uplift or the litho- spheric rift? (Bui Cong Que, Tran Tuan Dung, 2008; Nguyen Hoang and PhanTrong Trinh, 2009; Li S-L., et al., 2011; Tran Tuan Dung, 2013). The activity of basalt eruptions occurs quite commonly following the rest time of seafloor spreading in the East Vietnam Sea (from 32 to 17 Ma). In this region, Neogen - Quaternary basalt distributes mostly along the major fault system, appear after rift and later than 6 Ma. It is a combination of stone includ- ing tholeiite basalt, olivine basalt and alkaline basalt, little basalt-andesite and high alkaline basalt (Nguyen Hoang and Phan Trong Trinh, 2009). During the seafloor spreading process, ba- saltic magma of early phase was formed in the strong extension condition (high levels of par- tial melting), at a shallow depth (low pres- sure); In contrast, basalt of late phase was formed in limited extension condition (low levels of partial melting), at a deeper depth (high pressure). The difference of time and magnitude of the spreading process which de- cides to the characteristics the late Cenozoic basaltic activities and distribution in Vietnam: the strong eruptive activities in the southern part, as to the north as the eruptive activities decrease and mostly single eruption (Pham Tich Xuan et al., 1999, 2004). The volcanic activities occur in multi- phases and basalt eruption develops largely in the Spratly Islands and continental margins. In the Late Mesozoic, the eruptive volcanic bas- alt is main granite that distributes in the north- ern margin of the Spratly Islands, Indochina continental shelf and Paracel and Spratly is- lands. In the Cenozoic, types of the volcano are main basalt eruption, concentrate com- monly in the continental margins and oceanic crust areas. In general, the distributional char- acteristics of eruptive volcanic basalt have a close relationship with the geo-tectonics activ- ities in the Spratly Islands (Flower et al., 1992; McElhinny, 1973; Li S-L., et al., 2011; Tran Tuan Dung, 2013). In the ocean, eruptive volcanic basalt is the main source of the local gravity and magnetic anomalies. Based on the difference of density and magnetization between volcanic basalt and sedimentary rock, geoscientists have used the geophysical methods such as gravity, magnetic and seismic to determine the loca- tion of eruptive volcanic basalt. However, due to the complex tectonic activity, the difference in the Earth's crust structure, the diversity of rock types, variations of the geophysical anomaly at low latitudes, then the application of an individual geophysical method will not be able to bring out good results. Therefore, it is necessary to apply the interpretative meth- ods of gravity, magnetic with seismic and drilling data in combination (McElhinny, 1973; Li S-L., et al., 2011; Tran Tuan Dung et al., 2013). In order to eliminate the inaccuracy of magnetic anomalies at low-latitudes, which caused by magnetic declination and inclina- tion angle, some scientists have presented the methods of reduction to the magnetic equator (RTE) or pole in the spatial and frequency Vietnam Journal of Earth Sciences, 39(1), 1-13 3 domain web/declination). The application of low/high- frequency filters allows determining the lo- cal/regional gravity and magnetic anomaly. Here, the high-frequency anomalies are used to explain and determine the distribution of eruptive volcanic basalt (Lee et al., 1998; McElhinny, 1973). In 1988, SudhirJain has mentioned to a number studies on the magnetic field in his research, such as: Baranov (1957) described a technique for reducing the maps made any- where, except at very low latitudes. Into what they would be if the inclination of the magnet- ic field were 90 degrees. These maps are called reduced to the pole maps and the pro- cess is called reduction to the pole (RTP). Over the years, several refinements and other methods of RTP, such as techniques using Fourier transforms (Bhattacharyya, 1966: Spector and Grant, 1970) have been suggest- ed. Leu (1981) proposed reduction to the equator (RTE) and demonstrated that RTE is more reliable at high latitudes than RTP is at low latitudes (Sudhir Jain, 1988). In that research, Sudhir Jain gave out the equations and describe a model study to com- pare RTP (using conventional Baranov and Fourier transform operators) with RTE (using only Fourier transform operators). It is shown that RTE is more accurate than RTP even at high latitudes and that the Fourier transform provides more accurate RTP at lower latitudes than a conventional operator (Sudhir Jain, 1988). In general, reduction to the equator is preferable to reduction to the pole, more par- ticularly at the middle and lower latitudes. 2. Data The main gravity, magnetic and seismic data sources in the Spratly Islands are from the marine surveys of the Russian, Vietnam, French, German, United States and Japan geophysical companies. In the Vietnam Ma- rine Research Programs, which carried out by the Institute of Marine Geology and Geophys- ics in recent years, such as KHCN-06-12, KC- 09-02, KC09-11/11-15 are also collected, in- terpreted and added the new research results and the valuable geophysical data in the Spratly Islands and adjacent areas (Bui Cong Que, Tran Tuan Dung, 2008, Nguyen Hoang and Phan Trong Trinh, 2009; Tran Tuan Dung et al., 2013). In this paper, the authors have combined the satellite gravity and magnetic with ship- board data in order to bring out a data set of the satellite-shipboard-derived, which have a higher accuracy and uniform resolution, in the Spratly Islands. In recent years, United Stage scientists have built a new gravity, bathymet- ric grid with an interval of 1'×1' (David Sandwell V23.1) (Sandwell et al., 2013), (Figure 2a, 2b). The magnetic data (EMAG2) get from the NOAA's National Geophysical Data Center. EMAG2 is a global Earth Mag- netic Anomaly Grid that compiled from satel- lite, shipboard, and airborne magnetic surveys (Figure 3a). That is the result of global coop- erative projects on geophysical data, which are frequently added and updated. One can say that it is the geophysical data sources with the most uniform resolution, wide coverage, suitable accuracy and cost to use effectively for research the geological structure in the Spratly Islands (Sandwell et al., 2013; Tran Tuan Dung, 2013). In additions, this study also referring and using the seismic data from the oil and gas exploration surveys that carried out by the ge- ophysical companies inside and outside Vietnam, as projects VOR93, JMSU05, PK07-08 (Bui Cong Que, Tran Tuan Dung, 2008; Tran Tuan Dung et al., 2013). The re- search area was selected within 109°-118°E and 6-13°N (Figure 1). Tran Tuan Dung and Nguyen Quang Minh/Vietnam Journal of Earth Sciences 39 (2017) 4 Figure 1. Location of study area (red line rectangular) Figure 2. a) Bathymetry; b) Complete Bouguer gravity anomaly Vietnam Journal of Earth Sciences, 39(1), 1-13 5 Figure 3. a) Magnetic anomaly ΔTa; b) Magnetic anomalies ΔTa after reduction to the equator 3. Applied methods Research and predictively determination of the distribution of eruptive volcanic basalt (including eruption or intrusion) in the Spratly Islands is a complicated problem that cannot be solved only by a single method. The re- search must have an overall view of the geo- logical structure and the relationship between them and the eruptive volcanic basalts in the different phases. Therefore, it is necessary to combine geophysical methods to adjust the magnetic anomalies with their causing-source locations; to separate the anomalies, which could be caused by eruption volcanic basalts in the near Earth’s surface, from the observed gravity anomaly; to determine the ranges and spatial locations of the anomaly-causing sources. This paper presents some methods as follows: 3.1. Reduction to the magnetic Equator The quantitative analysis of magnetic data in the low-latitude region is encountered a dif- ficult problem causing by magnetic inclina- tion, that is, the anomaly locations and their sources are not coincident. Meanwhile, the study area is located in the low-latitudes near the equator, where magnetic anomalies have a poor correlation with anomaly-causing geo- logical objects. As a result, RTE method is used to solve the above-mentioned problem. This is a method applied to the low-latitude region to adjust the magnetic anomaly loca- tions to a best coincidence with their sources. It makes the interpretation of magnetic data more easier, which without losing their geophysical significance ( web/#declination; Karoly, 1990; Sudhir Jain, 1988; Ian N. MacLeod et al., 1993; Le Huy Minh et al., 2003). In this research, the Geosoft package software V7.01 was used to reduce the mag- netic field to equator transformation of an anomaly in the Fourier domain. The inclina- tion and declination angles of the ambient field were taken successively in the center points of each geographic degree in the study area at the date of July 2016 according to IGRF (International Geomagnetic Reference Field), respectively. Reduction to the magnetic equator is repre- sented by the following formula: ܨሼ∆ܶܽா௤ሺݔ, ݕሻሽ ൌ ܨሼܮሺሻሽܨሼ߂ܶܽሺݔ, ݕሻሽ(1) Where: F{} is the Fourier transform; Tran Tuan Dung and Nguyen Quang Minh/Vietnam Journal of Earth Sciences 39 (2017) 6 ΔTa୉୯ሺx, yሻ is the reduced magnetic filed to the equator with L(x,y) operator; Ta(x,y) is the observed magnetic data; L() is the operator of reduction to the magnetic equator. The operator of reduction to the magnetic equator is calculated: ܮሺߠሻ ൌ ሾୱ୧୬ሺூሻି௜∙ୡ୭ୱሺூሻ∙ୡ୭ୱሺ஽ିఏሻሿమൈൣିୡ୭ୱమሺ஽ିఏሻ൧ሾ௦௜௡మሺூೌሻା௖௢௦మሺூೌሻ.௖௢௦మሺ஽ିఏሻሿൈሾ௦௜௡మሺூሻା௖௢௦మሺூሻ.௖௢௦మሺ஽ିఏሻሿ (2) Where: D is the magnetic deviation angle; I is the magnetic inclination angle; q is the wave direction; Ia is the correction angle of magnetic inclination. The Figure 3b presents magnetic anoma- lies after reduction to the magnetic equator. Comparing the magnetic data in the figure 3b with the initial data in the Figure 3a, it can see that the magnetic anomalies after the reduc- tion have strongly changed in both the direc- tion and magnitude (Figure 3b). 3.2. Frequency filtering The frequency filtering separates out the gravity and magnetic anomalies caused by the shallow geological objects from the total ob- served anomalies in order to determine the ranges and the spatial locations of the eruptive volcanic basalts, which are formed in the up- per part of the Earth’s crust. Low-pass filter of gravity and magnetic anomalies are calculated by the following formula: Fሼ∆g୐୊ሺx, yሻሽ ൌ FሼHሺx, yሻሽFሼΔgሺx, yሻሽ (3) Where: F{} is the Fourier transform; Δg୐୊ሺx, yሻis the low frequency anomalies that filtered with H(x,y) operator; g(x,y) is the complete Bouguer gravity anomaly; H(x,y) is the Gaussian low-pass filter operator (Bracewell, R., 1999). The Gaussian low-pass filter operator as follows: Hሺx, yሻ ൌ eିୈሺ୶,୷ሻమ/ଶ஛మ (4) Where: D(x,y) is the distance between points (x, y) on the gird and the center of the filtering window; λ is cutoff wavelength. In the next step, residual gravity and mag- netic anomalies are calculated: ∆gୌ୊ሺx, yሻ ൌ ∆gሺx, yሻ െ ∆g୐୊ሺx, yሻ (5) After doing the filtering with different wavelength λ, the gravity, magnetic anoma- lies that correspond to these wavelengths will be used to calculate and interpret geological structures at different depths. Here, the Geosoft package software V7.01 is applied to do low-pass filter. The high fre- quency gravity and magnetic anomalies are calculated for the whole area using the Gauss- ian low-pass filter with the filter wavelength λ. For each wavelength λ, it will bring out a distribution of geophysical anomalies at dif- ferent depths. After comparing the results from the filtering in certain above-mentioned wavelengths λ, it can be seen that, the residual gravity, magnetic anomalies corresponding to the wavelengths λ=50km (Figures 4a, 4b) have a good correlation with the shallow geo- logical objects. As a result, they have been applied to build the 3D total gradient model (gravity and magnetic anomalies) to deter- mine the locations and ranges of the eruptive volcanic basalts. 3.3. 3D total gradient Method of 3D total gradient of gravity, magnetic anomalies is applied to determine the locations, ranges and depth of the anoma- ly-causing objects. However, this study refers only to their locations and ranges. There have been many studies on this problem, though, the method that presented by Nabighian, M. N., 1984, Roest et al.,1992; MacLeod et al., 1993; Phillips, J. D., 1997 are choosing to calculate the 3D total gradient of gravity, Vietnam Journal of Earth Sciences, 39(1), 1-13 7 magnetic anomalies in study area. 3D total gradient method is often used to determine the spatial location of the blocks of eruptive vol- canic basalt. The Geosoft package software V7.01 was used to calculate the 3D total gra- dient as follows: Graሺx, yሻ ൌ ටቀப୅ப୶ቁ ଶ ൅ ቀப୅ப୷ቁ ଶ ൅ ቀப୅ப୸ቁ ଶ (6) Where: A is gravity or magnetic anoma- lies. The 3D total gradient of gravity, magnetic anomalies bring out a clear picture of the spa- tial distribution of the eruptive volcanic basalt (in the upper part of the Earth’s crust). How- ever, to reduce the multi-solutions and im- prove the reliability of the method, it should have an overview analysis in combination with others geology-geophysics documents (seismic, drilling data...). The calculated result of the 3D total gradient is shown in Figure 5a, b. 4. Results In the area, the eruptive volcanic basalts have higher density and magnetization than the surrounding rocks. The eruptive volcanic basalts that have different components, then also have a relative change in density and magnetic value. Throughout the analysis and comparison with other data sources from this research, it has found that, in the area, the eruptive volcanic basalts have a residual grav- ity anomaly range from +10 to + 63mGal and the residual magnetic range from +15 to +185nT, respectively (Figure 4a, b). Figure 4. a) Residual gravity anomaly (filtering wavelength λ=50 km); b) Residual magnetic anomaly (filtering wavelength λ=50 km) From achieved results, the comparison among the gravity, magnetic anomalies, and their 3D total gradient is carried out to deter- mine the spatial-coincided coefficients be- tween them and the distribution of predictive eruptive-volcanic basalts (relative-coincident locations of the 3D total gradients with high blocks of residual gravity and magnetic anomalies). Here, the spatial-coincided coeffi- cients have a variable range from 0.0 to 5.39 (dimensionless) (Figure 6a). Based on the comparison of the coefficient (Figure 6a) with the seismic data (Figure 6b), the authors have evaluated, interrelationship-classified in order Tran Tuan Dung and Nguyen Quang Minh/Vietnam Journal of Earth Sciences 39 (2017) 8 to provide a detailed picture of the spatial distribution of the predictive eruptive volcanic basalt in the Spratly Islands and adjacent areas (Figure 7). Figure 5. a) 3D total gradient of residual gravity anomaly; b) 3D total gradient of residual magnetic anomaly Throughout the results of this research, comparing with the results of some previous study on the eruptive volcanic basalt in the Spratly Islands, which were used sparse borehole and seismic data, it has found that there are many remarkable coincidences be- tween them. The results from the above- mentioned methods have shown that the dis- tribution of the eruptive volcanic basalt main- ly concentrates along the Spratly Islands’ cen- ter, transitional crust, Palawan trench and at some large fault zones. In this research, the red-marked circles are the best correlation between the eruptive vol- canic basalts and geophysical fields, which have high correlation coefficients, they are supposed to be the spatial locations of the eruptive volcanic basalt, they have high densi- ty and magnetism (Figure 7). In Figure 7, one can see that the eruptive volcanic basalt distributes in the whole study area but not alike. Commonly, they are con- centrated mainly in the area of the continental slopes, slope of the deep basin and around the Spratly Islands. Otherwise, they are very so spare in the Phu Khanh and Vung May Ba- sins. One can see, in the Northern part, the eruptive volcanic basalts only appear incoher- ently on the oceanic crust of the East Vietnam Sea, but they concentrate as a large zone in the tail of the South-East oceanic crust. In the center area, the eruptive volcanic basalts are quite concentrated in the North and South part of the Spratly Islands. In the Western part, eruptive volcanic basalt appears along to Tuy Hoa shear zone, Tu Chinh, and Nam Con Son uplift. In the South of the study area, it is hard to find the volcanic basalt locations, except areas near Borneo-Palawan subduction zone. Similarly, in the Eastern part, they almost do not appear on the Reed Bank block and conti- nental shelf of Malaysia and Philippines. Vietnam Journal of Earth Sciences, 39(1), 1-13 9 Figure 6. a) Spatial-coincided coefficients between high-frequency gravity, magnetic anomalies and their 3D total gradient with distribution of predictive eruptive volcanic basalt; b) Eruptive volcanic basalt locations showing in the known seismic and drilling data Tran Tuan Dung and Nguyen Quang Minh/Vietnam Journal of Earth Sciences 39 (2017) 10 Figure 7. Predictive distribution of eruptive volcanic basalts in the Spratly Islands and adjacent areas 5. Discussions As above-mentioned, the results of this re- search completely based only on the determi- nation of gravity and magnetic data. The main purpose of the research is to bring out predic- tions about the potential structures of eruptive volcanic basalts in the area. This is the foun- dation for the detail geological researches in the next stages. Through this research it can take out some of the following discussions: The possible structures of the eruption vol- canic basalt are predicted by a comparative analysis of the multidimensional relationship between the high frequency of gravi- ty/magnetic fields and their 3D total gradient. However, there are still some difficulties in determining the correctly spatial location of these structures. On the Spratly Islands and adjacent area, based on the different charac- teristic between eruptive volcanic basalt and sedimentary rocks near the surface, it can be said that the blocks which are higher density and magnetization than those surroundings could be identified as eruptive volcanic basalt. The Gravity and Magnetic anomalies (at shallow layer) also change continuously in the whole Spratly Islands area (Figure 4a,b), here, the negative-positive anomalies are interleav- ed. The comparative analysis of the correla- tion between values of gravity and magnetic anomalies showing the clear differentiation in Vietnam Journal of Earth Sciences, 39(1), 1-13 11 their magnitude. There are some areas which have positive correlation (positive values) are more clearly than the negative correlation (negative value), (Figure 5a,b). These charac- teristics bring out a picture on the distribution of volcanic basalt structures (at shallow layer) in the islands and coral reefs are affected by geological formations. Figure 7 shows the distribution of some eruptive volcanic basalt in the deep water of the East Vietnam Sea, where the gravity and magnetic anomalies have the large change in a small range. Here, the local positive magnetic anomalies lay over the background negative values. The residual gravity anomalies are the separate closed positives that locate on the background negative values, too. The correla- tion values between gravity and magnetic anomalies have little changes at low ampli- tude, although some undersea mountains have a positive correlation related to their local ig- neous rocks. This implies the existence of the seafloor spreading and magmatic activities in the deep-water area of the East Vietnam Sea. In additions, throughout the spatial analy- sis of the gravity, magnetic anomalies as well as their correlation with 3D total gradient, faults distribution, it can see that the distribu- tion of the eruptive volcanic basalt could be also related to the deep faults. In figure 7, the distribution of eruptive volcanic rocks is con- trolled and affected by the deep crustal struc- ture and faults. These rocks are mostly con- centrated on the intersections of the faults (fault system in Figure 1) and transitional zone between crust’s types (Figure 7). The distribution of these rocks is mostly along a northeast-southwest direction, which relates to the deep fault’s direction in the area. It is can say that magmatic activity extended to the surroundings areas during and after spreading of the oceanic floor. 6. Conclusions This research has applied an effective geo- physical methods in combination to determine predictively the distribution of the eruptive volcanic basalts: Methods of reduction to the magnetic equator in low-latitudes to bring out a better correlation between magnetic anoma- lies and their causing-sources; High-frequency filtering is to separate gravity and magnetic anomalies as well as information about the volcanic basalts in the upper part of the Earth's crust; 3D total gradient is to define the spatial locations of high density and magnetic bodies. The method of multi-dimensional correla- tion analysis between the high-frequency gravity, magnetic anomalies with weighted total gradient 3D allows determining quite ac- curately the ranges as well as the spatial loca- tions of the eruptive volcanic basalts in the Spratly Islands and adjacent areas. The diversity of the formation and the complex physical properties of the eruptive volcanic basalt causing the determination of their characteristics more difficulties. In order to determine the distribution of the eruptive volcanic basalts more accurately and efficient- ly, it is necessary to apply the geophysical methods together with seismic, geochemistry and drilling data in combination. The eruptive volcanic basalts concentrate mainly in the area of the continental slopes and around the islands of the Spratly Islands. In the Northern part, they appear not only in- coherently on the oceanic crust of the East Vi- etnam Sea, but also concentrate as a large zone in the tail of the South-East oceanic crust. In the center area, they concentrate in the North and South part of the Spratly Islands. Additions, in the Western part, they appear along to Tuy Hoa shear zone, Tu Chinh, and Nam Con Son uplift. In the South of the study area, it is hard to find the volcanic basalt locations, except areas near Borneo- Palawan subduction zone. Similarly, in the Eastern part, they almost do not appear on the Reed Bank block and continental shelf of Ma- laysia and Philippines. Tran Tuan Dung and Nguyen Quang Minh/Vietnam Journal of Earth Sciences 39 (2017) 12 Acknowledgments The authors wish to thank the VAST’s Project No.VAST06.06/16-17 for funding this research. References Baranov, V., 1957. A new method for interpretation of aeromagnetic maps: Pseudo-gravimetric anomalies: Geophysics, 22, 359-383. Bhattacharyya, B. 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