MODELING OF MENTAWAI SUBDUCTION ZONE BASED ON GRAVITY DATA

MODELING OF MENTAWAI SUBDUCTION ZONE BASED ON GRAVITY DATA I. M. Fairayanti 1, Irham, MN 2, H. Danusaputro 3 1,2,3 Department of Physics, Faculty of Science and Mathematics, Diponegoro University, Semarang Abstract---Gravity data processing had been done to predict subduction model in Sumatera Mentawai segment. Modeling is carried out by doing upward continuation to the gravity anomaly in the flat plane to obtain the value of regional gravitational anomalies. Based on the modeling that has been done, obtained model of subduction system with features or elements such as trench, outer-arc ridge, outer-arc basin, volcanic arc, and foreland basin with constituent rock in the form of about 7 km thickness of oceanic crust with density 2.9 g/cm 3, Sedimentary rocks formed in the Cenozoic Age with a thickness of 1-2 km above the oceanic crust, Melange Wedge with density of 2.2 g/cm 3 and a thickness about 15 km, Pre-Tertiary rock or basement consists of Paleozoic and Mesozoic rocks complex, metamorphic rocks, igneous rocks, and carbonate rocks, have a density of 2.3 g/cm 3 and a continental crustal rock with a density of 2.67 g/cm 3. The dip of the subduction between 52 o to 5,6 o. Keywords--- Subduction zone, Mentawai, 2D Modeling, Gravity, Subduction dip I. INTRODUCTION Indonesia is a country with very high tectonic and volcanic activity because it is a meeting area of three main plates of the earth that is Indo-Australian Plate, Eurasian Plate, and the Pacific Plate. To the west, the converging movements of the Indo-Australian Plate moving relative to the north resulted in the formation of the subduction of the Sunda arc. The subduction zone of Sumatra is the area most often releases the earthquake energy caused by the movement of the Indo-Australian Plate which relative to the north. Mentawai Islands is a sedimentary seabed lifted by subduction activity. As a result, in Mentawai often happen big earthquake. The island of Sumatra is composed of two main parts, the west is dominated by the presence of oceanic plates, while the east is dominated by the presence of continental plates. Based on the gravitational force, magnetism, and seismic thickness of about 20 km, and the thickness of the continental plate is about 40 km [1]. During the Carbon to Perm Period, there is a subduction in western Sumatra that produces volcanic and pyroclastic rocks with compositions ranging from dacit to andesite in the Padang Plateau, Sangir and Jambi Plains [2]. The granitic intrusive rocks formed in Peninsular Malaysia, passing through Penang Island, and are thought to be continuous to the Riau Archipelago. Away off the west coast of Sumatra, plate subduction forms the Mentawai islands separated from the Sumatran plains by the Mentawai fault (MF) situated on the seafloor. Mentawai fault system is a horizontal fault formed from around Lampung continuously up to around Nias in the north. The activity of convergence of the plates is still active until now causing the activity of kegunungapian and seismicity along the fault lines and plate subduction. The tilted subduction process around the island of Sumatra resulted in the division/dissemination of tectonic vectors, ie slip-vectors that are almost perpendicular to the direction of the subduction zone accommodated by the mechanism of the factoring system. This is primarily in the accretion and slip-vector prisms in the direction of the subduction zones accommodated by the mechanism of the large fault system of Sumatra. The parallel slip-vector of this trough is not sufficiently accommodated by the Sumatran fault but also by other sliding fault systems along the Mentawai Islands, so it is called the Mentawai fault zone [3]. Figure 1 shows a cross-sectional scheme of subduction and arc of Sumatra Island and Mentawai Islands. DOI:10.23883/IJRTER.2018.4167.NXB5I 242

Figure 1. Cross-sectional Schemes of Sumatra and Mentawai [4] The method of gravity is a measurement made on the basis of a small variation of the gravitational field on the surface of the earth. A small difference or distortion in the gravitational field from one point to another on the surface of the earth is caused by the lateral variation in the mass of the earth's crust. Measurable variations are interpreted in terms of the possibility of subsurface mass distribution which then becomes the basis of how the geological conditions beneath the surface [5]. II. RESEARCH METHODS Flat field reduction is a process for bringing gravity data in topography to a flat plane of equal height. Reduction to plane aims to facilitate the process of interpretation and modeling. The method used in flat field reduction is the Dampney mass point method. The Dampney method brings the gravity anomaly object into an equivalent point mass source so that from the plane of the mass point it is obtained gravity anomaly data in a plane with the same height [6]. Upward appointment aims to focus on deep anomalies and eliminate local anomalies (shallow anomalies) by calculating potential field values at any point above the horizontal surface with known field values [7]. The data used in this research is secondary data, that is Bouguer Anomaly Complete data (ABL) which is accessed through web http://bgi.omp.obs-mip.fr on March 17, 2017 [8]. The data collection area is offshore West Sumatera with geographic area boundary of research area 1 o - 4 o LS and 95 o - 105 o BT. The research was conducted in Geophysics Laboratory of Diponegoro University Department of Physics. After obtaining the complete bouguer anomaly value, then the next data is taken to the plane by doing a flat field reduction. The flat field projection method used is the source method of the equivalent point of mass [9]. In this method an equivalent source of discrete mass points lies on a plane of depth with a depth under the boundary conditions beneath the surface of the spheroidal reference. Gravity anomalies are recalculated based on these mass points onto a plane with regular grids at a certain height. The next process is an upward continuation done to separate the regional and residual anomalies. Then do 2D modeling using polygon Talwani method [10]. III. RESULTS AND DISCUSSION After the process of flat field reduction and lifting up at an altitude of 36000 m, then modeled with four incisions above regional anomalies. @IJRTER-2018, All Rights Reserved 243

Figure 2. A-A ', B-B', C-C ', and D-D' sections on Contours of Regional Anomalies In this qualitative interpretation there is ambiguity because of the various models that can be produced, caused by the parameters of mass density, geometry, and depth. Therefore, it is necessary supporting data in the form of geological data of research area, data of rock mass meeting, drill data when needed, and other geophysical data. Figure 3. 2D cross-sectional model A-A' A-A' incision crossed the subduction path of Sumatra, Mentawai Islands, and parts of Sumatra Island. This makes the A-A model interpreted with the Indo-Australian Indducted subduction subduction infiltrated under the Eurasian Plate, the presence of the accretion prism, and the rear arc basin. The subduction system has features or elements of trough, outer-arc ridge, outer arc basin, volcanic arc, and front arc basin according to which Hamilton [11] prefers. The modeling results shown in Figure 4.6 show the presence of an oceanic plate with a density of 2.93 g/cm 3 with a thickness of about 7 km which floats above the mantle layer with a density of 3.49 g/cm 3. The oceanic plate is composed of alkaline igneous rocks such as perioditite, basalt, and gabbro that have a greater density than the rocks of the continental crust so that when a collision occurs, the oceanic plate will infiltrate beneath the continental plate. Due to the sedimentation process by sea water, formed a layer of sedimentary rock formed in the Cenozoic Age with a thickness of 1-2 km above the oceanic crust. In the northeast of the form of prism accretion due to the insistence of the oceanic @IJRTER-2018, All Rights Reserved 244

plate so that sedimentary rocks raised and deformed to form melan rock (Melange Wedge). Melange Wedge has a density of 2.2 g/cm 3 and a thickness of about 15 km composed of clay, broken formation, sepentinite, perioditite, basal and spilit, amphibol, greenschist and metasedimen methane mixed together. In Java and Sumatra, most of the sediments are deposited in the outer arc basin with a thickness of about 6 km. The basin is filled by sediments formed in the Cenozoic Age. Towards the northeast, the Sumatran basal rocks of Pre-Tertiary or basement rocks consist of Paleozoic and Mesozoic rock complexes, metamorphic rocks, igneous rocks and carbonate rocks, have a density of 2.3 g/cm 3 and below are continental crusts with a density of 2.6 g/cm 3.[12] Figure 4. 2D model cross-section of B-B ' Figure 4 shows the presence of an oceanic plate with a density of 2.9 g/cm 3 with a thickness of about 7 km which floats above the mantle layer with a density of 3.45 g/cm 3. The oceanic plate is composed of alkaline igneous rocks such as perioditite, basalt, and gabbro that have a greater density than the rocks of the continental crust so that when a collision occurs, the oceanic plate will infiltrate beneath the continental plate. Due to the sedimentation process by sea water, formed a layer of sedimentary rock formed in the Cenozoic Age with a thickness of 1-2 km above the oceanic crust. Cenozoic sedimentary rocks are also deposited in the outer arc basin with a thickness of about 6 km. In the northeast of the form of prism accretion due to the insistence of the oceanic plate so that sedimentary rocks raised and deformed to form melan rock (Melange Wedge). Melange Wedge has a density of 2.23 g/cm 3 and a thickness of about 15 km composed of clay, broken formation, sepentinite, perioditite, basal and spilit, amphibolite, greenschist and metasedimen rocks are mixed together. Headed northeast, the pre-tertiary or basement rocks of Sumatera Island consist of Paleozoic and Mesozoic rock complexes, metamorphic rocks, igneous rocks and carbonate rocks, have a density of 2.35 g/cm 3 and below are continental rocks with density 2.67 g /cm 3. Similar to the model on the A-A' and B-B incisions, the C-C incision shown in Figure 5 shows the presence of an oceanic plate with a density of 2.9 g/cm 3 with a thickness of about 8 km floating above the mantle layer with a density of 3, 46 g / cm 3. Layers of sedimentary rock formed in the Cenozoic Age with a thickness of 1-2 km above the oceanic crust and as a filler of the outer bow basin with a thickness of about 6 km. Melange Wedge has a density of between 2.2 g/cm 3 and a thickness of about 15 km composed of clay, broken formation, sepentinite, perioditite, basal and spilit, amphibol, greenschist and metasedimen methane mixed together. The pre-tertiary or basement rocks of Sumatera Island consist of Paleozoic rock and Mesozoic rocks, metamorphic rocks, igneous rocks and carbonate @IJRTER-2018, All Rights Reserved 245

rocks, have a density of 2.33 g/cm 3 and below is a continental crusted rock with a density of 2.67 g/cm 3. Figure 5. 2D model cross-section of C-C' The D-D incision shown in Figure 6 shows the presence of an oceanic plate with a density of 2.9 g/cm 3 with a thickness of about 6 km which floats above the mantle layer with a larger density of 3.39 g/cm 3. Sedimentary rock layers formed in the Cenozoic Age with a thickness of 1-2 km above the oceanic crust and fill the outer bow basin with a thickness of about 6 km. Melange Wedge has a density of between 2.2 g/cm 3 and a thickness of about 15 km composed of clay, broken formation, sepentinite, perioditite, basal and spilit, amphibol, greenschist and metasedimen methane mixed together. Pre-Tertiary or basement rocks of Sumatra Island consisting of Paleozoic rock and Mesozoic rocks, metamorphic rocks, igneous rocks and carbonate rocks, have a density of 2.33 g/cm 3 and below is a continental crusted rock with a density of 2.67 g/cm 3. Figure 6. Cross section Scheme by Model From the four modeling that have been done based on the regional anomaly map, can be made crosssectional transversal scheme in Mentawai segment shown in Figure 7. From Figure 7 it is known that the subduction system has elements such as ocean trenches, oceanic ridges, outer bow hollows, @IJRTER-2018, All Rights Reserved 246

volcanic arcs, and front arc basins that correspond to those proposed by Hamilton [13]. The Indo- Australian Lenpeng meeting with the Eurasian Plate starts at a depth of about 15 km. This causes the seabed sediments to rise to the surface forming the Mentawai Islands. To the northeast, shallow seas are formed separating the Mentawai Islands from the mainland of Sumatra Island. This shallow sea is a rear crater basin filled with Cenozoic sediment with a thickness of about 6 km and a density of 2.2 g/cm 3. The base rocks forming the island of Sumatra with a thickness varying between 5-10 km and below there is a layer of continental rock crust with a density of 2.3 g/cm 3. The magnitude of the subduction inclination angle can be calculated by knowing the point or coordinates of longitude and depth (x, z) using the inverse tangent relationship as shown in Fig. 8. Figure 7. Example of the inclination angle in the model Of the four subduction subduction structures of the subduction zones of the western offshore regions of Sumatra's Mentawai segment that have been made, it is known that the inclination angle of Indo- Australian plate subduction under the Eurasian Plate. Figure 8. Graph of Incision Subduction Slope A - A ' @IJRTER-2018, All Rights Reserved 247

FIG. 8 is a graphic example for obtaining gradient values or later subduction slopes from which the gradient can be calculated on the slope angle of Indo-Australian Plate subduction under the Eurasian Plate. Table 1. Calculation of inclination angle on track A-A ', B-B', C-C ', and D-D' Track Gradient (m) depth (km) Angle (θ=tan -1 (m)) A-A 0.0968 7 12.5 5.5 o B-B 0.0942 7 13.6 5.3 o C-C 0.0927 6.6 12.5 5.2 o D-D 0.0995 6 14.5 5.6 o The inclination angle obtained from the calculation of the subduction slant shows almost the same angle. The angle obtained from the calculation using inverse tangent relationship in accordance with that proposed by Hamilton (1979) that is the subduction angle in the trough Sunda 7 o while in Java between 5 o to 8 o. From these results it can be analyzed that the smaller the inclination angle the more gentle the subduction of the oceanic plate. Subduction angles are more gentle or less sharp, causing the formation of the accretion prism or the lifting of sediment under the sea surface. In Sumatra known prism akresi form Mentawai Islands composed of Bancuh Rock (Melange Wedge). In contrast to subduction in Java that does not show the appearance of the accretion prism on the surface. This is because subduction in Sumatra is more gentle than the subduction occurring in Java. IV. CONCLUSION From the modeling that has been done using gravity method with secondary data in offshore West Sumatera Mentawai segment is known existence of subduction Indo-Australian Plate and Eurasian Plate in Sumatera Mentawai segment. The elements or features of the subduction system are ocean trenches-the outer arch ridge-the outer arc-volcano basin-the island arch basin. With the constituent rocks of the subduction system in the form of oceanic crust rock, Cenozoic sedimentary rocks that are above the oceanic crust and as a filler basin of the bow, sedimentary rocks are lifted by the process of removal of the continental crust is urged by the oceanic crust called melange wedge (Melange Wedge), the foundation rock forming the island of Sumatra which is above the crust. Of the four modeling that has been done can be known large angle inclination subduction Indo- Australian plate under the Eurasian Plate on the Mentawai segment has a large between 5.2 o to 5.6 o. ACKNOWLEDGMENTS The authors would like to express heartfelt thanks to BGI (International Gravimetric Bureau http :// bgi.omp.obs.) over the use of the bouguer anomaly data, and Geophysics Laboratory, Physics Departement, Faculty of Science and Mathematics, Diponegoro University for providing support. REFERENCES I. Hamilton, W., 1979, Tectonics of The Indonesian Region, United States Geological Survey Professional Paper 1078. II. Handayani and Harjono, 2008, Tectonic Development of Sunda Strait Face Area and Its Relationship with Fault III. Zone of Sumatra, Journal of Geology and Mining Research, 10, 2, 31:40. Astawa,I. N., Silalahi, I.R.,and Rahardiawan, 2012, Geology Bottom of the Seabed Seabed Water Sheet Map 0421, Special Region of Aceh, Jurnal Kelautan, 10, 2, 101:116. IV. Katili, J.A.,1973, Volcanism and Plate Tectonics in Indonesian Island Arc, Tectonophys., 26.,p 165:188 V. C.S. Hutchison., 1973, Tectonic Evolution of Sundaland: A Phanerozoic Synthesis, Proceedings Regional Conference on the Geology of South East Asia, Geological Society of Malaysia, 6, 61:86. @IJRTER-2018, All Rights Reserved 248

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