Extensional Tectonic Regime of Garut Basin based on Magnetotelluric Analysis

Transcription

1 Indnesian Jurnal f Gelgy, Vl. 8 N. 3 September 2013: Extensinal Tectnic Regime f Garut Basin based n Magnettelluric Analysis Rejim Tektnik Ekstensi di Cekungan Garut berdasarkan Analisis Magnettelurik L. Handayani, Kamtn, and D. D. Wardhana Pusat Penelitian Geteknlgi - LIPI Jln. Sangkuriang, Bandung Crrespnding Authr: lina.handayani@gmail.cm Manuscript is accepted: March 07, 2013, revised: June 14, 2013, apprved: August 28, 2013 Abstract Garut Basin are is part f Bandung-Garut Greater Basin (Bandung Zne) characterized by a large basin surrunded by muntain ranges. Active vlcanes had distributed their material as pyrclastic depsits arund the uter brder f the zne and as lava flw depsit separating the tw basins. Buguer gravity anmaly data had als indicated the presence f several lw anmaly clsures at abut the area f Bandung and Garut Basins that were surrunded by high gravity anmaly znes. Tw magnettelluric surveys were cmpleted t acquire the subsurface mdel that might explain the tectnic evlutin f studied area. The first stage was characterized sby the presence f hrst - graben structures that might imply an extensinal regime f the area. The next stage f evlutinwas indicated by the hrizntal layering crrelated t the relative nn-active tectnic. In additin, a mst recent structure that appeared near the surface might suggest a pssible extensin frce as the current stage. Keywrds: Garut Basin, Bandung zne, extensin, magnettelluric, resistivity, subsurface mdel Abstrak Cekungan Garut merupakan bagian dari Cekungan Besar Bandung-Garut (Zna Bandung) yang memiliki karakteristik sebagai cekungan besar yang dikelilingi leh pegunungan. Prduk kegiatan gununggunung api aktif seperti endapan pirklastik yang tersebar di tepi Zna Bandung dan depsit aliran lava memisahkan Cekungan Bandung dan Cekungan Garut. Data anmali gayaberat Buguer juga menunjukkan adanya beberapa zna anmali rendah di sekitar kedua cekungan yang dikelilingi leh zna anmali tinggi. Dua survey pengukuran magnettelurik (MT) telah dilakukan di Cekungan Garut untuk memetakan area bawah permukaan yang dapat menjelaskan prses evlusi tektnik daerah penelitian ini. Mdel tahanan jenis bawah permukaan menunjukan adanya kemungkinan dua tahap evlusi tektnik di daerah ini. Tahap pertama yang ditandai leh adanya struktur hrst dan graben menunjukan adanya rejim ekstensi. Tahap evlusi selanjutnya ditunjukkan leh perlapisan hrizntal yang berkrelasi dengan tektnik yang relatif tidak aktif. Selanjutnya, tampak beberapa struktur di dekat permukaan yang menunjukan adanya gaya ekstensi yang kembali aktif di daerah ini. Kata kunci: Cekungan Garut, Zna Bandung, ekstensi, magnettelurik (MT), tahanan jenis, mdel bawah permukaan 127

2 128 Indnesian Jurnal f Gelgy, Vl. 8 N. 3 September 2013: Intrductin Java Island is an island arc prduced by the subductin f Indian-Australian Plate beneath Eurasian Plate. General feature f Java Island is the presence f vlcanes chain alng the island. Hwever, a clser bservatin shws that the tectnics f Western Java is nt entirely similar t the rest f Java Island. The different characteristic is indicated mainly by the spreading f muntainus regin. Figure 1 clearly shws the wide area f high elevatin at the Western Java area (Suthern Muntain), which is a remnant f ancient vlcanes (Van Bemellen, 1949). At the rest f the island, in general, small circular high elevatins spread spradically alng the middle f the island. This distributin f vlcanes and muntainus regin might indicate pssible variatin in tectnic prcesses alng the Java Island. Mre detailed tectnic studies at every part f the island are needed t understand the whle evlutin histry. Therefre, subsurface studies are essential t btain mre infrmatin f the tectnic f the regin. The subsurface infrmatin is expected t give sme new insights tward the tectnic evlutin that accuntable fr current gelgical and mrphlgical cnditin. In this research, we initiated the gephysical study in the western Java regin, by fcusing at Bandung and Garut Basins. Magnettelluric methd was emplyed due t its ability t explre a wide and deep reginal area. Reginal Gelgy and Mrphlgy Bandung and Garut Basins are tw large adjacent depressin znes that are actually main parts f the Bandung Zne (Van Bemellen, 1949). These basins are intra-muntain depressins frmed naturally between several active vlcanes. The hills and valleys f this area might indicate sme strng tectnic frces that frmed great flds and faults (Dam, 1994). The tectnic evlutin might invlve sme extensinal frce that pssibly frmed pull-apart basins as a prduct f at least tw parallel strike slip faults (Titus et al., 2002; Handayani and Harjn, 2012). Figure 1 shws the gelgical interpretatin frm Landsat Image f the studied area. Textures, clurs 6 42' S ' E ' E -6' 106' 108' 110' 112' 114' -8' N 0 10 km LEGENDS Pyrclastics Lava Flw/Yung Vlcanics Tertiary Sediments Tertiary-Quaternary Vlcanics Alluvium/Lake Depsits 7 24' S Faults Figure 1. Gelgical interpretatin frm Landsat Image (Suwijant, pers. cmm.) Insert: Tpgraphy Map f Java Island with red bx shwing the studied area. (Data surce: Smith and Sandwell, 1997)

3 Extensinal Tectnic Regime f Garut Basin based n Magnettelluric Analysis (L. Handayani et al.) 129 and pattern analyses f the Landsat Image were used fr its effectiveness in lithlgy and structural identificatin. As shwn in that gelgical map, the area was divided int five main znes based n their lithlgies. A cmbinatin f the Garut and Bandung Basins frms a hrse-she shape with the pening at the west. This hrse she cnfiguratin is utlined by pyrclastic depsits that cver muntain slpes (brwn regin in Figure 1). These pyrclastic muntainus ranges encircle Bandung and Garut Basins (blue regin) f which mst vlcanes at this muntainus regin are active. At the nrth, the basin is brdered by Burangrang - Tangkubanparahu - Bukittunggul - Manglayang Vlcanes. At the east side, there are Cikuray - Talagabdas - Galunggung Vlcanes. Papandayan, Malabar, and Cikuray Vlcanes are three f several vlcan chains brdered the suthern bundary f the basin. Bandung and Garut Basins are separated by a relatively yunger lava flw depsit (purple regin in Figure 1) that cvers a vlcanic cmplex f Guntur-Kamjang Vlcanes. Figure 2 shws a Buger gravity anmaly map (mdified frm Untung and Sat, 1978). This gravity anmaly map was btained frm a gravity survey by Untung and Sat (1978) n the main rads with 5 km interval between the statins. Altugh the data were widely dispersed, the map is still very useful fr reginal studies. Here, the gravity anmaly map was cmpared with the Landsat Image interpretatin (Figure 1). The gravity anmaly map indicates the dense cnturs with high anmaly values that framed bth basins (range regin). These high anmaly regin frames were rughly cincident with the pyrclastic depsits that encircle bth Bandung and Garut Basins. Inside thse high anmaly frames, there are several lw anmaly depressins, which might be cincident with the lcatin f the basins. N km 7 00' S G41 G39 G37 G35 G33 G31 G29 G27 G25 G23G22 G20 G18 G16 G15 G14 G13 G12 G11 G10 G9 G8 G7 G6 G5 G4 G3 G2 G ' E ' E m g a l Figure 2. Buguer gravity anmaly map (mdified frm Untung and Sat, 1978) and tw lines f Magnettelluric measurements.

4 130 Indnesian Jurnal f Gelgy, Vl. 8 N. 3 September 2013: Mst f the Bandung Zne is cvered by yung vlcanic and alluvial depsits except at the western bundary f the area, where there are sme Tertiary sedimentary rcks. The ldest vlcanic rck that had been identified as Middle Palecene age was fund at Cupunagara, Subang, nrth f Sunda Vlcanic Cmplex (Kartadinata et al., 2002), where there was als fund sme yunger vlcanic rcks were als fund frm Olig-Micene t Quarternary (Indart et al., 2006). The yunger vlcanic rcks (mstly Quarternary) are mre cmmn and fund in mst f the area, such as at Tangkubanparahu, Guntur, and Malabar Vlcanes (Brnt and Hartn, 2006; Dam, 1994; Kesemadinata and Hartn, 1981). The general pattern frm gravity anmaly and lithlgy abve had led us t pstulate that there were at least tw main perids f tectnic evlutins at this area. First, there was a frmatin f the frame r uter ring, which was characterized by pyrclastic sediments and high gravity anmaly area. The secnd perid was represented by the frmatin f Guntur vlcan cmplex that separated Bandung and Garut Basins. This area was cvered by lava flw depsits. Hwever, its crrelatin with the gravity anmalies was nt clear. Methds A subsurface mdeling is necessary fr further research f the Bandung Zne tectnics. The infrmatin f rck layers beneath the area might give sme indicatins n events during a range f time, as the deeper the infrmatin btained, the farther back in time the prcess culd be identified. In this research, a cmbinatin f magnettelluric (MT) and audi-magnettelluric (AMT) methds were emplyed due t its simplicity and cst effective t examine this substantially wide area. Furthermre, its ability t penetrate deeply make MT is a methd f ur chice fr the tectnic study. Magnettelluric (MT) methd is a gephysical passive measurement f the natural magnetic and electricity cntained in the earth, which has frequency between and Hz. Wide range f frequency allws this methd fr subsurface investigatin frm near surface (tens f meters) t thusands f meters beneath the surface. Cnductivity f subsurface infrmatin is btained frm the rati f electric t magnetic fields. Rati f a high frequency range gives infrmatin f shallw sub-surface, while the lwer frequency gives a deeper penetratin. The rati is then represented as MT-apparent resistivity with phase as the functin f frequency. Due t the limited resurces, the surveys were nly cmpleted within the Garut Basin. Magnettelluric sundings were cnducted alng tw west - east lines lcated at the Great Garut Basin (Figure 2). The first deplyment wrked alng the suthern line (SL) that cnsisted f 15 statins spaced abut 4 km apart. Then, the secnd deplyment ran thrugh the nrthern line (NL) that cmprised 26 statins with abut 2 km distance between each statins. Magnettelluric data were cllected in tw statins a day using tw units f Magnettelluric Instruments Phenix MTU-5a with 2 minutes sampling rate. During the secnd deplyment, the AMT was als measured (Audi-Magnettelluric) at each statin f NL t btain a near surface infrmatin. Result Data btained frm the surveys are apparent resistivities frm each statin frm the tw lines f surveys. Editing prcesses by MT-Editr were carried ut t eliminate nises and t create smth resistivity curve. Inversin prcesses were cmpleted with the WinGLink sftware. Figures 3 and 4 are the resistivity sectin mdels fr Nrthern and Suthern Lines, respectively. The Nrthern Line was extended at abut 50 km frm Cimnce (Sreang) t Leuwi Gng (Leles). The subsurface layering can be gruped int three grups f resistivity values: lw (less than 100 Ohm.m), medium (between 100 and 1000 Ohm.m), and high (mre than 1000 Ohm.m). The lw resistivity appears n the surface between statins 41 and 40 (west end) and between statins 24 and 22. The medium resistivity exists acrss the line t the depth f abut 2 km, and high resistivity appears t frm tw basins at the depth f mre than 2 km. The Suthern Line has a lwer reslutin mdel since the distances between statins are twice f thse at the nrth. Nnetheless, the infrmatin generally much the same: there are three grups f resistivity layers. Three small areas near the surface

5 Extensinal Tectnic Regime f Garut Basin based n Magnettelluric Analysis (L. Handayani et al.) 131 have the lwest resistivity. A layer f medium resistivity appears alng the line t the depth f abut 2 km and fills the basin shapes that were frmed by high resistivity layers. One feature that appears n bth mdels is the present f near surface lw resistivity areas crrespnd t the tp f the high resistivity parts beneath them. Discussin Garut Basin is ne f the tw depressins in Bandung Zne surrunded by a chain f active vlcanes. A preliminary deductin frm gravity anmaly and lithlgy interpretatin frm Landsat Image indicated tw pssible stages f evlutin. The first stage was the ne related t the lder pyrclastic spreading, and the secnd stage was the ne related t the basin structure and yunger vlcanic activity. Magnettelluric measurement was executed in this area t find a better subsurface data t supprt thse accunt. In general, bth subsurface mdels (Figures 3 and 4) have indicated the existence f basins. The lw resistivity areas (less than 100 Ohm.m r yellw t red areas in the figures) that appear at several places near the surface culd be assciated with lse sediment (Dbrin and Savit, 1988). The intermediate resistivity rcks (100 t 1000 Ohm.m r green areas in the figures) culd be sediment de- meter G41 G40 G39 G38 G37 G36 G35 G34 G33 G32G31 G30G29G28 G27 G26 G25 G24G23 G22 G21 G20 G19 G18 G17G km Ohm.m Figure 3. Tw dimensinal mdel f resistivity (West-East) f Nrthern Line sectin. meter 2000 G15 G13 G12 G11 G10 G14 G09 G08 G G06 G05 G04G03 G02 G km Ohm.m Figure 4. Tw dimensinal mdel f resistivity (West-East) f Suthern Line sectin.

6 132 Indnesian Jurnal f Gelgy, Vl. 8 N. 3 September 2013: psit and/r felsic igneus rck (Dbrin and Savit, 1988). And high resistivity rcks (mre than 1000 hm.m r blue-purple areas) culd be hard sediment depsit r igneus rck (Dbrin and Savit, 1988). This highest resistivity layer (purple) culd be lder igneus rck r basement. The presence f basins at the depth shwn in bth resistivity mdels indicates a pssibility f at least tw stages tectnic prcesses in this area. The idea f these tectnic evlutin stages f Bandung Zne is illustrated at Figure 5. First, the extensin regime began t affect the gelgical prcesses here by nrmal faults activities that develped sme hrst - graben cmplexes. These hrst-graben structures might be related t the pull-apart basins as a result f extensin in the end f Tertiary (Van Bemmelen, 1949). Then at the secnd stage, the extensin frce ceased. A relatively stable tectnic resulted in frmatin f hrizntal layering, which was nt nly filled the basins but als cvered the area. This stable tectnic appeared t cntinue until quite recently. A narrw high resistivity feature at the east end f Nrthern Line (Figure 3) indicates a disruptin f a smewhat cntinuus layering pattern. This upset culd be interpreted as a nrmal fault. Therefre, there is a pssibility that the extensin frce has been reappeared recently and frmed the nrmal fault as ne f the cnsequences (pssible third stage). Unfrtunately, these subsurface mdels culd nt further explain the wide Bandung Zne as had been expected. The tw line surveys were t little t cver the tectnic study f the area. Nevertheless, this new subsurface infrmatin had sufficiently revealed sme histries f Bandung Zne s tectnics Figure 5. Sketch f pssible tectnic evlutin f Bandung Zne at three stages. (1) the stages f extensin that develped hrst-graben structures, (2) the ceased f extensin, with fully develped hrst-graben that was then cvered by later depsitins, (3) pssible an early stage f anther extensin.

7 Extensinal Tectnic Regime f Garut Basin based n Magnettelluric Analysis (L. Handayani et al.) 133 Cnclusins Magnettelluric surveys acrss the Garut Basin (in Bandung Zne) cnfirmed the presence f basins at mre than 1 km depth, beneath a thick hrizntal layering f sedimentary depsit. The subsurface mdels might indicate at least tw stages f tectnic prcesses f the regin. The first stage was the extensin prcess that develped hrst-graben structures. The secnd stage was hrizntal layering that indicates a stable regime f tectnic. In additin, there is a pssible third stage that might have been ccurring recently. The extensin regime has appeared t be re-activated as was indicated by a nrmal fault that reach the surface at ne f the mdels. Acknwledgements---We wuld like t gratefully acknwledge financial supprt frm Research Center fr Getechnlgy LIPI during fiscal year f 2010 and This wrk was nly pssible with assistance frm Yayat Sudrajat, Suyatn, Sunardi, Karit L. Gal, and Djuprin during the fieldwrk. References Brnt, S. and Hartn, U., Ptensi Sumber Daya Gelgi di Daerah Cekungan Bandung dan sekitarnya. Jurnal Gelgi Indnesia, 1 (1), p Dam, A.A.C., The Late Quarternary Evlutin f the Bandung Basin, West-Java, Indnesia. Dissertatin. Universiteit Amsterdam. The Netherlands. Dbrin, M.B. and Savit, C. H., Intrductin t Gephysical Prspecting. McGraw-Hill, Singapre, 867pp. Handayani, L. and Harjn, H., Analisa Tektnik Jawa bagian Barat dan Implikasinya Terhadap Bencana Kegempaan di Daerah Bandung dan Sekitarnya. In: Anwar et al. (eds), Menyingkap Tabir Fenmena Bencana Seismik di Indnesia: Prespektif Pengurangan Resik Bencana Gempabumi dan Tsunami. Buku Bunga Rampai Penelitian Kmpetitif LIPI Sub Kegiatan Kebencanaan dan Lingkungan, Penerbit Andira dan LIPI, p Indart, S., Sudarsn, Setiawan, I., and Sumantri, T.A.F., Mdel Genesa Mineralisasi Hidrtermal di Daerah Cupunagara, Subang-Jawa Barat. Prsiding Peluang dan Peran Ilmu Kebumian Dalam Pembangunan Berkelanjutan. Pusat Penelitian Geteknlgi LIPI, p Kartadinata, M.N., Okun, M., Nakamura, T., Kbayashi, T., Eruptive Histry f Tangkubanparahu Vlcan, West Java, Indnesia: A Preliminary Reprt. Jurnal f Gegraphy, 111(3), p Kesemadinata, R.P., and Hartn, J., Stratigrafi dan Sedimentasi Daerah Bandung. PIT X Ikatan Ahli Gelgi Indnesia, Bandung. Smith, W.H., and Sandwell, D.T., Glbal seaflr tpgraphy frm satellite altimery and ship depth sunding. Science, 277. p Titus, S.J., Fssen, H., Pedersen, R.B., Vigneresse, J.L., and Tikff, B., Pull-apart frmatin and strikeslip partitining in an bliquely divergent setting, Leka Ophilite, Nrway. Tectnphysics, 354, p Untung, M. and Sat, Y., Gravity and Gelgical Studies in Jawa, Indnesia. Gelgical Survey f Indnesia & Gelgical Survey f Japan. Van Bemmelen, R.W., The Gelgy f Indnesia, 1A, General Gelgy f Indnesia and Adjacent Archipelages. Gvernment Printing Office, The Hague.