CHAPTER 1
INTRODUCTION
1.1 Background
In West Sumatra, many of small basins (sub-basins) are found in the volcanis arc or at the margin of Central Sumatra Basin area. Among that small basin, there are Ombilin Basin and Payakumbuh Basin which pertained intra-arc basin. Some of those basins are located at the margin of back-arc basin or near the volcanic arc such as Kiliran sub-basin, Kamang sub-basin, Peranap sub-basin, and many more which has not been named yet. Those basins were formed in Paleogene and generally loaded by lacustrine deposit and swampy deposit which contain coal and oil, as reported by previous researcher (Bachri et al., 2001; Suwarna et al., 2001; Wain and Jackson, 1995; op.cit. Bachri, 2006). These lacustrine deposit and swampy deposit, loaded the depressional area such as full graben and half graben as reported by previous researchers.
Central and South Sumatra Basin are located at southwestern of Sunda shelf which is the part of Southeast Eurasian Plate. According to Davis (1984; op), in Paleogene sub-period, extensional tectonic regime were held at western of Sunda Shelf which was caused by divergent-transform between Sunda Micro Plate and Hindia Oceanic Plate. This process made rift system on Early Tertiary (Eocene – Oligocene). Based on the previous reports, it is believed that the rifting system formed graben, half graben, and Paleogene horst in back-arc basin in Sumatra (Eubank & Makki, 1981; Heidrick & Aulia, 1993 ; Yarmanto drr., 1995 op.cit.Bachri, 2006). Considering graben in West Sumatra and Jambi found in small scale, so in this paper will presented more relevant analysis, that is related to existence of zones in influence of strike-slip tectonic regime.
The research area are located in area of Kamang, Northside of Kiliranjao, partly step into West Sumatra and the other into Jambi (picture 1). In this area, found some sub-basin which the part of Central Sumatra Basin.
Picture 1. Research area
1.2 Aims and Objectives
This paper is made to know how formed of West Sumatra and Jambi sub-basin.
1.3 Systematic
In this paper divided into some chapter. At the chapter one, introduction are about background, aims and object, systematic and methods that weared for writing this paper. Chapter 2 Research Area is an overview of geography and geology research area. Chapter 3 Research Data is all of research data area that cover deformation of strike slip research area. Chapter 4 is a discussion about data that get from research area and than get the conclusion of discussion on Chapter 5.
1.4 Methods
Writing methods this paper is by using literature such as journal of geology, books, note of lecture and also from internet.
CHAPTER 2
RESEARCH AREA
2.1 Geography Platform
Research area at this time reside at West Sumatra and Jambi that it is of course locate in Sumatra Island. West Sumatra and Jambi is the province that adjoin one another. West Sumatra is locate at 
and 
.
Picture 2. geographical map of West Sumatra (www.sumbar.org)
Picture 3. geographical map of Jambi (www.kotajambi.org)
Jambi’s located is at the south-southeast of West Sumatra. Jambi is locate at 
and 
. Jambi and West Sumatra is separated with Tiga Puluh Mountain. Tiga Puluh mountain also separate basins in Sumatra, that is Central Sumatra Basin and South Sumatra Basin.
Picture 4. Geology basins at Sumatra (lecture slide of Geology Indonesia)
2.1 Geology Platform
Sumatra Island is meeting area between Hindia Oceanic Crust on southside and southwest edge of Sunda self that Asian south-east continent or part of Eurasian Plate (picture 5).
Picture 5. Tectonic setting of Sumatra
Until now, subduction of oceanic crust to east direction still active, with average of speed 6.3 cm/years (le pichon, 1968). The pre-tertiary bed rock on Sumatra is alokton rock from Gondwana plate (Cameron drr.,1980 ; Pulunggono, 1983).
One of result from the subduction of oceanic plate under the continental plate on Sumatra, is formed some tertiary back arc basin, for example is North Sumatra Basin, Central Sumatra Basin, Sunda Basin, Northwest Java Basin, and also Eastern Java. That Basin is contain the economic hydrocarbon deposit.
The collision between Hindia Oceanic Plate on southside with Eurasian Continental Plate or Sunda Self at north formed some tectonic environment from northwest to southeast partake (picture 6) :
|
1. Trench that see band of active plunge along side Java Island
2. Outer arc non volcanic for example deputized by Mentawai Island, and some small island that parallel with this
3. Fore-arc basin that located between outer arc basin and intra arc basin
4. Volcanic arc that in Central Sumatra deputized by Barisan Mountain
5. Intra arc basin in Central of Sumatra deputized by Ombilin Basin and Payakumbuh Basin
6. Back arc basin in Central of Sumatra deputized by Central Sumatra Basin
Central Sumatra Basin and South Sumatra Basin dissociated by a high land so called Tigapuluh Mountain. Both of that basin initially is one unity of big basin that has many trench and gaben (De Coster, 1974). That basins in the southwest limited by fault and pre-tertiary rock that lifted alongside of bevel in east of Barisan Mountain; at north-east, limited by the end of precipitation of sediment in Sunda Self, and at south and east limited by high land of Lampung and an arc that parallel with East Coast Sumatra.
CHAPTER 3
RESEARCH DATA
Deformation of Strike Slip
Already many reported from former writer hit relationship between deformation strike slip with formed big basins called by pull-apart basin. With same mechanism with formed pull apart basins, Cowell (1974) explain mechanism of formed smaller basins in deformation zone of strike slip fault. In area that is at deformation zone formed many smaller fault that have relationship show pattern in weaved. In the range of that zone happened movement between block that have the character of convergent and than formed the highland, and movement divergent formed lowland called by fault-wedge basins (picture 7a and b).
7a. sketch of map showing upward at fault cause of convergent dextral strike slip (I) and also downward fault block cause of divergent dextral strike slip (II) (Crowel, 1974 op.cit. Syaiful Bachri 2006 )
7b. sketch of map showing transform regime with part formed pull-apart basins (L) and the part that relative go up (H) (Crowel, 1974 op.cit. Syaiful Bachri 2006 )
Study of geology structure area especially based of interpretation of radar image (picture 8), helped with field observation. Map of disseminated rock unit beside based of interpretation radar image also related to map of geology that already available at Silitonga dan Kastowo, 1995, with some modified.
Picture 8. Radar image of Kamang Area
From image radar analysis interpreted existence of couple of strike slip fault that have dextral character. Based its direction and size, this main fault trusted that have closely related or is part or segment of Sumatra’s fault system (picture 9).
Picture 9. Structure map of Kamang Area base on radar image interpretation
In zone that flanked by that main fault there is branches of fault that formed weaved pattern. The dextral convergent movement between block inside caused some lifted go up, whereas divergent dextral movement caused some of block lifted go down form depression area. The part that go down also resulted from existence of normal fault and strike slip fault that have downhill movement elementary body. Athwart cross section of this deformation zone show like horst and graben (picture 10) and also show on outcrop in Westside of Kamang (picture 11).
Picture 10. Scheme cross section crosscut strike slip faulting deformation as long as A-B line (see picture 9)
Picture11. Sketch of structure pattern at Km 47,5 TLK, that showing a group of convergent and divergent strike slip fault
On picture 11 show a number of fault with relative downwardses that pattern form like small horst and graben , but some fault show upwardses movement, as the same manner as seen at right and left. Relative upwardses movement of fault in the left side stand out with movement layer of sand stone, whereas upwadses fault movement on the right side shown by drag form on the layer of sand stone. As the same manner as interpretation at picture 9, this faults is a group of strike slip fault that have divergent movement until form some lowland, whereas fault on the cross section shown upwardses movement is strike slip fault with upward movement component, or movement with convergent movement.
Fault area that reside in field looks like normal fault, because of existence of normal displacement nevertheless without incisions indication, result of radar image (picture 9) is dextral strike slip fault. In the other word, boundary of small basins that parallel with main fault is dextral strike slip that have divergent component, that happened at paleogen era. Can not found paleogen sediment at the some low area anticipated because of already happened erosion, and since era of paleogen this area already happened lifting and folding as the same manner as shown at geology map Silitinga and Kastowo at 1995.
CHAPTER 4
DISCUSSION
In the Central of Sumatra, between volcanic arc and margin of Central Sumatra Basin found many coal mine that separated and have limited spread. If correlated with theory of forming depression area that correlated with strike slip faulting such as those which has been discussion in front of , so a long of area between volcanic arc and back arc basin, entered north and south of Sumatra, may be can found many small sub basin. That assumption taken because of strike slip fault by Sumatra’s fault happened along of Sumatra Island. That sub basin may be contain coal and chip of oil, like as founded at Central of Sumatra. Not many found coal that separated at the center part of back arc, is not mean that sub basins not happened at area referred. Because at the center of that back arc that initially is relative deeper marine and have bigger degradation than at the edge area, so intensive coal forming is impossible. At the specified period this area experience faulting of block or normal because extensional tectonic at the trench arc system.
Strain regime at paleogen in Sumatra to show with block faulting and graben and half graben forming, like researcher before has not know deeper already. Sumatra’s fault that parallel with zone of plunge now and also tertiary subduction zone, related sliver with oblique subduction in West Sumatra territorial water. That fault is the part of south-east Asia fault system, after the collision India continent with Himalaya. This strike slip regionally fault assumed have happened at paleogen that cause many of small sub basin forming in that zone by influence of that fault movement.
Convergent movement pattern in strike slip fault zone cause formed horst structure, that on the vertical cross section will show like positive flower structure. On the contrary, divergent pattern movement will produce notching dilasional have the shape of small basin that in the vertical cross section will show like negative flower structure. Existence of positive and negative flower structure pattern in Central Sumatra Basin drawn at data record of seismic like ever said by Yarmanto and Aulia at 1989.
CHAPTER 5
CONCLUSSION
Strike slip fault deformation has been done since Paleogen at West Sumatra and Jambi that south west outskirts Central Sumatra Basin. Deformation of sub basins that contain paleogen sedimentary rock more resulted by divergent movement of dextral strike slip fault, that formed because of dextral transtensional fault of Sumatra compared to extensional tectonic that resulted block faulting as do in back arc basin as same manner as said by researcher previously. Some faults with fault area can be parallel, in field show displacement indication that adversative (normal and go up). Result of this research show that a group of strike slip fault in certain segment has divergent pattern, in other segment can be convergent pattern.
REFERENCES
Bachri Syaiful ,2006. Peran Deformasi Pensesaran Mendatar Terhadap Pembentukan Beberapa Cekungan Kecil Paleogen di Sumatera Barat dan Jambi. Jurnal Survei Data Geologi vol. XVI No. 4 July 2006 page 232 – 240.
INTERNET
http://www.bgl.esdm.go.id/dmdocuments/
APPENDIX
Geological map on Central Sumatra :
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Structure analysis of research area :
