Structural Evolution of the Jambi Sub-basin

Jakarta06-OT-60

PROCEEDINGS, JAKARTA2006 INTERNATIONAL GEOSCIENCES CONFERENCE AND EXHIBITION Jakarta, August 14 – 16, 2006 STRUCTURAL EVOLUTION OF THE JAMBI SUB-BASIN: A ROTATED STRIKE-SLIP MECHANISM Margaretha E. M. Purwaningsih* Bambang Mujihardi** Lilik Prasetya*** Wah Adi Suseno**** Yarra Sutadiwiria*****

ABSTRACT

The activation of the great lateral dextral Sumatran strike slip fault related to the structural evolution in the Jambi sub-basin. Based on the strain ellipsoid analysis, it was recognized that the structural evolution of the Jambi sub-basin included three orders. First order was occurred when Sumatran strike slip was active in the southern part of South Sumatra basin at the Pre-Tertiary time. The WNWESE Lematang fault was resulted as a synthetic right lateral strike-slip fault relative to the Sumatran fault. The other ones, the NNE-SSW Setiti-Tembesi fault was active as an antithetic left lateral strike slip fault. The Kikim fault and Lembak fault were formed as an antithetic fault relative to the Lematang major fault on the second order at the Eocene to Oligocene time. The third order of the structural evolution at the Middle Miocene to Pliocene time was dominated by compression regime that results the NW-SE thrust and reverse fault. The structural evolution of Jambi sub-basin showed the block rotation of 45-degree clock wisely relatively to the great Sumatra strike slip fault. INTRODUCTION

The structural evolution of Tertiary basin, especially on the eastern part of Sumatra Island was started by rifting system because of the extension and continental thinning at the Eocene time. Morley (2002b) suggested that the basin formation in the Sumatra Island was occurred because of subduction rollback that was assumed as a resistance to the compression force causing rifting rifti ng and basin * ** *** **** *****

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formation. The models explained the breaking up of continental crust and move away by strike slip faults. Davies (1984) reported that Sumatra basins were formed as pull-apart among wrench fault with interchange orientation as effect of rotary movement Sunda Plate with some varies direction and subduction rate along Sumatra Island. Wortel and Cloetingh (1986) suggested that compression force was caused by the northward subduction of Indian oceanic plate below Sunda Plate, ensue the  N-S oriented basin formation was occurred. Detail analysis of fault lineation (normal fault and thrust fault) on the top of Talang Akar and Air Benakat formations was used statistically and the result was displayed on the roset diagram. Principal of strain ellipsoid was used to understand the structural history of the basin. The model of structural kinematics in the Jambi sub-basin can be explained using strain ellipsoid. STRUCTURES OF THE SOUTH SUMATRA BASIN Structural Grains

The NW-SE trending cross section of JA-16 seismic line shows the major normal fault dominated the structural grains in the Jambi sub-basin forming the series of half graben (Figure 1). The thickening of Talang Akar and Lahat formations were showed in the eastern part of major normal fault as synrift deposits. The Baturaja limestone followed the deposition which is showed by strong reflector on the top formation and underlain the postrift Gumai Formation. The anticlinal folds in the upper section indicated the inversion tectonic at the time deposition of Gumai to Air Benakat formations. Seismic line of JA-02 and JA-02W on the northern  part and parallel to the seismic line J A-16 shows the

segmentation of basement block going up and down that was caused by the extension of basin formation (Figure 2). There is no significantly impact of the inversion tectonic to the structural development in the northern part of Jambi sub-basin. Fault Trends

Detail analysis of the fault lineament (normal fault and thrust fault) was done to understand the stress regime forming structural evolution. Time structure map of the basement, Talang Akar, and Air Benakat formations are used for analysis. The trend of structural lineament was plotted in the roset diagram. a. Talang Akar Time Structure Map

Totally 43 faults of the Eocene to Oligocene Talang Akar Formation can be mapped in the Jambi sub-basin which are consisted of the normal and thrust fault. Thirty-two normal faults (74.4% of 43 faults) dominated the structural  pattern in the northern part of Jambi sub-basin that has trending range from N 0 0 E to N 150 E or  NNE-SSW direction. The remaining of thrust fault system occupied 25.6% of total faults dominantly the N 15 0  W to N 30 0  W or NNWSSE trending. The dominantly normal faults in the South Sumatra basin indicated the extension stress regime was acted as a major stress. b. Air Benakat Time Structure Map

Totally 27 faults of the Middle Miocene Air Benakat can be mapped in the Jambi sub-basin which are dominated by thrust faults. Twentyfour thrust faults (88.9% of 32 faults) dominated the structural pattern in the Jambi sub-basin that has trending range from N 45 0  E to N 60 0  E or  NNW-SSE direction. There are only three normal faults (11.1% of 32 faults), that were occupied the Middle Miocene structural pattern in the Jambi sub-basin. The dominated of NNWSSE trending thrust faults that mostly  perpendicular to the normal faults indicated compression regime at the Middle Miocene to Pliocene time. EVOLUTION OF THE STRUCTURES

Based on the seismic and structural map analysis, and reconstruction using strain ellipsoids, the structural evolution in the Jambi sub-basin can be recognized. There are three stages influencing the structural history or development.

a. First Order (Pre-Tertiary)

The first structural evolution was taken place when Sumatra right lateral fault moved actively in the southern part of the South Sumatra basin at the Pre-Tertiary. A couple of Lematang and Setiti-Tembesi fault were formed in this period. The right lateral Sumatra fault acted as the  principal displacement zone (PDZ) resulted the WNW-ESE and NE-SW trending faults. Within this period, the WNW-ESE trending Lematang fault was formed as synthetic right lateral strike slip fault relative to the Sumatra major fault. The  NE-SW trending Setiti-Tembesi fault was formed as antithetic left lateral strike slip fault relative to the Sumatra major fault (Figure 3). These faults developed a stress couple between normal and wrench fault.

b. Second Order (Eocene – Oligocene)

The second stage of structural evolution in the South Sumatra basin was occurred when the Lematang and Setiti-Tembesi faults acted as  principal displacement zone (PDZ) at the Eocene to Late Oligocene. In the southern part, the NESW rifting results relatively WNW-ESE horst and graben along the Lematang fault (Lematang Trend Fault) (Figure 4). The N-S Kikim, Klingi and Lembak faults developed as antithetic fault in the eastern tip of Lematang Fault at the second order. The PDZ of Setiti-Tembesi fault in the northern part formed the NNE-SSW horst and graben actively that dominated the Eocene to Late Oligocene structural pattern in the Jambi sub-basin (Setiti-Tembesi Trend Fault) (Figure 5). The extensional phase forming the relatively WNW-ESE and NNE-SSW horst and graben would be fill the thick the Eocene to Late Oligocene deposits.

c. Third Order (Middle Miocene – Pliocene)

The third structural evolution was occurred at the Middle Miocene to Pliocene time dominated by the NW-SE thrust and reverse faults that implication of compression phase (Figure 6). The compressional phase in the South Sumatra  basin formed the relatively NW-SE en echelon folds that occurred on the younger section. The Setiti-Tembesi strike slip fault also reactivated the older faults forming a stress couple of normal fault but it is not significantly. The dominantly thrust or reverse faults proved that the

compressional regime occurred in the third order at the Middle Miocene to Pliocene time. The structural development in the Jambi sub-basin that taken place during three stages reflected a rotation phenomenon at about 45 0  clock wisely relative to the Sumatra strike slip fault. CONCLUSIONS •

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The NNE-SSW trending normal fault dominated the structural geology in the Jambi sub-basin at the Eocene to Late Oligocene time. The NW-SE trending thrust and reverse faults dominated the active compressional regime at the Middle Miocene to Pliocene time relating the inversion tectonic. Two structural trends can be recognized that are Lematang trend and Setiti-Tembesi trend. The structural development involved three stages: (1) Pre-Tertiary Sumatra Fault period in the Pre-Tertiary, (2) Lematang and SetitiTembesi Fault period during the Eocene to Late Oligocene, and (3) North Jambi period in the Middle Miocene to Pliocene. The structural development from the first stage to the third stage shows a clockwise rotation in magnitude of 45 0  relative to the Sumatra great strike slip fault.

ACKNOWLEDGEMENTS

The paper is a summary of structural studies conducted by the authors when they studied in the Physic Department UI. The authors would like to give many thank to Mr Basuki Puspoputro who has supervised and given valuable discussion, guidance and suggestions during preparation this paper, and also made to our friends who provided very helpful input and support in order to finalize the paper.

REFERENCES CITED

Davies, P. R., 1984. Tertiary structural evolution and related hydrocarbon occurrences, North Sumatra Basin: Proceedings Indonesian Petroleum Association, 13th. Annual Convention, p. 19-49. Morley, C.K., 2002b. A tectonic model for the Tertiary evolution of strike-slip faults and rift basins in SE Asia: Tectonophysics, no. 347, p. 189-215. Pertamina/BEICIP, Indonesia.

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Wortel, M. J. R., and Cloetingh, S. A. P. L., 1986. On the dynamics of convergent plate boundaries and stress in the lithosphere. In Europe Union Geoscience, et. al., Origin of Arcs, International Conference in Elsevier Development Geotectonics Series, No. 21, Urbino, Italy.

Figure 1  - Seismic line 16JA(2) The structural model across Jambi sub-basin shows the right lateral that act as antithetic strike-slip relative to the Setiti-Tembesi major strike slip fault (modified after Pertamina/BEICIP, 1992).

Figure 2 - Seismic line JA-35 across the northern tip of Jambi sub-basin shows the left lateral strike slip that act a synthetic strike slip relative to the Setiti-Tembesi major strike slip fault (modified after Pertamina/BEICIP, 1992).

Figure 3 - Strain ellipsoid shows the first order of structural evolution in the South Sumatra basin.

Figure 4 - Strain ellipsoid shows the second order of structural evolution in the Lematang area, closed to the Palembang sub-basin.

Figure 5 - Strain ellipsoid shows the second order of structural evolution in the Setiti-Tembesi areas, Jambi sub-basin basin.

Figure 6 - Strain ellipsoid shows the third order of structural evolution in the Setiti-Tembesi areas, Jambi sub-basin.