ANALYSIS ON REVERSE FAULT STRUCTURE OF KENDENG ZONEIN NAMPU AREA, BOYOLALI REGENCY, CENTRAL JAVA

Hafidz Reyzananda, ST

Magister ProgramUPN “Veteran” Yogyakarta

Structural Geology Lab. UPN “Veteran” Yogyakarta

Dr. Ir. Jatmika Setiawan, MTUPN “Veteran” Yogyakarta

Structural Geology Lab. UPN “Veteran” Yogyakarta

Ir. Mahap Maha, MT

UPN “Veteran” Yogyakarta

Abstract

Introduction

Stratigraphic Analysis

Geology Map

Balanced Cross Section Analysis

Objective & Methods

Balanced Cross Section

Result

Acknowledgements

References

Discussion & Conclusions

Geological structure especially in rela�on to thrust fault is strongly related to tectonic compression which results in a deformated area. Kendeng Zone is a back-arc basin which has many thrust faults and folds in Java Island. Kendeng Lane is a fold-thrust lane. A fold-thrust lane is directly adjacent to volcanic arc. Asymmetric to reverse folds as well as thrusts oriented torela�vely west-east direc�on are o�en found and are typical characteris�c of this arc. Reconstruc�on of balanced cross-sec�ons is a very important procedure in making a good geological sec�on in order to get a sec�on close to the real condi�on. This method is very useful in conveying configura�on of geological structure of concerned area more informa�vely and communica�vely. In making balanced sec�on, deep knowledge of stra�graphy, fault sequence, and characteris�cs of faults (McClay, 2003). The restora�on done in sec�on showed that the results of shortened score was 31.30% (2.77 km) with construc�on ra�on of (L'/Lº) scoring 0.68 and strain (ε) of 0.31. The results of sec�on reconstruc�on showed that reverse fault system suitable with reverse fault in study area is the type of trailing imbricates fault.Key Words: Geological Structure, Kendeng Zone, Reconstruc�on, Restora�on

Geological structure is an interes�ng object to study, especially in rela�on to thrust fault since tectonic process happens in the form of compression which causes an area become deformated. Besides, Kendeng Zone is the back-arc basin which has many thrust faults and folds in Java Island.Thrust fault in Kendeng Zone o�en restricts the adjacent rock unit; for example, the contact between Pelang Forma�on and Kerek Forma�on as well as between Kerek Forma�on and Kalibeng Forma�on.The study area is located in the Kendeng Zone, specifically in the Boyolali Area, western Kendeng. At the present day, the Kendeng Zone is restricted by the Quaternary Volcanic Arc to the south and the Rembang Pla�orm to the north. The Kendeng Basin consists of Miocene volcanic turbidite deposits derived from the Southern Mountain Arc. The first upli� of the Kendeng Zone occurred during Late Pleistocene (De Genevraye and Samuel, 1972) due to northward movement of the Australian Plate, resul�ng in shortening of the basin and becoming an east-west trending foldthrust belt. This journal is the summary of the writer's research results supported by the previous writer, which includes: Geological arrangement of the research area, par�cularly studying the characteris�cs of thrust fault to obtain the appropriate cross sec�on model in accordance with the balancing cross sec�on reconstruc�on method.

Regional stra�graphy of Kendeng Zone according to some authors, Pringgoprawiro, 1983 (le�), Samuel and de Genevraye,1972 (mid), Smyth et al, 2005(right).

East Java Physiography map (Van Bemmelen,1949)

Geological Structure lineament analysis map of research area, (A) Before analysis, (B) A�er analysis

Direc�on Pa�er structure in eastern java (modifica�on from Sribudiyani et al., 2013)

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Calcareous-Sandstone

Calcareous-Claystone

Pelang Marl

Fault Contact

Flysch deposit, sandstone dominated

slopeturbidite fans

Flysch deposit, claystone dominated

slope-basinturbidite fans

Marly Claystone

Open marine-deep basin

Marl with calcareous-sandstone and calcareous-claystone intercalation

Marly Claystone

AlluvialDeposits

Stra�graphic propo�onality column from analysis result (author, 2014)

Foreland zone (red box) the tectonics of back arc, overtrust-fold lane forming zone (Boyer and Ellio�, 1982)

Reconstruc�on of balanced cross sec�on is a very important procedure in making good geological cross

sec�on in order to obtain the cross sec�on close to the real condi�on. This method id very useful in

conveying the geological structure configura�on of related area more informa�vely and

communica�vely. In making balanced cross sec�on, deep knowledge on stra�graphy, fault sequence,

and characteris�cs of faults are necessary (McClay, 2003). Balanced cross sec�on is also useful for

examining the validity of obtained structure geometry, including fault model analysis, length of rock layer

and consitency of cross sec�on area (Marshak and Mitra, 1988). One of the main keys in the procedure of

making balanced cross sec�on is cross sec�on restora�on, aiming at finding out the geological condi�on

before deforma�on occurs. The weakness of balanced cross sec�on restora�on in that it ignores strike

Ther results of cross sec�on restora�on in A-A' slide show that thrust fault system belongs to the type of trailing imbrica�on where the accommoda�on of displacement of main fault is distributed to smaller faults in the footwall, so the amount of the magnitude and the direc�on of movement sense becomes consistent (Boyer and Ellio�, 1982). The results of balanced restora�on was then used to do the calcula�on in order to find out the shortening value, contrac�on and strain ra�o in the research area. As for A-A' cross sec�on, the shortening value id 31% (2.7 km) with contrac�on ra�on (L'/Lº) of 0.68 β and strain (ε) of 0.31. (Table).

· Genevraye, P. De, & Luki Samuel, 1972, Geology of The Kendeng Zone (Central and East Java); Proceedings Indonesian Petroleum Associa�on (IPA) 1st Annual Conven�on, p. 17 – 30.

· Marshak, G. & Mitra, S., 1988, Basic Methods of Structural Geology, New Jersey: Pren�ce Hall.

· McClay, K.R., 1992, Glossary of Thrust Tectonic Terms, inK. R. McClay, eds., Thrust Tectonic, Chapman and Hall, p. 419 – 433.

· Pringgoprawiro, H., Sukido, 1992, Geologic Map of the Bojonegoro Quadrangle, East Java, Geological Research and Development Centre (GRDC).

· Sapiie, B., 2004, Short Course Balancing CrossSec�on, Departemen Teknik Geologi ITB, Bandung.

· Smyth, H., Hall, R., Hamilton, J., Kinny, P., 2005, East Java: Cenozoic Basins, Volcanoes and Ancient Basement, Proceedings Indonesian Petroleum Associa�on (IPA) 30th Annual Conven�on, p. 251 – 266.

· Van Bemmelen, R.W., 1949. The Geology of Indonesia, Vol. IA: General Geology of Indonesia and Adjacent Archipelagos. Government Prin�ng Office, The Hague, 732 p.

The author say thank you so much to Juwangi mapping team (Hari Irwanto, Abdurrahman Harits and Satrio Hapsoro) and Structural Geology Laboratory because their help, support and interes�ng discussion during prepara�on of this paper, and thank you so much to my lecturers ( Dr. Ir. Jatmika Se�awan, MT., Ir. Mahap Maha, MT., Dr. Ir. C Prastyadi, M.Sc.). Lastly, thank you for UPN “Veteran” Yogyakarta, especially thank you for the support from Magister Program.

1. Detail geological mapping resulted in five rock units from old to young: Pelang marl unit (Late Oligocene-Mid Miocene), Kerek calcareous-sandstone unit (Mid Miocene), Kerek calcareous-claystone unit (Mid Miocene-Late Miocene), Kalibeng claystone-limestone, and Alluvium Deposits (Holocene)

2. The geological structure developing in the research area consist of upthrust, wrench fault, and folds related to shi�ing-fold belt in foreland zone due to compression tectonic with rela�vity north-south directed stress, which is during the age of Plio-Pleistocene.

3. The reconstruc�on of cross sec�on resulted in upthrust system suitable for upthrusts type in the research area, which is trailling imbricates fault type (Boyer and Ellio�, 1982).

4. The result of cross sec�on restora�on was then used to do calcula�on to find out the shortening value, contrac�on ra�on, and strain in the research area. The results of restora�on in A-A' cross sec�on show that the shortening value is 31.10% (2.77 km) with contrac�on ra�on (L'/Lº) of 0.68 and strain (ε) of 0.31.

There some problems about structure in field research, such as fault and fold, so author can determine the rela�on between geological structure which develeop in field and that structure data can be a reference to make a cross sec�on restora�on

Methods

1. Geologi Structure

Such as force regimes, main tension, geological structure formed, geological structure

analysis such as kinema�cs and dinamics analysis and geological structure formed da�ng

2. Cross Sec�onReconstruc�on, such as restora�on, strain analysis, reverse fault characteris�c analysis and reverse fault system determina�on

Research Area

Research Area

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