Interpretation of gravity data over central Jawa, Indonesia

z-*--\'qî

The

University of

Adelaide Department of Geology and Geophysics

Interpretation of Gravity Data Over Central Jawa, Indonesia

Indra Budiman June 1991

A

thesis submitted to the

University

of Adelaide

in fulfilment of

the requirements

for

the degree

of

Master

of

Science.

Contents

Statement

Acknowledgement

L lntroduction

1-1

^The

Location of

the Study Area . . . . ^.

I-2

^Potential

for

^Research

X

X1

1-3

r-4

1-5

Aims of

the Research The

Previous'Work

The Organisation

of

the Research

Work

,.1 .1

..1

.5 .6 ..8 1-6

^Outcome

of

the Research

.10

2 Geologicallnformation

^.

..

^.

2-I

Introduction

2-2

^{Tectonic and} Geological Information on the Jawa Region

2-3

Geological

Information

on Central Jawa ^.

2-3.1 Physiography

.

..

2-3.2 Statigraphy

2-3.3

Tectonic

Setting

. . . . ^.

2-4

Geological

Information

on Four Areas of Interest

2-4.1 Lukulo Area

. . ^.

2-4.2

West Progo Mountains Area ^.

2-4.3

Ungaran Volcano Area

11

11

12

.14 .14 .18

..

¹⁸

..22

..22

..

²⁵

..26

..

²⁶

2-4.4

Merapi Volcano Area

CONTENTS

3

ii

.34

.

.34 .35

.36 ,.31 .40

.

.41

.

.4r

.

.44 3-3.1

Seismic

Datalnformation

^{. . . . .}

3-3.2 AtrborneMagneticDatalnformation ' ' ' ' ' '

^'

4

^Physical

ProPertY of

^the

Rock

4.I Introduction

4-2

^Procedure

4-3

^Density

of

^{the Rock}

4-4

Density of the Rock

in

Jawa and Madura Islands

5 The Evaluation of Gravity Data, Reduction and correction

^{. . .}

3-2.L

^{The Sunda}

Arc

^{Gravity Data}

Information

'

3-2.2 lawaGravity

^Data

Information

^.

3-2.2.1

Central Jawa Gravity Data

Information ' ' '

^'

Other Geophysical

Information

^{. .}

Introduction....

The Central Jawa GravitY Data

Gravity Data Reduction ^and Correction

5-3.1

Bouguer

Correction

^.

5-3.2 Tenain

Cor¡ection _{.. .} ^.

6 Computer

Processing and

Filtering Operations

Geophysical

Information

^{. .}

3-I Introduction....

3-2 Gravity Information

^{. . .}

5-J

5-1 5-2 5-3

6-1 6-2 6-3 6-4 6-s

Introduction...

.47 .47 .41 .48

.50

.54

.

.54

56 56 58 59

....61 ....61

.

...

⁶²

....66

....

⁶⁶

....69

Digitizing Gravity Data

. ^.

Digitizing Airborne

^Magnetic

Data

. . . . ^. Image

Artificial lllumination

^(Shaded

Relief)

. ^.

CONTENTS ilt

6-6 Filtering

Operations. ^.

6-6.1 Upward

Continuation

6-6.2

Trend Surface Residual

6-6.3 SpectralAnalysis

. . . . ^.

7 Interpretation

7

-2

The Qualitative Interpretation ^.

7-2-l Lukulo

Area

7-3 7-4

14 75 75 89

7-I Introduction....

...92 ...92 .. .93

..

¹⁰⁴

.rc]

..107

108 111

r12

113

7-2.2

West Progo Mountains Area ^.

7-2.3

Unga¡an Volcano Area

7-2.4

Merapi Volcano Area The Quantitative

Interpretation

. ^. Computer

Gravity Modelling

7-5 GRIN Program.

7-6

^Curve

Matching andAmbiguity Problems

^{. . . . .}

7-7

The Central Jawa Gravity

Modelling 7-1.1 Lukulo

^Area

7-7.2

West Progo Mountains Area ^.

7-7.3

Ungaran Volcano Area

l-'7.4

Merapi Volcano Area

8

Discussion

and Concluding Remarks

113 115 115

i16

116

tt7

I22

8-1 Introduction .

r22

8-2

^Regional

Gravity Anomaly

Pattern

..r23

8-2.1 TheMajorBoundary ...., "124

8-2.2 Typical

Gravity Features Over the Centrai Jawa

Volcanic region '

. .

\26 8-2.3

^The Causative Body Over Central Jawa Volcanic

Regions

^{. .}

.

¹²⁷

The Deep Structure Over Central

Jawa ' ' ' ' I29

8-3

CONTENTS iv

8-4

Concluding Remarks

Appendix A: Operating instruction for GRIN program

I Introduction

tr

^General

DescriPtion

. . ^.

III

^{Manual Commands}

rV

^{Graphical Commands}

V

Appendix B: Central Jawa 'raw gravity data'

References

. .

.¡'-2

.4.14

A-1

B-1

133

Demonstration

Tutorial

^'

A-19

List of Figures

1-1.

The location of the study area . ^.

The complex movements

of

the three great plates of the Indonesian region . . ^. The regional isostatic anomalies

of

Indonesian archipelago ^.

Flow

chart diagram

of

the organisation of the research

work

^.

I-2.

L-3.

t-4.

2 J 1 9

2-1.

The geological map

of

central Jawa ^.

2-2.

The physiographical map of

Jawa

^.

2-3.

The zone

division of

the physiographical map

of

Jawa

2-4.

The cross section

of

the

Lukulo

area . ^. z-5.

2-6.

2-7.

2-8.

2-9.

The schematic migrations

of

subduction zone during

Tertiary

The location

of

the

four

areas of interest include their cross sections Cross section

of

the'West Progo Mountains area

The development of

volcanic

structures

of

the transverse

row Ungaran-Merapi

. . ^.

Block

diagrams

of

the old Ungaran volcano

2-10. C¡oss section

of

the Ungaran volcano 2-11. Cross section

of

the Merapi volcano ^.

...13 ...15 ,...16 ..,...19

23 24 28 29 30 3T 32

3-L.

^The

simplified

^Bouguer anomaly contour map

of

Jawa and Madura

islands

^.

3-2.

^The general structure pattern map of Jawa and Madura islands as interpreted on the basis of the Bouguer anomaly map . . . ^.

3-3.

^The seismic velocities

of

crustal and uppermost mantle elements across the Jawa trench subduction system

3-4.

^Section

from

the Jawa trench crossing central

Jawa

^.

3-5. Thesimplifiedaeromagnetic contourmapof

theJawatransect

aIea..

^-

-....45

38

39

42 43

LIST OF FICURES vt

4-I.

The location map

of

the

rock

samples

which

were collected

from

Jawa and Madura islands

5-1.

^The

gravity

^{station positions} over central Jawa

The

original simplified

hand contoured Bouguer anomaly map

which

is used as the starting point

of

this resea¡ch . . ^.

The Bouguer anomaly map constructed by

digitising

the hand contoured map prepared by the GRDC. The mismatch between two maps is evident

The Bouguer anomaly contour map of the study area which is produced on the basis

of

the 'new data

file'

using the

individual glavity

stations The Bouguer anomaly grey-scale

digital

image over central Jawa

Total magnetic anomaly

gey-scale digital

image over Jawa Eansect area .

'

'

Shaded

relief

grey-scale

digital

image over central Jawa

with

..

⁵¹

6-1.

6-2.

6-3.

6-4.

6-5.

6-6.

55

63

64

65 67 68

elevation angle of 90o and azimuth 0o

6-1.

^Shaded

relief

grey-scale

digital

image over central Jawa

with

elevation angle

of

45o and azimuth

0o

^.

6-8.

^Shaded

relief

grey-scale

digital

image over central Jawa

with

elevation angle

of

45o and azimuth

1800

. ^.

6-.9.

Shaded

relief

grey-scale

digital

image ovef central Jawa

with

elevation angle of 45o and azimuth 30o

6-10 to

6-16

BouguerAnomalyMaps

6-10. Central Jawa upward continuation

with

elevation 500 metres 6-11. Central Jawa upward continuation

with

elevation 1000 metres 6-12. Central Jawa upward continuation

with

elevation 5000

metres

. . ^. 6-13. Central Jawa upward continuation

with

elevation 10000 metres . : . ^.-.

6-14. Central Jawa trend surface residuai

lst

orde 6-15. Central Jawa trend surface

residual2nd

^orde 6-16. Central Jawa frend surface residual 3rd orde

10

1I

72

t5

.

..16

..

⁷⁷

..

⁷⁸

...19 ..80 ...81 ...82 ...83

6-17. Western central Jawa trend surface residual 1st orde

LIST OF FIGURD,S vil

6-18. V/estern central Jawa trend surface residual2nd _{orde '} 6-19. Western central Jawa trend surface

residual3rd

orde _' 6-20. Eastern cenfral Jawa trend surface residual 1st orde . _' 6-21. Eastern central Jawa trend surface residual 2nd orde '

6-22.Eastem central Jawa trend surface residual 3rd orde

6-23. Centrai Jawa mean depth continuation

with

deep effect 11000 metres

7-7.

7-2.

7 -3.

1-4.

7 -5.

1-6.

1-7.

7-13. The

Lukulo gravity

^model

7-I4.TheWest

^Progo Mountains gravity model 7-15. The Ungaran volcano

gravity model

. . . ^. 7-16. The Merapi volcano

gravity

model

8-1.

The subduction lineament during the Cretaceous time

8-2.

The structural gravity model over central Jawa

84 85 86

.87

88

and shallow effect 2500 metres . . . ^.

The central Jawa Bouguer anomaly contour map

with

trend east-west

direction

^{. .}

.

95 Group

of

anomalies ^as

divided

on the basis of the amplitude anomaly

values

. . .

'

^{- 96}

A

strong steep gradient anomaly feature aiong the major

boundary

^{- - - 97}

The very high Bouguer anomaly

group ' ' ' '

⁹⁸

The very

low

Bouguer anomaly

group ' ' ' ' '

⁹⁹

The position

of

active volcanoes

in

central

Jawa . ' ' ' ' '

¹⁰⁰

The

A

gfoup occupies northern part

ofthe

strong steep gradient and the

B

group 91

at the southern part of that . . . ^.

7

-8.

The

A

and

B

groups

with

their subgroups

1-9.

^The

Lukuto

Bouguer anomaly conrour map with profrle

line

^.

7-10: The'West Progo Mountains Bouguer anomaly contour map

with profile line

^.

7-i

^1. The Ungaran volcano Bouguer anomaly contour map

with profile line

. .

' '

'

l-I2.'lheMerapi

volcano Bouguer anomaly contour map with

profile line

. _'

r02

..

¹⁰³

,..

¹⁰⁵

..

¹⁰⁶

...109

..

¹¹⁰

118 119

r20

T2T

..r25

..

¹³⁰

LIST OF FICURES

Appendix

A

1.

^{Axes used by}

GRIN 2.

^Typicat

MODEL PLAN

3. TypicaIPLOTPROFILE .. :...

4.

Viewpoints

for

the Profrle Plot

5. READ DATA form

^.

6. REGIONAL FIELD

^data

form

^{. . .}

7.

^SETÆDIT

BODY

^data

form 8.

^Data

form for

^a

2-D dYke

^.

9. Defînition of a2-D dyke

^.

10. Data

form for a2-D

^polygonal

prism

. _' 11.

Definition of

aZ-D polygonal prism ^{. . .}

l2.Dataform for

a 3-D rectangular prism . . ^. 13.

Definition of

^a 3-D rectangular prism ^{. . .} 14. Data

form for

^a 3-D polygonal prism ^. . . ^. 15.

Definition of

^a 3-D polygonal

prism

. . . .

16.

PROFILE

SCALES data

form

17.

PLAN

SCALES data

form

. ^. 18.

LX-80 RESOLUTION

^data

form

A-23 A-24

A25

A27 A28

vilr

A-34

. ^.

A-26

.

A-29

A-30

. A-31

A32 A33

A38

List of Tables

2-1. The stratigraphy

of

the central Jawa . ^,

2-2.The

age

of

the West Progo Mountains

4-1. The result of the density measurement of the central Jawa

rock

samples

Appendix

A

1.

^{Key menus}

flow

chart defined

by GRIN

and their relationship Example

of

a data

file

. ^.

Significance and legal ranges of parameters

for

dykes and rectangular prisms _' Significance and legal ranges

of

parameters

for 2-D

and3-D polygonal prisms

20 27

52

A-39 2.

-

J.

4.

. .

A-40

.

^A-41

. A-42

lx

Statement

This

thesis contains

no material which

has

been

accepted

for any other

^degree

or diploma in any university and, to the best of my knowledge, no'material previously

published or written

by

another person, except where due reference is made

in

the

text'

I consent to the thesis being made available for photocopying and loan

where applicàble

if

accepted

for

the award

of

the degree-

Indra Budiman

June, 1991

x

Acknowledgements

There

a-re

many people who should be thanked for their helpful

contributions throughout

this

research. Professor

David Boyd who

supervised

the work from start

to

finish and for the benefit of his geophysical insight, experience and particularly phylosophical way of ttrinking. Dr. John Paine and Mr Andy Mjtchell are gratefully

acknowiedged

for

^access

to

their excellent software programs on the computer.

I would like

to thank

my

colleagues,

Dr.

Shanti Rajagopaian, Robert

O'Dowd, Andrew Lewis,

Zhiqun

Shi,

Shaohua

Zhou and Nasir Syarif for their

discussion

and help on many

occasions, especially on computer ¡elated matters, and

for

permission

to

use many

of their

programs.

Also Dr.

John Fodden,

HamdanZ. Abidrn, Ukat

Sukamta and

M.

Syah

Arifin for

their helpful disscusion on geological related matters.

Other people

from the

department

that

^shouid

be

thanked

are Dr. Bill

^{Stuart, the}

Director

of

the

National

Centre

for

Petroleum Geology and Geophysics

for

the opportunity

to attend the 1988

courses.

John Willoughby, Phil McDuie and Rick Barret for

^their

technical assistance

of

the computer

facilities.

'

_{Numerous people}

from

_{outside the} Department of Geotogy and Geophysics were also

involved, these include

Dr.

Mohamad

Untung

former

Director of

the

Geological

Resealch and Development Centre, Indonesia who encourage the author to take this subject, Dr- Rab.

Sukamto the

Director of

the Geological Research and Development Centre, Indonesia

for giving

me leave

of

absence

from

my

duty

at that institute

for

the whole period

of

this study, and

in

permiting me to use the central Jawa gravity data

for

this research-

Mrs

Janice Laurie

from Advisory

Centre

for University

Education. The Australian International Development Assistance Bureau

for

^{granting me a} research scholarship.

Most

of ail, I

am

very

^gratefui

to my wive

and

my child¡en for their

sacrifice, ^great understanding and being patient during

my

study time in Adelaide.

xl

Abstract

Central Jawa, Indonesia,

is

a complex region resulting

from

the

collision of

the south eastern margin of Eurasia and the northward

drift of

the Australian continent, together

with

the westward

drift of

the Pacific plate.

The Bouguer anomaly contour map used

in

this study is the most detailed

gravity

map

of

central Jawa

yet to be

interpreted.

This

contour map was produced

on

the basis

of

the Bouguer reduction density

value

o12.61 _Erlcm3. However, the result

of

the Parasnis straight line method

for

Bouguer reduction density of the northern part of Purwokerto indicates ^that the density value

of 2.25

^{gr/cm3 is more suitable.} The interpretation

of this gravity

study is restricted

by certain limitations, for

example,

the actual gravity

observations

were

^not

available and there was very

limited

geophysical

contoi from

other methods.

However,

the results provide the starting

point for

further gravity study over cenlral Jawa.

The gravity map shows a major

boundary

which strikes in a roughly

east-west direction;

in

the Kebumen (Karangsambung) area this boundary coincides

with

the outcrop

of

a major

fault which

separates the Eocene basement

in

the south

from

^{younger sediment} in the north. This boundary is displaced by a north-west trending structure'

The

major

volcanoes

in

central Jawa such as the Slamet, Merbabu, Merapi, Ungaran and Rogojembangan volcanoes

are

along an east-west

line which is parallel to

the major fault descdbed in the above paragraph but displaced 30

km

north of

it.

The map shows that there are

two

types

of major

volcanoes, those

which

^{produce a} strong

gravity

anomaly and those

which

do not.

At

the Ungaran volcano, a

circular

feature

gravity

anomaly

with positive

values

reflect

^{a dense}

body

^whose

density

^conffast

is

0.5

g/cm3

higher than surrounding rocks. This body has a diameter

of 7 km

and extends

for

a

depth of 2 km

to

^{10 km.}

On the basis

of

the results

of

computer processing,

modelling

and analysis

of gravity

data, the effective interpretation

of

those anomalies provide us

with

better understanding

of

the major structural elements of island arcs.