Gravity Field and Structure of the Sorong Fault Zone Eastern Indonesia by Sardjono Thesis Submitted for the Degree of Ph.D. Department of Geological Sciences University College London University of London June 1998 ProQuest Number: 10042723 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10042723 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT Gravity surveys along coastlines of islands in the region Banggai-Sula, Eastern Sulawesi, Halmahera, Bacan and Obi were carried out as part of the Sorong Fault Zone Project. Results of the Surveys were integrated with gravity data previously acquired by other projects, including on-land gravity data from the Bird Head area Irian Jaya (Dow et al 1986), Seram Island (Milsom 1977), Bum Island (Oemar and Reminton 1993) and Central Sulawesi (Silver et al. 1983) as well as marine gravity information within and surrounding the Sorong Fault Zone (Bowin et al. 1980). Gravity expeditions of the Sorong Fault Zone Project also include measurements in Mayu Island and the island group of Talaud, situated further north in the Central Molucca Sea region. A total of one hundred and forty two gravity data were acquired in the region of Banggai-Sula islands, forty seven in eastern part of Central Sulawesi, about four hundred in Halmahera, Bacan and Obi, and seventy nine in Mayu and Talaud. Surveys in the eastern part of Central Sulawesi were carried out for the purpose of tieing the older gravity data obtained from Silver et al. (1983) and the more recent data of the Sorong Fault Zone Project. About one thousand thirty hundred and thirty gravity data were acquired as part of the Irian Jaya Geological Mapping Project (IJGMP) in the period of 1978-1983, a project commissioned by the Indonesian Geological Research and Development Centre (GRDC) and the Australian Bureau of Mineral Resources (BMR). The remoteness of the survey areas of the Sorong Fault Zone Project necessitated a careful planning for travel arrangements and provision of logistics. A wide range of magnitude of gravity field was observed in the Sorong Fault Zone, extending from values below -250 mGal recorded in the southern part of the Molucca Sea to values in excess of +320 mGal measured near to sea level in the coastal areas south of Mangole and north of Sulabesi, the two islands of the Sula Group. Steep gradients of free-air gravity were observed in south of Mangole (about 13 mGal/km) and west of Obi (about 15 mGal/km) but elsewhere were gentler. Analyses of gravity data along the Sorong Fault Zone in the region of Bar ggai-Sula Islan ds controlled in part by geological, reflection seismic and sidescan sonar data, have produced four models which suggest that the crustal structures beneath the zone consist predominantly of attenuated continental fragments, juxtaposed to thick layer of tectonic mélange and anomalous oceanic crusts. The continental fragments appear to be severely attenuated and limited in extent in the east but thicker and wider towards the west. The tectonic mélange is underlain by deep seated oceanic crust in the Molucca Sea region. The anomalously thin North Banda Sea crust appears to underlie a very thin layer of sediments and to have suffered some degree of arching. The deep seated oceanic crust and the thick layer of tectonic mélange are interpreted as the resuh of the shiMng of the lithospheric plate of the Molucca Sea. The descent of this :ç,hte :T.iy have producced bending forces which may have initiated flexure which propagates through the surrounding region. Depending on the rigidity of the crustal slab, arching and fracturing may have occurred in the crustal rocks. The arching of the oceanic crust of the North Banda Sea may have been one result of this process. The continental fragments of the Banggai-Sula region appear to dip northwards and this may, in addition to the effect of shear tectonics along the Sorong Fault Zone, also be interpreted as the response of the continental fragments to the sinking of the lithospheric plate of the Molucca Sea. In the Obi region, the gravity data suggest that most of the island is underlain by peridotitic and basaltic rocks. Continental crust appears to form the basement in the south and extend offshore south of the island and juxtaposed to oceanic rock. The ultramafrc and basic rocks appear to be emplaced on Obi by a high angle reverse fault which separates the continental block in the south from the oceanic material in the north. The exposed basaltic rocks could be a remnant of the oceanic crust of the Philippine Sea Plate. Ill ACKNOWLEDGMENT Financial support for this project was provided by the University of London Consortium for Geological Research in Southeast Asia. Equipment and personnel for gravity expeditions were provided by the Geological Research and Development Centre (GRDC), Bandung, Indonesia. My appreciation is due to Professor Robert Hall for bringing me to join the research team of the Sorong Fault Zone Project. My thanks are also due to Dr. Rah Sukamto the former director of GRDC and his successor. Dr. Irwan Bahar, who have granted me a temporary leave from the service. I truly thank Diane Cameron, the Administrator of the Southeast Asia Research Group, for helping me with all aspects of formalities. My thanks are also due to all academic and administrative staff as well as all members of the Birkbeck College and University College London Research School of Geological and Geophysical Sciences who have provided personal and professional assistance towards the completion of this study. Finally, my sincere gratitude is due to Dr. John Milsom for initiating and supervising this project. Dr. Milsom has always provided valuable assistance and practical advice in all aspects of this study and has spent a considerable amount of time in reading, criticising and subsequently making corrections to this thesis. IV CONTENTS ABSTRACT ii ACKNOWLEDGMENT iv CONTENTS V LIST OF FIGURES viii LIST OF TABLES x CONTENTS Chapter 1 INTRODUCTION 1 1.1 Background 1 1.2 Gravity Expedition in the Sorong Fault Zone 3 1.3 Organisation of the Thesis 10 Chapter 2 THE PRINCIPAL FACTS 15 2.1 Observed Gravity 15 2.2 Longitude and Latitude 18 2.3 Height 19 2.4 Accuracy of the Gravity Map 22 Chapter 3 GRAVITY REDUCTION AND INTERPRETATION TECHNIQUES 27 3.1 The Reduction of Gravity Data 27 3.1.1 The latitude correction 32 3.1.2 The free-air correction 3 3 3.1.3 The Bouguer correction 3 6 3.1.4 Terrain correction 39 3.1.5 Isostatic effects 42 3.1.6 Geological correction 43 3.1.7 Correction for shipbome observations 44 3.2 Interpretation Techniques 45 Chapter 4 TERRANE GEOLOGY OF THE SORONG FAULT ZONE 51 4.1 Introduction 51 4.2 Fault Strands of the Sorong Fault Zone 53 4.2.1 Sorong Fault 53 4.2.2 Koor Fault 54 4.2.3 Molucca-Sorong Fault 55 4.2.4 North Misool-Sorong Fault 56 4.2.5 Bum Fracture 56 4.2.6 Seram Trough 57 4.2.7 North Sula-Sorong Fault 57 4.2.8 South Sula-Sorong Fault 58 4.3 Oceanic Terranes ‘jg 4.3.1 Molucca Sea Collision Zone 59 4.3.2 North Banda Sea 60 4.4 Arc Terrane of the Philippine Sea Plate 62 4.4.1 West Halmahera-Tamrau Terrane 62 4.4.2 East Halmahera-Waigeo Terrane 63 4.4.3 Arfak Terrane 64 4.5 Continental Terranes 65 4.5.1 Banggai-Sula Platform 65 4.5.2 Buru-Seram Microcontinent 67 4.5.3 Misool Terrane 68 4.5.4 Kemum Terrane 68 4.5.5 Netoni Terrane 69 4.6 Amalgamated Terrane 69 4.6.1 East Sulawesi Terrane 70 4.6.2 Obi Terrane 70 4.6.3 Bacan Terrane 71 Chapter 5 SIDESCAN SONAR, SEISMIC IMAGES AND SEISMICITY 86 5.1 Sidescan Sonar 86 5.1.1 Eastern arm GLORIA coverage 88 5.1.2 Central region GLORIA coverage 91 5.1.3 Northern Arm GLORIA coverage 92 5.1.4 Southwestern arm GLORIA coverage 93 5.1.5 Southeastern arm GLORIA coverage 94 5.2 Seismic Images 95 5.2.1 Seismic images Segment 1 96 5.2.2 Seismic images Segment 2 97 5.2.3 Seismic images Segment 3 98 5.2.4 Seismic images Segment 4 98 5.2.5 Density analyses based on velocity of seismic wave 98 5.3 Seismicity 99 Chapter 6 GRAVITY FIELD AND STRUCTURE OF THE SORONG FAULT ZONE, EASTERN INDONESIA 114 6.1 Introduction 114 6.2 Provinces of Gravity Anomalies in Sorong Fault Zone 115 6.2.1 Gravity province of Kepala Burung (1) 116 6.2.2 Gravity province of the North Banda Arc (2) 117 6.2.3 Gravity province of the North Banda Basin (3) 119 6.2.4 Gravity province of Southeast Arm Sulawesi (4) 121 6.2.5 Gravity province of East Arm Sulawesi 122 6.2.6 Gravity province of the South Molucca Sea 123 6.2.7 Gravity province of Halmahera (7) 126 VI 6.2.8 Gravity province of Obi ( 8 ) 128 6.2.9 Gravity province of the Sula Group (9) 128 6.2.10 Gravity province of the Banggai Islands (10) 131 6.3 Structure of the Sorong Fault Zone 132 6.3.1 Crustal structure of Western Mangole Island 134 6.3.2 Crustal structure of the shelf region West Taliabu 138 6.3.3 Crustal structure of the Banggai Islands 141 6.3.4 Crustal structure of the Obi region 146 Chapter 7 CONCLUSIONS AND SUGGESTIONS FOR FURTHER STUDIES 189 REFERENCES 192 APPENDICES Appendix A Gravity anomalies in Talaud Islands, Central Molucca Sea 200 Appendix B Gravity data reduction spreadsheets 204 B.l.