The Gravity Field of the Papuan Fold Belt and its Geological Implications by David Harrison Thesis Submitted for the Degree of PhD. Department of Geological Sciences University College London University of London July 1991 ProQuest Number: 10611107 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 com plete 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 10611107 Published by ProQuest LLC(2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 2 Abstract The structure of the Papuan Fold Belt at both local and regional scales has been deduced from the interpretation of gravity data from the fold belt and its foreland basin. A digital terrain model of the fold belt region was created and used in the calculation of terrain corrections for the gravity stations in the fold belt. A computer program was developed to calculate such corrections for data from extensive areas of rugged terrain, and corrections were obtained for over 7000 existing gravity stations. In addition a field survey was undertaken and a further 300 gravity readings were taken in a previously unsurveyed area. Digital filtering techniques, including upward continuation and spectral analysis, were used to separate the regional and residual gravity anomalies, which were then used to constrain the lateral extent of both regional and local geological structures. 2D forward modelling was carried out along five SW-NE and two NW-SE profiles and provided the third dimension to the structural interpretation. In most areas the interpretation of the gravity data confirmed the existing models of the local structure based on geological fieldwork, though evidence of previously unidentified structures, notably in the Muller Anticline region, and beneath the northeastern fold belt, was discovered. On a regional scale it has been shown that the Mesozoic extensional structures of the Papuan Basin have a major controlling effect on the distribution of compressional structures within the fold belt, and on the isostatic compensation of the topographic load of the highlands. 3 Contents Abstract 2 Contents 3 List of Figures 8 List of Equations 12 List of Plates 12 List of Tables 12 Acknowledgements 13 Aims and Introduction 14 Chapter 1 - The Regional Geology of Papua New Guinea 15 Introduction 15 Regional Setting 15 Major Geological Provinces 15 1.3.1 The Southern Plains 18 1,3.2 The Central Orogen 18 1.3.3 Intermontane Depressions 20 1.3.4 North New Guinea Province 21 1.3.5 The Papuan Peninsula 22 1.3.6 New Britain 23 1.3.7 The Melanesian Arcs 23 1.3.8 The Bismarck Sea 24 1.3.9 The Solomon Sea 24 4 1.4 The Geological History and Evolution of Papua New Guinea 25 1.4.1 Mesozoic Geological History 25 1.4.2 Mesozoic Evolution 26 1.4.3 Cenozoic Geological History 28 1.4.4 Cenozoic Evolution 30 Chapter 2 - The Geology of the Papuan Fold Belt 33 2.1 Introduction 33 2.2 The Stratigraphy of the Papuan Fold Belt 33 2.2.1 Basement Lithologies 37 2.2.2 Upper Palaeozoic and Mesozoic Units 38 2.2.3 Cenozoic Units 41 2.2.4 Quaternary Stratigraphy 45 2.3 The Structure of the Papuan Fold Belt 46 2.3.1 Zone 1 - The Muller-Kutubu Zone 46 2.3.2 Zone 2 - The Oksapmin-Tari-Mt Murray Zone 48 2.3.3 Zone 3 - The Porgera-Nipa-Poroma-KarimuiZone 48 2.3.4 Zone 4 - The Laiagam-Giluwe-Kubor Zone 48 2.3.5 Zone 5 - The Darai Plateau 48 2.3.6 Zone 6 - The Aure Tectonic Belt 49 2.3.7 Relationships between Stratigraphy and Structure 49 2.4 The Geological History of the Papuan Fold Belt 50 2.4.1 The Papuan Basin 50 2.4.2 The Foreland Basin 51 2.4.3 The Central Orogen 51 5 Chapter 3 - Gravity Data 52 3.1 The Papua New Guinea Gravity Database 52 3.2 The Lavani-Juha Regional Profile 52 3.3 The Tiengo Gravity Survey 56 3.3.1 Introduction 56 3.3.2 Field Survey 56 3.3.3 Data Reduction 64 3.3.4 Data Presentation and Interpretation 70 Chapter 4 - Gravity Data Reduction 73 4.1 Introduction 73 4.2 Production of the Digital Terrain Model 73 4.2.1 Digitising 74 4.2.2 Data Quality 74 4.2.3 Data Presentation 75 4.3 Reduction and Terrain Correction of Gravity Data - Theory 75 4.3.1 Initial Data Format 75 4.3.2 Latitude Correction 80 4.3.3 Free-Air Correction 81 4.3.4 Topographic Correction 81 4.4 Reduction of Gravity Data - Method 85 4.4.1 Introduction 85 4.4.2 Reduction to Simple Bouguer Anomaly 86 4.4.3 Topographic Correction 86 4.4.4 Application of the Topographic Correction 94 4.4.5 Overview 102 6 Chapter 5 - Qualitative Interpretation and Further Processing 103 5.1 Extended Bouguer Anomaly Maps 103 5.1.1 General Subdivision 103 5.1.2 Qualitative Interpretation 107 5.2 Enhancement of Terrain Corrected Gravity Data 109 5.2.1 The Fourier Transform 109 5.2.2 The Digital Filtering Program 110 5.2.3 Source Depth Constraints 112 5.2.4 Trend Analysis 116 5.3 Results of Data Enhancement 116 5.3.1 Upward Continuation 116 5.3.2 Power Spectrum Analysis 118 5.3.3 Wavelength (bandpass) Filtering 122 5.3.4 Trend Analysis 127 5.4 General Comments on Enhanced Results 127 Chapter 6 - Forward Modelling 130 6.1 Forward Modelling Theory 130 6.2 Forward Modelling of the Papuan Fold Belt Data 131 6.3 Constraints on the Models 135 6.3.1 Surface Geology 135 6.3.2 Formal Gravity Interpretation 137 6.3.3 Density Determinations from Borehole Gravimetry 139 6.3.4 Density Values used for Modelling the Fold Belt Rocks 142 6.4 Overall Density Structure of the Models 145 6.5 Description of the Profiles and Models 146 6.6 Overview of Regional Profiles 170 6.6.1 Foreland Region 170 6.6.2 Fold Belt Region 170 6.7 Detailed Modelling 173 7 Chapter 7 - Geological Implications and Principal Conclusions 181 7.1 Structures affecting the sedimentary cover in the fold belt 181 7.1.1 Shallow Structures 182 7.1.2 The Muller Anticline 183 7.1.3 The Darai Plateau 185 7.1.4 Significance of Hydrocarbon Accumulations 186 7.1.5 Thin v thick skinned tectonics 187 7.2 The Foreland Basin 188 7.3 Regional Tectonics and Isostasy 189 References 200 Appendix A - Gravity Base Station Descriptions Gravity Meter G-513 Calibration Table 207 Appendix B - Tiengo Gravity Survey Local Terrain Corrections 212 Appendix C - Tiengo Gravity Survey Extended Bouguer Anomaly Data 215 Appendix D - FORTRAN program listings 224 Enclosure - PNG Gravity Study, extended Bouguer anomaly map (1:500000) 8 List of Figures Chapter 1 1.1 The Location of Papua New Guinea. 16 1.2 Plate tectonic steeing of Papua New Guinea. 16 1.3 Geological Provinces of Papua New Guinea. 17 Chapter 2 2.1 Geological summary map of the Papuan fold belt. 34 2.2 Diagrammatic relationship of the major stratigraphic units. 36 2.3 Major structural subdivisions of the fold belt. 47 Chapter 3 3.1 Gravity survey location map. 53 3.2 Tiengo survey location map. 57 3.3 Road quality map. 60 3.4 Tiengo survey station location map. 62 3.5 Tiengo and Kandep daily pressure readings. 66 3.6 Leru, Nipa and Mendi daily pressure readings. 67 3.7 Humidity correction to aneroid heights. 68 3.8 Tiengo extended Bouguer anomaly map. 71 3.9 Southern Highlands extended Bouguer anomaly map. 72 9 Chapter 4 4.1 Areas covered by the Digital Terrain Model. 76 4.2 Typical 1:50000 contour check plot. 77 4.3 The thin rod approximation. 84 4.4 Calculation of the angle subtended at the centre of the Earth. 84 4.5a Flowchart illustrating the logic of the one-stage correction program (pi). 88 4.5b Flowchart illustrating the logic of the one-stage correction program (p2). 89 4.6 Location of the Komo (8641) survey lines. 91 4.7 Profiles along Komo survey lines 1 and 2, showing both the one and two stage correction results using a standard crustal density of 2670kgm*3. 92 4.8 Difference between the results of the one and two stage corrections. 93 4.9a Plot of the difference between the one and two stage results against station elevation. 93 4.9b Plot of the difference between the one and two stage results against the terrain correction. 94 4.9c Plot of the difference between the one and two stage results against the total terrain correction. 94 4.10 Profiles along Komo survey lines 1 and 2 using variable density and uniform density of 2670kgm'3. 96 4.11 Profiles along Komo survey lines 1 and 2 using variable density and uniform density of 2500kgm'3. 97 4.12 Profiles along Komo survey lines 1 and 2 using variable density and constant density multiplied by a depth factor. 97 Chapter 5 5.1 Extended Bouguer anomaly map showing general subdivisions.