Chapter 2 Active tectonics of Myanmar and the Andaman Sea by R. A. Sloan, J. R. Elliott, M. P. Searle, and C. K. Morley Geological Society, London, Memoirs Volume 48(1):19-52 November 15, 2017 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Seismotectonic map of Myanmar (Burma) and surroundings. Seismotectonic map of Myanmar (Burma) and surroundings. Faults are from Taylor & Yin (2009) with minor additions and adjustments. GPS vectors show velocities relative to a fixed India from Vernant et al. (2014), Gahalaut et al. (2013), Maurin et al. (2010) and Gan et al. (2007). Coloured circles indicate Mw > 5 earthquakes from the EHB catalogue. Grey events are listed for depths <50 km, yellow for depths of 50–100 km and red for depths >100 km. The band of yellow and red earthquakes beneath the Indo-Burman Ranges represents the Burma Seismic Zone. The dashed black line shows the line of the cross-section in Figure 2.13. ASRR, Ailao Shan-Red River Shear Zone. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Seismotectonic map of Myanmar (Burma). Seismotectonic map of Myanmar (Burma). Faults are from Taylor & Yin (2009) with minor additions and adjustments. GPS vectors show velocities relative to a fixed Eurasia from Maurin et al. (2010). Slip rate estimates on the Sagaing Fault are given in blue and are from a, Bertrand et al. (1998); b, Vigny et al. (2003); c, Maurin et al. (2010); and d, Wang et al. (2011). Major earthquakes (Ms ≥7) are shown by yellow stars for the period 1900–76 from International Seismological Centre (2011) and by red stars for the period 1836–1900 from Le Dain et al. (1984). The location and magnitude of the Mb 7.5 1946 earthquake is taken from Hurukawa & Maung Maung (2011). Earthquake focal mechanisms are taken from the GCMT catalogue (Ekström et al. 2005) and show Mw ≥5.5 earthquakes, listed as being shallower than 30 km in the period 1976–2014. IR, Irrawaddy River; CR, Chindwin River; HV, Hukawng Valley; UKS, Upper Kachin State; SF, Sagaing Fault; KF, Koma Fault. The inset panel is an enlargement of the area within the dashed grey box. It shows the dense GPS network in this area. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Regional setting, and fault geometries and uplift distribution associated with the Sagaing Fault. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Pressure ridge west of the Yangon–Naypyidaw–Mandalay Expressway, south of Naypyidaw (18.479680° N, 96.373367° E): (a) view of the ridge; (b) more coherent boulders in rubble matrix; (c) sandstone boulder showing extensive cataclastic deformation bands. Pressure ridge west of the Yangon–Naypyidaw–Mandalay Expressway, south of Naypyidaw (18.479680° N, 96.373367° E): (a) view of the ridge; (b) more coherent boulders in rubble matrix; (c) sandstone boulder showing extensive cataclastic deformation bands. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Road cut in NW suburbs of Naypyidaw, illustrating strike-slip faulting and folding affecting Miocene Kyaukkok and Obogon formations. Road cut in NW suburbs of Naypyidaw, illustrating strike-slip faulting and folding affecting Miocene Kyaukkok and Obogon formations. Shale-prone sequences are abruptly juxtaposed by faulting with folded sandstones. Location: 19.773024° N, 96.048629° E. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

(a, b) Road cut in NW suburbs of Naypyidaw (same road, in a cutting west of Fig. 2.5), illustrating two unconformities related to transpressional deformation associated with the Sagaing Fault. (a, b) Road cut in NW suburbs of Naypyidaw (same road, in a cutting west of Fig. 2.5), illustrating two unconformities related to transpressional deformation associated with the Sagaing Fault. Irrawaddy Formation (Pliocene or possibly Late Miocene) overlies steeply dipping Obogon Formation (Miocene) unconformably. An internal unconformity also exists within the Irrawaddy Formation. Location: 19.77087° N, 96.042968° E. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

(a) Outcrop evidence of overpressure along the Sagaing Fault: sandstones injected into a bedded Miocene sandstone. (a) Outcrop evidence of overpressure along the Sagaing Fault: sandstones injected into a bedded Miocene sandstone. NW suburbs of Naypyidaw. Location: 19.474959° N, 96. 077197° E. (b) Thin-section of sandstone with cataclastic texture with clay matrix, indicating overpressure. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

(a) Simplified geological map modified from Mitchell et al. (a) Simplified geological map modified from Mitchell et al. (2012). Possible pinning points for restoration are numbered. (b) Restoration of the map in (a) with about 400 km displacement, showing how the Tagaung–Myitkina Belt ophiolites restore as a continuation of the Neotethys, Western Belt ophiolites. In this scenario the location of the Jade Mines Belt ophiolites remains problematic. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

GPS vectors, faults and earthquakes to the east of Myanmar. GPS vectors, faults and earthquakes to the east of Myanmar. GPS vectors are shown relative to stable Eurasia and are taken from Gan et al. (2007). In the second panel earthquake focal mechanisms are taken from the GCMT catalogue and coloured by mechanism (red for thrust events, blue for normal faulting events and grey for strike-slip faulting). Only Mw ≥ 5.5 events with a greater than 75% double component are shown. Faults are taken from Taylor & Yin (2009) with minor additions. ASRR, Ailao Shan–Red River Shear Zone; LS, Longmen Shan; SB, Sichuan Basin; XF, Xianshu-ihe Fault; GFZ, Gaoligong Fault Zone; WF, Wanding Fault; MF, Mengxing Fault; NMF, Nam Ma Fault; MPF, Mae Ping Fault; SF, Sagaing Fault. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

(a) Active fault map and hill-shaded DEM of Indo-China, highlighting the series of left-lateral strike-slip faults between the major right-lateral Saigon and Red River faults. (a) Active fault map and hill-shaded DEM of Indo-China, highlighting the series of left-lateral strike-slip faults between the major right-lateral Saigon and Red River faults. Inset boxes indicate location of river offsets. Faults are from Taylor & Yin (2009). (b) Landsat false colour mosaic (RGB 752) of the Nam Ma and Mengxing Faults. The Nam Ma Fault has a current 11 km left-lateral displacement in the river, consistent with recent earthquake focal mechanisms, but overall has c. 31 km of offset in a dextral sense (Lacassin et al. 1998). (c) Landsat false colour mosaic (RGB 752) of the Wanding Fault with a current 10 km left-lateral offset of the Salween River, but overall a right-lateral offset of 33–54 km (Lacassin et al. 1998). R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Lithosphere thickness and seismic anisotropy. Lithosphere thickness and seismic anisotropy. Lithosphere thickness values are taken from Priestley & McKenzie (2013). Values are calculated on a 2° by 2° grid and the model has c. 300 km horizontal resolution and c. 30 km vertically. Areas with lithosphere thicknesses of less than c. 100 km can be identified as thin, but cannot be further differentiated. Yellow lines indicate the fast polarization direction of the seismic anisotropy measured by Flesch et al. (2005) and Sol et al. (2007). Yellow dots indicate lower crustal earthquakes identified by Mitra et al. (2005) and Chen & Molnar (1990) in the vicinity of the Shillong Plateau (SP). The Indian Shield, and potentially the Sichuan Basin, are most likely underlain by ancient stable thick lithosphere, whereas the lithosphere beneath the Tibetan Plateau was most likely thickened by the ongoing collision. Faults map is taken from Taylor & Yin (2009) with minor alterations. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Earthquakes and GPS data west of Myanmar. Earthquakes and GPS data west of Myanmar. GPS velocities relative to stable India taken from Maurin et al. (2010), Gahalaut et al. (2013) and Vernant et al. (2014) and converted to a consistent reference from Vernant et al. (2014). Coloured dots indicate earthquake locations from Stork et al. (2008). Black circles indicate depths <20 km, blue dots depths of 20–60 km, yellow dots depths of 60–100 km and red dots >100 km. Earthquake focal mechanisms are shown in bold colours when obtained through body-waveform modelling (Chen & Molnar 1990; Mitra et al. 2005) and faint colours when taken from the GCMT catalogue with depths constrained by Stork et al. (2008) or depth-phase modelling by Copley & McKenzie (2007). The dark-grey low-angle thrust was found to lie at 5 km by both Copley & McKenzie (2007) and Stork et al. (2008). Light-grey earthquakes were selected from the GCMT catalogue because their mechanisms match the shortening direction of the anticlines. No conclusive depth identifications have been made for these events. Im, Imphal; CMF, Churachandpur-Mao Fault; TCF, Thahtay Chaung Fault; CI, Cheduba; RI, Ramree Islands; SP, Shillong Plateau; IBR, Indo-Burman Ranges; NH, Naga Hills; TFB, Tripura Fold Belt; GBD, Ganges–Brahmaputra Delta. The inset panel is an enlargement of the area within the dashed grey box, and shows the topography of the TFB after high-pass filtering with a 40 km threshold to pick out the anticlines. RA, Rashidpur Anticline; SE, Srimangal Earthquake. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

(a) GPS summary diagram showing the spatial variation in GPS vectors in this region. (a) GPS summary diagram showing the spatial variation in GPS vectors in this region. GPS velocities are taken from Maurin et al. (2010), Gahalaut et al. (2013) and Gan et al. (2007) and converted to a consistent reference frame by Vernant et al. (2014). Vectors show velocities relative to stable India. (b) Locations of the GPS stations used in (a). The colour changes with longitude, chosen to emphasize major tectonic features. Stations in the southern part of the area are surrounded by a black line in both panels. Dashed line marks the line of the cross-section in (c–e). (c) GPS velocities with respect to India oriented towards 190° (the opposite direction to India–Sunda Plate motion at the latitude of the TFB) in red, and 280° (perpendicular to plate motion) in black. SFZ and CMFZ mark the Sagaing and Churachandpur-Mao Fault Zones. VA, the volcanic arc in central Myanmar. (d) Topography of the Indo-Burman Ranges, Tripura Fold Belt and Myanmar Central Basin. (e) Earthquake focal mechanisms are shown as Northern Hemisphere projections. These mechanisms are taken from Chen & Molnar (1990), Mitra et al. (2005) or from the GCMT catalogue when the earthquakes have depth constraints from Stork et al. (2008). Red dots represent earthquakes from Stork et al. (2008) for which focal mechanisms are not available. The oval surrounds earthquakes that appear to occur above the subducting Indian Plate, apparently within the upper mantle or lower crust of the Myanmar Central Basin. Further study is needed to confirm these observations. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Regional tectonic setting of the Andaman Sea Region modified from Morley (2017). Regional tectonic setting of the Andaman Sea Region modified from Morley (2017). See text for explanation of labels A–E. The locations of Figures 2.15, 2.16, 2.17 are indicated. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Regional cross-section through the northern area of the Andaman Sea, offshore Myanmar, partially based on unpublished seismic reflection data. Regional cross-section through the northern area of the Andaman Sea, offshore Myanmar, partially based on unpublished seismic reflection data. See Figure 2.14 for location. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Map view and cross-section characteristics of the NE–SW-trending trench segment SW offshore Myanmar (modified from Nielsen et al. 2004). Map view and cross-section characteristics of the NE–SW-trending trench segment SW offshore Myanmar (modified from Nielsen et al. 2004). See Figure 2.14 for location. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Schematic cross-section from the eastern Alcock Rise across the East Andaman Basin, SE offshore Myanmar. Schematic cross-section from the eastern Alcock Rise across the East Andaman Basin, SE offshore Myanmar. The section illustrates a model for hyper-extended continental crust flooring much of the East Andaman Basin. See Figure 2.14 for location. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Geometry and key elements of the Central Andaman Basin and East Andaman Basin. Geometry and key elements of the Central Andaman Basin and East Andaman Basin. The strike-slip fault geometry on the eastern side of the basin and linkage with the Sagaing Fault is based on Diehl et al. (2013). This interpretation contrasts with the spreading-centre geometry of Curray (2005), which is superimposed. The figure is also based on data in Raju et al. (2004) and Morley (2013). R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Line drawing interpretation of a seismic line across the Central Andaman Basin, and putative spreading centre (original seismic line in Raju et al. 2004). Line drawing interpretation of a seismic line across the Central Andaman Basin, and putative spreading centre (original seismic line in Raju et al. 2004). See Figures 2.14 and 2.18 for location. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Map of the main basins and tectonic features in the northern part of the Andaman Sea. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved

Overview of major tectonic changes in the last c Overview of major tectonic changes in the last c. 20 Ma discussed in the text. Overview of major tectonic changes in the last c. 20 Ma discussed in the text. Red faults are from Taylor & Yin (2009) and thick grey dashed lines indicate the 150 km and 200 km contours of lithosphere thickness modelled by Priestley & McKenzie (2013). TFB, Tripura Fold Belt; CBB, Central Burma Basin; SF, Sagaing Fault and RRFZ, Red River Fault Zone; RL, right-lateral and LL, left-lateral respectively. R. A. Sloan et al. Geological Society, London, Memoirs 2017;48:19-52 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved