AOGS Conference Paper 2012

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responsibe for the individualization of a sliver plate (Jarrard, 1986; McCaffrey,. 1992). .... Global Tectonics, John Wiley & Sons, TheAtrium, Southern Gate,.
AOGS Conference Paper 2012 DYNAMIC EVOLUTION OF THE SAGAING FAULT Hla Hla Aung Myanmar Earthquake Committee MES Building, Hlaing University Campus,Hlaing Township Yangon, Myanmar Email: [email protected] Abstract The Sagaing fault is one of the largest right-lateral strike-slip faults and a major lithospheric fault in SE Asia. The Sagaing fault is an active fault as indicated by epicentral location of earthquakes along the fault. The interpretation of remote sensing images, regional topographic and geomorphic analysis are able to trace the geometry of the Sagaing Fault in detail. In satellite images , the Sagaing Fault can be divided into several fault segments based on the geometry of the rupture zones, geomorphic and structural features of the Sagaing Fault Zone. This geometry of the segments produced by strike-slip faulting or thrust faulting during repeated earthquakes, which also produced multiple segments separated by discontinuities that appear as step-overs or bends in map view. Along the trace of the Sagaing Fault Zone, both right-steps and left-steps occurred indicating the active strike-slip nature of the Sagaing Fault Zone. Motion is transferred across the gaps between the fault segments resulting in zones of localized extension and compression. These fault segments are easy to find when the individual earthquake struck on each fault. The Sagaing Fault is a plate boundary between Burma and Sunda plates and composed of numerous fault segments that represents a system in which earthquakes occurred. To improve the understanding of the geodynamics and evolution of the plate boundary, detailed studies of geomorphic features and geologic structures grow by repeated seismic events are conduted across the Sagaing Fault Zone. Geomorphic observations and previous documentation suggest that such kind of investigation are key for a better understanding of active tectonics and seismic hazards of the Sagaing Fault. Displacement on the fault is dominantly strike-slip, although in places it is associated with localized transpression and transtension. The geomorphic features developed by transtension ( Late Miocene) and transpression (Pliocene-Recent) appear to reflect two episodes of strike-slip deformaion as a result of change in motion from strike-slip to transform. Key words: strike-slip fault, transtension, transpression, pull-apart basin, pressure ridges, motion, deformation.

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Tectonic Setting Myanmar is composed of two different evolving continental blocks: the Burma plate and the Sundaland plate. The Sagaing Fault is interpreted as a recent dextral strike slip fault and a continental transform plate boundary that separates the Burma plate from the Sundaland plate (Curray et al., 1979; Le Dain et al., 1984; Yeats et al., 1997; Curray, 2005). The Sagaing fault is linked with Central Andaman spreading center to the south (Curray et al., 1979). The Andaman Sea has been formed by seafloor spreading along short ENE-striking spreading center that is offset by NNWstriking transform faults (Curray et al., 1978; Eguchu et al., 1979). Extension and rifting in the Central Andaman Basin began at around 11Ma and extension and sea floor spreading has been ongoing since 4-5 Ma (Khan & Chakraborty, 2005). Consequently this is the most possible age for the initiation of the Sagaing fault. Spreading in a 335°(N 25° W) direction, relative to present N, is at an average rate of 30 mm/yr the northward component is 27 mm/yr (Curray,2005). The rate of motion of the Burma plate with respect to the Sunda plate is 18-25mm/yr towards the north (Soquet et al.2006). The average trend of the Sagaing Fault is 351° (N9°W). The Sagaing fault accommodates part of the motion while the remainder of the motion is distributed on other faults within the fault zone (Vigny et al., 2003). These faults are located at short distances to the west of the Sagaing Fault in the Central Myanmar Basin. The NW-SE-oriented seafloor spreading direction agrees with the average trend of the Sagaing Fault. Moreover, the Sagaing Fault is a transform plate boundary with a right-lateral motion. The NW-SE-oriented spreading ridge segments in the Gulf of Mottama, are rifting the Burma Plate northward with respect to the Sunda plate. Further north, the Sagaing Fault System is essentially parallel to the relative motion so is largely a transform fault. Most of the motion is accommodated by faults that form a series of en echelon step-overs within a narrow, (3) to (11) km wide transform valley. At some places, the Sagaing Fault differs from the direction of plate motion, so that dip-slip faulting occurs. Rhomb-shaped grabens, elongate pull-apart basins, steep normal faults have formed where the fault segment steps to the right. One of the largest of these extensional features is Indawgyi Lake (26) km long (5) km wide. The pull-apart basins and normal faults along the Sagaing fault resemble those that characterize intracontinental rift basins in

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AOGS Conference Paper 2012 Central Myanmar Basin, but have different tectonic settings. Both types of basin typically are bounded by border faults and display strike segmentation.

1. Geometry of the Sagaing fault The Sagaing Fault is the primary plate boundary between the Burma Plate and the Sundaland Plate. It is the boundary along which most of the relative motion has occurred and will continue to occur for the geologic future. Rather than being a single major fault along which the plates move smoothly past one another, the plate boundary shares motion between fault segments that form a series of en echelon step-overs. To understand the complexity of the plate boundary system, first consider the geometry of the present day the Sagaing Fault. The crust is undergoing extension, a process by which the crust is slowly stretching and thinning as a by product of the long episode of subduction that defines the plate boundary before the Sagaing Fault came into existence. The Sagaing Fault is a major right-lateral strike-slip fault which has long and straight traces for 1000km along the entire length of Myanmar. This fault is the most conspicuous feature that can be seen on Landsat imagery of Myanmar. The fault is distinct between Mandalay and Thabeikyin, but southwards, where it crosses the lowlands of the Central Myanmar is much more difficult to observe on satellite images. Large-scale topographic maps, SRTM-DEM-based shaded relief maps and Landsat 7 band combination 742 as RGB (2003) and 1:24,000 scale aerial photographic maps are used for interpretation of morphological features of the Sagaing Fault and related tectonic structures. Numerous small fault segments have been indentified in map view which convey an impression of enechelon pattern. Most of the segments along its length, have ruptured recently, generating historical earthquakes. Individual fault segments are continuous along their trace until a small offset to one side, separates one segment from the next. The intervening gap between the two segments creates small zones of extension or compression depending on whether the step is to the right or to the left. Such zones of compression or extension also exist at fault bends in the fault zone. These faults are easy to find when the individual earthquakes struck on each fault. Fault geometry known as en echelon faulting leads to interesting consequences when faults rupture (Hough, 2004). Along the trace of

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AOGS Conference Paper 2012 the Sagaing Fault Zone both right-steps and left-steps occur indicating the active strike-slip nature of the Sagaing Fault Zone These segments are designated as the Indawgyi, Indaw, Htichaing, Thabeikkyin ,Singu,Yega, Yemethin, , Pyinmana, Swa, Phyu, Shwedan, Zwedeik, Kabauk and coastal segments. Where fault segments overlap, extensional forces have created linear depressions between them. Along the Sagaing Fault, Lake Indawgyi, Indaw lake, a sag pond near Hti-chaing, another sag pond near Singu, Yega In, Shwedan In, Zwedaik In and Kabauk In, Bagan In, and more sag ponds are tectonic lakes. Small linear zones of uplift called pressure ridges typically indentified compressional fault step to the left. The localized pressure ridges such as Pale, Desunpa, Magyigyin, Sagaing and Tagaung ridges in the Sagaing Fault Zone, have been uplifted in response to a combination of dextral motion and compression across the different parts of the Sagaing Fault (Fig.1).

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Fig.1. In large-scale topographic maps, SRTM-DEM-based shaded relief maps and Landsat 7 band combination 742 as RGB (2003) and aerial photographic maps, morphological features of the Sagaing Fault and related tectonic structures such as restraining bend, releasing bend, faultbounded lens-shaped basin, en echelon strike-slip fault segment are well-recognized and observed clearly. The tectonicgeomorphic features that can be observed on Landsat image of the nortern part of the Sagaing fault are created by lateral movement of the fault. The fault created a series of tectonic lakes:Lake Indawgyi, Indaw lake, Singu sag pond and Yega In and many other features in the northern part of Myanmar.

Within the neotectonic framework of Myanmar, the longitude of the Sagaing Fault is 96° 30’E (following the coastline in the south), 96°E (from Pyinmana to Tagaung) and 96°45E to the north. In other words, the fault is more westerly (NNW-SSE) in the south N-S in the central part, between Pyinmana and Thabeikyin, a distance estimated to be 300 km and then swings easterly (NNE-SSW) again north of Thabeikyin. When the trend of the Sagaing Fault differs from the direction of plate motion, dip slip faulting occurs and the plate motion becomes complex. Most of the NNW-striking faults that dominate the area appear to be presently active, as shown by recent seismicity, including historical earthquakes.

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AOGS Conference Paper 2012 Fans or Horsetail splays

To the north of Thabeikyin between lat.26°N and 27°N, the Sagaing Fault branches into various splays to form a 200-wide compressive horse-tail structure from east to west.

At the same time, the Indian Plate pushes the Burma Plate northeastwards.

This fault connects the Himalayan the Main Boundary Thrust in the north (Maurin, 2011). To the south at the latitude of Nyaunglebin-Bago( latitude 17°30’N), the Sagaing Fault branches into two main fault a few kilometers apart. In Mottama Gulf, the fault terminates as a horse-tail extensional system (Charmot-Rooke et al.2001). Extensional horsetail structures are also found in the Mottama Gulf with a recorded extensional deformation zone 250km wide from north to south (Rangin et al., 1999). The western branch of the fault, the West Sagaing Fault terminates southwards in a cluster of normal faults separated by a series of pull-apart basins and a system of oblique transform faults that connect southwards with the Andaman spreading centre (Curray et al.2005). The fault branch located immediately to the east, the East Sagaing Fault, is apparently covered by deep marine terraces and could be inactive.

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Fig. 2. The Sagaing fault runs through Myanmar region. Topographic map of Myanmar showing the epicenters of historical earthquakes. Black stars indicate the earthquakes of M7.0 that struck the whole region during eighty-two years prior to the 2012 Thabeikyin earthquake.

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Step-over/ Pull-apart basins The most striking feature of the fault zone is perhaps that it is remarkably straight and distinct on the satellite image extending for 1000 km along the entire length of Myanmar. For most of its course the fault zone is clearly defined by both tectonic and topographic features. At about latitude 25°N a fault segment step to another segment on the right forms an extensional pull-apart basin called Indawgyi Lake. At 24°N, north of Hti-chaing, there is another fault segment forming a sag pond called Indaw Lake. At 23°N-96°E, west of Htichaing there is a sag pond. Further to the south, at 22° 35’N, the Singu basin with 1.5 km wide and 2 km long, near Singu is observed. At latitude 21° 58’N, at 10km north of Sagaing there is a right-stepping fault segment forming an elongated basin called Yega In. Yega In is one of pull-apart basin with 1 km long and 500m wide, formed within the Sagaing Fault Zone. The Shwedan In is a tectonic depression or sag pong on the down-thrown side of the fault. The height of the fault scarp near the northern end of the Shwedan In is about 2.8m. A clear west-facing fault scarp extends south of Shwedan Village. North and south of the Paingkyun Chaung, an elongated linear depression about 10 m wide and l m deep extends along the fault. Farther south, the Sagaing fault bounds the eastern margin of the Zwegaik In. The Kabauk In is a pull-apart basin located at a right step of the Sagaing fault (Tsutsumi et al., 2009). These basins characterize a stepover where the intervening region has been thrown into tension. A large earthquake tends to nucleate from the tip of the fault segment and then propagate along the fault plane where the accumulated stress is high enough to cause slip on the fault plane. The pattern of seismicity is consistent with the transfer of slip on one fault within the fault zone onto another fault segment to the right (Hla Hla Aung, 2011)

Laterally offset surface features

Near the vicinity of Sagaing, the fault has brought Pegu sandstones on the west against the Irrawaddy Sandstone on the east, and in the north near West Taungnyo village, the fault brings sheared metamorphic rocks on the eastern side against the Pegu sandstones on the west, indicating lateral displacement of the fault. Between Kebwet and Thabeikkyin, the Sagaing fault forms a boundary between the older 8

AOGS Conference Paper 2012 Mogok Series in the east and the Plio-Pleistocene sandstone of the Irrawady Formation on the west. A straight narrow, north-south trending fault trace can be observed between the main Sagaing fault and the Min Wun ridge just north of Sagaing. Immediately north of Mandalay, at latitudes between 22°35’N and 22°45’N, the Singu lava plateau is dextrally displaced by the Sagaing Fault. In the central part of the Sagaing fault, the trace of the fault is rather linear and simple along a line of low hills from Kyaukse to Thazi-Pyawbwe area. Detailed study also show late-Cenozoic dextral slip along the NNW-SSE trending trace of the Sagaing fault. For example, the Sinthe Chaung (chaung means stream in Myanmar) is an offset stream channel that can be found along the Sagaing fault between latitude 20°00’ N and 20°10’ N. Seismic profiles for Swa (MOGE) show typical tectonic inversion feature which is normal faulting in deeper part and reverse faulting in the near surface. These ranges have been uplifted in response to a combination of dextral motion and compression across Sagaing Fault. An unconformity between the Pegu Group (Late Miocene) and the Irrawaddy Formation (Pliocene-Quaternary) separates an earlier transtensional regime from later transpressional regime. Miocene extension in NNW-SSE direction is dominated by predominantly NE-SW striking normal faults and Pliocene-Quaternary compression in ENE-WSW is NW-SE to NNW-SSE thrusting and folding.

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(a)

(b)

(c)

(d)

Fig.3 a, c, In 2D seismic profiles in E-W direction, with a scale of 1 inch=0.5km from MOGE, it is clearly visible that Sagaing Fault bifurcated into splay faults in the Swa region and line drawings of seismic profiles showing active reverse faults and positive flower structures (b) at NW of Swa and (d) at SW of Swa. These seismic profiles show typical tectonic inversion feature which is normal faulting in deeper part and reverse faulting in the near surface. These ranges have been uplifted in response to a combination of dextral motion and compression across Sagaing Fault. An unconformity between the Pegu Group (Late Miocene) and the Irrawaddy Formation (PlioceneQuaternary) separates an earlier transtensional regime from later transpressional regime. Miocene extension in NNW-SSE direction is dominated by predominantly NE-SW striking normal faults and Pliocene-Quaternary compression in ENE-WSW is NW-SE to NNW-SSE thrusting and folding.

Releasing / restraining bends

Pull-apart basins, zones of subsidence and deposition, and normal faults characterize releasing bends whereas restraining bends display thrust faults, folds, and push-ups. 10

AOGS Conference Paper 2012 These zones are similar to those that form in step-over situations. At diverging bends, a localized area is uplifted in response to a combination of dextral motion and compression across a portion of the fault that strikes more westerly than the general strike of the fault system. At a converging bend, a localized zone of subsidence occurs due to a combination of extension and dextral motion. These structures can be found on satellite images near Pyinmana and Phyu in the central part of the Sagaing Fault Zone. Releasing/restraining bends are located also at latitude 18° 30’N to 19° 35’N and latitude 17° 05’N to 17° 20’N. Linear fault scarps

There are many linear fault scarps and localized pressure ridges in the Sagaing Fault Zone, such as Desunpa, Magyigyin, Khendangyi, Sagaing ridge, Minwun ridge and Tagaung ridges. In the Bago area, the Sagaing Fault is identified by continuous westfacing fault scarps. Fresh west-facing scarps exist for a 2.5 km along the stretch between Kanbe Chaung and Wagadok Chaung (Tsutsumi et al.,2009). Fault scarp (Triangular faceted spurs) along splay faults of the Sagaing Fault Zone. GPS location N19 12.634’, E 96 14.374’ (Fig.5). The best location to observe pressure ridges within the Sagaing Fault Zone is the Mandalay region, just north of Sagaing township. Here, the Sagaing fault is highlined by two N-S trending parallel pressure ridges, the Minwun ridge on the west and the Sagaing ridge on the east. Shutter ridges (Sagaing ridge and Minwun ridge) are common tectonic landforms near Sagaing through which the Sagaing fault pass (Fig.3). . Dynamic Evolution of the Sagaing Fault Detailed structural analysis along the Sagaing Fault reveal several folds and faults, in varied directions along the fault, of which the NNW-direction is the most prominent. Dsplacement on the fault is dominantly strike slip, although in places it also is associated with localized transpression and transtension. The Sagaing Fault Zone is a major shear zone in Myanmar, by evidence like localized basins and uplifts and other geomorphic features such as releasing / restraining bends and flower structure.

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Fig.8. Fault scarp (Triangular faceted spurs) along splay faults of the Sagaing Fault Zone. GPS location N19 12.634’, E 96 14.374’

Fig.9. Shutter ridges (Sagaing ridge and Minwun ridge) are common tectonic landforms near Sagaing City through which the Sagaing fault pass.

Tectonic deformation has occurred from the Miocene to the present. From studies of fault characteristics within the Sagaing Fault Zone, the faults record both strike-slip deformation and dip-slip displacements that appear to reflect two episodes of strike-slip deformation. The first involved a combination of strike slip motion and extension on north-northwest trending faults, leading to the formation of localized pull-apart basins. The second involved strike-slip motion and compression forming folds, possibly as a result of a change in the direction of motion along the transform. On the basis of geomorphic investigations and kinematic elements discussed previously, it has been possible to reconstruct the large scale evolution of the Sagaing Fault. The Burma sliver plate is bounded eastward from the Sunda plate, by the rightlateral strike-slip Sagaing Fault (Curray et al. 1979, Le Dain et al., 1984; Guzman Speziale and Ni, 1996 ). The sliver plate results from the India/Sunda plate’s oblique convergence. Within this geodynamic framework, strain partitioning is 12

AOGS Conference Paper 2012 responsibe for the individualization of a sliver plate (Jarrard, 1986; McCaffrey, 1992). The Sagaing Fault is interpreted as a recent dextral strike slip fault and a continental transform plate boundary that separates the Burma plate from the Sundaland plate The Sagaing fault is linked with Central Andaman spreading center to the south. The Andaman Sea has been formed by seafloor spreading along short ENE-striking spreading center that is offset by NNW-striking transform faults. Extension and rifting in the Central Andaman Basin began at around 11Ma and extension and sea floor spreading has been ongoing since 4-5 Ma (Khan & Chakraborty, 2005). Consequently this is the most possible age for the initiation of the Sagaing fault. Prior to 5 Ma, there is a contact between the Burma plate and Sunda plate and form a plate boundary between them. The complex threedimensional geometry of the plates gave rise to a lateral relative motion rather than subduction. The Sagaing fault was born. The Sagaing Fault originated as a single fault strand produced within the context of oblique subduction of the India, beneath the Sunda Plate. It then developed several fault branches simulating the formation of a dextral duplex system through time, with the faults to the east developing earlier than those to the west. The Sagaing fault was a single fault strand along which the plates moved clearly pass one another and relative plate motion resulted mostly strike-slip movement on the fault. Right lateral strike-slip motion along the Sagaing fault resulted in brittle deformation of the upper crust and ductile deformtion at depth. Then the crust is undergoing a process known as extension since Late Miocene, by which the crust is slowly stretching, leading to the formation of pullaprt basins within the fault zone. The two plates cannot slide pass one another smoothly so that the plate boundary is essentially shearing or grinding, giving rise to folding and faulting, by a component of compression or squeezing since PlioPliestocene to Recent. The first deformation is by a combination of strike slip motion and extension on north-northwest trending faults, leading to the formation of localized pull-apart basins. The second deformation is by a combination of strikeslip motion and compression forming folds, possibly as a result of a change from strike-slip to transform motion. Evidence for this 5-My-old shift in plate motion is folding, thrusting and uplifting of crustal blocks, characterized by an ENE-WSW dominant stress direction. Stress orientation, determined from focal mechanism solution of earthquakes (Engdahl, 1998) and tensor plots of structural orientation of

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AOGS Conference Paper 2012 faults in the fault zone, indicate a maximum stress axis in ENE-WSW direction and minimum stress axis in NNW-SSE direction (Pivnik et al., 1998). These relationships illustate how large strike-slip faults like the Sagaing Fault typically evolve over timescales measured in millions of years.

Conclusion Earthquakes that occurred along the Sagaing Fault and some sub-parallel faults are caused by the movement of huge crustal blocks of the earth’s crust. Although detailed historic account of earthquakes exist, many questions regarding the mode of deformation from the point of view of the plate tectonics remains unresolved. The author gathered the information by reinterpretation of the historical earthquakes and add to the previous understanding and provides a fresh perspective on seismogenic zones. To improve the understanding of the geodynamics and evolution of the plate boundary, detailed studies of geomorphic features and geologic structures grow by repeated seismic events are conducted across the Sagaing Fault Zone. From satellite investigation, well- defined tectonic geomophic features have been indentified along the fault zone including fault scarps, tectonic depression and pressure ridges. The Sagaing Fault Zone illustrates many of the physiographic and structural features that are common to extensional step-overs and a series of contractional step-overs. These segments , each 50-180 km long and a half kilometer -5 km wide are linked by stepovers or bends with 125 km long. Tectonic-geomorphic features and seismic activity along the Sagaing fault characterize the fault segments. Many localized pull-apart basins and localized areas of uplift occur between these faults segments within the Sagaing Fault Zone. The Sagaing Fault Zone is a major shear zone in Myanmar. These studies of local tectonic geomorphic features and local geological structures show that all the historical earthquakes were associated with seismic amplification by small-scale topographic features such as localized scarps and localized basins. The seismic lines show typical tectonic inversion feature in Swa area, which is normal faulting in deeper part and reverse faulting in the near surface. These ranges have been uplifted in response to a combination of dextral motion and compression across Sagaing Fault. An unconformity between the Pegu Group (Late Miocene) and the Irrawaddy Formation (Pliocene-Quaternary) separates an earlier transtensional regime from later transpressional regime. The active deformation and tectonic

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AOGS Conference Paper 2012 evolution of the plate boundary between the Burma and Sunda plates is important in the sense that it may generate large earthquakes along the Sagaing Fault Zone. Detailed mapping of the Sagaing Fault reveals physiographic features such as linear fault scarps, laterally offset surface features, offset streams, step-over, push-ups and pull-apart basins, releasing/restraining bends and flower structures. These, and other observations, testify that the Sagaing fault is a major continental strike-slip fault and a continental transform fault and these relationships illustrate how large strike-slip faults typically evolve very rapidly on timescales of tens to hundreds of thousand years.

Data and Resources All data used in this article were obtained from published sources listed in the refrences and some from limited field investigations.

Acknowledgements I acknowedge anonymous reviewers for their comments, suggestions and criticism on the manuscript. Comments on an early version of the manuscript by these reviewers greatly improved the manuscript. Toe Win Thein prepared shaded relief maps and provide Landsat images. The seismic sections are shown in the manuscript with the permission of U Lin Myint (MOGE).

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AOGS Conference Paper 2012 evidences for ductile shear along the Sagaing fault or the northward migration of the eastern Himalayan syntaxis. Geophysical Research Letters 26, 7, 915-918. 5. Bonilla, M.G., (1979), Historic Surface Faulting—Map Patterns, Relation to Subsurface Faulting, and relation to Preexisting Faults. USGS Open – File Report 79-1239, 36-65. 6. Chhibber, H. L. (1934). The Geology of Burma Macmillan and Co., London, 538 pp. 7. Chamot-Rooke, N., C. Rangin, C. Nielsen, (2001). Timing and kinematics of Andaman basin opening. Eos Transactions Supplement 82 (20). 8. Curray, J.R. (2005). Tectonics and history of the Andaman Sea region, J. Asian Earth Sci. 25, 187-232. 9.

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AOGS Conference Paper 2012 15. Guzman-Speziale, M., J. Ni, (1993). The opening of the Andaman Sea. Geophysical Research letters 20, (2949-2952). 16. Hla Maung, (1983). A new reconstruction of Southeast Asia and Gondwanaland: Its relation to mantle plumes or hotspots. SEAPEX Proceedings, 6, 66-70. 17. Harding, T.P., (1985). Seismic characteristics and identification of negative flower structures, positive flower structures, and positive structural inversion. American Association of Petroleum Geologists, Bulletin 69, 582-600.

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