Abstract The Sagaing Fault (SF), one of the world's most active strike-slip faults, defines a plate boundary on the eastern West Burma Block margin, from the Andaman Spreading centre, northwards for >1600 km to the eastern Himalayas. In the Northern Andaman Sea the SF traverses the Late Miocene–Recent Moattama Basin. There, 2D and 3D seismic reflection data show highly unusual fault patterns, that overall resemble a giant (area >33 000 km 2 ) horsetail structure. A horsetail pattern typically implies loss of displacement at a fault tip, which is potentially incompatible with the SF forming a transform margin. In the thinner, northern part of the Late Miocene–Recent basin three branches of the SF can be identified. These become lost in the thickest (>7 km), central part of the basin, and two branches emerge to the south where the basin thins. The fault patterns are interpreted to represent a previously unknown interaction of thin- and thick-skinned styles, where relatively shallow detached structures and sediment loading have interacted with basement-involved strike-slip faults that form a releasing bend geometry at the basement level. The Moattama Basin demonstrates how very thick sedimentary basins can produce fault patterns that differ from classic structural models.

Abstract Due to the economic importance of its hydrocarbon and coal resources, more is known of the Tertiary than of any other part of the stratigraphic succession in Thailand. Tertiary sedimentary rocks occur both on- and offshore and they are generally associated with rift basins formed in extensional, transtensional or strike –slip settings (Fig. 10.1 ). In the Gulf of Thailand and Central Thailand the rift basins have been covered by extensive post-rift sag basins, while the Mergui Basin is in a post-rift passive margin setting. Thermal subsidence has yet to cover the rift basins of Northern Thailand. The basins trend north-south through the centre of the country onshore, and underlie much of the Gulf of Thailand and Andaman Sea. The Tertiary was a period of considerable tectonic activity and hence the timing of a basin’s subsidence, its location and the nature of its basin-fill and lateral facies variations are all tied to its structural development. The Tertiary structure is discussed by Morley et al. (2011) and further data on the Tertiary stratigraphy and structure are to be found in Ratanasthien (2011) and Racey (2011) ; the wider plate-tectonic context of the Tertiary structure is discussed by Searle & Morley (2011) . Information on the offshore Tertiary basins has been obtained entirely through hydrocarbon exploration and production activities since the late 1960s, and comprises seismic (both 2D and 3D) and borehole data. A number of the onshore basins in Northern and Central Thailand have

Abstract This chapter begins with a brief review of the data available for understanding the Cenozoic deformation of Thailand and a description of the different Cenozoic structural provinces of the country. The two dominant Cenozoic structural styles present in Thailand are strike-slip faulting (found predominantly in the western half of onshore Thailand and the Andaman Sea) and extension; folds, thrusts and inversion structures are also present in places (Fig. 11.1 ). The timing of deformation (Fig. 11.2 ) and typical structural styles present in outcrop and the subsurface are described. Particular attention is paid to the inheritance of fabrics on later fault geometries, since these are very distinctive and well developed in many areas. The structural geology of Thailand is very important for understanding Cenozoic orogenic processes marginal to the Himalayan orogen. For those less interested in regional tectonics, there are also individual examples of structures that are outstanding and could grace any structural geology textbook, including: A beautifully documented opencast coal mine (Mae Moh, Fig. 11.1) 3 × 4 km in area, that reveals Miocene rift-related normal faults. Extensively developed and large-scale strikeÂ∈"slip fault zones with multiple strikeÂ∈"slip duplex geometries. Cenozoic mid-crustal level mylonites and migmatites are exposed in places, with recently described outcrops along the Ranong Fault (Watkinson et al . 2008) that rival the Red RiverÂ∈"Ailao Shan Fault Zone in scale. The best examples seen anywhere on seismic reflection data of extensively developed, linked, high-length: displacement ratio normal faults (Gulf of Thailand).

Abstract Thailand occupies an important position for understanding the Cenozoic tectonic development of mainland SE Asia. It is located east of the indenting Indian Plate and is central to the region of southeastward extruding crust (Sundaland Block; Molnar & Tapponnier 1975 ; Tapponnier & Molnar 1977 ) following the Early Eocene India-Asia collision; and it is in the upper plate of the Sumatran-Andaman subduction zone. Figure 20.1 shows a 90 m Shuttle Radar Topography Mission (SRTM) digital elevation model of the topography of Thailand and the greater SE Asia region. The region is presently underlain by crust of variable thickness, ranging from moderately thick (c. 40-50 km) to thinned (c. 30 km), high heat flow and seismically slow mantle tomography (low shear velocities). Thailand is underlain by hot upper mantle with recent scattered alkali basaltic volcanism and mantle-derived helium isotopes from abundant hot springs. Earthquakes are in general restricted to the seismogenic upper crust, implying a weak and hot lower crust. The only exception is the Burma seismic zone, a narrow east-dipping zone of deep earthquakes (down to 230 km) where focal mechanisms reveal a combination of eastwards subduction of old oceanic crust combined with right-lateral shearing. During northwards indentation of India into Asia, the region east and SE of Tibet underwent large-scale right-lateral shear and clockwise rotation The thermal and structural evolution of Thailand and the greater SE Asia region is dominated by: (1) the separation of Cathaysia (North and South China and Indochina) from

Abstract Subsurface sediment mobilization (SSM) – which includes soft sediment deformations, sand injections, shale diapirs and mud volcanoes – is more widespread than previously thought. The ever-increasing resolution of subsurface data yielded many new observations of SSM, not only from regions obviously prone to sediment remobilization, such as an active tectonic setting or in a region with exceptionally large sediment supply, but also from tectonically quiescent areas. Until now, all the different aspects of SSM have largely been treated as separate phenomena. There is very little cross-referencing between, for example, studies of mud volcanoes and those of sand injections, although both are caused by sediment fluidization. Divisions according to sediment type, mobilization depth or triggering mechanism make little sense when trying to understand the processes of SSM. There is a gradation in mobilization processes that cause considerable overlap between categories in any classification. Hence, it is necessary to integrate our understanding of all types of SSM, regardless of scale, depth, location, grain size or triggering mechanism. In addition, polygonal faults are important in this context, as this non-tectonic structural style is closely associated with sedimentary injections and may also reflect large scale mobilization.

Abstract Shale dykes, diapirs and mud volcanoes are common in the onshore and offshore regions of Brunei Darussalam. Outcrop examples show that shale has intruded along both faults and tensile fractures. Conventional models of overpressure-induced brittle failure assume that pore pressure and total stresses are independent of one another. However, data worldwide and from Brunei show that changes in pore pressure are coupled with changes in total minimum horizontal stress. The pore pressure/stress-coupling ratio (Δσ h /ΔP p ) describes the rate of change of minimum horizontal stress magnitude with changing pore pressure. Minimum horizontal stress measurements for a major offshore field where undepleted pore pressures range from normal to highly overpressured show a pore pressure/stress-coupling ratio of 0.59. As a consequence of pore pressure/stress coupling, rocks can sustain a greater increase in pore pressure prior to failure than predicted by the prevailing values of pore pressure and stress. Pore pressure/stress-coupling may favour the formation of tensile fractures with increasing pore pressure rather than reactivation of pre-existing faults. Anthropogenically-induced tensile fracturing in offshore Brunei supports this hypothesis.

Abstract 3D seismic data in the Baram and Champion delta provinces offshore Brunei show that regions thought to be occupied entirely by chaotic seismic data and conventionally interpreted as shale diapirs, are regions of dimmed, but coherent reflectivity. Such data indicate shale diapir masses are not present, instead dimming can be attributed to sediment intrusive complexes, overpressured fluids and gas clouds, or processing artefacts. In this way significant delta structures are masked on 2D seismic data, which are important to interpret delta tectonic evolution. The Middle Miocene-Recent Champion and Baram deltaic provinces are characterized by typical gravity tectonics-related structures. However, being situated on an active margin they are also affected by episodic development of contractional structures, which are located on older reactive shale bulges and result in inversion of motion on some growth faults. The emplacement of shale pipes, gas clouds and intrusive complexes is generally relatively late (Pliocene) in comparison with the underlying reactive diapirs (Late Miocene) and their emplacement events may be separated in time by several million years. Late overpressured systems may be related to phases of pore fluid pressure increase during or following periods of inversion tectonics, which resulted in phases of enhanced fluid migration in the basin, where fluids were either expelled laterally oceanwards, or vertically.