Ian M. Watkinson

Ductile flow in the metamorphic rocks of central Sulawesi

Abstract Metamorphic rocks exposed along the Palu-Koro Fault of west-central Sulawesi, Indonesia, show abundant evidence of non-coaxial ductile deformation. The deformed rocks include gneisses, amphibolites and schists, that form part of a regionally metamorphosed basement complex of Mesozoic–Precambrian Australian (Gondwanan) origin. In the Palu and Neck regions of Sulawesi, ductile shear fabrics record low-angle westward extension. Further south in the Palu valley, extension is directed towards the south and SW, along with gently-dipping ductile thrust fabrics. Vergence exceptions are common at both outcrop and kilometre scale. Cross-cutting granitic dykes place some constraint on the timing of ductile foliation formation. In the neck region of Sulawesi, it occurred before c. 44–33.7 Ma. In the central and northern Palu valley, to the south, it occurred before 5–3.5 Ma. The timing and orientation of non-coaxial strain precludes its origin as a result of Palu-Koro Fault activity. Instead, ductile flow occurred during either Eocene–Miocene mid-crustal extension above a metamorphic core complex, Cretaceous subduction-related deformation in the over-riding plate, or intracontinental deformation within Gondwana.

Tectonic re-interpretation of the Banggai-Sula–Molucca Sea margin, Indonesia

Abstract High resolution multibeam bathymetric and seismic data from the area north of the Banggai-Sula Islands, Indonesia, provide a new insight into the geological history of the boundary between the East Sulawesi ophiolite, the Banggai-Sula microcontinent and the Molucca Sea collision zone. Major continuous faults such as the Sula Thrust and the North Sula–Sorong Fault, previously interpreted to bound and pass through the area are not seen. The south-verging Batui Thrust previously interpreted offshore to the east of Poh Head cannot be identified. In the areas where the thrust was interpreted there is a north-vergent thrust and fold zone overlain by almost undeformed sediments. Gently dipping strata of the Banggai-Sula microcontinent margin can be traced northwards beneath younger rocks. In the east, rocks of the Molucca Sea collision complex are deformed by multigenerational folds, thrusts and strike-slip faults. There is a series of small thrusts between the leading edge of the collision complex and the foot of the slope. In the west a zone of transpression close to the East Arm of Sulawesi is the termination of the dextral strike-slip Balantak Fault extending east from Poh Head.

Chapter 19 The Sagaing Fault, Myanmar

The Sagaing Fault is amongst the longest and most active strike-slip faults in the world (e.g. Molnar & Dayem 2010; Robinson et al. 2010; Searle & Morley 2011). It accommodates more than half of the right-lateral motion between Sundaland and India within the diffuse plate boundary along the eastern margin of India, which occupies much of Myanmar (e.g. Vigny et al. 2003; Nielsen et al. 2004; Socquet et al. 2006) (Fig. 19.1). Acting as a ridge-subduction transform (Le Dain et al. 1984; Guzman-Speziale & Ni 1996; Yeats et al. 1997), the 1500 km long Sagaing Fault links major thrust systems in the north such as the Naga, Lohit and Main Central thrust zones near the eastern Himalayan syntaxis to the Andaman Sea spreading centre in the south (e.g. Win Swe 1970; Le Dain et al. 1984; Guzman-Speziale & Ni 1993; Curray 2005). Acting as a sliver-bounding lithospheric strike-slip partition inboard of a 3700 km long section of oblique subduction at the Sunda Trench, it links to the dextral West Andaman and Sumatran faults in the south (e.g. Curray et al. 1979; Nielsen et al. 2004; Curray 2005; Searle & Morley 2011). Fig. 19.1. Tectonic setting of the Sagaing Fault. Modified after Morley et al. (2011), Soe Thura Tun & Maung Thein (2012), Wang et al. (2014) and Morley & Alvey (2015). Topographic scarps, lineaments, earthquake clusters and gravity anomalies along the trace of the Sagaing Fault have long been recognized as indicating the presence of an important north–south-trending structure west of the Shan Plateau (e.g. La Touche 1913; Coggin Brown & Leicester 1933; Chhibber 1934; Dey 1968; Aung Khin et al. 1970) (Fig. 19.2a). Large parts of the fault were first …

Fault systems of the eastern Indonesian triple junction: evaluation of Quaternary activity and implications for seismic hazards

Abstract Eastern Indonesia is the site of intense deformation related to convergence between Australia, Eurasia, the Pacific and the Philippine Sea Plate. Our analysis of the tectonic geomorphology, drainage patterns, exhumed faults and historical seismicity in this region has highlighted faults that have been active during the Quaternary (Pleistocene to present day), even if instrumental records suggest that some are presently inactive. Of the 27 largely onshore fault systems studied, 11 showed evidence of a maximal tectonic rate and a further five showed evidence of rapid tectonic activity. Three faults indicating a slow to minimal tectonic rate nonetheless showed indications of Quaternary activity and may simply have long interseismic periods. Although most studied fault systems are highly segmented, many are linked by narrow (<3 km) step-overs to form one or more long, quasi-continuous segment capable of producing M>7.5 earthquakes. Sinistral shear across the soft-linked Yapen and Tarera–Aiduna faults and their continuation into the transpressive Seram fold–thrust belt represents perhaps the most active belt of deformation and hence the greatest seismic hazard in the region. However, the Palu–Koro Fault, which is long, straight and capable of generating super-shear ruptures, is considered to represent the greatest seismic risk of all the faults evaluated in this region in view of important strike-slip strands that appear to traverse the thick Quaternary basin-fill below Palu city.

Chapter 21 The Kyaukkyan Fault, Myanmar

The Kyaukkyan Fault is an active dextral strike-slip structure that passes 510 km north–south across the western Shan Plateau (e.g. Chhibber 1934; Le Dain et al. 1984; Wang et al. 2014) (Fig. 21.1a). It lies broadly parallel to and about 100–150 km to the east of the central Sagaing Fault. Unlike the rather discrete Sagaing Fault, the Kyaukkyan Fault is characterized by a broad array of splaying segments and basins, dominated by the Inle Lake releasing bend and associated extensional fault systems (Fig. 21.1b). In the north the fault terminates within the complex intersection between the sinistral Kyaukme and Momeik faults, the largely inactive Shan Scarp Fault Zone and the Sagaing Fault. In the south the fault curves to the SW and links with the Mae Ping Fault in Thailand, which itself terminates as it passes east into Cambodia and offshore into the Gulf of Thailand (e.g. Lacassin et al. 1997; Morley 2004; Morley et al. 2011). Several other faults of the western Shan Plateau, such as the Nampun and Taungoo faults, also converge with the western Mae Ping Fault in the region of Papun, indicating that the Mae Ping Fault dissipates or transfers much of the dextral strain of the western Shan Plateau. Fig. 21.1. Kyaukkyan Fault location and overview. ( a ) Tectonic setting of the Kyaukkyan Fault, modified after Morley et al. (2011), Soe Thura Tun & Maung Thein (2012), Wang et al. (2014), Morley & Alvey (2015) and Soe Thura Tun & Watkinson (2017). ( b ) Main faults making up the Kyaukkyan Fault. Solid line: confident interpretation; dashed line: uncertain interpretation; dotted line: other related faults. Earthquake locations from the NEIC and IRIS catalogues 1972–2010. Map location shown in (a). Although it has been devoid of large seismic events for over …

Discriminating between the origins of remotely sensed circular structures; carbonate mounds, diapirs or periclinal folds? Purbeck Limestone Group, Weymouth Bay, UK

Many sedimentary rock successions contain plan-view circular structures, such as impacts, diapirs and carbonate build-ups. When remotely sensed, it can be difficult to discriminate between their formation mechanisms. Here we examine this problem by assessing the origins of circular structures imaged in high-resolution multibeam bathymetric data from Weymouth Bay, UK. The imagery shows 30–150 m across, concave-down structures within the upper Purbeck Limestone Group on the southern limb of the Purbeck Anticline. Similar structures have not been identified in the extensive outcrops around the bay. The morphology and geological setting of the structures are consistent with three different interpretations: carbonate mounds, periclinal folds and evaporite diapirs. However, none of these structures has been previously recorded in the upper Purbeck Limestone Group outcrops of this internationally renowned geological region. We apply a scoring system to 25 features of the circular structures to discriminate between these three alternative interpretations. This analysis indicates that evaporite diapirs are the least likely and carbonate mounds the most likely origin of the structures. The presence of carbonate mounds revises the upper Purbeck palaeofacies distribution in its type area and provides an analogue for the exploration for hydrocarbon reservoirs in lacustrine mounds. Supplementary material: The methods used in this paper and metrics of the circular structures are available at https://doi.org/10.6084/m9.figshare.c.4103840