Earthquakes and tsunamis caused by low-angle normal faulting in the Banda Sea, Indonesia

Cummins, Phil R.1,2,3, Pranantyo, Ignatius R.1,4, Pownall, Jon1,5, Griffin, Jonathan D.3,6, Meilano, Irwan7, Zhao, Siyuan1

1Research School of Earth Sciences, Australian National University, Canberra, Australia, 2Global Geophysics Research Group, Institut Teknologi Bandung, Bandung, Indonesia, 3Community Safety Branch, Geoscience Australia, Canberra, Australia, 4Dept. Civil and Environmental Engineering, Brunel University, London, United Kingdom, 5Dept. Geosciences and Geography, University of Helsinki, Helsinki, Finland, 6Dept. Geology, University of Otago, Dunedin, New Zealand, 7Faculty of Earth Science and Technology, Institut Teknologi Bandung, Indonesia

As the world’s largest archipelagic country in Earth’s most active tectonic region, Indonesia faces a substantial earthquake and tsunami threat. Understanding this threat is a challenge because of the complex tectonic environment, the paucity of observed data and the limited historical record. Here we combine information from recent studies of the geology of Indonesia’s Banda Sea with Global Positioning System observations of crustal motion and an analysis of historical large earthquakes and tsunamis there. Analysis of historical accounts of Banda Sea earthquakes from the 19th century and earlier show that past destructive earthquakes were not caused by the supposed megathrust of the Banda outer arc as previously thought, but are more likely due to a vast submarine normal fault system recently discovered along the Banda inner arc. Instead of being generated by coseismic seafloor displacement, analysis of tsunami observations shows that the tsunamis were more likely caused by earthquake-triggered submarine slumping along the fault’s massive scarp, the Weber Deep. This would make the Banda detachment representative not only as a modern analogue for terranes hyper-extended by slab rollback but also for the generation of earthquakes and tsunamis by a submarine extensional fault system. Our findings suggest that low-angle normal faults in the Banda Sea generate large earthquakes, which in turn can generate tsunamis due to earthquake-triggered slumping.


Biography

Phil Cummins has a PhD in Geophysics from U. California, Berkeley. He worked at ANU, U. Tokyo ad JAMSTEC, Japan,  before leading earthquake and tsunami research at GA. He now holds  a joint appointment, combining teaching and research at ANU with technical applications of earthquake and tsunami science at GA.

Cenozoic affinity of the Gondwanan rocks of eastern Timor: evidence from geo-thermochronometry

Duffy, B1, Lew, B1, Boland, K1, Kohn, B1, Matchan, E1, Maas, R1, Dixon, D1, Pedro, L2, de Carvalho, P2, Sandiford, M1

1School of Earth Sciences, The University Of Melbourne, Melbourne, Australia, 2Instituto do Petroleo e Geologia, Dili, Timor-Leste

Timor occupies a critical position within Wallacea and within the Indonesian throughflow, but its tectonic history remains poorly constrained compared to other parts of the region. Tectonic models typically divide the island into 1) Australian affinity rocks, including a thick sequence of Paleozoic and Mesozoic rocks and metamorphosed equivalents, and 2) Asian affinity rocks, made up of predominantly Cretaceous and Paleogene rocks and their metamorphosed equivalents, known as the Lolotoi Metamorphic Complex (LMC). New field mapping shows that the type-area of the LMC is pre-Permian basement exposed in an erosional window. Much of the previously mapped LMC is actually overlying alkaline Permian basalt. LA-ICPMS U-Pb ages for zircons, apatites and titanites from the LMC type-area are Precambrian and consistent with those from Gondwanan continental slivers that now form the basement of eastern Java and West Sulawesi. Such basement ages are also identified in peaks from inherited zircons from the LMC elsewhere in Timor. Basement faults separating the LMC from Triassic and Jurassic sediments contain white micas yielding Ar-Ar ages of c.38 Ma, which are within the age range of white micas from the Asian affinity Mutis metamorphic complex of West Timor. Zircon and apatite (U-Th)/He thermochronometric data and low vitrinite reflectance values across much of the study area do not support previous models suggesting Cenozoic overthrusting. However, close to the basement fault the thermochronometric data indicate rapid Eocene-Oligocene cooling and like the white mica ages, this is consistent with the thermal history of the Mutis complex of West Timor. On the basis of these data, we revive Barber’s (1978) interpretation that almost all of the pre-Neogene exhumed rocks of eastern Timor, including the Gondwanan rocks, resided in Sundaland during the Cenozoic. This finding strongly supports hydrocarbon prospectivity onshore in Timor and has implications for the geodynamics of the Banda Arc, reconstructions of Wallacea, and the Neogene paleogeography of the Indonesian Throughflow.


Biography

Brendan Duffy is a research fellow in structural geology and tectonics at The University of Melbourne. His research focuses on the structural and geomorphic development of convergent boundaries of the Australian plate, at single earthquake to Cenozoic timescales.

Geology of the Mutis Complex, Miomaffo, West Timor

Berry, Ron1, Goemann, Karsten2, Danyushevsky, Leonid1

1CODES, University of Tasmania, Hobart, Australia, 2Central Science Laboratory, University of Tasmania, Hobart, Australia

Mesozoic accretionary assemblages are widespread across Indonesia and these can be seen in the overthrust terranes on Timor. The oldest rocks in the allochthonous Mutis Complex, West Timor are Jurassic melange containing blocks of N-MORB basalt (194 ±5 Ma), amphibolite (metamorphism at 175 Ma), garnet and actinolite bearing schists, arkosic sandstone and volcanogenic material. The sedimentary component is sourced from Mesozoic island arc along the southern margin of Indonesia (≈200 Ma based on detrital zircon). Prehnite is common as a late metamorphic mineral in the matrix. The accretionary material is complexly deformed with a dominant foliation and cataclastic zones. This sequence is intruded by calc-alkaline andesitic dykes (possibly in the Eocene). To the west of the melange is a transition to high strain greenschist facies rocks (isoclinal folding and transposition layering) where the block in matrix texture can no longer be recognised, but the range of bulk composition is the same. Further west across several faults is an amphibolite facies metamorphic province. Amphibolites have the same N-MORB composition as the basalt in the east. The peak metamorphic conditions are 600-720 C, 0.6-1.1 GPa. This area has the same bulk composition as the melange including detrital zircon ages. The metamorphic assemblages overprint low T isoclinal folds. Peak metamorphism occurs at 38 Ma reflecting an event within the active margin of Sundaland. At about 5 Ma, NW Australia collided with this margin and the Mutis Complex was thrust over Timor. A late generation of cataclastic faults zones cuts all the pre-existing lithologies but is largely a result of extensional collapse. The complex is cut by a younger fault zone that affects the underlying unconsolidated sedimentary rocks and contains evidence of both strike slip and normal fault movement. The Mutis Complex is an excellent example of the complex geological history of rocks along the boundary between Indonesia and Australia.


Biography

Ron Berry has worked on the structure and tectonics of Tasmania, Indonesia and South East Asia for 40 years. Karsten Goemann and Leonid Danyushevsky are experts in petrology, mineralogy and mineral analysis and have provide the technical expertise to make this study possible

Seismic Structural Evidence of Australian Continental Lithosphere Beneath Timor at the Banda Arc Collisional Zone

Zhang, Ping1, Miller,As.Prof Meghan1

1Australian National University, Canberra, Australia

The active arc-continent collision in the Banda Arc is a result of convergence of the Indo-Australian plate with the Eurasian plate in Southeast Asia. There, the subduction of the Cretaceous to Jurassic age Indian Ocean lithosphere along the Sunda Arc transitions to the recent arc-continent collision at the Banda arc while the NW Australian continental margin and the active volcanic arc converge. Such complicated convergent tectonics leads to pronounced structural heterogeneity and complexity of both vertical and lateral extents along to the convergent margin. Therefore, understanding detailed knowledge of deep structure in the area is key to unravelling the dynamic processes that occur. In this study, we image detailed crustal and uppermost mantle structure utilizing ~4-years of broadband seismic data recently collected in the Timor-Leste and Nusa Tenggara Timor region of Indonesia. We apply three techniques, ambient noise tomography, teleseismic P-wave receiver function and coda autocorrelation, to resolve a 3-D Vs model and Moho structure. Our tomographic images show low-velocity anomalies (<30 km) beneath the outer arc island of Timor related to the underthrusted Gondwana Sequence from the incoming Australian plate, which are vertically offset by the high-velocity backstop of the Banda forearc terrane. The structure progressively changes along strike, reflecting different collisional stages developed as a result of the oblique convergence and incoming plate heterogeneity. At greater depth, we detect seismically fast lithospheric mantle (>30 km) and the arc-ward dipping Moho beneath Timor, both interpreted to be from the incoming Australian plate. Our findings provide direct seismic structural evidence of the subducted Australian continental margin at lithospheric depths beneath the Banda Arc collision zone.


Biography

Ping Zhang is a PhD student, working with As. Prof. Meghan Miller, at The Australian National University. Her PhD focuses on understanding the Banda Arc-Australian Continent collision through 3-D seismic imaging, based on a new seismic experiment carried out from March 2014 to August 2019.

There are two sides to every trough – the story of young plate collision and its shadow

Keep, M1., Haig, D. W.2

1School of Earth Sciences, The University of Western Australia, 2Oceans Institute, University of Western Australia

As the orogenic product of the collision between the Australian Plate and Banda Arc, the Timor Orogen provides a unique insight into the processes and dynamics of the early stages of plate collision. These early stages, overprinted during ongoing orogenesis, provide information and constraints as to the timing of elements of subduction, the episodic nature of collision, and the resulting regional geodynamics.  Investigating the tectonic, kinematic and geodynamic history of an orogen requires integration of a broad range of field and analytical techniques, the most important element of which must be that any models are consistent with the field geology. That requires a robust understanding of the age and nature of the exposures.

The collisional front of the Timor Orogen includes a severely dismembered sequence of stratigraphic units, which include a number of different pre-collision stratigraphic associations, as well as a structural melange and a synorogenic phase. Juxtaposition of different elements along young, high-angle strike-slip faults means that stratigraphic associations are often juxtaposed across vertical boundaries, commonly eroded. Most original fold-and-thrust geometries are largely dismembered. Biostratigraphy, used in the field as the main determinant of stratigraphic age and association, is perhaps the most important and robust tool in the geodynamic toolbox, although largely overlooked due a lack of familiarity. Painstaking and robust stratigraphic analysis of thousands of samples from across East Timor over the last two decades has facilitated reconstruction of the stratigraphy of the island, which is not easily recognised through lithostratigraphic approaches, allowing interpretations of original formation and subsequent deformation at the collisional front.

In addition, broad regional interpretation of extensive 2D and 3D seismic data from Australia’s North West Shelf over the last 25 years, tied closely to well data (biostratigraphic ages), has allowed correlation of regional events from the Timor Sea westwards to the Carnarvon Basin. These events, marked by uplifts, unconformities, inversion episodes and stratigraphic responses, relate closely to events unfolding at the collisional front. With strain partitioned more strongly at the leading edge, more subtle responses in the strain shadow are not overprinted, and less prone to misidentification. These events corroborate the timing of events seen at the collisional front, occur over a wide area, and tell a different part of the collisional story.

Reconstructing this young and complex orogen requires data from both sides of the Timor Trough.


Biography

Myra Keep and David Haig have been working in East Timor since early 2003, making many dozens of visits and collecting thousands of samples for dating. Together they have over 65 years of work history on the Australian NWS, from PNG to the Carnarvon and Perth basins.

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