Banda Arc Research Articles

The Margin‐Oblique Kumawa Strike‐Slip Fault in the Banda Forearc, East Indonesia: Structural Deformation, Tectonic Origin and Geohazard Implication

AbstractThe northeast Banda margin is affected by the arc‐continent collision between the Australian continental margin and the Banda arc. Unlike margin‐parallel strike‐slip faults in other subduction zones, the margin‐oblique Seram‐Kumawa Shear Zone (SKSZ) has greatly deformed the Banda forearc with 80 km displacement along plate boundary. It trends ∼ WNW‐ESE comprising the Kawa shear zone (KSZ, >200 km) on the Seram island and the Kumawa fault (KF, >200 km) offshore SE Seram. Using multibeam bathymetry, marine seismic reflection and earthquake data, we studied the active tectonics and driving mechanism of the SKSZ with focus on its offshore part (KF). Our results show: (1) the KF is a young (∼2 Ma), linear crustal structure with three segments from NW to SE: pull‐apart basin (dextral), diffuse shear zone (sinistral) and distinct fault trace (sinistral); (2) It propagates southeastward and cross‐cuts the entire upper plate with prominent seafloor expression, but without seismicity along it, similar to the Banda detachment caused by forearc extension; (3) By incorporating the oblique collision and regional tectonics with our results, we proposed that the KF is formed primarily by the upper plate extension induced by subduction rollback during the last 2 Ma, with minor contribution from oblique arc‐continent collision; (4) The slip rate of the KF has reduced from 40 mm/yr over 2 Ma to <14 mm/yr at present. These observations combined with the clear seafloor faulting, 80‐km displacement and occurrence of a Mw 5.2 earthquake on its horsetail fault suggest it is currently locked awaiting a large earthquake.

Upper Triassic carbonate-platform facies, Timor-Leste: Foraminiferal indices and regional tectonostratigraphic association

Timor is part of the non-volcanic Outer Banda Arc with chaotic geology in the late Neogene collision zone between Sundaland (Southeast Eurasia) and the Australian continent. Confusion in the distinction between Triassic, Jurassic and Cenozoic shallow-marine carbonate units has led to a lack of appreciation of the widespread extent of Triassic carbonate-platform facies, at least in Timor-Leste. It also resulted in misunderstanding the different tectonostratigraphic/palaeogeographic affinities of the shallow-water limestones. Foraminiferal assemblages are critical in stratigraphic discrimination of these units. This study records for the first time the foraminiferal microfauna of Upper Triassic shallow-water carbonate deposits in Timor. Many of the foraminiferal species are known from North Africa and Europe and date these units to the Carnian–Rhaetian. The Upper Triassic Bandeira Group represents extensive sheet-like carbonate-platform deposits (possibly on horst plateaus as well as shelf areas adjacent land in the intracratonic sea). Contacts between the hard limestone, forming fatus (limestone peaks) and ridges, and friable mud-dominated surrounding units are often obscured under deep tropical soil cover. At all localities the Bandeira Group is closely associated in outcrop with coeval basinal mud-dominated deposits (Babulu and Aitutu groups). With these it forms part of the East Gondwana Interior Rift Association (EGIRA), similar to the classical Dachstein Platform–Hallstatt Basin facies associations found in the European Alps. The basinal units contain turbiditic and debris-slide deposits that include clasts derived from the carbonate platform. Shallow-water carbonate facies attributed to EGIRA, are present throughout the East Gondwana Interior Rift from the Papuan Basin in the north to Exmouth Plateau, part of the Northern Carnarvon Basin, in the south. These were deposited before Gondwanan breakup along this rift system. The Triassic carbonate-platform deposits have a different dominant biogenic mineralogy (aragonite rather than calcite) and a different architecture to the bryozoan-crinoidal carbonate mounds present in the Permian of EGIRA in Timor-Leste. No Lower and Middle Triassic shallow-water carbonate-platform deposits have been recognized in Timor-Leste. The Bandeira Group, confined to the Upper Triassic, belongs to a stratigraphic association (EGIRA; autochthonous) different both in constituent units and outcrop coverage from that of the allochthonous Overthrust Terrane Association to which the Lower Jurassic carbonate-platform deposits (viz. Perdido Group) belong. The Bandeira Group has been confused not only with the Perdido Group but also with other shallow-water limestone units of the Permian and Miocene in some former studies of Timor. The distinction between these units solves a major tectonostratigraphic problem in Timor, and may apply elsewhere along the Outer Banda Arc.

Seismic Imaging of the Subducted Australian Continental Margin Beneath Timor and the Banda Arc Collision Zone

AbstractDetailed crustal and uppermost mantle structure is imaged for the first time utilizing ∼4 years of broadband seismic data newly 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 3D Vs model and Moho structure. Our tomographic images show low‐velocity anomalies (<30 km) beneath Timor related to underthrusted Gondwana sequence from the 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. 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 Australian plate. Our findings provide direct structural evidence of the Australian continental margin at lithospheric depths beneath the Banda Arc collisional zone.

Lithospheric mantle anisotropy from local events beneath the Sunda–Banda arc transition and its geodynamic implications

Shear wave splitting analysis to characterise lithospheric mantle anisotropy has been performed to provide better knowledge about lithospheric deformation and mantle flow beneath the Sunda–Banda arc transition, Indonesia. The tectonic setting of the study area is very complex characterised by the transition from subduction along Sunda arc to collision in Banda arc. The splitting measurements show lateral and vertical variation in the fast directions of the S-waves in this region. When the splitting results are analysed through 2D delay-time tomography and spatial averaging, systematic patterns in delay times and fast polarisation become more visible. In the subduction domain, the spatial averages of fast directions are dominated by two distinct fast polarisations: perpendicular and parallel to the plate motion for shallow and deep events, respectively. The results suggest that anisotropy in this area is not only controlled by anisotropic source related to the simple mantle flow model, but also by anisotropic fabric in the mantle deformed under influence of high stresses, high water contents and low temperatures. In addition, there might also be contribution from the anisotropic body in the upper layer. In the collision domain, spatially averaged fast directions show mostly perpendicular to the plate motion for all deep levels. For shallow level in this region, this trend is mainly governed by the lithospheric deformation process due to the continent-arc collision as also shown by delay time tomographic inversion. For deeper part of the region, the result of tomographic inversion and spatial averaging reveals a high anisotropy followed by rotational pattern of fast directions in the north of Timor. We suggest that this pattern might be related to the induced mantle flow due to lateral tearing of the slab.

The conceptual model of Wae Sano Geothermal field based on geology and geochemistry data

Wae Sano volcano is included in the inner Banda arc, Mount Wae Sano is a type C volcano and formed the Sano Nggoang crater lake. The magmatism activity produces geothermal manifestations such as; hot spring, rock alteration, and sulfur deposits, the hottest water temperature is 81 0C, with neutral pH, but the Sano Nggoang Lake water has acid pH. It becomes interesting to examine the characteristics of the geothermal system in that area. The research was conducted by Volcanostratigraphic studies to reconstruct the geological process and Geochemical sampling of hot springs, lake water, ground air, and the soil side to understand the subsurface characterization. The result showing some period of volcano products, with the youngest come from the product of Sano Nggoang 2 that spills its product to on the north-east side of Poco Dedeng volcano. The geochemical analysis shows all manifestations originate from one reservoir, chloride water type, NaCl type of the lake water with a few SO4 influence, presumably, the hot springs supply is influenced by seawater, the estimation of the reservoir has a temperature about ± 230 0C, with dacite and the rich organic sedimentary rock, and located at ± 1456 m from the manifestation, the isothermal section shows the rate of temperature increase at 97.07 m / 10 0C. The hypothetical resource is counted about 1,488.6 kWe.

Coastal and offshore provinces of Timor-Leste — Geophysics exploration and drilling

Regional geophysics research provides for prospect assessment of Timor-Leste, part of the Southeast Asia Archipelago in a region embracing the Banda Arc, Timor Island, and the northwest Australia Gondwana continental margin edge. Timor Island is a microcontinent with several distinct tectonic provinces that developed initially by rifting and drifting away from the Australian Plate. A compressive convergence began in the Miocene whereby the continental edge of the large craton collided with the microcontinent, forming a subduction zone under the island. The bulk of Timor Island consists of a complex mélange of Tertiary, Cretaceous, Jurassic, Triassic, Permian, and volcanic features over a basal Gondwana craton. Toward the north, the offshore consists of a Tertiary minibasin facing the Banda Arc Archipelago, with volcanics interspersed onshore with the basal Gondwana pre-Permian. A prominent central overthrust nappe of Jurassic and younger layers makes up the mountains of Timor-Leste, terminating south against an accretionary wedge formed by this ongoing collision of Timor and Australia. The northern coast of the island is part of the Indonesian back arc, whereas the southern littoral onshore plus shallow waters are part of the accretionary prism. Deepwater provinces embrace the Timor Trough and the slope of the Australian continental margin being the most prospective region of Timor-Leste. Overall crust and mantle tectonic structuring of Timor-Leste is interpreted from seismic and potential field data, focusing mostly on its southern offshore geology where hydrocarbon prospectivity has been established with interpretation of regional seismic data and analyses of gravity, magnetic, and earthquake data. Well data tied to seismic provides focal points for stratigraphic correlation. Although all the known producing hydrocarbon reservoirs of the offshore are Jurassic sands, interpretation of Permian and Triassic stratigraphy provides knowledge for future prospect drilling risk assessment, both onshore and offshore.

Introduction to this special section: Southeast Asia

This special section on Southeast Asia features geophysical topics that cover several of the magnificent geotectonic provinces of the region. Southeast Asia is the site of the world's largest archipelago, which features more than 20,000 islands extending east to west more than 3500 miles. The extent of the offshore regions of the archipelago is many times greater than its land area. The Sunda Shelf, with its numerous Tertiary basins in the western part of the archipelago, contains areas of India, Myanmar, Thailand, Laos, Cambodia, Vietnam, Malaysia, Singapore, the Philippines, Brunei, West Indonesia, and their offshore regions that extend from the Andaman Sea to the Makassar Strait. By contrast, eastern Sunda, with its pre-Tertiary basins, embraces the islands along and north of the Banda Arc, from Sulawesi to western Papua in Indonesia and Timor-Leste and surrounding seas ( Figure 1 ). The most distinguishing tectonic features of the archipelago are related to the collision of the Indo-Australian Plate with the Sunda Shelf and the areas east of it. Numerous volcanoes and earthquake epicenters trace an extensive arc of collision-related subduction zones, which makes this one of the most tectonically active regions in the world. Back-arc and other basins within the stable parts of the Sunda Shelf are the sites of significant hydrocarbon accumulation, primarily within the territorial boundaries of Indonesia and Malaysia.

Subducted Lithospheric Boundary Tomographically Imaged Beneath Arc‐Continent Collision in Eastern Indonesia

AbstractWe use traveltimes from a temporary seismic deployment of 30 broadband seismometers and a national catalog of arrival times to construct a finite‐frequency teleseismic P‐wave tomographic model of the upper mantle beneath eastern Indonesia, where subduction of the Indo‐Australian plate beneath the Banda Arc transitions to arc‐continent collision. The change in tectonics is due to a change from oceanic to continental lithosphere in the lower plate as inferred from geological mapping and geophysical, geochemical, and geodetic measurements. At this inferred transition, we seismically image the subducted continent‐ocean boundary at upper mantle depths that links volcanism on Flores to amagmatic orogenesis on Timor. Our tomographic images reveal a relatively high‐velocity feature within the upper mantle, which we interpret as the subducted Indo‐Australian slab. The slab appears continuous yet deformed as a result of the change in buoyancy due to the composition of the incoming continental lithosphere. Accordingly, there is a difference in dip angle between the oceanic and continental sections of the slab albeit not a gap or discontinuity. We suggest the slab has deformed without tearing to accommodate structural and kinematic changes across the continent‐ocean boundary as the two sections of the slab diverge. These results suggest that deformation in tectonic collisions can be localized along a continent‐ocean boundary, even at depth. We propose that future slab tearing may develop where we observe slab deformation in our study region and that a similar process may take place in collisions generally.

From subduction initiation to arc–polarity reversal: Life cycle of an Archean subduction zone from the Zunhua ophiolitic mélange, North China Craton

Subduction initiation and arc–polarity reversal have rarely been recognized in the Archean rock record. We document Neoarchean subduction initiation, fore-arc magmatism, and an arc–polarity reversal event from the Zunhua ophiolitic mélange along the eastern margin of the Central Orogenic Belt (COB) of the North China Craton (NCC). The Zunhua ophiolitic mélange is a mappable unit characterized by blocks of metamorphosed harzburgite/lherzolite, podiform chromite–bearing dunite, pyroxenite, amphibolite, metabasites (basalt and diabase) with rare intermediate volcanics, chert, and tectonic lenses of banded iron formation in a strongly sheared metapelitic matrix. New geochronological and geochemical analyses of magmatic blocks within the ophiolitic mélange show that the crustal magmatic rocks were produced in a fore-arc region at 2.55–2.52 Ga from depleted harzburgitic–lherzolitic mantle tectonites. Chemical, petrological, and temporal links between the depleted mantle blocks, and the suite of magmatic blocks derived from partial melting and metasomatism of these depleted mantle blocks, unequivocally shows that they represent part of the same original Neoarchean fore-arc ophiolite suite. After formation and accretion in the oceanic realm, the mélange was emplaced on the continental margin of the Eastern Block between 2.52 and 2.50 Ga, and underwent two stages of metamorphism at ca. 2.48–2.46 Ga and 1.81 Ga. Metamorphosed intermediate–mafic volcanic blocks exhibit systematic successive geochemical variations, from MORB-like to volcanic arc-like, and the N-MORB-like meta-basalts show remarkable similarity with the subduction initiation-related Izu–Bonin–Mariana (IBM) fore-arc basalts. We suggest that the Zunhua fore-arc complex records continuous geodynamic processes from subduction initiation to arc magmatism. The Zunhua ophiolitic mélange is part of a ca. 2.5 Ga suture between an oceanic arc of the COB and Eastern Block of the NCC. After the arc–continent collision, an arc–polarity reversal event initiated a new eastward–dipping subduction zone on the western side of the COB. This arc–polarity reversal is traced for more than 1600 km along the length of the orogen, similar in scale, geometry, and duration between collision and polarity flip to the present-day arc–polarity reversal of the Sunda–Banda arc during its ongoing collision with the Australia continent. This indicates that a life cycle of an Archean subduction zone, including birth (subduction initiation), maturity (arc magmatism), death (arc-continent collision) and re-birth (arc–polarity reversal), is recorded in the Zunhua ophiolitic mélange, and the geodynamics of plate tectonics at the end of the Archean was similar to those of today.

ZONASI SITE EFFECT DAN ANALISIS BAHAYA PENGUATAN GEMPA MENGGUNAKAN METODE DSHA UNTUK MENENTUKAN PGA DI KABUPATEN SUMBA BARAT DAYA

Southwest Sumba Regency is located in the Banda Arc area. Its position which is very close to the subduction area in the south of Indonesia caused a lot of tectonic activity and earthquake hazard. The arrangement of alluvium, coral, and thick deposits on the island of Sumba makes it prone to earthquake strengthening. Zoning of soil characters using site effects and DSHA method was carried out to determine the danger level of strengthening the Southwest Sumba Regency. Zoning of the site effect soil character uses microtremor data which is correlated with seismic hazard analysis to obtain the PGA values on bedrock and ground surface using the closest earthquake source. The area is dominated by Class 2 and 3 lands (f0 worth 1.333 - 5 Hz) according to the Kanai Classification (1983), with an amplification predominance of 3.8-8.3 times. This indicates that the research area is dominated by thick sediment deposits. The PGA value obtained from the DSHA method is known that Southwest Sumba Regency has a PGA soil of 0.075-0.19 g and a bedrock PGA of 0.067-0.085 g using earthquake record data in Sumba Subduction and Timor Subduction. With PGA estimation, it is known that the level of earthquake disaster vulnerability is high, located in the south of Southwest Sumba Regency which is suspected to be composed of thick sediments and close to the Sumba Subduction and Timor Subduction.Keywords: Site effect, predominant frequency, amplification, earthquake, DSHA, PGA

Direct Inversion of S‐P Differential Arrival Times for Ratio in SE Asia

AbstractSoutheast Asia lies within one of the most complex tectonic settings on Earth and exhibits a range of features, including strongly curved subduction zones, arc‐continent collision, and slab break‐off, which are not well understood. To help gain insight into mantle structure and processes beneath this region, we perform an inversion for variations in Vp, Vs, and structure using arrival time information from the ISC‐EHB catalog. The oceanic lithosphere subducting beneath Java is imaged as a positive dVp and negative d(Vp/Vs) anomaly. At 200 km depth, the forearc mantle beneath Sumatra and Java is revealed by positive dVp and d(Vp/Vs) anomalies which cease at Sumba island, where negative d(Vp/Vs) anomalies mark the presence of cold Australian lithosphere (down to 200–250 km depth) which is colliding with Sundaland. These negative d(Vp/Vs) anomalies depict a ∼WE trending structure that appears to correspond with the underthrusting of Australian continental crust. One notable salient has a location and shape which appears to coincide with those of ancient terranes or a Gondwana‐related microcontinent reconstructed by paleogeographic studies and may have been entrained in the subduction process. The velocity and d(Vp/Vs) patterns beneath the Banda Arc support the existence of a single curved subducting slab associated with rollback. The extreme extensional strike‐slip setting in Seram produces the highest positive d(Vp/Vs) anomalies in the model which may be due to one or more of widespread serpentinization, high concentrations of intraslab fluid‐filled faulting, and mantle upwelling.

Broadband seismic imaging around the Banda Arc: changes in the anatomy of offshore fold-and-thrust belts

Abstract The complicated geology of the Banda region results from complex collision between the Eurasia, Australia and Pacific plates, ongoing in the region since the Late Oligocene but particularly in the study area since the Middle Miocene (from 15 Ma). Regional 2D broadband seismic data have provided improved imaging of Mesozoic and Cenozoic sedimentary successions in the region. The region comprises the deep and ultra-deep Banda Sea enclosed by a magmatic inner arc and an outer deformed zone, comprising a series of orogens. This outer orogenic zone comprises islands with extended and sometimes hyperextended continental crust, a series of marginal foredeeps and intervening fold-and thrust belts. This paper illustrates how the offshore fold-and-thrust belts that bound the fore-deeps change in size, shape and degree of basement reactivation in a clockwise sense around the Banda Arc.

Vertical deformation analysis in Banda Arc deduced from GPS time series data 2008-2013

Banda arc is a product of complex interactions between the Eurasian, Australian, Pacific and Philippine Sea plates. As a result of this tectonic interaction, the Banda arc has undergone significant deformation of the crust, both in the horizontal and vertical directions. In this study, we performed a time-series data analysis of displacement in vertical components. A total of 16 permanents GPS observations from 2008 to 2013 were collected and analysed in this study. The GPS sites spread around the Banda arc area from Bali island in the south to Ambon in the north. This is the first research on vertical deformation analysis carried out in this area. The results of this study show that 3 GPS sites experience a subsidence, which are BANI, CAMB and CTOA, while the 13 others experience an uplift mechanism. The spatial pattern of vertical deformation obtained from this research will provide insight about tectonic conditions and can then be used as a basis for conducting seismic hazard analysis in the Banda arc.

The potential use of satellite gravity data for oil prospecting in Tanimbar Basin, Eastern Indonesia

One way to increase oil and gas production in Indonesia is through the geophysical study for new deposits in Basins frontier, i.e., Tanimbar basin in Province of Maluku, Eastern of Indonesia. The Tanimbar basin is located in an area known as the Outer Banda Arc. Gravity is a usually geophysical method used for the sedimentary basin. But for the regional scale, the method requires an expensive cost and long time-consuming. In this research, we use the satellite gravity data provided by Scripps Institution of Oceanography, University of California San Diego. This data has a low resolution of 1.5 km for 1 pixel and also free access. The satellite data will be compared with the gravity ground survey. The data was acquired by the University of London and Bandung Institute of Technology in 1987. The gravity satellite can show a more contrasting of geological structure compared to the ground survey, as well as syncline and anticline; the anomaly is indicated by the relatively high Bouguer data (3.5 to 25 mGal). The high anomaly is also influenced by tectonic activity from inner and Outer Banda Arc that exists in eastern Indonesia. The sub-sedimentary basin is represented by the low Bouguer anomalies (-40 to -25 mGal) that correlated well to ground survey data. Based on the result, it can be concluded; the satellite gravity is potentially used for delineating the sedimentary basin in Tanimbar Island.

Geological Aspect and Hydrocarbon Potential in West Timor

Geological condition of the West Timor is originated from the Australian Continent Terrane, Banda Arc Terrane and local deposited rocks. The geology of the study area specifically is divided into Banda Terrane units, Australian Passive Margin Sequence, Gondwana Sequence, Post-Orogenic Sequence, Syn-Orogenic Sequences and Bobonaro Clay Complex. This study aims to analyze the geology and hydrocarbon potential includes the source rock, reservoir rock and seal rock in the West Timor area. The Oligocene-Miocene Mutis Complex beginning the collision between the Asian Continent and the Australian Continent. In this study, it was shown that the highest collision activity occurred at the end of the Miocene - Early Pliocene and continued to recent. Outcrop samples were collected and GC-MS analyses performed in the laboratory for source rocks. The result shows that the main source rocks found was shale of the Aitutu Formation (Triassic) and shale from the Wailuli Formation (Early Jurassic) with Ro>0.7, they are indicating mature souce rocks. The source rocks are thought to be generated in a shallow marine environment and low energy condition with a mixture of terrestrial material. Halobia fossil was found out on the outcrop, which is play as marker fossil of the Triassic age. A reservoir rock is the sandstone from Oebaat Formation. Oebaat Formation consists of interbedded grainstone-wackestone. Belemnite fossil was found on the outcrops, which is marker as a fossil from Jurassic age. The secondary reservoir is a limestone of the Nakfunu Formation (Lower Cretaceous). The reservoir is thought to be generated in a high-density turbidity condition in shallow marine environment. Shale from Wailuli Formation (Early Jurassic) can be a potential seal rock.

Tectonic evolution of north‐eastern Tethyan Himalaya: Evidence from U–Pb geochronology and Hf isotopic geochemistry of detrital zircons

It is well established that the Himalayan Orogen was formed by successive amalgamation of continental slices to the Eurasian continent, with the final collision of the Indian continent. The Upper Triassic Langjiexue Group on the north‐eastern margin of the Tethyan Himalaya has been central to debates on the provenance with diverse models linking it with the northern India, Lhasa terrane, or multiple sources from surrounding terranes including Australia. In order to address this debate, here, we present U–Pb ages, trace element characteristics, and Hf isotope data of detrital zircons. The trace element data suggest that analysed zircons are mostly of igneous origin, with a broad affinity to mafic source rock, and some of the grains showing evidence for hydrothermal alteration. The zircon age spectra show distinct Permian to Triassic age peaks at 200–280 Ma with εHf(t) between −6.1 and 13.4, Neoproterozoic to Cambrian ages at 480–750 Ma with εHf(t) ranging from −24.7 to 8.5, and a broad Meso‐ to Neoproterozoic age range of 850–1,150 Ma with εHf(t) values of −8.1 to 10.1. The age spectra from the Langjiexue Group are in contrast with that of the Lhasa terrane which has pronounced age peaks of 300–325, 550–600, 1,150–1,350, and 1,750–1,900 Ma, suggesting that Lhasa might not be the source of Langjiexue Group detritus. In a similar way, north‐western Australia and the Banda Arc are excluded as sources of the Langjiexue Group. The Hf model age spectra show distinct peaks of 750–800 and 1,200–1,300 Ma, and the second peak agrees with that of the Tethyan Himalaya. We infer that the basin of Langjiexue Group formed within the basement of Tethyan Himalaya in the Triassic and conclude that the sediments were deposited at the passive margin of northern Indian continent. Based on the results, we also propose the palaeogeographic evolution of the Himalayan Orogen.

P and S wave travel time tomography of the SE Asia-Australia collision zone

The southeast (SE) Asia - Australia collision zone is one of the most tectonically active and seismogenic regions in the world. Here, we present new 3-D P- and S-wave velocity models of the crust and upper mantle by applying regional earthquake travel-time tomography to global catalogue data. We first re-locate earthquakes provided by the standard ISC-Reviewed and ISC-EHB catalogues using a non-linear oct-tree scheme. A machine learning algorithm that clusters earthquakes depending on their spatiotemporal density was then applied to significantly improve the consistency of travel-time picks. We used the Fast Marching Tomography software package to retrieve 3-D velocity and interface structures from starting 1-D velocity and Moho models. Synthetic resolution and sensitivity tests demonstrate that the final models are robust, with P-wave speed variations (~130 km horizontal resolution) generally recovered more robustly than S-wave speed variations (~220 km horizontal resolution). The retrieved crust and mantle anomalies offer a new perspective on the broad-scale tectonic setting and underlying mantle architecture of SE Asia. While we observe clear evidence of subducted slabs as high velocity anomalies penetrating into the mantle along the Sunda arc, Banda arc and Halmahera arc, we also see evidence for slab gaps or holes in the vicinity of east Java. In the Banda arc, we image the slab as a single curved subduction zone. Furthermore, a high-velocity region in the mantle lithosphere connects northern Australia with Timor and West Papua. The S-wave model shows broad-scale features similar to those of the P-wave model, with mantle earthquakes generally distributed within high-velocity slabs. The high velocity mantle connection between northern Australia and the eastern margin of the Sunda arc is also present in the S-wave model. While the S-wave model has a lower resolution than the P-wave model due to the availability of fewer paths, it nonetheless provides new and complementary insights into the structure of the upper mantle beneath southeast Asia.

Ocean current signals propagation along the Outer Banda Arcs

Equatorial Pacific Rossby waves may enter into the interior of the Indonesian Throughflow (ITF)’s eastern pathway, such as along the Outer Banda Arcs (OBA). This study aimed to investigate the dynamics of the ITF along the OBA channel from Ceram Sea to Aru Basin to Western Timor Passage. This is necessary because it can provide information about the physical oceanographic conditions, especially the dynamics of the current in the study area. Based on the Hovmoller diagram, a propagation of the signal from kinetic energy was found along the OBA which its phase speed theoretically appears to be trapped in Rossby waves, because of the similarity of the speed phase. The normal mode analysis shows that the oscillation in Ceram Sea, Aru Basin, and Timor Passage has the same pattern based on the potential density and Brunt-Vaisala frequency profiles. The transport’s coherence between Indo-Pacific meridional winds and transport volume in Aru Basin shows a strong coherence value (0.94) which is dominated by an annual variability with a period of 341 days. The results suggest that zonal winds along the equator Pacific Ocean act as a remotely forced Rossby waves that propagate westward, entering the interior of Indonesian Seas.

The use of gravity anomaly data to estimate the depth of mohorovicic discontinuity in bali area used power spectral analysis

Gravity is a geophysical method that can be used to measure the contrast difference in subsurface density for the interpretation of geological structures. Gravity is a non-destructive geophysical method so it is widely used in subsurface research. Bali is an area with a complex tectonic order because of its location which is part of the east Sunda arc. Bali is formed due to the subduction process of the Indo-Australian Ocean crust transition zone with Australia’s continental crust to the west and the Banda arc. This study aims to estimate the depth of sediment and limits of discontinuities of the island of Bali using the method of power spectral density analysis. The data used is gravity anomaly data from TOPEX on the island of Bali. Principle method of power spectral density analysis is to analyse the phenomenon of the harmonic oscillator in nature. The data of gravity obtained will be transformed using Fourier series so as to change the time domain into the frequency domain. From the results of processing obtained if the rocks on the island of Bali is a Miocene rock with an initial body density of 2.6 gr / cm3. The depth of shallow discontinuity ranges from 519.35 m hi to 5.92 km with an average depth of 2.63 km. While the depth of discontinuities in the range of 18.30 km to 69.62 km with an average depth of 43.39 km. It is estimated that the sediments of Bali island are shallow because of the rock age of the island of Bali which is still young adrift in the geological time scale.

Middle Eocene neritic limestone in the type locality of the volcanic Barique Formation, Timor-Leste: Microfacies, age and tectonostratigraphic affinities

Eocene shallow-water limestone is widespread in small isolated outcrops in Timor. Several occurrences of limestone in the Culocau River of Timor-Leste are significant because they lie within the type locality of the mainly volcanic Barique Formation. The tectonostratigraphic affinity of the Culocau River limestone is investigated by establishing the range of microfacies present (indicative of the inner neritic zone less than 40 m deep to the outer neritic Deep Euphotic Zone), the age of the limestone using planktonic and benthic foraminifers (late Middle Eocene, probably within the 37.8–43.6 Ma interval), and the similarity of the limestone–volcanic association to coeval occurrences elsewhere in Timor and the region. The field association of Middle Eocene neritic limestone with volcanic-volcaniclastic rocks is widespread in Timor, but is not known along the North West Shelf of Australia. Elsewhere in Timor the association of rocks found at the Barique Formation type locality also includes Late Eocene and latest Oligocene-earliest Miocene neritic limestone. This young, mainly volcanic association (Barique Group) is usually found in coherent areas of outcrop with the Late Mesozoic Palelo Group, oceanic facies (radiolarites to carbonate pelagites), Early Jurassic Bahaman-like carbonate-bank deposits (Perdido Group) and associated units of Gondwanan origin that form some high fatus surrounded by the Barique Group, and the Lolotoi/Mutis Metamorphic Complex. This is the Overthrust Terrane Association that is distinct in outcrop area and in lithostratigraphic makeup from the pre-collision Late Carboniferous to Middle Jurassic East Gondwana Interior Rift Association and the Late Jurassic to early Late Miocene Timor-Scott Plateau Association. The Cretaceous to Early Miocene units of the Overthrust Terrane Association, including the Barique Group, are similar to coeval units in Sumba and are considered to be fragments of the fore-arc of the Banda Arc.