Sulawesi and Borneo are tectonically complex islands with multistage subduction histories stretching back through the Cenozoic. Seismic studies have played an important role in helping to unravel this history, with spatial distributions of earthquakes tracking actively subducting slabs. In contrast, old or relict aseismic slabs are illuminated not by earthquakes within the slab, but rather by seismic waves that transmit through the slab and reveal them as regions of anomalously high velocity in the mantle via tomographic imaging. We use the relative arrival times of teleseismic body-waves from 1359 unique teleseismic earthquakes between 2019 and 2023, recorded by a network of 134 land and ocean bottom seismometers in the study region (corresponding to 72760 relative arrival times), to tomographically image the P wave velocity structure beneath Sulawesi and the Makassar Strait, which lies between eastern Borneo and Sulawesi. The resulting velocity model robustly uncovers a tabular high velocity anomaly at depths between ~300 and 700 km dipping NW beneath the Makassar Strait. By comparing tomographic models to plate tectonic reconstructions, we connect this anomaly to the hypothesised NW-Celebes slab, which actively subducted beneath northern Borneo between ~21 and 9 Ma. Geochemical evidence from contemporaneous volcanics suggests that slab break-off and the cessation of subduction may have been caused by the subduction of the Celebes mid ocean ridge. Comparison of our findings to pre-existing geodynamic modelling suggests that sinking of the broken-off NW Celebes slab may have helped to initiate S Celebes subduction via subduction polarity reversal.

The North Sulawesi Subduction Zone is one of the youngest active subduction systems in the western Pacific. In western Sulawesi, the Palu-Koro strike-slip fault connects with the westward-extending North Sulawesi Trench, forming a distinctive subduction-transform fault system. Understanding the crustal structure beneath the Celebes Sea and the geometry of the Palu-Koro fault is crucial for assessing regional deformation, rupture dynamics, and seismic hazards. In this study, we analyse data from nine ocean bottom seismometers (OBSs) deployed across the Palu-Koro fault using the receiver function H-kappa stacking method to estimate crustal thickness. Our results reveal a shallow Moho (similar to 8 km depth) beneath the Celebes Sea, in contrast to significantly greater depths (similar to 25 km) beneath eastern Kalimantan and northern Sulawesi. Sharp variations in Moho depth near the Palu-Koro fault suggest the juxtaposition of two distinct crustal blocks. Combining S-wave velocity structures and local seismicity catalogue, we infer that the Palu-Koro fault is a left-lateral, through-going strike-slip fault extending into the Celebes Sea. These findings provide new geophysical constraints on the interplay between strike-slip faulting and subduction retreat, with implications for the generation of tsunamis by submarine earthquakes in this tectonically complex region.

The Macquarie Ridge Complex (MRC), located at the boundary between the Australian, Macquarie, and Pacific plates south of New Zealand, is currently recognized as a dominantly transform plate boundary that evolved from a mid-ocean spreading ridge. We deployed five land stations and 27 ocean-bottom seismometers (OBSs) on and around Macquarie Island from 2020 to 2021 along the MRC in the Southern Ocean. From the waveforms recorded on successfully recovered OBSs and island stations, including permanent station MCQ, we generated a 3D S-wave velocity model of the crust and uppermost mantle using an adjoint waveform tomography method after five iterations based on surface waves (5–20 s) extracted from ambient seismic noise. The initial 3D model is constructed using real bathymetry, a water layer, and an optimal 1D model. During the inversion, we use the spectral element method to perform forward and adjoint seismic wavefield simulations with Specfem3D_Cartesian. The shortest resolvable period is about 1.35 s. The new S-wave velocity model reveals a pronounced increase in velocity across expected crustal and uppermost mantle depths between 7 and 12 km. Relatively high S-wave velocities (>3.8 km/s) in the uppermost lithosphere are consistent with the presence of upper mantle rocks at relatively shallow depths distributed along the ridge. Widespread high-velocity material may indicate that the uppermost lithosphere is not substantially deformed during obduction.

The South Yellow Sea and its environs are pivotal for unraveling the complexities of crustal dynamics and continental collision processes. A holistic assessment of deep structural variations from northern China to the Korean Peninsula is essential for a comprehensive and accurate determination of the tectonic affinity of the Korean Peninsula.Thus, we deployed a pioneering active-source seismic profile (Line2016) spanning the South Yellow Sea and the eastern onshore region of the Korean Peninsula, provides crucial insights into the collision dynamics between the Sino-Korean Block and the Yangtze Block. Our innovative approach, incorporating forward modeling, tomography, and finite-difference wavefield modeling, yielded a high-resolution crustal P-wave velocity model, addressing a significant knowledge gap in understanding the geological intricacies between northern China and the Korean Peninsula. The results confirmed and precisely located the West Marginal Fault of the Korean Peninsula, a significant crustalscale tectonic structure, likely representing the eastern boundary between the Yangtze Block and the Sino-Korean Block. The study advocates for classifying the Korean Peninsula as part of the Sino-Korean Block, presenting evidence for the one-part affinity hypothesis. This collision resulted in the creation of two distinct suture zones-an orogenic belt in the northern part and a significant strike-slip fault zone in the eastern part of the South Yellow Sea. The study emphasizes the pivotal role of block morphology in regulating plate convergence, providing valuable insights for understanding similar phenomena in other collision zones. (c) 2024 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

The North Sulawesi subduction zone is characterized by southward subduction of the Celebes Sea slab to a depth of similar to 250 km, mainly overlying the Sangihe slab that subducts west from the Molucca Sea and penetrates the mantle transition zone. The palaeo-subducted Sula slab dips northward and partially underlies both the Sangihe and Celebes Sea slabs. Adjacent subduction zones with horizontal overlapping subducting slabs in the upper mantle have unclear dynamic interactions. An extensive strike-slip fault forms the western boundary of the active North Sulawesi subduction zone, providing an ideal setting to study mantle flow between overlapping slabs. We use local S-wave and teleseismic S and SK(K)S waveform splitting analysis to measure seismic anisotropy in the northern Sulawesi region. Our observations reveal typical mantle wedge corner flow within the Sangihe subduction system. In the Gulf of Tomini, the observed trench-oblique fast-axis orientations above the Celebes Sea slab are likely a consequence of the interaction between two subducting slabs. The southernmost measurement with an E-W-trending fast direction in the mantle wedge might be related to the subduction of the Sula slab. Furthermore, fault-parallel fast-axis orientations of anisotropy near the southern segment of the Palu-Koro fault are attributed to large-scale shearing across this lithospheric-scale strike-slip fault system. Overall, our observations suggest that the strain caused by lithospheric and asthenospheric deformation is mainly confined within the microplate, displaying a restricted flow pattern and localized effects due to the size of the plate boundaries, such as the Palu-Koro fault. North Sulawesi is bordered by the Makassar Strait, Celebes Sea, Molucca Sea, and Gulf of Tomini and hosts the Palu-Koro fault. The region hosts two active subduction zones: the smaller, southward dipping North Sulawesi subduction zone and the larger westward dipping Sangihe subducting slab. Another fossilized submerged slab -Sula slab lies beneath both the Celebes Sea and Sangihe slabs to the south. Thus, it is an ideal area to investigate how the interaction between neighboring subduction zones can affect the upper-mantle deformation and whether a strike-slip fault could contribute to the mantle dynamics. We use shear-wave splitting analysis to measure seismic anisotropy to reveal the lithospheric deformation and upper mantle flow patterns of this interactive area. Our results show that the Celebes Sea slab controls mantle wedge dynamics in the North Sulawesi subduction zone, and the interaction between the Celebes Sea and Sangihe slabs occurs where two slabs entirely overlap. The strike-slip fault at the western boundary only appears to have a local influence on the regional strain field in western Sulawesi. We employ shear-wave splitting analysis to characterize the seismic anisotropy to infer upper-mantle deformation in north Sulawesi We find evidence for slab-slab dynamic interactions and constrain their influencing spatial range The Palu-Koro fault marks the west limit of lithospheric deformation imposed by the complex subduction system

Denoising is a critical step in signal processing. We develop a method for random noise reduction in active source seismic data using spectrum reconstruction. Two methods are developed for modifying the observed data’s amplitude spectrum: one substitutes it with the source wavelet’s amplitude spectrum, whereas the other involves multiplying the source wavelet’s amplitude spectrum with the observed data’s amplitude spectrum. By reconstructing the modified amplitude spectrum while preserving the observed data’s phase spectrum, noise suppression is achieved. Extensive testing with theoretical models, synthetic shot gathers, and field data indicate a notable improvement in the signal-to-noise ratio (S/N) compared with the traditional band-pass filtering method. This method proves particularly effective for enhancing the S/N in the context of active source wide-angle seismic data used in offshore structural studies, eliminating the need for data segmentation based on offset, and thereby improving processing efficiency. Our method relies solely on a single complete cycle of the source wavelet, making it a purely data-driven solution. It has broad applications in processing active source or controlled source data with consistent source wavelets, including but not limited to seismic exploration, acoustic detection, and signal denoising in various ground-penetrating radars used on Mars, the moon, and earth.

We deployed 27 ocean-bottom seismometers (OBSs) and five Macquarie Island stations between 2020 and 2021 along the Australian-Pacific plate boundary in the Macquarie Ridge Complex (MRC) of the Southern Ocean. From the waveforms recorded on successfully recovered OBSs and the island stations, including permanent station MCQ, we generated a 3-D S-wave velocity model for the crust and uppermost mantle using full-waveform ambient noise tomography. Distinct surface waves are identified via inter-station cross-correlation of the vertical components. The new S-wave velocity model reveals a pronounced increase in velocity across expected crustal and uppermost mantle depths. Relatively high S-wave velocities (>3.8 km/s at 7 km below sea level) are consistent with the presence of upper mantle rocks at relatively shallow depths spread out along the ridge. Overall, our findings provide initial insights on the sub-surface structure of the MRC, a complex tectonic setting of potential subduction initiation.

Supplementary material “SASSIER22: Full-waveform tomography of the eastern Indonesian region that includes surface topography and the fluid ocean” by Wehner, D., Rawlinson, N., Greenfield, T., Daryono, Miller, M.S., Supendi, P., Lü, ChuanChuan and Widiyantoro, S., for publication in Geochemistry, Geophysics, Geosystems. The material contains: Folder 1 MODELS Absolute values of the starting model for the SASSIER22 inversion and both final models (ocean layer — SASSIER22 — and ocean load inversion) as NetCDFs and HDF5 files, with the former being readable by e.g. xarray and the latter suitable for viewing with ParaView and interaction with Salvus. Folder 2 DATA The observed data (filtered and windowed) in MSEED format. Note that this data is provided for reproduction purposes only. The following additional information is provided in the Zenodo repository of Wehner et al. (2021), which can be found here. A Jupyter Notebook (MWE_data_processing.ipynb) that demonstrates the processing steps for the observed waveforms of an event used in the SASSY21 inversion (20181229_033914). A Jupyter Notebook (embrace_the_sass.ipynb) with a minimum working example on how to interact with the file formats mentioned above.

We present a new 3-D seismic structural model of the eastern Indonesian region and its surroundings from full-waveform inversion (FWI) that exploits seismic data filtered at periods between 15-150 s. SASSY21-a recent 3-D FWI tomographic model of Southeast Asia-is used as a starting model, and our study region is characterized by particularly good data coverage, which facilitates a more refined image. We use the spectral-element solver Salvus to determine the full 3-D wavefield, accounting for the fluid ocean explicitly by solving a coupled system of acoustic and elastic wave equations. This is computationally more expensive but allows seismic waves within the water layer to be simulated, which becomes important for periods <= 20 s. We investigate path-dependent effects of surface elevation (topography and bathymetry) and the fluid ocean on synthetic waveforms, and compare our final model to the tomographic result obtained with the frequently used ocean loading approximation. Furthermore, we highlight some of the key features of our final model-SASSIER22-after 34 L-BFGS iterations, which reveals detailed anomalies down to the mantle transition zone, including a convergent double-subduction zone along the southern segment of the Philippine Trench, which was not evident in the starting model. A more detailed illumination of the slab beneath the North Sulawesi Trench reveals a pronounced positive wavespeed anomaly down to 200 km depth, consistent with the maximum depth of seismicity, and a more diffuse but aseismic positive wavespeed anomaly that continues to the 410 km discontinuity.

Abstract The transition of a passive continental margin into a subduction zone remains a hypothesis because few geological cases have been reported. The North Sulawesi subduction zone is a 5–9 myr system in Southeast Asia that has evolved from a passive continental margin and has long been overlooked by studies of passive to active margin transitions. Here we compare geophysical evidence from the region with our numerical simulation results. We find that the initial subduction of North Sulawesi relies on horizontal stress, where the trench retreat depends on the negative buoyancy of the oceanic lithosphere. Furthermore, less space available for subduction leads to reduced mantle flow caused by subduction and slower trench retreat. These new dynamical constraints indicate that the negative buoyancy of the oceanic plate is a key factor for the trench retreat, even though subduction initiated was induced.

We deployed the first dense onshore‐offshore seismic survey profile across the ultrahigh/high‐pressure metamorphic Sulu Orogenic Belt (SLOB). By applying multiple seismic and gravity modeling, we obtained the crustal P wave velocity and density model across the SLOB and extending into tectonic units on both sides, including the eastern Sino‐Korean Block (SKB), and the lower Yangtze Block (YZB). A ∼120‐km‐wide high‐velocity and high‐density upper crust in the southern SLOB reveals the distribution of ultrahigh/high‐pressure metamorphism, which should be linking with the deep subduction, detachment, and exhumation of YZB midcrust. In the northern SLOB, the U‐shaped velocity contours throughout the crust and the low‐density midcrust reveals the downward bending of the crustal deformation due to the collision between SKB and YZB. The crustal deformation and detachment in the SLOB revealed by our results indicate that the orogenic spatial distribution should be wider than what we know before.

The Semail ophiolite, a thick thrust sheet of Late Cretaceous oceanic crust and upper mantle, was obducted onto the previously rifted Arabian continental margin in the Late Cretaceous, and now forms part of the United Arab Emirates (UAE)‐Oman mountain belt. A deep foreland basin along the west and SW margin of the mountains developed during the obduction process, as a result of flexure due to loading of the ophiolite and underlying thrust sheets. The nature of the crust beneath the deep sedimentary basins that flank the mountain belt, and the extent to which the Arabian continental crust has thickened due to the obduction process are outstanding questions. We use a combination of active‐ and passive‐source seismic data to constrain the stratigraphy, velocity structure and crustal thickness beneath the UAE‐Oman mountains and its bounding basins. Depth‐migrated multichannel seismic reflection profile data are integrated in the modeling of traveltimes from long offset reflections and refractions, which are used to resolve the crustal thickness and velocity structure along two E‐W onshore/offshore transects in the UAE. Additionally, we apply the virtual deep seismic sounding method to distant earthquake data recorded along the two transects to image crustal thickness variations. Active seismic methods define the Semail ophiolite as a high‐velocity body dipping to the east at 40°–45°. The new crustal thickness model presented in this work provides evidence that a crustal root is present beneath the Semail ophiolite, suggesting that folding and thrusting during the obduction process may have thickened the pre‐existing crust by 16 km.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing an older version [v1]Go to new versionCrustal structure of the UAE-Oman mountain range and Arabian rifted passive margin: new constraints from active and passive seismic methodsAuthorsSimonePiliaiDMohammed YAliiDMichael P.SearleiDAnthony B.WattsiDChuanChuanLüDavid AThompsonSee all authors Simone PiliaiDCorresponding Author• Submitting AuthorUniversity of CambridgeiDhttps://orcid.org/0000-0002-3805-9257view email addressThe email was not providedcopy email addressMohammed Y AliiDKalifa UniversityiDhttps://orcid.org/0000-0001-7502-3897view email addressThe email was not providedcopy email addressMichael P. SearleiDOxford UniversityiDhttps://orcid.org/0000-0001-6904-6398view email addressThe email was not providedcopy email addressAnthony B. WattsiDUniversity of OxfordiDhttps://orcid.org/0000-0002-2198-2942view email addressThe email was not providedcopy email addressChuanChuan LüUniversity of Cambridgeview email addressThe email was not providedcopy email addressDavid A ThompsonSchool of Earth and Ocean Sciences, Cardiff Universityview email addressThe email was not providedcopy email address

The region around the Celebes Sea, SE Asia, is evolving within a convergent tectonic environment involving the Pacific plate to the east and the Indian‐Australian plate to the south. It is arguably one of the most tectonically complex regions in the world and serves as an ideal setting to study dynamic interactions between the Pacific and Tethys tectonic domains. The issue of which subducting plate plays a leading role in governing the regional mantle flow is not well understood. Mantle flow can be characterized by seismic anisotropy observations, providing clues for understanding regional tectonics. We conducted SKS‐wave splitting analysis by using data from seven seismic stations located around the Celebes Sea. Our results, when combined with previous observations, suggest the presence of various types of mantle flow in this area, including (a) corner flow in the mantle wedge above the westward‐subducting Molucca Sea (Sangihe) slab, (b) two‐layer anisotropy related to the eastward‐subducting Molucca Sea (Halmahera) slab, (c) deflected flow due to proximity to the Sangihe slab's edges, (d) trench‐normal mantle flow beneath southeastern Borneo due to the subduction of the Indian‐Australian plate, and (e) Northwest Borneo‐Palawan trough‐parallel mantle flow beneath northeastern Borneo. Various types of mantle flow indicate that the dynamic interactions of adjacent subduction zones played crucial roles in influencing the regional upper mantle dynamics between the Tethys and western Pacific domains since the breakup of the Gondwana supercontinent in the Mesozoic.

Two major tectonic units in NE Asia are the Sims-Korean (or North China) and South China Blocks that collided in the Permo-Triassic periods. The South China Block is suggested to extend eastward to the Korean Peninsula across the southern Yellow Sea. The Gunsan Basin is a well-defined, fault-bounded sedimentary basin in the SE Yellow Sea and is regarded as a structural link of the South China Block to the Korean Peninsula. We collected deep seismic sounding data recorded on ocean bottom seismometers to investigate how the crustal structure and nature of the South China Block vary in the Gunsan Basin toward the middle part of the Korean Peninsula. The main part of the Gunsan Basin is underlain by relatively thin (similar to 10 km thick) upper and much thicker (similar to 20 km thick) lower crustal layers, indicating a close affinity to the South China Block; whereas it shows distinction from the Korean Peninsula underlain by upper and lower crustal layers with equal (similar to 15 km) thickness. A change in crustal structure is recognized under the eastern margin of the Gunsan Basin toward the Korean Peninsula, that features a transition to the Korean Peninsula. The thick lower crustal layer in the Gunsan Basin appears to be inherited from the evolution of the South China Block including crustal shortening associated with the assembly of Rodinia in the Neoproterozoic before the collision between the Sino-Korean and South China Blocks.

苏拉威西海具有独特的演化过程—位于北婆罗洲及苏禄岛弧下的苏拉威西海板块停止俯冲后,其俯冲板块的另一侧开始向苏拉威西岛俯冲,形成了诸如俯冲转换边缘、对向俯冲系统等独特的地质现象.针对这种同一板块一侧俯冲停止后在板块另一侧发育新俯冲的过程,科学界提出了众多的板块初始俯冲机制模式,如前陆盆地碰撞后发育新俯冲、大陆/岛弧边缘重力失稳后解耦后俯冲、岩石圈浅部小范围的地幔对流活动导致俯冲等14种初始模式.由于难以获得处于俯冲初始阶段俯冲带的深部岩石圈的地震观测资料,造成这些板块活动模式仍止步于推测.苏拉威西海区的天然地震观测资料不仅是研究苏拉威西海俯冲系统岩石圈深部构造及演化的基础,而且也是认识俯冲过程初始机制的关键一环.本文重点介绍了国家自然科学基金委员会重大研究计划"西太平洋地球系统多圈层相互作用"所属重点项目"苏拉威西海与古南海对向俯冲系统的三维地震观测与板块活动机制"在苏拉威西海区针对北苏拉威西海俯冲板片结构所开展的三维地震观测研究.该项研究实验以揭示俯冲系统深部结构与板块活动机制为目标,通过多边国际合作,于2019年8月15—25日在苏拉威西海区成功投放了27台国产宽频带海底地震仪(BBOBS),拟采集10个月的天然地震数据.同时计划利用印尼合作方的陆区地震台站数据,与OBS数据一同开展研究区岩石圈三维结构的地震学研究与数值模拟,认识俯冲下插板片的变形形态与特征、上覆地壳增厚程度及与俯冲系统持续作用过程之间的关系,讨论该地区独特俯冲系统的初始机制.

The residual mantle gravity anomaly (RMA) is a straightforward response of the deep mantle structure of the lithosphere. We acquired RMA from newly updated free-air gravity anomaly by incorporating abundant geophysical evidences, such as the International Ocean Drilling Program expedition 349, the interpretation of 10 seismic profiles and the bathymetric data in the Southwest Sub-basin (SWSB) of the South China Sea. By inverting the crustal thickness by the application of the isostatic flexure analysis, comparing with ocean bottom seismic tomography result, then determining a detail crustal thickness of the study region, we calculated the RMA of the oceanic basin in the SWSB. The main features of the RMA and their relation to the palaeo-spreading pattern are discussed, and the implications of the asymmetric lithospheric deformation due to variations in melt extraction are proposed. In the light of the asymmetric RMA at the north and south sides of the fossil spreading centre in the SWSB, we preferred asymmetric melt retention depth and extraction rate changes during the oceanic lithosphere creation of the South China Sea. Copyright (c) 2016 John Wiley & Sons, Ltd.

The continental margins of the southwest subbasin in the South China Sea mark a unique transition from multi-stages magma-poor continental rifting to seafloor spreading. We used reflection and refraction profiles across the margins to investigate the rifting process of the crust. Combining with the other seismic profiles acquired earlier, we focused on the comparative geological interpretation from the result of multichannel seismic analysis and wide-angle seismic tomography. Our result provides the evidence of upper crustal layer with abundant fractures below the acoustic basement with a P-wave velocity from 4.0 to 5.5 km s −1 . It indicates extensive deformation of the brittle crust during the continental rifting and can make a good explanation for the observed extension discrepancy in the rift margins of the South China Sea. The seismic chronostratigraphic result shows the possibility of the intra-continental extension center stayed focused for quite a long time in Eocene. Additionally, our evidence suggested that continental margin of the southwest subbasin had experienced at least three rifting stages and the existence of the rigid blocks is an appropriate explanation to the asymmetric rifting of the South China Sea.