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In the late 90's, some faults identified within oceanic crust were demonstrated to be artifacts arising from out-of-plane scattering along linear sediment-buried fault scarps. Symmetrical mantle reflections observed southwest northern Sumatra on seismic reflection profiles have been identified as faults cutting through the upper mantle down to unprecedented depths reaching ~45 km. Seawater being conveyed along sub-vertical re-activated fracture zones (FZs) to the upper mantle, the mantle portions of FZs are serpentinized and act as mirrors for seismic rays. We suggest that the mantle features are not faults but artifacts resulting from out-of-plane reflections on these mirrors. Two perpendicular seismic profiles crossing the same FZ display two dipping features down to 30 km, which cannot be explained as faults from recent tectonic and structural constraints but merely as out-of-plane reflections on this FZ. This result confirms that most of mantle reflections observed southwest northern Sumatra are fakes rather than faults. [ABSTRACT FROM AUTHOR]

An E-W oriented OBS2015-2 wide-angle refraction profile was shot in the northeastern South China Sea (SCS) between previous refraction profiles T1 and T2 in order to better understand the variability of the crustal composition and the role of specific tectonic features. P -wave velocity models established from forward and inversion modeling imaged a 12 to 15 km-thick thinned continental crust and a high velocity layer (HVL) in the lower crust which is interpreted as magmatic underplating. Profile OBS2015-2 cut across the Taiwan transfer zone (TTZ), which separates two consecutive rifted segments of different orientations in the northeastern SCS. The TTZ is a well-defined upper crustal feature characterized by a HVL with different thickness on both sides. We have defined the southward limit of the thinned continental domain with the typical oceanic domain as a continent-ocean boundary (COB). The COB between the drilling area of IODP legs 367–368 to the Manila trench is characterized by a sharp contrast between the low amplitude, irregular shape magnetic anomalies and the high amplitude, elongated shape magnetic anomalies, which are associated with the thinned continental domain and the oceanic domain, respectively. We further extend the COB into the 400 to 500 km wide unfolded Manila slab located east of the Manila trench by using mid-slab dVp tomographic velocities to define the boundary between subducted oceanic and thinned continental crust. The location of the reconstructed northeast SCS COB appears to step northwards toward Taiwan across a 400 km N-S segment, providing new constraints on SCS geodynamics at the end of spreading and Taiwan pre-collisional configuration. [ABSTRACT FROM AUTHOR]

Abstract: The Philippine Sea Plate (PHS) simultaneously subducts northwestward and collides eastward with the Eurasian Plate (EU) in northeast Taiwan. These two tectonic events induce high seismic activity, which makes northeastern Taiwan one of the most seismically active zones in the world. To understand the mechanical processes at work, we used existing geophysical data and the aftershocks recorded following a recent large strike-slip event occurring within the PHS oceanic crust. During this event, a NW–SE trending left-lateral sub-parallel to the PHS/EU convergence vector was active. As a consequence of the collision/subduction plate geometry, we show that the lithosphere of the northwestern corner of the PHS has been torn in a NW–SE orientation. This tectonic feature is associated with an abrupt tectonic stress boundary and could generate large intra-plate earthquakes. [Copyright &y& Elsevier]

The Ilan Plain is located at the southwestern tip of the Okinawa Trough backarc basin, which propagates westward into the Taiwan orogen. From three moderate earthquakes which occurred near the coastline of the Ilan Plain, one of magnitude 6.2 on 15 May 2002 and the other two of magnitudes 5.51 and 5.49 on 5 March 2005, we attempted to understand the relationship between the earthquake mechanisms and the geological context. Seven seismic reflection profiles collected roughly parallel to the coastline of the Ilan Plain were used in this work. A structural fault, with a significant normal faulting component trending approximately ENE-WSW, is identified to the east of the doublet earthquakes of 5 March 2005. Because this fault follows the ENE-WSW trend of the aftershock seismicity, we named it the Ilan ShelfFault, which might extend to the east beyond the Ilan continental shelf spur. However, the centroid seismic moment tensor solutions for the doublet earthquakes of 5 March 2005 are of a left-lateral strike-slip faulting type which are consistent with onland GPS observations. Hence, the Ilan ShelfFault may have a left-lateral strike-slip component. Based on GPS data, we suggest that the northern Central Range block, bounded in the west by the Lishan Fault and in the north by the Ilan Shelf Fault, rotates clockwise. This rotation is ascribed to the northwestward collision of the Luzon Arc against Taiwan. Thus, there is a left-lateral strike-slip and an extensional motion along the Ilan Shelf Fault. The northern Central Range block is tilting northward which may also induce the normal faulting component of the Ilan Shelf Fault. In short, the Ilan Shelf Fault could be a major tectonic and structural feature bounding the southern end of the Okinawa Trough backarc basin. [ABSTRACT FROM AUTHOR]

Intense magmatism in the form of widespread volcanoes and lava flows is identified in high-resolution 3-D seismic data over a postrift sequence of the northern South China Sea (SCS). Such a magmatism post-dates the end of seafloor spreading in the SCS by at least 6.8 m.y. A detachment (boundary) fault propagating into a deep-seated magma chamber provided the main vertical pathway for magma migration. In turn, normal faults and dykes constituted a shallow plumbing system through which the magma migrated from the boundary fault and was extruded onto the paleo- seafloor. Volcanism occurred in the study area from ca. 8.2 Ma to ca. 1.1 Ma in the form of two distinct events, dated ca. 5.2 Ma and ca. 2.8 Ma, which are correlated with the Dongsha Event. Extrusive magma formed volcano edifices and extensive lava flows; the latter of which were confined to the troughs of sediment waves or, instead, flowed along submarine canyons. As a corollary, this study shows that in the SCS: (1) young magmatism is widespread on the northern continental margin, (2) seafloor morphology greatly influences the architecture of deep-water volcanoes, and (3) syn-rift faults (especially detachment faults) reactivated by regional tectonics closely control the magma plumbing systems. [ABSTRACT FROM AUTHOR]

Forward and inverse P- wave velocity models along an E -W 280-km-long wide-angle reflection/refraction profile at 21°N across the northeastern South China Sea margin and Manila subduction zone reveal a 10–12 km thick thinned continental crust (TCC) located west of the Manila transcurrent fault (MTF) and Manila slab. The TCC is divided in two parts separated by the intra-continental velocity difference boundary (VDB). West of the VDB, the TCC is located above a high velocity layer (HVL). East of the VDB, the TCC consists of three layers: an undefined body with a velocity inversion at its base above the upper and lower crusts. The VDB and MTF merge east of the Hengchun peninsula showing that the TCC located east of the VDB decreases in size and disappears to the north. The distribution of earthquakes (EQs) shows that ∼80 km of crust have subducted east of the MTF since the onset of Luzon arc collision with Eurasia at ∼7 Ma. All published tomographies suggest the subduction of oceanic crust (OC). However, due to crustal corrections, up to a maximum width of 40 km of TCC might eventually exist in the upper part of the Manila slab instead of OC. Two end-members with 40 km of TCC or not in the upper part of the Manila slab were restored at ∼7 Ma. Since that time, the north Luzon island moved ∼600 km in the northwest while part of this motion was absorbed near the Hengchun peninsula by shrinking of Eurasia TCC, resulting in a ∼ 20° clockwise rotation of the Manila slab and MTF. Small Luzon arc blocks started to rotate 10°-30° clockwise at ∼7 Ma below northern Taiwan, 3–4 Ma in the Coastal range and 1 Ma in Lutao and Lanyu islands and not south of the Hengchun peninsula. Thus, both the clockwise rotation of the individual Luzon arc blocks and of the Manila slab and MTF might explain the collision process of the Luzon arc with the Eurasia continent. • Thinned continental crust (TCC) west of the Manila transcurrent fault and Manila slab. • Two adjacent stripes separated by an intra-continental velocity difference boundary composed the TTC. • The left-lateral motion guides the driving force controlling the collision of Luzon arc and Eurasian TCC. [ABSTRACT FROM AUTHOR]

We interpret seven two-dimensional deep-penetration and long-offset multi-channel seismic profiles in the northernmost South China Sea area, which were collected by R/V Marcus G. Langseth during the TAIwan GEodynamics Research (TAIGER) project in 2009. To constrain the crustal characteristics, magnetic inversion and forward magnetic modeling were also performed. The seismic results clearly show tilted faulting blocks in the upper crust and most of the fault plane connects downward to a quasi-horizontal detachment as its bottom in the south of the Luzon-Ryukyu transform plate boundary. North of the plate boundary, a small-scale failed rifted basin (minimum 5 km in crustal thickness) with negative magnetization probably indicates an extended continental origin. Significant lower crustal material (LCM) was imaged under a crustal fracture area which indicated a continent and ocean transition origin. The thickest LCM (up to 6.5 km) is located at magnetic isochron C15 that is probably caused by the magma supply composite of a Miocene syn-rift volcanic event and Pliocene Dongsha volcanic activity for submarine volcanoes and sills in the surrounding area. The LCM also caused Miocene crustal blocks to be uplifted reversely as 17 km crustal thickness especially in the area of magnetic isochron C15 and C16. In addition, the wide fault blocks and LCM co-existed on the magnetic striped area (i.e. C15-C17) in the south of the Luzon-Ryukyu transform plate boundary. Magnetic forward modeling suggests that the whole thick crustal thickness (>12 km thick) needs to be magnetized in striped way as oceanic crust. However, the result also shows that the misfit between observed and synthetic magnetic anomaly is about 40 nT, north of isochron C16. The interval velocity derived from pre-stack time migration suggests that the crust is composed of basaltic intrusive upper crust and lower crustal material. The crustal nature should refer to a transition between continent and ocean. Thus, the magnetic reversals may be produced in two possible ways: basaltic magma injected along the crustal weak zone across magnetic reversal epoch and because some undiscovered ancient piece of oceanic crust existed. The crustal structure discrimination still needs to be confirmed by future studies. [ABSTRACT FROM AUTHOR]

Submarine landslides or slumps may generate turbidity currents consisting of mixture of sediment and water. Large and fast-moving turbidity currents can incise and erode continental margins and cause damage to artificial structures such as telecommunication cables on the seafloor. In this study, we report that eleven submarine cables across the Kaoping canyon and Manila trench were broken in sequence from 1500 to 4000 m deep, as a consequence of submarine landslides and turbidity currents associated with the 2006 Pingtung earthquakes offshore SW Taiwan. We have established a full-scale scenario and calculation of the turbidity currents along the Kaoping canyon channel from the middle continental slope to the adjacent deep ocean. Our results show that turbidity current velocities vary downstream ranging from 20 to 3.7 and 5.7 m s-1, which demonstrates a positive relationship between turbidity current velocity and bathymetric slope. The violent cable failures happened in this case evidenced the destructive power of the turbidity current to seafloor or underwater facilities that should not be underestimated. [ABSTRACT FROM AUTHOR]

Deep seismic data and plate kinematic reconstructions help understand the mechanisms of rifting and opening of new oceans, basic principles of plate tectonic cycles. In this study, available deep wide-angle seismic velocity models from the Atlantic margins are reviewed and plate reconstructions used to define conjugate model pairs. The main objective was to study the question of how magma-rich and magma-poor margins develop and the role of inheritance in the break-up. We also studied the question of the mechanism of formation and the origin of transform marginal plateaus, which are typically found at the border of two ocean basins of different ages and are mostly characterized by at least one volcanic phase during their formation. The results of the study include the comparison of crustal thickness, oceanic plate thickness and the influence of volcanism along the Atlantic margins. The conjugate profiles image different degrees of asymmetry of the Atlantic Margin rifts. Marginal plateaus might form when rifting stops at barriers leading to the accumulation of heat in the mantle and increased volcanism directly before or after halting of the rifting. • Deep seismic velocity models, plate kinematic reconstructions and geological information are used. • Rifting styles and volcanic influence along the Atlantic Margins are studied. • Zones of exhumed and serpentinized upper mantle had been discovered only at magma-poor margins. • At magma-rich margins the volume of magma is largest close to transfer zones and decreases in the direction of rifting. • Sheared margins are often associated with transform marginal plateaus, which have undergone at least one volcanic phase. [ABSTRACT FROM AUTHOR]