Continent-ocean Boundary Research Articles

Subduction Initiation at the Corner of Small Oceanic Basins

AbstractIn Southeast Asia, emerging subduction zones often appear to begin at the corners of small oceanic basins, which have a triangular‐indenter continent–ocean boundary geometry. To investigate the influence of a triangular corner on subduction initiation, we performed a series of three‐dimensional numerical simulations with varying corner angles and base lengths. The results show that the apex of the corner constitutes the initial location of subduction, irrespective of the angle or the extent of the corner. Smaller angle corners are more likely to facilitate subduction initiation. At the same time, wide acute angle corners are difficult to form. Our findings suggest that triangular corner structures may facilitate subduction initiation in smaller basins; however, the role such corners in subduction initiation is limited in larger basins. Our results emphasize the importance of accounting for the three‐dimensional geometry of a subduction zone when examining its subduction dynamics and geological features.

Heat flow of the Mozambique channel: New data and predicted surface heat flow map

The Mozambique Channel, a complex domain composed of passive margins and oceanic crust affected by younger strike-slip and volcanic activity, is still poorly studied for surface heat flow. We present 33 new marine-type heat flow estimations acquired offshore in 2014–2015 during the MOZ cruises, completed with 4 new heat flow values derived from reappraised old well sites (PAMELA project). Offshore heat flow is generally low to normal and does not display strong variations. As poor data coverage persists in large parts of the area, we performed a heat flow prediction mapping using the similarity method. Based on an extrapolation of old and new data guided by 14 geological and geophysical proxies, we obtain on a 0.1° × 0.1° grid a predicted heat flow and its uncertainty from the statistics of all values at global grid points with similar proxies. We succeeded to obtain good predictability on 70% of pixels and an RMS error <2.3 mW/m2. The proxies selected for the prediction include a map of genetic domains, sediment thickness, topography and bathymetry, crustal ages, Curie point depth, Free air anomaly, distances to volcanoes, tectonic structures and continent-ocean boundaries. The new Mozambique heat flow map shows a predominantly low offshore heat flow of around 50 mW/m2, associated with the Jurassic and early Cretaceous age of the oceanic crust. Several heat flow anomalies standoff to this background heat flow with two main orientations: low heat flow anomalies with a N–S orientation and high anomalies with a mostly NE-SW orientation, which are linked mainly to tectonic structures.

ЛИТОЛОГИЯ И ОБСТАНОВКИ НАКОПЛЕНИЯ НИЖНЕМЕЛОВЫХ ОТЛОЖЕНИЙ СЕВЕРНОЙ ЧАСТИ ЖУРАВЛЕВСКОГО ТЕРРЕЙНА (ХАБАРОВСКИЙ КРАЙ)

The authors present the study and paleogeodynamic interpretation of the Lower Cretaceous deposits material composition from the northern part of Zhuravlevka terrane. In their composition, the sandstones correspond to petrogenic graywackes, formed due to the mechanical destruction of parent rocks. Interpretation of the results shows that sediments accumulated along the continent-ocean boundary in a basin, associated with strike-slip dislocations along transform faults. The sources area united the sialic land composed of acidic igneous and sedimentary rocks, as well as a mature, deeply eroded ensialic arc.

Continent‐Ocean Transition or Boundary? Crowd‐Sourced Seismic Interpretations of the East‐India Passive Margin

AbstractOn the edge of our continents, oceanic crust meets continental crust. At passive margins, those where there is no active tectonics, subduction or transform faulting, these crustal types are connected as sharp continent‐ocean boundaries (COB) or as diffuse continent‐ocean transition (COT) zones. Passive margins are hard to explore and consequently relatively little is known about their morphology or the geological processes of their formation. Here we elicit and analyze seismic image interpretations of the passive margin offshore East India conducted by 17 groups of geoscientists to better understand the differences, or lack therein, of COB or COT interpretations of the margin. The group interpretations provide a wide range of margin models, five of which are abrupt COB based and 11 which are diffuse COT based. However, interpretations within the COB set vary in the placement of the boundary line between continental and oceanic crust, the boundary placement lying within the range of interpreted COT zones, with the average COB location falling in the center of the interpreted COT zones. These crowd‐sourced results are then compared with 10 published interpretations across the margin, which show COB and COT zones falling in the same area. These findings raise questions as to the real differences in COB and COT models and the geological processes involved in their formation. Considering this, we discuss the implications for passive margin models and the use of Wisdom of Crowds‐type approaches in reflecting on both the range of interpretation‐based models and in the value of determining “average” model approaches.

The geoelectric field coast effect: Results from two dimensional finite element method and analytic solutions

The geoelectric fields induced during geomagnetic disturbances (GMD) may affect the normal operation of technological systems at the Earth’s surface. The non-uniform Earth conductivity structure, especially large lateral conductivity contrasts, can lead to the enhancement of geoelectric fields near boundaries such as a coastline. In this paper, we give an overview of the applicability of a two-dimensional finite element method (2D-FEM) for studying lateral variations of the Earth’s conductivity, which can be considered as one of magnetotelluric sounding forward modelling methods, including both the E and the H polarization cases. Then the spatial distribution of the geoelectric field near a continent-ocean boundary is calculated with different models. This is compared with analytic solutions for simple models to verify the accuracy of the FEM modelling, which is calculated by using the ANSOFT software. More realistic scenarios are then developed to emphasize the advantages of FEM. The factors influencing the coast effect are discussed. Results show that frequency, the shape of seafloor boundaries, and the depth of ocean have a significant impact on the geoelectric fields. Therefore, these factors should attract more attention when assessing risk for man-made conducting systems located within the coastal influence range.

From the Lebombo Monocline to the Mozambique Deep Basin, using combined wide-angle and reflection seismic data

The North Natal valley (NNV), South Mozambique margin, is a key area for the understanding of the SW Indian Ocean history since the Gondwana break-up as its crustal nature and geometry strongly impacted the reconstruction of the paleogeography before the rifting. It is also of considerable importance for the understanding of the evolution of a margin system as the NNV is situated at the transition between divergent and strike-slip segments and at the conjunction of Oxfordian-Kimmeridgian Indian Ocean and the Valanginian-Aptian Atlantic one. As one part of the PAMELA project (PAssive Margins Exploration Laboratories), the NNV and the East Limpopo margin have been investigated during the MOZ3/5 cruise (2016), through the acquisition of 7 intersecting wide-angle profiles and coincident marine multichannel (720 traces) seismic as well as potential field data. Simultaneously, land seismometers were deployed in the Mozambique coastal plain (MCP), extending six of those profiles on land for about 100 km in order to provide information on the onshore-offshore transition. Wide-angle seismic data are of major importance as they can highlight constraints on the crustal structure of the margin and the position of the continent-ocean boundary in an area where the crustal nature is poorly known and largely controversial. The MOZ3/5 data set therefore reveals new essential constraints for kinematic reconstructions. This work presents results on the crustal structure from P-waves velocity modeling along two E-W wide-angle profiles (MZ1 and MZ2) through the NNV, from the Lebombo Monocline to the Mozambique Basin (MB), and crossing the Mozambique Fracture Zone (MFZ).The new geophysical data reveals an upper sedimentary sequence characterized by low velocities generally not exceeding 3 km/s, and up to 3 km thick where a major contouritic structure was observed. This feature formes together with several other contouritic structures, a N-S alignment just west of the MFZ, which produces high positive gravity anomalies, previously thought to be related to the magmatism that built the Galathea and Dana Plateaus. High velocity lenses are locally identified through the sedimentary layers and interpreted as inter-bedded volcanic sills. Furthermore, from the NNV to the MFZ, the underlying sequence is formed of a 3.0–3.5 km thick volcano-sedimentary sequence presenting important lateral changes in its seismic signature and characterized by a large velocity range (4.4 to 5.8 km/s), which partly reflects variations in the volcanic/sedimentary ratio laterally and with depth. At depth, an initially smoother and reduced eastward thinning of the crust occurs to the West below the continental shelf, from 34 to 31 km thick. The crustal thickness remains relatively constant of about 28–29 km along the Central Domain (CD), whereas a second and major region of thinning (26 to 12 km thick) is imaged West of the MFZ, in the southward prolongation of the Limpopo Corridor (LC). By contrast, as the eastern extremity, the crust is <10 km thick when reaching the MB. Crustal velocities reveal low velocity gradients, with atypical high velocities, increasing to 7.3 to 7.6 km/s at the base of the crust, and globally in the whole crust in the LC, just West of the MFZ. We interpreted the velocity architecture combined with the evidence of volcanism at shallower depths as indicative of an intensively intruded continental crust in the NNV, and discuss the particular segmentation of the longest profile (MZ1) in the kinematic context of both divergent and strike-slipe segments offshore Mozambique.Combining wide-angle and reflection seismic observations along these two profiles and the other MOZ3/5 lines, the data shows a coherent segmentation of the E-W crustal architecture off South Mozambique. These results along MZ1 and MZ2 profiles, combined with the previously published profiles, gives a 3D-view of the NNV, which becomes one of the most passive margins covered in the world by deep wide-angle seismic data.

Geodynamic aspects of the metallogeny-magmatic zonality relationship of orogenic volcanic-plutonic belts

Research subject. The orogenic marginal-continental and collisional volcanic-plutonic belts, and the zonality of magmatism and metallogeny within them, as well as the relationship of endogenous metallogeny with magmatic zonality and geodynamics of belt formation.Materials and methods include the long-term studies of the Devonian and Upper-Paleozoic belts of Kazakhstan and the Cretaceous-Paleogenic belt of Sikhote-Alin; an analysis of the alterations in the material parameters of magmatites; the identification of the front and rear parts of the belts and the arrangement features of metallogenic zones within their limits; and the data on other belts of the world.Results. For the marginal continental belts, an asymmetric change of the tholeiitic and low-potassium magmatites of the calcareous-alkaline petrochemical series of the frontal zone of the belt into the magmatites of the high-potassium branch of the calcareous-alkaline and shoshonite series of the rear zone (Devonian and Upper Paleozoic belts of Kazakhstan) has been established. In the same direction, the elemental, as well as the genetic type of deposits changes: pyrite copper-polymetallic frontal zones are replaced by molybdenum-copper-porphyry rear zones (near the border with the frontal zone) and then by rare metal ones. This is also characteristic of the Andean and Okhotsk-Chukchi belts. The collision belts are characterised by symmetrical zonality with a change from rare metal deposits in the centre to copper-polymetallic deposits at the edges (Mongolia). In some cases (Sikhote-Alin and Japan), the continuity of the marginal-continental belt is disrupted as a result of the mantle diapir (or plume), symmetrical magmatic and metallogenic zonality occurs with the change of polymetallic manifestations (near the ocean–continent boundary) to rare-metal ones in the distance. Conclusions. The change in the material parameters of the magmatites of the marginal-continental belts correlates with the removal of their ranges from the ocean–continent transition zone to the interior of the latter. In this case, copper-polymetallic pyrite ore formations are replaced by copperporphyry and then by rare metal ones in this direction. This is due to subduction processes. The collision situation is characterised by a symmetrical zonality of material magmatic parameters with respect to the crowding zone and a symmetrical arrangement of metallogenic zones with a change of rare metal deposits in the crowding zone to copper and polymetallic along the frame. These data are important for metallogenic prediction, as well as for palaeotectonic reconstructions.

Structure and morphology of the Red Sea, from the mid-ocean ridge to the ocean-continent boundary

The diversity and complexity of existing models on the structure of the Red Sea is the result of a mobile sedimentary cover that not only hides most of the basement and oceanic structures but also hamper the geophysical signals. In this study, we combined published with new unpublished geophysical data to propose a revised interpretation of the structure of the lithosphere between the Hanish Islands (ca. 13.5°N) and the Dead Sea Fracture Zone (ca. 27.5°N). The recognition of allochthonous salt and overburden allows to propose a structural model with simple geometries that are consistent with the present-day kinematic constraints (GPS velocities and Euler Poles) and with the rules of plate's motion on a sphere (Euler Theorem).We mapped a continuous Mid Ocean Ridge segment parallel to the main rift orientation in the southern Red Sea that becomes highly discontinuous and more oblique towards the north as it gets closer to the Euler Pole. These segments all point towards the Euler pole and are consistently offset to the right by transform faults in the central and northern Red Sea cumulating an offset of 400 km. The transform faults are accommodating seafloor spreading in the Red Sea and cluster in three main fracture zones: (1) the Ikwan Fracture Zone near the Ikwan (or Brother) Islands, (2) the Zabargad Fracture Zone in the vicinity of the Zabargad Island and (3) the Jeddah Fracture Zone offshore the Jeddah/Port Sudan area (central Red Sea). Based on a new seismic reflection dataset and published data we propose an interpretation of the Ocean Continent Transition (OCT) in the Red Sea. We bring new evidence (magnetic lineaments, distance of the OCT to the ridge, basin stratigraphy) to support a Middle Miocene (ca. 14 Ma) age for the initiation of seafloor spreading throughout the Red Sea, and we discuss this event with the initiation of strike-slip motion in the Aqaba-Dead Sea transform zone. Finally, compiling all available data and literature, we propose a regionally coherent geodynamic reconstruction using two different poles of rotation for the rifting and seafloor spreading phases.

Abrolhos Magmatic Province petrogenesis and its link with the Vitória-Trindade Ridge, Southeast Brazilian Margin, South Atlantic Ocean

The Abrolhos Magmatic Province (AMP) is an example of an igneous province with about 63,000 km2 located at the Continent-Ocean Boundary (COB), into the Southeast Brazilian Margin. The AMP emerges as five small islands (Santa Bárbara, Redonda, Siriba, Sueste, and Guarita) which integrate the Abrolhos Archipelago located about 55 km offshore Brazil. The studied rocks belong to a transitional basalt series of alkaline affinity, with evolved rocks with high TiO2 contents. This work presents new detailed fieldwork mapping, petrographic, and whole-rock chemistry data, besides Sr–Nd isotopic compositions from AMP rocks. Mapped magmatic rocks in the Abrolhos Islands have been described as extrusive rocks, but we point out that they are shallow intrusions, mostly sills, and should be grouped into diabase units based on fieldwork and textural features. The AMP lithogeochemical and isotopic data suggest the operation of differentiation processes more complex than simple fractional crystallization or AFC. Our model mixing calculations based on new and compiled Sr-Nd-Pb isotopic data suggest a mixture with a prevailing depleted asthenosphere mantle source (represented by DMM; ca. 80%) metasomatized by an enriched mantle I (EMI) component (≤ 10%) and a possible HIMU-type constituent (up to 10%) in the AMP genesis. The assimilation of subducted slabs of the oceanic crust associated with the HIMU signatures is possibly linked to the Brasiliano Event due to the range of the AMP Nd TDM model ages, from 407 to 767 Ma. A viable mechanism for the EMI end-member contribution could either be a physical detachment of the South American subcontinental lithospheric mantle during the breakup of the Gondwana or lithospheric delamination of the South American plate caused by an edge-driven convection mechanism. The AMP and the Vitória-Trindade Ridge (VTR) show an eastward decreasing age pattern from the older ca. 60 Ma Abrolhos Province to the younger Martin Vaz and Trindade Islands (ca. 4 – 0.1 Ma), located ca. 1200 km away from the Brazilian coastline. The volcanic alignment between the VTR and AMP, along with the overlap of geochemical and isotopic data of their different igneous rocks, cannot be a random feature but instead represent the sampling of similar shallow mantle reservoirs, thus suggesting a cogenetic relationship. Finally, a possible petrogenetic link between the AMP and VTR magmatism is discussed.

Architectural elements, geometry, and magmatism of the Sub-Saharan Western African passive margin basins: Seismic and gravity data integration

The Western African Sub-Saharan continental margin includes a large region from Cameroon to South Africa comprising distinct margin types – magma-rich, magma-poor and transform margins. The integrated study of geological, seismic and potential field data allows the mapping of a structural framework subdivided in proximal, necking, distal, outer and oceanic domains, characterized by a set of comparable architectural elements and magmatism. These key domains display a strong and consistent continuity along the continental margin. Longitudinal transitional regions occur in a gradual way, but interactions between structural domains along transversal section may also occur, displaying spatial and temporal overprints. Because of this complexity, the outer domain that is the key feature in the magma-rich margin, tends to disappear northwards where the distal and proximal domains of the magma-poor segments become the most important domains of the margin. In the north of Gabon Basin, it is observed both the reappearance of the outer domain and the reworking of the distal domain by fracture zones. The basement of the distal domain within the magma-poor margin is interpreted as made by a hyperextended continental crust but with magmatic additions. Finally, the new proposed continent-ocean boundary (COB) in the region of the Namibe Basin (transitional zone between magma-rich and magma-poor margins) achieves a best fit in a reconstruction model with the limits recognized at the South America side. The results will imply in a drastic reduction on the gap proposed by other recent reconstruction models and also minimize the intraplate deformation required by some other models.

Breakup mechanism of the northern South China Sea: Evidence from the deep crustal structure across the continent-ocean transition

To improve constraints on breakup processes that eventually lead to seafloor spreading and the subsequent formation of new oceanic crust in the northern margin of the South China Sea (SCS), we collect a 172-km-long deep-penetration wide-angle seismic data OBS2018-L5. In the continental domain, the results present a gradually thinning crust with thickness ranging from ∼17 km to ∼8 km, with a slight thickening crust at the Outer Margin High. The attenuated continental crust is fairly rough and highly offset by distinctly different scale faults system. In the oceanic domain, the top of the basement is highly reflective with weak faulting and relatively constant crust thickness ranging from 5 km to 8 km (average 6 km). The continent-ocean boundary is characterized by an abrupt change in crustal thickness accompanied by a specific boundary between mostly positive gravity anomalies in the oceanic basin and grossly negative anomalies in the thinned continental crust, and between low amplitude, out-of-shape magnetic anomalies in the thinned continental domain and high amplitude, linear anomalies in the oceanic domain. A 0–5 km thick high-velocity layer (7.0–7.5 km/s) is present in the distal margin, which might represent mafic magma underplating. We also identify other magmatic features such as distinct buried volcanic edifices overprinting the original velocity structure. We propose that three mechanisms control the extension and breakup of the SCS: (1) the rapid transition from continental to oceanic crust, (2) elevated potential mantle temperature, and (3) the ductile lower crustal necking. Their interactions lead to the intense thinning of the continental lithosphere, decompression melting of the enriched asthenosphere followed by fast upwelling of mafic melt, relatively strong volcanic activities during the final stage of syn-rift and breakup, and finally generating a margin with intermediately rich magma, referred to as in intermediate-type margin.

Distribution of oceanic crust in the Enderby Basin offshore East Antarctica

SUMMARY Seismic reflection and refraction data were collected in 2007 and 2012 to reveal the crustal fabric on a single long composite profile offshore Prydz Bay, East Antarctica. A P-wave velocity model provides insights on the crustal fabric, and a gravity-constrained density model is used to describe the crustal and mantle structure. The models show that a 230-km-wide continent–ocean transition separates stretched continental from oceanic crust along our profile. While the oceanic crust close to the continent–ocean boundary is just 3.5–5 km thick, its thickness increases northwards towards the Southern Kerguelen Plateau to 12 km. This change is accompanied by thickening of a lower crustal layer with high P-wave velocities of up to 7.5 km s–1, marking intrusive rocks emplaced beneath the mid-ocean ridge under increasing influence of the Kerguelen plume. Joint interpretations of our crustal model, seismic reflection data and magnetic data sets constrain the age and extent of oceanic crust in the research area. Oceanic crust is shown to continue to around 160 km farther south than has been interpreted in previous data, with profound implications for plate kinematic models of the region. Finally, by combining our findings with a regional magnetic data compilation and regional seismic reflection data we propose a larger extent of oceanic crust in the Enderby Basin than previously known.

Exhumation history of the Hengchun Ridge and its implications for Taiwan orogenic processes

The orogenic evolution of Taiwan is thought to have occurred with a steady and southward propagating trend since the Late Miocene. Recent studies suggest a two-stage collision for the Taiwan orogen and that the collision occurred simultaneously along the entirety of the island of Taiwan. To test this hypothesis, we evaluated 270 bathymetry profiles normal to the trench to constrain variations in the width, length, and crest of the Taiwan accretionary prism from the northern Central Ridge to the southern Hengchun Ridge. South of Taiwan Island, a gradual increase in the width and elevation of the accretionary prism of the Hengchun Ridge is noted. Assuming that the uplift of Hengchun Ridge is dominated by the accretion of hyper-stretched continental crust (HSCC), we estimated the uplift rate of the ridge (ca. 0.3 km/Ma) based on a linear regression between the HSCC and the observed depth of the prism crest. Using this uplift rate, we forward modeled the prism crest depth variations from 19.7°N to 23.5°N, and compared these values to observations. The model gives a good match to observations of the Hengchun Ridge, but significantly deeper depths to the north of Hengchun Peninsula. This suggests that the Taiwan orogeny had two stages: the first stage was dominated by structurally underplated HSCC, and the second is a combination of the arrival of the continental shelf and arc–continent collision. In addition to the widely accepted arc–continent collision, our study suggests that both the location and orientation of the continent–ocean boundary play important roles in orogeny.

The transition from continental to lithospheric breakup recorded in proto-oceanic crust: Insights from the NW South China Sea

AbstractThe formation of a new plate boundary and creation of the first oceanic crust, two of the most important processes of plate tectonics, still remains little understood. While older studies used to assumed a sharp ocean-continent boundary between continent and ocean, recent studies suggest a progressive oceancontinent transition (OCT) between unequivocal continental and oceanic crusts. In the latter view, breakup is not instantaneous but a lasting phase, which raises questions about the nature of the OCT basement and the processes operating between continental and lithospheric breakup. Based on detailed observations of high-quality and yet unpublished reflection seismic data, we describe and interpret the characteristic structures of the NW-South China Sea OCT and their relationship with overlying syn-breakup phase sediments. We show that the OCT displays a transition from fault-dominated rifting to magma-dominated seafloor spreading. On its continent-ward side, the OCT is made of hybrid crust where tectonic thinning of continental crust is compensated by syn-extensional magmatic thickening. Oceanward, the hybrid crust evolves into a fully magmatic but fault-dominated proto-oceanic crust, and finally turns into a mature Penrose-type oceanic crust. Relying on the growth structures observed in the syn-breakup sedimentary sequences and magmatic additions, we propose a kinematic restoration of the breakup phase. We suggest out-of-sequence flip-flop faulting to explain the switch from asymmetrical, fault-dominated-extension, to fully magmatic and largely symmetrical syn-extension accretion recorded in the syn-breakup sedimentary sequences overlying the OCT.

What controlled the thickness of continental crust in the Archean?

Abstract Exposed continents are one of Earth's major characteristics. Recent studies on ancient ocean volume and exposed landmasses suggest, however, that early Earth was possibly a water world, where any significant landmass was unlikely to have risen above sea level. On modern Earth, the thickness of continental crust seems to be controlled by sea level and the buoyancy of continental crust. Simply applying this concept to the Archean would not explain the absence of exposed continents, and we suggest that a third element that is currently insignificant was important during early Earth: the strength of continental upper crust. Based on the pressure imbalance expected at continent-ocean boundaries, we quantified the conditions under which rock strength controls the thickness of continental crust. With the level of radiogenic heat production expected for early Earth, continents may have been too weak to have maintained their thickness against a deep ocean.

Continental-scale structural heritage from rift extension to postrift inversion: Implications for the central Brazilian Equatorial Margin evolution

The link between major basement anisotropies and basin evolution is crucial for understanding the role of tectonic inheritance on continental margins. However, it is still a matter of debate how and why it happens, especially in the Equatorial Atlantic, where the interplay between Precambrian fabric and the shearing of major oceanic fractures has been studied. Here, we investigate the offshore Ceará Basin and its basement in the central part of the Brazilian Equatorial Margin to (1) map basement anisotropies underneath marginal basins and show how Precambrian crustal domains influenced basin evolution; (2) determine the deformation style of reactivated basement faults; and (3) assess the interplay between reactivated faults and the oceanic Romanche Fracture Zone. The combined geological and geophysical data show that reactivation of NE-SW to ENE-WSW continental-scale ductile shear zones that acted as crustal weakness zones started in the Early Cretaceous under a normal stress field. Stress field inversion occurred in the Barremian during the continental breakup when the South American plate started to became an independent intraplate area under a strike-slip stress regimes. The shear zones arrest at the E-W-striking Romanche Fracture Zone, which marks the continental-oceanic boundary. The major fault offsets occurred along the Transbrasiliano Shear Zone, which represented a collisional boundary between paleocontinents in the Neoproterozoic. Fault reactivation also occurred in the eastern boundary of the Ceará Terrace marginal ridge along the Transbrasiliano Shear Zone, uplifted during tectonic inversion. We conclude that fault reactivation mainly occurred between contrasting Precambrian terrains separated by ductile shear zones. Additionally, the folds associated with the Romanche Fracture Zone were mainly generated along reactivated faults that affected syn- to postrift units on the Equatorial Margin.

A 3D Snapshot of Crustal Breakup Deduced From Seismic Analysis of the Tip of the NW South China Sea

AbstractBased on a high‐resolution seismic reflection data set, we describe and map rift domains at the tip of the V‐shaped NW‐South China Sea (NW‐SCS) Basin. We compare our rift domain map with seismic refraction, magnetic and gravity field data, and previously mapped Continent‐Ocean Boundaries (COB). We also define and map syn‐breakup sedimentary and magmatic sequences across the sediment‐rich NW‐SCS, which enables us to reconstruct the tectono‐stratigraphic evolution of the area. This work enables us to describe and discuss the 3D architecture and nature of the crust at the tip of the NW‐SCS, investigate how extensional and magmatic processes interacted during the breakup, and propose a kinematic model for late rifting, breakup, and finally stalling of the seafloor spreading system. This study provides exceptionally well‐documented insights into the spatial and temporal evolution at the tip of a so‐called “rift propagator,” and enables us to show that breakup in the NW‐SCS may rather correspond to a “retrogrador,” in which the extension zone retreats due to the reorganization of a failing rift system.

Structural control of Mesozoic orogens on SE Asia Basin opening

• Intrinsic relationship between Sundaland crustal inherited structures, and subsequent deformation through time since the Mesozoic. • The main rifting necking zones follow former sutures, and the direction of opening is perpendicular to them. • Counter Regional Normal Faults often reactivate former thrust faults and plutons boundaries. • Distal low angle normal faults mostly ocean dipping are dependant to the thermal state and boundary stress conditions. • Rifting of the Cenozoic basins was triggered and conditioned by the crustal collapse of Mesozoic orogens. A comparative study of basement structures, basins geometry and evolution, around SE Asia, highlights an intrinsic relationship between crustal inherited structures, and deformation through time from the Late Mesozoic to the Present. The present continental margin developed alongside of a large magmatic arc active from the Permian to the middle of the Cretaceous subsequently stretched during the Cenozoic. The Permo-Triassic crustal configuration was affected by a regional compression during the middle to late Cretaceous which appears to be mostly radial to the horse-shoe shape of the magmatic belt, and is best imaged by large regional shear zones, broad folds and densely distributed shear joints. A subsequent extension perpendicular to the margin followed in the Paleogene, which is firstly marked by an early stretching characterized by block faulting, dykes, and parallel extensional joint. We observed that the main necking zones follow former sutures, and the direction of opening is interestingly also perpendicular to them. The following thinning phase initiated with only minor discrepancies in the timing of initiation from E to W of SE Asia, marked by large crustal tilted blocks underlining a first thinning of the crust. This event was dominated by the occurrence of low angle normal faults bounding basins which capture most of the clastics derived from the formerly uplifted magmatic arc. These faults are frequently continent-dipping (counter-regional). Rooted into the lower crust, they often reactivate former thrust faults and plutons boundaries on the edges of the Mesozoic magmatic arc. Therefore, a link with the relaxation of the Late Mesozoic thickened crust is invoked. In a later stage, rifting developed across the outer edges of the previously thinned crust, outside the magmatic arc and up to the final continent-ocean boundary (COB of the South China Sea in the East and the Andaman Sea to the West). It is marked by distal low angle normal faults mostly ocean dipping (regional) that do not seem to be sensitive to margin fabric but more dependant to the thermal state and boundary stress conditions. Therefore, the pattern of basin structure and development seems conditioned by the intrinsic crustal configuration of SE Asia, specifically by the Triassic and Cretaceous orogenic structures (suture, crustal faults, and crustal thickness variations). The Late Mesozoic margins were indeed characterized by thickened crust which may have undergone collapse just before the onset of rifting sensus stricto . As a consequence, the general dip direction of these major crustal boundaries was often preserved during the Cenozoic extension that affected the marginal regions. Where the crustal thinning was strong enough, the fault plane dipped oceanward as a result from the crust/mantle relative motion. It is proposed that the rifting of the Cenozoic basins was triggered and conditioned by the relaxation of the orogens that assembled Sunda Plate in the past.

Geodynamic Characteristics in the Southwest Margin of South China Sea

The strike-slip fault system in the southwestern margin of South China Sea (SCS) lies on the transition zone between the continental shelf and slope of SCS, which is an important ocean–continent boundary. By using submarine heat flow data, a seismic shear wave tomography model, and gravity potential field data, this paper investigates the distribution of submarine heat flow in the southwestern margin of SCS, the thermal–rheological structure of the crust and mantle, the temperature–viscosity characteristics of the upper mantle Vs low-velocity layer, the tangential stress field of the rheological boundary layer at the lithosphere base, and the convective velocity structure of the mantle asthenosphere. Our new results show that the deep geothermal activity in the southwestern margin of SCS is intense, and the high heat flow area of the mantle with Qm/Qs &amp;gt;70% is distributed along an NNE-trending strip. Moreover, both the east and west sides of the strike–slip fault zone correspond to two low-value areas with a viscosity coefficient of 1021–1022 Pa⋅s at Moho depth, and beneath the Nansha Block are strong and cold blocks with a viscosity coefficient of 1024–1025 Pa⋅s. The northward and eastward shear stress components τN and τE of the rheological boundary layer at the base of the lithosphere mantle decrease with depth. At 65-km depth, both τN and τE are greater than 5.5 × 108 N/m2. At 100-km depth, both τN and τE are less than 1 × 108 N/m2. The calculation results based on the seismic shear wave model of the upper mantle and the gravimetric geoid model indicate that the depth of 120–250 km is the low-velocity layer, and the average temperature of the mantle at 180-km depth can be up to 1,300°C. Moreover, the average effective viscosity coefficient is close to 1018 Pa⋅s, which satisfies the temperature and viscosity conditions for partial melting or convective migration of mantle material. The mantle convection calculation results show that the average flow rate is 8.5 cm/a at 200-km depth and 2.2 cm/a at 400-km depth.

Mantle Flow Deflected by Arc–Continent Collision and Continental Subduction in Eastern Indonesia

Abstract We investigate seismic anisotropy across southeastern Indonesia where the Indo-Australian plate subducts beneath and collides with the Sunda-Banda arc. Geochemical, geodetic, and tomographic studies reveal an along-strike transition from oceanic subduction to continental subduction and collision near central Flores that is due to a change of lithospheric composition in the subducting plate. To investigate the anisotropic fabric and dynamics of the upper mantle surrounding this young (∼5 Ma) arc–continent collision, we perform shear wave splitting analysis on local and teleseismic S waves recorded by an array of broadband seismometers that crosses the subduction–collision boundary. Seismic anisotropy inferred from our local S dataset shows that anisotropic sources above the slab extend to depths exceeding 100 km. Analysis of teleseismic SKS and SKKS waves reveal a shift in subslab fast axes from trench-parallel to trench-perpendicular near the ocean–continent boundary in the lower plate, which we relate to regional subslab mantle flow being deflected around the subducted continental lithosphere. Along-strike variations in anisotropic fast axes from teleseismic phases overlap with distinct structural and tectonic boundaries that divide distinct regions of the collision, implying the effects of the collision transcend any one dataset and highlighting the complexity of collisional boundaries. These results shed light on the interaction between tectonic structure and mantle dynamics in an emergent collision, and help constrain the nature of upper mantle deformation in the early stages of collision.