High-velocity Lower Crust Research Articles

High velocity lower crust with anomalous rheological parameters under the Red Sea Passive Margin, SW Saudi Arabia – insight into the evolution of the Hijaz-Asir Escarpment Zone

SUMMARY The Red Sea Passive Margin (RSPM) in SW Saudi Arabia is characterized by two morphological provinces; the Red Sea Coastal Plains (RSCP) and the Hijaz-Asir Escarpment Zone (HAEZ) at the Arabian Shield Edge (ASE). Of these, the HAEZ is conspicuous by its seaward-facing heights of > 3 km. Based on the available receiver function analysis (REF) results from 25 seismic stations, we first present the Moho relief map underlying the RSPM to demonstrate that: (i) the transitional crust under the coastal region is 20–25 km thick which steadily increases landward to attain a thickness of ∼40 km below ASE, across an intervening region of depressed Moho with depths of 45–52 km. The latter exhibits significant regional extent and displays a close correlation with the topography of the HAEZ, suggesting a tectonic control on its formation. Eight rheological parameters for the upper as well as the lower crust are computed from the REF data, namely; Vp, Vp/Vs, density (ρ), Lame's first constant (λ), Lame's second constant (μ), Poisson's ratio (σ), bulk modulus (K) and acoustic impedance (AI). Their 2-D distribution for the lower crust unravels, for the first time, a high velocity lower crust (HVLC) underneath the HAEZ. The rheological character of the HVLC is further elucidated by investigating the relations between six rheological parameters: Vp–Vp/Vs, Vp/Vs–lower crustal thickness (hLC), Vs/ρ–hLC, λ–ρ, σ–ρ and λ–μ. The results indicate that lithology of the lower crust varies from diorite–felsic gneiss under RSCP to gabbro–diabase and mafic gneiss under HAEZ. We conjecture that the HVLC is an outcome of the exchange of materials from the subcrust at the hinterland of the RSPM directly underlying the HAEZ. Deep seismic control can only resolve the finer details of such crustal transformation.

Discontinuous Igneous Addition Along the Eastern North American Margin Beneath the East Coast Magnetic Anomaly

AbstractDetailed models of crustal structure at volcanic passive margins offer insight into the role of magmatism and the distribution of igneous addition during continental rifting. The Eastern North American Margin (ENAM) is a volcanic passive margin that formed during the breakup of Pangea ∼200 Myr ago. The offshore, margin‐parallel East Coast Magnetic Anomaly (ECMA) is thought to mark the locus of syn‐rift magmatism. Previous widely spaced margin‐perpendicular studies seismically imaged igneous addition as seaward dipping reflectors (SDRs) and high velocity lower crust (HVLC; >7.2 km/s) beneath the ECMA. Along‐strike imaging is necessary to more accurately determine the distribution and volume of igneous addition during continental breakup. We use wide‐angle, marine active‐source seismic data from the 2014–2015 ENAM Community Seismic Experiment to determine crustal structure beneath a ∼370‐km‐long section of the ECMA. P‐wave velocity models based on data from short‐period ocean bottom seismometers reveal a ∼21‐km‐thick crust with laterally variable lower crust velocities ranging from 6.9 to 7.5 km/s. Sections with HVLC (>7.2 km/s) alternate with two ∼30‐km‐wide areas where the average velocities are less than 7.0 km/s. This variable structure indicates that HVLC is discontinuous along the margin, reflecting variable amounts of intrusion along‐strike. Our results suggest that magmatism during rifting was segmented. The HVLC discontinuities roughly align with locations of Mid‐Atlantic Ridge fracture zones, which may suggest that rift segmentation influenced later segmentation of the Mid‐Atlantic Ridge.

Implications on crustal structure from the South Finland Coastal (SOFIC) deep seismic sounding profile

We present results from a deep seismic sounding (DSS) experiment carried out along the southern coast of Finland in summer 2015. Data used in the survey derived from industrial blasts recorded by temporary project stations and permanent network stations. The western 220 km part of the 450 km long Southern Finland Coastal (SOFIC) profile runs along the Uusimaa belt (UB) in the 1.7–1.9 Ga Southern Finland subprovince (SFS) of the Svecofennian domain, while the 170 km part in the east crosses the 1.62–1.65 Ga Wiborg rapakivi batholith (WRB). The farthest 60 km cross a geologically diverse area consisting of supracrustal rocks and granitoids of the Saimaa area (SA), an eastern extension of the SFS. Our results show that the Moho boundary depth varies significantly, from ca. 52–54 km below UB to 40–45 km below WRB. All three crustal layers (upper, middle, and lower) have their maximum depth in the contact zone between UB and the WRB. Below WRB, a lower crust with Vp ~6.7–6.9 km/s is observed. High velocity lower crust was observed below UB (Vp ~7.2 km/s) and possibly below SA (Vp ~7.35 km/s). The modelling was based on ray tracing, using the extrapolation of seismic wave arrival times with the help of travel times predicted from a one-dimensional velocity model. The resulting two-dimensional velocity model partly relies on data from the intersecting DSS profiles and supports previous observations of the lithospheric structure of southeastern Fennoscandia.

Geophysical insights on the crustal structure of Greenland's northern continental margin towards the Morris Jesup Spur

During expedition PS115/1, the German research vessel Polarstern acquired seismic refraction data along a 102-km-long profile crossing Greenland's northern continental margin and extending up to the southwestern limit of the Morris Jesup Spur. A P–wave velocity model is obtained and validated by gravity modelling. A nearby seismic reflection line provides insights on the structures within the sedimentary cover. Beneath a 2-km-thick sedimentary cover with velocities of 1.8 km/s to 3.4 km/s, an up to 1.5-km-thick layer is characterized by velocities of 4.2 km/s and is interpreted to consist of volcanic rocks. This is consistent with proposed volcanic activity on the Morris Jesup Spur and exposed volcano-sedimentary rocks of the nearby Kap Washington Group. Below the volcanic rocks, an up to 7 km-thick unit with velocities of 4.4 to 5.8 km/s is interpreted to consist of metasedimentary rocks still belonging to the deformed units of the Franklinian Basin. The seismic reflection data image a tectonic overprint of the acoustic basement and the lowermost sedimentary layers. Tectonic faults indicating the tectonic overprint do not reach to the uppermost sedimentary layers. This hints at an older fracture zone that is also observed in the velocity model and aligns well with observed anomalies in the regional gravity and magnetic fields. The seismic velocity model reveals a highly extended continental crust that shows a magmatic overprint at all crustal levels. The upper crust is 5 km thick with a velocity of 6.0 km/s. However, in three distinct zones its thickness is up to 8 km and the velocity increases to 6.3 km/s. These zones are interpreted as magmatic intrusions into the upper crust. The lower crust represents a 9-km-thick high-velocity layer (7.2 km/s) that is interpreted as magmatic underplating or lower crustal sill intrusions. Such a high-velocity lower crust is not present in models of adjacent Arctic margins.PACS: 0000, 11112000 MSC: 0000, 1111

The Neoproterozoic basement of the Parnaíba Basin (NE Brazil) from combined geophysical-geological analysis: A missing piece of the western Gondwana puzzle

A regional Pre-Silurian unconformity marks the erosive planar base of the Phanerozoic Parnaíba Basin (PB), and indistinctly truncates key tectonic units for the understanding of the Neoproterozoic evolution of western Gondwana in northeastern Brazil. We present a comparative analysis of geophysical and geological datasets in the PB and propose a new tectonic configuration for its pre-Silurian basement, composed of different terranes amalgamated during the Brasiliano orogeny. Using integrated seismic interpretation and gravity modelling, constrained by an updated crustal thickness map, well data and a compilation of recent geophysical studies, we have identified two main blocks in the center of the basin. They represent pre-Brasiliano inliers, surrounded by Brasiliano mobile belts, and assigned to two major crustal building blocks of western Gondwana: the Amazonian-West Africa (AWB; to the west/north) and the Central African blocks (CAB; to the east/northeast). The Grajaú block belongs to the AWB and is characterized by low gravity anomaly, thicker crust (41–45 km), transparent seismic character of the basement and a high velocity lower crust. The Teresina block belongs to the CAB and is characterized by slightly thinner crust (39–41 km), higher values of gravity and magnetic anomalies and by the presence of mid-crustal reflectors (MCR), observed in seven seismic lines and here interpreted as a remnant of a paleosuture zone between the two blocks. Along a NE-SW 500-km seismic and gravity profile, we interpreted the MCR as composed of crustal-scale thrust faults verging westwards and defining the Barra do Corda belt, which was formed by the closure of the Goiás-Pharusian ocean along the Transbrasiliano-Kandi corridor and subsequent collision of the Grajaú and Teresina blocks. This belt deforms the eastern margin of the Ediacaran Riachão foreland basin (RB), observed in seismic and well data beneath the SW portion of PB and overlying the southern Grajaú block. The western margin of RB is bounded by eastwards verging thrust faults, interpreted as a zone of back-thrusts and thinned crust (∼36 km) in the eastern prolongation of the Araguaia belt beneath PB. To the east, the limit between the Teresina block and the Borborema Province is marked by the NE-SW Transbrasiliano Shear Zone, along which Neoproterozoic mylonites were recovered from wellbores, and a system of narrow pull-apart basins is interpreted in the seismic data, triggered by Early Paleozoic transtensional reactivations of the TBSZ, prior to the deposition of the basal Ordovician-Silurian sequence of PB. This complex collisional tectonic setting of PB basement contradicts the idea of a stable cratonic block beneath it, inciting new formation models that account for the observed crustal and lithospheric heterogeneities.

The crustal structure in the Northwest Atlantic region from receiver function inversion – Implications for basin dynamics and magmatism

The Labrador Sea and Baffin Bay form an extinct Palaeogene oceanic spreading system, divided by a major continental transform, the Davis Strait, with the whole region defined as the Northwest Atlantic. The Davis Strait hosts the Ungava Fault Zone and is the central structural element of the Davis Strait Large Igneous Province (DSIP) that formed broadly coeval with continental breakup to its north and south. While constraints on the crustal structure in this region primarily exist in the offshore, crustal models are limited onshore, which makes an interpretation of regional structures as well as the extent, and therefore origin of the DSIP extremely difficult to ascertain. Here, we have collected all available teleseismic data from the Northwest Atlantic margins and applied a receiver function inversion to retrieve station-wise velocity models of the crust and uppermost mantle. We integrate the outcomes with published controlled-source seismic data and regional crustal models to make inferences about the crustal structure and evolution of the Northwest Atlantic. In particular, we focused on constraining the spatial extent and origin of high velocity lower crust (HVLC), and determining whether it is generically related to the Davis Strait Igneous Province, syn-rift exhumed and serpentinised mantle, or pre-existing lower crustal bodies such as metamorphosed lower crust or older serpentinised mantle rocks. The new results allow us to better spatially constrain the DSIP and show the possible spatial extent of igneous-type HVLC across Southwest Greenland, Northwest Greenland and Southeast Baffin Bay. Similarly, we are able to relate some HVLC bodies to possible fossil collision/subduction zones/terrane boundaries, and in some instances to exhumed and serpentinised mantle.

Spatial distribution and origin of the high-velocity lower crust in the northeastern South China Sea

The origin of seismically-defined high-velocity lower crust (HVLC) is an important scientific issue that has attracted much debate. This study compiles 12 published wide-angle seismic velocity models from the northeastern South China Sea (SCS) to define the spatial distribution and origin of the HVLC. We have compiled the P-wave seismic velocities and interface depths of sediment, crust (including the HVLC), and mantle from each seismic profile. A pseudo-3D seismic velocity model of the northeastern SCS is then built using Kriging interpolation. Sediment thickness is 4–12 km in the Southwest Taiwan Basin and is 8–12 km at the Hengchun Ridge due to an abundant source of sediments. On a regional scale, the crust gradually thins from continental shelf and Taiwan Island toward ocean basin. Moreover, a very high crustal stretching factor (5–7) within the northeastern Southwest Taiwan Basin shows an extremely extended crust that is interpreted as a failed rift. The HVLC is spatially uneven in distribution, and seems to be divided into two parts along the paleo-Pacific subduction zone. The northwestern HVLC, located in front of the paleo-Pacific subduction zone, is distributed in a continuous stripe shape and overlapped almost with high magnetic anomalies; whereas the southeastern HVLC is distributed in the form of discrete blocks. The former one is formed by residual magmatic materials in response to the paleo-Pacific subduction, and the latter one is inferred to originate from post-rift magmatic modification.

Crustal extension and magmatism along the northeastern margin of the South China Sea: Further insights from shear waves

The northern continental margin of the South China Sea (SCS) is a unique rifted margin that experienced weak magmatism in the syn-rift stage and intense magmatism in the post-rift stage. This area also exhibits a complicated relationship between crustal extension and magmatism. We present the tomographic inversion of seismic P-wave and S-wave velocity models, as well as the Vp/Vs ratio model, in order to delineate the crustal extension and post-rift magmatic features. The velocity structure is created by forward modelling (RayInvr) and travel-time tomographic inversion (Tomo2D). The results suggest that Mesozoic strata near the Dongsha area has a maximum thickness of ~4.6 km with a Vp of 3.5–5.5 km/s, a Vs of 1.9–3.1 km/s, and Vp/Vs ratios of 1.71–1.76, indicating a low porosity and high degree of diagenesis. High-velocity lower crust (HVLC) is imaged in our model, with a Vp of 7.0–7.5 km/s ± 0.05–0.25 km/s and Vp/Vs ratios of 1.70–1.82 ± 0.05. The composition of the HVLC is mainly mafic, we considered it is related to magmatic underplating due to decompression melting caused by crustal extension. The crustal anomalies with high Vp/Vs ratios of 1.80–1.85 ± 0.04 are identified, which are the product of post-rift magmatic intrusions. The seaward increase in continental lower crustal Vp/Vs ratios, suggests an increasing level of mafic intrusion into the continental crust. We attributed crustal hyperextension to increasing fault density, which caused intense magmatic intrusion and thinner HVLC beneath the continent-ocean transition (COT).

Mantle serpentinization beneath a failed rift and post-spreading magmatism in the northeastern South China Sea margin

SUMMARYThe post-spreading magmatic activities in the northeastern South China Sea (SCS) margin are very strong, evidenced by widely distributed high-velocity lower crust (HVLC) and numerous volcanoes. However, there are large contrasts in magmatic activities and crustal structure between the Southern Depression (TSD) of the Tainan Basin and the volcanic continental slope area further south. We analyse their crustal P-wave velocity structures based on a newly acquired wide-angle ocean bottom seismic data set. The Cenozoic strata below the TSD, a Cenozoic failed rift, are relatively thick (∼3–4.5 km) with velocities from 1.6 to 3.6–3.9 km s–1, whereas the Mesozoic strata are relatively thin (∼1–2.5 km) with velocities from 4.3 to 4.6–5.2 km s–1. In the TSD, magmatic activities are relatively weak and the crust is severely thinned (∼4 km). The crust is 9–15 km thick below the volcanic continental slope area, which shows extensive volcanism. We identified HVLC below the failed rift of the TSD (Zone 1) and attributed it to mantle serpentinization, whereas the imaged HVLC below the volcanic continental slope (Zone 3) and HVLC adjacent to the failed rift of the TSD (Zone 2) are due to post-spreading magmatic underplating/intrusions. At the model distance ∼90 km, lateral transition from magmatic underplating/intrusions to mantle serpentinization occurred abruptly. We concur that post-spreading cooling and thermal contraction in the nearby SCS oceanic lithosphere can trigger decompressive melting and deformation in the thinned continental slope zone. Our study shows that, in addition to mantle serpentinization in the continent–ocean transition (COT) zone, mantle can also be serpentinized below the rift during early-stage rifting. Weak syn-rifting magmatism and mantle serpentinization below the failed rift support that the northeastern SCS has a magma-poor margin.

Deep structure of the Demerara Plateau: From a volcanic margin to a Transform Marginal Plateau

The Demerara Plateau (offshore Suriname and French Guiana) is located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic Oceans. The study of its crustal structure is fundamental to understanding its tectonic history, its relationship with the adjacent oceanic domains and to enlightening the formation of Transform Marginal Plateaus (TMPs). This study presents two wide-angle seismic velocity models from the MARGATS cruise seismic experiment, and adjacent composite seismic reflection lines. The plateau itself is characterized by a 30 km thick crust, subdivided into three layers, including a high velocity lower crust (HVLC). The velocities and velocity gradients do not fit values of typical continental crust but could fit with volcanic margin or Large Igneous Province (LIP) type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is likely composed of extrusive volcanic rocks of the same magmatic origin, forming thick seaward dipping reflector sequences tilted to the west. This SDR complex was emplaced during hotspot related volcanic rifting preceding the Jurassic opening of the Central North Atlantic and forming the present-day western margin of the plateau. The internal limit of the SDR complex corresponds to the future limit of the eastern margin. The Demerara Plateau would therefore be an inherited Jurassic volcanic margin boarding the Central Atlantic. This margin was reworked during the Cretaceous at the eastern limit of the Jurassic SDR complex, creating the present-day northern transform margin and the eastern divergent margin along the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies such as hotspots and superposed tectonic phases in the history of TMPs, which share a great number of characteristics with Demerara.

Rift migration and transition during multiphase rifting: Insights from the proximal domain, northern South China Sea rifted margin

Multiphase rifting with tectonic quiescence intervals is one of the most distinguishing features during the evolution of rift basins and rifted margins, which always produce oceanward rift migration. However, this law seems to be challenged in the proximal domain of the northern South China Sea (SCS) rifted margin. Based on high-quality 2D/3D seismic reflection data and borehole data from the Zhu 1 Depression located in the northern SCS, we examine the temporal and spatial variations of rifting. The regional unconformity, cyclicity of faulting, and rotation in extension direction indicate that the Zhu 1 Depression experienced continuous two-phase rifting with four stages but without distinct tectonic quiescence intervals. In Stage 1 of Syn-rift 1, rifts were distributed across the whole depression. They began to migrate towards the continent in stage 2 of Syn-rift 1, which continued until stage 1 of Syn-rift 2, and then migrated to the east. Rifting was also accompanied by a clockwise rotation of the strikes of fault systems, which were ~N065°E and ~N080°E in stage 1 and stage 2 of Syn-rift 1, respectively, and then rotated to ~ S085°E during Syn-rift 2. We suggest that the interactions between the pre-existing basement fabrics and the regional stress fields rotating clockwise can account for this rotation and the eastward rift migration. The rift migration towards the continent is supposed to be caused by the emplacement of the high-velocity lower crust (HVLC) underneath the Dongsha Uplift, which may initiate in the Eocene. The distinctive multidirectional, multiphase, and multi-mechanical rift migration reflects the uniqueness of the SCS rifted margin. This study helps us understand the complicated rifting process that interacts with pre-existing fabrics, regional stress field, and the emplacement of high-velocity lower crust.

Full-Scale Crustal Interpretation of Kokkola\u2013Kymi (KOKKY) Seismic Profile, Fennoscandian Shield

The Kokkola–Kymi Deep Seismic Sounding profile crosses the Fennoscandian Shield in northwest-southeast (NW–SE) direction from Bothnian belt to Wiborg rapakivi batholith through Central Finland granitoid complex (CFGC). The 490-km refraction seismic line is perpendicular to the orogenic strike in Central Finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013. The campaign resulted in 63 usable seismic record sections. The average perpendicular distance between these and the profile was 14 km. Tomographic velocity models were computed with JIVE3D program. The velocity fields of the tomographic models were used as starting points in the ray tracing modelling. Based on collected seismic sections a layer-cake model was prepared with the ray tracing package SEIS83. Along the profile, upper crust has an average thickness of 22 km average, and P-wave velocities (Vp) of 5.9–6.2 km/s near the surface, increasing downward to 6.25–6.40 km/s. The thickness of middle crust is 14 km below CFGC, 20 km in SE and 25 km in NW, but Vp ranges from 6.6 to 6.9 km/s in all parts. Lower crust has Vp values of 7.35–7.4 km/s and lithospheric mantle 8.2–8.25 km/s. Moho depth is 54 km in NW part, 63 km in the middle and 43 km in SW, yet a 55-km long section in the middle does not reveal an obvious Moho reflection. S-wave velocities vary from 3.4 km/s near the surface to 4.85 km/s in upper mantle, consistently with P-wave velocity variations. Results confirm the previously assumed high-velocity lower crust and depression of Moho in central Finland.

The crustal structure of the southern Davie Ridge offshore northern Mozambique – A wide-angle seismic and potential field study

Some of the oldest surviving oceanic basins in the world, the Mozambique and West Somali basins, were created during the breakup of Gondwana, starting around 180 Ma. Between the two basins, relative movements of West Gondwana and East Gondwana, including Madagascar, created a shear zone, the Davie Fracture Zone (DFZ) with a topographic elevation (Davie Ridge - DR) marking its centre. The crustal composition of the DFZ and DR is a subject of speculation and debate.In this study, we present seismic refraction data across the prominent topography of the southern DR. Ray tracing of the wide-angle data as well as additional seismic amplitude modelling and 2.5D density modelling constrain its crustal structure and architecture.The data indicate that in the Mozambique Channel the DR consists of fragments of continental crust with a thickness of 10 to 12 km. An oceanic crust indenter extends northward from the Mozambique Basin into the area between the DR and the East African margin at 16.5°S. Northeast of the DR, at 41.8°W/14.5°S, the Somali Basin is probably floored by 6 km thick oceanic crust. Hence, the continental DR separates oceanic crust of the Somali and Mozambique basins.The transitional crustal area at the central Mozambican margin is underlain by high velocity lower crust (HVLC). The HVLC has velocities up to 7.3 km/s and extents along the margin, vanishing northward between 16.5° and 14.5°S. At the Madagascan side of the DR, at 16.5°S, the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller HVLC of 2.9 km thickness and an E-W extent of 120 km. The oceanic crust at 14.5°S represents the oldest part the Somali Basin, which formed after the initial NW-SE rifting between East and West Gondwana.

Transition from continental rift to back-arc basin in the southern Japan Sea deduced from seismic velocity structures

SUMMARY We obtain the crustal structure from active-source seismic surveys using ocean bottom seismographs and seismic shots to elucidate the evolutionary process from continental rifting to the backarc basin opening in the Yamato Basin and Oki Trough in the southern Japan Sea. Results show that the crust changes from approximately 14–15 km thick in the basin (the southern Yamato Basin) to 16.5–17 km in the margin of the basin (the southwestern edge of the Yamato Basin). The P-wave velocity distribution in the crust of the southern Yamato Basin is missing a typical continental upper crust with P-wave velocities of 5.4–6.0 km s–1, and is thought be a thicker oceanic crust formed by a backarc basin opening. By contrast, the crust of the southwestern edge of the Yamato Basin might have been formed by continental rifting because there is an unit with P-wave velocities of 5.4–6.0 km s–1 and with a gentle velocity gradients, corresponding to the continental upper crust in this area. This variation might reflect differences in mantle properties from continental rifting to backarc basin opening of the Yamato Basin. Because the Oki Trough has a crustal thickness of 17–19 km and having a unit with P-wave velocities of 5.4–6.0 km s–1, corresponding to the continental upper crust with a high-velocity lower crust, we infer that this trough was formed by continental rifting with magmatic intrusion or underplating. These crustal variations might reflect transitional stages from continental rifting to backarc basin opening in the southern Japan Sea.

Continental rifting and sediment infill in the distal part of the northern South China Sea in the Western Pacific region: Challenge on the present-day models for the passive margins

Abstract Well-known difficulties in imaging structures and sediment infill in the distal margins covered by deepwater and thick sediment in the western Pacific have limited our ability to interpret rifting-to-drifting processes of these margins. We used a wealth of multi-channel seismic profiles and drilling data to construct a regional stratigraphic framework for the Cenozoic strata in the Xingning-Jinghai Subbasin, which is now located to the north of the apparent oceanic crust in the South China Sea. Eight sequence-bounding unconformities segmented seven sequence stratigraphy can be confidently recognized in our survey using mostly standard criteria for identification. The particular arrangement of strata elements in the syn-rift succession presents two contrast geometry sedimentary packages, whose pattern is different with that bounded by high-angle faults on the continental shelf observed by the previous studies. In the Xingning-Jinghai Subbasin, strata pattern on the hangingwall of the low-angle faults shows that the faults bounding the depocenters were initially formed by high-angle faults, which is similar to the present-day high-angle faults on the continental shelf. Subsequently the high-angle faults in the deepwater region were rotated to be extensional detachment low-angle faults, which extremely thinned the crust from ∼33 km to ∼8–10 km. The low-angle detachment fault systems can be used to explain the magmatic addition during the post-rift stage on the hyper-extended crust in the northern margin. The hyper-extended crust of the South China Sea with the characteristics of landward-dipping low-angle faults, high-velocity lower crust and moderately magmatism challenges the magma-poor or magma-rich models constructed from the Atlantic margins.

The onset of Walvis Ridge: Plume influence at the continental margin

The opening of the South Atlantic is a classical example for a plume related continental breakup. Flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at Tristan da Cunha. To determine the effect of the proposed plume head on the continental crust, we acquired wide-angle seismic data at the junction of the Walvis Ridge with the African continent and modelled the P-wave velocity structure in a forward approach. The profile extends 430km along the ridge and continues onshore to a length of 720km. Crustal velocities beneath the Walvis Ridge vary between 5.5km/s and 7.0km/s, a typical range for oceanic crust. The crustal thickness of 22km, however, is approximately three times larger than of normal oceanic crust. The continent-ocean transition is characterized by 30km thick crust with strong lateral velocity variations in the upper crust and a high-velocity lower crust (HVLC), where velocities reach up to 7.5km/s. The HVLC is 100 to 130km wider at the Walvis Ridge than it is farther south, and impinges onto the continental crust of the Kaoko fold belt. Such high seismic velocities indicate Mg-rich igneous material intruded into the continental crust during the initial rifting stage. However, the remaining continental crust seems unaffected by intrusions and the root of the 40km-thick crust of the Kaoko belt is not thermally abraded. We conclude that the plume head did not modify the continental crust on a large scale, but caused rather local effects. Thus, it seems unlikely that a plume drove or initiated the breakup process. We further propose that the plume already existed underneath the continent prior to the breakup, and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts.

Structures and active tectonics of compressionally reactivated back-arc failed rift across the Toyama trough in the Sea of Japan, revealed by multiscale seismic profiling

Compressional reactivation of back-arc failed rifts in the Sea of Japan and its mechanism that mobilizes active tectonics based on the crustal structures remains poorly understood. Here, we show new results and interpretations of onshore-offshore deep seismic reflection profiles across the Toyama trough and its southern extension, located in the Sea of Japan back-arc rift, to reveal their deep to shallow structures of the crust and reactivated active faults. We observed (1) approximately 5km thick, syn- and post-rift Neogene sediments underlain by pre-Neogene basement rocks characterized by high velocity anomalies, and (2) crustal thrust wedges adjacent to the rift axis. Shallower high-resolution seismic reflection profiles also suggest Quaternary structural growth of the reactivated thrusts and thrust wedges, consistent with tectonic geomorphology of the middle to late Pleistocene marine and fluvial terraces. These findings demonstrate that the mechanical contrasts between the crustal thrust wedges, primarily composed of the pre-rift continental crust and high velocity lower crust caused by mafic intrusions in the rifting stage, have fundamentally controlled the styles of active deformation during the post-rift Quaternary compressional stress regime. These results also contribute to the construction of the seismic source fault models for tsunamis and seismic hazard estimations of this region. In addition, we emphasize that combining multi-scale seismic reflection profiles with tectonic geomorphology sheds light on complicated structural characteristics and recent activities of reactivated normal faults that are mostly blind because of the dynamic subsidence of failed rift structures.

Ferro buys Ferer (Turkey) & Al Salomi (Egypt)

Results are presented of a seismic wide-angle reflection/refraction survey along a profile between the Pannonian Basin (PB) and the East European Craton (EEC) called PANCAKE. The P- and S-wave velocity model derived can be divided into three sectors: the PB; the Carpathians, including the Transcarpathian Depression and the Carpathian Foredeep; and the south-western part of the EEC, including the Trans European Suture Zone (TESZ). Seismic data support a robust model of the Vp velocity structure of the crust. In the PB, the 22–23 km thick crust consists of a 2–5 km thick sedimentary layer (Vp = 2.4–3.7 km/s), 17–20 km thick upper crystalline crust (5.9–6.3 km/s) and an up to 3 km thick lower crustal layer (Vp = 6.4 km/s). In the central part of the Carpathians, a 10–24 km thick uppermost part of the crust with Vp ≤ 6.0 km/s may correspond to sedimentary rocks of different ages; several high velocity bodies (Vp = 5.35, 5.95 and 6.05 km/s) within the sedimentary flysch sequences may represent volcanic sequences. The Moho depth changes from 25 km to 45 km over ca. 100 km distance beneath the Carpathians, west of TESZ. The cratonic crust has a typical three layer structure with a pronounced thickening of the lower crust towards the Ukrainian Shield, where a high velocity lower crust (Vp > 7.2 km/s) is observed. Two low-velocity lenses in the upper crust of the EEC are interpreted beneath major sedimentary troughs (Lviv and Volyn-Podolsk). Mantle reflectors are observed at depths of ~ 45 km and ~ 75 km below the PB and 10–20 km below the Moho in the EEC. Sub-Moho (Pn) velocities increase from 8.0 km/s beneath the PB to 8.1 km/s beneath the Carpathians and to ~ 8.3 km/s beneath the EEC. S-waves of acceptable quality are recorded in the EEC; their signal-to-noise ratios increase towards the Ukrainian Shield.

High-resolution 3D seismic model of the crustal and uppermost mantle structure in Poland

In the area of Poland a contact between the Precambrian and Phanerozoic Europe and the Carpathians has a complicated structure and a complex P-wave velocity of the sedimentary cover, crystalline crust, Moho depth and the uppermost mantle. The geometry of the uppermost several kilometers of sediments is relatively well recognized from over 100,000 boreholes. The vertical seismic profiling (VSP) from 1188 boreholes provided detailed velocity data for regional tectonic units and for stratigraphic successions from Permian to the Tertiary and Quaternary deposits. These data, however, do not provide information about the velocity and basement depth in the central part of the Trans-European suture zone (TESZ) and in the Carpathians. So, the data set is supplemented by 2D velocity models from 32 deep seismic sounding refraction profiles which also provide information about the crust and uppermost mantle. Together with the results of other methods: vertical seismic profiling, magnetotelluric, allow for the creation of a detailed, high-resolution 3D model for the entire Earth's crust and the uppermost mantle down to a depth of 60km. The thinnest sedimentary cover in the Mazury–Belarus anteclise is only 0.3 to 1km thick, which increases to 7 to 8km along the East European Craton (EEC) margin, and 9 to 12km in the TESZ. The Variscan domain is characterized by a 1–4km thick sedimentary cover, while the Carpathians are characterized by very thick sedimentary layers, up to about 20km. The crystalline crust is differentiated and has a layered structure. The crust beneath the West European Platform (WEP; Variscan domain) is characterized by P-wave velocities of 5.8–6.6km/s. The upper and middle crusts beneath the EEC are characterized by velocities of 6.1–6.6km/s, and are underlain by a high velocity lower crust with a velocity of about 7km/s. A general decrease in velocity is observed from the older to the younger tectonic domains. The TESZ is associated with a steep dip in the Moho depth, from 30–35km in the Paleozoic Platform to 42–52km in the Precambrian craton. The new model confirms the Moho depth derived from previous compilations. In the TESZ the lower crust has a very high seismic velocity (>7.0km/s) which correlates to the high P-wave velocity (about 8.4km/s) in the uppermost mantle beneath the Polish Basin. The Cratonic area is generally characterized by high P-wave velocities (>8.2km/s), while the Phanerozoic area is characterized by velocities of ~8.0km/s. In the TESZ very high velocities of 8.3–8.4km/s are observed, and the southwestern limitation of this area coincides with a high velocity lower crust, and could be continued to the NW toward the Elbe line. The influence of the structure for teleseismic tomography time residuals of seismic waves traveling through the 3D seismic model was analyzed. Lithological candidates for the crust and uppermost mantle of the EEC and WEP were suggested by comparison to laboratory data. The presented 3D seismic model may make more reliable studies on global dynamics, and geotectonic correlations, particularly for sedimentary basins in the Polish Lowlands, the napped flysch sediment series in the Carpathians, the basement shape, the southwestern edge of the EEC, a high-velocity lower crust and the high-velocity uppermost mantle in the TESZ. Finally, the new 3D velocity model of the crust shows a heterogeneous structure and offers a starting point for the numerical modeling of deeper structures by allowing for a correction of the crustal effects in studies of the mantle heterogeneities.

Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

Abstract. High-velocity lower crust (HVLC) and seaward-dipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná–Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland–Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs. This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.