Hans Thybo

The Senja onshore-offshore seismic profile is located in the north-western part of Europe across the Norwegian coast into the North Atlantic ocean. A number of terranes and microcontinents collided to form this region from the Archean to the Paleoproterozoic. The Sveconorwegian (Grenvillian) and Caledonian orogenies significantly affected this region and created the major Caledonian mountain belt. Despite being far from any active plate boundaries, the Baltic Shield contains a mountain range called the Scandes that reaches heights of up to 2500 meters. This mountain range is oriented northeast-southwest and mainly correlates with the deformed Caledonian and Sveconorwegian part of the western North Atlantic coastal region.We present a crustal scale seismic profile along the northwest-to-southeast-directed Senja OBS Survey Profile in northern Scandinavia between 12°E and 20°E. This profile extends offshore and onshore for a total of ~300 kilometres across the Norwegian shelf in the No...

We calculate the depth to magnetic basement and the average crustal magnetic susceptibility, which is sensitive to the presence of iron-rich minerals, to interpret the present structure and the tecto-magmatic evolution in the Central Tethyan belt. Our results demonstrate exceptional variability of crustal magnetization with smooth, small-amplitude anomalies in the Gondwana realm and short-wavelength high-amplitude variations in the Laurentia realm. Poor correlation between known ophiolites and magnetization anomalies indicates that Tethyan ophiolites are relatively poorly magnetized, which we explain by demagnetization during recent magmatism. We analyze regional magnetic characteristics for mapping previously unknown oceanic fragments and mafic intrusions, hidden beneath sedimentary sequences or overprinted by tectono-magmatic events. By the style of crustal magnetization, we distinguish three types of basins and demonstrate that many small-size basins host large volumes of magmatic rocks within or below the sedimentary cover. We map the width of magmatic arcs to estimate paleo-subduction dip angle and find no systematic variation between the Neo-Tethys and Paleo-Tethys subduction systems, while the Pontides magmatic arc has shallow (∼15°) dip in the east and steep (∼50°–55°) dip in the west. We recognize an unknown, buried 450 km-long magmatic arc along the western margin of the Kırşehir massif formed above steep (55°) subduction. We propose that lithosphere fragmentation associated with Neo-Tethys subduction systems may explain high-amplitude, high-gradient crustal magnetization in the Caucasus Large Igneous Province. Our results challenge conventional regional geological models, such as Neo-Tethyan subduction below the Greater Caucasus, and call for reevaluation of the regional paleotectonics.

<p>The central Tethys realm including Anatolia, Caucasus and Iran is one of the most<br>complex geodynamic settings within the Alpine-Himalayan belt. To investigate the<br>tectonics of this region, we estimate the depth to magnetic basement (DMB) as a<br>proxy for the shape of sedimentary basins, and average crustal magnetic<br>susceptibility (ACMS) by applying the fractal spectral method to aeromagnetic data.<br>Magnetic data is sensitive to the presence of iron-rich minerals in oceanic fragments<br>and mafic intrusions hidden beneath sedimentary sequences or overprinted by<br>younger tectono-magmatic events. Furthermore, a seismically constrained 2D<br>density-susceptibility model along Zagros is developed to study the depth extent of<br>the tectonic structure.<br>Comparison of DMB and ACMS demonstrates that the structural complexity<br>increases from the Iranian plateau into Anatolia.<br>Strong ACMS show lineaments coincides with known occurrences of Magmatic-<br>Ophiolite Arcs (MOA) and weak ACMS zones coincide with known sedimentary<br>basins in the study region, including Zagros. Based on strong ACMS anomalies, we<br>identify hitherto unknown MOAs below the sedimentary cover in eastern Iran and in<br>the SE part of Urima-Dokhtar Magmatic Arc (UDMA). Our results allow for<br>estimation of the dip of the related paleo-subduction zones. Known magmatic arcs<br>(Pontides and Urima-Dokhtar) have high-intensity heterogeneous ACMS. We<br>identify a 450 km-long buried (DMB >6 km) magmatic arc or trapped oceanic crust<br>along the western margin of the Kirşehır massif in Anatolia from a strong ACMS<br>anomaly. We identify large, partially buried magmatic bodies in the Caucasus LIP at<br>the Transcaucasus and Lesser Caucasus and in NW Iran. Strong ACMS anomalies<br>coincides with tectonic boundaries and major faults within the Iranian plateau while<br>the ACMA signal is generally weak in Anatolia. The Cyprus subduction zone has a</p><p>strong magnetic signature which extends ca. 500 km into the Arabian plate to the<br>south of the Bitlis suture.<br>We derive a 2D crustal-scale density-susceptibility model of the NW Iranian plateau<br>along a 500 km long seismic profile across major tectonic provinces of Iran from the<br>Arabian plate to the South Caspian Basin (SCB). A seismic P-wave receiver function<br>section is used to constrain major crustal boundaries in the density model. We<br>demonstrate that the Main Zagros Reverse Fault (MZRF), between the Arabian and<br>the overriding Central Iran crust, dips at ~13° angle to the NE and extends to a depth<br>of ~40 km. The trace of MZRF suggests ~150 km underthrusting of the Arabian plate<br>beneath Central Iran. We identify a new crustal-scale suture beneath the Tarom<br>valley separating the South Caspian Basin crust from Central Iran. High density lower<br>crust beneath Alborz and Zagros may be related to partial eclogitization of crustal<br>roots at depths deeper than ~40 km.</p>

Earlier seismic studies of the Kalahari Craton in southern Africa infer deformation of upper mantle by flow with fast direction of seismic anisotropy being parallel to present plate motion, and/or report anisotropy frozen into the lithospheric mantle. We present evidence for very strong seismic anisotropy in the crust of the Kalahari craton, which is 30-40% of the total anisotropy as measured by SKS splitting. Our analysis is based on calculation of receiver functions for the data from the SASE experiment which shows strong splitting between the SV and SH components. The direction of the fast axes is uniform within tectonic units and parallel to orogenic strike in the Limpopo and Cape fold belts. It is further parallel to the strike of major dyke swarms which indicates that a large part of the observed anisotropy is controlled by lithosphere fabrics and macroscopic effects. The directions of the fast axes for the crustal anisotropy are parallel to the general directions determined fr...

This study presents seismic images of the crustal and lithospheric structure in Siberia based on the available broadband seismic data using teleseismic receiver functions (RFs). We invert P- and S-RFs jointly. The inversion technique is carried out by approach described by Vinnik et al. (2004). With this method, we determine seismic P- and S-velocities that are comparable to the results of teleseismic body wave and surface wave tomography techniques.TheRFmodel shows variations in the crustal thickness between 35 and 55 km. Intracrustal structures are identified, in particular using the high-frequency P-RF component as it has about an order of magnitude better resolution than S-RF. We find no indication for significant crustal anisotropy in the cratonic areas of Siberia. The preliminary crustal thickness results from the Hk stacking and from the inversion approach agree with a previous study of mainly controlled source results by Cherepanova et al. (2013). Here we also determine the Vp/...

<p>We interpret the paleotectonic evolution and structure in the Tethyan belt by analyzing magnetic data sensitive to the presence of iron-rich minerals in oceanic fragments and mafic intrusions, hidden beneath sedimentary sequences or overprinted by younger tectono-magmatic events. By comparing the depth to magnetic basement (DMB) as a proxy for sedimentary thickness with average crustal magnetic susceptibility (ACMS), we conclude:</p><p> (1) Major ocean and platform basins have DMB >10 km. Trapped ocean relics may be present below Central Anatolian micro-basins with DMB at 6-8 km and high ACSM.  In intra-orogenic basins, we identify magmatic material within the sedimentary cover by significantly smaller DMB than depth to seismic basement.</p><p>(2) Known magmatic arcs (Pontides and Urima-Dokhtar) have high-intensity heterogeneous ACMS. We identify a 450 km-long buried (DMB >6 km) magmatic arc or trapped oceanic crust along the western margin of the Kirşehır massif from a strong ACMS anomaly. Large, partially buried magmatic bodies form the Caucasus LIP at the Transcaucasus and Lesser Caucasus and in NW Iran.</p><p>(3) Terranes of Gondwana affinity in the Arabian plate, S Anatolia and SW Iran have low-intensity homogenous ACMS.</p><p>(4) Local poor correlation between known ophiolites and ACMS anomalies indicate a small volume of presently magnetized material in the Tethyan ophiolites, which we explain by demagnetization during recent magmatism.</p><p>(5) ACMS anomalies are weak at tectonic boundaries and faults. However, the Cyprus subduction zone has a strong magnetic signature which extends ca. 500 km into the Arabian plate.</p>

<p>The Baltic Shield is located in northern Europe. It was formed by amalgamation of a series of terranes and microcontinents during the Archean to the Paleoproterozoic, followed by significant modification in Neoproterozoic to Paleozoic time. The Baltic Shield includes a high mountain range, the Scandes, along its western North Atlantic coast, despite being a stable craton located far from any active plate boundary.</p> <p>The ScanArray international collaborative program has acquired broad band seismological data at 192 locations in the Baltic Shield during the period between 2012 and 2017. The main objective of the program is to provide seismological constraints on the structure of the lithospheric crust and mantle as well as the sublithospheric upper mantle. The new information will be applied to studies of how the lithospheric and deep structure affects observed fast topographic change and geological-tectonic evolution of the region. The recordings are of very high quality and are used for analysis by suite of methods, including P- and S-wave receiver functions for the crust and upper mantle, surface wave and ambient noise inversion for seismic velocity, body wave P- and S- wave tomography for upper mantle velocity structure, and shear-wave splitting measurements for obtaining bulk anisotropy of the upper and lower mantle. Here we provide a short overview of the data acquisition and initial analysis of the new data with focus on parameters that constrain the fast topographic change in the Scandes.</p> <p> </p>

<p>The Baltic Shield is located in the northern part of Europe, which formed by amalgamation of a series of terranes and microcontinents during the Archean to the Paleoproterozoic, followed by significant modification in Neoproterozoic to Paleozoic time. The Baltic Shield includes an up-to 2500 m high mountain range, the Scandes , along the western North Atlantic coast, despite being a stable craton located far from any active plate boundary.</p><p>We study a crustal scale seismic profile experiment in northern Scandinavia between 63<sup>o</sup>N and 71<sup>o</sup>N. Our Silverroad seismic profile extends perpendicular to the coastline around Lofoten and extends ~300km in a northwest direction across the shelf into the Atlantic Ocean and ~300km in a southeastern direction across the Baltic Shield. The seismic data were acquired with 5 explosive sources and 270 receivers onshore; 16 ocean bottom seismometers and air gun shooting from the vessel Hakon Mosby were used to collect both offshore and onshore.</p><p>We present the results from raytracing modelling of the seismic velocity structure along the profile. The outputs of this experiment will help to solve high onshore topography and anomalous and heterogeneous bathymetry of the continental lithosphere around the North Atlantic Ocean. The results show crustal thinning from the shield onto the continental shelf and further into the oceanic part. Of particular interest is the velocity below the high topography of the Scandes, which will be discussed in relation to isostatic equilibrium along the profile.</p>

The Palaeoproterozoic crust and upper mantle in the region between the Ukrainian and Baltic shields of the East European Craton were built up finally during collision of the previously independent Fennoscandian and Sarmatian crustal segments at c. 1.8-1.7 Ga. EUROBRIDGE seismic profiling and geophysical modelling across the southwestern part of the Craton suggest that the Central Belarus Suture Zone is the junction between the two colliding segments. This junction is marked by strong deformation of the crust and the presence of a metamorphic core complex. At 1.80-1.74 Ga, major late to post-collisional extension and magmatism affected the part of Sarmatia adjoining the Central Belarus Zone and generated a high-velocity layer at the base of the crust. Other sutures separating terranes of different ages are found within Sarmatia and in the Polish-Lithuanian part of Fennoscandia. While Fennoscandia and Sarmatia were still a long distance apart, orogeny was dominantly accretionary. The ...

SUMMARY We present a P-wave velocity model of the upper mantle, obtained from finite-frequency body-wave tomography, to analyse the relationship between deep and surface structures in Fennoscandia, one of the most studied cratons on the Earth. The large array aperture of 2000 km × 800 km allows us to image the velocity structure to 800 km depth at very high resolution. The velocity structure provides background for understanding the mechanisms responsible for the enigmatic and strongly debated high topography in the Scandinavian mountain range far from any plate boundary. Our model shows exceptionally strong velocity anomalies with changes by up to 6 per cent on a 200 km scale. We propose that a strong negative velocity anomaly down to 200 km depth along all of Norway provides isostatic support to the enigmatic topography, as we observe a linear correlation between hypsometry and uppermost mantle velocity anomalies to 150 km depth in central Fennoscandia. The model reveals a low-vel...

SUMMARY The seismic receiver function (RF) technique is widely used as an economic method to image earth's deep interior in a large number of seismic experiments. P-wave receiver functions (RFs) constrain crustal thickness and average Vp/Vs in the crust by analysis of the Ps phase and multiples (reflected/converted waves) from the Moho. Regional studies often show significant differences between the Moho depth constrained by RF and by reflection/refraction methods. We compare the results from RF and controlled source seismology for the Baikal Rift Zone by calculating 1480 synthetic RFs for a seismic refraction/reflection velocity model and processing them with two common RF techniques [H–κ and Common Conversion Point (CCP) stacking]. We compare the resulting synthetic RF structure with the velocity model, a density model (derived from gravity and the velocity model), and with observed RFs. Our results demonstrate that the use of different frequency filters, the presence of compl...

(by B. Xia, H. Thybo, I.M. Artemieva) We constrain the lithospheric mantle density of the North China Craton (NCC) at both in situ and standard temperature-pressure (STP) conditions from gravity data. The lithosphere-asthenosphere boundary (LAB) depth is constrained by our new thermal model, which is based on a new regional heat flow data set and a recent regional crustal model NCcrust. The new thermal model shows that the thermal lithosphere thickness is <120 km in most of the NCC, except for the northern and southern parts with the maximum depth of 170 km. The gravity calculations reveal a highly heterogeneous density structure of the lithospheric mantle with in situ and STP values of 3.22-3.29 and 3.32-3.40 g/cm 3 , respectively. Thick and reduced-density cratonic-type lithosphere is preserved mostly in the southern NCC. Most of the Eastern Block has a thin (90-140 km) and high-density lithospheric mantle. Most of the Western Block has a high-density lithospheric mantle and a thin (80-110 km) lithosphere typical of Phanerozoic regions, which suggests that the Archean lithosphere is no longer present there. We conclude that in almost the entire NCC the lithosphere has lost its cratonic characteristics by geodynamic processes that include, but are not limited to, the Paleozoic closure of the Paleo-Asian Ocean in the north, the Mesozoic Yangtze Craton flat subduction in the south, the Mesozoic Pacific subduction in the east, the Cenozoic remote response to the Indian-Eurasian collision in the west, and the Cenozoic extensional tectonics (possibly associated with the slab roll-back) in the center.

Formation of new oceans by continental break-up is understood as a continuous evolution from rifting to ocean spreading. The Red Sea is one of few locations on Earth where a new plate boundary presently forms. Its evolution provides key information on how the plate tectonics operates and how the plate boundaries form and evolve in time. While the new plate boundary has already been formed in the southern Red Sea where ocean spreading is active, the north-central segment still experiences continental rifting. The region also has west-east asymmetry: in the north-central Red Sea the rift-related magmatism is not located beneath the rift axis, as conventional models predict, but instead is offset by ca 300 km into Arabia. We propose a new geodynamic model which explains the enigmatic asymmetry of the Red Sea region and is fully consistent with various types of geological and geophysical observations. We demonstrate that the north-central rift is a transient feature that will not develop into coincident ocean spreading. Instead, the new plate boundary forms across Arabia. Our numerical experiments, supported by geological, seismic and gravity observations, predict that in 1-5 Myr the north-central extensional axis will jump ~300 km eastward into Arabia. The Ad Damm strike-slip fault, normal to the central Red Sea rift axis, will evolve into a transform fault between the ongoing ocean spreading in the southern Red Sea and the future spreading in north-central Arabia. We demonstrate that crustal-scale weakness zones control lithosphere extension and lead to long-distance jumps of extensional axes in continental lithosphere not affected by hotspots. Therefore, our model also provides theoretical basis for understanding dynamics and mechanisms of the transition from rifting to continental break-up at passive continental margins not affected by hotspots.

All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (Vp > 7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (Vp 

We present a new 2D crustal-scale model of the northwestern Iranian plateau based on gravity–magnetic modeling along the 500 km long China–Iran Geological and Geophysical Survey in the Iranian plateau (CIGSIP) seismic profile across major tectonic provinces of Iran from the Arabian plate into the South Caspian Basin (SCB). The seismic P-wave receiver function (RF) model along the profile is used to constrain major crustal boundaries in the density model. Our 2D crustal model shows significant variation in the sedimentary thickness, Moho depth, and the depth and extent of intra-crustal interfaces. The Main Recent Fault (MRF) between the Arabian crust and the overriding central Iran crust dips at approximately 13° towards the northeast to a depth of about 40 km. The geometry of the MRF suggests about 150 km of underthrusting of the Arabian plate beneath central Iran. Our results indicate the presence of a high-density lower crustal layer beneath Zagros. We identify a new crustal-scale...

The long-term stability of Precambrian continental lithosphere depends on the rheology of the lithospheric mantle as well as the coupling between crust and mantle lithosphere, which may be inferred by seismic anisotropy. Anisotropy has never been detected in cratonic crust. Anisotropy in southern Africa, detected by the seismological SKS-splitting method, usually is attributed to the mantle due to asthenospheric flow or frozen-in features of the lithosphere. However, SKS-splitting cannot distinguish between anisotropy in the crust and the mantle. We observe strong seismic anisotropy in the crust of southern African cratons by Receiver Function analysis. Fast axes are uniform within tectonic units and parallel to SKS axes, orogenic strike in the Limpopo and Cape fold belts, and the strike of major dyke swarms. Parallel fast axes in the crust and mantle indicate coupled crust-mantle evolution for more than 2 billion years with implications for strong rheology of the lithosphere.

The DOBRE-2 wide-angle reflection and refraction profile was acquired in June 2007 as a direct, southwestwards prolongation of the 1999 DOBREfraction'99 that crossed the Donbas Foldbelt in eastern Ukraine. It crosses the Azov Massif of the East European Craton, the Azov Sea, the Kerch Peninsula (the easternmost part of Crimea) and the northern East Black Sea Basin, thus traversing the entire Crimea–Caucasus compressional zone centred on the Kerch Peninsula. The DOBRE-2 profile recorded a mix of onshore explosive sources as well as airguns at sea. A variety of single-component recorders were used on land and ocean bottom instruments were deployed offshore and recovered by ship. The DOBRE-2 datasets were degraded by a lack of shot-point reversal at the southwestern terminus and by some poor signal registration elsewhere, in particular in the Black Sea. Nevertheless, they allowed a robust velocity model of the upper crust to be constructed along the entire profile as well as throug...