GD7.1

The central Tethys realm including Anatolia, Caucasus and Iran is one of the most
complex geodynamic settings within the Alpine-Himalayan belt. To investigate the
tectonics of this region, we estimate the depth to magnetic basement (DMB) as a
proxy for the shape of sedimentary basins, and average crustal magnetic
susceptibility (ACMS) by applying the fractal spectral method to aeromagnetic data.
Magnetic data is 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. Furthermore, a seismically constrained 2D
density-susceptibility model along Zagros is developed to study the depth extent of
the tectonic structure.
Comparison of DMB and ACMS demonstrates that the structural complexity
increases from the Iranian plateau into Anatolia.
Strong ACMS show lineaments coincides with known occurrences of Magmatic-
Ophiolite Arcs (MOA) and weak ACMS zones coincide with known sedimentary
basins in the study region, including Zagros. Based on strong ACMS anomalies, we
identify hitherto unknown MOAs below the sedimentary cover in eastern Iran and in
the SE part of Urima-Dokhtar Magmatic Arc (UDMA). Our results allow for
estimation of the dip of the related paleo-subduction zones. 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 in Anatolia from a strong ACMS
anomaly. We identify large, partially buried magmatic bodies in the Caucasus LIP at
the Transcaucasus and Lesser Caucasus and in NW Iran. Strong ACMS anomalies
coincides with tectonic boundaries and major faults within the Iranian plateau while
the ACMA signal is generally weak in Anatolia. The Cyprus subduction zone has a

strong magnetic signature which extends ca. 500 km into the Arabian plate to the
south of the Bitlis suture.
We derive a 2D crustal-scale density-susceptibility model of the NW Iranian plateau
along a 500 km long seismic profile across major tectonic provinces of Iran from the
Arabian plate to the South Caspian Basin (SCB). A seismic P-wave receiver function
section is used to constrain major crustal boundaries in the density model. We
demonstrate that the Main Zagros Reverse Fault (MZRF), between the Arabian and
the overriding Central Iran crust, dips at ~13° angle to the NE and extends to a depth
of ~40 km. The trace of MZRF suggests ~150 km underthrusting of the Arabian plate
beneath Central Iran. We identify a new crustal-scale suture beneath the Tarom
valley separating the South Caspian Basin crust from Central Iran. High density lower
crust beneath Alborz and Zagros may be related to partial eclogitization of crustal
roots at depths deeper than ~40 km.

How to cite: Teknik, V., Thybo, H., Artemieva, I., and Ghods, A.: Crustal structure in the central Tethys realm, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6210, https://doi.org/10.5194/egusphere-egu22-6210, 2022.

Εpithermal and porphyry-type mineralization is genetically associated with acidic dyke rocks in a part of the supra-detachment Western Thrace Basin. ⁴⁰Ar/³⁹Ar ages on biotite of an andesitic lava dome and on K-feldspar of quartz-feldspar porphyritic dykes were determined and thus, new temporal constraints on the age of volcanism and mineralization were obtained.           

Biotite of an andesitic lava dome yields a ⁴⁰Ar/³⁹Ar plateau age of 33.05 ± 0.07 Ma (P=0.12). The dated andesite is considered as representative of the andesitic-dacitic rocks of large volcanic and subvolcanic bodies in the Western Thrace basin (Mavropetra Formation, Kirki area). Andesitic rocks indicate affinities of calc-alkaline to high-K calc-alkaline series magmatism. They are coeval to the high-K calc-alkaline magmatic suite of Leptokarya – Kirki, which forms an ENE-WSW 30 km long magmatic dome, developed between the Rhodope metamorphics extending northwards and the overlying detached Melia non-metamorphic formations and Middle-Upper Eocene molassic clastics, extending southwards.

Smaller bodies of acidic dyke rocks (rhyolite and quartz-feldspar porphyry), crosscut the overall dome structure with the andesitic-dacitic volcanics, the Middle-Upper Eocene clastic sediments, the mafic rocks of the Melia unit, the metamorphics of the Kechros Unit of Rhodope and the Leptokarya - Kirki granitoids. They appear with planar subvertical boundaries following a general NNW-SSE trend, perpendicular to the main ENE-WSW dome structure. They are concentrated along a major fault zone  (Ag. Filippos fault), with high- to intermediate sulfidation epithermal polymetallic sulfide mineralization, as well as in a roughly 8 km long and 1 km wide fracture zone to the east and northeast of Aisymi village with porphyry-type mineralization. Structural observations document the mega-tension gashes nature of the dykes with pronounced sinistral strike-slip kinematic indicators of the Kirki mineralized tectonic zone. K-feldspars from quartz-feldspar porphyritic dykes at Kirki yield a ⁴⁰Ar/³⁹Ar plateau age of a 31.89 ± 0.12 Ma (P=0.08).  The acidic dyke rocks contain calc-alkaline to high-K calc-alkaline differentiation trends. They exhibit marked enrichment of LREE relative to the HREE, flat HREE pattern, negative Eu anomaly and Eu/Eu* values ranging between 0.32 and 0.82.

In conclusion, the ENE-SSW Leptokarya - Kirki granitic dome was developed contemporaneously with the andesitic-dacitic volcanics at the contact between the Rhodope metamorphics and the detached Melia formations and Middle-Upper Eocene clastics at about 33 Ma, followed by the NNW-SSE transverse faults and acidic dykes with epithermal and porphyry-type mineralization at about 32 Ma.

How to cite: Skarpelis, N., Jourdan, F., and Papanikolaou, D.: New time constraints from 40Ar/39Ar geochronology on andesitic-dacitic lavas and acidic dyke rocks: An attempt to date the associated mineralization in the Western Thrace supra-detachment basin (Kirki, NE Greece), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2159, https://doi.org/10.5194/egusphere-egu22-2159, 2022.

The Baltic Shield is located in the northern part of Europe. It 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 by the Sveconorwegian (Grenvillian) and the Caledonian orogenies. The Baltic Shield includes an up to 2500 m high northeast-southwest oriented mountain range, the Scandes, which mainly coincides with the Caledonian and Sveconorwegian deformed parts along the western North Atlantic coast, despite being located far from any active plate boundary.

We present a crustal scale seismic model along the WNW to ESE directed Silver Road profile in northern Scandinavia between 8^oE and 20^oE. This profile extends south of Lofoten for ~300km across the Norwegian shelf in the Atlantic Ocean and for ~300km across the onshore Caledonides and Baltic Shield proper. The seismic data were acquired with 5 onshore explosive sources and offshore air gun shots from the vessel Hakon Mosby along the whole offshore profile. Data was acquired by 270 onshore stations at nominally 1.5 km distance and 16 ocean bottom seismometers on the shelf, slope and into the oceanic environment. The results of this experiment will provide information on the origin of the anomalous onshore topography and offshore bathymetry at the edge of the North Atlantic Ocean.

We present results from ray tracing modeling and tomographic inversion of the seismic velocity structure along the profile. The crustal structure is uniform with a thickness of 45 km along the whole onshore profile including both the Caledonides and the shield part. The crust thins abruptly to ~25 km thickness towards the shelf around the coastline. Pn velocity is only ~7.6-7.8 km/s below the high topography areas with Caledonian nappes, and extending into the offshore part, whereas it is 8.4 km/s below the shield proper. By gravity modelling we find that the low Pn zone has a low density of 3.20 g/cm³, which we interpret as partially eclogitizised lower crust. The Svecofennian unit has a very high density of 3.48 g/cm³ in the shield with low topography. Isostasy to 60 km depth, as suggested by Receiver Functions, indicates a ~2 km topography which is ~1 km higher than observed. However, recent results from high-resolution seismic tomography shows a velocity change between the two onshore zones down to 120 km depth. Including this observations into the calculations allows us to explain the observed topography by isostasy in the crust and lithospheric mantle.

How to cite: Kahraman, M., Thybo, H., Artemieva, I., Shulgin, A., Hedin, P., and Mjelde, R.: Crustal Structure across Central Scandinavia along the Silver-Road refraction profile, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3139, https://doi.org/10.5194/egusphere-egu22-3139, 2022.

We present a new model for the density structure of the lithospheric upper mantle beneath the Siberian craton, based on a 3D tesseroid gravity modeling. Our model is based on a detailed crustal structural database SibCrust (Cherepanova et al., 2013) constrained by regional seismic data. The residual lithospheric mantle gravity anomalies are derived by removing the 3D gravitational effect of the crust. We next convert these anomalies to lithosphere mantle in situ densities. To evaluate chemical heterogeneities of the lithospheric mantle, thermal effects are removed based on the global continental thermal model TC1 (Artemieva, 2006). The resulting density model at SPT conditions shows a highly heterogeneous structure of the cratonic lithospheric mantle. Density heterogeneities reflect a complex geodynamic evolution of the craton, which still preserves parts of the pristine cratonic lithosphere in areas where the lithosphere has not been modified by metasomatism associated with the Siberian LIP, several pulses of kimberlite-type magmatism, and rifting at the peripheral parts.

How to cite: Shulgin, A. and Artemieva, I.: Lithospheric thermo-chemical heterogeneity and density structure of the Siberian craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5771, https://doi.org/10.5194/egusphere-egu22-5771, 2022.

Deep structure beneath the central part of the Balkan Peninsula was studied using P and S receiver function technique. Data from seismic stations from the Bulgarian National Seismological Network and several stations from neighbouring countries were used. Depth of Mohorovicic discontinuity has been estimated between 28–30 km in northern and central Bulgaria to 50 km in southwestern of Bulgaria. The 410 km mantle boundary is uplifted by 10 km relative to nominal depth in the area of Rhodopean Massif. In northern Bulgaria, the boundary is lowered by 10 km. Indications of a low-velocity layer are present at a depth exceeding 410 km. The thickness of the asthenosphere is estimated as 50 km and the depth of lithosphere-asthenosphere (LAB) boundary varies between 40 and 60 km.

The results of this study have been published in Vinnik et. al., Izvestiya, Physics of the Solid Earth, 2021, Vol. 57, No. 6, pp. 849–863. This research has been carried out as part of a joint project supported by the National Science Foundation of Bulgaria (grant no. KP-06-RUSIA/27.09.2019) and the Russian Foundation for Basic Research (RFBR, grant no. 19-55-18008 Bolg_a).

How to cite: Georgieva, G., Vinnik, L., Oreshin, S., Makeyeva, L., Dragomirov, D., Buchakchiev, V., and Dimitrova, L.: Upper mantle structure beneath Bulgaria obtained by receiver function analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9483, https://doi.org/10.5194/egusphere-egu22-9483, 2022.

Seismic discontinuities in the mantle are indicators of its thermo-chemical state and offer clues
to its dynamics. Ray-based imaging methods, though limited by the approximations made, have
mapped mantle transition zone (MTZ) discontinuities in detail, but have yet to offer definitive
conclusions on the presence and nature of mid-mantle discontinuities. We use a waveequation-
based imaging method to image both MTZ and mid-mantle discontinuities, and
interpret their physical nature. We focus on precursors to the surface-reflected seismic phases
PP, SS, PS, and SP to produce images of deep reflectors using reverse-time migration (RTM),
employing the full-waveform tomographic model GLAD-M25 for wavefield extrapolation. Our
adjoint-based inverse modeling accounts for more of the physics of wave propagation than raybased
stacking methods, which leads to improved accuracy and realistic precision of the
obtained images. The relative amplitude and location of the imaged reflectors are indeed well
resolved, but an interpretation of absolute amplitudes in terms of reflection coefficients
remains elusive. We observe a thinned mantle transition zone southeast of Mauna Loa, Hawaii,
and a reduction in impedance contrast around 410 km depth in the same area. These
observations coincide with anomalously low S-wavespeeds in the background tomographic
model, suggesting a hotter-than-average mantle in the region. Our new images furthermore
reveal a 4000—5000 km-wide reflector in the mid mantle below the central Pacific, at 950—
1050 km depth. This discontinuity displays strong topography and is marked by a polarity
opposite to that of the 660-km discontinuity, implying an impedance reversal near 1000 km.
We speculate that this mid-mantle discontinuity is linked to the mantle plumes rising from the
large low shear-velocity province (LLSVP) at the base of the mantle below this region. Some
seismic tomography models are in support of this interpretation, while others remain at odds---
a discrepancy that our observations may help resolve.

How to cite: Zhang, Z., Irving, J., Simons, F., and Alkhalifah, T.: Seismic evidence for a 1000-km mantle discontinuity under the Pacific, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13229, https://doi.org/10.5194/egusphere-egu22-13229, 2022.

Antarctica is losing ice mass by basal melting associated with processes in deep Earth and reflected in geothermal heat flux. The latter is poorly known and existing models based on disputed assumptions are controversial. Here I demonstrate that the rate of Antarctica ice basal melting is significantly underestimated: the area with high heat flux is double in size and the amplitude of the high heat flux anomalies is 20-30% higher than in previous results. Extremely high heat flux (>100 mW/m²) in almost all of West Antarctica, continuing to the South Pole region, and beneath the Lake Vostok region in East Antarctica requires a thin (<70 km) lithosphere and shallow mantle melting, caused by recent geodynamic activity. This high heat flux may promote sliding lubrication and result in dramatic reduction of ice mass. The results form basis for re-evaluation of the Antarctica ice-sheet dynamics models with consequences for global environmental changes. [Artemieva, I.M., 2022, Earth-Science Reviews]

How to cite: Artemieva, I. M.: Antarctica ice sheet basal melting enhanced by high mantle heat, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5722, https://doi.org/10.5194/egusphere-egu22-5722, 2022.

We present a thermal model- of lithospheric thickness and surface heat flow in Tibet and adjacent regions (74-110^o E, 26-42^o N) based on topography and seismic Moho. We interpret strong heterogeneity in lithospheric thermal structure to be caused by longitudinal variations in the northern extent of the subducting Indian plate, southward subduction of the Asian plate beneath central Tibet, and possible preservation of fragmented Tethyan paleo-slabs. Cratonic-type cold and thick lithosphere (200-240 km) with a predicted surface heat flow of 40-50 mW/m² typifies the Tarim Craton, the northwest Yangtze Craton, and most of the Lhasa Block that is likely refrigerated by underthrusting Indian lithosphere. We identify a ‘North Tibet anomaly’ (at 84-92^o E, 33-38^o N) with thin (<80 km) lithosphere and high surface heat flow (>80-100 mW/m²) in a region with anomalous seismic Sn and Pn propagation. We interpret this anomaly as the result of removal of lithospheric mantle and asthenospheric upwelling at the junction of the Indian and Asian slabs with opposite subduction polarities. Other parts of Tibet typically have intermediate lithosphere thickness of 120-160 km and a surface heat flow of 45-60 mW/m², with patchy anomalies in eastern Tibet.

How to cite: Xia, B., Artemieva, I., Thybo, H., and Klemperer, S.: Regional variability in the thermal structure of Tibetan Lithosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7161, https://doi.org/10.5194/egusphere-egu22-7161, 2022.

We present a P-wave velocity model of the upper mantle, obtained from finite-frequency body wave tomography, to analyze the relationship between deep and surface structures in Fennoscandia, one of the most studied cratons on Earth. The large array aperture of 2000 km by 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 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% 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 low velocity anomaly below the mountains underlain by positive velocity anomalies, which we explain by preserved original Svecofennian and Archaean mantle below the Caledonian/Sveconorwegian deformed parts of Fennoscandia. Strong positive velocity anomalies to around 200 km depth around the southern Bothnian Bay and the Baltic Sea may be associated with pristine lithosphere of the present central and southern Fennoscandian craton that has been protected from modification since its formation. However, the Archaean domain in the north and the marginal parts of the Svecofennian domains appear to have experienced strong modification of the upper mantle. A pronounced north-dipping positive velocity anomaly in the southern Baltic Sea extends below Moho. It coincides in location and dip with a similar north-dipping structure in the crust and uppermost mantle to 80 km depth observed from high resolution, controlled source seismic data. We interpret this feature as the image of a Paleoproterozoic boundary which has been preserved for 1.8 Gy in the lithosphere.

How to cite: Bulut, N. and Thybo, H.: Finite-Frequency Body-Wave Tomography in Scandinavia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10022, https://doi.org/10.5194/egusphere-egu22-10022, 2022.