The Bohai Bay Basin, located in northeast China, is a Meso-Cenozoic strike-slip extensional basin. A lot of research work carried out in Bohai Bay Basin, has shown that there is a huge potential of oil and gas resources. However, the proportion of known oil and gas reserves to the total estimated resources is not high, which means that this area still has broad exploration prospects. Although oil and gas resources are mainly distributed in sedimentary basins, their enrichment degree is largely influenced by the structure and development of the lithosphere. Based on lithospheric local isostasy theory and thermal conduction principle linked to temperature dependence of rock densities, the three-dimensional deep structure of the lithosphere under the Bohai Bay Basin is calculated by using geoid and gravity anomalies, topographic and existing geological-geophysical data. The results show that the lithosphere-asthenosphere boundary of Bohai Bay Basin gradually rises from the western onshore to the eastern offshore area from 90 to 110 km. The thinnest lithosphere is found under the Bozhong Depression in the southeast of the Bohai Bay Basin. It is concluded that the thinning of the lithosphere in the Bohai Bay Basin is closely related to the subduction of the Meso-Cenozoic Pacific plate, which led first to thickening followed by delamination of the North China Craton lithosphere, and then the magma upwelling led to slow uplift of the Earth’s surface and continuous stretching of the lithosphere. At the same time, favorable conditions of temperature, pressure, chemistry and structure were provided for the formation of oil and gas. In this wqy, the Bohai Bay basin developed into the present oil-rich basin. This study provides a new perspective for understanding the deep structure and hydrocarbon resource control mechanisms of Bohai Bay Basin.

How to cite: Ma, J., Wang, W., Zeyen, H., and Li, Z.: The Lithosphere Structure Of Bohai Bay Basin: Combining Gravity, Geoid, And Topography Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4710, https://doi.org/10.5194/egusphere-egu24-4710, 2024.

We investigate the properties of mantle discontinuities beneath the Arctic ocean and the Aleutian-Alaska subduction zone with underside reflections of PP and SS waves. The depth distributions of the 410-km and 520-km discontinuities suggest a relatively normal mantle transition zone beneath the Arctic ocean and a cold mantle transition zone with the subducted Pacific plate beneath Aleutian-Alaska subduction. The depth of the 660 km discontinuity shows normal behavior beneath the Arctic Ocean. However, the detection of deep reflectors with opposite polarities in depth range of 720~770 km beneath the eastern Aleutians introduces additional complexity for explaining the slab morphology.  We test several plausible compositions using mineralogical modeling along a subduction geotherm. The deep reflectors are interpreted as mid-ocean ridge basalt (MORB) crust associated with the Pacific slab that may deform or buckle at the bottom of the mantle transition zone beneath the eastern Aleutians.  Meanwhile, an uplifted 660-km discontinuity observed in the adjacent Alaska region suggests a different subduction depth, where the slab may penetrate the 410-km discontinuity but does not reach the 660-km discontinuity,  consistent with previous regional studies. Our observations thus depict a complex slab geometry along the Aleutian-Alaska trench, that is, the slab  may reach the top of the lower mantle beneath eastern Aleutian but remains at the base of the transition zone underneath central Alaska.

How to cite: Yuan, Y., Keith Magali, J., and Thomas, C.: Mantle discontinuities beneath Arctic ocean and Aleutian-Alaska subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4613, https://doi.org/10.5194/egusphere-egu24-4613, 2024.

The ratio between compressional and shear wave speeds (Vp/Vs) in the Earth's crust is crucial for gaining a better understanding of its chemical composition and geological evolution history. Many studies employed H-κ stacking of receiver functions to estimate the crustal Vp/Vs ratio. However, the Vp/Vs ratio obtained from H-κ stacking can be biased due to lateral variations in crustal structures and/or incorrect absolute wave speed assumption. In this study, we propose a novel method to estimate the crustal Vp/Vs ratio using P and S wave multiples near receivers, that is the PpPmp (where m represents the Moho) and SsSms phases. The absolute arrival times of PpPmp and SsSms are sensitive to crustal thickness and wavespeeds, but the ratio of their arrival times is most sensitive to the crustal Vp/Vs ratio. We first verify the new method using synthetic tests on various crustal models. Synthetic results show that in the presence of lateral variation in crustal structure, the new method gives more accurate Vp/Vs ratios than the conventional H-κ stacking of receiver functions. We further validate the new method using field data recorded by the broadband station HYB in the eastern Dharwar Craton. Our data analysis involved preprocessing and manual selection of teleseismic events. Ultimately, the observed PpPmp and SsSms phases from 351 teleseismic events were used to calculate the Vp/Vs ratio beneath the HYB station, resulting in a value of 1.737±0.016. We find that this value is comparable to research results obtained by previous researchers using receiver function inversion (Zhou et al., 2000). Our new method for estimating crustal Vp/Vs ratio can potentially to applied to many other regions of tectonic importance.
This study is supported by National Natural Science Foundation of China (43/K22431006).

How to cite: Meng, F., Yu, C., and Zeng, X.: A new method for constraining crustal Vp/Vs ratio using P and S wave multiples, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7153, https://doi.org/10.5194/egusphere-egu24-7153, 2024.

The depths of mineralogical phase transitions in the mantle (at ~410 and ~660 km depth) offer crucial insights into the thermal conditions of the mantle transition zone and, by extension, the upper mantle's state and circulation. Our approach involves conducting P-to-S receiver function analysis to determine the mantle transition zone's thickness and the absolute depths of the ~410 km and ~660 km discontinuities in the Central and Eastern European region.

Our workflow meticulously attends to each step, starting from data download, quality control, and culminating in the calculation of P-to-S receiver functions. We use data from multiple sources, including the AlpArray and AdriaArray Seismic Networks, the PACASE, Carpathian Basin, and South Carpathian Project temporary seismic networks, as well as the permanent stations of the Hungarian National Seismological network and of the neighboring countries. This analysis covers the time period from 2002 to 2023, involving over 860 seismological stations. Our extensive dataset, consisting of approximately 2 million three-component waveforms and over 120,000 high-quality P-to-S radial receiver functions, coupled with dense piercing-point coverage, allows us to achieve unprecedented resolution.

We present Common Conversion Point cross-sections migrated with a 3D tomographic velocity model underneath the Alps, Carpathians, and the Pannonian Basin. Additionally, we aim to offer new insights into the mantle transition zone's thickness beneath intriguing regions (e.g., Vrancea zone, Alpine Tethys Ocean zone, Eastern Alps–Pannonian Basin transition zone). For a precise understanding of geodynamic processes such as slabs, mantle plumes, and volcanism, it is imperative to accurately map these boundaries.

How to cite: Kalmár, D., Michailos, K., Petrescu, L., Hetényi, G., Bokelmann, G., and Working Groups, A. A. P.: Mantle Transition Zone structure beneath the Central and Eastern European region based on P-to-S Receiver Function analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7989, https://doi.org/10.5194/egusphere-egu24-7989, 2024.

We present the Moho depths in the Bohemian Massif and Western Carpathians derived from P-to-S receiver functions calculated from broad-band P-coda waveforms from teleseismic events recorded at temporary and permanent stations operated in a region within 10–23º E and 47.5–52º N during last two decades. By the Zhu and Kanamori method (2000) and the Ps time delays (Kvapil et al., 2021), we process data collected from running AdriaArray Seismic Network (since 2022), PACASE experiment (2019 – 2022), AlpArray Seismic Network (2015 – 2019) and its complementary experiment AlpArray-EASI (2014 – 2015), as well as from previous passive seismic experiments in the region – BOHEMA I-IV (2001 – 2014), PASSEQ (2006 – 2008) and EgerRift (2007 – 2013). By applying different methods, we aim at upgrading the current knowledge of the crust in the broader surroundings of the European Alps (Michailos et al., 2023), the Pannonian Basin (Kalmar et al., 2019), and the Carpathians. Locally, differences between Moho depth from individual methods could highly exceed 5 km, thus reflecting various sensitivities of individual methods to the local complex structure. An extended amount of data and regionally combined evaluation provide a homogeneous estimate of Moho depths, particularly for the usage in deep Earth studies, e.g., in applying crustal corrections in the upper mantle tomography of Central Europe.

How to cite: Kampfová Exnerová, H., Plomerová, J., Vecsey, L., and Working Groups and AdriaArray Seismology Group, A. A.-E. P.: Mapping the Moho in the Bohemian Massif and the Western Carpathians with P-receiver functions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8696, https://doi.org/10.5194/egusphere-egu24-8696, 2024.

We present a new combined model for the density structure of the lithospheric upper mantle beneath Europe and Siberia, based on a 3D tesseroid gravity modeling. Our results are based on the EuNaRho model (Shulgin & Artemieva, 2019) complimented by similar modeling approach for Siberia. For Siberia modeling is preformed based on a detailed crustal structural database SibCrust (Cherepanova et al., 2013) constrained by regional seismic data. The presented residual lithospheric mantle gravity anomalies are derived by removing the 3D gravitational effect of the crust. Later, these anomalies are converted 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, and distinct change at the transition between different tectonic units. We speculate on the origin of these anomalies.

How to cite: Shulgin, A. and Artemieva, I.: European and Siberian lithospheric thermo-chemical heterogeneity and density structure., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9631, https://doi.org/10.5194/egusphere-egu24-9631, 2024.

The Senja onshore-offshore seismic profile is located in the northwestern part of Fennoscandia, extending from onshore Norway into the North Atlantic Ocean. The Fennoscandian lithosphere has been formed by the amalgamation of terranes and microcontinents to an Archean core, primarily during the Palaeoproterozoic. The later Sveconorwegian (Grenvillian) and Caledonian orogenies had strong effect on the western part of Fennoscandia. The Scandia Mountain range extends along the west coast with elevation up to 2500 m, mainly coinciding with the surface outcrops of Caledonian deformed crust. Its location far from any active plate boundary makes this mountain range enigmatic. The offshore continental part of Fennoscandia experienced a long post-Caledonian extensional period for more than 200 My, and it now forms a continental shelf below sea level extending to the continent to ocean transition.

We present a crustal-scale seismic profile along the NW-SE striking Senja OBS Profile in northern Scandinavia between 12°E and 20°E. This profile covers both offshore and onshore domains over a total distance of ~300 km across the Norwegian shelf in the North Atlantic Ocean, Senja Island, and mainland Norway. Airgun shots from the vessel Hakon Mosby were used as sourced for the refraction/wide-angle reflection survey. The dataset includes recordings on 5 ocean bottom seismometers (OBS) on the shelf, slope, and oceanic environment, complemented by 68 onshore stations at 1.3-kilometre intervals. We present a seismic p-wave velocity model derived by ray-tracing modelling of P-wave arrivals along the profile.

The model includes a deep sedimentary basin extending to ~10 kilometres depth with velocities between ca. 2 km/s and 5.10 km/s, which gradually thickens from the coast to its maximum thickness of 10 km about 25 km from the coast. This deep sedimentary basin is very wide (approximately 8 km). Further offshore the sedimentary cover of the shelf and oceanic environment is relatively thin. The upper crustal velocity below the sedimentary sequence has velocities of ~ 6.0 km/s.

How to cite: Alemdar, R. E., Kahraman, M., Shulgin, A., Breivik, A., Artemieva, I., and Thybo, H.: Crustal Structure across the Northern Scandinavian margin along the Senja OBS Profile , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1098, https://doi.org/10.5194/egusphere-egu24-1098, 2024.

Sedimentary and crustal thickness constraints are important for a wide range of geological and geophysical applications, including: a) measuring dynamic topography; b) calculating heat flow; c) generating seismic tomographic models; d) improving predictions of resource distribution; and e) accurately assessing seismic hazards. In this contribution, we present the methodology and preliminary results of an ongoing study to improve sedimentary and crustal thickness constraints in the continental realm. Active-source seismic experiments and well data provide high-accuracy constraints for total sedimentary thickness. Interpolation between sedimentary thickness measurements is undertaken using a minimum curvature gridding algorithm. We investigate the impact of varying the grid resolution across a range of sedimentary basins, and demonstrate that a high-resolution grid (e.g., ~ 0.03 degrees) is crucial in order to capture lateral heterogeneity. We define crustal thickness as the vertical distance between the base of the sediment (i.e. top basement) and the Moho. Our new sedimentary thickness estimates constrain the top basement while measurements from a new publication of active- and passive-source seismic data are used to constrain Moho depth. Resulting crustal thickness estimates show relatively thin crust beneath a number of continental sedimentary basins. We investigate whether our new estimates of sedimentary and crustal thickness can improve predictions of surface heat flow. Our results demonstrate that constraints of the outermost layers of the Earth are important for understanding the interaction between crust, lithosphere and asthenospheric mantle.

How to cite: Holdt, M. and White, N.: Global Sedimentary and Crustal Thickness Constraints: Implications for Lithosphere-Asthenosphere Dynamics., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11624, https://doi.org/10.5194/egusphere-egu24-11624, 2024.

Cratons are usually considered ‘old and stable’ geological units and, therefore, the do not receive as much consideration by geophysical data acquisition as active tectonic regions. However, abundant evidence shows that ‘stable’ cratons are modified substantially during their existence as demonstrated by geophysical data imaging cratonic lithosphere in several cases:

(1) The Baltic Shield formed during the Svecofennian orogeny c. 1.7 Ga and its western parts were reworked by the Sveconorwegian/Grenvillian orogeny. Recent geophysical interpretations image a large body of crustal material in eclogite facies beneath the present Moho in the central shield. This body probably formed after the initial cratonization (Buntin et al., 2021).

(2) The isopycnicity hypothesis proposes that a trade-off between composition and temperature of the lithospheric mantle maintains constant topography in cratons (Jordan, 1978) based on kimberlite data from South Africa. However, gravity data from Siberia shows that kimberlite pipes solely modify cratons in isostatic equilibrium (Artemieva et al., 2019). Therefore, kimberlite sampling is nonrepresentative, and the real composition of most cratonic mantle lithosphere is unknown.

(3) Strong seismic anisotropy is observed in many cratons and is commonly attributed to the mantle due to frozen-in lithospheric features or asthenospheric flow. Recently it was demonstrated that a major part of the anisotropy resides in the crust of the Kalahara craton and that the fast axes are parallel to the strike of major dyke swarms and orogenic fabric (Thybo et al., 2019). This finding indicates significant craton modification by magmatic intrusion.

(4) Modification by external stresses and induced magmatism may even split existing cratons.  Integrated interpretation of existing data and geodynamic modelling show that a linear sequence of volcanic harrats in the Arabian craton potentially represents the formation of a new plate boundary (Artemieva et al., 2022). It is probable that the extension in the northern Red Sea rift will jump to the volcanic lineament, which eventually will develop into new ocean spreading and effectively split the existing craton.

References

Artemieva, I.M.., Thybo, H. & Cherepanova, Y, 2019. Isopycnicity of cratonic mantle restricted to kimberlite provinces. Earth Plan. Sci. Lett. 505, 13-19, doi:10.1016/j.epsl.2018.09.034 (2019).

Artemieva, I.M., Yang, H., Thybo, H. Incipient ocean spreading beneath the Arabian shield, Earth-Science Reviews, 226, 103955 (2022)

Buntin, S., Artemieva, I.M., Malehmir, A., Thybo, H. et al. Long-lived Paleoproterozoic eclogitic lower crust. Nat Commun 12, 6553 (2021).

Jordan, T. Composition and development of the continental tectosphere. Nature 274, 544–548 (1978)

Thybo, H., Youssof, M. & Artemieva, I.M. Southern Africa crustal anisotropy reveals coupled crust-mantle evolution for over 2 billion years. Nat Commun. 10, 5445 (2019)

How to cite: Thybo, H.: Cratons are not all that stable!, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11165, https://doi.org/10.5194/egusphere-egu24-11165, 2024.

Knowledge of elastic properties of the Earth's crust provide important constraints on its chemical composition, isostasy and tectonic evolution. However, accurate determination of crustal properties beneath sedimentary basins is challenging. This challenge mainly arises from interference caused by sedimentary reverberations, which may either mask desired signals or cause significant bias in parameter estimates. Some studies attempted to remove the sediment effect by applying wavefield downward continuation or resonance filters to conventional receiver functions, but successful applications were limited. Recently, a novel method utilizing Pn and its multiples, named as PnPn, has been developed and proven effective in imaging the Moho beneath sedimentary basins. Arrival times of Pn multiples are sensitive to crustal thickness (H) and P wave speed (Vp). In contrast, arrival times of converted phases in receiver functions are most sensitive to crustal thickness and Vp/Vs ratio. In this study, we apply a joint analysis of the newly developed Pn multiple method and conventional receiver functions to investigate the sedimentary and crustal structures of the northern Ordos basin along a west-east trending profile. We first apply a multi-frequency receiver function waveform fitting technique to constrain the shallow sediment structure. Then, we combine receiver functions and Pn multiples to determine the thickness, Vp and Vp/Vs ratio of the crystalline crust. Our results show that the interior of the Ordos basin is characterized by thick sediments, with the maximum thickness reaching 4.6 km. The sediment thickness shoals toward the eastern margin of the Ordos basin. The sediment structure in general is consistent with previous findings from active source studies and is of higher resolution than previous passive source studies. For the crystalline crust beneath the northern Ordos basin, the absolute Vp ranges from 6.45 to 6.57 km/s and the Vp/Vs ratio ranges from 1.73 to 1.78. These values suggest an overall intermediate crustal composition beneath the northern Ordos basin, in contrast to felsic crustal composition beneath the eastern North China Craton. The crustal thickness in the interior of the northern Ordos basin is remarkably flat, approximately 40 km, closely aligning with the Airy model. However, a deviation from Airy isostasy of approximately 5 km in crustal thickness is observed at the eastern margin of the Ordos basin, which could be due to increased bulk density of the crust accompanying the thinning of low-density sedimentary layer.

How to cite: Yin, W. and Yu, C.: Sedimentary and crustal structures beneath the northern Ordos basin constrained by receiver functions and Pn multiples, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7369, https://doi.org/10.5194/egusphere-egu24-7369, 2024.

    Based on the seismic data of 60 portable stations and 90 permanent CEA stations in the northeastern Tibet Plateau and adjacent regions, we utilized the wave equation post-stack migration method of S-receiver function to image the lithospheric structure of northeastern Tibet Plateau and the Sichuan Basin.

Fig. 1 Distribution of the seismic stations and imaging profile

    The imaging results show that the Moho in the northeastern Tibet Plateau is deeper than 50 km, and it gradually becomes shallow along the profile to the southeast until reaches about 45 km below the Sichuan Basin. The negative anomaly signals corresponding to the Lithosphere and Austhenosphee Boundry (LAB) are obvious in most areas, but under the Sichuan Basin, there are many strong negative anomaly signals in the migration images of different frequencies. In general, the LAB along the profile is undulating and discontinuous: The lithosphere is deeper in the southern Qilian Orogenic Belt, up to ~200 km, with no significant change at the boundary between the Qilian Orogenic Belt and the western Qinling Orogenic Belt. The lithosphere gradually thinned to ~150 km beneath the western Qinling Orogenic Belt, with a step of ~100 km at the tectonic boundary between the Qinling Orogenic Belt and the Songpan-Garze block, and the signal intensity is obviously weakened. LAB was maintained at this depth level until near the Longmenshan Fault, and the lithosphere thickened again to ~190 km after entering the Sichuan Basin. Moreover, there are two discontinuities within the lithosphere of the Sichuan Basin, with depths of ~100 km and ~140 km, respectively, and the latter becomes shallower to ~110 km in the western margin of the Sichuan Basin. Our observations of mid-lithospherci discontinuity (MLD) beneath the Sichuan Basin provide further evidence that the cratonic lithospheric mantle is generally stratified.

Fig. 2 The migration results of the profile

    It is proposed that the lithospheric thinning along the eastern margin of the Songpan-Garze Block may be related to the eastward flow of hot mantle materials beneath the eastern Tibet Plateau. Blocked by the Ordos block and the Sichuan Basin, which have preserved the ancient and rigid craton roots, the eastward flow of mantle materials from the Tibet Plateau will turn to the west of the two blocks. A small part of the blocked mantle material migrates eastward to the Qinling Orogenic Belt, while most of it migrates southward clockwise along the mantle flow path to the west of the Sichuan Basin. The lithosphere in the eastern margin of the Songpan-Garze block, heated by the mantle flow, will be subjected to thermochemical erosion and destruction in the process of collision with the Yangtze craton, and is more likely to be delaminated, resulting in significant thinning and destruction under long-term action.

How to cite: Zhang, Y., Chen, L., Ai, Y., Fang, H., and Wang, G.: Lithospheric Structure beneath Northeastern Tibet Plateau and Sichuan Basin revealed by S-Receiver Function Imaging , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13677, https://doi.org/10.5194/egusphere-egu24-13677, 2024.

The southern Sichuan-Yunnan block (SYB) is intersected by the NW-striking Honghe faults (HHF) and the nearly NS-trending Xiaojiang faults (XJF), providing an excellent zone for exploring severe crustal deformation and complicated tectonic movement. However, the crustal-mantle deformation mechanisms are still controversial, partially due to the lack of detailed information. With ambient noise data from several temporary seismic arrays and regional permanent seismic stations, we applied the direct surface wave tomography to obtain S-wave velocity and azimuthal anisotropy simultaneously. The crustal S-wave structures show complex heterogeneity both horizontally and vertically, relating to geologic settings and large faults. In the mid-lower crust, there are two significant low-velocity anomalies with strong azimuthal anisotropy, with the NNW-SSE direction near the northwest end of HHF and the NE-SW direction around the mid-south segment of XJF, respectively. The fast axis within the SYB shows approximately in the N-S direction, which differs from those in the low-velocity zones on its east and west sides. Therefore, we consider the ductile deformation in the mid-lower crust is more likely restricted by large faults. At the end of the wedged intersection, the southward mid-lower crustal flow could be blocked by the HHF, resulting in the weak materials distributed along the faults rather than crossing over at large-scale. Combining other independent studies, we conclude that there may be different deformation between the crust and the lithospheric mantle. This 3-D model provides important constraints for the regional deformations and plate tectonics of the large boundary faults [supported by NSFC Projects 42074065 & 41730212].

How to cite: Gao, Y., Li, Y., Yao, H., Tian, J., Fu, Y. V., and Wang, Q.: Crustal S-wave 3D azimuthal anisotropy beneath the southern Sichuan-Yunnan block of SW China from multiple seismic arrays, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14363, https://doi.org/10.5194/egusphere-egu24-14363, 2024.

Recent geological mapping in the Ronda peridotites (Betic Cordillera, S Spain) has unveiled a consistent field correlation between lower crustal metamorphic units and specific tectono-metamorphic domains of the ultramafic massif. Mylonitic and highly tectonized spinel ±garnet peridotites (i.e., Grt-Spl mylonite and Spl tectonite domains) –that are considered to originate from a thick continental lithosphere– are in contact with garnet-bearing gneisses (i.e., kinzigites of the Jubrique unit) along a narrow but continuous mylonitic shear zone. Phase equilibrium calculations indicate that these metamorphic rocks align with an initial continental setting characterized by normal crustal thicknesses, which underwent two melting events. The first melting occurred at the base of the lower crust, while the second one took place at shallower crustal conditions and led to a more restricted melt production. By contrast, the spinel ±plagioclase peridotites (i.e., Pl-tectonite domain) –that are stable only at shallowest mantle levels within a highly extended continental lithosphere– are consistently found exposed in contact with heterogeneous granites and migmatites that form part of the Guadaiza crustal unit. According to new thermodynamic modeling, this migmatitic series record a single melting event characterized by a moderate melt production at the base of an extremely thin continental crust. The systematic correlation observed between the crustal metamorphic units and specific ultramafic domains of the Ronda peridotites –consistently overlaying the mantle rocks– indicates that their juxtaposition primarily resulted from the severe extension of the continental lithosphere.

Previous and new U-Pb radiometric dating of zircons from gneisses, migmatites, and heterogeneous granites show that extensional processes, crustal anatexis, and melt stagnation occurred at around 280 Ma. Considering the structural position and correlation between mantle and crustal rocks, these radiometric ages suggest that a Permian high-temperature / low- to medium-pressure event uniformly affected the crustal units over the Ronda peridotites. This event coincided with the formation of characteristic ultramafic mineral assemblages in the Ronda massif, providing evidence for the interaction between upper mantle rocks and lower- to mid-crustal metamorphic rocks during that period.

This research received funding from the Agencia Estatal de Investigación of the Ministerio de Ciencia e Innovación (AEI, MICINN, Spain) under the grant no. PID2020-119651RB-I00/AEI/10.13039/501100011033.

How to cite: Hidas, K., Díaz-Alvarado, J., González-Menéndez, L., Azor, A., and Pedrera, A.: Tectono-metamorphic interaction between the upper mantle and lower crust during continental rifting in the western Betic Cordillera, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15642, https://doi.org/10.5194/egusphere-egu24-15642, 2024.

The thermal state of the Earth’s interior is a key factor in controlling various geological processes. However, our knowledge of the geotherm and its temporal and spatial variability is usually poorly constrained, as it is typically based on point-wise data. Specifically, for the lower continental crust (LCC), data on rock’s thermal properties are scarce, and therefore temperature estimates are uncertain.

We collect new data and provide new insights in this domain, realized in the frame of project DIVE (Drilling the Ivrea-Verbano zonE; www.dive2ivrea.org), which aims at a better understanding of the physical and chemical evolution and formation of the LCC. The first borehole in Ornavasso (DT-1B) has been successfully completed and reached a depth of 578.5 m with 100% core recovery; it provides continuous drill cores of mainly felsic metasedimentary and metamafic lithologies. The second borehole in Megolo di Mezzo (DT-1A) is ongoing and planned to be completed in Spring 2024.

The first results on the thermal characterization of lower crustal rocks are based on 17 fresh cores from DT-1B, sampling all the lithologies present in the borehole. We performed continuous, high-resolution (2 mm) thermal conductivity (TC) measurements using an Optical TC Scanner (Popov et al., 1999), profiling over 10 metres of rock cores in total. Our results show that TC can exhibit large variations even within a given lithology, as a result of mineralogical variability, indicating that this approach provides more representative results compared to conventional methods (e.g. needle-probe technique). We also measured the concentrations of heat producing elements (U, Th, K) using powder-based gamma spectrometry, and use (spectral) gamma borehole logs to evaluate the variability of heat production (A) in the borehole. The correlation of both TC and A with other petrophysical properties is analyzed.

Based on the new measurements, we investigate the consequences on LCC geotherms. The small-scale TC variations affect heat flow calculations and have implications for their uncertainty. These are quantified through model calculations as part of an upscaling procedure employing harmonic averaging. We aim to quantify the effect of continuous TC profiling and how our approach influences the level of uncertainties by applying many realizations of heat flow calculations. The probability distribution of heat flow can be determined by using Bullard’s approach (Bullard 1939; Beardsmore & Cull, 2001) and by randomly selecting rock’s thermal property data while calculating the geotherm. Further samples from DT-1B and a new set of samples from DT-1A will provide a representative dataset for the LCC.

References

Beardsmore, G. R. (Graeme R., & Cull, J. P. (James P. (2001). Crustal heat flow: a guide to measurement and modelling. Cambridge University Press.

Bullard, R. (1939). Heat flow in South Africa. Mon. Not. R. Astr. Soc., Geophys, 173, 229–248.

Popov, Y. A., Pribnow, D. C., Sass, J. H., Williams, C. F., & Burkhardt, H. (1999). Characterization of rock thermal conductivity by high-resolution optical scanning. Geothermics, 28(2), 253–276.

How to cite: Lemke, K. and Hetényi, G.: Thermal characterization of the lower continental crust: first results from the DT-1B borehole of project DIVE (Ivrea-Verbano zone, Italy) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14356, https://doi.org/10.5194/egusphere-egu24-14356, 2024.

The Precambrian terrains of the Eastern Indian Shield (EIS) comprise of Bundelkhand, Singhbhum, and Bastar cratons with intervening Proterozoic mobile belts such as Central Indian Tectonic Zone, Eastern Ghat Mobile Belt, Singhbhum Mobile Belt and Chotanagpur Granite Gneissic Complex. This region is also characterised by the presence of Proterozoic Mahanadi Rift, Chhattisgarh and Vindhyan Basins with significant coverage of Indo-Gangetic Plain sediments in northern part. In this study, we present the results of a seismically well-constrained 2-D multi-scale geopotential modelling to delineate lithosphere structure across different Precambrian terrains of the EIS. The joint interpretation of the potential field data reveals that i) mobile belts are bounded by the deep crustal faults with denser crust, ii) presence of thick underplated crust below Singhbhum craton, Singhbhum Mobile Belt, Chotanagpur Granite Gneissic Complex and the surrounding rift basin, iii) localised Moho upwarp at a depth of ~36-37 km below the Proterozoic basins, iv) the Lithosphere-Asthenosphere Boundary (LAB) varying between 90-200 km below the EIS region. The distinct crustal structure along with relatively deeper LAB (130-200 km) below the mobile belts suggests the Proterozoic amalgamation and lithosphere reworking. Below the Singhbhum craton, LAB is observed at a depth of ~145-155 km, which is comparatively thinner with respect to other cratonic areas elsewhere. The observed crustal underplating and thinner LAB below the Singhbhum craton indicate the lithosphere erosion and magmatic upwelling caused by the major Paleo-Mesoproterozoic and early- Cretaceous Large Igneous Province (LIP) events.  

How to cite: Sathapathy, S. K., Radhakrishna, M., and Giri, Y.: Multi-scale Potential Field Modelling to Delineate the Lithosphere Structure below the Eastern Indian Shield and its Tectonic Implications  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17947, https://doi.org/10.5194/egusphere-egu24-17947, 2024.

Olivine and its polymorphs are the dominant minerals in the upper mantle and transition zone. The olivine phase transitions, determined primarily by pressure and temperature, control mantle discontinuities and influence mantle dynamics. Pressure is a first-order control on olivine phase transition and relates primarily to depth; therefore, it is commonly used to interpret the depths of mantle discontinuities. However, mantle dynamic models predicted stress levels of 100-300 MPa or as high as 1 GPa. Previous work has provided a complete picture of how such stresses would affect the positions where mineral reactions occur (and hence large-scale mantle structure). In this work, we plan to focus on the feedback between pressure and stress on the olivine phase transition at grain scale, and then the results can be extrapolated and upscaled to mantle scale deformation.

We use the Open Phase Studio software based on the phase field model to simulate olivine phase transitions. The phase field model uses order parameters to distinguish different phases and describe their evolution. The parameter value of 1 indicates the bulk of the phase, and a value of 0 indicates the absence of this phase and is a smooth function of position. The smooth transition of a phase parameter indicates a diffuse interface between phases. The total free energies, interface properties, and microstructure control the phase field evolution. Open Phase Studio considers the Helmholtz free energies of each phase and uses their elastic energies to account for the pressure and stress effects on phase evolution. This software currently focuses on models of alloys, but appropriate values for silicates can be input. As a foundation, we first consider an Al-Li alloy to understand the behaviour of models. Then, we input olivine thermodynamic data via temperature-composition (T-x) phase diagrams for olivine composition and their elastic moduli to test the phase transition under different stress boundary conditions. We present our preliminary results here.

How to cite: Lu, L. and Wheeler, J.: Grain-scale simulation of olivine phase transition under stress: implications for mantle discontinuities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5239, https://doi.org/10.5194/egusphere-egu24-5239, 2024.