Quantitative analysis tools have become increasingly common in structural geology. Imaging techniques such as computed tomography are used to build highly accurate, three-dimensional models of geological structures. Structural measurements can be facilitated and often accelerated owing to photogrammetric methods of reconstructing the studied outcrops. Geological structures can then be classified using statistical methods. Experimental, analytical, and numerical techniques can be used to develop quantitative mechanical models of rock deformation processes, which are often coupled to chemical, hydrological or thermal processes. With the advent of modern computing power, high-resolution models and systematic simulations are nowadays feasible.

Public information:
In this session, we want to bridge the gap between observational methods and models through quantitative analysis/modelling. The displays are grouped into observational methods, methods that link the observations to processes, numerical models of deformation processes and link between different processes with theory and experiments.

Convener: Bernhard Grasemann | Co-conveners: Marcin Dabrowski, Stefan Markus Schmalholz, Marcel Thielmann
| Attendance Mon, 04 May, 08:30–10:15 (CEST)

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Chat time: Monday, 4 May 2020, 08:30–10:15

Chairperson: Stefan Schmalholz
D1436 |
Francois Renard, Jessica McBeck, and Benoît Cordonnier

Predicting the onset of system-size failure in rocks represents a fundamental goal in assessing earthquake hazard. On the field, seismological, geodetic and other monitoring data may record precursors to earthquakes. In laboratory experiments, such precursors often rely on monitoring acoustic emissions and this technique has some limitations in terms of spatial resolution and the lack of detection of aseismic strain. To overcome these challenges, we have performed a series of forty rock deformation experiments where we imaged, using synchrotron X-ray microtomography, rock samples as they deformed until brittle failure, at in situ conditions of pressure, high spatial micrometer spatial resolution, and through time. On the one hand, direct processing of the X-ray tomograms allow visualizing how precursory microfractures nucleate, grow, and coalesce until failure. From these data, we propose to characterize brittle failure as a critical phase transition, with evidence of several power-laws that characterize fracture growth. On the other hand, digital volume correlation techniques quantify the evolution of the local strain field inside each sample. We analysed the statistical properties of these strain fields using several machine learning techniques to predict the main parameters that control fracture growth (length, volume, shape, distance to the nearest fracture), and the features of the strain field that best predict the distance to failure. Our rock deformation experimental results show that, under laboratory conditions, precursors to brittle deformation exist. These precursors show predictable evolution when approaching system-size brittle deformation and we demonstrate that specific components of the strain field characterize this evolution to failure.

How to cite: Renard, F., McBeck, J., and Cordonnier, B.: Quantifying the precursors to brittle failure in rocks using synchrotron imaging and machine learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1659, https://doi.org/10.5194/egusphere-egu2020-1659, 2020.

D1437 |
Robert Faber and Gisela Domej

Theoretically, analysis of digital elevation data and remote sensing images allows for convenient mapping of geological settings. Using computer tools, geometric plane elements can be joined with elevation and image data to map structures in 3D. 
One disadvantage is that single geometric elements are processed consecutively, even if belonging to the same structure. Only in rare cases, it is possible to trace a geological structure in its entirety without gaps; moreover, various uncertainties during the mapping process (caused, e.g., by erosion or vegetation coverage) may let elements appear isolated or distorted. Therefore, creating an entirely consistent numerical model of a particular geological setting is highly time-consuming, usually cost-intensive, and – to some extent – error-prone.
We extended the existing 3D geological mapping tool FaultTrace (a module of TerraMath WinGeol) to allow for simultaneous processing of larger and more significant structural segments of faults and bedding planes. Geometric elements can influence each other now and the usage of optional time tables associated with the mapped structures helps to analyze more complex settings.  
The workflow is demonstrated using the example of the Richât Structure in the Sahara Desert of Mauritania.

How to cite: Faber, R. and Domej, G.: FABER, R., DOMEJ, G., 2020. Computer-Assisted Geological Mapping (CAGEM) in 3D with WinGeol by TerraMath: the Richât Structure in Mauritania., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2439, https://doi.org/10.5194/egusphere-egu2020-2439, 2020.

D1438 |
Christoph Kettler, Daniel Le Heron, Pierre Dietrich, Neil P. Griffis, and Isabel P. Montañez

Gathering statistically useful, quantitative structural data is always time intensive and laborious, and thus complete digital capture of an outcrop is ideal for the modern geologist. For instance, taking several hundreds of clast fabric measurements is tedious and time consuming, especially if material is lithified. For the “time poor geologist” under constant pressure, a solution is to digitize the entire outcrop and surfaces via ground based (hand held camera) or aerial surveying (Unmanned Aerial Vehicle) methods. The acquired imagery can then be processed to produce photogrammetric 3D models. This serves as the basis for both sedimentological and structural work, including bedding geometry, in appropriate 3D modelling software.

This paper presents both the workflow and the results of the digitization of multiple Late Carboniferous outcrops in Namibia. Each of these outcrops corresponds to Late Palaeozoic Ice Age (LPIA: 360-260) deposits that have only been subject to basic description, which exhibit varying degrees of structural complexity, and whose precise relationship to LPIA ice sheets across Gondwana remains unclear. A number of 3D models are presented, which provide vital new insights into the directions of ice movement. Some of this insight comes from diamictites deposited beneath the ancient ice sheets. This is because: (i) clasts tend to align themselves in a stress-field in the active layer below the ice, (ii) striated pavements can be seen as analogous to fault surfaces and (iii) diamictites may show evidence for complex internal shear planes. We measured the orientation of those clasts directly in the 3D models from several locations to acquire a precise understanding of ice flow over a wide (hundreds of km) area, which will serve as the basis for an ice sheet reconstruction. Integrating additional morphological data from numerous drone surveys, e.g. roches moutonnées, U-shaped valleys, striated pavements permits fresh insights into the ancient glacial environment. Thus, the digital outcrop approach underpins a truly interdisciplinary (structural, sedimentological, geomorphological) approach to unravelling the LPIA record.

How to cite: Kettler, C., Le Heron, D., Dietrich, P., Griffis, N. P., and Montañez, I. P.: A thousand pictures say a million words: digitisation and quantitative characterisation of structurally complex glaciogenic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21566, https://doi.org/10.5194/egusphere-egu2020-21566, 2020.

D1439 |
Xin Wang and Feng Gao

The knowledge of the strain/stress field evolution in time is important to seismic hazard assessment and risk mitigation, and is fundamental to the understanding of the earth dynamic system. Based on the principle that past tectonic stress should have left traces in the rocks, geologists have been trying to determine the paleostress history from evidence found in rocks for decades. Recent development of techniques for automatic extraction of fracture surfaces from digital outcrop models and estimation of historical shear deformation on rock fractures provide an efficient way of quantitatively acquiring large amount of high quality fracture/fault slip data (direction and sense of slip occurs on the fault plane) from outcrops. So unlike traditional paleostress inversion methods whose data is manually collected in the field, this high quality fracture/fault slip data provide an opportunity to develop fully automatic and quantitative methods for deciphering paleostrain. In this study, for slip on each fracture, the corresponding local strain tensor is calculated, then the local strain tensors are grouped into populations corresponding to far-field strain events and local strain events using a clustering analysis technique. The applications on outcrops in the eastern Tian Shan area give a clear picture of the paleostrain variation over space and time, and also throw light on the relationship between paleostrain, fracture development and the distribution of shear displacements in a thrusting environment.

How to cite: Wang, X. and Gao, F.: Quantitatively deciphering paleostrain from digital outcrops model and its application in the eastern Tian Shan, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3467, https://doi.org/10.5194/egusphere-egu2020-3467, 2020.

D1440 |
Katrin Heindel, Jochen Wolf, Philipp Chachaj, and Klaus Levin

At the slopes of the ‘Hersbrucker Alb’ nearby Happurg in Franconia (Germany), the so-called ‘Doggerwerk‘ has been projected as huge subsurface armaments factory (mainly for assembling aircraft engines). The name ‘Doggerwerk’ originated from the bedrock ‘Doggersandstein’, from what the tunnels have been excavated. The ‘Doggersandstein’ belongs to the geological stratigraphic formation ‘Eisensandstein’ (Dogger Beta, Middle Jurassic). The tunnel system has been realized only partly (around 3.9 km) until the late phase of the Second World War in 1944/45. The production of engines did never start at all. To date, only one of the originally eight entrances into the tunnel system, which are located at the steep, forested slope of the so-called ‘Houbirg’ mountain is accessible. The raw state of most of the tunnels - without a supporting inner shell - favoured the steady proceeding disintegration. Thus, the stability of most of the ‘Doggerwerk’ is not given. Consequently, the stabilisation of the tunnels in danger of 'imminent collapse' has been projected, resulting in several recovery measures since the beginning of the 1990th. The current stabilisation of overall 1.2 km long tunnels started in 2014 and was finalised in 2019.

After the evaluation of several technical approaches (securing by fencing, blasting etc.) and considering nature conservation guidelines, the gradually backfilling (single sections separated by brick-built walls) of the tunnels with cement suspension was favoured. The used filler (suspension of mainly cement, bentonite, slag sand and water) was specifically developed and certified for this project because of special requirements on viscosity (pumping distance of around 1.6 km), hardening and compressive strength (circa 1,6 N/mm2), as well as of the required environmental compatibility. For the estimation of the needed volume of filler and its potential loss in the fractures, faults and joints, the tunnel system was surveyed by using a quantitative visual application - the terrestrial laser scanning (TLS). Based on the laser scanning, a theoretical filling volume of around 12.918 m3 has been calculated for the 1.2 km long tunnels. Considering, however, the predominantly bad ground conditions, a volume of around 15.000 m3 was estimated. Additionally, the joint system was mapped classically. The joint system shows two general strikes, the Hercynian (110° - 157°) and the Rhenish (18° - 27°). The inclination of the joints is predominantly steep (circa 80°, ± vertical) and the bedding is mainly horizontal. Resulting from the laser scanning and the mapping, sections featuring particularly critical stability issues have been designated. Following the statically demands, these sections with critical ground conditions have been supported with a reinforced (two layers) shotcrete lining (minimum 20 cm thick) to guarantee the underground construction site safety during preparatory works before starting the backfilling.

The stabilisation of the whole ‘Doggerwerk’ tunnel system by gradually backfilling was successfully carried out by using in total 17.360 m3 cement suspension. A potential outflow of filler at the surface was monitored by verifying the actual injection volume with the calculated volume and visually also by the help of installed cameras. An outflow, however, has not been observed.

How to cite: Heindel, K., Wolf, J., Chachaj, P., and Levin, K.: Application of quantitative structural geology using Terrestrial Laser Scanning (TLS) for the stabilisation of the ‘Doggerwerk‘ tunnel system in Happurg (Franconia, Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3532, https://doi.org/10.5194/egusphere-egu2020-3532, 2020.

D1441 |
Caroline Modica Custódio, Maria Alcione Lima Celestino, Laís Vieira de Souza, Jaqueline Lopes Diniz, Leonardo Campos Inocencio, Juliano Bonato, Aline Fernanda Spaniol, Tiago Siqueira de Miranda, and Francisco Manoel Wohnrath Tognoli

The use of digital outcrop models (DOMs) in geosciences has been increasingly common since the beginning of the 2000s due to the technological advances in the positional and image-quality field equipment, data collection, and processing with practicality, agility, and high accuracy even in inaccessible sites. DOMs incorporate all the visual elements that geoscientists analyze in the outcrops as 3D models with a spatial resolution of a few millimeters per pixel and positional accuracy of less than 2 cm. The great advantage of DOMs is their daily availability for visual inspection and interpretation in the office, complementing the data analysis performed in the field. Therefore, the continuous development of high accuracy and dense image-based and point cloud models has been crucial for quantitative approaches using digital models. Another challenge in this process involves the development of tools and methodologies for interpretation of DOMs, especially analysis and interpretation of linear and planar features such as lineations, paleocurrents, joints, faults, and deformation bands. This study aims to systematize the manual and semi-automatic methods of plane extraction using tools (e.g., Compass and Facets) available in the open-source software such as the CloudCompare, and statistically analyze the structural measurements from the extracted data. In this work, we analyzed two 3D integrated ground-UAV photogrammetric models reconstructed with the Structure from Motion (SfM) technique. The study areas are part of the Araripe Basin basement, located in Northeastern Brazil, and represent two case studies involving joints and faults associated with the damage zone of the boundary fault. Initial results obtained by the automatic planes measured with the Facets plugin show distinct fracturing patterns. This is mainly due to the difference of the rock rheology and competence. In the metasedimentary outcrop, we identified 731 planes in phyllites, reduced to 459 real planes after noise remotion and visual inspection during interpretation. In this case, the data accuracy is 62% for plane recognition. The preferential orientation is N40-90E and N40-80W, with high dip angles, and subordinately N45E and N10W with low dip angles. In the metatonalite, 347 planes were recognized, but only 38 of them showed to be real planes, totalizing accuracy of 10,9%. The planes validated as real indicate a preferential orientation of N10-15W with high angles of dip. Both outcrops used the same processing routine and configuration. The difference observed in the number of planes automatically recognized in each outcrop is a consequence of the relationship between the plane orientation x outcrop orientation, spatial resolution of the model, and the degree of weathering. Besides that, positional accuracy and visual quality are crucial for accurate quantitative interpretation of structural features using digital outcrop models, as well as a well-defined data processing routine and careful inspection of the results by an expert. The data obtained from this methodological approach will contribute to quantitative approaches in structural geology based on robust datasets. 

How to cite: Modica Custódio, C., Alcione Lima Celestino, M., Vieira de Souza, L., Lopes Diniz, J., Campos Inocencio, L., Bonato, J., Fernanda Spaniol, A., Siqueira de Miranda, T., and Manoel Wohnrath Tognoli, F.: Quantitative structural analysis of fractures using digital outcrop models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11868, https://doi.org/10.5194/egusphere-egu2020-11868, 2020.

D1442 |
Steffen Abe and Hagen Deckert

The roughness of fracture surfaces is important for a range of geological processes such as the mechanical behaviour of faults or the fluid flow in jointed rocks or fault zones. However, the processes and parameters controlling the details of the fracture roughness are not fully understood yet. We therefore use numerical simulations based on the Discrete Element Method (DEM) to study the formation of fractures in triaxial deformation experiments under a wide range of stress conditions and to quantify the geometric properties of the resulting fracture surfaces. In the numerical experiments a DEM-model of a box-shaped rock sample is subjected to a displacement controlled load along its x-axis while a defined confining stress is applied to the other surfaces.

Based on the data from 131 numerical simulations the roughness of 388 fracture surfaces has been analysed. For this purpose the surface point clouds extracted from the Discrete Element models have been converted to height fields relative to a "best-fit" plane and the height distributions quantified. The results show that the heights are normally distributed. We observe no dependence on the confining stress except that models with equal confining stress in y- and z-direction show a higher standard deviation of the height distribution than those with differing y- and z-confinement. An analysis of the height-height correlation functions for those surfaces shows that they follow a power-law, demonstrating that the surfaces are self-affine. The Hurst exponent H describing the scaling of the roughness can be derived from the power-law relation. Values obtained are in the range H=0.2-0.6 for the full suite of experiments, while the mean of the Hurst exponents for each group of fracture surfaces generated under the same stress conditions is H=0.3-0.45. A weak decreasing trend of the Hurst exponent with increasing confining stress can be observed, but contrary to the standard deviation of the height distribution there is no dependence on the ratio of the confining stresses. There is also no difference between fractures generated in tensile (mode 1) or compressive conditions (mode 2).

Additionally, surfaces of rock samples fractured in triaxial tests in the laboratory have been analysed using the same methods. The surfaces show similar self-affine characteristics as those in the numerical experiments, although with significantly higher Hurst exponents H=0.6-0.8.

A comparison between our numerical models and laboratory experiments and data obtained from literature shows that natural and lab-created fracture surfaces and their numerically modelled counterparts are similar regarding the normally distributed heights and the self-affine scale, but the Hurst exponents do not match exactly. While the majority of field and experimental studies find significantly higher Hurst exponents of about 0.8, there are some studies, for example on Sandstone, which find H=0.4-0.5, falling into the range observed in our numerical experiments.

How to cite: Abe, S. and Deckert, H.: Roughness of fracture surfaces in Discrete Element Model triaxial deformation experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3035, https://doi.org/10.5194/egusphere-egu2020-3035, 2020.

D1443 |
Timothy Davis

In theory, pockets of fluid in brittle media can be transported large distances, provided that both the fluid volume is large enough, such that fluid pressures can fracture the rock, and that stress gradients exist causing asymmetric growth of the fracture's front. Currently, industrial injections are deemed safe based on empirical observations of volumes, rates and pressures from closed-access industrial data. Existing theoretical models are difficult to use a priori to predict the critical volume of fluid that will cause unhindered fracture ascent, as they are expressed in terms of the fracture’s length, which is hard to predict a priori and difficult to measure. Here we constrain scale-independent critical volumes as a function of only rock and fluid properties by supplementing simple analytical models with numerical simulations in three dimensions. We apply our model to laboratory and natural settings, showing that the volumes we estimate match well with laboratory data and can be used as a conservative estimate in geological applications.

How to cite: Davis, T.: Critical fluid volumes and the start of 'self-sustaining' fracture ascent, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10813, https://doi.org/10.5194/egusphere-egu2020-10813, 2020.

D1444 |
Filippo Luca Schenker, Marta Adamuszek, and Matteo Maino

Highly curvilinear folds develop during simple shear deformation due to perturbations in the velocity field around the inclusion heterogeneity. In the field, such structures may be recognized at the micro- and meso-scale within high-strain crustal-scale shear zones. However, at scarce outcrop conditions, fragments of these structures are often interpreted as generated by poly-phase deformation. The structural history becomes even more complex when the deformation within the inclusion is considered. In this inclusion-matrix deformation system, two end-member regimes has been already investigated: (i) a weak ellipsoidal inclusion that acts as a slip surface over which sheath folds develop and (ii) a rigid ellipsoidal inclusion that rotates within the matrix generating sheath folds in the back of the rotating ellipse in direction of the shearing. Between these two end-members, understanding the clast-matrix deformational regime is not trivial and the genesis of sheath fold is unexplored.

We employed 3D numerical models to study fold structure evolution around an ellipsoidal inclusion within a matrix during simple shear. Both inclusion and matrix were homogeneous and isotropic, and had linear viscous rheologies. We tested models with different (i) initial inclusion aspect ratio, (ii) viscosity ratio between the inclusion and the matrix, and (iii) strain. We identified three main deformation regimes that are closely related with the behaviour of the inclusion. In the first regime, the inclusion experiences massive stretching. In the second regime, we observe oscillatory motion of the principal inclusion axes and the deformation of the material lines within inclusion periodically changes from shortening to stretching conditions. In the third regime, principal inclusion axes rotate. The material lines within inclusion, similar as in the second regime, experience cyclic stretching and shortening, however, the amount of extension and shortening is significantly smaller. The transition between regimes is dependent of both initial inclusion aspect ratio and viscosity ratio. The first regime is characteristic for inclusions with small viscosity ratio. With increasing viscosity ratio, the regime changes to the second and eventually to the third. The change occurs at lower viscosity ratio for models with larger initial inclusion aspect ratio than for smaller once. All the models developed sheath folds around the inclusions.

The results of our simulations were compared with the deformation pattern derived from a main shear zone of the Cima-Lunga in the Central Alps. In the field, the elongated high-pressure ultramafic bodies are surrounded by folded amphibolite-facies paragneisses that locally depict sheath folds. The internal structures of ultramafic bodies are characterize by recumbent, sub-isoclinal folds and folded boudinaged mafic layers that suggest internal changes in stress direction. In a selected ultramafic body elongated sub-parallel to the shearing direction and with an aspect ratio a/c=3 and b/c=2, we estimate from a mafic boudinaged layer subparallel to the a/c axis a minimum stretching of 40%. This field data allowed us to establish that the viscosity ratio of the ultramafic body to the paragneisses at the time of the deformation of the shear zone was in the range of 4-11 and the strain was γ>13.

How to cite: Schenker, F. L., Adamuszek, M., and Maino, M.: Numerical models of sheath fold development in rheologically heterogeneous rocks of the Cima Lunga-Adula shear zone (Central Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18351, https://doi.org/10.5194/egusphere-egu2020-18351, 2020.

D1445 |
Paraskevi Io Ioannidi, Laetitia Le Pourhiet, Onno Oncken, Philippe Agard, and Samuel Angiboust

The physical nature and the rheology of a subduction shear zone play an important role in the deformation and the degree of locking along its interface with the upper plate. Inspired from exhumed subduction shear zones that exhibit block-in-matrix characteristics (mélanges), we create synthetic models with different proportions of strong clasts within a weak matrix and compare them to natural mélange outcrops. Using 2D Finite Element visco-plastic numerical simulations and simple shear kinematic conditions, we determine the effective rheological parameters of such a two-phase medium, comprising blocks of basalt embedded within a wet quartzitic matrix. We treat our models and their structures as scale-independent and self-similar and upscale published field geometries to km-scale models, compatible with large-scale far-field observations. Exhumed subduction mélanges suggest that deformation is mainly taken up by dissolution-precipitation creep. However, such flow laws are neither well-established yet experimentally nor of ample use in numerical modelling studies. In order to make our results comparable to and usable by numerical studies, we assume dislocation creep as the governing flow law for both basalt and wet quartz and by using different pressures, temperatures and strain rates we provide effective rheological estimates for a natural subduction interface. Our results suggest that the block-in-matrix ratio affects deformation and strain localization, with the effective dislocation creep parameters varying between the values of the strong and the weak phase, in cases where deformation of both materials is purely viscous. As the contribution of brittle deformation of the strong blocks increases, however, the value of the stress exponent, n, can exceed that of the purely strong phase.

How to cite: Ioannidi, P. I., Le Pourhiet, L., Oncken, O., Agard, P., and Angiboust, S.: Effective rheology of a two-phase subduction shear zone calculated by numerical simple shear experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11189, https://doi.org/10.5194/egusphere-egu2020-11189, 2020.

D1446 |
Mateusz Mikołajczak, Jan Barmuta, Małgorzata Ponikowska, Stanislaw Mazur, and Krzysztof Starzec

The Silesian Nappe in the westernmost part of the Polish Outer Carpathians Fold and Thrust Belt exhibits simple, almost homoclinal character. Based on the field observations, a total stratigraphic thickness of this sequence equals to at least 5400 m. On the other hand, the published maps of the sub-Carpathian basement show its top at depths no greater than 3000 m b.s.l. or even 2000 m b.s.l. in the southern part of the Silesian Nappe. Assuming no drastic thickness variations within the sedimentary sequence of the Silesian Nappe, such estimates of the basement depth are inconsistent with the known thickness of the Silesian sedimentary succession. The rationale behind our work was to resolve this inconsistency and verify the actual depth and structure of the sub-Carpathian crystalline basement along two regional cross-sections. In order to achieve this goal, a joint 2D quantitative interpretation of gravity and magnetic data was performed along these regional cross-sections. The interpretation was supported by the qualitative analysis of magnetic and gravity maps and their derivatives to recognize structural features in the sub-Carpathian basement. The study was concluded with the 3D residual gravity inversion for the top of basement. The cross-sections along with the borehole data available from the area were applied to calibrate the inversion.

In the westernmost part of the Polish Outer Carpathians, the sub-Carpathian basement comprises part of the Brunovistulian Terrane. Because of great depths, the basement structure was investigated mainly by geophysical, usually non-seismic, methods. However, some deep boreholes managed to penetrate the basement that is composed of Neoproterozoic metamorphic and igneous rocks. The study area is located within the Upper Silesian block along the border between Poland and Czechia. There is a basement uplift as known mainly from boreholes, but the boundaries and architecture of this uplift are poorly recognized. Farther to the south, the top of the Neoproterozoic is buried under a thick cover of lower Palaeozoic sediments and Carpathian nappes.

Our integrative study allowed to construct a three-dimensional map for the top of basement the depth of which increases from about 1000 m to over 7000 m b.s.l. in the north and south of the study area, respectively. Qualitative analysis of magnetic and gravity data revealed the presence of some  basement-rooted faults delimiting the extent of the uplifted basement. The interpreted faults are oriented mainly towards NW-SE and NE-SW. Potential field data also document the correlation between the main basement steps and important thrust faults.


This work has been funded by the Polish National Science Centre grant no UMO-2017/25/B/ST10/01348

How to cite: Mikołajczak, M., Barmuta, J., Ponikowska, M., Mazur, S., and Starzec, K.: Three-dimensional depth-to-basement modelling based on seismic and potential field data – basement configuration in the westernmost Polish Outer Carpathians, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7882, https://doi.org/10.5194/egusphere-egu2020-7882, 2020.

D1447 |
Fedor Yakovlev, Krzysztof Gaidzik, Viacheslav Voytenko, and Natalia Frolova

Talas Ridge forms the western part of the Tien Shan Caledonian structure. The sedimentary cover shows a thickness of about 10 km and consists of carbonate flysch and para-platform deposits metamorphosed under greenschist and lesser grade. This structure relates to the "hinterland" tectonic type, characterized by the abundance of many small and moderate-sized folds of the "similar" morphological type. Conventional cross-section balancing techniques developed for "foreland" structures, with large "parallel" folds cannot be applied correctly to such complicated structures. Thus, a special method based on the "geometry of folded domains" was developed for balancing of "hinterland" structures. To test the proposed method, we choose the westernmost Shilbilisaj profile of the Talas Ridge that consists of a large number of folds.

The proposed approach is based on the hierarchical system of hinterland fold structures, and on the accordance of the “folded domain” deformation to the strain ellipsoid, as described in detail in F. Yakovlev [2017]. On the first step the detailed structural profile is divided into a number of domains, 0.5-1 km wide; each domain consists of several folds of almost the same morphology. Consequently, a number of morphological parameters are measured, together with the axial surface dip angle and the interlimb angle that allow the construction of a strain ellipsoid for each domain. The core of the reconstruction method consists of three consecutive kinematic operations: 1) rotation, 2) horizontal simple shearing, and 3) horizontal stretching. As a result, a pre-folded form of a domain is produced, characterized by length and tilting of a domain segment that differ from the current profile parameter values. Sequential aggregation of all pre-folded domains leads to a complete pre-folded profile that allows the calculation of its shortening value. In the next step a few "structural cells" with a length approximately equal to the sedimentary cover thickness, are selected that combine several pre-folded domains. Taking into account the pre-folded and current lengths of such cells, their shortening values are determined. In the system of hierarchy of folded structures, folded domains and structural cells (and its strain parameters) belong to the third and fourth levels, respectively.

The first three project participants restored the structure of the section independently, starting with the domain selection procedure. The preliminary estimates of the shortening of the entire profile obtained by participants were close to each other and very high (K=L0/L1, where K – shortening value, L0, and L1 – pre-folded and current length in km, respectively): 4.49=118.5/26.4; 4.29=114.0/26.6; 4.67=119.1/25.5. The first participant allocated 63 domains and 12 structural cells, based on the thickness of the sedimentary cover. The shortening values for these cells varied along the profile from high in the southern cells to relatively small in the center and again to high in the northern parts (K=5.20, 4.47, 4.27, 3.79, 3.86, 3.93, 4.24, 4.91, 4.74, 5.53, 4.84, 4.9).

Yakovlev F.L. 2017. Reconstruction of folded and faulted structures in zones of the linear folding using structural cross-sections. Moscow, Published in IPE RAS, 60 p.

How to cite: Yakovlev, F., Gaidzik, K., Voytenko, V., and Frolova, N.: Extreme values of fold-related shortening in the hinterland structure of the Shilbilisaj section in the Talas Ridge (Tien Shan), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8080, https://doi.org/10.5194/egusphere-egu2020-8080, 2020.