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Tectonic strain in salt rock mass based on measurements

Szczerbowski Zbigniew, Niedbalski Zbigniew, Bednarek Łukasz

Geology, Geophysics and Environment

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2023

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Vol. 49, no. 2

157--173

EN

The measurement method with the application of an extensometer for the detection of the manifestation of tectonic strain is presented in this paper. The instrument is operated in underground construction for engineering purposes, and the authors applied it in a deeply placed underground old mine gallery in the Bochnia Salt Mine, just at the tectonic boundary of the Outer Carpathians which is commonly considered to be a tectonically active zone. The presented study is characterized by two basic features. The first is the placement of the measurements deep in an old mine which is an environment free of atmospheric factors disturbing the detection of a tectonic signal. The second is a combination of routine measurements carried out for engineering purposes and research measurements enabling the extension of the observation of displacements in the space outside underground workings, inside the rock mass that has been penetrated by extensometer probes. The extensometric measurements have been made using three 7-meter long sections. The results showed the differentiation in the displacement rates of points placed in the side walls: in the southern profile, the annual displacements are approximately 1.5 mm and in the northern one – approximately 1 mm. The combined result corresponds to the amount of the annual convergence value which has been determined by the classical surveys in the excavation where extensometric measurements have been made. What is more, the ongoing displacements in the southern side wall involve the entire part of the rock mass which is penetrated by an extensometric probe, but the displacements in the northern side are only observed in the first 2 m of the penetrated part of the rock mass. This differentiation is interpreted by the authors as being the result of tectonic strain acting from the south exerted by the Carpathians.

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Tectonic control on the distribution of onshore mud volcanoes in parts of the Upper Benue Trough, northeastern Nigeria

Musa O. K., Kurowska E. E., Schoeneich K., Alagbe S. A., Ayok J.

Contemporary Trends in Geoscience

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2016

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Vol. 5, iss. 1

28--45

EN

Onshore mud volcanoes are rare geological phenomena, which in Nigeria were reported for the first time few years ago in the Upper Benue Trough. In this study a detail geological mapping of the area of mud volcanoes occurrence was carried out, with the primary aim of defining their relationship, if any, to the structural geology there. The systematic field reconnaissance included field observations of the structural features, as well as analysis of the location and distribution of the onshore mud volcanoes, marking their locations on the topographic and geological maps, analysis of the aerial photographs and satellite images. The study area covered the central part of the Upper Benue Trough where the onshore mud volcanoes were found. The study area is the part of a sedimentary basin comprising Cretaceous clastic rocks that have been deformed intensively by a network of faults often embedded in the underlying Precambrian basement. This network of faults underwent a rejuvenation period from the Aptian to the Palaeocene. The most prominent tectonic structure in the study area is the NE – SW trending Kaltungo Fault Zone, however, there are other minor faults with N – S and NW – SE trends. This study shows that the mud volcanoes found in the study area are usually located near or within fault zones, within the outcropping Upper Cretaceous Yolde Formation and Upper Bima Sandstone, both of which were deformed by the Kaltungo faults, as well as by other minor faults. Worldwide, incidences of onshore mud volcano formation are usually attributed to areas of tectonic activity, rapid sedimentation or hydrocarbon occurrence. In this study, the interpretation of the field observations and mapping results, combined with information on the structural evolution of the study area and seismic pattern (very scarce), have led to the conclusion that the location of onshore mud volcanoes in the Upper Benue Trough, being located along the fault zones, is structurally controlled. The close relationship between mud volcano location and the structural framework of the area may be interpreted as one of several possible subsurface geological responses to present tectonic activity.

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Determination of tectonic stress using anisotropy of magnetic susceptibility (AMS) - a case study from the Krosno Beds (Silesian Unit, Polish Outer Carpathians)

Stachowska A., Ziółkowski P., Jankowski A. W., Wysocka A.

Geologia Sudetica

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2014

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Vol. 42

90--91

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Recent tectonic stress field investigations in Poland : a state of the art

Jarosiński M.

Geological Quarterly

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2006

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Vol. 50, No. 3

303-321

EN

The paper summarizes up-to-date knowledge of the contemporary tectonic stress field in Poland and compares the results of geophysical measurements with mathematical models. The extensive set of data provided by borehole breakout analyses is supplemented by hydraulic fracturing tests, earthquake focal mechanism solutions and preliminary resolution of regional intraplate motions from GPS measurements. Frequent breakout presence shows that tectonically driven anisotropy of horizontal stress is a common feature in the study area. Roughly N-S direction of maximum horizontal stress (SHmaxx) in Eastern Poland differs significantly from Western European stress domain. This difference is produced by tectonic push of Alcapa, which is successively compensated within the Teisseyre-Tornquist Zone (TTZ) and in the Upper Silesian segment of the Outer Carpathians. In the western part of Poland stress directions are ambiguous due to interplay of several additional tectonic factors. Most of hydraulic fracturing data and earthquake focal mechanism solutions indicate strike-slip stress regime in Eastern Poland where stresses are in equilibrium with preferentially oriented faults of Iow friction (0.16). Limited data from Western Poland suggest normal fault stress regime. Good conformity between directions of 5Hm", and intraplate motions occurs from comparison of breakout and GPS data. Finite element modelling shows that the most important factor shaping the stress field in Eastern and Central Poland is the Adria push transmitted through the Pannonian region. Secondary, but still notable factors are differentiation of loads along the Mediterranean collision zone and changes in magnitude of the ridge push force along the NW Continental passive margin of Europe. Results of rheological modelling indicate that the crust is entirely decoupled from the mantle in the Fore-Sudetic Platform, partial uncoupling in the base or the upper crust is possible in the TTZ while in the East European Craton (EEC) the whole lithosphere is coupled. The comparison of different set of data and models presented here provides a comprehensive geodynamic scenario for Poland, however, a number of unresolved questions still remains to be addressed.

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Reologiczny model litosfery w strefie szwu transeuropejskiego (TESZ) wzdłuż profilu sejsmicznego LT-7 (NW Polska - SE Niemcy)

Jarosiński M., Poprawa P., Beekman F.

Przegląd Geologiczny

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2002

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Vol. 50, nr 11

1073-1081

PL

Modelowanie reologii litosfery przeprowadzono wzdłuż przekroju sejsmicznego LT-7. Przecina on w poprzek strefę szwu transeuropejskiego (TESZ), cechującą się znaczną oboczną zmiennością struktury skorupy ziemskiej oraz reżimu termicznego. W zależności od warunków fizycznych oraz składu mineralnego deformowanego ośrodka skalnego, przypisywano mu styl deformacji kruchy lub podatny. Przyjęto, że wielkość naprężeń dyferencjalnych w warstwach kruchych ograniczona jest tarciem na powierzchniach uskoków, natomiast w warstwach podatnych oporem pełźnięcia dyslokacyjnego sieci krystalicznej. Jako dane do przeprowadzonych analiz wykorzystano sejsmiczny model prędkościowy wzdłuż refrakcyjnego przekroju LT-7, rozkład gęstości powierzchniowego strumienia cieplnego wzdłuż przekroju, a także, przez analogię do obszarów sąsiednich, miąższość mechanicznej litosfery oraz produkcję ciepła radiogenicznego. Dla pozostałych parametrów modelowania przyjęto wartości standardowe z literatury. Poszczególnym warstwom modelu sejsmicznego przypisano zgeneralizowany skład mineralny oraz stałe materiałowe, charakteryzujące deformacje podatne. W pierwszej kolejności wykonano profile temperaturowe litosfery, które następnie wykorzystano w modelach reologicznych. Jednowymiarowe modelowanie dla każdego profilu przeprowadzono dla wariantów zmiennych parametrów termicznych, zmierzając do uzyskania płynności zmian strumienia cieplnego z płaszcza i miąższości termicznej litosfery między profilami. Wyliczone wartości strumienia cieplnego z płaszcza są zmienne od 20 mW/m2 na kartonie wschodnioeuropejskim, przez 20-35 mW/m2 w strefie TESZ (z maksymalnymi wartościami w jej SW części), do 25-30 mW/m2 na platformie waryscyjskiej w Niemczech. Miąższość termicznej litosfery w poszczególnych, powyżej wymienionych strefach, wyniosła odpowiednio: 170-200 km, 90-160 km oraz 110-140 km. Główną cechą analizowanego profilu reologicznego, jest osłabienie litosfery w jego centralnej części, tj. na skraju platformy paleozoicznej i w sąsiadującej z nią części TESZ. Strefa tego osłabienia pokrywa się z obszarem podwyższonego strumienia cieplnego. W jej obrębie niemal zanika wytrzymałość górnego płaszcza. Wzdłuż całej pozakratonicznej części profilu, w obrębie środkowej i dolnej skorupy wyraźne zaznacza się strefa osłabienia, stanowiąca warstwę o miąższości ok. 20 km. Osłabienie to powoduje mechaniczne rozdzielenie górnej skorupy i górnego płaszcza. Jedynie na kratonie wschodnioeuropejskim poszczególne warstwy litosfery są mechanicznie spojone ze sobą. Obliczona całkowita wytrzymałość litosfery w kontrakcji zmienia się od 30-50 ×1012 N/m na kartonie wschodnioeuropejskim, przez 15-25 ×1012 N/m w strefie TTZ oraz poniżej 5 ×1012 N/m w SW części strefy TESZ, do 5-15 ×1012 N/m w na platformie waryscyjskiej. Analiza wrażliwości modelu wykazała, że zmienność parametrów termicznych w realistycznych granicach ma znaczny wpływ na wytrzymałość litosfery, lecz nie rzutuje na jej generalne rozwarstwienie reologiczne. Dodatkowo stwierdzono, że niezależne określenie miąższości mechanicznej litosfery jest podstawowym warunkiem sporządzenia wiarygodnego modelu reologicznego dla omawianego obszaru.

EN

The present study concerns rheological structure of the Trans-European Suture Zone (TESZ) and neighbouring tectonic units in Poland and SE Germany, along the LT-7 deep seismic sounding (DSS) profile. The SW-NE trending transect, 560 km in length, crosses the Variscan platform (VP - without its TESZ segment), part of the TESZ composed of external Variscan orogen and its foreland (VSZ- Variscan Suture zone), the Teisseire-Tornquist Zone (TTZ), and terminates on the western slope of the East European Craton (EEC; fig. 1). Both complex crustal structure and significant lateral changes in surface heat flow along the LT-7 profile make it an attractive object for study of the rheological differentiation of lithosphere. 1-D temperature and rheological modelling was performed for 10 sites located along the LT-7 profile. The most important and best-constrained input data are seismic wave velocity structure (fig. 2) and surface heat flow density (fig. 3). A simplified petrological model (fig. 4) based on P-wave velocity differentiation has been founded on a concept of quartz/diorite/diabase/pyroxenite/olivine layering of the lithosphere. Lithosphere temperature profiles for each site were derived by analytical solutions of Fourier 's law, applied to two layer crust model with the mantle being infinite half-space. For analysed sites, for each petrological defined layer constant value of radioactive heat production and thermal conductivity were assumed (fig. 4). For calculations of strength envelopes, Byerlee 'sfrictional law, for brittle layers and powerlaw creep for ductile layers were used. An assumption of wet rheology was generally applied. In order to narrow the range of possible solutions, the rheological models were to fulfill three principal conditions: (1) Thickness of thermal lithosphere should laterally vary from 70 km to 200 km (constrained by extrapolated seismological data). (2) Mantle heat flow and lithospheric thickness should change smoothly from site to site. (3) Cumulative strength of the lithosphere for the strain rate 10^-16 s^-1 should always be higher than 2-10^12 N/m (neotectonic quiescent of the analysed area indicates that the lithosphere sustains plate boundary forces). In spite of poor control on some input data and no restrictive principal conditions, possible solutions of the model fall into a narrow range of options. Performed modelling allowed to estimate cumulative lithospheric strength along the profile (fig. 5a), which changes of more than an order of magnitudefrom 30-50 *10^12 N/m at the edge of the EEC, through 15-25 -10^12 N/m in the TTZ and less then 5-10n N/m in the VSZ, to J 15-10^12 N/m in the VP. Calculated mantle heat flow (fig. 5b) varies in a range from 20 m W/m2 in the EEC, through 20-35 m W/m2 in I lie TESZ (with maximum values at its SW boundary), to 25-30 m W/m2 in the VP. For the same segments of the profile thickness of thermal lithosphere estimated on 170-200km, 90-160 km and 110-140 km (fig. 5c), respectively. Additionally, thermal modelling led also to some constraints on the radioactive heat production in the upper crust. The first-order feature of the obtained rheological section (llg. Sc) is that the transition zone from the VSZ to the VP is extremely weak. As evidenced from coincidence with high surface and manile heat /low, observed mechanical weakening is thermally controlled. The second significant outcome of the model is the existence of i hfological layering of the lithosphere. Apart of the EEC, only two strong layers were recognised, namely uppermost crust and uppermost manile. These layers are separated by extremely weak lower crust, more than 20 km thick. For the lithosphere of the EEC three or lour strong layers were recognised. Some of them might be mechanically welded to each other. Sensitivity of the rheological model to variability of radioactive heat production and surface heat flow has been also examined (fig. 6). Changes of these parameters in realis-lu range lead to significant differences in the shape of strength envelopes and thickness of thermal lithosphere. This leads to the con-i luston, that any independent control on lithospheric thickness would be crucial for improving quality of rheological model.