Mobility of local tectonic structures in western part of the Paczkow Graben (Sudetic Foreland) on the grounds of present investigations.
Investigations of present-day mobility of the upper Earth's crust in western part of Paczkow Graben tectonic structure have been undertaken in connection with plans of embankment dam construction above Kamieniec Zabkowicki (SW Poland). These studies consist of geodetic research providing quantitative data from GPS satellite measurements, observations of gravitational acceleration and comparison of precise levelling measurements. At the same time, available pieces of geological, hydrological, geotechnical, morphological and other relevant information concerning this area have been collected and analysed. Details of the dam engineering project and performed site investigations have also been included in the research. This paper presents analysis of vertical behaviour of crustal blocks based on precise levelling data and gravimetric measurements. Possible causes of the upper Earth's crust movements relating to recent tectonic history and geological and hydrological conditions have been discussed. Integration of satellite GPS measurements, in a subsequent part of this research, containing horizontal component of upper crustal movements should provide complete assessment of geodynamics in addition to the results presented here. Prognosis of dam influence on crustal blocks that compose the western part of the Paczkow Graben is also planned.
2. BACKGROUND INFORMATION
Increased load on uppermost layers of the Earth's crust resulting from dam construction and water storage always had an effect on reservoir's surroundings.
Deformations such as subsidence of the ground near dam body might take place. Such an effect has been observed for an embankment Nysa dam located in eastern part of Paczkow Graben (Cacori, Deeb, 1995). On larger scale a reservoir could have an effect on neighbouring crustal blocks, particularly in tectonically young regions (Gupta, 1992). Research work, a part of doctoral dissertation in preparation, follows studies initiated in the 90'ties with the intention of detecting any stress build up in the upper part of the Earth's crust. related to endogenous processes. Investigations will act as a reference record of undisturbed state of the ground, for future control measurements of dam performance and monitoring of neighbouring tectonic structures.
The planned site of "Kamieniec" Dam in the western part of a latitudinal depression zone known as the Paczkow Graben situated in Sudetic Foreland north of the Sudetic Marginal Fault (SMF) is characterised by complicated tectonic structure. Investigated part of the region consists of several crustal blocks separated by young faults. Two horsts (Braszowice and Kamieniec) with outcrops of old Palaeozoic rocks make up the northern border of the graben and are separated by a longitudinal Pawlowice Depression. The graben itself is divided into smaller, secondary blocks. All crustal units there show regular, trapezoid or rectangular form (Cwojdzinski, Jodlowski 1978; Dyjor 1982; Dyjor, Farbisz, 1993). Simplified tectonic structure is shown in Fig. 1.
[FIGURE 1 OMITTED]
The Paczkow Graben originated in Neogene times when the Sudetic Massif had been broken into Sudety Mts. and Fore-Sudetic Block along the SMF. It has shown tectonic activity of varying intensity throughout the remaining part of Tertiary and Quaternary. Recent studies, a part of the Global Seismic Hazard Assessment Programme, indicate that the Eastern Sudety Mts. with Sudetic Foreland are the most seismically prone area in Poland (Giardini et al., 1999; Schenk et al., 2001).
As the graben experienced tectonic processes, Tertiary and Quaternary sediments were deposited on top of old pre-Cenozoic formations in subsiding areas. Secondary subdivision into lowered and uplifted structures is mirrored in the morphology of underlying crystalline rocks and thickness of younger formations (Cwojdzinski, Jodlowski 1978; Dyjor, Oberc, 1981; Dyjor 1982).
The reservoir will fill up morphological depression in southern part of the Pawlowice Depression. The dam will be placed 200 to 300 m west of a longitudinal Kamieniec Fault running more less parallel to it. There are two more perpendicular faults east of Kamieniec Fault, separating southern stage of Kamieniec Horst subsiding into Paczkow Graben (Dyjor, Farbisz, 1993).
Interpretation of present-day processes, described in the following paragraphs, is limited by the lack of information about precise thickness and lithology of Cenozoic deposits in many parts of the investigated area. Available data come mainly from geophysical soundings and some scarce exploratory drillings. Publications of Polish Geological Institute list only about 40 exploratory boreholes in the whole of the western Paczkow Graben. These are also tightly grouped in several areas, near Bardo, in the limits of Braszowice Horst, as well as west and south of Kamieniec Zabkowicki. Only a few, and mainly those located in horst structures, reach pre-Cenozoic rock formations. There is clear lack of drillings in lowered zones subjected to subsidence in the Tertiary and Quaternary. Geological investigations of the proposed dam site that were performed for engineering project purposes consist of boreholes that only rarely reach Tertiary formations (Hydroprojekt, 1998).
Figures 3 and 4 present simplified geological sections showing the structure of Paczkow Graben and Pawlowice Depression with indicated tectonic faults and location of boreholes.
3. INVESTIGATIONS OF VERTICAL UPPER EARTH'S CRUST MOVEMENTS
Results of repeated measurements along national levelling lines of the 1st order and repeated determinations of acceleration of gravity in the local geodynamic network "Paczkow" were used with the aim at detecting movements of the upper part of the earth's crust.
3.1. PRECISE LEVELLING MEASUREMENTS
The authors had at their disposal results of four measurements along national levelling lines passing through or next to the area of future dam site. Characteristics of precise levelling data are given in table 1. Observations cover a period of almost 50 years. Records from 1953, 1975 and 1999 (2002) campaigns were used in the analysis. Results of two measurements along the levelling line (2nd order) running from Zabkowice Sl. through Bardo to Klodzko, were not considered, as only 8 points from 1953 were remeasured in 1989.
For the purpose of geometric analysis of benchmark height changes a fixed reference level was established. Nodal benchmark in Zabkowice Sl. was chosen as the reference point. Relative height changes of benchmarks were determined for the 1953-1975 and 1953-1999 (2002) periods. These were then correlated with geological and structural units of the ground along levelling lines. The results are presented in figures 5 and 6.
[FIGURE 2 OMITTED]
Along Zabkowice Sl.--Paczkow levelling line Quaternary (sands and gravels) deposits form the uppermost geological layer. At some places (Kamieniec Horst) pre-Cenozoic rock formations show up on the surface. The greatest benchmark movement (-13 mm) was detected near the fault separating Pawlowice Depression and the northern part of Kamieniec Horst. The horst is relatively stable; the observed benchmarks heights changes are below the double error value of their estimation. Slight subsidence of around -0.2 mm/year for the 1975-1999 period occurred NW of Paczkow.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Geological structure along the Paczkow-Klodzko levelling line is more complicated. Apart from Quaternary deposits near Paczkow, Tertiary formations (Neogene sands and gravels) occur on the surface near Kamienica. At some places Pleistocene muds also form the uppermost geological layer. The line passes along Zloty Stok fault and runs through the Sudetic Marginal Fault zone towards Makolno, where it enters the Sudety Mts.
Comparison of benchmark heights showed subsidence west of Paczkow of about -(0.2-0.3 mm/year) for the 1975-2002 period. This, together with results from the first levelling line, suggests downward movement of the graben's deepest part in the analysed region. However, latter part of this line shows gradual elevation of benchmarks along Zloty Stok and SMF zones.
Analysis of the mean groundwater level in wells from which data are available does not show any significant fluctuations for the periods of measurements considered.
3.2. GRAVIMETRIC OBSERVATIONS
Relative gravimetric measurements performed in local and regional research networks allow for determination of the Earth gravitational attraction values and studies of their short-term and long-term variations. Temporal changes of Earth's acceleration, may be caused by changeable atmospheric conditions, fluctuating ground water levels, etc. These may also be attributed to endogenous crustal movements, as well as large-scale human activity (Kamela et al., 1993).
In the area of planned dam site, 7 points of local geodynamic network "Paczkow Graben" are located (Cacon et al., 1998). These are used for gravimetric observations carried out almost every year in September. Relative measurements with Scintrex Autograv CG-3M gravimeter employ profile method of observations with reference station at Braszowice. Results of 8 observation campaigns spanning the last decade (1993-2002) were used for the purpose of vertical upper Earth's crustal movements' analysis (Barlik, Cacon, 2001, Barlik 2002) (1). Differences of temporal changes of acceleration of gravity with initial values from measurement carried out in 1993 are presented in Table 2. Significant changes exceeding double RMS error of their determination are marked in bold. Diagrams showing a graphic relationship between values of acceleration due to gravity and time are presented in Fig 7.
These were made for selected points inside the Paczkow Graben and tectonic blocks surrounding it. An attempt to analyse possible trends was made with placing best fitting polynomial regression curves. Value of [R.sup.2] coefficient changes in the 0-1 limits, where 1 shows the best fitting. Point Byczen situated on crystalline rocks in the Kamieniec Horst, north of the graben, shows initial decrease of gravity and then stabilisation. Observations at Sosnowa, to the east of future dam foreground and south of Kamieniec Horst, indicate continuous decrease of terrestrial gravity force. Value of acceleration of gravity has decreased since the onset of measurements by -0.118 mGal and -0.100 mGal at Byczen and Sosnowa respectively. Stations at Bardo and STOK manifest fluctuations of acceleration due to gravity. Point STOK, located in the Sudetic Marginal Fault zone, shows greatest variations with amplitudes in successive campaigns, reaching 0.172 mGal. These may indicate possible tectonic movements of endogenous character. Point Bardo, near Nysa Klodzka River, on the other hand shows relationship with high water levels in the river valley during the flood of 1997 (Fig 7c).
[FIGURE 7 OMITTED]
All of the locations, at which gravimetric observations were made, indicate general decrease of acceleration of gravity in the western part of the Paczkow Graben. If they were attributed to endogenous processes, they could have indicated indicate glacioisostatic relaxation of particular crustal blocks in the area. Thus, superposition of two phenomena may be inferred in the analysed region: subsidence of depression zones caused by endogenous tectonic movements and additional weight of Cenozoic sediments and glacioisostatic rebound. Comparison of values measured at stations to the east of the examined area, basing on available data (1996-2002), also show decrease of the acceleration of gravity (Barlik, Cacon, 2001). The results are presented in Table 3. However the value obtained for gravimetric station in Klodzko, which is located south of the Sudetic Marginal Fault and Paczkow Graben shows only negligible variation of 0.003 mgal for 1996-2002 period, well below the RMS error of its estimation, suggesting relative stability of this region.
4. PROPOSED LOCATION OF REFERENCE POINTS
Investigations of geodynamic activity in the described part of the Paczkow Graben and future dam location settings indicate the need for additional research points. The present network is made up of 7 points, shown in Fig. 1, used for periodic satellite GPS and gravimetric observations. New points are to be located in structures presently not covered with geodynamic measurements, such as Pawlowice Depression (KAM 2 and KAM 3) west of Kamieniec fault and subsiding stage of Kamieniec Horst (KAM 4), as well as in units framing the area for additional control of the network (KAM 1 and KAM 6). The project assumes that some of these points will be used as reference benchmarks for dam control measurements (satellite GPS and Total Station). The proposed layout (Fig. 1) was chosen with the aim at data acquisition for reliable assessment of geodynamic movements after thorough site reconnaissance and studies of available material.
Results of only vertical component of upper Earth crust have been analysed only. These are based on repeated measurements of national levelling lines during the last half century and observations of gravitational force during the last decade. The study area is a planned site of earth dam location.
Comparison of benchmark heights in successive measurement epochs reveals differentiated and varying crustal movements. Correlation of benchmark heights changes with local geology shows the greatest movements near Kamieniec faults that separate Pawlowice Depression, north of dam location, and the northern part of Kamieniec Horst. Area to the northwest of Paczkow, characterized by the thickest Cenozoic deposits in the studied area, experienced slight subsidence in the 1975-1999 period, of around 0.2-0.3 mm/year.
Values of Earth gravitational attraction obtained from gravimetric observations indicate a decrease of force of gravity to the east of future dam foreground and in Kamieniec Horst. Other locations show various degrees of gravitational force fluctuations. These could be attributed to the influence of flood in 1997 (point BARDO, near Nysa Klodzka river) or may suggest existence of mass movements (point STOK, situated in the Sudetic Marginal Fault zone).
Possible relationship between thinner Cenozoic sediments, decrease in the Earth gravitational attraction and elevation of benchmarks south of Byczen Depression was noticed near Sosnowa.
Analysis of Earth's gravitational force variations corresponds with the results obtained from comparison of benchmark height changes in the examined period. Benchmarks in national levelling lines show elevation near Sosnowa (south of Byczen Depression) and near STOO. There is no correlation for STOK station and the precise levelling line passing nearby. In other parts of the investigated area comparison of observations cannot be made.
The observed uneven and changing in character vertical movements of the upper Earth crust may point to either oscillating nature of possible tectonic processes or superposition of glacioisostatic rebound (glacial relaxation) and subsidence. This hypothesis, based on quantitative geodetic data, is compatible with conclusions of Przybylski (1998) drawn for those part of the Nysa Klodzka river valley, basing on geological and geomorphological research. Interpretation of geodynamic investigations and the planned construction of the dam raises the need for location of additional reference points. The authors propose to set up six more points in a layout that ensures coverage of all crustal blocks with geodynamic measurements. These points will also act as reference benchmarks for observations of future deformations of the planned dam and its surroundings.
This research has been partially supported by the State Committee for Scientific Research grant No. 8 T12E 010 20 ("Analysis and interpretation of the earth's crust surface layer deformation in the vicinity of planned water reservoir"). The authors would like to thank Assoc. Prof Dr. S. Dyjor for his contribution to this work.
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HYDROPROJEKT: 1998, Spatial and programme concept of Kamieniec Zabkowicki water reservoir, Warsaw, (in Polish).
Jan BLACHOWSKI (a) and Stefan CACON (a,b)
(a)--Division of Geodesy and Geoinformatics, Institute of Mining Engineering, Wroclaw University of Technology, 2 Teatralny Square, 50-051 Wroclaw, Poland, e-mail: firstname.lastname@example.org
(b)--Department of Geodesy and Photogrammetry, Wroclaw University of Agriculture, 53 Grunwaldzka Street, 50357 Wroclaw, Poland, e-mail: email@example.com
Table 1 Characteristics of available precise levelling data LEVELLING LINE CLASS LENGTH YEAR OF LINE Zabkowice Slaskie-- 1 27.5 km 1953, 1975, Kamieniec Zabkowicki-- 1994, 1999 Paczkow Paczkow--Zloty Stok--Klodzko 1 33 km 1953, 1975, Zabkowice Slaskie-- 1994, 2002 Bardo-Klodzko * 2 29 km 1956, 1989 LEVELLING LINE MEASUREMENT ERROR Zabkowice Slaskie-- [m.sub.0] = 0.4mm/km ('53) Kamieniec Zabkowicki-- [m.sub.0] = 0.4mm/km ('75) Paczkow Paczkow--Zloty Stok--Klodzko [m.sub.0] = 0.4mm/km ('99) Zabkowice Slaskie-- [m.sub.0] = 0.4mm/km ('02) Bardo-Klodzko * LEVELLING LINE NUMBER OF BENCHMARKS Zabkowice Slaskie-- 24 Kamieniec Zabkowicki-- Paczkow Paczkow--Zloty Stok--Klodzko 26 Zabkowice Slaskie-- Bardo-Klodzko * * 8 common points Table 2 Temporal variations of acceleration of gravity at local geodynamic network points located near planned dam site. POINT Gravity changes [DELTA]g [mGal] [DELTA]g94-93 [DELTA]g96-94 BARDO -0.050 -0.031 BYCZEN -0.020 -0.028 PRZYLEK -0.020 -0.017 SOSNOWA 0.000 -0.027 STOK -0.040 -0.019 STOO -0.010 0.011 m[DELTA]g 0.021 0.019 POINT Gravity changes [DELTA]g [mGal] [DELTA]g97-96 [DELTA]g98-97 BARDO 0.048 -0.010 BYCZEN -0.022 -0.044 PRZYLEK 0.012 -0.035 SOSNOWA -0.042 0.010 STOK -0.107 0.065 STOO -0.058 0.034 m[DELTA]g 0.019 0.021 POINT Gravity changes [DELTA]g [mGal] [DELTA]g00-98 [DELTA]g01-00 BARDO -0.030 0.008 BYCZEN -0.004 0.004 PRZYLEK 0.027 -0.012 SOSNOWA -0.015 0.005 STOK -0.037 0.042 STOO -0.024 0.023 m[DELTA]g 0.021 0.018 POINT Gravity changes [DELTA]g [mGal] [DELTA]g02-01 BARDO 0.021 BYCZEN -0.004 PRZYLEK 0.006 SOSNOWA -0.031 STOK -0.052 STOO -0.050 m[DELTA]g 0.014 Significant changes of earth gravitational attraction [greater than or equal to] 2m[DELTA]g are marked in bold "--" sign represents decrease in the force of gravity Table 3 Differences in value of the Earth's gravitational force measured at stations in eastern part of the Paczkow Graben between 1996 and 2002 [mGal] POINT JEDRZYCH0W OTMUCHOW NIERADOWICE [DELTA]g02-96 -0.078 -0.087 -0.107 POINT RADZIKOWICE BIALA NYSKA KAMIENNA GORA [DELTA]g02-96 -0.082 -0.080 -0.124
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|Author:||Blachowski, Jan; Cacon, Stefan|
|Publication:||Acta Montana. Serie A: Geodynamics|
|Date:||Jan 1, 2004|
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