Miscellanea geographica 11
KGE, Z U v Plzni, 2005
Contributions from geomorphological seminary Šumava ´05
Morphostructural analysis of the Hamerský Brook catchment
Alena Zemanová
[email protected]
Department of Physical Geography and Geoecology, Faculty of Science, Charles
University, Albertov 6, 128 43 Praha 2
Introduction
The morphostructural analysis is one of the chief parts of the
geomorphological analysis. It is used to recognize the relations of the
geological condition and the relief. The analysis is based on the knowledge of
antagonistic influence of endogenous and exogenous processes and allows the
recognition of the endogenous landforms. (BEZVODOVÁ et al. 1985) The
method was applied for recognition of the interdependence between the
tectonics and the relief of the central part of the Šumava Mts.
The studied area is located in the central part of the Šumavské plán Plains,
in the catchment of the Vydra River. It was delimited as the catchment of
Hamerský Brook (more than 80 % of total surface area) and both adjacent
valley slopes of Vydra River. It covers 25.9 km2. The span of the altitude is
358 m (895–1,253 m a.s.l.). The eastern part of the region has the
characteristic features of old peneplain relief. The dominant landforms of the
western part are the monadnock of Sokol Mountain, the deep Vydra River
valley and the Hamerský Brook valley, which is still deepen by the backward
erosion of Vydra River.
The main research objectives are the fissure systems’ influence on the
morphology of relief mezoforms, the range of the Vydra massif’s impact on
surrounding crystalline schist and tectonic predisposition of flow direction of
the Hamerský Brook. In particular, the analysis of fissure systems and valley
network was used within the morphostructural analysis.
Lithological conditions and microtectonics
The lithological conditions of studied area are not very diverse. Widespread
sillimanite-biotite and cordierit-biotite paragneisses and migmatites are
intruded by granite bodies. The Vydra massif is the biggest one; it forms the
western boundary of the area. The massif in the Czech part of the mountain
range is 12 km long and almost 3 km wide. The smaller granite bodies of the
region seem to be its branches. The main type of the intrusion is mediumgrained to coarse-grained biotitic granite (the variety of the Weinsberg type).
The migmatization of paragneisses decreases with the distance from the
Vydra massif (KODYM et al. 1961, MÜLLER ed. 1999). The boundary line of
each type of the paragneisses is covered with the quaternary sediments,
especially in the eastern part of studied area.
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Fig. 1. The position of the studied area in the upper part of Otava catchment
The microtectonic conditions are determined by the foliation. The foliation
system s3 is dominant element (MÜLLER ed. 1999). The foliation direction
bends around the granite massif of Vydra. In the northern part of the Vydra
River catchment the foliation runs in N–S direction with the inclination to the
east. There are two exceptions: the inclination to the SW between the Vydra
massif and the Prášilský massif and to the NW in the area south of the Zh í
Village. In the southern part of the Vydra River catchment the foliation runs
in N–S direction with the steep inclination to the west with the exception of
the right-hand bank of the Vydra River where it runs in NW–SE strike with
the inclination to NE.
Lithological features and microtectonics of each rock types affect their
resistance against exogenous processes. The differences in resistance of
granites and paragneisses (PAUK & HAB TÍN 1979) are evident in studied area.
The hard granite bodies strengthen practically all the upper part of the flat
elevations (localities of B emeno, U t í jedlí, Ranklov, P ilba, Jelení vrch and
B ezová hora) and the monadnock of Sokol Hill in the central part of the area.
The monadnock is consolidated by the granite intrusion and by the rest of the
resistant migmatite, a contact metamorphic rock in vicinity of this intrusion.
Cryogenic mezoforms of the relief occur only in highly migmatized
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paragneisses. This fact also can be caused by more intensive denudation of
western part of the studied area which is built by strongly metamorphosed
paragneisses. The considerable denudation is the result of the Pleistocene
changes of the river network of the Hamerský Brook and the Vydra River and
the shifting of their erosion basis (KETTNER 1923).
Fault analysis
The brittle tectonics affects only the border of the studied region (see Fig.
2), but outside the faults are plentiful. The Kvildský Fault in the east and the
Prášilský Fault on the west are the biggest faults in the region. They run in
NNE–SSW to NE–SW direction. Less significant faults with similar direction
break the Vydra massif, too. This massif has probably tectonically
predisposed eastern boundary, which controls the flow direction of Vydra
River.
Fig. 2. The faults in the neighbourhood of the studied area (according to
MÜLLER ed. 1999)
Fissure analysis
The fissures are the most abundant medium scale structure. Fissures are the
surfaces of disintegrated rock; they are less dense than foliation cleavage and
their minerals are not oriented parallel with the crack surface. The fissure
system has recorded the paleopressures from the period of granite massif
solidification and has copied into surrounding schists. The pressures of
granite body’s solidification are recorded by the primary fissure system; the
impact of the oriented pressure is recorded by the secondary one and the
impact of neotectonic is recorded by the tertiary one. (VOTÝPKA 1970)
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The shape, size and configuration of exogenous granite’s and migmatite’s
landforms are determined by the fissure systems. The exogenous processes
and the activity of the water act together and cause the developement of
primary (frost cliffs, nivation hollows) and secondary cryogenic landforms
(block fields, debris flows) (VOTÝPKA 1979). The movements on the fissure
planes are not as important and significant as the movements on the fault
planes (JAROŠ &VACHTL 1987).
Ideal collection of data for the statistic analysis of fissures systems would
be taken in the regular network, but that is usually impossible. All the fissure
measurements were done on the rock outcrops in western part of the area of
interest, because the relief of eastern part with the rests of old peneplain
hasn’t any rock outcrops. The measurements were performed on the localities
on the slopes of the Lišák Hill, on the top part of the Sokol Hill and the
B ezová Hill and on slopes of right bank of the Vydra River.
Altogether 1,464 measurements of fissure plane strike were performed;
with one exception all outcrops were built by paragneisses (95.5 % of all
measurements). The Cloose'
s rosette diagrams were used to evaluate the crack
systems’ orientation. The inclination of crack planes, the frequency and
character of crack surface were only described generally for each locality. In
studied area the analysis of fissure systems’ orientation showed that the
longer axis of the rock formation corresponds to the strike of S, the primary
fissure system, and the quartz dykes are connected with the secondary fissure
systems. The growth of the L fissure planes’ frequency to the depth was
validated on the granite outcrops, too. (ZEMANOVÁ 2005)
Two to three fissure systems were found in the studied territory (primary,
secondary and in some localities tertiary, too). Primary crack system has S
joints direction 0–40° and Q joints direction 100–140°. Secondary, generally
less abundant crack system has joints directions 150–170° and 50–90°. These
fissure systems are sometimes complicated (duplication or triplication of the
directions, the presence of third crack systems, etc.), but certain regularities in
their directions and in the dependence on the position towards the Vydra
massif were discovered.
First of all, the trend of fissure systems’ simplification with the increasing
distance between the locality of the measurement and the Vydra massif is
obvious. Furthermore, the third crack system can be seen as advancing
southwards (see Fig. 3).
The maximal extent of triple fissure systems is generally similar in the NW
and the SW margin of area of interest. It is probably caused by the decrease of
the intensity of the granite massif impact on the gneisses in its contact
aureole.
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Figure 3 Comparison of the changes and number of fissure systems in studied
localities in the area under consideration and its vicinity. (Cloose’s rosettes
outside the studied area were taken over from MAŠEK & Votýpka 1999. The
grey areas represent the granitic bodies. The numbers in brackets behind the
name of locality are the numbers of fissure measurement)
The cause of changes of fissure systems’ number can be explained by the
tectonics, chiefly by the presence of fault along the eastern boundary of Vydra
massif and its accompanying faults. One of the significant accompanying
faults runs across the Vydra massif along the contact of two different type of
massif’s granite (medium-grained to coarse-grained biotitic granite of the
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northern margin of the massif changes into its porphyric variety and then into
fine-grained to medium-grained muscovite-biotite granite in the southern
margin). The coarse-grained variety was consolidated deeper than the finegrained one. The occurrence of both varieties in vicinity could be caused by
vertical tectonic movements there. The described tectonic disturbance runs
near by the SW border of studied area (by the Modrava Village) and crosses
the fault of the Vydra River valley there, too. The fissure systems are most
complicated there and this feature covers a large area.
With the growing distance from the Vydra massif the fissure systems’
strikes change. From the diagrams of the localities of Kostelní vrch, Spálený,
Horní Hrádky and Turnerova chata (MAŠEK & VOTÝPKA 1999) can be
identified a gradual clockwise rotation of the fissure systems towards east.
The primary crack systems of the Kostelní vrch Hill and the Spálený Hill
localities trending towards 160–170° and 60–80° turns into direction of 0°
and 100 and 130° near the Turnerova Cottage. The difference is about 40°.
The comparison of Cloose'
s rosettes from the granite outcrops in the Horní
Hrádky locality and from the outcrops in the studied area (chiefly the Lišák
Hill and the Hamerský slope) shows again rotation within the same range
(approx. 30°) and in the same direction, too. In the outlying locality of the
B ezová Hill, the rotation reaches 40–50°. There are two exceptional
localities: the Sokol Hill and the Jelení Hill. On the locality Jelení Hill, the
fissure system is very similar to the locality of Horní Hrádky; the fissure
system of the Sokol Hill’s rock outcrops is rotated but the rotation is smaller
than would be expected considering the distance to the Vydra massif.
This described fissure systems’ rotation is probably the result of thermal
and pressure changes in the bedrock during granite body’s intrusion and their
transmission into the contact aureole of the intrusion (ZEMANOVÁ 2005). The
differences in trending of described fissure systems were caused by the
presence of small granite bodies. In the case of the locality Jelení Hill, the
granite body is an apophyse of the Vydra massif. The Sokol Hill has an
apophyse, too, but it intruded under slightly different conditions.
The dominant influence of the Vydra massif on the surrounding gneisses is
incontestable, but the fissure systems analysis supports the hypothesis of the
influence of the small side branches of the granitic intrusion on the area under
consideration. Cloose'
s rosettes of this locality and the diagram of all the
measurement in the whole Hamerský Brook catchment are very similar (see
Fig. 4). This fact is possible to explain by the predominant influence of the
Sokol Hill’s apophyses on surrounding paragneisses or by the uniform
evolution of the whole area to the east of Vydra massif (paragneisses of
contact aureole and granite apophyses together). The influence of
neotectonics is possible, too, but author considers it to be less significant
(especially in relation to the primary fissure systems).
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968 measurements (migmatite)
92 measurements (granite)-5×
enlarged
Figure 4 Comparison of Cloose’s
rosettes of migmatite outcrops in the
Hamerský Brook catchment with the
measurement of the Sokol Hill’s
granite outcrops
The analysis of the valley network
The analysis of valley network segments of Hamerský Brook catchment
was used to determine the tectonic predisposition extent. The orientation of
the valley network segments was extracted from the topographical maps (ZM
R 1 : 25,000). The length of main valley’s segments ranges from 75 m to
290 m (in average 130 m). The sums of segment lengths and the fissure
systems measurements (only the rock outcrops in Hamerský Brook
catchment, without the Vydra River valley’s slopes) were compared in
Cloose'
s rosette diagram.
Figure 5 shows the predominant valley network directions trending 100°
and 130°, which corresponds to the Q strike of primary crack system. The
other directions are: 40° (similar to S strike of primary system) and 0° and 60°
(the directions of the secondary fissure system). The directions are, as can be
seen from Fig. 5., quite similar, we can consider the valley network of
Hamerský Brook to be considerably tectonically predisposed.
rock outcrops (968 measur.)
valley segments (1scale
division = 300 m long
segment)
Figure 5 Comparison of the valley
segments directions with the fissure
systems directions
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Conclusions
The executed morphostructure analysis of enlarged Hamerský Brook
catchment brought some new information about geological structure and
geomorphological development of the central part of Šumava Mountains. The
attention was paid to the fissure systems analysis in particular.
This analysis proved the dependence of the morphology of rock outcrops
on fissure systems of bedrock. The rock outcrops’ walls are predisposed by
the fissure systems in the localities of Lišák, Sokol and B ezová hora and
Vydra River valley’s slopes especially. Furthermore, the important influence
of the Vydra massif’s granite body on surrounding schists was discovered.
The majority of the fissure systems of the studied area’s paragneisses are
affected by the granite massif of Vydra. There was observed some gradual
clockwise rotation of the fissure systems towards east. The recognition of the
rotation can be used by next research of the granite intrusion’s direction and
development. The influence of the Sokol Hill’s granite apophyses seems to be
considerable in the paragneisses of its contact aureole, too. But it could be
caused by common development of this apophyse and surrounding schist in
the Vydra massif’s contact aureole. The character of fissure system was
influenced by movements on faulted planes partly in the SW of studied area.
The analysis of valley network segments of Hamerský Brook proved that
the valley in question is tectonically predisposed. The strikes of valley’s
segments and the strikes of primary fissure systems of rock outcrops in
Hamerský Brook catchment are very similar.
The research showed the significant effects of granite intrusion in the
region under consideration – not only from structural-geological, but also
geomorphological point of view, because the fissure systems together with
cryogenic and fluvial processes shaped the prezent-day relief mezoforms. The
area of Hamerský Brook catchment and the upper part of the Vydra River
catchment was found to be highly interesting and the author would like to
continue the research in this area with the goal of detailed analyses of the fault
tectonics.
Acknowledgements
Author thanks Filip Hartvich from the Institute of Rock Structure and
Mechanics in Prague for review advices and remarks to make about this
paper.
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