Download Introduction The Mäntsälä Granodiorite Batholith Observations in the

Ductile and brittle deformation structures of the Mäntsälä granodiorite batholith in southern Finland; observation at three scales
Marit Wennerström1, Mira Markovaara-Koivisto2
1
Geological Survey of Finland, Finland
2
Department of Civil and Environmental Engineering, Aalto University, Finland
[email protected]
Introduction
Observations in the quarry
This study is a part of the co-operation project between the Geological Survey of Finland (GTK) and the Aalto University in Espoo
‘Rock quality classifications; the determination and presentation
of their uncertainty using statistical deduction and geological 3Dand GIS-softwares.’
The Mäntsälä Granodiorite Batholith
The Mäntsälä granodiorite batholith belongs to systematic domeand-basin structures in central southern Finland. The structural
pattern of the foliations suggests that the lower tectonic slices of
crust are exposed in the dome windows (Pajunen et al. 2008a). Migmatitic mica gneisses in the batholith centre provide evidence of
several folding generations prior to formation of the open rounded
dome structure. The dome series in Mäntsälä suggest a complex
fold interference pattern formed during DG+H+I.
The structural pattern is fragmented by brittle faults that partly follow the ductile shear zones (Pajunen et al. 2008a). In the west the
batholith abuts on the NE-trending fault which belongs to a major
brittle shear zone in southern Finland. To the south and southwest
steep strong shear zones border the batholith. Schistosity symbols
on the map show the general structural trend in the batholith (The
Bedrock Map Database; DigiKP, GTK).
343/22
Wall direction 333°
Fracture observations were gathered from the Mäntsälä quarry according
to a scanline mapping form, designed for later 2D and 3D illustration of the
results (Markovaara-Koivisto and Laine, in press), and for visualizing also
the undulative nature of discontinuities (seen on the left scanline). Schistosity and fault parameters were mapped according to the method described in
Pajunen et al. 2008b. In total 263 fractures were recorded along 7 scanlines,
and 13 schistosity and 11 fault measurements were observed on 6 rock wall
segments in two nearly perpendicular directions (40° and 333°). The above
figure shows the fractures along two scanlines in two nearly perpendicular
directions which are visualized with GOCAD. In the photographs beside
sections of the same rock walls are shown with foliation marked by dashed
lines.
The fracture observations shown in
the stereoplot were clustered with
Visual_Scanline3D
(MarkovaaraKoivisto and Laine, in press), which
uses iterative Kmeans method. Joints
show clustering behavior but many
random orientations occur, too. The
moderately NE dipping and gently SE
dipping schistosities are followed by
joints. The steep NW-trending jointing trails the fault orientations.
Joint (263), clusters colour coded
Fault (11)
Schistosity (13)
SG
SH
Quarry
Quarry
127/84
230/75
152/18
LEGEND
Observation point
Quartz feldspar schist
Biotite paragneiss
Amphibolite
Gabbro
Microcline granite
Uralite porphyrite
Granodiorite
60
Brittle faults
Vertical schistosity
Schistosity and dip
Ductile shear zones
Vertical foliation dominates in the centre of
the batholith (the map above) and foliation
dips steeply or moderately to the NE on the
northern and southern border zones tracing
the orientations of the outer contacts of the
batolith (the map above). During the field
work in this study (observations marked by
red stars on the map) steep and moderately
dipping E-W-trending foliations were detected. Also two thin seams of steeply southwest
dipping faults were recognized. In the batholith vertical joints trend mostly to the NE and cut the ductile structures. Subvertical or moderately to ENE or SW dipping fractures mostly follow the foliation trend or orientation of the brittle shear.
NE-trending fracture at microscale
A
B
N
Conclusion
Two main fracture types in the quarry
A
Wall direction 40°
In the oriented sample ’undulating’ fracture (white linear trace in figure
A) shows cutting behavior in N-S-direction and shortly parallels the cleavage in plagioclase trending NE at microscopic scale (figure B). Width of the
photo is 5.3 mm. (In figure B a straight crack is seen on the glass surface)
Schmidt’s Nett
Lower hemisphere
SI
023/73
Photo by K. Kinnunen
The Mäntsälä Granodiorite batholith in southern Finland has earlier been studied and modeled to define the rock quality in it. In the
present study the fracture orientations at three scales were examined. The batholith was mapped regionally using sparse observation across the batholith in N-S and E-W directions. In the northeastern part of the Mäntsälä Granodiorite in a quarry ductile and
brittle structures were mapped on the quarry walls and scanline
method was used to gather exact fracture information. The relatively homogenic migmatitic pattern of the batholith is a result of
lower crustal origin. In addition sparse fracturing in the granodiorite makes the rock suitable for natural stone use.
B
On the photographs two different types of fractures from the quarry
are shown.
Ductile and brittle deformation structures were studied in the
Mäntsälä Granodiorite Batholith to help in prediction of suitable
sites for natural stone extraction. The desirable texture of the rock
is a result of ductile deformation. Two dominant fracture orientations trending NE and NW are observed at the Batholith scale and
in the quarry, where they follow locally foliation or faulting orientations. At microscale the NE-trending fracture shows cutting behaviour. In the future studies these observation at three scales will
be structurally combined.
References
A) Subvertical shear plane dips to SW including marks of subhorizontal sinistral movement, determined by smooth surface and white carbonate minerals arranged perpendicular to the shear direction. On the
surface chlorite and hematite occur as well.
Lindqvist, K. And Laitakari, I. 1981. Palygorskite from Padasjoki, southern Finland.
Bull. Geol. Soc. Finland. 53 (2), 91-95.
B) In the NE-SW-direction joints with palygorskite infilling (XRDanalysis by M. Tiljander, GTK, 2010) cut the foliation orientation.
Lindqvist and Laitakari (1981) have described alike palygorskite in
an open fracture crystallized by hydrothermal activity in Padasjoki in
southern Finland.
Pajunen, M., Airo, M.-L., Elminen, T., Mänttäri, I., Niemelä, R., Vaarma, M., Wasenius, P. and Wennerström, M. 2008a. Tectonic evolution of the Svecofennian crust in
southern Finland. In: Tectonic evolution of the Svecofennian crust in southern Finland - a
basis for characterizing bedrock technical properties. Geological Survey of Finland. Special Paper 47. Espoo: Geological Survey of Finland, 15-160 + 1 app.
These two generations of fractures differ in orientation and type. The
chlorite covered fractures include shear component compared to the
open joints filled with palygorskite.
Markovaara-Koivisto M., Laine E. 2011 DOI: 10.1016/j.cageo.2011.07.010; Comput
Geosci (In press).
Pajunen, M., Airo, M.-L., Elminen, T., Niemelä, R., Salmelainen, J., Vaarma, M., Wasenius, P. and Wennerström, M. 2008b. Construction suitability of bedrock in the Helsinki
area based on the tectonic structure of the Svecofennian crust of southern Finland. In:
Tectonic evolution of the Svecofennian crust in southern Finland - a basis for characterizing bedrock technical properties. Geological Survey of Finland. Special Paper 47. Espoo:
Geological Survey of Finland, 309-326 + 1 app.
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