- Email: [email protected]
Magnetic fabric in folds of the easternmost Rheno-Hercynian Zone
Phys. Chem. Earth (A), Wol.25, No. 5, pp. 505-510, 2000 © 2000 ElsevierScienceLtd All rights reserved 1464-1895/00/$ - see front matter
Pergamon PII: S 1464-1895(00)00078-8
Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone F. Hrouda 1'2, O. Krej~i 3 and J. Otava 3 1AGICO Inc., Brno, Czech Republic 2Institute of Petrology and Structural Geology, Charles University, Praha, Czech Republic 3Czech Geological Survey, Brno, Czech Republic Received 1 July 1999; accepted 18 October 1999
Abstract. The magnetic fabric in folds was investigated in the easternmost Rheno-Hercynian Zone, the Nizk~ Jesenik Mts. In their eastern areas, the rocks show signs of only weak anchimetamorphism and very gentle ductile deformation; SE vergent buckle folds of long wavelength are developed whose magnetic fabric can be easily unfolded geometrically. In the central areas, spaced cleavage and NW vergent buckle folds can be found; the folds can be unfolded mostly only partially. In the western areas, NW vergent cleavage folds and very well developed slaty cleavage occur. The magnetic fabric in the folds is homogeneous, the folds cannot be unfolded at all. The cleavage is transformed into metamorphic schistosity at the western border of the area. © 2000 Elsevier Science Ltd. All rights reserved.
senting the easternmost Rheno-Hercynian Zone is exposed in a NNE-SSW trending and northwards widening belt stretching from the surroundings of the town of Brno in the south to the Odra Lineament in the north (southern Poland). Culm Basin sediments are exposed in two main areas, southern Drahany Upland and northern Nizk~ Jesenik Mts., now separated by the Elbe Lineament. The sampled area comprises the Culm of the Nizl~ Jesenlk Mountains. The referred part of the basin is bounded to the
~]
Moravice Fm
1 Introduction Pre- and Post-Culm Fins
Magnetic anisotropy is a very good tool for investigating the origin of folds. First attempts to use this method in studying the origin of folds in the Rheno-Hercynian Zone on the territory of the Czech Republic were made by Hrouda (1978, 1981); the folds in the central and western part of the Nizl~ Jesenlk Mts. were investigated. Recently, folds from the easternmost areas were added. The purpose of the present paper is to show how the origin of the folds can differ according to metamorphic and deformation histories of the formation in which the folds occur.
~'.,".*"~
k.
ranlCe
Fig. 1 Geologicalschemeof the Nizk32Jesenik Mts. in the easternmost Rheno-HercynianZonewith plotted locationsof the folds investigated. west by Devonian volcano-sedimentary complex of the Vrbno Group metamorphosed in greenschist facies grade The studied sequence belongs to the foreland flysch basin (conglomerates, greywackes, siltstones, mudstones, shales and related rhythmites and laminites) of Vis6an to Lowermost Namurian age. The flysch sequence of over 7 km thick sediments was deposited in a deep marine
2 Geological Setting The Lower Carboniferous Moravian-Silesian Culm Basin located in the eastern part of the Czech republic and repreCorrespondence to: F. Hrouda
505
506
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
environment and comprises an axial turbidite system sourced mostly from the south of the basin. It is conformably overlain by paralic coal-bearing coastal plain deposits of Namurian to Westphalian age. The whole Palaeozoic area is bounded to the east by Tertiary and Mesozoic sediments of the Carpathian Foreland. The basin-fill was strongly deformed during the Mid- to Late Carboniferous Variscan Orogeny. Deformation primarily affected the western margin of the basin resulting in extensive faulting and folding, the development of penetrative cleavage and metamorphism. A progressive decrease in deformation and metamorphic grade is seen from the west to the east across the basin. The Cuim of the Nizl~ Jesenik Mts. can be very roughly subdivided into western and eastern parts. The two parts are approximately separated by an antiform belt of mostly Devonian to Lower Carboniferous volcano-sedimentary complex (~temberk-Homi Bene~ov Belt). The western Culm subbasin of Vis6an age include And61sk~i Hora and Horni Bene~ov Formations. The westernmost And61sk~ Hora Formation is composed mostly of shale rhythmites and laminites. Structurally it is characterized by NW vergent cleavage folds and a well developed slaty cleavage which is transformed into metamorphic schistosity at the western border of the Formation. Mostly subvertical slaty cleavage and cleavage folds are developed in the sandstone dominated Horni Bene~ov Formation. The eastern Culm subbasin is of Upper Vis6an age and it comprises Moravice and Hradec-Kyjovice Formations. Older Moravice Formation is characterized by dominance of pelitic sediments and occurrence of spaced cleavage and generally E vergent buckle folds. The upper part of the Moravice Formation and the Hradec-Kyjovice Formation underwent only weak anchimetamorphism (eg. Dvo~fik and Wolf, 1979) and relatively gentle deformation by east vergent folds and faults. Lower part of the formation represents over 1 km thick sandstone dominated sequence, while the upper half of it is mostly of fine grained laminated facies.
3 Magnetic fabric
The mean bulk susceptibility in all investigated folds is relatively low, in the lower part of the order of 10-4 [SI]. The AMS is carded mostly by phyllosilicates (Hrouda et al., 1993). ~ Locality of Star6 Ves u Bilovce (154). In this locality, a large fold is exposed. Its fiat limbs are very large, whereas the steep limb is short. The upper fold hinge is sharp (like in a kink fold), the lower one is curved. The oriented specimens were taken from three positions in the fold. One group of specimens was taken from the fiat fold limb (Group A), the second group from the curved part of the fold hinge (Group B), and the third group from the short and steep fold limb (Group C) (Fig. 2a). The degree of AMS is relatively low and the susceptibility ellipsoid shape
ranges from neutral to very oblate (Fig. 2b). The ellipsoids of the specimens taken from both the flat and steep limbs (A,C) are moderately to strongly oblate, while those of the specimens taken near the lower curved fold W
E
Fig. 2a Fold in the locality of Star~i Ves u Bilovce with sampling sites plotted.
hinge (B) range from neutral to moderately oblate. T
NJI54
1.0
0.5
0 !.00
! .135
~ 1.10
P
-0.5
-1.0 Fig. 2b Magnetic anisotropy P-T plot for the fold in the locality of Star~i Ves u Bilovce.
In the geographic coordinate system, the magnetic foliation poles are very near the bedding poles and create three distinct groups corresponding to the sample positions in the fold (Fig. 2c). The poles lying near the center of the projection net represent the specimens taken from the fiat, large fold limb. The poles plunging moderately NW represent the specimens taken in the vicinity of the curved lower fold hinge, and the poles plunging moderately SE represent the specimens taken from the short and relatively steep hinge. The orientations of magnetic lineations do not differ too much in the individual groups of specimens. In the palaeogeographic coordinate system (obtained through rotation of the bedding into its horizontal position about the bedding strike), all the magnetic foliation poles are near the center of the projection net and the magnetic lineations lie near the net circumference, not differing substantially between the above specimen groups (Fig.2d). The interpretation of the above magnetic fabric is as follows. The relatively low degree of AMS and oblate magnetic fabric correspond to those characteristic of the Hradec Kyjovice Formation showing predominantly sedimentary magnetic fabric (Hrouda, 1979). In addition, the virtual parallelism of the magnetic foliation to the bedding suggests that the magnetic fabric is in principle
N,I154~
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone ~ ~ , x m
507
GEO
A
B
Fig. 3a Fold in the locality of Jakub6ovice with sampling sites plotted.
The ellipsoids of the groups B,C are moderately to strongly oblate, while those of the groups A,C range from neutral to moderately oblate.
T 1.0
Fig. 2e Orientation of magnetic fabric and mesoseopic fabric elements in the fold in the locality of Starer Ves u Bilovce in geographical coordinate system. Equal-area projection on lower hemisphere. Legend: closed square - magnetic iineation, closed circle - magnetic foliation pole, open circle bedding pole.
NJ1S4
NJI53
.1
0.5
0 1.o0
PAl
1,05
l 1.1o
P
-0.5
-1.0 Fig. 3b Magnetic anisotropy P-T plot for the fold in the locality of Jakub~ovice. 8
In the geographic coordinate system, the magnetic foliation poles are near the bedding poles and create two groups according to the sample positions in the structure (Fig. 3c). N,1153
~
Fig. 2d Orientations of magnetic fabric elements in the fold in the locality of Stani Ves u Bilovce after tilt correction (after rotation of the bedding to horizontal position about the fold axis). Equal-area projection on lower hemisphere.
sedimentary in origin. During folding, the magnetic fabric mostly rotated as a rigid body, being strained intemaUy only very weakly. The only exception is the part near the lower curved hinge where the magnetic fabric is less flattened than in the other parts of the fold and may represent a superposition of the deformational magnetic fabric associated with the fold buckling on the sedimentary magnetic fabric.
Locality of Jakubdovice (153). The structure investigated in this locality is overthrust of a slab of beds over another bed slab (Fig 3a). The oriented specimens were sampled is such a way that a group of specimens came from the overthrusting slab (A), while the other group came from the slab being overthrusted (B,C).The degree of AMS is relatively low and the susceptibility ellipsoid shape ranges from neutral to very oblate (Fig. 3b).
GEO
\
/
/
/
Fig. 3e Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Jakub~ovice in geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
The poles lying very near the center of the projection net represent the specimens of the groups A,B. The poles plunging gently to moderately SE represent the specimens of the groups A,C. The orientations of magnetic lineations do not differ too much in the two groups of specimens. In
508
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
the palaeogeographic coordinate system, the magnetic foliation poles are relatively near the center of the projection net, only the specimens from the most inclined part of the overthrusting slab show remarkable delection. The magnetic lineations mostly lie near the net circumference, not differing substantially between the above specimen groups (Fig. 3d). NJ153
and well developed girdle, but these girdles are not identical, being considerably shifted each with respect to the other (Fig. 4c). The magnetic lineations are well W
............
E
......
PAl
Fig. 4a Fold in the locality of Hrab~vka with sampling sites plotted. NJ152
T
1.0
0.5 t •
0
1 1,0~
1.00 Fig. 3d Orientations of magnetic fabric elements in the fold in the locality of Jakub~oviee after tilt correction (after rotation of the bedding to horizontal position about the fold axis). Equal-area projection on lower hemisphere.
The interpretation of the above magnetic fabric is as follows. The relatively low degree of AMS and oblate magnetic fabric correspond to those characteristic of the Hradec - Kyjovice Formation showing predominantly sedimentary magnetic fabric. In addition, the virtual parallelism of the magnetic foliation to the bedding in the most specimens suggest that the magnetic fabric is in principle sedimentary in origin. During creation of the overthrusting structure, the magnetic fabric was mostly preserved, without being affected by observable internal strain. The only exception is the overthrusting slab where the magnetic fabric is less anisotropic and less flattened than in the other parts of the structure and may represent a superposition of the deformational magnetic fabric associated with overthrusting on the sedimentary magnetic fabric.
mm
•
I ml.lO
i 1.15
-0.5
-1.0 Fig. 4b. Magnetic anisotropy P-T plot for the fold in the locality of Hrabfivka.
NJ152
GEO
D# ab :
: ,ao: I ~
a
Locality of Hrab~vka (152). In this locality, a near hinge area of a fold is exposed. The oriented specimens were mostly taken along the fold curve (Fig. 4a).The degree of AMS varies widely, from very low values in some specimens to relatively high in the others. The shape of the susceptibility ellipsoid is also very variable, ranging from very prolate to moderately oblate (Fig. 4b). However, the variations both in the degree of AMS and in the shape of the susceptibility ellipsoid are independent of the position of the specimen in a fold. In the geographic coordinate system, the magnetic foliation poles create a very narrow and relatively well developed girdle. The bedding poles create also a narrow
Fig. 4e Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Hrabfivka in geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
concentrated, being approximately perpendicular to the girdles in magnetic foliation poles and bedding poles. In the palaeogeographic coordinate system, the magnetic lineations take more or less the same position, but the magnetic foliation poles split into two groups. One group is characterized by very good parallelism of the magnetic foliation to the bedding, while the second group comprises specimens in which the magnetic foliation is virtually perpendicular to the bedding (Fig. 4d). The specimens of
F. Hrouda
Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
et al.:
both groups do not depend on their positions in the fold. NIl52
509
between linear and planar, while that in the second exposure is moderately to strongly planar.
PAl T
NJ4
0.5 ~
t
Ira" "
I
~
illlei)
1.0
1/
"
I I'
•"
J 1.2
1.3
P
-0.5
-1.0
1
Fig. 4d Orientations of magnetic fabric elements in the fold in the locality of Hrabfivka after tilt correction (after rotation of the bedding to horizontal position about the fold axis). Equal-area projection on lower hemisphere.
The interpretation of the above magnetic fabric is not easy. Relatively high degree of AMS in some specimens and frequent prolate susceptibility ellipsoids and almost perpendicular magnetic foliations to the bedding in some specimens indicate doubtless deformational origin of the magnetic fabric in this locality. The magnetic fabric can be unfolded only in some specimens. Consequently, the folding was represented by buckling of the beds in principal, but this was accompanied by another ductile deformations which gave rise to the magnetic foliations almost perpendicular to the bedding.
Fig. 5b Magnetic anisotropy P-T plot for the fold in the locality o f Paseck~, ~Jeb Open symbols represent the specimens from the W limb, closed symbols represent those of the E limb.
In the NW exposure with bedding and cleavage, the magnetic lineations are well parallel to the bedding/ cleavage intersection lines and the magnetic foliations are very near the slaty cleavage. In the SE exposure with bedding parallel schistosity, the magnetic lineation is relatively badly defined, while the magnetic foliation is virtually parallel to the bedding parallel schistosity (Fig.5c). NJ4
GEO
Locality of Paseck~ ~leb (4). This locality consists of two partial exposures. In the NW exposure, bedding, and slaty cleavage can be observed, while the rocks of the SE exposure show only one foliation (very fissile bedding covered by recrystallized phyllosilicates - probably bedding parallel schistosity). Both the exposures are presumed to be parts of a fold whose hinge is not preserved (Fig.5a). The degree of AMS is moderate and the susceptibility ellipsoid shape ranges from neutral to very oblate (Fig. 5b).
NW
-" ~C~'s---~:: " \ ~ "-
i
i
I
SE
•
•
x
10 I
20 m r
Fig. 5a Fold in the locality of Paseck~ Zleb with sampling sites plotted.
The magnetic fabric in the first exposure is on the transition
/ Fig. 5c Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Paseck~, ~leb i n geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
In this locality, the formation of the magnetic fabric was evidently controlled by slaty cleavage generation rather than by folding. The slaty cleavage developed perpendicular to the maximum shortening direction, either during the last stage of folding or after folding. The magnetic fabrics in both exposures was formed by the same stress field, i.e. that forming the slaty cleavage. Owing to differently oriented mechanical anisotropies in both fold limbs, the external forces were differently distributed throughout the rocks in each limb. In the places where the angle between slaty cleavage and bedding was large the slaty cleavage cut the bedding, while in the places where this angle was small the
510
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
slaty cleavage developed more or less parallel to the bedding giving rise to the bedding parallel schistosity.
NJ23
~ . E, 00,~, O , 0 ~1 E
W
I
I
I
Fig. 6a Fold in the locality of Skrbovice with sampling sites plotted.
Fig. 6c Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Skrbovice in geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
Locality of Skrbovice (23). In this locality, a mildly curved fold in bedding was investigated showing very steep and well-developed slaty cleavage (Fig. 6a). The degree of AMS is moderate and the susceptibility ellipsoid shape ranges from slightly prolate to moderately oblate, not differing in both the fold limbs (Fig. 6b). T 1,0
N J23
,S
0.5
0
1.00
F
m~'
1.05
1.10L-~
/
i
I
1.15
1.20
P
-0.5
-1.0 Fig. 6b Magnetic anisotropy P-T plot for the fold in the locality of Skrboviee. Open symbols represent the specimens from the W limb, closed symbols represent those of the E limb.
The magnetic fabric is oriented homogeneously over the fold regardless of which limb the specimens under consideration come. The magnetic foliations are virtually parallel to the slaty cleavage showing no relationship to the bedding, the magnetic lineations are near the fold axis and the intersection lines between bedding and slaty cleavage (Fig. 6c). This fold can be classified as homogeneous fold from the point of view of the magnetic fabric. The magnetic fabric is evidently deformational in origin and was formed in association with the slaty cleavage formation rather than through folding.
4 Concluding remark The magnetic fabric in folds of the easternmost RhenoHercynian Zone varies considerably according to the metamorphic and deformation history of the formation in which the folds occur. In the easternmost Hradec - Kyjovice Formation which underwent only very weak anchimetamorphism and weak ductile deformation, the magnetic fabric in the folds can be partly or entirely unfolded by rotation about the fold axis: It is evident that the folds originated through buckling of the beds and the magnetic fabric is mostly older than the buckling. In the westermost Andrlsk~i Hora Formation, characterized by strong anchimetamorphism to epizonal metamorphism and occurrence of slaty cleavage, the magnetic fabric in folds is homogenous and cannot be unfolded. In these folds the magnetic fabric formation was controlled by slaty cleavage generation rather than by buckle folding. References DvorAk, J., and M. Wolf, 1979.Thermal metamorphism in the Moravian Palaeozoic (Sudeticum, (~.S.S.R), Neues Jahrb. Geol. Palaeontol. Monatsh., 1979, 596-607. Hrouda, F., 1978. The magnetic fabric in some folds, Phys. Earth Planet. lnter., 17, 89-97. Hrouda, F., 1979. The strain interpretation of magnetic anisotropy in rocks of the Nizk~, Jesenik Mountains (Czechoslovakia). Sb. Geol. V~d, UG, 16, 27-62. Hrouda, F., 1981. On the mechanism of the fold generation in the nizk2? Jesenik Mts. (in Czech). Vdst. (]st~. (]st. Geol., 56, 271-278. Hrouda, F., Pros, Z. and Wohlgemuth, J., 1993. Development of magnetic and elastic anisotropies in slates during progressive deformation. Phys. Earth Planet. Inter., 77, 1993, 251-265.
Pergamon PII: S 1464-1895(00)00078-8
Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone F. Hrouda 1'2, O. Krej~i 3 and J. Otava 3 1AGICO Inc., Brno, Czech Republic 2Institute of Petrology and Structural Geology, Charles University, Praha, Czech Republic 3Czech Geological Survey, Brno, Czech Republic Received 1 July 1999; accepted 18 October 1999
Abstract. The magnetic fabric in folds was investigated in the easternmost Rheno-Hercynian Zone, the Nizk~ Jesenik Mts. In their eastern areas, the rocks show signs of only weak anchimetamorphism and very gentle ductile deformation; SE vergent buckle folds of long wavelength are developed whose magnetic fabric can be easily unfolded geometrically. In the central areas, spaced cleavage and NW vergent buckle folds can be found; the folds can be unfolded mostly only partially. In the western areas, NW vergent cleavage folds and very well developed slaty cleavage occur. The magnetic fabric in the folds is homogeneous, the folds cannot be unfolded at all. The cleavage is transformed into metamorphic schistosity at the western border of the area. © 2000 Elsevier Science Ltd. All rights reserved.
senting the easternmost Rheno-Hercynian Zone is exposed in a NNE-SSW trending and northwards widening belt stretching from the surroundings of the town of Brno in the south to the Odra Lineament in the north (southern Poland). Culm Basin sediments are exposed in two main areas, southern Drahany Upland and northern Nizk~ Jesenik Mts., now separated by the Elbe Lineament. The sampled area comprises the Culm of the Nizl~ Jesenlk Mountains. The referred part of the basin is bounded to the
~]
Moravice Fm
1 Introduction Pre- and Post-Culm Fins
Magnetic anisotropy is a very good tool for investigating the origin of folds. First attempts to use this method in studying the origin of folds in the Rheno-Hercynian Zone on the territory of the Czech Republic were made by Hrouda (1978, 1981); the folds in the central and western part of the Nizl~ Jesenlk Mts. were investigated. Recently, folds from the easternmost areas were added. The purpose of the present paper is to show how the origin of the folds can differ according to metamorphic and deformation histories of the formation in which the folds occur.
~'.,".*"~
k.
ranlCe
Fig. 1 Geologicalschemeof the Nizk32Jesenik Mts. in the easternmost Rheno-HercynianZonewith plotted locationsof the folds investigated. west by Devonian volcano-sedimentary complex of the Vrbno Group metamorphosed in greenschist facies grade The studied sequence belongs to the foreland flysch basin (conglomerates, greywackes, siltstones, mudstones, shales and related rhythmites and laminites) of Vis6an to Lowermost Namurian age. The flysch sequence of over 7 km thick sediments was deposited in a deep marine
2 Geological Setting The Lower Carboniferous Moravian-Silesian Culm Basin located in the eastern part of the Czech republic and repreCorrespondence to: F. Hrouda
505
506
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
environment and comprises an axial turbidite system sourced mostly from the south of the basin. It is conformably overlain by paralic coal-bearing coastal plain deposits of Namurian to Westphalian age. The whole Palaeozoic area is bounded to the east by Tertiary and Mesozoic sediments of the Carpathian Foreland. The basin-fill was strongly deformed during the Mid- to Late Carboniferous Variscan Orogeny. Deformation primarily affected the western margin of the basin resulting in extensive faulting and folding, the development of penetrative cleavage and metamorphism. A progressive decrease in deformation and metamorphic grade is seen from the west to the east across the basin. The Cuim of the Nizl~ Jesenik Mts. can be very roughly subdivided into western and eastern parts. The two parts are approximately separated by an antiform belt of mostly Devonian to Lower Carboniferous volcano-sedimentary complex (~temberk-Homi Bene~ov Belt). The western Culm subbasin of Vis6an age include And61sk~i Hora and Horni Bene~ov Formations. The westernmost And61sk~ Hora Formation is composed mostly of shale rhythmites and laminites. Structurally it is characterized by NW vergent cleavage folds and a well developed slaty cleavage which is transformed into metamorphic schistosity at the western border of the Formation. Mostly subvertical slaty cleavage and cleavage folds are developed in the sandstone dominated Horni Bene~ov Formation. The eastern Culm subbasin is of Upper Vis6an age and it comprises Moravice and Hradec-Kyjovice Formations. Older Moravice Formation is characterized by dominance of pelitic sediments and occurrence of spaced cleavage and generally E vergent buckle folds. The upper part of the Moravice Formation and the Hradec-Kyjovice Formation underwent only weak anchimetamorphism (eg. Dvo~fik and Wolf, 1979) and relatively gentle deformation by east vergent folds and faults. Lower part of the formation represents over 1 km thick sandstone dominated sequence, while the upper half of it is mostly of fine grained laminated facies.
3 Magnetic fabric
The mean bulk susceptibility in all investigated folds is relatively low, in the lower part of the order of 10-4 [SI]. The AMS is carded mostly by phyllosilicates (Hrouda et al., 1993). ~ Locality of Star6 Ves u Bilovce (154). In this locality, a large fold is exposed. Its fiat limbs are very large, whereas the steep limb is short. The upper fold hinge is sharp (like in a kink fold), the lower one is curved. The oriented specimens were taken from three positions in the fold. One group of specimens was taken from the fiat fold limb (Group A), the second group from the curved part of the fold hinge (Group B), and the third group from the short and steep fold limb (Group C) (Fig. 2a). The degree of AMS is relatively low and the susceptibility ellipsoid shape
ranges from neutral to very oblate (Fig. 2b). The ellipsoids of the specimens taken from both the flat and steep limbs (A,C) are moderately to strongly oblate, while those of the specimens taken near the lower curved fold W
E
Fig. 2a Fold in the locality of Star~i Ves u Bilovce with sampling sites plotted.
hinge (B) range from neutral to moderately oblate. T
NJI54
1.0
0.5
0 !.00
! .135
~ 1.10
P
-0.5
-1.0 Fig. 2b Magnetic anisotropy P-T plot for the fold in the locality of Star~i Ves u Bilovce.
In the geographic coordinate system, the magnetic foliation poles are very near the bedding poles and create three distinct groups corresponding to the sample positions in the fold (Fig. 2c). The poles lying near the center of the projection net represent the specimens taken from the fiat, large fold limb. The poles plunging moderately NW represent the specimens taken in the vicinity of the curved lower fold hinge, and the poles plunging moderately SE represent the specimens taken from the short and relatively steep hinge. The orientations of magnetic lineations do not differ too much in the individual groups of specimens. In the palaeogeographic coordinate system (obtained through rotation of the bedding into its horizontal position about the bedding strike), all the magnetic foliation poles are near the center of the projection net and the magnetic lineations lie near the net circumference, not differing substantially between the above specimen groups (Fig.2d). The interpretation of the above magnetic fabric is as follows. The relatively low degree of AMS and oblate magnetic fabric correspond to those characteristic of the Hradec Kyjovice Formation showing predominantly sedimentary magnetic fabric (Hrouda, 1979). In addition, the virtual parallelism of the magnetic foliation to the bedding suggests that the magnetic fabric is in principle
N,I154~
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone ~ ~ , x m
507
GEO
A
B
Fig. 3a Fold in the locality of Jakub6ovice with sampling sites plotted.
The ellipsoids of the groups B,C are moderately to strongly oblate, while those of the groups A,C range from neutral to moderately oblate.
T 1.0
Fig. 2e Orientation of magnetic fabric and mesoseopic fabric elements in the fold in the locality of Starer Ves u Bilovce in geographical coordinate system. Equal-area projection on lower hemisphere. Legend: closed square - magnetic iineation, closed circle - magnetic foliation pole, open circle bedding pole.
NJ1S4
NJI53
.1
0.5
0 1.o0
PAl
1,05
l 1.1o
P
-0.5
-1.0 Fig. 3b Magnetic anisotropy P-T plot for the fold in the locality of Jakub~ovice. 8
In the geographic coordinate system, the magnetic foliation poles are near the bedding poles and create two groups according to the sample positions in the structure (Fig. 3c). N,1153
~
Fig. 2d Orientations of magnetic fabric elements in the fold in the locality of Stani Ves u Bilovce after tilt correction (after rotation of the bedding to horizontal position about the fold axis). Equal-area projection on lower hemisphere.
sedimentary in origin. During folding, the magnetic fabric mostly rotated as a rigid body, being strained intemaUy only very weakly. The only exception is the part near the lower curved hinge where the magnetic fabric is less flattened than in the other parts of the fold and may represent a superposition of the deformational magnetic fabric associated with the fold buckling on the sedimentary magnetic fabric.
Locality of Jakubdovice (153). The structure investigated in this locality is overthrust of a slab of beds over another bed slab (Fig 3a). The oriented specimens were sampled is such a way that a group of specimens came from the overthrusting slab (A), while the other group came from the slab being overthrusted (B,C).The degree of AMS is relatively low and the susceptibility ellipsoid shape ranges from neutral to very oblate (Fig. 3b).
GEO
\
/
/
/
Fig. 3e Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Jakub~ovice in geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
The poles lying very near the center of the projection net represent the specimens of the groups A,B. The poles plunging gently to moderately SE represent the specimens of the groups A,C. The orientations of magnetic lineations do not differ too much in the two groups of specimens. In
508
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
the palaeogeographic coordinate system, the magnetic foliation poles are relatively near the center of the projection net, only the specimens from the most inclined part of the overthrusting slab show remarkable delection. The magnetic lineations mostly lie near the net circumference, not differing substantially between the above specimen groups (Fig. 3d). NJ153
and well developed girdle, but these girdles are not identical, being considerably shifted each with respect to the other (Fig. 4c). The magnetic lineations are well W
............
E
......
PAl
Fig. 4a Fold in the locality of Hrab~vka with sampling sites plotted. NJ152
T
1.0
0.5 t •
0
1 1,0~
1.00 Fig. 3d Orientations of magnetic fabric elements in the fold in the locality of Jakub~oviee after tilt correction (after rotation of the bedding to horizontal position about the fold axis). Equal-area projection on lower hemisphere.
The interpretation of the above magnetic fabric is as follows. The relatively low degree of AMS and oblate magnetic fabric correspond to those characteristic of the Hradec - Kyjovice Formation showing predominantly sedimentary magnetic fabric. In addition, the virtual parallelism of the magnetic foliation to the bedding in the most specimens suggest that the magnetic fabric is in principle sedimentary in origin. During creation of the overthrusting structure, the magnetic fabric was mostly preserved, without being affected by observable internal strain. The only exception is the overthrusting slab where the magnetic fabric is less anisotropic and less flattened than in the other parts of the structure and may represent a superposition of the deformational magnetic fabric associated with overthrusting on the sedimentary magnetic fabric.
mm
•
I ml.lO
i 1.15
-0.5
-1.0 Fig. 4b. Magnetic anisotropy P-T plot for the fold in the locality of Hrabfivka.
NJ152
GEO
D# ab :
: ,ao: I ~
a
Locality of Hrab~vka (152). In this locality, a near hinge area of a fold is exposed. The oriented specimens were mostly taken along the fold curve (Fig. 4a).The degree of AMS varies widely, from very low values in some specimens to relatively high in the others. The shape of the susceptibility ellipsoid is also very variable, ranging from very prolate to moderately oblate (Fig. 4b). However, the variations both in the degree of AMS and in the shape of the susceptibility ellipsoid are independent of the position of the specimen in a fold. In the geographic coordinate system, the magnetic foliation poles create a very narrow and relatively well developed girdle. The bedding poles create also a narrow
Fig. 4e Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Hrabfivka in geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
concentrated, being approximately perpendicular to the girdles in magnetic foliation poles and bedding poles. In the palaeogeographic coordinate system, the magnetic lineations take more or less the same position, but the magnetic foliation poles split into two groups. One group is characterized by very good parallelism of the magnetic foliation to the bedding, while the second group comprises specimens in which the magnetic foliation is virtually perpendicular to the bedding (Fig. 4d). The specimens of
F. Hrouda
Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
et al.:
both groups do not depend on their positions in the fold. NIl52
509
between linear and planar, while that in the second exposure is moderately to strongly planar.
PAl T
NJ4
0.5 ~
t
Ira" "
I
~
illlei)
1.0
1/
"
I I'
•"
J 1.2
1.3
P
-0.5
-1.0
1
Fig. 4d Orientations of magnetic fabric elements in the fold in the locality of Hrabfivka after tilt correction (after rotation of the bedding to horizontal position about the fold axis). Equal-area projection on lower hemisphere.
The interpretation of the above magnetic fabric is not easy. Relatively high degree of AMS in some specimens and frequent prolate susceptibility ellipsoids and almost perpendicular magnetic foliations to the bedding in some specimens indicate doubtless deformational origin of the magnetic fabric in this locality. The magnetic fabric can be unfolded only in some specimens. Consequently, the folding was represented by buckling of the beds in principal, but this was accompanied by another ductile deformations which gave rise to the magnetic foliations almost perpendicular to the bedding.
Fig. 5b Magnetic anisotropy P-T plot for the fold in the locality o f Paseck~, ~Jeb Open symbols represent the specimens from the W limb, closed symbols represent those of the E limb.
In the NW exposure with bedding and cleavage, the magnetic lineations are well parallel to the bedding/ cleavage intersection lines and the magnetic foliations are very near the slaty cleavage. In the SE exposure with bedding parallel schistosity, the magnetic lineation is relatively badly defined, while the magnetic foliation is virtually parallel to the bedding parallel schistosity (Fig.5c). NJ4
GEO
Locality of Paseck~ ~leb (4). This locality consists of two partial exposures. In the NW exposure, bedding, and slaty cleavage can be observed, while the rocks of the SE exposure show only one foliation (very fissile bedding covered by recrystallized phyllosilicates - probably bedding parallel schistosity). Both the exposures are presumed to be parts of a fold whose hinge is not preserved (Fig.5a). The degree of AMS is moderate and the susceptibility ellipsoid shape ranges from neutral to very oblate (Fig. 5b).
NW
-" ~C~'s---~:: " \ ~ "-
i
i
I
SE
•
•
x
10 I
20 m r
Fig. 5a Fold in the locality of Paseck~ Zleb with sampling sites plotted.
The magnetic fabric in the first exposure is on the transition
/ Fig. 5c Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Paseck~, ~leb i n geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
In this locality, the formation of the magnetic fabric was evidently controlled by slaty cleavage generation rather than by folding. The slaty cleavage developed perpendicular to the maximum shortening direction, either during the last stage of folding or after folding. The magnetic fabrics in both exposures was formed by the same stress field, i.e. that forming the slaty cleavage. Owing to differently oriented mechanical anisotropies in both fold limbs, the external forces were differently distributed throughout the rocks in each limb. In the places where the angle between slaty cleavage and bedding was large the slaty cleavage cut the bedding, while in the places where this angle was small the
510
F. Hrouda et al.: Magnetic Fabric in Folds of the Easternmost Rheno-Hercynian Zone
slaty cleavage developed more or less parallel to the bedding giving rise to the bedding parallel schistosity.
NJ23
~ . E, 00,~, O , 0 ~1 E
W
I
I
I
Fig. 6a Fold in the locality of Skrbovice with sampling sites plotted.
Fig. 6c Orientation of magnetic fabric and mesoscopic fabric elements in the fold in the locality of Skrbovice in geographical coordinate system. Equal-area projection on lower hemisphere. For legend see Fig. 2c.
Locality of Skrbovice (23). In this locality, a mildly curved fold in bedding was investigated showing very steep and well-developed slaty cleavage (Fig. 6a). The degree of AMS is moderate and the susceptibility ellipsoid shape ranges from slightly prolate to moderately oblate, not differing in both the fold limbs (Fig. 6b). T 1,0
N J23
,S
0.5
0
1.00
F
m~'
1.05
1.10L-~
/
i
I
1.15
1.20
P
-0.5
-1.0 Fig. 6b Magnetic anisotropy P-T plot for the fold in the locality of Skrboviee. Open symbols represent the specimens from the W limb, closed symbols represent those of the E limb.
The magnetic fabric is oriented homogeneously over the fold regardless of which limb the specimens under consideration come. The magnetic foliations are virtually parallel to the slaty cleavage showing no relationship to the bedding, the magnetic lineations are near the fold axis and the intersection lines between bedding and slaty cleavage (Fig. 6c). This fold can be classified as homogeneous fold from the point of view of the magnetic fabric. The magnetic fabric is evidently deformational in origin and was formed in association with the slaty cleavage formation rather than through folding.
4 Concluding remark The magnetic fabric in folds of the easternmost RhenoHercynian Zone varies considerably according to the metamorphic and deformation history of the formation in which the folds occur. In the easternmost Hradec - Kyjovice Formation which underwent only very weak anchimetamorphism and weak ductile deformation, the magnetic fabric in the folds can be partly or entirely unfolded by rotation about the fold axis: It is evident that the folds originated through buckling of the beds and the magnetic fabric is mostly older than the buckling. In the westermost Andrlsk~i Hora Formation, characterized by strong anchimetamorphism to epizonal metamorphism and occurrence of slaty cleavage, the magnetic fabric in folds is homogenous and cannot be unfolded. In these folds the magnetic fabric formation was controlled by slaty cleavage generation rather than by buckle folding. References DvorAk, J., and M. Wolf, 1979.Thermal metamorphism in the Moravian Palaeozoic (Sudeticum, (~.S.S.R), Neues Jahrb. Geol. Palaeontol. Monatsh., 1979, 596-607. Hrouda, F., 1978. The magnetic fabric in some folds, Phys. Earth Planet. lnter., 17, 89-97. Hrouda, F., 1979. The strain interpretation of magnetic anisotropy in rocks of the Nizk~, Jesenik Mountains (Czechoslovakia). Sb. Geol. V~d, UG, 16, 27-62. Hrouda, F., 1981. On the mechanism of the fold generation in the nizk2? Jesenik Mts. (in Czech). Vdst. (]st~. (]st. Geol., 56, 271-278. Hrouda, F., Pros, Z. and Wohlgemuth, J., 1993. Development of magnetic and elastic anisotropies in slates during progressive deformation. Phys. Earth Planet. Inter., 77, 1993, 251-265.