International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 02 46
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Buckling of a Migmatized-Gneiss at Ago-
Sunmonu, Southwestern Nigeria. *Omosanya, K.O., Ajibade O.M*, & Akintola, A.I*, Adio. N*
*Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye, Ogun State.
E-mail: [email protected]
Abstract - The Migmatised-Gneiss outcrop at Ago Sunmonu
has undergone different phases of deformation, as evident by
the numerous folds found on it. Folds generally are formed
through three (3) basic mechanisms buckling, passive folding
and bending; this work was done to classify, and investigate
the mechanisms responsible for the formation of the folds
found in this rock exposure.
Biot’s buckling hypothesis was tested on four hundred and
ninety-eight different (498) folds and ten (10) fold groups
found in the study area; parameters measured include
wavelength (λ), thickness (t), Amplitude (A), and the
interlimb angle (i).
The folds in this rock were formed as a result of compression
of the foliations; they have high, intermediate, and low
competence contrasts, and were categorised as disharmonious
Gentle fold based on their interlimb angles and vergence
towards the N, NE, and NW direction; minor occurrences of
Open, Close and Tight folds were recorded.
The relationship between the measured parameters
corroborates results from Biot’s experiment and previous
workers. The buckling hypothesis thus accounts for the folds
found in the Migmatized Gneiss in Ago-sunmonu and
invariably provide impetus for investigating buckling of folds
in other mixed rocks (igneous and metamorphic).
Index Term-- Folds, Buckling, interlimb angle, competence
contrast.
I. INTRODUCTION
Folds are obvious structures, seen on satellite views of the
planet, in cliffs, in thin sections, rock exposures, at all
scales. They exist in those rocks with layers of contrasting
stiffness, through three principal mechanisms, buckling,
passive folding, and bending. Buckling accounts for
majority of folds found in Igneous, metamorphic and
Sedimentary rocks. Their occurrence is well documented in
stratified rocks as results from the application of
compressive stresses parallel to a competent layer (Twiss
and Moore, 2007). Most of folds found in the basement
complex of Nigeria exhibits mechanism similar to buckling,
it is pertinent to test the buckling hypothesis in order to
provide information on the evolution of these folds.
The study was carried out within the area specified by
70211N-70241N and 30411-30451E and emphasis was placed
on an outcrop exposure at Ago-Sunmonu which lies on
702311711N and 304214511. The aim of this study is to test the
buckling hypothesis for folds seen in a Migmatized Gneiss
outcrop in other to determine their mode of formation.
II. REGIONAL SETTING
The general geology of Nigerian have been studied by
various workers, Rahaman (1971), Oyawoye (1972), Cooray
(1972), Elueze (1981), Caby (1981), Dada (1998,1999);
Nigeria is covered nearly equal proportion by both
crystalline and sedimentary rock, The crystalline rocks are
divided into the Basement complex, the younger Granites
and Tertiary-recent volcanic, the basement complex is
distributed in (i) a triangular area in south western Nigeria,
(ii) a roughly circular area in North-central Nigeria and (iii)
a rectangular area broken up into 3 zones by sedimentary
rocks on the eastern border of Nigeria with Cameroon
Republic.
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Omi- Onigbagbo
Ago-Sunmonu
Elesa AremuAdeyemo
Omiyolo
Badipe
IjaiyeSch
Disp.
Stream/Drainage
Settlement
Contour Line
Vegetation Cover
Major Road
Footpath
Legend
3 45 E0 13 42 E
0 1
7 21 N0 1
7 24 N0 1
N
0 1 km Fig. 1. Topographical Map of the Study Area
The study area lies within the basement complex of south-
western Nigeria. Rocks previously described in this region
include A polycyclic Migmatized-Gneiss complex that is
characterised by grey foliated Biotite acid/Biotite
Hornblende quartz feldspathic gneiss of tonalitic to
granodioritic composition (Rahaman, 1981);Mafic to
ultramafic component which outcrops as discontinuous
boudinaged lenses or concordant sheet of amplibolites with
minor amount of biotite-rich ultramafite; and Felsic
component, a varied group comprised of pegmatite, aplite
quartz-oligoclase veins, fine-grained granite gneiss, and
porphyritic granite.
The outcrop exposure at Ago Sunmonu covers a length of
~1880cm from N-S and 1222cm from E-W, with an altitude
of 197.4m. The area of study is located in southwestern
Nigeria, some few meters to Bakatari. Regionally, the
geology of the area belongs to the basement complex of
Nigeria, the area is characterised by metamorphic rocks
which include Migmatized Gneiss, Banded Gneiss and
Porphyroblastic Gneiss. The MGn are restricted to the N and
NNW part of the study area, this rock unit are slightly
foliated, displaying a medium degree of metamorphism with
minerals such as Quartz, feldspar, Hornblende, Biotite. The
strike of these rock units exhibit a NW-SE trend except in
exposure along the main road some 350 meters SE of
Bakatari that trend in the NE-SW direction. The BGn in the
study are strongly deformed, the foliation in the rock are
conspicuously folded which attest to the degree of
deformation recorded in the rocks, the minerals in this rock
are not different from those of the MGn. Apart from the
strongly folded foliation on these rocks, the amount of
extension of the rock is represented by the number joints
found in this rock. This unit has undergone episodes of
compression and extension has revealed by the structural
features identified in the rock. The PGn is from a
porphyritic Granite protolith, with metamorphism
consequently causing alignment of the feldspar phenocryst
found in this rock. These rock units belong to the
Migmatized-Gneiss complex of Nigeria that is thought to be
Achaean (3.5Ga) in age (Dada, 1989). Like other Achaean
rocks elsewhere the Banded Gneiss occur as a window in
the Porphyroblastic Gneiss (Fig.2)
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Ago-Sunmonu (Ago-Tente)
Aremu
Om i-Onigbabo
Om iyolo
540
240
200
160
320
640
220
Mgn
Pgn
Bgn
Pgn
Bgn
Mgn
3 45 E0 1
3 42 E0 1
7 21 N0 1
7 24 N0 1
A
B
N
Pgn
A(SW) B(NE)BgnPgn Pgn Mgn
Geological Cross sect ion along line ABLegend
Pgn - Porphyroblastic Gneiss
Bgn - Banded Gneiss
Mgn- Migmatized Gneiss
Geological Boundary
Settlement
Strike & Dip Values
Not to Sca le
Fig. 2. Geological Map of the Study Area
The area is densely vegetated with an average annual
temperature of 32°C / 89.6°F. The relief is undulating from
the Northern end to the south; a common stream regionally
drains the area.
III. METHODS
Initial study of rock types was done by carrying out a
detailed geological mapping of the entire region within the
coordinate specified by Map 1 through traversing and
positioning with the aid of a GPS. This preliminary study
was done in order to understand the geology of the area and
the regional structural pattern.
The foliations found in the rock of the study area have been
folded, parameter measured on the fold include thickness of
the folds, designated as t, the wavelength i.e. distance
between successive crests, λ, the vergence of the fold, the
interlimb angle, i, and the amplitude, A, all measurements
taken in centimetres (cm).These parameters were measured
on four hundred and Ninety Eight (498) folds that were
grouped into ten (10) basic fold sets/groups from North to
South of the outcrop. A fold set may contain one or more
folds that run from the east to the west of the outcrop. The
folds were labelled G1a, G3a which means, the fold is the
first fold in Group of 1 or 3. Consequently, description of
each fold set was done from the east to west of the outcrop,
and west to east for the next fold after.
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Fig. 3. Parameters measured on the fold include i- interlimb angle, t-thickness, λ-wavelength, V-vergence and A, Amplitude.
IV. DATA ANALYSIS, RESULT AND
DISCUSSION
Parameter measured were averaged across each fold
set/group and the average values were used in testing the
different hypothesis put forward by Biot for buckling
Mechanism. This hypothesis was experimentally
determined by Biot in 1957 for a single layer in his work
“Folding instability of a layered viscoelastic medium ....”
and later modified in 1968 in “Experiments of buckling of
multilayers which permit interlayer” for multilayering of
rocks.
λ = 2π t [(μ1 /6 μ2)1/3] where μ1 > μ2.
Where μ1 / μ2 is the ratio of viscosities of the competent
layer to the incompetent layers, λ is the wavelength of the
fold that grows most rapidly, frozen in as the arc length,
W,of the mature fold; t is thickness. This equation assumes
that gravity and inertia are unimportant, the layer is thin, the
fold has low amplitude, and there is Newtonian viscosity
and Plane Strain. The ration of μL/μM can be used to
determine the competence contrast between layers; ratio >
50 implies high competence contrast, while values < 10
describes low competence contrast. Values between 10 and
49 were described as intermediate competence contrast,
though this term has not been previously used, it would not
be out of order to use it in this study. The interlimb angle of
the folds was measured to characterise the folds using table
I.
TABLE I
TIGHTNESS OF FOLDING (MODIFIED AFTER FLEUTY, 1964)
Descriptive terms Folding angle Interlimb angle
Acute
Gentle 0<Φ<60 180>i>120
Open 60<Φ<110 120>i>70
Close 110<Φ<150 70>i>30
Tight 150<Φ<180 30>i>120
Isoclinal Φ =180 i= 0
Other modification to this classification may include
description of folds based on interlimb angle as: A Gentle
(1800- 1350), Open (1350-900), close (900-400), Tight (400-
10) and Isoclinal folds (0).
Biot predicts that the ratio λ/t has preferred value which is
constant for any given pair of lithologies under similar
conditions, in this instance, the λ/t ratio is relatively constant
across each fold profile (Table III), by profile we mean the
E-W or W-E occurrence of a fold on the outcrop, since the
folds occur along foliations, we assume that the petrological
units along the foliation reflects a single mineral
assemblage, which makes it plausible to conclude that the
relatively constant value of the ratio is a function of the
homogeneous mineralogical composition along the foliation.
On the average the entire fold set exhibit a λ/t ratio of 2-19.
Folds created by buckling are periodic, this characteristic is
displayed by folds seen on this rock exposure, most of the
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folds are periodic, they are recurring and are characterised
by similar shape from one point to the other. Folds die out
rapidly away from the competent layer, most of the
pronounced buckle folds are seen at the centre of the field,
while majority of the fold are sinusoid at the tip. This
observation entails that shortening is maximum at the
centre and by Biot's prediction the shortening is
perpendicular to the axial surfaces.
The thickness of the competent layer is approximately
equal along a profile but unequal on the average; this factor
may not be unrelated to the crystalline nature of the rock.
A plot of average thickness against wavelength exhibits a
near-linear relationship and positive correlation (Table IV
and Figure 5a&b), this agrees with Biot’s prediction that
wavelength is directly proportional to thickness of the
competent layer. Though, the correlation on individual
basis is fair to poor, there were strong correlation in fold
group G6a, G6b,G9a, G9b; averaging these parameters per
group the relationship exhibits a strong correlation value of
0.8. These strong deviation may be attributed to error due
to measurement, parallax and the difference in competence
contrast of each of the fold group. Overall, most of the
folds exhibits fair correlation of 0.5 across most of the
group.
Using competence contrast, the folds exhibit values
between <10 - >50 and were characterised as low,
intermediate, and high competence contrasts layers. The
terms intermediate connotes values within the range of 10-
50; this could be a function of the mineralogical
composition of the rock which is thought to vary from one
part of the outcrop to the other because of the mixed nature
of the rock.
These folds are thought to have evolved from compression
of the different layers as shown by the variation in the
competence contrast recorded in the rocks.
TABLE II
THE AVERAGE VALUE FOR ALL THE PARAMETERS MEASURED ON EACH FOLD SET/GROUP.
FOLD
NAME t (cm) λ(Cm)
Amp
(Cm) λ/t µL/µM
Competence
i FT
1a 17.2 69.3 18.9 4.0 1.6 L 164.0 GENTLE
1b 2.0 41.9 3.0 20.8 216.9 H 163.3 GENTLE
1c 2.1 21.0 4.6 9.8 23.0 I 158.2 GENTLE
2a 6.8 27.2 9.4 4.0 1.6 L 130.0 GENTLE
2b 3.9 15.6 6.5 4.0 1.5 L 134.0 GENTLE
2c 3.7 13.2 8.3 3.6 1.1 L 150.0 GENTLE
3a 14.9 66.6 18.5 4.5 2.1 L 148.2 GENTLE
3b 1.1 31.6 4.1 28.3 549.2 H 136.2 GENTLE
3c 1.2 28.9 3.7 24.4 349.7 H 148.5 GENTLE
4a 17.6 73.0 27.4 4.2 0.7 L 136.7 GENTLE
4b 3.3 20.5 7.4 6.3 1.3 L 126.7 GENTLE
4c 3.6 21.5 6.2 6.0 5.8 L 104.7 OPEN
4d 4.2 17.3 5.1 4.1 ~ O 118.8 OPEN
5a 1.7 20.2 6.4 16.8 273.7 H 120.6 OPEN
5b 2.0 33.1 6.5 19.0 102.6 H 122.1 GENTLE
5c 2.4 21.9 6.2 13.0 143.6 H 123.1 GENTLE
5d 2.8 24.3 6.6 12.1 109.1 H 123.8 GENTLE
6a 4.1 25.8 9.3 7.2 14.6 I 127.0 GENTLE
6b 4.2 32.3 6.5 12.2 111.4 H 141.7 GENTLE
6c 4.1 20.5 8.4 12.7 233.7 H 137.7 GENTLE
6d 4.6 30.2 9.8 8.7 116.6 H 130.3 GENTLE
7 20.1 34.8 27.5 1.9 0.2 L 149.8 GENTLE
8a 3.2 25.5 5.9 10.4 157.6 H 145.0 GENTLE
8b 9.5 26.6 10.3 5.3 40.5 I 154.6 GENTLE
8c 9.0 30.0 7.1 4.3 5.6 L 136.4 GENTLE
9a 16.1 59.4 40.9 4.0 3.2 L 138.0 GENTLE
9b 1.1 21.2 4.6 21.3 391.4 H 132.8 GENTLE
9c 11.6 51.2 17.6 4.9 7.9 L 119.6 OPEN
10a 7.2 36.3 8.0 5.7 9.2 L 137.7 GENTLE
10b 10.3 65.9 5.0 6.9 11.4 I 151.9 GENTLE
10c 8.2 41.1 3.7 6.0 10.0 L 171.6 GENTLE
10d 5.6 42.4 7.2 9.8 47.1 I 140.9 GENTLE
H-High competence contrast, L- low competence contrast, I- intermediate competence contrast
TABLE III
PARAMETERS MEASURED FOR EACH OF TEN (10) FOLDS IN G1A
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S/N FOLD NAME t (cm) λ(Cm) Amp (Cm) λ/t
1 G1a 15 53.5 20.5 4
2 19.5 72 17.4 4
3 17 60 13.4 4
4 19 110 15.5 6
5 19 79 18.2 4
6 20 66.7 17.3 3
7 19 85 24.7 4
8 10.5 62 17.5 6
9 16.4 62.7 25.2 4
10 16.5 42.2 16.5 3
AVERAGE 17.19 69.31 18.86 4
Fig. 4. Average Thickness Variation along each of the Fold group
Fig. 5a. Average Thickness Against Wavelength per group.
TABLE IV CORRELATION BETWEEN THICKNESS AND WAVELENGTH IN EACH OF THE FOLD GROUP
R² = 0.5728
1.0
10.0
100.0
1.0 10.0 100.0
Thic
kne
ss (
cm)
Wavelength (cm)
Average Wavelength VsThickness
Linear (AverageWavelength Vs Thickness)
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Fold Group R2 R Correlation Parameter
G1 0.221 0.5 Fair
G1b 0.292 0.5 Fair
G1c 0.057 0.2 Poor
G2a 0.075 0.3 Poor
G2b 0.239 0.5 Fair
G2c 0.95 1.0 Poor
G3a 0.06 0.2 Poor
G3b 0.071 0.3 Poor
G3c 0.266 0.5 Fair
G4a 0.114 0.3 Poor
G4b 0.005 0.1 Poor
G4c 0.006 0.1 Poor
G4d 0.025 0.2 Poor
G5a 0.1 0.3 Poor
G5b 0.165 0.4 Poor
G5c 0.025 0.2 Poor
G5d 0.063 0.3 Poor
G6a 0.321 0.6 Fair-Strong
G6b 0.518 0.7 Strong
G6c 0.152 0.4 Poor
G6d 0.217 0.5 Fair
G7 0.286 0.5 Fair
G8a 0.072 0.3 Poor
G8b 0.013 0.1 Poor
G8c 0 0.0 No correlation
G9a 0.456 0.7 Strong
G9b 0.402 0.6 Fair-Strong
G9c 0.01 0.1 Poor
G10a 0.002 0.0 No correlation
G10b 0.042 0.2 Poor
G10c 0.034 0.2 Poor
G10d 0.235 0.5 Fair
Average thickness and
wavelength per group 0.572 0.8 Strong
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Fig. 5b. Correlation between Thickness and Wavelength within each of the fold group.
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Fig. 6. (a-d) Growth/development of G4 folds from a sinusoid to being rounded. (e)A thin foliation characterised by symmetrical fold on top of a larger
folded foliation characterised by asymmetrical limbs (f).Folding of a Quartzo-feldspathic intrusion with its large amplitude.
In most places the shape of the inner arc of the fold was not
different from the outer arc; this behaviour contradicts
Biot’s prediction that the inner arc and outer arc will display
different shape. However, this assumption may work better
on stratified rocks. An initial development of the fold begins
with a sinusoid shape and later develops into rounded from
one tip of the fold to the other (Fig: 6).
The interlimb angle of the folds were measured to
characterise the folds(table II), the dominant fold type in
this area are Gentle folds, which are verging in the N, NE,
and NW direction, with minor occurrences of Open, Close
and Tight folds recorded. Because these folds were verging
in different direction it signifies a disharmony among
them, because they occur at different sizes/scales, they
grew at the same time, with different thickness and
competence, and the fold could be described as parasitic
folds (after Ramberg, 1963). Apart from harmony and
competence contrast of the rock, most of the folds display
both symmetrical and asymmetrical limbs (fig.6)
V. CONCLUSION
The Biot’s hypothesis works well for the mixed rock at
the study area except in one instance when the
hypothesis failed to account for the harmonious
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behaviour of inner arc and outer arc of the folds. This
folds developed form initial sinusoid and later became
rounded, the size of the foliation controls the Amplitude
of the folds, as thin foliation displays low amplitude
while larger foliation have high amplitude. From this
study, it has been established that the Buckling
hypothesis can be applied to both stratified rocks and
mixed rock (Igneous and Metamorphic) and that these
folds were formed through buckling which is evident
from the difference in competence contrast across the
outcrop. Similar Migmatized-gneisses found in outcrops
around Bakatari, iloko, Mokorode, and Imala around
Alabata, all within the radius of the study area show the
same pattern of folding and invariably are thought to
have developed through the same mechanism.
ACKNOWLEDGEMENT
We acknowledge the assistance of the 2006/2007 300
level students in gathering the data during the field
exercise, and the department of Earth Sciences Olabisi
Onabanjo University, Ago-Iwoye during the mapping
exercise.
REFERENCES [1] Biot, M. A.,1968, Experiments of buckling of multilayers
which permit interlayer .Tectonophysics Volume 5, Issue 2,
Pages 89-105
[2] Caby, Betrand, J.M.L and Black, R. (1981)., Pan-African ocean closure and continental collision in the Hoggar-Iforas
segment. Central Sahara, In Precambrian Plate Tectonics,
edited by A. Kroner, Elsevier Amsterdam, 407- 434. [3] Cooray,P.G. (1972)., A note on the Charnockites of the Ado-
ekiti area, western state, Nigeria in Geology of Africa, edited
by T.F.J Dessauvagie and A.J whiteman, pp 45-54, University Ibadan.
[4] Dada, S.S (1989): Evolution de la Croute continentale au
Nord Nigeria: apport de la Geochemie, de la geochronology U-Pb et des traceurs isotpiques Sr, Nd et Pb. These de l’
Universite de Montpelier 11, Montpelier, France.
[5] Dada, S.S (1998): Crust Forming ages and Proterozoic Crustal evolution in Nigeria: A reappraisal of current
interpretation. Precambrian Research 87. pp 65-74
[6] Dada, S.S (1999): Pb-Pb and Sm-Nd isotope study of metaigneous rocks of Kaduna region: implications for
Archaean Crustal development in Northern Nigeria. Global
Journal of Pure and applied sciences Vol. 6 No. 7 [7] Elueze, A.A (1981), Dynamic metamorphism and Oxidation
of amphibolites of Tegina area, northwest Nigeria.
Precambrian Res., 14, 379-388. [8] Fleuty, M.J. 1964. The description of folds. Proc. Geol.
Assoc. 75: 461-492.
[9] Oyawoye, M.O (1972): The Basement complex of Nigeria, In African Geology Ibadan 1970 eds Dessauvagie and
whiteman. Geol. Dept Univ. Ibadan, Nigeria p. 67-99
[10] Rahaman, M.A., (1971): Classification of rocks in the Nigeria Precambrian Basement complex. Paper read at
Annual conference of Nigerian Mining geological and
Metallurgical society Dec. 1971. Kaduna [11] Rahaman, M.A (1981): Recent advances in the study of the
basement complex of Nigeria. First symposium on the
Precambrian Geology of Nigeria, Summary. [12] Ramberg, H. (1963): Fluid dynamics of viscous folding.
Amer. Assoc.Petrol. Geol. Bull. 47: 484-515.
[13] Twiss, R.J and Moores, E.M., (2007) Structural Geology, 2nd edition, Freeman and Company Newyork, 364,
About the Authors Omosanya, K.O, M.Sc Structural Geology with Geophysics (Leeds), B.Sc Geology (OOU), a Lecturer in the Department of Earth Sciences,
Olabisi Onabanjo University. He is a Structural Geologist, Basin analyst,
and a Seismic interpreter, His research interests include field and/
laboratory-based project with emphasis on structural studies of
sedimentary, metamorphic, and igneous rocks.
Adio, N. A, B. Sc Geology (OOU), a graduate of Geology from Olabisi
Onabanjo University. He is currently studying for his postgraduate degree at The University of Aberdeen, United Kingdom.
Ajibade, O.M, M.Sc Applied Geochemsitry (UI), B.Tech Applied Geology (FUTA), a Lecturer in the Department of Earth Sciences,
Olabisi Onabanjo University. He is a petrologist and a Geochemist with
over 10 years experience in the academics. His research interest covers Geochemistry, Environmental Geology and Hydrogeochemistry.
A.I. Akintola, holds an M.Sc. in Mineral Exploration with Option in
Economic and Mining Geology (UI) and a B.Sc. Geology (Ogun). He is
a Lecturer in the Department of Earth Sciences, Olabisi Onabanjo
University. His research interests include mineral exploration and
environmental ecology. He is a specialist in the study of granitic
intrusion especially pegmatites.