Buckling of a Migmatized-Gneiss at Ago- Sunmonu ... 11 I 02/113602-2727 IJBAS-IJENS.pdfOther modification to this classification may include description of folds based on interlimb

International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 02 46

113602-2727 IJBAS-IJENS © April 2011 IJENS I J E N S

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.



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)



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.



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



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



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)



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



Fig. 5b. Correlation between Thickness and Wavelength within each of the fold group.



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



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.

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Buckling of a Migmatized-Gneiss at Ago- Sunmonu ... 11 I 02/113602-2727 IJBAS-IJENS.pdfOther modification to this classification may include description of folds based on interlimb