Structural mapping adjacent to the 'Woman-in-White amphibolite' in the Olary Domain, South Australia.
Ashley David Taylor B.Sc.
Department of Geology and Geophysics The University of Adelaide
This thesis is submitted as partial fulfillment for the Honours Degree of Bachelor of Science
November 1999
Australian National Grid Reference (SI 54 - 2)
ABSTRACT
A structural study of Palaeoproterozoic Willyama Supergroup rocks to the southeast of Old Boolcoomata approximately 20 kilometres north of Olary, South Australia, discloses a complex history of deformation. This includes an axial planar S1
schistosity and several intersecting locally developed subsequent planar fabrics. The main findings support previous unpublished company studies. Structural maps were produced at various scales in areas surrounding the ‘Woman-in-White’ amphibolite and all available observations were used to form a chronology of events and tectonic model for the geometric and kinematic evolution of the area.
In a domain east of the 'Woman-in-White' amphibolite the S1 is parallel to the axial plane of a major isoclinal synform closing to the east. S2 is axial planar to tight to open class 2 and class 1c F2 folds that trend generally north to northeast. Regionally, and particularly in the vicinity of the 'Woman-in-White' amphibolite, a third
deformation is very intensely developed generating two fabrics. The S3 schistosity is the axial planar fabric to tight to isoclinal F3 folds trending consistently east-west.
The S3 fabric is also expressed as a crenulation of the S1 regional schistosity. These pre-Adelaidean structural elements are recognised as comprising the Olarian Deformation.
Fold interference is present on all scales. Olarian deformation events two and three have given the flat lying western limb of the principal F1 synform a luniform, dome and basin morphology. Type 2 and type 3 interference patterns are the most common in the area mapped. The occurrence of the two interference patterns is due to the variable angle between OD2 and OD3 compressions, which is commonly
approximately 40° in the west-southwest part of the mapped area.
This work conforms closely in complexity to previous regional studies and has been supplemented by other new investigations of an important northeast-southwest trending shear zone corresponding to OD3, lying further to the north, and a geochemical investigation of the 'Woman-in-White' amphibolite indicating its probable mantle origin and possible emplacement before all deformations occurred.
TABLE OF CONTENTS
CHAPTER 1 - Introduction
1.1 Introduction
1.2 Polyphase Deformation 1.3 Aims of Study
1.4 Methods
CHAPTER 2 - Regional Geology
2.1 Geological Setting 2.2 Stratigraphy
2.2.1 Willyama Supergroup 2.2.2 Adelaidean Supergroup 2.3 Deformation
2.3 Metamorphism
CHAPTER 3 - Lithological variation and aerial distribution within the area surrounding the 'Woman-in-White
amphibolite.'
3.1 Introduction 3.2 Metasediments 3.3 Intrusive rocks
CHAPTER 4 - Structural Analysis
4.1 Introduction
4.2 First Deformation (OD1) 4.3 Second Deformation (OD2) 4.4 Third Deformation (OD3)
4.5 Fourth and Fifth Deformations (DD4 and DD5) 4.6 Discussion
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1 2 2 3
4
4 5 5 7 7 8
9
9 9 15
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17 17 18 19 20 20
CHAPTER 5 - Description of Fold Interference
5.1 Introduction
5.2 Review of Polyphase Folding 5.3 Small Scale Interference 5.4 Mesoscopic Fold Interference 5.5 Large Scale Interference 5.6 Discussion
CHAPTER 6 - Tectonic Model
6.1. Introduction
6.2. Tectonic Model for the 'Woman-in-White' Area 6.3. Delamerian Analogue
6.4. Discussion
CHAPTER 7 – Conclusion Acknowledgments
References
Apendices
1. Location map for samples, photos, gridmaps and cross sections.
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21 21 25 25 26 27
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28 28 29 29
30 31 32 34
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List of Figures
Chapter 1
Figure 1.1 Location of the field area within the Willyama Complex.
Chapter 2
Figure 2.1 Interpreted correlation of the Willyama Supergroup sequence in the Olary Domain with the Broken Hill Domain (Ashley et al., 1998).
Figure 2.2 Metamorphic grade and timing of the major deformational events for the Olary Domain (Flint and Parker, 1993).
Figure 2.3 Metamorphic assemblages through the Olary and Broken Hill Domains based on porphyroblasts (Clarke et al., 1986).
Chapter 3
Figure 3.1 Geological map of the 'Woman-in-White' area, Olary Domain.
Figure 3.2
(a) Muscovite-sillimanite schist with graphitic interbeds unit in thin section. It shows a fabric defined by biotite and muscovite and recrystallised quartz ribbons.
(b) Hinge schist thin section showing a dominant fabric defined by muscovite and minor biotite with weak crenulations.
(c) Bimba Formation showing a strong fabric defined by heavy minerals. The outcrop generally is white (calcareous) however the rock is red to yellow when broken.
(d) Thin section of the Bimba Formation showing the stronger fabric (defined by heavy minerals and biotite) cross-cut by a weaker fabric (defined by magnetite, muscovite, biotite and quartz ribbons).
(e) Outcrop of the calcsilicate unit showing strong laminations with dark red and white layering.
(f) Laminated feldspar-biotite rock outcropping in a creek in the interference zone (subarea B). The fresh rock is grey in colour.
(g) Psammopelitic gneiss with interbedded migmatite showing intense folding. The rock commonly has an interbedded appearance with psammitic and pelitic layers along with migmatite.
(h) Thin section of the albitite unit showing a compositional layering, magnetite-quartz- feldspar, that has been recrystallised.
Chapter 4
Cross section 1 Cross section 2
Figure 4.1 Sub areas with bedding(So) and schistosity (S1) data.
Figure 4.2 Analysis of F2 folds found in a sample of psammopelitic gneiss with interbedded migmatite.
Figure 4.3 Subareas with L2 lineation data.
Figure 4.4
(a) Open F2 folding in Psammopelitic gneiss with interbedded migmatite unit, plunging east in the interference zone (subarea B).
(b) Tight F3 fold in Psammopelitic gneiss with interbedded migmatite, plunging east and verging to the north.
(c) Sample containing a small F3 fold and shearing. The plunge of the fold is defined by the rodding lineation of the migmatite around the rim of the sample.
(d) Fold isogon analysis of the fold from (c). It shows the fold is class 2 (Ramsay and Huber, 1987) with parallel isogons.
(e) Thin section from the limb of the fold in (c). It shows the F3 axial planar fabric cross cutting the stronger S1 schistosity.
(f) F3 crenulations in the hinge schist unit plunging east at approximately 20 degrees. Lens cap is approx. 4cm in diameter.
(g) S-C mylonite from the MacDonald Shear Zone.
Figure 4.5 Subareas with L2 lineation data.
Figure 4.6 Deformed lineation loci: lineation geometry arising during folding of an intially planar surface which contained an initially rectilinear structure. X, Y, and Z indicate the principle strain axes of the superposed homogeneous strain, a is the slip direction of simple shear in the shear fold and a’ the modified direction after imposing a homogeneous strain.
(Ramsay and Huber, 1987)
Chapter 5
Figure 5.1 (a) The four principle types of three-dimensional fold forms arising by the superposition of shear folds on pre-existing fold forms.
(b) Summary of the main types of two-dimensional interference patterns resulting from the horizontal sectioning of the three-dimensional forms.
The type numbers are shown in the top left of each box (Ramsay and Huber, 1987).
Figure 5.2 Photos showing various fold interference patterns.
Figure 5.3 Three dimensional expanded view of two boundaries from the layered sample.
Figure 5.4 Series of maps and three dimensional analyisis of an area that was grid-mapped (30 x 40m).
Figure 5.5 Fold axial traces of major F1, F2 and F3 folds.
Chapter 6
Figure 6.1 Tectonic model.
Figure 6.2 Tectonic model in 3-D.
