Deformation of Pillowed and Massive Metabasalts Iin The

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TH 1844 DEFORMATION OF PILLOWED AND MASSIVE METABASALTS IIN THE EARLY PROTEROZOIC CAPE SMITH TECTONIC BELT, NEW QUEBEC, CANADA DEr-CRMATION OF PILLOWED AND MASSVE MPTAPASALTS iN THE. EARLY PROTPRO73IC CAPE SMITH e"- CI 4 ;at— NEW QUEBEC; cANAnA

PAUL BUDKEWITSCii

M.Sc. 1990 -4, r8v-1

Deformation of pillowed and massive metabasalts in the Early Proterozoic Cape Smith Tectonic Belt, New Québec, Canada

Paul Budkewitsch

A Thesis submitted in conformity with the requirements for the Degree of Master of Science in the University of Toronto

The Fox Lake area, studied in this thesis, is a 30 km2 area near the geometric centre of the Cape Smith Tectonic Belt, New Québec. The rocks exposed in the area are predominantly metavolcanics of the Chukotat Group. This work is mainly concerned with (1) the description of primary features in metavolcanics and of their deformation, and (2) the detailed geological and structural description of the map area.

Undeformed columnar joint polygons (basalt polygons) from the Giant's

Causeway, Northern Ireland, viewed along paleohorizontal sections were analyzed and proved to have an essentially isotropic crack pattern. Using the shape parameters calculated from a simple geometric model of basalt polygons, a new model is proposed for the growth of columnar structures. The VOPONUCE (Voronoi polygons nucleated on the centroid) model is a geometric model of the evolution of the crack pattern as a cooling front propagates inward into the basalt sheet. The rules of the model crudely mimic how the joint face separating two columns would propagate as cooling proceeds.

The VOPONUCE model seems to describe well the evolution of the polygonal pattern as it advances.

Basalt polygons from deformed basalt flows are shown to be reliable and useful quantitative sectional strain gauges Basalt polygons from the Chukotat Group were documented to be deformed, yielding sectional strain ratios, R, .in the range of 1.21 to 2.42.

(2) pillow selvages, (3) pillow-shelves, (4) columnar jointing, (5) sheet joints and

(6) felsic varioles. The high strain zones are interpreted as faults splaying off a major décollement at the base of the Chukotat Group. Detailed structural mapping of folds and stratigraphic discordances was possible in deformed metavolcanics by measuring the orientations of primary structures that mark the paleohorizontal.

The structural history of the Fox Lake area can be described by two nearly coaxial folding events, with fold axes plunging gently east or west, parallel to the trend of the Belt. Minor crenulations developed transverse to the earlier folds are also recognized. The effects of folding is best observed in the highly deformed

Povungnituk Group which underlies less deformed Chukotat rocks. Thrust faults of several generations have predominantly southward directions of transport. An early imbrication of thrust slices at the base of the Chukotat, stacks a previously formed syncline of metavolcanics. This folded structure, called the Fox Lake syncline, is further tightened by the second folding event and is also truncated to the north by late, high angle thrust faults. ACKNOWLEDGEMENTS

I would like to express my great appreciation to Dr. Pierre-Yves Robin, my thesis supervisor, for numerous discussions (scientific and otherwise) and for his thorough and insightful questioning of ideas that arose during this thesis. I benefitted greatly from Pierre- Yves' suggestions and criticisms, however, any errors in this thesis are entirely my own. The brief, but intensive three days of field consultations we had in the early summer of

1986 in Ungava taught me valuable techniques that quantitative measurements of strain requires in the field. I also learned that if one needs a strain indicator and searches hard enough, one can usually can discover one.

During academic residence at the University of Toronto, I gratefully acknowledge financial support received from a University of Toronto Open Fellowship (1986-1987), W.W. Moorehouse Scholarship (1986) and an Ontario Graduate Scholarship (1987-1988). Additional support was provided by NSERC operating grants to Dr. Pierre-Yves Robin. A study of deformation in the metavolcanics of the Cape Smith Tectonic Belt

(Fosse de L'Ungava) was originally conceived by Daniel Lamothe of the Ministère de l'Énergie et des Ressources du Québec. The field work for this project was entirely supported by the Ministère de l'Énergie et des Ressources through a contract awarded to the writer and I am indebted to the MER for the opportunity I had to examine this problem. I would also like to express my sincerest gratitude to Daniel Lamothe, for this project would not have been possible without his support and interest. Field excursions in the Cape Smith

Tectonic Belt in during the 1986 and 1987 field seasons and discussions with Daniel Lamothe and James Moorhead (MER) were very instructive. Paul Arscott (Concordia) was a competent and energetic field assistant, sometimes under difficult conditions, and I thank him for his good spirited company and for keeping the foxes at large. Je remerci

également tous les géologues et les assistants au lac Lemming pour l'esprit chaleureux pendant l'été de 1986 et 1987. iv

Enlightening and beneficial discussions on computer applications, strain analysis, structural geology and volcanology were particularly important, and I thank Dr. Paul Clifford (McMaster), Dr. Henry Halls, Dr. Bill Pearce, Dr. Pierre-Yves Robin, Dr. Fried

Schwerdtner, Dr. Bob Stesky, John Stix (St. George) and the participants of the Spring 1988 GLG2101H graduate course. Chapter 2 of this thesis grew out of a term paper for Bob Stesky's graduate course, GLG2104H. Chapter 4 was originally a rough manuscript which benefitted considerably from constructive reviews in GLG2101H.

I honestly enjoyed and learned a great deal from numerous discussions and by osmosis, being surrounded by a helpful and enthusiastic group in the lab, at lunch or out for dinner and a beer: thanks Dave Ball, Bill Barclay, Dr. Martin Bates (PDF), Louise

Corriveau (St. George), Paula Mackinnon (McMaster), Matt Manson, Dr. Barb Murck,

Sebastian Pfleiderer, Lynn Pryer, Bill Shanks and Marty Van Kranendonk (St. George). Frank Fueten introduced me to computer-aided draughting and other computer applications in geology. I thank Frank and Bill Pearce for their tips and frequent help, as they were always willing to interrupt their own work in order to debug problems I would inevitably encounter alone on the PCs. Mr. Steve Jaunzems (Erindale Media Services) is thanked for expertly reproducing the photographs for the thesis and many slides of artwork used for various oral presentations related to this work. My friend of many years, Glen Newton (Waterloo) is acknowledged for pointed out to me the similarities of Smalley's (1965) model for basalt polygons to the construction of

Voronoi diagrams. In particular I would like to extend my sincere thanks to Martin, Matt,

Glen (and Marika) and Sebastian for their generous hospitality in Ontario during the 'late stages' of this thesis while I was living in Montréal.

Diane Joyal, my fiancée, deserves a special thank you (and more) for enduring my idiosyncrasies and tardiness, nevertheless supportive of my pursuits.

Software: In house, PC based computer programs developed by the Tectonic Studies Group at the Erindale Campus and Quattro were used to produce the contoured stereograms, circular histogram and calculate the strain analysis results presented in this thesis. Several figures and the geological map were prepared using AutoCAD release 10 and the text was processed on WordPerfect 5.0.

Table of Contents

Chapter 1: INTRODUCTION PURPOSE 1-1 OUTLINE 1-1 LOCATION OF STUDY AREA 1-2 REGIONAL GEOLOGY OF THE CAPE SMITH TECTONIC BELT 1-3

Chapter 2: SHAPE ANALYSIS OF POLYGONAL OUTLINES FROM COLUMNAR JOINTS AND A NEW MODEL FOR THEIR DEVELOPMENT ABSTRACT 2-1 INTRODUCTION 2-2 MODELS OF COLUMNAR JOINT POLYGONS 2-3 The regular hexagonal model 2-4 The Voronoi polygon model 2-5 SHAPE ANALYSIS OF POLYGONAL MOSAICS 2-6 Quantitative results using the inertia tensor method 2-7 1. The Voronoi polygon model 2-8 2. Basalt polygons of the Giant's Causeway 2-8 PREVIOUS INVESTIGATIONS OF COLUMNAR STRUCTURES AND POLYGONAL PATTERNS 2-9 THE VOPONUCE MODEL: A FEED-BACK MODEL FOR THE INCREMENTAL CRACK PROPAGATION PATH OF COLUMNATED STRUCTURES 2-11 Static columnar development 2-12 Evolving columnar structures 2-13 DISCUSSION 2-14 CONCLUSIONS - 2-16 APPENDIX 2.1: Construction of the isotropic Voronoi polygon model 2-32 APPENDIX 2.2: Difficulties in applying statistical tests to geometric models 2-35

Chapter 3: STRAIN ANALYSIS OF POLYGONAL OUTLINES FROM COLUMNAR JOINTED BASALT FLOWS OF THE CHUKOTAT GROUP ABSTRACT 3-1 INTRODUCTION 3-1 PRACTICAL STRAIN ANALYSIS OF BASALT POLYGONS: AN EVALUATION 3-2 1. Basic assumptions 3-3 2. Implementation of the inertia tensor method 3-3 3. Effect of sample size 3-4 An example from the Fox Lake area 3-5 COMPARISON OF STRAIN METHODS ON BASALT POLYGONS 3-6 The inertia tensor method (Robin, submitted) 3-7 The method of diameter ratios (Robin 1977) 3-7 Centre distribution methods (e.g. Fry 1979) 3-8 CONCLUSIONS 3-8

Chapter 4: RECOGNITION OF TECTONIC STRAIN INTENSITY IN GREENSCHIST FACIES MASSIVE AND PILLOWED BASALTS OF THE CHUKOTAT GROUP ABSTRACT 4-1 INTRODUCTION 4-1 Semi-quantitative subdivisions of strain 4-2 PILLOWED BASALTS 4-3 1. Primary shapes of pillow outlines 4-3 2. Pillow selvages 4-4 3. Pillow-shelves 4-5 DEFORMATION OF PILLOWED BASALT FLOWS 4-7 1. Pillow outlines 4-8 2. Pillow selvages 4-9 3. Pillow-shelves 4-9 vii

COLUMNAR BASALT STRUCTURES 4-10 1. Colonnade structures 4-12 2. Entablature structures 4-13 DEFORMATION OF COLUMNAR STRUCTURES 4-14 Compatibility of paleohorizontal and paleovertical markers 4-14 SHEET JOINTS AND THEIR DEFORMATION 4-15 PIPE AMYGDALES 4-16 FELSIC VARIOLES AND THEIR DEFORMATION 4-17 THE POSSIBLE ORIGINS OF "LAYERING" IN SILLS AND FLOWS 4-18 DISCUSSION 4-19 DISTRIBUTION OF STRAIN FABRICS IN THE FOX LAKE AREA 4-19 CONCLUSIONS 4-20

Chapter 5: GEOLOGY OF THE FOX LAKE AREA ABSTRACT 5-1 INTRODUCTION 5-2 THE CHUKOTAT GROUP 5-2 THE POVUNGNITUK GROUP 5-3 THE POVUNGNITUK - CHUKOTAT DISCORDANCE 5-4 STRUCTURAL EVOLUTION OF THE FOX LAKE AREA 5-5 Chukotat deformation 5-6 D, Structures 5-7 D2 Structures 5-8 Povungnituk deformation 5-9 D, Structures 5-9 D2 Structures 5-9 D, Structures 5-11 CONCLUSIONS 5-11 APPENDIX 5.1 (map of outcrop locations examined) 5-26 APPENDIX 5.2: Paleo-flow directions in the Chukotat Group, Fox Lake area 5-26

References 7p viii

List of Figures

Figure 1.1 1-8 Figure 1.2 1-9

Figure 2.1 2-19 Figure 2.2 2-20 Figure 2.3 2-21 Figure 2.4a 2-22 Figure 2.4b 2-22 Figure 2.5 2-23 Figure 2.6 2-24 Figure 2.7 2-25 Figure 2.8 2-26 Figure 2.9a 2-27 Figure 2.9b 2-27 Figure 2.10 2-28 Figure 2.11 2-28 Figure 2.12 2-29 Figure 2A.la 2-34 Figure 2A. lb 2-34

Figure 3.1a 3-11 Figure 3.1b 3-11 Figure 3.2 3-12 Figure 3.3 3-13 Figure 3.4 3-14

Figure 4.1 4-22 Figure 4.2 4-27 Figure 4.3 4-28 Figure 4.4 4-28 Figure 4.5a 4-29 Figure 4.5b 4-29 Figure 4.6 4-30 Figure 4.7 4-31 Figure 4.8 4-30 IC

Figure 4.9 4-32 Figure 4.10a 4-33 Figure 4.10b 4-33 Figure 4.11 4-34 Figure 4.12a 4-35 Figure 4.12b 4-35 Figure 4.13 4-34 Figure 4.14 4-36 Figure 4.15 4-36 Figure 4.16 4-37 Figure 4.17 4-38

Figure 5.1 5-15 Figure 5.2 5-15 Figure 5.3 5-16 Figure 5.4 5-16 Figure 5.5 5-17 Figure 5.6 5-17 Figure 5.7 5-18 Figure 5.8 5-18 Figure 5.9 5-19 Figure 5.10 5-19 Figure 5.11 5-20 Figure 5.12 5-20 Figure 5.13 5-21 Figure 5.14 5-21 Figure 5.15 5-22 Figure 5.16 5-22 Figure 5.17 5-23 Figure 5.18 5-23 Figure 5.19 5-24 Figure 5.20 5-24 Figure 5.21 5-25 Figure 5.22 5-25 Figure 5A.2a 5-28 Figure 5A.2b 5-29 Figure 5A.2c 5-30 x

List of Tables

Table 2.1 2-31 Table 2.2 2-31 Table 2.3 2-31

Table 3.1 3-15

Separates

Figure 5A.1

Geological map of the Fox Lake area

* * * Chapter 1: INTRODUCTION

PURPOSE

This thesis reports on a study of the deformation of metavolcanic rocks of the

Chukotat Group in a 30 km2 area of the Early Proterozoic Cape Smith Tectonic Belt.

Structural mapping of the Chukotat Group metavolcanics is often difficult due to the lack of bedding planes from sedimentary sequences, absent in the study area.

However, deformation elements such as strain markers, tectonic veins, and slickensides on minor slip-surfaces, contributed significantly towards the interpretation of the geological map. Recognition of primary volcanic structures enabled paleovenical and paleohorizontal markers to be identified as well. Indicators of strain in the metavolcanics are described and used, notably pillows and columnar joint polygons, for identifying zones of high strain. The Cape Smith Tectonic Belt provides excellent three-dimensional exposures of these primary structures and of how they respond when strained.

Another part of the thesis deals with columnar jointing in basalts. A geometric model, the VOPONUCE model, for the evolving polygonal crack pattern is described.

This model explains slight adjustments observed in the pattern as viewed along successive sections perpendicular to the column axis.

OUTLINE

The geology and tectonic history of the Cape Smith Tectonic Belt is reviewed in the present chapter. Introduction... 1-2

For quantitative investigations of deformation, an understanding of the initial geometry of strain markers is required. One strain gauge for quantitative estimates identified in the field was well developed columnar jointed basalts. In Chapter 2, the geometrical properties from an undeformed example of columnar joint polygons is characterized and a new model (VOPONUCE model) for the formation of columnar joints in cooling basalt flows proposed.

Chapter 3 presents the results of strain analysis from columnar joints in basalt flows from the Chukotat Group in the study area. The method of diameter ratios

(Robin 1977) was carried out directly from outcrops while the inertia tensor method

(Robin, submitted) and the method of Fry (1979) was performed on an orientated photograph of columnar joint polygons.

In addition to the columnar jointing in the metabasalts, other primary volcanic structures in massive and pillowed basalts are present in the Chukotat rocks. Chapter 4 assesses their potential as paleohorizontal or paleovertical markers for geological mapping. Qualitative observations of deformation affecting the volcanic primary structures are described, with emphasis given to the different style of deformation encountered in several varieties of pillowed basalt forms.

Chapter 5 presents an interpretation of the geology of the area examined and of its structural evolution, based on the data of strain fabrics and strain markers.

LOCATION OF STUDY AREA

The area of study lies near the geometric centre of the Cape Smith Tectonic

Belt, about 90 km south of the village of Salliait and about 10 km ENE of Chukotat

Lake (Figure 1.1). The area is approximately 7 x 4 km and will be referred" to as the Introduction... 1-3

Fox Lake area. The Fox Lake area lies entirely within the Chukotat Lake Region mapped by Moorhead (1986a, 1989) at a scale of 1:50 000 (Figure 1.2), contained on the topographic sheet 35G/5-EST, Lac Chukotat.

The Fox Lake area was chosen because of the local structural complexity and the outcrop density is favourable (15 to 20% in the Chukotat Group and 5-10% in the

Povungnituk Group) for detailed study of this problem. This area also hosts the only location on Moorhead's (1986a) map where the base of the Chukotat Group is exposed, overlying the Povungnituk Group (Moorhead 1986, pers. comm.).

Two months of field work were carried out during the summer of 1986. The camp was established in a valley, on a sandy knoll, about 1 km west of Fox Lake as a fly-camp from the main Lac Lemming camp of the Ministère de l'Énergie et des

Ressources. Field equipment was transported to the site by helicopter and provisions from Lemming Lake about every 10 days. The field project was entirely supported by the Ministère de l'Énergie et des Ressources as part of their 1/50 000 scale mapping objective of the western part of the Cape Smith Tectonic Belt (la Fosse de l' Ungava).

Subsequent verifications were permitted in the study area during a four day period while employed by the Ministère de l'Énergie et des Ressources in 1987.

REGIONAL GEOLOGY OF THE CAPE SMITH TECTONIC BELT

The Cape Smith Tectonic Belt is a 350-km long orogenic belt that is exposed across the northern part of the Ungava peninsula in a WSW direction. Together with the Belcher Belt and the Labrador Trough, it forms part of the Circum-Superior Belt

(Baragar & Scoates 1981). The rock assemblage of the Belt has been divided into the

Povungnituk, Chukotat, Watts and Parent tectonostratigraphic Groups (e.g. Bergeron Introduction... 1-4

1959, Lamothe et al. 1984, Lamothe 1988, pers.comm.). Hudsonian deformation led to an overall southward thrusting and folding of the supracrustal Belt rocks.

The southernmost, Povungnituk Group has been divided into two Sub-groups by

Lamothe et al. (1984): the southern Lamarche Group and the northern Beauparlant

Group. The Lamarche consists largely of a thick, imbricated metasedimentary sequence overlying, in fault contact, the Archean granite - granodiorite basement or a few metres of autochthonous arkosic conglomeratic sandstone and iron formation (Hoffman 1985,

Moorhead 1986b, St-Onge et al. 1986). The upper part of the Beauparlant Subgroup is dominated by continental-rift related tholeiitic volcanics, interbedded with various volcaniclastic deposits and shallow water clastics or intraformational breccias (Moorhead

1986b).

The central, Chukotat Group occupies the core of the Proterozoic trough, overlying the Povungnituk Group to the South in thrust fault contact and being truncated along its northern limit by the Bergeron Fault. Internally, it is divided into a series of North dipping homoclinal blocks consisting almost entirely of mafic metavolcanics and related intrusives (Hynes & Francis 1982). In their petrogenetic study, Francis et al. (1981) were able to identify three main basaltic lava types in the

Chukotat Group: a) olivine-phyric basalt, b) pyroxene-phyric basalt, and c) plagioclase- phyric basalt based upon their compositional variations. Each of the three types of basalts can be distinguished in the field by several criteria, the most reliable being the proportion and assemblage of phenocrysts in the chilled margins of basalt pillows

(Francis et al. 1981, Hynes & Francis 1982). No similar field criteria are known for discriminating among the massive flow varieties however. The olivine-phyric variety are typically komatiitic basalts while the plagioclase-phyric basalts are comparable to

MORB tholeiites (Francis et al. 1981). The pyroxene-phyric basalts are transitional Introduction... 1-5 between the olivine and plagioclase-phyric basalts and also have a tholeiitic affinity

(Francis et al. 1981). Evidence, in the form of a Proterozoic ophiolite suite, suggests that this Group may represent the formation of true oceanic crust (St-Onge et al. 1988).

North of the Bergeron Fault lies the Watts and Parent Groups (Lamothe 1988, pers. comm., Lamothe et al. 1984), and they are possibly the least understood of all the tectonostratigraphic units; consisting of metamorphosed volcanics and limited sedimentary units respectively, with a wide compositional range of intrusives.

Significant amounts of ultramafic to mafic and intermediate intrusive rocks, consanguineous to both the Povungnituk and Chukotat Groups been recognized throughout the entire Belt (e.g. Gélinas 1962, Lamothe et al. 1984, Hervet 1986,

Tremblay 1986, St-Onge et al. 1987). The ultramafic sills are often layered and have

been known for some time to be rich in Ni-Cu sulfides and, more recently, to be enriched in platinum group elements as well (e.g. Barnes et al. 1982, Giovenazzo 1985.

1986). A third late- and post-tectonic Narsajuaq phase of alkaline rich stocks also

intrude the Watts Group (Hervet 1986).

Perhaps the most controversial aspect of the Cape Smith Tectonic Belt has been

its overall tectonic evolution. Wilson (1968) first proposed that the linear belt could be the site of an ancient continent - continent collision. The exact location of the geosuture has long been considered to lie rooted beneath the supracrustals (Dimroth et al. 1970, Baragar & Scoates 1981, Hynes & Francis 1982). However, an Archean

basement can be traced completely around the eastern end of the Belt (Schimann 1978).

Incorporating the geological and geophysical data of the Ungava Peninsula, Hoffman

(1985) reinterpreted seemingly contradictory data and suggested that the actual geosuture

lies to the North of the belt. In his mode), the supracrustal rocks of the Cape Smith Introduction... 1-6

Tectonic Belt are the obducted remnants of the collisional event, preserved as a synformal klippe in the foreland.

Recent age determinations given by Parrish (1989) provided an age of 1998 ±2

Ma (U-Pb, zircon) for the Purtiniq ophiolite in the Watts Group. The Povungnituk and

Chukotat sequences gave an age of rhyolite volcanism as 1960 ±3 Ma (U-Pb, zircon) for the timing of continental rifting whose subsequent fill deposits are intruded by 1922

+9/-8 Ma (U-Pb. baddeleyite) mafic-ultramafic sills (Parrish 1989). Late stage granite

plutons range in age from 1900 to 1840 Ma cross cut splay faults and movement along the sole thrust of the Belt also continued to post-date these stocks (Parrish 1989).

These new data should impose some constraints on possible tectonic models for the

evolution of the Cape Smith Tectonic Belt.

The metamorphic history and structural build up of the orogen have been the subject of recent detailed investigations by St-Onge and co-workers (1988). On a

regional scale, metamorphic grade increases northward from lower greenschist to amphibolite facies, but isograds also curve around the eastern end of the Belt (Gélinas

1962, Westra 1978, Schimann 1978, St-Onge et al. 1986). In brief, the structure is the

result of two main Belt parallel folding events and southward thrust faulting, followed

by a third phase of NNW cross folding, confined to the eastern part of the Belt (Hynes

& Francis 1982, St-Onge et al. 1986). The majority of structures are south-vergent and

the Povungnituk Group is most strongly affected by the deformation. The style of

deformation in the Chukotat is characterized by several very long (in the order of 100

km) layer-parallel to subparallel thrust faults which extend along the entire length of the

Belt. Folded structures are relatively uncommon features in the Chukotat Group. Introduction... l-7

Figure 1.1 1987 geological compilation of the Cape Smith Tectonic Belt, illustrating the major lithological units, major faults and important mineral occurrences. Rectangle outlines the Chukotat Lake area of Figure 1.2. This map first appeared in Lamothe (1986) and has been updated since (document de promotion 87-03, Ministère de L'Énergie et des Ressources, Québec). 1-8

Figure 1.2 Simplified geological map of the Chukotat Lake area highlighting three main lithotectonic groups and the location of major fault and fold trends. The study area (Fox Lake area) covers the Povungnituk - Chukotat contact. The basal Chukotat thrust sheet is exposed in oblique section and numerous fold structures are present in the Povungnituk (after Moorhead 1986a). 1-9 Gouvernement du Québec Ministère de rÉnergie et des Ressources Direction générale de l'Exploration géologique et minérale

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APHÉBIEN

ROCHES INTRUSIVES GROUPE DE CHUKOTAT Basalte coussiné, un peu de roches sédimentaires PHASE NARSAJUAQ (post - tectonique) Gabbro. diorite AKULIVI j/ GROUPE DE POVUNGNITUK Sous -Groupe de Beauparlant Granite, granodiorite • Pyroclastite PHASES CHUKOTAT ET POVUNGNITUK (pré - ou syntectonique) Phyllade, quartzite, calcaire, dolomie Filons-couches mafiques et / ou ullramafiques différenciés Gîtes minéraux et indices significatifs Basalte massif ou coussiné PHASE POVUNGNITUK ( pré- ou syntectonique) Gabbro, diorite Sous -Groupe de Lamarche • Ni-Cu, Cu-Ni Phyllade, quartzite, dolomie, formation de fer Hornblendite • Ni-Cu-Pd-Pt ■ ® Zn-Pb-Ag Granodiorite, granite ARCHÉEN ® Zn-Ag-Pt

♦ Pt-Pd GROUPE DE WATTS Granodiorite gneissique ou folié. paragneiss, paraschiste. amphibolite A Amiante Métavolcanites non subdivisées Q Au Métabasalte - Faille de chevauchement D 1 60°00' • Stéatite 60° 00'

Métapyroclastite - Faille de chevauchement D 2

0 15 Km Roches métasédimentaires Faille de type non défini 78°00' 72°00'

Pro 87-03 Compilé par D. Lamothe, 1987 Géologie et minéralisations de la Fosse de l'Ungava Figure 1.1

Introduction... 1-9

Location map: CHLIKOTAT LAKE AREA (after Moorhead, 1988 ) 61°30' •x •

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0 ARCHEAN 0 POVUNGNITUK CHUKOTAT ea WATTS Figure 1.2 Chapter 2: SHAPE ANALYSIS OF POLYGONAL OUTLINES FROM COLUMNAR ,JOINTS AND A NEW MODEL FOR THEIR DEVELOPMENT.

ABSTRACT

Undeformed columnar joint polygons, referred to as basalt polygons, from the

Giant's Causeway (Northern Ireland), were analyzed and proved to have an isotropic crack pattern to within 2.5%. Comparisons between the topological properties of regular and irregular Voronoi polygons with actual columnar basalts viewed along paleohorizontal sections, indicate that a irregular polygons are more representative of true basalt polygons.

Using shape parameters calculated from model basalt polygons, such as (1) their anisotropy, the location of their (2) Voronoi centres and (3) centroids, a new model is proposed for the growth of columnar structures. The VOPONUCE (Voronoi polygons nucleated on the centroid) model is a geometric model of the evolution of the crack pattern as a cooling front propagates inward into the basalt sheet. The rules of the model crudely mimic how the joint face separating two columns would propagate as cooling proceeds. Successive transverse sections across a specific column would therefore display a series of evolving polygons. The VOPONUCE model seems to describe well the evolution of the polygonal pattern of columnar jointed basalts. Columnar jointing...

INTRODUCTION

Polygonal crack networks have been observed from columnar basalts by naturalists for several centuries. Early ideas focused on ideal, regular polygons to explain the two-dimensional pattern of joints.

Columnar structures in rocks form a network of long, slender joint planes which are all subparallel to a common direction (the axis of the column) and are therefore interconnected along their edges such that all intersections are subparallel. Transverse sections perpendicular to the axis of the column reveal the true width of each joint surface and true angles of intersection between meeting joints. In this section, the joints form a network of definite line segments that outline a mosaic of interlocking polygons.

The prismatic fabric that results can be found in a variety of rock types from quartz arenites (Moreno & Latorre 1982) to tuffaceous volcanics (Boyd 1961) but are best developed in basalt flows (e.g. Tomkeieff 1940, Spry 1962, BelIon et al. 1985).

The intriguing morphology and geometrical regularity of the columnar structures have inspired such names as the Giant's Causeway (Northern Ireland) and the Devil's

Postpile (California) to some well known localities. Tomkeieff (1940) reviewed the controversial and fascinating historical perceptions concerning the phenomena of columnar jointing. Columnated basalt flows typically consist of two or more 'tiers' of joints. A tier is a laterally extensive interval characterized by a particular style of columnar jointing distinct from those of the overlying or underlying levels (Figure 2.1).

A more complete description and nomenclature of these structures can be found in

Chapter 4 .

More recently, Ryan & Sammis (19786, b) explained the origin of banded striae or "chisel marks" (Iddings 1886, James 1920) across column faces as a product of Columnar jointing... 2-3 incremental crack advance due to cyclic release of tensile stress during cooling. They

interpreted smooth and subsequent rough zones across each band as the result of crack initiation and propagation through brittle material followed by crack arrest in hotter, more ductile lava. From detailed examination of surface features on the transverse

bands, DeGraff & Aydin (1987) described the kinematics of crack nucleation and crack propagation. In addition, they addressed the problem of crack-crack interactions at triple junctions, a key issue for understanding the polygonal evolution in the columnar jointing process.

The purpose of this chapter is: (1) to demonstrate that Voronoi polygons centered on random close-packed points in a plane, as originally suggested by Smalley

(1965), do account well for the major geometrical properties of columnar joints.

(2) to propose a model, called the VOPONUCE model, which reproduces the expected evolution and maturation of a polygonal columnar joint pattern as it is understood to evolve from an initial crack pattern which might develop at the upper or lower contacts of a cooling lava flow.

MODELS OF COLUMNAR JOINT POLYGONS

Iddings (1886) introduced the hexagonal model, later popularized by Holmes

(1965). This model interpreted the jointing as a honeycomb-like arrangement approximating regular hexagons. As thoughts on the subject progressed, a descriptive approach to the irregular geometry of the pattern emerged. An alternative model, referred to here as the Voronoi polygon model, was originally proposed by Smalley

(1965) to account for the irregularity of the bisalt polygons. Columnar jointing... 2-4

This chapter discusses only the topology of columnar jointing as it appears in two-dimensional sections, perpendicular to the column axis. The two models of polygonal cracking, the hexagonal model and the Voronoi polygon model, are purely geometrical, although based on certain principles of crack formation. As the lava cools and solidifies from the upper and lower surfaces of the flow, uniform thermal contraction of the solidifying lava occurs in planar isotherms parallel to the flow contacts. Uniform contraction within isothermal planes can be accomplished through localized shrinkage towards imaginary 'contraction centres' distributed over the entire surface. Lines that link neighbouring centres define the directions of maximum local tensile stress. Cracks will likely occur along perpendicular bisectors to these lines to relieve the accumulating stress most effectively. As the isotherms migrate into the thickness of the lava, concomitant incremental propagation of the preestablished crack network imparts the familiar columnar fabric to the solidifying basalt.

The essential difference between the two models lies in the criteria used to determine the spatial distribution of contraction points. Uniform shrinking in a plane can be achieved by ordering the location of the imaginary system of contraction centres according to an ideal regular closed-packed arrangement. This is described below in the hexagonal model. In the Voronoi polygon model, however, the distribution of contraction centres is only approximately closed-packed; instead there is an anticlustered, random distribution of centres within the plane.

The regular hexagon model

The preponderance of 6-sided polygons in columnar basalt led Iddings (1886) and later Holmes (1965) to adopt a model of Uniform thermal contraction in a plane towards equally spaced contraction centres. The model imposes a close-packed plane Columnar jointing... 2-5 arrangement of contraction. At contact points between circles, cracks occur at a tangent and meet at triple junctions forming 120° angles (Figure 2.2). The resultant regular hexagonal network of fractures releases the tensile stress most efficiently per unit crack length (Price 1968, p.160).

The actual topology of basalt columns in cross-section (see Figure 2.3, discussed

below) is not adequately described by a simple hexagonal pattern. Several authors have described the pattern exhibited by transverse sections of columns as an intimate arrangement of 3 to 8 sided interlocking polygons. Beard (1959) presented data to suggest that a similar distribution of 3 to 8 sided prismatic structures are encountered at several different lava flows worldwide. Data from Beard (1959) and other sources are shown in Figure 2.4a. Data collected from three sites of columnar jointed basalts in the

Chukotat were also found to have similar distributions of polygon type (Figure 2.4b).

Figure 2.4 shows that hexagonal polygons account for only about half the basalt

polygons present, an observation which is commonly ignored in the literature and many textbooks of structural geology (e.g. Billings 1972, p. 172, Ryan 1974, Hobbs et al.

1976, p. 295, Getis & Boots 1978, p.9, Ryan & Sammis 1978b, Reiter et al. 1987).

The Voronoi polygon model

In light of Beard's observations, Smalley (1965) proposed an alternative model

that recognized the role of crack nucleation and growth. He argued that in cooling lava

flows, it is unlikely that many cracks forming independently would interconnect into a

regular hexagonal pattern. It would seem more plausible that some cracks would

release local tensile stress around themselves, thus restricting the possible locus for

subsequent cracks. His model calls for a random close-packed plane of contraction 1

Columnar jointing... 2-6 nuclei instead of a close packed array. The resultant pattern (Figure 2.5) is a assemblage of contiguous, irregular polygons.

A mathematical derivation for boundaries had already been formulated by

Voronoi (1908) for defining spatial relationships between neighbouring points. Ahuja

(1982) describes in detail the areal properties of Voronoi polygons (also called Thiessen polygons) from point clusters that divide up a Euclidean surface. In order to compare the geometrical properties of actual basalt polygons and Voronoi polygons, a sample pattern of 30 isotropic Voronoi polygons was genereated. Selecting the locations of points for an isotropic distribution is arrived at in the present paper by using a modified version of Smalley's (1965) recommendations. We refer to this method for generating an isotropic pattern of irregular polygons as the Voronoi polygon model for columnar joints. The rules for generating this model of columnar joint outlines by Voronoi tessellation is given in Appendix 2.1.

SHAPE ANALYSIS OF POLYGONAL MOSAICS

O'Reilly (1879) was aware of the problem of simply extracting numerical data from basalt columns without the benefit of their spatial arrangement by stating "Indeed without these drawings the angular measurements could be but imperfectly utilised for the discussion of the problem." Table 2.1 presents a summary of the topological properties of the two crack models and an actual transverse section of columnar structures at the Giant's Causeway (Figure 2.3).

By inspection, the polygonal mosaic generated with Voronoi polygons shown in

Figure 2.5, is strikingly similar to the columnar joint pattern of Figure 2.3. The

Voronoi polygon model does not suffer from the geometrical limitations of the Columnar jointing... 2-7

hexagonal version. The irregular polygons that result may have an elongatedness and

variable number of sides. Overall, the topological properties of the Voronoi polygon

model appears to reflect adequately the characteristics of the polygonal pattern created

by the process of columnar jointing. In order to evaluate properly the Voronoi diagram

as a viable model, direct comparisons must be made with actual patterns of columnar

joint polygons. Additional quantitative tests are thereby required which take into

account the spatial properties of the individual polygons.

Quantitative results using the inertia tensor method

The inertia tensor method (Robin, submitted) can be used to calculate a best-

fitting ellipse to each polygon, i. From that ellipse can be calculated an axial ratio r;, a

measure of the anisometry of the eh-polygon. For regular polygons, such as the ideal

hexagons in Idding's model, the axial ratio is always 1; these regular polygons are said

to be isometric (no principal directions can be defined). Irregular polygons posses an

elongatedness which can be expressed by the r; parameter. For a population of N

polygons, two different means can be calculated: a scalar mean

r = 1/N S r, (2-1)

and a tensor mean, R, which is the axial ratio of an ellipse calculated from a tensor sum of the N individual polygon tensors (Robin 1989, pers. comm.). It is easy to see

that R should always be smaller than r. Unlike previous quantitative studies of basalt

columns (e.g. Beard 1959, Smalley 1965, Weaire & O'Carroll 1983) the important

advantage of this approach is that directional properties of each polygon in the network

are conserved. From the set of polygons of a given pattern, the calculated parameters

provides the basis for the statistical comparison of the models with an actual mosaic of

basalt polygons. Columnar jointing... 2-8

1. The Voronoi polygon model: From the results of shape analysis of the Voronoi

polygon model shown in Table 2.2, it is clear that the irregular polygons display a wide

range of diameter ratios: r; = 1.024 to 1.534. As expected, the mean diameter ratio

(1.247 ± 0.133) is significantly greater than the tensor mean of 1.032 for the set. For

the 30 Voronoi polygons of Figure 2.5, the diagram is demonstrably isotropic since

R = 1 (Table 2.2). It is expected that with larger sample sizes this value may approach

even closer to 1.

2. Basalt polygons of the Giant's Causeway: Undoubtedly, one of the best developed

examples of columnar jointing is found in the Tertiary lavas of North Antrim, Ireland,

known as the Giant's Causeway. Well exposed plan surfaces of the vertical columns

enabled O'Reilly (1879) to accurately survey over 200 individual polygons. It is

noteworthy that O'Reilly reported each polygon side to the nearest 0.5 cm and internal

angles to the nearest 0.5° (the average of two separate measurements in many cases!),

thus permitting shape analysis to be performed directly from his scale map, reproduced

as Figure 2.3 . As pointed out by Weaire and O'Carroll (1983), data from this detailed study is invaluable and should be taken into account when discussing any geometrical

model of basalt column patterns.

The shape attributes for the 201 component polygons of Figure 2.3 are plotted in

Figure 2.6 and display a reasonably uniform distribution. Shape analysis of this data

set, summarized in Table 2.2, are comparable to the shape parameters of the Voronoi

diagram. The mean (virtual) shape tensor gave an R,, = 1.025 (Figure 2.7) and suggests

that the joint system responsible for dividing the lava into columnar units is essentially

isotropic. These findings support the hypothesis that thermally dominated tensile

stresses are essentially equal in all directions for two-dimensional cracking of columnar

basalts. Columnar jointing... 2-9

The peculiar clustering of the last 150 successive tensor means plotted in

Figure 2.7 suggests however, that the small departure of R,, from 1 is significant.

Tomkeieff (1940) described the depressed surface of the lower contact to represent a river meander in-filled by lava. One might speculate that during cooling of the lava from the lower contact, columns developed at near right-angles to a convex downwards isotherm that continued to propagate upward as the isothermal surface migrated inward and became more and more planar. This shrinking area of the isothermal surfaces from a trough to a plane may result in an anisotropic tensile stress field (R. M. Stesky, 1989, pers. comm.). This would manifest itself by a slight anisometry of polygon shapes: promoting a longer growth of cracks in orientations near normal to the greater stress direction. Surprisingly, the calculated SE elongation direction of R,, for the basalt polygons is in agreement with the symmetry of the depression as illustrated by

Tomkeieff (1940, Figure 2).

PREVIOUS INVESTIGATIONS OF COLUMNAR STRUCTURES AND

POLYGONAL PATTERNS

Upon extrusion, the flow-top surface of fresh lava flows quickly chills and begins to develop cooling cracks which grow long and irregular, often meeting at orthogonal intersections (Peck & Minakami, 1968). Later cracks further divide up the solidified crust into smaller plates, forming many Y-type crack intersections (Figure 2.8).

Gray et al. (1976) speculated that columnar joints are the product of a complex interactive cracking process gradually evolving from 4-sided polygons to 6-sided polygons. DeGraff & Aydin (1986, 1987) observed in columnar basalt flows of the western United States, a progression from predominantly T and X-type crack junctions Columnar jointing... 2-10 near the margins of the flow to almost exclusively Y-type triple junctions towards the interior, confirming the hypothesis of Gray et al. (1976).

Gray et al. (1976) demonstrated that for infinite plane nets containing space- exhaustive n-sided polygons (convex polygons), the average number of sides per polygon, n, follows the relation;

n = 2(2J0-3Jy+4Jx)/(JT+Jy+2Jx) (2-2) where IT. and Jy are the number of T-type and Y-type triple junctions respectively. J, represents the number of four fold vertices in the network. Equation 2-2 can be tested on the diagrams of convex polygons (Figures 2.2 & 2.5).

Near the contacts of a lava flow, abundant T and X-type crack junctions would bias n to low values, between 4 and 5. A highly evolved network of cracks (near the interior of the flow) consisting entirely of Y-type triple junctions would yield n = 6.

This does not strictly imply that the polygons are all hexagons. A complete selection of 4 to 8 sided polygons are still expected due to the disordered arrangement of contraction centres.

Basalt columns are very high aspect ratio (length/diameter) structures created out of a numerous stacking succession of relatively wide, but short discrete crack segments

(DeGraff & Aydin 1987). A system of hydrothermal convection circulating throughout much of the crack network appears to be largely responsible for maintaining constant temperature gradients in the solidifying lava near the advancing crack front

(Hardee 1980, Reiter et al. 1987). The writer is in favour of Ryan & Sammis' (1978b) idea that incremental thermal cracking keeps pace with the constant rate of cooling and solidification of the lava flow.

Once the established crack network has- propagated far enough into the interior of the flow, the orientation of subsequent cracks is governed by the edges •of the Columnar jointing... 2-11 previous cracks (DeGraff & Aydin 1987). However, the crack pattern that eventually produces the prismatic structures is not purely static. The polygonal crack pattern, as will be described, gradually evolves from one incremental level to the next. This is documented by slight changes in orientation and in width of the column faces along their length. The collateral colonnade structures in basalts are therefore not perfectly

'cylindrical' prismatic forms. Current topological discussions of columnar jointed structures do not take into account these important observations.

THE VOPONUCE MODEL: A FEED-BACK MODEL FOR THE

INCREMENTAL CRACK PROPAGATION PATH OF COLUMNATED

STRUCTURES

The VOPONUCE model (Voronoi polygons nucleated on the centroid) attempts to describe the evolution of the pattern of columnar sections as cooling proceeds into the lava flow. From a starting pattern which may be quite irregular (e.g. Figure 2.8), and far from the ideal hexagonal distribution of centres, the VOPONUCE model should mimic the evolution of the pattern toward a mature one such as that of the Giant's

Causeway (Figure 2.3). The operation of the model appears to be consistent with the way in which lava flows cool, based on the observations made by Hardee (1980) and

Reiter et al. (1987):

1. Rapid removal of heat from the solidifying lava is achieved by convection along the conduits of the penetrating crack network.

2. Additional heat is more slowly transferred from the interior of the cooling columns to the cooler perimeter by thermal conduction (Figure 2.9a). Columnar jointing... 2-12

The first condition permits a cracking front to carry planar isotherms (at the scale of the flow) inward to the solidifying lava. More effective cooling along the cracks proper causes local deflections of the isotherms away from the immediate crack- tip (Figure 2.9b). In three-dimensions, the isotherms protrude inside the newly formed

portion of the columns as a series of nested paraboloids. An isothermal 'plane' can be

envisaged as similar in shape to the surface of an egg-carton.

Condition 2 implies that the temperature gradient from the centroid to the

perimeter of the polygon is controlled by the thermal conductivity of the basalt. Figure

2.9a is a planar cross section along the crack-tips of a single column. The transected

paraboloidal isothermal surfaces are symmetrically disposed around the centroid of the

polygon. If heat is convected away equally from all the column faces, the centroid of the polygon approximates the location of the highest temperature (Tmu).

Lines connecting the neighbouring centroids between polygons define maximum

thermal stress gradients, across which subsequent cracks are predicted to occur.

Centroids of polygons at one level define the Voronoi nuclei of the next level of

polygons. The distance separating these two points can be considered as a measure of

the polygon's eccentricity. The eccentricity is independent of the ellipticity and the two should not be confused. The local changes in thermally controlled principal stress

directions within each cooling column is characterized by the eccentricities of two

neighbouring polygons and is the reason provided by the VOPONUCE model for

changes in crack orientation from one increment to the next.

Static columnar development

Voronoi tessellation around a pattern of nuclei in a regular hexagonal

arrangement (Figure 2.2) gives rise to Voronoi polygons whose nuclei are coincident Columnar jointing... 213 with the centroid of the hexagon. According to the VOPONUCE model, for the case of hexagonal plane nets, no change in shape or orientation of the successive polygons occurs. For lava flows cooling with such an initial crack pattern, the individual polygon outlines from one crack increment to another would remain static and thus produce cylindrical colonnades.

Evolving columnar structures

Voronoi polygons constructed from an anticlustered, random distribution of nuclei (e.g. Figure 2.5). consist of polygons whose Voronoi nuclei are not coincident with the polygon's centroid. During incremental advance of the isotropic crack network. the new crack orientations (extending the columns) are controlled by the new lines connecting neighbouring centroids from the polygons of the previous crack advance (as shown in Figure 2.9a). The resultant polygon from the new set of cracks is not identical to the outline of its predecessor.

The rotation of the subsequent crack orientation is governed by the amount of shift each corresponding neighbouring pair of nuclei (centroids) exhibit. The new fractures link to form new Voronoi polygons whose centroids lie once again in a different location. Some sides of the polygons remain unchanged, but most diverge a few degrees from prior orientations.

Throughout the development of a colonnade tier, the width of column faces may increase or decrease. Column faces which narrow may even pinch out at newly formed four-fold vertices. Conversely, new column faces may emerge as pairs of Voronoi neighbours switch at a four-fold vertex which splits into two, three-fold vertices (see

Figure 2.10). This dynamic model may help "to explain the divergence of the crack plane observed by DeGraff & Aydin (1988) and how column-faces are consumed by Columnar jointing... 2-14 two adjoining column-face orientations (Figure 2.11), as described by Faust (1978) and

Ryan & Sammis (1978a).

DISCUSSION

During inward propagation of the crack network, uniform cooling around the perimeter of the columns causes a thermally controlled migration of the 'stress centre' from the Voronoi centre of the basalt polygon, to its centroid. As a result, the subsequent crack advance deviates slightly from the orientation of the previous crack plane. The isothermal surfaces are again modified by convective cooling along the new crack fronts and the centroid of the new polygon becomes the Voronoi centre for the next crack increment. This process is cyclic, continually being maintained by a constant thermal gradient required for subsequent crack advances.

The VOPONUCE model predicts that under ideal conditions (isotropic cooling), the crack pattern would rapidly reach a steady-state where the eccentricities of individual polygons approaches zero. Perfect stability occurs when a Voronoi nucleus

(and all neighbouring nuclei) are coincident with the polygon's centroid. Colonnades under such conditions may continue to develop as cylindrical structures similar to the static progression expected for the regular hexagonal pattern. Three successive polygonal crack patterns were constructed according to the VOPONUCE model and are illustrated in Figure 2.12. The results of shape analysis presented in Table 2.3 indicates that the average ellipticity and the tensor mean of the Voronoi diagrams do not vary significantly from one level to the other. It is thought that several 10's of iterations are required in order to observe any overall trend, supporting the long term stability associated with the process of columnar jointing. Columnar jointing... 2-15

Geometrical models for evolving polygonal crack patterns are difficult to accept

or reject using simple statistical tests (see Appendix 2.2). Table 2.2 shows the

calculated shape attributes of the basalt polygon outlines surveyed by O'Reilly (Figure

2.3), to be quite similar to the values obtained from the Voronoi polygon model. To

obtain a better match for the average number of sides per polygon in the crack model,

Smalley (1965) eliminated some "very short sides" and re-adjusted the lines to meet at a

four-fold vertex. In the writer's view, this modification was unnecessarily justified for several reasons:

1. Smalley's data is based on diagrams consisting of less than 20 polygons and hence

is not representative enough to determine a reasonable average number of sides per

polygon.

2. Very short sides of polygons, defined here as less than 5% of the polygon

perimeter, are fairly numerous in Figure 2.3 . On the other hand, only 4 occurrences of

four-fold X-type crack junctions are encountered. For the most part, very short sides

are transient features: they have been observed in the Proterozoic volcanics of the Cape

Smith Belt to grow into wider joint faces or disappear by merging into adjoining,

widening column faces during the advance of the evolving crack front (Figure 2.11).

3. The contiguous assemblage of basalt columns (Figure 2.3) are a reliable set of polygons to calculate an average since all are accounted for in a given area, thus reducing the possibility of selective sampling. For the 201 polygons represented, the

average number of sides is 5.93, higher than Beard's (1959) findings of 5.66 . Another continuous set of 153 columns examined by O'Reilly (1879) yields an average of 5.86. Columnar jointing... 2-16

CONCLUSION

Previous attempts to quantify patterns of columnar joint polygons based on their average number of sides have been inconclusive. Basalt columns examined along their length in the Cape Smith Tectonic Belt have shown to possess a variable number of sides, similar to other known examples. This stems from the fact that columnar structures are the product of an evolving system of intimately associated cracks. The average number of sides is not a measure of a model but rather a measure of the degree of evolution or maturation of the jointing process. The shape attributes calculated through shape analysis provides a better means of evaluating models of variable and evolving crack networks like columnar joints. Conditions promoting ideal development of columnar structures may not necessarily be present in all solidifying lava flows. One of the most evolved examples of columnar jointing is found at the

Giant's Causeway, where n = 5.86-5.93.

The topology expressed from isotropic Voronoi polygons displays significant geometric similarities to the process of columnar jointing exemplified in basalts of the

Giant's Causeway. The VOPONUCE model of crack advance may explain the observed departure in colonnades from a true prismatic form, caused by the continual readjustment of the width and orientation of column-faces along the axis of columns. Columnar jointing... 2-17

Figure 2.1 Idealized profiles of columnar jointed basalt flows. Top profile consists of a two-tiered flow with upper and lower colonnades. Bottom profile reveals a three-tiered flow with a central entablature zone. 2-19

Figure 2.2 Hexagonal crack network formed as a result of a close- packed plane arrangement of imaginary contraction centres. Each point is surrounded by 6 equidistant neighbours. (Modified after Iddings 1886). 2-20

Figure 2.3 A surveyed plan view of 201 contiguous columnar joint polygons. The network of basalt polygons is part of the Giant's Causeway, North Antrim, Ireland. (After O'Reilly 1879). 2-21

Figure 2.4 a) Frequency histogram of polygon type from several well- known columnar jointed lava flows. Data from O'Reilly (1879) and Beard (1959). b) Three sites a,b, & c of basalt polygons from the Chukotat are plotted on a frequency vs. polygon type and exhibit similar distributions.

Figure 2.5 Voronoi model of columnar joint polygons. Fractures are constructed on an infinite plane net according to Voronoi tessellation (solid lines) for an isotropic distribution of points (random close-packed). This pattern of 30 complete convex polygons (11=5.93) bears strong resemblance to the pattern observed at the Giant's Causeway. 2-23

Figure 2.6 Circular plot of the principal a; eigenvector direction and log ratio of alb; for each component basalt polygon shown in Figure 2.3. These values were calculated by inertia tensor method (Robin, submitted) using the digitized polygons from the scale map in Figure 2.3. 2-24

Figure 2.7 Circular plot displaying a progression of the changing resultant mean a eigenvector directions and R values with increasing number of measurements successively added to the mean. Note how rapidly the series converges to the final result (see text for discussion). 1--)5

Columnar jointing... 2-18

Figure 2.8 An example of and immature cooling-crack pattern on the surface of Alae Lake, Hawaii. Polygons have curved boundaries and have variable sizes. Crack-crack intersections form predominantly T- or X-type junctions. (Simplified from Peck & Minakami 1968). 2-26

Figure 2.9 a) Transverse section across a hypothetical column structure displaying the polygonal outline (solid lines) and it's corresponding Voronoi nuclei and Voronoi neighbours (solid circles). Concentric cooling relocates the stress maxima to the polygon's centroid (cross) which act as the following Voronoi nuclei for the subsequent crack advance (interrupted lines). b) Longitudinal section of columnar structures illustrating the centro-symmetric nested isothermal surfaces. Crack propagation is from bottom to top. Isothermal surfaces increase in temperature from bottom to top as well. 2-27

Figure 2.10 A block diagram of a transition from one polygonal crack pattern (top) to the next level (bottom) according to the VOPONUCE model. Note the four-fold crack junction (*) which bifurcates from (top) into two Y-type junctions (bottom). Many column faces also change width from narrow to wider ones, or the inverse. 2-28

Figure 2.11 Two adjacent basalt columns showing the nature of cyclic striae from smooth (blank) to rough (hatched) and the gradual assimilation of one column-face into another (From Ryan & Sammis 1978b). '--)8

Figure 2.12 A transverse section across model basalt polygons as viewed along the axis of propagation. Three successive Voronoi polygon patterns as predicted by the VOPONUCE model. (1) Solid line is the initial Voronoi polygon network of 30 polygons given in Figure 2.5. (2) Interrupted line is the second VOPONUCE pattern, followed by (3) a third VOPONUCE of the second pattern shown by dashed lines. This represents only three of hundreds of such crack advances expected to occur in columnar structures. 2_29

Columnar jointing... 2-19

upper colonnade

lower colonnade

upper colonnade

entablature

lower colonnade

Figure 2.1 Columnar jointing... 2-20

Figure 2.2 Columnar jointing... 2-21

A N

1 2 metres

Figure 2.3

Columnar jointing... 60

n=50 50 n=200

40 %)

( n=400 ncy 30 ue n=200 freq 20. n=100

10. n=67

r 3 5 6 8

number of sides Figure 2.4a

60 a n=4C 50 al= b n=15 40 c n=20

0 3 4 5 6 7 Figure 2.4b number of sides Columnar jointing... 2-23

Figure 2.5 Columnar jointing... 1.14

log a/b

1.00 090 1.05

1.15

1.25

1.35

1.45

n=201 1.55 180 tensor mean (A/B)

Figure 2.6 Columnar jointing... 2-25

A/B

090

n t =201

180 tensor mean (A/B) = 1.025

Figure 2.7 Columnar jointing... 2-26

Figure 2.8

Columnar jointing... 2-27

The VOPONUCE (VOronoi POlygon NUcleated on the CEntroid) model for sequential crack advance in columnar structures. t

+ • Voronoi nuclei + • • Voronoi polygon (crack increment A) • centroid (T max) ,- • VOPONUCE (crack increment 6) isotherms •

• cross—section Figure 2.9a

------

------... ., .~' ~ . ---- ~'~ --- ,, ,,

♦C

, ,, 1...•

. ,

• A

< column > <

longitudinal—section • crack—tip

Figure 2.9b 2-2g Columnar jointing...

Figure 2.10

~\\\\\N.\\\\\ \\\\A\\\\\\\\\\\\VY

SO\\\\\\\\\\\\\\\\\\ NAM%%UWVA

~~••\\"‘—`

ru \\\u\\\\ •\•\\•\\\\\\\.\ \•\\\•\\\\\\\\\ \\\\ •\\\\\\\\ ‘,, a\\\\\\ \\\\\a\\\\\\\\\\\\\\\\\\\~\r a••\\\\\\. ~..a.•..~\~~

cm‘ \\""`"""4'.\\N,

\\•\u\\\\~ \\\\\\\\\\m,‘‘

COL Um Z

® ROUGH ZONE

Q SMOOTH ZONE

Figure 2.11 Columnar jointing... 2-29

Figure 2.12 Columnar jointing... 2-30

Table 2.1 A comparative look at some topological properties of the two-dimensional crack patterns discussed in the text.

Table 2.2 The results of shape analysis from the Voronoi polygon model (1) and actual (2) basalt polygon patterns. The tensor average for both patterns is essentially isotropic (less than 5% anisotropy) and the shape characteristics of the individual polygons are also remarkably similar.

Table 2.3 The same shape analysis carried out for the VOPONUCE for two iterations after the initial crack pattern. The mean anisometry of the individual polygons in the second and third crack pattern are consistent with the mean alb from the Giant's Causeway (Table 2.2). One VOPONUCE step appears to be sufficient in order to reduce the very anisometric polygons, however the tensor means do not change significantly between the three levels of patterns. Columnar jointing... 2-31

Table 2.1 crack pattern crack orientation crack length nature of crack junctions polygon types

1) Hexagonal model confined to three directions rued unit length all crocks intersect at mutual regular he.ogans (figure 2.2) ( 060•; 120'; 180') angles of 12(/ at Y-type triple junctions.

2) Voronoi model statistically Occurs in all variable range of cracks meet at venous acute 4 - 8 sided (Fqun 2.5) directions. crack lengths passible, to obtuse angles of mainly Y-type irregular polygons and rarely X-type junctions.

3) Giant's Causeway occurs in almost at various crock lengths crocks meat of acute to obtuse 4 - 8 sided (Fqure 2.3) directions in equal ( 4 to 52 cm ). angles ( 60 to 175° ) of r-type. irregular polygons proportions. and rarely X or r-type junctions.

Table 2.2

crack pattern range of a/b mean a/b tensor mean (A/B)

1) Voronoi polygon model 1.024 to 1.534 1.247 ± 0.133 1.032 (Figure 2.5) n = 30

2) Giant's Causeway 1.012 to 1.548 1.187 ± 0.121 1.025 (Figure 2.3) n = 201

Table 2.3

VOPONUCE model (Figure 2.12) rcnge of a/b mean a/b tensor mean (A/B)

A) initial crock pattern 1.024 to 1.534 1.247 ± 0.133 1.032

B) second crack advance 1.013 to 1.402 1.192 ± 0.118 1.036

C) third crack advance 1.023 to 1.393 1.173 ± 0.104 1.044 Columnar jointing... 2-32

APPENDIX 2.1: Construction of the isotropic Voronoi polygon model

Rules for generating an isotropic distribution of contraction nuclei or "stress circles" is described in Smalley (1965). Determination of neighbourhoods by Voronoi tessellation is described by Ahuja (1982). For convenience, the procedure for constructing the Voronoi polygons from an isotropic distribution of nuclei is summarized below. Figure 2A.la shows the construction of a Voronoi diagram for random close- packed centres, presented in the text as Figure 2.5. A simple arrangement of hexagonal close-packed centres with the minimum radii and the same construction lines is also shown (Figure 2A.1 b) for comparison.

Note: It is theoretically possible for a nuclei to be located at any point on the array, not just at integer coordinates. To minimize the effect of grid spacing and the problems encountered by Smalley (1965) concerning very short sides of polygons, it is recommended to have the effective radius of "stress circles" less than 1/10th the array dimensions.

A more realistic representation of basalt polygons can be made in the Voronoi polygon model if the radii of initial "stress circles" is varied by ±10% (Robin, 1988, pers. comm.). This can be randomly introduced just as the selection of their location in the array is determined.

1. Contraction nuclei are loaded into an array by randomly choosing coordinates.

All nuclei have a fixed effective radii, each forming a unit "stress circle". In order to form a continuous pattern, portions of the circles falling outside the array boundaries are continued in on the opposite side. A new data point introduced whose effective radius overlaps with a previously located circle is not permitted into the array. The array is complete when the remaining void spaces are too small to accommodate any more stress circles.

2. Neighbouring points are defined by Delaunay triangulation (see Ahuja, 1982),

By convention, only closest point pairs are chosen to complete the net. Each pair of Columnar jointing... 2-33 neighbouring points is joined by a line. None of the lines are permitted to cross one another. The result produces a network of lines which divide the array (an infinite plane) completely into a net of triangular areas.

3. Perpendicular bisectors are constructed across the triangulation lines of step 2.

The bisectors are considered to be the direction of crack growth and can be extended in both directions until they meet other lines. Most lines will meet at Y triple-junctions, rarely at X or T junctions. The locus of line junctions are equidistant to each of the three (or four) surrounding points. This completes the isotropic Voronoi polygon model of basalt polygons.

Figure 2A.1 a) Pseudo-randomly located centres in a random close- packed array. The working grid selected was 64 x 64 and a minimum fixed radius of 5 units was chosen for the closest near neighbours. In all, a maximum of 30 centres (solid circles) were able to fit without overlap in this particular example. Dashed lines represent Delaunay tessellation, solid lines are the result of Voronoi tessellation. b) Similar construction lines are shown for a higher density of centres achieved in the hexagonal close-packed array with interrupted lines defining the "stress circles" where no two circles are permitted to overlap. 2-34 Columnar jointing... 2-34

Figure 2A.1a

Figure 2A. 1 b Columnar jointing... 2-35

APPENDIX 2.2: Difficulties in applying statistical tests to geometrical models

An example of this problem can be found in Getis & Boots (1978, p.136-137):

To form an expected frequency standard, Getis & Boots (1978) grouped

Smalley's (1965) three small sample size (n = 17, 16, 18) examples (each with quite variable distributions of each polygon type). These authors then apply a comparative X test on the number of sides from natural examples compared to the number of sides of the standard. The field examples yield a value of X' which they consider too high and reject the hypothesis that the field examples are described by the standard.

The weakness of their argument is apparent if any one of the three individual model examples are tested in the same manner against the combined expected set

(Robin, 1989, pers. comm.): The Xz test fails for two of Smalley's three examples. contradicting their own statistical test. In addition, their data is incorrectly normalized to the number of specimens rather than the required normalization of data to frequency. Chapter 3: STRAIN ANALYSIS OF POLYGONAL OUTLINES FROM COLUMNAR JOINTED BASALT FLOWS OF THE CHUKOTAT GROUP.

ABSTRACT

Basalt polygons from deformed basalt flows are shown to be reliable and useful

quantitative sectional strain gauges. Eight locations of basalt polygons from columnar

jointed flows in the Fox Lake area were suitable for sectional strain analysis and in all

cases they were found to be deformed, yielding sectional strain ratios in the range of

R = 1.21 to 2.42. Three methods of strain analysis were tested on the basalt polygons,

(1) the method of diameter ratios (Robin 1977), (2) the inertia tensor method (Robin,

submitted) and (3) the method of Fry (1979). In most cases, the results were similar to

within 10 to 20% of one another. Empirically, 20 to 25 basalt polygons are necessary

to provide reliable estimates of strain using the inertia tensor method. The dm-size of

basalt polygons limits the number of exposed polygons per unit area of outcrop, making

it difficult to collect large samples demanded by most other strain methods.

INTRODUCTION

This chapter describes the results and limitations of quantitative sectional strain

analysis of columnar jointed lavas from two-dimensional sections, parallel to cross-

joints. Such sections are perpendicular to the columnar axis in the undeformed state;

once deformed, cross-joints and the columnar axis are no longer necessarily

perpendicular.

Several authors (e.g. Dimroth et al. 1978, Ramsay & Huber 1983, Martinez

1986) have suggested that columnar joints may be suitable strain markers. Harvey & Strain analysis of basalt polygons... 3-2

Ferguson (1981) proposed an eigenvector method for determining sectional strain from deformed desiccation cracks. Mudcracks have similar geometrical properties to columnar joint polygons and their method, although implied for regular polygons, is also applicable to irregular polygons. These authors have implicitly or explicitly assumed that these polygons are statistically isotropic, a necessary requirement for these methods.

The validity of this assumption was demonstrated in Chapter 2.

The geometric properties of basalt polygons are indeed quite amenable to several well known finite strain analysis techniques (e.g. Harvey & Ferguson 1981, Fry 1977,

Robin 1977, Sanderson 1977). Martinez (1986) put forth a geometrical method for determining sectional strain from regular hexagons of equal dimensions. The application of this method is severely limited to natural honeycomb-like strain markers (e.g.

Favosite corals, see Ramsay & Huber 1983, p.135). Transverse sections of columnar joints exhibit irregular polygons of unequal size. Therefore, they can not be treated as

Martinez (1986) previously suggested. Even though basalt polygons meet the criteria for several strain measurement methods, the inertia tensor method (Robin, submitted), maximizes the use of information provided by the marker.

PRACTICAL STRAIN ANALYSIS OF BASALT POLYGONS: AN EVALUATION

The inertia tensor method (Robin, submitted) can be used to estimate the amount of two-dimensional shortening by evaluating the ellipticity, R, of the sectional strain from a set of markers

R=A/B (3-1) where A and B are the principal axes of the calculated sectional strain for the sample. Strain analysis of basalt polygons... 3-3

1. Basic assumptions

Sectional strain measurements recorded from deformed basalt polygons in the

Fox Lake area are based upon three assumptions: (1) In true section, the columns initially formed an isotropic mosaic of closed contours. In Chapter 2, the original fabric of undeformed Tertiary columnar jointed basalts from the Giant's Causeway was found to have a virtual strain (Robin 1977), R„ = 1.025. (2) The joints are thought to behave as passive markers, which simply track the strain path. (3) Over the scale of the sampled polygons, deformation has been homogeneous. Although no evidence was found, a minor contribution by sliding along column-face boundaries is plausible.

2. Implementation of the inertia tensor method

The inertia tensor method applied to basalt polygons, requires an accurately surveyed diagram of the position, length and orientation of each polygon segment.

Recording this information directly from outcrops of large sample sizes (n = 30 to 50) can be very arduous. A more efficient way of recording this data is by taking orientated polaroid photographs. The image is instantly scaled and available for on site examination and clarification with a draughting pen where joint segments are unclear.

This maximizes the number of useful specimens on the photograph, closing any otherwise lost contours. If non-orthogonal photographs of the outcrop are taken, simple geometrical corrections can be made. Cooper & Bamford (1987) describe how true perspective photographs can be obtained from orientated negatives by correcting the positive image during the development process. Their method was not used in the present study.

Planar outcrops of columnar joint polygons were very often encountered due to the development of primary cross-joints, providing planes of weakness along which Strain analysis of basalt polygons... 3-4 sections of columns are preferentially exposed. The stereogram of Figure 3.1a illustrates

the general orthogonal relationship observed between column-face intersections (a

measurable approximation of the column axis) and the cross-joint orientations. Often

the outcrop surface is stepped from column to column (Figure 3.1b). Cross-joints are

not trans-columnar structures and only segment individual columns. Where cross- jointing is particularly well developed, exposures may easily disaggregate into angular

blocks, often referred to as diced lava (Spry 1962).

From orientated photographs, the a/b ratio was determined for each polygon.

The tensor mean of all a/b values provides an estimate of the principal directions of sectional strain, denoted here as A/B. The average reproducibility of results using the digitizing apparatus for individual markers was found to be ±0.011 for a/b ratios and

+2° for their principal directions. For the purposes of strain analysis from any natural strain gauges, these uncertainties are within the margin of error prone to most strain methods.

3. Effect of sample size

For practical strain analysis, it is important to know how quickly the averaging process converges to a 'stable' estimate of the finite strain ellipse. One question the geologist must ask is: how many data measurements are necessary in order to obtain a reasonable quantitative determination of the strain? Shape analysis of an undeformed population of strain markers, described in the previous chapter, may provide some empirical insight to this problem.

Plotting the cumulative average for successive measurements provides an empirical examination for how rapidly the series converges. Results discussed previously from Figure 2.7 indicate that from 201 irregular basalt polygons of the Strain analysis of basalt polygons... 3-5

Giant's Causeway, a virtual anisotropy of 2.5% is detected. It is important to note that the cumulative total anisotropy, R„', remains below 1.05 after only 10 measurements despite average basalt polygon r; values of 1.187. Only about 35 measurements of r; are needed to achieve a consistent estimate of R; about 45 total measurements to eliminate the 'noise' for the principal directions. The 10 additional measurements required is explained by the very low R,, of the population. Compared to the tensor mean, individual samples have a relatively high r; values and directional scatter of a;.

Each subsequent r, measurement added will tend to deflect the a principal direction of

R,,' towards the a;-direction of the last r; measurement. The amount of angular change from the previous cumulative average to the new average is greatest when the number of averaged measurements is small and/or r; of the new specimen added is high and with its a,-direction orientated at 45° to the a-direction of the current tensor mean.

Furthermore, it is clear that beyond 45-50 measurements of r, no significant improvement of R,, is obtained. The additional measurements indicate the level of sample noise around the tensor average associated with this set of polygons. The scatter of intermediate tensor means (from n; = 45 to 201) is an informal indicator of the level of confidence one can assign to the estimate. Figure 2.7 indicates that for the basalt polygons of the Giant's Causeway these values are very low indeed: approximately ±0.010 for R,, and ±4° for the principal directions.

An example from the Fox Lake area

Low strain deformation of basalt polygons is shown in Figure 3.2a and quantified in Figure 3.2b. For passive strain markers in deformed rocks, the long axis of deformed markers are generally concentrated towards the direction of maximum extension. On the circular plot (Figure 3.2b), the dispersion of points (r; vs. a;) art' Strain analysis of basalt polygons... 3-6 somewhat concentrated around their mean tensor. The same plot (Figure 2.6) of the individual basalt polygons from the Giant's Causeway have a distinctly random orientation of points. It appears that fewer measurements are required for deformed markers to arrive at a stable tensor mean.

Regardless of what strain method is employed, practical strain analysis is a time consuming process both in gathering field data and its numerical treatment. It is useful, particularly for low strains, to determine how many individual markers should be recorded to obtain a reasonable estimate of finite strain. Empirically, 20 measurements of r; are all that are needed to obtain a strain ratio within 5% of R =1.37 for the 43 measurements shown in Figure 3.2b. This example illustrates the low levels of sectional strain that can be successfully estimated.

COMPARISON OF STRAIN METHODS ON BASALT POLYGONS

Eight outcrop areas in the Fox Lake area offered suitable exposures for strain analysis. 19 sectional strain values were calculated; 6 from orientated photographs using the inertia tensor method (Robin, submitted); 12 measured directly in the field by diameter ratios (Robin 1977); 1 example from a photograph using the method of Fry

(1977). The compiled results of the calculated strain ratios are presented in Table 3.1.

A comparison between methods gives an indication of the confidence one may give to these R values. Note the consistently higher values from the method of diameter ratios. determined in the field, over the values of R from the inertia tensor method, calculated from photographs. The implementation of each method is described briefly below. Strain analysis of basalt polygons... 3-7

The inertia tensor method (Robin, submitted)

As discussed previously, this method is ideally suited to scale diagrams or orientated photographs, thereby minimizing the actual field time used for data collection.

With the aid of a digitizing tablet, many geometrical parameters are quickly determined in the laboratory. The advantage of this method is the ability of the inertia tensor to utilize the information contained in the area of the marker. By consequence. only relatively small sample sizes are needed to obtain good estimates of sectional strain.

The method of diameter ratios (Robin 1977)

If the direction of maximum extension is known in a section containing a collection of strain markers, the amount of accumulated strain can be estimated. With respect to a known direction of extension, two orthogonal measurements are taken of the strain marker. A practical description of this method is presented using columnar joint polygons.

Apparent flattening of columnar structures appears to be concordant with the trace of a weakly developed foliation. The location of the centroid was visually estimated for each of the basalt polygons. A visual approximation of the polygon's centroid appears to give reasonable results, not significantly different from results obtained from the calculated centroids (Robin 1977). The length of the polygon through its centroid and parallel to the trace of foliation was recorded as a;, the direction and length of maximum elongation. Maximum shortening, b;, of the polygon was measured perpendicular to a;, such that b, bisects a,. For a set of polygons measured, all a; are parallel to one another as are all b; measurements. An estimate of the sectional strain

(A/B) can be determined from the geometrical average of the alb; ratios (Robin °1977). Strain analysis of basalt polygons... 3-8

Figure 3.3 is a photograph of part of an outcrop area where 52 moderately strained basalt polygons were measured using the method of diameter ratios. A value of 1.66 for R was determined. For the photograph containing a subset of 28 basalt polygons, the inertia tensor method yielded a similar result of R = 1.55 (see Table 3.1).

Centre distribution methods (e.g. Fry 1979)

Centre-to-centre methods are popular diagrammatic strain analysis techniques. In order to obtain a valid estimate of strain from the elliptical cloud, it is inherent that the dispersion of studied markers is initially an isotropic anticlustered distribution. These methods are best suited to relatively small strain markers, such as oolites, where a few hundred are contained within a manageable area, such as a thin section or polished slab.

From a photograph containing only 20 basalt polygons (Figure 3.3), an estimate of strain using the method of Fry (1979) yielded similar results to the inertia tensor estimate. With such small numbers, however, fitting of an ellipse through the zone of highest density of points becomes somewhat subjective and much less reliable than the inertia tensor method's accuracy.

CONCLUSIONS

Basalt polygons from deformed flows are proven to be reliable sectional strain gauges, assuming the columnar jointing in basalts passively tracks the deformation path.

Quantitative evaluation of undeformed strain markers may provide empirical insight as to the precision expected from strain analysis techniques. For the purpose of strain analysis, the initial 3% anisotropy of the columnar assemblage is negligible. Using weakly deformed columnar jointed metavolcanics, 20-25 measured columns are sufficient Strain analysis of basalt polygons... 3-9 to provide a reasonable estimate of strain with the inertia tensor method. Columnar joints are not restricted to basaltic lavas alone and therefore any topologically similar structures are potentially useful for accurate quantitative strain determinations of even very low strain. If deformed entablature joints are also present, it may be possible from several different orientated sections to assemble a strain ellipsoid. Strain analysis of basalt polygons... 3-10

Figure 3.1a The effect of low strain (R = 1.66) on the expected orthogonal relationship between columnar joint intersections and cross-joint orientations. The calculated intersection average is 162°-461° and the average pole to cross-joints is 225°/19°NW. In this case, the two averages differ by only 14°, but the angular change is dependant upon the initial orientation of the markers to the axes of the deformation ellipsoid. [PB86-216] 3-11

Figure 3.1b Plan view of weakly strained basalt polygons. Segments of colonnades are exposed in a stepwise fashion from column to column along well developed cross-joints. Scale bar (10 cm) is orientated north-south. [PB86-2161 3-11

Figure 3.2a Line drawing of basalt polygons traced from a polaroid photograph of deformed columnar joints. [PB86-69] 3-12

Figure 3.2b Circular plot of: (1) the 43 individual polygon ellipticities (r4; open circles) and principal a, eigenvector direction. (2) the changing tensor average (R;; linked closed circles) with each successive measurement. The final average (Ri) pitches 18°SE on the exposed surface (95°/80°SW). Note the non-uniform distribution of the individual markers and rapid convergence of cumulative tensor averages towards a stable estimate of R. 3-12

Figure 3.3 Moderately strained basalt polygons are shown in the photograph. The method of diameter ratios (Robin 1977) for 62 basalt polygons yielded R = 1.66, while the inertia tensor method (Robin, submitted) gave R = 1.55 for a subset of 28 basalt polygons traced from the orientated photograph. The principal axis of the sectional strain ellipse lies in the trace of S1, roughly parallel to the fold axis of the Fox Lake syncline. [PB86-212] 3-13

Figure 3.4 The method of Fry (1977) performed on 20 basalt polygons using the locations of their estimated centroid from the photograph. Note the low density of the cloud plot associated with such small sample sizes. The best fit ellipse was determined subjectively. This method estimated R = 1.29 and the inertia tensor method yielded R = 1.21 for the same photograph (n=17). [PB86-251 1 3-14 Strain analysis of basalt polygons... 3 -1 1

cross joints and colonnade axes

Figure 3.1a

+ column-face intersections (n=20) ❑ average intersection (column axes) O cross-joints (n=15) • average pole to cross joints.

Figure 3.1b Strain analysis of basalt polygons... 3-12

3.2a

18°

R f = 1.37

• cumulative strain tensor (Ri )

O individual "best—fitting" ellipse for each sectional marker (r i )

32b

Figure 3.2 Strain analysis of basalt polygons... 3-13

R = 1.66

Figure 3.3 Strain analysis of basalt polygons... 3-14

R = 1.29

Figure 3.4 Strain analysis of basalt polygons... 3-15

Table 3.1 Comparative table of sectional strain analysis results of 14 different basalt polygon samples from 8 outcrop areas. Three strain analysis methods were implemented. Sample 43 represents random selection of specimens taken over 75 m of expOsure, while 43a to 43d were contiguous basalt polygon samples taken at intervals over the same 75 m long discontinuous exposure.

Table 3.1

inertia tensor method diometre ratios centre distribution method (Robin 1989, in press) (Robin 1977) (Fry 1979) outcrop n R n R n R 43 56 1.72 43a 43 2.13 43b 31 1.70 43c 29 1.82 43d 40 1.56 69 43 1.37 118 22 1.57 29 1.90 118 60 1.51 212 28 1.55 52 1.66 213 33 1.51 215 21 2.21 29 2.42 216 46 1.66 251 17 1.21 20 1.29 251 29 1.26 37 1.52

Chapter 4: RECOGNITION OF TECTONIC STRAIN INTENSITY IN GREENSCHIST FACIES MASSIVE AND PILLOWED BASALTS OF THE CHUKOTAT GROUP

ABSTRACT

Results from strain analysis and other less quantitative strain observations from primary structures help to delineate zones of high strain in the metavolcanics. The primary structures examined and described are (1) the outlines of pillows, (2) pillow selvages, (3) pillow-shelves, (4) columnar jointing, (5) sheet joints and (6) felsic varioles. Strain gradients were delineated using qualitative and quantitative indicators of deformation in the Chukotat metavolcanics. The high strain zones are interpreted as faults splaying off a major décollement at the base of the Chukotat Group.

INTRODUCTION

For structural mapping in volcanic terrains, an understanding of the origin of primary volcanic features is essential. Features such as pillow-shelves, columnar joints, sheet joints and pipe amygdales are potentially useful where measurable 'bedding' or paleohorizontal surfaces are required. Knowledge of the in situ orientation of primary physical structures with respect to the paleohorizontal or paleovertical prior to deformation is paramount. This chapter intends to describe each primary volcanic structure encountered in the Fox Lake area and to discuss their potential as indicators of the paleohorizontal or paleovertical. A second objective is to provide a description of the strain fabric(s) associated with some of these primary structures in deformed metavolcanic rocks. Strain fabrics in metavolcanics... 4-2

The Chukotat Group is suitable for such a study since it consists almost entirely of komatiitic to tholeiitic pillowed and massive basalt flows (and related intrusives).

The Fox Lake area exposes the floor-thrust of the Chukotat in oblique section, permitting a rare look at various structural levels within a single thrust sheet. The qualitative descriptions of the progressive levels of strain are intended to complement the more rigorous results of finite strain analysis presented in the preceeding chapter.

The advantage of recognizing a progression of deformation in primary structures is that very often, quantitative analysis is not possible. The origin of each volcanic feature recognized in the Fox Lake area and their potential use for structural mapping and strain analysis are discussed separately below.

Semi-quantitative subdivisions of strain

In this Chapter and the next, qualitative states of strain will be described from observed deformation fabrics in primary structures of the metavolcanics. Attempts are made to correlate the observations with quantitative strain determinations. The subdivisions are somewhat arbitrary, but provide a working basis to compare different sources of deformation indicators. RXZ is defined as the ratio of the principal axes of the finite strain ellipse lying in the XZ-plane of the strain ellipsoid.

1. No finite strain is used to describe rocks with RXZ less than 1.05.

2. Very low strain applies to rocks with RXZ values between 1.05 and 1.50.

3. Low strain lies within the range 1.50 < RXZ < 2.00.

4. Moderate strain in litho]ogical units are considered to have RX, values between 2.00 and 5.00. Strain fabrics in metavolcanics... 4-3

5. High strain is used as a qualitative descriptor of lithological units whose primary structures are attenuated, yet still recognizable. Strict finite strain analysis may be difficult; however, when possible, it should yield Rx2 greater than 5.00.

6. Very high strain is used to describe zones where tectonic fabrics (ie., C-planes) and recrystallization have obliterated all mesoscopically visible primary structures.

PILLOWED BASALTS

There is much literature on the morphology, development and mechanism of formation of pillowed and layered lava flows (e.g. Lewis 1914, Moore et al. 1971,

Greeley 1972, Hekinian et al. 1973, Moore et al. 1973, Bellaiche et al. 1974, Moore

1975, Ballard & Moore 1977, Dimroth et al. 1978, Hargreaves & Ayres 1979, Wells et al. 1982, Baragar 1984, Yamagishi 1985). However, only a few authors (Wilson

1951, Clifford 1972, Borradaile & Poulsen 1981, Borradaile 1982, 1985) have studied the strain fabric in deformed pillowed basalt sequences.

The terminology used in this paper for the various forms of pillowed lavas follows the proposals of Ballard & Moore (1977). Baragar (1984) discusses the historical perspectives on the pillow structures and provides an excellent detailed description on the formation of pillowed basalt flows from the Circum-Superior Belt.

1. Primary shapes of pillow outlines

Upon extrusion, pillowed lava may develop into a wide variety of three dimensional forms. Several varieties of pillows have been observed and described by

Ballard & Moore (1977) and their terminology is adopted here. The three principal

(primary) directions of pillow forms are defined as X„_Y„>_Z„; where X0 is parallel to Strain fabrics in metavolcanics... 4-4 the axis of the lava tube. Y„ and Zo lie in a plane perpendicular to X0, where Y„ lies in a horizontal plane (pillow width) and Zo (pillow height) is perpendicular to Y„. The volcanic accumulation consists of an intimately interwoven mass of pillow forms that may be represented by two geometrical end members: elongate pillows and bulbous pillows.

Elongate or feeder-tube pillows have dimensions X„»Yo>_Z„. Partially exposed pillow-tubes are frequently observed in the Fox Lake area to be 2 to 5 metres long, indicating a minimum length. In newly erupted lava flows, Moore et al. (1973) reported elongate pillows routinely 10 to 15 m long and some as long as 100 ni. The

X0Y0 plane of elongate pillows give good estimates of the depositional slope.

Budding from main feeder tubes are oblate to sub-spherical bulbous pillows where Xo—Yô Z„. The depositional slope is not as accurately estimated from the X„Y„ equatorial plane of bulbous pillows because of their low anisotropy. The actual depositional slope may be a shallow angle or inclined more steeply as in foreset-bedded volcanics. Moore et al. (1971, 1973) have reported foreset slopes in recent volcanic piles of 30° to 90° from recent submarine eruptions.

2. Pillow selvages

Pillow selvages are often visible in cross sections of preserved pillowed basalt.

Quenched basalt forms an outer rind 1 to 4 cm in thickness, characterized by an aphanitic texture and the presence of occasional microphenocrysts (Figure 4.1),

Following metamorphism, the rinds or selvages are often much paler in color than the interior portions of the pillow. The internal contact is sharply defined and in most cases, the selvage maintains a uniform thickness around undeformed pillows. Strain fabrics in metavolcanics... 4-5

3. Pillow-shelves

Inside a cooling lava tube or pillow, the planar interface that separates the quenched basalt of a standing lava level from the overlying drainage cavity is defined here as a pillow-shelf, shown in Figure 4.2. Ballard & Moore (1977) suggested that when the lava source to feeder tubes has been cut off, lava may continue to break through the outer crust and flow out. The molten lava is most likely replaced by water in the partially hollow tube by entering through cracks in the frozen margin and consequently chilling the surface of the new lava level to form the pillow-shelf. This process may repeat itself as long as rupturing of the outer skin occurs and molten lava remains in the tube (Moore et al. 1973, Ballard & Moore 1977). As a result, several shelves within a single sectional pillow may occur, separated from each other by septa of quenched lava (Figure 4.2).

The cross section of pillows with a single pillow-shelf display a large crescentic- shaped cavity located at the upper part of the pillow. , The convexity of the cavity is controlled by the curvature of the pillow selvage. Any subsequent drainage cavities appear rectangular in cross-section, beneath each earlier formed cavity and septum pair.

All cavities in the pillowed basalts of the Fox Lake area are presently mineralized with carbonate and variable amounts (5-95%) of white quartz.

The top surface of the septum is generally smooth. The undersurface on the other hand, is more irregular with occasional miniature lava stalactites (about 1 cm in length) present. These features are similar to those illustrated from recent pillows by

Ballard & Moore (1977) and Bryan et al. (1979).

Viljoen & Viljoen (1969) noted the potential usefulness of what are now recognized as pillow-shelves for top determinations in pillowed lavas. More recently, in the Cape Smith Belt, Sawyer et al. (1983) illustrated the practical application of Strain fabrics in metavolcanics... 4-6 measuring the orientations of planar pillow-shelves to unravel fold structures in the deformed volcanics. The best estimates of volcanic layering should be measured from planar, unfolded pillow-shelves as shown in Figure 4.3. In outcrop, the carbonate

(± quartz) amygdales overlying the pillow-shelves are often weathered out. This permits accurate measurements of the tectonic rotation with respect to the horizontal, by inserting a thin board along the pillow-shelf and using an orientation compass. In the

Chukotat Group of the Fox Lake area, about one in fifteen to twenty pillow sections encountered contain one or more pillow-shelves. When mapping, there is seldom difficulty in finding a suitable pillow-shelf for structural measurement.

In some pillows, numerous pillow-shelves occupy the entire internal pillow section. (It is quite likely that these pillows are sections across an extensive lava tube).

The factors limiting the number of pillow-shelves in a given pillow are the height of the pillow, thickness of the top and bottom pillow selvage and the height of each pillow- shelf and cavity unit. 25 to 30 pillow-shelves have been observed in large pillow sections 1 to 1.5 m in height.

Within an outcrop area of 10 to 20 m 2 , pillow-shelf orientations from one individual pillow to another are comparable; Figure 4.4 gives an indication of the scatter of data associated with this indicator of the paleohorizontal plane. This scatter of data is comparable to that of measured bedding planes from a sedimentary sequence.

Evidence for significant adjustments of the volcanic pile due to gravity settling or pressure upheavals between drainage events may be identified by occasional large measurable angular differences between successive planar shelves in a single pillow or from two individual pillows (see Figure 4.5a). In the instance of Figure 4.4b, no tectonic rotation of the pillows with respect to; one another was recognized. Strain fabrics in metavolcanics... 4-7

DEFORMATION OF PILLOWED BASALT FLOWS

The presence of interpillow void spaces (filled with clay minerals or hyaloclasite material), chilled margins and internal cavities creates a heterogeneity in pillowed flows.

For this reason, deformation of pillows is also expected to occur heterogeneously. The following discussion will describe the manifestations of strain on basalt pillows based upon field observations. The difficultly in assessing strain quantitatively can be appreciated at the outcrop scale where some individual pillows appear to retain their original outlines (the ratio of their long, a, to short, h, axes appear comparable to the typical Yo and Z0 primary axis of undeformed pillows). Often, apparently undeformed pillows are juxtaposed against other pillows displaying more asymmetrical outlines and internal deformation fabrics. Several deformation mechanisms, revealed by various strain fabrics in the pillowed basalts, must contribute to the overall bulk strain.

Evaluation of bulk strain in heterogeneously deformed bodies requires knowledge of the strain contribution from each deformation mechanism (Schwerdtner, 1988, pers. comm.).

For heterogeneous deformation experienced by the Chukotat pillowed basalts, the integration of strain from each deformation mechanism is a difficult task. For this reason, quantitative integration of all the deformation mechanisms is not attempted in this study of the Fox Lake area.

For each of the following primary structures, the low strain exhibited by the metavolcanics are subdivided into three levels, differentiated by their respective strain fabrics. Descriptions are based upon observations of well exposed sub-vertical Q-joint surfaces trending NNW that provide cross-sections of the regional structure (Figure 4.6).

In the study area, three dimensional exposures; from several locations reveal the X'„-axis Strain fabrics in metavolcanics... 4-8 of elongate pillows and tubes to be largely belt parallel (east-west). Therefore, Q-joint surfaces provide numerous near orthogonal sections for the majority of pillows observed.

1. Pillow outlines

The extreme variation in the X0 dimension of three-dimensional forms of pillow structures described previously, does not permit the use of this principal axis for determining the shape and orientation of the tectonic strain ellipse or ellipsoid. For example, the pillow dimensions X'0:Y'0:Z'a estimated by Gold (1962) as 8:4:3 in the metavolcanics of the eastern part of the Belt do not indicate whether or not the pillows have been strained.

In this discussion, the simple term pillow will refer to the outline of the three- dimensional pillowed basalt structure as seen in a random section. For pillow sections orientated approximately normal to the X,-axis of the tube, the proportions of Y, to Z„ do not vary as greatly as oblique sections would in a group of tubes (Borradaile 1985).

Yo/Zo pillow sections may be partially helpful for determining sectional strain. Clifford

(1972) and Borradaile (1982) considered model cross sections of undeformed pillows to have Y0/Z, axial ratios of 3:2 to 4:3. At high tectonic strains, where the long to short axes of deformed pillows, a/b, are about 30:1, Clifford (1972) was able to identify a strain gradient in proximity to granitic plutons. Such applications are limited to high strain zones and are clearly not reliable for low levels of strain.

As shown by Borradaile & Poulsen (1981), when acute angles between the maximum axis of the strain ellipse, A, and Zo exist, Z'0 may become greater than Y', if sufficiently strained. Figure 4.8 shows an example of such a case. The direction of A can be estimated to lie within the acute angle _formed between a and Z',, of an initially bilaterally symmetrical pillow section (Borradaile & Poulsen 1981). In the Fox Lake Strain fabrics in metavolcanics... 4-9 area, the trace of the S, foliation with respect to the a and Z'0 directions suggests that

S, appears to be a principal plane schistosity.

2. Pillow selvages

Borradaile & Poulsen (1981) described a more reliable method for determining strain in pillows from their selvages and discussed the practical restrictions in a subsequent paper by Borradaile (1985). Although numerous outcrops with Q-joint surfaces provided suitable cross sections necessary for the analysis, the ubiquitous coating of encrusting lichens on the weathered basalt obscured the selvages. At one location, water washed blocks revealed selvages clearly, and gave R = 2.12 from 3 pillows. This method maybe more widely applicable in the coastal part of the belt where outcrops are polished by the actions of waves and ice. The types of pillowed basalt best suited for the selvage method of strain analysis are those which display little or no internal structural or compositional variation (è.g. Figure 4.7a, discussed in detail below).

3. Pillow-shelves

The critical relationship for the following discussion is that the paleohorizontal of the basalt pillows lies at near right angles to the plane of flattening (S1 ). A summary sketch of the progressive stages of strain observed in the pillowed basalts of the Chukotat Group is presented in Figure 4.7.

As described earlier, undeformed pillow-shelves are planar interfaces; the deforming structures are the basalt septa. Folded pillow-shelves are evidence of shortening across the width of the pillow. Tht basalt septa in multi-shelved pillows

(Figure 4.7f & j) deform by buckling, as shown in Figures 4.9. The constant thickness Strain fabrics in metavolcanics... 4-10 of the folded septa indicate that they behaved as the competent member and the carbonate- and quartz-filled cavities were the incompetent member. The interlimb angle, or more precisely, the amount of shortening (LAO, can be used as indicators of the progressive stages of strain experienced by the pillow (Figure 4.7f,g,h & 4.7j,k).

Moderate levels of strain, are reached when Z'„/Y'„ > 1 and simple tensional or conjugate shear fractures (Figure 4.7d,f) begin to develop into crack-seal type veins.

Figure 4.7g is one example which also displays a collapse of the upper part of the pillow due to the internal heterogeneity when pillow-shelves are restricted to the top half.

At high levels of strain, previously formed crack-seal veins (Figure 4.7f,g) now begin to show signs of buckling similar to that of the pillow-shelves. With continued flattening across the width of the basalt pillows, pillow-shelves are tightly folded and often are broken in their hinge zones, the limbs stacked one against the other with very little of the cavities' carbonate-quartz mineralization remaining (Figure 4.7h,k & Figure

4.10b). It is possible that much of the incompetent carbonate-quartz member was remobilized by pressure solution into dilatant veins or by plastic flow into the hinge zones of buckle folds.

COLUMNAR BASALT STRUCTURES

Columnar joints are tensional fractures, often occurring in ponded lava flows and generated during the cooling history of the basaltic lava (Peck & Minakami 1968,

Hardee 1980). Jaeger (1961) proposed that columnar joints are a variety of thermal cracks, orientated perpendicular to the direction of maximum tensile stress which, for simple geometrical bodies, may lie within isothermal planes. Most lava flows can be Strain fabrics in rnetavolcanics... 4-1 I modeled by a simple tabular geometry, cooling primarily from their upper and lower surfaces.

Characterization of the internal structural levels or 'architecture' of columnar jointed flows was first put forth by Tomkeieff (1940) and later expanded upon by Spry

(1962) (see Figure 2.1). Thin flows, generally less than 50 m, often develop a lower and upper set of colonnade columnar joints orientated at near right angles to flow-tops or lower flow bottoms. For such cases, it appears that during cooling from upper and lower surfaces, a homogeneous stress distribution exists parallel to isothermal surfaces and promotes cracking due to contraction of the solidifying lava.

Recently, the mechanics of basalt fracturing associated with the cooling of lava flows has received much attention in the literature. Ryan & Sammis (1978a) and

DeGraff & Aydin (1987) documented evidence to support a mechanism of incremental growth for the upper and lower colonnades in basalt flows. From their investigations on the directions of successive crack propagations On columnar joint faces, it appears that fracturing begins at the flow contacts and migrate inwards. For two-tiered basalt flows (Figure 2.1a), upper and lower colonnades of different diameters meet along a quasi-planar interface towards the interior of the flow (e.g. Songnian 1980). The termination of the crack front along such an interface implies that within each colonnade tier, the pre-established crack network advanced together: adjacent cracks being never more than an increment or so behind one another (Ryan & Sammis 1978a, 1978b).

For simple two-tiered colonnade flows there is general agreement on their evolution; however, there are several different hypotheses concerning the origin of entablature style joints in multi-tiered flows exist (Justus 1978, DeGraff & Aydin 1987,

Philpotts & Burkett 1987, Long & Wood 1987). Thick, multi-tiered flows often have Strain fabrics in metavolcanics... 4-12 the entablature consisting of internal tiers of complexly jointed stnictures not easily explained by simple cooling models (Cf. Schmincke 1967, Long & Wood 1986).

Ideal colonnade joints define prisms which remain parallel within their tier over the extent of the igneous body. Tomkeieff (1940) regarded colonnades to be perpendicular to the upper and lower surfaces of the tabular body. Indeed such arrangement would be expected from Jaeger's (1961) model of joint formation perpendicular to isothermal surfaces.

1. Colonnade structures

The axes of colonnade joints are potentially useful primary indicators of the paleovertical in volcanic rocks if the body can be shown to horizontal upper or lower contacts. For such a case, cross-joints developed perpendicular to the column axis provide a marker for the paleohorizontal. For these reasons, the primary fabric of columnar jointed rocks are of interest to the structural geologist.

Actual colonnades are not perfect polyhedral prismatic structures. Column A of

Figure 2.11 shows individual joint-faces that decrease or increase in width along the column axis and eventually merge into or consume adjacent joint-faces respectively. As a result, successive sections across a single column reveals polyhedral outlines with different lengths and number of sides. In practice, the column axis can be found to a good approximation by taking the average orientation of several columnar joint intersections (see Figure 3.1a). The greater the degree of colonnade parallelism, the smaller the angular scatter of column-face intersection lineations.

In the basaltic flows and ultramafic sills of the Chukotat Group, good examples of well defined colonnades are frequently encountered in the study area. The presence of remarkable colonnade joints in the Cape Smith Tectonic Belt has been noted by Strain fabrics in metavolcanics... 4-13 numerous previous workers (Gunning 1933, Shepherd 1959, Beall 1959, 1960, Stam

1961, Gold 1962, Fujiwara & Schwarz 1975, Barnes et al. 1982, Hynes & Francis

1982, Moorhead 1986, St-Onge et al. 1987).

At the western end of the Belt, Gunning (1933) recognized that the columnar structures lay approximately perpendicular to top or bottom flow contacts. Beall (1960) however, did not include the structures among his criteria for inferring the orientation of igneous units. This relationship was examined at several well exposed outcrop areas in the least deformed parts of the Fox Lake area and it was found that the colonnade axes are approximately perpendicular (2 to 10° of true angular difference) to upper or lower contacts.

2. Entablature structures

In contrast to the distinct recognition of colonnade joints in the Cape Smith

Tectonic Belt by the many earlier workers listed above, none, to the authors' knowledge, have reported observations of entablature style joints. Only one example of entablature joints was found in the Fox Lake area (Figure 4.11). Thick lava flows are more likely to develop three-tiered joint systems (Figure 2.1b) than thinner lava flows

(and intrusive sills ?) which generally consist of a two-tiered arrangement of colonnades.

Metavolcanics of the Chukotat Group are made up of a sequence of interlayered pillowed flows and massive flows ranging from 2 to 30 m and typically 5-10 ni thick.

The scarcity of entablatures in the Cape Smith Tectonic Belt may be due to the dominance of relatively thin flows and perhaps selective erosion.

In addition to massive flows and sills, crude radiating columnar joints have been observed in some large pillows 1-2 m across.: The radiating display of columns 5 to 10 cm across is similar to rosettes illustrated by Spry (1962) and appear to be controlled Strain fabrics in metavolcanics... 4-14 solely by the outline of the individual host pillow. Internal radiating columns are not an uncommon occurrence in pillows and similar structures have been described from the

Precambrian (Dimroth et al. 1978, p. 907) to Recent volcanics (Snyder & Frazer 1963, p. B3; Moore et al. 1971, Fig. 2c, p. C91).

In conclusion, meaningful estimates of the paleohorizontal cannot he determined from cross-joints in the entablatures due to the inherent directional irregularity of the structures.

DEFORMATION OF COLUMNAR STRUCTURES

The previous chapter discussed the methods and results of sectional strain measurements from basalt polygons taken from sections of colonnade joints. It is appropriate to note here that the relative homogeneity of massive flows makes these strain gauges good markers of sectional strain at the scale of the flow unit.

Compatibility of paleohorizontal and paleovertical markers

Schwerdtner (1978) called attention to the fact that in strained rocks, paleohorizontal and paleovertical markers lose their initial orthogonal relationship. As strain increases, their angular deviation from 90° become more significant. Indeed where strain was higher in the deformed metavolcanics of the Fox Lake area, the angular difference between the colonnade axis and cross-joints increased. It is therefore preferable to infer the orientation of volcanic layering from an average of several cross- joint orientations, structures which track the paleohorizontal throughout the deformation.

At a fortuitous location where both cotonnade joints and pillow-shelves were present, the column axis from the underlying flow was found to be 68° to an overlying Strain fabrics in metavolcanics... 4-15 pillow-shelf orientation (Figure 4.12). This example illustrates the limitations of using the colonnade axis to estimate the volcanic layering since the independent markers are no longer orthogonal. However, to be conclusive about the angular relationships, it is preferable for several orientations to be averaged from each marker.

SHEET JOINTS AND THEIR DEFORMATION

Near the margins of some massive basalt flows are sheet joints, oriented parallel to the lower or upper surface. Sheet joints or platy joints (MacDonald 1968, p. 43) are considered primary in origin, forming as a result of shear along flow-planes during solidification in a semi-solid crystal mush.

In lava flows or intrusive sills of the Chukotat Group, sheet joint planes are regularly spaced every 2 to 5 cm and somewhat discontinuous, often occurring for up to a metre above or below the quench-solidified upper or lower contacts. Beall (1959.

1960) regarded the joints as "a valuable criterion" for inferring the orientation of sills in the volcanic-intrusive sequences. The joint surfaces are often observed to be tectonically deformed into concentric folds or the surfaces may be used as shear planes near shear zones (Figure 4.13).

Concentric folding of sheet joints is commonly encountered in the Chukotat basalt flows as shown in Figure 4.14. Folds may be open to tight, according to the degree of shortening, and provide fold axes directions that coincide with the regional axis of the major structure. During folding, flexural-slip occurs between layers defined by the joint surfaces. A flexural-slip mechanism is interpreted from the presence of fibrous mineral growths (amphibole ?) aligned; along the a kinematic direction in the limbs of the fold. Strain fabrics in metavolcanics... 4-16

Dilatant hinge zones in the flexural-slip folds are filled with fibrous quartz, amphibole and interstitial carbonate. The solution weathering of carbonate exposes the

mineral fibres, revealing a fanning of preferred long axis directions within the ac plane.

The orientations suggest a progressive rotation from dominantly slip motion along the a direction in the limbs to pure extension along the c direction at the hinge line proper.

Dilatant radial joints (Hobbs, Means & Williams 1976, Fig. 7.4, p. 294) near hinge

zones (Figure 4.14) are occupied by coarse quartz rods a few mm in diameter, oriented approximately perpendicular to the vein walls. The radial joints are perpendicular to the

folded layer and appear to have opened in a purely tensional fashion.

PIPE AMYGDALES

Pipe amygdales are long tubular cavities that are formed by rising gas bubbles in cooling lava and later filled in by secondary mineralization. Pipe vesicles usually occur at the base of volcanic flows that host them and form vertically in stagnant, cooling

lava (Walker 1987, Waters 1960). Pipe amygdales are not ubiquitous in basalt flows, and their absence does not necessarily indicate the lava was moving during solidification.

Pipe amygdales in the Chukotat Group are known to occur in other parts of the

Belt (Paul Barrette pers. comm., 1987), but none were found in the basalt flows of the studied area. Strain fabrics in metavolcanics... 4-17

FELSIC VARIOLES AND THEIR DEFORMATION

The origin of the varioles in basaltic lava remains the subject of much controversy (for discussion see Fowler et al. 1987). For structural work, the genesis of the felsic bodies is of little consequence since whether formed by liquid immiscibility or

products of spherical crystallization. There is general agreement that their geometry, in the initial state, is approximately spherical. Basaltic rocks containing the spherical masses of felsic material are referred to as variolites.

Variolites in the Cape Smith Tectonic Belt are characteristic of the pyroxene- phyric basalts of the Chukotat Group, described by Francis et al. (1981), Hynes &

Francis (1982) and in the Chukotat Lake area by Moorhead (1986). Individual felsic varioles are typically 1 cm in diameter and range from 2 mm to almost 3 cm in diameter. The value of varioles for structural work is their potential use as three- dimensional strain markers.

In the Fox Lake area, varioles are most commonly encountered in the pyroxene- phyric pillowed basalts. In NNW trending cross sections to the regional structure, sectional outlines of varioles indicate a heterogeneous strain distribution within pillows:

1.) Where pillow selvages are subparallel to S„ varioles near the outer margins are flattened in the trace of S,. 2.) The varioles close to the portion of the selvage orientated at high angles to S,, by contrast, suffered little or no apparent strain (Figure

4.7b). At progressively higher levels of strain, the effect of strain shadows within

pillows is less significant and ellipsoidal varioles are often fractured by tension gashes.

These tensional fractures are only present in the varioles, indicating a more competent

behaviour of the felsic masses in a less competent basaltic matrix. As a result of this viscosity contrast, deformed varioles are minimum strain gauges for the basalts. Strain fabrics in metavolcanics... 4-18

THE POSSIBLE ORIGINS OF "LAYERING" IN SILLS AND FLOWS

The interpretation of primary igneous layering in differentiated intrusives remains highly problematic (Campbell 1978). Primary sequences of rhythmic layering have been described from the vertical side walls of intrusive bodies by Philpotts (1968) and

McCall & Peers (1971). Rhythmic layering of felsic-rich layers alternating with mafic- rich bands display excellent planar interfaces that do not necessarily represent horizontal gravity settling horizons (Campbell 1978).

Some layered gabbroic to dunitic sills of the Chukotat Group exhibit strongly parallel and laterally continuous mineralogical layering. The Katiniq Sill in the eastern part of the Cape Smith Tectonic Belt described by Barnes et al. (1982) is an example.

The uncertainty of the primary orientation of these layers does not permit their use as paleohorizontal indicators. Greater uncertainty arises when attempting to use the magnetite-rich horizons in serpentinized ultramafic units as markers of primary igneous layering; many are oxides confined to remarkable (rhythmic-like) regularly spaced (1-3 cm) tectonic tension fractures. The trails of oxide resemble primary chromite or magnetite seams, but in thin section the associated serpentine (± chrysotile) occurs in optical continuity from veinlet to veinlet. The magnetite in the veinlets exist as elongated grains and in chains of euhedral to subhedral grains oriented perpendicular to the walls of the fractures. 1 to 3 mm thick serpentine-magnetite veinlets also cross cut large oikocrysts of pyroxene. These observations support a tectonic origin for the oxide rich veinlets. Strain fabrics in metavolcanics... 4-19

DISCUSSION

In thick (t > 10 m) semi-rigid slabs, contact-parallel faulting is most often observed, with little internal deformation. Occasionally, such faulting occurs along major internal zones of weakness, such as the interface between upper and lower colonnades (Cf. Figure 2.1) in columnar jointed flows. A smaller component of strain is also partitioned internally, as evidenced by the folding of sheet joints and the deformation of basalt columns as determined from sectional strain analysis, described in

Chapter 3. These observations suggest that the tabular or lenticular bodies appear to act as buttresses, focusing a large component of strain to their upper, lower or sometimes internal contact surfaces.

Since pillowed basalts deform heterogeneously by several deformation mechanisms, they can be thought of as a heterogeneous variety of basalt. At moderate to high levels of strain, some strain is partitioned into an anastomosing network of C- surfaces of limited slip around the selvages of individual pillows (recall Figure 4.1).

This fabric is pervasive throughout the thickness of the pillowed unit (Figure 4.15) and has been described in the eastern part of the Cape Smith Tectonic Belt as a 'wrap- around' shear fabric by Sawyer et al. 1983.

DISTRIBUTION OF STRAIN FABRICS IN THE FOX LAKE AREA

The limited number of sectional strain ratios in a similar orientation

(subhorizontal) does not permit a lengthy interpretation of their significance to the structural history of the Fox Lake area. The long axis of the sectional strain ellipses are approximately parallel to the trend of the Fox Lake syncline and in general, to the Strain fabrics in rnetavolcanics... 4-20

Cape Smith Tectonic Belt. The only other observation noted in this study is a general tendency for sectional strain ratios (R = A/B) to increase in proximity with splay faults or the basal thrust fault of the Chukotat Group.

Two summary sketchs of the deformation fabrics encountered in the Fox Lake area are presented, but further description of the fault and fold styles are left to

Chapter 5. A part of the geological map shown in Figure 4.16, suggests that strain

ratios increase with increasing proximity to mapped and suspected fault traces. This observation is supported by similar qualitative increases of deformation in pillows and other primary volcanic structures, particularly at the base of the Chukotat Group as illustrated in Figure 4.17.

CONCLUSIONS

Progressive strain fabrics developed in pillowed and massive basalt flows delineated the zones of "highest" strain around observed thrust planes. Strain gradients are recognized in close proximity (up to 20 metres away) to the basal Chukotat thrust plane and to a lesser degree, along associated splay faults. Qualitative and, when possible, quantitative strain gradients in the metabasalts were used to help identify suspected fault zones where exposure was poor. Stratigraphie discordances across the high strain zones, especially around the presumed splay faults, supported the inference of faults.

The strain state of the basalts, together with their orientation data, aid in better determining the pattern of faults in the deformed Chukotat metavolcanic rocks in the

Fox Lake area. A cursory examination of the ;metabasalts tends to underestimate the bulk strain when visually evaluating a single aspect, like flattening for instance. A Strain fabrics in tnetavolcanics... 4-? 1 more complete inventory of the qualitative deformation fabrics and quantitative strain information allows for a better understanding of the bulk strain and the style of deformation affecting the rocks. Strain fabrics in metavolcanics... 4-22

Figure 4.1 Photograph of a fresh surface on the outer margin of an olivine-phyric basalt pillow. Field of view is 10 x 7 cm (near actual size). A horizontal, 2 cm thick portion of the pillow selvage is exposed. Dark (green), mm sized specks of serpentine pseudomorphically replaces olivine microphenocrysts in the light (bluish-gray) colored, aphanitic matrix of the selvage. High strain tangent to the selvage not only occurs along interpillow contacts at the outside of the selvage, but along the internal contact of the selvage as well. Near the contacts of the selvage the serpentine grains are deformed into a selvage parallel foliation; the internal part of the selvage is not visibly affected. Right side of the photograph: varioles (light colored) several mm in diameter are visible in the dark crystalline interior (not common in the olivine-phyric variety).

Strain fabrics in metavolcanics... 4-23

Figure 4.2 Sketch showing a cut-away view into a lava tube, revealing the horizontal, elliptical shape of four successive drainage events (from top to bottom). The curved lava tube plunges gently to the lower right. True Y.Z0 sections, AA' and BB', across the same lava tube reveal different outlines and internal features. Basalt septa often host fine, closely spaced (1 to 5 mm) cooling joints. In outcrops, pillow-shelves may be fortuitously exposed in sections such as BB'. Profiles like AA' does not necessarily preclude the possiblity that no pillow-shelves exist the lava tube in some other section. 4-27

Figure 4.3 A basalt pillow containing numerous pillow-shelves dipping steeply to the north. Planar basalt septa indicate a lack of internal deformation to the basalt pillow. 4-28

Figure 4.4 Equal area stereogram of 15 pillow-shelves taken from a single outcrop area exposing approximately 5 m laterally and 2 m in thickness of a pillowed basalt layer. Most poles lie 4° to 14° from the average orientation (127°/23°SW). The largest angular deviation measured is 21°. 4-28

Figure 4.5a Photograph of two adjacent basalt pillows with a distinct angular discordance between drainage events from a small diameter pillow, below (n=2), to the larger overlying one (n=3). No tectonic deformation is apparent. The angular difference between the two sets of poles shown in the stereogram below is attributed to settling of the volcanic pile during accumulation. 4-29

Figure 4.5b Equal area stereogram of pillow-shelf orientations from the two contiguous basalt pillows measured from the photograph of 4.5a (shown above). Two pillow-shelf orientations (open circles) are from the lower, smaller pillow and the three pillow-shelf orientations (closed circles) are from the upper, larger pillow. 4-29

Figure 4.6 Pillowed Chukotat metabasalts are exposed in section along well developed. metre spaced Q-joints (ac-joints). Facing south, Q-joints are subvertical and strike NNW. perpendicular to the fold and thrust structures of the Cape Smith Tectonic Belt. (Assistant, Paul Arscott, is standing centre, right). 4-30 Strain fabrics in metavolcanics... 4-24

Figure 4.7 A diagrammatic summary of the style of deformation observed in pillowed basalts of the Chukotat Group. A series of Y0'Z0' sections through four kinds of pillows is illustrated. The implied conditions of deformation is that the plane of flattening is roughly parallel to the X0Z, principal plane of the pillow (shortening parallel to the Y,, axis). Sketchs a,c,e,i show model undeformed pillows in Y0Zo sections. The vertical axis of the diagram represents increasing strain. See text for discussion. 4-31

Figure 4.8 An eastward view on a subvertical exposure (338°/87°NE) of deformed metabasalts. Pillows are flattened into 'molar' shapes by lateral compression across the width of the pillows: pillow-shelves dip moderately to the NW while S, dips moderately SE. For the 28 pillow outlines recorded, the average ratio of Zo'1Y0' = 1.32. A component of lateral shortening is suspected when Z„'/Y0' > 1. Some dilation, creating large interpillow spaces, and interpillow-slip occurs between pillows previously moulded against one another. Compare with Figure 1.3 (10 cm scale bar is at centre, right of the photograph). 4-30

Figure 4.9 A Yo'/Zo' section of a basalt pillow with 6 pillow- shelves exhibits internal deformation by buckling. Carbonate (weathered away by dissolution) ± quartz infilling the cavities behaved as the incompetent member. The relatively competent behavior of the basalt septa is testified by the constant thickness of the folded septa. 4-3?

Figure 4.10 Several N-S striking Q-joints provide a series of Yo'/Zo' exposures across east trending, subhorizontal pillow tubes. Serial sections across the X0'-direction of the pillow tube serve to illustrate the heterogeneous deformation within the same 'pillow'. (a) Foreground, shows a large (1.5 m diameter) pillow tube containing numerous pillow-shelves, fractured with only minor folding (Scale bar is 10 cm long, background shows inset of Figure 4.10b). (b) 5 ni along strike, the same pillow tube exhibits tight buckle folding to complete hinge rupture and imbrication of basalt septa. Dissolution of large amounts of the incompetent carbonate rich cavity fillings must have occured during the folding. The intensity of deformation is seen in the lower right where a continued flattening of a septum is achieved by folding along 1/2 cm spaced fractures. 4-33 Strain fabrics in metavolcanics... 4-25

Figure 4.11 An inverted fan (Spry, 1962) is exposed from the entablature tier of a columnar jointed basalt flow in the Chukotat Group. Photograph was taken facing north (Field book, right, is 12 x 19 cm). Note the well developed tectonic Q-joints cutting across all primary columnar structures.

Figure 4.12a The upper, irregular surface of a pillowed basalt flow is directly overlayen by a younger, columnar jointed flow (upper left). Pillow-shelf dips moderately north and lies to the immediate upper right of the 10 cm scale bar. 4-35

Figure 4.12b Equal area stereogram of a columnar joint intersection and the pillow-shelf orientations taken approximately 1 m apart in Figure 4.12a. The orientations are comparable, but several examples of each structure are required for a more accurate assessement due to the scatter of data associated with each type of marker (see Figures 3.1a & 4.4). 4-35

Figure 4.13 Near the basal contact of a massive metabasalt flow (hammer), closely spaced (19 per metre) but discontinuous contact parallel fractures are developed. They are possibly sheet-joints. 1.5 m above the base of the outcrop, a narrow zone of high strain is exhibited by a strong C- fabric. Outcrop is located near the base of the Chukotat Group. 4-34

Figure 4.14 Facing eastward, down plunge. Concentric folding of primary sheet-joints in a massive flow. Flexural slip is the perceived folding mechanism which operated along the joint surfaces. Radial joints rupture individual sheets (centre) and dilate with quartz rods, growing roughly perpendicular to the fracture wall. 4-36

Figure 4.15 Near normal and oblique sections of basalt pillows are exposed in a N-S section. Strong developement of slickenside-striations around the outer margin of pillow selvages suggest some degree of tectonic movement between pillows. This anastamosing foliation around the pillows is particularly well developed in this outcrop. Compare with the lesser deformed pillows shown in Figure 4.8 and the virtually undefotméd pillows of Figure 1.3. 4-36 Strain fabrics in metavoicanics... 4-26

Figure 4.17 A composite sketch of the basal 10 m of pillowed basalts in the Chukotat Group. Metabasalt (chlorite schist) with a very strong C-fabric occurs at the base, overlying the carbonate rich metasediments of the Povungnituk. Overlying the chlorite schists, a fault parallel fracture cleavage and slip-surfaces cut indiscriminately across partly discernable pillows, decapitating some along pillow- shelves. Above this zone, pillows outlines are preserved and deformation is internal or along pillow - pillow contacts. Very localized brittle, antithetic shears of limited displacements (less than 1 m) are also present. 4-38 Strainf abrics inmet avolcanics... ~ Figure 4.2 basalt septum drainage cavity pillow shelf selvage B Strain fabrics in metavolcanics... 4-28

Figure 4.3

15 pillow-shelves from

one outcrop area

N 15 k 5.3 Vinv E 2.8 s 0.9 Peak: value 13.9 position 377 67°

~ — Figure 4.4 Strain fabrics in metavolcanics... 4-29

Figure 4.5a•

angular discordance

between 2 basalt pillows

Figure 4.5b Strain fabrics in metavolcanics... 4-30

Figure 4.6

Figure 4.8 DEFORMATION FEATURES OBSERVED IN PILLOW BASALTS S

OF THE CHUKOTAT GROUP t r ai n f Massive pillow (spherulitic) Vesicular (amygdaloidal) core Multi-level pillow-shelf development ab ri cs i n , n et av ol c a ni c s in tra s ing reas inc

J\ k Figure 4.7

h Strain fabrics in metavolcanics... 4-32

Figure 4.9 Strain fabrics in metavolcanics... 4-33

Figure 4.10a

Figure 4.10b Strain fabrics in metavolcanics... 4-34

Figure 4.11

Figure 4.13 Strain fabrics in metavolcanics... 4-35

Figure 4.12a

ili • pillow shelf olonnade axis

All

Figure 4.12b Strain fabrics in metavolcanics... 4-36

Figure 4.14

Figure 4.15 Strain fabrics in metavolcanics...

Chukotat pillowed and massive meta—basalts

41, Povungnituk meta—sediments and meta—volcanics

Figure 4.16 A sketch map showing the location of sectional strain measurements (centre of ellipse) and the magnitude of strain (R) on gently dipping (maximum 25°) to subhorizontal planes. Dashed lines in the Chukotat Group mark the trace of volcanic layering. For the 5 available measurements, greater sectional strain is recorded in proximity to basal décollement of the Chukotat and help to infer the presence of splay faults. The décollement and splay faults are shown with a continuous line and closed triangles. Strain fabrics in metavolcanics... 4-38

Composite Fault Zone Profile

NNW SSE

10 m

ics Chukotat lcan antithetic shear ta-vo me

Fault contact en

im Povungnituk d ta-se me Figure 4.17

Chapter 5: GENERAL GEOLOGY OF THE FOX LAKE AREA

ABSTRACT

The Fox Lake area studied is about 30 km=, lying near the geometric centre of the Cape Smith Tectonic Belt, New Québec. The rocks exposed in the area are predominantly metavolcanics of the Chukotat Group. Detailed structural mapping of folds and stratigraphic discordances was possible in deformed metavolcanics by measuring the orientations of primary structures that mark the paleohorizontal.

The structural style in the Fox Lake area is different for the Chukotat and

Povungnituk Groups: evidence for faulting is predominant in the Chukotat metavolcanics, while the effects of folding are best observed in the layered Povungnituk rocks. The folding can be described by two nearly coaxial folding events, with fold axes plunging gently east or west, parallel to the trend of the Belt. Minor crenulations developed transverse to-the earlier folds are also recognized.

Thrust faults of several generations have predominantly southward directions of

transport. An early imbrication of thrust slices at the base of the Chukotat, stacks a

previously formed syncline of metavolcanics. This folded structure, called the Fox Lake

syncline, is further tightened by the second folding event and is also truncated to the

north by late, high angle thrust faults.

INTRODUCTION

This chapter presents a description and interpretation of the geology in the Fox

Lake area, with emphasis on the structural evolution. Excellent petrographic and field Structure of the Fox Lake area... 5-2 descriptions of the lithological units has been documented by Moorhead (1989) and only a brief description of the units locally encountered is included in this work.

The area under investigation is located within the southernmost fault block of the Chukotat Group, directly overlying the Povungnituk Group (see Figure 1.2). The interpretation of the Fox Lake area is presented on the geological map and the numbered outcrop stations are given on a separate map with UTM coordinates

(Appendix 5A.1). A reinterpretation of the structural evolution of the Fox Lake area is based upon detailed field mapping and strain analysis. The major structures identified by Moorhead (1986a. 1989) are largely confirmed. Several secondary splay faults, however, which stack and truncate the Fox Lake syncline have been identified in this study. The complex deformation in the Povungnituk can be broadly described from stereograms of the structural elements, but field relationships due to uncertain stratigraphy remain problematic.

THE CHUKOTAT GROUP

Each thrust sheet of the Chukotat Group is about 2.5 to 6 km thick and laterally continuous for several tens of kilometres. No interflow sediments were found in the 15 km 2 of Chukotat exposure in the study area. Five kilometres to the west, Moorhead

(1986a, 1989) identified phyllites at the base of the thrust. The Fox Lake syncline is composed mainly of weakly strained pillowed and columnar jointed basaltic and komatiitic basalt flows and consanguineous sills.

The three varieties of Chukotat basalts described in Chapter 1 were distinguished in the Chukotat Lake region by Moorhead (1986a, 1989) and recognition of each variety can be a useful mapping tool. In the Fox Lake area, only the olivine- and pyroxene- Structure of the Fox Lake area... ;_3 phyric basalts were encountered. Approximately 2/3 of the volcanic stratigraphy consists of pillowed basalts (Figure 5.1) and 1/3 as 2 to 30 m thick massive basalt flows (Figure 5.2). Accurate estimates of the proportion of pillows to massive flows is difficult because of the preferential exposure of pillows and lateral facies gradations between the two. The majority of massive flows were encountered in the lower part of the Fox Lake syncline. About 80% of the outcrops of pillowed basalt are olivine- phyric. The two varieties are not distinguished on the geological map. Pyroxene-phyric pillowed basalts are most commonly found in Domain C, but are generally confined to narrow sequences a few metres thick in the hangingwall of certain splay faults and along the basal décollement of the Chukotat.

Excellent three-dimensional exposures of pillowed basalts permitted sonic paleo- flow directions to be determined using the direction of the lava tubes. Results presented in Appendix 5.2 indicate that the average flow direction was towards 050°, suggesting a western source for the basalts of the Fox Lake area.

THE POVUNGNITUK GROUP

The Povungnituk rocks are represented in the Fox Lake area by the Beauparlant

Sub-group. Locally, the Beauparlant consists of a wide variety of metavolcaniclastic deposits interlayered with metabasalts and relatively thin metasedimentary beds.

Chlorite-rich ash to lapilli metatuffs are the most abundant rock types. The only additional unit encountered not described by Moorhead (1986a, 1989) is a subaerial to shallow water basalt flow, shown in Figure 5.3. This unit is characterized by highly vesicular, irregular pahoehoe toes and an internal concentric zonation of vesicles (Phil

Thurston 1987, pers. comm.). The matrix is a light gray to white crystalline carbonate, Structure of the Fox Lake area... 5-4 with rare fragments of light gray to buff coloured epiclastic beds found interstitially between the irregular pahoehoe toes.

Some of the limestone and dolostone units in the Beauparlant Sub-group described by Moorhead (1986a, 1989) are reinterpreted in this study. Thin-section examination revealed many of the clastic grains to be angular shard-like fragments or crystal fragments in a carbonate matrix. These observations suggest that they may be

pyroclastic deposits which have undergone extensive carbonate replacement. Some of these deposits were eroded and redeposited as intraformational conglomerates (Cf.

Moorhead 1989) which locally cap the Beauparlant stratigraphy. Further study of the sedimentological character of these deposits would provide more insight on the nature of the Povungnituk depositional basin.

THE POVUNGNITUK - CHUKOTAT DISCORDANCE

The nature of the Povungnituk - Chukotat contact in the western part of the

Cape Smith belt was described by Bergeron (1957, 1959) as an angular unconformity.

Beall (1959, 1960) and Gold (1962) agreed with Bergeron's findings in other parts of the Belt. In the Cross Lake area, which lies some 50 km east of the Fox Lake area,

Beall (1959) described the contact as an angular unconformity with "the possibility of some southward slippage of the Chukotat Group over Povungnituk rocks".

Shepherd (1959) and Stam (1961) were the first to describe the deformation at the contact between what is now recognized as the Chukotat - Povungnituk boundary.

They disagreed with Bergeron's earlier findings and concluded that the contact \vas a

north dipping thrust fault. Regional mapping -by Stevenson (1968), Taylor (1982),

Lamothe et al. (1984), Roy (1985) and Moorhead (1986a, 1989) as well as Structure of the Fox Lake area... 5-5

investigations by Hynes & Francis (1982) and St-Onge et al. (1986, 1987) all show a

tectonic fault contact at the base of the Chukotat Group. In this study, evidence of a

penetrative foliation and chloritization of the metabasalts, shown in Figure 5.4, was

found at the base of the Chukotat to support the presence of a fault in the Fox Lake

area as well.

The Fox Lake syncline appears to be the western equivalent of the Cross Lake

syncline described by Shepherd (1959), Beall (1959) and St-Onge et al. (1987). In the

Fox Lake area, the volcanic pile of the Chukotat Group is folded into an open syncline,

plunging gently to the WSW. It is exposed in oblique section (Figure 5.4) and provides

a rare opportunity to examine how the layered metavolcanics deform in various

locations around the fold in response to the deformation.

STRUCTURAL EVOLUTION OF THE FOX LAKE AREA

The Chukotat volcanics have been described by most previous investigators as an

undeformed. or a weakly deformed coherent assemblage of mafic metavolcanics at

greenschist facies. Their emplacement is generally believed to be the result of tectonic

tilting into a parallel set of north dipping homoclinal blocks, separated by thrust faults.

The displacements along these faults have been interpreted to be very limited (e.g. Beall

1959) or up to the order of 60 to 90 km (Hoffman 1985). The relative 'pristine' nature

of the Chukotat basalts is largely due to their juxtaposition against the moderate to

highly deformed and metasomatically altered metavolcanics of the Povungnituk Group.

Deformation textures described quantitatively in Chapter 3, and qualitatively in

Chapter 4, reveal the presence Of a distinct strain gradient above the basal thrust fault in

the hanging wall Chukotat rocks. This strain gradient is indicated quantitatively in Structure of the Fox Lake area... 5-6

Figure 4.16 by a progression of increasing sectional strains in the basalt polygons. A summary of the qualitative deformation indicators from pillowed Chukotat basalts was presented in a composite sketch (Figure 4.17) from the fault contact with the underlying

Povungnituk rocks. In the absence of direct evidence from outcrops, such as a strong foliation or C-fabric, faults were interpreted to occur along zones that corresponded with three criteria: 1) an aerial photograph lineament, 2) a stratigraphic or lithological discordance and 3) the deformation textures described above.

Chukotat Deformation

Three late stage thrust faults (termed out-of-sequence faulting in St-Onge et al.

1987) truncate the Fox Lake syncline to the north. The third fault to the north. runs the length of the map and carries a thick (300 to 1000 m) peridotite-pyroxenite sill.

The metavolcanic succession overlying the thick sill, faces northward and dips steeply to the north. A strong foliation is associated with this faulting and is accompanied by extensive chloritization of the basalts. Primary features of the host rock, such as selvages, pillow-shelves and the sac-like forms typical of basalt pillows (Figure 5.1), are completely obliterated. This intensive strain is restricted to several metres on either side of the fault and crenulates any previous S1 fabric that may be present.

The basal thrust of the Chukotat is folded into a 2-km wide. open syncline that is overturned to the north. The structure is best described as a stacked, doubly plunging syncline forming a crude basin (Figure 5.5). The opposite vergence of the structure, with respect to the rest of the belt, appears to be controlled in part by the amount of imbrication of thrust slices in the southern limb (Figure 5.6). Structure of the Fox Lake area... 5-7

The structural elements of the Fox Lake syncline in the Chukotat are presented on equal area stereograms (Figures 5.7 to 5.15), contoured by the method of Robin &

Jowett (1986).

D, Structures: The orientation of poles to pillow-shelves (an equivalent measurement to bedding planes) in Figure 5.7 is somewhat complex, and can be simplified by classing the data into four different spatial domains, A, B, C, and D (shown on the Map). Their boundaries are controlled by fault planes or. in the case of Domains B and C, a zone delineating zero plunge of the Fox Lake syncline. In general, the main S, data peak reflects moderate NNW dipping strata. The thrust slices of the Chukotat are generally homoclines, dipping quite steeply to the NNW across the Belt (Hynes & Francis 1982).

In in the Fox Lake area, the southern most thrust sheet is folded, as shown by the following stereograms.

Two great circles may be traced through the highest peak and each of the two

'secondary' peaks in Figure 5.7. Poles to these two great circles plunge very gently to the ENE and WSW. These two fold axes reflect the reversal of plunge affecting the

Fox Lake syncline. Indeed, the WSW plunging Domain B (Figure 5.12) represents the eastern half, while the ENE plunging Domain C (Figure 5.13) lies at the western half of the map area.

Figure 5.8 represents the measured orientations of a poorly developed, but fairly consistent (and weakly convergent) axial planar foliation developed in the Chukotat metabasalts. An average orientation of this foliation, called S, is 069°/49°SE. A broad. shallow second peak is also present with an orientation similar to the layer orientation maxima of Figure 5.7. This secondary peak may be a weakly developed C-fabric

parallel to bedding. Structure of the Fox Lake area... 5-8

All the overturned pillowed basalts that were encountered (Figure 5.9) are northward (and down) facing. They are mainly associated with more or less upright blocks found in Domain A (Figure 5.11). These blocks were thrust (possibly back- thrust) into place by the late out-of-sequence faults (St-Onge et al. 1987) truncating the northern limb of the Fox Lake syncline.

A slight concentration of poles near the SE margin of the stereogram (Figure

5.12) is interpreted to be the result of a counter-clockwise rotation of layers along oblique ramps due to imbrication in the southern limb of the Fox Lake syncline, shown in Figure 5.5. By contrast, no imbrication of the synclinal limbs in Domain C is observed. Similarly, no anomalous concentrations of poles outside the great circle girdle of data is seen (Figure 5.13).

The existence of a Domain D was inferred from the orientation of data which is different from that in the adjacent Domain C. The orientation of pillow-shelves plotted in Figure 5.14 show orientations comparable to those of the secondary peak in Domain

B (Figure 5.12). The boundary between Domains C and D is assumed to be a fault. but the exact location and nature of this boundary is not known.

D, Structures: A second foliation, 52 (Figure 5.15), is the only D2 structure observed in the Chukotat metabasalts. In Domain C, occasional mm- to cm-scale crenulations (S2) were developed along chloritized pillow selvages or in chlorite-rich hyaloclasite filled interpillow spaces. This S2 cleavage is most likely a C-fabric related to the late, out-of-sequence thrusts. Structure of the Fox Lake area... 5-9

Povungnituk Deformation

The structure of the Povungnituk Group in the Fox Lake area is not well understood owing to an uncertain stratigraphy, facies changes and relatively poor exposure for the degree of structural complexity. Two main phases of deformation, labelled D, and D2, are recognized. The local structural elements are similarly summarized in a series of stereograms (Figures 5.16 to 5.20).

D Structures: The first recognized deformation produced tight to isoclinal folding with an axial planar (S,) cleavage that is well developed in the schistose volcaniclastics. The

S, cleavage is marked by recrystallization into platy sheets; the rock may or may not be fissile along S1. F, folds range from small, dm-sized folds, to large recumbent folds about 30 m in wavelength that can be traced almost through the entire map length.

Figure 5.21 is an example of the D, structures present in outcrop.

D Structures: A second foliation, S2, crenulates S, penetratively; it is axial planar to the second generation of folds, F2. F2 folds can be seen mesoscopically folding the S, foliation as well F, isoclines (Figure 5.22). The regional F2 folds are identified by systematic changes in S, orientations. F2 folds with wavelengths in the order of 1 km can be seen in the Fox Lake area. They are typically south-vergent folds which are present in the Povungnituk Group throughout the entire Cape Smith Tectonic Belt, characterizing the structure of the belt. S2 exhibits little directional variation

(Figure 5.19).

The mean S2 orientation of 243°/52°NW in the Povungnituk Group is quite similar to the S2 mean of 246°/57°NW obtained in the Chukotat Group. Measured F2 axes are quite consistent throughout the Fox Lake area, and cluster tightly about their mean orientation. The unimodal stereograms for S2 and F2 indicates that F2 is Structure of the Fox Lake area... 5- 1 0 essentially cylindrical and that no significant post-D, tectonic rotational events deformed the Fox Lake area.

The mean axis direction of very gently ENE or WSW plunging F, folds (Figure

5.18) is exactly co-incident with a subsequent coaxial refolding F, axis orientated

256°-49° (Figure 5.20). The F, axes, however, display a greater directional variation.

Note in Figure 5.22, the northward vergence of the F, folds due to refolding about F,.

A Type 3 fold interference pattern (Ramsay 1967) is expected from these orientations. although no mesoscopic example was observed.

Figure 5.16 is a stereogram of bedding plane orientations of the Povungnituk

Group in the Fox Lake area. A best-fitting great circle can be traced through the contoured data to yield a fold axis plunging parallel to the common F,/F, axes.

Evidence of refolding is not immediately apparent on the stereogram, but two pairs of data peaks are noticed in the girdle, noted as a,a' and b,b' on Figure 5.16. Where the contours narrow between the peak pairs, a plane can be traced out which corresponds in orientation to the S2 (243°/52°NW) axial plane. Each peak pair therefore. corresponds to each limb of the FZ folds: the mid points between peaks representing the average

limb orientation. Since the earlier F, folds have been refolded by the F2 folding, the F,

folds should be found on both limbs. It appears that the peak values a and a' are each

limb of the F, folds on the steeply South dipping F2 limb while b and b' are the F, fold

limbs on the opposite, gently dipping FZ limb. Field evidence thus far tends to support

such an interpretation of the stereogram data. Figure 5.22 is an outcrop example

illustrating this kind of refolding event.

The above interpretations are based on the assumption that the sampling of data

has been taken uniformly across -the area. Orie might carry the speculations further by

noting that asymmetric folds, whose orientations are sampled uniformly, would favour Structure of the Fox Lake area... 5-11 the long limb over the short limb in terms of their peak values on the stereogram. This appears to be case of the southward vergent F2 folds on Figure 5.16, where the gently north dipping (b,b') limb is more strongly represented. If we now look at the relative strengths of the F, fold limbs (still assuming uniform representation) b' is a larger peak than b and possibly a' is greater than a. This geometry suggests that the F, folds are also asymmetric and that the F,, and F,b folds are both 'S'-folds (WSW plunge), on both sides of the D2 structure.

D Structures: The effects of a D, cross-folding, such as described from the eastern part of the Belt by Gold (1962), Hynes & Francis (1982), Lamothe et al. (1984). and

St-Onge et al. (1986) are not pervasive in this part of the Belt. Rare crenulations to dm wavelength folds are found plunging moderately to the SE with a NNW trending axial plane. The location and symmetry of these folds with respect to the D, structures suggests that this third direction of folding could be explained as a response to crowding in the basins or domes where F2 synforms or antiforms, respectively, reverse their plunge.

CONCLUSIONS

The general synclinal aspect of the basal Chukotat thrust mapped by Moorhead

(1986a) has been confirmed. In detail however, the internal structure of the Fox Lake syncline appears to be more complex. consisting of an imbricate metavolcanic stack.

The northward vergence and local overturning of the syncline, figured in the Structure of the Fox Lake area... 5-12 stereograms, is interpreted to be the result of the imbrication in the southern limb. The

presence of splay faults has been described for the first time within the regional, km- scale structural blocks of the Cape Smith Tectonic Belt identified by Hynes & Francis

(1982). Observations on the deformed primary volcanic structures and sectional strain

measurements from basalt polygons provided important information about the distribution of strain in the Chukotat rocks. Identification of some faults was possible

through recognition of increasing strain gradients towards high strain zones defining the

trace of the fault. For these reasons and because of the striking contrast in structural style above and below this contact, the boundary between the Chukotat and Povungnituk

Groups can correctly be described as a major décollement zone.

In the Fox Lake area, the structural geology of the Povungnituk and Chukotat

Groups can be described by two main, nearly coaxial ENE-WSW deformations. The

first phase produced extensive close to isoclinal folds mainly affecting the Povungnituk

Group and km-scale open folds in the Chukotat. Apparent right-lateral offsetting of the

Fox Lake synclinal axis by the oblique splay faults suggests that thrust faulting of the

Chukotat Group over the Povungnituk Group occurred during the latter part of this

event.

The second deformational event refolded the earlier fault and fold structures

developed in the Povungnituk rocks. Evidence is scarce that a second deformation

affected the Chukotat rocks beyond a possible tightening of the Fox Lake synclinal structure. Structure of the Fox Lake area...

Figure 5.1 Photograph of a cross sectional exposure through a coherent pile of subvertically tilted basalt pillows of the Chukotat Group. These pillows are closely moulded against one another and show no evidence of differential movement between pillows. Pillow outlines indicate a facing (tops) direction to the right. Well developed radial cracks of primary origin initiated around the pillow margins figure prominently. [Domain A]

Figure 5.2 Photograph of a three-dimensional exposure of a massive basalt flow in the Chukotat Group. Upper right, reveals a moderately north dipping ropy lava surface. In section, centre, the upper most 1 m of the lava flow is massive, below which a tier of slender colonnettes (a style of columnar jointing) is developed at near right angles to the flow top. Field book (lower left) for scale measures 12 x 19 cm. [PB8738]

Figure 5.3 Photograph of highly vesicular (shallow water - subaerial) pahoehoe basalt, outlined in white, displaying good concentric zoning of the vesicles. Centre right reveals a laminated sedimentary clast between pahoehoe toes. [PB86-178]

Figure 5.4 Highly strained pillowed basalt (unrecognizable) at the base of the Chukotat. Closely spaced cross-joints (Q- joints) intersect the C-fabric to form crude pencil structures. [PB86-224]

Figure 5.5 The synclinal hinge zone of the basal thrust of the Chukotat Group is shown in this oblique aerial photograph facing WSW. The southern limb dips 80°-85°NNW and the northern limb dips 20°S, resulting in a northward vergence of the structure. [Domain B] '

Figure 5.6 Facing NNE along the basal (subhorizontal) Chukotat thrust. A differentiated mafic sill overrides a southeastward thickening wedge of pillowed metabasalt. The Povungnituk rocks, foreground, are not exposed. [PB86-223] Structure of the Fox Lake area... 5-14

Figure 5.7 to 1.20 Equal area stereograms of the structural elements in the Chukotat and Povungnituk Groups.

5.7 Pillow-shelves (bedding) in Chukotat basalts. 5-18 5.8 S, foliation in the Chukotat basalts. 5-18 5.9 Downward-facing pillow-shelves. 5-19 5.10 Upward-facing pillow-shelves. 5-19 5.11 Domain A (Chukotat north) pillow-shelves. 5-20 5.12 Domain B (Chukotat east) pillow-shelves. 5-20 5.13 Domain C (Chukotat west) pillow-shelves. 5-21 5.14 Domain D (Chukotat northwest) pillow-shelves. 5-21 5.15 S2 crenulations in foliated Chukotat chlorite-schists (metabasalt). 5-22 5.16 Bedding planes in the Povungnituk Group. 5-22 5.17 S1 foliation in the Povungnituk Group. 5-23 5.18 F1 fold axis orientations of minor parasitic folds in the Povungnituk Group. 5-23 5.19 S, crenulation, strain-slip, or fracture cleavage in the metavolcanics of the Povungnituk Group. 5-24 5.20 F2 minor fold axes in the Povungnituk Group. 5-24

Stereoaram abbreviations: N is the number of orientation measurements. k is the kurtosis of the conical, Gaussian-weighted counting function. E is the expected density of data, assuming the data are uniformly distributed over the entire surface of the hemisphere. s is the standard deviation from E. Density contours are traced starting at E, E+2s, E+4s, E+6s, ... Orientations on the stereogram above E+1.8s are considered significant (Robin & Jowett 1986).

Figure 5.21 An example of D1 deformation in the Povungnituk Group. Facing eastward (up plunge) on a subvenical exposure of tightly folded (carbonate-replaced) metavolcaniclastics. The F1 axis on the north vergent syncline measures 267°-21° with an axial surface of 101°/63°SW. The steeply N dipping limb is truncated by a dip-slip fault orientated 109°/29°SW. The hangingwall consists of strongly foliated ash tuff, subparallel to the shear zone (S1: 111°/43°SW). [PB8707] 5-25

Figure 5.22 A synoptic view of D2 structures affecting the Povungnituk Group. Facing westward (down plunge) on a south- vergent F2 antiform. The core of the fold contains an earlier (F1) isoclinally folded 'Z'-fold in the carbonate rich mudstones(?) or ash tuffs. The F1 fold axis is orientated 241°-*11°. S1 surfaces in the overlying chlorite-rich ash tuff are folded by F2 with an orientation of 268°-a2°. The S2 axial planar cleavage is developed only in the ash tuff as a fracture cleavage orientated 267°/43°NW. [PB8708] 5-25 Structure of the Fox Lake area... 5-15

Figure 5.1

Figure 5.2 Structure of the Fox Lake area... 5-16

Figure 5.3

Figure 5.4 Structure of the Fox Lake area... 5-17

Figure 5.5

Figure 5.6 Structure of the Fox Lake area... 5-18

Chukotat Group pillow-shelves

N 452 k 102.4 E 4.4 s 1.5 Peak: value 32.9 position 146°/ 57°

Figure 5.7

S1 foliation

N 245 cc k 56.4 E 4.3 s 1.4 Peak: value 42.3 position 339°/ 51°

Figure 5.8 Structure of the Fox Lake area... 5-19

Chukotat Group inverted pillow-shelves

N 40 k 10.9 E 3.7 s 1.2 Peak: value 21.4 position 344°/ 20°

Figure 5.9

normal pillow-shelves

N 412 k 93.6 E 4.4 s 1.5 Peak: value 35.2 position 1467 57°

Figure 5.10 Structure of the Fox Lake area... 5-20

Chukotat Group Domain A pillow-shelves

N 56 k 14.4 E 3.9 s 1.3 Peak: value 21.7 position 353°/ 11°

Figure 5.11

Domain B pillow-shelves

c N 239 k 55.1 E 4.3 ‘111LF- s 1.4 1 Peak: value 43.4 •0 position 146°/ 57°

Figure 5.12 Structure of the Fox Lake area... 5-21

Chukotat Group

Domain C pillow-shelves

N 144 k 34.0 E 4.2 s 1.4 Peak: value 39.1. position 2077 80°

Figure 5.13

Domain D pillow-shelves

N 13 k 4.9 E 2.7 s 0.9 Peak: value 9.8 position 63°/ 70°

Figure 5.14 Structure of the Fox Lake area... 5-22

Chukotat Group S2 foliation

N 27 k 8.0 E 3.4 s 1.1 Peak: value 14.9 position 156°/ 33°

Figure 5.15

Povungnituk Group bedding planes

N 202 k 46.9 E 4.3 s 1.4 Peak: value 25.8 position 135°/ 71°

Figure 5.16 Structure of the Fox Lake area... 5-23

Povungnituk Group S1 foliation

N 377 k 85.8 E 4.4 s 1.5 Peak: value 34.0 • position 3547 48°

Figure 5.17

Fl fold axes

111111 N 59 k 15.1 E 3.9 s 1.3 Peak: value 19.1 position 2567 9°

Figure 5.18

Structure of the Fox Lake area... 5-24

4 _ Povungnituk Group S2 foliation

N 100 k 24.2 E 4.1 s 1.4 Peak: value 41.3 position 153°/ 38°

Figure 5.19 ‘II7

F2 fold axes

N 123 k 29.3 E 4.2 s 1.4 Peak: value 53.1 position 256°/ 9°

Figure 5.20 Structure of the Fox Lake area... 5-25

Figure 5.21

Figure 5.22 Structure of the Fox Lake area... 5-26

APPENDIX 5.1: Location of outcrops examined (Figure 5A.1, in envelope) is on a

UTM grid from the Lac Chukotat 1/50 000 topographic sheet 35G/5 EAST.

APPENDIX 5.2: Paleo-flow directions in the Chukotat Group, Fox Lake area.

Results of a paleo-flow direction study was presented by Budkewitsch & Robin

(1989), the abstract is given below as well as key figures.

DEFORMATION OF PILLOWED BQ_SALTS IN THE UNGAVA TROUGH (CAPE SMITH BELT): BUCKLED PILLOW-SHELVES AS INDICATORS OF PALEO-FLOW DIRECTIONS

Thick sequences of pillowed basalt in volcanic piles are often incorporated into tectonic belts. Some of the best preserved examples can be found in the Precambrian greenstone belts of the Canadian Shield. Well exposed komatiitic to tholeiitic volcanics of the Chukotat Group, in the Lower Proterozoic Ungava Trough, permitted a combined detailed structural and volcanological study of a 4 x 7 km area in the central part of the Belt. A pillow-shelf is defined as the interface between a planar floor of quenched basalt and its overlying tabular drainage cavity, recording a partial draining of a lava tube during its cooling history. Episodic draining events within a single lava tube are preserved as a series of stacked basaltic septa and largè cavities, mineralized with quartz-carbonate. The shelves are somewhat elliptical in plan view, the long axis being in the direction of the lava feeder tube. The ellipticity of the shelf is controlled by the ellipticity of the tube itself and by its inclination. Mechanically, the multi-shelved pillows deform by buckling. From two-dimensional exposures, measured folc axis directions of the internal buckle folds gives the elongation direction of the tube. The elongation direction of the pillows are assumed to be parallel to the local paleo-flow direction. In the area studied, trends of local flow lines were found to be 045° to 070° by this method. Rare three- dimensional exposures of elongated pillowed lavas and tubes corroborated these findings. Branching lava tubes suggest that the actual direction of lava flow was from West to East in this area. Structure of the Fox Lake area... 5-27

Figure 5A.2a A diagrammatic view of how pillow-shelves in lava tubes deform. Mechanically, the pillow-shelf can be envisaged as an elliptical membrane clamped along its edges. If the lava tube is shortened across its width, the edge restriction tends to favour the formation of buckle fold axis along the long axis of the elliptical shelf. In this model, the maximum direction of shortening is not required to be perpendicular to the Y-direction of the tube. Therefore, the fold axis gives the direction of the lava tube, not necessarily the regional fold structure. 5-28

Figure 5A.2c A circular histogram of the three indicators for paleo-flow directions in lava tubes indicates a significant NE trend. Data is from the deformed metabasalts of the Chukotat Group in the Fox Lake area. 5-30

o J ap

MODEL: od x nnultiiaye, buckling with edge restrictions 77 d:p n Pai high low

' ~ plan ~|~9w Croc, gect,on

Figure 5/\2u Structure of the Fox Lake area... 5-29

-O-- bifurcated lava tubes buckled pillow shelves — axes of rotation (tilted pillow shelves) 1 km

Figure 5A.2b A sketch map of a single fault bounded block of the Chukotat Group from the geological map. Domain A and part of Domain B are represented. The strike of each tube axis indicator ((1) bifurcated lava tubes, (2) fold axis of pillow-shelves and (3) the axis of rotation between planar pillow-shelves (not discussed in text)) are located. Rotation corrections for local plunge and dip were found to be less than 5° and therefore considered negligible. Structure of the Fox Lake area... 5-30

estimated paleo-flow trends

n = 52 E = 3.79 a = 2.75 k = 120

Peak value = 10.82 Peak Orientation = 50.0°

E -- - - E±1.8a

Figure 5A.2c REFERENCES CITED

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