GEOLOGY AND THERMAL HISTORY OF AN AREA NEAR LAKE, SOUTHERN .

by Gary Allan Medford B.Sc.(Hon.) McGill University 1968 M.Sc. McGill University 1970

A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

in the Department of Geological Sciences

We accept thi.s thesis as conforming to the required standard.

THE UNIVERSITY OF BRITISH COLUMBIA January, 1976 In presenting this thesis in partial fulfilment of the requirements for

an advanced degree at the University of British Columbia, I agree that

the Library shall make it freely available for reference and study.

I further agree that permission for extensive copying of this thesis

for scholarly purposes may be granted by the Head of my Department or

by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my

written permission.

Depa rtment

The University of British Columbia Vancouver 8, Canada

Date i

ABSTRACT

Five phases of deformation are recognized in Shuswap (Monashee Group) gneiss in an area east of , southern British Columbia.. The first is delineated by north trending mesoscopic structures. The second comprises a south closing megascopic synform with a horizontal ESE axial direction. This structure has in turn been coaxially refolded into a more open phase 3 synform. The second deformation was associated with extensive introduction of synkinematic quartz monzonite and granodiorite that comprises much of the area, and culminated with amphibolite grade metamorphism. Phase 3 deformation was followed by extensive local recrystal1ization and meta• somatism which destroyed earlier fabric elements of the gneisses. Phases 1 to 3 are pre-mid-Carboniferous based on poor fossil evidence whereas phases 4 and 5 are Tertiary. Phase 4 comprises open flexural slip folding about NE trending axes and Phase 5 consists of broad warps about horizontal ESE axes. These deformational events were associated with high level thermal and hydrothermal activity which appears to be most intense in areas of high grade Shuswap gneiss, where it has reset K-Ar dates to about 50 million years (paper no. 1). Thermally sensitive fission track apatite dates indicate that the high thermal gradients can be traced into the plutonic rocks west of the Okanagan Valley in which K-Ar dates have been much less affected and range between 130 and 200 million years. Thus perhaps only the oldest dates represent minimum emplacement dates. The statistical methods used in acquiring the apatite dates are discussed and developed beyond that available in the literature (paper no. 2). i i

TABLE OF CONTENTS Page ABSTRACT i TABLE OF CONTENTS ii LIST OF FIGURES: GEOLOGY OF THE AREA v LIST OF TABLES: GEOLOGY OF THE OKANAGAN MOUNTAIN AREA vii LIST OF PLATES: GEOLOGY OF THE OKANAGAN MOUNTAIN AREA vii LIST OF TABLES: PAPER NO. 1 vii LIST OF PLATES: PAPER NO. 1 vii LIST OF TABLES: PAPER NO. 2 vii ACKNOWLEDGEMENTS vi i i

A. GEOLOGY OF THE OKANAGAN MOUNTAIN AREA 1 1 - INTRODUCTION TO THE FIELD STUDY AND RELATED PAPERS 1. General Introduction 1 General Geology 8 2 - STRUCTURAL SUCCESSION 14 Unit 1: Laminated Amphibolite 16 Unit 2: Hornblende (biotite)Granitoid Gneiss 18 Unit 3: Augen Gneiss 19 Unit 4: Undifferentiated Granitoid Paragneiss 21 Unit A: Leuco-quartz Monzonite 21 Unit B: Foliated Granodiorite 23 Unit C: Diorite 25 Unit D: Unfoliated Granodiorite 25 Unit E: Quartz Monzonite Dikes 28 Unit F: Protoclastic Quartz Monzonite 28 Breccias and Ultramylonites 30 i i i

Page 3 - STRUCTURE 31 Phase 1 (earliest) 34 Phase 2 37 Phase 3 43 Subset 3a 45 Subset 3b 54 Phase 4 54 Phase 5 62 Summary 67 4 - METAMORPHISM AND THE EMPLACEMENT OF IGNEOUS ROCKS 63 Early metamorphism 68 Phase 3 metamorphism 70 Late or post phase 3 metamorphism Tertiary metamorphism 72 5 - DISCUSSION 77 Correlation with nearby areas 77 Timing of deformation and metamorphism 80 Origin of sediments 82 Regional interpretation 83

B. PAPER NO. 1: K-Ar AND FISSION TRACK GEOCHRONOMETRY OF AN EOCENE THERMAL EVENT IN THE KETTLE RIVER (WEST HALF) MAP AREA, SOUTHERN BRITISH COLUMBIA 86 Abstract 87 Introduction 88 Analytical techniques 88 Distribution of K-Ar dates 89 Fission track analysis 92 Correlation of the Tertiary event with geophysical evidence 96 Regional aspects 98 Summary 99 iv

Page

C, PAPER NO, 2; ON THE COMPUTATION OF STATISTICAL ERROR IN FISSION TRACK ANALYSIS 101 Introduction 101 Fission track age equation 102 Techniques of fission track analysis 103

D. REFERENCES 116

E. APPENDIX 1: Fission-track and K-Ar analytical techniques 122 APPENDIX 2: Cooling-rate calculations 126 V

LIST OF FIGURES; GEOLOGY OF OKANAGAN MOUNTAIN

Page

1-1 Generalized geology of southeastern B.C. 2 1-2 Structural elements of the Canadian Cordillera 3 1-3 Location map of areas of study 9 1- 4 General geology of the Okanagan Valley 11

2- 1 Unit 1. Photo of hand specimen 17 2-2 Unit 2. Photo of hand specimen 17 2-3 Unit 3. Photo of hand specimen 20 2-4 Unit 3. Photo of hand specimen 20 2-5 Unit A. Photo of hand specimen 22 2-6 Unit A. Photo of hand specimen 22 2-7 Unit B. Photo of outcrop 24 2-8 Unit B. Photo of outcrop 24 2-9 Unit C. Photo of hand specimen 26 2-10 Unit D. Photo of outcrop 27 2-11 Unit D. Photo of hand specimen 27 2- 12 Unit E. Photo of hand specimen 29

3- 1 Structural domains, south part of map area 35 3-2 Photo of phase 1 fold deformed by phase 2 fold. Phase 3 cleavage 36 3-3 Photo of phase 1 fold in unit 2 36 3-4 Photo of hand specimen of a phase 2 fold 38 3-5 Thin section of nose of Fig. 3-4 39 3-6 Thin section of nose of Fig. 3-5 39 3-7 Stacked rootless phase 2 isoclines in outcrop of unit 2 40 3-8 Phase 2 folds in laminated amphibolite. Hand specimen 40 3-9 Phase 2 folds developed in unit 3 42 3-10 Stereoplot of phase 1 and 2 structures 44 3-11 Isometric view of phase 2 and 3 megastructures 46 3-12 Stereoplot of phase 3 structures 47 3-13 Synopsis of phase 2 and 3 axial directions 48 vi

Page 3 -14 Photo of phase 3a folds in mylonitized unit 3, Outcrop 50 3 -15 Photo. Detail of fold in Fig, 3-14 50 3 -16 Photo of phase 3a fold in augen gneiss. Unit 3 51

3; -17 Detail of phase 3a fold illustrating cleavage F3a 51 3--18 Photo of phase 2 fold deformed by phase 3 fold in outcrop of unit 1 52 3--19 Photo of fold mull ion in amphibolite (unit 1) outcrop 52 3--20 Photo of phase 3a fold in unit B 53 3--21 Photo of phase 3 fold in unit B 53 3--22 Photo of subset 3b fold in laminated amphibolite 54 3--23 Photo of phase 3 slide in laminated amphibolite 55 3--24 Stereoplot of poles to F2 and compositional layering 56 3--25 Schematic illustration of phase 3 structures in unit 1 57 3--26 Photo of phase 4 fold 58 3-•27 Stereoplot of phase 4 distortion of phase 2 elements 60 3-•28 Stereoplot of phase 4 structural elements 61 3-•29 Stereoplot of measured fractures 63 3-•30 Photo of a phase 5 fold 64 3-•31 Photo of protoclastic dike (unit F) cutting phase 4 fold 64 3-•32 Stereoplot of phase 5 dikes 65 3-•33 Cross-section illustrating phase 5 structures 66

4- •1 Photomicrograph of amphibolite grade assemblage in unit 1 69 4- •2 Photomicrograph of hornblende alignment parallel to l_2 69 4- •3 Photomicrograph of diopside alignment parallel to L2 71 4- •4 Photo of quartz monzonite sill, unit A, intruding gneiss 71 4- •5 Photomicrograph of strained quartz caused by phase 2 73 4- •6 Photomicrograph of F3 strain pattern in quartz in nose of fold of unit 3 73 4- •7 Photomicrograph of F3 strain pattern in quartz in granodiorite, unit A 74 4- •8 Photomicrograph of chlorite veining and sericite alter• ation caused by Tertiary hydrothermal activity 74 4- 9 Photomicrograph of sensitization of feldspar caused by Tertiary hydrothermal activity 75 5- 1 Correlation of structure in the south Okanagan. Cross- section 79 VI 1

LIST OF TABLES: GEOLOGY OF OKANAGAN MOUNTAIN

3-1 Nomenclature of structural elements

LIST OF PLATES: GEOLOGY OF OKANAGAN MOUNTAIN (in pockot) Map Gb. 1 Geology of the Okanagan Mountain area 2 Vertical cross-section of the south portion of the Okanagan Mountain area

LIST OF TABLES: PAPER NO. 1

1 Potassium-argon analytical data 2 Fission track analytical data

LIST OF PLATES: PAPER NO. 1 (4n pocket) flu? C*b > S"

1 Geochronology of the Kettle River (west half) map sheet

LIST OF FIGURES: PAPER NO. 2

1 Track density ratios + 3 standard deviations 2 Observed versus expected track frequencies

LIST OF TABLES: PAPER NO. 2

1 Computed errors using Poisson model and equation 11 2 Comparison of errors determined in this paper with those stated in the literature 3 Suggested error values for data presented in the 1iterature vi i i

ACKNOWLEDGEMENTS

The author would like to thank Dr. J.V. Ross for suggesting the field problem and supplying funds through National Research Council of Canada Grant

A-2134.

Thanks are also extended to Mrs. V. Bobik and Mr. J. HarakaT for assistance with the K-Ar analyses, and to Dr. P. Christopher for introducing the author to the fission track technique. The author would like to express appreciation to all those who gave of time and interest during the preparation of the thesis, especially the technical staff of the department.

The author acknowledges aid provided through two National Research Council

Scholarships, 1970-72, and a University of British Columbia Graduate Scholar• ship, 1972-73. Further support was obtained through Teaching Assistantships in the Department of Geological Sciences, 1970-1974. 1

INTRODUCTION

General Introduction

This study was undertaken to examine the structure of the Okanagan Mountain region, and the nature, extent and significance of a Tertiary thermal event in the Kettle River map area (west half), which includes the Okanagan Mountain region. Okanagan Mountain is located on the east side of the Okanagan Valley of south central British Columbia in an area of high-grade metamorphic rocks known as the Shuswap Metamorphic Complex (Fig. 1-1). The Shuswap Metamorphic Terrain, as originally described by G.M. Dawson (1877), is an area of high grade schists and gneisses in the vicinity of Shuswap Lake, some 80 miles north of the Okanagan Mountain map area. This survey was followed by the publication of his "Shuswap Sheet" in 1898 (Dawson, 1898). Since that time the Shuswap has been expanded to include a belt of high-grade (generally sillimanite grade) metamorphic rocks (Reesor, 1970) occupying the southern part of the core zone of the Eastern Cordilleran Fold Belt (Fig . 1-2) extending into north-central . This core zone, known as the Omineca Geanticline (Wheeler, 1970) consists of northerly trending groups of metamorphic rocks bounded on the west by the predominantly volcanic Inter-Montane Zone and on the east by the complexly folded Kootenay Arc (Ross, 1970). Daly (1912) in his geological survey of the 49th parallel of the North American Cordillera, mapped metasedimentary, volcanic, and plutonic rocks but he did not associate them with the Shuswap complex. In 1911, Daly re-examined the area previously mapped by Dawson and was the first to extend the term Shuswap Terrain to cover metamorphic and granitic rocks farther to the south. He (Daly, 1915, 1917) considered the area to be a product of load metamorphism 2

GENERALIZED GEOLOGY SOUTHEASTERN B.C.

Okanagan Lake

Granitic Rock GNEISS DOMES - Tertiary Volcanics 1. Frenchman's Cap 2. Thor-Odin M Mesozoic - Upper Paleozoic Rock 3. Valhalla

Shuswap Metamorphic Complex

Lower Paleozoic- Windermere Strata

Belt Purcell Strata FIGURE l-l 3

STRUCTURAL ELEMENTS CANADIAN CORDILLERA

FIGURE 1-2 4

induced by deep burial, and resulting in schistosity and bedding "which nearly always lie rigorously parallel" (Daly, 1915, p. 45). Gilluly (1934) disagreed with this concept after completing a petrofabric analysis of two of Daly's samples and suggested differential movement or shear parallel with the layering to be of prime importance.

Many conflicts pertaining to the ages of rocks involved and the timing of metamorphic events arise because much of the terrain has been mapped only on a reconnaissance basis (Little, 1961) and its structure and stratigraphy is poorly known. Correlation across the regional strike is difficult because of the high grade of metamorphism which has obliterated fossils and the original character of the rock. Ages determined by isotopic investigations are not abundant and are predominantly by the K-Ar method. These have given information on more recent metamorphic and plutonic events, but not the age of the rock involved in the metamorphism. Dawson (1898) believed the rocks of his investigation to be Archean because of their similarity to Grenville Series rocks of eastern

Canada, and Daly (1915) lent credence to this suggestion by proposing that

Precambrian Belt sedimentary rocks rest unconformably on Shuswap rocks in

Albert Canyon, east of Revelstoke, British Columbia. This unconformity was later disproved by Gunning (1928) although no serious opposition to the Shuswap as a Precambrian metamorphic terrain was voiced until Cairnes (1939) had completed his examination of the complex. In opposition to Dawson and Daly,

Cairnes believed the rocks to be of several ages and the metamorphism related to the intrusion of numerous Mesozoic plutons. His criterion for delineating

Shuswap rock was based on the presence of structures conformable to bedding which excluded less metamorphosed formations characterized by more upright structures. The differences were ascribed to variation in the depth of burial 5

during Mesozoic deformation and metamorphism.

In the Vernon map area, Jones (1959) mapped the Shuswap as unconformably overlain by Cache Creek rocks of possible Carboniferous and Permian age. He redefined the stratigraphy and divided the Shuswap into the Mt. Ida, Monashee, and Chapperon Groups whose stratigraphic relations to one another have not been yet established. Jones recognized younger and older periods of deform• ation and believed these all to be pre-Permian and possibly Precambrian.

Preto (1964) examined three of the areas mentioned by Jones where Cache

Creek rocks might unconformably overlie the Shuswap but was unable to find evidence for unconformities except at Salmon River.

Little (1961), in his examination and compilation of the Kettle River

(west half) map sheet, revised Cairnes' work with emphasis on stratigraphy and structure but did not obtain sufficient structural data within the area to delineate folds. He mapped the intrusives and he separated Nelson "Plutonics from the slightly younger Valhalla Plutonics (both of Mesozoic age) based on petrologic similarity to these rocks in their type area some 60 miles to the east. The layered gneiss of the map area was referred to as the Monashee

Group by Jones (1959) who designated the oldest rocks in his map area by this name.

At the eastern margin of the complex Hyndman (1968) noted Triassic

rocks of the Slocan group to have undergone the same phases of deformation as

the underlying Shuswap rocks thus indicating at least a pre-Triassic time

of sedimentation but post-Triassic deformation.

Ross (1968) suggested that Hudsonian basement rocks are involved in the

core of the Frenchman's Cap gneiss dome on the eastern margin of the complex

(Fig. 1-1), forming wedges in the cores of refolded recumbent isoclines, and has 6

identified three phases of deformation in rocks known to be of late Protero- zoic and early Paleozoic age. He suggested that two phases of deformation and metamorphism predate the Triassic (Ross, 1970).

Reesor (1970) postulated the mantles of the series of domes along the eastern margin of the complex to have been twice deformed in post-Mississ-

ippian, possibly as late as post-Triassic time. The core gneiss was considered to be metasomatically derived'from sediments as old as late Precambrian

(Windermere Group). These domes have received much attention and include

Frenchman's Cap gneiss dome (Ross, 1968; Fyles, 1970; McMillan, 1970, 1973;

Blenkinsop, 1972) the Valhalla gneiss dome (Reesor, 1965) and the Thor-Odin

gneiss dome (Reesor, 1970; Reesor and Moore, 1971).

Timing of some earliest phases of deformation in the southwestern part

of the complex has recently been shown to be at least pre-mid-Carboniferous

(Ross and Barnes, 1972) and possibly related to the Caribooan orogeny.(White,

1959). The evidence is based on the presence of non-metamorphosed sedimentary

rocks in the vicinity of Keremeos, British Columbia which contain poorly

preserved fossils of late Mississippian to early Pennsylvanian age. This

sequence overlies rocks with structures similar to those of the Vaseaux

Formation, the most westerly exposed part of the Shuswap in the Okanagan

Valley of southern British Columbia (Ross and Christie, 1969; Christie, 1973).

Farther north near Shuswap Lake, however, Fyson (1970) finds metamorphism

and four phases of deformation in rocks believed to be of Paleozoic age.

Isotopic work accomplished to date is not extensive. The K-Ar method

has been most frequently employed and has generally failed to give information

on the emplacement ages of many rocks because of the complicated metamorphic

history of the terrain, and the occurrence of a widespread thermal event in 7

the early Tertiary. A few dates, chiefly on overlying Tertiary volcanics, have been reported by Mathews (1964), White et al. (1968) and Church (1970). Fair- bairn and Hurley (1964) attempted some Rb-Sr work but were unsuccessful because of low Rb/Sr ratios. Ryan (1973) has achieved somewhat more success in the Okanagan Valley immediately north of the international border but the oldest whole-rock age obtained, 170 million years, is clearly not that of even the latest pre-Carboniferous deformation (Ross and Barnes, 1972).

Three occurrences of gneiss on the east margin of complex in the

Revel stoke vicinity have been interpreted as crystalline basement (Ross, 1968;

Campbell, 1968; Giovanella, 1968; Campbell and Campbell, 1970); these have given Rb-Sr isochrons of about 800 million years (Blenkinsop, 1972) and a 207 206

Pb /Pb age of 722 million years (Campbell, 1973). These ages are older than associated stratified rocks and as such indicate basement. Data from the lead age suggests the system has suffered a loss of lead and hence the gneiss could be older than the quoted 722 million years.

Clearly, both stratigraphic and isotopic investigations do not give a consistent and unified picture of the timing of metamorphism and deformations nor relative ages of rocks comprising the Shuswap Complex. These events may be post-Triassic in some areas and pre-Cenozoic in others (Reesor and Moore,

1971). The only unifying aspect is that all areas examined in detail display at least one intense deformation resulting in recumbent folding with variable axial trends, and involve amphibolite or more specifically sillimanite grade metamorphism.

The thesis area occupies part of the southwestern margin of the Shuswap

Complex where a number of detailed projects have been carried out at the University 8

of British Columbia (Ross and Christie, 1969; Okulitch, 1969; Ross and Barnes, 1972; Chrisite, 1973; Ryan, 1973). By concentrating on a relatively small area of the complex it is possible to avoid some of the correlation problems alluded to in the previous paragraphs. To this end detailed structural, lithologic, and isotopic investigations have been carried out. Specific details of the work are furnished in the following section.

General Geology

Okanagan Mountain is located near the Okanagan Valley of south-central British Columbia centered at latitude 49°43' N and longitude 119°36' W. The map area, comprising about one hundred square miles, is located between the major Okanagan cities of in the north and in the south (Fig. 1-3). Good access is provided to the boundaries of the area from both cities. A dirt road winds through the area via Chute Lake, and is commonly in disrepair from Chute Lake northwards. Another dirt track via Wildhorse Canyon is frequently impassible at the south end, and the road to the top of Okanagan Mountain is usable only by four wheel drive vehicle near the top. Access to the southern portion of the area is partly facilitated by the C.P.R. Kettle River Valley line which switchbacks up the mountain, as well as a few logging and Naramata Water Board roads. The area in which Tertiary heating was investigated is also outlined in Fig. 1-3. The borders correspond to those of the Kettle River Valley (west half) four mile geological map of Little (1961).

Field work was carried out over a total period of about six months during the summers of 1971, 1972 and 1973 using air photographs and 4"=1 mile base maps enlarged from 1:50,000 national topographic maps. The area north of Okanagan mountain was not covered as extensively as the southern section because of poor 9 10 exposure, difficult access and a generally homogenous lithology. The map area forms part of the created by the dissection of an early Tert• iary erosion surface (Holland, 1964). Okanagan Mountain, the highest peak in the area, is about 5200 feet in elevation or approximately 4100 feet above the mean level of Okanagan Lake (1123'). Topography is moderately steep within about one or two miles of the lake but flattens above 3000 feet. A northerly trending fracture pattern, accentuated by glaciation, has resulted in an extrem• ely clefty terrain which makes traversing in an east-west direction very tedious in many parts of the area. Evidence of glaciation is present at all elevations in the form of polished bedrock exposures and glacial deposits rimming Okanagan Lake. The glacial history of the area has been discussed by Nasmith (1962).

The area under consideration is part of the Monashee Group as mapped by Little (1961). The portion centred about Okanagan Mountain is shown as Nelson Plutonics but is best considered part of the Monashee Group as in this study no pluton has been found. The local geology is illustrated in Fig. 1-4. Farther south Oku!itch (1969), Ross and Christie (1969), Christie (1973), and Ryan (1973) have delineated polyphase deformation in the Kobau, Vaseaux, and Anarchist Formations (see Bostock, 1940, 1941a) respectively. Little (1961) mapped the latter two formations as part of the Monashee Group and these have since been found to exhibit consistent structure as well as lithologic similarity. The structure in the vicinity of Okanagan Mountain is also com• parable but lithologic correlation is dangerous as the area is located some twenty miles north. Within the limits of detailed study to date it appears that the Monashee group has undergone an early phase of deformation which the Kobau and adjoining (Late Paleozoic ? see Neugebauer, 1965) Old Tom and Shoe• maker Formations did not experience. In all, five phases of deformation are recognized. The first three are characteristically isoclinal recumbent structures 11

LEGEND Sedimentary and Volcanic

Lower Tertiary.

5 | Upper Triassic.

Triassic. Anarchist Gp.

Pennsylvanian and Permian. Cache Cr. Gp., Blind Cr. Fm. (3A).

Kobau Gp.

Lower Palaeozoic and/or Pre-Cambrian. Shuswap

Complex, Monashee Gp.

Intrusive Jurassic and/or Cretaceous. Okanagan Batholith Complex including (a), Similkmeen (b), Colville(c), Oliver(d), Krugerfe), Fairview(f) intrusive bodies.

————— Faults

MILES

10 2i0

49c 120°

GENERAL GEOLOGY of the OKANAGAN VALLEY FIGURE 1-4 12

of probable pre-late-Mississippian age (Ross and Barnes, 1972). The last two are open structures of Eocene age and associated with widespread block faulting, volcanic, and hydrothermal activity.

Rocks of the map area grade eastward into a foliated granodioritic terrain of homogenous lithology. On the west and north they appear to be fault bounded by Mesozoic intrusives of the Okanagan Composite Batholith (Daly, 1912;

Peto, 1971, 1973a,b) with the exception of a thin sliver of gneiss on the west shore of Okanagan Lake. To the south they merge with the 1ithologically similar Vaseaux Formation. Early Tertiary sediments and volcanics are present .•t in the region but are not found on the Okanagan Mountain map area.

The Okanagan Mountain area has been interpreted as a gneiss dome arising from the upwelling of a granite magma within its core with subsequent injection into flat lying gneiss layers of the complex (Brock, 1934). Several other mountains (e.g. Little White Mountain) in the area were believed to be formed by a similar mechanism. The first portion of this thesis was thus undertaken in the anticipation that the dome might be explained with a set of structures consistent with those found in nearby areas which have been studied in detail.

The second part of the study was initiated because of a 50 million year K-Ar date obtained from Okanagan Mountain on a hornblende sample extracted from the gneiss. If the rocks concerned were deformed and metamorphosed in pre- mid-Carboniferous time (Ross and Barnes, 1972) or Jurassic time (Monger et al.,

1972) then the apparent age is low and has been reset in the early Tertiary, coeval with extensive volcanism in the area (Mathews, 1964; Church, 1970).

Because many tertiary erosion surfaces are found in surrounding areas there is the suggestion that, in the absence of large displacement block faulting and uplift of some areas relative to others, significant heating must have 13

occurred at very high levels in the crust. Fission track dating and more extensive K-Ar work was undertaken to gain more knowledge of the spatial distribution and intensity of the event within the Kettle River (west half) map sheet (Little, 1961) and to see if the use of dating methods with different

susceptibilities to thermal resetting could give a better picture of the

recent thermal history of the area. The results of this work are submitted

as paper #1. In the course of analysis of the fission track data ii was

found that in some instances errors exist in the statistical processing of

fission track counting data and in others improvements could be made.

Alternative approaches which differ from those currently in the literature

have been developed and these are presented as paper #2. 14

STRUCTURAL SUCCESSION

The Monashee Group was not subdivided by Little (1961) in his reconnaiss• ance survey. It includes the Vaseaux Formation located about thirty miles south of Okanagan Mountain (Fig, 1-3), This formation has since been subdivided by

Ross and Christie (1969), They have outlined a structural succession exceeding

4000 feet in present thickness. Five map units were defined of which two, an amphibolite and a schist, form lithologically distinct markers. Ryan (1973) has observed similar lithology and structure farther south in the area bordering

Osoyoos Lake.

Three paragneiss units are recognized in the Okanagan Mountain map area, of which only one is distinctive but of limited distribution (see Plate 1,2).

This is an amphibolite, unit 1, which appears to be similar to those observed by Ross and Christie, and Ryan. No schist is found anywhere in the area. The

other two units are difficult to distinguish and are characterized by the

presence of abundant hornblende and minor biotite in one (unit 2), and characteris•

tic large orthoclase augen in the other (unit 3). These units are confined to

the southern part of the map area.

The north section bordering the lake is an undifferentiated paragneiss

sequence (unit 4) composed of gneiss, mylonite, and minor massive amphibolite.

No major continuous units are observed here and the contact with the central

granodiorite (unit B) is gradational. It is impossible to correlate any parts

of unit 4 with those in the south but it is likely that it represents the remains

of a paragneiss sequence into which abundant later synkinematic (phase 2) grano•

diorite (unit B) was intruded. The topography, for the most part, is sub-

parallel to the dip of the foliation of the unit so very little vertical section 15

is exposed to view. Minor amounts of units A and C (see below are also found in this area.

Intrusive igneous units are lettered A to E. Unit A is spatially confined to the area of paragneiss and is believed to be an early second-phase synkinematic intrusive of leuco-granite to leuco-quartz monzonite. The central portion of the area is composed largely of granitoid rock which probably represents a late second-phase intrusive (unit B). This unit contains scattered screens and stringers of amphibolite. It is impossible, however, to define any units within this relatively homogeneous mass of granitoid gneiss.

Units C to F are cross-cutting igneous rocks whose emplacement postdated phase 2 deformation. Unit C is a diorite sill complex which predates at least the fourth phase of deformation whereas unit D is a coarsely crystalline grano• diorite which postdates phase three and is a result of recrystallization and metasomatic alteration of unit B. Units E and F are leuco-quartz monzonite dikes probably of Tertiary age and believed associated with the Coryell intrusives to the east.

Mylonitization has produced a mappable phase within unit 3 but is not extensive enough to delineate elsewhere in the area. Brecciation is common throughout the area and is associated with local high angle faults.

The absolute ages of the map units cannot be defined so they are described by numbering outwards from the core of the earliest recognized large scale structure (phase 2). The units are illustrated in Plate 2.

The present thickness of units is obtained from cross sections in Plate 2.

Unit 1 is 200 or 300 feet thick. Unit 2 does not exceed 600 feet and where in contact with unit 1 thins to only half that amount. Because of antipathetic variation in the thickness of units 1 and 2, the aggregate thickness of both is 16

essentially constant and about 600 - 700 feet. Unit 3 is by far the thickest.

The contact of this unit with one structurally lower is not found in the area and it is thereby known to have a minimum thickness of about 3000 feet. The relationship of the present thickness to the original thickness is well disguised by the rival processes of tectonic thinning during deformation and dilation because of synkinematic igneous activity.

The units are described in the following sections to give a general

impression of their appearance. Detailed information pertaining to microscopic and mesoscopic structures is given in section 3. In general, the rocks are composed of various combinations of hornblende, biotite, feldspar, and quartz.

No other distinctive mineralogy can be used to separate formations and this leads to problems where gradational changes occur. Textural consideration can be

invoked in some cases.

Unit 1 - Laminated Amphibolite

This unit is of limited area! extent and forms a band striking south of

east across the southern portion of the map area. It is easy to recognize in

the field and weathers dark grey to rusty brown but is otherwise a deep green.

More importantly, it possesses a characteristic striped appearance caused by

alternating layers of hornblende and plagioclase (Fig. 2-1, 2-13). Banding is on the scale of from 1/8 to 3 inches. The grain size of the constituent hornblende

is variable and ranges from 1/16 to 1/4 inch. This mineral commonly outlines a

penetrative lineation but often is present as a mosaic of interlocking grains.

Mesoscopic fold hinges are common and the banding, where very thin, usually

defines numerous small rootless isoclinal folds. Locally the banded material

grades to coarse-grained massive amphibolite. 17

2-1 Unit 1. Laminated amphibolite. Light col• oured rock at left is part of a Tertiary (?) dike. Scale bar = 1 cm.

2-2 Unit 2. Hornblende-biotite granitoid gneiss illustrating folded compositional layering common in this unit. Scale bar = 1 cm. 18

In thin section the dark bands are composed of more than 60 percent dark green hornblende with the remainder plagioclase (An4g). Dark brown biotite may constitute 10-15 percent of the dark minerals and sometimes is segregated into bands with almost complete absence of hornblende. The lighter bands are composed chiefly of plagioclase (An4g, occasionally An5g). Many contain laths of anhedral poikiloblastic diopside which is sometimes partially replaced by hornblende and plagioclase. Occasionally the plagioclase is altered to clinozoisite and the diopside shows sporadic development of clusters of epidote. Sphehe is partic• ularly abundant in some light coloured pods within the amphibolite. These pods, which sometimes core small isoclinal folds, also contain abundant diopside.

Mesoscopic structures are well preserved in the unit, especially those associated with deformation phases 2 and 3, such as the fold mull ion illustrated in Fig. 3-19. Folds are sometimes outlined by biotite rich layers within other• wise massive amphibolite and wh'ere these structures are weathered out. a micaceous sheen is imparted to the outcrop.

Unit 2 - Hornblende (biotite) Granitoid Gneiss, Granulite

Unit 2 is defined by the presence of 40 to 60 percent mafics with hornblende predominating over biotite as well as the common presence of macroscopic rootless isoclines of between a few inches and a few feet in size. The leucocratic con• stituents normally include modal orthoclase as well as plagioclase and quartz.

The percentages, however, are variable. The unit weathers dark brown but it often

possesses a pink hue where orthoclase is abundant. For the most part the unit is

coarsely crystalline and does not readily preserve structures such as foliation and

cleavage caused by post phase 2 deformation. A photograph of a sample from this

Textural term having no metamorphic facies connotation. 19

unit is given in Fig, 2-2 as well as Fig. 3-7.

No other distinctive minerals were observed. The unit is approximately granodiorite in bulk composition but its provenance cannot be ascertained.

The abundant folded mafic layering strongly suggests a metasedimentary ancestry but the possibility of metamorphic segregation layering cannot be discounted. A variation of this unit includes rock containing thin biotite- rich layers separated by quartz-plagioclase rich bands. Such outcrops contain small folds which tend to weather out in relief as opposed to the aforemen•

tioned crystalline material which weathers uniformly (see Fig. 3-4).

Unit 3 - Augen Gneiss

The salient difference between this unit and unit 2 as far as field

recognition is concerned is the abundance of orthoclase augen (Fig. 2-3).

Although about 50 percent of this material appears dark grey to black, the

mafic mineral content is relatively low. Hornblende is not common. The

darker regions are generally composed of strained and recrystallized quartz

and feldspar rimmed by a thin network of biotite and minor chlorite. The

matrix feldspar is almost entirely orthoclase (with plagioclase An25-35) as

are the porphyroclasts. The lighter bands are those in which quartz and

feldspar predominate and the grain size is sometimes larger in these bands.

The observed foliation is associated with transposition developed

chiefly during the second phase of deformation. Highly appressed remnant fold

hinges are sometimes found along with a penetrative lineation associated with

hornblende growth, alignment of feldspar porphyroclasts, and quartz rodding.

This unit is extremely mylonitized in one area and forms a mappable band

(Plate 1). A photo from this band is presented in Fig. 2-4. 20

2-3 Unit 3. Augen gneiss. Plane of photo is perpendicular to Lg. Despite the abundant dark material the mafic mineral content is low ( = 10%). Scale bar = 1 cm.

2-4 Unit 3. More intensely mylonitized variety of the augen gneiss. 21

Unit 4 - Undifferentiated Granitoid Paragneiss

Unit 4 is a combination of granitoid gneiss, amphibolite lenses and

bands, mafic screens, mylonite and ultramylonite*. No continuous lithology was

found and hence no attempt was made to examine the area in as much detail as in the south.

Unit A - Leuco-quartz monzonite

Unit A is a series of leuco-quartz monzonite sills emplaced during the latter stages of the second phase of deformation. As such they are strongly foliated and penetratively lineated by the development of quartz rodding and the alignment of elongate orthoclase megacrysts. The rock is composed of 35 to 50 percent quartz, the remainder being feldspar of which significantly more than half is orthoclase. The plagioclase is about An35. Two textural varieties are present: The porphyritic variety (Fig. 2-5) contains potassium feldspar mega• crysts which appear to be replacements of original plagioclase phenocrysts.

Where these clasts are in contact with plagioclase, myrmekitic intergrowths are

present. The other variety (Fig. 2-6) is similar but has smaller feldspar grains.

It may simply be a more highly mylonitic form of the same rock. The variety with the large feldspar clasts is dominantly pink whereas the other has a cream to bleached white colour.

Unit A occurs as discrete sills (Plates 1,2) but is more pervasive than

indicated on the map. Close examination of some of the areas of unit 2 and 3 reveals that this material is disseminated throughout the country rock. Because of this unit's rather pallid and subdued colour, it is frequently difficult to perceive.

*Nomenclature according to Higgins, 1971. 22

2-5 Unit A. Porphyritic variety of leucoquartz monzonite. Plagioclase phenocrysts have been almost entirely replaced by orthoclase. Stained for pot• assium. Light grey is orthoclase, white is plagio• clase. Scale bar = 1 cm.

2-6 Non-porphyritic variety of the above. Lower slab has been stained for potassium. Light grey is orthoclase, white is plagioclase. Scale bar = 1 cm. 23

In the sills of unit A, most of the feldspar reveals incipient sensi• tization and randomly distributed secondary fractures filled with epidote, . which imparts a distinct greenish cast to the rock. This alteration is believed to have occurred much later than emplacement and to be associated with a Tertiary thermal event.

Unit B - Foliated Granodiorite

This grey to cream-white unit covers the central part of the map area

(Plate 1). It consists predominantly of foliated hornblende granodiorite and grades to hornblende quartz monzonite in areas where large subhedral porphyro- blasts comprise much of the rock. The porphyroblasts, along with elongate

hornblende, give rise to a penetrative lineation associated with the second

phase of deformation and lie rigorously within a foliation plane otherwise defined by lenticles of feldspar and quartz. The texture of this unit is

illustrated in Fig. 2-7, 2-8.

Mineralogy of this unit is simple and consists mainly of orthoclase,

plagioclase (An25-35), and quartz. Hornblende is the principal mafic consti•

tuent with biotite absent or in low abundance. Sphene, apatite, and zircon

are common accessories and diopside after hornblende is noted occasionally.

In the area bordering units to the south biotite appears to increase in

quantity but does not produce a distinctly mappable phase. In addition, the

abundance of large orthoclase crystals appears greater within this region

but more detailed mapping would be necessary to substantiate this observation.

Dark grey to black screens up to a foot thick are found occasionally within 24

2-7 Unit B. Plane of photograph is parallel to

F2. The outcrop is filled with large crystals of orthoclase which trend 110°.

2-8 Non-porphyritic variety of unit B. Plane of photograph is parallel to F?. Hornblende mineral lineations trend 110°. Pod'of amphibolite is trun• cated by movement on one of many northerly trending fractures. 25

this unit and consist primarily of hornblende and biotite, They are conformable with the foliation of the granodiorite but are much more deformed internally,

The foliation developed within the screens parallels that of the surrounding granodiorite,

Unit C - Diorite

Unit C is a sill complex consisting of diorite which has been intruded parallel to the foliation developed in the second phase of deformation. The rock is equigranular, of grainsize about 1/16", and consists of an unstrained mosaic of plagioclase (An^) and hornblende with about one percent interstitial oxides. These sills have been affected by the fourth phase of deformation but have apparently developed no observable associated penetrative structures. The unit, which has a characteristic salt and pepper texture (Fig. 2-9), often does not occur in large enough bodies to map. A few of these sills are indicated on

Plate 1.

Unit D - Unfoliated Granodiorite

Unit D is of limited distribution and appears to be a result of static recrystallization or metasomatism of unit B, and contacts with unit B are gradational over several feet. It possesses no penetrative fabric but commonly includes disoriented stringers of amphibolite (Fig. 2-10). Fig. 2-11 portrays the texture and grainsize of the material. The sample contains 10 to 15 percent hornblende, 10 percent quartz and about equal amounts of plagioclase (An^) and orthoclase. Sphene and apatite are accessories. The average grain size exceeds

1/8". 26

2-9 Unit C. Diorite. Equigranular unstrained mosaic of hornblende and plagioclase. Scale bar = 1 cm. 27

2-10 Unit D. At top of Okanagan Mountain. Dis• oriented stringers of amphibolite suggest attainment of a liquid phase.

2-11 Unit D. Recrystallized or remelted portion of unit B illustrating the granitic texture which has succeeded strongly foliated (F2) granodiorite. 28

This material forms a few small- patches in the map area, one of which is at the centre of Okanagan Mountain. Brock's (1934) interpretation of a pine- tree structure for the mountain was influenced strongly by the presence of this material.

Unit E - Quartz Monzonite Dikes

Unit E consists of quartz monzonite dikes which range in colour from pink to white and are believed related to the Coryell plutonics (Little, 1961). The

sample shown here (Fig. 2-12) consists of phenocrysts of plagioclase (An24) up to 1" in length commonly almost completely replaced by orthoclase. These two minerals constitute about 80 percent of the rock and quartz the remainder.

Diopside and sphene are present in trace quantities.

Apparently the surrounding rocks became hot during emplacement as there are no chilled contacts and where the laminated amphibolite has been .intruded, local distortion and contortion of the layering has occurred (Fig. 2-13).

Unit F - Protoclastic Quartz Monzonite

This unit is found as a series of shallow south dipping dikes, dark grey in appearance and frequently very difficult to distinguish from the host rock at a glance. The rock possesses a very strong foliation and lineation formed by mineral alignment and is composed of about 40 percent plagioclase (An3g), 25 percent orthoclase, and 25 percent quartz. Oxides account for about 5 percent of the volume and secondary mica rimming grains the remaining 5 percent. An example of this material is illustrated in Fig. 3-3V. 29

2-12 Unit E. Tertiary (?) dike intruding lamin• ated amphibolite (see below). Stained for potassium. Dark grey is orthoclase, white is plagioclase. Scale = 1 cm.

2-13 Unit 1. Laminated amphibolite shouldered aside and deformed by invading Tertiary (?) dikes. 30

Breccias and Ultramylonites

Microbreccias derived from the various granitoid rocks in the area are common. They are observed where faults are present and are otherwise locally associated with NNE trending fractures which are found throughout the area.

Just northeast of Squally Point they are especially abundant and coincident with extensive development of fracturing. Mylonites and ultramylonites are present and have been noted especially in the northwest part of the map area.

Hand specimens of this material are dark grey to black, possess a subconchoidal fracture and may well be of pseudotachylitic origin, perhaps a result of shearing in the waning stages of phase two deformation or associated with flexural-slip folding in the Teritary. 31

STRUCTURE

Five phases of deformation are recognized in the Okanagan Mountain map area. Most information is derived from the layered rocks south of Okanagan

Mountain as structural features are more plentiful within these units than in the homogenous granitoid ones. This section outlines the evidence for the interpreted sequence of structural events and gives a description of the geometries and interference relationships of the successive phases of folding.

The earliest recognizable deformation (phase 1) resulted in isoclinal folding about northerly trending axes, This was followed, perhaps closely in time, by isoclinal folding about a west-northwesterly axis (phase 2) and extensive late-kinematic intrusion of granitoid rock. Amphibolite grade metamorphism was attained during phase 2 deformation. Phase 3a folds developed approximately co-axially with those of phase 2 about shallow south dipping axial planes. Folding was considerably more open in style. Subset 3b folds formed later about north dipping axial planes. Phase 4 resulted in concentric folds about north-northeasterly axes with near vertical axial planes sometimes delineated by closely spaced fractures. Finally, and perhaps coevally with phase 4 deformation, warping about a west-northwesterly axis (phase 5) inter• fered with phase 4 folds to impart a crude domical shape to the area.

The sequence of deformation is deduced, in part, from interference relationships of mesoscopic folds. Evidence of all periods of folding is not present at any one location and so folds associated with a particular struc• tural event must of necessity be related by style and orientation. In this way folds and associated fabric elements have been organized into groups which are considered representative of a specific period or phase of deform• ation. The phases of deformation may or may not have overlapped in time. 32

The earliest recognisable period of deformation (phase 1) is by far the most poorly represented and resolved in the preceding scheme. The orientation of associated mesoscopic structures, however, is consistent with those of the earliest phase of deformation outlined by Ross and Christie (1969), Christie

(1973), and Ryan (1973). The sequence of development of phases 2, 3 and 4, on the other hand, is readily deduced. Warping associated with phase 5 deforma• tion cannot definitely be resolved as post-phase 4 from information obtained within the map area, although evidence presented later in this section strongly supports such an interpretation. In addition, the orientation and style are similar to folds mapped by Christie (1973) in an adjoining area where the sequence can be demonstrated.

A resume'of the structural elements, their orientation and nomenclature

is presented in Table 3-1. The presence of original bedding, FQ, is question• able as no relict sedimentary structures are found. Best examples of possible sedimentary layering are found in unit 2 (Fig. 3-5,6) and consist of thin biotite-rich bands alternating with quartz-plagioclase layers. The lamina• tions within unit 1 may also represent primary (igneous ?) layering but the possibility of metamorphic segregation cannot be ruled out. Other planer structures F-j-F4, are defined in Table 3-1 and elaborated upon in the sections that follow. Mesoscopic lineations L-j-L4 include three types: those formed by mineral growth (mostly hornblende and orthoclase), fold hinges of minor folds, and crenulations or relict micro-hinges.

Megascopic folds associated with phases 2 and 3 can only be delineated in the layered units in the southern part of the map area. There is thus a limited region in which data have been acquired for domain analysis. Specific• ally, domains homogenous with respect to the effects of phases 2 and 3 can be 33

TABLE 3-1

DEFORMATION AND AXIAL TREND STYLE MEASURED STRUCTURES

FQ - original bedding (?)

D] N-S Recumbent, isoclinal L-| - crenulations, mineral growth, folds (?) fold hinges

D2 E-SE Recumbent, isoclinal L2 - mineral growth, mainly folds hornblende and orthoclase - quartz rodding, fold hinges.

F2 - axial planes minor folds, cleavage. Associated folia• tion in igneous units.

D3 E-SE Recumbent, more open L3 - fold hinges

folding than D2

F3A- axial planes minor folds, cleavage.

F3g- axial plane minor fold conjugate (?) to F3A

D4 N-NE Upright flexural-slip L4 - fold hinges, statistically open warps statisti• defined fold axes. cally defined, and tighter mesoscopic F4 - axial planes. folds.

D5 E-SE Flexural-siip open L5 - statistically defined mega• warps, statistically scopic fold axis. defined. F5 - estimated from megascopic fold. 34

outlined by use of the axial traces of major phase 2 and 3 folds (Fig. 3-1) as domain boundaries. Because of access difficulties and poor outcrop only two of these areas (1 and 2) permitted acquisition of a reasonable number of structural measurements. In addition, phase 4 folding is extremely well developed within the region, and, because of the presence of folds such as illustrated in Fig. 3-26, early structural elements have been reoriented.

Phase 5 has also contributed to their reorientation but to a lesser extent.

Thus, effective domain analysis is precluded and reorientation of earlier structures by successive phases of folding cannot be defined rigorously.

Further information pertaining to the above is presented in the following sections in which the five phases of deformation are described in chronolog• ical order. The associated emplacement of igneous rock into the succession and concommitant metamorphism are discussed in a later section.

Phase 1 (earliest)

Phase 1 is poorly defined and is delineated by the presence of north trending penetrative mineral lineations and crenulations found on fine-grained and micaceous compositional layering. These elements are not abundant and are found only within units 1 and 2. Lineations, L], are plotted with structural data associated with phase 2 deformation in Fig. 3-10 and trend approximately 010°. Because measurements of L] at the noses of phase 2 minor folds were not obtained, and because of limited data, no systematic reorien• tation of L-| by phase 2 deformation can be deduced. Similarly the effect of phase 3 cannot be assessed because L-j measurements have not been observed in domains 2 and 4.

Figure 3-2 illustrates a refolded set of phase 1 folds in the core of STRUCTURAL DOMAINS -south part of map area- 36

3-2 Phase 1 folds in the core of a phase 2 isocline developed in unit 2. F3 cleavage is faintly visible. Facing east.

3-3 View along hinge of phase 1 fold developed in crystalline part of unit 2. Facing south. Axis trends 188°, axial plane dips west because of phase 4 folding. 37

a recumbent phase 2 isocline in unit 2. The style of these folds (Fig. 3-3)

is similar to those described below which developed in phase 2 deformation.

The axis of the fold illustrated in Fig. 3-3, however, is oriented almost at

right angles to the axial directions of nearby phase 2 folds.

Ross and Christie (1969) and Christie (1973) have mapped a megascopic

phase 1 fold which closes to the west and brings about repetition in the

succession near Vaseaux Lake (Fig. 1-3), and Ryan (1973) has outlined a few

large, poorly defined phase 1 structures. The absence of distinctive lith•

ology and limited area of outcrop of layered rock precludes such success

in the Okanagan Mountain map area. Nevertheless, the evidence obtained does

support the presence of pre-phase 2 deformation similar in orientation to

that outlined in nearby map areas. Destruction of most of the early structure

of the area by subsequent deformation has thwarted any attempt to outline the

geometry of phase 1 structures.

Phase 2

Phase 2 folds are recumbent isoclines found in a range of sizes. The

largest is a south-closing synform (Plate 2) at least four miles in amplitude, whereas the smallest folds have amplitudes measuring but a few inches. Meso•

scopic folds are best represented in unit 2 where compositional layering

(FQ/F-,) is well developed (see section 2). Many folds weather out and are

similar to that illustrated in Fig. 3-4 (these are class Ic according to the geometrical classification of Ramsay, 1967). Here the fold is defined by thin lamellae of brown biotite in a quartz-plagioclase rich rock (Fig. 3-5,

Fig. 3-6). Axial plane foliation (F2) is visible in thin section but is not readily observed in hand samples. Fig. 3-7 illustrates a stacked set of

39

3-5 Thin section of nose of fold illustrated in Fig. 3-4. Cut perpendicular to L.£. Dark bands are

biotite rich. F2 is well displayed. Cross polar• ized light. Field of view measures 4 cm across.

3-5 Close-up of hinge of fold in Fig. 3-5. Bio•

tite has not generally grown parallel to F2 but rather is bent around the nose of the fold and thus may represent layering FO/FT. Cross polarized light. Field of view measures 5 mm across. 40

3-7 Stacked rootless isoclines developed in the crystalline part of unit 2. Facing east.

1). Folds commonly contain cores of light coloured material composed mainly of plagioclase and diopside. 41

rootless isoclines in the more crystalline part of unit 2. Phase 2 folds developed in unit 1 are similar in appearance, with more angular hinges in most cases. They are sometimes cored by discontinuous pods and lenses of i lighter coloured plagioclase-diopside aggregates (Fig. 3-8). In general the folds have subrounded to subangular hinges, are near- isoclinal and are frequently rootless, making it impossible to deduce sense

of movement. F2 cleavage is not well developed and is probably often indis• tinguishable from compositional layering (Fg/F-|) because of extreme trans• position. Where sensibly developed, is defined by porphyroblastic or porphyroclastic orthoclase, and mica. It is thus extremely penetrative

and outlines an east-southeasterly plunging lineation, L2, defined by elongate

mineral growth, quartz rodding, and intersection of F2 with FQ/F-|. Mesoscopic fold closures are seldom found in unit 3. Where observed they are defined by thin tightly folded discontinuous stringers of fe.lds- pathic minerals (Fig. 3-9). The preponderant mylonite within this unit is one reason for the lack of minor folds and is a result of intense phase 2 deformation. This deformation has resulted in a strong penetrative lineation,

L2, defined by elongate feldspar porphyroclasts, occasional hornblende mineral lineation, and streaking of minerals. Within unit B which covers the central part of the map area no phase 2 closures are found as this material was introduced as a late kinematic

intrusion. Nevertheless, a strong foliation (F2) and a penetrative lineation

(L2) formed by the growth of tabular and elongate minerals is found in this area. The relationship between this fabric and phase 2 deformation is deduced by the parallelism of the foliation plane to the axial plane of phase 2 folds in adjoining layered units. Similarly the lineation in unit B, which is 42

3-9 Rootless isoclines developed in unit 3. Phase 3 fracture cleavage is visible. Photo measures about 15 cm across. Facing east.

» 43

usually a result of alignment of hornblende and orthoclase megacrysts, is

parallel to the axial directions of minor folds. Although phase 3 folds

are essentially coaxial with phase 2 folds, the plane F2 in unit B is

observed to be folded into phase 3 folds and so no ambiguity exists in

assigning this distinctive foliation and lineation to phase 2 of deformation.

The present dip of F2 for the megascopic synform in the southern part

of the area is about 20° south. In Fig. 3-10 poles to axial planes of phase

2 folds in domain 1 (Fig. 3-1) are plotted (open circles) and poles of

measurements of F2 from unit .B north of the phase 5 megafold (Plate 1) are

plotted as dots. These poles form two distinct groups which are a result of

phase 5 folding. Furthermore they show a spread caused by phase 4 folding

about the indicated axis obtained from structural analysis in Fig. 3-28.

This spread is more pronounced in the data from the south dipping layered

rock, and in the field this observation is correlated with higher periodicity

in the development of phase 4 folds. L2 from the north and south sides of

the phase 5 megafold which passes through the top of Okanagan Mountain are

intermixed on the stereoplot and, indeed, systematic reorientation of l_2 by

Lg would be anticipated to be minimal compared with measurement error as both

lineations are essentially coaxial. On the stereoplot, those L2 which plunge

steeply are measurements that have been taken from the flanks of phase 4 folds.

In spite of reorientation of L2 by later folding, the axial direction of phase 2 structures can be seen to be approximately 105°. The axial plane of the megascopic phase two synform now dips south at about 20°.

Phase 3

Phase 3 folds are wel1-developed in the layered rocks of the southern Phases 182 a L i from domain I + L 2 north of phase 5 mega-fold n H n II II F 2 * L 2 south of phase 5 mega-fold r- II II II II II o p 2

FIGURE 3-10 45

portion of the map area but are not as readily perceived in the homogeneous

widespread granodiorite (unit B). For convenience of description they can

be divided into subsets 3a and 3b, of which subset 3a is more common.

Subset 3a.

The megascopic expression of this fold set is the Robinson Creek

synform, defined by well-developed congruent minor folds on both limbs (Fig.

3-11). The axial surface dips 20-30° to the south-southwest (Fig. 3-11,

Plates 1,2) and has an axial direction, L3, similar in direction to L2 (about

105°). As with phase 2, later deformations have caused reorientation of

data. This reorientation is best displayed by examination of the distribution

of poles to axial planes of observed phase 3 minor folds. The measurements

were taken from the lower limb of the Robinson Creek synform, which are

homogeneous with respect to phase 2 and 3 megafolds (domain 1). The data is

plotted in Fig. 3-12 which also illustrates the theoretical distortion caused

by phase 4 and phase 5 folding. The scatter of poles appears to be accounted

for by these two phases of deformation. The position of measured phase 3 fold

hinges (Fig. 3-12), when compared with similar phase 2 data (Fig. 3-10), dictates that the folding associated with phases 2 and 3 of deformation must be considered coaxial within the bounds of measurement error, and scatter caused by later deformation (Fig,. 3-13).

Minor folds associated with this deformation are considerably more open than phase 2 folds and are equally well represented in all the paragneiss units. As a consequence of their open form, the sense of movement can almost always be deduced whereas this is never so with phase 1 and 2 mesoscopic folds.

In units 2 and 3 the fold hinges are generally rounded to subrounded (Fig. 3-14

FIGURE 3-12 FIGURE 3- 13 49

to 3-17). Axial-plane cleavage, F3a, is generally visible only as discontin•

uous fractures (Fig. 3-2, 3-15, 3-17), Measured linear structures, L.3a, are

from the hinges of third phase minor folds. These folds are not characterized

by mineral growth parallel to L.3a (e.g. hornblende mineral lineation) as in

phase 2 deformation. Unit B also exhibits folds about F3a, but the lack of

compositional layering makes their presence more difficult to perceive (Fig.

3-20). In some instances they are outlined by alkali feldspar porphyroblasts

(Fig. 3-21).

The style of phase 3 folds in unit 1 differs markedly. They are generally extremely open, large arcuate folds (Fig. 3-18) terminated by slides* (Fig. 3-23) and frequently weather out as fold mullion (Fig. 3-19).

The slides are believed associated with formation of subset 3b folds and their formation is discussed in the following section.

Most of the southern part of the map area is part of the lower limb of the Robinson Creek synform (Fig. 3-11) and contains minor folds congruent with the megascopic fold. Towards the core the sense of movement is lost but the folds are not appreciably tighter here than elsewhere. On the upper limb the folds are similar in appearance to those on the lower limb but with the opposite sense of movement.

Poles to axial plane surfaces of phase 2 structures (F2) and compositional layering (FQ) were taken from domains 1 and 2 and plotted in Fig. 3-24 in order to examine the reorientation of these structures by phase 3 folding.

These poles do not define a great circle about L3 and, as is the case with phase 3 axial surfaces (Fig. 3-12), better reflect the effects of phases 4 and 5 folding.

* a fault formed in close connection with folding, conformable with the fold limb or axial surface; Fleuty (1964). 50

3- 14 Phase 3a folds deform mylonitic lamination/ compositional layering of unit 3. Facing west. Sense of movement indicates lower limb of the Robinson Creek synform which closes to the south.

3-15 Detail of Fig. 3-14 illustrates the limited development of F3a in this unit. Folded layers are compositional layering, mylonitic lamination, or both. 51

3-]6 Phase 3a fold developed in augen gneiss, unit 3. Light coloured bands occasionally preserve phase 2 folds. Facing east.

3-17 Detail of phase 3a fold in unit 3. Axial plane cleavage is faintly visible. 52

3-18 Phase 2 fold in amphibolite taken around a broad open phase 3a fold in unit 1. Photo facing east.

3-19 Development of fold mullion in amphibolite of unit 1 by phase 3 folding. Hinges trend about 110°. 53

3-20 Faintly visible phase 3a fold in unit B formed by folding of earlier foliation, F2. Hammer lies along axial plane.

3-21 Phase 3 fold in unit B accentuated by growth of orthoclase porphyroblasts. 54

Subset 3b.

Subset 3b folds are confined to unit 1 and only two or three have been

noted. They have the opposite sense of movement to 3a folds and are developed about a north-dipping axial plane; possibly bearing a conjugate relationship to (Fig. 3-22, 3-25). The 3b axial planes appear to represent incipient slides which are common in unit 1 (Fig. 3-23), truncate phase 3a folds, and are thus post subset 3a in time. The development of these slides is schematically depicted in Fig. 3-25 which also illustrates how structures similar to those in Fig. 3-23 may have formed by movement on 3b surfaces.

Phase 3b structures have not been observed in the other units and it would appear that they are restricted to the amphibolite (unit 1) which is compositionally the most distinct rock type. The occurrence of structures related to this subset are too limited to assess their effect on earlier structural elements. There is no evidence of an associated megascopic expression of this event.

Phase 4

Phase 4 flexural slip folding trends approximately 035° with a 70° east dipping axial plane. Large open warps are found in the northern part of the map area with up to 150° between the limbs. In the south the folds are better developed on a mesoscopic scale and measure 110° or less between the limbs (Fig. 3-25). The sense of movement of these folds indicates that the area occupies the west flank of an anticlinorium which apparently closes east of the map area. Such mesoscopic fold development has not been observed in the northern and central part of the map area. Here broad open flexures

have been delineated by careful measurement of the orientation of F? which, 55

3-22 Subset 3b fold in laminated amphibolite. North dipping axial plane intruded by a Tertiary dike. Facing east.

3-23 Slide in laminated amphibolite terminates arcuate phase 3 fold. Facing east. L.

4 A

+ •

L3 'from

.El. (Fig.3-12)

Domain I Domain 2 * + Poles to compositional layering

Poles to F2

FIGURE 3-24

58

3-26 Phase 4 fold. Axial plane dips steeply east. Photo taken facing north. 59

in this area, has not been extensively reoriented by phase 3 folding. An example is the folding defined south of Squally Point (Plate 1) in Fig. 3-27.

Folds may be tighter in the south because of the presence of layering and facility of fold formation by reactivation of this layering. A cross-section is presented in plate 2.

The distribution of poles to axial planes, F^, defines an average axial plane of 036/70E which contains the associated south plunging measured fold axes, L4 (Fig. 3-28). Those L4 plunging north (because of phase 5 folding) do not exactly lie in this plane. This could be caused by measurement error but as all poles fall oh one side of the axial plane (Fig. 3-28) the cause could be small circle rotation of L4 about L^. Possible dispersion by concentric folding of L4 by Lg is designated by the dashed small circle on the stereoplot.

Predominant fracturing in the area trends about 015° and is vertical

(Fig. 3-29). The maxima of poles to fractures do not correspond to the observed positions of poles to phase 4 axial planes, although a certain small component of the measured fractures may be representative of F4. Most of the fracturing is probably related to failure towards the end of phase 4 folding. Some movement is indicated by offset structures (Fig. 2-8) and occasional cataclasite in zones parallel to these fractures, but the amount of offset cannot be deduced. The majority of fracture planes have developed obliquely to the average position of F4 (i.e. about 30° away from F4). A similar situation was also suspected by Christie (1973) in the Vaseaux Lake area but was not proved because the exact trend of phase 4 folding could not be delineated. Thus Christie's conclusion that the fractures represent a set of shear fractures related to failure at the final stage of phase 4 Phase 4

+ Lz mineral lineations • Poles to F2

FIGURE 3-2 7 61

Phase 4

+ l_4 Fold axes o Poles to F4 • l_2 on flanks of phase 4 meso-folds

FIGURE3-28 62

folding seems tenable.

Phase 5 .

The most obvious expression of phase 5 deformation is the large gentle antiformal structure with 'its almost horizontal axis passing through Okanagan

Mountain, and trending about 105°. This fold, by interference with phase 4 open folds, has imparted a crude domical structure to the area which is evident from Plate 1. The folding is very open and gentle and not always readily delineated mesoscopically in the field, but can be outlined easily on a larger scale (Plate 1). Fig. 3-30 illustrates a phase 5 flexure defined by a foliation and further enhanced by parting parallel to this foliation.

Phase 5 folding appears related to a series of protoclastic* quartz monzonite dikes consistently emplaced with a shallow southerly dip of between

10 and 40 degrees. A few of these dikes are mapped on Plate 1. They range in thickness from 2 to 60 feet and possess a streak lineation which parallels

L5 (Fig. 3-32). Fig. 3-31 illustrates one of these dikes cutting a phase 4 fold. It is likely these dikes occupy one set of fractures of a conjugate set whose intermediate stress axis parallels L5 (Fig. 3-33). Perhaps fort• uitously,, the orientation of L5 is perpendicular to the average plane of fracturing in the Tertiary (Fig. 3-29). Otherwise, the orientation may have been dictated by complex competition between the necessity of bending L4 fold axes and the jostling of discrete blocks, defined by phase 4 fracturing, against one another in order to release the stresses during phase 5 deformation.

* Nomenclature according to Higgins, 1971. 63 L

,' \ Principal concentration

^s cf poles to F4

Poles to Fractures (Tertiary)

186 points Contoured at 135,8.0, 2.7 Percent

FIGURE 3-29 64

3-30 Phase 5 warp defined by parting parallel to faint foliation (F2). Taken in area transitional between unit 3 and unit B.

3-31 Grey protoclastic quartz monzonite (unit F) cross-cuts phase 4 fold. Photo taken facing approx• imately NE. kes associated with phase 5 deformation • poles * lineations

FIGURE 3-32 PHASE 5 STRUCTURES

mm.

en phase 4 flexures, trending 035° Corrugated with phase 4 folds, trending 055°. SOUth North , °PEN PH

max. max Okanogan Lake Naramata Okanagan Mountain

Open folding Failure by frocturing -with emplacement of dikes.

SCALE: I inch = 2miles CD C JO mm. rn OJ i w Ol 67

Summary

The following structural events have occurred in the area:

1) Phase 1 folding occurred along axes now possessing a north to north•

easterly trend. They may have been originally isoclinal or later closed

by phase 2 folding.

2) Phase 2 folding obliterated much of the evidence of the earlier deformation

as recumbent isoclinal folds formed about an axis trending about 105°. The

axial plane of the megascopic fold now dips 15 - 20° south but its original

orientation is unknown.

3) Phase 3 deformation resulted in much more open folding about an axis

essentially parallel to phase 2. The megascopic expression is the

Robinson Greek synform developed about a 25 - .30° south dipping axial plane.

4) Phase 4 deformation caused open folding in the north, and tighter flexural

slip folds to form in the south. The difference in degree might have

been caused by facility in reactivating existing slip surfaces formed by

earlier deformations in layered rock to the south. Fracturing on steeply

dipping northeasterly trending planes followed this folding and resulted •

in the final relief of stress. j

5) Phase 5 deformation, essentially coaxial with phases 2 and 3, produced

broad warps, the largest one being that through the centre of Okanagan

Mountain. Interference with phase 4 folding imparted a crude domical

structure which culminates at the top of Okanagan Mountain. If phase 5

folding was really a result of lateral restriction that is commonly found

in thrust systems, then phase 4 and 5 folding may have been contemporaneous,

and associated with a single episode of deformation (i.e. Laramide orogeny). 68

METAMORPHISM AND THE EMPLACEMENT OF IGNEOUS ROCKS

The structural history described in the previous section was punctuated by the emplacement of large quantities of granodiorite as well as some quartz monzonite and granite. Most of this material was introduced during the second phase of deformation, associated with amphibolite grade metamorphism of the meta-sediments. After phase 3 deformation local recrystallization destroyed earlier structures in the rock, especially those within the granitic units.

Finally, in the Tertiary, chloritization and epidotization (principally within fractures) occurred throughout the area.

The restricted range of bulk composition of all units in the map area precludes a fruitful investigation of the metamorphic geology. Information of metamorphic grade is found only in limited areas of unit 1, and is assoc• iated with phase 2 of deformation. In the other units there is ample evidence of mineral growth related to this deformation but little more information can be obtained readily.

Early metamorphism.

Information on early metamorphism is derived from assemblages that formed during the second deformation. They are representative of amphibolite grade metamorphism and are found within the pods frequently located in the cores of phase 2 folds in laminated amphibolite (unit 1). They comprise: hornblende

+ plagioclase (An4o) + diopside + biotite + epidote/clinozoisite + sphene, as might be expected in, but not confined to, metamorphosed basic rocks (Fig.

4-1). No other distinctive assemblages or index minerals have been found in the 69

4-1 Unit 1. Assemblage including hornblende (hb), diopside (di), epidote (ep), biotite (bt), plagioclase (pi). Cross polarized light. Field of view is 1.2 mm across.

4-2 Unit 1. Hornblende (hb) aligned parallel to L-2. Grid cleavage is rarely visible as section is cut parallel to L2. Plane-polarized light. Field of view is 4.5 mm across. 70

examination of about 100 thin sections of all units.

Although information about the metamorphic grade attained is minimal,

extensive growth of minerals took place during phase 2 deformation. In the

amphibolites (unit 1) hornblende has grown rigorously parallel with L2 as

illustrated in Fig. 4-2. Within the light coloured pods in the cores of

phase 2 folds diopsidic-pyroxene also parallels L2 (Fig. 4-3). In the other

units, the salient feature of phase two deformation and metamorphism is the

pervasive growth of hornblende and orthoclase parallel to L2.

Phase two deformation was closely associated with late-stage conformable

plutonism. Intrusion of leuco-quartz monzonite (unit A, Fig. 4-4) was followed

by granodiorite (unit B) which forms part of a large batholithic (orthogneiss)

complex extending east of the area. The foliation in these sills is nowhere

deformed into phase 2 folds thus indicating intrusion in the latter stages

of the event.

Information of metamorphism associated with phase 1 has not been found

in the area.

Phase 3

Metamorphic assemblages that may have formed during phase 3 metamorphism have not been observed. The degree of mineral growth is minimal in comparison with that during phase 2. Axial directions of phase 3 folds are not character• ized by mineral lineations (i.e. hornblende,orthoclase) and those phase 2 mineral assemblages found in unit 1 have not responded perceptibly to the event.

Extensive development of phase three mesoscopic folds dictates that at 71

4-3 Unit 1. Thin section cut parallel to L2. Pyroxene porphyroblast is part of a series of crystals parallel to L2. Cross-polarized light. Field of view is 5 mm across.

4-4 Unit A. Leuco-quartz monzonite sill injected parallel to F2. 72

least some mineralogic reorganization or growth must have occurred. Evidence

for this has only been observed microscopically in the form of pronounced

strain patterns in quartz parallel to F3 (Fig. 4-5, 6, 7). Occasional growth

of biotite parallel to F3 indicates that lower or middle greenschist metamorphism

may have prevailed during phase 3 deformation.

Late or post phase 3 metamorphism.

The next event resulting in local mineralogical reconstitution of the

gneiss post-dated phase 3 folding, or may represent a continuation of the

phase three metamorphic event following the cessation of deformation. A

few small areas (unit D, Plate 1; Fig. 2-10, 11) contain non-foliated, non-

lineated granodiorite which grades within a few feet into rock bearing the fabric of phase 2 and 3 deformation. One of these areas occupies the top of

Okanagan Mountain (Fig. 2-10) and was in part responsible for Brock's (1934) interpretation that the domical shape of Okanagan Mountain was caused by upwelling of granitic material into the core of the structure.

Tertiary metamorphism.

Tertiary metamorphism was essentially a hydrothermal low pressure event associated with hot spring development and extensive alteration in the area mapped by Christie (1973). In the Okanagan Mountain area the event is characterized by the presence of epidote and chlorite veins within fractures

(Fig. 4-8) often imparting a greenish stain or caste to the rock. Sericiti- zation, kaolinization and other alteration of minerals is locally common

(Fig. 4-9). 73

4-5 Extreme deformation in unit 3 caused by straining of quartz (qt) during phase 2 (Fo). Deformation caused by phase 3 is also present (F3).

Section cut perpendicular to L2. Feldspar (f) has remained rigid. Cross polarized light. Field of view is 4.5 mm across.

4-6 Strain pattern in quartz outlines F3 in nose of phase 3 fold in unit 3. Cross polarized light. Field of view is 3 mm across. 74

4-7 Faint development of F3 in granodiorite, unit A, outlined by strained quartz. Cross polarized light Field of view is 3 mm across.

4-8 Chlorite veins and strong sericitization, etc. of other minerals attest to hydrothermal activity in the early Tertiary. Epidote veins are also common. This section is one of those most intensely altered. Plane polarized light. Field of view is 4.5 mm across 75

4-9 Sen" ci ti zation of feldspars records Tertiary hydrothermal activity in unit A. Plane polarized light. Field of view is 4.5 mm across. 76

Tertiary volcanism and associated high level plutonism was coeval or

shortly followed this alteration and involved the emplacement of pink to white

quartz monzonite or granite dikes (unit E). These dikes occupy fractures,

axial planes of early folds (Fig. 3-22) and are generally localized along

planes of weakness. The host rocks appear to have been hot during injection

as indicated by the distortion of laminated amphibolite (Fig. 2-13). With

one exception, the dikes were found to be unfoliated indicating emplacement

after phase 4 deformation. The single exception was a north trending vertical

dike with a strong vertical foliation.

Phase 5 deformation was associated with the emplacement of protoclastic

quartz monzonite dikes (unit F) which differ significantly in colour, texture,

and orientation (see Sections 2,3) from those of unit E. Contrary to the

view expressed by Christie (1973) that phase 5 fracturing was important in

localizing hydrothermal alteration, it is evident that alteration had ceased

and the country rock had cooled prior to the injection of protoclastic quartz monzonite dikes during phase 5 deformation. These dikes are remarkably fresh with no replacement of their well zoned plagioclase phenocrysts as is found

in other Tertiary dikes (see Fig. 2-12). It may thus be concluded that the stresses associated with phase 5 deformation outlasted any alteration or hydrothermal effects associated with the Tertiary thermal event. 77

DISCUSSION

Five phases of deformation in order of occurrence have been deduced within

the Okanagan Mountain map area. In addition, macroscopic folds associated with

phases 2, 3 and 5 have been outlined.

Three metamorphic events have occurred in the area. The most intense was

that associated with phase 2 of deformation. Extensive recrystallization and

growth of new minerals accompanied this metamorphism and resulted in a strong

penetrative fabric throughout the map area. Amphibolite facies metamorphic

grade was attained, and was associated with synkinematic emplacement of gran•

itoid rock. Evidence of Phase 1 deformation therefore has not been found in

the granitoid rock and has been largely obliterated by the second deformation

in the other rocks. The second metamorphic event followed phase 3 deformation

and resulted in local recrystal1ization of the gneisses, thereby destroying

their fabric. This metamorphism may have been a continuation of that accompany•

ing phase 3 deformation, or a later event. The final metamorphism was associated

with phase 4 of deformation. It involved shallow level heating and hydrothermal

activity and resulted in extensive chloritization and epidotization of existing

fractures in the area.

Correlation with nearby areas.

Three adjoining areas have been mapped in detail some thirty miles south of the present location (Ross and Christie, 1969; Okulitch, 1970; Christie, 1973;

Ryan, 1973) as part of detailed structural study of the west margin of the Shuswap metamorphic terrain. From this a reasonably consistent structural history has emerged. Correlation of lithology on a unit to unit basis has not been possible 78

between the areas but compositional similarities suggest equivalent lithology.

The Okanagan Mountain map area bears a strong structural resemblance to the areas east of the Okanagan Valley both in style and orientation of fold sets, and in the number of distinct phases of deformation (5) recognized. The lower grade

Kobau Group west of the Okanagan Valley as mapped by Okulitch (1969) does not show evidence of the first deformation. A map and cross-section are presented in

Fig. 5-1. The striking difference between the present study area and those to the south is the preponderance of second phase synkinematic granitoid rock. The relative structural position of the map area cannot definitely be established although it might be speculated that it represents a deeper level which was more thoroughly permeated and digested during phase 2 deformation. Post phase 3 intrusions are not represented in the map area although they may in fact be present at a shallow depth (Fig. 5-1).

The south-southwesterly vergence of phase 2 folding agrees with.that found to the south. Further information can be established regarding phase 3 folding.

The relatively tight phase 3 megafold mapped by Christie (1973) contrasts strongly with the correlative more open phase 3 fold defined by Ryan (1973). Christie believed the fold to have been closed by injection of a phase 3 pluton (Fig. 5-1).

This interpretation was supported by Ryan (1973) who thereby accounted for the openness of his folding because of its structural position above the pluton. The style of the folding was thus believed to be strongly controlled by local events.

The similar style of the phase 3 megafold in this study to that mapped by Christie

(1973) suggests that development is much more regional in extent and not necessarily controlled by emplacement of sheets of phase 3 igneous rock.

80

Timing of deformation and metamorphism.

The absolute timing of the early deformational events outlined in this study

remains uncertain. Isotopic work does not. provide satisfactory information because

of the complex thermal history of the terrain. In conjunction with structural

studies, however, some constraints can be defined.

It is apparent that the five phases of deformation recognized in map areas east of the Okanagan Valley include structures consistent in both style and orientation. West of the Valley and north of Keremeos, Ross and Barnes (1973) have suggested that deformations believed equivalent to phases 2 and 3 are pre-mid-

Carboniferous, based on poor fossil evidence in undeformed rock found above an unconformity. This evidence would necessarily place phases 2 and 3 as pre-mid-

Carboniferous but little more can be said of the absolute timing of the first 3 phases. Certainly, isotopic work has not given any pre-mid-Carboniferous dates.

Ryan's (1973) oldest Rb-Sr date is appreciably younger, only 170 million years.

Christie (1973), prior to the work by Ross and Barnes (1973), placed phase

3 deformation at pre-144 million years as this was the age obtained by White et al. (1968) on the Oliver quartz monzonite which intrudes the Shuswap gneisses.

This age is probably low, because of an intense thermal event about 50 million years ago which has completely or partially reset K-Ar dates throughout the area.

The 170 million year Rb-Sr age is probably also affected somewhat but the extent is unclear. The present author favours the interpretation that the oldest ages obtained by the K-Ar method in the surrounding Okanagan and Similkameen Plutonic

Complex (about 200 million years) should at present be considered minimum ages for the termination of phase 3 folding. Uranium-lead dating may prove to be the only method of getting older dates in these rocks as high strontium concentrations limit the usefulness of Rb-Sr (Ryan, personal communication, 1973). Post phase 3 81

plutonism may thus at least straddle the Permo-Triassic boundary and possibly extend back into the Permian or Carboniferous. In effect, older dates that might coincide with metamorphism associated with the pre-mid-Carbonif- erous deformation suggested by Ross and Barnes (1973) may have been obliterated by a rather complex thermal history which followed deformation. Conceivably, the magmattsm following the Caribooan orogeny could have been continuous over this period and may have in fact continued right into the Jurassic and Cretaceous

(Columbian orogeny) reflected in currently available K-Ar dates.

Phase 4 structures are similar in style and orientation to those in the map areas to the south, Christie (1973) has deduced phase 4 deformation to have occurred in the early Tertiary as rhomb porphyry dikes and sills similar in appearance to volcanic rocks of the Marron Formation (K-Ar date 51.6 million years;

Church, 1970) are deformed by phase 4 structures. Emplacement of other light to dark coloured igneous rocks (petrologically equivalent to the younger parts of the

Marron volcanics) was strongly controlled by the north trending fractures in his area. These fractures are similar in orientation to those found in the Okanagan

Mountain map area and their formation was probably coeval, although emplacement of dikes within the fractures was rarely observed. The above evidence suggests that the north trending fractures are late phase 4. Data from the Okanagan Mountain i

area show ? in addition, that there is a difference in orientation between axial planes to phase 4 folds and the average position of the north trending fracture set (see Fig. 3-28).

Phase 4 deformation was apparently associated with an intense hydrothermal event (Ross, 1974). Most of the fractures in the region are chloritized and epidotized, and Christie (1973) has noted intense hydrothermal alteration in part of his map area. He suggested that fracturing associated with phase 5 deformation 82

may have provided channels for the movement of fluids. He has found alteration

in the gneisses only as high as the base of the overlying Tertiary volcanics and

so hydrothermal activity must have terminated before extrusion of the volcanics.

The emplacement of protoclastic dikes associated with phase five folding in the

Okanagan Mountain map area, however, suggests that the host rock was at least cool enough at that time to support fracturing. Furthermore, the freshness of the dike rock suggests hydrothermal alteration had ceased prior to emplacement.

In summary the evidence available thus far from detailed studies at the southwest margin of the Shuswap Complex would relegate phases 1 to 3 to pre-mid-

Carboniferous time based on structural considerations. This timing, however, is

in disagreement with information which is available in the Vernon Map area (Jones,

1959), approximately 40 miles north of Okanagan Mountain, in which evidence for

two penetrative phases of deformation is present in rocks of probable Triassic age (Okulitch, personal communication, 1974). This disparity is at present

inadequately resolved and is predicted upon the availability of only poor fossil evidence. At any rate, these early events were followed by plutonism which extends possibly into the Cretaceous but evidence for this warrants further examination.

Finally high level heating of the crust associated with hydrothermal activity occurred between 50 and 45 million years ago and was closely related to phases 4 and 5 of deformation.

Origin of sediments.

Chrisite (1973) favoured the interpretation that the metamorphosed rock had a greywacke affinity, and included shales and basic volcanics as part of the succession. Ryan (1973), using isotopic evidence, supported the suggestion that the amphibolites were derived from volcanics as opposed to lime-rich pelites and moreover showed that the amphibolites had affinities with andesites found on the 83

continental side of subduction zones. He also found an isolated structure in the amphibolite which may have been originally volcanic.

Regional Interpretation.

The high grade gneisses found east of the Okanagan Galley display a similar structural history. To the west, the Kobau Group mapped by Okulitch lacks evi• dence of the earliest deformation but exhibits consistent later structures.

Kobau rocks may thus project structurally above those to the east and Okulitch

(1969) has suggested that the Kobau sediments were derived from rising nappe structures developed during phase 1 folding of the rocks to the east. Whatever the reason for the absence of the early deformation, it seems clear that this deformation is not present in any lower grade rocks to the west of the Monashee

Group, including the Old Tom and Shoemaker Formations (Ross and Barnes, 1972;

Ryan, 1973, p.168). Moreover, farther to the west at Hedley, the Triassic Hedley

Formation (Bostock, 1940) does not contain isoclinal recumbent folds which might be equivalent to phase 3 folds (Ryan, 1973). Thus phases 1, 2 and 3 of folding may only be present in older (pre-mid-Carboniferous) rocks.

In terms of current plate-tectonic models the deformational history of the

Cordillera is still somewhat confused and controversial. Phases 4 and 5 of deformation, however, are much more amenable to discussion and are considered first. Both phases have occurred about 50 million years ago (Ross, 1974) and as such are correctable with the extensive Laramide phase of Cordilleran tectogenesis. Coney (1972) has attributed this orogeny to major reorganization of plate motions resulting from the separation of Europe and North America which commenced about 80 million years ago and resulted in southwesterly rotation of the North America plate away from Europe. Subsequent to about

40 million years ago plate motion was reduced and 34

deformation ceased because of slowing of spreading in the North Atlantic. The

associated subduction model for the west coast is more elaborately discussed by

Atwater (1970) and involves interaction of the Farallon plate with western North

America, rapid subduction, and associated andesitic volcanism. The related

effects include folding, as encountered in the western part of the continent, and

thin-skinned deformation in the Rocky Mountains. The area studied would occupy

the inner arc in the context of Miyashiro's(l972) paired metamorphic belt, and the

Coast Mountain uplift (Culbert, 1971) would correspond to events in the arc -

trench gap. The only dissatisfying aspect in the application of this model is

the rather great width (200 miles) of the metamorphic pair in B.C. as opposed to

those discussed by Miyashiro(60 miles). Coney (1972) explains this discrepancy

by emphasizing the importance of the rotation of North America in causing deform•

ation, in contrast with simple subduction where stresses are transmitted over a

relatively short distance.

The time gap between pre-mid-Carboniferous and Tertiary deformation (Ross

and Barnes, 1972) is large within the areas studied in detail. Elsewhere, however,

evidence for the Columbian orogeny of Jurassic and Cretaceous age is found (Monger

et al., 1972). Folding which may be associated with this deformation has been

noted by Ryan (1973) in Triassic rocks of the Hedley Formation some 30 miles west

of the Okanagan Valley. Monger et al. (1972), in fact, favour the hypothesis that

the main metamorphism in the Omineca Belt is Triassic in the north and Jurassic in

the south.

Specific details of the position of subduction zones are few. The Pinchi-

Teslin lineament (Coney, 1972; Paterson, 1973) is at least one possibility for a

suture in Permo-Traissic time but probably only one of a series of parallel, west-migrating subduction zones which existed throughout the Phanerozoic. Unfor- 85

tunately evidence for the relative motions of interacting oceanic plates early on in the development of the Cordillera connot now be found within the present day

Pacific ocean, for, as pointed out by Coney (1972), the Pacific oceanic plate was not in contact with the North American plate during the time of interest. 86

K-Ar and fission track geochronometry of an Eocene thermal event in the Kettle River (west half) map area, southern British Columbia.

PAPER NO. 1

G.A. Medford Department of Geological Sciences The University of British Columbia Vancouver, British Columbia 87

ABSTRACT

The Okanagan and Similkameen plutonic complexes west of the Okanagan

Valley of south-central British Columbia yield K-Ar dates that range from

185 to 133 million years. East of the Okanagan Valley Shuswap gneisses

into which the plutonics intrude, and which may be as old as pre-mid-

Carboniferous in age (Ross and Barnes, 1972), yield K-Ar dates between 59.9 and 47.4 million years. This abrupt change, which approximately coincides with the Okanagan Valley, is a consequence of an intense thermal event in the early Tertiary which has reset K-Ar dates in the gneisses at shallow depths. Comparison of K-Ar, sphene and apatite fission track dates demon• strates that the heating affected the plutons west of the Okanagan Valley and that cooling of the Shuswap gneisses occurred at a rate in excess of 25°C. per million years. The scatter observed in the older K-Ar dates of the plutonic complexes could be caused by post-emplacement heating with variable partial argon loss rather than by separate magmatic events. Thus only the oldest K-Ar dates obtained from the plutons may be significant as minimum ages for emplacement. 88

Introduction

This investigation was begun when an early Eocene date (49.9 million years) was obtained for a hornblende sample (1-150) extracted from ortho- gneiss of the Shuswap metamorphic complex east of Okanagan Lake. This date was unexpected because Jurassic plutons are known to intrude the gneiss and because many nearby patches of Eocene volcanics, ranging in age from about

50 to 45 million years (Mathews, 1964; Church, 1970), sit upon fanglomerates resting on an early Tertiary erosion surface. In the absence of block faulting with large displacement and deep erosion of uplifted blocks, the hornblende date must have been reset at a high level in the crust perhaps by a nearby unobserved body of Coryell granite (Little, 1961).

In this paper, the nature, extent, and significance of the high level

Tertiary thermal event are outlined in the area contained within the Kettle

River (west half) map sheet. This event has lowered K-Ar dates, especially those obtained east of the Okanagan Valley where local intense hydrothermal alteration has been observed (Ross, 1974). An apatite fission track study reveals that this thermal event affected areas where K-Ar dates have not been greatly altered. A few sphene dates were also obtained and comparison of these dates with those from the apatites indicates very rapid cooling of the rock when the thermal event ended.

Analytical Techniques

The K-Ar analytical procedure used is described by White et al. (1967).

In addition samples were baked to about 130°C for approximately 18 hours to eliminate or reduce the effects of atmospheric argon contamination. Fission 89

track dates were calculated using the equation in Naeser (1967). Details of

the analytical methods are given in Appendix 1.

Distribution of K-Ar Dates.

A few K-Ar dates west of the Okanagan Valley have previously been reported

for the Okanagan and Similkameen Complexes (see Peto, 1973a,b) which intrude

gneisses of the Shuswap metamorphic complex. White et al.(1968) found the

Oliver quartz monzonite (Plate 1) to have a muscovite K-Ar date of 144 million

years. Farther west near Hedley, Roddick et al. (1972) obtained dates between

140 and 180 million years for various units of the Okanagan and Similkameen

Complex.

K-Ar dates on hornblende and biotite obtained in this study on gneisses

east of the Okanagan Valley all fall in the interval of 51 to 47 million years

(Plate 1, Table 1) with one exception (sample 3-10) at 59.9 million years. In

addition, concurrent Rb-Sr work by Ryan (1973) in the gneisses just north of

the International Boundary east of Osoyoos indicates resetting of minerals and

some schists to dates of between 30 and 50 million years. On the other hand, dates from the plutonic rocks on the west side of the Okanagan Valley range between 185 and 133 million years (Plate 1, Table 1). A sharp break in K-Ar dates which approximately follows the Okanagan Valley is thus coincident with the contact between the area containing plutonic bodies and the Shuswap gneisses.

The Okanagan Valley has long been considered fault controlled (see Little,

1961). Naturally, the young dates on the east side might reflect uplift and erosion which have exposed an area sufficiently deeply buried and hot enough to reset K-Ar dates during the lower Eocene. Available evidence does not support this interpretation. Widespread patches of volcanic rocks ranging 90 TABLE 1 POTASSIUM-ARGON ANALYTICAL DATA Ar40r PERCENT Ar40r TO"5 Ar40r APPARENT No. LOCATION ROCK TYPE,UNIT,MINERAL K+S A740T ccSTP/g ((40 AGE M.y. 1- 50* 49°36.7'N South shallow dipping dike 3.18i0.003 0.937 0.6038 0.002805 47.4*1.5 119°34.6'W Highly sheared Whole rock

1-160* 49"39.2'N Coarse-grained granodio• 1.19+0.003 0.792 0.2339 0.002904 49.0+1.6 119°33.8'W rite dike Whole rock

1-148* 49"43.9'N Recrystallized granodio• 1.24+0.003 0.469 0.2411 0.002873 48.5+1.5 119°31.3 W rite gneiss. Monashee Group - Hornblende

1-150* 49°44.5'N Granodiorite gneiss. 1.1U0.004 0.744 0.2220 0.002955 49.9±1.7 119°31.0'W Hornblende lineated. Monashee Group Hornblende

1-178* 49°42.6'N Granodiorite gneiss. 1.05±0.004 0.713 0.2145 0.003033 51.1*1.6 119o36.0'M Hornblende lineated. Monashee Group Hornblende

3- 3 49"25.2'N Similkameen quartz diorite. 3.82±0.034 0.912 2.9456 0.0T13924 185 +6.6 119047.2'W Similkameen Complex. Biotite

3- 5 49"51.6'N Augen gneiss. 1.33±0.003 0.735 0.2788 0.003097 52.2+1.5 119°17.2'W Hornblende lineated. Hornblende

3- 7 49"03.5'N Kruger Syenite. 0.58±0.006 0.775 0.4098 0.010476 171 +6.4 119°38.5'W Similkameen Complex. Hornblende

3- 8 49"47.5'N Paragneiss. 1.49±0.012 0.831 0.3197 0.003170 53.4±1.9 119°08.0'W Monashee Group. Hornblende

3- 10 49"16.5'N Vaseaux Formation para• 1.40±0.009 0.730 0.3374- 0.003560 59.9+2.0 119°31.2'W gneiss. Monashee Group. Hornblende

3- 13 49"36.0'N Granodiorite gneiss. 1.34+0.000 0.839 0.2816 0.003104 52.3±1.4 119°34.5'W Monashee Group. Hornblende

3- 18 49"36.6'N Similkameen quartz diorite. 2.50+0.081 0.712 1.7103 0.010107 165 +9.7 119047.8'W Okanagan Complex. Biotite - 25% Hornblende

3- 20 49"42.5'N Valhalla Granodiorite. 6;44+0.023 0.950 3.5024 0.008035 133 ±4.1 119°48.8'W Okanagan Complex. Biotite

U 3- 21 49 42.8'N Granodiorite gneiss. 4.85+0.019 0.596 1.0089 0.003073 51.8±1.6 119016.5'W Monashee Group. Biotite

v 3- 22 -49 15.0'N Valhalla Granodiorite. 6.78±0.026 0.919 1.4086 0.003070 51.8±1.6 ~119°10.5'W Biotite Argon analyses by J.Harakal (*) and G.Medford using MS-10 mass spectrometer. 91

in age from 50. to 45 million years are found unconformably overlying the

gneisses and plutonic rocks throughout the area. It is possible, of course,

that the area east of the Okanagan Valley was uplifted at a greater rate prior

to the Tertiary and displays the effects of uplifted isotherms. This area might then have been quickly eroded just before the extrusion of the Eocene volcanics. In the absence of an abnormally steep geothermal gradient, the uplift must have been both rapid and have involved relative large-scale dis• placement. Although this possibility cannot be entirely precluded, evidence obtained from detailed mapping near Oliver in the south argues against it.

Ross and Christie (1969), Christie (1973) and Ryan (1973) do not note large displacement faulting in their map areas and Ross and Christie (1969) have mapped continuous lithology and structure across the Okanagan Valley.

It is considered more likely that the intensity of the shallow thermal event diminishes sharply to the west. Although White et al. (1968) obtain a

144 million year K-Ar date on muscovite for the Oliver quartz monzonite, they also report several biotite ages of between 118 and 82 million years from nearby parts of the same pluton. Similarly, at the Brenda Mine in the north• west corner of the area (Plate 1), White et al. (1968) obtain a biotite K-Ar date of 148 million years whereas hornblende yields 168 million years. This discrepancy, however, may only reflect dating of secondary biotite associated with mineralization. In any case, there is some evidence that Tertiary heating may have occurred west of the Okanagan Valley, but it was apparently insuffic• ient to reset the K-Ar dates as has happened to the east. In order to see if evidence of such heating could be detected in the west several apatite fission track dates were determined. 92

Fission Track Analysis and Cooling-Rate Determination

Apatite is the most thermally sensitive mineral used in fission track

dating at present. Track-loss curves have been experimentally determined as

a function of temperature and time (Naeser and Faul, 1969). Track retention

(for a period of 10^ to 10^ years) is essentially complete in apatite at

temperatures below about 50°C and in sphene below 275°C. On the other hand,

a conservatively low estimate of the closure temperature of the hornblende K-Ar

clock is about 150°C (see Damon, 1968, p.23, 24).

Because of its sensitivity to resetting, apatite has been used to date

recent uplift. Specifically, the age obtained represents the time at which the

rock passed through the critical isotherm for track retention, which in many

instances may represent only 3 or 4 kilometers of burial. For high level

intrusion of magma into cool rock (rapid cooling), however, fission track dates concordant with other isotopic methods have been obtained (Christopher,

1973). Similarly a high level regional thermal event which is sufficiently

intense to reset K-Ar biotite and hornblende dates might be expected to result in concordance between the K-Ar and apatite ages, as cooling upon cessation of the heat input would be expected to be relatively rapid. In the following paragraphs, a reasonable minimum c^obling-rate for the Shuswap gneisses is calculated. Details of the calculation are presented in Appendix 2.

Apatite dates obtained in this study are presented in Table 2 and Plate 1.

On the east side of the Valley at each location the apatite and sphene dates are concordant with the K-Ar dates, within the stated limits of error. The mean hornblende K-Ar date is 51.2 million years (excluding sampleQ^lOjihich is unusually high), the mean sphene date is 53.6 million years and the mean apatite date is 48.4, not very much less. On the west side of the Okanagan

Valley, however, the mean apatite date is 54.3 whereas the K-Ar dates vary but TABLE 2 FISSION TRACK ANALYTICAL DATA

TRACK DENSITY APPARENT No, LOCATION MINERAL ROCK TYPE.UNIT RATIO AGE

1-145 49 41 . 2 ' N Apatite Granodiorite gneiss, 0 . 591±0.108 41 .2 + 7.7 119°34.4'W Monashee Group.

1-148 49"43.9'N Apatite Recrystal1i zed 0.686+0.034 47.8+3.1 119 31 .3'W granodiorite gneiss, Monashee Group.

1-150 49U44.5'N Apatite Granodiorite gneiss. 0.609±0.038 42.413.1 1 1 9° 31 .O'W Monashee Group.

1-154 49U39.7'N Apatite Granodiorite gneiss, 0.889+0.094 61 .9 + 7.2 119°34.3'W Monashee Group.

1-173 49U39.9'N Apatite Granodiorite gneiss. 0.770*0.087 53.6+6.4 119°38.0'W Monashee Group.

2- 83 49^37.6'N Apatite Granodiorite gneiss 0.629+0.028 43.8+2.7 119°34.0'W Monashee Group.

3- 0 49U11.9'N Apatite 01i ver i ntrus i ve. 0.646+0.062 45.0+4.6 119°35.3'W Similkameen Complex,

3- 1 49°01.8'N Apatite Granodioritic intru• 0.774+0.081 53.9+6.0 119°41 .1 'W sive. Similkameen Complex.

3- 5 4 9 51.6'N Apatite Augen gneiss. 0.666+0.060 46.4+4.6 • 119°17.2'W Monashee Group.

3- 6 49 27.9'N Apatite Jura granodiorite. 0.978+0.085 68.0i6.5 119°41 . 2 ' W Okanagan Complex.

3. 8 49U47.'5'N Apatite Paragneiss. 0.761+0.055 53.0+4.4 119°08 . 0 ' W Monashee Group.

3- 10 49°16.5'N Apatite Vaseaux Formation 0.688+0.079 47 . 9±5 . 8 119°31.2 ' W paragnei ss. Monashee Group.

3- 1 2 49u16.1 'N Apatite Orthognei ss. 0.907±0.145 63.1±10 . 4 119°22.3'W Monashee Group,

3- 13 49 26.0'N Apatite Granodiorite gneiss 0.649+0.048 45.2+3.9 11 9°34.5'W Monashee Group.

3- 14 49°28.6'N Apatite Coryell intrusive. 0.822+0.064 57.2+5.0 119u38.1'W Granite.

3- 16 49°32.7'N Apatite Paragneiss 1 .01610.066 70.6+5.4 1 1 9 38.1 'W Monashee Group.

3- 17 49°31.O'N Apatite Augen gneiss 0.51OiO.045 35.6+3.5 119U32.7'W Monashee Grouf FISSION TRACK ANALYTICAL DATA TRACK DENSITY APPARENT No. LOCATION MINERAL ROCK TYPE,UNIT RATIO AGE

3 - 18 49°36. 6'N Apati te Similkameen quartz 0..698+0 . 046 48,.6+3. 8 119°47. ,8'W diorite. Okanagan Complex.

3 - 20 49°42. 5 ' N Apati te Valhalla granodiorite 0..544+0 . 026 37,,9+2. 4 119°48. ,8'W Okanagan Complex.

3 - 21 49°42. 8' N Apati te Granodiorite gneiss. 0. 677±0. 049 47 .,2+3. 9 119°16. 5'W Monashee Group.

3 - 22 -49°15. 0 ' N Apati te Valhalla granodiorite 0.,689+0 . 059 48.,0 +4.6 -119°10. 5'W

1 -178 49°42. 6'N S p h e n e: Granodiorite gneiss. 0.,816+0 . 026 56,,8±7. 8 119°36. O'W Monashee group.

3 - 5 49°51 ., 6 ' N Sphene Augen gnei ss . 0,,662t0 . 1 66 46,.1+11. 5 119°17. ,2'W Monashee Group.

3 - 13 49°26. .O'N Sphene Granodiorite gneiss. 0,.745+0 . 085 51 .9 + 11 .4 11 9°34...5' W Monashee Group.

3 - 20 49°42, . 5 ' N Sphene Valhalla granodiorite 0 . 569i0. 1 20 39 .7+8.4 119°48 ;8'W Okanagan Complex.

3 - 22 ~49°1 5 .O'N Sphene Valhalla granodiorite 0 .855+0. 175 59 .5+12.2 -119°10 .5'W

Thermal neutron fluence: (11.4±0.5)x1015neutrons/cm2 Quoted error: one standard deviation Model age is calculated according to Naeser ( 1 967-)

Etch conditions: Apatite; 75 percent HN0?, 20 seconds. Room temperature.

Sphene; 1HF,2HN0v3HC1,6H?0, 12 minutes. Room temperature. 95

are always considerably higher. It would seem, then, that the increased

thermal gradients were common to both sides of the Valley, but that heating was

less intense in the west so that the K-Ar dates are only slightly reduced.

The difference between the mean hornblende K-Ar date and the mean apatite

date for samples east of the Okanagan Valley is 2.8 million years. During this

time the area must have cooled from at least 150°C. to a temperature below 75°C.

(see Appendix 2). Hence a cooling rate in excess of 25°C. per million years is

implied. Similarly, the difference in mean dates for sphene and apatite is 5.2

million years during which time the area cooled over a range of 200°C, or at a

rate of about 40°C. per million years. This estimate is independent of errors

in decay constants and flux calibration. The cooling rates calculated above

may be compared with those calculated for the uplift-cooling model of the Alps

(Clark and Jager, 1969). In the high-grade core of the Alps rocks have cooled

from a maximum of 400°C. to 650°C. to surface temperatures in about 30 million years, and this is compatible with a denudation rate of 0.4 to 1.0 mm per year.

Despite the continuous, rapid uplift and erosion displayed by the Alps the

cooling rate there is only 13°C. to 22°C. per million years, somewhat less than

that estimated for the Shuswap rocks east of the Okanagan Valley.

Examination of individual dates allows a number of interesting observa•

tions to be made. Sample 3-20 yields the lowest K-Ar date (133 million years) obtained in this study west of the Okanagan Valley. The apatite date, 37.9 million years, is also relatively low here and is well determined (+2.4).

A sphene date at this location is similar to the apatite date (39.7 million years). The oldest K-Ar date obtained (185 million years), on the other

hand, is found at location 3-3 which is about 1000 feet below a known Tertiary erosion surface. In addition, nearby apatite dates are relatively high

(samples 3-6 and 3-16). This general region may thus have escaped most of 96

the effects of Tertiary heating. If this is so then the 185 million year K-Ar date, which is among the oldest obtained in surrounding areas (Hibbard, 1971;

Roddick et al., 1972; Rinehart and Fox, 1972) may really be a minimum age for the plutonic complex and the others are scattered due to partial resetting. It is interesting to note that Peto (1970, 1973a,b) postulated a genetic similarity between intrusive units of the Okanagan Complex based or> chemical arguments but retracted the hypothesis because of the data of Roddick et al.(1972) which suggested a time difference of 20-30 million years between intrusive events.

In view of the above discussion Peto's original conclusion may be valid.

The scatter of K-Ar dates in areas affected by Tertiary heating probably reflects partial argon loss, although the possibility of argon gain in a region peripheral to an area being degassed by an intense thermal event cannot be overlooked. Sample 3-20 may reflect this phenomenon as the sphene date is quite low here (39.7 m.y.) whereas the biotite date is significantly higher

(133 m.y.). The anomalously high hornblende date at location 3-10 (59.9 m.y.) might be explained similarly. To date, however, Ar gain has only been observed in minerals recrystallized within basement terrains where an Ar overpressure has developed (see for example, Bocquet et al., 1974). Notwithstanding, the resolution of emplacement dates might best be left to other techniques.

A pair of well-determined low apatite dates (samples 3-17, 35.6 + 3.5; and 3-20, 37.9 + 2.4) provide evidence that heating continued locally long after the main event about 50 million years ago.

Correlation of the Tertiary Event with Geophysical Evidence

The crust is known to be relatively thin between the Rocky Mountain

Trench and the Hope Fault, not more than 30 kilometers as opposed to 45-55 kilometers elsewhere in the Cordillera (Berry et al., 1971). In addition, 97

a highly conductive layer below this zone is now interpreted to be caused by

hydration and possible partial melting, leaving a rigid crust of only 10-15

kilometres. At the present time a thermal anomaly in the northwestern United

States outlined by Blackwell (1969) appears to extend into Canada (Jessop

and Judge, 1971) and may be associated with continued or intermittent

heating following the main thermal event in the early Eocene. Later heating

is suggested by a few relatively low (35 million year) dates obtained on

apatite.

Circulating water is a convenient method of thermal energy transport.

Ross (1974) has noted extensive alteration in the southeast part of the

map area and has associated this with a high level hydrothermal event coin•

cident with extensive plutonism (e.g. Coryell plutonics; Little, 1957, 1961)

and volcanic activity (Kamloops Group or Princeton Group volcanics; see

Mathews, 1964) during the interval 50 to 45 million years ago. Much.of the

area dated at 50 million years did not show evidence of such extreme altera•

tion, but common chloritization and epidotization of fractures may represent

correlative phenomena. Otherwise, heat may have been introduced by shallow

intrusion of large convecting bodies of Coryell granitic rock. A few small stocks are found within the area (Plate 1) as well as numerous dikes and these conceivably tap a large batholith similar to that unroofed in the

Kettle River (east half) map area (Little, 1957). The rather abrupt decrease in the intensity of the event at the Okanagan Valley (which is commonly considered fault controlled, albeit of probably minor displacement), may reflect structural control on the movement of the magma.

The change from andesitic volcanism to extrusion of alkali-basalts in the Miocene throughout British Columbia (Souther, 1970) is not reflected in 98

isotopic ages obtained thus far. This is reasonable considering the source

of basalts is deep and they are quickly transferred to the surface with

contact effects only next to feeder dikes. In contrast, the disturbance

discussed thus far is a shallow crustal phenomenon of regional extent.

Regional Aspects.

It has become apparent' that heating associated with early Tertiary

igneous events is widespread and found in many parts of British Columbia and

the northwest United States. Armstrong (1974), Fox et a]., (1973), and

Miller (1974) noted comparable regional resetting in K-Ar dates in plutonic

rocks of and Washington, and Birnie (personal communication, 1974) has

obtained several Tertiary K-Ar dates from Shuswap gneiss near Revelstoke,

as well as discordant hornblende (146 million years) and biotite (51 million years) dates from an intrusive into the gneisses. Examination of the dates

in south central British Columbia compiled by Uanless (1969) reveals a large

number of early Tertiary dates throughout the south-central Cordillera as well as frequent discordance in the older measurements. It seems apparent

that the resolution of distinct plutonic episodes may be rendered quite

difficult if Tertiary heating is widespread and intense. Erroneous time

relationships may be proposed, for example, if dikes intruding other rocks

are dated without ensuring that both dike and host rock have not been reset.

The delineation of general areas strongly affected by the Tertiary

thermal event must await rather extensive dating of the plutonic and gneiss

complexes of the province. The thermally sensitive apatite fission track

technique can prove useful in detecting areas where modification of K-Ar dates is likely to have occurred. Obviously an old apatite date which is 99

concordant with a K-Ar date on a oogenetic mineral supports the absence of

later partial resetting.

The spatial correlation of the 50 million year K-Ar dates (i.e.,

areas of most intense heating) with high grade gneisses of the Shuswap

metamorphic complex, originally metamorphosed in Triassic/Jurassic (Monger et

al., 1972) or possibly pre-mid-Carboniferous time (Ross and Barnes, 1972),

is remarkable within the Kettle River (west half) map sheet. Whether or not

this correlation is consistent over a larger region is a question that

requires much more isotopic work, although the dates (mentioned above)

obtained by Birnie, and those compiled by Wanless (1969) are in agreement with this; possibility. Armstrong (personal communication, 1974) has noted

a similar coincidence in Idaho. The apparent spatial overlap of the Eocene

thermal event and the high grade gneisses of the Shuswap Complex is at present unexplained.

Summary.

Evidence has been presented to suggest that a high level thermal event which was less intense west of the Okanagan Valley has reset K-Ar dates in the older Shuswap gneisses to about 50 million years and has probably altered some of the K-Ar dates of the Triassic and Jurassic plutonics of the Okanagan and Similkameen complexes. Apatite fission track dating has traced the event into areas where K-Ar radiometric dates have been much less affected.

In the Kettle River (west half) map area only the oldest K-Ar dates obtained may thus have any real significance, and, assuming no excess argon is present,

represent a minimum age of intrusion. Obviously difficulty will always be encountered in deducing distinct magmatic events in areas where later 100

heating has occurred. Apatite fission track dates are useful as detectors of thermal events that are too weak to totally reset K-Ar dates yet responsible for significant degrees of Ar loss. 101

On the computation of statistical error in fission track analysis.

PAPER NO. 2

G.A. Medford Department of Geological Sciences The University of British Columbia Vancouver, British Columbia. 102

Introduction

This paper is concerned with two aspects of defining error limits in fission

track analysis. Firstly it illustrates that in some instances operator variance

can exceed that predicted by Poisson statistics and that this should be taken

into account when deriving error limits for fission track dates. Secondly it

deals with combination of errors in the variables Ps, Pi, and 0 (defined below)

and illustrates difficulties that can arise if care is not taken. The discussion

that follows was prompted by the nature of data collected by the author in the

course of fission track analysis. These data, as well as that from other pub-

lishea papers, are used to demonstrate the methods employed. The statistics for the different procedures of fission track analysis (see below, cases 1, 2 and 3) are discussed separately.

Fission track age equation.

The standard equation used to determine date based on fission tracks is

(Maeser, 1967)

9 (1) A - 6.49 x 10 In (1 + 9.45 x 10-18 £|. 0) where A = age (million years)

Ps = spontaneous track density tracks/unit area

Pi = induced track density from tracks/unit area neutron irradiation

0 = thermal neutron fluence neutrons/cm

The variables Ps, Pi and 0 are all determined by counting fission tracks. The quantity 0 usually is obtained by counting tracks on a glass standard of known uranium content for which a conversion factor relating track density to neutron fluence is available. 103

Precision of the calculated value, A, is related to the errors assigned to

the variables Ps, Pi and 0: namely os, aj and O0. Because counting data are

involved, Poisson statistics (Freund, 1962) have been invoked to estimate these

parameters.

Techniques of fission track analysis.

Three different techniques are employed in determining a fission track age

and these are discussed separately in the following sections. Particular

attention is given to determining the error in Ps/Pi. Combination of this error

with that in 0 is considered later.

Case I

One determines Ps by counting the number of tracks per unit area in the

polished surfaces of a number of grains. The value of Pi is obtained similarly

in grains that have been annealed to remove spontaneous tracks, and then irradiated,

Assuming xs and x-j track counts are obtained on each sample, one can obtain suit•

able error limits using Poisson statistics. The appropriate Poisson parameters

(Bennett and Franklin,1967) are ns _ zjxs_

X Ax, = i 1 i ni

where ni and ng are the number of observations (or grids counted). The variances 2 2 o s and a are

2 _ ,»s

s np (3) s 2 _ Ai a i n. l 104

For n>50 the central limit theorem applies (Bennett and Franklin, 1967, p.119) and

the limiting form of the Poisson distribution is the normal distribution with 2 2 - variances a and a .. and means Pi and Ps where

(4) Ps = AS

Pi = A . i

It is necessary to derive the resultant error in the quotient Ps/Pi and this can be readily accomplished by Taylor series expansion (Greenwalt and

Schultz,1962).

Defining:

- Pi (5) z = z = the track density ratio Ps

and the percentage error in z is

(7) a % = -^zr X 100 z From equation (6) it is evident one can also write

2 2 2

(8) c< z% ~ a j% + a %

This formula, in fact, has been used by most fission track workers (CW. Naeser, written communication, 1973).

In order to investigate the possibility that the estimate of a using

Poisson statistics might at times be too low, each sample of a suite being dated by the author was counted on three different occasions over a period of about two 105

months. The track ratios, obtained on each trial are plotted in Fig. 1 as well

as error bars representing three standard deviations (3oz)- Given one estimate of z it is unlikely that any subsequent determination would fall outside — the probability of such an event being about .003%. Fig. 1, however, indicates that several samples do exhibit an unduly large variation in i. More than half are within the limits of variation expected assuming the Poisson estimates of o . and

0 s are valid, and a few are marginal. The anomalous samples could be a result of the following:

1) The count-data are not Poisson distributed (e.g. inhomogenous uranium

content)

2) Interpretational variation is larger than that predicted by Poisson

statistics (e.g. faintly etched tracks might be included in the count at

some times and not others) i.e. operator variance is higher in some

instances.

In order to check for the first possibility all data were obtained using a 6 x 6 gridocular in which the number of tracks appearing in each subdivision was recorded. Those subdivisions containing grain imperfections were omitted from the count. It was thus possible to construct a frequency diagram for each sample to check the distribution of the data. Two samples which showed anomalous behaviour were selected and the data are illustrated in Fig. 2. The observed data do not depart significantly from that expected under the assumptions of Poisson distribution at the Chi-square 0.01 percent level of significance. Hence it must be concluded that an additional component of variance exists in the data (i.e. operator variance).

It is thus necessary to tailor the method of obtaining to the quality of the data collected. In those cases where Poisson assumptions hold, the best 106

TRACK DENSITY RATIOS

No 0.5 0.6 0.7 0.8 0.9 1.0 l.l 1.2

I - 145 ^ • i « i I - 148 ' ' t . .' • i i i

I- 150 ' ' t • - —• \

1-154 ' i'- ( »' t

1- 173 ' • ' ' " i ' * ' i '

2- 83 ', ' " ' '. "'

3- 0 ' i' , » ,

3- I ' ' t - ' • " J '

3" 5 ' • =^ 3- 6 i

3" 8 J '.' ,

3- 10 '» ' . - 1 (

3- 12 ' 1

3- 13 ' • f „..',—. (

3- 14 *t • • t

i , 1 3- 16 I— « I 0

3- 17 , ' T~"*t- „ -t

3- 18 ' 1 ' 1 * . " ' i • »

3- 20 >t «t t

3- 21 i f » t ' • -i

3- 22 , ' « > ' —'

3- 24 ! ' » t '

3- 25 ' >f • ^ (

3- ST ' t ' ~ '

FIGURE I 107

OBSERVED VERSUS EXPECTED TRACK FREQUENCIES

200h Sample 3- 0 Jl

lOOh

0 12 3 4 ' Z 3 4 S 6 C 1 2 i 4 S 0 1 2 J i 5 0 12 3 4 0 1 2 3 4 5 6 m- 0.86 1.56 0.92 1.24 0.92 1.41 412 510 368 453 472 379

trial trial 2 trial 3

4001 Sample 1-145

200

1 LJD L 0 12 3 0 12 3 0 1 2 0 12 3 !E UJJ 0 1 2 0 12 3 0.33 0.56 0.23 0.56 0.38 0.49 499 457 525 51 5 523 506

trial I trial 2 trial 3

m= estimated mean track density n = number of fields counted S- spontaneous tracks I= induced tracks

Observed freq: U Expected freq:

FIGURE 2 108

estimate of A. (or ^ ) is (Bennett and Franklin 1954, p.608)

E N p. (qN _ j ij for determinations on j occasions M E • n.. n.. = number of fields counted on J J sample i on occasion j Pi = average track density per field of sample i on occasion

j from which suitable error limits can be derived:

(10) / 1 ~ E 1 . n. . J iJ Otherwise, it is probably better to consider each track ratio, z. (determined on a particular occasion), as an independent estimate from a distribution with larger variance. That is

2= = J J-l z = the overall mean for deter- a (ID z j ruinations on j occasions

In cases where two values of z. are close together and one remote, it might be best to count a fourth or fifth sample before deciding which of the above pro• cedures to use. Any outlier can then be discarded. Table 1 illustrates the raw data obtained and results obtained by using both of the above models. It is most conservative to use the method which generates the largest percentage error. Note that in some cases estimated percentage errors differ by a factor of 2.

The preceding discussion illustrates the usefulness of counting the same sample several times as opposed to counting very many tracks (high n) in a sample on one occasion. In this study, about fifteen grains were counted at each time and involved between 100 and 400 tracks, depending on the uranium content of the sample. Anomalous variability in the data was then readily detected. EQUATION 11 POISSON MODEL i Ps SAMPLE Z Z Z Z 0% . Ps Xs Pi Xi Z= — 0% l 2 3 °Z °Z Pi

] 145 .5914 .4041 - .7763 0 .591 .108 18.2 .3115 481 .5338 789 0 .584 .034 5.8

1- 148 .5696 .7542 .7355 0 .686 .034 5.0 .5711 884 .8027 1233 0 .712 .031 4.4

150 .5916 .6043 .6582 0 .618 .020 3.3 .2827 406 .4642 738 0 .609 .038 6.2

154 .7070 1.0176 .9431 0 .889 .094 10.5 .4310 653 .4769 754 0 .904 .048 5.3

173 .6139 .9134 .7814 0 770 .087 11.3 .3629 560 .4611 712 0 .787 .045 5.7

2- 83 .6622 .6598 .6209 0 648 .013 2.0 .6303 762 1.0020 1530 0 .629 .028 4.4

3- 0 .6533 .7491 .5368 0 646 .062 9.5 .9022 1134 1.4266 1710 0 634 .024 3.8

3- 1 .9171 .7677 .6379 0 774 .081 10.4 .3177 338 .4345 663 0 731 .049 6.7

3- 5 .5666 .6559 .7742 0 666 .060 9.0 .1328 234 .2062 413 0 644 .053 8.2

3- 6 1 .0642 1 .0610 .8081 0 978 .085 8.7 .5798 759 .6230 985 0 931 .045 4.8

3- 8 .7061 .7061 .8704 0 761 .055 7.2 .6735 982 .8788 1254 0 766 .033 4.3

3- 10 .8470 .5947 .6258 0 688 .079 11.5 ' 3628 747 .5143 1076 0 705 .034 4.8

3- 12 1 .0560 .6175 1 .0488 0 907 .145 16.0 2088 208 .2321 344 0 900 .079 8.8

3- 13 .5599 .7240 .6640 0 649 .048 7.4 4128 637 .6281 998 0 657 .033 5.1

3- 14 .8904 ..7888 .8519 0 844 .027 3.5 2373 267 .2888 428. 0 822 .064 7.8

3- 16 .9089 1.1358 1 .0042 1. 016 .066 6.5 3555 625 .3622 648 0 982 .055 5.6

3- 17 .5782 .4247 .5257 0. 510 .045 8.9 3138 381 .6178 868 0. 508 .031 6.1

3- 18 6863 .7690 .6363 0. 697 .039 5.6 4555 302 .6525 892 0. 698 .046 6.7

3- 20 5537 .5071 .5749 0. 545 ' .020 3.7 4371 660 .8041 1371 0. 544 .026 4.7

3- 21 6493 .6075 .7739 0. 677 .049 7.2 2676 393 .4000 628 0. 699 .043 6.4

3- 22 6496 .8073 6216 0. 693 .058 8.3 2301 202 .3342 403 0. 689 .059 8.6

3- 24 7937 .7718 7244' 0. 763 .021 2.7 ;466 462 .4479 499 0. 774 .050 6.5

3- 25 7754 .6913 6581 0. 708 .035 J,]9 378 .3425 509 0. 730 .050 6.8

Symbols defined in text Xs,Xi=number of tracks counted 110

Case II

Another method of obtaining Ps and Pi involves counting the spontaneous

track density on several grains and then pressing a detector plate against the

polished grain surfaces. Induced tracks are then counted in the detector plate

after irradiation.

The statistical procedure in this case is identical to that involving

equations (9) and (10). In the literature, however, one can find ages based on each grain count which are then averaged to obtain the sample age. The standard error is obtained using an equation analogous to equation (11). This is incorrect for two reasons. Firstly, the fluence error, a^, is common to each age estimate and cannot be reduced by j-1 (see equation 11) as for independent estimates.

Thus the track ratios, z, must be treated separately. In general, one can determine (or the track ratio of any given sample) as precisely as possible or desired but this error must later be combined with the fluence error o^.

Otherwise the error a^, common to each grain estimate, is reduced without justi• fication. Secondly, the use of equation (10) is not warranted unless the varia• bility in z from grain to grain is higher than that predicted by the Poisson distribution, as discussed in the preceding section.

To illustrate the arguments set out above one can consider the data on apatite obtained by Stuckless and Naeser (1972) (Table 2). Each grain age has an error of between 8.7 and 16 million years. These figures presumably include the fluence error, the exact value of which is not stated in the paper. The standard error of the mean age (times 2) is calculated to be + 0.8 million years which is very small considering the errors on the individual estimates based on

Poisson statistics. Assuming for the moment that there is no fluence error and in

TABLE 2

APATITE AP-202 mi 11 ion years

49.9 ± 8.8

48.9 ±16 5QJ 50.5 + 8.7 51.0 + 12

Data from Stuckless and Naeser 1972

0.8 = two standard errors of the mean

From this study suggested age is

50.1 ± 5.9

5.9 = one standard error of the mean 112

the age errors are proportional to the track ratios, z, the individual ages

could be combined as

(12) Mean Age = E indiv1dua1 a9es J n

with error

(12) 2 _ " ,1.2 2 o~ • - variance in each Age * 0 mean . ,V " j J estimate on a grain n = 4 in this case

The variance estimate is easily obtained from the Taylor series expansion

(Greenwalt and Shultz 1962) with equation (12) as the source function. The

standard error obtained from the apatite data is + 5.9, a more realistic figure.

It is interesting to note that using an equation similar to (11) gives a very

small standard error. Nevertheless, the grain ratios z are each dependent on

the parameters Ps and Pi which can only be known, at the best of times, as

precisely as specified by the appropriate Poisson distributions. The individual

grain ratio estimates perhaps lie close together because grains to be used can be preselected and variability caused by poorly polished, slightly altered (etc.) grains is eliminated using the detector plate technique. In addition, the same area is examined when determining Ps and Pi and hence variability because of inhomogeneity in uranium is removed. This is not always the case, however.

The data presented by Nagpal and Nagpaul (1973) illustrate further the discrep• ancies which can occur depending on the method of analysis. The results from four samples dated in that paper are reproduced in Table 3. In addition the stat• istics calculated by the present author are included for comparison. Nagpal and

Nagpaul (1973) state that the quoted overall error for each sample is 'the standard deviation from the mean value" although the numbers seem to be most 113

TABLE 3

Data from Nagpal & Nagpaul (1973)

Sample Equation 13 j-i

Table 1, #7

756 + 30 750 + 22 747 + 6* 16 744 + 28

Table 1, #8 803 + 40 746 + 74 768 + 25 31 18 36 755 + 67

Table 2, #1 538+16 518 + 25 519 + 15 19 11 12 501 + 20

Table 2, #3 457 + 9 300+15 515 + 10 428 + 77 68 31 384 + 12 485 + 7

Standard Standard deviation error of the mean. See Equation 11. Original data: mean + Calculations in this paper. "standard deviation of the mean". tactual mean appears to be 750, not 747. 114

similar to the square root of the sample variance as calculated by the author.

The standard error of the mean (Eg. 11) is also calculated as well as the error according to Eq. 13 which is based purely on counting statistics. If the suggestion made in this paper is followed, the largest of these two errors would be quoted. These numbers are marked with asterisks in Table 3 and, in some cases, can be seen to differ greatly from each other and from those given by Nagpal and

Nagpaul.

Case III

Here spontaneous track densities are determined on polished surfaces of several grains. The grains are then irradiated and re-etched to reveal induced tracks. The induced track density is obtained by subtracting the spontaneous track density from the total track density after irradiation (see Bigazzi and

Ferrara 1971). In this case the track density ratio is

(14) z = Ps Ps = spontaneous track density

Pt - Ps Pt = total track density

The propagated error is given by Taylor series expansion (Greenwalt and Shultz

1962). Here

2 2 I 6iV 2 (15) -1 °s + a a\= p t from which it can be shown

16 2 2 < > oz%J__h , .s + P - Ps °t Ps (Pt-Ps)J It s

Interestingly, as Pt approaches Ps (this can happen if an unsuitable neutron 115

fluence is used for any given age) the error increases quite drastically because

of the subtraction in the denominator of equation (14) which results in a small

number with a large error. For this reason it is probably best to avoid using

this method in favour of detector plates. If used it is necessary to choose a

suitable reactor fluence which results in as low a ratio as possible (see equation

(1): 0

of the sample which is not always available. The parameters Ps and Pi along with

their standard errors can be obtained using equations (9) and (10).

Combination of track ratio error and fluence error.

As pointed out earlier (see case II) it is necessary first to deal with

the track ratio error and then to combine in the fluence error. This can be done

with the following formula:

2 2,2 + (15) a Age% = a 2% a Q% which is a simple extension of equations (5) and (6). Strictly speaking the

error obtained from the right hand side of equation (15) should be applied within

the argument of equation (1). Since, however, the function In varies almost

linearly within the range of arguments used the percentage error can be applied

directly to the age with only a fraction of a percent error.

An interesting sidelight to the combination of the ratio and fluence error

involves discordancy of ages. Obviously if one wishes to look at two ages the

same reactor run it is unnecessary to combine in the fluence error which is

common to both. These samples can be compared in terms of their track density

ratios. On the other hand, two ages from different runs must be compared after

the appropriate fluence errors have been incorporated. 116

REFERENCES

Armstrong, R,L., 1974. Geochronometry of the Eocene volcanic « plutonic episode in Idaho, Northwest Geology, 3, pp. 1-15.

Atwater, T., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America, Geol. Soc. Am., bull. 81, pp. 3513- 3536.

Bennett, CA. and Franklin, N.L., 1954. Statistical analysis in chemistry and the chemical industry, John Wiley and Sons, Inc., New York.

Berry, M.J., Jacoby, W.R., Niblett, E.R. and Stacey, R.A., 1971, A review of geophysical studies in the Canadian Cordillera. Can. J. Earth Sci. 8, pp. 788-801.

Bigazzi, G., Ferrara, G. and Innocenti, F., 1971. Fission track ages of gabbros from northern Appennines ophiolites. Earth and Planet. Sci. Letters, 14, pp. 242-244.

Birnie, D.J., M.Sc. thesis (in preparation). Department of Geophysics and Astronomy. The University of British Columbia.

Blackwell, D.D., 1969. Heat-flow determinations in the northwestern United States. J. Geophys. Res. 74, pp. 992-1007.

Blenkinsop, J., 1972. Computer assisted mass spectrometry and its applications to Rb-Sr geochemistry. Unpublished Ph.D. thesis, Department of Geophys• ics and Astronomy. The University of British Columbia.

Bocquet, J., Delaloye, M., Hunziker, J.C. and Krummenacher, D., 1974. K-Ar and Rb-Sr dating of blue amphiboles, micas, and associated minerals from the western Alps. Contrib. Mineral. Petrol. 47, pp. 7-26.

Bostock, H.S., 1941. Okanagan Falls, British Columbia, Geol. Surv. Can. Map 627 A.

Brock, B.B., 1934. The metamorphism of the Shuswap terrane of British Columbia, J. Geol.42jpp. 673-699.

Cairnes, C.E., 1939. The Shuswap rocks of southern British Columbia. Proc. Sixth Pacific Sci. Congress, 1, pp. 259-272.

Campbell, R.B., 1968. Canoe River, British Columbia, Geol. Surv. Can., Map 15-1967.

Campbell, K.V. and Campbell, R.B., 1970. Quesnel Lake map-area, British Columbia. Jjn Rept. of Activities, Pt. A, Geol. Surv. Can., Pap. 70-1, pp. 32-35. 117

Campbell, R.B., 1973. Structural cross?section and tectonic model of the southeastern Canadian Cordillera. Can, J. Earth Sci., 10, pp, 1607-1620.

Christie, J.C, 1973. Geology of Vaseaux Lake area. Unpublished Ph.D. thesis, The University of British Columbia, Vancouver, B.C.

Christopher, P.A., 1973. Application of apatite fissions-track dating to the study of porphyry type mineral deposits. Can. J. Earth Sci., 10, pp, 846- 851.

Church, B.N., 1970. The Geology of the White Lake Basin. Geology, Exploration and Mining in British Columbia, B.C. Dept. of Mines and Petrol. Res., pp. 396-402.

Clark, S.P.(Jr.) and Jager, E., 1969. Denudation rate in the Alps from geochronologic and heat flow data. Amer. J. Sci. 267, pp. 1143-1160.

Coney, P.J., 1972. Cordilleran tectonics and North America plate motions. Amer. J. Sci. 272, pp. 603-628.

Culbert, R.R., 1971. A study of tectonic processes and certain geochemical abnormalities in the Coast Mountains of British Columbia. Unpublished Ph.D. thesis, Department of Geophysics and Astronomy, University of British Columbia.

Daly, R.A., 1912. Geology of the North American Cordilleran at the forty- ninth parallel. Geol. Surv. Can. Mem., 38.

Daly, R.A., 1915. A geological reconnaissance between Golden and Kamloops, British Columbia. Geol. Soc. Can. Mem., 68,

Daly, R.A., 1917. Metamorphism and its phases. Geol. Soc. Amer. Bull., 28, pp. 375-418.

Damon, P.E., 1968. Potassium-argon dating of igneous and metamorphic rocks with application to Basin Ranges of Arizona and Sonora. Jji Radiometric dating for geologists (E.I. Hamilton and R.M. Farquhar, Editors . New York, Interscience Publishers.

Dawson, G.M., 1877. Preliminary report on the physical and geological features of the southern portion of the Interior of British Columbia. Geol. Surv. Can. Report of Progress, 1877.

Dawson, G.M., 1898. Report on exploration in the southern British Columbia: Shuswap sheet. Geol. Surv. Can., Map 143A.

Fairbairn, H.W., Hurley, P.M. and Pinson, W.H., 1964. "Initial Sr 87/86 and possible sources of granitic rocks in southern British Columbia, Jour. Geophys. Res. 69, pp. 4889-4893. Fleuty, M.J., 1964, Tectonic slides, Geol, Mag, 101, pp. 452*455,

Fox, K.F., Rinehart, CD., and Engels, J.C, 1973. Mesozoic plutonic history of Okanogan County, north-central Washington. Geol, Soc. Am,, abstr, 5, p. 44.

Freund, J.E,, 1962. Mathematical statistics. Prentice-Hall Inc., N.J.

Fyles, J.T., 1970. Structure of the Shuswap Complex in the Jordan River area, northwest of Revelstoke, British Columbia. IJT_ Structure of the Southern Canadian Cordillera. Geol, Assoc. Can. Special Paper, 6, pp. 87-98.

Fyson, W.K., 1970. Structural relations in metamorphic rocks, Shuswap Lake area, British Columbia. Geol. Assoc. Can. Spec. Paper, 6, pp. 107-122.

Gilluly, J.A., 1934. Mineral orientation in some rocks of the Shuswap Terrane as a clue to their metamorphism. Amer, Jour. Sci,, 228, pp. 182-201.

Giovanella, C.A., 1968. Structural studies of the metamorphic rocks along the Rocky Mountain Trench at Canoe River, British Columbia. J_n Rept of Activities, Pt. A, Geol. Surv. Can., Pap. 68-1, pp. 27-30.

Greenwalt, CR. and Shultz, M.E., 1962. Principles of error theory and carto• graphic applications. U.S. Aeronautical Charts Information Centre. ACIC report #96.

Gunning, H.C, 1928. Geology and mineral deposits of Big Bend map area, British Columbia. Summary Rept. Geol. Surv. Can., pp. 136-193.

Hibbard, M.J., 1971. Evolution of a plutonic complex, Okanogan Range, Washington, Geol. Soc. Am., Bull. 82, pp. 3013-3048.

Higgins, M.W., 1971. Cataclastic rocks, U.S. Geol. Surv. Prof. Paper, 687, 97 p.

Holland, S.S., 1964. Landforms of British Columbia. A physiographic outline, B.C. Dept of Mines and Petrol. Res. Bull., 48.

Hyndman, D.W., 1968. Mid-Mesozoic multiphase folding along the border of the Shuswap Metamorphic Complex. Geol. Soc. Amer. Bull.,. 79, pp. 575-588.

Ingamells, CO., Engells, J.C, and Switzer, P., 1972. Effect of laboratory sampling error in geochemistry and geochronology: Section 10, Intern. Geol. Congress, Montreal.

Jessop, A.M. and Judge, A.S., 1971. Five measurements of heat flow in southern Canada. Can. J. Earth Sci. 8, pp. 711-716.

Jones, A.G., 1959. Vernon map area, British Columbia. Geol. Surv. Can. Mem., 296.

Little, H.W., 1957. Geology of the Kettle River (East Half), British Columbia. Geol. Surv. Can., Map 6-1957.

Little, H.W., 1961. Geology of the Kettle River (West Half), British Columbia, Geol. Surv. Can., Map 15-1961. 119

Mathews, W,H,, 1964, Potassium-Argon age determinations of Cenozoic volcanic rocks from British Columbia, Geol, Soc, Am,, Bull, 75, pp, 465-468,

McMillan, W.J., 1970. West flank, Frenchman's Cap gneiss dome, Shuswap terrane, British Columbia, JJT_ Structure of the Southern Canadian Cordillera. Geol. Assoc. Can, Special Paper, 6, pp. 99-106.

McMillan, W.J,, 1973. Petrology and structure of the west flank, Frenchman's Cap gneiss dome, near Revelstoke, British Columbia. Geol, Soc. Can. Paper 71-29.

Miller, F.K., 1974, Distribution and trends of discordant ages of the plutonic rocks of northeastern Washington and Northern Idaho. Geol. Soc. Am., Abstr. 6, p. 220.

Miyashiro, A., 1972. Metamorphism and related magmatism in plate tectonics. Amer. J. Sci. 272, pp. 629-656.

Monger, J.W.H., Souther, J.G., and Gabrielse, H., 197-2, Evolution of the Canadian Cordillera: a plate-tectonic model. Amer, J. Sci, 272, pp, 577-602.

Naeser, C.W., 1967. The use of apatite and sphene for fission track age determinations. G.S.A. Bull. 78, pp. 1523-1526.

Naeser, CW. and Faul, H., 1969. Fission-track annealing in apatite and sphene. J. Geophys. Res. 74, pp. 705-710.

Nagpal, M.K. and Nagpaul, K.K., 1973. Thermal history of Rajasthan pegma• tites of India as revealed by fission track studies. Can. J. Earth Sci. 8, pp. 711-716.

Nasmith, H., 1962. Late glacial history and surficial deposits of the Okanagan Valley, British Columbia. B.C. Dept. of Mines and Petrol. Res. Bull, 46.

Neugebauer, H.E.O., 1965. Lithology and structure of the Late Paleozoic rocks of the Apex Mountain area, British Columbia: Unpublished M.Sc. thesis, University of Oregon.

Okulitch, A.V., 1970. Geology of Mount Kobau. Unpublished Ph.D. thesis. Department of Geology. The University of British Columbia.

Okulitch, A.V., 1974. Stratigraphy and structure of the Mount Ida Group (82L), Seymour Arm (82M), Bonaparte Lake (92P) and Kettle River (82E) Map-areas, British Columbia. Geol. Surv. Can., Paper 74-1, Part A.

Paterson, I.A., 1973. The geology of the Pinchi Lake area, central British Columbia. Unpublished Ph.D. thesis. Department of Geological Sciences, University of British Columbia.

Peto, P., 1970. The petrology of the Similkameen Batholith. Unpublished M.Sc. thesis. The University of Alberta. 120

Peto, P., 1973a. Petrochemical study of the Similakmeen Batholith, British Columbia, Geol. Soc. Am., Bull 84, pp. 3977-3984.

Peto, P., 1973b. Potassium-argon ages of igneous rocks from the area near Hedley, southern British Columbia: Discussion, Can. J. Earth Sci. 10, pp. 1357-1359.

Preto, V.A., 1964. Structural relationships between the Shuswap terrane and the Cache Creek Group in southern British Columbia. Unpublished M.Sc. thesis, Department of Geology, the University of British Columbia.

Reesor, J.E., 1965. Structural evolution and plutonism in Valhalla gneiss complex, British Columbia. Geol. Soc. Can. Bull. 129.

Reesor, J.E., 1970. Some aspects of structural evolution and regional setting in part of the Shuswap Metamorphic Complex. Geol. Assoc. Can. Spec. Paper, 6, pp. 73-86.

Reesor, J.E. and Moore, J.M., 1971. Petrology and structure of the Thor Odin gneiss dome, Shuswap Metamorphic Complex, British Columbia. Geol. Surv. Can. Bull., 195.

Richards, G.G., 1968. Petrology of the Oliver Quartz Monzonite, Unpublished B.Sc. thesis, the University of British Columbia, Vancouver, B.C.

Rinehart, CD. and Fox, K.F., 1972. Geology of Loommis Quadrangle, Okanogan County, Washington. State of Washington, Dept of Natural Res., Division of Mines and Geol. Bull. 64. Roddick, J.C. and Farrar, E., 1971. High initial argon ratios in hornblendes. Earth and Planet. Sci. Letters, 12, pp. 208-214.

Roddick, J.C, Farrar, E., and Procychyn, E.L., 1972. Potassium-argon ages of igneous rocks from the area near Hedley, southern British Columbia. Can. J. Earth Sci. 9, pp. 1632-1639.

Ross, J.V., 1968. Structural relations at the eastern margin of the Shuswap Complex, near Revel stoke, southern British Columbia. Can. Jour. Earth Sci., 5, pp. 813-849.

Ross, J.V., 1970. Structural evolution of the Kootenay Arc, southeastern British Columbia. Geol. Assoc. Can, Spec. Paper, 6, pp. 53-65.

Ross, J.V., 1974. A Tertiary thermal event in south-central British Columbia, Can. J. Earth Sci. 11, pp. 1116-1122.

Ross, J.V. and Christie, J.C, 1969. Polyphase deformation within the Shuswap Terrane of the southern Okanagan Valley, British Columbia. Geol. Soc. Am., abstr. Part 3, p. 57.

Ross, J.V. and Barnes, W.C, 1972. Evidence for "Caribooan Orogeny" in the Southern Okanagan region of British Columbia. Can. J. Earth Sci. 9, pp. 1693-1702. 121

Ryan, B.D., 1973. Geology and Rbr-Sr Geochronology of the Anarchist Mountain area, southcentral British Columbia. Unpublished Ph.D. thesis, the University of British Columbia, Vancouver, B.C.

Souther, J.G., 1970. Volcanism and its relation to recent crustal movements in the Canadian Cordillera, Can, J. Earth Sci. 7, pp. 553-568.

Stuckless, J.S. and Naeser, C.W., 1972. Rb-Sr and fission-track age determin• ations in the Precambrian plutonic basement around the Superstition volcanic field, Arizona. U.S. Geol. Surv. Prof. Pap. 800-B, pp. 191-194.

Turner, F.J., 1968. Metamorphic petrology, mineralogical and field aspects. McGraw-Hill Inc.

Wanless, R.K., 1969. Isotopic age map of Canada, Geol. Surv. Can., Map 1256A.

Wheeler, J.O., 1970. Summary and discussion. Geol, Assoc. Can. Spec. Paper, 6, pp. 155-166.

White, W.H., 1959. Cordilleran tectonics in British Columbia. Amer. Assoc. Petrol. Geol. Bull., 43, pp. 60-100. White, W.H., Erickson, G.P., Northcote, K.E., Dirom, G.E., and Harakal, J.E., 1967. Isotopic dating of the Guichon batholith, British Columbia: Can. J. Earth Sci. 4,pp.677-690.

White, W.H., Harakal, J.E., and Carter, N.C., 1968. Potassium-argon ages of some ore deposits in British Columbia. Canadian Min. and Met. Bull, pp. 1326-1334.

Winkler, H.G.F., 1967. Petrogenesis of metamorphic rocks. Springer-Verlag New York Inc., New York. 122

APPENDIX 1

Fission track and K-Ar analytical techniques.

The calculation required to obtain a fission track age is given in Naeser

(1967), namely:

Age (yr.) = 6.49 x 109ln(l + 9'45 * R0\ v 1(T

where R = the ratio of natural track density (ps) to induced track density (fxO 0 = total neutron flux (thermal) x 10^

Samples of apatite which had been annealed of spontaneous tracks by heating at

600°C for 24 hours were sent to the Nuclear Science and Technology Facility State University of New York at Buffalo Rotary Road Buffalo, New York 14214 15 with a request for a total thermal neutron exposure of approximately 1 x 10 2 neutrons per cm . Sphene samples which had been pre-polished and etched to reveal natural tracks were also included. Each sample was wrapped in an aluminum foil packet and, together, all samples occupied a volume of approx- 2 imately 8 cm .

In order to determine the actual neutron exposure, 3 standards consisting

of apatite of a known age were included. Two of these apatite samples were

obtained from the Kitley Lake Member of the Marron Formation (Church, 1970)

with a K-Ar date of 51.6 + 1.8 m.y. A third apatite was obtained from Dr. C.W.

Naeser of the U.S. Geological Survey. This sample was taken from a tuff on which concordant dates (27.2 +0.7 m.y.) on the minerals plagioclase, horn•

blende, biotite, and sanidine had been obtained. Measurement of the ratio of

natural to induced track densities for these samples allowed estimation of the

neutron exposure by rewriting the age equation as follows: 123

By substituting the appropriate age (A) for a given sample and measuring the

track density ratio (R), 0 could be measured. The results were as follows:

R 3% 0 x 1014

C.W. Naeser1s apatite .398 +7.56 11.40+.84 Kitley Lake apatite (1) .763 + 6.71 11.07 + .74 Kitley Lake apatite (2) .708 +6.99 11.92 + .83 Mean 0 = 11.4 + 0.5 x 1014 neutrons/cm2

Samples were counted using a magnification of 1200 with a 6 x 6 square gridocular. Statistical methods of treatment of the data are presented in

Paper #2. Approximately 30 mineral grains of apatite containing natural tracks were examined. Track density within each grid subfield was recorded.

Subfields containing imperfections were omitted from the count. The track density ratio was subsequently calculated (tracks per field). In the case of the sphene samples, polished grains were pre-etched to reveal natural tracks.

These grains were then re-etched after irradiation and the induced track density obtained by subtraction. In this case, the induced track density had to be doubled to account for tracks that would have been contributed from the part of the mineral removed by polishing.

The K-Ar analyses were carried out using the mass spectrometry system of the University of British Columbia located in the Department of Geophysics and

Astronomy, and operated jointly by the above department and the Department of

Geological Sciences. The system is identical to that described by White et al. (1967) except that the sample and entire fusion system were baked at 130°C for 18 hours to eliminate or reduce atmospheric argon contamination (Roddick and Farrar, 1971). A schematic diagram of the argon extraction and analytical system is presented on page 124. TO PUMPS AIR PIPET

ELECTROMETER Mg r

Ar 30 ((0)) A^-s \ SAMPLE IN SPIKE PIPET I o g Mo CRUCIBLE -J o ° / o ION o PUMP MASS A SPECTRO• METER c c WW 1 TRAP TRAP Ti IN Si02 TUBE —fp. m 7 / ivy-Vp rr / nr / t z PTT^^ r-7—r~r-/ J / /1 j LEAK VALVE rMETAL VALVES

I TO PUMP SC I i T I TO PUMPS! a

UBC K/Ar LABORATORY

ARGON EXTRACTION a ANALYT1CAI SYSTEM 125

Potassium analyses were obtained by flame photometry as described in

White et al. (1967). The number of replicate analyses was selected such that the standard error of the mean was less than about 3%. This was necessary as impurities in some samples resulted in variable sampling errors (Ingamells, et al., 1972)(see footnote of Table I, p. 90). 126

APPENDIX 2

Cooling-rate calcuations.

In order to obtain a minimum estimate of the cooling-rate of the Shuswap

gneisses, it was necessary to select a closure temperature for the argon clock

which is conservatively low, and a track retention temperature limit for fissioi

track dates which is conservatively high. Thus, the temperature range over

which cooling occurs was made as small as possible.

In the first cooling-rate estimate, the hornblende blocking temperature

was selected as 150°C. This estimate was taken from "the fractional argon

loss" versus "duration of elevated temperature" curve (case I) computed by

Damon (1968). This estimate is quite a reasonable one as it can be observed

that heating hornblende for 100 m.y. at 155°C would result in less than

approximately 2% argon loss. On the other hand, the temperature of complete

track retention in apatite was selected as 75°C, or 25°C higher than the

estimate presented by Naeser and Faul (1969). Thus a minimum cooling interval

of 75°C was obtained.

The time over which cooling occurred has been estimated by comparing

2jthe mean hornblende K-Ar date (51.2 + 4.0 m.y.), excluding sample 3-10 (59J^

m.y.) which is unusually high, possibly because of Ar gain with the mean

apatite date (48.4 +7.5 m.y.), including sample 3-17 (35.6) which is unusually

low probably because of later heating. Inclusion and exclusion of these

numbers makes a small difference in the means (e.g. 51.2 becomes 52.5, and

48.4 becomes 49.3 m.y.) and does not appreciably alter the cooling-rate estim•

ate. As it is physically reasonable that the mean K-Ar date should be greater

than mean apatite date, one sided confidence limits for difference between the

means (i.e. 2.8 m.y.) results in a range of possible values of 0 to 7.3 m.y. at 127

the 5% level of significance. Using these extremities the cooling-rate varies

between an infinite value and 10°C/m.y. The calculated difference, 2.8 m.y.,

however, is nevertheless a reasonable estimate and results in a minimum

cooling rate of approximately 25°C/m.y.

A similar argument using the sphene mean date (53.6 +_ 5.9 m.y.) and the

apatite mean date (48.4 + 7.5 m.y.) results in difference of 5.2 m.y. with

one-sided 95% confidence limits of 0 and 11.8 m.y. Using the 11.8 m.y.

extremity, and a temperature interval of 200°C (somewhat smaller than that

given by the data of Naeser and Faul, 1969), the cooling rate must be at

least 17°C/m.y.. The observed mean difference, on the other hand, results

in an estimate of approximately 40°C per m.y.

Thus, regardless of how one wishes to manipulate the data, rapid cooling

of the Shuswap gneiss is an inescapable conclusion and is consistent with a model involving shallow level heating of the crust followed by quick conduc• tive cooling upon cessation of the heat input. NORTH SOUTH VERTICAL CROSS-SECTIONS OKANAGAN MOUNTAIN AREA ELEVATION FEET

- SOOO

- 4000

Unit I : Laminated amphibolite, minor mossive amphi• bolite, granulile. - 3000

2000 Unit 2 = Hornblende (biotite)granitoid gneiss.granulite A'

Unit 3 : Augen gneiss

3000

12000

OKANAGAN LAKE IOOO

Unit A Leuco-quartz monzonite ,early synkinematic phase 2.

Unit 8 • Foliated Granodiorite, mainly hornblende granodiorite (Bi) gradational into biotite- rich hornblende granodiorite (Bz), late syn• kinematic phase 2. Hsooo

H2000

IOOO MILES

• 2 3 4 5~ FEET THOUSANDS

isooo

H4000

i3000

2000

OKANAGAN LAKE IOOO

PLATE 2 50° 00'

2 ;

'!> 5«Kh52.2±l 5 A 46.4M 6 S 46. I-l I 5 12

/ / l/l

3 •Kh53.4i|.9 GEOCHRONOLOGY A 530-4.4

KETTLE RIVER (WEST HALF) \ II \

3-2i«Kb5l.8*1.6 A 47.2*39 L_1 2 PRINCETON GROUP VOLCANiCS 10

CORYELL INTRUSIVES

10 VALHALLA GRANODIORITE

NELSON PLUTONICS \ SHUSWAP GNEISS \

EMPRESS GRANITE / McNULTY CREEK QT/.MONZONITE r

JURA GRANODIORITE u SIMILKAMEEN OTZ.DIORITE

SUMMERLAND DIORITE 4 9 3 30'

KIRTON DIORITE

NICOLA GROUP 10

0:0LD TOM .SHOEMAKER, BAR SLOW FM. b KOBAU ^iROUP c MONASHEE GROUP, ANARCHIST GROUP

NOTE Geology generalized from mop by Little (1961). Intrusive contacts west of Okanogan Lake are from Peto(l973a,b).

10 K Potassium-Argon date b-biotite h hornblende m muscovite wwhole rock 3-10 Kh59.9-2.0 A Apatite dote

S Sphene date 3-22» Kb54.8*1.6 A 480-46 Underlined dates from White et al (1968) S 59.5*12.2

Km "44*6 Kb 82,99,118

'3-o»A45.0*4.6 Id I// / \ < SHUSWAP GNEISS / / I OLIVER O O lb < / '2 /

< ic / < ic < < > o r> w z <

b > \ 12 Khl7l*6.4

ic s

SIMILKAMEEN COMPLEX 3-i«A 53 9*6.0 Ax) 5?

49°00

I20°C0' l9°0O'