Geometry of a convergent zone, Central British Columbia, Canada

Geometry of a convergent zone, Central British Columbia, Canada

Tectonophysics, 119 (1985) 285-297 Elsevier Science Publishers 285 B.V., Amsterdam in The Netherlands - Printed GEOMETRY OF A CONVERGENT ZONE, CE...

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Tectonophysics, 119 (1985) 285-297 Elsevier Science Publishers

285

B.V., Amsterdam

in The Netherlands

- Printed

GEOMETRY OF A CONVERGENT ZONE, CENTRAL BRITISH COLUMBIA, CANADA

J.V. ROSS, J. FILLIPONE,

J.R. MONTGOMERY,

DC.

ELSBY and M. BLOODGOOD

Geology Department, University of British Columbia, Vancouver, B.C. (Canada) (Received

January

14, 1985; accepted

January

24, 1985)

ABSTRACT

Ross,

J.V., Fillipone,

J., Montgomery,

J.R.,

convergent

zone, central

British Columbia,

Tectonics:

Deformation

of Continental

The Canadian Orogen

Cordillera

and the easterly within

Rocky

the Omineca

Orogen-more towards

specifically

well-exposed

analysis

basement

verging

Lake, central

across

verging

folds that deform

common

Change

in vergence

to subduction subduction

transport

has

There taken

place

or evidence

verging

easterly

units-the

Pacific

Crystalline

over

Belt

of Jurassic

terranes

two belts

these

age

of the Pacific

the Omineca and

two terranes

Belt

this margin

wherein

indicates

of deformation convergence

by a second

is

common

the present

regional

that

the

and metamor-

is accomplished,

is

phase having

westerly

map pattern.

A final

of transport

related

folds.

as resulting

from change

onto the Omineca

in direction craton,

followed

later by easterly

below the craton. points

to convergence

to be no evidence

parallel

of westerly these

between

and control

easterly

of Quesnellia

direction(s)

appears

tectonic

metamorphism

have several phases

phase of deformation,

is interpreted

Quesnellia

of transport

Collision

into the Omineca

was thrust between

folds that are superposed

first abduction

of an oceanic

of convergence. convergence

direction

process:

All evidence

cover sequences

produced

of a

(Editor),

British Columbia.

the zone of convergence

phase of defo~ation

trending

separated

that

the margin

the zone of convergence

The initial common

by easterly

northerly

with accretion

terrane-

mark

M., 1985. Geometry

and S. Uyeda

Belt. Synkinematic

to be associated

rocks

and the accreted

in common.

characterized

Fold and Thrust

Bloodgood,

Tectonophysics, 119: 285-297.

into two major

the Quesnellia

Mylonitic

near Crooked

Structural phism

Mountain

and

In: N.L. Carter

with the latter further

Belt is thought

the craton.

cratonic

Orogen.

D.C.

Lithosphere.

is separable

and the Columbian

Elsby, Canada.

to the strike

of this motion

occurring

at very high angles to the zone

parallel

with the strike of the zone. If

of transport of the zone,

has been destroyed

during

then

this

transport

occurred

before

the convergence.

INTRODUCTION

The Canadian Cordilfera (Fig. 1) may be separated into two major northerly trending tectonic units-the Pacific Orogen and the Columbian Orogen, with the latter further subdivided into the Omineca Crystalline Belt and the easterly Rocky Mountain Thrust and Fold Belt (Wheeler and Gabrielse, 1972). Deformation and

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@ 1985 Elsevier Science Publishers

B.V.

2Rh

OMWECA CRYITALLINE BELT

OUESNELLIA

2

+

52N

12ow

I-~g. 1. Locatmn map showing the configuratmn of the boundary between @E.VI&I~ Tcrranc ( IntermontaneBelt) and the Omineca Crl;stalline Belt. The position of this region around Crooked Lake. central British Columbia, is indicated with respect to the major tectonic units of the Canadian CordiNera.

287

metamorphism

of mid-Jurassic

associated

with accretion

specifically

the Quesnellia

towards the characterized

age within

of more westerly

terranes

Belt is believed

of the Pacific

to be

Orogen-more

that was thrust easterly over the Omineca Belt of these terranes are craton (Monger et al., 1982). The margins by extensive mylonite zones, and it is the aim of this present paper to

give a short description

terrane-

of the structural

relations

Belt boundary and to attempt to integrate tectonic evolution of the whole region. REGIONAL

the Omineca

across

these into

the Quesnellia-Omineca a coherent

picture

of the

SETTING

A generalized geological map of the region around Crooked Lake, central British Columbia, shows the configuration of the Omineca Belt-Quesnellia boundary (Fig. 2). This boundary outlines a series of northwesterly shallow plunging antiformal folds that are cored by metasediments of the westernmost part of the Omineca Belt. These metasediments comprise members of the Snowshoe Group of rocks whose age may range from Hadrynian through lower Paleozoic (Campbell, 1978; Struik, 1984). Included within these Snowshoe Group metasediments are several bodies of granitic gneiss, one of which, the Quesnel Gneiss, has yielded zircons of mid-Palaeozoic age. They have all been variably

metamorphosed

through

amphibolite

facies and belong

in part to the northernmost extremity of the Shuswap Complex which is the largest of the metamorphic complexes in the Omineca Belt (Read and Brown, 1981). Structurally basic volcanic Mountain

overlying the Snowshoe Group is a variable thickness (15-250 m) of and ultramafic rocks that may be the southern equivalents of the Slide

Group,

known

locally

as the

Antler

Formation,

composed

of basic

volcanics, minor ultramafics and chert of late Paleozoic age (Struik, 1982). This volcanic sequence is in turn overlain by black phyllites of Upper Triassic age that pass upwards Jurassic age.

through

sandstone

into volcaniclastics

Each of these major groups is separated

and massive volcanics

from its neighbour

of Lower

by a well-defined

fault

zone that is usually outlined by mylonitic rocks. The Snowshoe Group metasediments are thus considered to be basement to the overlying cover of younger late Palaeozoic and early Mesozoic volcanic and sedimentary rocks. STRUCTURAL

RELATIONS

Basement The earliest recognizable structures within the Snowshoe Group are mesoscopic isoclinal folds that are outlined by compositional layering. These folds are commonly rootless, have ubiquitous mineral lineations parallel with their hinge lines and a well-developed axial foliation. It is this foliation that is the earliest recognizable

288

foliation observed within the Quesnel Lake Gneiss and the other smaller gneissic bodies, but as yet this foliation and associated folds have not been related to any large-scale

regional

able structure Earliest

is a transposed

regional

and probably

structure.

the earliest

foliation

within

the main

phase

the Snowshoe

Group

is associated

with ductile

axial-foliation

that dips variably

snowskae ecw Habynian- E. Pz Graraii

recognized deformation

and associated

behaviour

gneissic

of all rock types

to the west near

boundary

/

Eady Fc#atiin St-cover SP-basement

y

second Phase Liiation

_-

gneiss

Antler Fm Late Pz

cl

Phylltte LateTriassic

cl

Third Phase Axial Trace + kantlomlal synformal

Massive volcanicsvolcanidastii Early Jurassic

_---

Shear Zone D6coltement

Surface

-

- --

on all scales that is everybodies.

This

producing

the boundary

part of the area under consideration.

Convergence

any recogniz-

nature.

of penetrative

pronounced

to the east in the eastern

that outlines

2 deformation-are

deformation variably

surface

of a regional

structures--Phase

represent

where recognized

Thus,

a and

The foliation

289

(b)

NE

SW

\ h

Fig. 2. Generalized of the different

boundary structural

geologic between

map of the Crooked Quesnellia-Omineca

levels indicate

the geometry

Lake area, central Crystalline

British Columbia.

Belt is outlined;

of the zone and adjacent

three

The configuration cross-sections

at

sequences.

is axial planar to major and minor folds and varies in style according to lithology. Fold axes, outlined by mineral lineations, have variable northerly and southerly plunges and invariably have girdle distributions-that is, they frequently are curvilinear on all scales. Minor folds show consistent easterly vergence related to a major synformal structure that closes to the southwest and whose core is the local for small bodies of highly deformed granitic gneiss, similar in style and composition to that of the Quesnel Gneiss (Fig. 2). Associated with this penetrative deformation is a metamorphism, whose mineral assemblages are characteristic of middle amphibolite facies and micro-probe analyses of garnet-biotite pairs developed synkinematically indicate a pressure of 4.5 kbar and temperature of 600°C. Microscopic textures indicate that this metamorphism peaked and outlasted the second phase of deformation and began to wane during the third phase down to middle greenschist facies at the end of phase 3. Third phase structures, in terms of an axial foliation, are well-developed throughout the region, dip variably to the east and are associated with mesoscopic folds having a westerly vergence. Close to the Omineca-Quesnellia boundary large-scale third phase folds are apparent and have a characteristic form of lobate

Fig.

3. a. Synclinat

sliver of Upper

Triassic

phyllite

mechanically intercalated between gncisses

Showshoe Group. Note folded contact (outlined in ink) and that this contact transects the foliation the gneiss. h. Transposed hinges with Upper Triassic

earliest folds withtn the Upper Triassic phyllite.

phyllite.

c. ‘Transposed

c

.he

u

lin

limbs and

,rn

291

upward closing folds in the basement separated by cuspate downward closing folds of cover rocks. It is these large-scale structures having this geometry that control the regional map pattern and the local configuration of the Omineca-Quesnellia boundary. Ductile shear zones of variable width and high flattening strain are located at the cuspate closures and extend downwards below these closures. Close to the boundary these ductile shear zones are cored with younger Upper Triassic phyllite (Fig. 3a). Phase 2 easterly verging folds are refolded by these almost coaxial westerly verging third folds and their associated axial-foliation. It is the superposition of this third phase stage on the pre-existing phase 2 folds that gives rise to the curvilinear nature of these phase 2 axial structures and results in regional elongate domes and basins having the phase 2 trend (Fig. 2). Wherever it is possible to determine the orientation of the phase 3 slip direction from the locus of the distorted phase 2 linear features (Fig. 4), this slip direction makes a high angle with the third axial direction, almost parallel with the dip direction of its associated axial foliation and not parallel to the strike of the boundary. The final phase of deformation, phase 4, is represented by non-penetrative small scale crenulations often accompanied by a well-developed crenulation cleavage associated with biotite growth. These cleavages all dip at shallow angles to the southwest across the previously developed surfaces and display a consistent north-

293

BASEMENT/C~VER SLIP DIRECTIONS ASSOCIATED WlTH CONVERGENCE ZONE SHORTENING

Fig. 4. Dots are early axial-structures basement/cover

shortening.

Calculated

having

a curvilinear

slip direction

with their fold axes (L) on the highly flattened

form resulting

(a) associated

limb regions,

from distortion

with this shortening

associated

with

make high angles

but are close to their fold axis directions

in

the hinge region.

easterly sense of vergence. This deformation is particularly concentrated near, but by no means restricted, to the Omineca-Quesnellia boundary and appears to become less well developed, at structurally lower levels, away from the boundary. Thus, the Snowshoe Group, basement to the late Paleozoic-early Mesozoic cover displays a variably oriented series of structures that change their vergence direction with time, firstly to the northeast, then to the southwest and finally to ihe northeast again.

294

immediately deformed

above

volcanic

thickness

unit

(15-250

in tectonic

believed

m) within

tion that is parallel discrete

and

contact

with

to be of late

the basement

Paleozoic

age.

It has ;i variable myhitic

this region and has a well-developed

with gross composition

rootless isocIina1 folds. Structurally

layering

is the highly lamina-

and within which are contained

above this volcanic

unit is the Mesozoic

package of mixed sedimentary and volcanic rocks, the lowest of which is the Upper Triassic black phyllite. This whole package shows no gross structural inversion and the lowest member, the phyllite, is in sharp contact, probably tectonic, with the underlying late Paleozoic volcanic unit. Near this contact the phyltite has a transposed foliation (Fig. 3b) with local recognizable mesoscopic inversions, parallel with the transposed foliation within the underIying volcanic unit. Both of these earliest recognizable axial foliations are parallel with the phase 2 axial foliation recognized within the Snowshoe Group basement rocks. Some 100 m structurally above the phyllite-volcanic contact the transposition of the foliation within the phyllite is less so, and isoclinal limbs

and

folds gradually

rounded

hinges

develop

measureable

of up to 60”--70’

angles

with

between

transposition

their planar features

OI$J

apparent in limb regions of larger folds (Fig. 3~). Total transposition of these earliest cover folds is only seen again in a narrow zone immediately below the Lower Jurassic voI~anic-sedimental package that lies above the black phyllite. This latter zone is truncated abruptly by the overlying volcanic sequence which is deformed into open buckle folds having a well-developed axial foliation. Throughout the whole of the cover sequence and especiarly within the black phyllite the axial foliation is the locus of pressure solution and the folded bedding surfaces contain extensive development of plicated fractures filled with quartz (Fig. 3d). Whereever apparent. these earliest cover folds- transposed or open -.-exhibit a northeasterly vergencc. All of the early cover folds. described above, are refolded on all scales by a second fold set that also has its own well-developed axial foliation characterized by pressure

solution

surfaces

or spaced

cleavage

vergence. It is this fold set that controls Omineca-Quesnellia boundary its present Lastly. a mesoscopic

crenulation

and

has a consistent

the regional map pattern regional configuration.

and crenulation

southwesterly and gives the

cleavage, heterogeneously

formed

across all of the previous surfaces, is well-developed in the west and adjacent to the boundary, but becomes less well-developed at structurally higher levels within the cover. These small scale plications show a consistent vergence to the northeast. DISCUSSION

It is apparent that both basement and cover have common phases of deformation with the basement having an extra phase of deformation that is no where seen within the cover. However, the conditions wherein the c~~mmon phases are accomplished

295

are different

for the basement

and the cover. The junction

and cover is not folded by the earliest

phase common

between

earliest common phase (phase 2 within the basement) prograde synkinematic within the basement, peaking at middle amphibolite

metamorphism is facies and outlast-

ing phase 2 and waning

during

phase 3, whereas

the cover deformation

earliest

during

initial

accompanied

time is achieved

dewatering

the basement

to both. At this time of the

during

by pressure

this

solution

and hydraulic fracturing of the cover sedimentary pile. It is only at the second common phase of deformation of the basement-cover complex that the junction between junction occupies

the two is deformed. Because of extreme ductility contrast across this during shortening, the less competent cover is drawn down into and the cores of tightly cuspate synclines between lobate upward closing folds

of the more competent basement. Below these cuspate closures basement shear zones contain slivers of cover graphitic-phyllite that are laced with quartz-filled hydraulic fractures

with some of these phyllites

supporting

growth

of garnet

and staurolite

produced during the waning metamorphism. These basement shear zones and cuspate downwarps are probably conduits for heat flow up into the cover during dewatering producing increased pore pressure within the cover by thermal expansion that allows the overlying early Jurassic volcanic sequence to be thrust further eastwards independent of the underlying cover phyllite. Thus, the effect of basement and cover reaction is restricted within the cover to the phyllites that are separated from the overlying volcanics by a decollement surface. The following discussion shows the simplest interpretation of this body of data, which combines the above geometry together with vergence directions of the different deformation phases. The very fact that the first common phase of deformation within the basement and cover involves the development of easterly verging folds without any visible shortening of the junction of the basement and cover indicates that these two packages of rocks had likely undergone an initial phase of convergence (Fig. 5a). This convergence involving the emplacement of late Paleozoic volcanics together with early Mesozoic

sediments

by the continental Hadrynian converging package. A change deformation,

in the transport wherein

westerly

and volcanics

was likely an abduction

rocks were subducted direction verging

is inferred

westwards

process where

below

the easterly

from the next common

folds are developed

throughout

phase of both

base-

ment and cover rocks and the basement-cover junction is markedly shortened. These westerly verging folds, shortening of the junction together with the development of east-dipping shear zones within the basement appear to indicate a reversal of the direction of subduction that may also have allowed continuation of eastward thrusting of the early Jurassic volcanics (Fig. 5b). The development of the late crenulation cleavage is likely a consequence of late eastward thrusting of the early Jurassic volcanics during the late dewatering stages of the underlying phyllites. Certainly, the upper mantle signature presently below this region is one of oceanic

/ /

a

b Fig. 5. Cartoons

showing

tectonic

evolution

tion as in Fig. 2. a. Initial convergence shown

as essentially

verging

middle Jurassic westerly

planar

fold structures;

affinity,

fold-set geometry

supporting

The tectonic direction.

and continued

this common

easterly

thrusting

zone of convergence. basement

upon an already Basement

and cover develop developed

easterly

fold-set. b. Later

and cover folds now refolded

of the basement/cover

by

zone with characteristic

of the cover volcanic/volcaniclastic

sequence.

(Ross, 1983).

above is simple involving tectonic

Omamenta-

time, with main zones of active movement phase,

direction.

in shortening

such a proposition regional

Jurassic

are superposed

in transport

that also results

model described

However,

During

those in the basement

time, showing shange

verging

lobate-cuspate

features.

of the Omineca-Quesnellia

at early-mid

models

proposed

only a change in transport by Monger

et al. (1982)

involve not only convergence but also large-scale lateral translation associated with the accretion process. In any convergent model the shortening direction observed in rocks is not likely to be related directly to the absolute direction of convergence, rather it is perpendicular to the zone of convergence even when the process is oblique. Often, the component of convergence parallel with the zone is evidenced by large-scale strike-slip displacement in the arc region behind the subduction zone. No such strike-slip displacement has been recognized within the region under discussion, and no evidence of lateral regional extension parallel with the zone of convergence has been seen. All evidence of movement related to convergence appears to be at a very high angle with the strike of the zone. Thus, if large-scale translation is involved in the accretion processes then, either the evidence for same has been destroyed

291

during

the covergence

outside

of this zone of convergence.

process,

or, more likely,

translation

is taken

up elsewhere

ACKNOWLEDGEMENTS

This work was made possible

by financial

support

from N.S.E.R.C.

to J.V. Ross

(A-2134). REFERENCES Campbell,

R.B..

Geological Monger,

1978.

J.W.H..

and plutonic

P.B. and Brown,

Monashee

map

R.L., 1981. Columbia

complexes,

from peridotite

southern

xenoliths.

L.C., 1982. Bedrock

(93 A/13).

map-area

(NTS

D.J., 1982. Tectonic

welts in the Canadian

Cordillera.

93 A), British

accretion

Geology,

River fault zone: southeastern

British Columbia.

Ross. J.V., 1983. The nature and rheology Struik,

of the Quesnel

Columbia.

Open File 574.

Price, R.A. and Templeman-Kluit,

major metamorphic Read,

Geological

Survey of Canada,

margin

of the Shuswap

and

Can. J. Earth Sci.. 18: 1127-1145.

of the Cordilleran

Tectonophysics,

and the origin of two 10: 70-75.

upper mantle of British Columbia:

inferences

100: 321-358.

geology

of Cariboo

and Wells (93 H/4)

map-areas,

Lake (93 A/14), central

British

Spectacle

Lakes (93 H/3),

Columbia.

Geol.

Surv. Can.,

Swift River Open

File

Rep., 858. Struik,

L.C.. 1984. Bedrock

Columbia. Wheeler,

J.O. and Gabrielse,

Douglas

geology of Quesnel

Lake and part of Mitchell

Lake map-areas,

central

British

Geol. Surv. Can., Open File Rep.. 962. (Editors),

H., 1972. the Cordilleran

Variations

in Tectonic

structural

Styles in Canada.

province.

In: R.A. Price and R.J.W.

Geol. Assoc. Can., Spec. Pap., 11: l-81.