A model of compressional tectonics for the origin of the Vredefort structure

115

~ecr~~o~~ysic~, 171 (1990) 115-118 Else&r

S&we

Publishers B.V., Amsterdam - Printed in The Ne~~r~~ds

A model of compressional tectonics for the origin of the Vredefort structure W.P.

COLLISTON

~ep~r~rneni of Geology,Uitiuersipof the Orange Free State, P. 0. Box 339, Bl~rnjon~~~~9300 {South Ajtica)

(Revised version accepted February 25,1989)

This article highlights some of the main aspects presented in a poster at the “International Workshop on Cryptoexplosions and Catastrophes in the Geological Record” in Parys, South Africa (Colliston et al., 1987). The core of the Vredefort structure has been recognised as a ~~atite complex which forms part of a larger complex within the early Archaean basement rocks of the Kaapvaal craton. Deformation episodes in the basement fall into two main groups, events and structures predating the Dominion Group, and those that postdate it. Pre-Dominion Group events comprise a middle to the late Archaean anatexis and migmatisation of the basement with subhorizontal fabric development. This is followed by pervasively developed NE- and ~-trend~g, subvertical conjugate ductile shear zones. The shear zones are truncated by the Dominion Group, thereby revealing their involvement with tectonic events in the basement which pre-date the deposition of the late Archaean-early Proterozoic cover sequence, This precludes the shear zone event being associated with the genesis of the - 2.0 Ga (Walraven et al., this volume) Vredefort structure. Post-Dominion Group events include a phase of subvertical shearing at Broodkop (southeastern Vredefort dome) (Fig. 11, followed by mafic dyke intrusion, pseudotachylite development {of which two ages of generation are recognised), and finally a joint-fracture event. The mafic dyke event post-dates the development of the Vredefort structure and may be associated with the late igneous activity of the Bushveld Igneous Complex. Pseuoo40-1951/90/$03.50

0 1990 Elsevier Science Publishers B.V.

dotachylite veins containing xenoliths of an earlier pseudotachylite generation transect this dyke and represent an event which post-dates the Vredefort structure (Reimold et al., 1988). Preliminary results across the contact between the basement and the late Archaean-early Proterozoic cover sequence in the northwest of the dome indicate the presence of foliations in lower Witwatersrand rocks and a shear zone at the basement-Dominion Group contact. These structures post-date the intrusion of the alkali granite intrusions and associated contact metamorphism, and are provisionally interpreted as being related to the evolution of the dome. Considerable geometric agreement between preand post-Do~on shear zones, joints, and pseudotachylite younger than 2.0 Ga (cf. also Reimold et al., this volume) is apparent in the basement, with northeasterly and northwesterly trends being most common. The coincidence of these structures suggests a geometric control on their development, possibly by basement anisotropy. It is inferred that stress regimes in the basement varied through geological time, with easterly and northerly compressive trends common. A tectonic hypothesis in the form of a deep crustal subsurface shear zone model is proposed for the Vredefort structure and is an alternative to both the endogenic/exogenic catastrophic shock hypotheses and to diapirism. The hypothesis is based on preliminary structural studies on the basement and cover rocks of the dome (Colliston et al., 1987; Reimold et al., 1988; Colliston and

W.P. COLLISTON

116

WitwatersranctSupergroup Exposed Central Rand Group Exposed West Rand Group Exposed Granitic Gneisses Greenstones Covered West Rand Group

P

Parys

V Vredefort K ,,‘$

Koppies Approximate position of the transition between OGG and ILG

Fig. 1. Vredefort

geology

and section

line for the structural

profile (see Fig. 2). OGG-Outer Leucogranofels.

OGG-Outer movement strata (d)

subhorizontal

Granitic

and subsurface

Gneiss;

(flat-ramp-flat

geometry

is developed partial

in the forelimb.

“shock

by lower grade developed

Stage

metamorphites)

in a subvertical

Smges

at the crust-mantle

(g), the crust is thickened

3. Advanced

of the crust, deformation

uplift

has taken

given rise to an increase phenomena.

could be explained of the structure attitude

boundary.

place.

of the Vredefort

Uplift

Decompression by melting,

in stored

weathering

ILG-Inlandsee

(isothermal

in the dome (a granulite

release,

the structure. is explained duplex

in previous

publications

and of the

give rise to the core surrounded

The crenulation

exhibit

(j)

movement

this could

by lateral

structures.

of the structure

begins,

form in granulites

with progressive

Upon

by Iater developing

been reported

reactions

decompression)

(c) and the first cleavage

energy.

through

margin

is initiated

together

dome (schematic).

of shear zone (b) with thrust

of strata

unconformity,

in the southern

as has erroneously

evolution

I and 2. Initiation

locally, local melting

distribution

Group-basement

may be explained

part of the dome only. Strata

and not a vertical

The present

by differential

shear zone at the Dominion

of the forelimb

to the northern attitude,

regime

metamorphic”

dome. Overfolding restricted

antiform

for the tectonic

Zone.)

melting occurs (k). The increase in volume in the rocks caused

shear in the seismogenic so-called

Metamorphic

(e and f) possibly

are bulged up into a flat-topped

further

shear zone model

SMZ-Steynskraal

Gneisses;

The model involves a NW-trending, SE-dipping subsurface ductile shear zone. The shear probably follows zones of metamorphic phase changes (Boyer and Elliott, 1982), with its lower boundary possibly the crust-mantle interface (cf. Coward, 1980, 1983; Ramsay, 1980). The general sense of thrust displacement is towards the north, as indicated by the asymmetry of the structure. Decoupling at lower levels gives rise to the bulging up of

Reimold, 1989), as well as on regional tectonic and geophysical findings as reported in the literature (e.g. Corner et al., 1987). The proposed model showing the evolution of the dome is schematically shown in Fig. 2; the section line is indicated in Fig. 1. The cross section was geometrically constructed using the fault-bend fold model of Suppe (1983) and is based on geometric data measured along the line of section.

Fig. 2. Deep crustal

Granitic

stretching

cleavage around

(h), the

Note that overturning a subhorizontal

S-dipping

(e.g. Hart et al., 1981).

is

VREDEFORT

STRUCTURE

COMPRESSIONAL

117

TECTONICS

;ion of this structure would be dome-like (see 1Boyer and Elliott, 1982). Rapid uplift of the fold itructure and denudation would give the present

the overlying strata into a flat-topped antiformal fold structure, forming above a thrust ramp at depth (Fig. 2, stages 1 and 2). The surface expres-

Dominion Group and WWRSupergroup

SE

NW

Unconformity

km

-0

Upper

Migmatitlc gneiss (OCC) Amphfbolite, tonaltte granul f te (SK?)

Crust Middle to Lower Crust

Leucogranofels Upper Mantle

-60

(a)

(b)

Archaean-age Initiation

subvertical of

Interface;

shear

zone

at

of

(12).

intrusion

(cl

Alkaligranites

(d)

Development

shear

contact

zones. crust-mantle

metamorphism.

-20

-

(e)

20

of

first

cleavage

Footwall dary

flat

at

(f)

Footwall

-60

(g)

Structurally and

(h) VAAL RIVER

shear

in

necessary

front

fold

of

footwall

lower

boun-

overlying

strata

formed ramp; and

above locally

thickening

crust.

Crenulatlon

cleavage

in

tween OCC and Dominion

(i)

zone

interface.

ramp.

lifting the

of

crust-mantle

-40

-40

by flexural

slip.

km

-0

Oversteepening cate

of

shear

zone

be-

Group.

limb

by further

imbri-

development.

-20

(_!I

Decompression core

-0

-20

km

of

footwall

(k)

reactions

antiformal

Partial

melting

body at ly

depthi

by

dyke

activity -60

km

20 10

of

10

20

km

lower

crust

emplacement intrusion

in

above

a

caused

by

fracturing),

rise

possible

to

by uplift of magmatic

of melts

local-

local

seismic

development;

(melting, phenomena.

0 F

granulites

forming

ramp.

(decompression): -40

in

fold,

increase

in

locally shock

volume giving

deformation

W.P. COLLISTON

118

exposure of the various crustal levels. These include a core of grant&es and an outer rim of relatively lower grade metamorphites (Fig. 2, stage

gement given during manuscript.

3).

References

The model also suggests an explanation for some of the characteristic features of the Vredefort dome, such as the overturning of the northern limb of the structure, microdeformation features, so-called multiply-striated joints (including “shatter cones”) and intrusive rocks. Certain aspects related to the model, however, require further clarification (e.g. lateral ramps on the margins of the structure) which is part of an ongoing study. In conclusion it is suggested that the Vredefort structure could be viewed as a response to regional erogenic events in the Witwatersrand basin, probably peaking at post-Transvaal times.

Boyer,

SE. and Elliott,

D., 1982. Thrust

Colliston,

W.P. and Reimold,

the Vredefort

Dome:

this structure. Transvaal 40-43

In: Jt. Conf.

Branch.

report

and isotopic SE Vredefort

S. Afr.,

pp.

A.S., 1987. A

structural,

of the Broodkop

geochemical

Migmatite

In: Int. Workshop

in the Geological

Complex,

Cryptoexplosions

Record

R.J. and Nicolaysen,

(Parys,

South

Craton.

In:

Africa).

in

L.O., 1987. Grav-

studies reveal a unified

Structure,

Catastrophes

Witwatersrand

Int. the

Workshop

framework

Basin

Record

for

and Kaap-

Cryptoexplosions

Geological

and

(Parys,

South

Sect. Cl, 7pp.

M.P., 1980. The thrust

and shear zones of the Moine

thrust zone and the NW Scottish London,

Caledonides.

J. Geol. Sot.

140: 795-811.

M.P., 1983. Shear zones in the Precambrian

southern Hart,

Western

Johannesburg,

Sect. C3, 22 pp.

the Vredefort

Coward,

origin for

Div. and

W.U. and Robertson,

Dome.

G., Durrheim,

Coward,

Tectonics

Sot.

studies on

of an erogenic

on a detailed

study

and Catastrophes Africa). Corner,

Geol.

W.P., Reimold,

preliminary

Africa.

J. Struct.

R.J., Nicolaysen,

in the deep profile

at Vredefort,

evolution.

J. Geophys.

crust of

Geol., 2: 19-27.

L.O. and Gale, N.H.,

basement Ramsay,

I am indebted to Uwe Reimold for kindling my interest in the Vredefort dome and for the many hours of inspiring discussion during cooperative field and laboratory studies. I would also like to thank John Bristow, Chris Roering and Prof. H.S. Yoder, Jr. for constructive criticism and encoura-

Am. Assoc.

W.U., 1989. Structural

Implications

ment concentrations

Acknowledgements

systems.

(Abstr.)

Colliston,

vaal

It is considered that two-dimensional structural analyses are somewhat inadequate in solving the Vredefort dilemma and need to be supplemented by geophysical methods involving seismic reflection techniques and drilling at selected sites. Testing of the shear zone hypothesis can only be carried out by seismic reflection.

of this

Pet. Geol. Bull., 66: 1196-1230.

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Recommendation

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with implications Res., 86 (Bll):

J.G., 1980. Shear zone geometry:

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through

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for early crustal

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W.U.,

Chronological In: Geocongr. 501-504 Suppe,

Colliston,

W.P.

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and

Robertson,

A.S.,

work in the Vredefort Sot.

S. Afr.,

1988. Dome.

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pp.

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fold-

(Abstr.).

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