DEFINITION OF STRUCTURAL ELEMENTS OF GREENSTONE BELTS

The earliest structures found in greenstone belts are those that

formed while the rocks were being deposited. In most greenstone

belts, the mafic volcanic/plutonic section is older than

the clastic sedimentary section, so penecontemporaneous structures

are older in the magmatic rocks than in the sedimentary

rocks. Unequivocal evidence for large-scale deformation of the

magmatic rocks of greenstone belts during their deposition is

lacking. Structures of this generation typically include broken

pillow lavas grading into breccias, and possible slump folds

and faults in interpillow sedimentary horizons.

From the Jamestown ophiolite complex within the Barberton

greenstone belt, Maarten de Wit describes two types

of early extensional shear zones that were active prior to

regional contractional deformation. The first are low-angle

normal faults located along the lower contacts of the ophiolite-

related cherts (so-called Middle Marker) and cause

extensive brecciation and alteration of adjacent simatic

rocks. The faults and adjacent cherts are cut by subvertical

mafic rocks of the Onverwacht Group, showing that these

faults were active early, during the formation of the ophiolitic

Onverwacht Group. The second type of early faults in

the Barberton greenstone belt occur in both the plutonic and

the extrusive igneous parts of the Jamestown ophiolite, and

they may represent steepened extensions (root zones) of the

higher-level extensional faults, or they may represent transform

faults. Other possible examples of early extensional

faults have been described from the Cameron River and Yellowknife

greenstone belts in the Slave Province, and from the

Proterozoic Purtuniq ophiolite in the Cape Smith Belt of the

Ungava Orogen. In the Purtuniq ophiolite, early sinuous

shear zones locally separate sheeted dikes from mafic schists,

causing rotation of the dike complex. In other places, dikes

intrude this contact, showing that the shear zones are early

features. Although not explicitly interpreted in this way,

these shear zones may be related to block faulting in the

region of the paleoridge axis.

Detailed mapping in a number of greenstone belts has

revealed early thrust faults and associated recumbent folds.

Most do not have any associated regional metamorphic fabric

or axial planar cleavage, making their identification difficult

without very detailed structural mapping. Examples are

known from the Zimbabwe and Kaapvaal cratons, the Yilgarn

craton, the Pilbara craton, the Slave Province, and the

Superior Province. In these cases, it is apparent that early

thrust faulting and recumbent folding are responsible for the

overall distribution of rock types in the greenstone belts and

also account for what were, in some cases, previously interpreted

as enormously thick stratigraphic sequences. In some

cases, early thrust faults are responsible for juxtaposing

greenstone sequences with older gneissic terrains. Few of

these early thrust faults are easy to detect; they occupy thin

poorly exposed structural intervals within the greenstone

belts, some are parallel to internal stratigraphy for many kilometers,

and most have been reoriented by later structures. In

many greenstone belts there are numerous layer-parallel fault

zones, but their origin is unclear because they are not associated

with any proved stratigraphic repetition or omission.

Although it is possible that these faults are thrust, strike-slip,

or normal faults, the evidence so far accumulated in the few

well-mapped examples supports the interpretation that they

are early thrust faults. In some cases, late intrusive rocks have

utilized the zone of structural weakness provided by the early

thrusts for their intrusion.

Emplacement of the early thrust and fold nappes typically

is not associated with any penetrative fabric development

or any regional metamorphic recrystallization, making

recognition of these early structures even more difficult.

Delineation of early thrusts depends critically on very

detailed fieldwork with particular attention paid to structural

facing, vergence, and patterns of lineations. Determining

the sense of tectonic transport of early nappes is critical

for tectonic interpretation, but it is also one of the most elusive

goals because of the weak development of critical lin

eations, and reorientation of the earliest fabric elements by

younger structures. Kinematic studies of early “slide” zones

have received far less attention than they deserve in Archean

greenstone belts.

Early penecontemporaneous structures within the younger

metasedimentary sections appear in general to be related to

thrust stacking of the mafic volcanic/plutonic section, as

shown best by the deformed flysch and molasse of the Pietersburg,

South Africa, and Point Lake, Northwest Territories,

greenstone belts.

Folds are in many cases the most obvious outcrop to

map-scale structures in greenstone belts. Several phases of

folding are typical, and fold interference patterns are commonplace.

Some greenstone belts show a progression from

early recumbent folds (associated with thrust/nappe tectonics),

through two or more phases of tight to isoclinal

upright folds, which are associated with the most obvious

mesoscopic and microscopic fabric elements and metamorphic

mineral growth. One or more generations of late open

folds or broad crustal arches, with associated crenulation

cleavages also affect many greenstone belts. Fold interference

patterns most typically reflect the geometry of F2 and

F3 structures because they have similar amplitudes and

wavelengths; F1 recumbent folds are best recognized by

reversals in younging directions, or downward-facing F2

and F3 folds.

In many examples, the relationships of individual fold

generations to tectonic events is poorly understood, and the

orientations of causal stresses are poorly constrained, largely

because of uncertainties associated with correctly unraveling

superimposed folding events. The relative importance of

“horizontal” v. “vertical” tectonics has been debated, in part

because it is difficult to distinguish granite-cored domes produced

by the interference of different generations of folds

from domes produced by diapirism. Two generations of

upright folds with similar amplitudes and wavelengths will

produce a dome and basin fold interference pattern that very

much resembles a diapir pattern.

Many granite-greenstone terrains are cut by late-stage

strike-slip faults, some of which are reactivated structures

that may have been active at other times during the history of

individual greenstone belts. Strike-slip dominated structural

styles are known from the Superior Province, the Yilgarn craton,

the Pilbara craton, and southern Africa.

Large-scale lineaments of the Norseman-Wiluna Belt

near Kalgoorlie and Kambalda show late reverse motion

related to regional oblique compression. However, their

length, often greater than 62 miles (100 km), and consistent

indicators of a sinistral component of motion suggest that

they are dominantly strike-slip structures. These corridor

bounding structures are not just late cross-cutting faults but

lateral ramps, present through the deformation history that

bound domains within which unique structures are developed.

These major faults may represent reactivation of terrane

boundaries, since they separate zones of contrasting

stratigraphy and structure.

The Superior Province consists of a number of faultbounded

sub-provinces containing rocks of different lithological

associations, ages, structural and metamorphic

histories. The greenstone terrains consist of several types,

including tholeiitic-komatiitic lava, tholeiitic to calc-alkaline

complexes, and shoshonitic/alkalic volcanic rocks with associated

fluvial deposits. Belts of variably metamorphosed volcanogenic

turbidites, interpreted as accretionary prisms,

separate the older individual volcanic belts. A general southward

younging of central parts of the Superior Province,

together with the contemporaneity of deformation events

along strike within individual belts suggests that the Superior

Province represents an amalgam of oceanic crust and

plateaux, island arcs, continental margin arcs, and accretionary

prisms, brought together by dextral oblique subduction,

which formed the major sub-province–bounding

strike-slip faults. Continued late orogenic strike-slip motion

on some of the faults localized alkalic volcanic and fluvial

sedimentary sequences in pull-apart basins on some of these

strike-slip faults.

In the Vermillion district of the southern Superior

Province, the deformation history begins with early nappestyle

structures, which are overprinted by the “main” fabric

elements related to dextral transpression. This sequence of

fabric development is interpreted to reflect dextral-oblique

accretion of island arcs and microcontinents of the southern

Superior Province. A combination of north-south shortening

together with dextral simple shear led to the juxtaposition of

zones with constrictional and flattening strains. The constrictional

strains in this area were previously interpreted to be a

result of “squeezing” between batholiths, necessitating

reevaluation of similar theories for the origin of prolate

strains in numerous other greenstone belts.

In some cases, strike-slip faults have played an integral

role in the localization and generation of greenstone belts in

pull-apart basins. Several strike-slip fault systems associated

with the formation of greenstone belts in pull-apart basins

are known from the Pilbara craton. The Lalla Rookh and

circa 2,950 Ma Whim Creek belts are interpreted as “secondcycle

greenstones,” because they were deposited in strike-slip

related pull-apart basins that formed in already complexly

deformed and metamorphosed circa 3,500–3,300 Ma rocks

of the Pilbara craton. Thus, although these fault systems form

an integral part of the structural evolution of the Pilbara craton,

they postdate events related to initial formation of the

granite-greenstone terrain. One problem that may be

addressed by future studies of greenstone belt structure is

understanding the nature of the transition from brittle to ductile

strains in pull-apart regions along strike-slip fault systems,

such as those of the Pilbara.

Late stage major ductile transcurrent shear zones cut

many cratons, but few have well constrained kinematic or

metamorphic histories. An exception is the 186-mile (300-

km) long Koolyanobbing shear zone in the Southern Cross

Province of the Yilgarn craton. The Koolyanobbing shear is a

four- to nine-mile (6–15-km) wide zone with a gradation

from foliated granitoid, through protomylonite, mylonite, to

ultramylonite, from the edge to the center of the shear zone.

Shallowly plunging lineations and a variety of kinematic indicators

show that the shear zone is a major sinistral fault, but

regional relationships suggest that it does not represent a

major crustal boundary or suture. Fault fabrics both overprint

and appear coeval with late stages in the development

of the regional metamorphic pattern, suggesting that the

shear zone was active around 2.7–2.65 Ga.

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