Geophysical surveys have shown that greenstone belts are
mostly shallow to intermediate in depth, some have flat or
irregular bases, and many are intruded by granitic rocks.
They are not steep synclinal keels. Gravity models consistently
indicate that greenstone belts rarely extend to greater than
six miles (10 km) in depth, and seismic reflection studies
show that the steeply dipping structures characteristic of
most greenstone belts disappear into a horizontally layered
mid to lower crustal structure. Seismic reflection surveys have
also proven useful at demonstrating that boundaries between
different “belts” in granite-greenstone terrains are in some
cases marked by large-scale crustal discontinuities most easily
interpreted as sutures or major strike-slip faults.
Just as greenstone belts are distributed asymmetrically
on cratons, many have asymmetrical distributions of rock
types and structural vergence within them, and in this respect
are very much like younger orogenic belts. For example, the
eastern Norseman-Wiluna belt in the Yilgarn craton contains
a structurally disrupted and complex association of tholeiites
and calc-alkaline volcanic rocks, whereas the western Norseman-
Wiluna belt contains disrupted tholeiites and komatiites.
In other belts, it is typical to find juxtaposed rocks from different
crustal levels, and facies that were originally laterally
separated. One of the long-held myths about the structure of
greenstone belts is that they simply represent steep synclinal
keels of supracrustal rocks squeezed between diapiric granitoids.
Where studied in detail, there is a complete lack of continuity
of strata from either side of the supposed syncline,
and the structure is much more complex than the pinchedsynform
model predicts. The structure and stratigraphy of
greenstone belts will only be unraveled when “stratigraphic”
methods of mapping are abandoned, and techniques commonly
applied to gneissic terrains are used for mapping
greenstone belts. Greenstone belts should be divided into
structural domains, defined by structural style, metamorphic
history, distinct lithological associations, and age groupings
where these data are available.
One of the most remarkable features of Archean greenstone
belts is that structural and stratigraphic dips are in most
cases very steep to vertical. These ubiquitously steep dips are
evidence for the intense tectonism that these belts have experienced,
although mechanisms of steepening may be different in
different examples. Some belts, including the central Slave
Province in Canada and Norseman-Wiluna belt of Western
Australia, appear to have been steepened by imbricate thrust
stacking, with successive offscraping of thrust sheets steepening
rocks toward the hinterland of the thrust belt, whereas
greenstone belt rocks on the margins of plutons and
batholiths have commonly been further steepened by the
intrusions. Examples of this mechanism are found in the Pilbara
and northern Zimbabwe cratons. Late homogeneous
strain appears to be an important steepening mechanism in
other examples, such as in the Theespruit area of the Barberton
Belt. Tight to isoclinal upright folding, common in most
greenstone belts, and fold interference patterns are responsible
for other steep dips. In still other cases, rotations incurred in
strike-slip fault systems (e.g., Norseman-Wiluna belt, Superior
Province) and on listric normal fault systems (e.g., Quadrilatero
Ferrifero, Sao Francisco craton) and may have caused
local steepening of greenstone belt rocks. These are the types
of structures seen throughout Phanerozoic orogenic belts.
Structural vs. Stratigraphic Thickness of Greenstone Belts
Many studies of the “stratigraphy” of greenstone belts have
assumed that thick successions of metasedimentary and
metavolcanic rocks occur without structural repetition, and
that they have undergone relatively small amounts of deformation.
As fossil control is virtually nonexistent in these
rocks, stratigraphic correlations are based on gross similarities
of lithology and poorly constrained isotopic dates. In pre-
1980 studies it was common to construct homoclinal
stratigraphic columns that were 6–12 miles (10–20 km) or
more thick, but recent advances in the recognition of usually
thin fault zones, and precise U-Pb ages documenting olderover-
younger stratigraphies makes reevaluation of these
thicknesses necessary. It is rare to have intact stratigraphic
sections that are more than a couple of kilometers thick in
greenstone belts, and further mapping needs to be structural,
based on defining domains of like structure, lithology, and
age, rather than lithological, attempting to correlate multiply
deformed rocks across large distances.
An observation of utmost importance for interpreting
the significance of supposed thick stratigraphic sections in
greenstone belts is that there is an apparent lack of correlation
between metamorphic grade and inferred thicknesses of
the stratigraphic pile. If the purported 10–20-kilometer-thick
sequences were real stratigraphic thicknesses, an increase in
metamorphic grade would be detectable with inferred
increase in depth. Because this is not observed, the thicknesses
must be tectonic and thus reflect stratigraphic repetition in
an environment such as a thrust belt or accretionary prism,
where stratigraphic units can be stacked end-on-end, with no
increase in metamorphic grade in what would be interpreted
as stratigraphically downward. Other mechanisms by which
apparent stratigraphic thicknesses may be increased are folding,
erosion through listric normal fault blocks, and progressive
migration of depositional centers.
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