Greenstone Belt Geometry

Geophysical surveys have shown that greenstone belts are mostly shallow to intermediate in depth, extending to 3-15 miles (5-20 km). Some have flat or irregular bases, and granitic rocks intrude many of them. They are not steep synclinal keels. Gravity models consistently indicate that greenstone belts rarely extend to greater than 6 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 in 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 they are much like younger orogenic belts. For example, the eastern Norse-

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Map showing the main structural elements of greenstone belts: dark green are granites; light green are greenstones; tan are metasediments; red lines are foliations (modified from T. Kusky, and J. Vearncombe, 1997)

man-Wiluna belt in the Yilgarn craton contains a structurally disrupted and complex association of oceanic-type mafic and island arc-type volcanic rocks, whereas the western Norseman-Wiluna belt contains disrupted rocks of predominantly oceanic affinity. 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 misconceptions about the structure of greenstone belts is that they simply represent steep synclinal keels of volcanic and sedimentary 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 pinched-synform model predicts. The structure and stratigraphy of greenstone belts will be unraveled only when "strati-graphic" 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 litho-logical 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 steep dips are evidence of the intense deformation 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 the Norseman-Wiluna belt of Western Australia, appear to have been steepened by a series of thrust faults stacking the rocks end over end. Successive offscraping of the greenstone from oceanic crust in thrust sheets steepens rocks toward the interior of the thrust belt. In other cases intrusions have steepened greenstone belt rocks on the margins of plutons and batholiths. Examples of this mechanism are found in the Pilbara craton of Australia and northern Zimbabwe cratons of Africa. Shortening of the entire crust appears to be an important steepening mechanism in other examples, such as in the Theespruit area of the Barberton Belt of southern Africa's Kaapvaal craton. 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, San Francisco craton) may have caused local steepening of greenstone belt rocks. These are the types of structures present throughout Phanerozoic orogenic belts.

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