Structural v Stratigraphic Thickness Of Greenstone Belts

Many studies of the "stratigraphy" of greenstone belts have assumed that thick successions of metamorphosed sedimentary and volcanic 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 broad similarities of rock types and poorly constrained isotopic dates. In pre-1980 studies it was common to construct single stratigraphic columns that were 6-12 miles (10-20 km) or more thick, but recent advances in the recognition of thin fault zones and precise geochronological ages documenting older rocks thrust over younger rocks in the stratigraphy of some belts makes reevaluation of these thicknesses necessary. Intact stratigraphic sections more than a couple of miles thick in greenstone belts are rare. Further mapping needs to be structural, based on defining domains of similar structure, lithology, and age, rather than lithological, attempting to correlate multiply-deformed rocks across large distances.

An observation of great 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 6-12-mile (10-20-km) 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 strati-graphic 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 meta-morphic grade in what would be interpreted as strati-graphically downward. other mechanisms by which apparent stratigraphic thicknesses may be increased are by folding, erosion through listric normal fault blocks, and progressive migration of depositional centers.

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