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Structural disharmonies between rock masses lying above and below the salt-bearing Lower Fars formation are characteristic of the Alpine foothill folds of the Zagros mountain belt in parts of northern Iraq and southwestern Iran.

Most impressive and what appear to be chaotic structural discrepancies have been described from the Iran sector of the saliferous Fars basin, where depositional thicknesses of salt were great. Salt deposits were much thinner in Iraq, and lacked the spectacular disharmonies of the Iran deposits. The typical disharmonic habit in northeastern Iraq is the overthrusting of the northeastern flanks of the long and strongly developed anticlines.

Overthrusting has generally been attributed to tectonic compression in the later phases of folding, or to southwestward gliding of the post-salt cover, as a unit, off the rising “high” of the Zagros. Both interpretations are inadequate in some particulars to account for some features of the overthrusts.

The thesis is advanced that the overthrusts resulted from gliding adjustments of each synclinal fill to gravitational changes caused by isostatic recovery of the Zagros after the fold-producing orogeny. Overthrusting essentially postdated the completion of folding, being facilitated by decreases in competency of the anticlinal crests as these became deeply eroded. It was controlled by the excess of load in the northeast of each syncline, which resulted from development of high relief during progressive regional tilting.

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