Dr William Bosworth writes: Two recent papers by Alan Gibbs (1983, 1984a) have helped introduce rigorous geometric techniques to the analysis of extensional tectonic terranes. Several of the more salient points of Gibbs’ discussion include (1) straight-forward areal balancing techniques can be applied to cross-sections through extensional basins to obtain such information as depth-to-detachment in listric fault systems and to assess the geometric plausibility of the proposed sections; (2) geoseismic cross-sections are thereby produced that can be integrated with well data and thermal-subsidence models of basin evolution to produce detailed, predictive views of basin geology; and (3) extensional tectonic settings develop structures with numerous analogies to contractional settings, including such details as depth-to-detachment (c. 10–18 km), lateral discontinuities in structural geometry (transfer zones of Gibbs 1984a), and, potentially, the formation of ramp–flat and duplex geometries. These concepts have far-reaching implications for future studies of all areas of crustal extension. I would like to state several small points of disagreement concerning the technique of balancing sections in extensional settings, which could become important in actually applying Gibbs’ approach to some specific basins or portions thereof.

Gibbs’ fundamental assertion is that any cross-sectional area created during the formation of a rift basin (and preserved as a syn- to post-rift sedimentary package) must be accounted for elsewhere in the geologic interpretation. This is illustrated in fig. 1 of Gibbs (1983), where the cross-sectional area of the newly formed basin becomes equal to the product of the net crustal extension (l1l0)

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