The reconstruction of ancient mobile belts reveals the presence of old continental margins. The reconstructions are rather hazardous and uncertain, but they do permit us to compare and contrast ancient with modern continental margins. Information from folded belts derives mostly from surface and subsurface observations, whereas continental margins are generally described in geophysical terms. This contrast in the nature of information helps to focus on a number of problem areas, which are listed below:

(1) Passive margins appear to be emplaced on a previously attenuated crust characterized by extensive rifting. Several intriguing geophysical models have been proposed to explain the attenuation process, but they remain to be documented. Sedimentation and erosion during the rifting process can be observed in a number

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of ancient fold belts (Alps, Caledonides). The nature of volcanism associated and immediately following the attenuation process should be studied in ancient analogs.

(2) Reflection seismic work on passive margins reveals stratigraphic geometries which should inspire a new look at the stratigraphy of miogeosynclinal sequences.

(3) Much has been written about marginal highs on passive margins, but equivalents in folded belts are difficult to document.

(4) The angularity of the outline of passive margins is often due to transform faults. Pattern of mobile belts reflects former angular outlines, but the anatomy of the relation remains to be studied.

(5) Ancient marginal seas have been postulated in many areas (S. Andes, Western Cordillera, eastern Australia). Marginal seas appear to be destined for destruction by folding process, yet there is little evidence for this in modern marginal seas.

(6) Active margins continue on land, where "accretionary wedges" can be studied in substantial detail both in surface and subsurface.

(7) The lithospheric and thermal regime in subduction zones has been modeled by geophysicists, but remains to be documented on the landward side of subduction zones.

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