Gravity Collapse Structures and Mountain Ranges, as exemplified in South-Western Iran

The object of the present paper is to recapitulate our previous work on gravity-formed anomalous structures, to describe further examples, and to propound arguments based upon structures in the Swiss Alps and upon theoretical considerations. In south-western Iran, post-Miocene orogenesis produced a series of approximately symmetrical folds, in section nearly sine curves, trending north-west and south-east. Vigorous erosion followed, revealing the following column of strata:— The limestones now often protrude as strike ridges and dome-like ranges with the marl groups weathered away between and around them. Dissection, measured from the mountain summits to the strike stream beds, reaches as much as 8000 feet; earlier in the cycle it probably reached 12,000 feet. When erosion left a knife edge upstanding or a dome unsheathed, the beds at first remained unaffected, but when the pile exceeded about 2000 feet in height they tended to bend and break, and to produce such secondary structures as roof-and-wall, cascade, flap, or slip-sheet. In every case, an unmoved control bed can be seen within the original fold; the form which these secondary structures took depended upon the attitude of the rock under the progressive denudation of its cover and support. A roof-and-wall structure is produced from a sine-curve fold when the crest is notched by erosion, and the two sheets so produced slip down and outwards, bending at first into a knee-fold. This knee then breaks; the lower limb remains nearly vertical whilst the upper rests upon it with a sudden change in dip, and separated