Dear All:
For me the term Nappe generally contained the implication of overturned
beds (Monte Rosa; Blue Ridge; Iltay etc. ). Extensive allochthonous
sheets separated from their roots are termed Slides (Bailey);
allochthonous sheets without overturning are Thrusts; and a reverse
fault is that part of a thrust plane with hade less than 45 degrees.
However, there is a relativity problem. Before plate tectonics it was
generally assumed that the allochthonous sheet was a result of
compression, diapiric uplift and gravity gliding off the resulting
geotumour. After Bert Bally defined the B-type subduction zone, the
situation became more complicated, because after detachment,
compression was minimal and the mountain belt could be regarded as a
relatively passive spectator to under-thrusting by a tectonically
virgin continental plate.
Perhaps new tectonic terms are needed to distinguish between thrusts
driven by gravity gliding and thrusts driven by underthrusting.
Malcolm McClure
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