EXOTIC NAPPES IN EXTERNAL PARTS OF OROGENIC BELTS

In many orogenic belts, large nappes or thrust sheets, often preserved in klippen, overlie parts of the more standard fold-and-thrust belts that form the external zones of the orogens, Commonly the stratigraphi c sequences in these nappes cover all or a large part df the same time span as the strata in the underlying fold-and-thrust belt but are of a very different-''exotic''-facies evidently deposited somewhere else, presumably in more internal parts of the orogenic belt. In most cases strata of the fold-and-thrust belt were deposited in shallow water on continental basement near the continental margin, whereas those in th e overlying nappes represent deeper water, commonly starved sequences, In certain regions, several such nappes are superposed; in some such regions one of the highest nappes Includes ultramafic rocks-ophiolite- probable representing part of the oceanic crust on which some of the e xotic sedimentary sequences were deposited, In certain cases the exoti c materials were actually intercalated like ''sediments'' into the hig hest part of the underlying, more ''normal'' stratigraphic sequence an d then underwent its later deformational history, Some of the youngest examples of such nappes (for example, the Prerif of Morocco, the argi lle scagliose or Ligurian ensemble of Italy) seem to have been displac ed into their ''exotic'' position at least partly by gravity (sliding or spreading), from uplifts in internal zones of the orogenic belt int o downwarped parts of its external zones, In older examples (for examp le, the Prealpes the Taconic klippen), the role of gravity is more equ ivocal, and more ''standard'' subterranean thrusting may account for m uch or all of the displacement.