Two of the great puzzels in Adirondack geology are (1) what caused the high pressures indicated by experimental studies of mineral assemblages that characterize Precambrian rocks there, and (2) what caused enormous nappes (large folds with subhorizontal axial surfaces) to develop in these rocks. Studies of pyroxenes from the Mount Marcy area indicate that the Precambrian units of the Highlands were subjected to pressures of 7 to 10 kbar (Whitney-and McLelland, 1973; Jaffee and others, 1978; Boone and others, 1978; Bohlen and Essene, 1978). Furthermore, pressures of 5 to 7 kbar have been proposed for the supracrustal rocks of the lowlands (Bohlen and Essene, 1978). Nappes up to 80 km in amplitude with a coherent “stratigraphy” have been mapped in the southern Adirondacks (McLelland, 1977), and large nappes have been postulated elsewhere in the Adirondacks (de Waard, 1967; Romey and others, 1978, 1979).
Dewey and Burke (1973) proposed that the Grenville Province (of which the Adirondacks form a part) resulted from a collision of two continents about 1,000 m.y. ago and that one of these continents thickened as a result of lateral compression. They proposed that the collisional suture lies to the southeast of the present exposures of Grenville basement. Deep erosion of a thickened continent could explain how rocks once subjected to the extreme pressures inferred for the Precambrian rocks of the Adirondacks could be exposed at the Earth's surface. However, it is difficult to explain the origin of the large nappes with this model. Simple lateral compression will tend to produce folds with steeply inclined rather than subhorizontal axial sufaces, because axial surfaces tend to develop perpendicular to the maximum principal stress.