Critical Aspects of the Plate Tectonics Theory, Vol II: The Importance of Transcurrence Phenomena in Mountain Building
KeyWords: Orogeny, Earth Expansion, Transcurrence, Subduction
Ophiolites are not true oceanic crust and subduction is a questionable phenomenon, supported only by circumstantial (i. e., geophysical) evidence. Geological evidence generally disfavours involvement of subduction-type processes in either generation of arc volcanism or formation of accretionary prisms. Moreover, recent geophysical and geodetical measurements strongly suggest that the lithosphere is welded to its asthenospheric substratum and that the Earth is expanding. In full agreement with these data, orogens are considered to be the result of large transcurrent lithospheric motions in response to non-convective differential displacements of the yielding asthenosphere. In a first stage, shearing along transcurrent systems generates distension and subsidence, giving birth to elongated troughs, i. e., geosynclines. Heat steadily accumulated due to friction initiates then, in a second stage, thermal expansion and diapiric uplift along the strike of such systems. Concomitant with heat consumption during diapirism, a pressure gradient occurs that eventually triggers compression of the activated thermal bulge. This characterizes stage three in the evolution of an orogen. A final stage, implying epirogenic uplift due to isostatic readjustment, concludes a complete four-stage orogenic cycle.