On the increasing size of the orogens moving from the Alps to the Himalayas in the frame of the net rotation of the lithosphere

Abstract

The tectonic equator represents the great circle of the non-random mainstream of plate motions and it is inclined about 30° relative to the geographic equator. Divergence or convergence rates among plates are in average faster along the tectonic equator and they tend to decrease toward the polar areas. Moving from Western Europe to eastern Asia, the mainstream is roughly oriented southwest northeast. Here we show how this pattern may have played a role in determining the dimension of the Alpine-Himalayas orogenic belt, which is increasing in size and thickness moving from west-northwest to east-southeast, i.e., moving from high-latitude to low-latitude of the tectonic mainstream of plates. The Alps are in average 200–250 km wide, whereas the Himalayas are regularly > 1000 km wide. Moreover, due to the “westerly” polarization of the lithospheric mainstream relative to the mantle, either the net-rotation or the westward drift of the lithosphere, the subduction zones can be differentiated into two types, 1) increasing or 2) decreasing the lithospheric thickness. The Alpine-Himalayas system pertains to type 1 and it may represent a prototype of the continental lithosphere growth since the Archean. The increasing size of the orogens moving from the Alps to the Himalayas is presently concentrated in the northern hemisphere of the tectonic mainstream because subduction type 2 dominated the western margin of the Pacific ocean, hence preventing continental growth in the southern hemisphere in that longitude range. Therefore, the largest growth of continental crust and mantle lithosphere should have occurred along the tectonic equator, but only where type 1 subduction was generated.