great downbuckle of the Earth's crust into the mantle below deep-sea trenches that are filled with marine sediments. The sediments become crumpled and folded within the deep downbuckle; some portions are squeezed downward into the mantle, while others are squeezed upward in highly deformed and dislocated strata. According to the theory of Dutch geophysicists working in Indonesia under the leadership of Felix Andries Vening Meinesz in the 1930s, the development of a tectogene is a necessary precursor to orogenic, or mountain-building, activity. According to this theory, the folded and dislocated trench sediments are incorporated into island arcs, which in turn become the site of Andean-type and, finally, Himalayan-type orogenic belts. See also geosyncline; orogeny.