1895, probably a back-formation from adjective geosynclinal (1879); see geo- + synclinal. Geosynclinal was used as a noun meaning "a region of depression" from 1873.
Entries linking to geosyncline
geo-
word-forming element meaning "earth, the Earth," ultimately from Greek geo-, combining form of Attic and Ionic gē "the earth, land, a land or country" (see Gaia).
synclinal adj.
"sloping downward on both sides," 1833 (in Lyell), from -al (1) + Latinized form of stem of Greek synklinein "to incline, lean," from syn- "together" (see syn-) + klinein "to slope," from PIE root *klei- "to lean."