SIGNALS OF CONTINENT-CONTINENT COLLISION: WHEN DID ARABIA AND EURASIA COLLIDE?

Using the Arabia-Eurasia collision as a laboratory to understanding basic cause-and-effect relationships between plate boundary forces and plate motions hinges on a precise knowledge of: (1) the age of collision between the Arabian and Eurasian margins in Iran, (2) the original width and closure history of Neotethys, (3) the magnitude of post-collisional shortening of the Arabia and Eurasia margins. Suggestions for the age of collision range from late Cretaceous to Pliocene, based on a wide variety of presumed geologic responses. However, early Miocene synorgenic strata with growth structures adjacent to the main Zagros fault, early Miocene overlap strata over post-collisional thrusts and mid-Miocene growth strata within the fold belt all argue for collision in the late Oligocene to early Miocene. Africa-North America-Eurasia plate circuit rotations, combined with Red Sea rotations provides precise estimates of the relative positions between the northern Arabian margin and the southern Eurasia margin and indicates ~1500 km of subducted ocean crust since 68 Ma with the latest possible time of collision at ~10 Ma. A critical step in determining the onset of continent-continent collision using plate reconstructions is setting limits on the location of the edges of each of the continents in question. The northern margin of Arabia has shortened 90-160 km (including estimates from the Zagros proper, 45–90 km, and the Zagros “crush” zone, 50-70 km). Shortening within Eurasia is estimated to be 53–75 km through the Kopet Dagh and Alborz Mountains, and 18 km across Central Iran. These estimates suggest that the orogen has shortened 143 to 235 km since the early Miocene. Without accounting for post-collisional shortening, plate reconstructions indicate that Arabia and Eurasia were 660 km ± 80km apart from each other at ~27 Ma. Taking into account the maximum shortening, a 50 km wide passive margin (not preserved) on Arabia and Eurasia and the minimum distance from plate reconstructions, Arabia and Eurasia were still 245 km apart at 27 Ma and could have been separated by a distance as great as 500 km. These estimates suggest either 1) subduction of continental crust without any surface record, 2) shortening through the orogen must be at least double of that previously calculated or 3) timing of collision is slightly younger (~23-20 Ma).