SOUTHERN BORDERLAND OF TRIASSIC LAURASIA IN NORTHEAST IRAN

Results obtained by Iranian and European geoscientists in the critical area to the north-east of the North Iran Suture east of Mashhad are d esribed and discussed. A slightly metamorphosed ophiolite belt, outcro pping as the south easterly continuation of the previously known ophio lites of Mashhad along the north eastern perimeter of the Fariman - To rbat-e-Jam depression, proved to be either the remnant of a Permian oc ean floor or more likely the remnant of a narrow ocean trough. There i s as yet no proof of a Triassic age for this ophiolitic belt. To the n orth of this ophiolitic belt an epicontinental Triassic sequence is ex posed at the southern edge of Laurasia in the erosional Window of Aghd arband. This is the result of intermittent sedimentation in a pull-apa rt basin along sinistral strike-slip faults. The Triassic of Aghdarban d has much in common with other deposits of the Triassic Tethys; howev er, it shows a few unique features, e.g. the Early Anisian Nicomedites fauna of a palaeobiogeographic North Tethyan Subprovince, or volcanog enic sedimentation during the late Anisian and the entire Ladinian. Pe rmian ophiolites outcropping at the south-west corner of the Aghdarban d erosional Window are transgressively overlain by basal conglomerats of this Triassic sequence. Hence the existence of a Triassic ocean sou th of Laurasia is very unlikely. This is an agreement with paleomagnet ic data which suggest that the Central Iranian microcontinent was in d irect contact with Laurasia during Triassic times. These palaeomagneti c data also suggest a clockwise rotation of the Central East Iran micr oplate during Triassic times (contrary to the anti-clockwise rotation of this microplate in post-Triassic times). The sinistral strike-slip faulting and compression from the south-west which controls the struct ure of the Triassic may be derivative sequels to this clockwise rotati on. All Eo-Cimmerian deformations of the Triassic rocks (e.g. folding, thrust faulting, strike-slip faulting) had stopped by Rhaetian times.