THE SOUTHERN BORDERLAND OF TRIASSIC LAURASIA IN NE-IRAN

The results recently gained by Iranian and European geoscientists in the critical area just to the Northeast of the "North-Iran-Suture" east of Mashhad are summarily described and discussed.

A slightly metamorphosed ophiolite belt, outcropping as the southeasterly continuation of the previous known ophiolites of Mashhad along the northeastern perimeter of the Fariman- Torbat- e-Jam depression, proved to be either the remnant of a Permian ocean floor, ore- more likely- the remnant of a merely narrow ocean trough. There is as yet no proof of a Triassic age in this ophiolitic belt.

To the North of this ophiolitic belt, an epicontinental Triassic sequence is exposed at the southern edge of Laurasia in the erosional window of Aghdarband. This is the result of intermittent sedimentation in a pull- apart basin along sinistral strike- slip faults. The Triassic of Aghdarband has much in common with other deposits of the Triassic Tethys; however, it shows a few singularities, e.g. the Early Anisian "Nicomedites fauna" of a "Paleobiogeographic North Tethyan subprovince", or a volcanogenic sedimentation during the late Anisian and the entire Ladinian.

Permian ophiolites outcropping at the SW- corner of the Aghdarband erosional window are transgressively overlain by basal conglomerates of this Triassic sequence. Hence the existence of a Triassic ocean south of Laurasia is most unlikely. This is in agreement with paleomagnetic data which suggest that the Central Iranian microcontinent was in direct contact with Laurasia during Triassic times. These paleomagnetic data suggest also a clockwise rotation of the" Central- East- Iran microplate" during Triassic times (contrary to the anticlockwise rotation of this microplate in post-Triassic times). Both, the sinistral strike- slip faulting and the oblique compression from SW which controls the structure of the Triassic, may be derivative sequels to this clockwise rotation. All eo-Cimmerian deformations of the Triassic rocks (e.g. folding, thrust faulting, strike-slip faulting) have already ceased during Rhaetian times.