تعيين وضعيت تنش به روش برگشتي از صفحه هاي گسلي لبه جنوبي البرز مركزي

Determination of Stress State and Direction by Inversion of Fault-Slip Data in the Southern of Flank of Central Alborz

The present study covers the area between longitudes 51°E to 52.5°E. The Central Alborz is an arcuate fold- and thrust belt towards Central Iran. By means of inversion of 181 striated fault planes and related striations the stress evolution from Eocene (rock formation) up to young Quaternary units, (alluvium) cropping out in northern part of Tehran city was studied. The good condition of outcrops in the northern part of Tehran city allowed us to conduct the majority of measurements in alluvium of Plio-Pleistocene (A-Formation) and early Quaternary (B-Formation). The obtained results in Plio-Pleistocene unit up to Holocene show three distinct stress directions. It changes from NW-, N-S- to NE-directed s, in a clockwise manner. Each of this stress state is associated with two tectonic regimes namely compressive and strike-slip.Using the cross cutting relations of individual fault planes in A and B-Formations and differentiation of events, the ages of stress changes were determined.The south flank of Central Alborz indicates a regional NW-SW-directed compression that lasted during the Eocene to Plio- Pleistocene. The NW-SE-directed stress was; responsible for forming of faults and related folds with E-W orientation. This event was followed by a strike-slip that we interpret as being coeval with sedimentation of B-Formation. The stress tensor deduced on Mosha fault ( Holocene in age)is compatible with focal mechanisms obtained in this region. It shows a NE-SW-directed stress associated with a strike-slip regime. By differentiation of the youngest event (NE-SW-directed s,) and oldest event (NW-SE-directed s1) it remains the N-S directed s1 which could have prevailed during late Quaternary (C-Formation).Each of the three mentioned events is associated with compressive and strike -slip regime.