عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Structural evolution of Alborz has already been mentioned by many researchers all over the world. Central Alborz is located in the bending of the eastern and the western Alborz. Damavand volcano is situated in the bending part along the great and active faults such as Mosha, North of Baijan, Ask, Nava and Ira. The Nava and Ira faults are in the eastern side of Damavand volcano with a trend of ESE parallel to the general tenor of faulted and fractured part of western Alborz.
Based on the geological map, both faults are active reverse structures which are hidden under lavas. With regard to the structural studies, Ira and Nava are active and related to the other unknown faults with the same trend and also with a trend of WNW that show the new established transtension mechanisms prevailing on the Alborz.
Geologically, it is believed that some new tectonic events affect the structural evolution in the region, such as new extensional system with the WNW trend activated during 5±2 m.y. In this study we extensively applied geophysical methods combined with former structural data to find any event of the neo-tectonic systems in the area. Due to being more applicable, magnetometry was used for surveying the unconformities.
The field study concentrated on the faulted and fractured sedimentary bedrock of Alborz, east of Damavand. The average height level is about 4000 meter. Because of the hard topographic conditions, we could design only two North-South profiles. Total magnetic field variations were measured using a moving proton magnetometer and one system as the remote base. More than 286 data points were collected and processed to extract the best model out of the reduction to the pole transform, first vertical deviation, upward continuation and analytical signal. The model of the first vertical deviation is the best reliable output to show the anomaly of tectonic signature.
With this filter, amplitude spectra were enhanced as well as the wave numbers. Another advantage of this method is the detecting any type of geological and subsurface block movements caused by the faults, folding or other tectonic events.
First vertical deviation proves the best model compared to the other models and with correlation to the geological map, it presents many important insights of minor and major faults that were hidden before.
In our founding, two NWN junction faults are remarkable which verify the activation of new transtensional system due to having the sign of normal-strike slip movements in the tectonic of the Eocene units. It seems that they are minor repeatedly faults with the normal movement of the hanging wall towards SW.
In general we recognized, approximately eight fault mechanisms at subsurface whose signatures are not shown on the geological maps of the region. Some of them belong to the former reverse system and two of them are in accordance with the new conventional transtension system with WNW tenor and the normal movement.