عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The Zagros fold- thrust belt as a part of Alpine- Himalayan orogenic belt, is one of the most active continental collision zones on the earth, which extends from the Tarus mountains in south eastern Turkey to the Minab fault in the east of the Strait of Hormoz in southern Iran. Structurally, its formation is related to the continuing convergent movement between the Arabian plate to the southwest and the Central Iranian Microcontinent to the northeast, resulting from the north- northeastward drift of Afro- Arabia against Eurasia.
The northeastern boundary of the Zagros coincides with the Main Zagros Reverse Fault and the Main Recent Fault. The Main Zagros Reverse Fault has a NW- SE strike from western Iran to the area north of Bandar Abbas. To the northwest, the boundary feature consists of a series of right-lateral strike-slip faults called the Main Recent Fault. The Main Recent Fault is a major structure broadly parallel but quite distinct from and younger than the Main Zagros Reverse Fault which transects it in several places. Earthquakes of larger magnitudes mostly nucleate along different segments of the Main Recent Fault with a prominent northwest trending right-lateral strike-slip mechanism along the northeast margin of the Zagros ( For example, the Silakhor earthquake of 23 January 1909, Ms=7.4, on Dorud Fault segment, is the largest event recorded in the Zagros).
The most recent tectonic deformation, and in particular the seismicity along the MRF between latitudes 33 and 35ْ N, is summarized in the context of the seismotectonic history of the region. The relation between the seismicity and the individual fault segments forming the MRF is studied and interpreted in terms of a continuing right-lateral strike- slip deformation. The Main Recent Fault is not a single structure but a narrow zone formed by a succession of individual fault segments, often arranged in a right-lateral en-echelon pattern. Thus in the southeast, the subsidence of the Silakhor Valley, probably a consequence of Quaternary right-lateral movements on the Dorud fault and on small subparallel faults, was renewed during the 1909 and 2006 earthquakes. Near the center of the region, the Nahavand plain is similarly limited by two strike-slip faults, the Nahavand and Garun Faults, both of which were reactivated, at least along their northern sections, during the 1958/08/16 (Ms=6.6) and 1963/03/24 (Ms=5.8) earthquakes. In the northwest, the Sahneh Fault with a long history of seismic activity (the Dinavar earthquakes of 1008, Ms=7.0; 1107, Ms=6.5; and 2002/04/24 and 2002/12/24, by Mw=5.4 and Mw=5.2 respectively is characterized by its exceptional direction, which is at about 20ْ to the other faults studied here, but contained in a region limited by the extensions of the Nahavand and Morvarid Faults. Practically all these segments fall into the three categories of Riedel shears, P shears and tension structures. The Riedels which were formed during the first stages of the deformation are represented by the Dorud, Nahavand and the Morvarid Faults. The P shears, which were formed at a later stage in the structural evolution, are represented mainly by the three sections of the Sahneh Fault and Ghilabad Fault. The Sahneh Fault is a good example of Positive Flower Structure (palm-tree structure) on the MRF. The tension structures, of which the Qaleh Hatam Fault, and the subsidence of the Silakhor Valley and Nahavand plain are the best examples, are much shorter and less numerous, and seem to be located near the intersections of the Riedels and P shears. The partitioning of oblique regional convergence into effectively pure thrusting and pure strike-slip is the most likely explanation for the different mechanisms of the 2002 mainshocks on the Sahneh fault.