ABSTRACT

The Sanandaj-Sirjan Ranges constitute the southwestern border of the Central Iranian Plateau and lie between the Main Zagros fault and Zagros Mountains on the southwest and the Urmia-Dokhtar volcanic belt on the northeast. The ranges occupy a northwest-trending belt in which the Zagros structural grain is overprinted on the typical central Iran structural and stratigraphic framework. Detailed investigation of the tectonic evolution of part of the belt revealed four major northwest-striking, high-angle, crustal fault zones in which most deformation was concentrated throughout Cenozoic convergence. Main mechanisms of crustal shortening on these faults were high-angle reverse faulting and dextral translation.

Upper Cretaceous northeast-southwest convergence resulted in interblock underthrusting of the Persian plate along these major faults within or bordering the Sanandaj-Sirjan Ranges and the formation of intrablock folds and thrusts. Continued convergence during the Tertiary resulted in the episodic subsidence of elongate northwest-oriented troughs southwest of the high-angle reverse faults and the parallel uplift of the adjacent blocks on the northeast side of the faults. The depressions served as the spatially restricted depocenters for the Eocene, Miocene, and Pliocene-Pleistocene sediments.

Pre-Miocene, northwest-striking normal faults on the northeastern flank of one of the uplifted blocks are thought to have resulted from post-Paleocene uplift due to intrablock arching. Post-Miocene northeast-striking normal faults are structurally, spatially, and temporally related to at least one of the northwest-trending depressions (e.g., the Gavkhuni depression) and appear to be important in its formation and evolution. Continued compression has resulted in the pervasive development of near-vertical fractures in Pliocene-Pleistocene cemented gravels. These fractures indicate Holocene reactivation of the northwest and northeast fault zones, as do focal-mechanism solutions of shallow seismic events.

Right-lateral strike-slip faulting on large northwest faults was previously thought to be entirely post-Miocene. However, the presence of older, second-order structures on interblock dextral faults and of older intrablock strike-slip faults suggests that this style of crustal shortening was also active in the early Tertiary.

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