Inversion of Cimmerian extensional structures: insights on the tectonic evolution of the Central Alborz, Iran

Central Alborz is a significant example of inversion tectonics related to the long and complex evolution which accompanied this poorly known segment of the Alpine-Himalayan orogenic system. Extensional structures forming grabens were activated during Late Triassic just south of the collision zone in the stable foreland of the North Iranian block, possibly due to peripheral bulging, in response to the Cimmerian orogeny. Normal faulting at the end of the Cimmerian orogeny, producing block tilting and horst and graben structures before the deposition of the Shemshak Formation, has been also recognized in the of the Iranian block long time ago (STOECKLIN, 1968). In Central Alborz, extensional structures were inverted as sub-parallel right lateral strike-slip or highangle reverse faults. These pre-existing discontinuities can be responsible for the partitioning of strain along faults parallel to the trend of the belt, in response to an oblique convergence, as suggested by the present-day evolution of the belt, where both strike-slip and wrench faults are active (JACKSON et alii, 2002). The reactivation of pre-existent faults as strike-slip structures was also hypothesized by ALLEN et alii (2003), but it was never demonstrated before. The evolution recognized around the «Sharistanak klippe» can be extrapolated also to other parts of the belt, where «anomalous» faults stack young units above older rocks. This is the case of several faults occurring along the axial zone of Central Alborz, which result from the inversion of pre-existent normal faults. In the Sharistanak and Gajere grabens stratigraphic evidence suggests that the extensional structures were coeval to the Cimmerian orogeny. In other cases normal faults can be attributed to older extensional events related with the evolution of the Paleo- and -Tethyan passive margins, as indicated by important facies changes and thickness variations across the belt, as in the case of the Kandevan Fault. Elsewhere, the younger post-Triassic extension related to the opening of the Paleo-Caspian basin (BRUNET et alii, 2003), or which accompanied the deposition of the Eocene volcanics, can be responsible for the activation of extensional structures. The reconnaissance of inversion processes changes the general interpretation of the Central Alborz and can explain several of its features. The gentle deformation observed in the axial part of the Central Alborz north of the Kandevan fault suggests that strain was localized along the high-angle pre-existing tectonic discontinuities, which especially outcrop in the southern part of the belt. In this way mild or total inversion occurred along inherited high-angle normal faults. Tight folds with vertical axial planes developed within the paleo-fault blocks due to buttressing effects induced by the obstacle created by pre-existent high angle normal faults. This phenomenon, which has been analysed in detail in the «Sharistanak klippe», is also evident just south of the Kandevan fault, where Eocene and Miocene successions, detached from their substratum, are affected by isoclinal folds with very steep axial planes showing no clear vergence. Due to the occurrence of steep normal faults, the formation of shortcut thrust planes is favoured, producing «floating horses» (BUTLER, 1989) of Cambrian to Pre-Cambrian rocks along the paleo-graben shoulders, which are stacked above the Tertiary succession in the external part of the belt. In this new interpretation, high to