Geological setting
The oldest rocks in the study area (Fig.2) are the dolomite, sandstone and limestone of the Triassic period which are exposed in the north parts of the region. These rocks are followed by the formations given below:
1. The Sangestan formation. This formation is composed of a very thick sequence of red sandstone and conglomerate. The Shir-Kuh batholith is overlain unconformably by this formation (Khosrotehrani and Vazirimoghadam, 1993).
2. The Taft Formation. The Taft Formation consists of massive to bedded dark grey marine carbonates (Khosrotehrani and Vazirimoghadam, 1993). These rocks were probably deposited on a continental shelf. A Barremian- Lower Albian age was reported for this formation.
3. The Eocene association. The Eocene conglomerate and volcano-sedimentary rocks overlain the Mesozoic sedimentary sequences. Andesitic lavas, trachyandesite and pyroclastic rocks are the main products of Eocene volcanic activity.
4. The Oligocene-Miocene-Pliocene rocks. The Oligo-Miocene plutons consist of granite, granodiorite, tonalite, quartzmonzodiorite and diorite (exposed in the west and northwest of the study region) that intrude into the Mesozoic and Paleocene units (Zarasvandi et al., 2004). The Neogene dacitic and rhyodacitic domes and flows are the youngest magmatic activity in this region. These volcanic rocks crop out in the north (Surk, Kuhe-Mile), central (ShamsAbad, Ostaj, Abdollah, G-Ali Abad) and southern (Buragh, Darreh Zereshk, Turan Posht, Ernan) parts of the study area. Rhyodacitic and dacitic flows display flow banding, autobrecciated upper carapaces and columnar jointing. They have numerous sub-rounded, vesicular enclaves of varying size (few millimeters to few centimeters) with granular and fine-grained textures.
5. The youngest rocks in these regions are the Quaternary deposits large tabular and dome like travertine deposits (Hajmolaali and Alavi, 1993) generated by geothermal activity. The Dehshir-Baft dextral strike-slip fault and related faults are the major structural features that seperate the Mesozoic Formations (Cretaceous limestone) from the Cenozoic Formation (Eocene andesite and tuffs).