Geometric investigation and kinematic analysis of the Tuyeh-Darvar pop-up structure (in Southern slope of Eastern Alborz)

Introduction
Alborz Mountains range in northern Iran is an arc that extends from the end of Talesh in the west to its intersection with the Kopeh Dagh Mountains in the east (Jackson et al, 2002). (Alavi, 1996) introduced the mountains as a multi-orogen belt that has been influenced by the Cimmerian and Alpine orogenies from Late Triassic to Oligo-Miocene. Detailed structural studies in the mountains indicate it has been suffered inversion tectonics (Zanchi et al, 2006; Yassaghi and Madanipour, 2008; Gholami et al, 2016).
 
Materials and Methods
The geological map of the study area has been constructed based on the present geological maps of Kiasar (Akbarpour and Saeedi, 1991) and Jam (Alavi and Hamedi, 1997), investigation of Landsat 8 satellite images with 15 meters spatial resolution, and detaile field mapping. Using the constructed geological map in this study, detailed field mapping, and Digital Elevation Model (DEM)¹ data, two cross-sections perpendicular to the general trend of the structures have been constructed. For fault kinematic analysis, Riedel shear fractures, fault steps, crescentic fractures, mineral fiber growth on fault surfaces, as well as stereographic analysis have been employed.
 
Results and Discussion
- Pop-up structure of Tuyeh Darvar Mountains:
The Tuyeh Darvar Mountains are bounded by the Mila Fault in the north and the Giv Fault in the south. These faults with opposite dip directions thrust the Lower Paleozoic and Mesozoic rocks over the younger Tertiary units. The Giv and Mila faults, with opposite dips, have uplifted the Paleozoic rocks and present a geometry similar to one introduced as inlier structure in the Taleghan Mountains (Annels et al, 1975). Structurally this geometry is known as pop-up structure, which is mostly formed by faulting during inversion tectonics (Mcclay, 1995). Since the displacement of backthrust is greater than that of the main fault in the inverted area (Conney et al, 1996), this geometry can be considered a characteristic of faulting related to inversion tectonics in the Tuye Darvar area. As a result of Mila's fault activity, in the eastern part, Upper Paleozoic rock units (Jeirud Formation) were thrust over Cenozoic rock units (Fajan Conglomerate) and in the western part, Mesozoic rock units (Lar Formation) were thrusted over Cenozoic rock units (Eocene marls). From east to west, the strike of the fault changes from east-west to northeast-southwest. In the hanging wall of the Mila fault, the Tuyeh Fault has formed, causing the lower Paleozoic rock units to be thrust over the upper Paleozoic units in the western part, and over the Triassic rock units in the eastern part.
The NE-striking Giv Fault with approximately 20 km in length forms the southern border of Tuyeh Darvar Mountains. As a result of the Giv fault activity, the deposits of the Shamshak Formation (Jurassic) and the Cretaceous limestone units have been thrusted over the Eocene units. The dip angle of the Giv fault plane varies between 60 and 75 degrees to the north-west and has a left-lateral reverse mechanism.
The Darvar Fault in AA' section, has a dip of 75 degrees in the north-west of the Giv fault and has a left-lateral reverse mechanism. The Darvar Fault, as a hanging-wall branch of Giv fault, has caused Triassic rock units (Elika formation) to thrusted over Jurassic rock units (Shamshak Formation).
- Tectonic evolution of Tuyeh Darvar Mountains: 
Because of the early Paleozoic extension or Permian-Triassic peripheral bulge and formation of normal faults, the Giv Fault could have been formed during this process.  Following the compressive phase in the Upper Cretaceous, reactivated the pre-existing normal faults and inverted them to reverse faults. The inversion effect of the Giv Fault is associated with the development of the Darvar Fault in its hangingwall. Consequently, the Mila Fault has formed as a backthrust to the Giv Fault and the Tuyeh Fault has also formed in the hangingwall of the Mila Fault. The inversion mechanism has been continued until Upper Eocene by considering the outcrops of the Paleocene and Eocene rocks in the footwall of the Mila and Giv faults. Continuation of convergence, since Miocene, through westward movement of the southern Caspian Block relative to central Iran causes reactivation of the left-lateral strike-slip faults. 
Therefore, the Giv Fault has been formed during the early Paleozoic extensional phase or as a result of environmental uplift and the formation of normal faults in Eastern Alborz during the Permian-Triassic age. Closure of the Neo-Tethys Ocean (Upper Cretaceous), applied the compressive phase that renewed activity of the pre-existing normal faults and their inversion. Therefore, the Giv Fault could have started to invert from this time. The effect of inversion of the Giv Fault is associated with the development of the Darvar Fault in its outer wall, and with the continuation of this inversion, the Mila Fault is formed as a backthrust of the Giv Fault and the Tuyeh Fault in the outer wall of the Mila Fault. According to the Paleocene (Fajan conglomerate) and Eocene (Karaj Formation) outcrops and their location in the footwall of the Mila and Giv faults, it can be inferred that the inversion mechanism continued until the upper Eocene.
It seems that the evolution of Alborz crust at the end of the Cenozoic (Miocene), was more compressive and accompanied by generally strike-slip movement (Allen and et al, 2003).  The westward movement of the South Caspian basement relative to Central Iran has caused left-lateral movement of the faults. Accordingly, the Alborz mountain range is currently under oblique left-lateral shortening, while the faults in its eastern parts has greater amount of left-lateral than its western parts (Jackson et al, 2002).
 
Conclusion
Tuyeh Darvar Mountains are asymmetric pop-up structure that thrust the older Paleozoic-Mesozoic rock formations over the Tertiary rock units by the Giv and Mila faults, which have opposite dip directions. The Giv Fault as the main thrust and the Mila Fault as its back thrust are southern and northern boundaries of the pop-up structure, respectively. The Giv Fault as initial normal fault has formed during the Early Paleozoic extension phase or during the Permo-Triassic uplift in the eastern Alborz. Compressive phase in Upper Cretaceous, related to Neotethys closure, reactivated the pre-existing, e.g., the Give Fault, normal faults and invert them to reverse faults. Inversion mechanism has been continued until Upper Eocene by considering the outcrop of Paleocene (Fajan conglomerate) and Eocene rocks (Karaj Formation) in the footwall of the Mila and Giv faults. The post-Miocene to Quaternary left-lateral-shear in the eastern Alborz is proposed to be related to the southern Caspian westward movement.