Structure and Evolution of the Eastern Kopet Dagh and Southern Amu Darya Basin Since Triassic Time

Letouzey, Jean ¹; Muller, Carla ¹; Sherkati, Shahram ²; Hosseini, Abolfazl ²; Piryaei, Alireza ²; Masibegi, Masoud ²
(1)ISTEP, University P. & M. Curie - CNRS, UMR 7193, Paris, France. (2) NIOC Directorate, Teheran, Iran, Islamic Republic of.

The studied area was focused in the eastern Kopet-Dagh Mountains, which is the eastern prolongation of the South Caspian Sea, and southern part of the Amuh-Darya Basin in Iran.The aim of the study was to understand the structural style and the structural evolution of the eastern Kopet-Dagh in Iran. Exploration and gas production are active in this area, with a recent new discovery. 2D and 3D seismic, subsurface data and field work studies, allowed understanding the transition between the Kopet-Dagh structures and series, with the hydrocarbon rich, Amu-Darya Basin. The whole Triassic is exposed in the Aghdarband Erosional Window, affected by folding and thrusting, and uncomformably covered by Lower(?)-Middle Jurassic Kashafrud Formation. In the south of the window, bellow the mid-Jurassic conglomerates, ophiolites are outcropping. Theses ophiolites are in tectonic contact with the uppermost Triassic. This Late Triassic-Early Jurassic tectonic event correspond to the collision and suture between the Central Iranian Block and the Eurasian-Turan Plate, with obduction of ophiolites folding and faulting, followed by a strong erosion. Seismic sections show that the Mid-Jurassic Kashafrud Formation was deposited in a rift system, with conglomerates at the base, and absence or nearly absence above pre-Jurassic basement high. Break-up unconformity is marked by an erosional surface at the base of the Upper Jurassic formations. Facies and thickness variations during the Upper Jurassic and Lower Cretaceous prove the existence of gentle paleo-highs in the eastern part of the Kopet-Dagh. Uplift and folding of the Kopet-Dagh mountains occurred during Tertiary, linked with oblique inversion of the Kashafrud grabens and oblique reactivation of the “Main Kopeh Fault Zone” which extends from the Caspian sea to the Afghan border). En echelon patterns of the folds along these zones, as well as en echelon fault pattern observed on 3D seismic blocks, demonstrate a dextral shear component during the folding and inversion. Main reservoirs are Late Jurassic carbonates and Early Cretaceous clastics. Depositions are strongly controlled by presence of paleo-highs. Cap-rock is Neocomian anhydrites or salt, and main source rocks are considered as mid-Jurassic, with possible contribution from Triassic coal beds. Close to the mountain main structures are above inverted Jurassic graben and within thrust folds or duplex within a triangle zone.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.