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Title: The tectonic significance of the Kazerun fault zone, Zargos fold-thrust belt, Iran
Author: Sepehr, Mohammad
Awarding Body: Imperial College London (University of London)
Current Institution: Imperial College London
Date of Award: 2000
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The N-S trending Kazerun Fault zone obliquely crosses the NW-SE trending Zagros fold thrust belt in southwest of Iran. This still active fault zone is an ancient structural lineament, which has controlled the structure, sedimentation and subsidence of part of the Zagros mountain belt since the Cambrian. This study presents a new interpretation of the Kazerun Fault zone, and recognises it to be made up of four north-south trending segments, termed from north to south the Sisakht, Yasuj, Kamarij and Burazjan segments respectively. The Kazerun Fault limited the distribution of the Cambrian Hormuz salt with considerable thickness forming to the east of the fault (i.e. in the Fars region) and little or none to the west (i.e. in the Dezful Embayment). This thick salt layer decouples the cover rocks from the basement structures. The salt significantly reduces the cohesion and frictional resistance at the base of the sedimentary cover and enables deformation to the east of the fault to propagate 100km further to the southwest than in the Dezful Embayment to the west. This basal decollement and the rheological differences in the cover rocks each side of the Kazerun Fault is the reason for the arcuate plan view of the fold-thrust belt in the Fars region. There is a remarkable difference in the distribution of deformation in the cover rocks in the two regions which are separated from each other by the Kazerun Fault zone. The Sisakht, Kamarij and Burazjan segments of the Kazerun Fault acted as lateral ramps linking the three major Zagros deformation fronts, the High Zagros, the Mountain Front and the Zagros Frontal Faults respectively. These segments are characterised by huge vertical displacements with downthrow to the west. However, it is argued that the Yasuj segment only became active relatively lately, during the folding and unlike the other segments did not control the post collision sedimentation of the Zagros. A study of the digital elevation model of the Sarbalesh anticline (formed by combination of aerial photographs, the TM satellite image and topographic maps) and field data were used to analyse this anticline and its associated structures. It is suggested that synchronous folding and faulting (both thrusting and normal faulting) occurred during the formation of this boomerang shaped anticline which is situated in the junction between the Kamarij segment of the Kazerun Fault and a segment of the Mountain Front Fault. A detailed review of the sedimentation of the Iranian sector of the Zagros fold-thrust belt reveals the structural evolution of the Kazerun Fault and other major structural elements of the Zagros and shows the intimate interplay between sedimentation and tectonics. The Zagros basin experienced rifting during the Permo-Trias and the resulting horsts and grabens had a NW-SE trend i.e. parallel to the present mountain belt. These structures controlled the subsidence of the basin during this period. During the Jurassic-Early Cretaceous the area corresponding to the present day Kazerun and Izeh Fault zones was the site of transition between the two entirely different basins which occupied the Fars and Lurestan regions. Important movements along the Kazerun-Izeh Fault zone in the Mid-Cretaceous produced a series of N-S trending horst and graben structures which are marked by significant sedimentary change across them. During the Tertiary the NW-SE trending Mountain Front Fault (i.e. introduced in this study) was the major deformation front, controlling the deposition and geometry of the Zagros foreland and piggyback basins to the southwest and northeast of the fault respectively. The different deformational styles of the cover rocks in the Zagros (e.g. on either side of the Kazerun Fault zone), relates to inherited basement structures, the different rheological profiles and particularly, the presence and position of the detachment horizons within the cover rocks. In fact, these differences are mainly the result of activity along the Kazerun Fault in the Cambrian, the Kazerun-Izeh Fault in the Jurassic- Cretaceous and the Mountain Front Fault in the Tertiary. It is also argued that Zagros anticlines (the major hydrocarbon traps in the Zagros) are the result of different mechanisms. These include classical buckle folds, en echelon folds along strike-slip faults and fault related folds above thrusts and reactivated normal faults. These differences related directly to the rheological profile of the cover rocks which was controlled by activity along the Kazerun, Jzeh, Bala Rud and Mountain Front Faults. Based on this study a clearer picture of the interplay between structures, sedimentation and deformation of the Zagros fold-thrust belt has emerged. This makes it appropriate to revise the entire Zagros belt from the point of view of new hydrocarbon targets, which include folds, faults and sedimentary traps.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available