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Remote Sensing applications in the Fars Region of the Zagros Mountains of Iran
Abstract
The Zagros Mountains in Iran are an active fold-and-thrust belt that contains world class examples of folding and salt tectonics. The Fars Region is an excellent location for testing remote sensing methods and applications of satellite data due to its dry climate and superb exposure of geological features. Recent advances in satellite technology and image acquisition have resulted in high quality global datasets at increasing resolution that provide a highly valuable source of data for structural analysis of fold-and-thrust belts. Oblique lineaments like the Razak Line in eastern Fars can be identified by changes in the dimensions of anticlines, presence of deflected folds, offsets in the alignment of structures, location of anticlinal plunges and exposures of salt. Drainage analysis can show propagation and coalescence of anticlines. We review several methods using optical data, digital elevation models, and interferometric synthetic-aperture radar. We integrate those methods and generate detailed geological maps, structural cross-sections, identify structural steps and hidden basement features, and compare to seismological data and their effect on the exposed geology.
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A series of scaled analogue models are used to study (de)coupling between basement
and cover deformation. Rigid basal blocks were rotated about a vertical axis in a ‘bookshelf’ fashion,
which caused strike-slip faulting along the blocks and in the overlying cover units of loose sand. Three
different combinations of cover–basement deformations are modelled: (i) cover shortening before
basement faultmovement; (ii) basement fault movement before cover shortening; and (iii) simultaneous
cover shortening with basement fault movement. Results show that the effect of the basement faults
depends on the timing of their reactivation. Pre- and syn-orogenic basement fault movements have a
significant impact on the structural pattern of the cover units, whereas post-orogenic basement fault
movement has less influence on the thickened hinterland of the overlying belt. The interaction of
basement faulting and cover shortening results in the formation of rhombic structures. In models with
pre- and syn-orogenic basement strike-slip faults, rhombic blocks develop as a result of shortening
of the overlying cover during basement faulting. These rhombic blocks are similar in appearance to
flower structures, but are different in kinematics, genesis and structural extent. We compare these
model results to both the Zagros fold-and-thrust belt in southwestern Iran and the Alborz Mountains
in northern Iran. Based on the model results, we conclude that the traces of basement faults in cover
units rotate and migrate towards the foreland during regional shortening. As such, these traces do not
necessarily indicate the actual location or orientation of the basement faults which created them.
The Middle East contains some 65% of the world’s remaining oil reserves and some 35% of its gas reserves. The vast majority of these reserves are located on the Arabian Plate. In 2000, daily world oil production averaged 76 million barrels of oil per day (MM bopd) (H.E. Rilwanu Lukman, personal communication, 2000). Some 30 MM bopd (c. 40%) was produced by OPEC and some 22 MM bopd (c. 30%) supplied by the Middle East, primarily OPEC members Iran, Iraq, Kuwait, Saudi Arabia and the United Arab Emirates. In the 4th quarter of 2000, daily world oil production averaged 78.4 MM bopd (P. Shammas, personal communication, 2000), close to global supply capacity.
The current OPEC World Economic Model (OWEM) predicts that global oil demand will rise by 27 MM bopd (35%) to 103 MM bopd by 2020 (H.E. Rilwanu Lukman, personal communication, 2000). The model predicts OPEC taking more than 50% market share between 2007 (M. Naraghi, personal communication, 2000) and 2016 (H.E. Seyed Mehdi Mirmoezi, personal communication, 2000). Should this forecast transpire, Middle East oil production will need to be raised by at least 15 MM bopd, and possibly by as much as 22 MM bopd (i.e. double current levels) to meet this demand. The speedy and efficient development of this oil production capacity will be essential to power the global economy. Understanding the relationships between the reservoirs producing this oil, and their respective source and seal lithologies, is thus of paramount importance.
This GeoArabia Special Publication 2 presents the results of a study of the sequence stratigraphy of the late Precambrian and Phanerozoic sedimentary successions of the Arabian Plate, carried out by LASMO plc, a British independent oil company.
The sequence stratigraphic and chronostratigraphic interpretation is supported by a tectonostratigraphic review of the plate, and the identification, dating and correlation of 63 Maximum Flooding Surfaces (MFS). These should be regarded as candidate surfaces, as it is yet to be proved that they are isochronous and, in some cases, that they represent true maximum flooding. Although conceptually they are here each regarded as a surface, perhaps more accurately they should be considered as “maximum flooding intervals” - full integration of seismic, outcrop and well data being required to accurately identify the surface itself. The interpretation allows the existing disparate lithostratigraphic schemes across the plate to be placed for the first time within a unifying, sequence stratigraphic framework.
Maximum flooding surfaces were chosen as the primary layering tool due to their relative ease of identification, dating and correlation in the subsurface and outcrop (instead of sequence boundaries sensu Vail et al., 1977). The sequence stratigraphic interpretation here is thus presented using genetic stratigraphic sequences (GSS) sensu Galloway (1989). The study is based on a comprehensive review of the literature, together with the experience and observations of the authors. It is illustrated with numerous figures, together with a large-scale tectonic elements map and a plate-wide chronostratigraphic cross section from the Mediterranean coast of Lebanon to the Arabian Sea coast of Oman.
Until quite recently in geological terms (c. 34 million years ago) the Arabian Plate lay on the southern margin of Neo-Tethys, which stretched from northwest Africa, across Arabia and northern India to northern Australia. GeoArabia Special Publication 2 is the first publication of this nature to cover a significant segment of this important continental margin. Should the majority of the MFS presented here prove to be mainly eustatic in origin, as appears likely, the results of this study will have implications for the many other petroleum systems that are developed along the length of this ancient continental margin.
The primary benefit of the study is that for the first time it allows the plate-wide sedimentary succession to be sub-divided into isochronous packages of sediment, placing oil source rocks, reservoirs and seals within their systems tract positions. At the exploration scale, this allows greater prediction accuracy (i.e. reduces risk) in the likely geometric juxtaposition of source, reservoir and seal facies. This should lead to improved exploration well locations targeting structural and stratigraphic traps. At the field development scale, it allows the insertion of reservoir models into their appropriate systems tract position, and will lead to better definition of reservoir flow units and flow barriers, and thereby lead to reduced costs and improved recoveries. This should in turn allow more efficient field developments.
The structural review of the Arabian Plate identifies eleven tectonostratigraphic megasequences (TMS) separated by major unconformities. These megasequences reflect the evolution of the north and north-eastern plate margin from an intra-cratonic setting, through back-arc, to passive margin, and finally to the active margin setting of today. Within each of these megasequences the general pattern of accommodation space is interpreted to have remained essentially the same, allowing an understanding to be developed of the sedimentary response to fluctuating relative sea levels and sediment supply. The definition of megasequences thus allows the identified MFS to be placed within an appropriate structural framework.
The 63 MFS have been identified using biostratigraphic data, integrated with sedimentological and lithological data (e.g. the presence of shallow water limestones, representing transgression, within an otherwise proximal coarse clastic succession), and by the application of sequence stratigraphic concepts. They are located between upward-deepening (i.e. transgressive systems tracts) and upward-shallowing (i.e. highstand systems tracts) successions of sediments, and may be represented by a variety of lithologies, but most commonly by either deeper water outer shelf shales, or shallow water limestones. Each MFS has first been assigned a geological age, and then attributed a chronometric date following the time scale of Gradstein and Ogg (1996) for the Phanerozoic and Harland et al. (1990) for the late Precambrian. In general terms, the present day distributions of Mesozoic and Tertiary MFS are a function of both original extent and subsequent erosion, whereas the distribution of Palaeozoic and late Precambrian MFS are also a function of more limited well penetrations.
Of the 63 MFS, 33 are interpreted as being preserved over most of the Arabian Plate and provide the most reliable ‘lower order’ cyclicity within the succession. Of the remainder, 25 are interpreted as being preserved ‘sub-regionally’ over large areas of the plate, and 5 preserved only ‘locally’. These MFS are believed to be effectively isochronous and result primarily from global eustatic fluctuations. Subsidence probably controlled the development of MFS in the early period of each megasequence, when following a phase of plate-wide structuring, an extensional regime tended to pre-dominate. Progressive inversion and/or uplift towards the end of each megasequence is also thought to have played a role in the deposition and erosion of MFS. It is hoped that critiques and amendments of this new framework will provide further impetus to the unravelling of the subsurface architecture of this unique and economically important hydrocarbon province.
This study summarizes the regional structure of the Zagros fold and thrust belt in the Kurdistan Region of northern Iraq. The fold and thrust belt is dominated by a 10 to 12 km thick suc-cession of Late Proterozoic to present-day strata that have been deformed into a series of NW-SE-to E-W-trending and SW-to NE-verging doubly-plunging folds and associated thrust faults. Regional structural styles, topography and the distribution of recent seismicity in the region suggest the struc-ture of the fold belt is in part controlled by basement-involved structures. Iraq holds oil reserves of approximately 115 billion bar-rels, the third largest in the world. It is estimated that the Zagros fold and thrust belt in the Kurdistan Region of northern Iraq may contain up to 45 billion barrels of oil (Kurdistan Regional Government, 2007). However, unlike its Iranian counterpart, the fold and thrust belt in the Kurdistan has received very limited geological attention in the past decades due to major geopolitical problems. In addition, because of the scarcity of subsur-face data, the structure of the area remains poorly con-strained. This study has used remote sensing strati-graphic and structural mapping techniques to docu-ment the regional structure of the fold and thrust belt.
Synthetic aperture radar (SAR) is a coherent active microwave imaging method. In remote sensing it is used for mapping the scattering properties of the Earth's surface in the respective wavelength domain. Many physical and geometric parameters of the imaged scene contribute to the grey value of a SAR image pixel. Scene inversion suffers from this high ambiguity and requires SAR data taken at different wavelength, polarization, time, incidence angle, etc. Interferometric SAR (InSAR) exploits the phase differences of at least two complex-valued SAR images acquired from different orbit positions and/or at different times. The information derived from these interferometric data sets can be used to measure several geophysical quantities, such as topography, deformations (volcanoes, earthquakes, ice fields), glacier flows, ocean currents, vegetation properties, etc. This paper reviews the technology and the signal theoretical aspects of InSAR. Emphasis is given to mathematical imaging models and the statistical properties of the involved quantities. Coherence is shown to be a useful concept for system description and for interferogram quality assessment. As a key step in InSAR signal processing two-dimensional phase unwrapping is discussed in detail. Several interferometric configurations are described and illustrated by real-world examples. A compilation of past, current and future InSAR systems concludes the paper.
The main purpose of this study in the High Central Zagros area near the townships of Lordegan and Yasuj was to investigate the nature of the High Zagros Fault (HZF) and the structural styles in the adjacent foreland. A second task was to determine whether the prolific petroleum plays productive further out in the Zagros Foreland would extend into the study area and if new petroleum plays could be recognized. Two selected cross-sections across and one just behind the HZF provide evidence for its overthrust nature. In some places the HZF exhibits a low-angle thrust plane, locally with a substantial amount of thrusting over the foreland. The interpretation is based on surface geological mapping, available well data, remote sensing from satellite data, aeromagnetic and high resolution gravimetry data, magnetotelluric, WARRP (wide angle reflection refraction profiling) and reflection seismic data. The integration of this unique set of newly acquired geophysical data complements recently published papers, which were based more on surface geology and rather limited subsurface data. The data were integrated to construct a regional balanced cross-section and to develop structural petroleum play concepts for the area investigated. Play potential may exist both in the overthrust terrain of the High Zagros as well as in the subthrust. Analysis of the width of the surface anticlines (or more precisely: their frequency) gives a first indication of the depth of the décollement plane. Within the adjacent Zagros Foreland evidence was found for the presence of multiple detachment planes above the ubiquitous deep Hormuz level, i.e. at the level of the Kazhdumi and the Pabdeh-Gurpi Formations. These intermediate décollement planes are thought to be responsible for the generation of deeper structural plays unrelated to the structuring that can be observed at the surface.
We present the first complete balanced cross section across the southeastern Zagros fold-thrust belt (ZFTB). The following main structural features emerge from this section: (1) In the south of the ZFTB, the Proterozoic-to-Recent sedimentary sequence has been decoupled from its Panafrican basement along the ductile basal evaporites and folded into a series of large detachment anticlines. Ongoing shortening of these structures has resulted in migration of the basal salt layers into the cores of the anticlines and propagation of forelimb thrusts. (2) In the north of the ZFTB, deep-seated ramps have folded the hanging wall rocks and produced imbrications and duplex structures within the higher levels of the sedimentary sequence. (3) Out-of-sequence thrusts, linked to major seismogenic basement faults, have cut through the structures in the cover of the ZFTB. A three-step incremental restoration of the section shows that two main phases of deformation can be separated in the tectonic evolution of the ZFTB: a Mio-Pliocene thin-skinned phase, in the course of which most of the structures in the cover were generated, followed by a Pliocene to Recent thick-skinned phase, expressed as out-of-sequence faulting in the cover, which is currently underlined by the seismicity within the basement. In plan view, the initial structures of the southeastern ZFTB developed with a curved shape essentially controlled by the shape and thickness of the underlying Proterozoic salt basin (i.e., the “Jura style”). In the following basement-involved phase, out-of-sequence thrusts cut at oblique angles through the preexisting structures of the cover. The total shortening absorbed in the cover amounts to at least 45 km, corresponding to a ratio of ∼22%.
The Zagros Mountains result from the ongoing collision between the Arabian and central Iran plates. The main features of the eastern Zagros are (1) numerous emerged or buried salt diapirs, made up of Late Precambrian Hormuz salt and (2) the irregular along-strike shape of the collision-related detachment folds with frequent bending. To understand this layout, four geological cross sections have been constructed from the Persian Gulf foreland basin to the inner part of the Zagros Fold-and-Thrust Belt. Shortening in the deformed parts of the sections is less than 10% and is mainly accommodated by detachment folding. We show that late Cenozoic folding occurred in a region that was already punctuated by salt domes and diapirs. In fact, almost continuous halokinesis developed since the earlier Paleozoic, i.e., just short time after the deposition of the Hormuz salt, and continued up to the Present. These preexisting salt structures and their relevant local thickening strongly influenced both the localization and the direction of folds.
In the current Zagros Fold Belt of Iran and in its contiguous offshore, five petroleum systems caused an impressive gathering of oil and gas fields that represent some 8% and 15% of global oil and gas reserves, respectively. Almost all the oil fields are located in the relatively small Dezful Embayment, which extends over 60 000 km2, whereas most of the gas fields are concentrated in Central and Coastal Fars and in the contiguous offshore area. This paper describes the functioning of the various petroleum systems through time, each petroleum system having its own specificity, and reconstructs the succession of events that explains the current location of the oil and gas fields and the reservoirs in which oil and/or gas accumulated. In addition to the classical description of the petroleum systems (distribution and organic composition of the source rocks, evolution of their maturity through time, geometry of drains and reservoirs, and trap availability at the time of migration), the influence of tectonic phases (Acadian, Hercynian, Late Cenomanian to pre-Maastrichtian, and Late Miocene to Pliocene Zagros phases) on the various systems are discussed. As the time of oil and/or gas expulsion from the source rocks is necessary to reconstruct migration paths and to locate the traps available at the time of migration, extensive modelling was used. The timing of oil or gas expulsion was compared with the timing of tectonic events. For the older systems, namely the Palaeozoic (Llandovery source rocks), Middle Jurassic (Sargelu), Late Jurassic (Hanifa-Tuwaiq Mountain-Diyab) and Early Cretaceous (Garau), oil and/or gas expulsion occurred before the Zagros folding. Oil migrated over long distances, according to low-angle geometry, towards large-scale low-relief regional highs and salt-related structures. In the current Zagros Fold Belt, oil and gas remigrated later to the closest Zagros anticlines. In contrast, for the prolific Middle Cretaceous to Early Miocene System (Kazhdumi, Pabdeh), oil expulsion occurred almost everywhere in the Dezful Embayment after the onset of the Zagros folding. Oil migrated vertically towards the closest anticlines through a system of fractures. A comparison was made between the oil expelled from the source rocks, as calculated by the model, and the initial oil in place discovered in the fields. Oils were grouped into families based upon isotopic composition (carbon and sulphur), and biomarkers. Correlation between pyrolysates and oils verifies the origin of the oils that was proposed to explain the current location of the oil (and gas) fields.
Longitudinal components of the Zagros mountain chain change in character and width across N-S trending zones of strike-slip transfer faults lying between 51° and 54° E. To the northwest, a fold-thrust belt with consistent SW vergence has a width of c. 220 km in front of an imbricate belt c. 160 km wide. To the southeast, an imbricate zone c. 80 km wide is fronted by a gently tapering festoon of upright periclines that is c. 350 km wide and punctured by over a hundred emergent salt diapirs. Pre-Zagros stages of the transfer zones in the Zagros are preserved on the Arabian platform and the two most obvious bound what we call the incipient Qatar syntaxis. This is at an early stage of one of the many syntaxes that compartmentalize the Alpine-Himalayan mountain chain. We use structures in the Hormuz salt to map and gauge the tectonic pulse of basement blocks that jostled as ocean basins opened and closed diachronously like zip fasteners along the Tethyan margin of Gondwana. This incipient syntaxis was a lithospheric key that went up while others went down during the rifting and riffling, but not drifting, of the still-bora Hormuz basin we call Proto-Tethys.
The Zagros (Iran) developed during Mio-Pliocene times in response to Arabia-Eurasia convergence. The western Fars highlights a major bend of the deformation front and displays a remarkable set of nearly N-S right-lateral strike-slip faults (the Kazerun-Borazjan/Karebass/Sabz-Pushan/Sarvestan faults) oblique at high angle to the belt. The region likely plays a major kinematic role by accommodating the change in shortening modes from partitioned in the western central Zagros to nonpartitioned in the eastern Zagros. The inversion of focal mechanisms from small and moderate earthquakes shows a consistent N020°-030° compression with a low ratio between differential stresses. This regime accounts for the combination of strike-slip and thrust-type mechanisms through likely sigma2/sigma3 permutations. Fault slip analysis reveals two successive late Cenozoic regional compressional trends, NE-SW then N020°. The latter is in good agreement with the present-day stress. The significance of the NE-SW compression is discussed alternatively in terms of stress deviations or block rotations in relation to the strike-slip fault system. Fieldwork and satellite imagery suggest that these faults behave first as transfer faults during folding of the cover and later as strike-slip faults, in agreement with the succession of stress regimes and the evolution of the dominant deformation style from thin-skinned to thick-skinned. The first-order stability of the collision-related state of stress since ~5 Ma supports that the Arabia-Eurasia convergence did not give rise to partitioning in the western Fars but rather was (and is still) accommodated by distributed deformation involving both shortening and strike-slip motion throughout the cover and the basement.
In the Zagros Fold-Thrust Belt (ZFTB) of Iran it is firmly established that the basement is involved in the deformation. The strongest line of evidence for this assertion comes from the relatively intense mid-crustal seismic activity. On one hand, the main basement structures such as the Main Zagros Fault (MZT) and High Zagros Fault (HZF) reach the surface and are therefore well identified. On the other hand, basement faults south of the HZF are hidden by sedimentary cover and their location is uncertain. In the Eastern Zagros, basement control on surface structures occurred only at a late stage of the tectonic evolution. In other words, the current thick-skinned style of Zagros deformation succeeded a more general thin-skinned phase of orogeny. This chronology is particularly well illustrated by spectacular interference patterns, in which early detachment folds are cut by late oblique basement faults. We present a combined morphological and structural analysis of such structures, and we explore their impact on the river network. We confirm that basement involvement occurred at a late stage of deformation and we show that thick-skinned deformation progressively propagated towards the foreland. An overview of basement steps throughout the Zagros based on published cross-sections allows us to conclude that although basement deformation is concentrated on two major faults in the Central Zagros, it is distributed on several segmented faults in the Fars Arc. This segmentation increases to the SE towards the so-called Oman line and the transition to the Makran accretionary prism.
Deformation styles within a fold-thrust belt can be understood in terms of the spatial organization and geometry of the fold structures. In young fold-thrust belts such as the Zagros, this geometry is reflected topographically by concordant landform morphology. Thus, the distribution of deformation structures can be characterized using satellite image analysis, digital elevation models, the drainage network and geomorphological indicators. The two distinct fold types considered in this study (fault-bend folds and detachment folds) both trending NW-SE, interact with streams flowing NE-SW from the High Zagros Mountains into the Persian Gulf. Multiple abandoned stream channels cross fault-bend folds related to deep-seated thrust faults. In contrast, detachment folds, which propagate laterally relatively rapidly, are characterized by diverted major stream channels and dendritic minor channels at the fold tips. Thus these two fold types can be differentiated on the basis of their geometry (fault-bend folds, being long, linear and asymmetrical, can be distinguished from detachment folds, which tend to be shorter and symmetrical) and on their associated geomorphological structures. The spatial organization of these structures in the Zagros Simply Folded Belt indicates that deformation is the result of the interaction of footwall collapse and the associated formation of long, linear fault-bend folds, and serial folding characterized by relatively short periclinal folds. Footwall collapse occurs first, followed by serial folding to the NE (i.e. in the hanging wall of the fault-bend folds), often on higher detachments within the sediment pile.
Maps of the paleography of Iran are presented to summarize and review the geological evolution of the Iranian region since late Precambrian time. On the basis of the data presented in this way reconstructions of the region have been prepared that take account of the known major movements of continental masses. These reconstructions, which appear at the beginning of the paper, show some striking features, many of which were poorly appreciated previously in the evolution of the region. They include the closing of the 'Hercynian Ocean' by the northward motion of the Central Iranian continental fragment(s), the apparently simultaneous opening of a new ocean ('the High-Zagros Alpine Ocean') south of Iran, and the formation of 'small rift zones of oceanic character' together with the attenuation of continental crust in Central Iran.With the disappearance of the Hercynian Ocean, the floor of the High-Zagros Alpine Ocean started to subduct beneath southern Central Iran and apparently disappeared by Late Cretaceous – Early Paleocene time (65 Ma). From this time the compressional motion between Arabia and Eurasia has been accommodated in Iran by shortening and thickening of the continental crust. This crustal thickening is accompanied by a progressive, though eventful, transition from marine to continental conditions over the whole region.A striking feature highlighted in this study is the existence of extensive alkaline and calc-alkaline volcanics, which appear to be unrelated to subduction. The intrusion of these rocks started in Middle Eocene time (45 Ma) and extended to the present. It is clear that some major fault systems have played a continuous but varied role from the Precambrian until the present, and whatever controlled the original fold orientation at the onset of continental compression (65 Ma) apparently still controls the orientation of contemporary folding.
Many map-scale folds in sedimentary sequences are formed by the bending of fault blocks as they ride over non-planar fault surfaces. These structures, here called fault bend folds, include 'reverse-drag' or 'rollovers' associated with normal faults that flatten with depth and the bending of thrust sheets as they ride over steps in decollement. This paper presents a number of geometric and kinematic properties of parallel fault-bend folds, the most important of which is a relationship between fault shape and fold shape for sharp bends in faults. These relationships are useful tools for developing internally consistent cross sections in areas of suspected fault-bend folding, particularly in fold-and-thrust belts. -from Author
It is here proposed that some thrust faults do not propagate rapidly as clean fractures but rather propagate gradually as slip accumulates. At each instant during propagation, slip goes to zero at the fault tip and is consumed in folding. This kinematic process is here called fault-propagation folding and is put forward as an explanation for the common association of asymmetric folds with one steep or overturned limb adjacent to thrust faults. Two quantitative geometric and kinematic models of fault-propagation folding are derived which encompass the principal expected properties of fault-propagation folding under brittle conditions, one based on conservation of layer thickness and bed length, and the other allowing thickening or thinning of beds in the steep front fold limb. -from Authors
The Shuttle Radar Topography Mission (SRTM) collected elevation data over 80% of earth's land area during an 11‐day Space Shuttle mission. With a horizontal resolution of 3 arc sec, SRTM represents the best quality, freely available digital elevation models (DEMs) worldwide. Since the SRTM elevation data are unedited, they contain occasional voids, or gaps, where the terrain lay in the radar beam's shadow or in areas of extremely low radar backscatter, such as sea, dams, lakes and virtually any water‐covered surface. In contrast to the short duration of the SRTM mission, the ongoing Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is continuously collecting elevation information with a horizontal resolution of 15 m. In this paper we compared DEM products created from SRTM data with respective products created from ASTER stereo‐pairs. The study areas were located in Crete, Greece. Absolute DEMs produced photogrammetricaly from ASTER using differentially corrected GPS measurements provided the benchmark to infer vertical and planimetric accuracy of the 3 arc sec finished SRTM product. Spatial filters were used to detect and remove the voids, as well as to interpolate the missing values in DEMs. Comparison between SRTM‐ and ASTER‐derived DEMs allowed a qualitative assessment of the horizontal and vertical component of the error, while statistical measures were used to estimate their vertical accuracy. Elevation difference between SRTM and ASTER products was evaluated using the root mean square error (RMSE), which was found to be less than 50 m.
Data gleaned from the literature on the Zagros have been compiled and used in conjunction with new interpretations to provide a better picture of the structures, sedimentation history and deformation of this hydrocarbon rich Mountain Belt. The belt parallel Mountain Front, the N–S trending Kazerun, Izeh and E–W trending Bala Rud Fault Zones are defined as the master structural elements of the Belt. These fault zones divide the Zagros basin into zones with different stratigraphic successions and different rheological profiles. This resulted in different structural styles developing along the belt during the subsequent collision.The Kazerun and Izeh Fault Zones acted as the depositional system transition zone between the Lurestan and Fars regions in Jurassic–Cretaceous time and partly controlled the distribution of the Kazhdumi Formation (one of the major source rocks). By the end of the Cretaceous the NW–SE trending Mountain Front Fault divided the present Folded Belt of the Zagros into a major foreland basin to the southwest, and a piggyback basin to the northeast. Activity along other major (transfer) fault zones including the Izeh, Kazerun and Bala Rud Fault zones occurred at this time and controlled the sedimentation and subsidence of the Dezful Embayment the main target region for hydrocarbon exploration. These fault zones controlled the thickness of the Asmari Formation (one of the main reservoirs) and also the considerable thickness and facies changes of the Gachsaran Formation (the major seal to the Asmari Formation). The present morphology of the mountain belt is interpreted as being mainly related to the pinning of the Mountain Front Fault to the northwest of the Dezful Embayment and to the north of the Strait of Hormuz.
This is a handbook of practical techniques for making the best possible interpretation of geological structures at the map scale and for extracting the maximum amount of information from surface and subsurface maps. The 3-D structure is defined by internally consistent structure contour maps and cross sections of all horizons and faults. The book is directed toward the professional user who is concerned about both the accuracy of an interpretation and the speed with which it can be obtained from incomplete data. Quantitative methods are emphasized throughout, and numerous analytical solutions are given that can be easily implemented with a pocket calculator or a spreadsheet. Interpretation strategies are defined for GIS or CAD users, yet are simple enough to be done by hand. The user of this book will be able to produce better geological maps and cross sections, judge the quality of existing maps, and locate and fix mapping errors.
The central part of the Zagros Fold-Thrust Belt is characterized by a series of right-lateral and left-lateral transverse tear fault systems, some of them being ornamented by salt diapirs of the Late Precambrian–Early Cambrian Hormuz evaporitic series. Many deep-seated extensional faults, mainly along N–S and few along NW–SE and NE–SW, were formed or reactivated during the Late Precambrian–Early Cambrian and generated horsts and grabens. The extensional faults controlled deposition, distribution and thickness of the Hormuz series. Salt walls and diapirs initiated by the Early Paleozoic especially along the extensional faults. Long-term halokinesis gave rise to thin sedimentary cover above the salt diapirs and aggregated considerable volume of salt into the salt stocks. They created weak zones in the sedimentary cover, located approximately above the former and inactive deep-seated extensional faults. The N–S to NNE–SSW direction of tectonic shortening during the Neogene Zagros folding was sub-parallel with the strikes of the salt walls and rows of diapirs. Variations in thickness of the Hormuz series prepared differences in the basal friction on both sides of the Precambrian–Cambrian extensional faults, which facilitated the Zagros deformation front to advance faster wherever the salt layer was thicker. Consequently, a series of tear fault systems developed along the rows of salt diapirs approximately above the Precambrian–Cambrian extensional faults. Therefore, the present surface expressions of the tear fault systems developed within the sedimentary cover during the Zagros orogeny. Although the direction of the Zagros shortening could also potentially reactivate the basement faults as strike-slip structures, subsurface data and majority of the moderate-large earthquakes do not support basement involvement. This suggests that the tear fault systems are detached on top of the Hormuz series from the deep-seated Precambrian–Cambrian extensional faults in the basement.
Structural analysis of surface and subsurface data in the Dezful Embayment, the northern Fars, and the High Zagros provinces shows that the presence of the Eocambrian Hormuz and the Miocene Gasharan salt layers have a direct control on the structural style. Both are levels of major disharmony and decollement during the Neogene Zagros folding.
There is some evidence that Hormuz salt doming started before the Neogene Zagros orogeny. Permo-Triassic Tethysian rifting along High Zagros northwest-southeast trends and Cretaceous-Paleogene obduction and compressive events associated with basement reactivation of north-south Arabian trends could have initiated some episodic salt diapir activity in the Central Zagros province. However, in the absence of high-quality, deep seismic imaging in most of the Zagros fold zone, early Paleozoic or Hercynian salt movements are not excluded.
The Hormuz complex is known from emergent halite and anhydrite plugs in the Fars and High Zagros areas. The emergence of Hormuz evaporite plugs is closely associated with major thrusts parallel to the fold trend, such as the Dinar thrust. Plugs also occur along tear faults or where space is created by pull-apart along the north-south trending strike-slip faults. These faults and the associated salt plugs are clearly related to the Zagros folding event, even if they are sometimes located above reactivated paleo-structures. The analysis of the deformation of sandbox models using X-ray tomography suggests that the initiation of thrust and wrench faults is influenced by pre-existing salt domes (weak zones). The driving mechanism of Hormuz halokinesis and extrusion was the squeezing of pre-existing salt domes. Local pull-apart and wrench fault deflection probably also allowed for rapid rising of the evaporites.
In the Fars and High Zagros areas, the Hormuz salt series played the role of a low friction, basal décollement level, and influenced fold style by free development of fore-thrusts and back-thrusts without any preferred vergence. The high competency contrasts within the sedimentary pile favored the development of “fish tail” structures and caused axial shifting of anticlinal crests from surface to depth.
The Neogene sedimentary sequence begins with the deposition of the evaporitic Gachsaran Formation above the Asmari limestone reservoir. The lateral extent of this facies is restricted to the Dezful zone, marking the evolution of the area towards a fold belt and its associated flexural basin. Thickness variations and early diapirism show that this syntectonic deposit is contemporaneous with folding in this area. The Gasharan salt is a major level of décollement and disharmony in the north Dezful Embayment zone, south of the Mountain Front Fault. This interpretation implies that the surface expression of the structures does not reflect their geometry at depth. The Gachsaran evaporitic sequence also plays an important role in sealing Asmari reservoirs.
Early orogenic movements resulted in consolidation of Precambrian basement and formation of vast Iranian platform considered to be extension of Arabian shield; only epeirogenic movements affected region during Paleozoie, which is represented by typical platform deposits; however, most of Iran went through all stages of complete Alpine orogeny in spite of prevailing platform character in preorogenic time; important trends in Alpine structural plan clearly were inherited from Precambrian structures; numerous structural zones are recognized which differ in structural development and present tectonic style.
This paper evaluates four different methods that have been proposed for the estimation of
the detachment depth beneath detachment folds. Guidelines are presented in order to use the most
suitable method in a particular region deformed by detachment folds. These guidelines are constructed
considering the assumptions of each method, the influence of different parameters on the estimation
of the detachment depth (folding mechanisms, cross-sectional area variations, position and orientation
of the regional datum, number of stratigraphic horizons, bed length and thickness, dip and position
of the bounding lines, initial thickness of the ductile unit, single anticlines or fold trains) and the
available data (ductile unit thickness, regional datum, detachment depth). Moreover, a new, simple
method based on two previous methods is presented. The advantages of this new method are that the
calculations are extremely simple and it uses information from more than one stratigraphic horizon;
therefore, it does not depend strongly on the accuracy of the data at one level. The precision of the
proposed methods is tested by their application to a number of natural and experimental single anticlines
and fold trains. It appears that better predictions are obtained when analysing a complete
detachment fold train than when dealing with a single detachment anticline. The reason might be that
ductile material flow beneath the folds along the cross-sectional plane is taken into account when dealing
with a long-enough cross section. We also suggest some hypotheses concerning what the shape and
dimensions of the detachment folds indicate about the detachment depth in those cases where insufficient
data are available to apply the methods proposed.
The Pavlov Hills are formed by separated limestone blocks previously identified as klippen. A new flat-ramp-flat thrust model of the Pavlov Hills is formulated in this paper. The main tectonic detachment is located at the base of the limestone plate and other subsidiary detachments are located within the nodular limestone horizon and also at the base and top of the Upper Cretaceous deposits. The ramps are situated in the Klentnice Fm and Ernstbrunn Lst. The ramp angle was determined by structural evidence combined with interpretation of seismic profiles. Two parallel antiformal structures plunging to the NE are recognized within the study area. The antiformal fold axes are gently plunging to the NE so the anticlines are not ideal for 3-point hydrocarbon trap setting. These anticlines were subsequently cut by sinistral strike-slip faults perpendicular to the fold axis which resulted in the formation of a large-scale pseudo en-echelon structure in an approximate north-south direction.
We describe a kinematic approach to simulate folds above listric propagating thrusts. The model is based on a pre-defined circular thrust geometry with a maximum central angle beyond which the thrust is planar, inclined shear above the circular thrust, and trishear in front of the thrust. Provided the trajectory of thrust propagation is established, the model can be run forward and backwards. We use this last feature to implement a global simulated annealing, inverse modeling strategy. This inverse modeling strategy is applied to synthetic folds as well as two real examples in offshore Venezuela and the Niger Delta toe-thrust system. These three examples illustrate the benefits of the algorithm, particularly in predicting the possible range of models that can fit the structures. Thrust geometry, depth to detachment level, and backlimb geometry have high impact in model parameters such as backlimb shear angle and fault slip; while forelimb geometry is critical to constrain parameters such as fault propagation to fault slip ratio and trishear angle. Steep to overturned beds in forelimb areas are often not imaged by seismic, so in the absence of additional well data, considering all possible thrust-fold geometries is critical for the modeling and whatever prediction (e.g. hydrocarbon trap integrity) is made from it.
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A leading text for undergraduate- and graduate-level courses, this book introduces widely used forms of remote sensing imagery and their applications in plant sciences, hydrology, earth sciences, and land use analysis. The text provides comprehensive coverage of principal topics and serves as a framework for organizing the vast amount of remote sensing information available on the Web. Featuring case studies and review questions, the book's 4 sections and 21 chapters are carefully designed as independent units that instructors can select from as needed for their courses. Illustrations include 29 color plates and over 400 black-and-white figures.
New to This Edition
Reflects significant technological and methodological advances.
Chapter on aerial photography now emphasizes digital rather than analog systems.
Updated discussions of accuracy assessment, multitemporal change detection, and digital preprocessing.
Links to recommended online videos and tutorials.
We investigate the depth of faulting and its connection with surface folding in the Zagros Simply Folded Belt of Iran. Our focus is a sequence of earthquakes (Mw 5.7, 5.5, 5.2, 5.0, 4.9) that struck the Fin region, in the south-eastern Simply Folded Belt, on 2006 March 25. Modelling ground displacements measured with radar interferometry, we find that either N- or S-dipping model reverse faults can reproduce the observed fringe patterns. Despite the uncertainty in fault orientation, we can constrain the vertical extents of rupture to between a top depth of ~5-6 km and a bottom depth of ~9-10 km, consistent with the ~8 km centroid depth of the largest earthquake. We suggest that the faulting ruptured the thick `Competent Group' of Paleozoic and Mesozoic conglomerates and platform carbonates, which makes up the lower part of the sedimentary cover. The rupture probably terminated within the Precambrian Hormuz salt at its base, and the Cretaceous Gurpi marls at its top. These mechanically weak layers act as barriers to rupture, separating faulting within the Competent Group from deformation in the layers above and below. The pattern of coseismic surface uplift is centred on the common limb of the Fin syncline and Guniz anticline, but is oblique (by 20°) to the trend of these open, symmetric, `whaleback' folds, and also overlaps a section of the Fin syncline axis. These observations suggest that locally, surface folding is decoupled from the underlying reverse faulting. Although the Fin syncline and Guniz anticline are symmetric structures, some other nearby folds show a strong asymmetry, with steep or overturned southern limbs, consistent with growth above N-dipping reverse faults. This suggests that the Simply Folded Belt contains a combination of forced folds and detachment folds. We also investigate the distribution of locally recorded aftershocks in the weeks following the main earthquakes. Most of these occurred at depths of ~10-30 km, with a particularly high concentration of events at ~20-25 km. These aftershocks therefore lie within the crystalline basement rather than the sedimentary cover, and are vertically separated from the main rupture. This study confirms earlier suggestions that earthquakes of Mw 5-6 are capable of being generated within the thick `Competent Group' of Paleozoic and Mesozoic sediments, as well as in the basement below the Hormuz Salt Formation.
This study investigates the quality (in terms of elevation accuracy and systematic errors) of three recent publicly available elevation model datasets over Australia: (i) the 9 arc second national GEODATA DEM-9S ver3 from Geoscience Australia and the Australian National University; (ii) the 3 arc second SRTM ver4.1 from CGIAR-CSI; and (iii) the 1 arc second ASTER-GDEM ver1 from NASA/METI. The main features of these datasets are reported from a geodetic point of view. Comparison at about 1 billion locations identifies artefacts (e.g. residual cloud patterns and stripe effects) in ASTER. For DEM-9S, the comparisons against the space-collected SRTM and ASTER models demonstrate that signal omission (due to the ∼270 m spacing) may cause errors of the order of 100–200 m in some rugged areas of Australia. Based on a set of geodetic ground control points over Western Australia, the vertical accuracy of DEM-9S is ∼9 m, SRTM ∼6 m and ASTER ∼15 m. However, these values vary as a function of the terrain type and shape. Thus, CGIAR-CSI SRTM ver4.1 may represent a viable alternative to DEM-9S for some applications. While ASTER GDEM has an unprecedented horizontal resolution of ∼30 m, systematic errors present in this research-grade version of the ASTER GDEM ver1 will impede its immediate use for some applications.
Fault-bend folding, fault-propagation folding, and detachment (or décollement) folding are three distinct scenarios for fold-thrust interaction in overthrust terranes. Simple kink-hinge models are used to determine the geometric associations implicit in each scenario. Bedding maintains constant thickness in the models except in the forelimb of the fold. The forelimb is allowed to thicken or thin without limit. The models address individual folds, and the calculated fold geometries are balanced structures.Each mode of fold-thrust interaction has a distinct set of geometric relationships. Final fold geometry is adequate in itself to discern many fault-bend folds. This is not the case for fault-propagation and detachment folds. These two fold forms have very similar geometric relationships. Some knowledge of the nature of the underlying thrust or décollement zone is usually needed to distinguish between them. The geometry of a fold is altered, in a predictable fashion, by transport through an upper ramp hinge and by fault-parallel shearing of the structure. The shearing results in a tighter fold, whereas transport through the ramp hinge produces a broader fold.The viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the Canadian Cordillera. The geometric analysis is also used to evaluate cross-sections through subsurface structures. In an example from the Turner Valley oil field, the analysis indicates how the interpretation should be altered so as to balance the cross-section. The analysis reveals hidden assumptions and specific inconsistencies in structural interpretations.
The basement-involved active fold-thrust belt of the Zagros in southwest Iran is underlain by numerous seismogenic blind basement thrust faults covered by the folded Phanerozoic sedimentary rocks. Meizoseismal regions of moderate- to large-magnitude earthquakes in the Zagros are localized and concentrated along particular structural-geomorphological features and topographic fronts at the surface. The study reveals at least four active SW-vergent segmented master blind thrusts in the Zagros collisional belt, along which different morphotectonic units are thrusting over the deforming regions. These boundary thrusts, which make contiguous frontal asymmetric anticlines, prominent escarpments and Quaternary deformation, mark topographic fronts at the surface, and have vertically displaced geologic marker beds for more than 6000 m, include: the High Zagros (with a maximum recorded historic earthquake of Ms = 6.0 at Daryan); the Mountain Front (Ms = 7.0 at Khurgu); the Dezful Embayment (Ms = 5.7); and the Foredeep (Ms = 6.5 at Ahwaz) thrusts. Three other seismogenic blind thrusts responsible for the Qir (Ms = 6.9), the Lar (Ms = 6.5) and the Beriz-Dehkuyeh (Ms = 5.7) earthquakes are also documented in this study. The master faults, as evidenced by deformation of the asymmetric anticlines in the hanging wall of the blind thrusts, are segmented and discontinuous, and are separated by gaps in faulting that have presumably controlled the extent of rupture and the magnitude of earthquakes. The master seismic thrusts are displaced right-laterally by deep-seated active transverse faults of Kazerun (Io = VIII), Sarvestan (Ms = 6.4?) and Sabz Pushan. The study shows that active deformation in the Zagros is dominated by: (1) prevalent subsurface blind thrusting; (2) occasional surface strike-slip faulting; (3) coseismic asymmetric folding and uplift of sedimentary cover; and (4) surface thrusting ramping up from at least two regional upper (Miocene Gachsaran) and lower (Lower Cambrian Hormoz) décollement detachments.The active master thrust faults have implications for seismic hazard assessment that were not previouly appreciated. The possibility of large compressive earthquakes (Ms ∼ 7.0) along the introduced blind thrusts must be considered. Locations of other unknown segmented blind thrusts in the belt, which have distinct effects on the surface morphotectonics and topography, and on the structures at depth, could be easily based on meizoseismal maps of the earthquakes combined with active morphotectonic features, morphometric analyses and accurate aftershock sequence studies.
A statistical analysis of reserves in fold and thrust belts, grouped by their geological attributes, indicates which of the world's fold and thrust belts are the most prolific hydrocarbon provinces. The Zagros Fold Belt contains 49% of reserves in fold and thrust belts and has been isolated during the analysis to avoid bias. Excluding the Zagros Fold Belt, most of the reserves are in thin-skinned fold and thrust belts that have no salt detachment or salt seal, are partially buried by syn- or post-orogenic sediments, are sourced by Cretaceous source rocks and underwent their last phase of deformation during the Tertiary. A significant observation is that the six most richly endowed fold and thrust belts have no common set of geological attributes, implying that these fold belts all have different structural characteristics. The implication is that deformation style is a not critical factor for the hydrocarbon endowment of fold and thrust belts; other elements of the petroleum system must be more significant. Other fold and thrust belts may share the structural attributes but the resource-rich fold belts overwhelmingly dominate the total reserves in that group of fold belts. There is nothing intrinsic in fold and thrust belts that differentiates them from other oil- and gas-rich provinces other than the prolific development of potential hydrocarbon traps. Many of the prolific, proven fold and thrust belts still have significant remaining exploration potential as a result of politically challenging access and remote locations.
The Zagros fold-thrust belt in Iran forms the external part of the Zagros active orogenic wedge. It includes a sequence of heterogeneous latest Neoproterozoic-Phanerozoic sedimentary cover strata, similar to 7 to 12 km thick and composed of alternating incompetent and competent layers, overlying Precambrian crystalline basement with a complex pre-Zagros structural fabric. Balancing structures of the cover statigraphic units in the Zagros fold-thrust belt requires in-sequence and out-of-sequence involvement of the Precambrian basement in the deformation. Six detailed balanced and retrodeformable cross-sections, which are constructed based on geological and geophysical data across various sectors of the belt, show fault-bend and fault-propagation folds interpreted to have formed by slip on their subjacent thrusts. Out-of-sequence, basement-rooted thrusts, as the interpretations in the constructed cross-sections suggest, have breached the cover/basement interface and, using incompetent cover strata for propagation, cut across the cover structures and have created associated new folds superimposed upon the pre-existing structures. This style of deformation, which has resulted in structural complexity of the belt, characterizes a similar to 200 to 300 km-wide zone of distributed, partly synchronous, deformation with along-strike and across-strike variations. It also implies that the Zagros fold-thrust belt, as the external part of the orogenic wedge, is still in its subcritical condition with internal deformation to achieve a critical taper. In creating structural complexity, in addition to out-of-sequence thrusting, mechanically weak layers (evaporites and mud-stones) of the cover strata have played a significant role by providing several detachment horizons. Shortening estimates across the belt are variable; based on the constructed cross-sections and their restorations, minimum shortening estimates range from 16 percent to 30 percent in different sectors of the belt.
Maps of the paleography of Iran are presented to summarize and review the geological evolution of the Iranian region since late Precambrian time. On the basis of the data presented, reconstructions of the region have been prepared that take account of the known major movements of continental masses. These reconstructions show some striking features, many of which were poorly appreciated previously in the evolution of the region. With the disappearance of the Hercynian Ocean, the floor of the High-Zagros Alpine Ocean started to subduct beneath southern Central Iran and apparently disappeared by Late Cretaceous-Early Paleocene time. From this time the compressional motion between Arabia and Eurasia has been accommodated in Iran by shortening and thickening of the continental crust. This crustal thickening is accompanied by a progressive, though eventful, transition from marine to continental conditions. A striking feature highlighted in this study is the existence of extensive alkaline and calc-alkaline volcanics, which appear to be unrelated to subduction. Refs.
It has long been recognised that the deposition and deformation of the Phanerozoic cover in the Zagros Basin (mountains plus foreland) was strongly influenced by the reactivation of old tectonic fabrics in its basement. Facies boundaries and structures trending north‐south and NW‐SE can be attributed to the reactivation of Pan‐African sutures and Najd faults which are exposed in the Nubian‐Arabian Shield. However, to the east of a projection of the Oman line SWwards into the Rhub Al Khali Basin, cover structures have a NE‐SW trend which is not seen in Arabia. This boundary may overlie a Pan‐African suture between Arabia and India (Somalia or Pakistan).
Data including magnetic intensities, geothermal gradients and isopach maps are used here to distinguish old faults which were reactivated in the basement from more recent faults formed in the cover by Zagros shortening. Old faults trending NW‐SE are interpreted as having reactivated episodically since the Permo‐Triassic opening of Neo‐Tethys; perhaps more significantly, the basement faults that reactivated in the East Arabian Block since then trend north‐south. The basement configuration is clarified by extending a modified East Arabian Block across the Zagros to an “East Arabian‐Zagros block” in which the NW trend of the Zagros lies between two syntaxes. This suggests a new tectonic framework for the region. The repeated reactivation of basement faults throughout the East Arabian‐Zagros Block controlled source rocks, traps and seals for the supergiant and giant oil and gas reserves which are present at various stratigraphic levels in different areas.
Lateral offsets in the pattern of seismicity along the Zagros fold and thrust belt indicate that transverse faults segmenting the Arabian basement are active deep-seated strike-slip faults. The dominant NW-SE trending features of the belt have undergone repeated horizontal displacements along these transverse faults. These reactivated basement faults, which are inherited from the Pan-African construction phase, controlled both deposition of the Phanerozoic cover before Tertiary-Recent deformation of the Zagros and probably the entrapment of hydrocarbons on the NE margin of Arabia and in the Zagros area. We have used observations of faulting recognized on Landsat satellite images, in conjunction with the spatial distribution of earthquakes and their focal mechanism solutions, to infer a tectonic model for the Zagros basement.
The Zagros mountains of SW Iran are one of the most seismically active intra-continental fold-and-thrust belts on Earth, and an important element in the active tectonics of the Middle East. Surface faulting associated with earthquakes is extremely rare, and so most information about the active faulting comes from earthquakes. We use long-period teleseismic P and SH body waves to determine the orientation and depth of faulting in 16 new earthquakes, and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the Zagros. We use this information to investigate the style and distribution of active faulting in the Zagros, and how it contributes to the N–S shortening of the Arabia–Eurasia collision. When the data are ranked in quality and carefully evaluated, simple patterns are seen that are not apparent when routine catalogue data are taken at face value. An important change in the fault configuration occurs along strike of the belt. In the NW, overall convergence is oblique to the trend of the belt and the surface anticlines, and is achieved by a spatial separation (‘partitioning’) of the orthogonal strike-slip and shortening components on separate parallel fault systems. By contrast, in the SE, overall convergence is orthogonal to the regional strike and achieved purely by thrusting. In the central Zagros, between these two structural regimes, deformation involves parallel strike-slip faults that rotate about vertical axes, allowing extension along the strike of the belt. The overall configuration is similar to that seen in other curved shortening belts, such as the Himalaya and the Java–Sumatra trench. All the Zagros earthquakes we have been able to check have centroids shallower than ∼20 km and are confined to the upper crust. Many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover, which is active even beneath areas of known shallow structural decollement such as the Dezful embayment. The dominant style of shortening is high-angle reverse faulting with dips >30° though some lower-angle thrusting occurs in places. Active thrust and reverse faulting is relatively confined to the lower topography on the SW edge of the belt today, and only strike-slip faulting affects the higher topography. Profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the Simple Folded Belt, but has progressively migrated SW over the last 5 Ma. There is no evidence for a seismically active structural decollement, such as a low-angle thrust, beneath the Zagros, nor is there any seismic evidence for active subduction, either beneath the Zagros or beneath central Iran. Instead the Arabian margin seems to have shortened by distributed thickening of the basement. Only in the syntaxis of the Oman Line, at the SE end of the Zagros, is there any evidence for a low-angle thrust of regional extent. Here, earthquakes continue 50 km north of the Zagros Thrust Line (the geological suture between the Arabian margin and central Iran) reaching depths of ∼30 km, and may represent thrusting of Arabian basement beneath central Iran to this extent.
The present day seismicity of the Zagros seems to occur on high angle reverse faults distributed across the whole width of the belt. It does not indicate activity on a single inclined thrust surface and there do not seem to have been any well located events at intermediate depths. Modelling of the long period teleseismic body waves of seven large earthquakes presented here shows their focal depths to be in the range 8–15 km. This is thought to indicate faulting in the uppermost basement beneath the sedimentary cover, though the absence of published seismic refraction results renders the sediment thickness uncertain. Faulting of this type and depth may occur on inherited normal faults which have subsequently been reactivated as thrusts. Such reactivation allows considerable shortening to take place in the early stages of continental collision without the subduction or excessive thickening of continental crust.
The latest Neoproterozoic through Phanerozoic stratigraphy of the Zagros fold-thrust belt of Iran has been revised in the light of recent investigations. The revised stratigraphy consists of four groups of rocks, each composed of a number of unconformity-bounded megasequences representing various tectonosedimentary settings. In the lowest group, ranging in age from latest Precambrian to Devonian(?), the uppermost Neoproterozoic to middle Cambrian rocks constitute a megasequence of evaporites, siliciclastic deposits, and interlayered carbonates, which were deposited in pull-apart basins that developed by the Najd strike-slip fault system. This mega-sequence is overlain by a second one, Middle to Late Cambrian in age, which consists of shallow, marine siliciclastic and carbonate rocks representing deposition in an epicontinental platform. The overlying shales, siltstones, and partly volcanogenic sandstones of Ordovician, Silurian, and Devonian(?) age are local remnants of stratigraphic units that were extensively eroded during development of several major unconformities. The second group consists of two megasequences, one Permian and the other Triassic, composed of widespread, transgressive basal siliciclastic rocks and overlying evaporitic carbonates of an equatorial, epi-Pangean, very shallow platformal sea. The third group is composed of four megasequences formed of shallow-and deep-water carbonates with some siliciclastic and evaporite deposits, which accumu-lated on a Neo-Tethyan continental shelf during earliest Jurassic through late Turonian time. The fourth group comprises siliciclastic and carbonate deposits of a largely underfilled, NW-to SE-trending, forward and backward migrating, late Cretaceous to Recent proforeland basin, which has evolved as an integral part of the Zagros orogen. This last group consists of three megasequences (IX, X, and XI) with distinctive lateral and vertical facies variations, which reflect specific tectonic events. Megasequence IX comprises uppermost Turonian to middle Maastrichtian prograding and retrograding siliciclastic and carbonate deposits, whose accumulations reflect emplacement ("obduc-tion") of ophiolite slivers and subsequent collisional events in the Zagros orogen. Megasequence X consists of uppermost Maastrichtian to upper Eocene siliciclastic and carbonate rocks, which deposited first progradationally in front of the Zagros orogenic wedge with reduced contractional tectonic activity, and then retrogradationally due to intensified thrust stacking in the interior parts of the orogen. Megasequence XI consists of Oligocene and lower Miocene carbonate strata deposited retrogradation-ally shortly after a period of intensified late Eocene thrust faulting in the deforma-tional wedge, and an overlying succession of upward-coarsening, northeasterly-derived siliciclastic deposits of lower Miocene to Recent age which are composed of erosional byproducts of the southwest-vergent Zagros thrust sheets.
A central question in structural geology is whether, and by what mechanism, active faults (and the folds often associated with them) grow in length as they accumulate displacement. An obstacle in our understanding of these processes is the lack of examples in which the lateral growth of active structures can be demonstrated definitively, as geomorphic indicators of lateral propagation are often difficult, or even impossible to distinguish from the effects of varying lithology or non-uniform displacement and slip histories. In this paper we examine, using the Zagros mountains of southern Iran as our example, the extent to which qualitative analysis of satellite imagery and digital topography can yield insight into the growth, lateral propagation, and interaction of individual fold segments in regions of active continental shortening. The Zagros fold-and-thrust belt contains spectacular whaleback anticlines that are well exposed in resistant Tertiary and Mesozoic limestone, are often > 100 km in length, and which contain a large proportion of the global hydrocarbon reserves. In one example, Kuh-e Handun, where an anticline is mantled by soft Miocene sediments, direct evidence of lateral fold propagation is recorded in remnants of consequent drainage patterns on the fold flanks that do not correspond to the present-day topography. We suggest that in most other cases, the soft Miocene and Pliocene sediments that originally mantled the folds, and which would have recorded early stages in the growth histories, have been completely stripped away, thus removing any direct geomorphic evidence of lateral propagation. However, many of the long fold chains of the Zagros do appear to be formed from numerous segments that have coalesced. If our interpretations are correct, the merger of individual fold segments that have grown in length is a major control on the development of through-going drainage and sedimentation patterns in the Zagros, and may be an important process in other regions of crustal shortening as well. Abundant earthquakes in the Zagros show that large seismogenic thrust faults must be present at depth, but these faults rarely reach the Earth's surface, and their relationship to the surface folding is not well constrained. The individual fold segments that we identify are typically 20-40 km in length, which correlates well with the maximum length of the seismogenic basement faults suggested from the largest observed thrusting earthquakes. This correlation between the lengths of individual fold segments and the lengths of seismogenic faults at depth suggest that it is possible, at least in some cases, that there may be a direct relationship between folding and faulting in the Zagros, with individual fold segments underlain by discrete thrusts.
The geometric properties of folds are considered to be of prime importance in fold classification and in the analysis of natural folds. Methods of precise description of fold geometry are necessary for natural fold analysis, and they enable a link between theoretical and experimental work and natural folds to be made.Various methods of analysing data for the spatial attitude of folds are briefly mentioned, and a detailed examination of methods of representing the geometric form of folds in profile section is made in two parts, one dealing with the forms of single layers and the other with the shapes of individual surfaces. In both cases existing methods of geometrical analysis are critically appraised and many are found to be impracticable.Folded layer geometry can most usefully be represented by two parameters that are both functions of apparent dip. These are thickness, t, and a new parameter, φ, which derives from and is used in conjunction with dip isogons. A refinement of geometrical fold classification is made in terms of these parameters, their interrelationship is examined and their relative merits are considered.In the case of single folded surfaces, a simple application of harmonic analysis provides a useful means of describing the fold shape. The most basic and suitable segment of a folded surface for analysis is a “quarter-wavelength” unit between adjacent hinge and inflexion points. Such a choice of unit leads to a harmonic series consisting only of the odd terms of a sine series. The first few harmonic coefficients are sensitive parameters of fold shape, most information being contained in the first two coefficients, b1 and b3. Plots of b3 against b1 and log bn against log n are useful means of representing and comparing coefficients for different folds. Many natural folds seem to be closely matched in shape by a member of an ideal series of mathematical forms, and this leads to a rapid visual method of harmonic analysis that involves no measurements. This has proved most useful in studies of the comparative morphology of folded rocks.Theories of fold development are discussed with particular reference to the development of folds by buckling in isolated competent layers embedded in a less competent matrix. The geometric properties of folds predicted by the theories are examined in terms of the descriptive parameters discussed in the first part of the paper. Harmonic analysis is used to describe the progressive changes in fold shape predicted by recent theory for the buckling of a layer starting as a low-amplitude sine wave.The geometric properties of passive folds are briefly considered, and the problem of “similar” folds is discussed.The geometric forms taken up by two idealised models of a buckled layer are calculated and compared. The clearest distinction between the models is brought out in the isogon patterns or by the thickness variations with dip.The effect of compression in modifying the geometry of folds is considered. Plots of thickness or φ against apparent dip for parallel folds that have been uniformly “flattened” can be so adjusted as to give straight line relationships. Most natural fold shapes are closely represented by straight lines on such adjusted graphs, and the slope or intercept of the best-fit straight line to natural fold data is an empirical parameter of folded layer shape.A simulated effect of simultaneous buckling and flattening of a layer is described that predicts relationships between thickness and dip that have been observed in natural and experimentally produced folds.
Extensive field observations over a large tract of continuous rock outcrops in the Zagros Mountain Range of southwest Iran have yielded a wealth of stratigraphic and structural detail. In the region structural anomalies are frequently associated with similar facies distribution patterns. In the eastern portion of the region emergent salt plugs of infra-Cambrian age exhibit the same alignment patterns. Such trends bear no apparent genetic relationship to the Tertiary folding responsible for the present fold belt grain of the Zagros Range but rather indicate affinity with linear basement features which are readily observable on Landsat imagery and air photographs.Superimposed on the eastern region's mode of facies trends and structure are localized variations which are directly attributed to pulses of salt diapiric activity. Thus stratigraphic data acquired from deep sections associated with salt domes can lead to erroneous overviews of regional facies distributions while anomalous dome-shaped structural features associated with elongate fold, so common to the fold belt, can only be attributed to near surface diapiric structures.The recognition of features related to basement tectonics and the realization of their implication in the control and modification of geological processes is an important adjunct to the search for hydrocarbon accumulations in the region. Indeed it can be shown that renewed movements on basement trends directly affect ooil production patterns as a consequence of the enhancement of fracture porosity and permeability in Tertiary carbonate reservoir structures. These constitute some of the world's largest oil-producing fields.
Genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting, base level of erosion, complex response, threshold of critical power, diachronous and synchronous response times, and static and dynamic equilibrium. Climatic and tectonic stream terraces are major terraces below which flights of minor complex-response degradation terraces can form.These three types of terraces can be summarized by describing a downcutting-aggradation-renewed downcutting sequence for streams with gravell bedload. By tectonically induced downcutting, streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion. Lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream-terrace landform. Aggradation events record brief reversals of long-term tectonically induced downcutting because they raise active channels. They may be considered as major (the result of climatic perturbations) or minor (the result of complex-response model types of perturbations). Climatically controlled aggradation followed by degradation leaves an aggradation surface; this type of fill-terrace tread is the fundamental climatic stream-terrace landform. Aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces. Raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form; first in alluvial fill and then in underlying bedrock along tectonically active streams. These are complex-response terraces because they result from interactions of dependent variables within a given fluvial system. Pauses in degradation to a new base level of erosion, and/or minor episodes of backfilling, lead to formation of complex-response fill-cut and strath, or of fill terraces. Fill-cut terraces are formed in alluvium; they are complex-response terraces because they are higher than the base level of erosion. Good exposures and dating are needed to distinguish static equilibrium complex-response minor strath terraces from dynamic equilibrium tectonic (major) straths. Strath terraces may be regarded as complex-response terraces where degradation rates between times terrace-tread formation exceed the long-term uplift rate for the reach based on ages and positions of tectonic terraces.Late Quaternary global climatic changes control aggradation events and even the times of cutting of major (tectonic) straths, because the base level of erosion can not be attained during times of climatically driven aggradation-degradation events.Most terrace soils form on treads of climatic and complex-response terraces. Aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence. Each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads. Multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces. Pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous - variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors. Climatic terraces of adjacent watersheds also should be roughly synchronous (correlatable) - variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds. Such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times, and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations. Soils on climatic terraces of distant watershedswill not be synchronous if their respective aggradation events occur during full-glacial times and interglacial times. Soils on some complex-response terraces may be diachronous within a given fluvial system, and typically are diachronous between watersheds.