Zagros Deformation Research Articles

Crustal deformation analysis in Zagros and Makran zones, based on GPS permanent network data

We present an analysis of strain based on the evaluation of crustal deformation in Zagros and Makran, which has implications in terms of the dynamics of the study area. We have used data from 17 permanent GPS stations of the Cartographic Centre of Iran spanning the period 2011–2013. The raw observed data were in RINEX format. The elements of strain tensor were calculated by a 2D isoparametric method and the inversion of strain equation. Then the dilatation (extension and compression) and shearing were obtained through the analysis of eigenvalue and eigenvector of tensors. In most cases compression has overcome the extension, which can be expected due to the convergence of the Arabia and Eurasia plates. The compression axis is nearly vertical along the Zagros causing reverse and thrust faults in Zagros. Due to the N–S trending of the Sabzvaran–Jiroft–Kahnuj fault system and to the direction of compression and extension, the system will be of strike–slip mechanism. Sudden changes of shearing can be used to identify the strike–slip faults. As observed in this study, there were sudden changes in shearing of the Rafsanjan and Jiroft–Kahnuj faults. The P/T axis rotates significantly in the Zagros–Makran transition zone. Moreover, the Makran's P/T axis is smaller than the P/T axis of Zagros, which demonstrates that the impact of the Arabian plate, moving towards Iran, is greater than the impact of the Indian subcontinent plate movement. The values of the counter-clockwise rotation rate at stations located east of the Zagros–Makran transition zone are higher than at other stations. Also the velocity vectors at stations were determined while using the software GAMIT/GLOBK.

THE CONTRIBUTION OF EARTHQUAKES TO THE DEFORMATION OF ZAGROS TECTONIC PROVINCE

Abstract. In this study, we investigate the contribution of earthquakes to the deformation of Zagros province and compare the seismicity and the density of earthquakes in different parts of the province. The mathematics used in this research is based on calculations of moment rates. The seismic moment rate is the average amount of seismic energy releases from the tectonic province in each year. The geodetic moment rate is the average amount of energy which is consumed every year to make deformation in Zagros. The ratio of these two moment rates expresses the contribution of earthquakes in making deformation in Zagros province. According to the calculations, this ratio is estimated to be 13.06%. Along with the information obtained from the moment rates, we can also obtain the shear and the dilative strain rates from the strain rate tensors, which show the volumetric changes and the deformation rate in different parts of the Zagros, respectively. The data used in this study include the focal coordinates of the Zagros earthquakes with their magnitude and the velocity vectors of the Zagros geodynamic network, which are used to calculate the seismic and the geodetic moment rates.

Temporal evolution of the Darmadan salt diapir, eastern Fars region, Iran

A wide range of diapirs crop out along the eastern Fars region. They are constituted by the Pre-Cambrian to Early Cambrian Hormuz Fm. and located in the transition from the frontal structures of the Zagros fold-and-thrust belt and the Oman Ranges. While, in different previous works about the Persian Gulf, off-shore diapirs the deformation history has been determined on the basis of subsurface data, onshore evidence for salt tectonics activity is often limited to post-Oligocene stratigraphic units and is significantly overprinted by shortening. The Darmadan diapir is one of the few salt structures where is possible to observe halokinetic sequences involving the pre-collision Mesozoic succession in the Fars region of the Zagros fold-and-thrust belt. The objective of our study was to characterize on the base of field data, the stratigraphic and structural relationships between the Hormuz salt and its overburden in order to establish its temporal evolution. The Darmadan diapir is interpreted as a salt stock connected to a salt wall structure at depth evolving to a passive diapir, at least since Late Jurassic-Early Cretaceous times. Expansional geometries toward the diapir reveal that primary welding was already occurred at the Albian time. The onset of contractional deformation in the internal part of the Zagros and Oman orogenic systems is recorded by the reactivation of the Darmadan diapir during the Late Cretaceous. The Darmadan diapir continued to be exposed during the Paleogene and was squeezed during the Neogene to recent Zagros deformation. Second-order contractional features and related growth geometries constrain the timing of the secondary welding during the early stages of the late Miocene Darmadan anticline development. The explained structure remarks that the present structural trends of the Zagros fold-and-thrust belt in the eastern Fars region are the result of the reactivation of preexistent salt structures.

Evolution of the Karebas Fault System and adjacent folds, central Zagros fold-and-thrust belt, Iran: Role of pre-existing halokinesis (salt structures and minibasins) and detachment levels

In the central part of the Zagros fold-and-thrust belt of Iran, several diapirs of the Latest Precambrian–Early Cambrian Hormuz Salt locally decorate the surface exposures of a series of N–S to NNW–SSE trending right-lateral tear fault systems. The Karebas Fault System (KBFS), as one of these fault systems, is associated with five salt diapirs. There are also several NW–SE trending anticlines, which are either cross-cut by the fault system or terminated against it and the adjacent salt diapirs. The Neogene Zagros folds are clearly affected by the surface expression of the Karebas Fault System, suggesting that the fault system is coeval with or post-dates the Neogene deformation. On the contrary, halokinesis as old as at least Late Jurassic is documented by near-diapir stratigraphic exposures, indicating pre-orogenic existence of the diapirs. Although the lack of older rock outcrops precludes tracking the pre-Jurassic records of diapirism, the geometry of strata, which flank these diapirs, indicates long-term pre-folding salt rise driven by downbuilding of lateral minibasins and progressive draping of sedimentary layers over the flanks of the rising diapirs. Therefore, presence of old salt structures along the young (Neogene) tear fault system suggests an important role played by the presence of early-formed salt structures in inducing later strike-slip deformation along zones of preferential weakness, which was prepared by the pre-folding salt structures and thinner sedimentary pile overlying them. Salt diapirs and related changes in stratigraphic thicknesses (i.e. minibasins) also effectively grappled with the process of folding. Detachment folding is typically the main structural style across the area, and was mainly developed above the Hormuz Salt as the basal décollement and partly by involvement of the intermediate detachment levels. However, preferential weakness and squeezing of the diapirs on one hand and greater strength of the minibasins on the other hand gave rise to further complexities of deformation during shortening. Such first-order controls could led to perturbations, at the scale of diapirs and minibasins, in the stress pattern as the southwestward-advancing front of the Zagros deformation migrated across the stratigraphic section and the salt structures. It occurred because strain is localized at the diapirs and squeezes them whereas tends to avoid folding the thick, and hence stronger, minibasins into anticlines. In such a context, the localization and the longitudinal propagation of the folds in the cover sequence could be remarkably influenced and irregular. The consequence was variable fold geometries (location and dimension) and fold-axes rotations with respect to the diapirs and minibasins.

Salt tectonics and tear faulting in the central part of the Zagros Fold-Thrust Belt, Iran

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 Study and Analytical Modeling of Mangerak Salt Diapir (South West of Firozabad Fars-Iran)

Mangerak salt diapir is in the South West of Firuz Abad in Fars province, southern Iran and structurally, it is exposed in the simple folded belt of Kohzad Zagros. This diapir, now, is located in a transtentional zone in the overlapping parts of Koreh Bas right fault zone. The origin of this diapir is evaporative series of Hormuz, at the beginning of the Cambrian and ending Precambrian age. In this paper, we investigated tectonic structures around the salt dome of Mangerak. By promoting this salt dome, a variety of structures, such as marginal reverse faults, radial normal faults, folds and caves have been developed. In addition, many changes can be seen in the status line, and the amount and direction of the strata dip, thickness of rock units, and facies change, that all showed the downbuilding phenomenon in the diapir. The method was based on structural desert surveys and relevant measurements. The results show that salt domes above were rising during the Late Cretaceous-Paleocene, and pre-deformation of Zagros and in connection with basement Fault of Korebas, and probably, when sedimentation was in the Zagros basin, they have been exposed in the form of an island. Zagros deformation at the same time, the pressure released from the collision zone on the north side of the East and its wave motion, to the South West, exerts more pressure on the salt horizons and helps them to erupt. About Mangerak salt dome, which is exposed on the side of the Sayakh anticline axis and Basement fault of Korebas, two phenomena are effective in its exposing. Analytical modeling shows the life of the outcrop 31,000 years that this age is consistent with the effects of salt dome Neotectonic.

Depositional environments of the Kalhur Member evaporites and tectonosedimentary evolution of the Zagros fold–thrust belt during Early Miocene in south westernmost of Iran

The Oligocene–Miocene deposits of the Asmari Formation are the most important oil reservoirs in the Zagros fold–thrust belt in southwest Iran. The Aquitanian evaporites of Kalhur Member of the Asmari Formation were deposited in a depression situated nearly between the Mountain Front Fault and the Dezful Embayment Fault, which borders northern and southern margins, respectively. Detailed field, petrographic studies and wireline log analyses were used to investigate the depositional facies and sedimentary environments of the Kalhur Member. The occurrence of evaporites between pelagic facies and the presence of slump folding, considerable continuity of laminated gypsum/anhydrite along with progradation of clinoforms towards depositional side of evaporites, suggest the presence of relatively deep and isolated depressions during deposition of Kalhur Member. Different facies and thickness variation along with various geometries were formed as the result of overthrust loading event related to the Zagros deformation during Rupelian–Chattian. As the result of this tectonic event, depocentres shifted laterally and relatively narrow deep depressions were filled by evaporites under relative sea-level lowstand and arid conditions and sea-level falls during Aquitanian. Sea-level rise during Burdigalian resulted in sedimentation of the Asmari Formation carbonates throughout the Lurestan and Dezful Embayment, meanwhile with deeper facies towards north. This was along with basin reconfiguration and creation of carbonate ramp.

Structure, timing and morphological signature of hidden reverse basement faults in the Fars Arc of the Zagros (Iran)

Abstract 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.

Displacement fields and finite strains in a sandbox model simulating a fold-thrust-belt

This thesis uses geodetic satellite data to measure present-day crustal deformation in the Zagros fold-thrust belt (SW Iran). Geodetic-type measurements are also used in down-scaled models that simulate the surface deformations seen in convergent settings like the Zagros fold-thrust belt.Global Positioning System (GPS) measurements of three surveys between 1998 and 2001 indicate 9 ± 3 mm/yr and 5 ± 3 mm/yr shortening across the SE and NW Zagros respectively. GPS results show that in addition to the different rates and directions of shortening on either side of the NS trending Kazerun fault, local along-belt extension occurs to the east. Differential SAR interferograms of ERS1 & 2 images between 1992 and 1999 detect 8 ± 4 mm/yr uplift rate across a newly recognized fault in SW Qeshm Island. This can be attributed to a steep imbricate thrust that may still represent the local Zagros deformation front.The salt diapirs in the Zagros rise from a source layer that acts as a low-frictional decollement that decouples the deformation of the cover sediments from their basement in the eastern Zagros whereas the cover to the west deforms above a high-friction decollement. Physical models were prepared to simulate cover deformation in the Zagros by shortening a sand pack above adjacent high- and low-frictional decollements (represented by a ductile layer). The strain distributions differed above the two types of decollements; it was more heterogeneous above the salt where local extension in the shortening direction was dominant. A separate work also investigated systematically the role of basal friction on cover deformation in convergent settings. Accurate height measurements of the model surface by laser-scanner indicated a deformation front more distal than usual, particularly in the low-basal frictional models. The volume reduction in our shortened sand models correlated directly with their basal friction.

Mechanical modelling of oblique convergence in the Zagros, Iran

SUMMARY Recent GPS surveys indicate that the Zagros kinematics corresponds to an oblique convergence between a rigid central Iranian plateau and the Arabian plate at ∼ 7m m yr −1 at the longitude of the Persian Gulf. Convergence is almost frontal in the SE Zagros and oblique (45 ◦ )i n the NW part of the range. It has been proposed that internal deformation of the NW Zagros occurs in a partitioned mode. In such a view, the Main Recent Fault (MRF) bordering the Iranian plateau accommodates all the tangential motion, while shortening happens by pure thrusting within the fold and thrust belt as suggested by the focal mechanisms within the range. We use a 2.5-D mechanical finite element model of the Zagros to understand the influence on the Zagros deformation of (1) the obliquity of convergence, (2) the rheological layering of the lithosphere (strong upper crust, weak lower crust, strong or weak uppermost mantle) and (3) a possible weakness of the MRF. Surprisingly, a fully partitioned mode occurs only when the collision is very oblique. In the case of the NW Zagros, we find that the MRF can accommodate only ∼25 per cent of the whole tangential motion. As the GPS inferred range parallel motion across the NW Zagros is ∼ 5m m yr −1 , the present day slip rate of this fault could be as low as 1.2 mm yr −1 . In agreement with the observed strain in SE Zagros, our model predicts that no strike-slip activity takes place on a vertical fault when the direction of convergence is more frontal. We discuss the implications of our model on the seismicity and kinematics of oblique collision zones by comparing our results with the available data in the Zagros and other oblique convergence domains.

Role of the Kazerun Fault Zone in the formation and deformation of the Zagros Fold‐Thrust Belt, Iran

The Kazerun Fault is a N‐S trending fault zone, which obliquely truncates the NW‐SE trending Zagros Fold‐Thrust Belt. This active fault zone is an ancient structural lineament which has controlled the structure, sedimentation, and subsidence of the central part of the Zagros since the Early Cambrian and has had an influence on the formation of the hydrocarbon system of the belt. The Kazerun Fault limited the distribution of the Cambrian Hormuz salt (the major decollement horizon that separates the Precambrian basement from the thick sedimentary cover) to the west. Its later reactivation with other N‐S trending fault zones (e.g., the Izeh Fault Zone) during the Cretaceous resulted in major sedimentary thickness and facies variations along the belt. This study presents a new interpretation for the evolution of the Kazerun Fault Zone based on field evidence and a review of various published and unpublished data. It is argued that at the surface, the fault zone is delineated by four north‐south trending segments and that marked differences occur in the distribution of deformation on either side of the fault segments in the Late Tertiary. During this time these segments acted as transfer faults (lateral ramps) linking different segments of the Zagros deformation fronts.

Tectono-sedimentary framework of the Gachsaran Formation in the Zagros foreland basin

Continental collision between Iranian and Arabian plates resulted in the formation of the Zagros fold–thrust belt and its associated foreland basin. During convergence, pre-existing faults in the basement were reactivated and the sedimentary cover was shortened above two different types of basal decollement (viscous/frictional). This led to heterogeneous deformation which segmented not only the Zagros fold–thrust belt but also its foreland basin into different compartments resulting in variation in facies, thickness and age of the sediment infill. Based on this concept, a new tectono-sedimentary model is proposed for one of the most important syn-tectonic sedimentary unit, the Gachsaran salt in the Zagros foreland basin. In this proposed model, it is argued that differential propagation of the deformation front above decollements with different mechanical properties (viscous versus frictional) results in along-strike irregularity of the Zagros deformation front whereas movement along pre-existing basement faults leads to development of barriers across the Zagros basin. The irregularity of the deformation front and the cross-basin barriers divided the Zagros foreland basin into six almost alternating sub-basins where Gachsaran salt and its non-salt equivalents are deposited. In the salt sub-basins, two different processes were responsible for the deposition of Gachsaran salt: (1) evaporation, and (2) dissolution of extruding Hormuz salt and its re-precipitation as Gachsaran salt. Re-precipitation was probably the most significant process responsible for the huge deposit of Gachsaran salt in the extreme south-east part of the Zagros foreland basin.

Accommodation of Arabia‐Eurasia convergence in the Zagros‐Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system

At Iranian longitude, the Arabian plate is moving northward relative to Eurasia (∼20 mm yr−1 according to GPS). To the east, this relative motion is accommodated by northward subduction under the E‐W Makran emerged accretionary prism. To the west, it is accommodated partly by the Zagros fold‐and‐thrust belt and partly by the Alborz/Kopet Dagh deforming zones further north. This work investigates the NNW striking transition zone that connects Zagros and Makran: the Minab‐Zendan fault system. Satellite images, and structural and geomorphic field observations show a distributed deformation pattern covering a wide domain. Five north to NW trending major faults were identified. They exhibit evidence for late Quaternary reverse right‐lateral slip, and correspond to two distinct fault systems: the western one transferring the Zagros deformation to the Makran prism, and the eastern one northward transferring the deformation to the Alborz/Kopet Dagh. Tectonic study and fault slip vector analyses indicate that two distinct tectonic regimes have occurred successively since the Miocene within a consistent regional NE trending compression: (1) an upper Miocene to Pliocene tectonic regime characterized by partitioned deformation, between reverse faulting and en echelon folding; (2) a NE trending σ1 axis transpressional regime homogeneously affecting the region since upper Pliocene. The change is contemporaneous with major tectonic reorganization regionally recorded. Therefore this study provides evidence for active deformation that is not localized, but distributed across a wide zone. It accommodates the convergence and transfers it from collision to subduction by transpressional tectonics without any partitioning process in the present‐day period.

Crustal scale geometry of the Zagros fold–thrust belt, Iran

Balanced cross-sections across the Zagros fold–thrust belt in Iran are used to analyze the geometry of deformation within the sedimentary cover rocks, and to test the hypothesis of basement involved thrusting throughout the fold–thrust belt. Although the Zagros deformation front is a relatively rectilinear feature, the sinuous map-view morphology of the mountain front is a result of a 6 km structural step in the regional elevation of the Asmari Limestone that produces a pronounced step in topography termed the ‘mountain front flexure’. Although the height of the mountain front flexure is sufficient to permit basement fault–bend folds at the front of the Lorestan and Fars regions, the taper of the orogen, low percentage of shortening, and consistent structural elevation from the mountain front flexure to the hinterland suggest that Lorestan and Fars segments of the fold and thrust belt are completely detached on lower Cambrian salt and that basement-involved thrusting occurs only in the hinterland of the orogen. Mass balance constraints necessitate that detachment folds throughout the fold–thrust belt are cored by faults that branch from the basal detachment. The steep dips of these faults and their depth within the lower Paleozoic sedimentary rocks can account for recorded earthquakes. This suggests that the ∼11±4-km-deep earthquakes throughout the fold–thrust belt may be nucleating within sedimentary rocks rather than in the basement as previously proposed. Total shortening in the Zagros fold–thrust belt is 70±20 km, which corresponds to ∼20% shortening of the Arabian block.

The interplay of faulting and folding during the evolution of the Zagros deformation belt

Abstract In this short paper satellite images, aerial photographs and seismic sections are used to show that pure buckle folds, pure forced folds and folds intermediate between the two have all formed, and are still forming, in association with the compression tectonics currently occurring in the Zagros deformation belt which is situated along the northeastern margin of the Arabian plate. The type of folding and its distribution can be linked directly to the distribution of ancient basement faults and to the rheological profile of the cover sequence.

Origin of the Hafit structure: Implications for timing the Tertiary deformation in the Northern Oman Mountains

The Hafit structure belongs to a group of large-scale, post-obduction periclines that deform the Late Cretaceous-Tertiary sediments of the Oman Mountains Foredeep. Due to one-sided compression from the ENE, the Hafit structure grew as a detachment fold above a basal decollement which is probably located in the Upper Cretaceous Lower Fiqa shale. Its reversed vergence, back thrust and the folds superposed on its limbs, are explained as being caused by simple shear modification of the original structure. The interpretation that the Hafit structure and other foreland folds in the Northern Oman Mountains have resulted from a Zagros-related late Eocene-Miocene compressional event, is at variance with the stratigraphical and sedimentological evidence from the Hafit area. This evidence shows that the Hafit structure grew synchronously with sedimentation from just before the Middle Eocene, until the end of the Miocene, whilst the related structures to the east of it were initiated at the beginning of the Palaeocene. With further evidence of a similar early start to the Tertiary deformation emerging from other parts of the Northern Oman Mountains, it becomes necessary to push back the date of initiation of the Tertiary compressional event to the Palaeocene and to set it apart from the Plio-Pleistocene Zagros deformation.

Regional joint sets in the Arabian Platform as indicators of intraplate processes

The eastern part of the Saudi Arabian craton is a platform underlain by gently tilted and horizontal Mesozoic‐Cenozoic sedimentary rocks everywhere more than 250 km distant from the Zagros deformation front. Despite its tectonic simplicity, the platform contains several laterally extensive, but subdued, structures and lineaments belonging to more than one system of fault zones and flexures. New microtectonic observations of dominant sets of extension and conjugate hybrid joints permit regionally significant extension directions to be inferred and the origin of the macrostructures to be assessed. The central Arabian arch, the E‐W crest of which coincides with an axis of Mesozoic subsidence, developed during the Late Cretaceous to Eocene when the basin inverted as a consequence of the emplacement of ophiolite‐bearing nappes on the Zagros and Oman margins. During arching, there was strike‐parallel elongation of beds. At the same time as the arch amplified, the central Arabian graben system evolved as a result of the northward displacement of an East Arabian block or sheet that became detached across an arcuate separation zone, which in the south is coincident with the crest of the arch. An important Neogene response in the Arabian platform to NE‐SW shortening in the Zagros ranges was the formation of a swarm of NE striking master joints and lineaments. These structures reflect the diffuse and peripheral expansion of the foreland beyond the Zagros deformation front.

Basic Framework, Deformation, and Petroleum in Middle East: ABSTRACT

The most prolific oil-producing area of the Middle East lies on the southern flank or extension of the ancient Asiatic Tethyan geosyncline and was at least partly subjected to the same tectonic forces which produced the Alpine-Taurus-Himalayan Mountains. The importance of these tectonics is second only to depositional conditions in the Arabian-Persian Gulf geosyncline during which occurred the proper distribution of salt and anhydrites in time and place. To emphasize the importance of the evaporites, most if not all of the large gentle structures of Arabia and the southern gulf are salt-generated and the remaining structures in Iran and Iraq were created or modified by plastic deformation of salt and anhydrite. The Hith Anhydrite caps Jurassic oil in Arabia; the Fars Anhydrite caps Cretaceous-Miocene oil in Iran and Iraq. Late Tertiary deformation and fracturing of the Cretaceous Bangestan and Miocene Asmari Limestones created the long lines of story-book folds of the Zagros Mountains and oil reservoirs in Iran and Iraq. Fractured Bangestan oil reservoirs leaked much of their contents upward into fractured Asmari reservoirs where oil finally was trapped by the plastic Fars Formation. End_Page 469------------------------------ Zagros deformation not only determined the size and shape of reservoirs but also their permeability and porosity. The economic importance of this area is obvious. The Middle East, especially the Arabian-Persian Gulf and its periphery, is presently producing more than one third of the world's oil. Recent discoveries and extensions of known reservoirs have increased production to the point that the area produces more than North America. More than half of the world's proved reserves are in this area. Communist-block countries, in spite of tremendous potential, are contributing less than one-sixth of total world oil. Doubtless this is partly a result of the fact that these countries still have not attained the same wide use of petroleum products as the free world. Also, oil commerciality depends on the proper balance between a stable growing market and cheap transportation. The unique accessibility and availability of Middle East oil from this gulf guarantees its rising importance in world-energy needs. A plethora of oil is here and available; transportation is cheap. Extensive oil discoveries in new, inaccessible, or distant inland areas, even in the Middle East, world conditions permitting, should in no way threaten this source for many years to come. End_of_Article - Last_Page 470------------