Foreland propagation folding and structure of the mountain front flexure in the Pusht-e Kuh Arc (NW Zagros, Iran)

Abstract

The focus of this work is to determine the folding characteristics including geometry, interaction of the surface and subsurface geometries, kinematic evolution of the structures and timing of deformation in Pusht-e Kuh Arc in Lurestan Province of Zagros fold and thrust belt in Iran.<br/>Multidisciplinary methods have been used for different objectives in this thesis. Structural characteristic and fold geometry in two different units have been studied in the Zangul anticline. The Zangul anticline is a four closures anticline, with an open box folding shape and slightly verging to the SW. Its amplitude is of about 1.5 km and its half wavelength is ~4.5 km. The geometric construction of several cross-sections showing a regular position of the axial traces intersections indicates that the Triassic Dashtak evaporites may represent one of the major intermediate detachment levels in the sedimentary pile. The significant result is that the grouping of Asmari folds forms synforms (SE termination of the Zangul anticline) and antiforms (NW termination), which is so important for oil exploration.<br/>The Anaran anticline on top of the Mountain Front Flexure represents the most external fold of the Pusht-e Kuh Arc. This anticline is asymmetric with a long and gently dipping backlimb and a very steep forelimb. However, the most characteristic tectonic feature is the large amount of normal faults that cut the crestal and forelimb domains of the anticline. These normal faults, formed by layer-parallel extension during folding, limit a crestal graben and are not very deep. In addition to these normal faults, the potential tectonic decoupling across the intermediate Gachsaran detachment level and the lack of reflections imaging the Anaran anticline forelimb in newly acquired seismic lines preclude the understanding of the geometry of the fold at depth. We propose a geometric and evolution model for the Anaran anticline with the help of sand box models and growth strata ages. We also explore the potential effects of erosion and sedimentation coeval to folding in the development of the Anaran anticline. The characteristic geometry of the Anaran anticline is directly related to its singular position on top of the Mountain Front Flexure. However, the proposed model may be applied to other folds on top of this major basement-related thrust as for example the Siah Kuh and Khaviz anticlines in Pusht-e Kuh Arc and Dezful Embayment domains.<br/>The magnetostratigraphy dating technique applied to the syntectonic detrital sediments of the Agha Jari Formation in two locations across the Pusht-e Kuh Arc. The first location is the Afrineh syncline in the center part of the folded belt displaying field evidences of growth strata in the uppermost part of the Agha Jari Formation. The second section is located in front of the High Zagros Fault across the Chaman Goli syncline with the same stratigraphy as in Afrineh syncline. The magnetostrigraphy dating technique in this study provides with the ages of the Agha Jari units and permits to constrain the sequence of the folding associated to growth strata across this particular part of the Zagros fold belt. Both sections are showing good paleomagnetic results with sequence of normal and reverse polarity intervals. They show very good correlation to the Global Polarity Time Scale. The correlation to the GPTS shows the base of the growth in Afrineh syncline dated at about 11.8 Ma. The onlap geometry in Afrineh syncline is indication of deformation pulse associated to the folding. This phase of folding is about 5.4±0.5 Ma earlier than folding in the front of the Pusht-e Kuh Arc. The correlation of the magnetic polarity sequence to the GPTS shows age of ~13.9 Ma in Afrineh and ~17.2 Ma in Chaman Goli synclines for the base of the Agha Jari Formation. It shows that both Agha Jari and Bakhtyari prograde from hinterland to foreland in agreement to other foreland basin in the World. The age of the folding becomes younger towards the foreland and therefore implying a foreland ward sequence of deformation that started at about 20 Ma in hinterland, reached the frontal folds at about 7.6 Ma and continued to about 2.5-1.5 Ma.

<i>El Zagros representa la culminación de la colisión continental entre las placas tectónicas de Arabia y Euroasiática. Las montañas del Zagros se extienden por más de 2000 kilómetros desde Turquía hasta la parte suroccidental de Irán. Los movimientos orogénicos se iniciaron durante el Cretácico superior con el emplazamiento de unidades ofiolíticas sobre el margen de Arabia. A partir del Cretácico superior la continua convergencia entre las dos placas tectónicas produjo un acortamiento traducido en un sistema de cabalgamientos y pliegues que finalmente formó el plegamiento tal y como lo conocemos actualmente. Este plegamiento, sin embargo, ocurrió principalmente durante el Mioceno.ç<br/><br/>La parte más externa de las montañas del Zagros presentan unos afloramientos de gran calidad que permiten estudios muy detallados de la estructura y permiten recoger muestras para datar los sedimentos sintectónicos que permitirán determinar la edad de la deformación tectónica. La datación del sistema de pliegues es muy importante para la exploración de hidrocarburos en una región en la que existen importantes reservas de petróleo y gas.<br/><br/>Los objetivos de investigación de esta tesis doctoral son: 1) construir la geometría 3D de una región plegada del Zagros, en concreto del anticlinal de Zangul, en donde coexisten pliegues de gran amplitud y pliegues de menor amplitud al nivel de las calizas de la formación de Asmari; 2) determinar la geometría de la parte frontal del Zagros donde coexisten el anticlinal de Anaran y el frente del Pusht-e Kuh Arc levantado por encima del "Mountain Frontal Flexure" (MFF); 3) determinar la edad de las diferentes formaciones litológicas que rellenan los sinclinales del Pusht-e Kuh Arc y así determinar la edad de la deformación y la secuencia de esta deformación. La datación de estos sedimentos sintectónicos continentales se ha realizado por medio de técnicas magnetoestratigráficas. </I>