The Zagros fold-and-thrust belt in the Fars province (Iran): II. Thermal evolution

Accepted Manuscript The Zagros fold-and-thrust belt in the Fars province (Iran): II. Thermal evolution L. Aldega, S. Bigi, E. Carminati, F. Trippetta, S. Corrado, A.M. Kavoosi PII:

S0264-8172(18)30122-3

DOI:

10.1016/j.marpetgeo.2018.03.022

Reference:

JMPG 3287

To appear in:

Marine and Petroleum Geology

Received Date: 28 September 2017 Revised Date:

14 March 2018

Accepted Date: 15 March 2018

Please cite this article as: Aldega, L., Bigi, S., Carminati, E., Trippetta, F., Corrado, S., Kavoosi, A.M., The Zagros fold-and-thrust belt in the Fars province (Iran): II. Thermal evolution, Marine and Petroleum Geology (2018), doi: 10.1016/j.marpetgeo.2018.03.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Reconstruction of the eroded thickness by paleothermal indicators

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THE ZAGROS FOLD-AND-THRUST BELT IN THE FARS PROVINCE (IRAN): II.

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THERMAL EVOLUTION

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ALDEGA L.1*, BIGI S.1, CARMINATI E.1, TRIPPETTA F.1, CORRADO S.2 and KAVOOSI

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Dipartimento di Scienze della Terra, Sapienza Università di Roma, P.le Aldo Moro, 5, 00185 Roma

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Dipartimento di Scienze, Sezione di Geologia, Università degli Studi Roma Tre, L.go S. Leonardo Murialdo 1, 00146 Roma

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National Iranian Oil Company, Tehran, Iran

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*corresponding author e-mail: [email protected] tel. +39 06 49914547

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Abstract

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Temperature-dependent clay minerals and vitrinite reflectance data, surface and subsurface

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geological constraints were used to unravel the burial evolution of the Ordovician-Quaternary

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sedimentary successions from the inner to the outer zones of the Zagros fold-and-thrust belt in the

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Fars province (Iran). These sedimentary successions were buried to their thermal maxima during

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early to late diagenesis, achieving temperatures corresponding to the immature to early mature

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stages of hydrocarbon generation. They experienced low levels of thermal maturity in the Interior

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Fars, corresponding to vitrinite reflectance values between 0.38 and 0.66%, to mixed layers illite-

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smectite (I-S) with an illite content between 30 and 75% and to KI values between 0.97 and 1.18

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°∆2θ. In the Central and Coastal Fars, vitrinite reflectance ranges between 0.35 and 0.51%, the illite

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content in I-S displays values between 20 to 87% and KI data are between 0.71 and 1.30 °∆2θ. In

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individual anticlines, mixed layers I-S show an increase of the illite content as a function of

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ACCEPTED MANUSCRIPT stratigraphic age (depth), suggesting that levels of thermal maturity are controlled by sedimentary

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burial. One dimensional thermal history models allowed us: (i) to estimate the maximum burial

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experienced by the sedimentary successions and the amount of the sedimentary pile currently

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removed by erosion, (ii) to determine the thickness of the ophiolite units obducted during Late

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Cretaceous time in the High Zagros, and (iii) to define the onset of oil generation for the Albian

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source rocks throughout the Zagros belt. Paoleothermal data were used to constrain the geometry of

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eroded structures in a 253 km long cross-section extending from the High Zagros to the Coastal

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Fars. Along the cross-section, lithostatic load slightly decreases towards the foreland (e.g., from

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3.65 km to 3.2 km for the Bangestan Group) and the amount of the eroded material varies between

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~6 km (above anticlines in the Central and Interior Fars) and ~200 m (above synclines in the

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external part of the belt).

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KEYWORDS: mixed layers illite-smectite, vitrinite reflectance, thermal maturity, 1D thermal

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modelling, Fars province, Zagros fold-and-thrust belt

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1. Introduction

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Contractional deformation in thrust belts can either affect the sedimentary cover detached from the

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underlying basement in the frontal portion of the belt or may also involve basement rocks along

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crustal-scale ramps in the inner part of the belt, thus resulting in different mechanical behavior and

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in thin- or thick-skinned tectonics (e.g., Chapple, 1978; Coward, 1983; DeCelles and Mitra, 1995;

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Calabrò et al., 2003; Poblet and Lisle, 2011). Both tectonic styles may develop through time and

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superimpose to each other, implying large differences in the amounts of shortening in various

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portions of thrust belts. Generally, either surface (e.g., attitude of beds and tectonic structures,

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thickness of the stratigraphic pile) or subsurface (e.g., seismic lines, borehole data) geological and

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geophysical data are used to constrain the orogen structure. In this regard, several works

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investigating the structure of the Zagros in the Fars province (Iran) provided crustal scale cross

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ACCEPTED MANUSCRIPT sections using such constraints (e.g., McQuarrie, 2004; Molinaro et al., 2005; Mouthereau et al.,

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2007; Alavi, 2007) and concluded that the belt is characterized either by thin-, thick- or by a

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combination of thin- and thick-skinned tectonics.

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An open issue for the Zagros belt is that surface and subsurface data do not exclude the past

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occurrence of overthrusts now removed by erosion and provide partial information on the amount of

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burial experienced by the sedimentary succession during mountain building. Such constraints can

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be provided either by low temperature thermochronology (e.g., apatite fission track and U-Th/He

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dating) or from organic and inorganic thermal indicators, including vitrinite reflectance (Ro%) and

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illite content in mixed layer illite-smectite (I% in I-S).

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In particular, these latter paleothermal indicators allow quantifying the maximum burial thickness

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of the stratigraphic section and therefore provide an estimation as to the thickness of the

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stratigraphic section removed by erosion and/or tectonics (Botti et al., 2004; Aldega et al., 2011, Di

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Paolo et al., 2012; Meneghini et al., 2012; Carlini et al., 2013; Perri et al., 2016; Schito et al., 2017).

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In other cases, paleothermal indicators allow estimating the original thickness and extent of thrust

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units (Caricchi et al., 2015; Aldega et al., 2017). This is important for the frontal part of thrust belts,

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where tectonic burial is generally limited to a few kilometers and very few techniques may be

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applied to both sedimentary and crystalline rocks to detect such tectonic thickening (e.g., Corcoran

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and Dorè, 2005 for a review), and for the inner part of orogens where tectonic overburden may be

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totally removed by erosion (e.g., Ring et al., 1999; Doré et al., 2002).

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The integration of organic and inorganic thermal indicators with surface and subsurface geological

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data were used: (i) to constrain the burial evolution of the Ordovician-Quaternary sedimentary

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sequences exposed in the inner and outer zones of the Zagros fold-and-thrust belt in the Fars

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province, (ii) to define the onset of oil generation for the Albian source rocks throughout the Zagros

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belt, (iii) to determine the thickness of the ophiolite units obducted in Late Cretaceous time in the

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internal part of the belt. These new constraints were included in the 253 km long cross-section

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(described in Bigi et al., 2018) extending from the High Zagros to the Coastal Fars to provide

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information on the original thickness of the thrust belt and its eroded part.

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This

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thermochronological datasets, and can help to substantially reduce the number of acceptable

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geometric, thermal and kinematic models on the basis of the amount of maximum burial that

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characterizes different portions of the thrust wedge.

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2. Geological setting

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The Zagros fold-and-thrust belt is the result of the Cenozoic convergence and Alpine-type

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continental collision between the Central Iran domain and the Arabian plate (e.g., Mouthereau et al.,

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2012; Navabpour and Barrier, 2012). The Zagros fold-and-thrust belt is constituted by two main

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zones, the High Zagros Imbricated Zone to the north and the Zagros Simply Folded Belt to the

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south, bounded by the High Zagros fault (HZF in Fig. 1A; Mouthereau et al., 2007). The Zagros

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Simply Folded Belt is separated from the Dezful Embayment by the Mountain front fault (MFF;

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Sepehr and Cosgrove, 2004). In addition to these tectonic subdivisions, the Zagros belt can be

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divided into three main geological provinces bounded by major N-S trending basement faults

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(Bahroudi and Talbot, 2003), re-sheared as strike-slip faults during the orogenic phase (e.g.,

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Sherkati and Letouzey, 2004; Sepehr and Cosgrove, 2005; Lacombe et al., 2006; Ahmadhadi et al.,

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2007; Carminati et al., 2014). From north-west to south-east, they are the Lorestan, the Dezful

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Embayment and the Fars provinces (Fig.1A).

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Crustal scale cross-sections across the Zagros (e.g., McQuarrie, 2004; Sherkati et al., 2006;

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Mouthereau et al., 2007) and subsurface data from wells reveal an imbricate-fan geometry,

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constituted by an uppermost Neoproterozoic to Phanerozoic sedimentary succession up to 12 km

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thick (Fig. 2, Alavi, 2004). In detail, the Precambrian basement rocks are covered by the Hormuz

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series constituted by evaporites (halite and anhydrite), gray trilobite-bearing dolostones with

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interlayered mafic and felsic volcanic rocks in its upper part (Alavi, 2004). The tectono-sedimentary

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ACCEPTED MANUSCRIPT setting of the Ordovician-Carbonifeorus rocks is poorly constrained because of the scarcity of

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outcrops and the presence of major regional unconformities related to several Paleozoic erosional

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episodes (Alavi, 1994). In general, the Paleozoic succession is characterized by thickness and facies

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changes likely associated with early Cambrian salt diapirism, basement faulting and eustatic sea

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level changes (Berberian and King, 1981; Callot et al., 2007; Jahani et al., 2009).

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The Permian-Triassic succession marks a significant change in sedimentation from dominantly

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Paleozoic clastic sediments to carbonate rocks deposited in a shallow marine environment (Alavi,

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2004). This succession contains volcanic rocks associated with the Permo-Triassic rifting and the

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opening of the Neo-Tethys ocean. Following the rifting episode, sedimentation took place in a

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passive continental margin and different successions were deposited in the Lorestan and Dezful

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basins to the NW and the Fars basin to the SE. As an example, the Fars region was characterized by

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shallow water sedimentation until Late Cretaceous time, whereas coeval deep marine sediments

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were deposited in the Lorestan and Dezful Embayment areas (Berberian and King, 1981; Casciello

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et al., 2009).

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During Early Triassic time, the marine carbonate sedimentary regime persisted with the deposition

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of the Kangan Fm. in the Fars Province (Szabo and Kheradpir, 1978). Regressive conditions

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occurred in the Middle-Late Triassic, resulting in deposition of the evaporites of the Dashtak

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Formation (Berberian and King, 1981) and the dolomitic limestones of the Khaneh Kat Fm. (Pyriaei

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et al., 2010).

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The Early Jurassic was characterized by terrigenous clastic sedimentation and by transitional to

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open marine deposits of the Neyriz Fm. (Szabo and Kheradpir 1978), followed by limestones and

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marls of the Khami Group (Surmeh, Fahliyan, Gadvan, Daryan; Middle Jurassic-Aptian).

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In the Interior Fars, siltstones and iron/glauconite rich sandstones occurring in the upper parts of the

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Fahliyan and Dariyan Fms suggest marine regression, emergence, and erosion in Neocomian and

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late Aptian times (Setudehnia 1978; Navabpour et al., 2010). The shallow marine shales and

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ACCEPTED MANUSCRIPT limestones of the Bangestan Group (Kazhdumi and Sarvak Fms; Albian-Turonian) contain late

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Turonian conglomerates and breccias, suggesting tectonic-related uplift (Berberian and King 1981).

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The Late Cretaceous sedimentation in the Interior Fars was affected by the southwestward

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obduction of ophiolites in a subduction setting (e.g., Breton et al. 2004), which caused significant

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variations in sedimentary facies, sedimentation patterns and accommodation space, and determined

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the shift of depocentres (Pyriaei et al., 2010). During this time, sedimentation was characterized by

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neritic carbonates (Ilam Fm.) followed by deeper water conditions with the deposition of marls and

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shales of the Gurpi Fm. in most of the Zagros. Since Maastrichtian time, marly sedimentation of the

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Gurpi Fm. was coeval with rudist-dominated platform carbonate sedimentation (Tarbur Fm.;

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Setudehnia 1972; Vaziri-Moghaddam et al., 2005) in the Interior Fars. This lateral facies change has

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been related to the uplift associated with ophiolites emplacement (Pyriaei et al., 2010).

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During the Cenozoic, sedimentation changed from an open marine to a continental environment

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with increasing clastic input. The Tarbur limestones were overlain by the uppermost Maastrichtian

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to Paleogene evaporitic deposits of the Sachun Fm., consisting of gypsiferous limestones,

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dolostones and red marls deposited in a sabkha environment (Alavi, 2004). Heteropic to the Sachun

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evaporitic limestones are the neritic shales and marls of the Pabdeh Fm., deposited on top of the

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Gurpi basinal sediments in the Coastal Fars (Fig. 2).

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During the Eocene-Oligocene, shallow water limestones of the Jahrum and Asmari Formations

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were deposited in the Fars Province. Supratidal, sabkha-like conditions, developed locally in the

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Early Miocene, as the Razak Formation of the Interior Fars.

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The Middle Miocene clastic sedimentation marks the transition of the Fars province to a foreland

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basin setting (Khadivi et al., 2010) with the deposition of marls, shales and sandstones of the

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Mishan Fm., conglomerates, calcarenites and cross-bedded sandstones of the Agha Jari Fm. and

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molasse-type conglomerates (Bakhtiyari Fm., Fig. 2).

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Strontium isotope stratigraphy shows that marine foreland deposits of the Mishan Fm. are strongly

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diachronous with ages ranging between 17 Ma and 1.1 Ma becoming progressively younger from

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ACCEPTED MANUSCRIPT the Dezful Embayment to the Fars province (Pirouz et al., 2015). Magnetostratigraphic and

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chronostratigraphic analysis pointed out that the onset of continental clastic sedimentation was

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largely diachronous as well throughout the Zagros foreland basin. The deposition of the Bakhtiyari

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conglomerates began in the early Miocene in the High Zagros (Fakhari et al., 2008), after 14.8 Ma

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close to the High Zagros fault in the NE Fars (Khadivi et al., 2010), after 3.6-3.2 Ma in the Central

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Fars (Ruh et al., 2014) and an age of 3.0 Ma was proposed for the coastal areas (Homke et al.,

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2004).

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2.1 Stratigraphy: thickness variations

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Thickness variations of the main lithostratigraphic units can be inferred from borehole stratigraphic

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logs shown in figure 3. Along NE-SW and N-S-trending directions, main thickness changes occur

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in the Triassic Dashtak Fm, the Khami Group and the Asmari-Jahrum Fms.

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The pre-folding thickness of the Dashtak Fm. varies from 550 m in the Coastal Fars to 850 m in the

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Interior Fars indicating a northward thickening of the Triassic sediments (Figs. 3B and C). In the

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Sefid Zakhur-1 and Cham-e-noori-1 wells, located at the crest of anticlines, about 1,400 to 2,000 m-

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thick successions of shallow-marine limestones and dolomitic limestones interbedded with

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evaporites and local intercalations of shales belonging to the Dashtak Fm. have been intercepted.

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These large thickness variations (∆ = 600 m in Fig. 3B, ∆ = 1,300 m in Fig. 3C) are also reported in

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the isopach and facies distribution maps by Koop and Stoneley (1982) and Pyriaei et al. (2010) and

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are interpreted as the result of thrust faulting in the crestal domain of anticlines (Najafi et al., 2014).

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The Khami Group shows an increasing thickness toward the Interior Fars similar to that observed

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for the Dashtak Fm. In the Varavi-1 well, the Jurassic to Lower Cretaceous succession is 1050 m

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thick whereas it reaches 1324 m and 1446 m of thickness in the Aghar-2 and Sefid Zakhur-1 well

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respectively (Fig. 3).

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The Asmari-Jahrum Fm. displays a more complex thickness variability. Maximum values were

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measured in the Kuh-e-Sim anticline where a 700m thick succession is exposed (Carminati et al.,

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ACCEPTED MANUSCRIPT 2013) and in the Aghar-2 well where 652 m of Nummulites-bearing limestones and dolomitic

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limestones were intercepted. Twenty-five kilometers to the south, the thickness of the Asmari-

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Jahrum Fm. reduces to 274 m in the Sefid-Zakhur-1 well and to about 150 m in the Safid Baghun-1

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well. In the Coastal Fars, thicknesses ranging from 250 to 350 m were calculated from Hangam

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(sheet 20861E), Dehram (20861W) and Kangan (20867W) geological maps (scale 1:100.000).

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No major thickness changes occur in the coastal areas along a WNW-WSE trending direction

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parallel to the strike of anticlines for the Triassic-Upper Cretaceous succession (Fig. 3D)

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3. Methods

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3.1 X-ray diffraction of clay minerals

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Clay minerals contained in sedimentary rocks are heterogeneous assemblages of detrital material

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coming from various source rocks, and, at paleotemperatures >70°C (Środoń, 1999), of

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superimposed diagenetic modifications of this sediment. Clay minerals undergo diagenetic and very

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low-grade metamorphic reactions when sedimentary basins subside in response to burial and/or

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tectonic loading. Reactions in clay minerals are irreversible under normal diagenetic and anchizonal

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conditions, so that uplifted and exhumed sequences generally retain indices and fabrics indicative of

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thermal maturity and maximum burial (Árkai, 2002). Clay minerals are mainly sensitive to

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temperature, and the use of mixed layers illite-smectite (I-S) and the transformation sequence

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dismectite-random-ordered mixed layers I-S (R0)-ordered mixed layers I-S (R1 and R3)-illite-di-

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octahedral K-mica (muscovite) as indicator of maximum paleotemperature condition is generally

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accepted (Burst, 1959; Hower et al., 1976; Pollastro 1990; Aldega et al., 2007a, b, Corrado et al.,

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2010 a, b; Izquierdo-Llavall et al., 2013, Schito et al., 2016). In fact, changes in the composition of

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mixed layering, layer expandability, and I-S ordering are empirically related to temperature changes

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due to burial (Hoffman and Hower, 1979; Pollastro and Barker, 1986; Botti et al., 2004).

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Another parameter successfully applied worldwide for determining the grade of diagenesis and

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very-low metamorphism of clay-rich and clastic sedimentary rocks is the Kübler index (Kübler,

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ACCEPTED MANUSCRIPT 1967; KI). It has been commonly used as an empirical measure of the changes in sharpness of the

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X-ray 10 Å basal reflection of illite-dioctahedral K-white mica. The 10Å peak width at half-peak-

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height is commonly considered to be primarily a function of the average illite crystallite thickness

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normal to (001) and several authors have shown how KI values decrease as metamorphic grade

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increases (e.g., Jaboyedoff et al., 2001; Warr and Cox, 2016; Potel et al., 2016).

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X-ray diffraction (XRD) analysis of the <2 µm grain size fraction has been carried out on 38 surface

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samples with a Scintag X1 X-ray system (CuKα radiation) at 40 kV and 45 mA. Oriented air-dried

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and ethylene-glycol solvated samples were scanned from 1 to 48 °2θ and from 1 to 30 °2θ

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respectively with a step size of 0.05 °2θ and a count time of 4 s per step. The illite content in mixed

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layers I-S was determined according to Moore and Reynolds (1997) using the delta two-theta

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method after decomposing the composite peaks between 9-10 °2θ and 16-17 °2θ. The I–S ordering

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type (Reichweite parameter, R; Jagodzinski 1949) was determined by the position of the I001-S001

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reflection between 5 and 8.5 °2θ (Moore and Reynolds 1997).

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The KI determinations, measured on oriented air-dried (AD) and ethylene-glycol solvated (EG) <2

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µm mounts were made by first subtracting the background from the raw data, followed by peak

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fitting using Pearson 7 functions. The FWHM (full-width-half-maximum) of the deconvoluted

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~10Å peak was used as expression of illite “crystallinity”. Half-peak widths were converted to the

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Crystallinity Index Standard (CIS) scale (Warr and Rice, 1994).

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Peaks in relative close position were selected for clay mineral quantitative analysis in order to

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minimize the angle-dependent intensity effect. Composite peaks were decomposed using Pearson

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VII functions and the WINXRD Scintag associated program. Integrated peak areas were

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transformed into mineral concentration by using mineral intensity factors as a calibration constant

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(for a review, see Moore and Reynolds 1997).

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3.2 Organic matter optical analysis

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ACCEPTED MANUSCRIPT Vitrinite is derived from the thermal degradation of organic macerals of continental origin that are

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present in post-Silurian sediments (Stach et al., 1982). Its reflectance strictly depends on the thermal

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evolution of the host sediments and is correlated with the stages of hydrocarbon generation and

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other thermal parameters in sedimentary environments (Durand, 1980). Thus, it is one of the most

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widely used parameter to calibrate basin modeling and provides consistent and reliable constraints

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on maximum burial depths (Dow, 1977; Mukhopadhyay, 1994; Corrado et al., 2009; 2010a).

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Specimens for vitrinite reflectance were prepared according to standardized procedures described in

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Bustin et al. (1990). Picked kerogen particles were cold set into epoxy resin blocks and polished

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using carborundum papers and isopropanol as lubricant. After washing the sample in order to

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remove debris, three alumina powders of decreasing grain size (1, 0.3, 0.01 µm) were used to polish

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the samples. Random reflectance was measured under oil immersion (ne 1.518, at 23ºC), with a

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Zeiss Axioskop 40 A pol microscope-photometer system and calibrated against standards of

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certified reflectance. On each sample, measurements were performed on vitrinite or bitumen

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unaltered fragments. Mean vitrinite (Ro%) and bitumen (Rb%) reflectance values were calculated

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from the arithmetic mean of these measurements. Rb values have been converted into vitrinite

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reflectance equilvalent values (Roeq%) according to Jacob and Hiltmann (1985).

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4. Results

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4.1 Clay mineralogy

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High Zagros

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X-ray diffraction analyses refer to the Lower Cretaceous Kazdhumi and Gadvan Fms (Tab.1). The

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Kazdhumi Fm. (KAZ2 and KAZ3) in the Arsenjan area shows a mineralogical assemblage

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composed of kaolinite (72% mean value), illite (15%), mixed layers I-S (7%) and chlorite (6%).

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The underlying Gadvan Fm. (GAD1) consists of an illite- and kaolinite-rich assemblage (48% and

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38% respectively) with subordinate amounts of mixed layers I-S (14%). Traces of calcite and

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gypsum have been observed in the XRD pattern.

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Both the Kazdhumi and the Gadvan Fms display short range-ordered (R1) mixed layer I-S with an

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illite content of about 75% and Kübler index data ranging from 0.92 and 1.01 °∆2θ which

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corresponds to the first stages of the late diagenetic zone (Fig. 4A and Tab. 1; Merriman and Frey,

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1999).

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Zagros Simply Folded belt: Interior Fars

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A suite of 10 samples, belonging to the Upper Cretaceous-Miocene portion of the sedimentary

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succession, has been collected in the Sarvestan area (Tab.1). Seven samples are from the

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hangingwall of the Sarvestan fault (Fig. 1B) whereas three samples (MIS9, RAZ2, RAZ1) have

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been collected some 40 km SW of the fault.

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In the hangingwall units, we observe an increase of the illite content in mixed layer I-S and a

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decrease of Kübler index values as a function of stratigraphic age (depth; Fig. 4A). Random ordered

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I-S (R0) with high expandability (30-45% of illitic layers), that characterize the Miocene deposits,

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converts into short range ordered structures (R1) with an illite content of 60-70% in the Upper

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Cretaceous Gurpi Fm. Kübler index measurements display values ranging from 0.97 to 1.18 °∆2θ

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consistent with early to late diagenetic conditions (Tab. 1). Forty kilometers SW of the Sarvestan

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fault, the Razak Fm. shows random ordered I-S with low illite content (30%) indicating early

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diagenetic conditions (Fig. 5A).

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Zagros Simply Folded belt: Central Fars

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Clay mineralogy data for the Central Fars are from the Kuh-e-Sim (Aldega et al., 2014), Kuh-e-

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Meymand and Kuh-e-Surmeh anticlines (Tab.1; Fig. 3A for anticlines location). In the surroundings

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of the Kuh-e-Sim anticline, Oligocene-Miocene lithologies of the Mishan, Guri and Champeh Fms

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and the Mol member of the Gachsaran Fm. display random-ordered (R0) mixed layers I-S with an

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illite content between 21% and 52% (Fig. 4A) indicating early diagenetic conditions according to

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Merriman and Frey (1999). 11

ACCEPTED MANUSCRIPT The Mishan Fm. shows an illite- and chlorite-rich composition with subordinate amounts of

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palygorskite and mixed layer I-S. Red shales from the Mol Member are mainly characterized by

286

phyllosilicates (illite, chlorite and mixed layer chlorite-smectite), carbonate group minerals (calcite

287

and ankerite), quartz and albite. In the Guri Fm., illite and chlorite are the most abundant clay

288

minerals (85%) and prevail on mixed layer I-S (15%). Marls from the Champeh Fm. show an illite-

289

rich composition (40-51%) followed by chlorite (24-34%) and low contents of random ordered

290

mixed layer I-S (3-5%). Palygorskite, kaolinite and mixed layer chlorite-smectite (C-S) were

291

occasionally observed.

292

Four samples from the Pabdeh, Champeh and Mishan Fms and the Mol member (Tab. 1) were

293

collected in the surroundings of the Kuh-e-Meymand anticline. The Mishan Fm. is composed of

294

illite (35%), chlorite (28%) and mixed layers I-S (11%) and C-S (26%). The underlying Mol

295

member (MOL2) is characterized by an illite-rich composition (63%) and subordinate amounts of

296

chlorite (18%) and mixed layers C-S (19%) with a chlorite content of 55%. The mineralogical

297

assemblages of the Champeh Member is mainly constituted by abundant illite and mixed layer I-S

298

and subordinate amounts of chlorite, palygorskite and kaolinite (Tab. 1). The greenish silt-rich

299

marls of the upper part of the Pabdeh Fm. are composed of illite (50%), short range-ordered I-S

300

(26%), chlorite (17%) and kaolinite (7%).

301

Mixed layers I-S in the Paleocene to Miocene succession of the Kuh-e-Meymand anticline are

302

either random ordered (R0) or short range ordered (R1) structures with an illite content between

303

50% and 70% indicating early to late diagenetic conditions (Tab. 1). Kübler index measurements

304

show values between 0.95 and 1.11 °∆2θ (Tab. 1), which are consistent with levels of thermal

305

maturity observed by mixed layers I-S.

306

In the Kuh-e-Surmeh anticline, we collected five samples from the Mishan Fm. down to the Silurian

307

shales of the Ghakum Fm. Mixed layered minerals indicate an increase of the illite content in mixed

308

layer I-S as function of the stratigraphic age (depth). High expandable random ordered I-S (R0)

309

typical of the Mishan Fm. progressively evolves to structures with higher amount of illite layers (up

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ACCEPTED MANUSCRIPT to 50-55%) in the Pabdeh and Gurpi Fms (PAB6, GUR7; Fig. 5B), to short-range ordered R1 I-S in

311

the Gadvan Fm. (GAD2) and to long-range ordered R3 I-S in the Ghakum Fm. (GAH1). The illite

312

content in mixed layer I-S at the bottom of the succession is about 85% indicating late diagenetic

313

conditions (Fig. 4A and Tab. 1; Merriman and Frey, 1999). KI data decrease as level of thermal

314

maturity increases with values ranging from 0.71 to 1.08 °∆2θ (Tab. 1).

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315

Zagros Simply Folded belt: Coastal Fars

317

Most samples were collected in the surroundings of the Kuh-e-Asalujeh anticline (Fig. 3A for

318

anticline location) spanning from the Kazdhumi to the Mishan Fms (Tab.1).

319

The Kazdhumi Fm. is composed of a kaolinite-rich assemblage that constitutes at least the 50% of

320

the overall composition, and subordinate amounts of illite, mixed layers I-S and chlorite (Fig. 5C).

321

Mineralogy of the <2µm grain-size fraction of the overlying Gurpi Fm. shows illite (38%) and

322

mixed layer I-S (62%). Moving up in the stratigraphic column, younger deposits display the

323

occurrence of palygorskite, which represents the main component of the clay fraction (82% mean

324

value) in the Pabdeh Fm. The remaining 18% is made up of illite and mixed layers I-S.

325

The Mishan Fm. is constituted by palygorskite (25-54%), chlorite (14-22%), illite (11-19%) and

326

mixed layers I-S (5%). Smectite occurs in the sediments cropping out in the north-western part of

327

the Kuh-e-Asalujeh anticline with amounts of 50%.

328

Random ordered mixed layers I-S or discrete smectite characterize the Miocene deposits indicating

329

low levels of thermal alteration which correspond to the early diagenetic zone. KI values between

330

0.52 and 0.60 °∆2θ point to higher levels of thermal maturity. We interpreted these values as the

331

signature of detrital K-micas produced from uplift and erosion of the Zagros belt. Thus, KI data for

332

Miocene deposits cannot be used for the reconstruction of the burial history of the Kuh-e-Asalujeh

333

anticline but provide information on provenance and thermal condition of the source rock.

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The Cretaceous samples of the Kazdhumi Fm. at the core of the anticline display short-range

335

ordered mixed layers I-S with an illite content ranging from 70 to 75% and KI values of 0.95-1.01

336

°∆2θ which indicate the beginning of the late diagenetic zone (Fig. 4A).

337

4.2 Organic matter optical analysis

339

A suite of 22 samples was collected from the Cretaceous to Miocene deposits throughout the Fars

340

Province. Optical analysis provided results for 10 samples as the organic matter content was very

341

low as indicated by the scarce presence of bitumen and vitrinite-like fragments (Tab.2). Samples

342

from the Arsenjan area in the High Zagros are devoid of vitrinite-like fragments as are those

343

collected in the coastal areas close to the Kuh-e-Asalujeh anticline. In the Interior Fars, rare

344

fragments of reworked vitrinite-huminite and inertinite macerals and stains of bitumen are observed

345

in the Gurpi (GUR3, GUR4, GUR5; Tab.2) and in the Mishan Fms (MIS9). Vitrinite macerals

346

display mean vitrinite reflectance values between 0.38-0.49% indicating immature to early mature

347

stages of hydrocarbon generation consistently with Roeq% values between 0.55 and 0.66% of

348

bitumen.

349

In the Central Fars, the Champeh Fm. and the Mol member are very poor in organic matter content

350

and vitrinite-huminite macerals are scarce. Very few collinite fragments of small size display a

351

mean reflectance value of 0.39% (Tab. 2) indicating the immature stage of hydrocarbon generation.

352

The Mishan Fm. (MIS6, MIS7, MIS10) contains both inertinite and vitrinite-huminite macerals

353

associated with finely dispersed pyrite. Mean vitrinite reflectance values (between 0.32% and

354

0.50%) indicate the immature/early mature stage of hydrocarbon generation.

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355 356

4.3 Synthesis of paleothermal data

357

Organic and inorganic paleothermal indicators indicate that the Late Cretaceous to Miocene

358

sedimentary succession experienced similar levels of thermal maturity in early-late diagenetic

359

conditions independently from their position in the fold-and-thrust belt (Fig. 4 A and C). Focusing 14

ACCEPTED MANUSCRIPT on Upper Cretaceous-Eocene source rocks, a slight decrease of levels of thermal maturity toward

361

the coastal areas is observed (Fig. 4B). Miocene deposits generally show immature stages of

362

hydrocarbon generation suggesting shallow burial depths (Fig. 4C). In each anticline structure,

363

mixed layers I-S show a general increase of the illite content as function of the stratigraphic age

364

(Fig. 4A).

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365

5. Thermal modeling

367

Simplified reconstructions of the burial and thermal history of the Lower Cretaceuos succession in

368

the Arsenjan area and of the Ordovician to Quaternary sedimentary successions in the Kuh-e-

369

Surmeh and Kuh-e-Asalujeh anticlines were constrained by inorganic thermal indicators and

370

performed using the software package Basin Mod® 1-D (Figs. 6, 7 and 8). The main assumptions

371

for modeling are that: (1) rock decompaction factors apply only to clastic deposits, according to

372

Sclater and Christie’s method (1980); (2) seawater depth variations in time are assumed as not

373

relevant, because thermal evolution is mainly affected by sediment thickness rather than by water

374

depth (Butler, 1992); (3) thermal modeling is performed using LLNL Easy %Ro method based on

375

Burnham and Sweeney (1989) and Sweeney and Burnham (1990); and (4) geothermal gradients

376

between 15 and 24°C/km were tested on the basis of thermochronometric studies in the High

377

Zagros (Gavillot et al., 2010), results from tectonic modelling (Mouthereau et al., 2006),

378

microthermometry of fluid inclusions (Ceriani et al., 2011) and measurements from deep wells for

379

the Fars area (Bordenave, 2008).

380

Thicknesses have been calculated from geological maps and/or from subsurface stratigraphy. Age

381

constraints for Miocene to Quaternary clastic deposits are from Pirouz et al. (2015), Khadivi et al.

382

(2010), Ruh et al. (2014), and Homke et al. (2004). Mixed layers I-S from outcrop samples were

383

projected to a pseudo/paleo-depth based on stratigraphic position and measured thicknesses of the

384

units. Then mixed layers I-S, representative of individual samples, were converted into vitrinite

385

reflectance-equivalent values by the correlation of vitrinite reflectance and expandability (reverse of

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illite content in I-S) data based on the kinetic model of vitrinite maturation of Burnham and

387

Sweeney (1989) and the kinetics of the I-S reaction determined by Hillier et al. (1995).

388

5.1 High Zagros

390

Remnants of thrust sheets of ophiolitic mantle and crustal rocks and of their sedimentary cover are

391

preserved in the Neyriz area (Fig. 1B). In the Arsenjan area, ophiolite units occur only as

392

radiolarite-rich covers and clasts of pillow to massive lavas are observed in the Gurpi Fm. (Haines

393

and Reynolds, 1980). From these geological evidences, in order to determine the thickness of

394

ophiolite units in the inner sector of the Zagros fold and thrust belt, we propose a burial and thermal

395

model for the Arsenjan area that includes overthusting of ophiolite bodies during the Late

396

Cretaceous and their partial erosion (Figs. 6A and B).

397

The reconstructed evolution depicted in figure 6A begins in the Jurassic with sedimentation of 900

398

m thick continental-shelf deposits of the Khami Group followed by 560 m of shallow marine shales

399

and limestones of the Bangestan Group during the Cretaceous. The Late Cretaceous sedimentation

400

was followed by the emplacement of 1,100 m thick ophiolite units that overthrust atop the

401

Bangestan Group during the latest Turonian. This thrusting event exposed the ophiolite units to

402

erosion, removing about 200 m of rocks during the Maastrichtian and leading to the accumulation

403

of ophiolite clasts in the Upper Cretaceous-Paleocene conglomerates and in the Gurpi Fm. Erosion

404

has been set in the Maastrichtian as rudist-dominated platform carbonate sedimentation of the

405

Tarbur Fm caps the ophiolite body in the Neyriz area (James and Wind, 1965; Moghadam and

406

Stern, 2015).

407

During the Paleocene, ~300 m-thick red shale and evaporites of the Sachun Fm. were deposited in

408

the Arsenjan area. They graded up with dolostones of the Jahrum Fm. in the Eocene and evolved to

409

shallow marine Nummulites-bearing limestones of the Asmari Fm documenting persistent sabkha-

410

like conditions until Oligocene time.

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ACCEPTED MANUSCRIPT From the Early Miocene onwards, a large amount of marls and siliciclastic rocks (Razak, Agha Jari,

412

and Bakthiyari Fms; Khadivi et al., 2010) buried the sedimentary succession at depths of ~5.1 km.

413

Maximum burial took place in the Late Miocene when both the Bangenstan and the Khami Groups

414

(in particular the Kazdhumi and the Gadvan Fms) experienced early mature stages of hydrocarbon

415

generation (Fig. 6B). The onset of hydrocarbon generation occurred in the Early Eocene for the

416

Gadvan Fm. and during the middle Miocene for the Kazdhumi Fm.

417

Since the Late Miocene, a 3650 m-thick overburden composed of Late Cretaceous to Miocene

418

sedimentary cover and ophiolite units began to be eroded and the outcropping succession exhumed.

419

The burial history has been reconstructed by a constant geothermal gradient of 20°C/km which

420

provide the best fit matching paleothermal data (Fig. 6C) and is consistent with surface and

421

subsurface thickness of the stratigraphic section and (U‐Th)/He thermochronometric data by

422

Gavillot et al. (2010).

423

A sensitivity analysis was also performed assuming different geothermal gradients (between 15 and

424

24°C/km) derived from thermochronological data from the High Zagros (Gavillot et al., 2010),

425

results from tectonic modelling (Mouthereau et al., 2006), microthermometry of fluid inclusions

426

(Ceriani et al., 2011) and measurements from deep wells for the Fars area (Bordenave, 2008). The

427

overburden calculated atop the Bangestan Group varies from 3,000 m to 5,250 m as a function of

428

decreasing geothermal gradient (Tab. 3). A 5250 m-thick overburden (case A in Tab.3) is not

429

geologically valid as it implies the deposition of more than 3 km-thick siliciclastic deposits eroded

430

since the Early Pliocene. An overburden of 3000 m (case C) is stratigraphically acceptable but the

431

thermal maturity curve calculated by a geothermal gradient of 24°C/km overestimates levels of

432

thermal maturity of the Bangestan Group. Simulating ophiolite thrust sheets thicker than 1,100 m

433

during the Late Cretaceous overestimates I-S derived vitrinite reflectance equivalent values, leading

434

to a present-day thermal maturity curve that does not match paleothermal constraints.

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5.2 Central Fars 17

ACCEPTED MANUSCRIPT The burial history of the Kuh-e-Surmeh anticline in the Central Fars begins in the Late Ordovician

438

with the deposition of the ca. 400 m-thick Gahkum Fm. (Fig. 7A). Hiatuses and erosional episodes

439

influenced Silurian to Carboniferous sedimentation in several areas of the Zagros and Central Fars

440

as a consequence of sea level changes (Alavi, 1994). During Permian and Triassic times,

441

sedimentary facies in the Central Fars changed from clastic to carbonate platform sediments with

442

the deposition of about 1,300 m of oolitic shallow-water carbonates interbedded with evaporites

443

belonging to the Dehram Group and 300 m of dolomitic limestones and evaporites of the Dashtak

444

Fm. In the Early Jurassic, about 200 m of thin-bedded dolostones and shales of the Neyriz Fm. were

445

followed by a 500 m-thick succession of Jurassic-Lower Cretaceous continental shelf deposits

446

belonging to the Khami Group. Anoxic conditions prevailed until the Early Cretaceous leading to

447

the deposition of dark bituminous shales (Kazhdumi Fm.).

448

Late Cretaceous sedimentation was characterized by neritic conditions with the deposition of the

449

youngest formations of the Bangenstan Group (Ilam-Sarvak Fms) followed by deeper water

450

conditions with marls and shales of the Gurpi and Pabdeh Fms (about 900 m).

451

During Oligocene time, the marls and shales of the Pabdeh Fm. graded up and interfingered the

452

shallow marine limestones of the Asmari Fm. From Early Miocene onwards, sedimentation rates

453

increased and a large amount of marls and siliciclastic rocks (Gachsaran, Mishan and Agha Jari

454

Fms) buried the sedimentary succession of the Kuh-e-Surmeh anticline at depths of ~6.3 km in the

455

early Pleistocene. At that time, maximum levels of thermal maturity were experienced and recorded

456

by mixed layer I-S (Fig. 7C). Upper Cretaceous-Miocene rocks were thermally immature whereas

457

the underlying deposits of the Khami Group experienced early mature stages of hydrocarbon

458

generation at depths of 3.5 km (Fig. 7B). The Gahkum Fm. experienced maximum temperatures of

459

145 °C in mid-mature stage of hydrocarbon generation at time of maximum burial while it entered

460

the early mature stage of hydrocarbon generation during the Late Cretaceous (at about 80 Ma) prior

461

to Zagros folding.

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ACCEPTED MANUSCRIPT This burial reconstruction, modelled assuming a geothermal gradient of 20°C/km (case B in tab. 3),

463

accounts for the erosion of 700 m-thick Agha Jari Fm. since the early Pleistocene and indicates that

464

sedimentary load was the main factor affecting levels of thermal maturity. Furthermore, thermal

465

modelling of figure 7A is consistent with magnetostratigraphic data for growth strata for the Kuh-e-

466

Ghol Ghol anticline (about 40 km to the SE of the Kuh-e-Surmeh anticline) where initiation of

467

deformation took place in the Late Pliocene and continued until the early Pleistocene (Ruh et al.,

468

2014).

469

Other attempts to simulate the burial evolution of the Kuh-e-Surmeh anticline lead to a less accurate

470

thermal maturity curve implying: i) the accumulation of large amounts of siliciclastic deposits (>2

471

km thick) from Miocene to early Pleistocene (case A in tab. 3), or ii) the absence of erosion during

472

the Quaternary (case C in tab. 3). Both hypothesis do not satisfy geological data such as the present-

473

day thickness of the Agha Jari Fm. in adjacent areas (~1000 m thick) and the exhumation of the

474

Kuh-e-Surmeh anticline that is very unlikely to have occurred without erosion.

475

The most accurate reconstruction indicates a maximum overburden of 3,200 m atop the Bangestan

476

Group.

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5.3 Coastal Fars

479

Similar burial and thermal conditions can be reconstructed for the Cretaceous to Quaternary

480

sedimentary succession cropping out in the Kuh-e-Asalujeh anticline in Coastal Fars.

481

Figures 8A and 8B display that the base of the Bangestan Group and the top of the Khami Group

482

experienced burial depths in the order of 3.2 km in early mature stage of hydrocarbon generation

483

during the early Pleistocene when large supply of clastic material lead to the deposition of about

484

1,000 m of the Bakthiyari Fm. Upper Cretaceous to lower Pleistocene deposits overlying the

485

Bangestan Group were thermally immature as indicated by the present-day maturity curve

486

constrained by inorganic thermal indicators (Fig. 8C). Erosion in Quaternary times removed about

487

200 m of the sedimentary succession. The type of evolution outlined here allows the best calibration

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against measured data, suggesting that levels of thermal maturity are controlled by sedimentary

489

burial. Also for this area, burial and thermal models built by assuming geothermal gradients of 15

490

and 24°C/km do not satisfy geological and paleothermal data (Tab. 3; Fig. 8C).

491

6. Discussion

493

6.1 Maximun ophiolite thickness in the Arsenjan area

494

Short range ordered I-S for the Kazdhumi and Gadvan Fms in the Arsenjan area and thermal model

495

of figure 6A allow us to conclude that a 3,650 m thick overburden atop the Kazdhumi Fm. was the

496

main factor responsible for the observed thermal maturity values.

497

In the Arsenjan area, ophiolite thrust sheets mainly consist of radiolarites and lack of oceanic

498

mantle and crustal rocks, whereas the present-day Neyriz ophiolite crustal sequence is represented

499

by harzburgites and by a ~700–900 m thick, highly fragmented sheeted dike complex with pillow to

500

massive lavas associated with radiolarites and Upper Cretaceous pelagic limestones. The occurrence

501

of different portions of ophiolite units (sedimentary vs. magmatic rocks) in the two areas suggests

502

that the Arsenjan ophiolites were close to the obduction front (to the E-SE of Arsenjan no more

503

ophiolites crop out), whereas the Neyriz ophiolites were in a more internal position.

504

Short-range ordered I-S measured in the Arsenjan area allow us to constrain the original thickness

505

of the ophiolite sheet close to its front. In fact, inorganic paleothermal indicators display levels of

506

thermal maturity consistent with a 1,100 m thick ophiolite thrust sheet that was partially eroded

507

during the Maastrichtian (Fig. 6A). Erosion has been estimated in 200 m, explaining the

508

accumulation of ophiolite clasts in the Upper Cretaceous-Paleocene conglomerates and within the

509

Gurpi Fm. Simulating the emplacement of thicker ophiolite bodies during the Late Cretaceous or

510

higher amounts of erosion, leads to an overestimation of the I-S derived vitrinite reflectance

511

equivalent values and related levels of thermal maturity. This result allows us to exclude the

512

obduction of large and thick slices of ophiolite units in the Interior Fars and to suggest that the

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extent of the Maastrichtian erosional phase, was negligible in comparison to the removal of 3,500

514

m-thick ophiolite units in Oman (Aldega et al., 2017).

515

6.2 Maximum burial in the Fars province

517

Inorganic and organic thermal indicators show that the sedimentary cover experienced similar levels

518

of thermal maturity from the internal to the external zones, consistent with early-late diagenetic

519

conditions. Only a slight reduction of the overburden thickness can be observed from hinterland to

520

foreland. In the most internal area, where the Bangestan Group (Kazdhumi Fm.) crops out, a

521

currently eroded overburden of 3.65 km has been calculated (case B in tab. 3).

522

In the Sarvestan area, Carminati et al. (2016) pointed out similar burial depths for the Bangestan

523

Group through the interpretation of seismic lines and 1D thermal modelling. In particular, the

524

Sarvak Fm. was buried to depths of 3.6 km by a large amount of marls and siliciclastic rocks in

525

Early Pliocene times.

526

In the Central and Coastal Fars, organic and inorganic indicators from the Paleocene to Miocene

527

deposits display low levels of thermal maturity in early diagenetic conditions and slightly lower

528

burial depths. In fact, in the Kuh-e-Surmeh and Kuh-e-Asalujeh anticlines, 1D thermal models

529

evidenced a lithostatic load of about 3.2 km for the top of the Bangestan Group occurring in early

530

Pleistocene times. The decrease of sedimentary load and of the associated levels of thermal maturity

531

towards the foreland is consistent with the findings of recent papers regarding source rocks

532

evaluation in the Persian Gulf. Mashhadi et al. (2015) pointed out that the Kazdhumi Fm. and the

533

overlying Gurpi and Pabdeh Fms are immature in the Central Persian Gulf with vitrinite reflectance

534

values <0.5% suggesting lower burial depths than those recorded in the Coastal Fars.

535

Sfidari et al. (2016), on the basis of vitrinite reflectance measurements and Rock-Eval pyrolysis

536

data, reconstructed the thickness of the sedimentary pile from the Kazdhumi Fm. to the Mishan Fm.

537

prior to the onset of folding in the Zagros Simply Folded Belt indicating a decreasing thickness

538

from the Interior Fars towards the Persian Gulf. These thicknesses (3000-3500 for the Interior Fars,

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ACCEPTED MANUSCRIPT 539

1500-2000 for the Coastal Fars and <1000 m for the Central Persian Gulf) are lower than those

540

calculated by our thermal models because the thickness of syn-orogenic deposits (Agha Jari and

541

Bakhtiyari Fms) eroded during the uplift of the anticlines is not considered.

542

6.3 Timing of thermal maturation

544

Even if the Bangestan Group (in particular the Kazdhumi Fm.) experienced similar burial depths

545

(3.65 km in the Arsenjan area, 3.2 km in the Central and Coastal Fars) and levels of thermal

546

maturity (%I in I-S between 68 and 75%, Fig. 4A and Tab.1) in different sectors of the Zagros fold-

547

and-thrust belt, 1D thermal models point out that the onset of hydrocarbon generation was largely

548

diachronous throughout the Zagros foreland basin depending on timing of deformation and the age

549

of synorogenic deposits.

550

In particular, the onset of hydrocarbon generation for the Kazdhumi Fm. occurred in middle

551

Miocene times at about 13 Ma in the Arsenjan area (Fig. 6B) and at 11 Ma in the Interior Fars

552

(Carminati et al., 2016). In the Kuh-e-Sim anticline, hydrocarbon generation occurred after the early

553

Pliocene (Aldega et al., 2014) rejuvenating towards the external part of the belt. In the Kuh-e-

554

Surmeh anticline, the Kazdhumi Fm. enter the early mature stages of hydrocarbon generation in the

555

early Pleistocene (Fig. 7B) and in the late Pleistocene in the Coastal Fars (Fig. 8B).

556

This is a consequence of foreland propagating thrusting inducing progressive flexural subsidence

557

from the High Zagros to the Cosatal Fars with sedimentation of the Bakthiyari conglomerates (14.8

558

Ma close to the High Zagros fault in NE Fars; Khadivi et al., 2010; 3.6-3.2 Ma in the Central Fars,

559

Ruh et al., 2014; 3.0 Ma for the Coastal Fars, Homke et al., 2004).

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560 561

6.4 Geological and paleothermal constraints for reconstructing the geometry of eroded portion of

562

the Zagros fold-and-thrust belt

563

22

ACCEPTED MANUSCRIPT Surface (geological maps and stratigraphic sections) and subsurface (borehole) stratigraphic data

565

were used to draw a cross section across the Zagros fold-and-thrust belt using 3DMove 2014

566

software by Midland Valley (http://www.mve.com/software/move) that is extensively described in

567

Bigi et al. (2018).

568

The surface geological data were extracted from a series of 1:100.000 sheets of the Geological Map

569

of Iran: Kangan (no. 20867W), Hangam (no. 20861E), Firoozabad (no. 6547), Kushk (no. 6647),

570

Sarvestan (no. 6648), Arsenjan (no. 6649) and Abadeh-e-Tashk (no. 6649) and derive from 2011

571

and 2012 field campaigns. Measured stratigraphic sections and boreholes stratigraphy from NIOC

572

(National Iranian Oil Company) internal reports and thickness and facies information from the

573

paleogeographic maps of Koop and Stoneley (1982) and Pyriaei et al. (2010) integrate the dataset.

574

Borehole stratigraphic data come from several wells drilled in the area such as: Assaluyeh West-1

575

well (located 11 km ESE of the section), Kangan-1A well (40 km NW), Nar-1 well (6 km NW),

576

Varavi-1 well, Safid Baghun-1 well (9 km ESE), Cham-e-Noori-1 well (45 km ESE), Sefid Zakhur-

577

1 well (6 km ESE), Aghar-2 well (25 km ESE), Sim-1 well (1 km ESE), Toudej-1 well (71 km

578

ESE), Sarvestan-1 well (11 km ESE) (Fig. 3).

579

The cross-section of figure 9 highlights a geometry of the Zagros fold-and-thrust belt controlled by

580

a complex tectonic evolution that includes thin-skinned shortening of a thick sedimentary pile

581

characterized by facies and thickness variations associated with pre-subduction tectonics.

582

Major thickness changes occur in the Dashtak Fm., Khami and Bangestan Groups, beneath the Kuh-

583

e-Asalujeh and Pazan anticlines in the Coastal Fars, beneath the Daryau anticline in the Central

584

Fars, and in the Sarvestan plain in the Interior Fars. These thickness variations are thought to be

585

associated with E-W and NNW-SSE normal fault systems (Navabpour et al., 2010) generated by

586

Paleozoic-Mesozoic rifting-related extension. In the Interior Fars (from the Sarvestan plain to the

587

north), lateral facies and thickness changes were due to differential flexure of the Arabian plate

588

controlled by the Cretaceous obduction of ophiolites, cropping out in the Neyriz and Arsenjan area

589

(Pyriaei et al., 2010; Carminati et al., 2016). In agreement with Bigi et al. (2018), we propose that

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ACCEPTED MANUSCRIPT thin-skinned thrust faults within the Gachsaran and Dashtak Fms either inverted pre-existing

591

extensional structures or rotated them as in the case of the Kuh-e-Surmeh, Kuh-e-Gareh and Kuh-e-

592

Amhadi anticlines (Fig. 9). In these areas, back-thrusts developed in response of thickness

593

variations of the Bangestan Group and Dashtak Fm. which are controlled by Cretaceous and

594

Triassic normal faults as extensively described by Bigi et al. (2018).

595

Paleothermal indicators and 1D thermal modelling were included in the cross-section providing

596

maximum burial depths experienced by the sedimentary successions from the inner to the outer

597

zones of the Zagros fold-and-thrust belt. Eroded portion of structural units has been reconstructed

598

by projecting the calculated overburden onto the geological cross-section. This allowed us to infer

599

the geometry of eroded structures in different areas of the chain providing information on the

600

original thickness of the thrust belt (Fig. 9).

601

Paleothermal data exclude the occurrence of significant overthrusting during growth of the orogenic

602

wedge, coherently with the low shortening value (7% on average) that characterizes the cross-

603

section (Bigi et al., 2018) and consistent with shortening evaluations from previous works (e.g.,

604

Alavi, 2007; Mouthereau et al., 2007). Burial and thermal model of figure 8A clearly indicates 3.2

605

km of overburden for the Bangestan Group exposed at surface in the Kuh-e-Asalujeh anticline, thus

606

constraining exhumation and erosion to more than 3 km. This anticline is surrounded by synclines

607

characterized by much lower exhumation (down to 200 m, Fig. 9) and outcrops of Miocene-

608

Pleistocene rocks. In the Kuh-e-Surmeh anticline, thermal modelling (Fig. 7A) indicates similar

609

overburden (3.2 km) for the Bangestan Group and documents more than 6 km of exhumation north

610

of the cross section trace (Fig. 1B) where Silurian strata crop out. This amount of exhumation is

611

consistent with the vertical throw of the Surmeh fault involving the basement, in agreement with

612

previous studies (Mouthereau et al., 2006; 2007; Bigi et al., 2018) and with active seismicity

613

(Berberian, 1995; Ansari and Zamani, 2014; Mouthereau et al., 2012).

614

From the Kuh-e-Surmeh anticline to the Sarvestan plain, the eroded portion of the thrust belt

615

increases progressively from ca. 2 km to ca. 4.5 km. The Sarvestan plain represents a structural low

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where eroded overburden varies between ca. 400 m and 2.3 km. North-east of the Sarvestan plain,

617

structural elevation is constant, with erosion of 3.5 km, until the Kuh-e-Siah anticline characterized

618

by 5 km of erosion and higher structural elevation.

620

7. Conclusions

621

Main findings can be summarized as follows:

622

•

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Vitrinite reflectance measurements and temperature-dependent clay minerals show that the sedimentary successions of the Zagros fold-and-thrust belt, in the Fars province (Iran),

624

experienced similar levels of thermal maturity from the internal (Interior Fars) to the

625

external zones (Coastal Fars) in early-late diagenetic conditions. •

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1-D thermal models display a slightly decreasing lithostatic load towards the foreland from 3.65 km to 3.2 km for the Bangestan Group and indicate that sedimentary burial is the main

628

factor affecting thermal maturity. Only, in the Arsenjan area of the High Zagros, the

629

overburden includes thin tectonic slices of ophiolite units, maximum 1,100 m-thick. •

deposits more than on the amount of maximum burial.

631 632

The onset of oil generation depends on timing of deformation and the age of synorogenic

•

Organic and inorganic thermal indicators were used to reconstruct the eroded portion of the

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Zagros thrust belt that varies between 200 m in synclines and 6 km in anticlines, confirming

634

minor shortening amounts (average shortening is 7%).

635

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Acknowledgements

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Financial support from the Darius Programme and Progetti di Ateneo 2017 to Eugenio Carminati,

638

from Progetti di Ateneo 2016 to Luca Aldega, and from PRIN2015 (Project 2015EC9PJ5_001) to

639

C. Doglioni are acknowledged. The National Iranian Oil Company is thanked for assistance in

640

fieldwork organization and logistics and for allowing the publication of this work and of the

641

associated data. Ali Shaban and Hossain Narimani are thanked for sharing the fieldwork campaign. 25

ACCEPTED MANUSCRIPT 642

Midland Valley is acknowledged for providing educational license for Move software. Frederic

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Mouthereau, Dale R. Issler and Jeremy Powell provided constructive criticisms to an early version

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of the manuscript. We are grateful to A. Ceriani and three anonymous reviewers for detailed

645

revisions and helpful suggestions.

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Tectonic and Stratigraphic Evolution of Zagros and Makran during the Mesozoic–Cenozoic.

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Geol. Soc. London Spec. Publ. 330, p. 211–251.

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Poblet, J., Lisle, R.J., 2011. Kinematic evolution and structural styles of fold-and-thrust belts. Geol. Soc. London Spec. Publ. 349, 1–24.

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Pollastro, R.M., 1990. The illite-smectite geothermometer. Concepts, methodology, and

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applications to basin history and hydrocarbon generation. In: Nuccio, V.F., Barker, C.E., (Eds.),

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Applications of thermal maturity studies to energy exploration: SEPM Rocky Mountain section,

859

p. 1-18.

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Pollastro, R.M., Barker, C.E., 1986. Application of clay-mineral, vitrinite reflectance, and fluid

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inclusion studies to the thermal and burial history of the Pindale anticline, Green River Basin,

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Wyoming. In: Gautier, D.L., (Ed.), Roles of organic matter in sediment diagenesis: SEPM

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Special Publication, v. 38, p. 73-83.

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Potel, S., Maison, T., Maillet, M., Sarr, A.C., Doublier, M.P., Trullenque, G., Ferreiro Mählmann

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R., 2016. Reliability of very low-grade metamorphic methods to decipher basin evolution: Case

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study from the Markstein basin (Southern Vosges, NE France. App. Clay Sci. 134, 175-185.

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Ring, U., Brandon, M.T., Willett, S.D., Lister, G.S., 1999. Exhumation processes. In: Ring, U.,

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Brandon, M.T., Lister, G.S., Willett, S.D., (Eds.), Exhumation Processes: Normal Faulting,

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Ductile Flow and Erosion: Geol. Soc. London Spec. Publ. v. 154, p. 1–27. Ruh, J.B., Hirt, A.M., Burg, J.-P., Mohammadi, A., 2014. Forward propagation of the Zagros

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Simply Folded Belt constrained from magnetostratigraphy of growth strata. Tectonics 33, 1534–

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1551.

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Sclater, J.G., Christie, P.A.F., 1980. Continental stretching: An explanation of post–Mid Cretaceous subsidence on the central North Sea Basin. J. Geophys. Res. 85, 3711–3739.

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Schito A., Corrado, S., Aldega, L., Grigo, D., 2016. Overcoming pitfalls of vitrinite reflectance

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measurements in the assessment of thermal maturity: The case history of the lower Congo basin.

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Mar. Petrol. Geol. 74, 59-70.

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Schito, A., Corrado, S., Trolese, M., Aldega, L., Caricchi, C., Cirilli, S., Grigo, D., Guedes, A.,

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Romano, C., Spina, A., Valentim, B., 2017. Assessment of thermal evolution of Paleozoic

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successions of the Holy Cross Mountains (Poland). Mar. Petrol. Geol. 80, 112-132.

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Petrol. Geol. 21, 829–843.

Sepehr, M., Cosgrove, J.W., 2005. Role of the Kazerun Fault Zone in the formation and

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Sepehr, M., Cosgrove, J.W., 2004. Structural framework of the Zagros Fold–Thrust Belt, Iran. Mar.

deformation of the Zagros Fold‐Thrust Belt, Iran. Tectonics 24 (5), TC5005. Setudehnia, A., 1972. Iran du Sud-Ouest: Lexiqu Strat. Internat., Centre Nat.Rech. Scientifique,

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881

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Paris, III, Asie, Fasc.9b, p. 289-376. Setudehnia, A. 1978. The Mesozoic sequence in south-west Iran and adjacent areas. J. Petrol. Geol. 1, 3–42.

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Sfidari, E., Zamanzadeh, S.M., Dashti, A., Opera, A., Tavakkol, M.H., 2016. Comprehensive source 890

rock evaluation of the Kazhdumi Formation, in the Iranian Zagros Foldbelt and adjacent 891

offshore. Mar. Petrol. Geol. 71, 26-40. 892

Sherkati, S., Letouzey, J., 2004. Variation of structural style and basin evolution in the central 35

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insights from seismic data, field observation, and sandbox modeling. Tectonics 25, TC4007. Środoń, J., 1999. Nature of mixed-layer clays and mechanisms of their formation and alteration. Annu. Rev. Earth Planet. Sci. 27, 19-53.

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Sherkati, S., Letouzey, J., Frizon de Lamotte, D., 2006. Central Zagros fold-thrust belt (Iran): New

Stach, E., Mackowsky, M.Th., Teichmüller, M., Taylor, G.H., Chandra, D., Teichmüller, R., 1982. Stach’s Textbook of Coal Petrology. Berlin-Stuttgart, Gerbrüder Borntraeger, 535 p.

Szabo, F., Kheradpir, A., 1978. Permian and Triassic stratigraphy, Zagros basin, southwest Iran. J.

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Zagros (Izeh zone and Dezful embayment), Iran. Mar. Petrol. Geol 21, 535–554.

Petrol. Geol. 1, 57–82.

Sweeney, J.J., Burnham, A.K., 1990. Evaluation of a simple model of vitrinite reflectance based on

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chemical kinetics. Am. Assoc. Petroleum Geol. Bull. 74, 1559–1570. Vaziri-Moghaddam, H., Safari, I.A., Taheri, A., 2005. Microfacies, paleoenvironments and

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sequence stratigraphy of the Tarbur formation in kherameh area, SW Iran. Carbonates and

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Evaporites 20, 131-137.

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Warr, L.N., Rice A.H.N., 1994. Inter-laboratory standardization and calibration of clay mineral crystallinity and crystallite size data. J. Metamorph. Geol. 12, 141–152. Warr, L.N., Cox, S.C., 2016. Correlating illite (Kübler) and chlorite (Árkai) “crystallinity” indices

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with metamorphic mineral zones of the South Island, New Zealand. Appl. Clay Sci. 134, 164-

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174.

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Figure and table captions

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Fig.1 - A) Structural setting of the Zagros fold-and-thrust belt showing the major fault zones, the

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geological provinces and the fieldwork area (modified and redrawn after Pirouz et al., 2011). HZF:

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High Zagros fault, MFF: Mountain front fault, ZF: Zagros front. B) Geological maps of the Fars

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province with sampling location (modified and redrawn after Mouthereau et al., 2007). 36

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Fig.2 - Stratigraphic correlation chart of the Fars province showing lateral lithology and facies

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changes (modified after Sepehr and Cosgrove, 2004 for the Mesozoic-Cenozoic part).

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Fig. 3 – Lithotratigraphic correlation charts of borehole data across the Fars province. (A) boreholes

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location, (B-C) thickness variations along NE-SW and N-S trending directions, (D) thickness

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changes in the Coastal Fars along WNW-ESE trending direction. For the Sim-1 well, data are

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available from depths deeper than 2,000m. Values rely to measure depths and the ground level is

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not considered (courtesy of NIOC).

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Fig. 4 – Distribution of organic and inorganic thermal indicators across the Zagros fold-and-thrust

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belt. (A) illite content in mixed layers illite-smectite in the Fars province; (B) illite content in mixed

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layers illite-smectite for Albian to Eocene rocks; (C) vitrinite reflectance for Miocene rocks. Data

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for the Kuh-e-Sim anticline are from Aldega et al., 2014.

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Fig. 5 - Representative ethylene-glycol-solvated (grey line) and air-dried (black line) diffraction

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patterns of the <2 µm grain-size fraction: A) Razak Fm.; B) Gurpi Fm.; C) Kazdhumi Fm.

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Acronyms: Chl-chlorite; I-illite; I-S-mixed-layer illite-smectite; K-kaolinite; Pal-palygorskite; Qtz-

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quartz; Cal-calcite; Gy-gypsum; Ab-albite. R0 and R1 refer to mixed layers illite-smectite stacking

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order.

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Fig. 6 - One-dimensional burial and thermal models of the outcropping sedimentary succession of

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the Arsenjan area in the High Zagros. A-B) Burial evolution in the last 200 Ma and 20 Ma

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estimating the thickness of the ophiolite thrust sheet and the amount of Maastrichtian erosion C)

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Present-day thermal maturity data plotted against maturity curves calculated by a series of

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geothermal gradients. 37

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Fig.7 - One-dimensional burial and thermal models of the outcropping sedimentary succession of

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the Kuh-e-Surmeh anticline and adjacent areas: A) in the last 450 Ma; B) in the last 20 Ma; C)

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Present-day thermal maturity data plotted against maturity curves calculated by a series of

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geothermal gradients.

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Fig. 8 - One-dimensional burial and thermal models of the outcropping sedimentary succession of

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the Kuh-e-Asalujeh anticline and adjacent areas: A) in the last 180 Ma; B) in the last 20 Ma; C)

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Present-day thermal maturity data plotted against maturity curves calculated by a series of

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geothermal gradients.

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Fig. 9 – Cross section along the Zagros fold-and-thrust belt (refer to Bigi et al., 2018 for a full

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discussion on geological constraints). The currently eroded portion of sedimentary successions

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(dashed line when inferred) and maximum burials (bars) were constrained by inorganic and organic

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thermal indicators from this study.

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Tab. 1- X-ray quantitative analysis of the <2µm grain-size fraction. Sm=smectite, Pal=palygorskite;

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I= illite; I-S= mixed layer illite-smectite; C-S = mixed layer chlorite-smectite; K= kaolinite; Chl=

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chlorite; Qtz= quartz; Cal= calcite; Dol= dolomite; Ab= albite; Ank= ankerite, Hem= hematite;

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Kfs= k-feldspar; Gy= gypsum; Go=goethite; Hem=hematite; R= stacking order (Jagodzinski,

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1949); %I in I-S= illite content in mixed layer illite-smectite; %C in C-S= chlorite content in mixed

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layer chlorite-smectite; KI =Kübler index data for air dried (AD) and ethylene-glycol-solvated (EG)

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mounts; N.D.=not determined. Data from the Kuh-e-Sim anticline and partly from the Sarvestan

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area are from Aldega et al. (2014) and Carminati et al. (2016) respectively.

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Tab. 2. Organic matter maturity and petrographic analysis. Ro% values with an asterisk indicate

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mean reflectance of reworked fragments or too scarce data, both not suitable for 1D thermal

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modeling; Roeq% values in italics. – absent; s.d. = standard deviation; nr = number of

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measurements.

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Tab. 3. Synthesis of maximum overburden atop the Bangestan Group and thickness of the eroded

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units derived from 1D thermal modelling and calculated by a series of geothermal gradients

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available from the literature.

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R 1 1 1 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 1 0 1 0 0 0 1 3 0 1 0 0 0 1 1

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Arsenjan Arsenjan Arsenjan Sarvestan Sarvestan Sarvestan Sarvestan Sarvestan Sarvestan Sarvestan Sarvestan Sarvestan Sarvestan Kuh-e-Sim Kuh-e-Sim Kuh-e-Sim Kuh-e-Sim Kuh-e-Sim Kuh-e-Sim Kuh-e-Sim Kuh-e-Sim Kuh-e-Meymand Kuh-e-Meymand Kuh-e-Meymand Kuh-e-Meymand Kuh-e-Surmeh Kuh-e-Surmeh Kuh-e-Surmeh Kuh-e-Surmeh Kuh-e-Surmeh Kuh-e-Asalujeh Kuh-e-Asalujeh Kuh-e-Asalujeh Kuh-e-Asalujeh Kuh-e-Asalujeh Kuh-e-Asalujeh Kuh-e-Asalujeh Kuh-e-Asalujeh

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Kazdhumi Kazdhumi Gadvan Agha Jari Mishan Razak Razak Razak Sachun Pabdeh Gurpi Gurpi Gurpi Mishan Mishan Mishan Mishan Mol Guri Champeh Champeh Mishan Mol Champeh Pabdeh Mishan Pabdeh Gurpi Gadvan Gahkum Mishan Mishan Gachsaran Pabdeh Pabdeh Gurpi Kazdhumi Kazdhumi

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KAZ2 KAZ3 GAD1 AJ6 MIS9 RAZ2 RAZ1 RAZ3 SAC1 PAB2 GUR4 GUR3 GUR5 MIS4 MIS5 MIS6 MIS7 MOL1 GURI1 CH1 CH2 MIS10 MOL2 CH3 PAB3 MIS15 PAB6 GUR7 GAD2 GAH1 MIS14 MIS12 GAS8 PAB5 PAB4 GUR6 KAZ4 KAZ6

X-ray quantitative analysis of the <2µm grain-size fraction (%wt.) Sm Pal I I-S C-S K Chl Other 13 8 72 7 Qtz, Cal 16 7 72 5 Qtz, Cal 48 14 38 Cal, Gy 5 51 9 35 Qtz, Cal 29 27 14 30 Cal, Qtz, Ab 5 45 15 7 28 Cal, Qtz, Ab 44 21 11 7 17 Cal, Qtz, Ab 5 42 6 12 35 Qtz, Cal 91 6 1 2 Dol, Ab, Gy 34 53 6 7 Cal, Qtz 27 37 36 Cal, Qtz 20 24 43 7 6 Qtz, Cal 49 11 40 Qtz, Cal 45 18 37 Qtz, Cal, Dol, Ab 17 40 16 27 Qtz, Cal, Dol, Ab 40 24 3 33 Qtz, Cal, Dol, Ank Qtz, Cal, Ank, Ab 17 37 4 42 53 21 26 Qtz, Cal, Ank, Hem 47 15 38 Qtz, Cal, Dol, Hem 13 40 5 18 24 Qtz, Cal 51 3 12 34 Qtz, Cal, Ank 35 11 26 28 Qtz, Cal 63 19 18 Qtz, Ank 16 30 34 6 14 Qtz, Cal 50 26 7 17 Qtz, Cal 43 22 5 9 21 Cal, Qtz, Ab, Gy 46 24 19 5 6 Qtz, Cal 22 42 36 Cal, Qtz, Gy 20 10 64 6 Cal 32 7 61 Qtz, Kfs, Go 50 25 11 14 Cal, Qtz, Ab 54 19 5 22 Cal, Qtz, Ab, Dol 61 11 18 10 Cal, Qtz, Ab, Dol 85 5 10 Qtz, Cal 80 6 14 Qtz, Cal 38 62 Cal, Qtz, Gy 10 14 69 7 Cal, Qtz, Ank 33 12 50 5 Cal

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%I in I-S 75 75 75 30 30 30 40 45 50 60 60 70 38 21 40 46 52 43 32 60 50 70 20 50 55 80 85 20 60 35 40 40 70 75

%C in C-S 60 55 70 52 55 70 55 -

KI (°∆2θ) AD EG 1.01 0.98 1.01 0.97 0.97 0.92 N.D. N.D. 1.18 1.15 N.D. N.D N.D N.D. N.D. N.D. N.D. N.D. 1.08 1.06 1.06 1.00 1.09 1.04 0.99 0.97 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 1.04 1.00 1.30 1.25 1.11 1.06 0.98 0.95 0.92 0.88 1.05 1.02 1.08 1.03 0.86 0.83 0.73 0.71 0.60 0.54 0.56 0.52 1.06 1.00 1.12 1.10 1.10 1.07 1.12 1.10 1.01 0.99 0.99 0.95

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Latitude

Longitude

Formation

Microscopic description of organic content

KAZ2 KAZ3 AJ5 RAZ3 GUR3

29.74359 29.84496 29.20972 29.20478 29.16244

53.17986 53.29696 53.34074 53.32447 53.31382

Kazdhumi Kazdhumi Agha Jari Razak Gurpi

GUR5 GUR4

29.44980 29.17279

53.28020 52.95759

Gurpi Gurpi

MIS9 MIS4

29.98442 28.85008

52.93960 52.86995

Mishan Mishan

MIS6

28.73050

52.94020

Mishan

MIS7 MOL1 CH1 CH2

28.62716 28.70915 28.79090 28.71220

53.12663 53.03830 52.85669 53.03550

Mishan Mol Champeh Champeh

MIS10 PAB3 PAB6 PAB5

29.67107 28.69535 28.52659 27.94757

52.75314 52.74102 52.68062 52.15743

Mishan Pabdeh Pabdeh Pabdeh

GUR6 KAZ6 KAZ4 KAZ5

27.95062 27.92971 27.61396 27.61329

52.16220 52.35970 52.53680 52.53554

Gurpi Kazdhumi Kazdhumi Kazdhumi

Scattered impregnated areas Scattered impregnated areas Barren Barren Bitumen and scarse solid fragments of reworked vitrinite and inertinite macerals Scattered fragments of vitrinite-huminite macerals Small and squared huminite-vitrinite fragments, and inertinite macerals with reflectance >0.9% Rare migrated bitumen Scarce dispersed organic matter of continental origin with both inertinite (Ro% >1%) and huminite-vitrinite group fragments (collinite) Scarce organic matter of continental origin with inertinite (Ro% >2%) and huminite-vitrinite group fragments Rare fragments of fusinite and collinite Few fragments of small sized collinite Barren Rare inertinite group macerals in small fragments, one fragment of collinite Collinite Barren Barren Scarce migrated material with abundant framboids of pyrite Scarce fragments of inertinite macerals R>0.7% Barren Barren Stains of migrated bitumen, inhomogeneous and small-sized

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Ro% and Roeq% +s.d. 0.66±0.05

nr

0.38±0.14 0.49±0.11

3 9

0.55±0.10 0.34±0.07

3 5

0.50±0.03

5

0.32±0.06 0.39±0.06 0.51*

4 3 1

0.35* -

1 -

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High Zagros

Central Fars

Coastal Fars

Geothermal gradient (°C/km)

Thickness of maximum overburden atop the Bangestan Group (m)

15 20 24 15 20 24 15 20 24

~ 5250 ~ 3650 ~ 3000 ~ 4700 ~ 3200 ~ 2800 ~ 5500 ~ 3200 ~ 3000

Timing of maximum burial and subsequent erosion

Late Miocene

early Pleistocene

early Pleistocene

A

Thickness of eroded units (m) ~ 5250A ~ 3650B ~ 3000C ~ 2200A ~ 700B ~ 0C ~ 1500A ~ 200B ~ 0C

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the model is not geologically acceptable as implies 1500 to 3000 m thick eroded siliciclastic deposits best solution, constrained by paleothermal data and consistent with surface and subsurface thickness of the stratigraphic section C the solution is stratigraphically acceptable but overestimates levels of thermal maturity for Cretaceous and older rocks

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Thermal evolution of the Zagros fold-and-thrust belt Decrease of lithostatic load towards the foreland A quantitative approach to determine the amount and extent of eroded structures

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