3D geometry of a shale-cored anticline in the western South Caspian Basin (offshore Azerbaijan)

Accepted Manuscript 3d geometry of a shale-cored anticline in the western South Caspian basin(offshore Azerbaijan) Idaira Santos Betancor, J.I. Soto PII:

S0264-8172(15)30021-0

DOI:

10.1016/j.marpetgeo.2015.06.012

Reference:

JMPG 2281

To appear in:

Marine and Petroleum Geology

Received Date: 11 February 2015 Revised Date:

20 June 2015

Accepted Date: 23 June 2015

Please cite this article as: Betancor, I.S., Soto, J.I., 3d geometry of a shale-cored anticline in the western South Caspian basin(offshore Azerbaijan), Marine and Petroleum Geology (2015), doi: 10.1016/ j.marpetgeo.2015.06.012. 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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Folding in the western SCB (Santos Betancor & Soto)

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3D GEOMETRY OF A SHALE-CORED ANTICLINE IN THE WESTERN SOUTH CASPIAN BASIN

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(OFFSHORE AZERBAIJAN)

Idaira SANTOS BETANCOR (a) *and J.I. SOTO (a)

(a)

Departamento de Geodinámica and Instituto Andaluz de Ciencias de la Tierra (C.S.I.C.-

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Universidad de Granada), Facultad de Ciencias, Universidad de Granada, Campus de

Corresponding author. Tel.: +34 958243351/+34 659698707; fax: +34 958 248527. Email address: [email protected]

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*

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Fuentenueva s/n, 18071-Granada, Spain.

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ABSTRACT The internal structure of one of the common fold structures in the western margin of the South Caspian Basin (SCB) has been characterized in 3D using a depth-migrated seismic cube in offshore Azerbaijan. The fold corresponds to a NNW-SSE anticline with a

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basinward vergence; i.e., eastwards, because in this direction of the margin the SCB is floored by a probable oceanic crust. The anticline has two culminations cut by mud-diapirs and is bounded by two parallel rim synclines with contrasting sedimentary thickness. This anticline deforms congruently the thick Productive Series (PS; Messinian to Late Pliocene),

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whereas the most recent sequences (<3.1 Ma; e.g., Akchagyl and Apsheron units) onlap or thin towards the fold crest.

We reconstruct the existence of two episodes of folding. During deposition of the uppermost

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PS sequences (ca. 3.5-3.4 to 3.1 Ma), fold uplift initiated with a significantly lower rate than sedimentation. During this epoch, folding was accompanied by basin tilting and by faulting in a basinward normal fault with a limited right lateral, strike-slip component. Motion along this fault zone promoted the downdip flow of a weak layer formed by fluid- and mud-rich sediments (Maykop Formation), which also migrated along strike to build-up the growing

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anticline. Henceforth, fold growth accelerated and sedimentary units like the Akchagyl (3.11.7 Ma) were deposited preferentially in the subsiding flanks. Seafloor upwarping due to folding conditioned the sediment transport, and large deltas adapted their prograding pattern to the growing anticline crest.

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This structure resembles a detachment fold with a leading, East-vergent forelimb. Nevertheless, the occurrence of progressive tilting accompanying sedimentation and

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folding, or the mud inflation of the fold core by deep flow parallel to the anticline axis, make this example in the SCB a special example of this fold type.

Keywords: 3D fold geometry, mud diapirism, syn-sedimentary folding, South Caspian Basin, detachment folds, basin tilting, deep faulting, hydrocarbon.

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1. INTRODUCTION Most of shale-cored anticlines are located in compressive settings as folds-and-thrusts belts, and also in distal areas of passive margins, where a ductile and overpressured shale-rich unit occurs at shallow crustal levels (e.g., Morley and Guerin, 1996; Bally, 2000; Rowan et al., 2004; Wu and Morley et al., 2011). Shortening promotes mud diapirism and also the

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development of a bedding-parallel fault within the shale layer, called as décollement or detachment surface. Displacement is also transferred into folding in the decoupled overlying sediments according to a buckling mechanism (Dahlstrom, 1969; Hudleston, 1973) along the termination of the thrust plane, above the tip line or within the interior of the thrust

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sheet, where a sharp decrease in faulting rate occurs (e.g., Poblet and McClay, 1996; Shaw et al., 2005).

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In many regions, detachment folds form hydrocarbon traps and developed diverse geometries, depending on the folding kinematics (e.g., Medwedeff, 1989; Poblet and McClay, 1996; Poblet et al., 2004). Detachment folds shape is asymmetric and independent of the fault shape, in contrast to other fault-related folds, like fault-bend folds (Rich, 1934) and fault-propagation folds (Dahlstrom, 1970; Mitra, 2002). When detachment folds interact

2010).

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and link, they have a final similar aspect to fault-propagation folds (e.g., Burberry et al.,

Simple geometrical models of detachment folds assume that no thickness variations or shear within layers occur in the competent multilayer unit (e.g., Mitra and Namson, 1989; Poblet,

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2004). Complexity increases if it is considered in three dimensions (e.g., Jamison, 1987; Mitra and Namson, 1989; Epard and Groshong, 1993a; Poblet and McClay, 1996). To unravel folding kinematics it is commonly used the geometry of the pre- and syn- growth

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strata, i.e., the sediments deposited before and during folding, respectively. Different geometrical methods are also used to calculate shortening rates and to estimate the depth of the detachment level using the pre-growth unit (e.g., Chamberlin, 1910; Epard and Groshong, 1993b; Mitra, 2003; Wiltschko and Groshong, 2012). The anticlinal structures in the South Caspian Basin (SCB; Fig. 1) are interpreted to be buckle folds overlying a regional ductile detachment zone that are commonly pierced by mud diapirs and volcanoes (e.g., Devlin et al., 1999; Allen et al., 2003; Brunet et al., 2003). These structures have been extensively studied due to their interest as stratigraphic traps for hydrocarbons. In the western SCB, folding is a response of the Caucasus shortening that 3

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occurred from the Pliocene to Present, simultaneously to a rapid basin subsidence and sedimentation (Nadirov, 1985; Chumakov et al., 1988; Sobornov, 1996; Devlin et al., 1999; Allen et al., 2003; Brunet et al., 2003). We study the Kurdashi-Araz Deniz (KAD) anticline, a complex fold structure NNW-SSEdirected settled in the western margin of the SCB, to the South of the Kura River mouth, in

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offshore Azerbaijan (Figs. 1 and 2). We have accomplished a detailed interpretation of a 3D seismic dataset migrated in depth, provided by REPSOL Exploración S.A. This fold was generated during the Pliocene-to-Recent times and deforms both, the reservoir unit of the Productive Series (PS; Late Miocene to Late Pliocene) and the youngest sediments up to the

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seafloor surface, here referred as the Post-PS units. Recent folded structures of this type have been widely documented in the region, although a detailed reconstruction of their

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evolution, associated deformation rates or their kinematics are far from being well established. In addition, we believe that the studied case deviates from the classical detachment fold-type. One of the differences consists on the participation of overpressured mud that cut the sedimentary sequences shaping complex diapir-like structures. The objectives of this contribution are: (1) to analyse the three-dimensional geometry of the KAD anticline; (2) to precise the timing of folding episodes through the characterization of

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the pre- and syn-growth geometries; (3) to examine the different fault structures to advance in the discussion on how they relate to folding processes; and (4) to reconstruct the shape of the mud diapirs and the relationship to the folded structure, to infer the role played by mud

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during deformation.

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2. GEOLOGICAL SETTING OF THE SCB The SCB is a presumable relict fragment of the Tethyan ocean generated in the Late Mesozoic during the N-S convergence between Arabia and Eurasia (Berberian, 1983; Nadirov, 1985; Zonenshain and Le Pichon, 1986; Lerche et al., 1996; Mangino and Priestley, 1998; Brunet et al., 2003; Yusifov, 2004). According to geophysical data, an oceanic-type crust probably floors the SCB. Structures in the western SCB are affected by the orogenic processes that built up the Caucasus fold-and-thrust belt (Sobornov, 1996; Nadirov et al., 1997; Allen et al., 2003; Brunet et al., 2003). The deep, West Caspian Fault (Fig. 1) also promoted deformation partitioning into strike-slip and reverse slip (Khain et al., 1966; Nadirov, 1985; Alsop and 4

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Holdsworth, 2002; Jackson et al., 2002; Allen et al., 2003). Fold structures are commonly cored by mud intrusions, forming mud diapirs and mud volcanoes (Fig. 2; Yusifov and Rabinowitz, 2004; Davies and Stewart, 2005). These folds in the sedimentary cover seem to be decoupled from the basement surface. Axial traces that switch from NW-SE to NNWSSE, along the western margin of the SCB (Fig. 1; e.g., Nadirov, 1985; Nadirov et al., 1997;

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Tagiyev et al., 1997; Lyberis and Manby, 1999; Jackson et al., 2002; Engdahl et al., 2006). Folding is Pliocene to Present in age and is represented by abundant detached and upright anticlines with domal culminations along the offshore Azerbaijan (Fig. 2).

Flexure and subsidence of the oceanic floor of the SCB have promoted exceptionally thick

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depocentres. The sedimentary section in the western SCB contains more than 20 km of

Jurassic-to-Present sediments (Zonenshain and Le Pichon, 1986; Abrams and Narimanov,

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1997; Nadirov et al., 1997; Mangino and Priestley, 1998; Brunet et al., 2003; Knapp et al., 2004). Thick, organic-rich shales constituting the Maykop Formation (~36-16.5 Ma) were deposited during the Oligocene to the Miocene in a euxinic shallow-water environment that was probably connected with the Black Sea (Inan et al., 1997; Hudson et al., 2008; Afandiyeva et al., 2009).

Up to 10 km of the sedimentary infill in the SCB correspond to the rapid deposition (~2

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mm/y) of different shallow-marine-to-continental sediments during the last ~6 my (Nadirov et al., 1997; Brunet et al., 2003; Knapp et al., 2004). A significant part of this section (over 5-6 km) is the PS sequence, which consist of alternating sandstone and shale intervals

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deposited between 5.9 and ~3.4-3.1 Ma; i.e., Late Messinian to Late Pliocene (Berberian, 1983; Devlin et al., 1999; Brunet et al., 2003; Morton et al., 2003; Hinds et al., 2004). The PS sequence is commonly divided into Lower PS (5.9-5.2 Ma) and Upper PS (5.2 to ~3.4-

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3.1 Ma; Fig. 3). Lower PS is composed by the KAS, PK, KS, NKP and NKG formations, whereas the Upper PS is formed by the Pereryva (5.2-4.9 Ma), Balakhany (4.9-4.0 Ma), Sabunchi (4.0-3.7 Ma) and Surakhany units (3.7 to ~3.4-3.1 Ma). The top of the PS, referred in this work as the PS-top surface, corresponds to a regional unconformity, which shows significant evidence of erosion in the basin. Recent studies suggest that the PS-top is diachronous in the western margin of the SCB, becoming slightly younger from the offshore domain (3.2 Ma) to the Kura basin (up to 2.7 Ma; Van Baack et al., 2013; Forte et al., 2015). The Post-PS sequence is divided here into three distinctive stratigraphic units, 5

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corresponding to the Akchagyl (~3.4-3.1 to ~1.7-1.6 Ma), Apsheron (~1.7-1.6 to ~0.8-0.7 Ma) and Gelasian sequences (~0.8-0.7 to Present) (e.g., Abdullayev, 2000). This group comprises sediments deposited between the Late Pliocene to Early Pleistocene (Fig. 3). High sedimentation rates from the Pliocene to Quaternary resulted in excessive fluid pressure within the sediments throughout the basin. Overpressure conditions in the Maykop

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Formation originated subsurface mobilization, mud diapirism and mud volcanism in the SCB (Bredehoeft et al., 1988; Dadashev et al., 1995; Tagiyev et al., 1997). Rapid burial also created low temperatures gradients (14-16 ºC/km; Devlin et al., 1999) suitable for

hydrocarbon generation and preservation in the Maykop Unit, which is the main source rock

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for hydrocarbons in the SCB (e.g., Korchagina et al., 1988). The PS represents the major reservoir in the basin (e.g., Smith-Rouch, 2006). The SCB is part of the Great Caspian

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hydrocarbon province, which is a major producing region with a total proven oil and gas reserves ranging from 15 to 31 million bbl and 230 to 360 Tcf, respectively (Belopolsky and Talwani, 2007; EIA, 2013).

3. DATASET AND METHODS

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This study is carried out by the seismic interpretation of a pre-stacked seismic cube migrated in depth, which was provided by REPSOL Exploración S.A. In addition to the prestack depth migration, it was applied a 3D continuity algorithm to improve the seismic resolution (Bertello et al., 2001). The 3D seismic covers approximately 653 km2 and

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illuminates the so-called KAD structure. Maximum depth penetration is 8990 m, with 900 samples per trace and a sample interval of 10 m. Seismic interpretation has been conducted

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with KingdomSuite® and Petrel® softwares and it has been tied using logging and chronostratigraphic information from two wells. This area is located South of the Kura River mouth (Figs. 1 and 2), in offshore Azerbaijan, with a water depth between 30 and 770 m. Most of the geographical information of the seismic block and the detailed stratigraphy has been removed due to the data confidentiality. The 3D seismic interpretation is mainly based on the characterization of the tops of different seismostratigraphic units within the KAD anticline (Fig. 3). The seismic units are defined according to their seismic facies and the stratigraphic division is based on the general correlation to other studies in both, offshore and onshore Azerbaijan. Interpretation was mainly conducted using the KingdomSuite® software. Within the PS 6

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sequences we have firstly distinguished 10 key, seismic units. Seismic reflections H1 to H10 are labelled upwards (Fig. 3). The Post-PS package has been characterized by picking the seismic tops of the Akchagyl and Apsheron formations. Picking workflow is supported by crosscutting relationships between seismic lines and reinforced with well data correlation. Faults were later exhaustively interpreted in KingdomSuite®, whereas mud structures were

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depicted in Petrel® by picking the mud-diapir limits in consecutive in-lines and cross-lines. The seismic interpretation of the external boundaries of the mud diapirs has been conducted manually, drawing the approximate boundary between the layered seismic facies and the domain with chaotic reflections (Fig. 4). The position of the mud diapirs and the overall

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geometry of their limits have been also constrained by using filters like variance and

coherence. Figure 4 illustrates the criteria we have used to establish the external shape of the

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mud diapirs in the region.

4. SEISMIC UNITS

The seismic facies of the different units we have distinguished in the KAD structure are

hereafter.

4.1. Productive Series

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summarized in Figure 3 and their bounding reflections (e.g., H1 to H10) are described

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We have distinguished two major sequences within the PS; the lower package, comprised between the PS-bottom and the H7 reflection, and the upper sequence that ends with the PS-

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top reflection.

The overall PS sequence is composed by parallel reflections with a high lateral continuity. Seismic reflections are locally displaced by faults or interrupted by sub-vertical contacts with mobilized muds. The PS multilayer is a sand-shale sequence deposited under deltaic to lacustrine conditions in the Miocene-to-Pliocene transition during ~3 my (e.g., Reynolds et al., 1998). The PS-bottom is a strong-to-intermediate negative reflection, locally disrupted in centralto-eastern domains of the KAD fold. Intermediate and positive amplitudes characterize the highly continuous H7 reflection. They bound thick intervals of transparent facies with some highly reflective layers that thin upwards. According to Morton et al. (2003), horizons 1 to 5 7

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within the lower portion of the PS, correspond to thick sand intervals deposited in a delta. These sandstone layers are intercalated with mudstone-prone levels, which have been interpreted as lacustrine facies and stages of decreased fluvial discharge (Reynolds et al., 1998; Hinds et al., 2004). The proportion of muds increases upwards (between H5 and H7). The succession from the PS-bottom to the H7 probably corresponds to Late Miocene-Early

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Pliocene age.

The H7-to-PS-top package shows a general thinning-and fining-upward sequence. Lithology is dominated by mudstones and siltstones and comprises scarce and very thin sandstone layers, probably reflecting the repeated emergence of this interval of the PS, which occurred

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in the Late Pliocene (Reynolds et al., 1998; Morton et al., 2003; Hinds et al., 2004).

The upper succession terminates in the upper limit of the PS, identified as a remarkable

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reflection with strong, positive amplitudes. It corresponds to an unconformity regionally dated at ~3.4-3.1 Ma, which is more pronounced in the fold core that erodes the upper PS package with a progressive thinning of the H7 to PS-top sequences.

4.2. Post-Productive Series

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The Post-PS seismic package includes the Akchagyl, Apsheron, and Gelasian units (Fig. 3), although the latter unit has not been distinguished in this work. This sequence is represented by variable acoustic response with abundant irregular reflections and some stratified reflections with medium amplitudes. The Post-PS group usually depicts two wedges that

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thin towards fold crest.

The Akchagyl Unit onlaps the PS-top in the central areas of the KAD anticline. Onlap

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geometries are also found within the Apsheron Unit and towards fold culminations, whereas reflections are commonly discontinuous and even disappeared in the flanks. The Apsheron and Akchagyl units are also characterized by numerous sigmoidal bodies, which are internally stratified.

According to numerous authors, the Post-PS sequences have been deposited in the last ~3.43.1 Ma in a marginal shelf. They occurred in a close basin formed by terrigenous input provided by turbidity currents and deltas (e.g., Abdullayev, 2000).

4.3. Mud diapirs 8

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Complex diapiric structures occur in the fold core of the KAD anticline and are fed by overpressured muds, which produce a characteristic low-velocity zone and a shallow seismic attenuation (Lee et al., 1998; Gherasim et al., 2015; Hill et al., 2015). Most of the mud-diapir bodies are developed within the PS units, with a diapir head situated just below the PS-top, although the more mature structures occurring in the KAD fold culmination

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achieve exceptionally the Akchagyl Unit. In the studied area, we have not found active mud volcanoes in the seafloor surface.

Mud diapirs have a low-reflective, chaotic seismic facies with some intermediate-amplitude reflections that may correspond to remnant fragments of country sediments. The transition

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between the mud body and the surrounding layered rocks occur through a wide zone with a progressive reduction of the reflectivity and in the reflectors continuity (Fig. 4). The absence

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of a sharp seismic boundary with an associated reflectivity contrast is one of the common characteristics of mud diapirs. This probably reflects the existence of a wide contact region with pervasive fluid migration from the mobilized, overpressured muds to the country sediments (e.g., Van Rensbergen et al., 1999; Soto et al., 2010, 2014). The deep portion of the mud diapirs is not clearly seen in the seismic cube due to the abundance of crosscutting diffractions and noise. In some cases, we interpret these domains

dipping flanks (Fig. 4).

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as sub-vertical feeder channels, coinciding with a narrow fold hinge that separates highly

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5. FOLD GEOMETRY

The KAD anticline is an open to gentle anticline composed by two culmination domains,

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called here as Southern and Northern culminations. We present a series of vertical and parallel seismic profiles, oriented SW-NE, and one roughly perpendicular (NW-SE) arbitrary section. Vertical sections are named as Southern, Central and Northern and they are shown in Figures 5, 6 and 7, respectively. These sections correspond to dip-lines with respect to the KAD fold. Seismic correlation is shown with a central arbitrary line (Fig. 8). Additionally, two horizontal sections illustrate the deformation style at 4 and 2 km depths (Figs. 9 and 10, respectively).

5.1. Vertical seismic sections 9

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5.1.1. Southern dip-line Seismic section in Figure 5 shows a continuous, double-hinge anticline that deforms completely the stratigraphic section. The structure resembles a box-like anticline with a fold wavelength of ~6-7 km and a fold amplitude of ~0.7 km measured along the PS-top surface.

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Fold limbs pass laterally to two wide synclines. The PS sequences describe a symmetric fold with a geometry that evolves upwards from a narrow, angular fold hinge for the PS-bottom (at 5.5 km depth), to a rounded hinge with a wide and sub-horizontal fold culmination for the PS-top (~4 km-long at 1.2-1.4 km depth).

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This culmination has two narrow and planar limbs, dipping 30ºW and 45ºE, in the western and eastern limbs, respectively, measured in the PS-top. The eastern domain is wider and deeper than the western one. Syncline crest is almost flat in the West whereas it dips to the

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inner basin in the East. The interlimb angle in the anticline is ~110º corresponding to an open to gentle anticline (Fleuty, 1964). The axial surface is a single plane in the PS-bottom (dipping ~80ºW), describing an upright fold, which branches up-section into two convergent surfaces dipping oppositely (Fleuty, 1964). The overall fold vergence is to the East. The PS units thicken globally across the structure, having a stratigraphic thickness of ~4.6

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km in the western syncline and 5.15 km-thick in the eastern syncline. In the fold hinge, the complete stratigraphic thickness is ~4.3 km. The PS sequences limited by the PS-bottom and the H7 evidence a progressive eastward thickening, varying from 2.8 km in the West, 2.95 km in the anticline hinge, and 3.25 km in the eastern flank. In contrast, the sequence

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bounded by H7 and the PS-top thins towards the fold hinge. In this domain, it has a stratigraphic thickness of 1.35 km, whereas in the nearby synclines is 1.8 and 1.9 km-thick

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in the West and in the East, respectively. The architecture of the Post-PS sedimentary layers; i.e., Akchagyl to the seafloor, consists of two opposite wedges than thin and onlap towards the anticline crest. The fold also deforms the seafloor. This sequence changes progressively from 1.4 to 1.6 km-thick, from West to East, respectively, whereas above the anticline hinge it has a minimum stratigraphic thickness of ~0.5 km. Two contrasting fault families occur in this section. One disrupts the PS sequences as reverse, high-angle faults at the fold core, and the other corresponds to normal faults that displace the H8 and propagate upwards to the seafloor. The latter faults occur in the outer arc of the fold and are slightly displaced form the position of the fold culmination. Thrusting 10

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occurs along double-vergence faults developed along the two, kink-like anticline axial zones (dipping 45-60º), describing a pop-up structure. The normal faults close to the fold crest, have high dips (~50º) and show mutual cross-cutting relationships corresponding to conjugated, synchronous faults (e.g., Ferrill et al., 2000). The boundary between normal and

is on the key characteristics of buckling folds (Ramsay, 1967).

5.1.2. Central dip-line

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reverse faulting describes a layer-parallel neutral surface (e.g., Bertello et al., 2001), which

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In the Southern Culmination area, the structure corresponds to a single-hinge fold intruded by a mud diapir (Fig. 6). Fold has ~6 km of wavelength and the crest is at 0.7 km depth. The fold is markedly asymmetrical anticline for the PS package, with an angular-to-rounded fold

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hinge. PS-top changes from a concordant to a slightly discordant reflection from the flanks towards the fold crest, showing a subtle toplap geometry with respect to the upper part of the PS. Western flank dips up to 45ºW, whereas the eastern limb dips ~30ºE (measured for the PS-top). Axial surface is near vertical, corresponding to an upright anticline (Fleuty, 1964).

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Mud diapir shows teardrop geometry, with a nearly flat diapir head placed just below the PS-top surface at ~0.6 km depth. The diapir body contains discontinuous reflections parallel to the layering of the surrounding layers in the upper PS sequences (above H6). At 1.5 km depth, the diapir narrows and becomes < 0.1 km wide. Downwards, the structure seems to

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be disconnected from the source layer. The downward termination of the diapir is interpreted to occur along a high-angle sub-vertical fault, which has a reverse and probably

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a strike-slip component. We interpret that faulting determines the generation of a weld, which is the remnant of a depleted sub-vertical vertical feeder channel. Apparently, reverse faulting does not affect the uppermost horizons of the PS sequences (H6 to PS-top), which are clearly displaced by a discrete normal fault. A reverse fault is also observed in the western flank, crosscutting the sequences bounded by the horizon H8 and the PS-top surfaces. The stratigraphic thickness of the PS series in the western anticline limb is 4.2 km, increasing eastwards to finally have a total stratigraphic thickness of 5.35 km. In detail, the thickness change occurs in the package between PS-bottom and H7, because it is 2.55 km thick in the West and 3.35 km in the East. The overlying sequence, up to the PS-top surface, 11

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has a total stratigraphic thickness of 1.65 and 2.0 km, in the western and eastern synclines, respectively. The geometry of the Post-PS sediments resembles the structure seen in the Southern dip-line (Fig. 5), with a significant thinning towards the fold crest. Onlap geometries are observed within both the Apsheron and Akchagyl units. The Akchagyl reflector truncates the

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underlying H10 and PS-top horizons in the eastern limb. The Post-PS sequence has a total stratigraphic thickness of 1.45 and 1.65 km in the West and East, respectively. On the fold

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hinge, the Post-PS units achieves a minimum thickness of <0.5 km.

5.1.3. Northern dip-line

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The section shown in Figure 7 illustrates the internal structure of the Northern Culmination of the KAD anticline. This section shows a collapsed fold crest with another mud diapir. Fold wavelength and amplitude are here ~5.2 km and 0.6 km, respectively, and is an Evergent anticline.

Fold culmination is cut by two conjugate normal faults that seem to limit the mud diapir. The diapir shows again teardrop geometry, and the top coincides with the faulted PS-top

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surface. At around 1 km depth, this structure is approximately 2.2 km-wide and 2.5 km-tall. It is not found evidence of a diapir root because is not seen the connection to the mud source. We postulate the occurrence of a sub-vertical fault plane with a probable strike-slip

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component under the mud diapir, which coincides with a probable weld. The anticline has a narrow hinge for the sequence between the PS-bottom to H4, whereas

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the crest becomes rounded upwards. With respect to the PS-top surface, the two flanks dip 20ºW and 35ºE, in the western and eastern limbs, respectively, being deeper the eastern syncline. This is a gentle and upright anticline with an interlimb angle of ~125º and a subvertical axial surface (85ºW). The PS multilayer has a complete stratigraphic thickness of 4.35 and 5.2 km in the western and eastern synclines, respectively. The sequence between the PS-bottom and H7 surfaces is 2.65 and 3.3 km-thick in the same domains. The overlying PS units achieve a maximum thickness of 1.7 and 1.9 km in these areas. There are two onlapping, opposite wedges within the Post-PS sequences. These units are collapsed in the fold crest and some of the faults deform up to the seafloor. The Akchagyl 12

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Unit is almost absent on the crestal domain, above the mud diapir, and shows onlap geometries in both flanks. The total thickness of the Post-PS units is 2.2 km in the western domain, thinning progressively towards the crest (0.85 km), whereas the eastern limb is 2.3 km-thick. Two main conjugate normal faults limit the crestal graben developed in the Post-PS

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sequences. They are high-angle faults (dip >65º) that converge downwards up to 2 km, near the position of H7. The western fault seems to be the master fault of the graben and it

propagates upward to shallower levels (0.1 km), whereas the eastern faults are sealed by the

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post-Apsheron sequence.

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5.1.4. Arbitrary line

An arbitrary line has been constructed along the fold crest (Fig. 8). This section shows a double-culminated anticline and correlates the Northern (Fig. 7) and Southern (Fig. 6) culminations. An intermediate saddle zone with a total length of 6.3 km separates both culminations. The complete fold wavelength is <4 and 17 km in the northern and southern domains, and fold amplitude is approximately constant (0.55 km) in both culminations.

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Mud diapirs with teardrop geometries pierce each culmination (cf., Figs. 6 and 7). The northern diapir is limited by discrete normal faults, which bound a symmetric collapse graben sealed by a flat PS-top surface at 1 km depth. This diapir terminates at 2.5 km depth,

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between horizons 4 and 5. The Southern Culmination has a shallow bulb-shaped mud diapir with vague limits. This body has some scarce reflections, parallel to the laminated fabric of the country sediments. The base of both diapirs occur at 2.5 km depth and it is not seen here

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a potential mud feeder-channel or a pedestal. The overall fold structure corresponds to open-to-gentle anticlines with interlimb angles varying from 150º to 170º in the Northern and Southern culminations. These culminations are inclined to upright folds, respectively, because the axial planes dip between 70º to 85ºNW. Fold vergence in both cases is gently to the SE. The total stratigraphic thickness of the PS sequence varies along the fold crest, with 4.3 and 5.0 km-thick in the Northern and Southern culminations, whereas it has a minimum thickness of 3.4 km in the saddle. There is also a remarkable variation within the PS, expressed because the PS-bottom to H7 package changes from 2.9, 2.7 to 3.3. km from 13

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North to South, whereas the upper package bounded by H7 and PS-top varies simultaneously from 1.4, 0.7 to 1.7 km. The Post-PS units describe a double-sedimentary wedge that thins significantly towards the Southern Culmination. The Akchagyl Formation accomplishes most of this thinning and shows onlap and toplap geometries in the gentle flanks of this culmination (Fig. 8b). The

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Apsheron Unit is significantly thicker in the Northern Culmination and tapers the Southern Culmination.

Faulting occur as conjugate faults with normal dip-slip in the outer arc of the anticline

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culminations and as divergent reverse faults in the inner arcs. Normal faults tend to be

restricted to the upper PS sequence, particularly above the H6-H7 horizons. Some of these faults continue downward as reverse faults (e.g., northern flank of the Southern

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Culmination; Fig. 8b). The lower PS-sequences, i.e., between PS-bottom and H1, thicken towards the inner core of both culminations. These observations suggest collectively the occurrence of a transition from layer-parallel extension and crestal grabens in the outer arcs, to shortening with conjugate reverse faults and layer ductile flow of the shale-rich sedimentary sequences (Lower PS) towards the inner arc of the fold. The neutral surface occurs at about the H6-to-H7 horizons in the Northern Culmination, whereas it is

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significantly shallower in the South (between H8 to H9).

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5.2. Horizontal seismic sections 5.2.1. Depth slice at 4 km

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This depth slice shows an elongated and tight anticline with a domal shape and a sigmoidal axial trace, trending NW-SE (Fig. 9). The structure has an elliptical shape, ~40 km-long (measured along the axial trace) and is up to 11 km-wide in the Southern Culmination area, narrowing up to 3 km in the southernmost, periclinal domain. The two fold culminations are well recognized along the fold trace and are separated by a small (<5 km-long), intermediate saddle zone (e.g., Fig. 8). Maximum length of both culminations is ~8.5 and 3 km in the North and South, respectively, and the fold has a constant width of about 2 km. The anticline is bounded by two parallel, rim synclines with a diversely plunging axis. This explains the occurrence of two elongated depocentres in every rim syncline, corresponding

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to the double-plunging domains. The trough domains coincide approximately with the twoanticline culminations. The most significant faults at this depth are parallel to the KAD crest and the main structure is a NW-SE normal fault situated in the fold core (Fig. 9b). This fault is ~17 km long and has a curve-to-planar trace. In detail, this fracture changes along strike, being mostly a high-

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angle normal fault dipping westwards (~70º) in the Northern Culmination, and a sub-

vertical fracture along the Southern Culmination. Southwards, the fault-dip diminishes

progressively (30-40ºE) and it finally vanishes in the periclinal region. In this sector, when the KAD fold has a kink-like profile, parallel, NW-SE, conjugate reverse faults run parallel

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to both limbs (e.g., Fig. 5). The saddle domain of the KAD anticline has different high-angle concave segments (60ºE) of reverse and eastwards faults.

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At this depth there are also two SW-NE-trending faults in both flanks of the KAD anticline. They are curved normal faults with an opposite concavity that prolong towards the Southern Culmination, in the vicinities of the location of Figure 6.

5.2.2. Depth slice at 2 km

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The fold becomes wider at 2 km depth and has a total wavelength of 7.5 km (Fig. 10). For example, fold width measured for the H7 surface is 5.5 km and 3.7 km around the Southern and Northern culminations, respectively (e.g., Figs. 6 to 8).

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The mud diapirs correspond here to domains with chaotic seismic facies and occur in the central domain of both culminations (Figs. 6 to 8). The northern mud diapir has an

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elongated shape of 2x0.75 km (Fig. 10b). The southern one has a near circular section with ~0.75 km diameter.

Most of the deeper faults identified at 4 km depth prolong upwards and are also seen at 2 km depth (cf., Figs. 9 and 10). At this shallower level, new normal faults trending SWNE to W-E occur in the crest of both culminations. The axial, NW-SE normal fault characterized at 4 km become complex upward (Fig. 9b). In the Southern Culmination, it consists of a single normal fault with a total length of 9-10 km that dip westwards with a high angle (60º). Towards the Northern Culmination, it diverges into two splay normal faults that partially bound a symmetric graben and the northern mud

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diapir (fault dips <40º). The reverse faults running along the flanks in the Southern Culmination diverge upward and appear at 2 km depth as concave faults (Fig. 10b). The Southern Culmination shows here the two W-E symmetric grabens in both flanks. The fault strands has a contrary concavity in both regions and do not crosscut the KAD

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culmination (Fig. 10b).

5.3. Surface structural maps

We have chosen two contoured surfaces for key horizons to complement the interpretation.

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The structural maps correspond to the H7 (Fig. 11a) and the PS-top (Fig. 11b) surfaces.

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5.3.1. Horizon 7 surface

The structural contour map of the Horizon 7 (H7) surface depicts a tight and non-cylindrical fold with a sigmoidal trace that trend NNW-SSE, similar to the orientation of the rim synclines (Fig. 11a). The H7 surface describes a double-plunged anticline with two elongated culminations, being significantly larger the Southern compared to the Northern

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Culmination. The ratio between their long, NW and the short, SW axes is 13:8 and 4:2 km, respectively. The intermediate saddle zone is seen here as an elongated depression, with a maximum elevation of about 2.2 km depth. The Southern Culmination has a long periclinal closure due to the gentle dip of the fold axis towards the South. Steep and narrow limbs

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surround the KAD anticline, resulting parallel to the rim, broad synclines. The two mud diapirs have an elongated shape in both anticline culminations. At this

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structural depth they have a ratio of 2.3:0.6 and 4.2:1.5 km in the Northern and Southern culminations, respectively. The faulting pattern of H7 coincides with the descriptions conducted at 4 and 2 km depths (Figs. 9 and 10). The geometry and nature of the faults reinforce our previous interpretations, with long faults running parallel to the fold core and connecting both culminations, throughout the saddle domain. This system has a total length of 13 km and the different strands dip around 55ºW corresponding mostly to normal faults. In the Northern Culmination, these faults define a N-S symmetric graben with conjugate, concave fractures that dip in average 60º. They correspond to a crestal collapse graben (Figs. 5 and 6).

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Two pairs of NW-SE conjugate reverse faults occur in both fold flanks of the Southern Culmination. They are long, nearly planar fractures that dip oppositely of about 45º. The curved fractures trending SW-NE identified in the Southern Culmination are also normal faults with an opposite concavity in both flanks. Whereas in the western flank, they are nearly planar or slightly concave to the South, in the eastern flank they are markedly

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concave to the North. In both regions they have an average dip of 65º.

5.3.2. PS-top surface

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The structural map of the PS-top depicts a folded surface that becomes wider upwards (Fig. 11b). The elongated Southern Culmination (with a ratio of 24:8 km) contrasts to the rounded

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Northern Culmination (ratio 8:7.5 km). PS-top is not present in crestal domains in both culminations due to the significant erosion to this regional unconformity (e.g., Fig. 6). In these domains, the area occupied by the mud diapirs is significantly smaller and is elongated parallel to the axial trace. At this level, mud diapirs occupy ~40 km2 and 5 km2 at the core of the Southern and Northern culminations, respectively.

Most of the faults crosscutting the H7 surface continue towards the PS-top, branching

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upwards. The most important difference between the faulting patterns in both surfaces concerns to the faults oriented parallel to the KAD axial surface. In the Southern Culmination the PS-top is cut by two concave trends of normal faults. Towards the saddle

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domain, the fault strands are majorly planar and short, describing an overall sigmoidal graben. The largest fault is extends a maximum length of 15 km with an average dip of 65ºW. To the South, normal faults are markedly concave to the West and run along the flank

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of the fold crest. They resemble the keystone, collapse faults developed above some diapirs (e.g., Rowan et al., 1999), although in our case they are clearly deviated from the fold hinge.

6. DISCUSSION

6.1. 3D fold geometry To illustrate the 3D geometry of the KAD fold, it is presented a seismic chair-cut view, including the H6 and the PS-top surfaces (Figs. 12 and 13). The fold shape in both horizons evidences an open to gentle anticline in the South with a symmetric box-like section, containing two, convergent axial traces that merge forming a deep, single and sub-vertical 17

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surface (e.g., Southern dip-line in Fig. 5). The fold axis plunges both southwards and northwards, describing a general elongated dome. Northwards, the folded structure evolves to a gentle fold with two culminations and a single, sub-vertical axial plane (e.g., Figs. 6 to 8). Fold geometry has a broad, sub-horizontal crest in the South that becomes relatively angular to the North.

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The KAD anticline is asymmetric along the strike, with a shallow western limb faced

against a deeper eastern fold flank. Nearby synclines are also decoupled in both fold limbs. Flank lengths for the H6 and PS-top surfaces are 1.0-1.8 km in the West, and 1.1-1.6 km in the East. In the same domains, dip limbs for these surfaces are 20-45ºW and 30-40ºE. The

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interlimb angle of the KAD fold slightly increases northwards from 105º to 125º, whereas the axial plane dip varies slightly from ~80ºW to 90º. The overall geometry corresponds

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therefore to an upright anticline with a general eastward vergence.

The faulting pattern changes significantly from deeper to shallower levels. In the H6 surface large NW-SE reverse and planar faults accompany kink-band folding in the South (e.g., Fig. 9). Faulting in the fold flanks is replaced upwards by curved N-S normal faults that affect the PS-top surface (e.g., Fig. 10). In both surfaces, the saddle region of the fold corresponds to a narrow and linear N-S graben. Towards the North, small planar faults depict a collapsed

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fold culmination, which are abundant at the PS-top level and scarce at the H6 depth (Figs. 12 and 13). These faults usually progress upwards displacing the Akchagyl and Apsheron units and even the seafloor (see Figs. 7 and 10). Large SW-NE normal faults disrupt the

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continuity of both surfaces in the western limb. They correspond to conjugate structures that bound a symmetric graben, and it seems that the South-dipping fault represents the master structure. In any case, both faults have a long-lived history of deformation because they

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extend upwards deforming the Post-PS and even the seafloor surface. Summarizing, the KAD is a non-cylindrical fold with four periclinal closures and a sigmoidal, NNW-SSE trace. Independently of the selected surface, the KAD anticline tends to have a constant asymmetric geometry and a general, East-vergence, whereas, fold style and attitude of the axial trace might change along strike and in depth. Fold geometry evidences that the studied anticline resembles a detachment-fold type, and we recognize the eastern flank as the leading forelimb, whereas the western flank corresponds to the dragged backlimb. Although the studied seismic block does not achieve a deep structural level to recognize the detachment surface, we postulate that a thick layer of overpressured shales, 18

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like the deep Maykop formation, could act as a detachment layer during folding. This inference is also based on the occurrence of mobilized shales feeding both fold culminations. Mud teardrop diapirs are consistent with deep squeezed levels that upwards become bulged or culminated areas. We interpreted mud diapirism is controlled by local deformations,

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where both compressional and extensional domains simultaneously occur at different

structural levels of the fold. The absence of the source layer or shale pedestal suggests mud depletion in the source rock or withdrawal to out of plane flow.

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Our observations within the PS demonstrate a different in fault behaviour depending on the structural level of the fold; whereas deep horizons are displaced by reverse faults, the youngest units are commonly affected by normal faults. The evolution from layer-parallel

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shortening in the inner core domain, to sub-horizontal extension in the outer fold arc, reflects a common observation in buckling folds with a marked rheological contrast between layers (e.g., Ramsay, 1967; Cosgrove et al., 2000). The different fold sections support that the neutral surface during folding, which separate shortening from extension, tends to be located at around H6-H7 in the Northern Culmination, becoming shallower to the South, because it occurs there between H8 and H9. The occurrence of kink-like geometries in

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specific regions, like the southern domain of the KAD (e.g., Fig. 5), may also indicate that folding style is controlled by general variations of the layer thickness or even the sedimentary grain-size within the basin, which tend to decrease southwards in the studied

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area.

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6.2. Growth sequences and isopach maps To unravel the folding history of the KAD structure we have accomplished two analyses. The interpretation of some of the selected seismic lines has been flattened with respect to the PS-top surface (Fig. 14). Additionally, we have computed true-stratigraphic thickness (TST) or isopach maps of the PS and the Post-PS sequences (Fig. 15).

6.2.1. Flattened seismic interpretation Three of the most representative interpretations of dip-line sections have been flattened to analyse the growth history of the KAD fold (Fig. 14). Flattening is done in the Southern 19

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(Fig. 5) and Northern (Fig. 7) dip-lines, which respectively correspond to sections in Figure 14a and c. The unfolded section in Figure 14b is closely coincident to the Central dip-line (Fig. 6). The Post-PS sequences thin significantly towards the anticline hinge, demonstrating that they clearly correspond to a syn-growth epoch. The uppermost PS sequences, particularly

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the package bounded between the H7-H8 and the PS-top, thin also towards the fold crest (Figs. 14b and c). This indicates that fold started to grow before the regional unconformity marked by the PS-top surface. The central section of the KAD also reinforces the

occurrence of a high-angle fault in the inner core of the anticline, with a reverse fault

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displacement (Fig. 14b).

Extrapolating linearly the age of the PS-top and PS-surfaces; i.e., 5.9 and ~3.1 Ma,

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respectively (e.g., Inan et al., 1997; Abreu and Nummedal, 2007; Forte et al., 2015), we estimate that the H7-H8 interval has an age of 3.5-3.4 Ma. This is the estimated time for the onset of folding and the syn-growth folding extends henceforth to Present (Fig. 14).

6.2.2. PS-bottom to H7 isopach map

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TST estimate has been conducted using smoothed surfaces of the key folded horizons, to avoid local errors in the seismic signal, and has been measured always perpendicular to the top surface of each unit. Within the PS sequences, we have selected the TST of two

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intervals, separated by the H7 surface as a key level during folding (Fig. 15). Isopach maps also include the structural interpretation of the different upper-bounding surfaces. For this analysis we have not applied a decompaction correction and in consequence, these maps

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represent minimum thickness values. Figure 15a shows two major thickness domains separated approximately by the actual position of the trace of the fold hinge. The western fold limb is characterized by a thinner platform of 2.2-2.5 km that thickens to the South up to 3.1 km, following a NW-SE direction. Thickness in the eastern domain of the fold remains almost constant with 3.1-3.2 km of sediments, whereas it reaches a maximum thickness of 3.5 km in the SE syncline. In detail, the transition zone between fold limbs take place in a narrow and elongated zone with a maximum thickness of 3.5 km. Southern depocentres are locally separated by tight thinned areas parallel to the NW-SE kink-band structures.

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With this map, we infer the existence of a linear structure along fold hinge that promoted a major sinking of the deeper PS sequence (PS-bottom-to-H7; 5.9 to 3.5-3.4 Ma) in the eastern with respect to the western flank, with differences of up to 1-1.5 km. This narrow domain is interpreted to be the upward continuation of a buried normal fault dipping to the East. The syn-sedimentary faulting above a buried tip-line of a growth fault creates a tri-

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shear domain with a horizontal width and geometry that depends on the dip, depth, and slip of the buried normal fault (e.g., Hardy and McClay, 1999). Accordingly to these in tri-shear models of deformations above normal faults, we infer the existence of a buried, high-angle fault dipping eastwards along the axial trace of the KAD anticline. Faulting along this

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buried fault occurred simultaneously to the deposition to the lower part of the PS sequence, at least up to the H7 horizon. According to the general pattern of TST in the KAD structure,

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we cannot rule out the contribution of some right-lateral displacement to the general eastward-downthrown of the master fault, but we interpreted that this strike-slip component of the fault, which has been postulated in the region (e.g., Khain et al., 1966), is limited.

6.2.3. H7 to PS-top isopach map

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The TST of the uppermost interval of the PS units is shown in Figure 15b. The resulting pattern departs slightly from the deeper interval (Fig. 15a), suggesting a change in the folding history, i.e. since 3.5-3.4 to 3.1 Ma. A narrow (0.8 km-wide) and slightly curved domain separates both anticline culminations and flanks (Fig. 15b). The thickness difference

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between fold flanks is less evident, although it is recognisable in the central and northern domains of the KAD fold because the mean value of the western limb is about 1.5-1.7 km

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and the eastern has a constant value over 2.0 km. During the deposition of the uppermost package of the PS sequence; i.e., H8 to H10 units, the previous fault drape and step in the TST domain, becomes a growing and narrow bulge that is located between the Southern and Northern culminations. The main difference with the deeper interval of the PS is seen southwards of the Southern Culmination, because the two rim synclines appears as subsiding and narrow domains bounding an incipient growing anticline with a broad and elongated crest (cf., Figs. 15a and 15b). We infer that the KAD fold started to grow just after H7, with a fold hinge that migrated probably southwards, and consequently the H7-to-PS-top interval should be considered as the initial syn-growth sequence. In the northern domain the structure had had a subsiding eastern flank, and fold 21

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started to grow above the deep, growth normal fault. To the South, folding propagated trough a migrating hinge that tend to vanish southwards.

6.2.4. Post-PS isopach map

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The Post-PS map describes a completely different scenario and clearly reflects a growing anticline bounded by two sinking rim-synclines (Fig. 15c). The crest region is draped by a thin package of Post-PS sediments, especially in the Southern Culmination, where this

package is <0.25 km-thick. Values in the Northern Culmination and in the saddle domain

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are similar (~0.75 km-thick). Two great depocentres are registered in the northernmost

sectors (2.25 and 2 km-thick in the NE and NW, respectively), whereas they are thinner in

differences of about 0.5 km.

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the South (~1.5 km). Eastern limb is generally thicker than the western flank, with

The TST estimate reinforces the seismic interpretation; i.e., the Post-PS units were deposited during fold growth, thus representing the syn-growth epoch of folding, with sedimentation rates greater than the vertical anticline uplift for the last 3.1 my. The overall thickness distribution, in addition to the observations conducted in strike seismic lines (e.g.,

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Fig. 8), indicate that most of the Post-PS sediments prograded southwards. In the KAD area, sediment progradation interacted with the growing anticline, accumulating preferentially the sediments in the sinking, rim synclines, whereas thinner sediments draped the uplifting anticline crest. During the Post-PS epoch the basin margin experienced a subtle tilting

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one (Fig. 15c).

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towards the East, because the western syncline is slightly thinner than the paired, eastern

6.3. Fold evolution

Our interpretation of the KAD evolution is shown schematically in Figure 16. This figure summarizes the different evolutionary stages of this structure in the western margin of the SCB, since the deposition of the Maykop Unit (labelled W) above an underlying sequence and/or basement (labelled collectively as B). The different panels combine also a schematic 3D perspective of the KAD structure and the representation of the structures in plan view. The Maykop Unit and the underlying (B) sequence in this margin of the SCB dip gently basinwards; i.e., towards the East, where the basin was floored by a presumable oceanic 22

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crust (Fig. 1). Accordingly to the numerous descriptions and evidence, both onshore and offshore, we interpret that the Maykop Unit behaved ductile and flow during subsequent deformations (e.g., Sobornov, 1996; Yusifov and Rabinowitz, 2004; Smith-Rouch, 2006;). Since the first stage, we consider the occurrence, below the weak layer, of different fault strands dipping basinwards. Due also to its great thickness, the weak layer constitutes a

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decoupling level during deformation, limiting the connection or propagation during

sedimentation and burial, of the deep structures with the new ones formed above it. We believe that these suggestions help to explain some of the singular features identified in the KAD, although the seismic observations have never achieved such deep levels of the

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structure.

At the first stage, the shale-rich Maykop Unit (<36 Ma) is deposited above a gentle eastward

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ramp of the pre-Oligocene basement. Deep faulting occurs through small and disconnected fault segments below the weak unit, which define an incipient normal fault zone (stage 1 in Fig. 16). Fault propagation determines the enlargement of the different fault segments in length and depth, and the possible generation of overlapping and bend zones (stage 2 in Fig. 16). We postulate that since deposition of the post-Maykop units, normal faulting, with some component of right-lateral shear, occurred along the high-angle fault zone, promoting

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the generation of drape folds above the upper-tip of the faults (stage 3 in Fig. 16). We also suggest that the KAD fold nucleated above the overlapping fault segments, and near to the projection of the trace of the buried normal fault (inset in stage 3, Fig. 16). The incipient fold would have an abrupt bend that mimics the domain with overlapping strands.

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Accompanying the fault propagation and linkage, the weak, Maykop layer spread probably downdip. These suggestions could explain some of the observations we have done in the

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deeper PS sequences (e.g., Fig. 15a): (1) the occurrence of an abrupt increase of their sedimentary thickness in the actual position of the KAD anticline hinge; (2) the thicker domains always occur in the eastern flank; and (3) the subtle variation of their thickness along the KAD strike, showing some reduction towards the actual fold culminations, where mud diapirs pierce actually the folded structure (e.g., Fig. 6). The stage corresponding to the deposition of the higher PS sequences, i.e. the ones bounded by the H7 and the PS-top surfaces (ca. 3.5-3.4 to 3.1 Ma), is illustrated by stages 4 and 5 in Figure 16. The KAD fold started to growth. We inferred that folding initiated just after the H7. Shortening deformed congruently the pre-H7 sequences, whereas the younger units, up to the PS-top surface, thin progressively towards the KAD hinge (Fig. 14). This observation 23

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indicates that sedimentation rates were always smaller than the fold uplift (e.g., Poblet et al., 1997; González-Mieres and Suppe, 2011). Due also to the occurrence of some layer thinning in these sequences along the KAD strike and towards the fold culminations (e.g., towards the Southern Culmination in Fig. 8), we suggest the existence of deep shale migration, parallel to the fold strike, to inflate locally these domains (cf. stages 4 and 5 in

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Fig. 16).

The PS-top surface marks a change in the folding history. The overlying sequences (<3.1 Ma), particularly within the Akchagyl Unit, onlap progressively to the KAD crest, which is locally cover only by the overlying Apsheron Unit. This geometry can be seen either in dip

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sections (e.g., Figs. 5 to 7) or along strike (Fig. 8). They reflect that the vertical fold growth was greater than the sedimentation rate during the Akchagyl interval (3.1-1.7 Ma;

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represented by layer #4 in stage 6, Fig. 16). Subsequently, since the onset of the Apsheron Unit (1.7-0.8 Ma) and throughout the Gelasian Unit (<0.8 Ma), folding rate diminished and the sedimentation rate exceeded the fold uplift up to nowadays (both units are represented schematically by layer #5 in stage 6, Fig. 16).

We suggest that fold culminations coincide with domains of preferential upward migration and piercing of the overpressured sediments of the Maykop Unit. The flow of these weak,

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fluid- and mud-rich sediments probably occurred downdip, following the basement slope and tilting, but also along the KAD strike, to feed and upbuild the fold domains with major

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horizontal shortening.

7. CONCLUSIONS

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1. Using a 3D seismic dataset migrated in depth in offshore Azerbaijan; we have characterized the internal structure of the KAD structure. This results to be a NW-SE anticline with a sigmoidal trace and changing fold-profile along strike, although it generally verges eastwards, following the possible dip of the basement in the western margin of the South Caspian Basin (SCB). The anticline corresponds to a non-cylindrical upright fold, pierced locally by overpressured muds, which deform Late Miocene-to-Present sediments. The outer arc is the locus of collapse grabens, which reflect layer-parallel extension, whereas the inner arc is cut by numerous, high-angle reverse faults. A marked difference also exists between the shallower western limb with respect to the deeper and thicker, eastern limb. This difference also reflects a general tilt of the SCB margin towards the East, 24

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as a consequence of the occurrence of a presumably oceanic crust flooring the deepest areas of the SCB. 2. The KAD anticline folds congruently a thick sedimentary sequence formed by the Productive Series (PS) of the SCB; i.e., a thick package (4.5 to 5 km) of marine sediments deposited since the Late Miocene to the Late Pliocene (5.9 to ~3.4-3.1 Ma); and is covered

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by younger, Post-PS sediments (<3.1 Ma) with progressive onlap. These geometries

demonstrate the occurrence of two major episodes of folding. The limit between both

epochs is marked by a regional discordance, the PS-top surface (~3.4-3.1 Ma), which shows

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local erosion in the fold culmination domains.

3. The upper part of the PS sequence, the one situated above the H6-H7 reflections, shows a subtle thinning towards the anticline crest. This progressive thinning of the youngest PS

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sequences is observed both in dip (WNW-ESE) and strike (SSE-NNW) fold sections. We postulate that folding initiated during this epoch (ca. 3.5-3.4 to ~3.1 Ma), with a sedimentation rate that was greater than the upbuilding of the fold. 4. Accordingly to the pattern of layer-thickness variations within the upper PS (i.e., post H7 reflection), it is suggested that folding nucleated above a buried fault-tip. Sedimentation of

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the post-H7 sequence occurred simultaneously to deep faulting along a high-angle fault zone. This fault zone is inferred to have a main, normal (basinward) displacement with a limited strike-slip (right-lateral) component. Syn-sedimentary faulting accompanied margin tilting towards the East, and it was probably conducted through propagation and overlaps or

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bend between nearby fault segments. We postulate that this deformation occurred below a weak, mud-rich layer (Maykop Unit), which flowed both, perpendicular and parallel to the

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fold axis; i.e., basinwards and subhorizontally, respectively. 5. The major pulse of deformation occurred just after the formation of the PS-top surface and coinciding with the Akchagyl Unit (3.1-1.7 Ma). During this epoch, sedimentation rates were significantly lower that the vertical fold uplift. Mass-instability processes accompanied sedimentation during this epoch, and were promoted by the local emersion of the fold crest in an upwarped seafloor. Since this epoch, and up to present-day, folding rates tend to diminish progressively, and the KAD anticline is progressively draped by the Apsheron and Gelasian units (1.7-0.8 Ma and younger, respectively). 6. One of the singularities of the KAD anticline is that overpressured muds pierce the fold culminations, shaping complex diapirs with multiple inter-fingering and a general teardrop 25

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shape. The connection with the source layer or the pedestal of the mud diapirs has not been identified in the seismic cube. It is interpreted that overpressured mud comes from a deep layer formed by the Maykop Formation (~36-16.5 Ma). The final geometry depicts isolated mud-teardrops rooted by a sub-vertical weld. Welding occurred most probably during

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folding and is seen with some component of dip- and strike-slip (right-lateral) components.

ACKNOWLEDGEMENTS

This research would not be possible without the data survey from REPSOL Exploración

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S.A. We acknowledge specially the support given to this study in this company by T. Zapata and C. Macellari. We are grateful to IHS for the Kingdom Suite® software and to Schlumberger for Petrel®, which are used through academic agreements. Thanks to L.

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Lonergan at the Imperial College of London for hosting and supervising I.Santos Betancor during different stages of this study. H. Dobrova from HIS is acknowledged for providing hydrocarbon licensing information of the Caspian Sea. We also appreciate Gabor Tari for a constructive review of the manuscript. This research has been funded by the project TRACE TRA2009_0205 (Ministerio de Ciencia e Innovación) and the research group RNM-376 of

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the Granada University.

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FIGURE CAPTIONS Figure 1: Tectonic map of the South Caspian Basin (SCB) and part of the Central Caspian Basin (CCB) showing the main folds and thrusts. The study area is situated over 140 km South of Baku, in offshore Azerbaijan, in a transitional area between the Cimmerian block and the oceanic-type crust. The limit of this crust is taken from Berberian, 1983. In

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this margin, the trend of Pliocene anticline folds changes progressively from NW-SE, in the vicinity of the Caucasus Mountains, to NNW-SSE, in offshore Azerbaijan. Pliocene anticline culminations are taken from Priestley et al. (1994), Inan et al. (1997), Nadirov et al. (1997), Tagiyev et al. (1997). Geology is compiled from Devlin et al. (1999), Allen

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et al., (2004), Smith-Rouch (2006), Khain et al. (2007), Hollingsworth et al. (2008). Inset shows the political divisions of the area. Large rectangle marks the detailed map shown

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in Figure 2. Abbreviations: Armenia (A), Araks Fault (AF), Apsheron Ridge (AR), Azerbaijan (AZ), Georgia (G), Greater Caucasus Mountains (GC), Kazakhstan (K), Kura Basin (KB), Kabateh Fault (KF), Khazaar Fault (Kh.F), Lesser Caucasus Mountains (LC), Main Caucasian Thrust (MCT), Russia (R), Racha-Lechkhumy Fault (RLF), South Balkhan Fault (SBF), Turkmenistan (T), Uzbekistan (U), West Caspian Fault (WCF), West Kopeh Dagh Fault (WKDF), and West Turkmenistan Basin (WTB).

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Figure 2: Detailed map of the western margin of the South Caspian Basin (SCB), showing the distribution of prospective anticline culminations and of the recent (active and Quaternary) mud volcanoes. Sources for mud volcanoes are: Nadirov et al. (1997),

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Kadirov (2000), Planke et al. (2003), Etiope et al. (2004), Guliev and Panahi (2004), Yusifov (2004), Yusifov and Rabinowitz (2004), Mellors et al. (2007), Evans et al. (2008), and Roberts et al. (2010). Anticlines are from Fowler et al. (2000), Aliyeva

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(2005), Baganz et al. (2012), Imbert et al. (2014), and H. Dobrova (IHS, personal communication). Digital topography is from SRTM30 plus (Becker et al., 2009), with a grid size of 1 km.

Figure 3: Seismostratigraphic panel summarizing the lithology and seismic expression of the distinguished units in this study. On the right-hand column, formation tops and lithologies of the different units are described. The left-hand column describes the main sequences and the geological ages. Mud seismic facies are shown in the lower panel. The PS-bottom and the PS-top represents the boundaries of the Productive Series (PS). The tops of different units are expressed as horizons H1 to H10. The Akchagyl and Apsheron 27

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reflections also correspond to the tops of homonymous units. The Post-Productive Series are referred as Post-PS, and the sequences deposited before the PS are called as the PrePS units. Within the PS units, we distinguish the Lower PS (L. PS) and the Upper PS sequences. Figure 4: Seismic examples of mud diapirs, illustrating their internal fabric and the criteria

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used to delimit the mobilized-mud bodies from the country sediments. (a)Vertical, arbitrary section and (b) depth-slice at 1500 m depth. They illustrate the seismic expression of the mud diapir in the Southern Culmination (Fig. 6).

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Figure 5: Seismic dip-line in the southern area of the studied block non-interpreted (a) and interpreted (b). Vertical scale is in real depth (in meters) with no vertical exaggeration. Inset shows the position of this seismic section. Seismic units are summarized in Figure

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3. Abbreviations: horizon (H), bottom (PS-bottom) and top (PS-top) of the Productive Series.

Figure 6: Seismic dip-line in central areas of the KAD fold that crosscut the Southern Culmination domain, non-interpreted (a) and interpreted (b). The chaotic seismic facies at the fold culmination comprises a teardrop diapir and the mud body is in brown. Black-

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and white-filled circles represent the inferred component of strike-slip movement, towards and forward the observer, respectively. Vertical scale is in real depth (in meters) with no vertical exaggeration. Inset shows the position of this seismic section. Seismic units and other symbols are detailed in Figure 3.

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Figure 7: Seismic dip-line in the Northern Culmination domain non-interpreted (a) and interpreted (b). A collapsed mud diapir is found in the fold crest, bounded by normal

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faults. Vertical scale is in real depth (in meters) with no vertical exaggeration. Inset shows the position of this seismic section. Seismic units and other symbols are detailed in Figure 3.

Figure 8: Composite arbitrary line along the structure non-interpreted (a) and interpreted (b). Two domes called as Northern and Southern culminations comprise the KAD anticline. Both domains are cored by mud diapirs that depict teardrop shapes. Vertical scale is in real depth (in meters) with x2.5 vertical exaggeration. Inset shows the position of this seismic section. Seismic units and other symbols are detailed in Figure 3.

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Figure 9: Seismic depth slice of the KAD area at 4 km depth non-interpreted (a) and interpreted (b). Horizontal dips in (b) are expressed as red crosses. Seismic units and other symbols are detailed in Figure 3. Location of Figures 5 to 8 is also included. Figure 10: Seismic depth slice of the KAD area at 2 km depth non-interpreted (a) and interpreted (b). Two small and elongated mud structures are depicted in (b) in the core of

Seismic units and other symbols are detailed in Figure 3.

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both fold culminations. Black-filled circle represents the tip line of fault structures.

Figure 11: Contour structural maps for (a) Horizon 7 (H7) and (b) PS-top surfaces. The

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white-coloured areas in b) represent those areas where the PS-top surface is absent. Contour interval is 500 m.

Figure 12: 3D perspective view of the seismic cube with the interpreted surface of the

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Horizon 6 (H6). The position of the Horizon 8 (H8) and the PS-top (PS-t) surfaces is also included for reference. Depth boundaries of the seismic cube are 2.2 and 5 km. Figure 13: 3D perspective view of the seismic cube with the interpreted surface of the PStop (PS-t). The position of the horizons 6 and 8 (H6 and H8, respectively), and the Akchagyl top (Ak.) is also included for reference. Depth boundaries of the seismic cube

to those in Figure 12.

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are 1.5 and 5 km. The vertical limits of the seismic cube and the perspective are similar

Figure 14: Fold profile interpretations flattened with respect to the PS-top surface. The fold

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sections (a) and (c) correspond to the Southern and Northern dip-lines shown in Figures 5 and 7, respectively, whereas section (b) is nearly coincident with Central dip-line in

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Figure 6. Seismic units and other symbols are detailed in Figure 3. Figure 15: True stratigraphic thickness (TST) or isopach maps of (a) the interval between the PS-bottom and the Horizon 7 (H7); (b) the interval from Horizon 7 and the PS-top; and (c) the complete package of the Post-PS series. Details of the calculations are presented in the text.

Figure 16: Schematic panels to show the 3D evolution of the KAD structure from just after the deposition of the Maykop Unit to the Present. The frontal side of the blocks is roughly oriented parallel to the fold strike (i.e., NNW-SSE), whereas the left side represents the perpendicular dip section (i.e., WSW-ENE). It is postulated the occurrence of: (1) a deep and buried, high-angle fault, with a displacement behaviour that control the 29

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deformation of the cover sequences, and (2) a variable amount of horizontal extension between the sedimentary cover and the deeper sediments (including basement), due to the occurrence of a thick, mud-rich layer. This layer is labelled W (weak layer) and represents the Maykop Unit, whereas the sub-Maykop units, including the basement, are labelled collectively as B. PS subunits are represented by numbers #1 to #3 and the Post-

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PS structure is sketched as layers #4 (representing mostly the Akchagyl Unit) and #5 (corresponding to the Apsheron Unit). βw shows the dip of the base of the weak, shale layer (W). Inset schemes show in every panel the plan-view of the structures, with faults

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in black and folds in red colour.

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REFERENCES

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Abdullayev, N.R., 2000. Seismic stratigraphy of the Upper Pliocene and Quaternary deposits in the South Caspian Basin. Journal of Petroleum Science and Engineering 28, 207-226. doi: 10.1016/S0920-4105(00)00079-6. Abrams, M.A., Narimanov, A.A., 1997. Geochemical evaluation of hydrocarbons and their potential sources in the western South Caspian depression, Republic of Azerbaijan. Marine and Petroleum Geology 14, 451-468. doi:10.1016/S0264-8172(97)00011-1. Abreu, V., Nummedal, D., 2007. Miocene to Quaternary sequence stratigraphy of the South and Central Caspian Basin, in: Yilmaz, P.O., Isaksen, G.H. (Eds.), Oil and gas of the Greater Caspian Sea. AAPG Studies in Geology 55, pp. 65-86. doi: 10.1306/1205845St553000. Afandiyeva, M.A., Babayev, R.G., Johnson, C.L., 2009. Stratigraphy of the Maikopian series of Azerbaijan, in: Peryt, D., Kaminski, M.A. (Eds.), Seventh Micropalaeontological Workshop MIKRO-2009. Grzybowski Foundation, Special Publication, Sw. Katarzyna, Poland. Aliyeva, E.G.-M., 2005. Reservoirs of the Lower Pliocene Productive Series at the western flank of the South Caspian Basin. Lithology and Mineral Resources 40, 267-278. doi: 00244902/05/4003/0267. Allen, M.B., Jackson, J., Walker, R., 2004. Late Cenozoic reorganization of the Arabia-Eurasia collision and the comparison of short-term and long-term deformation rates. Tectonics 23, 1-16. doi: 10.1029/2003TC001530. Allen, M.B., Vincent, S.J., Alsop, G.I., Ismail-zadeh, A., Flecker, R., 2003. Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone. Tectonophysics 366, 223-239. doi: 10.1016/S0040-1951(03)00098-2. Alsop, G.I., Holdsworth, R.E., 2002. The geometry and kinematics of flow perturbation folds. Tectonophysics 350, 99-125. doi: 10.1016/S0040-1951(02)00084-7. Baganz, O.W., Bagirov, E., Michael, G.E., Shultz, A., 2012. Productive series play of the Paleo-Volga Delta, South Caspian Basin: Exploration history, sedimentation, and petroleum system, in: Baganz, O.W., Bartov, Y., Bohacs, K., Nummedal, D. (Eds.), Lacustrine sandstone reservoirs and hydrocarbon systems. AAPG Memoir 95, pp. 57-70. doi: 10.1306/13291384M953444. Becker, J.J., Sandwell, D.T., Smith, W.H.F., Braud, J., Binder, B., Depner, J., Fabre, D., Factor, J., Ingalls, S., Kim, S-H., Ladner, R., Marks, K., Nelson, S., Pharaoh, A., Trimmer, R., Von Rosenberg, J., Wallace, G., Weatherall, P., 2009. Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS. Marine Geodesy 32, 355-371. doi: 10.1080/01490410903297766. Belopolsky, A.V., Talwani, M., 2007. Assessment of the greater Caspian region petroleum reserves and their role in world energy, in: Yilmaz, P.O., Isaksen, G.H. (Eds.), Oil and gas of the Greater Caspian Sea. AAPG Studies in Geology 55, pp. 5-7. doi: 10.1306/1205836St552723. Berberian, M., 1983. The southern Caspian: a compressional depression floored by a trapped, modified oceanic crust. Canadian Journal of Earth Sciences 20, 163-183. doi: 10.1139/e83015 Bertello, F., Cooper, C., Castellano, P., Ronchitelli, G.P., 2001. Improved characterization of a complex structure in the Azerbaijan offshore revealed by 3D pre-stack depth migration and depth continuity cube applications, SEG Annual meeting, U.S.A., SEG Technical Program Expanded Abstracts, pp. 617-620.

31

Folding in the western SCB – ms Santos-Betancor & Soto, 2015 ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Bredehoeft, J.D., Djevanshir, R.D., Belitz, K.R., 1988. Lateral fluid flow in a compacting sandshale sequence: South Caspian basin. American Association of Petroleum Geologists Bulletin 72, 416-424. Brunet, M.-F., Korotaev, M.V., Ershov, A.V., Nikishin, A.M., 2003. The South Caspian Basin: a review of its evolution from subsidence modelling. Sedimentary Geology 156, 119-148. doi: 10.1306/1205830St553247. Burberry, C.M., Cosgrove, J.W., Liu, J.G., 2010. A study of fold characteristics and deformation style using the evolution of the land surface: Zagros Simply Folded Belt, Iran, , in: Leturmy, P., Robin, C. (Eds.), Tectonics and Stratigraphic Evolution of Zagros and Makran during the Mesozoic-Cenozoic. Geological Society, Special Publication 330, pp. 139-154. doi: 10.1144/SP330.8. Chamberlin, R.T., 1910. The Appalachian folds of central Pennsylvania. Journal of Geology 18, 228-251. Chumakov, I.S., Byzova, S.L., Ganzey, S.S., 1988. Meotian-pontian geochronology of the eastern paratethys. Doklady Earth Sciences Sections 303, 66-69. Cosgrove , J.W., Ameen, M.S., 2000. A comparison of the geometry, spatial organization and fracture patterns associated with forced folds and buckle folds, in: Cosgrove, J.W., Ameen, M.S. (Eds.), Forced folds and fractures. Geological Society, Special Publication 169, pp. 721. doi:10.1144/GSL.SP.2000.169.01.02. Dadashev, F.G., Guseynov, R.A., Aliev, A.I., 1995. Map of mud volcanoes of the Caspian Sea (Explanaotry Notes). Geological Institute of Azerbaijan, Baku, p. 20. Dahlstrom, C.D.A., 1969. The upper detachment in concentric folding. Bulletin of Canadian Petroleum Geology 17, 326-346. Dahlstrom, C.D.A., 1970. Structural geology in the eastern margin of the Canadian Rocky Mountains. Bulletin of Canadian Petroleum Geology 18, 332-406. Davies, R.J., Stewart, S.A., 2005. Emplacement of giant mud volcanoes in the South Caspian Basin: 3D seismic reflection imaging of their root zones. Journal of the Geological Society of London 162, 1-4. doi: 10.1144/0016-764904-082. Devlin, W.J., Cogswell, J.M., Gaskins, G.M., Isaksen, G.H., Pitcher, D.M., Puls, D.P., Stanley, K.O., Wall., G.R.T., 1999. South Caspian Basin: young, cool, and full of promise. Geological Society of America Today 9, 1-9. EIA, 2013. Overview of oil and natural gas in the Caspian Sea region. U.S. Energy Information Administration, p. 25. Epard, J.L., Groshong, R.H.J., 1993a. Detachment folds; excess area and depth to detachment, Seventh meeting of the European Union of Geosciences; abstract supplement. Blackwell Scientific Publications Oxford, International, University of Alabama, United States. Epard, J.L., Groshong, R.H.J., 1993b. Excess area and depth to detachment. American Association of Petroleum Geologists Bulletin 77, 1291-1302. Etiope, G., Feyzullayev, A., Baciu, C.L., Milkov, A.V., 2004. Methane emission from mud volcanoes in eastern Azerbaijan. Geology 32, 465-468. doi: 10.1130/G20320.1. Evans, R.J., Stewart, S.A., Davies, R.J., 2008. The structure and formation of mud volcano summit calderas. Journal of the Geological Society 165, 769-780. doi: 10.1144/001676492007-118. Ferrill, D.A., Morris, A.P., Stamatakos, J.A., Sims, D.W., 2000. Crossing conjugate normal faults. American Association of Petroleum Geologists Bulletin 84, 1543-1559. Fleuty, M.J., 1964. The description of folds. Proceedings of the Geologists' Association 75, 461-492. doi: 10.1016/s0016-7878(64)80023-7. Forte, A.M., Sumner, D.Y., Cowgill, E., Stoica, M., Murtuzayev, I., Kangarli, T., Elashvili, M., Godoladze, T., Javakhishvili, Z., 2015. Late Miocene to Pliocene stratigraphy of the Kura Basin, a subbasin of the South Caspian Basin: implications for the diachroneity of stage 32

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JMPG-D-15 00068, June 2015

AC C

EP

TE D

M AN U

SC

RI PT

boundaries. Basin Research, 27, 247-271. doi: 10.1111/bre.12069. Fowler, S.R., Mildenhall, J., Zalova, S., Riley, G., Elsley, G., Desplanques, A., Guliyev, F., 2000. Mud volcanoes and structural development on Shah Deniz. Journal of Petroleum Science and Engineering 28, 189-206. Gherasim, M., Viceer, S., Brusova, O., Naumenko, J., Gullette, K., Dingler, J., Hill, A.W., 2015. Application of an integrated workflow for shallow-hazard characterization using a 3D high-resolution survey offshore Azerbaijan. The Leading Edge 34, 390-396. doi: 10.1190/tle34040390.1. Guliev, I., Panahi, B., 2004. Geodynamics of the deep sedimentary basin of the Caspian Sea region: paragenetic correlation of seismicity and mud volcanism. Geo-Marine Letters 24, 169-176. doi: 10.1007/s00367-004-0174-1 Hardy, S., McClay, K, 1999. Kinematic modelling of extensional fault-propagation folding. Journal of Structural Geology 21, 695-702. Hill, A.W., Arogunmati, A., Wood, G.A., Attoe, D., Fiske, M., Dingler, A., Dingler, J.A., Hobson, M., Robertshaw, A., Allinson, C., Kjos, E., Higson, M., Manning, T., Kassarie, K., Lewis, S.M., 2015. Slicing and dicing HR seismic acquisition: Varied approaches to delivery of high-resolution 3D seismic data volumes for drillinghazard studies. The Leading Edge 34, 380-388. doi: 10.1190/tle34040380.1. Hinds, D.J., Aliyeva, E., Allen, M.B., Davies, C.E., Kroonenberg, S.B., Simmons, M.D., Vincent, S.J., 2004. Sedimentation in a discharge dominated fluvial-lacustrine system: the Neogene Productive Series of the South Caspian Basin, Azerbaijan. Marine and Petroleum Geology 21, 613-638. doi: 10.1016/j.marpetgeo.2004.01.009. Hollingsworth, J., Jackson, J., Walker, R., Nazari, H., 2008. Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma. Geology 36, 763-766. doi: 10.1130/G25008A.1. Hudleston, P.J., 1973. Fold morphology and some geometrical implications of theories of fold development. Tectonophysics 16, 1-46. doi: 10.1016/0040-1951(73)90129-7. Hudson, S.M., Johnson, C.L., Efendiyeva, M.A., Rowe, H.D., Feyzullayev, A.A., Aliyev, C.S., 2008. Stratigraphy and geochemical characterization of the Oligocene–Miocene Maikop series: Implications for the paleogeography of Eastern Azerbaijan. Tectonophysics 451, 4055. doi: 10.1016/j.tecto.2007.11.045. Imbert, P., Geiss, B., Fatjó de Martín, N., 2014. How to evacuate 10 km3 of mud: saturate with gas and decrease the pressure! Geo-Marine Letters 34, 199-213. doi: 10.1007/s00367-0140357-3 Inan, S., Yalçin, M.N., Guliev, I.S., Kuliev, K., Feizullayev, A.A., 1997. Deep petroleum occurrences in the Lower Kura Depression, South Caspian Basin, Azerbaijan: an organic geochemical and basin modeling study. Marine and Petroleum Geology 14, 731-762. doi: 10.1016/S0264-8172(97)00058-5. Jackson, J., Priestley, K., Allen, M.B., Berberian, M., 2002. Active tectonics of the South Caspian Basin. Geophysical Journal International 148, 214-245. doi: 10.1046/j.1365246X.2002.01588.x. Jamison, W.R., 1987. Geometric analysis of fold development in overthrust terranes. Journal of Structural Geology 9, 207-219. doi: 10.1016/0191-8141(87)90026-5. Kadirov, F.A., 2000. Application of the Hartley transform for interpretation of gravity anomalies in the Shamakhy-Gobustan and Absheron oil- and gas-bearing regions, Azerbaijan. Journal of Applied Geophysics 45, 49–61. doi: 10.1016/S0926-9851(00)000185. Khain, V.E., Gadjiev, A.N., Kengerli, T.N., 2007. Tectonic origin of the Apsheron Threshold in the Caspian Sea. Doklady Earth Sciences 414, 552-556. doi: 10.1134/S1028334X07040149. 33

Folding in the western SCB – ms Santos-Betancor & Soto, 2015 ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Khain, V.Y., Grigoriants, B.V., Isayev, B.M., 1966. West Caspian fault and some factors controlling the transverse faults in geosynclinal folded regions. Byulleten' Moskovskogo Obshchestva Ispytatelei Prirody 41, 115-124. Knapp, C.C., Knapp, J.H., Connor, J.A., 2004. Crustal-scale structure of the South Caspian Basin revealed by deep seismic reflection profiling. Marine and Petroleum Geology 21, 1073-1081. doi: 10.1016/j.marpetgeo.2003.04.002. Korchagina, Y.I., Guliev, I.S., Zeinalova, K.S., 1988. Hydrocarbon and Cenozoic deposits of the South Caspian Basin, in: Problems in the oil and gas content of the Caucasus. Nauka, Moscow. Lee, S., Shaw, J., Ho, R., Burger, J., Singh, S., Troyer, B., 1998. Illuminating the shadows: Tomography, attenuation, and pore-pressure processing in the South Caspian Sea. The Leading Edge 17, 777-782. doi: 10.1190/1.1438053 Lerche, I., Bagirov, E., Nadirov, R.S., Tagiyev, M., Guliyev, I., 1996. Evolution of the South Caspian Basin: geologic risks and probable hazards. Nafta-Press, Azerbaijan Academy of Sciences, p. 625. Mangino, S., Priestley, K., 1998. The crustal structure of the southern Caspian region. Geophysical Journal International 133, 630-648. doi: 10.1046/j.1365-246X.1998.00520.x. Medwedeff, D.A., 1989. Growth fault-bend folding at southeast Lost Hills, San Joaquin Valley, California. American Association of Petroleum Geologists Bulletin 73, 54-67. doi: 10.1306/703C9AE6-1707-11D7-8645000102C1865D Mellors, R., Kilb, D., Aliyev, A., Gasanov, A., Yetirmishli, G., 2007. Correlations between earthquakes and large mud volcano eruptions. Journal of Geophysical Research 112, doi:10.1029/2006JB004489. Mitra, S., 2002. Structural models of faulted detachment folds. American Association of Petroleum Geologists Bulletin 86, 1673-1694. doi: 0.1306/61EEDD3C-173E-11D78645000102C1865D Mitra, S., 2003. A unified kinematic model for the evolution of detachment folds. Journal of Structural Geology 25, 1659-1673. doi: 10.1016/S0191-8141(02)00198-0. Mitra, S., Namson, J., 1989. Equal-area balancing. American Journal of Science 289, 563-599. doi:10.2475/ajs.289.5.563. Morley, C.K., Guerin, G., 1996. Comparison of gravity-driven deformation styles and behavior associated with mobile shales and salt. Tectonics 15, 1154-1170. doi: 10.1029/96TC01416 Morley, C.K., King, R., Hillis, R., Tingay, M., Backe, G., 2011. Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectivity: A review. Earth-Science Reviews 104, 41-91. doi: 10.1016/j.earscirev.2010.09.010. Morton, A., Allen, M., Simmons, M., Spathopoulos, F., Still, J., Hinds, D., Zadeh , A.I., Kroonenberg, S., 2003. Provenance patterns in a neotectonic basin: Pliocene and Quaternary sediment supply to the South Caspian. Basin Research 15, 321-337. doi: 10.1046/j.13652117.2003.00208.x. Nadirov, R.S., 1985. The geological structure and oil and gas presence of the Talysh-Vandam gravitational maximum area. Master thesis (in Russian). Baku, p. 30 Nadirov, R.S., Bagirov, E., Tagiyev, M., Lerche, I., 1997. Flexural plate subsidence, sedimentation rates, and structural development of the super-deep South Caspian Basin. Marine and Petroleum Geology 14, 383-400. doi: 10.1016/S0264-8172(96)00054-2. Planke, S., Svensen, H., Hovland, M., Banks, D.A., Jamtveit, B., 2003. Mud and fluid migration in active mud volcanoes in Azerbaijan. Geo-Marine Letters 23, 258-268. doi: 10.1007/s00367-003-0152-z. Poblet, J., 2004. Geometría y cinemática de pliegues relacionados con cabalgamientos. Trabajos de Geología 24, 127-146. 34

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Poblet, J., Bulnes, M., McClay, K., Hardy, S., 2004. Plots of crestal structural relief and fold area versus shortening: a graphical technique to unravel the kinematics of thrust-related folds, in: McClay, K.R. (Ed.), Thrust tectonics and hydrocarbon systems. AAPG Memoir 82, pp. 372-399. Poblet, J., McClay, K., 1996. Geometry and kinematics of single-layer detachment folds. American Association of Petroleum Geologists Bulletin 80, 1085-1109. Poblet, J., McClay, K., Storti, F., Muñoz, J.A., 1997. Geometries of syntectonic sediments associated with single-layer detachment folds. Journal of Structural Geology 19, 369-381. doi: 10.1016/S0191-8141(96)00113-7. Priestley, K., Baker, C., Jackson, J., 1994. Implications of earthquake focal mechanism data for the active tectonics of the South Caspian Basin and surrounding regions. Geophysical Journal International 118, 111-141. doi: 10.1111/j.1365-246X.1994.tb04679.x. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill Book Company, New York, p. 568. Reynolds, A.D., Simmons, M.D., Bowman, M.B.J., Henton, J., Brayshaw, A.C., Ali-Zade, A.A., Guliyev, I.S., Suleymanova, S.F., Ateava, E.Z., Mamedova, D.N., Koshkarly, R.O., 1998. Implications of outcrop geology for reservoirs in the Neogene Productive Series: Apsheron Peninsula, Azerbaijan. American Association of Petroleum Geologists Bulletin 82, 25-49. Rich, J.L., 1934. Mechanics of low-angle overthrust faulting as illustrated by Cumberland Thrust block, Virginia, Kentucky, and Tenesse. American Association of Petroleum Geologists Bulletin 18, 1584-1596. Roberts, K.S., Davies, R.J., Stewart, S.A., 2010. Structure of exhumed mud volcano feeder complexes, Azerbaijan. Basin Research 22, 439-451. doi: 10.1111/j.13652117.2009.00441.x. Rowan, M.G., Jackson, M.P.A., Trudgill, B.D., 1999. Salt-related fault families and fault welds in the northern Gulf of Mexico. American Association of Petroleum Geologists Bulletin 83, 1454-1484. Rowan, M.G., Peel, F.J., Vendeville, B.C., 2004. Gravity-driven fold belts on passive margins, in: McClay, KR. (Ed.), Thrust tectonics and hydrocarbon systems. AAPG Memoir 82, 157182. doi: 10.1306/M82813C9. Shaw, J.H., Connors, C.D., Suppe, J., 2005 (Eds.). Seismic interpretation of contractional faultrelated folds. AAPG Studies in Geology 53, p. 156. Smith-Rouch, L.S., 2006. Oligocene–Miocene Maykop/Diatom total petroleum system of the South Caspian Basin Province, Azerbaijan, Iran, and Turkmenistan. U.S. Geological Survey Bulletin 2201, 1-27. Sobornov, K.O., 1996. Lateral variations in structural styles of tectonic wedging in the northeastern Caucasus, Russia. Bulletin of Canadian Petroleum Geology 44, 385-399. Soto, J.I., Fernández-Ibáñez, F., Talukder, A.R., Martínez-García, P., 2010. Miocene shale tectonics in the Northern Alboran Sea (Western Mediterranean), in: Wood, L.J. (Ed.), Shale Tectonics. AAPG Memoir 93, pp. 119-144. doi: 10.1306/13231312M933422. Soto, J.I., Santos-Betancor, I., Fernández-Ibáñez, F., Talukder, A.R., 2014. Procesos tectónicos en cuencas con formaciones arcillosas: Estructuras, condiciones e implicaciones para la exploración petrolera, in: Hernández Romano, U., Aranda-García, M. (Eds.), Congreso Mexicano del Petróleo, Acapulco, México, pp. 91-101. Tagiyev, M.F., Nadirov, R.S., Bagirov, E.B., Lerche, I., 1997. Geohistory, thermal history and hydrocarbon generation history of the north-west South Caspian Basin. Marine and Petroleum Geology 14, 362-382. doi: 10.1016/S0264-8172(96)00053-0. Van Baak, C.G.C., Vasiliev, I., Stoica, M., Kuiper, K.F., Forte, A.M., Aliyeva, E., Krijgsman, W., 2013. A magnetostratigraphic time frame for Plio-Pleistocene transgressions in the 35

Folding in the western SCB – ms Santos-Betancor & Soto, 2015 ACCEPTED MANUSCRIPT

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South Caspian Basin, Azerbaijan. Global and Planetary Change 103, 119-134. doi: 10.1016/j.gloplacha.2012.05.004. Van Rensbergen, P., Morley, C.K., Ang, D.W., Hoan, T.Q., Lam, N.T., 1999. Structural evolution of shale diapirs from reactive rise to mud volcanism: 3D seismic data from the Baram delta, offshore Brunei Darussalam. Journal of the Geological Society of London 156, 633-650. doi: 10.1144/gsjgs.156.3.0633. Wiltschko, D.V., Groshong Jr, R.H., 2012. The Chamberlin 1910 balanced section: Context, contribution, and critical reassessment. Journal of Structural Geology 41, 7-23. Wu, S., Bally, A.W., 2000. Slope tectonics-comparisons and contrasts of structural styles of salt and shale tectonics of the Northern Gulf of Mexico with shale tectonics of Offshore Nigeria in Gulf of Guinea, in: Mohriak, W., Talwani, M. (Eds.), Atlantic Rifs and Continental Margins. American Geophysical Union, Geophysical Monograph, Washington, DC, pp. 151-172. doi: 10.1029/GM115p0151. Yusifov, M., 2004. Seismic interpretation and classification of mud volcanoes of the South Caspian basin, offshore Azerbaijan. Master thesis. Texas A&M University, p. 104. Yusifov, M., Rabinowitz, P.D., 2004. Classification of mud volcanoes in the South Caspian Basin, offshore Azerbaijan Marine and Petroleum Geology 21, 965-975. doi: 10.1016/j.marpetgeo.2004.06.002. Zonenshain, L.P., Le Pichon, X., 1986. Deep basins of the Black Sea and Caspian Sea as remnants of Mesozoic back-arc basins. Tectonophysics 123, 181-211.

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The studied anticline is detached from overpressured muds in the South Caspian Sea. Geometrical analysis evidences two folding episodes and one mud diapirism event. Folding is nucleated above a buried fault with normal and right-lateral components. Subsurface mud flow and basin tilting accompany the deposit of the Productive Series. Mud teardrop diapirs are isolated in fold culminations rooted by sub-vertical welds.