Disharmonic Folding Research Articles

Seismic stratigraphy and structural analysis in the determination of petroleum play within Salin basin, Myanmar

<p>This study presents a new interpretation of the seismic stratigraphic unit equivalent to the Late Cretaceous to Pliocene formations in the Salin basin. Many researchers investigated the outcrops of the stratigraphic succession of the Salin basin but there is a lack of comprehensive analysis by seismic interpretation. This study provides a comprehensive description of the structural development within the Salin basin, enhancing our comprehension of the regional stratigraphic evolution of the examined region. Seismic sequences were delineated using a combination of horizon mapping, internal reflection configuration, termination patterns, and thickness analysis. We identified two distinct mechanisms for lateral fault seals: (1) primary juxtaposition seals and (2) secondary fault rock seals (also known as membrane seals). Once the hydrocarbons are matured within the source formations (Late Cretaceous to Early Oligocene shales units): Kabaw (KB); Laungshe (LA); Tabyin (TA); Pondaung (PO); Yaw (YA); and Shwezettaw (SZT), they are migrated towards the reservoir formations (Pondaung (PO), Shwezettaw (SZT), Padaung (PA), Okhmintaung (OHK), and Kyaukkok (KK)) through these predicted paths. Generally, the primary migration takes place along the fault or fractured planes or pores within rock units. In the southwestern limb of the Yenangyaung anticline, the disharmonic folding and upward bending of the KK, PY, OHK, and PA formations resulted in the deformation of ductile strata within the PA Formation, forcing them towards the peripheral synclines. The main deep fore-thrust in the southwestern limb of the Chauk anticline rooted within the PA Formation flattened up while approaching the OHK and PY formations. Chauk anticlinal axis above the PY and OHK detachment level are more migrated toward the northeast. As a result, above the crest of the deep anticline, the shallow low-angle fore-thrust pushed up into the growth strata, leading to the uplift of the YA and SZT formations within the Letpando anticline. YA Formation crops out locally along the axis of the structure and is the major seal, with sandstones in the underlying Eocene PO Formation being the main reservoir.</p>

Effect of inherited structural highs on the structure and kinematics of the South Dezful Embayment, SW Iran

AbstractUnderstanding the structural evolution of the South Dezful Embayment (SDE) in the Zagros Fold-and-Thrust Belt (ZFTB) is significant for hydrocarbon exploration and production. The structural evolution of this area has been controlled by the reactivation of basement structures related to the oblique convergence along the ZFTB. In this work, we study the effect of the pre-contractional interaction between the basement step and overlaid salt layer superimposed by the Zagros shortening on the structural style and evolution of the sedimentary basin in the SDE. We use multidisciplinary approaches involving surface and subsurface data and analogue models. Disharmonic folds with small wavelength formed over the Kharg-Mish Palaeo-high (KMPH) where the sedimentary cover is thinner. On the other hand, faulted detachment folds with large wavelengths developed in the adjacent depocenters. Furthermore, the KMPH affected the geometry and kinematics of the frontal part of the Zagros Belt, which is characterized by the Mountain Front Flexure (MFF) topographic step. Long-lasting active deformation of the sedimentary cover over the frontal ramp of the KMPH developed the South Izeh Promontory (SIP). Localized contraction in the SIP was accommodated temporally by trishear fault-propagation folding and resulted in > 5 km elevation difference between this promontory and two adjacent local basins. Supporting scaled analogue models show that structural evolution, folding and along-strike variations in fold style of the SDE have been controlled by thickness variations of the sedimentary cover and the geometry of the inherited pre-salt structure. Based on the results of this study, the effect of inherited structural highs on the structure and kinematics of the SDE provide important insights for hydrocarbon entrapment, migration and exploration.

THE ROLE OF MULTIPLE WEAK LITHOLOGIES IN THE DEFORMATION OF COVER UNITS IN THE NORTHWESTERN SEGMENT OF THE ZAGROS FOLD‐AND‐THRUST BELT

The geometry, kinematics and dynamics of fold‐and‐thrust belts are strongly influenced by the mechanical behaviour of the basal décollement. However, many fold‐and‐thrust belts also include mechanically weak lithologies such as evaporites and marls or mudstones at different levels within the shortened stratigraphy. The kinematics and dynamic evolution of these thrust belts are controlled by the mechanical behaviour both of the basal décollement and of the weak units embedded within the overlying stratigraphic succession. In the Zagros fold‐and‐thrust belt (ZFTB), the shortened sedimentary cover is between 7 and 12 km thick and mechanically weak lithologies compartmentalize the stratigraphic column at shallow and intermediate levels. In this paper, satellite, field and seismic data from the Kurdistan Region of Iraq are used to identify structures of different sizes and surface traces. The observations are used to underline the role of mechanically weak horizons within the Zagros stratigraphy and the decoupling of deformation both laterally and with depth in the belt.The decoupling between shallow and deeper structures observed in seismic profiles from the Kurdistan Region of Iraq is also reported from field observations from the Iranian part of the Zagros fold‐and‐thrust belt, where folds with different surface traces occur. Decoupling between shallow and deep layers by incompetent lithologies at intermediate depths (e.g. marls, mudstones and evaporites) results in the formation of disharmonic folds. The geometry, size and location of such folds may differ between outcropping and subsurface structures. Decoupling may have a significant impact on hydrocarbon exploration in different parts of the Zagros fold‐and‐thrust belt due to potential offsets between outcropping and subsurface structures and their associated traps.

Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From text{ca.;26,Ma} onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults (text{ca.} 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than 50^{circ} already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions.

Detalied magnetotelluric study of the northern part of Subandean fold belt, Bolivia

A detailed magnetotelluric survey was carried out in the northern part of Subandean fold belt, Bolivia, with the main goal of better understanding of the geological structure of the sedimentary basin and petroleum system in the study area. An improvement of magnetotelluric technology was developed aimed at better imaging of resistivity structures in fold belt. A new static shift correction procedure and a new multistage unconstrained and constrained 2D inversion workflow was suggested and successfully implemented. In addition, the original 3D inversion approach was developed and its efficiency for fold belt geological settings was shown.Advances in magnetotelluric technology made it possible to obtain new geological information about the structure of the sedimentary cover in the study area. Two resistivity structural levels differing in resistivity were outlined. The boundary between levels corresponds to the top of the Carboniferous-Devonian formations, which is one the principle detachments in the northern part of Subandean fold belt. The topography of the top of the lower structural level does not correlate with structures in the upper structural level and with structures outlined by surface geology in most of the survey area. A large numbers of fault zones are outlined in the upper structural level, narrow anticlines are characterized by complicated structure (palm-tree structures).Magnetotellurics gives the most reliable position of the buried axes of anticlinal disharmonic folds, which opens up the possibility of constructing more reliable geological models using joint inversion of magnetotelluric data together with other geophysical data, first of all seismic data. Application of magnetotellurics in fold basins as complimentary to 2D seismic geophysical technology gives possibility to increase reliability of geological interpretation.

Structural style and kinematic analysis of folding in the southern Dezful Embayment oilfields, SW Iran

Compression of a region containing multiple detachment layers produces large-scale disharmonic folding, where individual competent units separated by detachment fold independently within the succession. Here, we present a case study of the kinematic evolution of three subsurface structures (the Karanj, Paranj and Parsi oilfields) bounded by multiple detachments, located in the southern Dezful Embayment, a major petroleum province of the Zagros Fold-Thrust Belt in SW Iran. Interpretation of seismic sections and the construction of corresponding balanced cross-sections reveals different geometries along the folds and the complete decoupling of these structures from the surface geology. This is controlled by the reactivation of basement faults, and flow of the Gachsaran incompetent units (Miocene) as the upper detachment, and the Kazhdumi (Upper Cretaceous),Pabdeh-Gurpi and Dashtak Formations as the middle detachments, respectively. Detachment folding with limb rotation started in the Middle Miocene and continued with the migration of the basal detachment horizon (equivalent Hormuz series) into the core of the anticlines. Flow of both the basal and upper detachments continued until no more material is available, whereupon the shortening is accommodated by fault development. The development of a decoupled, disharmonic folding style was controlled by the upper detachment surface through migration from the crests of the anticlines to the synclinal areas and by the reactivation of the basement faults. From NW to SE part of the area The total shortening amounts are changed between 21% and 12%.

Partitioned tectonic shortening, with emphasis on outcrop-scale folding and flattening, Pindos fold-and-thrust belt, Peloponnese, Greece

Shortening estimates for fold–thrust belts seldom take into consideration outcrop-scale folding, especially folding related to prethrusting layer-parallel shortening (LPS) and flattening. The Pindos belt of the Peloponnese contains products of Maastrichtian to Paleocene tectonic shortening amenable to assessing strain partitioning. Shortening there initiated with LPS, including outcrop-scale folding, which was superseded by thrusting and macrofolding, with both macromechanisms producing additional outcrop-scale folding and (or) form-modification of initial LPS outcrop-scale folds. Skourlis and Doutsos (2003. International Journal of Earth Sciences, 92: 891–903) concluded that total shortening in this region was 68%, the summation of ∼15% LPS, ∼6% fault-related macrofolding, and ∼47% thrust–slip translation. But even this degree of shortening may be underestimated when outcrop-scale folding is considered. Two limestone-dominated Cretaceous formations and one Cretaceous–Eocene formation of limestone and mudstone display spectacular outcrop-scale folding produced by true multilayer behavior, internal buckling, and disharmonic quasi-flexural folding. Fold forms vary from chevron buckling of multilayers to more rounded folding of true multilayers to disharmonic flattened folds in pseudo-bedded limestone. Ramsay analysis of layer thickness variations across individual disharmonic folds underscores the disharmonic fold forms and intense degree of pressure dissolution generated fold flattening. Line-length shortening for well exposed “packages” of LPS outcrop-scale folding is as high as 30%, but overlying and underlying panels may be more modestly folded and (or) flattened, suggesting that shortening transfer mechanisms may have caused some sections to be bypassed. Achieving more meaningful shortening estimates of the outcrop-scale folding within an entire progressive deformation will require understanding of the partitioning of distribution of this folding regionally.

Slump sheets as a record of regional tectonics and paleogeographic changes in South China

Slumps of various scales are recognized in the late Katian (late Ordovician) mixed carbonate and siliciclastic slope successions in the Changshan area, western Zhejiang Province, South China. In order to understand their deformational processes, triggering mechanisms, paleo-slope orientation and geological implications, a detailed sedimentological study was carried out on four outcrop sections. In the southern Changshan (the Putangkou, Zhoutang and Shikeng sections), we recognized three slumping episodes (ca. 3.5 m, 11 m, and 73 m in thickness in stratigraphically ascending order) and co-occurred debris flows. At the Sanqushan section (~11.5 km north of Changshan), thirteen slump units, each being tens of cm to 6 m in thickness, are recognized. Facies analyses and diagenetic characteristics of these slumps and their internal deformation structures suggest that they formed as a result of plastic and brittle deformation, and liquefaction and fluidization during slumping. The rip-up blocks, cracks and ruptured beds in the slumps of southern Changshan indicate extensional features, whereas recumbent and disharmonic folds in the slumps of northern Changshan represent compressional features. These spatially different features suggest that the slumps may have been generated on a paleo-slope dipping generally to the north. The case study of slumps and debris flows from the Putangkou section indicates a significant mass flow transition from a slumped mass into debris flow deposits. Autogenic triggering mechanisms of these slumps, e.g., rapid deposition, storm-wave loading, and tidal shear, are ruled out by the results of facies analysis. Instead, the slumps might have been triggered by relative sea-level changes or tectonically-induced, progressively steepening of the slope, which is likely related to the contemporaneous Kwangsian Orogeny.

Structural styles and decoupling in stratigraphic sequences with double décollements during thin-skinned contractional tectonics: Insights from numerical modelling

Six series of particle-based numerical experiments were performed to simulate thin-skinned contractional tectonics in stratigraphic sequences with double décollements during horizontal shortening. The models were assigned with varying rock competence, depth and thickness of the upper décollement, which resulted in significantly different styles of deformation and decoupling characteristics above and below the upper décollement. The models composed of the least competent material produced distributed sinusoidal detachment folds, with many shallow structures profoundly decoupled from the deep-seated folds. The models composed of a more competent material are dominated by faulted box folds, with minor disharmonic folds developed in their limbs. In contrast, the results of models composed of the most competent material are characterised by localised piggyback thrusts, fault-bend folds and pop-up structures with tensile fractures developed in fold hinges. Depth of the upper décollements also plays an important role in controlling structural decoupling, i.e. the shallower the upper décollements, the higher the degree of decoupling becomes. Thicker upper décollements can provide sufficient mobile materials to fill fold cores, and contribute to the formation of secondary disharmonic folds, helping enhance structural decoupling. Our modelling results are comparable to the structural features exhibited in the Dezful Embayment of the Zagros Fold-and-Thrust Belt with the Miocene Gachsaran Formation acting as the shallow upper décollement, and the Fars with the Triassic Dashtak Formation as its intermediate décollement. This study demonstrates that rock competence, depth and thickness of the upper décollements can jointly affect the structural styles and decoupling. Our modelling results are instructive for structural interpretation of deep zones in fold-and-thrust belts that exhibit distinct structural decoupling features.

Earthquake-induced soft sediment deformation (SSD) structures from the Bilara limestone formation, Marwar basin, India

The Neoproterozoic Bilara limestone Formation of the Marwar Group, Rajasthan, India exposes metres-thick layers of soft sediment deformation (SSD) structures at different stratigraphic levels which could be traced over hundreds of metres on the outcrop scale. The SSD structures include disharmonic folds, low-angle thrusts, distorted laminae, fluidisation pipes, slump and load structures, homogeneities, diapirs, etc. Whereas SSD structures suggesting tensional stress, viz., intrastriatal graben, fluidisation, slump, etc. dominate in the lower part of the Bilara succession, features implicating compression, viz., folds, low-angle thrust are prevalent in the uppermost part. Since SSD structures are mostly confined within the algal laminites, we interpret that enhanced micritic fluid pressure below early cemented algal carbonate played a major role in laminae deformation. Depending on the degree of lithification and pore-water pressure, deformation features formed either plastically or led to diapiric injection at enhanced pore water pressure. Separated by near-horizontal underformed strata, the SSD layers, traceable over hundreds of metres, are interpreted as products of seismic shacking. Considering the time frame of the Marwar basin, i.e., the Precambrian–Cambrian transition, the SSD horizons present within the Bilara succession may hold the potential for the correlation with SSD structures reported from the time-correlative stratigraphic successions present in erstwhile adjoining tectonic terrains, e.g., China, Siberia, etc.

Evolution of the NW Zagros Fold-and-Thrust Belt in Kurdistan Region of Iraq from balanced and restored crustal-scale sections and forward modeling

We present the first regional balanced and restored sections across the northwestern part of the Zagros Fold-and-Thrust Belt in Kurdistan Region of Iraq and a 2D kinematic model that illustrates the evolution of the belt since Late Cretaceous time. The balanced cross-section, based on surface and sub-surface data, is characterized by multi-detachment folds detached above a Lower Triassic basal ductile level, with intermediate detachment levels that induced internal complexities like accommodation thrusting and/or disharmonic folding. Our work suggests that the two main structural steps in the detachment level in the High Folded Zone may be related to low-angle thrusts rooted at the brittle/ductile transition. Growth strata of Late Cretaceous and Paleocene times have been recognized for the first time in the Kurdistan Fold-and-Thrust Belt. This allows us to constrain timing of deformation and to estimate the evolution of the shortening and the advance of the deformation front since Late Cretaceous. Deformation of the Zagros belt is characterized by a combination of thin- and thick-skinned tectonics that reactivated the Late Cretaceous-Paleogene obduction belt.

Archean controls on basin development and mineralisation in the Southern Capricorn Orogen

Basins along the northern margin of the Yilgarn Craton developed in response to extensional and compressional processes during the Paleoproterozoic. Early extension resulted in the formation of the Yerrida Basin as a large single basin over the northern Yilgarn Craton. Subsequent rifting led to voluminous volcanism in the northern part of the basin, within two depositional centres - the c. 2.03 to 1.96 Ga Bryah and Mooloogool Sub-basins. Cu-Au VMS deposits formed in the Bryah Sub-basin. Yilgarn Craton crust can be mapped using gravity and magnetic data beneath the Yerrida Basin, and Mooloogool Sub-basin. However, the Yerrida Basin can’t be mapped below much of the Bryah Basin, implying the formation of an ocean in this region. The degree of rifting of the Yilgarn Craton, and resulting architecture influenced subsequent basin development, and deformation in the region. For example, in areas where Yilgarn Craton crust can’t be mapped beneath basin sediments deformation is pronounced with the formation of disharmonic folds, refolded folds, and anastomosing shear zones. The southern part of the Yerrida Basin and the Earaheedy Basin formed shallow depositional centres over the Yilgarn Craton, and subsequent deformation in these regions is less intense. Base metal mineralisation in the region can, in part be related to the presence of deep crustal scale structures that initially developed in the Archean, and were re-activated during the Proterozoic. However, the location of c. 1800 Ma orogenic Au mineralization in the Bryah Sub-basin may not have been influenced by localisation of fluid flow around deep crustal-scale faults that formed as 200 Ma earlier, or even as early as the Archean.

Folding style of the Dezful Embayment of Zagros Belt: Signatures of detachment horizons, deep-rooted faulting and syn-deformation deposition

Defining the governing factors on the geometry of the Dezful Embayment folds, a major oil province in the Zagros Fold-Thrust Belt in SW Iran, reduces the risks inherent in hydrocarbon exploration. The goal of the current study is to find a reasonable relationship between folding and faulting in the northern part of the Dezful Embayment. The role of major and minor detachment horizons and deep-rooted faults on structural style is also a matter of investigations. To achieve these goals, geological surface information and the available well and 2D and 3D seismic data were used to construct a balanced structural cross-section. The area is subdivided into NE and SW sectors based on dissimilar physiographic features of the surface and structural and stratigraphic characteristics. In the both sectors, the Miocene Gachsaran Formation acted as the upper detachment horizon. In the NE sector, the role of the Gachsaran Formation as major detachment horizon in decoupled folding styles of overlying and underlying units is prominent. The Gachsaran Formation becomes less effective as a detachment horizon towards the SW as its thickness decreases and its facies change. The Triassic Dashtak Formation is most probably a major detachment horizon in the study area and evidence of another deeper detachment horizon (such as the pre-Cambrian-Cambrian Hormuz Series) is not apparent based on available data. The Lower Cretaceous Garau, Upper Cretaceous Gurpi and Paleocene-Eocene Pabdeh formations and the Oligocene Kalhur Member are subordinate detachment horizons. The role of major faults in the area, such as the Mountain Front Fault in driving thick-skinned deformation is taken into account along with the dominant thin-skinned deformation in the study area. The Kuh-e Asmari Anticline is the only emergent anticline exposing the Paleocene strata due to the activity of the deep-rooted Lahbari Thrust. This study reveals the commanding role of the upper detachment Gachsaran Formation and the overlying syn-deformation fluvial clastics of the Aghajari (Miocene) and Bakhtyari (Pliocene) formations on disharmonic folding styles.

New insights into the glacial history of southwestern Poland based on large-scale glaciotectonic deformations – a case study from the Czaple II Gravel Pit (Western Sudetes)

This paper presents the results of structural and sedimentological studies carried out in the outcrops of Quaternary (Middle Pleistocene) deposits near the village of Czaple in Lower Silesia, Western Sudetes. Fluvial sands, gravels and glacial tills traditionally assigned to the Middle Polish Pleistocene Glaciations (Odranian Glaciation) crop out in the active gravel pit Czaple II. In these deposits, the authors have recognised and documented numerous mesoscale glaciotectonic deformation structures that were previously undescribed from the mountainous part of the Sudetes. These structures represent effects of sediment deformation in both proglacial and subglacial settings, and include such features as asymmetrical and disharmonic folds, thrusts, steeply inclined reverse faults, normal faults and conjugate sets of fractures. Based on the orientation of kinematic indicators associated with the faults and fracture planes (slickensides, hackles, grooves, R-shears) within the fine-grained sediments and in sands and gravels, an S- and SE-directed horizontal compression due to ice-sheet advance is postulated. This direction is consistent with some relevant other data from the Sudetes, indicating that the Odranian ice sheet advanced into the area of the Sudetes from the north, north-east and north-west.

Kilometre-scale structural setting of ore shoots in the Frasers gold deposit, Macraes mine, New Zealand

The Hyde-Macraes Shear Zone in southern New Zealand contains the circa 10 million ounce Macraes gold deposit, one of the larger Phanerozoic orogenic gold deposits discovered to date globally. Approximately 50% of this 10 million ounce resource is hosted by 5 major ore shoots up to 400m wide and 1500m long in the Frasers area at the southern end of the mine. Higher grade (>1.5g/t Au) ore shoots are located along and immediately below the Hangingwall Shear, the principal strand of the Hyde-Macraes Shear Zone at the Frasers deposit. They typically trend parallel to the intersection of the shear and foliation in the underlying schist, commonly where the foliation dips more steeply that the overlying Hangingwall Shear. Especially thick zones of higher grade mineralised rock are located between the Hangingwall Shear and underlying second order splay shears whose position correlates with minor right-hand bends in the strike of the overlying Hangingwall Shear. Lower grade (<1.2g/t Au), but economically significant, ore shoots are located within mineralised schists below the Hangingwall Shear. Outer margins of these lower grade ore shoots are generally parallel to the strike of the foliation in the host schist. They are most extensive where open disharmonic folding has resulted in the strike of the foliation diverging from that of the overlying Hangingwall Shear. No correlation exists between the position of any ore shoots and gently dipping jogs in the Hangingwall Shear, despite mineralisation occurring during reverse movement on the Hyde-Macraes Shear Zone. Instead the angular relationship between various strands of the Hyde-Macraes Shear Zone at Frasers and foliation in underlying schists is the most consistent structural feature likely to predict the location, extent, and orientation of ore shoots within the Frasers segment of the Hyde-Macraes Shear Zone.

Glass Microbeads in Analog Models of Thrust Wedges.

Glass microbeads are frequently used in analog physical modeling to simulate weak detachment zones but have been neglected in models of thrust wedges. Microbeads differ from quartz sand in grain shape and in low angle of internal friction. In this study, we compared the structural characteristics of microbeads and sand wedges. To obtain a better picture of their mechanical behavior, we determined the physical and frictional properties of microbeads using polarizing and scanning electron microscopy and ring-shear tests, respectively. We built shortening experiments with different basal frictions and measured the thickness, slope and length of the wedges and also the fault spacings. All the microbeads experiments revealed wedge geometries that were consistent with previous studies that have been performed with sand. However, the deformation features in the microbeads shortened over low to intermediate basal frictions were slightly different. Microbeads produced different fault geometries than sand as well as a different grain flow. In addition, they produced slip on minor faults, which was associated with distributed deformation and gave the microbeads wedges the appearance of disharmonic folds. We concluded that the glass microbeads may be used to simulate relatively competent rocks, like carbonates, which may be characterized by small-scale deformation features.

Intermediate decollement activation in response to the basal friction variation and its effect on folding style in the Zagros fold-thrust belt, an analogue modeling approach

Although the role of various basal and intermediate decollement levels on structural style is well documented individually in many folded terrains, the interaction between basal and intermediate decollements is poorly constrained. This study uses results of two scaled sand-box models shortened from one end to study the variation in structural development in response to varying basal friction and its consequent interaction with intermediate decollement horizons. Two models with similar incompetent intermediate decollement, but with different basal friction (with and without a thick basal decollement), were prepared analogous for the eastern and the western parts of the Razak basement fault in the Fars Region of the eastern part of the Zagros fold thrust belt (ZFTB). Combined results of scaled models with geological observations are used to argue that the basal decollement friction characteristics govern propagation of deformation front. In addition, model results, analogues to north-south direction, show that deformation complexity and disharmonic folding exist in the section where the intermediate decollement has been activated in response to the shortening without the basal decollement (throughout the western part of the Razak basement fault where less thickness of the Hormuz series as the basal decollement has been documented compared to its eastern part). In other words, the complexity in deformation is less portrayed along sections where basal friction beneath the model decreases (e.g. the eastern part of the Razak basement fault). We argue here that, in addition to other parameters (not presented in this study) interaction of intermediate decollement levels with basal decollement friction characteristics could explain decoupling between structures within the sedimentary column of the Fars Region of the eastern part of the Zagros fold thrust belt.

A semi-balanced section in the northwestern Zagros region: Constraining the structural architecture of the Mountain Front Flexure in the Kirkuk Embayment, Iraq

ABSTRACT The Mountain Front Flexure or Fault (MFF) of the Zagros Mountains separates the foreland or foothills area from the morphological apparent mountain belt. Across this feature the regional elevations of Mesozoic to Neogene stratigraphic horizons substantially rise towards the mountain belt. Thin-skinned and thick-skinned structural styles have been proposed for this rise in other parts of the Zagros region. In our study area, in the Kurdistan Region of Iraq (KRI), we integrated surface and subsurface data and constructed a (semi-) balanced cross-section across the MFF. The section features duplex structures in the deeper subsurface, related to a deeper Palaeozoic and a shallower Triassic decollement horizon. On a smaller scale, layer-parallel shortening and intense deformation is observed in the incompetent lithologies, leading to an incipient disharmonic folding. Restoration of the section reveals a distinct imbalance between shortening in the upper part of the stratigraphic section (approximately 4 km or 16% on top Jurassic level) to the lower part (approximately 20 km or 49% on top Permian level). The imbalance can only be equalised on a regional section if the shortening is transferred from the lower to the higher decollement levels, which is connected to folds and thrusts in the foothills area. Based on observations from the mechanical stratigraphy, geometric relationships in map and cross-section, as well as morphological considerations, we argue that the origin of the MFF in the area of the considered section is related to active roof duplexes rather than basement-involved thrusting.

Gridino melange zone of the Belomorian eclogite province: Succession of tectonic events and structural position of mafic dyke swarms

Based on relationships between Paleoproterozoic mafic dykes, lithotectonic complexes, and tectonic structures of the Gridino Zone in the Belomorian eclogite province of the Fennoscandian Shield, deformations have been divided into groups differing in age and the succession of tectonic events has been reconstructed. The formation of Neoarchean eclogite-bearing melange was related to disintegration of large eclogite sheets in the course of near-horizontal ductile flow accompanied by syntectonic granitoid magmatism, multiple migmatization, and granulite-to amphibolite-facies metamorphism. The exotic blocks, including eclogites, were incorporated into TTG gneisses as sheets and lenses up to a few hundreds of meters in thickness and oriented conformably with gneissic banding. As a result of ductile flow, the lithotectonic complexes were transported at the level of discrete brittle-ductile deformations expressed as strike-slip faults and associated folds. Under conditions of a relatively rigid medium, individual structural elements underwent rotation approximately through 90° in plan view. Under the extension regime in the Early Paleoproterozoic, several swarms of mafic dykes were injected into the already cold framework rocks, as is evident from dyke morphology. The dykes crosscut all predated structures, included turned blocks, and are therefore important reference points for subdivision of Neoarchean and Paleoproterozoic processes. The Svecofennian postdyke tectonic activity was accompanied by local shearing and boudinage of metabasic rocks, development of quartz and pegmatite veins along tension cracks, disharmonic folding, and discrete retrograde metamorphism up to amphibolite-facies conditions. The postdyke deformations did not exert a substantial effect on the previously formed regional structure.

Uncommon and Impact-Suspicious Geologic Phenomena across Jordan and Adjacent Areas, Arabian Plate

For the discovery and analysis of Jebel Waqf as Suwwan (JWS) Impact Crater, Jordan pushed the authors to consequently focusing on other unusual geologic phenomena such as circular/oval structures (some of “crypto-volcanic” origin), disharmonic folding, horizontal stylolites, and a broad stripe of Sanidine-Hornfels-Facies, all of them hosted in Upper Cretaceous/Paleogene carbonate rocks exposed across Jordan and adjacent areas. Shatter cones are the most useful tools during field work in the realm of circular structures. In addition to the impact-geologic data of JWS Impact Structure hitherto available, the cooling process of melted Lower Cretaceous Kurnub-Sandstone could be verified by microscopically identified SiO2-modifications between the melting point (1714°C) and low temperatures. In comparison with the Suffield 500 tons explosion tests [20] and with the Ries Impact Crater, Germany, excavation and vaporization processes of target rocks demand drilling between Central Uplift and Inner Ring of the JWS Impact Structure (“Chert-Carbonate-Impact-Chess Game”). In a scenario: “Impacting meets Plate Tectonics”, phenomena like disharmonic folding, horizontal stylolites, and an abundance of circular/oval structures of high diameter variation through northwest Jordan are discussed under aspects of gravitational gliding, effects of seismic surface waves (Love-), transpressional structures related to Jordan Rift-Tectonics, and possible impact processes of unknown number occurred on the Arabian Plate in southeastern direction with northwest-directed impulse. The so-called “Mottled Zone” of Jordan and Palestine owning a high number (~100) of mineral neoformations with formation temperatures up to ~1120°C(pseudo-wollastonite = β-CaSiO3), is to be interpreted as of “Sanidine-Hornfels-Impact Facies” under ricochet conditions along an “Avenue of Ricochet-Impact-Thermo-Cataclysm”. Marble-related travertines represent a low temperature facies of the impact event. The temporal projection of patterns like Rapid Climate Change (RCC), astrophysical data [Hoyle’s Comet: 47], acid events in ice-cores, tektite fall, Rise and Fall of Neolithic/Bronze Age Cultures, reveals an amazing “Ensemble” of coincidences among all of these parameters since the Middle/Upper Pleistocene boundary throughout Holocene. Ten short Interval Cases are discussed as being “Rare Event”- suspicious. Myths like the “Gilgamesch Epos” and “John’s Apocalypse” surprisingly provide realistic hints on impact events comparable with modern observations and experiments, which are: approach of extraterrestrial bodies, impact cratering, earthquakes and faulting, supercyclonic whirl-storms (wildfires), effects of hot ejecta, destruction of fauna, flora, and cities, long-lasting flooding, darkness caused by ashes and dust circulating in atmosphere and stratosphere (sintwinter), and environmental pollution. Such hints were handed down in myths of ~180 ethnic peoples around the globe. “High probability Cases” focus on impact events on ~9600 yr B.P and on ~6000 yr B.P. Other “Rare Events” may have possibly occurred about ~8000 yr B.P. and on ~3200 yr. B.P. (all radiocarbon yr.cal.). Core-drilling in the pull-apart basins of the Jordan Rift System (Dead Sea, Lake Tiberias) as traps having possibly preserved volcanic and impact ejecta would be a challenging international and interdisciplinary project.