Fold Amplification Research Articles

Two Fold Amplification of Paste Electrode with Pt/Graphene Nanocomposite and Room Temperature Ionic Liquid to Monitoring of Vitamin B6 in Environmental and Food Samples

Two Fold Amplification of Paste Electrode with Pt/Graphene Nanocomposite and Room Temperature Ionic Liquid to Monitoring of Vitamin B6 in Environmental and Food Samples

Shale 3D Flow and Interaction With Basement Faults in the Niger Delta Deep‐Water Fold and Thrust Belt

AbstractBased on a large 3D seismic data set in the deep‐water domain of the Niger Delta, this study challenges previous interpretations involving the occurrence of multiple detachments and extensive thrust flats, illustrating timing and mode of shales flow at the toe of the gravity system. Five units of syn‐kinematic sediments, reaching a maximum thickness of ∼800 m, accumulated in the tectonically subsiding synclines during fold amplification between ∼9.5 and ∼1.4 Ma. The volumes of syn‐kinematic units roughly balance those of the shales accumulated in the thickened cores of WNW trending anticlines. This feature is consistent with folding resulting from buckling controlled by the competence contrast between isopach Cenozoic units and underlying overpressured shales of the Akata Formation. A dense network of NE‐SW striking oblique extensional faults offsets a prominent anticline characterized by a NE‐SW trend (which is almost perpendicular to the regional fold trend). These faults form a narrow, continuous deformation zone extending for tens of kilometers along and beyond the length of the anticline. The faults, rooting within the shales of the Akata Formation, formed since ∼5 Ma and deform the seabed. Displacement distribution suggests mechanical interaction between isolated fault segments within the deformation zone. The latter is interpreted as the shallow expression of a deep‐seated fault zone inherited from the segmented passive margin and marked by gravity and magnetic data. Our results, providing a comprehensive picture of active deformation features and their relationships with deep‐seated faults, shed new light into the modes of interaction between gravity systems and underlying basement structures.

Contractional fold amplification through bed-parallel gypsum vein (“beef”) formation

Field observations and structural analysis of a 50-m-wavelength gentle anticline document bed-parallel fibrous gypsum veins (“beef”) and calcite veins associated with contractional folding in the Boquillas Formation, east of Langtry, west Texas. Gypsum beef veins with bed-perpendicular fibers accommodated vertical extension of 13% in the anticline, diminishing to 0% in adjacent synclines. Bed-parallel calcite veins accommodated 0–4% vertical extension across the structure, with no clear correspondence of vein distribution to structural position. Homogenization temperatures from two-phase fluid inclusions in calcite veins indicate trapping depths of 2.0–2.7 km. Source rock analysis from unoxidized organic-rich mudrock at the site shows that the rocks never reached hydrocarbon maturation conditions, therefore vein formation cannot be explained by in situ hydrocarbon expulsion. Bed-parallel veins with vertical or bed-perpendicular fibers document a thrust faulting stress regime during contractional folding near the frontal limit of Laramide deformation. Gypsum vein formation may be related to meteoric water interacting with pyrite or H2S to form sulfuric acid, dissolution of carbonate, and deposition of gypsum as an auxiliary mineral. Switching between calcite and gypsum mineralization suggests opening and closing of fault and fracture controlled fluid pathways during deformation.

Detachment folding, growth mechanism and seismic potential in the Jammu Sub-Himalaya

The Surin Mastgarh anticline (SMA) marks the active deformation front in the northwest Sub-Himalaya near Jammu. The southern limb of the SMA is not truncated by an emergent thrust, unlike other frontal folds along the Himalayan front. We investigate the structural data, deformation pattern, and seismic potential of the SMA using field surveys and re-interpretation of seismic profile. Strath profiles along the Chenab and the Munavar Tawi river valleys indicate active growth of the SMA by layer parallel shortening, and limb rotation accompanied by flexural slip, suggesting an early phase of its development. Lateral migration of the southern limb suggesting limb lengthening and bending at the hinge are the dominant mechanisms of fold amplification with strain localization at the fold core above the floor thrust tip. Bending moment faulting at the hinge led to the formation of crestal grabens or lakes in the southeastern section of the SMA. Our results suggest the existence of a weak, less viscous layer beneath a brittle sedimentary detachment. The SMA initiated as a detachment fold and sequentially deformed by passive roof thrusting. Therefore, the seismotectonic model of the Jammu Sub-Himalaya is different from that of the central and eastern Himalaya.

Deformation history of a foredeep basin during the incorporation of its deposits within an advancing orogenic wedge: The case of the Oligocene-Early Miocene Macigno Costiero Formation, southern Tuscany, northern Apennines, Italy

The Upper Oligocene Macigno Formation cropping out in the coastal section of SW Tuscany represents the sedimentary fill of a foredeep basin developed during the collisional stages that led to the construction of the Apennine mountain belt of peninsular Italy. The stratigraphic sequence consists of alternating sandstones and siltstones, that are affected by km-scale contractional structures, namely SW-dipping thrusts and related NE-verging folds. An original field survey carried out along a superb coastal exposure, integrated with analysis of mesoscopic fabrics and their overprinting relationships, makes it possible to unravel a complex deformation history. Four main deformation stages are recognized: i) layer-parallel shortening A; ii) top-to-the-foreland shear B; iii) folding C, comprising the fold nucleation sub-stage C1 and the fold amplification sub-stage C2; and iv) thrust propagation D. The sequence of recognized stages indicates a progressive deformation history of the Macigno Formation as it was incorporated within the evolving Apennine orogenic system. The kinematic history inferred from deformation fabrics and their overprinting relationships within Macigno Fm. deposits in SW Tuscany shows remarkable analogies with the structural evolution of foredeep sediments from other fold-and-thrust belts, yet with little deviations mainly consisting of pureshear vs. simple shear fluctuations in the early history of accretion. These provide original information on the modes of accretion of foredeep deposits within evolving collisional belts, thus contributing to an enhanced understanding of orogenic dynamics during mountain building with a classical example whose evolution is unravelled in detail.

Photoluminescence amplification of cerium incorporated graphene oxide nanoparticles by photoinduced reduction: A mechanistic study highlighting structural orderness

This work demonstrates a strategy to overcome the limitation of weakly luminescent graphene oxide (GO) based materials by using a simple one pot synthesis of enormously luminescent cerium ion incorporated graphene oxide nanoparticles (GO-Ce NPs). Apart from more than 100 fold amplification of photoluminescence intensity compared to graphene oxide, the origin of such enhancement of GO-Ce NPs is investigated by exploring binding interaction between cerium ions and GO NPs and the structural orderness of the GO NPs in presence of cerium. XPS confirms the existence of two oxidation states of cerium in GO-Ce NPs. Experimental observations enable us to conclude a mechanism involving photoinduced reduction of non emissive Ce (IV) to form luminescent Ce (III) accompanied with energy pumping of cerium by photoexcited GO NPs in the structurally ordered GO-Ce nanoparticles. Again, the interaction between the cerium ions and the oxygen containing functional groups of GO leads to decoupling of the functional groups and there by weakens the intensity of luminescence originated from disordered induced defect states which results a non radiative chanel to generate trivalent cerium via single electron transfer from the orbitals of the functional groups to the vacant 4f orbitals of tetravalent cerium.

Variations of thick-skinned deformation along tumuxiuke thrust in Bachu uplift of Tarim Basin, northwestern China

The curved Tumuxiuke thrust is the northeastern boundary of the Bachu uplift in the Tarim Basin. Based on the interpretation of 2D seismic reflection sections, thick-skinned deformation along the Tumuxiuke structural belt is characterized by monoclinal fault-propagation folds that alternate with push-up structures. The localization and patterns of thick-skinned deformation were controlled by pre-existing normal faults. The forward trishear modeling process further revealed the existence of short-cut thrusts branched from pre-existing normal faults and tested their effect on folding deformation. Monoclines decapitated by low-angle short-cut thrusts coexist with anticlines decapitated by high-angle inverted faults due to the selective inversion of pre-existing normal fault segments. Furthermore, the fault propagation to fault slip ratio (P/S) and the combined effect of multiple propagating thrusts determine the variations in the fold amplification rate and the growth-stratal architecture. The coeval activity of the western Kunlun orogen and the Tumuxiuke thrust in the Cenozoic indicates the far-field transmission of orogenic stress into the distal foreland. Under the approximately N–S-directed compressive stress, inverted structures in nearly W-E-striking segments exhibit monoclinal fault-propagation folds controlled by forethrusts; for the oblique NW-SE-striking segments, the transpressional deformation was partitioned into the frontal contractional deformation zone and the backlimb strike-slip zone.

Late-stage tectonic evolution of the Al-Hajar Mountains, Oman: new constraints from Palaeogene sedimentary units and low-temperature thermochronometry

AbstractMountain building in the Al-Hajar Mountains (NE Oman) occurred during two major shortening stages, related to the convergence between Africa–Arabia and Eurasia, separated by nearly 30 Ma of tectonic quiescence. Most of the shortening was accommodated during the Late Cretaceous, when northward subduction of the Neo-Tethys Ocean was followed by the ophiolites obduction on top of the former Mesozoic margin. This shortening event lasted until the latest Santonian – early Campanian. Maastrichtian to Eocene carbonates unconformably overlie the eroded nappes and seal the Cretaceous foredeep. These neo-autochthonous post-nappe sedimentary rocks were deformed, along with the underlying Cretaceous tectonic pile, during the second shortening event, itself including two main exhumation stages. In this study we combine remotely sensed structural data, seismic interpretation, field-based structural investigations and apatite (U–Th)/He (AHe) cooling ages to obtain new insights into the Cenozoic deformation stage. Seismic interpretation indicates the occurrence of a late Eocene flexural basin, later deformed by an Oligocene thrusting event, during which the post-nappe succession and the underlying Cretaceous nappes of the internal foredeep were uplifted. This stage was followed by folding of the post-nappe succession during the Miocene. AHe data from detrital siliciclastic deposits in the frontal area of the mountain chain provide cooling ages spanning from 17.3 to 42 Ma, consistent with available data for the structural culminations of Oman. Our work points out how renewal of flexural subsidence in the foredeep and uplift of the mountain belt were coeval processes, followed by layer-parallel shortening preceding final fold amplification.

Stress and strain evolution during single-layer folding under pure and simple shear

Folds in rocks are commonly used in geology for strain analysis. They contain information about rock rheology, and the kinematics and mechanics of deformation. The systematic analysis of the evolution of stress and strain during rock folding can provide key information on the mechanical behaviour of rocks and the efficiency of the folding process. In order to investigate the evolution of rock rheology during fold development, a series of two-dimensional simulations of single-layer folding are presented here. They are run using the finite element method BASIL, integrated within the software platform ELLE, to simulate linear and non-linear viscous deformation. The kinematics of deformation, the competence ratio between the folding layer and surrounding matrix as well as the stress exponent of the power-law viscous material are systematically varied. The results allow comparing the stages of folding under different deformation kinematic conditions. For all simulations the folding amplification process starts when the second invariant of the strain rate tensor in the competent layer deviates from the theoretical strain rate curve for a homogeneous material, which corresponds to a viscosity ratio between layer and matrix of m = 1. The relative time when the fold amplification starts is determined by the viscosity ratio between the competent layer and its surrounding matrix, the initial layer orientation with respect to the shear plane, the kinematics of deformation and the stress exponent. The folding process is more effective in cases with high viscosity ratio, non-linear rheology and layers initially oriented at a low angle with respect to the shear plane, because the second invariant of the strain rate tensor at the layer deviates earlier from the theoretical curve. The results also show differences depending on the boundary conditions, where (1) folding a competent layer requires less work in simple shear than in pure shear, and (2) the geometrical softening experienced by the competent layer due to fold development is followed by a hardening stage in pure shear and by a major softening phase in simple shear. The simulation results suggest that the decrease of stress of a competent layer without decreasing the mechanical strength has a direct influence on the behaviour of a lithospheric layer around the crust-mantle boundary, which may experience geometrical softening depending on the tectonic settings rather than material softening due metamorphic reactions or grain size reduction.

Long-wavelength topography on Mercury is not from folding of the lithosphere

Previous work suggested that the lithosphere of Mercury could undergo folding in response to global contraction, and indeed observations from the MESSENGER mission revealed several regions where long-wavelength topography is present. Here, we test, via finite-element simulations that use a more realistic rheological model than that earlier work, lithospheric folding as a formation mechanism for long-wavelength topography on Mercury from interior secular cooling over the last 3.8 Gyr. This radial contraction has been estimated from geological observations to be less than 10 km, which translates into small amounts of horizontal shortening of < 0.3%. Under expected surface temperatures of ∼440 K, the development of even modest fold amplification in such low strain environments is untenable. The scenarios under which there is this positive fold amplification begin with a fully compensated crust, but amplifications are small (factors < 1.1). Under other, non-compensated scenarios (e.g., a constant thickness crust), the collapse to isostasy overwhelms any folding instability. In order to produce lithospheric fold amplitudes that match those observed on Mercury, unrealistically large amounts of horizontal shortening (in excess of 10%, corresponding to hundreds of kilometers of radius change) are required. Therefore, we find that lithospheric folding cannot produce the observed long-wavelength topography on Mercury, and conclude that this topography must be buoyantly supported.

From axial parallel to orthogonal groundwater flow during fold amplification: insights from carbonate concretion development during the growth of the Quattro Castella Anticline, Northern Apennines, Italy

This paper is based on a multidisciplinary field and laboratory study of carbonate concretions developed in poorly lithified Quaternary, syn-kinematic sediments along the Quattro Castella Anticline, Northern Apennines, Italy. The studied concretions consist of both tabular (parallel to bedding) and elongate single to coalescent concretionary bodies oriented at different angles to the bedding throughout the exposed stratigraphic succession. The dimensions of the concretions range from a few centimetres for single elongate concretions up to several metres for tabular to coalescent concretions. Field observations and petrophysical data indicate that the concretions developed preferentially in sediments characterized by mean grain sizes of 90–290 μm and a permeability ranging from 7 × 10 2 to 7 × 10 4 mD. Carbon and oxygen stable isotope analyses in conjunction with the petrographic investigations indicate that the precipitation of concretionary calcite occurred in a meteoric vadose realm during early eogenesis and subsequently in a meteoric phreatic environment. Diagenetic data and concretion patterns in syn-tectonic sediments suggest they formed during the lateral propagation of the anticline, which, in turn, promoted a change in the local topographic–hydraulic gradient from fold axial parallel to fold orthogonal. The integrated analysis of carbonate concretions provides a useful tool with which to unravel the palaeo-fluid flow history and therefore to predict fluid circulation patterns in folded siliciclastic rocks. Supplementary material: Complete isotopic data from both hand samples and thin sections, together with statistical analysis, scanning electron microscope cement images and grain size distributions with the standard operative procedure tests performed, and detailed permeability measurements are available at https://doi.org/10.6084/m9.figshare.c.4087019

Episodic gold mineralisation correlated with discrete structural events at Ballarat East, southeast Australia

New 40Ar/39Ar geochronological data suggest orogenic gold mineralisation at the Ballarat East deposit, southeast Australia, occurred in three main episodes at ca. 445–435Ma, ca. 420–415Ma and ca. 380–370Ma. The gold mineralisation is localised in muscovite-bearing quartz and quartz-carbonate veins hosted in the steep faults (70–90°), on limbs of tight and isoclinal folds in an Ordovician turbidite sequence, and within west-dipping (≤45°) faults, historically known as leather jacket lodes. Initiation of the ≤45° faults that are confined to fold culminations, begins at ca. 445Ma, with peak metamorphic conditions at 440Ma. The earliest vein sets (V1), were emplaced on limb thrusts at ca. 445–435Ma and are characterised by arsenopyrite-dominated quartz veins. These V1 veins parallel arsenopyrite-rich shale units, historically referred to as ‘indicator beds’. Both the steep and ≤45° faults were reactivated during fold amplification with deposition of the V2 auriferous veins at ca. 420–415Ma. A later set of auriferous veins (V3–V4) with ages of 380–370Ma, dominated by pyrite-sphalerite-galena-white-mica quartz-(V3) or carbonate-rich (V4) veins are predominantly associated with reactivation of the ≤45° west-dipping faults. This new geochronological data constrains the local kinematic history of the Ballarat East deposit and has regional implications. The V1–V2 vein development appears to be synchronous across the entire western section of the Lachlan Orogen, where previous studies have suggested that initial gold mineralisation was linked to orogenesis at ∼440Ma, asa result of metamorphic devolatilisation reactions in the lower crust. In contrast, a close spatial and temporal relationship exists between the felsic dykes and the mineralisation recognised in the V3–V4 veins. The deformation that accompanies V3–V4 vein development is attributed to small, localised events during east-west shortening, utilising pre-existing fold and fault structures. The origin of the fluids producing the V3–V4 veins may be metamorphic devolatilisation associated with widespread felsic magmatism that occurred at this time across central Victoria.

Structural Constraints and Localization of Gold Mineralization in Leather Jacket Lodes, Ballarat, Victoria, Australia

Gold mineralization at the Ballarat East deposit, central Victoria, Australia, is hosted in lodes that are historically known as “leather jackets.” These are quartz-dominant vein arrays related to low-displacement, W-dipping faults (≤45°) that transect the core and/or eastern limb of tight, asymmetric N-S-trending anticlines. The leather jacket lodes typically have dip extents from 5 to 65 m, widths of ≤20 m, and strike lengths up to hundreds of meters, but their along-strike continuity is disrupted by oblique, low-displacement listric faults known as “cross courses.” The gold lodes are characterized by distinct phases of sulfide paragenesis with minor gold + arsenopyrite + pyrite defining the early sulfide stage. Late-stage coarse gold was precipitated with galena + sphalerite ± pyrrhotite ± chalcopyrite (late pyrite also occurs). The gold mineralization events are linked to low-strain mineralized fracture networks, which are closely related to the final deformation stages and the amplification of the major folds enclosing the lodes. This amplification produced domal fold culminations, with plunges ≤30°, and localized minor parasitic folds with shallower plunges (≤10°). A network of dilation sites, on the W-dipping faults, preferentially developed in the cores of anticlines, particularly in zones where there are changes in strike of bedding or fault bifurcation and refraction through contrasting sandstone and interbedded packages of sandstone and shale. Numerical three-dimensional simulations were undertaken to test our geologic observations and replicate conditions controlling the emplacement of the leather jacket lodes. Two different scenarios were investigated: first, to determine how changes in the local stress field orientation influences dilation and fluid infiltration; secondly, to test variations in fault geometry during the last stages of deformation—that is, within the final 2% of shortening, when most of the mineralized sites were created. Results show that strain and fluid flow localized along refracted sections of faults and around changes in dip, specifically on the shallower dipping sections within subvertical sandstone units. This is consistent with the observation that high-grade gold-bearing quartz is associated with localized changes in fault dip in thicker sandstone and sandstone-shale packages. There was also a component of strike-slip motion and near-field NW-to-SE or N-to-S stress fields, which can be attributed to the development of a component of out-of-plane motion during the development of fold culminations. The preferred model for the distribution of the high-grade auriferous vein arrays defining the leather jacket lodes is one of fold amplification and extension parallel to the fold axes, which produced an increasing out-of-plane relaxation. The main fluid conduits responsible for the leather jacket style of mineralization involve infiltration along steep bedding-parallel faults and veins that link up with the arrays of low-displacement W-dipping faults.

Parasitic folds with wrong vergence: How pre-existing geometrical asymmetries can be inherited during multilayer buckle folding

Parasitic folds are typical structures in geological multilayer folds; they are characterized by a small wavelength and are situated within folds with larger wavelength. Parasitic folds exhibit a characteristic asymmetry (or vergence) reflecting their structural relationship to the larger-scale fold. Here we investigate if a pre-existing geometrical asymmetry (e.g., from sedimentary structures or folds from a previous tectonic event) can be inherited during buckle folding to form parasitic folds with wrong vergence. We conduct 2D finite-element simulations of multilayer folding using Newtonian materials. The applied model setup comprises a thin layer exhibiting the pre-existing geometrical asymmetry sandwiched between two thicker layers, all intercalated with a lower-viscosity matrix and subjected to layer-parallel shortening. When the two outer thick layers buckle and amplify, two processes work against the asymmetry: layer-perpendicular flattening between the two thick layers and the rotational component of flexural flow folding. Both processes promote de-amplification and unfolding of the pre-existing asymmetry. We discuss how the efficiency of de-amplification is controlled by the larger-scale fold amplification and conclude that pre-existing asymmetries that are open and/or exhibit low amplitude are prone to de-amplification and may disappear during buckling of the multilayer system. Large-amplitude and/or tight to isoclinal folds may be inherited and develop type 3 fold interference patterns.

Structural controls of fluid flow and gold mineralization in the easternmost parts of the Karagwe-Ankole Belt of north-western Tanzania

Gold mineralization in the Biharamulo region of western Tanzania is confined to the sheared, low-angle basement-cover contact between Archaean basement gneisses of the Tanzania Craton and the structurally overlying, low-grade metamorphic metasediments of the Mesoproterozoic Karagwe-Ankole Belt. Regional-scale fluid flow along this detachment is indicated by the pervasive silicification and retrogression of wall rocks to pervasively foliated phyllonites and pyritization of particularly metasediments, commonly graphite-rich, in the hanging wall of the shear zone. Gold mining centres on specific structural sites along the detachment, but also in stratigraphically higher sections in the structurally overlying metasediments. Zones of gold mineralization along the detachment correlate with NE trending ramp structures (dip angles 20°–35°) that are most ideally orientated for slip and reactivation within the low-angle phyllonitic detachment. Repeatedly overprinted auriferous quartz-vein stockworks in quartzofeldspathic gneisses immediately below the detachment indicate brittle fracturing of the competent footwall lithotypes during slip along the weaker detachment. In cases of massive silicification, up to 50m thick quartz blows are formed along the contacts between detachment phyllonites and footwall gneisses. The multiple overprinting relationships of successive quartz-vein generations in these zones of massive silicification suggests that the quartz blows acted as competent blocks in the weak detachment, causing the repeated overprint of earlier silicification by later fracturing and quartz-veining events. Gold mineralization above the detachment and in stratigraphically higher metasediments is closely associated with fold structures that form part of the low-grade metamorphic fold-and-thrust belt. Veining is particularly abundant in competent lithotypes, such as quartzite and chemically reactive ferruginous mafic sills. Overprinting relationships between quartz vein sets illustrate fluid flow during fold amplification and, importantly, the final lock-up stage of folds, during which much of the mineralization was introduced. Oxygen isotope values for quartz veins indicate fluids were likely derived from clastic, mainly metapelitic sedimentary sequences of the Karagwe-Ankole Fold Belt. The data also implies that the partially reworked Archaean granitoid-greenstone basement of the Tanzania Craton has not contributed to the fluid evolution and possibly gold mineralization. The extent (>100km) of the basement-cover detachment and associated alteration is indicative for a regional-scale fluid system. Gold mineralization is, however, controlled by local structures and lithological contrasts that require the detailed mapping and sampling of the regional structure.

3D fold growth in transpression

Geological folds in transpression are inherently 3D structures; hence their growth and rotation behavior is studied using 3D numerical finite-element simulations. Upright single-layer buckle folds in Newtonian materials are considered, which grow from an initial point-like perturbation due to a combination of in-plane shortening and shearing (i.e., transpression). The resulting fold growth exhibits three components: (1) fold amplification (vertical), (2) fold elongation (parallel to fold axis), and (3) sequential fold growth (perpendicular to axial plane) of new anti- and synforms adjacent to the initial fold. Generally, the fold growth rates are smaller for shearing-dominated than for shortening-dominated transpression. In spite of the growth rate, the folding behavior is very similar for the different convergence angles. The two lateral directions always exhibit similar growth rates implying that the bulk fold structure occupies an increasing roughly circular area. Fold axes are always parallel to the major horizontal principal strain axis (λ→max, i.e., long axis of the horizontal finite strain ellipse), which is initially also parallel to the major horizontal instantaneous stretching axis (ISA→max). After initiation, the fold axes rotate together with λ→max. Sequential folds appearing later do not initiate parallel to ISA→max, but parallel to λ→max, i.e. parallel to the already existing folds, and also rotate with λ→max. Therefore, fold axes do not correspond to passive material lines and hinge migration takes place as a consequence. The fold axis orientation parallel to λ→max is independent of convergence angle and viscosity ratio. Therefore, a triangular relationship between convergence angle, amount of shortening, and fold axis orientation exists. If two of these values are known, the third can be determined. This relationship is applied to the Zagros fold-and-thrust-belt to estimate the degree of strain partitioning between the Simply Folded Belt and the bordering strike-slip fault-system.

Paleofluid Evolution In Fault-Damage Zones: Evidence From Fault–Fold Interaction Events In the Jabal Qusaybah Anticline (Adam Foothills, North Oman)

Abstract Fault zones are known to play a key role in controlling fluid migration, including hydrocarbons, at local and regional scales. In this contribution physicochemical data from veins are used to document the extent and maintenance of fluid conductivity associated with fault-damage zones in an anticlinal structure occurring in the frontal part of a foreland fold and thrust belt. Migration of fluids during major deformational events has been recorded by several generations of calcite and localized dolomite cementation in veins in marine Cretaceous carbonates of the Jabal Qusaybah Anticline (North Oman). Applying detailed petrographic and paragenetic relations, geochemical analysis (stable carbon and oxygen isotopes, strontium isotopes, and trace elements), and microthermometry, coupled with structural studies of fault zones and associated calcite veins, made it possible to link fluid circulation to the structural evolution of the Jabal Qusaybah Anticline. Based on the structural framework and fault systems, three groups of veins were distinguished. Group 1 veins are interpreted to be linked to burial and early-tectonic deformation in association with E–W extensional fault zones. Calcite cementation in this group occurred in a closed to semiclosed fluid system, which originated as a result of intraformational fluid movement along a low-connectivity fracture network. The δ13C values of these veins range from 3‰ to 3.5‰, and δ18O values vary from –10.3 to –8.1‰ V-PDB. Group 2 veins are related to syntectonic deformation along NE–SW strike–slip and incipient N–S extensional faults linked to culmination of the Jabal Qusaybah Anticline. From this stage onward, fluids correspond to progressive fault connectivity and dilation related to interaction of fault–folding systems. The cements precipitated during this stage have more variable δ13C values ranging from 0.9 to 3.2‰ and possess a wide range of δ18O values varying between –11.2 and 0.3‰ V-PDB. The cements in Group 2 veins originated from fluids characterized by a relatively broad range of precipitation temperature (75–107°C) and salinities evolving from seawater to moderately saline fluids (3.4–12.8 eq. wt% NaCl), with H2O–NaCl composition. The source of salinity is inferred to be evaporitic brines that originated from the Ara Group, and the large range of salinity is interpreted as the result of mixing between those brines and modified seawater. Group 3 veins are considered to be related to late–tectonic deformation corresponding to late fold amplification of the anticline hinge, transition from NE–SW left-lateral strike–slip to N–S extensional faulting, progressive regional and local uplift, and exhumation. Stable-isotope data support the occurrence of bacterial methanogenesis or CO2 degassing at the end of this stage. Their associated cements are characterized by higher δ13C and δ18O values compared to other groups of veins, ranging from 0.6 to 8.0‰ and –3.6 to 1.5‰ V-PDB, respectively. Fluids responsible for calcite precipitation in this group evolved from a moderate saline to a more saline (8.5–16 eq. wt. % NaCl) H2O–NaCl–CaCl2 brine. The progressive 18O enrichment relative to seawater and preceding cements and its covariation with fluid salinity in the later stage suggest the higher contribution of the evaporitic brines. This hypothesis is further supported by the increase in their 87Sr/86Sr ratios compared to both middle to Upper Cretaceous marine carbonates and other groups of veins. Although during this episode calcite cementation occurred at temperatures below 50°C, their geochemical signatures reveal that the faults were sufficiently conductive to transfer extraformational fluids late in the deformation history of this structure. The interference between strike-slip and extensional fault systems and their interaction with fold amplification is invoked to be responsible for enhanced conductivity in fault-damage zones.

Evolution of the Nourooz anticline (NW Persian Gulf) deciphered using growth strata: Structural inferences to constrain hydrocarbon exploration in Persian offshore anticlines

The structural features and kinematic evolution of the Nourooz anticline, an oil producing structure located in the NW Persian Gulf, are unravelled based on the interpretation of a seismic profile grid and well data, construction of a 3D depth model and sequential restoration of a 2D geological section. The Nourooz anticline is a kilometre-scale, slightly asymmetrical fold that comprises pre-growth and growth strata. The main stratigraphic feature is an Upper Cretaceous unconformity. The amplification of the Nourooz anticline from Upper Cretaceous to present-day probably took place by hinge migration with minor contribution of limb rotation assuming that the structure resulted from horizontal compression and maintained constant bed length. The estimated average fold-shortening rate is 0.002 mm/a, assuming that the structure resulted from horizontal shortening, and the average fold-uplift rate calculated is 0.019 mm/a, although the fold amplification rates decreased with time. The anticline does not seem to be related to a detachment at depth and its growth may have been caused by motion along a basement fault, that favoured movement of deep evaporite bodies underneath the anticline, and buckling caused by propagation of the Zagros belt deformation into the foreland basin. The processes involved in the formation of this anticline coincide with those documented for an adjacent structure and seem to be the main processes that controlled the structural style in the Persian Gulf basin.

Regional dome evolution and its control on ore-grade distribution: Insights from 3D implicit modelling of the Navachab gold deposit, Namibia

We introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3D implicit modelling in data-rich environments with field observations. We apply this approach to the early Palaeozoic Navachab gold deposit which is located in the Damara orogenic belt, Namibia. Compared to traditional modelling methods, 3D implicit modelling reduces user-based modelling bias by generating open or closed surfaces from geochemical, lithological or structural data without manual digitisation and linkage of sections or level plans. Instead, a mathematically defined spatial interpolation is used to generate 3D models that show trends and patterns that are embedded in large drillhole datasets. In our 3D implicit model of the Navachab gold deposit, distinctive high-grade mineralisation trends were identified and directly related to structures observed in the field. The 3D implicit model and field data suggest that auriferous semi-massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome, where shear strain reached peak values during fold amplification. This setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit. Both bedding-parallel and highly discordant sets of auriferous quartz-sulphide veins are interpreted to have formed during the later lock-up stage of the regional scale dome. Additionally, pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set. We propose that kilometre-scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures (traps and pathways for fluids) at different preferred sites and orientations. We also propose that areas that experience high shear strain, located along the inflection lines of folds can act as preferred sites for syn-deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts. Our research also suggests that examination of existing drillhole datasets using 3D implicit modelling is a powerful tool for spatial analysis of mineralisation patterns. When combined with fieldwork, this approach has the potential to improve structural understanding of a variety of ore deposits.

Enigmatic structures within salt walls of the Santos Basin—Part 1: Geometry and kinematics from 3D seismic reflection and well data

Understanding intrasalt structure may elucidate the fundamental kinematics and, ultimately, the mechanics of diapir growth. However, there have been relatively few studies of the internal structure of salt diapirs outside the mining industry because their cores are only partly exposed in the field and poorly imaged on seismic reflection data. This study uses 3D seismic reflection and borehole data from the São Paulo Plateau, Santos Basin, offshore Brazil to document the variability in intrasalt structural style in natural salt diapirs. We document a range of intrasalt structures that record: (i) initial diapir rise; (ii) rise of lower mobile halite through an arched and thinned roof of denser, layered evaporites, and emplacement of an intrasalt sheet or canopy; (iii) formation of synclinal flaps kinematically linked to emplacement of the intrasalt allochthonous bodies; and (iv) diapir squeezing. Most salt walls contain simple internal anticlines. Only a few salt walls contain allochthonous bodies and breakout-related flaps. The latter occur in an area having a density inversion within the autochthonous salt layer, such that upper, anhydrite-rich, layered evaporites are denser than lower, more halite-rich evaporites. We thus interpret that most diapirs rose through simple fold amplification of internal salt stratigraphy but that locally, where a density inversion existed in the autochthonous salt, Rayleigh–Taylor overturn within the growing diapir resulted in the ascent of less dense evaporites into the diapir crest by breaching of the internal anticline. This resulted in the formation of steep salt-ascension zones or feeders and the emplacement of high-level intrasalt allocthonous sheets underlain by breakout-related flaps. Although regional shortening undoubtedly occurred on the São Paulo Plateau during the Late Cretaceous, we suggest this was only partly responsible for the complex intrasalt deformation. We suggest that, although based on the Santos Basin, our kinematic model may be more generally applicable to other salt-bearing sedimentary basins.