Drag Folds Research Articles

Early Precambrian eclogites in the Belomorian Province, eastern Fennoscandian Shield

This study focuses on a boudin of Archean amphibolite that occurs in the Gridino eclogite-bearing mélange (Belomorian Province, eastern Fennoscandian Shield) and contains two varieties of eclogite. Eclogite-1 is banded, retrogressed (omphacite is totally replaced by clinopyroxene-plagioclase symplectite), and deformed by recumbent tight folds identical to those in the TTG-amphibolite host. Zircon groups are dated at 2721 ± 26 Ma (Zrn-I, fir-tree zoning, no or small negative Eu anomalies, flat HREE patterns, 700–730 °C, 14–15 kbar), ca. 2.70 Ga (Zrn-II, rims around Zrn-I, grains, 750–900 °C, 11–14 kbar, high-P granulite facies), and ca. 2.70 Ga (Zrn-III, rims around Zrn-II). Newly formed Paleoproterozoic zircon crystals are lacking. Retrogressed eclogite-1 in which amphibolization and epidotization totally obliterated garnet and plagioclase-clinopyroxene symplectites is intruded by 2646 ± 46 Ma old granodiorite. Eclogite-2 is massive, contains preserved omphacite-garnet-quartz-rutile assemblages, and comprises two patches whose boundaries truncate folds in eclogite-1. A Paleoproterozoic age of eclogite-2 is evidenced by previously published 1.90 Ga ages of zircon rims around Archean zircon and newly formed crystals with omphacite inclusions. The largest patch contains tabular enclaves of three varieties of amphibolites. One variety is identical to some bands of totally retrogressed eclogite-1 changed into monomineral amphibolites in both the Neoarchean and Paleoproterozoic. The orientation and attitude of these tabular enclaves are coherent with those of the banding on limbs of recumbent tight folds in eclogite-1 within the boudin and later drag folds at its margins and form one and the same structural carcass or skeleton. These data along with recent findings of inherited 2.68 Ga old zircon with inclusions of omphacite in eclogite-2 indicate that this eclogite developed after Neoarchean eclogite-1 due to fluid infiltration along reactivated Neoarchean shear zones enveloping the boudin. These results favor the idea that the transition to the modern-style plate tectonics started not later than in the Neoarchean.

Imaging along-strike variability in fault structure; insights from seismic modelling of the Maghlaq Fault, Malta

Seismic modelling studies of outcrop analogues have proven a powerful method to provide a link between geology as observed at outcrop, and the interpretation of subsurface geology from seismic data. Seismic imaging of fault zones is inherently challenging, as they may be associated with steep dips, narrow zones of heterogeneously deformed rocks, and complex geometries at the limit of seismic resolution. We here investigate how along-strike variations in fault geometry and structural style are imaged in reflection seismic images, based on seismic modelling of well-exposed outcrops of the Maghlaq Fault hosted in the Miocene-Oligocene carbonate rocks in Malta. Using two-dimensional seismic modelling, selected structural cross-sections have been modelled, focusing on variations in fault- and hangingwall bed geometries, and dominant seismic signal frequency. The seismic models show great variability in resolution and, thus, level of resolved geologic detail. The modelled seismic images show that, for all signal frequencies, the fault itself is not imaged, but is manifested by terminations of footwall reflections as well as fault-related strain in the form of drag folding of hangingwall beds, in addition to fault-proximal seismic imaging artefacts. Rotated and ‘drag-folded’ hangingwall beds along the fault produces a high-amplitude fault parallel reflection bundle in the immediate hangingwall of the fault. The illumination issues that dominate in low dominant frequency seismic models are less prevalent in the higher-resolution, higher dominant frequency models. The results of this outcrop-to-seismic study offer insight to the relationship between faults and seismic images, which may improve our ability to accurately interpret faults in the subsurface form reflection seismic data.

Morpho-structural signatures of neotectonic activity along the HFT bound Himalayan mountain front in Kathgodam-Chorgallia sector of NW Himalaya, India

Thrusting supports the development of diverse landforms and deformational structures in the Himalayan Frontal Thrust (HFT) region between Kathgodam and the Chorgallia sector, according to this study. The influence of the active tectonic condition in the area was analyzed using seven geomorphic indices, which also underlined the importance of morpho-structural study utilizing remote sensing, geomorphic indices, tectono-geomorphic landforms, field evidence, and luminescence dating. The investigation of sub-basins surrounding the HFT reveals moderate to high tectonic activity, with tectonic activity concentrating in the eastern half and decreasing toward the west. A topographic discontinuity was also observed in the region as indirect active tectonic evidence. Hanging wall rocks have well-preserved features that govern tectonic transport direction, and the HFT in this region is emergent. Shear sense indications point to southerly tectonic migration, with a few modest northward shear senses indicating northward movement, which might be attributable to reverse thrusting along the HFT at some time during the Outer Himalaya's tectonic history. Petrographic investigations reveal sustained frontal deformation at various intervals, with well-preserved thrust-induced deformation fingerprints in deformed rocks throughout the Jam Raula and Sukhi nadi sections. Micro thrust duplexes, or micro-scale asymmetrical drag folds, are shown in thin section investigations as a result of HFT-related brittle-ductile deformation at moderate temperature and pressure. The sandy unit from the Quaternary sediments yields 10 ± 0.8 ka, 11.8 ± 0.7 ka, 17 ± 1 ka, and 56 ± 3 ka quartz OSL dates, demonstrating the migration of hanging wall rocks on footwall post-Siwalik fluvial deposits along the HFT plane and reactivation of the HFT in the study area.

Structural Classification, Discharge Statistics, and Recession Analysis from the Springs of the Gran Sasso (Italy) Carbonate Aquifer; Comparison with Selected Analogues Worldwide

The relative importance of karst conduits and fractures in driving groundwater flow affects the discharge of springs and the long-term availability of water resources. Applying statistics to the hydrographs of the discharge and studying the recessions provide information on the degree of reliability and variability of the springs and, therefore, the flow regime within the saturated part of the carbonate aquifers. This approach was applied to six springs at the Gran Sasso aquifer in Central Italy. These springs were divided into three structural geological groups that determined the position of the permeability thresholds. The type of tectonic structures and the pattern of the permeability thresholds allow a correlation with the computed statistics. The studied springs were associated with the presence of thrusts, overturned drag folds, and a normal fault. The computed statistics describe a general scenario of reliability and steadiness for the springs. The Flow Duration Curves for the springs show limited groundwater flow through the conduits through a comparison with analogues in Slovakia. Joints and bedding plane fractures dominate the groundwater flow, fitting both the relative steadiness of the discharges and the pattern of the Flow Duration Curves. The recessions are also characterized by more gentle slopes with respect to nearby areas fitting a conceptual model of dominant fracture flow. This mathematical scenario depicts groundwater resources, which have limited exposure to episodes of summer droughts. The proposed approach is a holistic combination of structural geology and hydrologic elements and can be successfully exported to other tectonized carbonate areas for the sustainable management of groundwater resources worldwide.

Non-coaxial deformation of foreland basement involved in a fold-and-thrust belt: a strain partitioning approach to the Eastern Variscan orogen

The general SW–NE course of the Variscan orogen in Europe is abruptly bent to the N–S course at its eastern margin, where an oblique convergence occurred. The main suture in this part of the Variscan orogenic belt is called the Moldanubian Thrust, characterized by a dominant dextral strike-slip kinematics and a minor thrust component. The deep level of erosion and the good exposure of this structure allowed us to study the mechanisms of oblique convergence and the incorporation of the foreland basement into the orogenic belt. The combination of small-scale structures with the anisotropy of magnetic susceptibility studies allowed the recognition of two deformations in the studied rocks: dextral simple shearing and drag folding. Due to oblique convergence, the deformations induced by this mechanism were non-coaxial; therefore, their contributions can be easily distinguished. Finally, an overturned, almost recumbent large-scale synformal fold structure in the footwall and an antiformal structure in the hanging wall of the Moldanubian Thrust were formed. These two folds can be interpreted as structures formed by dragging along the Moldanubian Thrust. The previously described sinistral simple shearing in the upper limb of the synform resulted from the original dextral strike-slip shearing, which was overturned during progressive deformation.

Oblique lateral extrusion during dextral transpression along the Beardmore-Geraldton belt, Canada

The Beardmore-Geraldton belt (BGB) is a boundary zone between the Wabigoon and Quetico subprovinces of the Archean Superior Province, Canada. It consists of interleaved metavolcanic and metasedimentary panels that were imbricated during an early D1 thrusting event and regionally folded during a D2 event. D1 and D2 steepened stratigraphy and produced contact-parallel shear zones and a regional bedding-parallel cleavage, which controlled the development of a D3 dextral transpression zone. During D3, the panels deformed by pure shear and were overprinted by a second regional cleavage. Bedding and the early cleavage acted as subvertical planes of anisotropy that were reactivated during D3 dextral shearing. Z-shaped drag folds and pre-existing D1 and D2 folds are parallel to a shallowly-plunging (20°–30°) stretching lineation, which together with the fold axes rotated towards the extensional flow apophysis during D3 shearing. As the lineation and fold axes are oblique to the sub-horizontal shear direction expressed by the intersection between synthetic shear bands and the early cleavage, the reactivated shear zones are transpression zones with shallowly-plunging lateral extrusion and triclinic symmetry. Thus, the shear zones are transpressive within a regional tectonic transpression zone that extends across the BGB and over 500 km along the Quetico-Wabigoon boundary.

Structural core observations in a siliciclastic reservoir-scale framework

Abstract Detailed knowledge of subseismic structures and their influence on reservoir production performance is important for optimal reservoir management. Predicting subseismic structures from seismic-scale structural interpretations is inherently difficult without the use of core data. Cores allow direct measurements of porosity and permeability of deformed rocks and enable researchers to make detailed investigation of deformation mechanisms and cementation processes. Frequency, distribution and sometimes orientation of planar structures can be constrained. Such observations are then used together with location relative to seismically mapped fault and fold structures, and with respect to lithology and stratigraphy. However, a significant gap exists between the scale of core observation and the size of structures mappable from seismic data. Bridging this gap requires a sound general understanding of the different structures that occur in reservoirs, which, in addition to faults, includes drag folds, veins, fractures and the many types of deformation bands that can exist in porous rocks. Proper understanding of such subseismic structures is primarily based on outcrop-based observations and analyses, aided by physical experiments and numerical modelling. We stress that integrating such cutting-edge knowledge with core, seismic and other case-specific subsurface data in an appropriate tectonic context is paramount for realistic reservoir characterization and successful reservoir management.

TECTONIC DEFORMATIONS AND PALEOSTRESS FIELDS IN THE ROCK COMPLEXES OF THE PIENINY KLIPPEN BELT IN THE CITY OF SZAWNYCTA (POLAND)

Purpose of the paper is study of faults and folds and reconstruction of paleostress fields in rock complexes of the Pieniny Klippen Belt in Pieniny Mts. area (Southern Poland). Methods of field geological and structural research were involved, including the kinematic method with subsequent reconstruction of paleostress fields using the program Win-Tensor. The results. On the right bank of the Danube River in the south-western outskirts of the city of Szczawnytsia (Southern Poland), numerous fold and fault deformations, their kinematic types and structural patterns were studied in the large rootless block of the PKB. Paleostress fields of the thrust, normal and strike-slip kinematic types have been restored. In the inverse stress field, thrusts were formed on the border of competent and incompetent rocks. Drug-folds, mélange and broken formations zones were formed in the more ductile rocks of the lower block. In the normal-type stress field, the former thrusts were activated as normal faults and cuted by the ortogonal and oblique sinistral strike-slip faults. The kinematic sence of the thrusts changed to an oblique normal faults with a component of the sinistral strike-slip, in the bodering zones of the activated faults, drag folds ans s-type folds were formed. In the youngest stress field, the former thrust surfaces were activated as strike-slip ones. Shear deformations were also recorded along thrust-limiting orthogonal and oblique surfaces. Kinematic types of tectonic deformation and folded structures in the different parts of the studed rootless block vitnesses the rotation of the fault wings in the horizontal and vertical planes. The wavy shape of the surface of the movement of the rootless block may be the reason for the simultaneous existence of a field of compression and tension in its various segments. Scientific novelty. As a result of the research, the kinematic types of fault, their relationships and features of development in various paleostress fields were determined. The features of the structure of the thrust and the faults that cut them, the relationships of folded and faulted deformation are established. Changes of the kinematic types of faults in time from the inverse to the strike-slip type have been established. Practical impact. The obtained results complement the history of the formation of the Pieniny Klippen Belt and can be used to refine tectonic maps and select a geodynamic model of the region. Key words: Pieniny Klippen Belt, paleostress, thrust, strike-slip, mélange.

The Broken Hill Block, a new structure and mineral exploration target areas

The Broken Hill Block contains the Paleoproterozoic Willyama Supergroup, host to the world-renowned lead–zinc–silver deposits known as the Line of Lode. A new large-scale geological structural interpretation has been developed based on the construction of 11 regional-scale cross-sections and one longitudinal section. Over an extended period of major folding during the Olarian Orogeny, two distinct sets of regional folds were produced. The longitudinal folds are of similar-type and mostly trend northeast. The cross-folds are subtle, mostly trend at right angles to the longitudinal folds and are open-style. Longitudinal folds in or near the Line of Lode have been reinterpreted. A newly interpreted major slump fold, central to the Line of Lode, allows correlation of rock units from east to west. The well-known Western Anticline and Eastern Syncline, which host the main Line of Lode, are normal drag folds, formed during the initial folding with their axes folded by the later slump folding. The lead lodes are older than the zinc lodes. We interpret previously mapped longitudinal folds, as formed by successive pulses within the one tectonic event. This resulted in increased folding complexity on a local scale, but not a regional scale, and with no substantive impact on the regional structure. Pitch and plunge data were used to identify 23 cross-folds, some more than 50 km long. Away from the Line of Lode, the new structure is simpler than previous interpretations and with no tectonic inversion of the stratigraphy, other than gravity slumping, and no regional-scale nappe structures. The combination of cross-folds and longitudinal folds has produced regional-scale domes and troughs throughout the block. The intersection of longitudinal fold axes with cross-fold crests are proposed to be particularly prospective for metallic sulfides with 25 named and prioritised mineral exploration target areas identified. The cross-folding has caused the ore bodies to plunge steeply to great depths north of the Line of Lode. Further to the northeast, the plunge is reversed bringing strata, containing potential northern extensions of the Line of Lode, closer to the surface. The retrograde schist zones, which are regional-scale faults or shears that have undergone retrograde metamorphism, postdate the longitudinal and cross-folding and are not considered to be related to thrust structures. Displacement of strata and structures is evident across the zones and is generally well understood. KEY POINTS A new set of regional folds, termed cross-folds, has been identified. In the Line of Lode, large-sale longitudinal folds include a major slump fold. The stratigraphic sequence is not inverted; nappes are for the most part not present. Broken Hill-type lead–zinc–silver deposits are preferentially found close to the intersection of longitudinal and cross-fold axes.

Structural geometry, kinematics and paleostress of the Main Boundary Thrust: New constraints from the Hazara-Kashmir syntaxis, Northwest Himalayas, Pakistan

• The two separate faulting phases are preserved along the shear zone of the Main Boundary Thrust. • These fault zone fabrics help in relative time reconstruction of tectonic events based on displaced relationships, cross-cutting, and overprinting. • The stress regime changes along the Main Boundary Thrust (MBT) compressional in the east and transpressional in the west. • The variation in fold axis trendings, axial planes, and inclined to steep plunges of folds results from deformation and cross folding. The Himalayan fold and thrust belt in Kashmir is investigated for structural geometry, kinematics, and Paleostress analysis of Main Boundary Thrust along the eastern limb of the Hazara Kashmir syntaxis, Northwest Himalayas , Pakistan. The Precambrian to Cenozoic cover stratigraphic unit of the Indian plate is imbricated and faulted along the Main Boundary Thrust (MBT). The Main Boundary Thrust separates the Late Proterozoic Dogra Formation and Permo-Triassic Panjal Group of rocks from the Murree Formation molasses of Early Miocene . The faults and folds along Main Boundary Thrust are developed due to the Tertiary Himalayan collision. The regional strike of the area is northwest-southeast, and dip changes from northeast to southwest. The brittle lineations found at the surface of faulted rocks are deformed and displaced. Slickensides are mainly calcite and quartz coated. The coated surfaces of slickenside with the calcite fiber crystal growth or quartz reveal earlier origin prior to the slickenside surfaces, uncoated or fresh polished. These older slickensides show older thrust movement along the Main Boundary Thrust Fault , and uncoated fresh slickensides horizontal to sub-horizontal in trend shows strike-slip movement. The features like offset streams (Neelum River), displaced drainages (Panjkot nalla), triangular facets, displaced ridges, uplifted terraces, nick points, and seismicity associated with Main Boundary Thrust show that fault is an active fault. In the footwall of the Main Boundary Thrust, the Z-type drag folds developed as a result of extreme deformation at various localities. These Z-type folds along fault indicate pure thrust movement subordinate strike-slip sense of shear. The paleostress analysis and shear indicators yield the detailed kinematics history of the Main Boundary Thrust. Due to stress in this region, the movement's characteristics, together with outcrop structures, indicate clearly that brittle deformation exists along the Main Boundary Thrust. The present analysis of kinematics indicators fault-related slickensides, conjugate fractures, folding, dragging, faulting, shearing, and paleo stress analysis shows that pure thrust with subordinate strike-slip movements present in the eastern limb of Hazara Kashmir Syntaxis along the Main Boundary Thrust.

The Regional Extents of Local Thrust Systems in Jabbari and Rupper Town, South East of Hazara Pakistan

Geological traverse between the town of Jabbri and Rupper, southeast Hazara delineates the stratigraphic and structural element of the area presented by high resolution geological map and cross section of the area. Oldest rocks in this section is Precambrian Hazara formation which is disconformably overlain by a Jurassic to Eocene sequence including Samana Suk, Chichali, Lumshiwal, Kawagarh, Hangu, Lockhart, Patala, Margalla-hill-limestone, Chorgali and Kuldana formations. Located in Lesser Himalayas, under the influence of main boundary thrust (MBT), this area developed several thrust systems from north to south including Hazara thrust, Haro thrust, Sangoda thrust, Rupper thrust. The main regional Hazara thrust has Precambrian rock in its hanging wall thrusted over the Jurassic Samna Suk formation. In lower detachment, there are several local thrust systems within Jurassic-Eocene strata.These thrusts trends in NE-SW direction indicating NW-SE compressional regime with the hanging wall Drag folds showing southward vergence. The stereographic plot of mesoscopic scale folds shows at least three folding events and varying plunging directions between the range of 240° and 290°. Most of the folds are plunging towards west and this is yet another indicator of east-west compressional forces. These folds orientations suggest transpressional deformation rather than pure compression. The balanced restore section shows that there is about 2.5 km shortening along 15 km restored section which makes about 16% shortening along the section.
 

Mapping of fracture zones and structural lineaments of the Gulf of Guinea passive margins using marine gravity data from CryoSat-2 and Jason-1 satellites

Satellite gravity datasets have been widely used in understanding of the Earth's internal structure and processes. These datasets are effective in providing insights into the lithospheric structure of the Earth. Gravity data from the satellite products CryoSat-2 and Jason-1 have been used to investigate the Gulf of Guinea sedimentary basins and structural lineaments in this under-explored marginal sea region through the application of various edge filters. Present-day filtering methods such as the analytical signal, tilt angle of the gradient amplitude, NTilt gradient amplitude and softsign function were evaluated for their performance on synthetic gravity anomalies with and without noise prior to their application to gravity data covering the Gulf of Guinea. The softsign filter outputs obtained from synthetic examples result in higher resolution and more explicit edges while preventing fictitious edges production in the findings. The results of the data interpretation disclosed deep-seated and shallow structural features within the Gulf of Guinea oriented mainly in the WNW-ESE, NW-SE, NE-SW and NNE-SSW directions and seen to be concentrated in the upper slope domain. This observation is in good agreement with the occurrence of drag folds and the apparent concentration of wrench faulting features across the ridge top and bordering the upper slope. These results bring new facts ruling our understanding of this atypical regional tectonic setting in close agreement with the NE–SW-striking Pelusium Megashear Zone.

Geologic observations in the San Marcos area, Coahuila, Mexico: the case for sediment-hosted stratiform copper–silver mineralization in the Sabinas basin during the Laramide orogeny

The sediment-hosted stratiform copper–silver mineralization in the San Marcos area of Coahuila, northeastern Mexico occurs predominantly at an Early Cretaceous redox boundary between footwall siliciclastic red beds of the San Marcos Formation and hanging-wall carbonate strata of the Cupido Formation in the Sabinas basin. The hypogene mineralization is mainly present as chalcocite-group minerals, with additional bornite and chalcopyrite, and everywhere occurs in both disseminated and vein/veinlet forms. Supergene copper-bearing oxides (malachite, chalcanthite, azurite, chrysocolla) are, however, the dominant surface expression of the mineralization. Additional sediment-hosted stratiform copper–silver mineralization also occurs, albeit erratically, in lower strata of the Sabinas basin as well as in veins in basement granitoids, thus spanning ~3000 m of basin stratigraphy. Where best developed, the stratiform mineralization displays intense structural control proximal to the regional San Marcos fault system. This major bounding fault, regional in nature and with numerous periods of activity, controlled the evolution of the Sabinas basin. Structural controls on mineralization include stacked, shallow-angle, bedding-parallel, northeast-vergent thrust faults and associated drag folds, in addition to numerous, steeply-dipping, northeast-trending copper-bearing veins and veinlets. The mineralized veins and veinlets, and the bedding-parallel thrusts display mutually crosscutting relationships. These elements are all consistent and in harmony with a regional northeast-trending direction of horizontal shortening accompanying reverse motion of the San Marcos fault system. Inversion along the San Marcos fault system, and the entire Sabinas basin in the Paleogene from ~60 to 40 Ma, resulted from wholesale contractional deformation during the Laramide (Mexican) orogeny. Hence, emplacement of the sediment-hosted stratiform copper–silver mineralization at San Marcos, and elsewhere in the larger Coahuila region, is interpreted as a natural corollary of large-scale, metal-bearing fluid expulsion, migration, and precipitation resulting from orogenic shortening, lithostatic loading, and squeezing of the Sabinas basin during Mexican orogen construction. Although sedimentation of the host strata in the Sabinas basin took place in a pericratonic setting associated with the opening of the Gulf of Mexico, sediment-hosted stratiform copper-silver mineralization occurred during orogenic uplift and conversion of the original basin into an orogen-foreland pair, with similarities to some of the world´s largest sediment-hosted stratiform copper provinces.

Drag folds in sedimentary rocks hosting kimberlites, West Yakutia

Drag folds were revealed in Lower Paleozoic sedimentary strata of Mirny, Nakyn and Syuldyukar diamondiferous fields, West Yakutia. They consist of minor anticlinal forms (3-5 to 15-20 cm thick) and cut marl, clayey limestone and dolomite bands located between monolith seams of carbonate rocks. Some folds as monocline, flexures and S- or Z-shaped folds reflect the degree of shearrelated interlayer offset. Drag folds are among shear zone occurrences including microfaults, slickensides with slip groove horizontal planes, and schistosity zones. Drag folds reflect local extension points while schistosity zones indicate compression points. For Syuldyukar field, detailed mapping results for drag folds and schistosity zones are provided at 3 scales: across 20 × 20 m observation grid within a local 2 km2 site; across 200 × 200 m grid within 20 km2 area; across 500 × 500 m grid within ~100 km2 territory. For all scales, drag fold halos are restricted to schistosity zones. Within a local site adjacent to kimberlites, drag folds mark ore-hosting fault controlling long axes of kimberlite areal projections. Within large areas, drag fold halos are 1,2-2 km, which compares with kimberlite group areal parameters. Drag fold halos reflect shear junctions, with some of them hosting kimberlites. Local occurrences of drag folds mark a major shear hosting a kimberlite body. Drag fold analysis combined with other evidence should be used as an indirect prospecting indicator of concealed shears and local extension areas controlling kimberlites.

Structure of the Golets Vysochaishy gold deposit (Northern Transbaikalia)

The article describes the fold-thrust structure of the Golets Vysochaishy deposit located at the Baikal-Patom Upland in the Marakan-Tunguska megasyncline. The latter is composed of terrigenous-carbonate carbonaceous rocks metamorphosed in greenschist facies conditions. The deposit is detected in the hanging wing of the asymmetric Kamenskaya anticline. In a cross section, the anticline is an S-shaped structure extending in the latitudinal direction. The main feature of the Golets Vysochaishy deposit is the development of interlayer sulfidization zones (pyrite, pyrrhotite), including gold-bearing ones. Its gold-ore zones tend to occur in layered areas of interlayer sliding in the rocks of the Khomolkhinskaya suite.Four structural markers revealed within the deposit area are indicative of repeated deformation processes: (1) sublatitudinal folding, cleavage of the axial surface and its subsequent transformation into schistosity; (2) crenulation cleavage; (3) interlayer sliding and rock breakdown with interlayer drag folds, parallel microfractures and polished slickensides; (4) large quartz veins and veinlets that cross cut the main structural elements in plan.

Mineral chemistry and deformation in a temperature gradient in the Sierras Pampeanas of Córdoba (Argentina): The Chicamtoltina Tonalite–Trondhjemite Orthogneiss

On the eastern margin of the Sierra Chica, in a gneissic host rocks metamorphosed under medium to high amphibolite facies (‘Sil-in’ isograd) during the Neoproterozoic–Cambrian M2-D2 event, a body of tonalitic-trondhjemitic composition was discordantly emplaced, cutting the S2 metamorphic foliation and the ‘Sil-in’ isograd. A strong deformation, with along the temperature gradient of the host rocks, transformed the igneous rocks into an orthogneiss during a M3-D3 event. Based on textural, microstructural and mineral chemistry changes along the mentioned metamorphic transition and age determination, the orthogneissification process is related to the deformational and thermal history in the Sierras Pampeanas of Córdoba. The M3-D3 event would have reworked the S2 foliation, generating an S3 foliation associated with folding and boudinage in the gneissic host rocks, forming a major intrafoliar fold in the tonalitic-trondhjemitic body, with its flanks on both sides of the ‘Sil-in’ isograd. The M3-D3 event is recorded in the textural and mineral chemistry variations, following a medium-high temperature gradient during deformation, from the southeast to the northwest of the body. In that direction, the ‘core and mantle’ texture in plagioclase increased the proportion of the recrystallized mantle, which was also enriched in anorthite content. Quartz textures evolved from polycrystalline ribbons, with internal deformation bands, to dismembered ones, with internal chessboard pattern. Recrystallization of the entire plagioclase-quartz aggregate ended in an interlobate to amoeboid texture, implying high-temperature conditions (~650-750 °C) along the mentioned temperature gradient during the M3-D3 event. Regarding micas, the primary biotite progressively increased in Mg and decreased in Fe towards the northwest, without appreciable textural changes. Concomitantly, the primary Ti-rich ferrimuscovite increases its content in Mg and becomes poor in Ti, at the same time that it developed symplectitic quartz-plagioclase intergrowths. During a final D4 deformational event, also under high ductility conditions, asymmetric intrafolial folds (drag folds, with geometrical S4 axial planes) were developed, associated with imbrication and folding of boudins within the S3 (// S2) foliation. The D2, D3, and D4 deformation events are part of a continuum (progressive deformation) in a combined simple shear and flattening regimen (transcompression = convergent oblique shear). The contrasting rheology between the stiffer Chicamtoltina igneous body and its gneissic host rocks explain the different behavior during high ductility deformation. Internally, this small igneous body, with plagioclase as stress-supporting mineral framework (plagioclase-quartz ratio 2:1), mirrored the rheological behavior of the middle to lower crust deformation. A concordia age of 535.81 ± 0.79 Ma (U–Pb LA-ICP-MS on zircon) obtained for the Chicamtoltina orthogneiss is interpreted as the crystallization age of the tonalitic-trondhjemitic protolith (igneous event Ig1). By correlation with the tonalitic orthogneisses from the Sierra Chica and Sierra Norte of Córdoba, this protholith is linked to the Pampean magmatic arc, with an age range of 541–515 Ma and a peak of magmatic activity at 530–535 Ma.

Tectono-stratigraphic evolution of a cretaceous intracontinental rift: Example from Rio do Peixe Basin, north-eastern Brazil

The Rio do Peixe Basin is part of a set of aborted rifts with NE-SW trend along the NE region of Brazil. This axis was originated in response to the stress caused by the fragmentation of Pangea during the Mesozoic, and effectively installed during the Cretaceous event that defined the Brazilian equatorial margin. The detailed structural framework allowed the recognition of the Brejo das Freiras fault zone which presents NE-SW trend and, locally, ENE-WSW. This fault zone, strongly influenced by the basement fabric, deformed the layers and originated drag folds, fault-bending folds, and fault-propagation gentle folds. The half-graben fault margin developed alluvial fans, triggered by tectonic activity and geologically instantaneous creation of accommodation space. At the central portion it predominates a lacustrine sedimentation, while fluvial systems are responsible for carrying sedimentary supply from the flexural margin towards the depocenter. Since tectonics controls sedimentation on rifts, the tectonic systems tracts correspond to an arrangement of contemporaneous depositional systems associated with basin evolutional stages. In the syn-rift sequence, corresponding to Rio do Peixe Group nine stratigraphic units were recognized, bounded by flooding surfaces and drowning unconformities which record cycles of expansion and contraction of the lacustrine system. The tectonic systems tracts related to the initial stages of the rift installation were identified, followed by moments of high and low tectonic activity. The seismic and the well data support both the tectono-sedimentary evolution of the Brejo das Freiras Sub-Basin and the seismic-stratigraphic model proposed in this work. Different correlations between accommodation and sedimentation along a half-graben basin are expected in moments of tectonic activity and quiescence. The use of seismic attributes and filters improved the visualization and extraction of geological features, in both, structural (using pseudo-relief and curvature attributes) and stratigraphic interpretations (RMS amplitude and cosine phase). The understanding of spatial and temporal distribution of depositional systems responsible for a basin filling allows predicting potential petroleum systems.

Precise Location and Mapping of the Main Central Thrust Zone in Reference to Micro-Structures and Deformation along Khudi-Tal Area of Marsyangdi Valley

Geological mapping was carried out along Marsyangdi valley in the Khudi - Dahare -Tal area on a scale of 1: 50,000 covering about 142 square kilometers. Recent study aims to locate the Main Central Thrust (MCT) precisely based on lithostratigraphy, micro-structures, deformation, and metamorphism. Several thin sections were observed to study the metamorphism, deformation, and micro-structures developed in the rocks. The rocks sequences in both the Higher Himalaya and the Lesser Himalaya have undergone polyphase metamorphism and deformation. The Lesser Himalaya experienced first burial metamorphism (M1) followed by garnet grade inverted metamorphism related to the MCT activity (M2) followed by retrograde metamorphism (M3) whereas the Higher Himalaya has undergone regional high-pressure/ high-temperature kyanite/ sillimanite- grade prograde regional metamorphism (M1) followed by the (M2) related to ductile sharing which in turn is overprinted by the later post-tectonic retrograde garnet to chlorite grade metamorphism during exhumation. The polyphase deformation is indicated by the cross-cutting foliation and many other features. The deformation phase D1 is associated with the development of the bedding parallel foliation due to burial in both the Higher Himalaya and the Lesser Himalaya. Isoclinal folds and crenulation cleavage were developed before the collision is categorized as D2. Development of nearly N- S trending mineral and stretching lineation, south vergent drag folds, folded S2 cleavage and microscopic shear sense indicators, rotated syn- tectonic garnet grains, etc. were developed during the deformation D3 related to the ductile shearing through the MCT. Various brittle faults and shear zones cross-cutting all earlier features were developed during D4 during the upheaval. The rocks in the MCT zone are affected by intense sharing and mylonitization as indicated by the presence of many mylonitic structures in the thin sections throughout the Lesser Himalaya in the area. Features like polygonization and ribbon quartz with evidence of sub-grain rotation, mica fish, syn-tectonic rotated garnet grains indicate the ductile shearing in the MCT area suggesting the dynamic recrystallization in the MCT zone whereas rocks of the Higher Himalaya show the evidence of recrystallization under static condition. The MCT zone was mapped precisely based on the microstructures and deformation.

Tectonic‐magmatic framework identified by remote sensing analyses and geochronology in West Junggar, Central Asian Orogenic Belt

The temporal and spatial evolutions of the Darbut Fault deformation patterns were analysed for multiple deformation stages in this study by using a regional geological survey, quantitative remote sensing image analyses, and structural analyses. Remote sensing images indicating that the Karamay granitoid batholith is greenish‐grey, the Hongshan granitoid batholith is yellowish‐green along the Darbut Fault, and the Miaoergou, Akebasitao, and Hatu granitoid batholiths are round in shape and luminous yellow colour. The remote sensing images of the Darbut Fault, Hongshan granitoid batholith, and data from previous research suggest that the upright folds (F1) in the Carboniferous strata near the Darbut Fault and northeast‐southwest (NE‐SW) striking sub‐vertical dextral shear zones in the mélanges represent the regional dextral strike‐slip shear zones. The NE‐SW striking transcurrent sinistral strike‐slip faults and ‘S‐shaped’ drag folds (F2) overprinted the dextral shear zones. The Hongshan batholith is a complex pluton, which was composed of alkaline feldspar granite, syenite granite, and monzogranite. The LA‐ICP‐MS U–Pb 311–307 Ma of zircons from the ductilely deformed Hongshan alkaline feldspar granite and monzogranites are formed during the dextral shear zone of the Darbut Fault. The 302 Ma Hongshan alkaline feldspar granite is contemporarily formed during the sinistral strike‐slip stage of the Darbut Fault. The Hongshan complex pluton has a prolonged formation age and witnesses the whole process of ocean‐land conversion from the capture of island arc‐related rocks, granite formed in the post‐collision stage, and dyke swarms formed in the intracontinental extension stage.

TECTONIC AND GEOLOGICAL STRUCTURES OF GUNUNG KROMONG, WEST JAVA, INDONESIA

The Gunung Kromong complex is a solitary hill which physiographically located within the Jakarta Coastal Plain Zone. The research is mainly formed of primary data from observations and measurements of all geological elements especially morphological aspects and geological structures to reveal the tectonic background and its relation to the oil seepage. Sedimentary rocks outcrops appear in the western part around the cement plant surrounded by well exposed of igneous rocks. Oil seepage, gas and hot springs are found in limestone which has high fracture intensity and at low elevation. The distribution of sedimentary rocks is not only controlled by the folds and fault structures but also influenced by the igneous rock intrusion. The alignment of the structure in West Palimanan is formed by steep slopes on dacite igneous rock which is concluded as fault scarp. There are found abundance of fault indications in the form of slickenside, fault breccias, millonites, drag folds and have high intensity of shear joint. The KRG-7, KRG-8 and KRG-9 observation points, a slickenside with the same plane was found but has different slicken line, each shows a thrust fault and normal faults. The direction of slickenside divided into 4 units of fault groups. The series of hills in the south of the Baribis Fault occur because of the lifting through the fold and the thrust faults mechanism. The thrust fault in the Gunung Kromong complex is the hanging wall section of the Baribis thrust fault, so that it belongs to the leading thrust system.