Fold Interference Patterns Research Articles

Gold concentration during polyphase deformation: Insights from Boulanger Project, French Guiana

The Boulanger gold deposit is hosted within the Rhyacian greenstone belts from the Guiana shield located within the North Guiana Through, French Guiana. The Boulanger deposit is characterized by a complex polyphase deformation and mineralization history involving a coaxial shortening phase, D1, followed by a transcurrent deformation event, D2. The main ore bodies consist of a series of quartz-tourmaline-pyrite-carbonate extension and shear veins in response to NNE-SSW shortening event (D1), and adjacent altered host-rocks. The proximal alteration bordering the vein system is characterized by pyrite-tourmaline (main ore assemblage). This shortening event is followed by transcurrent deformation, D2. Locally, D2 is expressed by the formation of N020-030 narrow zones (F2 axial trace) along which D1 structures are reoriented. The structural analysis performed on both S1/0 foliation and on geophysical data interpretation highlights the presence of Type-1 interference patterns. These zones are associated with significant gold enrichment and the presence of a new ore assemblage dominated by pyrrhotite-pyrite. Quantitative structural analysis approach, based on measured foliations (S1/0) and gold grade demonstrates that D2-related folding is associated with important gold endowment. These findings suggest that the characterization of the exact fold interference pattern is fundamental for the targeting of favorable enriched zones at the deposit scale.

Enhanced structural analysis through a hybrid analogue-digital mapping approach: Integrating field and UAV survey with microtomography to characterize metamorphic rocks

This study focused on collecting structural data orientations of a crustal-scale shear zone (Palmi Shear Zone, PSZ, southern Calabria, Italy) by integrating various analytical and field-based techniques. The PSZ consists of deformed metamorphic rocks (migmatitic biotitic paragneiss, marbles, and skarns) showing multiple folding phases, and Hercynian tonalites and pegmatites (306-290 Ma), crosscut by Late Hercynian leucocratic dykes (ca. 290 Ma). Multi-sized clasts composed of different lithologies are preserved on clean outcrop surfaces, and are sheared into both σ - and δ -type objects that collectively suggest opposing senses of shear. The study incorporates structural analysis of folds, field and aerial surveys (UAV), digital mapping, and microcomputed tomography. Various kinematic indicators were observed in the PSZ, indicating a mix of factors influencing the shear strain patterns (e.g. fold interference patterns, different rock types with high viscosity contrast). The findings suggest a clear consistency between structural data inferred from 3D VOM (Virtual Outcrop Model) and those collected directly in the field, confirming the occurrence of both sinistral and dextral shear in the PSZ, providing important insights into the tectonic evolution of the Calabrian-Peloritani Terrane.

Formation of a dome-and-basin fold interference pattern by granite intrusions

Folding of layered sequences is influenced by the presence of igneous intrusions, which form competent bodies that control fold distribution. Here, we show an example from the Broken River Province (northeastern Australia), where macroscopic folding affected sedimentary rocks and felsic intrusions, forming a dome-and-basin pattern. Field relations indicate synkinematic emplacement of the granitic intrusions, which occur as folded sill-like microgranitic bodies, mildly deformed stocks at structural domes, and undeformed granite that cuts across the folded sedimentary sequence. U-Pb zircon ages from the various intrusions yielded an age range of 335−320 Ma, thus constraining the timing of folding. The deformed zone is restricted to an ∼30-km-wide corridor bounded by two crustal-scale shear zones. Kinematic indicators show that folding was generated by dextral transpression, giving rise to the dominant orientation of folds. Domes and basins are generally concentrated in the proximity of the granitic intrusions, indicating that their development was likely controlled by the synkinematic emplacement of these plutons. The volume of granite added to the exposed section could have altered the local strain regime, switching the horizontal stretching direction during transpression to a shortening direction. We therefore suggest that forced intrusion of synkinematic plutons into a confined transpressional zone (dominated by flattening strain) can create local constrictional strain where shortening occurs in two broadly orthogonal horizontal directions. The results demonstrate a link between Carboniferous granitic dome-and-basin patterns and syn-transpressional magmatism. It is plausible that similar processes were responsible for the development of granitic dome-and-basin patterns in Archean terranes.

The Late Mesozoic Intracontinental contraction–Extension Transition in the Beishan Fold‐Thrust Belt, Central Asia: Constraints From Structural Analysis and Apatite (U–Th)/He Thermochronology

AbstractThe late Mesozoic Beishan fold‐thrust belt occupies a key position in central Asia, figuring importantly in our understanding of intracontinental deformation in response to far‐field plate‐boundary forces. To determine when and how the Beishan fold‐thrust belt was developed, we conducted detailed field mapping, structural analysis, and apatite (U–Th)/He (AHe) thermochronology in the Hongliudaquan area, southeastern Beishan Range. The study area exposes the Hongliudaquan fold and thrust system, consisting of the southern Hongliudaquan thrust nappe and autochthonous fold sets. The nappe experienced >16 km northward thrust‐induced translation. The autochthonous fold sets strongly deformed Lower–Middle Jurassic strata showing diverse trends of kilometer‐scale basins and ranges. We interpret that this fold interference pattern was produced in a constrictional deformation field, which underwent synchronous shortening in two orthogonal directions. This stain history was related to synchronous imbricate thrusting, duplex formation, and strike‐slip faulting at different structural levels. Restoration of the Hongliudaquan fold and thrust system yields a minimum of ∼27% (∼10.1 km) north–south shortening strain. Thrusting and nappe emplacement within the Beishan fold‐thrust belt may have started in the late Middle Jurassic and persisted until the Early Cretaceous (∼140–133 Ma). Our AHe data also record ∼124–115 Ma cooling related to footwall exhumation during subsequent normal faulting, indicative of the late Mesozoic intracontinental contraction–extension transition. The Beishan fold‐thrust belt initiated in central Asia via the late Mesozoic reactivation of the Paleozoic–early Mesozoic Beishan orogen as a result of the far‐field responses to multi‐directional plate convergence around Asia.

Fold interference pattern and crustal decoupling in northern Tanzania

High-temperature granulite facies metamorphism and deformation evolved through Neoproterozoic tectonic processes in large parts of southeastern Africa. The Eastern Granulites of Tanzania, one example for these extreme conditions, show progressive decoupling between lower and middle crust together with progressive cooling and rheology stratification. Coupled deformation during earliest stages of deformation arises from extremely small viscosity contrast of all minerals and rocks at granulite facies metamorphic conditions around 850 °C. With cooling of the orogen rheology stratification increases and deformation decouples into horizontal stretching through gravity driven channel flow in the lower crust and thickening and horizontal west-east shortening in the middle/upper crust. As soon as a stiff and cold indenter, represented by external parts of the Archean Tanzania Craton (Western Granulites) enters the system, the indenter acts as lower crustal ramp along which deep crustal units are exhumed and displaced towards the foreland. During this orogenic period the crust was fully stratified and a second phase of decoupling, detachment over crustal scale ramp, occurs. Superposition with late orogenic north-south shortening structures resulted in type 1 fold interference pattern and formation a of dome-and-basin geometry in northeastern Tanzania. Structural modelling of fold superposition helps to explain the complex structural pattern and to predict sites of decollement horizons. By comparison with fold interference pattern exposed in southern Madagascar we define different types of dome-and-basin formation. Those with neglectable rheology contrast are viscosity determined where mixing of upper and lower crustal material is enabled by vertical flow folding. Those with strong viscosity contrast are mechanically determined where domes and basins are formed by changes in regional principal stress directions.

Differential fold interference patterns in an intracontinental orogen: Insights from superposed buckle folding in southern Jiangxi Province, South China Block

The South China Block (SCB) experienced multi-stage tectono-magmatic events during the Mesozoic, forming a broad and episodic intracontinental orogenic belt. It is controversial whether the driving force of the Mesozoic intracontinental orogeny in the SCB is related to the far-field effects of plate convergence. To better understand the driving mechanism of intracontinental orogeny, we conducted a detailed structural investigation of the Xingguo area in southern Jiangxi Province, located in the central part of the SCB. Two regional-scale buckling superposed folds were identified as the Chayuan arcuate syncline of the fold axis protruding to the north (Type 2a interference pattern) and the Xiefang syncline of the fold axis extending NW–SE (Type 1d interference pattern). Although there are differences in fold interference patterns, the Chayuan arcuate syncline and Xiefang syncline were formed by the superimposition of the Middle–Late Triassic NE–SW shortening and the Middle–Late Jurassic nearly E–W shortening. This phenomenon of the differential fold interference patterns in the same tectonic setting is determined by the difference in geometric characteristics of their initial folds. Combined with the variation of the Mesozoic paleostress field, it is considered that the Mesozoic intracontinental orogeny in the SCB is mainly controlled by the far-field stress propagation generated by plate interactions. Based on the analysis of tectonic architecture, we propose that the Mesozoic tectonic evolution of the SCB experienced a transformation from multi-plate convergence in the Triassic to Andean-type subduction in the Jurassic. This tectonic transformation finally resulted in the reactivation of the Precambrian multi-terrane collage of the SCB.

Structural analysis of layered evaporites using internal deformation patterns: Examples from Santos and Campos basins, Brazil

The Santos and Campos basins, offshore southeastern Brazil, contain layered evaporite sequences (LES) deposited during the Late Aptian, marking the end of lacustrine and beginning of restricted marine conditions. Density and velocity differences between evaporite minerals such as halite, anhydrite, carnalite, tachyhydrite and other bittern salts result in strong seismic impedance contrasts and therefore enable excellent stratal imaging of internal salt deformation. Different acoustic responses allow a detailed analysis of the internal deformation of evaporites, with the corresponding mechanical properties providing constraints on regional salt flow in the Santos and Campos basins. Detailed mapping and analysis of deformed LES support conclusions from cross-section restorations and provide insight on local salt flow and deformation mechanisms. This study utilizes kinematically restored regional sections to characterize overall basin-wide salt flow, while leveraging salt facies and their control on internal salt fold styles to demonstrate localized salt deformation. The important results from this study are the following. a) Santos and Campos basins are linked extension-contraction systems, however pre-existing basin-wide changes in paleo-topography represents a fundamental control in the overall salt flow, accounting for much of the downdip contraction in the middle of the basins rather than the toe of the system. b) Salt mobilization due to gravity initiated early and there is evidence that it was contemporaneous with evaporite deposition in many parts of the basins. Intrasalt unconformities and stratigraphic features are presented to support this conclusion. c) Mechanical stratigraphy within the LES play an important role in controlling intra-salt structural styles, and their fold interference patterns can be used to understand salt deformation in great detail. These results can be applied to better investigate a spectrum of salt tectonic problems in the basins and elsewhere, such as regional and localized salt flow, sub-regional prediction of weld kinematics and local prediction of weak salt units for drilling hazard analysis.

Along-strike variation of fault-related inversion folds within curved thrust systems: The case of the Central-Northern Apennines of Italy

The along-strike variation of fault-related inversion folds within curved thrust systems is documented by presenting cases from the Central-Northern Apennines of Italy. Fold structures are reconstructed by integrating geological field data and structural restoration of various key balanced cross-sections, which are also constrained by the detailed interpretation of panoramic views, geological field mapping, and structural analysis. The lateral variation from fault-bend to fault-propagation folding is promoted by selective reverse reactivation of precursor, mainly rift-related, normal faults preferentially located along the oblique and frontal ramps. Their space variability is identified within single curved thrust systems of three different size testifying a scale-invariant phenomenon. Fault-bend reactivation and fault-propagation shortcut folding produce a characteristic fold interference pattern at the salient apex of the curved thrust system. The transition style depends on the pattern of the inherited normal fault template and on the amount of shortening achieved in the salient zone. The modest amount of shortening resulting from fault-related inversion folding along regional curved thrust systems is consistent with a basement-involved thick-skinned thrusting style. Recognition of fault-related inversion fold styles and of their along-strike variations represents a useful predictive tool when unravelling the kinematic evolution of curved orogenic belts resulting from positive inversion. Inversion fold development modes and their diagnostic features are also important for investigations of subsurface Earth resources.

Fold interference patterns in Meso-Proterozoic Champaner fold belt (CFB) Gujarat, western India

This paper documents three phases of folding within the Meso-Proterozoic Champaner Fold Belt (CFB) located at the eastern part of Gujarat, western India. The first phase ( $$\hbox {F}_{1})$$ displays WNW plunging $$\hbox {F}_{1}$$ fold of moderately inclined nature in the schists of the Khandia Formation. The second phase ( $$\hbox {F}_{2})$$ refolded $$\hbox {F}_{1}$$ along a similar trend to produce folds of tight isoclinal nature in the Khandia and Narukot quartzites. Additionally, these $$\hbox {F}_{1}$$ folds depict second-order tight, and $$\hbox {F}_{2}$$ folds as first-order open type in the younger sequences of the CFB with varying amplitude vs. wavelength ratio. The ratio for $$\hbox {F}_{1}$$ folds ranges from 2:1 to 3:1, obtained along 3–4 m length across 3–6 $$\hbox {m}^{2}$$ area, whereas for $$\hbox {F}_{2}$$ folds the ratio ranges from 1:4 to 1:5 and is obtained along 1–2.5 km length across 0.5–1.5 km $$^{2}$$ area. The fold interference pattern developed on account of $$\hbox {F}_{1}$$ and $$\hbox {F}_{2 }$$ folds has resulted into Type-III or hook-shaped geometry regionally. The last phase of folding is characterised by N–S trending $$\hbox {F}_{3}$$ folds of open type over kilometre long limbs of $$\hbox {F}_{1}$$ and $$\hbox {F}_{2}$$ folds. The superposition of $$\hbox {F}_{1-3 }$$ folds developed map scale Type-I or Dome and Basin geometry over Type-III superposed folds. The overall compressional direction for $$\hbox {F}_{1}$$ and $$\hbox {F}_{2}$$ folds ranges from N–S to NNE–SSW and for $$\hbox {F}_{3}$$ ranges from E–W to ESE–WNW.

Interference fold patterns in regional unidirectional stress fields: A result of local kinematic interactions

Folds are commonly found in association with ductile shear zones. However, superimposed folding contemporaneous with shearing is rarely reported. Here, we describe a macroscopic fold interference pattern with geometry intermediate between types 1 and 2 related to development of adjacent transcurrent shear zones. The interference fold pattern is located in a compartment bounded, on the eastern side, by an ENE-trending, dextral shear zone that connects with a NE-trending, sinistral shear zone, and, on the western side, by a NE-trending sinistral shear zone. A first folding episode produced NW-trending, SW-verging inclined folds and a later one NE-trending, upright folds. Folds of the first event are attributed to a local contractional strain field induced by the growth of the shear zones with opposed kinematics whereas the later folding episode and nucleation of the western shear zone reflects regional NW-SE contraction. Folding of a macro-scale NW-trending fold by NE-trending folds produced the fold interference structure. In contrast with most cases of types 1 and 2 fold interference patterns, in the present case no change on the orientation of regional stress axes is required.

Superimposed deformation of the Solonker Belt and nearby regions in western Inner Mongolia, China

AbstractThe deformation of the Solonker Belt and nearby regions is helpful for understanding the tectonic evolution of the Central Asian Orogenic Belt. This study carried out structural analysis in the Mandula and Ganqi areas of western Inner Mongolia, including the Solonker Belt, the Southern Orogenic Belt and the northern Yinshan Belt (Langshan range). Our results reveal that the Solonker Belt, the Southern Orogenic Belt and the northern Yinshan Belt underwent two stages (D1 and D2) of deformation during the Mesozoic period. The D1 stage produced the NNE-directed thrusts and asymmetric folds, indicating a NNE–SSW contraction. The northern Yinshan Belt, the Southern Orogenic Belt and the Solonker Belt formed as a series of NNE-verging tectonic nappes. The D2 stage developed the NE-trending folds and the SE- or NW-directed thrusts that cross-cut the D1 structures. The two events of nearly orthogonal or oblique shortening gave rise to the superimposed structures (e.g. fold interference patterns). The quartz veins that filled the fractures of the D1 deformation contain zircons of Middle Triassic U–Pb ages. The new dating data, along with the regional sedimentary hiatus, led us to infer that the D1 stage of deformation occurred in Middle Triassic time and the D2 stage occurred in Late Jurassic time. We consider that the D1 stage of deformation resulted from a convergent event, which might be related to the closure of the Palaeo-Asian Ocean or limited, narrow ocean basins; and the D2 stage of deformation was the far-field result of subduction of the Palaeo-Pacific Ocean and the closure of the Mongol-Okhotsk Ocean.

Folded Radiolarite Unit as a Kinematic Indicator of the Zagros Collision Processes, Southwestern Iran

Radiolarites have an important role in the reconstruction of the paleogeography, bathymetry and their coexistence with ophiolites gives an opportunity to determine the tectonic history of collisional zones. The radiolarite units of the southwestern Neyriz are part of the Zagros accretionary prism and positioned beneath the thick bedded carbonate sediments of Tarbur Formation. The existing various structural elements within this unit give a unique fortunate for investigation on the deformation history and studying of the kinematics of the Zagros collision zone. The numerous evidence show that this unit has experienced different deformation conditions during ongoing evolution, including ductile, brittle-ductile and brittle deformation conditions. The main strike of E-W for axial planes of folds, eastward trend and plunge of fold axes, boudins’ neck axes and Type III of the fold interference patterns are indicators of formation and evolution of folds during transpressional deformation. Structural evolution of the study area has been affected by an NE-dipping subduction zone of Neo-Tethyan oceanic lithosphere below the Iranian microcontinents. These structural elements suggest that high mechanical anisotropy and two successive generations of folding simultaneously with thrusting and extrusion of this unit had led to formation and evolution of this highly asymmetrical folded unit on top of the subducted oceanic crust.

Orthogonal switching of AMS axes during type-2 fold interference: Insights from integrated X-ray computed tomography, AMS and 3D petrography

We applied X-ray computed microtomography (μ-CT) in combination with anisotropy of magnetic susceptibility (AMS) analysis to study metamorphic rock fabrics in an oriented drill core sample of pyrite-pyrrhotite-quartz-mica schist. The sample is extracted from the Paleoproterozoic Martimo metasedimentary belt of northern Finland. The μ-CT resolves the spatial distribution, shape and orientation of 25,920 pyrrhotite and 153 pyrite grains localized in mm-thick metapelitic laminae. Together with microstructural analysis, the μ-CT allows us to interpret the prolate symmetry of the AMS ellipsoid and its relationship to the deformation history. AMS of the sample is controlled by pyrrhotite porphyroblasts that grew syntectonically during D1 in subhorizontal microlithons. The short and intermediate axes (K3 and K2) of the AMS ellipsoid interchanged positions during a subsequent deformation (D2) that intensely crenulated S1 and deformed pyrrhotite, while the long axes (K1) maintained a constant position parallel to the maximum stretching direction. However, it is likely that all the three AMS axes switched, similar to the three principal axes of the shape ellipsoid of pyrite porphyroblasts from D1 to D2. The superposition of D1 and D2 produced a type-2 fold interference pattern.

Fold-interference patterns in the Bowen Basin, northeastern Australia

ABSTRACTDeformation patterns of Paleozoic and Mesozoic strata in eastern Australia are evidence of a structural and tectonic history that included multiple periods of deformation with variable strain intensities and orientations. Detailed analysis of structural data from the Bowen Basin in northeastern Australia reveals previously undescribed, north–south elongate, Type-1 fold-interference patterns. The Bowen Basin structures have similar orientations to previously described interference patterns of equivalent scale in upper Paleozoic strata of the New England Orogen and Sydney Basin of eastern Australia. The east Australian folds with north–south-trending axes most likely formed during late stages of the Permian–Triassic Hunter–Bowen Orogeny, and they were subsequently refolded around east–west axes during post 30 Ma collision of the Indo-Australian plate with the Eurasian and Pacific plates. The younger, east–west-trending folds have orientations that are well aligned with the present-day horizontal stress field of much of eastern Australia, raising the possibility that they are active structures.

Interference folding and orocline implications: A structural study of the Ponga Unit, Cantabrian orocline, northern Spain

The origin of the Cantabrian orocline of the Variscan orogen in NW Iberia remains a topic of debate. We present a structural study of the Ponga Unit, a Cambrian to Carboniferous tectonostratigraphic package within the West European Variscan belt foreland fold-and-thrust belt that lies within the core region of the orocline. Our primary goal was to determine if W-plunging folds of the fold-and-thrust belt are attributable to formation of the Cantabrian orocline, or if they reflect lateral ramps in the underlying Variscan thrust faults. The major lithologic units of the Ponga Unit are the rheologically competent Lower Ordovician Barrios quartzite, and the less-competent, Carboniferous Barcaliente limestone and Beleno shale and sandstone formation. Our mapping and structural analysis within the Ponga Unit focused on the Laviana, Rioseco, and Campo de Caso thrust sheets, and associated bounding thrusts. Over 800 structural orientation measurements were collected across the study area. These data, coupled with data compiled from regional geological maps, allow for analysis of the crustal structure. West-plunging folds of the Laviana, Rioseco, and Campo de Caso thrust sheets form kilometer-scale anticline-syncline pairs, producing a complex fold interference pattern that is characteristic of the Ponga Unit. Our analysis shows that: (1) the geometry of the W-plunging folds is inconsistent with a lateral ramp model; (2) the map pattern defines a mushroom-type fold interference pattern, indicating two distinct deformational events characterized by principal compressive stresses oriented at a high angle (perpendicular) to one another; and (3) paleomagnetic data from the study area are consistent with the secondary model of orocline formation and indicate that there was a short window of time between the end of Variscan orogenesis and the onset of oroclinal buckling. Our results indicate that early N-S–trending folds, which resulted from Variscan orogenesis, were refolded during a post-Variscan orogen-parallel compression event attributable to formation of the Cantabrian orocline.

Fold interference pattern in thick-skinned tectonics; a case study from the external Variscan belt of Eastern Anti-Atlas, Morocco

Conflicting views are expressed in literature concerning fold interference patterns in thick-skinned tectonic context (e.g. Central Anti-Atlas and Rocky Mountains-Colorado areas). Such patterns are referred to superimposed events with distinct orientation of compression or to the inversion of paleofaults with distinct strike during a single compressional event. The present work presents a case study where both types of control on fold interference are likely to be combined. The studied folds occur in the Tafilalt-Maider area of eastern Anti-Atlas, i.e. in the E-trending foreland fold belt of the Meseta Variscan Orogen in the area where it connects with the SE-trending, intracontinental Ougarta Variscan belt. Detail mapping documents unusual fold geometries such as sigmoidal and croissant- or boomerang-shaped folds associated with a complex major fault pattern. The folded rock material corresponds to a 6–8 km-thick Cambrian-Serpukhovian sedimentary pile that includes alternating competent and incompetent formations. The basement of the Paleozoic succession is made up of rhomboedric tilted blocks that formed during the Cambrian rifting of north-western Gondwana and the Devonian dislocation of the Sahara platform. The latter event is responsible for an array of paleofaults bounding the Maider and South Tafilalt Devonian-Early Carboniferous basins with respect to the adjoining high axes. The Variscan Orogeny began during the Bashkirian-Westphalian with a N-S direction of shortening that converted the NW-trending Ougnat-Ouzina paleogeographic high into a mega dextral shear zone. Folds developed on top of a moving mosaic of basement blocks, being oriented en echelon on the inverted paleofaults or above intensely sheared fault zones. However, a dominantly NE-SW compression responsible for the building of the Ougarta belt also affected the studied area, presumably during the latest Carboniferous-Early Permian. The resulting fold interference pattern and peculiar geometries (J. Tijekht croissant-shaped fold) would exemplify a dual control of deformation by both the variably oriented basement paleofaults and the evolution of the regional shortening direction with time.

Superimposed folding and thrusting by two phases of mutually orthogonal or oblique shortening in analogue models

Orogens may suffer more than one phase shortening resulting in superposition of structures of different generations. Superimposition of orthogonal or oblique shortening is studied using sandbox and centrifuge modelling. Results of sand models show that in orthogonal superimposition, the two resulting structural trends are approximately orthogonal to each other. In oblique superimposition, structures trend obliquely to each other in the relatively thin areas of the model (foreland), and mutually orthogonal in areas where the model is thickened during the first phase of shortening (i.e. the hinterland). Thrusts formed during the first shortening phase may be reactivated during the later shortening phase. Spacing of the later phase structures is not as wide as expected, considering they across the pre-existing thickened wedge. Superposition of structures results in formation of type 1 fold interference pattern. Bedding is curved outwards both in the dome and basin structures. Folded layers are dipping and plunging outwards in a dome, while they are dipping and plunging inwards in a basin. In the areas between two adjacent domes or basins (i.e. where an anticline is superimposed by a syncline or a syncline is superimposed by an anticline), bedding is curved inwards, and the anticlines plunge inwards and the synclines outwards. The latter feature could be helpful to determine the age relationship for type 2 fold interference pattern. In tectonic regions where multiple phases of shortening have occurred, the orogenic-scale dome-and-basin and arrowhead-shaped interference patterns are commonly formed, as in the models. However, in some areas, the fold interference pattern might be modified by a later phase of thrusting. Similar to models results, superimposition of two and/or even more deformation phases may not be recorded by structures all over the tectonic area.

Mega arrowhead interference patterns in the central part of the Yanshan Orogenic Belt, North China

The Chengde-Pingquan region is located in the central part of the Yanshan Orogenic Belt (YOB). At Daheishan and Pingquan in the central YOB, thrusts and folds of variable trends are displayed in 2 km-scale fold interference patterns. Detailed field mapping was conducted to decipher the geometry of these two superimposed structures. Map-view geometry and stereonet plots for outcrop-scale folds indicate that the superimposed structures form arrowhead interference pattern where NW-SE-trending F1 folds are refolded by later ENE-WSW F2 folding. After remove the effects of later faulting, restored map-views of the superimposed structures show that when the F1 folds have inclined axial surfaces but with no an overturned limb, an arrowhead interference pattern (here called modified type-2 pattern) can form. Our field data and reinterpretation of the findings of previous studies suggest that five major shortening phases have occurred in the Chengde-Pingquan region. The first two phases, which formed the superimposed folds, occurred earlier than the Late Triassic (D1) and during the Late Triassic to Early Jurassic (D2). These two phases were followed by three deformation phases that are mainly characterized by thrusting and strike-slip faulting, which strongly modified the large-scale fold interference patterns.

Deformation and Metamorphism of a Schistose Terrane Accreted with the Proterozoic Pakhal Sequence of Godavari Valley near Kothagudem, Andhra Pradesh

Abstract From the recent research by several workers it is becoming evident that the geology , particularly the structure and metamorphism of the Proterozoic sedimentary basins are not simple , which can at times be compared with fold-thrust belts. In the southern part of the Pranhita-Godavari (PG) basin in its close proximity with the Eastern Ghat Mobile belt (EGMB) in the east and Eastern Dharwar craton (EDC) in the west there is a supracrustal association exposed near Kothagudem in Andhra Pradesh, which was earlier referred in published literature as Mulug and Mallampalle Groups of lower Pakhal sequence. From the field and petrological studies we have attempted to enumerate here the deformation and metamorphic characteristics and the tectonic setup of this schistose unit. It is revealed from our studies that this lithotectonic unit represents a strongly deformed and metamorphosed deeper crustal supracrustal assemblage, which is in sharp contrast with the unmetamorphosed and weakly deformed true Pakhal sequence occurring in the western part of the area. In order to explain the spatial association of these two contrasting litho-tectonostratigraphic units we propose a westerly verging thrust model, supported by convincing structural and metamorphic evidences, whereby the schistose terrane is tectonically juxtaposed with the unmetamorphosed Pakhal sequence. Imprints of polyphase deformation and metamorphism could be recognized to document the complex history of the schistose terrane. At least three generation structures (D1, D2, D3) are identifiable in quartzite-metapelite-carbonate assemblages. D1 and D2 are quite pronounced and conspicuous by the development of respective planar and linear elements, while D3 is relatively mild in intensity. Development of different generations of folds (F1, F2, F3) and associated axial planar cleavage/ schistosity (S1, S2, S3) together with different types of fold interference patterns, demonstrated mainly in carbonate rocks, bear testimony to successive events of fold formation as a result of superposed deformations. The tectonic trend defined by N-S to NNESSW (approx.) elongation of the belt results from the combined effects of all the three deformations. The structural pattern indicate a possible combination of predominantly pure shear and subordinate simple shear transpressional type of tectonism accompanied by considerable E-W (approx.) tectonic shortening across the belt. The rocks show low to medium grade metamorphism with the characteric appearances of chlorite, biotite, andalusite, chloritoid, staurolite and garnet porphyroblasts in pelitic rocks. From a study on ‘Si-Se’ tectonites in different mineral assemblages we have elucidated the history of deformation and metamorphic mineral growth in this assemblage. A variety of relationship exists between ‘Si’ and ‘Se’ demonstrating multi-stage growth of porphyroblasts, particularly of andalusite and staurolite, which took place during at least two deformational events. The important mineral assemblages include: (i) garnet-bearing andalusite- biotite schist, (ii) garnet free andalusite-biotite schist, (iii) staurolite-andalusite-garnet-mica schist, (iv) andalusite-chloritoid-muscovite schist, and (v) garnet-chloritoid-chlorite-muscovite schist. The study establishes a prograde metamorphic evolution in each rock type followed by a retrograde history. But the relative timing of prograde and retrograde reactions with respect to successive phases of deformation varies widely.

Multidetachment analogue models of fold reactivation in transpression: The NW Persian Gulf

Two deformation events have been documented in the NW Persian Gulf during the Late Cretaceous and the Late Cenozoic. The most distinctive feature in this part of the Persian Gulf is the reactivation of the Late Cretaceous NNE-SSW Arabian trending folds by NE-SW shortening during the Late Cenozoic Zagros orogeny. In general, if a set of folds with horizontal axes is shortened roughly parallel to its fold axis, a dome-basin fold interference pattern is produced. In the NW Persian Gulf, reactivation of some old folds occurred instead of a fold interference pattern. Reactivation may be influenced by the following factors: i) the presence of incompetent layers (i.e. evaporites), ii) a variable overburden, iii) basement faults, and iv) obliquity between the younger deformation shortening axis and fold axis. It is this last factor that we investigated by means of analogue modelling. The experimental apparatus is similar to that commonly used for experiments with brittle-ductile systems at the Laboratory of Experimental Tectonics of Geosciences Rennes. The model consisted of an alternation of ductile and brittle horizontal layers with a stratigraphy similar to the one found in the NW Persian Gulf. The model was deformed by two deformation events with an angle α between the two directions of shortening. After deformation, the resulting structure resembled a fold facing the static wall with internal thrust faults and detachment faults arranged into a geometry similar to a fish tail. In the second shortening event, the fold was reactivated without formation of an interference pattern. Moreover, the displacement on both the reactivated and newly formed faults varied between almost pure thrust faults for low α and oblique thrust faults with a strike-slip component for high α. The models suggest that the presence of incompetent layers plays an important role in fold reactivation and confirm that basement faults are not necessary.