Role Of Structural Inheritance Research Articles

Exploring spatial variations in geomorphic features of a young intracontinental orogenic belt: A case study of the High Atlas range

Morphotectonic parameters are sensitive to crustal and/or mantle dynamics, especially in young convergent margins and contractional orogenic systems. In northwest Africa, the High Atlas orogen is a natural example where tectonics interact with mantle dynamics to shape the orogen's topography.The High Atlas Mountains are an intracontinental segment of the peri-Mediterranean belt in northwest Africa, formed during the Late Cretaceous-Cenozoic period as part of the Alpine orogeny. An important aspect in the evolutionary history of the High Atlas belt is the presence of uplifted asthenosphere beneath the range, overlaying the Late Cretaceous-Cenozoic crustal shortening. This study aims to investigate the geomorphic and topographic features that reflect the influence of tectonics and mantle dynamics in the High Atlas orogen. A morphotectonic approach is employed, utilizing analysis of geomorphic indices such as relief, slope, channel steepness index, and stream length gradient. Additionally, eleven 30 km wide and 150 to 230 km long swath profiles are constructed to illustrate the variations in topography and geomorphology along the range.The geomorphic indices display a similar pattern of evolution across the High Atlas, with higher values of these indices associated with the primary faults that control the structure of the range. By combining the geomorphic indices and swath profiles, it is evident that the eastern part of the orogen exhibits geomorphic asymmetry, with the northern border experiencing greater neotectonic activity and surface uplift. The central part of the range displays more geomorphic symmetry, while the western part is asymmetrical, characterized by higher elevations and elevated values of geomorphic indices along the southern border. This variation in geomorphology is attributed to the oblique intersection of the High Atlas orogen and a region of thinned lithosphere, which facilitated increased neotectonic activity and surface uplift in the northern part of the Eastern High Atlas and the southern part of the Western High Atlas. In the westernmost portion of the range, the role of structural inheritance is critical in the neotectonic activity. At a local scale, the role of lithology in the drainage reorganization is evident especially in basement exposures such as Mougueur inlier, Skoura inlier and Ouzellarh salient.This study underscores the robustness of tectonic geomorphology as a tool to understand the complex interplay between crustal processes and mantle dynamics, together with surface processes, in young orogenic belts like the High Atlas.

High-resolution geophysical data unravel the post-Variscan structural history of the NW Cotentin inner shelf (Central English Channel)

The Cotentin Peninsula (CP) is one of the only area in Europe which contains records of a > 2.5 Ga-lasted geological history including three orogenic events (Archaean, Cadomian and Variscan) followed by a polyphase basin/inversion evolution during Meso-Cenozoic times. The CP area sensu lato is thus a suitable place for discussing how the structural configuration of the basement might have influenced the development of part of the southern shelf margin of the Central English Channel, even if sediments and post-Variscan tectonic records are limited at this place. This issue is addressed through an onshore/offshore structural approach combining newly-acquired high resolution bathymetric data and reflection seismic profiles, further constrained by lateral correlations onshore. The resulting Land-Sea Digital Elevation Model and corresponding geological map reveal a number of fault-bounded blocks involving a relatively thin package of Jurassic to Plio-Quaternary sequences, locally involved in slightly compressional deformations. These specific sedimentary and tectonic features typically characterize the southern shelf margin of the Central English Channel. They are discussed in terms of basin development and inversion processes in relation with basement structures and then integrated in the English Channel basin framework. Special attention is paid to three major structural features, i.e. the La Hague Offshore Fault, the La Hague Deep Fault network and the La Hague Deep, which emphasize, respectively, the role of structural inheritance and erosion/incision/deposition events during the post-Variscan tectono-sedimentary history of the southern elevated shoulder of the Central English Channel.

Role of inherited fault reactivation in accommodating multistage deformation in the southwestern margin of the Ulleung Basin, East Sea (Sea of Japan): Insights from 3D seismic interpretation

The southwestern margin of the Ulleung Basin, the East Sea (Sea of Japan), preserves complex structural and stratigraphic architectures produced by a multistage tectonic evolution in response to the Cenozoic convergence of the Indian and Pacific/Philippine Sea Plates to the Eurasian Plate. However, the origin, geometry, kinematics, and interrelationship of geological structures that evolved through the multistage tectonic deformation remain elusive, particularly regarding the role of structural inheritance in the evolution of the basin. Based on comprehensive 2D and 3D multichannel seismic reflection and well data, we present structural interpretations of regional-scale faults, folds, and growth strata in the southwestern margin of the Ulleung Basin, with a focus on inherited fault reactivation. In addition, a revisited deformation framework is proposed, involving back-arc extension/normal faulting (stage 1: ca. 23 to 15 Ma), positive inversion/reverse faulting (stage 2: 15 to 10.6 Ma), and strike-slip faulting/local transpression (stage 3: 10.6 Ma to present). Here, we illustrate how the reverse reactivation and propagation of segmented NE–SW normal faults and subsequent strike-slip movement have led to the complex structural and stratigraphic architectures presently evident in the study area. Notably, the final overprinting of a strike-slip deformation on the inherited NE–SW faults during deformation stage 3 has caused significantly damaged stepovers and tips, in which secondary faults and folds might control hydrocarbon migration/trapping and leakage. In this context, we suggest that the gentle antiformal Gorae Ⅰ structure is linked in origin to the Gorae Ⅴ structure, bearing a commercially viable gas trap as a detachment fold system formed by local transpression during deformation stage 3. Our comprehensive 3D structural seismic interpretation increases the understanding of Cenozoic multistage deformation, highlighting the impact of different tectonic drivers affecting the eastern marginal sea of the Eurasian Plate.

Structural and tectonic assessment of the western Huincul High, Neuquén Basin (Argentina)—The role of structural inheritance and mechanical stratigraphy in inversion systems

AbstractThe Neuquén Basin is a major Mesozoic sedimentary depocentre located in the retroarc foreland of the Argentinian Andes. The basin hosts world renowned inversion systems that have been the target of georesource exploration for the last three decades. The Huincul High is a structurally and economically prominent ca. 270 km long, E–W trending feature that formed by the accretion of exotic Palaeozoic terranes, influencing subsequent Mesozoic deformation in the basin. Exploration in the Huincul High has been mainly focused on the shallow part of the inversion structures leaving a limited understanding of the deep structural architecture and early tectonic evolution, particularly in the western reaches of the high. This research reveals that Late Triassic extensional faulting was followed by widespread thermal subsidence in the Early Jurassic, as shown by the occurrence of an extensive ca. 60 km long, ca. 20 km wide, NE–SW‐trending, central depocentre. In the Early Jurassic, as contraction ensued across this regional sag basin, atypical inversion geometries were developed. These exhibited prominent thickening in the hanging‐wall and, strikingly, in the footwall of reactivated faults. The style of inversion was also markedly influenced by the mechanically weak stratigraphy of the thick, Lower Jurassic, Los Molles formation that promoted broad inversion folding, inhibited shortcut fault creation, and decoupled post inversion deformation from earlier faulting. Quantitative fault analysis suggests that the reactivated faults originated during the Late Triassic extensional phase as separate ca. 10 km long fault segments. The analysis also indicates that segmentation of extensional faults, as well as their orientation to the later contractional vector (SH), spatially dictated style and magnitude of inversion. This research highlights the critical role played by structural inheritance and mechanical stratigraphy in the development of inversion in the Neuquén Basin, which might be of relevance for characterising inversion systems elsewhere. This research also proposes an evolutionary model for the western reaches of the Huincul High that suggests crustal weakening and thermal sag in the Early Jurassic. Moreover, the model highlights a previously unknown late Early Cretaceous transtensional phase that overprints the main Early Jurassic–Early Cretaceous inversion.

The role of structural inheritance in the tectonic configuration of the Moroccan Meseta Coastal Block: Insights from morpho-structural and aeromagnetic data

The Coastal Block (CB) of the Western Meseta (Morocco) is located between the Atlantic passive margin and the Middle and High Atlas Mountains. In this contribution, a morpho-structural analysis using topographic data and an aeromagnetic data interpretation have been integrated to map tectonically related structures and to unravel the surface-subsurface faults interaction and their geodynamic implications in the CB. The morpho-structural analysis allowed the structural lineaments mapping through the processing of the topographic and hydrographic data. The application of derivatives operators on the magnetic anomaly map reduced to the pole allowed the assessment of the tectonic framework. The visual integrated analysis of surface and subsurface structures allowed to establish a synthetic structural map of the CB. Several N–S to NNE-SSW, NW-SE and NE-SW oriented fault systems were identified, some of which were confirmed and others were newly outlined. Four geometrically horst and graben type blocks were well distinguished, including the Triassic Basins of Berrechid to the north and of Doukkala to the south separated by the Casablanca Paleozoic block. Meanwhile, the Sahel represents an uplifted zone separating the Doukkala Basin from the continental shelf to the west. Rooted faults bounding these blocks, which are heavily inherited from the pre-Mesozoic tectonic events, record variable reactivation indicators related to the Neogene-Quaternary Atlasic shortening.

The interplay between basement fabric, rifting, syn‐rift folding, and inversion in the Rio do Peixe Basin, NE Brazil

AbstractThe Rio do Peixe Basin is part of a series of aborted Cretaceous rifts formed within the Proterozoic Borborema Province, onshore NE Brazil in response to rifting between Africa and South America. The basin is remarkably well‐imaged and comprises fault‐bounded depocentres, the main ones being the NE‐oriented Brejo das Freiras and the E–W‐oriented Sousa half‐grabens. These grabens and their bounding faults are influenced by Neoproterozoic basement shear zones and present a complex framework of secondary normal faults and folds. Recent workers also interpret large reverse faults and regional post‐rift shortening driven by far‐field stresses from the Andes. For those reasons, the basin represents an ideal setting to investigate the multiphase history of rift basins. We thus combine borehole‐calibrated 2D and 3D seismic and magnetic data with section restoration and numerical modelling to investigate the architecture and evolution of this basin. We aim to understand: (i) the controls of the basement fabric in 3D fault architecture and kinematics and (ii) how syn‐rift faults controlled the geometry and development of fault‐related folds. By doing this, we also investigate the timing, kinematics, and magnitude of inversion in the basin to explore its multiphase history. We demonstrate that (i) the basement fabric controlled not only the strike of faults but also their geometry and polarity at depth, (ii) folds in the syn‐rift sequence are attributed simply to syn‐rift extension along stepped and/or curved faults, and (iii) inverted and/or reverse faults occur within the basin, but these are minor and appear to have formed during rifting. We explain this minor inversion by a change in plate kinematics related to the onset of the nearby transform margin to the north. These results have implications for understanding the 3D evolution of oblique grabens, the role of structural inheritance, and the recognition of inversion‐ versus extension‐related folds in rift basins worldwide.

New onshore insights into the role of structural inheritance during Mesozoic opening of the Inner Moray Firth Basin, Scotland

The Inner Moray Firth Basin (IMFB) forms the western arm of the North Sea trilete rift system that initiated mainly during the Late Jurassic–Early Cretaceous with the widespread development of major NE–SW-trending dip-slip growth faults. The IMFB is superimposed over the southern part of the older Devonian Orcadian Basin. The potential influence of older rift-related faults on the kinematics of later Mesozoic basin opening has received little attention, partly owing to the poor resolution of offshore seismic reflection data at depth. New field observations augmented by drone photography and photogrammetry, coupled with U–Pb geochronology, have been used to explore the kinematic history of faulting in onshore exposures along the southern IMFB margin. Dip-slip north–south- to NNE–SSW-striking Devonian growth faults are recognized that have undergone later dextral reactivation during NNW–SSE extension. The U–Pb calcite dating of a sample from the synkinematic calcite veins associated with this later episode shows that the age of fault reactivation is 130.99 ± 4.60 Ma (Hauterivian). The recognition of dextral-oblique Early Cretaceous reactivation of faults related to the underlying and older Orcadian Basin highlights the importance of structural inheritance in controlling basin- to sub-basin-scale architectures and how this influences the kinematics of IMFB rifting. Supplementary material: Analytical protocols and analysed sample details are available at https://doi.org/10.6084/m9.figshare.c.5635432

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

Role of structural inheritance and salt tectonics in the formation of pseudosymmetric continental rifts on the european margin of the hyperextended Mauléon basin (Early Cretaceous Arzacq and Tartas Basins)

Arzacq and Tartas were two Cretaceous basins developing in the Bay of Biscay as secondary depocentres in the larger Aquitaine Basin. They belong to an Albian rift system, where recent studies have stressed the role of simple-shear thinning on the genesis of regional asymmetric basins. However, the two basins differ from recent models in having a pseudo-symmetric morphology. Here, we address the Early Cretaceous tectono-sedimentary evolution of the Arzacq and Tartas rift basins and the lithospheric processes associated with their genesis. These basins did not suffer major brittle deformation during rifting stage, but nevertheless record a major change in their configuration between the Aptian and the Albian. The Arzacq Basin shifted from a symmetric to a slightly asymmetric configuration. This change was controlled by gravity sliding of the pre-rift sedimentary cover in the eastern Arzacq Basin moving its depocenter south, and by salt diapirism in the western Arzacq Basin moving its depocenter north. Several inherited Paleozoic structures were reactivated as transfer zones and appear to be responsible for this depocenter shift, the emplacement of salt diapirs, at same time regionally limiting gravity sliding. The Tartas Basin maintained a symmetric regime throughout the Early Cretaceous, except along the Barlanès transfer zone where the Lussagnet salt diapir originated. Two drivers of subsidence are thus envisaged in the Arzacq Basin: a Neocomian-Aptian crustal thinning stage, followed by reactivation of Paleozoic faults in Albian time accompanied by salt diapirism and gravity cover sliding. The Mauléon North Pyrenean Basin and its north adjacent Tartas and Arzacq basins thus formed part of a series of rift basins that progressed in maturity southward, from a simple sag basin (Tartas) to a slightly asymmetric basin (aborted detachment, Arzacq) to a hyperthinned basin (Mauléon). Our results offer new insights on the role of: (1) structural inheritance in partitioning extensional strain, (2) transfer zones as able to influence the sedimentary record of basins during crustal thinning, and (3) pre-rift salt décollements as able to separate the structural styles of sub- and supra-salt successions. • Pseudo-symmetric rift basins resulting from Early Cretaceous hyperextension. • Barremian-Aptian sag basins, decoupled deformation between upper and lower crust. • Albian-Cenomanian asymmetric rifting stage. • Synrift salt tectonic and basinward cover gliding along Jurassic salt décollement. • Role of inherited transverse transfer zones during rift formation.

Quantitative impact of structural inheritance on present-day deformation and seismicity concentration in intraplate deformation zones

Structural inheritance (i.e. paleo-tectonic) areas, acting as weakened domains, appear to be a key element localizing the seismicity in intraplate deformation zones. However, the impact of structural inheritance on the observed present-day seismicity and strain rate concentration remains to be quantified. In this study, we quantify through 2D numerical modeling the localization and amplification factor of upper crustal strain rates induced by structural inheritance. Our 2D models are constrained by intraplate velocity boundary conditions and include rheology laws that accounts for inherited strain weakening in both the brittle and ductile layers of the lithosphere. The role of structural inheritance is investigated for different localization of the weakened domain in the lithosphere. For an average intraplate geotherm (Moho temperature ca. 500°C), brittle weakening (i.e. inherited faults) alone induces a limited amplification factor of upper crustal strain rates of ca. 4. Ductile weakening can increase the amplification factor to ca. 7 when localized in the lower crust, but has no effect when localized in the lithospheric mantle. Overall, the amplification factors of upper crustal strain rates vary between 1 and 27 depending on the location of the weakened area in the lithosphere and on the different possible net driving forces, crustal strengths, amounts of weakening, and geotherms. These model amplification factors are in reasonable agreement with those derived from GPS and seismicity data over large spatial scale (several hundreds of kilometers) in North America.

Role of structural inheritance in the gravitational deformation of the Monviso meta-ophiolite Complex: the Pui-Orgiera serpentinite landslide (Varaita Valley, Western Alps)

ABSTRACT Depending on its chemical and mineralogical composition, the serpentinite represents a peculiar rocky material, which plays a significant role in influencing pedogenetic weathering, vegetation ecology, geo-mechanical and tectonic processes, as well as gravitational slope failure. In serpentinite-bearing environments the response to gravitational stress is more effective than in other rock types. In the Monviso meta-ophiolite Complex (Western Alps), a close relation between landsliding and serpentinite rock occurrences is particularly evident. Our geological map (Main Map), at a scale 1:10,000, illustrates the geology and geomorphological features of the Pui-Orgiera giant (2.98 km2) complex landslide, located on the southern slope of the Monviso Massif. This map clearly documents that the characteristics and kinematics of the landslide are closely associated to the anomalous thickening and widening increase of the Baracun Shear Zone, a remnants of an intra-oceanic detachment fault which separates serpentinite and meta-intrusives from metabasalt and metasediments of the Monviso meta-ophiolite Complex.

The role of structural inheritance in continental break-up and exhumation of Alpine Tethyan mantle (Canavese Zone, Western Alps)

The Canavese Zone (CZ) in the Western Alps represents the remnant of the distal passive margin of the Adria microplate, which was stretched and thinned during the Jurassic opening of the Alpine Tethys. Through detailed geological mapping, stratigraphic and structural analyses, we document that the continental break-up of Pangea and tectonic dismemberment of the Adria distal margin, up to mantle rocks exhumation and oceanization, did not simply result from the syn-rift Jurassic extension but was strongly favored by older structural inheritances (the Proto-Canavese Shear Zone), which controlled earlier lithospheric weakness. Our findings allowed to redefine in detail (i) the tectono-stratigraphic setting of the Variscan metamorphic basement and the Late Carboniferous to Early Cretaceous CZ succession, (ii) the role played by inherited Late Carboniferous to Early Triassic structures and (iii) the significance of the CZ in the geodynamic evolution of the Alpine Tethys. The large amount of extensional displacement and crustal thinning occurred during different pulses of Late Carboniferous–Early Triassic strike-slip tectonics is well-consistent with the role played by long-lived regional-scale wrench faults (e.g., the East-Variscan Shear Zone), suggesting a re-discussion of models of mantle exhumation driven by low-angle detachment faults as unique efficient mechanism in stretching and thinning continental crust.

Post-Caledonian extension in the West Norway–northern North Sea region: the role of structural inheritance

Abstract The northern North Sea region has experienced repeated phases of post-Caledonian extension, starting with extensional reactivation of the low-angle basal Caledonian thrust zone, then the formation of Devonian extensional shear zones with 10–100 km-scale displacements, followed by brittle reactivation and the creation of a plethora of extensional faults. The North Sea Rift-related approximately east–west extension created a new set of rift-parallel faults that cut across less favourably orientated pre-rift structures. Nevertheless, fault rock dating shows that onshore faults and shear zones of different orientations were active throughout the history of rifting. Several of the reactivated major Devonian extensional structures can be extrapolated offshore into the rift, where they appear as bands of dipping reflectors. They coincide with large-scale boundaries separating 50–100 km-wide rift domains of internally uniform fault patterns. Major north–south-trending rift faults, such as the Øygarden Fault System, bend or terminate against these boundaries, clearly influenced by their presence during rifting. Hence, the North Sea is one of several examples where pre-rift basement structures oblique to the rift extension direction can significantly influence rift architecture, even if most of the rift faults are newly-formed structures.

Interpretation and modelling of new Browse Basin airborne magnetic data for igneous rocks and basement

SUMMARY The Browse Basin on Australia’s North West Shelf is a NE-trending Paleozoic to Cenozoic depocentre that contains more than 15 km of sediments. These sediments host significant hydrocarbon reserves, some of which are currently under development. The basin also has the potential to store large volumes of carbon dioxide. Recently-acquired aeromagnetic data over the Browse Basin provide new impetus for studies of the nature of basement, the role of structural inheritance and controls on the distribution of volcanic rocks. Initial interpretation of the new magnetic data has utilised magnetic source polygons and depth estimates derived from the tilt-angle filter. Exploration wells that intersect mainly volcanic flows or tuffaceous rocks tend to lie on or adjacent to source polygons. Computed tilt depths show that these sources tend to coincide with the depth to the top of volcanics in wells and that tilt depths extend deep into the basin (up to ~10 km). The magnetic susceptibility distribution inferred from minimallyconstrained, regional-scale inversion models also indicates that magnetic anomalies arise from features deep in the basin and within basement. These results highlight the importance of understanding the role of volcanic rocks in basin evolution and their influence on reservoirs that may host hydrocarbons or that may be suitable for CO2 storage.

The role of structural inheritance in oblique rifting: Insights from analogue models and application to the Gulf of Aden

The geometry and kinematics of rifts are strongly controlled by pre-existing structures that may be present in both the crust and the mantle lithosphere. In the Gulf of Aden, the Tertiary oblique rift developed through inherited Mesozoic extensional basins that trend orthogonal to the direction of Oligo-Miocene divergence. Such inheritance may produce lateral variations in crustal thickness and thus in rheology of the crust and mantle lithosphere. How can such variations influence the present-day geometry of oblique rifts? May they locally overcome the impact of the oblique rheological weaknesses that in certain cases control the overall trend of the rift system? Moreover, we observe that major fracture zones systematically crosscut the inherited basins: may such inheritance influence the localization of major fracture zones by shifting the initial spreading centers? The brittle-ductile multilayer analogue models presented in this contribution reproduce oblique rifts. We tested the effect of an initial oblique weakness in the lithospheric mantle (i.e. structural or thermal inheritance) by introducing an oblique rheological weakness, which is alternatively initially imposed or dismissed. We tested the effect of the orthogonal Mesozoic inheritance by adding an elongated thicker brittle mantle, orthogonal to the direction of extension, in two models. These models mainly show en-echelon patterns with orthogonal faults and few rift-parallel faults (parallel to the rift obliquity), suggesting that the inherited orthogonal discontinuity is more influential than the oblique weakness in the lithospheric mantle. These results suggest that the presence in the lithosphere of an inherited basin could constitute a barrier to the deformation and sufficiently offset the spreading centers to lead to independent rift systems separated by major fracture faults. Moreover, the genetic nature of the obliquity (boundaries-driven or inherited) could control the length of the transform offset between two spreading centers and thus, the length of the future transform margin (continental domain affected by a transform fault).

Variscan Tectonics in the Malaguide Complex (Betic Cordillera, Southern Spain): Stratigraphic and Structural Alpine versus Pre‐Alpine Constraints from the Ardales Area (Province of Malaga). II. Structure

Unraveling structural features related to different orogenic cycles is fundamental for a better understanding of collisional orogens, particularly concerning the role of structural inheritance and extent of crustal reworking. In the Internal Domains of the Betic Cordillera, the geometry and kinematics of pre‐Alpine contractional deformation in the Paleozoic basements are rather poorly known. At Ardales, a strongly deformed area corresponding to part of the Alpine suture zone of the Betic Cordillera, the Malaguide Complex consists of two distinct, superposed Alpine tectonic units, including both Paleozoic basement and Mesozoic‐Tertiary cover rocks. Within the Alpine footwall unit, stratigraphic data document a tectonic superposition of two subunits made of different Paleozoic successions, which was later fossilized by Triassic redbeds. Structural analysis indicates that the contact between these subunits is an east‐directed overthrust and that the internal structure of the lower subunit is consistent with bulk noncoaxial strain dominated by top‐to‐the‐E shearing. These features show a marked contrast with the well‐known Alpine nappe transport direction to the N‐NW, with tectonic features developed along the Internal‐External Domain boundary and with the structural trends obtained from the analysis of Mesozoic‐Tertiary Malaguide rocks of the area. This study demonstrates that east‐directed thrusting and shearing within Paleozoic Malaguide rocks resulted from Variscan tectonics and that careful, detailed, and integrated stratigraphic and structural analysis is necessary to avoid confusion between older and newer orogenic cycles in the Internal Domain Units of the Betic Cordillera.

Variscan tectonics in the Holy Cross Mountains (Poland) and the role of structural inheritance during Alpine tectonics

The present study was carried out in the Holy Cross Mountains (HCM) of south-central Poland and includes computation of palaeostresses following Angelier's method and field structural analysis. The Palaeozoic basement of the HCM comprises two tectonic units separated by the major WNW–ESE-striking Holy Cross Fault (HCF). Fold analysis indicates a N–S to NNE–SSW direction of Variscan shortening. Micro-structures and fold analysis from Upper Devonian rocks further reveal: (1) a brittle tectonic event due to a NW–SE compression preceding folding that could be related to pre-Late Carboniferous tectonics, due to block transport within the Tornquist–Teisseyre Zone (TTZ), and (2) polyphase Variscan folding comprising (a) an early stage of N–S shortening marked by north-verging ramps, (b) a main folding event and axial cleavage formation involving N–S to NNE–SSW shortening, and (c) a late stage of shortening deforming older folds and cleavage. A mainly extensional tectonic regime dominated from the Permian until the Cretaceous, during which time the HCF was reactivated as a normal fault. Large NW–SE faults bordering the Mid-Polish Trough (MPT) developed. Subsequent tectonic inversion of the MPT resulted in basin uplift (`Mid-Polish Swell', MPS). Palaeostress computations from Mesozoic strata suggest a NE–SW direction for the main Maastrichtian–Paleocene shortening phase, in addition to two minor brittle events resulting from N–S and E–W compression. Analysis of local folds in the Mesozoic cover indicates a causal relationship with the Maastrichtian–Paleocene reactivation of older faults. In particular, en-échelon folds in the Radomsko Elevation suggest a sinistral reactivation of the Palaeozoic HCF. Folds in the southwestern part of the HCM argue for reactivation in the reverse mode of a NW–SE-trending fault bordering the MPS that originated in the Mesozoic. In Palaeozoic strata, post-Variscan brittle deformation and micro-fault reactivation are attributed to the tectonic events of Maastrichtian–Paleocene age. Reactivation of N–S fractures is demonstrated in the Rzepka Quarry, where fractures in Devonian dolomites are filled by calcite, probably formed during a late Variscan event, and by younger Late Permian–Early Triassic sediments. They were subsequently reactivated as dextral strike-slip faults during the NE–SW Maastrichtian–Paleocene compression.