Nappe Research Articles

Contrasting along-strike deformation styles in the central external Dinarides assessed by balanced cross-sections: Implications for the tectonic evolution of its Paleogene flexural foreland basin system

The Cenozoic foreland fold and thrust belt of the external Dinarides along the eastern shore of the Adriatic Sea shows an overall NW-SE structural strike that changes to E-W in its central part, between Zadar and Mostar. This change is associated with a dextrally transpressive zone, named Split-Karlovac Fault. Segments of this fault straddle the Eo-Oligocene flexural foreland basin system of the external Dinarides. Two balanced cross-sections across the northern and southern segments of this transverse fault zone document highly contrasting styles of deformation and amount of shortening. The northern segment across Velebit Mountain is characterized by a triangle structure with a minimum shortening of 44 km. This triangle structure is formed by a set of thick-skinned top-SW thrust duplexes, topped by a thin-skinned passive top-NE roof backthrust. By contrast, the southern segment, traversing north of Split, shows a thin-skinned top-SW to top-S directed nappe stack, associated with a minimum shortening of 127 km and rooting into a basal detachment in the Paleozoic basement. To better understand parameters controlling these contrasting styles of deformation, we modelled the flexural response of the foreland lithosphere that provided the accommodation space for the Eocene to Oligocene syn-orogenic Promina Beds, a prevailingly carbonate-clastic and conglomerate-bearing sequence making a part of the foreland basin fill. Tectonic load exerted by the hinterland nappe stack was estimated using the two balanced cross-sections. Estimates of tectonic load allowed modelling the amount of flexural response that yielded depth and shape of the basin containing the Promina Beds. The best fitting modelling results provided additional confidence in the validity of the balanced cross-sections. It is shown that shortening in the two structurally contrasting segments of the thick-skinned thrust belt were contemporaneous with dextral strike-slip along the thick-skinned Split-Karlovac Fault and coeval with sedimentation of the Promina Beds. We propose that the Velebit triangle structure acted as a back-stop that prevented the propagation of the deformation front towards the SW. Instead shortening was transferred along the Split-Karlovac Fault into the southern transect, where deformation propagated towards the foreland along a successively shallowing stepped detachment. The contrasting style of Eo-Oligocene deformation probably resulted from the interplay of Permo-Triassic evaporites with the reactivation of inherited Middle Triassic normal faults to both sides of the transpressive zone. • Nappe stack vs. triangle structure in the external Dinarides fold and thrust belt. • Substantial along-strike difference of crustal shortening. • Effects of mechanically weak lithologies on the style of deformation. • Lithosphere deflection modelling in combination with balanced cross-sections. • Impacts of a transverse zone on the flexural foreland basin system geometry.

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

Moine metasedimentary rocks of northern Scotland are characterized by arcuate map patterns of mineral lineations that swing progressively clockwise from orogen-perpendicular E-trending lineations in greenschist facies mylonites above the Moine thrust on the foreland edge of the Caledonian Orogen, to S-trending lineations at higher structural levels and metamorphic grades in the hinterland. Quartz c-axis fabrics measured on a west to east coast transect demonstrate that the lineations developed parallel to the maximum principal extension direction and therefore track the local tectonic transport direction. Microstructures and c-axis fabrics document a progressive change from top to the N shearing in the hinterland to top to the W shearing on the foreland edge. Field relationships indicate that the domain of top to the N shearing was at least 55 km wide before later horizontal shortening on km-scale W-vergent folds that detach on the underlying Moine thrust. Previously published data from the Moine thrust mylonites demonstrate that top to the W shearing had largely ceased by 430 Ma, while preliminary isotopic age data suggest top to the N shearing occurred at ~470–450 Ma. In addition, data from the east coast end of our transect indicate normal-sense top down-SE shearing at close to peak temperatures at ~420 Ma that may be related to the closing stages of Scandian deformation, metamorphism and cooling/exhumation.

Zircons from Collisional Granites, Garhwal Himalaya, NW India: U–Th–Pb Age, Geochemistry and Protolith Constraints

In the present work, we studied zircons from the less foliated granites of the Chail Group, which form a thrust sheet of the Lesser Himalayan Sequences, Garhwal region. Compositionally, these granites are S–type, formed in a collisional tectonic setting. Zircons possess an internal structure, mineral inclusions, and geochemical characteristics typical of magmatic origin. The U–Th–Pb geochronology and geochemistry were assessed using the laser ablation multi–collector inductively coupled plasma spectrometry (LA–ICP–MS) technique. U–Th–Pb isotope dating of zircons from two different samples revealed their age, estimated from the upper intersection of the discordia, to be 1845 ± 19 Ma. Zircons from one sample contained inherited cores belonging to three age groups: Paleoarchean (3.52 Ga), Neoarchean (2.78 Ga and 2.62 Ga), and Paleoproterozoic (2.1 Ga). Zircons with ages of 3.52, 2.62, and 2.1 Ga were interpreted as magmatic based on their geochemical characteristics. The 2.78 Ga core was interpreted as metamorphic. The observed inheritance is consistent with the melting of sedimentary rocks. The inherited zircons could have originated from Aravalli and Bundelkhand Craton and Paleoproterozoic Aravalli Fold Belt rocks. This confirms that the studied granites are S–type and could have been formed in a collisional environment at 1.85 Ga on the western flank of the Columbia Supercontinent.

Centrifuge modelling of thrust systems in the brittle crust: Role of frictional décollement geometry

Centrifuge analogue modelling has provided significant insights into the evolution and architecture of fold and thrust systems. However, all previous works focused on the deformation of viscous and/or plastic layers, and did not analyze the development of structures in the brittle crust. In this work, we present the results of analogue centrifuge models reproducing compression of purely brittle upper crustal layers. We run enhanced gravity models investigating the role exerted by frictional décollements on the evolution and architecture of thrusting and compared the results with new normal-gravity models with similar set-up. In line with the results of previous experimental works, our models show that the presence of a low-friction basal décollement significantly influences the evolution and pattern of thrust systems. By reducing the basal friction of the experimental wedge or the sector of the wedge where the low-friction décollement is located, this layer promotes experimental wedges with low tapers and low heights in the inner portion. Moreover, when a low-friction basal décollement occurs, the tectonic wedge is characterized by an increase in wavelength of thrust sheets toward the foreland, compared to the purely brittle models. Results show a good comparison between centrifuge and normal gravity models, indicating that the centrifuge technique can be successfully used to model shortening in the brittle crust and therefore to analyze the evolution and architecture of thrust systems.

Apatite and biotite thermochronometers help explain an Arctic Caledonide inverted metamorphic gradient

Biotite generates significant radiogenic Sr and is thus suitable for Rb–Sr dating, which can be achieved directly in thin section via laser ablation and provides a means to directly date growth of this important fabric-forming mineral or its cooling. In contrast, apatite has an extremely low Rb/Sr ratio and hence offers a useful tool to estimate source 87Sr/86Sri. Plagioclase also contains significant Sr and may further assist in understanding the rocks Sr isotopic system. However, the fluid source of growing or recrystallizing apatite, plagioclase, and biotite need not be the same. Compounding the complexity, apatite may gain radiogenic Sr from biotite during recrystallization or dissolution–reprecipitation that accompanies metamorphism. In this contribution, we present a case study from the Kalak Nappe Complex in the Scandinavian Caledonides, Arctic Norway, where a combined geochronology program, critically supported by biotite Rb–Sr dating, helps to resolve the conspicuous inverted metamorphic field gradient found upwards through the nappe stack in this orogen. Apatite U–Pb ages of 417–413 Ma in the upper nappes are interpreted as cooling through the Pb retention zone. Such ages contrast with the suggestion of older, partially reset apatite ages in the lower nappes. Biotite Rb–Sr ages of c. 413 Ma in the upper nappes are similar to U–Pb ages of coexisting apatite but conspicuously different from c. 433 Ma biotite Rb–Sr ages in the lower nappes. Ages of c. 433 Ma are comparable to muscovite 40Ar/39Ar and titanite and monazite U–Pb ages throughout the complex and may directly date mineral growth at peak thermal conditions during the Scandian orogenic event. Apatite 87Sr/86Sr ratios also track the influence of metamorphism and imply more radiogenic signatures relative to relic plagioclase in samples which have greater overprinting in the upper nappes of the complex. Phase equilibrium modelling establishes Palaeozoic peak P–T conditions of 610–750 °C at <12 kbar in the upper nappes, comparable to peak conditions previously established for the lower nappes. Combined thermal modelling of the post-peak Scandian orogenic history based on published 40Ar/39Ar, paired with new Rb–Sr, and U–Pb data, implies a slower cooling pathway in the upper nappes relative to the lower nappes. We reconcile this thermal history with a geodynamic model of obduction of a hot back-arc, as represented by the Silurian Magerøy Nappe, onto the upper nappes of the Kalak Nappe Complex at c. 430 Ma. These results demonstrate that in-situ laser ablation Rb–Sr dating of biotite provides a powerful tool for resolving orogenic tectonothermal histories in orogens flushed with excess argon.

Geology of the Northern Apennines nappe stack on eastern Elba (Italy): new insights on the Neogene orogenic evolution of the Northern Tyrrhenian Sea

ABSTRACT We document the tectonic and metamorphic evolution of thrust nappes of the eastern island of Elba. The area exposes a natural cross section of the Northern Apennines hinterland, from the metamorphic basement units to the overlying continent- and ocean-derived nappes. We integrated mapping, analysis of structures and microstructures, and the interpretation of drill core logs with lithostratigraphic, metamorphic, and geochronological constraints, producing a novel geological map of eastern Elba (1:5’000 scale). We show that the area experienced polyphase Oligocene – Pliocene contractional tectonics marked by in-sequence and out-of-sequence thrusting accompanied by folding and overprinted by faulting in the Pliocene. Magmatism occurred during contraction with post-magmatic thrusting ultimately coupling HP-LT and LP-HT units. Drill core logs allow for the first time the reconstruction of the N-dipping character of the Zuccale Fault, which represents the youngest (late Miocene – early Pliocene) large-scale structure in the area.

Multiple fluid flow events from salt‐related rifting to basin inversion (Upper Pedraforca thrust sheet, SE Pyrenees)

AbstractFluid systems in inverted rifted margins are challenging to interpret because fractures formed before compression were often reactivated acting as fluid pathways as new ones formed. Deciphering the fracture and fluid flow history in such complex settings has key implications for the prediction of the distribution of mineral resources. As an example, we reconstruct the fluid flow evolution of a portion of the inverted Pyrenean rift, the Upper Pedraforca thrust sheet, from the Mesozoic extension to the Alpine orogeny. We combine structural analysis and petrographic, geochemical and geochronological data obtained from 87 samples of fracture‐filling carbonate cements. During the Late Jurassic‐Early Cretaceous, low‐temperature seawater produced dolomitization of Jurassic and Lower Cretaceous limestones in an extensional setting. During the Early Cretaceous salt‐related extension, formation waters, probably evolved seawater or fluids that interacted with Triassic evaporites, at temperatures from 125 to 149°C migrated through fractures. The formation of breccias within post‐salt rocks in primary weld zones facilitated the upward migration of formation waters that interacted with pre‐salt rocks with high 87Sr/86Sr ratios. Formation waters at temperatures of 80°C migrated during the emplacement of the Upper Pedraforca thrust sheet in the Late Cretaceous‐Palaeocene. These fluids interacted with Upper Cretaceous Carbonates and/or Triassic evaporites. In contrast, the influence of meteoric fluids increased in shallower positions due to the exhumation of the SE Pyrenees during the Eocene‐Oligocene. Coevally, hot dolomitizing fluids migrated along diapir walls during the formation of secondary welds. Supergene ores documented in diapiric areas worldwide related to meteoric fluids, and the similar meteoric percolation occurred in the Upper Pedraforca thrust sheet, suggest that supergene mineralization could be found in the Pyrenees. Brecciation of rocks in primary weld zones and evaporite detachments, where mineralization accumulate in fractures from basement‐derived fluids, also suggests that similar ores could be found in the Pyrenees.

The interplay of structure and metamorphism at Broken Hill, NSW, Australia

Downward younging of stratigraphy in several large F2 fold structures in the Paleoproterozoic Willyama Supergroup in the Broken Hill area has previously been explained by very large-scale F1 nappe folds followed by relatively upright F2 and F3 folds. In three major structures, the Mt Robe–East Eldee Structure, the Paps Synform and the Bijerkerno Synform, the andalusite-sillimanite metamorphic isograd parallels stratigraphy for tens of kilometres. Given the conformability of these segments of isograd in plan view, it is reasonable to suggest that in two of the structures, sillimanite-grade rocks rest on andalusite-grade rocks. In all cases the younger stratigraphic units are at lower metamorphic grade than the older stratigraphic units, whether the stratigraphy is upright, as in the Bijerkerno Synform, or inverted, as in the Mt Robe-East Eldee Structure and the Paps Synform. Major bodies (mini-batholiths) of pegmatite are confined stratigraphically to below the Sundown Group, whether the stratigraphy is upright or overturned. In the D1 nappe model the andalusite-sillimanite isograd and the emplacement of pegmatite mini-batholiths must both predate the D1 nappe folding. There is evidence that andalusite preceded D1, but not sillimanite, which is a defining mineral in the first-generation schistosity (S1). The large-scale nappe model is also not compatible with calculated metamorphic pressures. An alternative model involving extensional D1, followed by D2 sheath folds, explains downward-facing metamorphic facies but is not viable. Large-scale fold structures in the Mt Robe Block can be explained by recumbent folding at a moderate scale, followed by inclined sheath folding. The downward-facing stratigraphy and metamorphic facies in the Mt Robe and Paps synforms are interpreted to lie on the underside of inclined F2 sheath-like folds. KEY POINTS In major fold structures in the Willyama Supergroup, upper stratigraphic units are at a lower metamorphic grade than lower stratigraphic units, whether the fold is upward-facing or downward-facing. In two large F2 folds sillimanite-grade rocks showing melting are situated above andalusite-grade rocks. Pegmatitic mini-batholiths show semi-regional stratigraphic control below the top of the Broken Hill Group, whether they are in upward or downward-facing fold limbs. A new structural model with D1 recumbent folding, followed by D2 sheath-like folding, is based on structures in the Mt Robe Block.

Thermotectonic constraints on the geodynamic evolution of the Pindos fold‐and‐thrust‐belt by low‐temperature (U–Th)/He thermochronometry

This study presents new thermochronometric data to constrain the cooling history of the Pindos fold‐and‐thrust‐belt (PFTB) on the NW Peloponnese, and the timing of nappe stacking in the external Hellenides in western Greece. Low‐temperature thermochronology was performed on apatite and zircon minerals extracted from basal exposures of the PFBT in Mt. Erymanthos (NW Peloponnese) using the (U‐Th)/He system. Three bedrock samples collected from a profile across the strike of the PFBT returned thermochronometric ages for the apatite (AHe) between 15 and 9 Ma, while for the zircon (ZHe) the ages are interpreted as unreset (or partially reset). The AHe ages are interpreted as cooling ages related to the Cenozoic geodynamic evolution of the PFTB. Thermal models generated in this research demonstrate two phases of geodynamic evolution of the PFBT during the Cenozoic: an early one associated with the heating of the fold‐and‐thrust belt during 40–20 Ma and a later one associated with fast cooling from 20 Ma onwards. Heating is attributed to the structural thickening of the PFTB while cooling is associated with enhanced uplift and exhumation, which became widespread in Erymanthos by 9 Ma. The initiation of cooling on Mt. Erymanthos echoes the substantial geometric remodelling that underwent the Hellenic arc during the Middle Miocene, associated with slab rollback and arc extension, and is connected to the underplating of the internal segments of the external platform (Gavrovo‐Tripolis unit) by the external metamorphic segments (Mani and Arna units). The current inverse thermal modelling offers the first continuous description of the geodynamic evolution of the most prominent fold‐and‐thrust belt of the Hellenides, from the oceanic stage through basin inversion to uplift and orogen decay.

Imaging Neoarchean crustal structures: An integrated geologic-seismic-magnetotelluric study in the western Wabigoon and Winnipeg River terranes, Superior craton

The Neoarchean is considered as a pivotal era during the Earth’s evolution in terms of cratonization and development of plate tectonics. The supporting crustal processes, however, remain elusive due in part to limited comprehensive imaging of crustal structures underlying Neoarchean terranes. This study uses new seismic, magnetotelluric, and geologic data from the Metal Earth Sturgeon transect (70 km) and existing seismic and aeromagnetic data to investigate the Neoarchean crustal architecture underlying the greenstone-dominated western Wabigoon terrane (WWT) and the tonalite-trondhjemite-granodiorite-dominated Winnipeg River terrane (WRT) of the Superior craton. The results suggest that (1) the Sturgeon Lake greenstone belt of the WWT is 5–10 km thick and separated by a thrust fault from the underlying basement, (2) the basement is characterized by gneissic fabrics from the mid- to lower-crust of probable Paleo to Mesoarchean WRT rocks, and that (3) the crust was extensively reworked by Neoarchean post-tectonic magmatism that was characterized by carbon- and/or hydrogen-rich silicates and likely caused by partial melting of mantle/crustal rocks triggered by subduction-related fluids/melts. With the reprocessed Lithoprobe seismic data from the same region, a 3D crustal architecture is reconstructed and reveals a convergent belt between the main WRT to the north and a continental margin promontory of the WRT to the south. This Neoarchean convergent belt is characterized by an allochthonous greenstone belt as a thrust sheet in the upper crust, a collision zone in the mid- to lower-crust, an apparent crustal root of 3–5 km relief, and subcreted crustal rocks beneath a mantle wedge. Published data suggest that an outboard, north-dipping subduction zone associated with the Wawa terrane provided the fluids/melts for the post-tectonic magmatism in the WRT-WWT crust. This study favors a model of subduction followed by collision over a collision-only model, which indicates that multiple subduction zones were operative synchronously in a periodduring the assembly of the western Superior craton. Cratonization by accretionary orogenesis due to plate tectonics in the Neoarchean, therefore, is imaged and characterized in this study using a 3D crustal architecture.

Geological and seismic evidence for the tectonic evolution of the NE Oman continental margin and Gulf of Oman

AbstractLate Cretaceous obduction of the Semail ophiolite and underlying thrust sheets of Neo-Tethyan oceanic sediments onto the submerged continental margin of Oman involved thin-skinned SW-vergent thrusting above a thick Guadalupian–Cenomanian shelf-carbonate sequence. A flexural foreland basin (Muti and Aruma Basin) developed due to the thrust loading. Newly available seismic reflection data, tied to wells in the Gulf of Oman, suggest indirectly that the trailing edge of the Semail Ophiolite is not rooted in the Gulf of Oman crust but is truncated by an ENE-dipping extensional fault parallel to the coastline. This fault is inferred to separate the Semail ophiolite to the SW from in situ oceanic Gulf of Oman crust to the NE. It forms the basin margin to a “hinterland” basin formed atop the Gulf of Oman crust, in which 5 km of Late Cretaceous deep-water mudstones accumulated together with 4 km of Miocene and younger deep-water mudstones and sandstones. Syndepositional folding included Paleocene–Eocene folds on N-S axes, and Paleocene to Oligocene growth faults with roll-over anticlines, along the basin flank. Pliocene compression formed, or tightened, box folds whose axes parallel the modern coast with local south-vergent thrusts and reversal of the growth faults. This Pliocene compression resulted in large-scale buckling of the Cenozoic section, truncated above by an intra-Pliocene unconformity. A spectacular 60-km-long, Eocene(?) to Recent, low-angle, extensional, gravitational fault, down-throws the upper basin fill to the north. The inferred basement of the hinterland basin is in situ Late Cretaceous oceanic lithosphere that is subducting northwards beneath the Makran accretionary prism.

Blueschists and Blue Amphibole Schists-Gneisses Associated with the Nan River Mafic – Ultramafic Belt, North Thailand

The discovery of an areally limited outcrop of blueschists in Huai Sak, near Ban Huai Lao, SE Nan Province in the early 1980s has not been followed up until now. This is despite these blueschists being used in many regional tectonic compilations as unequivocal evidence for the presence of a former subduction zone along the trend of the Nan river. We report additional structural information and mineral analyses for the Huai Sak blueschists and the discovery nearby, in Huai Phi Rong, of structurally similar schists containing the Na-Ca amphibole barroisite. In addition, we report the discovery in Huai Phi Rong of float samples, believed to be derived from a thrust sheet of serpentinite matrix melange, consisting of coarse schists and gneisses that contain a variety of blue amphiboles associated with garnet. EPMA analysis confirms these blue amphiboles to be variously riebeckite, glaucophane, winchite and barroisite.

Geology of the Magura Nappe, south-western Gorce Mountains (Outer Carpathians, Poland)

ABSTRACT The studied area, located in the south-western part of the Gorce Mountains, is built of flysch deposits representing the Krynica and Bystrica Subunits of the Magura Nappe. Here, in the Late Cretaceous–Early Miocene part of the lithostratigraphic succession of the Krynica Subunit, four formations containing several members were distinguished, whereas in the Bystrica Subunit, only one Middle Eocene–Oligocene formation was identified. The Turbacz and Kudłoń Thrust Sheets in the tectonic structure of the Krynica Subunit and the southernmost fragment of the Tobołów Thrust Sheet in the Bystrica Subunit are prominent. Several folds occur in the studied area. Numerous transverse and oblique faults are also present. The result of the present geological investigations in the study area was the creation of a geological map of the Magura Nappe deposits in the Gorce Mountains. In general, a high-resolution digital elevation model contributed significantly to the progress of this geological research.

Construction of the Lesser Himalayan–Subhimalayan thrust belt: The primary driver of thickening, exhumation, and high elevations in the Himalayan orogen since the middle Miocene

Abstract Documenting the structural evolution of the Himalayan orogen is fundamental for understanding the dynamics of collisional orogenesis. We argue that the importance of deformation in the frontal, Lesser Himalayan–Subhimalayan (LH-SH) portion of the Himalayan thrust belt for driving crustal thickening over the past ~15–13 m.y. has long been overlooked. To quantify its contribution to thickening, we measured parameters from 22 published cross sections that span the length of the orogen. The mean structural uplift accomplished by the LH-SH thrust belt increases from 10–15 km in the eastern half of the orogen to 15–23 km in the western half. An antiformal culmination constructed by LH duplexing is observed across the orogen and increases in structural height (to as much as 15–20 km) and north-south width moving westward. Construction of the culmination was the primary mechanism for building and maintaining wedge taper. The westward scaling of culmination size is accompanied by doubling and tripling of LH-SH shortening and accretion magnitude, respectively; when combined with a consistent orogen-wide modern taper angle (11° ± 2°), this indicates that duplexing facilitated the growth of an overall larger orogenic wedge moving westward. Following the initial southward propagation of deformation into LH rocks at ca. 15–13 Ma, the Himalayan orogenic wedge has been characterized by stacking of multiple thin, small-displacement thrust sheets to develop a high-taper orogenic wedge. Thus, LH-SH deformation has had a profound effect on driving thickening, exhumation, and the attainment of high elevations since the middle Miocene.

Buckle folding in the Northern Calcareous Alps - Field observations and numeric experiments

We report the results of a numerical modelling study based on folds in the western Northern Calcareous Alps (NCA) fold-and-thrust belt. We study boundary conditions of folding based on the mechanical properties of the rocks involved. One key control on the model is stratigraphy that can be simplified to three layers: (1) an incompetent base along the décollement of the thrust sheet, and (2) an up to 3 km thick competent layer, which is overlain by (3) an incompetent top layer. The incompetent base layer needs to be very weak to facilitate folding and is a salt-bearing evaporitic unit. Another key is erosion prior to folding. It needs to remove half of the competent layer to allow the creation of folds with limb lengths comparable to field observations. The results of the numeric model contribute to the understanding of the structural development in the western NCA. Folding within the upper thrust sheet was only possible above a salt-bearing décollement and after a decrease in lithostatic pressure related to Upper Cretaceous and/or Paleogene erosion.

Late Palaeozoic extensional volcanism along the northern margin of Gondwana in southern Turkey: implications for Palaeotethyan development

The Late Palaeozoic–Early Mesozoic Tethyan development of the Eastern Mediterranean region remains debatable, especially in Turkey, where alternative northward and southward subduction hypotheses are proposed. Relevant to this debate, new whole-rock geochemical data are provided here for early Carboniferous (Late Tournaisian-Late Visean; c. 340–350 Ma) tuffaceous sedimentary rocks within the Çataloturan thrust sheet (Aladağ nappe), eastern Taurides. The tuffs accumulated from evolved alkaline volcanism, variably mixed with terrigenous and radiolarian-rich sediments. In addition, Late Palaeozoic meta-volcanic rocks, c. 150 km farther NE, within the Binboğa (= Malatya) metamorphics (a low-grade high-pressure unit), are indicative of a within-plate setting. An impersistent geochemical subduction signature in these volcanics may represent an inherited, rather than contemporaneous, subduction influence, mainly because of the absence of a continental margin arc or of arc-derived tuff. Both the Binboğa metamorphics and the Çataloturan thrust sheet (Aladağ nappe) restore generally to the north of the relatively autochthonous Tauride carbonate platform (Geyik Dağ), within the carbonate platform bordering north-Gondwana. The Çataloturan thrust sheet is interpreted, specifically, as a c. E–W, deep-water, volcanically active rift that progressively infilled. Regional geological evidence suggests that melange units (Konya Complex, Afyon zone), Teke Dere unit, Lycian nappes), and Chios–Karaburun melange, E Aegean) accreted to the north-Gondwana continental margin during the late Carboniferous; this was coupled with localised calc-alkaline granitic magmatism (Afyon zone of Anatolide crustal block). We propose an interpretation in which Late Devonian–Carboniferous alkaline intra-plate volcanism relates to extension/rifting along the north-Gondwana margin. In contrast, the melange accretion and granitic magmatism could relate to short-lived late Carboniferous southward subduction that accompanied the diachronous closure of Palaeotethys.

Timing of Alpine Orogeny and Postorogenic Extension in the Alboran Domain, Inner Rif Chain, Morocco

AbstractThe variety of temporal and structural constraints on the Alpine tectonometamorphic signature of the metamorphic core of the Betic‐Rif orogen (Alboran Domain) has supported a long‐lasting debate regarding the Alpine tectonic and geodynamic evolution of the Western Mediterranean region. Uncertainty still exists on the timing and tectonic significance of (a) the Alpine orogenic construction; (b) exhumation of the deep roots of the orogen; and (c) transition from orogenic shortening to crustal extension. In this study, we address these major geological issues by focusing on the lower‐grade units of the Alboran Domain (Upper Sebtides and Ghomarides) exposed in the Rif belt of northern Morocco. Through a multidisciplinary approach that integrates mesostructural and microstructural investigations with X‐ray diffraction, quantitative mineral chemistry, and 40Ar/39Ar geochronology, a 20 Ma long tectonic history is reconstructed, which involves burial of the tectonic units at depth (late Eocene) and postorogenic exhumation under brittle conditions in the upper crust (early Miocene). We document a Priabonian (∼37‐34 Ma) D1/M1 progressive compressional deformation, during the formation of a SW‐verging orogenic wedge (present coordinates), accreted toward the Africa plate. Brittle extensional detachment tectonics operated during the Burdigalian (∼18‐17 Ma), controlling the thinning of the previously structured Alboran Domain nappe stack and the final exhumation of the Alpine orogenic units. We propose that transition from orogenic build‐up to collapse in the hinterland of the Betic‐Rif orogen occurred when the retreat of the Apennine‐Maghrebian subduction was efficient to drive transition from shortening to extension in the back‐arc domain of the western termination of the Apennine‐Maghrebian subduction zone.

Korotaikha Composite Tectono-Sedimentary Element, Northwestern Russia

Abstract The Korotaikha Composite Tectono-Sedimentary Element constitutes the northeastern part of the Timan–Pechora petroleum province. It has evolved through rifting, continental margin development, and a two-stage collisional phase related to the Uralian Orogeny in the Late Artinskian–Late Permian and the Pay-Khoy Orogeny in the Late Triassic. In the course of the later phase, the sedimentary cover was delaminated along the Upper Ordovician evaporites, which produced general uplift. The complex tectonostratigraphic history resulted in the deposition of an up to 15 km-thick sedimentary section that has all the essential ingredients for a prolific petroleum system, including the world-class Domanik source rock and a wide variety of structural and stratigraphic traps. Nevertheless, up to now, no commercial discoveries have yet been made. A review of the geological setting and petroleum habitat suggests that the petroleum systems development was more complex than was assumed previously. The amount of uplift and erosion related to the updip displacement on the salt detachment was underestimated. Reinterpretation of the available data shows the presence of large untested exploration opportunities including duplex thrust sheets and subsalt structures related to squeezed diapirs.

Variation in structural styles within fold-and-thrust belts: Insights from field mapping, cross-sections balancing, and 2D-kinematic modelling in the Jura mountains (Eastern France)

Fold-and-thrust belts developed above an evaporite décollement host structural traps where geofluids, including hydrocarbons, can accumulate. A valid and accurate structural style intepretation is therefore crucial to understand the three-dimensional geometries in the subsurface, in order to better model geofluid reservoirs and to increase success during exploration. Here, we present a multidisciplinary approach to unravel the subsurface structural styles of the Jura fold-and-thrust belt (eastern France), combining geological mapping, from regional to outcrop scale, with cross-section balancing and 2D-kinematic forward modelling. Results show that the Jura fold-and-thrust belt is characterized by mixed structural styles. Evaporite-cored folds are similar to those commonly observed in fold-and-thrust belt characterized by a thick salt-bearing basal detachment (e.g. Zagros fold-and-thrust belt), while fault-bend fold geometries and the emplacement of thrust sheets are similar to those occurring in fold-and-thrust belts characterized by multiple and thin detachment horizons within the sedimentary sequence (i.e. Canadian Rocky Mountains). In the Jura, the flow of evaporites in most cases is inhibited, as anhydrites and salt form thin layers and are mixed with mechanically stronger rocks, such as marls and dolostones. Results from this study can be used as a benchmark to compare structural styles within other fold-and-thrust belts characterized by lithological variation within the evaporite-bearing basal detachment at the regional scale and by multiple detachment layers within the sedimentary sequence.

Development of a Mississippian–Lower Pennsylvanian isolated carbonate platform within the basinal griotte facies of the Cantabrian Mountains, NW Spain

The Valdediezma platform consists of upper Tournaisian to lower Bashkirian (Carboniferous) shallow-water carbonates deposited in the core of the Picos de Europa Province (Cantabrian Mountains, northwest Spain). Although faulted in several thrust sheets, it is the only preserved platform developed in the Mississippian starved basins of the southern branch of the Variscan Orogen that is characterised also by pelagic sedimentation. This unusual platform provides an exceptional opportunity to study the lateral variation from the platform to the typical condensed griotte limestones developed in a starved basin, the origin of such a platform in a particularly unfavourable setting for carbonate accumulation, as well as the nucleation of the subsequent widespread Pennsylvanian carbonate platforms of the Cantabrian Mountains. Sixteen carbonate microfacies are differentiated in the Valdediezma Limestone, from shallow-water to slope to basinal environments. The carbonate production is related to the submarine topography and the rapid rates of microbial mound growth and accumulation, particularly from the upper Viséan to the lower Serpukhovian. A high-elevation platform and steep southern margin occurred during the deposition of condensed cephalopod-bearing limestones in the basin. A higher rate of carbonate accumulation is recognised from the upper Serpukhovian and younger, with similar thicknesses in shallow- and deeper water settings. The thickest part of the succession was coeval with the larger subsidence resulting from the migration of the Variscan deformation at the margin of the foreland basin of the Cantabrian Zone. The migration of deformation along the foreland uplifted the Valdediezma platform from the lower Bashkirian and caused its partial erosion. The Pennsylvanian carbonate platform developed on an exhumed Mississippian platform. Tectonic overloading due to the emplacement of nearby thrust sheets caused the subsidence and burial of the Valdediezma platform in the upper Moscovian.