Nappe Contacts Research Articles

Implications of New Geological Mapping and U‐Pb Zircon Dating for the Barrovian Tectono‐Metamorphic Evolution of the Lepontine Dome (Central European Alps)

AbstractThe Barrovian metamorphism of the Lepontine dome is manifested by isograds that cross‐cut tectonic nappe contacts, which is commonly interpreted as metamorphism that occurred after nappe emplacement. However, the pervasive mineral and stretching lineation in amphibolite facies, associated with top‐to‐foreland shearing, suggests that peak Barrovian conditions are coeval with nappe‐overthrusting. Here, we combine mapping and U‐Pb zircon dating to better constrain the relation between metamorphism and overthrusting. Metamorphic zircon rims show two age populations at 31–33 and 22–24 Ma. The younger population is locally observed in post‐foliation dikes (and associated metasomatism) likely sourced from deep‐migmatites exhuming along the Alpine backstop. The older population occurs regionally and is found in syn‐kinematic migmatites which occur along a crustal‐scale shear zone. Below this shear zone, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a tectonic mélange with Mesozoic fragments, was a pre‐Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was strongly sheared along the top of the Simano nappe during overthrusting of a rock pile here‐termed Maggia‐Adula nappe. This large‐scale nappe emplacement imprinted the regional lineation and peak temperatures until 31–33 Ma. Péclet (1–10) and Brinkman (0.002–1.8) numbers, estimated for the overthrusting, suggest an advection‐dominated heat transfer caused by rock exhumation, with some diffusion (conduction) during nappe emplacement. Diffusion contributed to Barrovian isograds discordant to the thrust. Shear heating was important if stress times shearing rate >∼5·10−6 W·m−3 within the nappe. The thermal evolution after overthrusting was spatially heterogeneous until ca. 22 Ma.

Stretched Thin: Oligocene Extrusion and Ductile Thinning of the Basal Unit Along the Evia Shear Zone, NW Cyclades, Greece

AbstractWe present field‐based structural, geochronological, and petrological evidence for a major new Oligocene extensional shear zone, the Evia Shear Zone (ESZ), on the island of Evia in the NW Aegean Sea. Strain related to this structure occurs in a diffuse zone structurally underlying the nappe contact between the Cycladic Blueschist Unit and Basal Unit, within rock previously attributed to the Tsakei Unit or Basal Unit metaflysch. Structures within the ESZ record stretching in both the X and Y directions under ductile and brittle‐ductile conditions, indicative of a component of oblate coaxial strain during overall top‐to‐E general shear. New white mica 40Ar/39Ar geochronology from the ESZ shows that the structure was accommodating ductile deformation in the late Oligocene. Thermodynamic modeling shows that rock within the ESZ records greenschist facies retrogressive conditions of 310 ± 15°C and 7 ± 1 kbar. Zircon (U‐Th)/He thermochronology demonstrates that both the CBU and Basal Unit had exhumed into the brittle crust by the middle Miocene, slightly earlier than indicated by low‐temperature thermochronology for the HP‐LT units throughout the Aegean. We interpret these data as evidence that the Basal Unit underwent rapid exhumation via syn‐orogenic processes persisting until at least the early Miocene, facilitated in part by normal‐sense displacement along the ESZ. The occurrence of structures representative of coaxial deformation in the shear zone alludes to a major role for ductile thinning in controlling exhumation of HP rocks in the Cyclades, perhaps influencing the relative rates of exhumation observed across the region.

Tectonic Evolution of the Nevado‐Filábride Complex (Sierra de Los Filábres, Southeastern Spain): Insights From New Structural and Geochronological Data

AbstractThe high‐pressure metamorphic Nevado‐Filábride Complex (NFC) in the Betics mountain range of southeastern Spain exhibits continental and ocean‐derived tectonic units, which are key for understanding the geodynamic evolution of the Western Mediterranean. We address the current debate in the definition of tectonic units, the emplacement of (ultra)mafic rocks, and the timing of burial metamorphism by conducting a structural study combined with single grain fusion 40Ar/39Ar dating of white micas in structurally critical outcrops of the eastern Sierra de Los Filábres. One older 40Ar/39Ar age population (38–27 Ma) is found at distance from the main shear zones in the relics of an early foliation, while a younger 40Ar/39Ar population (22–12 Ma) is dominant in the vicinity of these shear zones, where the early foliation is obliterated. Both age groups are interpreted as the record of deformation or fluid‐induced recrystallization during distinct fabric‐forming events, while alternative scenarios are discussed. A key observation is the presence of an ophiolitic mélange, which—together with new and published geochronological data—allows for a new tectonic hypothesis. This considers Paleogene subduction beneath a Jurassic oceanic lithosphere, followed by the continued subduction of NFC and overlying ophiolites below the Alpujárride Complex. Exhumation during westward slab roll‐back led to the formation of an extensional detachment system that obliquely cut nappe contacts. Although the timing constraints for high pressure‐low temperature (HP‐LT) metamorphism in the NFC remain inconclusive, the new tectonic hypothesis provides a solution that can account for both Paleogene and Miocene ages of HP‐LT metamorphism.

Petrology and kinematic vorticity constraints of the Al Amar region, Northern Ar-Rayn terrane, Eastern Arabian shield, Saudi Arabia

The Neoproterozoic basement rocks in Ar Rayn terrane set as parts in the Eastern Arabian Shield. It focus the petrological and kinematic vorticity for high deformed volcanic and granitoids rocks in the Al-Amar locality. The volcanic rocks divide into two main series, the eastern sequence cycle represents dacite, rhyodacite, rhyolite and ignimbrites, and the western sequence cycle characterizes andesite and pyroclastics. Furthermore, the granitoids rocks content monzodiorite, tonalite, granodiorite and alkali-feldspar granite. For assessing the proportions of pure and simple shear contributions in penetrative deformed rocks, the kinematic vorticity number (Wm) to the mean appreciation by applying porphyroclast, rotating in a flowing matrix is crucial. In the Ar Rayn terrane of Arabian shield, kinematic vorticity numbers for high deformed metavolcanics and granitoids gneisses vary from 0.4 to 0.6. The deformation in the studied area varied from simple shear, according to vorticity and strain studies. Nappe stacking getting ready during the thrusting activity only through the Al Amar –Idsas fault, according to the results, most likely due to brittle imbrication. A penetrative sub-horizontal foliation formed sub-parallel to the nappe contacts with the beneath laying nappes, showing that the ductile strain was superimposed on the construction of the nappe under high pressure. The Ar Rayn terranewas created sub-horizontal foliation by the buildup of ductile strain during under plating, which was induced by a vertical shortening component. This foliation was formed in the majority of cases during the thrusting nappes upon each other, implying that nappe stacking was related to vertical shortening due to a massive scale by the active Al Amar –Ideas fault.

Torn Between Two Plates: Exhumation of the Cer Massif (Internal Dinarides) as a Far‐Field Effect of Carpathian Slab Rollback Inferred From 40Ar/39Ar Dating and Cross Section Balancing

AbstractExtension across the southern Pannonian Basin and the internal Dinarides is characterized by Oligo‐Miocene metamorphic core complexes (MCCs) exhumed along mylonitic low‐angle extensional shear zones. Cer MCC at the transition between Dinarides and Pannonian Basin occupies a structural position within the distal‐most Adriatic thrust sheet and originates from two different tectonic processes: Late Cretaceous‐Paleogene nappe‐stacking during a continent‐continent collision with Adria in a lower plate position, and exhumation related to Miocene extension driven by the Carpathian slab‐rollback. Structural data and a balanced cross section across the Cer massif show linking of the exhuming shear zone to a breakaway fault, which reactivated the early Late Cretaceous most internal nappe contact. Paleozoic greenschist‐to amphibolite‐grade lithologies surround a polyphase intrusion composed of I‐ and S‐type granites and were exhumed along a shear zone characterized by top‐N transport. Thermobarometric analyses indicate an intrusion depth of 7–8 km of the Oligocene I‐type granite; cooling below ∼500°C occurred at 25.4 ± 0.6 Ma (1σ) yielded by 40Ar/39Ar dating of hornblende. Biotite and white mica from this intrusion as well as from the mylonitic shear zone yield 40Ar/39Ar cooling ages of 17–18 Ma independent of the used techniques (in situ laser ablation, single‐grain total fusion, single‐grain step heating, and multi‐grain step heating). White mica from the S‐type granite yield an 40Ar/39Ar cooling age of 16.7 ± 0.1 Ma (1σ). Associated dikes intruding the shear zone were also affected by N‐S extension resulting in the exhumation of the MCC, which was triggered by the opening of the Pannonian back‐arc basin in response to the Carpathian slab‐rollback.

Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400 °C near the dome, and 230 °C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont color alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming event. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300 °C and 360 °C could be explained by the presence of granitic plutons under the foreland basin.

The connection between the Alps and the Carpathians beneath the Pannonian Basin: Selective reactivation of Alpine nappe contacts during Miocene extension

Abstract The Cretaceous basement of the broader NW Pannonian Basin, including the Vienna Basin, has an Alpine nappe structure that can be correlated between the Eastern Alps and the Western Carpathians. For the first time in this region, based on the systematic integration of petroleum industry vintage 2D reflection seismic data with well data from Austria, Hungary and Slovakia, we document various modes of interaction between pre-existing thrust and superimposed normal faults. Whereas in some cases the pre-existing Cretaceous nappe contacts became extensional low-angle normal faults, the typical scenario is that the Miocene normal faults obliquely cut across them. The style of this negative inversion, i.e. contraction followed by extension exploiting the same fault system, is based on the orientation and geometry of the Alpine nappes in relation to the successor extensional basins. We expand our earlier findings made in the Hungarian sector of the NW Pannonian Basin where Neoalpine (Neogene) low-angle normal faults did indeed interact with abandoned Eoalpine (Cretaceous) and Mesoalpine (Paleogene) thrust fault planes, but in a more selective manner than expected before. The subsurface examples of this paper are located in the Austrian part of the Vienna Basin, the Hungarian, Austrian and Slovakian segments of the broader Danube Basin. Our new regional and local scale subsurface observations also helped to define a very large, but so far unrecognized Eocene antiformal nappe stack beneath the Danube Basin. Presently this is a highly dismembered and dominantly subsurface structure concealed beneath the NW Pannonian Basin between the Eastern Alps and the Western Carpathians. This finding has important implications for the understanding of the Alpine evolution of the broader region.

Evolving temperature field in a fossil subduction channel during the transition from subduction to collision (Tauern Window, Eastern Alps)

AbstractWe investigate the evolution of the three‐dimensional thermal structure of a palaeo‐subduction channel exposed in the Penninic units of the central Tauern Window (Eastern Alps). Structural and petrological observations reveal a sheath fold with an amplitude of some 20 km that formed under high‐P conditions (~2 GPa). The fold is a composite structure that isoclinally folded the thrust of an ophiolitic nappe derived from Alpine Tethys Ocean onto a unit of the distal European continental margin, also affected by the high‐P conditions. This structural assemblage is preserved between two younger domes at either end of the Tauern Window. The domes deform isograds of the T‐dominated Barrovian metamorphism that itself overprints the high‐P metamorphism partly preserved in the sheath fold. Using Raman spectroscopy on carbonaceous material (RSCM), we are able to distinguish peak‐temperature domains related to the original subduction metamorphism from domains associated with the later temperature‐dominated (Barrovian) metamorphism. The distribution of RSCM temperatures in the Barrovian domain indicates a lateral and vertical decrease of peak temperature with increasing distance from the centres of the thermal domes. This represents a downward increase of palaeo‐temperature, in line with previous studies. However, we observe the opposite palaeo‐temperature trend in the lower limb of the sheath fold, namely an upward increase. We interpret this inverted palaeo‐temperature domain as the relic of a subduction‐related temperature field. Towards the central part of the sheath fold's upper limb, RSCM temperatures increase to a maximum of ~520°C. Further upsection in the hangingwall of the sheath fold, palaeo‐peak temperatures decrease to where they are indistinguishable from the peak temperatures of the overprinting Barrovian metamorphism. Peak‐temperature contours of the subduction‐related metamorphism are oriented roughly parallel to the folded nappe contacts and lithological layering. The contours close towards the northern, western and eastern parts of the fold, resulting in an eye‐shaped, concentric pattern in cross‐section. The temperature contour geometry therefore mimics the fold geometry itself, indicating that these contours were also folded in a sheath‐like manner. We propose that this sheath‐like pattern is the result of a two‐stage process that reflects a change of the mode of nappe formation in the subduction zone from thrusting to fold nappe formation. First, thrusting of a hot oceanic nappe onto a colder continental nappe created an inverted peak‐thermal gradient. Second, sheath folding of this composite nappe structure together with the previously established peak‐temperature pattern during exhumation. This pattern was preserved because temperatures decreased during retrograde exhumation metamorphism and remained less than the subduction‐related peak temperatures during the later Barrovian overprint. The fold ascended with diapir‐like kinematics in the subduction channel.

Finite strain analysis for deformed Neoproterozoic rock associations of the Al Amar fault zone, Eastern Arabian Shield, Saudi Arabia

The Al Amar area has played important role to effect the Al Amar–Idsas fault zone, which consists of a sequence of volcanic rocks and sedimentary detrital. Also, it is characterized by a difference in facies with intruded granitoid bodies. The present work has used the finite strain techniques to investigate the relationship of the different lithologies and nappe contacts, which recorded in the Al Amar area. Furthermore, the microstructural analysis has investigated the Abt schist, deformed granitoids, and metavolcanic-sedimentary rocks, which represented the Al Amar area. The axial ratio results average from 1.55 to 2.87 for the Rf/φ method and 1.43 to 7.51 for the Fry method in the XZ sections for metamorphic rocks. In the finite strain data, the long axes direction (X) displays grouping along a NW–SE trend with shallow plunging. The direction of short axes (Z) displays subvertical and accompanied with a subhorizontal foliation. Our strain measurement reveals that strain symmetry shows an oblate shape (flattening) and the strain magnitudes do not display any valuable increasing towards the tectonic nappe contacts. Also, it is indicated that the volume change in the different rock units was unassociated with the accumulation of finite strain. In the investigated area, the subhorizontal foliation in the different lithologies was parallel to thrusting. Furthermore, it shows mainly the similar attitudes for tectonic contact nappes. In the present work, it is suggested that the ductile strain is accumulated during thrusting and involves as vertical shortening component with the subhorizontal foliation. The field observations with finite stain data are not sufficient to decide that the tectonic contact in the orogeny is formed as the simple-shear deformation.

A regional Alpine graphite décollement level beneath the NW Pannonian Basin

The so-called Transdanubian Conductivity Anomaly (TCA) of the Hungarian part of the NW Pannonian Basin has been well known for more than five decades. The exceptionally low resistivity (i.e. 1–2 Ωm) zone has a very large areal extent (on the order a few thousand km2) and it is an entirely subsurface anomaly occurring at depth between circa 3–15 km, with no corresponding outcrops. Various geological explanations of this enigmatic crustal-scale geophysical anomaly range from invoking sub-horizontal Alpine nappe contacts to sub-vertical dikes with graphite and/or saline fluid content. Only one possible analogue outcrop area was considered for the high conductivity anomaly so far, namely the Drauzug/Gailtal area of the Eastern Alps in Austria, some 300 km to the West from the TCA area. Previous attempts to find correspondence between the TCA and prominent seismic reflectors seen on 2D seismic reflection profiles were based on data acquired by research institutions. This study systematically correlates, for the first time, the TCA with 2D industry seismic reflection data in the same area. Our new results show a very strong correlation between the subsurface extent and location of the TCA with various sub-horizontally oriented Cretaceous Alpine nappe surfaces. In addition, we draw on the latest structural correlation of the Alpine nappe stack of the Transdanubian Range with its proper tectonic counterpart in the Eastern Alps.At the southern edge of the Upper Austroalpine units in northern Styria, in the Veitsch Nappe of the Greywacke Zone, numerous graphite localities are known historically. These laterally extensive graphite units in NW Styria formed as the result of greenschist-grade metamorphism of a Carboniferous coal sequence during the Cretaceous. For the first time, we describe here one well penetration of possibly age-equivalent graphitic units in NW Hungary. Correlation of the magnetotelluric anomaly with the distinct reflection seismic signature suggests that the same Palaeozoic graphitebearing Upper Austroalpine units should be present at 3–15 km depth in our study area.Therefore we propose that the best explanation for the observed extent and geometry of the TCA is the presence of graphite in subhorizontal, tectonically thinned detachment surfaces at the base of the Upper Austroalpine nappe edifice of NW Hungary

Deformation within the Cycladic subduction–exhumation channel: new insights from the enigmatic Makrotantalo nappe (Andros, Aegean)

New detailed geological/structural mapping coupled with microstructural investigations and amphibole chemistry analyses are used to elucidate the enigmatic tectonic evolution of the Makrotantalo nappe on northern Andros (Aegean region). On the basis of the new geological map, we re-evaluate the structural position of samples with previously published metamorphic ages to show that the Makrotantalo nappe has been affected by the early Eocene high-pressure (HP) metamorphism. Therefore, the Makrotantalo nappe is an integral part of the Cycladic Blueschist Unit (CBU). The early, main ductile deformation phase recognized in the mapped area, is associated with the emplacement of the Makrotantalo nappe over the North Cyclades nappe at deep subduction levels during early Eocene. The subsequent deformation phase occurred at the middle Eocene–Oligocene synchronous with the decompression of blueschist rocks from the stability field of glaucophane to that of actinolite. During this phase, the nappe pile was folded by map-scale folds, which were locally transposed by ductile shear zones. These shear zones were related with NE-directed oblique thrusting and, locally, either restacked the nappe pile or reworked the early nappe contact. Our results in combination with previous studies in the northern Cyclades show that the top-to-the-NE shearing was dominant at a large part of the CBU during exhumation. Opposite, SW-directed shearing was restricted at the upper structural levels of the CBU. We suggest that the observed spatial variation in shear sense is compatible with NE- rather than SW-directed ductile extrusion of the CBU.

Strain analysis and deformation history of Zalm area, Arabian shield, Saudi Arabia

The present work deals with the strain analysis and deformation of the Zalm area which is located in the central part of the central Nabitah suture belt, which separates the continental Afif terrane from the ensimatic arc terranes of the western Arabian Shield. This study carried out through field investigation, strain parameters, and microscopic examination for understanding the nature of deformation in the Zalm area. On the basis of the field investigation and the geometry of the contact, it is more likely that the contact between ultramafic rocks and gabbroic rock complex is low-angle thrust faulting. The granitoid rocks are divided into the quartz monzodiorite, monzogranite, alkali-feldspar granite and leucocratic-granodiorite. The Rf/ϕ and Fry techniques on feldspar and mafic grains from 53 samples representative of each rock type were collected in the field. Strain data shows that the deformation of the granitic, metavolcanic and amphibole schist samples range from high to moderate. The axial ratios (XZ section) range from 1.60 to 4.10 for the Rf/ϕ method and 2.80 to 4.90 for the Fry method. Furthermore, the finite strain data does not dispay any significant difference in the deformation behavior between Rf/ϕ and Fry methods. We concluded that the strain magnitude has the same order of deformation in the deformed granitoid and amphibole schist rocks. The majority of samples are in a zone of flattening symmetry, while a few samples are showing constrictional symmetry. In addition, it is noted that the short axes (Z) are sub-perpendicular related with a sub-parallel foliation. Furthermore, the nappe contact are designed during the intrusion of plutons associated with faults in the Zalm regions under brittle to semi-ductile deformation conditions.

Nappe stack of Piemonte–Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

New results of structural and lithostratigraphic investigations performed in the Urtier Valley, located south of Aosta Valley at the northern flank of the Gran Paradiso Massif, shed new light on the nappe stack architecture of the Western Alps. Two distinct Piemonte–Ligurian units were recognized in the area: (a) the blueschist facies Broillot unit, with a well‐preserved lithostratigraphic succession that includes ophiolites and their metasedimentary cover and (2) the eclogitic Bardonney unit with clasts and/or slices of ophiolites embedded in a carbonate/quarzitic and/or metabasic matrix. Three major deformation stages have been recognized (D1, D2, and D3). D1 phase occurred during the peak metamorphic conditions (i.e., eclogite or blueschist facies). D2 and D3 phases are related to multistage exhumation processes, from early stacking to late refolding of former nappe contacts. Despite the intensity of deformation and metamorphism, the original lithostratigraphic succession of the Broillot unit allowed us to use a lithostratigraphic approach to reconstruct the structures in the study area. For the first time, differences in the nappe stack exposed south and north of the Aosta Valley are reported. In fact, contrary to what is observed in the north, the eclogitic units exposed in the Urtier Valley lay above and not below the blueschist units. A new model for the tectonic evolution, which ranges from stages of subduction to large‐scale nappe refolding, is proposed. It is able to explain the difference in the architecture of the north and south of the Aosta Valley.

South Menderes Monocline: Low-temperature thermochronology constrains role of crustal extension in structural evolution of southwest Turkey

We report apatite and zircon fission-track data across the contact zones between the Menderes nappes, the Cycladic blueschist unit, and the Ören nappe, in the Anatolide belt of southwest Turkey. These data resolve previous debate on the deformation history of these Cretaceous to Eocene nappe contacts, including whether they were reactivated during late Oligocene to Miocene crustal extension. Apatite fission-track ages range from 18 to 28Ma in the Menderes nappes, and 31 to 42Ma in the Ören nappe. Zircon fission-track ages are 29 to 31Ma in the Menderes nappes, 30 to 33Ma in the Cycladic blueschist unit, and between 93Ma and 129Ma in the Ören nappe. The data reveal that the tectonic contacts within the Menderes nappes, and with the overlying Cycladic blueschist unit have been sealed since ~35–30Ma. In the Ören nappe, zircon fission-track ages are distinctly older (with mixed single grain ages between ~300 and 75Ma) reflecting partial to full resetting during late Cretaceous high-pressure metamorphism. The contact between the Cycladic blueschist unit and the Ören nappe shows no significant tectonic movement in the brittle crust after ~70Ma. These data reveal that the nappe boundaries in the southern Menderes Massif have undergone no significant differential offset relative to the Earth's surface since the Oligocene and therefore were not reactivated during late Oligocene to Miocene continental extension. We interpret the steeply dipping portion of the nappe pile in the southern Menderes Massif as a tilted crustal section, which we name the ‘South Menderes Monocline’. We speculate this monocline formed by differential uplift in the Miocene, either at the hinge zone of a plateau, or by unloading of the Simav detachment footwall.

Late Cretaceous extension and exhumation of the Stong and Taku magmatic and metamorphic complexes, NE Peninsular Malaysia

Fragmentation of large continental areas by post-orogenic extension requires favourable geodynamic conditions and frequently occurs along pre-existing suture zones or nappe contacts, as exemplified by the Stong and Taku magmatic and metamorphic complexes of northern Peninsular Malaysia. For this case, we have employed a field and microstructural kinematic study combined with low temperature thermo-chronology to analyse the tectonic and exhumation history. The results show that the Late Palaeozoic - Triassic Indosinian orogeny created successive phases of burial related metamorphism, shearing and contractional deformation. This orogenic structure was subsequently dismembered during a Cretaceous thermal event that culminated in the formation of a large scale Late Santonian - Early Maastrichtian extensional detachment, genetically associated with crustal melting, the emplacement of syn-kinematic plutons and widespread migmatisation. The emplacement of these magmatic rocks led to an array of simultaneously formed structures that document deformation conditions over a wide temperature range, represented by amphibolite- and greenschist- facies mylonites and as well as brittle structures, such as cataclastic zones and normal faults that formed during exhumation in the footwall of the detachment. The formation of this detachment and a first phase of Late Cretaceous cooling was followed by renewed Eocene - Oligocene exhumation, as evidenced from our fission track ages. We infer that an initial Cretaceous thermal anomaly was responsible for the formation of an extensional gneiss dome associated with simple shear and rotation of normal faults. These Cretaceous processes played a critical role in the establishment of the presently observed crustal structure of Peninsular Malaysia.

From nappe stacking to extensional detachments at the contact between the Carpathians and Dinarides – The Jastrebac Mountains of Central Serbia

Reactivation of inherited nappe contacts is a common process in orogenic areas affected by back-arc extension. The amount of back-arc extension is often variable along the orogenic strike, owing to the evolution of arcuated mountain chains during stages of rapid slab retreat. This evolution creates low rates of extension near rotation poles, where kinematics and interplay with the pre-existing orogenic structure are less understood. The amount of Miocene extension recorded by the Pannonian Basin of Central Europe decreases SE-wards along the inherited Cretaceous – Paleogene contact between the Dinarides and Carpathian Mountains. Our study combines kinematic data obtained from field and micro-structural observations assisted with fission track thermochronological analysis and U-Pb zircon dating to demonstrate a complex poly-phase evolution in the key area of the Jastrebac Mountains of Serbia. A first event of Late Cretaceous exhumation was followed by latest Cretaceous – Eocene thrusting and magmatism related to a continental collision that sutured the accretionary wedge containing contractional trench turbidites. The suture zone was subsequently reactivated and exhumed by a newly observed Miocene extensional detachment that lasted longer in the Jastrebac Mountains when compared with similar structures situated elsewhere in the same structural position. Such extensional zones situated near the pole of extensional-driven rotation favour late stage truncations and migration of extension in a hanging-wall direction, while directions of tectonic transport show significant differences in short distances across the strike of major structures.

Relationship between finite strain and tectonic setting in the Ar-Ruwaydah area, eastern Arabian Shield, Saudi Arabia

Our work deals with structural characteristics of the Ar-Ruwaydah area, which lie between the Al Amar–Idsas fault and the Halaban belt. Our goals are accomplished through field reconnaissance and finite strain analysis, as well as microstructural investigation to estimate the tectonic evolution and structures in the Ar-Ruwaydah area. Our strain data show that Abt schist, metavolcano-sedimentary rocks, and granitic gneiss are characterized by weak to moderate deformation, and the axial ratios for the XZ section range from 1.55 to 2.22. The long axes for ellipsoids (X) have NW–SE and W–E trend directions in the Ar-Ruwaydah area while their short axes (Z) are subhorizontal to subvertical foliations. The strain magnitude does not display any increase towards the tectonic contacts for varying rocks units. Most of the obtained data indicate a dominant oblate with minor prolate strain symmetries in the Abt schist and metavolcano-sedimentary and metagranite rocks. The strain data also display flattening with some constriction. It is concluded that Abt schist, granitic gneiss, and metavolcano-sedimentary rocks have the same deformation behavior. Our finite strain accumulated during the metamorphism which was investigated by thrusting activity in the studied rocks. Furthermore, the nappe contact for different types of rock formed during thrusting activity and intrusion of granitic plutons under semi-brittle to ductile deformation conditions in the Ar-Ruwaydah area.

Integrated Remote Sensing and Structural Analysis Studies of Tayyib Al-Ism Area, Northwestern Arabian Shield, Saudi Arabia

Geological and structural mappings of Tayyib Al-Ism area were carried out using the rocks finite strain data, the Landsat-7 Enhanced Thematic Mapper Plus (ETM+) data and the field based observations. To analyze the finite strain in the studied rocks, the Rf /ϕ and Fry methods are applied to feldspar porphyroclasts and mafic grains from nine metavolcano-sedimentary samples (Hegaf Formation), four diorite-gabbros suite samples (Sawawin Complex), two meta-granite samples (Ifal suite) and five Zuhd alkali granite samples. The obtained data indicate traces of high to moderate level of deformation in the meta-granite and metavolcano-sedimentary rocks. The axial ratios along the XZ section range from 1.70 to 4.80 for the Rf/ϕ method and from 1.50 to 4.50 for the Fry method. A sub-vertical trend of short axes in association with sub-horizontal foliation is also observed. These informations allow us to conclude that a finite strain in the deformed granitic rocks is of the same order of magnitude as in the metavolcano-sedimentary rocks. The contacts between the metavolcano-sedimentary and granitic rocks in Tayyib al Ism area were formed during the granitic intrusions along some of the faults under brittle to semi-ductile deformation conditions. These faults have significantly influenced the geometry and style of rifting in the Red Sea during the Neogene. The finite strain was accumulated in the area during the process of deformation, which superimpose the already existed nappe structure. It indicates that the nappe contacts formed during the accumulation of finite strain. In addition to finite strain analysis, band ratio images (3/1, 5/3, 7/5) and Principal Component Analysis (PCA) technique have been used, which proved effective in mapping geological and structural features of various rock bodies exposed in the study area.

Microstructural and strain analysis in the Tanumah area, Arabian Shield, Saudi Arabia

The magnitude and orientation of the finite strain ellipsoid are critical for constraining displacement across ductile deformation. Finite strain was studied in granitic gneiss and metavolcano-sedimentary rocks of the Tanumah area. Finite strain shows its relationship to the nappe contacts between the granitic gneiss and metavolcano-sedimentary rocks and explains the nature of the subhorizontal foliation typical for the Tanumah area. It used the Rf/φ and Fry methods on quartz, feldspar porphyroclasts and mafic grains (such as hornblende and biotite) from two granite, four tonalite and granodiorite, one gneissic quartz diorite, three biotite quartz diorite, six metasedimentary, one meta-andesite and two metavolcanic rocks. The obtained finite strain data show that the granitic gneisses and metavolcano-sedimentary rocks studied were moderately to highly deformed; the X/Z axial ratios range from 1.70 to 4.80 for Rf/φ method and from 1.20 to 4.50 for Fry method. The direction of finite strain for the long axes shows N to WNW trend and plunges shallowly to the WNW in most studied samples. The Z-axes are subvertical. The data indicate oblate strain symmetry (flattening) in the Tanumah area, and the strain magnitudes do not increase towards the tectonic contacts. It is assumed that the granitic gneisses and metavolcano-sedimentary rocks studied have similar deformation behaviour. It is suggested that the accumulation of finite strain was not associated with any significant volume change. The penetrative subhorizontal foliation is subparallel to the tectonic contacts with the overlying nappes, and foliation was formed during thrusting and intrusion of granite rocks. It is concluded that the nappe contacts formed during progressive thrusting under semi-brittle to ductile deformation by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation in the Tanumah area.

Structural and Mechanical Controls on Intrusion Related to Mahd Ad Dahab Area, Arabian Shield, Saudi Arabia

Mahd Ad Dahab area is located on the western side of the Arabian Precambrian Shield. In the study area, Mahd group rocks are deposited along margin of active extensional volcanotectonic basin. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Precambrian basement. Field geology, finite strain and microstructural studies were carried out and the relationships between the lithological contacts and major/minor structures have been studied. The Rf/φ and Fry methods on the metavolcano-sedimentary samples from 3 andesite samples, 2 lower agglomerate samples, 7 lower rhyolite samples, 3 upper agglomerate, 1 upper tuff sample and 2 rhyolite porphyry samples were used in Mahd Ad Dahab area. Finite-strain data shows that a low to moderate range of deformation of the metavolcanosedimentary samples and axial ratios in the XZ section range from 1.10 to 2.50 for the Rf/φ method and from 1.15 to 2.35 for the Fry method. Furthermore, the short axes are subvertical associated with a subhorizontal foliation. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. Furthermore, contacts formed during intrusion of plutons with some faults in the Mahd Ad Dahab area under brittle to semi-ductile deformation conditions. In this case, finite strain accumulated during superimposed deformation on the already assembled nappe structure. It indicates that the nappe contacts formed during the accumulation of finite strain.