Latest Pliocene to recent thick-skinned tectonics at the Upper Rhine Graben - Jura Mountains junction

The southernmost Upper Rhine Graben and adjacent Jura experienced basement-rooted shortening that occurred after the deposition of the Pliocene fluvial "Sundgau gravels”. Folds affecting the base of these gravels systematically trend NE to ENE. Combined evidence from reflection seismic lines and contour maps of the base-Tertiary and base-Pliocene levels indicates that these folds probably formed by thick-skinned reactivation of both NNE-SSW and WSW-ENE-striking faults. This thick-skinned shortening is NW-SE oriented, i.e. parallel to the maximum horizontal stresses inferred from seismotectonics. NNE-SSW-striking faults (paralleling the Upper Rhine Graben) have been reactivated in sinistral strike-slip mode. However, dextrally transpressive reactivation of the WSW-ENE-trending faults that belong to the Rhine-Bresse Transfer Zone is interpreted to predominate. Deflections of recent river courses around the crests of en-échelon-aligned surface anticlines suggest that the deformation is ongoing at present. Retro-deformation of the folds affecting the base of the Sundgau gravels indicates horizontal displacement rates of about 0.05 mm/a. This corresponds to a minimum strain rate in the order of 2·10−16 s−1, given the maximum time span of 2.9 Ma for this deformation, i.e. the biostratigraphically determined minimum age of the gravels. A change from thin-skinned tectonics, that prevailed during the main phase of Jura folding, to very probably still ongoing thick-skinned tectonics is inferred to have occurred in the Late Pliocene. We speculate that this change might be linked to the incipient inversion of Permo-Carboniferous troughs within the Alpine foreland in general. This inversion in dextrally transpressive or purely compressive mode along a WNW-ESE-trending basement fault, that is part of the Rhine-Bresse Transfer Zone, which in turn was prestructured during the formation of the Permo-Carboniferous troughs, could have triggered the 1356 Basel earthquak

Reactivation of pre-existing crustal discontinuities: the southern Upper Rhine Graben and the northern Jura Mountains : a natural laboratory

This thesis is devoted to the analysis of faults that were repeatedly reactivated under changing stress fields. The investigations were carried out at the junction between the Upper Rhine Graben and the Jura Mountains. This area represents the intersection between part of the western European Cenozoic rift system and the northern Alpine foreland. It has been shaped by an interplay between extensional and compressional tectonic forces and involved the repeated reactivation of crustal-scale faults since the Late Palaeozoic. At present, it is characterised by increased seismicity, giving proof of ongoing tectonic activity. Therefore, studying the tectonic evolution of this area can improve the understanding of various tectonic precesses that shape the earth's crust. Extension in the Upper Rhine Graben initiated in the Late Eocene under W-E- to WNW-ESE-oriented extension. Its southern end connects into the Rhine-Bresse transfer zone. This transfer zone linked the simultaneous extension in both the Upper Rhine Graben and the Bresse Graben, another branch of the European Cenozoic rift system. The transition from mainly rift-perpendicular extension in the Upper Rhine Graben to sinistral transtensive movements in the Rhine-Bresse transfer zone was predetermined by ENE-trending basement faults delimiting a Late Palaeozoic trough system in the subsurface. Rifting involved the reactivation of these ENE-trending basement faults as sinistral transtensive strike-slip faults, simultaneously with normal faulting along NNE-oriented faults. In the sedimentary cover, this was manifested in simultaneous extensional flexuring above ENE-trending faults and growth faulting above NNE-oriented faults. The interaction of these differently oriented faults led to cumulative throw and localised depocenters in the Late Eocene to Early Oligocene. The formation of the thin-skinned Jura Mountains in the Late Miocene to Early Pliocene initiated under NW-SE- to N-W-oriented compression and was related to Alpine orogeny. In the study area, the detached sediments encountered a fault pattern inherited from Palaeogene rifting. While ENE- to E-trending faults led to the nucleation of frontal thrusts and anticlines, NNE-trending faults paralleling the Rhine Graben served as transfer zones. Along these transfer zones, sinistral oblique ramps developed and allowed the detached sediments to be transferred sinistrally to the north. As a result, the northern Jura Mountains reveal a geometry that largely mimics the structural pattern inherited from Palaeogene extensional tectonics. Thin-skinned Jura folding was followed by post-Late Pliocene thick-skinned fault reactivation, as evidenced in the spatial coincidence between subsurface faults and surface anticlines. This youngest tectonic phase characterised by horizontal shortening rates below 0.1 mm/a, is most presumably ongoing at present, as evidenced by faulted Late Pliocene gravels and deflected rivers. Dynamically scaled sand-silicone models showed that within wrench systems, the reactivation of faults in a sand cover, separated from a basal discontinuity by a viscous décollement layer, is strongly controlled by the mechanical coupling between "basement" and "cover" across a décollement layer. The coupling in turn depends on the displacement rates applied to the basal plate. The experimental setup was inspired from the Rhine-Bresse transfer zone (RBTZ) that connected the Upper Rhine Graben with the Bresse Graben. This transfer zone bears evidence for the reactivation of basement-rooted structures in Neogene times. The experimental results suggest not only that fault reactivation i the RBTZ has occurred under low displacement rates, but they also provide an explanation for partial stress decoupling between cover and basement

Simultaneous normal faulting and extensional flexuring during rifting: an example from the southernmost Upper Rhine Graben

The southern end of the Upper Rhine Graben (URG) is formed by a major continental transfer zone, which was localised by the reactivation of ENE-oriented basement faults of Late Palaeozoic origin. A combination of subcrop data (derived from exploration wells and reflection seismic lines) and palaeostress analysis provided new constraints on the timing and kinematics of interacting basement faults. Rifting in the southern URG began in the Upper Priabonian under regional WNW-ESE-directed extension, oriented roughly perpendicular to the graben axis. In the study area, this led to the formation of NNE-trending half-grabens. Simultaneously, ENE-trending basement faults, situated in the area of the future Rhine-Bresse Transfer Zone (RBTZ), were reactivated in a sinistrally transtensive mode. In the sedimentary cover the strike-slip component was accommodated by the development of en-échelon aligned extensional flexures. Flexuring and interference between the differently oriented basement faults imposed additional, but locally confined extension in the sedimentary cover, which deviated by as much as 90° from the regional WNW-ESE extension. The interference of regional and local stresses led to a regime approaching radial extension at the intersection between the URG and RBT

Graben width controlling syn-rift sedimentation: the Palaeogene southern Upper Rhine Graben as an example

Eocene to Early Oligocene syn-rift deposits of the southern Upper Rhine Graben (URG) accumulated in restricted environments. Sedimentation was controlled by local clastic supply from the graben flanks, as well as by strong intra-basinal variations in accommodation space due to differential tectonic subsidence, that in turn led to pronounced lateral variations in depositional environment. Three large-scale cycles of intensified evaporite sedimentation were interrupted by temporary changes towards brackish or freshwater conditions. They form three major base level cycles that can be traced throughout the basin, each of them representing a stratigraphic sub-unit. A relatively constant amount of horizontal extension (ΔL) in the range of 4-5km has been estimated for the URG from numerous cross-sections. The width of the rift (L f ), however, varies between 35 and more than 60km, resulting in a variable crustal stretching factor between the bounding masterfaults. Apart from block tilting, tectonic subsidence was, therefore, largely controlled by changes in the initial rift width (L 0). The along-strike variations of the graben width are responsible for the development of a deep, trough-like evaporite basin (Potash Basin) in the narrowest part of the southern URG, adjacent to shallow areas in the wider parts of the rift such as the Colmar Swell in the north and the Rhine Bresse Transfer Zone that delimits the URG to the south. Under a constant amount of extension, the along-strike variation in rift width is the principal factor controlling depo-centre development in extensional basin

Pseudotachylites along the Pustertal-Gailtal-Line, eastern Periadriatic Fault system, Austria

The Pustertal-Gailtal Line (PGL) belongs to the dextrally transpressive Periadriatic Fault system and forms the border between Southern and Eastern Alps. Although part of the ongoing convergence between Adria and Europe appears to be accommodated by this fault system, it reveals little instrumental and historical seismicity. In our study, we attempted to find evidence for past seismic activity along the PGL by investigating pseudotachylite occurrences. We investigated an area of c. 19 km2 to either sides of the PGL around Maria Luggau (Austria). We identified cataclasites and fault gouges along the fault core zone, from which we investigated only the cohesive rocks. Cataclastic, foliated Oligocene granitoids as well as garnet-mica schists of the Austroalpine basement are crosscut by cm- to dm-scale veins containing black fault rocks, which were sampled for further analyses (Fig. 1). Polarisation microscopy reveals that the vein-forming black fault rocks are often optically isotropic, testifying to their origin as quenched melts. Sharp margins of mm- to cm-sized injection veins against the surrounding host rock, well-rounded quartz and feldspar clasts, the absence of hydrous minerals in the matrix, as well as spherulites are further hints at a seismogenic origin of the studied fabrics. Some of the optically isotropic veins are internally foliated; their in-situ µ-XRF analysis of major element concentrations revealed chemical composition variations in the foliation. Even if this foliation might suggest overprinting by aseismic creep, our observations indicate a seismogenic origin of the studied fabrics as pseudotachylites

Late Pleistocene‐Holocene Slip Rates in the Northwestern Zagros Mountains (Kurdistan Region of Iraq) Derived From Luminescence Dating of River Terraces and Structural Modeling

Abstract A significant amount of the ongoing shortening between the Eurasian and Arabian plates is accommodated within the Zagros Fold‐Thrust Belt. However, the spatial and temporal distribution of active shortening within the belt, especially in its NW part, is not yet well constrained. We determined depositional ages of uplifted river terraces crossing the belt along the Greater Zab River using luminescence dating. Kinematic modeling of the fault‐related fold belt was then used to calculate long‐term slip rates during the Late Pleistocene to Holocene. Our results provide new insight into the rates of active faulting and folding in the area. The Zagros Mountain Front Fault accommodates about 1.46 ± 0.60 mm a −1 of slip, while a more external basement fault further to the SW accommodates less than 0.41 ± 0.16 mm a −1 . Horizontal slip rates related to detachment folding of two anticlines within the Zagros Foothills are 0.40 ± 0.10 and 1.24 ± 0.36 mm a −1 . Basement thrusting and thickening of the crust are restricted to the NE part of the Zagros belt. This is also reflected in the regional topography and in the distribution of uplifted terraces. In the southwestern part, the deformation is limited mainly to folding and thrusting of the sedimentary cover above a Triassic basal detachment. In the NE, deformation is associated with slip on basement thrusts. Our study sheds light on the distribution of shortening in the Zagros Mountains and helps to understand the regional tectonic system. Our results may be the foundation for a better seismic hazard assessment of the entire area.Plain Language Summary In active mountain belts, river terraces found above the present‐day river level can be indicative of differences in uplift rates due to the thickening, faulting, and folding processes in the Earth's crust. These processes, driven by the motion of tectonic plates, are responsible for the formation of mountain belts. Here, we took sediment samples from uplifted river terraces along the Greater Zab River that crosses the Zagros Mountains in the Kurdistan Region of Iraq. We determined their deposition age using luminescence dating. From their age and elevation, we calculated uplift rates. We built a geometrical model of the fault zones in the area and determined how fast the slip occurs on these faults based on the uplift rates. Our results indicate that there were less than two millimeter per year of slip on these faults on average during the last 60 thousand years. This motion is a result of the convergence between the Arabian and Eurasian plates. With studies like this we can measure how fast fault blocks move, even if they were not associated with large earthquakes in the recent past. This approach helps to better assess the potential earthquake hazard in the area under investigation.Key Points We estimated fault slip rates in the NW Zagros Mountains by luminescence dating of river terraces and structural modeling There is c. 1.46 mm a −1 slip on the Mountain Front Fault and c. 1.64 mm a −1 slip from detachment folding in the NE part of the Foothill Zone Crustal thickening and basement thrusting occur in the NE parts of the Foothill Zone and only cover deformation occur in the SW part

Quaternary Seismic Slip in the Eastern Alps: Dating Fault Gouges from the Periadriatic Fault System Using Trapped Charge Dating Methods

The Periadriatic Fault System (PAF) is among the largest post-collisional structures of the Alps. Recent studies using GPS velocities suggest that Adria-Europe convergence is still being accommodated in the Eastern Alps. However, according to instrumental and historical seismicity records, earthquake activity is mostly concentrated along structures in the adjacent Southern Alps and adjacent Dinarides. Apart from ambiguous historical events, the PAF has little to no earthquake record. Electron spin resonance (ESR) and Optically Stimulated Luminescence (OSL) are dating methods that can be applied as ultra-low temperature thermochronometers (closing temperature below 100 °C), with a Quaternary dating range of a few decades up to ~2 Ma. Both are potentially applicable to date shear heating during earthquakes in slowly deforming fault zones. Since the saturation dose of the quartz ESR signals is larger than that of quartz and feldspar OSL, ESR enables establishing a maximum age of the events (assuming the resetting during seismic events was at least partial), while OSL allows finding their minimum age when the signal is in saturation. We analyzed fault gouge samples from 4 localities along the easternmost segment of the PAF (east of the Giudicarie Fault), and 5 localities along the southernmost segment of the Lavanttal Fault. For ESR, we measured the signals from the Al center in quartz, comparing the results from the single aliquot additive dose (SAAD) and single aliquot regenerative dose (SAR) protocols. Different grain size fractions were measured (SAR protocol) to establish a grain-size age plateau. For OSL, we measured the Infrared Stimulated Luminescence (IRSL) signal at 50 °C (IR50) and the post-IR IRSL signal at 225 °C (pIRIR225) on potassium feldspar. Additionally, experiments of thermal activation of the OSL signal in quartz were performed to observe the shear heating effect in different grain size fractions. For the PAF, the OSL shear heating sensitivity experiments show that quartz has been thermally activated to temperatures below 300 °C, corroborating that shear heating was sufficient for at least a partial system reset. The ESR grain size plateaus suggest that the most effectively reset fraction is 100-150 µm. In general, our dating results indicate that the studied segment of the PAF system accommodated seismotectonic deformation within a maximum age ranging from 1075 ± 48 to 349 ± 17 ka (ESR SAR) and a minimum age in the range of 196 ± 12 to 281 ± 16 ka (pIRIR225). The obtained ages and the current configuration of the structure suggest that the studied segment of the PAF could be considered a potentially active fault at least. In the case of the Lavanttal fault, the ESR dose-response curves were either close to or in saturation, allowing to obtain only minimum ages of ca. 4 Ma for the last total reset of the system. This could be the result of insufficient shear heating by low magnitude earthquakes, or the fault has not seen significant activity since then. Altogether, our results show that large structures in the Eastern Alps such as the PAF have accommodated part of the Adria-Europe convergence during the Quaternary and can potentially host earthquakes in the future

Late quaternary tectonic activity of the Udine-Buttrio Thrust, Friulian Plain, NE Italy

The NW-SE trending Udine-Buttrio Thrust is a partly blind fault that affects the Friulian plain southeast of Udine in NE Italy. It is part of a wider fault system that accommodates the northward motion of the Adriatic plate. Although seismic reflection data and morphological evidence show that the fault was active during the Quaternary, comparably little is known about its tectonic activity. We used high-resolution digital elevation models to investigate the surface expression of the fault. Measured vertical surface offsets show significant changes along strike with uplift rates varying between 0 and 0.5 mm/yr. We then analyze a topographic scarp near the village of Manzano in more detail. Field mapping and geophysical prospections (Georadar and Electrical Resistivity Tomography) were used to image the subsurface geometry of the fault. We found vertical offsets of 1–3 m in Natisone River terraces younger than 20 ka. The geophysical data allowed the identification of deformation of the fluvial sediments, supporting the idea that the topographic scarp is a tectonic feature and that the terraces have been uplifted systematically over time. Our findings fit the long-term behaviour of the Udine-Buttrio Thrust. We estimate a post-glacial vertical uplift rate of 0.08–0.17 mm/yr recorded by the offset terraces. Our results shed light on the Late Quaternary behaviour of this thrust fault in the complicated regional tectonic setting and inform about its hitherto overlooked possible seismic hazard

A map-view restoration of the Alpine-Carpathian-Dinaridic system for the Early Miocene

A map-view palinspastic restoration of tectonic units in the Alps, Carpathians and Dinarides reveals the plate tectonic configuration before the onset of Miocene to recent deformations. Estimates of shortening and extension from the entire orogenic system allow for a semi-quantitative restoration of translations and rotations of tectonic units during the last 20Ma. Our restoration yielded the following results: (1) The Balaton Fault and its eastern extension along the northern margin of the Mid-Hungarian Fault Zone align with the Periadriatic Fault, a geometry that allows for the eastward lateral extrusion of the Alpine-Carpathian-Pannonian (ALCAPA) Mega-Unit. The Mid-Hungarian Fault Zone accommodated simultaneous strike-perpendicular shortening and strike-slip movements, concomitant with strike-parallel extension. (2) The Mid-Hungarian Fault Zone is also the locus of a former plate boundary transforming opposed subduction polarities between Alps (including Western Carpathians) and Dinarides. (3) The ALCAPA Mega-Unit was affected by 290km extension and fits into an area W of present-day Budapest in its restored position, while the Tisza-Dacia Mega-Unit was affected by up to 180km extension during its emplacement into the Carpathian embayment. (4) The external Dinarides experienced Neogene shortening of over 200km in the south, contemporaneous with dextral wrench movements in the internal Dinarides and the easterly adjacent Carpatho-Balkan orogen. (5) N-S convergence between the European and Adriatic plates amounts to some 200km at a longitude of 14° E, in line with post-20Ma subduction of Adriatic lithosphere underneath the Eastern Alps, corroborating the discussion of results based on high-resolution teleseismic tomography. The displacement of the Adriatic Plate indenter led to a change in subduction polarity along a transect through the easternmost Alps and to substantial Neogene shortening in the eastern Southern Alps and external Dinarides. While we confirm that slab-pull and rollback of oceanic lithosphere subducted beneath the Carpathians triggered back-arc extension in the Pannonian Basin and much of the concomitant folding and thrusting in the Carpathians, we propose that the rotational displacement of this indenter provided a second important driving force for the severe Neogene modifications of the Alpine-Carpathian-Dinaridic orogenic syste

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

Abstract Extension 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 40 Ar/ 39 Ar dating of hornblende. Biotite and white mica from this intrusion as well as from the mylonitic shear zone yield 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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.Plain Language Summary Horizontal stretching of continental plates induces thinning of the crustal upper part, melting of rocks, the sinking of the land surface, and formation of large basins. One of the world's best‐studied basins formed by such a process is the Central European Pannonian Basin. This basin is surrounded by the mountain belts of the Alps, Carpathians, and Dinarides. We have studied rocks between the Pannonian Basin and the southerly adjacent Dinaride Mountains, where rocks deposited in the basin are found right next to rocks that were initially about 7–8 km deep in the crust. These rocks are separated by a shear zone, along which they were brought to the surface. We have dated the activity of the shear zone by measuring concentrations of radioactive isotopes and their decay products contained in deformed minerals. The shear zone was active at a time when the Pannonian Basin started to open due to tectonic processes further NE underneath the Carpathian mountain chain. We also found evidence that the shear zone, which brought metamorphic rocks upwards was formerly one that brought rocks downwards into the crust during an earlier phase of mountain building, predating basin formation.Key Points Activity along the shear zone exhuming Cer metamorphic core complex in the internal Dinarides was dated by 40 Ar/ 39 Ar geochronology to ∼17 Ma Exhumation was facilitated by extensional reactivation of Late Cretaceous‐Paleogene nappe contacts resulting from Adria‐Europe collision Extensional reactivation of the thrusts is interpreted as a far‐field effect of Oligo‐Miocene Carpathian slab rollbac