Miocene Extension Research Articles

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.

Interpretation of vintage 2D seismic reflection data along the Austrian-Hungarian border: Subsurface expression of the Rechnitz metamorphic core complex

In the border zone between Austria and Hungary, the Miocene extension of the Pannonian Basin was characterized by extreme, large-magnitude upper crustal extension accommodated along low-angle detachment faults. Although some of these prominent normal faults have already been described using 2D seismic data sets and well data on the Hungarian side, we offer the first systematic interpretation using the Austrian and Hungarian vintage seismic data sets acquired in the 1970s and 1980s. The refinement of the previously proposed metamorphic core complex (MCC) style, east-northeast–west-southwest-trending very high-strain extension provides a modern understanding of back-arc extension in this part of the Pannonian Basin system as the result of the collapse of the Alpine orogen. Although previous interpretations could not achieve the subsurface correlation of major structural elements across the border, we did systematically map these for the first time. Numerous exploration wells, drilled on both sides of the border, were integrated with reflection seismic data to differentiate between the lower versus upper plates of the major low-angle detachment faults, including the largest one responsible for the formation of the Rechnitz MCC. Based on our new interpretation, the regionally mapped Rechnitz detachment fault has an unexpectedly large subsurface extent, on the order of 1000 km2. Moreover, the unusually large number of industry 2D seismic profiles (approximately 50) used to map this and other prominent faults, in the Austrian and Hungarian sides, makes the Rechnitz MCC possibly the best constrained one in the world in terms of subsurface definition by reflection seismic data.

Seismic evidence for failed rifting in the Ligurian Basin, Western Alpine domain

Abstract. The Ligurian Basin is located in the Mediterranean Sea to the north-west of Corsica at the transition from the Western Alpine orogen to the Apennine system and was generated by the south-eastward trench retreat of the Apennines–Calabrian subduction zone. Late-Oligocene-to-Miocene rifting caused continental extension and subsidence, leading to the opening of the basin. Yet it remains unclear if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a 130 km long seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, a Priority Programme of the German Research Foundation (DFG) and the German component of the European AlpArray initiative, and trends in a NE–SW direction at the centre of the Ligurian Basin, roughly parallel to the French coastline. The seismic data were recorded on the newly developed GEOLOG recorder, designed at GEOMAR, and are dominated by sedimentary refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Mohorovičić discontinuity (Moho) reflection. The main features share several characteristics (e.g. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are complemented by gravity data and yield a ∼ 6–8 km thick sedimentary cover and the seismic Moho at 11–13 km depth below the sea surface. Our study reveals that the oceanic domain does not extend as far north as previously assumed. Whether Oligocene–Miocene extension led to extremely thinned continental crust or exhumed subcontinental mantle remains unclear. A low grade of mantle serpentinisation indicates a high rate of syn-rift sedimentation. However, rifting failed before oceanic spreading was initiated, and continental crust thickens towards the NE within the northern Ligurian Basin.

Shear-wave splitting properties of the upper crust, during the 2013–2014 seismic crisis, in the CO2-rich field of Florina Basin, Greece

The Florina basin (NW Greece) embodies sources of natural CO2 emissions, which are aligned parallel to the NE-SW striking regional major normal faults. The study area is characterized by low to moderate seismicity, as historical information and instrumental data reveal. During February 2013 to January 2014 an outburst of seismic activity emerged, consisting of more than 2000 located earthquakes, with a mainshock of Mw = 4.1 on 17 February 2013 (16:48 UTC). A local temporary network was installed in July 2013, whose recordings provided the necessary data for performing a Shear-wave Splitting (SwS) study of the upper crust. After the application of strict selection criteria, 532 SwS measurements were obtained for two stations, using a mixture of manual and fully automatic methods. Two polarization directions of the fast shear-wave were identified at one of the stations, with the main one striking NE-SW and the secondary in a NW-SE direction. The first direction is in agreement with the local stress field, as it is identified by focal mechanisms, and with the strike of faults along which soil gas CO2 concentrations are distributed and can be explained by the Anisotropic Poro-Elasticity (APE) model. The second direction is consistent with the strike of mapped local faults, inherited since the Late Miocene extension, which are responsible for the formation of the Florina basin. The SwS results at the second station revealed an almost N-S dominant polarization direction, which is not consistent with the local stress field. This direction, obtained from events belonging to the southern group of the sequence, that was probably triggered by fluid intrusion, might be interpreted by spatiotemporal patterns of seismicity and CO2 upwelling.

Layering and structural inheritance controls on fault zone structure in three dimensions: a case study from the northern Molasse Basin, Switzerland

Mechanical heterogeneity of a sedimentary sequence exerts a primary control on the geometry of fault zones and the proportion of offset accommodated by folding. The Wildensbuch Fault Zone in the Swiss Molasse Basin, with a maximum throw of 40 m, intersects a Mesozoic section containing a thick (120 m) clay-dominated unit (Opalinus Clay) over- and underlain by more competent limestone units. Interpretation of a 3D seismic reflection survey indicates that the fault zone formed by upward propagation of an east–west-trending basement structure, through the Mesozoic section, in response to NE–SW Miocene extension. This configuration formed an array of left-stepping normal fault segments above and below the Opalinus Clay. In cross-section a broad monoclinal fold is observed in the Opalinus Clay. Folding, however, is not ubiquitous and occurs in the Opalinus Clay where fault segments above and below are oblique to one another; where they are parallel the fault passes through the Opalinus Clay with little folding. These observations demonstrate that, even in strongly heterogeneous sequences, here a four-fold difference in both Young's modulus and cohesion between layers, the occurrence of folding may depend on the local relationship between fault geometry and applied stress field rather than rheological properties alone.

Origin and Dynamics of Saltwater Intrusion in a Regional Aquifer: Combining 3‐D Saltwater Modeling With Geophysical and Geochemical Data

AbstractWorldwide, aquifers in low‐lying coastal areas are threatened by saltwater occurrence, as a result of small head gradients, high groundwater abstraction rates, and drain management of the landscape, which is likely to intensify with climate change. Numerical models can serve as tools to identify the sources of the salt and thus to increase understanding of the driving mechanisms and important parameters controlling the extent of saltwater intrusions. This way, areas vulnerable to sea level rise can be identified and managed. Challenges include unknown initial salt concentrations, heterogeneous geology, and anthropogenic alterations. In this study, hydrogeological, geophysical, and geochemical data are used to develop a numerical density‐dependent groundwater flow and transport model with the objective to understand the history of a saltwater‐affected groundwater system and its likely response to historic and future changes. The extent of the simulated saltwater intrusion compares well with Airborne Electromagnetic data that show salt water up to 20 km inland. The results reveal that the salt water originates from a combination of laterally intruding seawater and vertically infiltrating transgression water. Main features controlling the progression of the modern seawater into the coastal aquifers are high permeable, deep Miocene sand aquifers, buried valleys that provide preferential flow paths in combination with extensive Miocene clay layers that delay saltwater intrusion. Anthropogenic activity enhances the saltwater inflow from the ocean and induces transient conditions. Future scenarios show that saltwater progression due to nonstationarity leads to enhanced contamination of the deeper aquifers. Climate change affects primarily the shallow aquifer systems.

Kinematics of Foreland‐Vergent Crustal Accretion: Inferences From the Dinarides Evolution

AbstractOne of the most common observation in Mediterranean areas is the migration of contractional deformation and associated slabs through time toward external orogenic areas, associated with lower plate crustal accretion. The Dinarides orogen of Central Europe is an optimal place to study such a sequence of contractional deformation. Compared with other areas, contraction in the Dinarides was less overprinted by subsequent extension, while a remnant of the subducted slab is observed in a far external orogenic position. Understanding the deformational evolution of the Dinarides is hampered by the reduced availability of kinematic studies. Therefore, we have performed a surface kinematic study in the external parts of the Dinarides. By correlating with available geophysical and evolutionary constraints, we constructed two large‐scale, kinematically controlled regional transects. The results demonstrate a long‐lived evolution of shortening that affected the Dinarides lower orogenic plate. While the Late Jurassic‐earliest Cretaceous deformation was associated with an earlier obduction moment, the latest Cretaceous onset of continental collision has gradually focused deformation at inherited rheological weakness zones. We show that shortening was interrupted by a period of Miocene extension that affected all orogenic areas and created the Dinarides Lake System. The extension was followed by renewed shortening, which started during the latest Miocene and remains presently active, whose kinematics in the central and SE part of the Dinarides is revealed for the first time by our study. These results indicate a lower plate crustal accretion mechanism that was spatially and temporally connected with gradual slab retreat in the Dinarides.

Tectonic strain recorded by magnetic fabrics (AMS) in plutons, including Mt Kinabalu, Borneo: A tool to explore past tectonic regimes and syn-magmatic deformation

Tectonic strain commonly overprints magmatic fabrics in AMS (Anisotropy of Magnetic Susceptibility) data for plutonic rocks produced by both compressional and extensional regimes. Mt Kinabalu, Borneo, is a composite pluton with an exceptional vertical range of exposure and clearly defined internal contacts. We show that tectonic fabrics are recorded pervasively throughout the intrusion, even near contacts, and present a workflow distinguishing compressive and extensional syn-magmatic deformation. At Mt Kinabalu this reveals a pervasive tectonic fabric indicating NW-SE Miocene extension in Borneo at 7.9–7.3 Ma, later than previously recognised, oriented NW-SE at 319° ±13.1°. Comparing data from Mt Kinabalu with data from globally distributed studies shows that tectonic strain is commonly recorded by plutons. Therefore, AMS fabric can be used to identify the syn-magmatic tectonic setting and combined with both geochronology and evidence for paleomagnetic rotation to provide a powerful tool for accurate determination of syn-magmatic tectonic regimes and strain orientations within temporal frameworks.

Compressional evolution of the PNG margin from an orogenic transect from Juha to the Sepik

A crustal-scale, fully restored section across the PNG orogenic belt reveals the Oligocene to Recent compressional deformation of the margin. The northern end of the section comprises the Landslip Metamorphics, an accreted continental terrane, separated from the main part of the Fold Belt by the Jurassic April Ultramafics and Om Metamorphics, interleaved with Eocene volcanics which together constitute an accretionary prism. Existing maps show that the suture is overlain by distal Miocene sediments indicating Oligocene docking and probable compression prior to Early Miocene subsidence. The latter is consistent with Early Miocene extension in PNG and the emplacement of metamorphic core complexes in the Sepik area, but is also related to dynamic topography causing subsidence of the whole northern margin of Australia. Neogene compression commenced around 12 Ma with ~70km shortening in the Om terrane and ~38km shortening in the Fold Belt. Existing thermochronology data indicate shortening of ~12mm/year from 12-4 Ma, but only 2.5mm/year from 4-0 Ma, consistent with a change in structural style in the Fold Belt from thrust to more ductile, fold-dominated deformation. The model also requires substantial thickening of the continental crust beneath the Muller Ranges, here represented by ‘basement’ underthrusting. Gravity modelling indicates the presence of sedimentary graben up to 10 km deep beneath the Fold Belt, which were strongly inverted, such as beneath the Lavani Valley. A key issue is when this inversion occurred, in the Oligocene or Pliocene, as this has a significant influence on the timing of hydrocarbon generation and migration.

Stratigraphic correlation and structural position of Lower Cretaceous flysch-type deposits in the eastern Southern Alps (NW Slovenia)

Lower Cretaceous syn-orogenic sediments derived from the obducted ophiolites of the Meliata–Maliac–Vardar (Neotethys) Ocean are typically found in the Dinarides and the Austroalpine units. Correlative flysch-type deposits linking both regions through the Southern Alps had been reported from the Bohinj area (NW Slovenia), but their stratigraphic and structural framework remained poorly known. Our research focused on stratigraphic and structural field studies in a 50 km2 area between Lake Bled and Lake Bohinj in the Julian Alps. The mixed carbonate–siliciclastic sediments, informally named the Studor formation, range in age from the Valanginian (possibly late Berriasian) to the Aptian. They occur on top of two different stratigraphic successions, which we assign to two separate nappes. The first succession consists of deep-water Middle Triassic to Lower Cretaceous deposits of the Bled Basin and belongs to the Pokljuka Nappe, which is the uppermost nappe of the Julian nappe stack. The second succession consists of Upper Triassic to Lower Jurassic platform carbonates and a thin Jurassic–Cretaceous deep-water sequence. This succession was deposited in the marginal area of the Julian Carbonate Platform/Julian High and now belongs to the underlying Krn Nappe. The original (Dinaric) thrust contacts between the Pokljuka and Krn nappes are obliterated by younger deformations. The present-day boundaries between these two nappes are steep NE–SW and younger NW–SE trending faults. The post-nappe deformation sequence characterizing the Alps–Dinarides transition zone has been recognized: (1) Oligocene–Early Miocene NW–SE contraction; (2) Early–Middle Miocene extension; and (3) Late Miocene to recent inversion and transpression.

On the tectono-stratigraphic evolution and hydrocarbon systems of extensional back-arc basins: inferences from 2D basin modelling from the Pannonian basin

Two-dimensional basin modelling was carried out in the Pannonian basin of Central Europe to investigate the Miocene extension, post-rift evolution, and subsequent basin inversion and associated sedimentation. A tectono-sedimentary evolutionary model was constrained by seismic and well data. The simulated basin and petroleum systems model performed with petroleum systems modelling software package integrates the spatial and temporal variations of episodes of subsidence and uplift, sedimentation and erosion, and the dynamics of biogenic and thermogenic gas generation, migration, accumulation and loss. This high-resolution approach analysed the impact of the shelf-margin slope progradation and sequential sediment loading on mechanical compaction, pore pressure development, source rock maturation and hydrocarbon charge. Generation and migration processes were genetically controlled by the deposition of the SSE-ward prograding Pannonian (s.l.) shelf-margin slope sediments, and repeated tectonic inversions along the Mid-Hungarian Fault Zone. We tested different maturation kinetic models and compared the impact of different generation reaction schemes on charge. Biogenic gas generation was associated with the deposition of almost the entire sedimentary succession in the studied Jaszsag and Bekes sub-basins. However, the preservation of the gases was limited in time and space. Most of the thermogenic gas was generated by the deepwater marls in the Bekes sub-basin, whereas the depocentre of the Jaszsag sub-basin, mainly filled with Miocene rhyolites or rhyolitic tuffs connected to the intense volcanic activity, prevented the generation of significant amounts of hydrocarbon there.

Fluid composition changes in crystalline basement rocks from ductile to brittle regimes

Abstract The relationships between deformation and fluid flow have been investigated in the Paleozoic basement of an isolated horst of the Catalan Coastal Ranges. A structural, petrological and geochemical study has been performed in a complex fracture network that resulted from a long-lived tectonic history (from Carboniferous to Miocene). Nine fracture types, developed from a ductile regime in the greenschist facies to a shallow brittle regime, have been characterized in order to establish P, T and fluid compositions during the evolution of the horst. Syn-cleavage and late-cleavage quartz veins (qtz1-chl1 ± mu and late qtz2-chl2-dol1) formed during the Hercynian ductile deformation. These minerals precipitated from metamorphic fluids, possibly evolved from seawater, at temperatures between 240 and 340 °C. En-echelon albite vein arrays (ab-qtz3-chl3 ± ti-an) and NE-SW normal faults generating breccias mark the change from ductile to brittle, from compression to extension and from a closed to an open hydrologic regime. This paragenesis precipitated from the mixing of metamorphic and magmatic fluids at temperatures between 180 and 290 °C during the early Permian extension. Dolomite veins (dol2-chl4-qtz4), precipitated at 210–280 °C from the mixing of previous fluids with hypersaline oxidizing external brines, possibly during the late stage of the early Permian extension. Reverse faults and calcite veins (Cc1-ba) formed either during the Paleogene compression or during the Langhian to early Serravallian minor compression. Calcite and barite precipitated from meteoric or marine waters in an open hydrological system at temperatures below 50 °C. The Miocene extension is represented by NE-SW normal faults with fault gouges and NNW-SSE normal faults cemented by calcite 2 that precipitated at temperatures below 50 °C from meteoric fluids in an open basin-scale hydrological system. The studied horst was part of a relay zone between two segments of the NNW-SE Llobregat fault during the early Permian, explaining the high fracture density and the fast upflowing of hydrothermal fluids at that time, thus controlling the development of albite veins exclusively in this area.

Porphyry Copper Potential in Japan Based on Magmatic Oxidation State

AbstractThe formation of large porphyry Cu deposits requires shallow intrusion of felsic stocks that are injected into the focused sites of the mineralization. The parental magmas for these igneous rocks are oxidized, with a fO2 above FMQ + 2 (2 logarithmic units above fO2 of fayalite‐magnetite‐quartz buffer), which enables the magmas to transport large amounts of S and metals. The Japanese islands contain continental and island‐arc magmatic belts of various ages, from Mesozoic to recent, that were formed in response to westward subduction of oceanic plates. Mesozoic–Paleocene magmatism formed arcs on the eastern margin of Eurasia before the opening of the Sea of Japan. The back‐arc rifting that formed the Sea of Japan separated these igneous rocks from Eurasia, with their relics exposed as volcano–plutonic belts in Japan. Among these, the San‐in belt is the only representative of an upper crustal section that includes magnetite‐bearing granitic rocks. The opening of the Sea of Japan was accompanied by the asthenospheric mantle upwelling and eruption of oceanic magma. This was followed by extensive Miocene submarine volcanic activity that formed the Green Tuff belt. The current configuration of the Japanese islands was established in Pliocene. Young arcs, such as the Kuril, Izu‐Bonin, and Ryukyu arcs, are largely composed of mafic magmas, with accompanying asthenospheric upwelling in back‐arc extension. As the asthenospheric mantle and associated magmas have a reduced fO2, below FMQ (fayalite‐magnetite‐quartz buffer), continual upwelling of asthenospheric mantle creates limited areas of potential porphyry Cu mineralization in Japanese islands. Possible prospective areas include young stratovolcanoes in Honshu and Kyushu islands. Fertile rocks that may be associated with porphyry Cu mineralization can be identified by evaluating their magmatic oxidation conditions through mineralogical and mineral chemistry studies.

The role of the slab pull force in the late Oligocene to early Miocene extension in the Southern Central Andes (27°-46°S): Insights from numerical modeling

Although extensional deformation plays a significant part of Andean history, the causes behind its driving mechanisms and its impact throughout the geological record remain controversial. Through the aid of numerical modeling of subduction zone dynamics, we were able to reproduce a brief period of intra-arc basin formation that affected the Southern Central Andes (27°-46°S) during late Oligocene and early Miocene times. The results of the model show that, after a period of slow subduction (6–8 cm/yr), the oceanic plate approaches the mantle transition zone at ca. 23 Ma, triggering the slab pull force. The addition of this slab pull force generates a progressive increase in convergence velocity (reaching ∼20 cm/yr) and the retreat of the trench hinge away from the upper plate, resulting in the steepening of the slab. Effects observed in the upper plate are the formation of a basin located 200–300 km east of the trench and an asthenospheric influx beneath an 800 km wide zone east of the oceanic and continental plate's boundary. A series of parameters extracted from our model, such as the basin depth and the stretching factor, indicate that crustal stretching, basin formation, convergence velocity and asthenospheric influx would have reached their climax approximately at 20 Ma. These results are in good correlation with the convergence rate obtained through plate reconstructions and the geological record along the Southern Central Andes, where a series of extensional intra-arc basins were created and mantle derived magmatic processes affected a wide area ranging between the present fore-arc and retroarc areas during late Oligocene to early Miocene times. However, differences in extension magnitude, magma composition and basin fill depositional environment are observed, indicating that the impact of the slab pull force was stronger towards the southern basins. Possible causes that could explain these differences are variations in crustal thickness before the influence of the slab pull force and the effect of toroidal mantle flow near the southern lateral slab edge. This would indicate that although the main parameter controlling tectonic regime is the absolute motion of the overriding plate, the slab pull force may leave its imprint along the evolution of subduction-type orogens such as the Andes.

Fluid‐Enhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the Carpathian‐Pannonian Extensional Basin System

AbstractMantle xenoliths from the Styrian Basin Volcanic Field (Western Pannonian Basin, Austria) are mostly coarse granular amphibole‐bearing spinel lherzolites with microstructures attesting for extensive annealing. Olivine and pyroxene CPO (crystal‐preferred orientation) preserve nevertheless the record of coeval deformation during a preannealing tectonic event. Olivine shows transitional CPO symmetry from [010]‐fiber to orthogonal type. In most samples with [010]‐fiber olivine CPO symmetry, the [001] axes of the pyroxenes are also dispersed in the foliation plane. This CPO patterns are consistent with lithospheric deformation accommodated by dislocation creep in a transpressional tectonic regime. The lithospheric mantle deformed most probably during the transpressional phase after the Penninic slab breakoff in the Eastern Alps. The calculated seismic properties of the xenoliths indicate that a significant portion of shear wave splitting delay times in the Styrian Basin (0.5 s out of approximately 1.3 s) may originate in a highly annealed subcontinental lithospheric mantle. Hydroxyl content in olivine is correlated to the degree of annealing, with higher concentrations in the more annealed textures. Based on the correlation between microstructures and hydroxyl content in olivine, we propose that annealing was triggered by percolation of hydrous fluids/melts in the shallow subcontinental lithospheric mantle. A possible source of these fluids/melts is the dehydration of the subducted Penninic slab beneath the Styrian Basin. The studied xenoliths did not record the latest large‐scale geodynamic events in the region—the Miocene extension then tectonic inversion of the Pannonian Basin.

Slip Inversion Along Inner Fore‐Arc Faults, Eastern Tohoku, Japan

AbstractThe kinematics of deformation in the overriding plate of convergent margins may vary across timescales ranging from a single seismic cycle to many millions of years. In Northeast Japan, a network of active faults has accommodated contraction across the arc since the Pliocene, but several faults located along the inner fore arc experienced extensional aftershocks following the 2011 Tohoku‐oki earthquake, opposite that predicted from the geologic record. This observation suggests that fore‐arc faults may be favorable for stress triggering and slip inversion, but the geometry and deformation history of these fault systems are poorly constrained. Here we document the Neogene kinematics and subsurface geometry of three prominent fore‐arc faults in Tohoku, Japan. Geologic mapping and dating of growth strata provide evidence for a 5.6–2.2 Ma initiation of Plio‐Quaternary contraction along the Oritsume, Noheji, and Futaba Faults and an earlier phase of Miocene extension from 25 to 15 Ma along the Oritsume and Futaba Faults associated with the opening of the Sea of Japan. Kinematic modeling indicates that these faults have listric geometries, with ramps that dip ~40–65°W and sole into subhorizontal detachments at 6–10 km depth. These fault systems can experience both normal and thrust sense slip if they are mechanically weak relative to the surrounding crust. We suggest that the inversion history of Northeast Japan primed the fore arc with a network of weak faults mechanically and geometrically favorable for slip inversion over geologic timescales and in response to secular variations in stress state associated with the megathrust seismic cycle.

Insights into the emplacement of upper-crustal plutons and their relationship to large silicic calderas, from field relationships, geochronology, and zircon trace element geochemistry in the Stillwater – Clan Alpine caldera complex, western Nevada, USA

Geologic mapping, new U-Pb zircon ages, and new and published 40Ar/39Ar sanidine ages document the timing and extent of Oligocene magmatism in the southern Stillwater Range and Clan Alpine Mountains of western Nevada, where Miocene extension has exposed at least six nested silicic calderas and underlying granitic plutons to crustal depths locally ≥9km. Both caldera-forming rhyolitic tuffs and underlying plutons were emplaced in two episodes, one from about 30.4–28.2Ma that included the Deep Canyon, Job Canyon, and Campbell Creek calderas and underlying plutons, and one from about 25.3–24.8Ma that included the Louderback Mountains, Poco Canyon, and Elevenmile Canyon calderas and underlying plutons. In these two 1–2m.y. periods, almost the entire Mesozoic upper crust was replaced by Oligocene intrusive and extrusive rocks to depths ≥9km over an estimated total area of ~1500km2 (pre-extension). Zircon trace element geochemistry indicates that some plutonic rock can be solidified residual magma from the tuff eruptions. Most plutons are not solidified residual magma, although they directly underlie calderas and were emplaced along the same structures shortly after to as much as one million years after caldera formation. Magma chambers and plutons grew by floor subsidence accommodated by downward transfer of country rocks. If other Great Basin calderas are similar, the dense concentration of shallowly exposed calderas in central Nevada is underlain by a complexly zoned mid-Cenozoic batholith assembled in discrete pulses that coincided with formation of large silicic calderas up to 2500–5000km3.

Subsurface temperature model of the Hungarian part of the Pannonian Basin

Abstract Hungary is one of the most suitable countries in Europe for geothermal development, as a result of large amounts of Miocene extension and associated thermal attenuation of the lithosphere. For geothermal exploration, it is crucial to have an insight into the subsurface temperature distribution. A new thermal model of Hungary is presented extending from the surface down to the lithosphere-asthenosphere boundary (LAB) based on a new stochastic thermal modeling workflow. The model solves the heat equation in steady-state, assuming conduction as the main heat transfer mechanism. At the top and the base, we adopt a constant surface temperature and basal heat flow condition. For the calibration of the model, temperature measurements were collected from the Geothermal Database of Hungary. The model is built up in a layered structure, where each layer has its own thermal properties. The prior thermal properties and basal condition of the model are updated through the ensemble smoother with multiple data assimilation technique. The prior model shows a misfit with the observed temperatures, which is explained fundamentally by transient thermal effects and non-conductive heat transfer. Other misfits can be attributed to a-priori assumptions on thermal properties, boundary conditions, and uncertainty in the model geometry. The updated models considerably improve the prior model, showing a better fit with measured records. The updated models are capable to reproduce the thermal effect of lithospheric extension and the sedimentary infill of the Pannonian Basin. Results indicate that the hottest areas below 3 km are linked to the basement highs surrounded by deep sub-basins of the Great Hungarian Plain. Our models provide an indication on the potential sites for future EGS in Hungary and can serve as an input for geothermal resource assessment.

Multiple Metasomatism beneath the Nógrád–Gömör Volcanic Field (Northern Pannonian Basin) Revealed by Upper Mantle Peridotite Xenoliths

Peridotite xenoliths from the Nograd-Gomor Volcanic Field (NGVF) record the geochemical and structural evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. The xenoliths are divided into a lherzolitic and a wehrlitic series based on their modal composition and textural features. This paper focuses on the lherzolite series and presents petrography and major- and trace-element geochemistry for xenoliths 51 selected from all xenolith-bearing localities of the NGVF. The xenoliths consist of olivine, orthopyroxene, clinopyroxene, spinel ± amphibole. Protogranular textures are most frequently found in the northern part, while to porphyroclastic and equigranular textures are more dominant in the central and southern parts. The xenoliths have undergone varying degrees (<5-30 %) of partial melting with overprinting by different metasomatic processes. Based on heir Mg#, the spinel lherzolite xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 92, whereas Group II has Mg# <89, and significant enrichment of Fe and Mn in olivine and pyroxenes, and of Ti in spinel. Trace-element contents of the xenoliths vary widely, allowing a further division based on LREE enrichment or depletion in pyroxenes. REE patterns of amphiboles match those of clinopyroxenes in each xenolith where they appear, and depict different origins based on their Nb (and other HFSE) content. It is inferred that Nb-poor amphiboles record the oldest metasomatic event, caused by volatile-bearing silicate melts, followed by at least two further metasomatic processes: one that resulted in U-Th-(Nb-Ta)- LREE-enrichment and crystallization of Nb-rich amphibole, affecting selective domains under the entire NGVF, and another evidenced by Fe-Mn-Ti-enrichment which overprinted previous geochemical signatures. We suggest that the metasomatic agents in both cases were basaltic silicate melts, compositionally similar to the host basalts. These melts were generated during the Miocene extension of the Pannonian Basin. The effects of heating and subsequent cooling are evident in significantly different equilibration temperatures.

The link between tectonics and sedimentation in asymmetric extensional basins: Inferences from the study of the Sarajevo-Zenica Basin

The coupled tectonic and depositional history of extensional basins is usually described in terms of stratigraphic sequences linked with the activity of normal faults. This depositional-kinematic interplay is less understood in basins bounded by major extensional detachments or normal fault systems associated with significant exhumation of footwalls. Of particular interest is the link between tectonics and sedimentation during the migration of normal faulting in time and space across the basin. One area where such coupled depositional-kinematic history can be optimally studied is the Late Oligocene - Miocene Sarajevo-Zenica Basin, located in the Dinarides Mountains of Bosnia and Herzegovina. This intra-montane basin recorded Oligocene – Pliocene sedimentation in an endemic and isolated lake environment. We use field kinematic and sedimentological mapping in outcrops correlated with existing local and regional studies to derive a high-resolution evolutionary model of the basin. The novel results demonstrate a close correlation between moments of normal faulting and high-order sedimentological cycles, while the overall extensional basin was filled by a largely uni-directional sediment supply from the neighbouring mountain chain. The migration in time and space of listric NE-dipping normal faults was associated with a gradual shift of the sedimentological environment. Transgressive-regressive cycles reflect sequential displacements on normal faults and their footwall exhumation, defining a new sedimentological model for such basins. This Early - Middle Miocene extension affected the central part of the Dinarides and was associated with the larger opening of the neighbouring Pannonian Basin. The extension was preceded and followed by two phases of contraction. The Oligocene - Early Miocene thrusting took place during the final stages of the Dinarides collision, while the post-Middle Miocene contraction is correlated with the regional indentation of the Adriatic continental unit. This latter phase inverted the extensional basin by reactivating the inherited basal listric detachment.