Molasse Units Research Articles

Tectono-structural framework of the Indo-Myanmar Ranges: Implications for the structural development on the geology of the Rakhine Coastal Region, Myanmar

The Indo-Myanmar Ranges (IMR), known as an accretionary prism, lies between the Kaladan Fault (subduction suture) in the west and Kabaw Fault (the boundary between flysch units and molasse units) in the east is very important to interpret the paleogeographic evolution of the major continents, restoring the paleogeography and plate boundary evolution between the India Plate and West Myanmar Block. The present study explained compression along the Shillong Massif into the IMR with a conceptual model that involves 6-stages of deformation processes. The Kabaw Fault along the eastern rim of Kabaw Valley or the western border of the Kabaw Formation occurs as both an east-dipping thrust and a dextral strike-slip fault. The tectonic model proposed for the IMR is a double-wedge system, in which retro-wedge and east-dipping Kabaw Fault are interrelated to form Nappe of Triassic unit (Thanbaya Formation) with double plunging synclines of Chindwin and Minbu-Salin sub-basins. The Jurassic unit (Kyauksitchaung Formation) is newly found along the eastern base of the IMR as a lithostratigraphic unit and a window structure under the Nappe of the Triassic unit. The alignment of the Kaladan Fault is newly proposed as a suture zone revealed by the window structures of the Oligocene flysch unit under the Nappe of Neogene units on the onshore area, and localities of the mud volcanoes and structural trend of small islands at the offshore area. The formation of circular structures on the Ramree Archipelago is explained by superimposed folding with heterogenous simple shear deformation of the dome and basin structures due to the NW movement of the Broken-Block of IMR colliding with foreland basins including Chittagong-Tripura Fold Belt (CTFB) and Baronga (Boronga) Islands.

Improving Absolute Hypocenter Accuracy With 3D Pg and Sg Body‐Wave Inversion Procedures and Application to Earthquakes in the Central Alps Region

AbstractAccuracy of hypocenter location, in particular focal depth, is a precondition for high‐resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub‐kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter‐velocity inversion procedure restricted to direct, first‐arriving, mainly upper‐crustal Pg and Sg phases. On the basis of synthetic tests and selected ground‐truth events we demonstrate that a sub‐kilometer hypocenter accuracy can be achieved by regional‐scale, three‐dimensional Pg and Sg velocity models combined with dynamic phase selection and a non‐linear location algorithm. The tomographic inversion uses about 60,000 Pg and 30,000 Sg quality‐checked phases of local earthquakes in the Central Alps region. The derived models image the VP and VS structure of the Central Alps upper crust at unprecedented resolution, including small‐scale anomalies such as those caused by Subalpine Molasse units below the Alpine front. The relocation procedure is applied to more than 18,000 earthquakes and the relocated hypocenters reveal previously unrecognized seismogenic structures, for instance in the Swiss Molasse basin south of Bern. The ML 4.6 Urnerboden earthquake of 2017 is used as an example to demonstrate how the derived 3D velocity structure and relocated hypocenters can be jointly interpreted to constrain the lithology hosting upper‐crustal seismicity in the Central Alps.

Active out-of-sequence thrusting in the Molasse Basin constrained by a multidisciplinary approach (Eastern Alps, Austria)

The current tectonic activity at the frontal part of the Eastern Alps is so far barely assessed due to the lack of geologic record and the scarcity of earthquakes in the external units of the fold-thrust belt and its foreland. This paper presents the results of a multidisciplinary study of the Austrian sector of the Imbricated Molasse Basin, near the Alpine floor thrust front, where independent geologic data proves active thrusting at different out-of-sequence thrusts.The integration of structural borehole image interpretation, in-situ stress analysis, geomechanical modeling, 3D seismic interpretation and surface geology shows that out-of-sequence thrusting had a significant impact on the local structural evolution of the frontal part of the Alpine belt. Stress data from boreholes penetrating two of the out-of-sequence thrusts in the Imbricated Molasse revealed stress perturbations indicating that these thrusts are currently active. This finding is supported by 1D geomechanical modeling that proved a thrust regime in the Molasse units and the overlying Rheno-Danubian Flysch.The observation of active thrusting of the imbricated Molasse units is in line with Pleistocene out-of-sequence thrusts in the external tectonic units of the Eastern Alps (Helvetic units and Rheno-Danubian Flysch). There, Pleistocene fluvial terraces of the Mindel glaciation (MIS 12) are vertically displaced by two different out-of-sequence thrusts, while the younger terraces appear undeformed. One of these thrusts is imaged in 3D seismic allowing to calculate a Pleistocene slip rate of 0.33 to 0.43 mm/a from fault dip, terrace ages and vertical offset.In sum, our data proves that Quaternary shortening activated several pre-existing out-of-sequence thrusts in the frontal part of the Eastern Alps. Data, however, does not allow deciding whether Quaternary thrust shortening is a continuous process or occurs episodically, with each shortening episode reactivating different pre-existing structures.

S-wave experiments for the exploration of a deep geothermal carbonate reservoir in the German Molasse Basin

There are many successful geothermal projects that exploit the Upper Jurassic aquifer at 2–3 km depth in the German Molasse Basin. However, up to now, only P-wave seismic exploration has been carried out. In an experiment in the Greater Munich area, we recorded S-waves that were generated by the conventional P-wave seismic survey, using 3C receivers. From this, we built a 3D volume of P- to S-converted (PS) waves using the asymptotic conversion point approach. By combining the P-volume and the resulting PS-seismic volume, we were able to derive the spatial distribution of the vp/vs ratio of both the Molasse overburden and the Upper Jurassic reservoir. We found that the vp/vs ratios for the Molasse units range from 2.0 to 2.3 with a median of 2.15, which is much higher than previously assumed. This raises the depth of hypocenters of induced earthquakes in surrounding geothermal wells. The vp/vs ratios found in the Upper Jurassic vary laterally between 1.5 and 2.2. Since no boreholes are available for verification, we test our results against an independently derived facies classification of the conventional 3D seismic volume and found it correlates well. Furthermore, we see that low vp/vs ratios correlate with high vp and vs velocities. We interpret the latter as dolomitized rocks, which are connected with enhanced permeability in the reservoir. We conclude that 3C registration of conventional P-wave surveys is worthwhile.

Cyclic variations of sulfate and boron concentrations and isotopes in deep groundwaters in the Aquitaine Basin, France

Concentrations and isotope contents of major and trace elements are principal factors in determining the origin of the chemical composition of groundwater. This paper focuses specifically on the use of sulfur and boron isotopes to characterize the origin of cyclic variations in the deep Eocene aquifer in the Aquitaine sedimentary basin (southwest of France). It is part of a multi-layer system mainly composed of sands and sandstone deposits. Groundwater contained in this deep reservoir is known to present stable chemical compositions, allowing its use for various purposes like drinking water, geothermal energy, thermal activity and agricultural irrigation. However, among the dozens of wells exploiting this aquifer and despite the reservoir's substantial depth, variations in sulfate concentration have been identified in a limited area of the reservoir. These fluctuations are cyclic and they seem to be correlated with water level variations due to gas storage activities nearby in the same aquifer. Regular water samplings and analyses of major and trace elements and their isotopes have identified that bore concentration variations are correlated with sulfate variations. A geochemical modelling approach based on water mixes elucidates the causes of these variations in chemical composition, especially the boron and sulfate concentrations and their respective δ11B and δ34SSO4 values. From these numerical results, we identify that different sources explain the variations of boron concentration, one part coming from the silicates alteration (present in sub-layers of the exploited aquifer) and the other part coming from the evaporites alteration (present in the underlying molasse unit). These results also confirm the existence of mass and potentially water transfers between the different sub-layers of the reservoir and with the underlying molasse aquitard, implying new constraints for the future hydrogeologic modelling.

A basin thermal modelling approach to mitigate geothermal energy exploration risks: The St. Gallen case study (eastern Switzerland)

In sedimentary basins geothermal resources may coexist alongside hydrocarbon resources. The latter can represent a risk to geothermal exploration as experienced during the drilling of the deep St. Gallen geothermal well in eastern Switzerland. In this case, the unexpected occurrence of substantial amounts of natural gas, along with other external factors, prevented the continuation of the geothermal project. Therefore, this work aims at resolving the origin of the gas alongside evaluating the processes activating the petroleum system in the study area.In order to characterize the petroleum system of St. Gallen, we performed a basin analysis study aimed at reconstructing the thermal history of the basin and quantifying the main variables controlling the temperature in the basin: the paleo-heat flow and the magnitude and timing of the most relevant erosion events. The findings from this study indicate that the thermal conditions attained in the area were mostly controlled by the deposition of the Molasse units during the Oligocene-Miocene time. Older thermal events could not be detected by organic paleothermometers. An erosion thickness of 1800−2000 m was estimated for the Molasse deposits, related to the uplift of the Northern Alpine foreland, occurring in the area most likely at 8−5 Ma.Results of thermal modelling revealed favourable conditions for the activation of a petroleum system in the St. Gallen area. The source rocks, located most likely in the Permo-Carboniferous grabens, are in the gas window. The model suggests that most of the hydrocarbons generated in the study area migrated northward, because of the southward dipping of the basin. According to the model only a small percentage of hydrocarbons were trapped in reservoirs, mostly located in the uppermost Permo-Carboniferous and basal Mesozoic units. Accumulations were simulated a few hundred meters below the final depth reached by the geothermal St. Gallen GT-1 well. From here the gas likely migrated into the overlying Mesozoic units, the target area of the well, where effectively the model predicts high petroleum saturation levels. This work demonstrates that the basin-scale thermal modelling approach adopted in this study should be incorporated into the feasibility and planning phase of future geothermal exploration campaigns to de-risk the subsurface manifestation of hydrocarbons.

Increased uranium concentrations in ground and surface waters of the Swiss Plateau: A result of uranium accumulation and leaching in the Molasse basin and (ancient) wetlands?

Increased uranium (U) concentrations are found in certain ground and surface waters of the Swiss Plateau. Analysis of more than 100 public fountains revealed that increased 238U concentrations frequently occur close to the interface between the Lower Freshwater Molasse and the Upper Marine Molasse, cropping out in the western part of the Swiss Molasse Basin. Out of these locations, Mont Vully, situated ca. 20 km west of Berne, was studied in detail. As this hill consists of the two aforementioned stratigraphic Molasse units, it represents an ideal case study. Two springs at the northern slopes of Mont Vully exhibit the highest 238U concentrations with more than 300 mBq/L and were thus monitored for almost two years in order to screen possible seasonal variations. Further water samples were collected from spring captures, creeks and drainage pipes. The pipes drain the farmland north of Mont Vully showing 238U concentrations with more than 600 mBq/L. In order to discover the reason for the duplication in concentration, gamma dose rate measurements were accomplished on the farmland, revealing elevated dose rates of up to 160 nSv/h. These are located above ancient pathways of creeks that originated from Mont Vully. At these locations with elevated dose rates, three shallow sediment drill cores were taken and analyzed for their U content. The sediment cores can essentially be divided into three parts: (i) an upper soil with common U concentrations of about 30 Bq/kg 238U, (ii) an U-rich peat horizon with concentrations of up to 500 Bq/kg 238U, and (iii) an impermeable clay unit that acts as an aquitard with again minor 238U concentrations. Radiocarbon dating of the U-rich peat horizon reveals ages younger than 8.1 kyrs. This study suggests that a wetland was formed on top of the impermeable clay layer after the last glaciation during the Holocene. The stream waters with 238U concentrations of more than 300 mBq/L originating from Mont Vully contributed significantly to the water supply for the development of the wetland. Due to the reducing conditions that are present in wetlands, the dissolved U in the incoming streams was reduced and adsorbed onto organic matter. Accordingly, an entrapment for U was generated, persisting for at least 6 kyrs – a sufficient time to accumulate up to 500 Bq/kg. In the course of the last century, numerous wetlands in Switzerland were drained by capturing streams and installing drainage pipes to make the land suitable for agriculture. This kind of melioration was also done at the wetlands north of Mont Vully resulting in a subsequent change in redox conditions within the soil. The solubility of U depends on its oxidation state and U can be oxidized by oxygen-rich rainwater. The rainwater leached the U adsorbed on the peat yielding 238U concentrations of more than 600 mBq/L. Consequently, the duplication of 238U concentrations in the drainage water as compared to the original concentration that creeks brought into the ancient wetlands has been clarified during this study. Twenty of the analyzed public fountains in the Swiss Plateau exhibited a 238U concentration of more than 50 mBq/L. All of them could have contributed to the formation of a wetland after the last glaciation, which leads to the assumption that the situation at Mont Vully is not a singularity in the Swiss Plateau.

Thermal history of the westernmost Eastern Alps (Penninic Rhenodanubian Flysch nappes, Helvetic nappes, and Subalpine Molasse thrust sheets)

The frontal part of the westernmost Eastern Alps comprises from top to bottom of the Austroalpine and Penninic nappes, Ultrahelvetic slices, and two Helvetic thrust sheets, thrust upon the northern Alpine Molasse Basin. The thermal evolution of the Penninic Rhenodanubian Flysch nappes, the Helvetic nappes, and the allochthonous part of the Alpine Molasse Basin is constrained by vitrinite reflectance measurements and apatite fission track dating and implemented in a tectonic evolution scheme. Within the Helvetic nappes, vitrinite reflectance increases regionally from north to south and stratigraphically from the Campanian–Maastrichtian Wang Formation to the Toarcian Mols Member. Apatite fission track ages from Penninic and Subalpine Molasse units are consistently younger than the deposition age. They indicate therefore a post-depositional thermal overprint exceeding approximately 120 °C, the upper temperature limit of the apatite partial annealing zone. 1D thermal modelling suggests that the Penninic nappes attained deepest burial between the latest Cretaceous and Early Palaeocene with the Penninic basal thrust being located at approximately 8 km in the north compared to approximately 12 km in the south. Deepest burial of the upper Helvetic nappe occurred between the latest Eocene and Early Miocene. Its base was buried down to approximately 10.5 km in the north compared to 11.5 km in the south. Exhumation of the entire nappe stack started in the Early to Middle Miocene. For both Penninic and Helvetic models, a heatflow minimum during the Cenozoic deformation (max. 27–32 mW/m2), followed by an increase from the Middle Miocene onwards (up to 60 mW/m2), was assumed.

The last erosional stage of the Molasse Basin and the Alps

We present a synoptic overview of the Miocene-present development of the northern Alpine foreland basin (Molasse Basin), with special attention to the pattern of surface erosion and sediment discharge in the Alps. Erosion of the Molasse Basin started at the same time that the rivers originating in the Central Alps were deflected toward the Bresse Graben, which formed part of the European Cenozoic rift system. This change in the drainage direction decreased the distance to the marine base level by approximately 1,000 km, which in turn decreased the average topographic elevation in the Molasse Basin by at least 200 m. Isostatic adjustment to erosional unloading required ca. 1,000 m of erosion to account for this inferred topographic lowering. A further inference is that the resulting increase in the sediment discharge at the Miocene–Pliocene boundary reflects the recycling of Molasse units. We consider that erosion of the Molasse Basin occurred in response to a shift in the drainage direction rather than because of a change in paleoclimate. Climate left an imprint on the Alpine landscape, but presumably not before the beginning of glaciation at the Pliocene–Pleistocene boundary. Similar to the northern Alpine foreland, we do not see a strong climatic fingerprint on the pattern or rates of exhumation of the External Massifs. In particular, the initiation and acceleration of imbrication and antiformal stacking of the foreland crust can be considered solely as a response to the convergence of Adria and Europe, irrespective of erosion rates. However, the recycling of the Molasse deposits since 5 Ma and the associated reduction of the loads in the foreland could have activated basement thrusts beneath the Molasse Basin in order to restore a critical wedge. In conclusion, we see the need for a more careful consideration of both tectonic and climatic forcing on the development of the Alps and the adjacent Molasse Basin.

Lower Miocene structural evolution of the central Vienna Basin (Austria)

The tectonic evolution of the Vienna Basin overlying the Alpine-Carpathian fold and thrust belt includes two stages of distinct basin subsidence and deformation. The earlier phase contemporaneous with thrusting of the Alpine-Carpathian floor thrust is related to the formation of a wedge-top basin (“piggy-back”), which was connected to the evolving foreland basin (Lower Miocene; c. 18.5–16 Ma). This stage is followed by the formation of a pull-apart basin (Middle to Upper Miocene; c. 16–8 Ma). Sediments of the latter unconformably overly wedge-top basin strata and protected them against erosion. Analyses of subcrop data from the central part of the basin focus on the complex stratal geometries and tectonics of the Lower Miocene wedge-top basin. During that period the overriding allochthon is characterised by syntectonic subsidence and sedimentation, out-of-sequence fold-thrusting, normal faulting and sinistral wrenching. All types of faults are dated as Lower Miocene by the occurrence of growth strata and the age of overlying post-tectonic sediments related to the pull-apart basin stage. Faults mapped in 3D seismic are compared to faults along the SW margin of the Vienna Basin. They prove that complex Lower Miocene deformation occurred in large parts of the Austroalpine overriding nappes concurrent with thrusting at the Alpine floor thrust and in the Molasse unit. The structural analysis of 3D seismic data shows that Lower Miocene sediments of the Ottnangian and Karpatian stage unconformable overly the Alpine-Carpathian thrust nappes with a thickness of up to 1500 m. Geometries include onlaps to the basin floor, internal unconformities and toplaps to the overlying Middle Miocene sediments. These toplaps are the results of tilting and subsequent erosion of the sediment pile by out-of-sequence thrusting in the central part at the Karpatian–Badenian transition (c. 16.3 Ma) prior to the onset of pull-apart deformation.

New constraints on the India–Asia collision: the Lower Miocene Gangrinboche conglomerates, Yarlung Tsangpo suture zone, SE Tibet

Lower Miocene conglomerates crop out along the length of the Yarlung Tsangpo suture zone on the southern margin of the Lhasa terrane. These conglomerates, known by various local names, are correlated herein as the Gangrinboche conglomerates. All units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the Lhasa terrane is ubiquitous. At most localities the tops of sections are either removed by erosion or truncated by north-directed thrusts. These conglomeratic molasse units developed in response to the India/Asia collision and record aspects of its development. In all units initial clast derivation was from the Lhasa terrane on the northern margin of the Yarlung Tsangpo suture zone. Up-section the first appearance of clasts derived from terranes within the suture zone and the northern margin of India, all of which lie to the south of any outcrops of Gangrinboche conglomerates, is observed. Although these units were previously thought to be Eocene, analysis of fossil and structural constraints indicates Early Miocene deposition. As development of the Gangrinboche conglomerates records a significant phase in the evolution of the India–Asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models.

New insights into the evolution of the Yarlung Tsangpo suture zone, Xizang (Tibet), China

Detailed investigations along the Yarlung Tsangpo suture zone, Tibet result in the following conclusions. (1) Arc, forearc, and subduction complex elements of at least one intra-oceanic island arc which once lay within Tethys and accreted to India prior to its collision with Asia are preserved within the suture. (2) Collision-related tectonic melange is widespread. (3) Temporallydistinct conglomeratic molasse units were developed; they are related to different collision events. (4) Improved radiolarian fossil data place constraints on timing of the development of intra-oceanic terranes and melange formation. (5) A simple unconformable contact occurs between Upper Oligocene–Lower Miocene conglomerates and the southern Lhasa terrane; the socalled south-directed Gangdese thrust does not exist. These new results suggest the former existence of an intra-oceanic subduction system within Tethys and indicate that considerable revision of existing models for the evolution of the Yarlung Tsangpo suture zone is required.

Finite element models of stress in eastern Austria during the early Miocene

The transition zone between the Alps and the Carpathian mountains in eastern Austria is characterized by a change of strike of the tectonic units from E-W to SW-NE, resulting in the formation of the Alpine-Carpathian arc. This study investigates the effect that a depression of the eastern edge of the Bohemian Massif, filled with Mesozoic and Tertiary sediments, may have had on the advancing Alpine nappes in the early Miocene. Two-dimensional linear-elastic plane stress finite element models were used to simulate the patterns of stress, faults and seismicity resulting from such a situation. Three different materials were used in the reconstruction of the geological-tectonic setting during the early Miocene: a limestone unit striking east-west, a molasse unit, and a crystalline unit. Whereas the northward movement of the limestone unit in the west is restricted by the crystalline unit, which acts as a buttress, in the east the movement is only slightly retarded by the molasse unit. Calculations show that following such a situation in the early Miocene, pre-existing zones of weakness in the crystalline unit can be reactivated. East of the point where northward movement of the limestone material is no longer restricted, in the area of the present Vienna Basin, the development of a pull-apart type basin is suggested. The main faults for this process are probably reactivated basement faults. A similar basin is predicted 200 km to the east in the region of the West Danube Basin. The results also predict the establishment of a zone of seismic activity, which seems to be reflected by the pattern of seismicity observed in eastern Austria today. Overall, the models show that the present seismic and tectonic patterns can be traced back to the events in the early Miocene, when Alpine nappes draped around the Bohemian Massif.

Middle Paleozoic to Cenozoic geology and tectonic evolution of the northwestern Himalaya

Previously published data and data collected by the authors from the northwestern Himalaya are summarized and assessed. The account focuses on Kohistan and the Karakorum Range in Pakistan, and on Ladakh and the Zanskar Range in India. Five palaeotectonic units are recognized. From south to north these are: 1. (1) Palaeozoic-Eocene continental shelf sequence on the northern margin of the Indian continent; 2. (2) a Triassic-Jurassic continental slope sequence: the Lamayuru unit of the Indus zone and of the Spongtang klippe; 3. (3) the Cretaceous-Tertiary Kohistan-Ladakh magmatic arc, which includes the Kokistan sequence, the Ladakh batholith, the Dras unit, the Spongtang klippe, the Indus molasse unit and the Shyok zone; 4. (4) the Karakorum batholith: a late Tertiary zone of granitoid magmatism and metamorphism; 5. (5) a PalaeozoicMesozoic continental shelf and slope sequence deposited on the southern margin of an Asian continent. The following sequence of events is tentatively proposed: 1. (1) Permian-Carboniferous continental rifting; 2. (2) Triassic-Jurassic collapse of continental shelf, and formation of deep (oceanic?) basin; 3. (3) Cretaceous formation of Kohistan-Ladakh magmatic arc, in response to subduction from the south as ocean closed; 4. (4) late Cretaceous-Eocene emplacement of Kohistan arc and of thrust sheets of Indus zone (magmatic arc units and Lamayuru unit) southwards onto Indian continental margin, as India approached and collided with magmatic arc; 5. (5) silicic phases of Ladakh batholith (Palaeocene-Eocene) and Karakorum batholith (Miocene) may be related to subduction of oceanic basin between magmatic arc and Asian margin. Alternatively, they are late- and post-collisional products; 6. (6) the Shyok zone, between the Ladakh and Karakorum batholiths, was emplaced to the south in Oligocene or Miocene time; 7. (7) final collision between India, the magmatic arc and Asia in Oligocene-Miocene time was followed by continued crustal shortening which disrupted thrust units.

Carpathian Foreland Fold and Thrust Belt and Its Relation to Pannonian and Other Basins

The foreland fold and thrust belt of the Carpathians can be traced from Austria through the western and eastern Carpathians to the south Carpathian bend in Romania where most of its structural units plunge beneath younger Pliocene-Pleistocene cover. Folds in the youngest rocks at the south Carpathian bend continue westward, until all surface expression disappears before reaching the Danube River. The fold and thrust belt is flanked by elements of the European, Russian, and Moesian cratonal areas which are overlain by a variable width and thickness of foredeep deposits that are partly involved within the external fold and thrust belt. Internal to the fold and thrust belt are older parts of the Carpathian orogene formed mainly on continental crust. These older structures have significant differences in evolution between the western and eastern Carpathians. The foreland fold and thrust belt consists predominantly of flysch in its inner parts and molasse in its outer parts. These sedimentary rocks are cut into a series of thrust sheets that have moved relatively toward the craton. Oldest flysch units are Late Jurassic and the youngest units are Miocene. Some of the flysch units of the inner part of the belt may have been underlain by oceanic crust, whereas the younger, more external molasse units were underlain by continental crust. Timing of thrusting is only well-constrained locally, but suggests that deformation in the western Carpathians developed during Oligocene to Miocene time. In the eastern Carpathians, structural activity continued into the Pliocene, and deep earthquake activity at Vrancea suggests subduction. Deformation is sti l locally active. A volcanic arc of Miocene to Holocene age lies internal to the fold and thrust belt. Magmatic activity appears to have migrated externally and eastward in time. Contemporaneous with thrusting was the development of Neogene basins in the intra-Carpathian region. These basins exhibit a two-phase subsidence history (except for the Pannonian and Transylvanian basins)--fast initial subsidence, followed by a period of slower linear subsidence. The Pannonian basin shows only a reasonably fast linear subsidence. Structural and thermal models indicate the basins were formed by about 100% stretching. The convex east loop of the Carpathian foreland fold and thrust belt is a result of the collision of continental fragments against a recess in the combined European-Moesian plate. In this region, subduction or downbending of the subducted slab results in an extensional stress field, suggesting that thrusting and apparent compression along the mountain belt are only thin-skinned, superficial effects due to the inability to subduct light upper crustal material and detachment of the crust from the underlying lithosphere. The dominant forces seem to be those acting on the downgoing slab, which produce shortening along the plate boundary, and plate geometry that inhibits rigid plate convergence.