Low-viscosity Layer Research Articles

Direct Numerical Simulations of Three-Layer Viscosity-Stratified Flow

Two-dimensional simulations of flinstability at the interface of a three-layer, density-matched, viscosity-stratifi ed Poiseuille fl ow are performed using a front-tracking/fi nite difference method. This is an extension of the study for the stability of two-layer viscosity-stratifi ed fl ow of Cao et al., Int. J. Multiphase Flow, 30, 1485-1508 (2004). We present results for large-amplitude non-linear evolution of the interface for varying viscos- ity ratio m, Weber number We, and phase difference between the perturbations of the two interfaces. Strong non-linear behaviour is observed for relatively large initial perturbation amplitude. The higher viscosity fl uid is drawn out as a fi nger that penetrates into the lower viscosity layer. The fi nger originates at the crest of the perturbation at the interface. The simulated interface shape compares well with previously reported experiments. Increasing interfacial tension retards the growth rate of the interface as expected, whereas increasing the viscosity ratio enhances it. The sinuous instability appears to evolve faster than the varicose one. For certain fl ow parameters the high-viscosity fi nger displays a bulbous tip, which is also seen in our previously conducted experiments and two-layer results, although it is less pronounced. The low-viscosity intruding fi nger does not display this curious bulbous tip. Drop formation is precluded by the two-dimensional nature of the calculations.

High-harmonic geoid signatures related to glacial isostatic adjustment and their detectability by GOCE

Abstract The Earth’s asthenosphere and lower continental crust can regionally have viscosities that are one to several orders of magnitude smaller than typical mantle viscosities. As a consequence, such shallow low-viscosity layers could induce high-harmonic (spherical harmonics 50–200) gravity and geoid anomalies due to remaining isostasy deviations following Late-Pleistocene glacial isostatic adjustment (GIA). Such high-harmonic geoid and gravity signatures would depend also on the detailed ice and meltwater loading distribution and history. ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission, planned for launch in Summer 2008, is designed to map the quasi-static geoid with centimeter accuracy and gravity anomalies with milligal accuracy at a resolution of 100 km or better. This might offer the possibility of detecting gravity and geoid effects of low-viscosity shallow earth layers and differences of the effects of various Pleistocene ice decay scenarios. For example, our predictions show that for a typical low-viscosity crustal zone GOCE should be able to discern differences between ice-load histories down to length scales of about 150 km. One of the major challenges in interpreting such high-harmonic, regional-scale, geoid signatures in GOCE solutions will be to discriminate GIA-signatures from various other solid-earth contributions. It might be of help here that the high-harmonic geoid and gravity signatures form quite characteristic 2D patterns, depending on both ice load and low-viscosity zone model parameters.

Modelling a flow of a highly viscous liquid with the boundary layer of low viscosity in an extruder

Features of the flow of a highly viscous liquid in an extruder with boundary layer of low viscosity liquid at the cylinder wall and at the endless screw surface are considered. The liquid flow is modeled under conditions of isothermal simple shift using cylindrical model of a screw canal. The profiles of rates for the stream of a highly viscous liquid with the ring boundary layer of low-viscous liquid at the cylinder wall and at screw in the both presence and absence of a counter pressure are calculated. Consideration of computer calculations shows that increase in the flow rate is possible when the counter pressure tends to zero and the low-viscous liquid forms a ring boundary layer at the cylinder wall.

Core-annular flow through a horizontal pipe: Hydrodynamic counterbalancing of buoyancy force on core

A theoretical investigation has been made of core-annular flow: the flow of a high-viscosity liquid core surrounded by a low-viscosity liquid annular layer through a horizontal pipe. Special attention is paid to the question of how the buoyancy force on the core, caused by a density difference between the core and the annular layer, is counterbalanced. From earlier studies it is known that at the interface between the annular layer and the core waves are present that move with respect to the pipe wall. In the present study the core is assumed to consist of a solid center surrounded by a high-viscosity liquid layer. Using hydrodynamic lubrication theory (taking into account the flow in the low-viscosity liquid annular layer and in the high-viscosity liquid core layer) the development of the interfacial waves is calculated. They generate pressure variations in the core layer and annular layer that can cause a net force on the core. Steady eccentric core-annular flow is found to be possible.

Channel flow and the Himalayan–Tibetan orogen: a critical review

The movement of a low-viscosity crustal layer in response to topographic loading provides a potential mechanism for (1) eastward flow of the Asian lower crust causing the peripheral growth of the Tibetan Plateau and (2) southward flow of the Indian middle crust to be extruded along the Himalayan topographic front. Thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface. Isotopic constraints on the timing of fault movement, anatexis and thermobarometric evolution of the exhumed garnet- to sillimanite-grade metasedimentary rocks support mid-crustal channel flow during the Early to Mid-Miocene. Exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface, defined by the South Tibetan Detachment System, and strain softening along the base, bounded by the Main Central Thrust. Neotectonic extrusion, bounded by brittle Quaternary faults south of the Main Central Thrust, is positively correlated with the spatial distribution of precipitation across a north–south transect, suggesting climate–tectonic linkage over a million-year time scale. A proposed orogen-wide eastward increase in extrusion rate over 20 Ma reflects current precipitation patterns but climate–tectonic linkage over this time scale remains equivocal.

Rheology of surface granular flows

Surface granular flow, comprising granular material flowing on the surface of a heap of the same material, occurs in several industrial and natural systems. The rheology of such a flow was investigated by means of measurements of velocity and number-density profiles in a quasi-two-dimensional rotating cylinder, half-filled with a model granular material – monosize spherical stainless-steel particles. The measurements were made at the centre of the cylinder, where the flow is fully developed, using streakline photography and image analysis. The stress profile was computed from the number-density profile using a force balance which takes into account wall friction. Mean-velocity and root-mean-square (r.m.s.)-velocity profiles are reported for different particle sizes and cylinder rotation speeds. The profiles for the mean velocity superimpose when distance is scaled by the particle diameterdand velocity by a characteristic shear rate$\dot{\gamma}_C = [g\sin(\beta_m-\beta_s)/d\cos\beta_s]^{1/2}$and the particle diameter, where βmis the maximum dynamic angle of repose and βsis the static angle of repose. The maximum dynamic angle of repose is found to vary with the local flow rate. The scaling is also found to work for the r.m.s. velocity profiles. The mean velocity is found to decay exponentially with depth in the bed, with decay length λ = 1.1d. The r.m.s. velocity shows similar behaviour but with λ = 1.7d. The r.m.s. velocity profile shows two regimes: near the free surface the r.m.s. velocity is nearly constant and below a transition point it decays linearly with depth. The shear rate, obtained by numerical differentiation of the velocity profile, is not constant anywhere in the layer and has a maximum which occurs at the same depth as the transition in the r.m.s. velocity profile. Above the transition point the velocity distributions are Gaussian and below the transition point the velocity distributions gradually approach a Poisson distribution. The shear stress increases roughly linearly with depth. The variation in the apparent viscosity η with r.m.s. velocityushows a relatively sharp transition at the shear-rate maximum, and in the region below this point the apparent viscosity η ∼u−1.5. The measurements indicate that the flow comprises two layers: an upper low-viscosity layer with a nearly constant r.m.s. velocity and a lower layer of increasing viscosity with a decreasing r.m.s. velocity. The thickness of the upper layer depends on the local flow rate and is independent of particle diameter while the reverse is found to hold for the lower-layer thickness. The experimental data is compared with the predictions of three models for granular flow.

Analytical approach for the toroidal relaxation of viscoelastic earth

This paper is concerned with post-seismic toroidal deformation in a spherically symmetric, non-rotating, linear-viscoelastic, isotropic Maxwell earth model. Analytical expressions for characteristic relaxation times and relaxation strengths are found for viscoelastic toroidal deformation, associated with surface tangential stress, when there are two to five layers between the core–mantle boundary and Earth's surface. The multilayered models can include lithosphere, asthenosphere, upper and lower mantles and even low-viscosity ductile layer in the lithosphere. The analytical approach is self-consistent in that the Heaviside isostatic solution agrees with fluid limit. The analytical solution can be used for high-precision simulation of the toroidal relaxation in five-layer earths and the results can also be considered as a benchmark for numerical methods. Analytical solution gives only stable decaying modes—unstable mode, conjugate complex mode and modes of relevant poles with orders larger than 1, are all excluded, and the total number of modes is found to be just the number of viscoelastic layers between the core–mantle boundary and Earth's surface—however, any elastic layer between two viscoelastic layers is also counted. This confirms previous finding where numerical method (i.e. propagator matrix method) is used. We have studied the relaxation times of a lot of models and found the propagator matrix method to agree very well with those from analytical results. In addition, the asthenosphere and lithospheric ductile layer are found to have large effects on the amplitude of post-seismic deformation. This also confirms the findings of previous works.

Fold amplification rates and dominant wavelength selection in multilayer stacks

A combination of thin- and thick-plate theories, and finite element models is used to systematically analyze folding in multilayer stacks. We show that if the interlayer spacing is large, individual layers fold as single layers, if the spacing is small the entire stack folds as one effective single layer. In between, a third folding mode exists that is characterised by a dominant wavelength that scales with n 1/3, irrespective of total number of layers, n. The maximum growth rates in the true multilayer-folding mode are higher than the corresponding single layer growth rates, increase with n and are bounded by a saturation value that is directly proportional to the viscosity contrast. This growth rate saturation as well as the applicability of the true multilayer-folding mode with respect to interlayer spacing can be explained by the normal and inverse contact strain theory. The true multilayer-folding mode is expected to be the most frequent mode in nature, because it exhibits the highest growth rates and has a relatively large applicability range with respect to interlayer spacing. The increased growth rates in multilayer folding are especially important for systems where the corresponding single layer values are not sufficient to drive the folding instability, such as folding in low-viscosity contrast layers and detachment folding.

Observation of a low-viscosity interface between immiscible polymer layers

X-ray photon correlation spectroscopy was employed in a surface standing wave geometry in order to resolve the thermally driven in-plane dynamics at both the surface/vacuum (top) and polymer/polymer (bottom) interfaces of a thin polystyrene (PS) film on top of Poly(4-bromo styrene) (PBrS) and supported on a Si substrate. The top vacuum interface shows two relaxation modes: one fast and one slow, while the buried polymer-polymer interface shows a single slow mode. The slow mode of the top interface is similar in magnitude and wave vector dependence to the single mode of the buried interface. The dynamics are consistent with a low-viscosity mixed layer between the PS and PBrS and coupling of the capillary wave fluctuations between this layer and the PS.

Mechanism for the void formation in the bright spot of a fiber fuse

A simple mechanism for the formation of a chain of voids (cavities) behind the spot of the laser optical discharge in an optical fiber is proposed. This mechanism is related to the motion of liquid in the opposite direction with respect to the propagation direction of the laser radiation between the isotherms that bound the charge-separation region and low-viscosity region. The motion is caused by the extrusion of the low-viscosity layer owing to the excessive pressure induced by the charge repulsion. The void shape (a bullet moving along the laser radiation) is determined by the isotherm sharpness. The bridge formation temperature (about 5000 K) is estimated based on the comparison of the extrusion velocity of the low-viscosity layer and the velocity of the bright spot.

A simple model of high Prandtl and high Rayleigh number convection bounded by thin low-viscosity layers

SUMMARY Motivated by recent numerical results on convection in the Earth’s mantle in the presence of a low-viscosity zone, an analytical model is derived for 2-D steady convection with symmetric low-viscosity layers at the upper and lower boundaries. The asymptotic limits of high Rayleigh and high Prandtl number are assumed. The low- viscosity layers carry with minimal dissipation most of the horizontal component of the convection flow and thereby increase the efficiency of the convective heat transport. A Nusselt number (Nu)‐Rayleigh number (R) relationship of the form Nu ∼R 1/3 (�/δ ) 2/3 is obtained for an aspect ratio 2� of the convection cell of order unity or less. Here δ denotes the fraction of the layer depth occupied by each of the low

Izu detachment hypothesis: A proposal of a unified cause for the Miyake-Kozu event and the Tokai slow event

Abstract Based on the fact that interseismic deformation of collision zones is generally described by slip along a detachment at depth, I attempt to interpret the deformation of the Izu collision zone in terms of a detachment model. The systematic deviation of the GPS velocities of the Izu Peninsula (Nov. 1998–June 2000) from the Philippine Sea-Eurasian relative plate motions is fitted by the slip on the detachment at a depth of 15–20 km with a rate of 3 cm/yr. On June 26, 2000, seismo-magmatic activity that started near Miyakejima expanded NW by 20 km close to Kozushima in association with dike intrusion over a few months. The horizontal movements associated with this event, however, spread over wide areas in central Honshu. Simple dike intrusion models cannot explain these movements. To explain these, I hypothesize that a 20 cm of rapid slip occurred on the detachment at the time of this event. The abnormal crustal movements in the Tokai-central Honshu-Kanto region then started after the event. I propose that they represent delayed diffusive transfer of the slip on the detachment over surrounding low viscosity layers, such as nearby rupture zones of great earthquakes.

Influence of a low-viscosity layer between rigid inclusion and viscous matrix on inclusion rotation and matrix flow: A numerical study

We have used 2-D finite element modelling to investigate the influence of a permanent low-viscosity layer between matrix and inclusion on matrix flow and inclusion rotation under viscous simple shear flow. Rigid inclusions of different shape (circle, square, ellipse, lozenge, rectangle and skewed rectangles) and aspect ratio ( R) were used. The calculated matrix flow pattern is neither bow tie nor eye-shaped. It is a new flow pattern that we call cat eyes-shaped, which is characterized by: (i) straight streamlines that slightly bend inwards at the inclusion's crests; (ii) elongate eye-shaped streamlines on each side of the inclusion; (iii) stagnation points in the centre of the eyes; (iv) absence of closed streamlines surrounding the inclusion; (v) changes in flow configuration with inclusion orientation; the lines of flow reversal bend and tilt, closed streamline circuits may disappear, and streamlines may bend outwards at the inclusion's crests. Concerning inclusion rotation, the numerical results show that: (i) a low-viscosity layer (LVL) makes inclusions with R = 1 rotate synthetically, but the rotation rate depends upon shape (circle or square) and orientation. Therefore, shape matters in the slipping mode. (ii) All studied shapes with R > 1 rotate antithetically when starting with the greatest principal axis ( e 1) parallel to the shear direction ( ϕ = 0°); (iii) rotation is limited because there is a stable equilibrium orientation ( ϕ se) for all studied shapes with R > 1. (iii) There is also an unstable equilibrium orientation ( ϕ ue), and both ϕ se and ϕ ue depend upon inclusion's R and shape. The present numerical results closely agree with previous results of analogue experiments with a permanent low viscosity interface. Only minor deviations related with small shape differences were detected.

Three-dimensional rheological structure of the lithosphere in the Ordos block and its adjacent area

The 3-D rheological structure of the lithosphere in the Ordos block and its adjacent area (32°–43°N, 102°–116°E) is obtained using iterative and averaging methods, based on the seismic velocity structure, thermal structure and composition of the lithosphere. The thermal structure is obtained from the newly published surface heat flow data sets. The brittle fracture mechanism is taken into account when the rheological structure is calculated. The strain rate obtained from GPS observation in the area under study is used to calculate the strength and the viscosity of the lithosphere. The results show that there are low-viscosity layers at the bottom of the upper crust, the lower crust and the bottom of the lithosphere both under the Ordos block and its adjacent regions, and the viscosity varies horizontally. The viscosity in grabens surrounding the Ordos block is one to two orders of magnitude lower than that in the Ordos block. The results also show that the rheological structure coincides with the major geological structure, below the major fault systems the viscosity is relatively low. The earthquakes with Ms≥ 4.0 occurred along the large active faults. Most of the hypocentres of the earthquakes are in the upper and middle crust with high viscosity. The maximum depth of the earthquake is above the viscosity contour line of 1022 Pa s. Numerical studies show that the active geological structures must be taken into account when the 3-D rheological structure is used for geodynamics study.

On the effect of a low viscosity asthenosphere on the temporal change of the geoid—A challenge for future gravity missions

New satellite technology to measure changes in the Earth’s gravity field gives new possibilities to detect layers of low viscosity inside the Earth. We used density models for the Earth mantle based on slab history as well as on tomography and fitted the viscosity by comparison of predicted gravity to the new CHAMP gravity model. We first confirm that the fit to the observed geoid is insensitive to the presence of a low viscosity anomaly in the upper mantle as long as the layer is thin ( ∼ 200 km) and the viscosity reduction is less than two orders of magnitude. Then we investigated the temporal change in geoid by comparing two stages of slablet sinking based on subduction history or by advection of tomography derived densities and compared the spectra of the geoid change for cases with and without a low viscosity layer, but about equal fit to the observed geoid. The presence of a low viscosity layer causes relaxation at smaller wavelength and thus leads to a spectrum with relatively stronger power in higher modes and a peak around degrees 5 and 6. Comparing the spectra to the expected degree resolution for GRACE data for a 5 years mission duration shows a weak possibility to detect changes in the Earth’s gravity field due to large scale mantle circulation, provided that other causes of geoid changes can be taken into account with sufficient accuracy. A discrimination between the two viscosity cases, however, demands a new generation of gravity field observing satellites.

Effect of time on tensile bond strength of resin cement bonded to dentine and low-viscosity composite

The purpose of this study was to evaluate the tensile bond strength (TBS) of Panavia F resin cement (PF) applied on dentine pre-treated with ED Primer (ED) and Clearfil Liner Bond 2V (CLB) coated with a layer of low-viscosity composite Protect Liner F (PLF) at 10 min, 24 h and 12 months after curing. The labial surfaces of 60 bovine lower incisors were ground to obtain a flat dentine surface, allowing a demarcation of a 4.0 mm-diameter area with adhesive tape. The teeth were randomly divided in six groups; ED was applied in groups A I, A II and A III and CLB was applied, followed by PLF, in groups B I, B II and B III. A resin composite rod with a wire loop was luted directly to the prepared surface of each group with PF. The specimens of groups A I and B I were submitted to TBS test after 10 min. Groups A II and B II were submitted to TBS test after 24 h storage and groups A III and B III were submitted to TBS test after 12 months storage. Each specimen was inspected by SEM and classified according to the failure mode. Additionally, two representative specimens of each failure mode were sectioned for a composite/dentine interface SEM evaluation. No significant statistical differences were observed among the groups at 10 min and 24 h. Groups A III and B III presented the lowest TBS values (p<0.05) after 12 months storage. PF on resin-coated dentin (PLF) showed the highest TBS values and was statistically different to PF on dentine for all the groups. The fracture pattern was generally cohesive on the adhesive/hybrid layer for groups A I, A II and A III and cohesive on composite resin for B I, B II and B III. The use of a less hydrophilic self-etching system to pre-treat dentine, coating with a low-viscosity composite layer prior luting with resin cement, may provide a protection of the hybridised complex, allowing a dentine seal during the 12 months storage period.

Sensitivity of glacial isostatic adjustment models with shallow low-viscosity earth layers to the ice-load history in relation to the performance of GOCE and GRACE

Abstract The GOCE satellite mission, which is planned by ESA for launch in August 2006, is designed to map the static global gravity field with centimeter accuracy in geoid height at 100 km or better resolution. Such a global high resolution gravity field might be able to constrain properties of shallow low-viscosity zones (LVZs) using glacial isostatic adjustment (GIA) models. In (L.L.A. Vermeersen, The potential of GOCE in constaining the structure of the crust and lithosphere from post-gracial rebound, Space Sci. Rev. 108 (2003) 105–113.) and (W. van der Wal, H.H.A. Schotman, L.L.A. Vermeersen, Geoid heights due to a crustal low viscosity zone in glacial isostatic adjustment modeling; a sensitivity analysis for GOCE, Geophys. Res. Lett. 31 (2004) 10.1029/2003GL019139.) it is shown that a crustal low-viscosity zone (CLVZ) introduces variations in geoid height up to several decimeters with spatial scales down to hundred kilometers underneath and just outside formerly glaciated areas. In (W. van der Wal, H.H.A. Schotman, L.L.A. Vermeersen, Geoid heights due to a crustal low viscosity zone in glacial isostatic adjustment modeling; a sensitivity analysis for GOCE, Geophys. Res. Lett. 31 (2004) 10.1029/2003GL019139.) it is shown that the response is sensitive to both changes in the properties of the CLVZ and the Late Pleistocene ice-load history. In this study we quantify the sensitivity to ice-load history, and investigate the effect of an asthenospheric low-viscosity zone (ALVZ) just below the lithosphere. We show, using spherical harmonic degree amplitudes, that GOCE is predicted to be sensitive to differences in the load history up to degree 130 for a CLVZ and degree 70 for an ALVZ. The sensitivity of GRACE, using the realized performance over a 111-day period (GGM01S, (B.D. Tapley, S. Bettadpur, M. Watkins, C. Reigber, The gravity recovery and climate experiment: Mission overview and early results, Geophys. Res Lett. 31 (2004) 10.1029/2004GL019920.)) is limited to lower degrees. This means that GOCE, and to a lesser degree GRACE, could provide information on the ice-load history in the presence of an LVZ, provided that our knowledge of the earth mode is sufficient. To extract information about the LVZ from the degree amplitudes, we show that it is possible to largely remove the influence of the ice-load history and thus compute spectral signatures for different properties of the LVZ. We focus on the continental shelf-areas of western Europe to show the sensitivity of estimates of present-day sea level change to the inclusion of an LVZ.

Vibrational Dynamics of a Centrifuged Fluid Layer

The dynamics of a low-viscosity fluid layer inside a rotating cylinder under transverse translational vibration relative to the rotation axis is investigated experimentally. A novel vibrational effect, the generation of intense azimuthal fluid flows with velocities comparable with the cavity rotation velocity, is revealed. The structure and intensity of the vibrational flows and the flow transformation with variation of the determining dimensionless parameters (frequency and vibrational acceleration) are studied.

Injection molding of long sisal fiber-reinforced polypropylene: Effects of compatibilizer concentration and viscosity on fiber adhesion and thermal degradation

A two-step process was used to obtain long sisal fiber-polypropylene (SF/PP)–reinforced thermoplastic composites, using maleic anhydride grafted polypropylene (MA-g-PP) as a compatibilizer. At a first stage, modified polypropylenes (mPP) were used for an extrusion impregnation process, for the preparation of composite pellets containing about 70 wt% of SF. SF/mPP pellets with a large aspect ratio were prepared by continuous extrusion impregnation of a continuous SF yarn, using a single screw extruder and an adequate impregnation die. The mPP used were MA-g-PP and regular polypropylene (PP), modified by reaction with different amounts of an organic peroxide. The composite pellets were thus dry blended with regular PP pellets in an injection machine hopper, and injection molded to obtain composite tensile specimens with a minimum quantity of modified polypropylene, minimum fiber breakage and thermal degradation, and excellent mechanical properties. It is shown that the fiber breakage is reduced to a minimum, even for recycled composites, due to the presence of the low-viscosity polymer layer wetting the SF fibers. The bulk composite effective viscosity and the fiber breakage extent and thermal degradation during the injection-molding step are found to be closely related. Blending with much less expensive mPP at the impregnation stage optimizes the amount of expensive MA-g-PP. POLYM. ENG. SCI., 45:613–621, 2005. © 2005 Society of Plastics Engineers

Lithospheric-scale analogue modelling of collision zones with a pre-existing weak zone

Abstract Lithospheric-scale analogue experiments have been conducted to investigate the influence of strength heterogeneities on the distribution and mode of crustal-scale deformation, on the resulting geometry of the deformed area, and on its topographic expression. Strength heterogeneities were incorporated by varying the strength of the crust and upper mantle analogue layers and by implementing a weak plate or part-of-a-plate between two stronger ones. Three (brittle crust/viscous crust/strong viscous upper mantle) and four (brittle crust/viscous crust/brittle upper mantle/strong viscous upper mantle) layer models were confined by a weak silicone layer on one side in order to contain but not oppose lateral extrusion. Experimental results show that relative strength contrasts between converging plates and intervening weak plates control the location and the shape of deformation sites taken as ‘collision orogens’. If the contrast is small, internal deformation of the strong plates through fore- and backthrusting occurs early in the deformation history. However, the bulk system is dominated by buckling that nucleates on the weak plate whose antiformal topography is highest; model Moho of the bordering stronger plates is deepest under these conditions. If the contrast is large, deformation remains localized within the weak plate for a larger amount of shortening and develops a root zone below a narrow deformation belt, which coincides with the locus of maximum topography. Implementing a buoyant, low-viscosity layer above the model Moho of the weak plate favours the development of asymmetric model orogens notwithstanding the initial symmetric setup. Once the asymmetry is established strain remains localized in thrust faults and ductile shear zones documenting foreland directed displacement of the model orogen. Such laterally and vertically irregular configurations have applications in continent-continent collision settings such as the Eastern Alps. First-order mechanical boundary conditions recognized from modelling to be favourable to the post early Oligocene tectonics of the Eastern Alps include: (1) subtle rather than high-strength contrasts between the Adriatic indentor and the strongly deformed region comprising Penninic and Austroalpine units to the north of it; (2) decoupling of Penninic continental upper crust from its substratum to allow for crustal-scale buckling of the Tauern Window; (3) weak mechanical behaviour of the European lower crust during collision to account for its constant thickness along the TRANSALP deep seismic transect; and (4) the direct continuation of the basal detachment underlying the fold and thrust belt in the hangingwall of the European plate with a wide ductile shear zone in the core of the orogen, which separates the European from the Adriatic plate.