experiments simulating surface deformation induced by pluton emplacement abstract we present centrifuge experiments to study the surface deformation induced by shallow pluton emplacement in a rheologically stratified crust sand simulates the topmost brittle crust plastilina and denser silicone represent more and less competent crustal portions respectively lighter silicone simulates a buoyant intrusion in the models density differences affect the rate of intrusion but not their evolution or shape whereas viscosity and strength stratifications control both the shape and rate of the intrusions with a higher viscosity contrast 10 2 4 between the intrusion and the embedding media the rise of the lighter silicone induces a laccolith like intrusion responsible for doming and thinning of the overburden an apical depression may form inducing silicone extrusion conversely with a lower 10 1 viscosity contrast the overburden and the intrusion exhibit a lens shaped form with a broad central depression bordered by an upward flexure towards the periphery a sag in the floor of the intrusion is commonly observed no silicone extrusion occurred the intrusion is a hybrid between a laccolith and a lopolith the comparison with nature 1 confirms roof uplift as an important means of accommodating space during pluton emplacement and 2 suggests that where roof uplift plays a major role pluton emplacement can induce a well correlated sequence of events at surface doming the development of a depression and extrusion

flow and fabric development during the diapiric rise of magma scaled isothermal experiments of the internal circulation induced by viscous drag at a diapir country rock interface are compared with published theoretical studies of flow induced finite deformation inside a rising sphere and new numerical results are presented for the incremental strain pattern developed in spherical diapirs the results predict a time dependent deformation history within rising diapirs that produces finite strains characterized by strong oblate flattening strains at the margin and strong vertical prolate strains in the center with an intervening zone of low plane strain finite strains are discussed in terms of the different sub fabrics expected to form in granitoids after considering the history of rheology and temperature distribution in rising magma strong rheological and temperature gradients are predicted for the margins of internally circulating diapirs that can lead to the formation of strong penetrative foliations at the margin and magmatic or even isotropic fabrics in the i

constraining the relative strengths of high grade metamorphic rocks using foliation refraction angles an example from the northern new england appalachians foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite facies metaturbidites in the presidential range of the white mountains in eastern new hampshire an early formed foliation developed during km scale nappe folding consistently displays larger bedding foliation angles in metapelitic units than metapsammitic units suggesting that the metapelitic units had higher effective viscosities during this deformation we collected bedding foliation angle measurements from a combination of outcrops and locally derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers which we attribute to the presence of large up to 15 cm long abundant up to 30 vol effectively rigid andalusite porphyroblasts in the metapelitic layers we present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types address the practical limitations we encountered and suggest this method is an easy way of estimating the strength contrast between different rock types finally we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis

estimation of magma depth for resurgent domes an experimental approach post collapse resurgence is a phenomenon affecting many calderas attributed to a renewed magma rise the process is still poorly understood and the associated source parameters remain poorly constrained a set of experiments has been conducted to gain insight into the structural evolution of caldera resurgent domes a sand plaster mixture was chosen as an analogue for the brittle pile of volcanic rocks and silicone putty simulates the ductile behavior of the intruding magma resurgence is driven by the vertical uplift of the silicone with variable shape and depth similarity conditions are achieved through eight dimensionless numbers which are of the same order of magnitude in both nature and experiments results show that extension due to doming is in many cases accommodated by one axial graben opposite master faults of this graben intersect at depth at the junction with the rising viscous silicone the simplicity of the geometry of the whole analogue system provides equations which allow the estimation of the silicone depth from surface parameters these equations are then used in some field examples to assess the magma depth beneath natural resurgent domes

the analogue shear zone from rheology to associated geometry the geometry of ductile strain localization phenomena is related to the rheology of the deformed rocks both qualitative and quantitative rheological properties of natural rocks have been estimated from finite field structures such as folds and shear zones we apply physical modelling to investigate the relationship between rheology and the temporal evolution of the width and transversal strain distribution in shear zones both of which have been used previously as rheological proxies geologically relevant materials with well characterized rheological properties newtonian strain hardening strain softening mohr coulomb are deformed in a shear box and observed with particle imaging velocimetry piv it is shown that the width and strain distribution histories in model shear zones display characteristic finite responses related to material properties as predicted by previous studies application of the results to natural shear zones in the field is discussed an investigation of the impact of 3d boundary conditions in the experiments demonstrates that quantitative methods for estimating rheology from finite natural structures must take these into account carefully

the control of overburden thickness on resurgent domes insights from analogue models abstract resurgent doming consists of the uplift usually accompanied by volcanic activity of part of a collapse caldera analogue models were used to investigate the architecture of resurgent domes dry sand simulates the brittle crust uprising silicone located at the base of the sand pack simulates magma the deformation pattern depends mainly upon 1 the ratio aspect ratio between the thickness of the sand overburden and the width of the silicone intrusion 2 the duration of experiment for aspect ratios 1 two concentric domes develop the first formed outer dome is bordered by inward dipping reverse ring faults while the inner dome by outward dipping normal ring faults the layers inside the dome are uniformly dipping for aspect ratios 0 4 the dome shows a crestal depression surrounded by radial fractures followed by an apical extrusion of silicone the internal structure of the dome is made up of domed layers independently from the aspect ratio the duration of the experiment enhances silicone extrusion a consistent structure is observed in most resurgent domes in nature the comparison between experiments and nature suggests that two distinct resurgence modes occur mainly depending on the aspect ratio thickness width of the crust overlying the magma chamber aspect ratios 1 develop a resurgent block with uniformly dipping layers and peripheral volcanic activity ischia and pantelleria type aspect ratios 0 4 develop a resurgent dome with a crestal depression domed layers within and peripheral and internal volcanic activity valles and long valley type

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

zoned minerals in garnet peridotite nodules from the colorado plateau implications for mantle metasomatism and kinetics some garnet peridotite nodules from the thumb a minette neck on the colorado plateau in the southwestern united states contain zoned minerals zoning does not exceed 1 5 wt for any oxide but some relative changes are large in one garnet tio2 and cr2o3 ranges are 0 05 0 65 and 3 5 5 0 wt respectively in two porphyroclastic nodules garnet rims are depleted in mg and enriched in fe ti and na compared to cores and one garnet is irregularly zoned in ti and cr olivine crystals in these rocks are unzoned and pyroxene zoning is slight yet matrix olivine and pyroxene contain more fe and ti and less mg and cr than inclusions of these phases in garnet in three coarse nodules garnet rims are ti rich compared to cores and ca fe mg and cr zoning patterns are complex several nodules appear to have partially equilibrated near 1200 c and 35 kb and under these conditions cation mobility in pyroxene was greater than in garnet

channel flow extrusion model to constrain dynamic viscosity and prandtl number of the higher himalayan shear zone constraining magnitudes of mechanical and thermo mechanical parameters of rocks and shear zones are the important goals in structural geology and tectonics talbot in j struct geol 21 949 957 1999 such parameters aid dynamic scaling of analogue tectonic models ramberg in gravity deformation and the earth s crust in theory experiments and geological applications 2nd edn academic press london 1981 which are useful to unravel tectonics in further details schultz ela and walsh in j struct geol 24 247 275 2002 the channel flow extrusion of the higher himalayan shear zone hhsz higher himalaya can be explained by a top to s sw simple shear i e the d2 deformation in combination with a pressure gradient induced flow against gravity presuming its newtonian incompressible rheology with parallel inclined boundaries the viscosity of this shear zone along a part of the himalayan chain through india nepal and bhutan is estimated to vary widely between 1016 and 1023 pa s and its prandtl number pr within 1021 1028 the estimates utilized ranges of known thickness 6 58 km of the hhsz that of its top subzone of ductile shear of normal shear sense stdsu 0 35 9 4 km total rate of slip of its two boundaries 0 7 131 mm year 1 pressure gradient 0 02 6 kb km 1 density 2 2 3 1 g cm 3 and thermal diffusivity 0 5 10 6 2 1 10 6 m s 2 along the orogenic trend considering most of the parameters specifically for the sutlej section india the calculated viscosity and the prandtl number pr of the hhsz are deduced to be 1017 1023 pa s and pr 1022 1028 the upper limits of the estimated viscosity ranges are broadly in conformity with a strong tibetan mid crust from where a part of the hhsz rocks extruded on the other hand their complete ranges match with those for its constituent main rock types and partly with those for the superstructure and the infrastructure the estimated mechanical and thermo mechanical parameters of the hhsz will help to build dynamically scaled analogue models for the himalayan deformation of the d2 phase

finite strain effects in experimental mullions 1986 abstract mullions are cylindrical structures developed at the interface between rocks of different competence shortened along the layering natural examples include phyllosilicate rich meta igneous sheets and massive sulfides in silicate country rocks in axial profiles they consist of rounded lobes convex into the less competent material which alternate with sharp cusps pointing toward the more competent material mullions have been produced experimentally in incompetent single layers of silicone putty viscosity l 2 9 x 104 pa s and power law exponent n 1 embedded in a strain rate softening host of plastilina silicone putty mixture h 5 5 10 pa s and n 7 1 the initiation development and changes in geometry of mullions in the two interfering interfaces of a single incompetent layer shortened along its length by pure shear are reported up to 10 bulk shortening bs gentle flexures develop first in one interface then by 17 bs in both these rounded irregularities become cuspate at 25 bs where a lobe develops on one contact a cusp is slightly preferred on the opposite side such antisymmetric mullions contrast with the symmetrical pinch and swell mullions predicted by smith but are like natural examples and those figured in the literature the amplitudes of the cusps increase while their wavelength range decreases with finite shortening it has been found that mature cusps distort but do not transgress the original contact even if the viscosity contrast is 1 1000 beyond 40 bs cusps either mature to flames or combine to horns and after 60 bs even the last surviving mullions cease to amplify dynamically and the layer only thickens uniformly most stages seen in the models can be matched by natural field examples

coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer plane strain coaxial deformation of a competent plasticine layer embedded in an incompetent plasticine matrix was carried out to improve our understanding about the evolution of folds and boudins if the layer is oriented perpendicular to the y axis of the finite strain ellipsoid the rock analogues used were beck s green plasticine matrix and beck s black plasticine competent layer both of which are strain rate softening modelling materials with a stress exponent n ca 8 the effective viscosity of the matrix plasticine was changed by adding different amounts of oil to the original plasticine at a strain rate dot e of 10 3 s 1 and a finite strain e of 10 the effective viscosity of the matrix ranges from 1 2 106 to 7 2 106 pa s the effective viscosity of the competent layer has been determined as 4 2 107 pa s if the viscosity ratio is large ca 20 and the initial thickness of the competent layer is small both folds and boudins develop simultaneously although the growth rate of the folds seems to be higher than the growth rate of the boudins the wavelength of both structures is approximately the same as is suggested by analytical solutions a further unexpected but characteristic aspect of the deformed competent layer is a significant increase in thickness which can be used to distinguish plane strain folds and boudins from constrictional folds and boudins

magma mixing in mantle xenolith bearing calc alkalic ejecta ichinomegata volcano northeastern japan abstract the latest pleistocene ejecta from the ichinomegata volcano ne japan composed of three maars show a wide compositional variation ranging from basalt 51 sio 2 to dacite 65 sio 2 these have the same phenocryst assemblage of olivine augite hornblende biotite quartz plagioclase and titanomagnetite upper mantle peridotite xenoliths are found not only in basalts but also in andesite whole rock major chemistry and modal compositions indicate that the chemical and modal variations have been caused by the mixing of three magmatic components primitive high alumina basalt sio 2 51 wt mgo 8 5 wt with olivine fo 88 and augite phenocrysts differentiated basaltic andesite sio 2 54 wt mgo 5 wt which has the same but slightly fe rich phenocryst phases as the primitive basalt and porphyritic dacite sio 2 65 wt mgo 2 0 wt with phenocrysts of hornblende biotite quartz plagioclase and titanomagnetite the upper mantle peridotite bearing andesite may not be derived directly from the upper mantle and its peridotite xenoliths may have been carried by the primitive basaltic magma the estimated density for three magmatic end components requires a high flow velocity of basaltic magma and a small volume 5 m 3 of the dacite magma body to facilitate breaking of the stratification in the dacite magma body this is consistent with the retention of peridotite xenoliths in the mixed magma and the preserved disequilibrium petrographic features

rheological and physical characteristics of crustal scaled materials for centrifuge analogue modelling abstract we characterize a set of analogue materials used for centrifuge analogue modelling simulating deformation at different levels in the crust simultaneously specifically we improve the rheological characterization in the linear viscoelastic region of materials for the lower and middle crust and cohesive synthetic sands without petroleum binding agents for the upper crust viscoelastic materials used in centrifuge analogue modelling demonstrate complex dynamic behaviour so viscosity alone is insufficient to determine if a material will be an effective analogue two series of experiments were conducted using an oscillating bi conical plate rheometer to measure the storage and loss moduli and complex viscosities of several modelling clays and silicone putties tested materials exhibited viscoelastic and shear thinning behaviour the silicone putties and some modelling clays demonstrated viscous dominant behaviour and reached newtonian plateaus at strain rates 2 s 1 while other modelling clays demonstrated elastic dominant power law relationships based on these results the elastic dominant modelling clay is recommended as an analogue for basement cratons inherently cohesive synthetic sands produce fine detailed fault and fracture patterns and developed thrust strike slip and extensional faults in simple centrifuge test models these synthetic sands are recommended as analogues for the brittle upper crust these new results increase the accuracy of scaling analogue models to prototype additionally with the characterization of three new materials we propose a complete lithospheric profile of analogue materials for centrifuge modelling allowing future studies to replicate a broader range of crustal deformation behaviours

analogue models of melt flow networks in folding migmatites we have modelled the formation and the layer parallel shortening of layered stromatic migmatites the model consists of thin superposed layers of partially molten microcrystalline wax the melt 30 vol has a negative buoyancy and a high viscosity contrast with its solid matrix as soon as the shortening begins melt filled veins with high aspect ratios open along foliation the melt is segregated into the veins forming a stromatic layering during incipient folding crescent shaped saddle reefs open at the hinges of open sinusoidal folds further shortening and melt enhanced shear displacements on interlayer interfaces cause chevron folds to develop and the saddle reefs to become triangular in comparison a melt free experiment shows only a few layer parallel openings and no saddle reefs in chevron folds on the basis of our experimental results we propose that in migmatites 1 mesoscale melt migration is a combination of flow in immobile veins and movements of veins as a whole 2 the changes in the geometry of the mesoscale melt flow network create the pressure gradients that drive melt migration 3 the melt flow network does not need to be fully interconnected to allow local expulsion 4 melt expulsion is episodic because the temporal evolution of the network combines with the spatial heterogeneity of the deformation

emplacement and rheomorphic deformation of a large lava like rhyolitic ignimbrite grey s landing southern idaho the miocene grey9s landing ignimbrite reaches 70 m thick and covers at least 400 km 2 in the central region of the snake river plain it shows particularly intense welding and rheomorphic deformation and although parts are eutaxitic most is lava like with flow banding and no fiamme a near ubiquitous penetrative flow lamination associated with a well developed elongation lineation is folded into small intrafolial tight to isoclinal oblique and sheath folds which are refolded by larger folds in the upper parts structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot 1000 c high mass flux pyroclastic density current that flowed westward across a graben faulted landscape as hot particles were deposited they rapidly agglutinated and coalesced and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current deposit interface the shear zone is interpreted to have been less than 2 m thick it produced and deformed the rheomorphic fabric and it migrated upward with the rising current deposit interface during aggradation so that it transiently affected all levels of the resultant thick ignimbrite deformation was progressive and after the density current had dissipated viscous spreading and downslope flow continued and involved an increasingly thick portion of the sheet this folded the flow banding and f 1 intrafolial isoclines into larger sheath folds and into more upright periclines near the top of the ignimbrite we demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites

the structure of composite volcanoes unravelled by analogue modelling a review abstract volcanic edifices are unstable accumulations of eruptive and intrusive products with complex internal structures a documentation of these structures facilitates the assessing of the various hazards associated with volcanoes e g flanks instabilities and eruptive events however structures are not easily assessed in the field as volcanoes are regularly covered by eruptive products and are rapidly eroded in the case of recent and active volcanoes internal structures are generally extrapolated from the external morphology of the edifice which can be documented using digital elevation models dem and geodetic data interpretation of such data greatly benefits from analogue models which are idealised representations of reality used to test the impact that various processes may have on the shape of a volcanic edifice this short review focuses on the contribution of analogue models to our comprehension of the internal structure of composite volcanic cones the other volcanic morphologies e g caldera are excluded from this review as they have distinct structure and evolution that have already been thoroughly reviewed the reviewed models were used to investigate the impact that regional faults magma injections and ductile substratum among other processes have on the internal structure and surface morphology of many volcanic edifices these models were also used to locate unstable flanks and eruption sites

the competition between rates of deformation and solidification in syn kinematic granitic intrusions resolving the pegmatite paradox abstract while fully crystallized granites rich in feldspar generally serve to strengthen the continental crust their precursor melts are assumed to be important agents of crustal weakening many syn tectonic granitic pegmatites are deformed within shear zones but ubiquitously preserve undeformed primary magmatic textures implying that they were largely molten during shearing yet the shapes of pegmatite bodies indicate that they deformed with a greater competence than their surroundings this co located pair of material behaviours is paradoxical we interpret field relationships in a typical pegmatite shear zone association torrisdale nw scotland and propose a mechanism by which syn tectonic granitic melts may in effect act as competent bodies while not yet fully crystallized competence was rapidly increased by preferential crystallization on intrusion margins that served to encapsulate residual melt inside stiff rinds further crystallization may have been pulsed as the concentrations of crystallization inhibitors fluxes increased in residual fluids postulating the existence of initial stiff rinds also consistent with modern estimates for rates of feldspar crystallization cms yr from undercooled hydrous silicic magma to form pegmatites these greatly outpace strain rate estimates for shear zones thus fully liquid granitic melts may only be present fleetingly and have little opportunity to weaken deforming crust before crystallization begins

application of rheology gauge to determine rock competency in taili western liaoning ne china abstract the intensive deformation along with the rheological behavior of the tectonic units in taili eastern north china craton ncc are recognized as the manifestation of the presence of the ne sw striking ductile shear zone in the region the presence of two types of granitic veins aplitic and pegmatitic in taili exhibit obvious strain refraction and boudinage structures ten samples from different rock types are collected to analyze their deformation characteristics and the rheological properties the analysis is performed by measuring the rock competency following the methods of mainly two types of rheology gauges 1 pinch and swell structures and 2 refraction of the veins we conclude that two main factors control the deformation patterns in taili i e grain size and proportion of the mineral component grain size is found to be one of the dominant factors controlling the deformation pattern i e the larger the grain size the higher the rock viscosity is the proportion of the mineral content is another important parameter which is critical to infer the rheological contrast along with the deformational style of the studied region the rock viscosity of the samples is inversely proportional to the mica and feldspar content of the same micro structural observation suggests that these rocks experienced strong extensional deformation at a low to middle temperature condition around 450 550 c during the mesozoic this event transformed the granitic veins into boudins associated with intensive strain refraction which are also responsible for the formation of voluminous felsic rocks with high viscosity contrast in this region

ductile shear zones as counterflow boundaries in pseudoplastic fluids abstract ductile shear zones with different styles are attributed to rocks having deformed as pseudoplastic power law fluids with different exponents n 1 of the stress sensitivity of the strain rate profiles of velocity and accumulated displacements recorded by passive markers in pseudoplastic fluids flowing steadily and continuously along no slip boundaries have robust geometries distinctive of the n value what have previously been considered as single shear zones are forward modelled as pairs of back to back boundary shears coupled across individual counterflow boundaries displacement gradients in natural shear zones are shown to fit theoretical displacement curves for steady flows of pseudoplastic fluids along no slip boundaries in a variety of rock types of different age environment and scale general fits of these curves indicate that strain was homogeneous along natural counterflow boundaries so that specific values or ranges of n value can be assigned to the rocks when they sheared whatever deformations and deformation mechanisms were involved fits that are only local indicate inhomogeneous strains along natural boundary shears and require more detailed analysis

can field data constrain rock viscosities abstract in the 1960s it looked as though the ratio of wavelength to thickness of folds along shortened competent single layers might allow constraint of the viscosity ratios between the layers and their hosts when they deformed together in the 1970s the possibility arose that simple field measurements of boudins and mullions might also constrain rock viscosity ratios and thereby distinguish deformation facies and map rock viscosities in pressure temperature time space even more potential tools for constraining rock viscosities appeared in the 1980s but since then progress appears to have stagnated in a welter of problems an attempt is made to refocus attention on direct retrospective measurements of rock rheologies during natural deformations by reviewing the potential field tools for constraining rock viscosities discussing some of their problems and by a crude application of the most developed approach further advances are likely to come from iteration between modellers and structural geologists working in a variety of tectonic settings as well as constraining the pressure temperature time paths of our rocks we should also be attempting to measure their viscosities

stress state interpretation in light of pressure solution creep numerical modelling of limestone in the eastern paris basin france abstract this paper presents an interpretation of the present stress state in the eastern paris sedimentary basin france based on considerations that regard the long term limestone rheology a comprehensive stress profile was acquired in this region and it was determined that the minimum horizontal stress in an argillite layer is much larger than that in the under and overlying stiffer limestone layers while the major horizontal stress seems almost insensitive to lithology in a previous study the genesis of this intriguing stress state was modelled by considering the limestone to be elastic however the calculation implied the use of unrealistic mechanical characteristics for this rock extremely low long term elastic modulus and poisson s ratio from the study it was concluded that some dissipative viscous process of unknown origin might be at work in the limestone herein we propose that the deformation micro mechanism that is responsible for this time dependent behaviour is pressure solution creep psc we show that this hypothesis is mechanically realistic by adapting an existing model of psc which is based on the description of mass transfer within grain to grain contacts dissolution diffusion and precipitation we deduce a set of long term rheological characteristics for the limestone and use them in the numerical model of the stress genesis in the paris basin the results show that psc is a possible explanation for the present stresses and total strains in this region the fact that limestone should be considered as visco elastic over geological time scales with a viscosity ranging between 10 17 and 10 21 pa s and not as purely elastic is at the core of this work although it is probably ubiquitous in nature psc is notoriously difficult to assess in the laboratory thus our effort of coupling small scale processes in a quantitative manner to large scale deformation and in situ measurements provides a useful tool for gaining insight into the role of psc in the long term behaviour of carbonated rocks and its effect on sedimentary basin evolution

mullions in the high ardenne slate belt belgium numerical model and parameter sensitivity analysis in the high ardenne slate belt belgium pre existing layer perpendicular quartz veins in psammite layers acted as mechanical boundaries due to the difference in competence between vein quartz and psammite this caused the formation of mullions during layer parallel shortening in this paper the process of mullion formation is modelled using finite element techniques a parameter sensitivity analysis of the mullion model demonstrates that the stress exponent of psammite the total horizontal shortening of the psammite layers the initial aspect ratio of the psammite segments between the veins and the competence contrast between psammite and vein quartz are the controlling parameters for the shape of the mullions our results suggest that the morphology of the mullions can be used to constrain the rheology of psammite deforming in the middle crust moreover the parameter sensitivity analysis illustrates the range of layer parallel shortening extension structures associated with layer perpendicular quartz veins that can be expected in nature e g dogbones inverted mullions and creates future perspective for the use of the model as a paleorheological gauge for these structures

strain ellipsoids from incompetent dykes application to volume loss during mylonitization in the singo gneiss zone central sweden we describe how to constrain bulk strain ellipsoids by distinguishing the attitudes of mullioned from planar contacts in incompetent dykes our method is demonstrated by using metabasaltic dykes to

the history and dynamics of a welded pyroclastic dam and its failure the 2 360 bp eruption of mount meager british columbia began as an explosive dacitic sub plinian eruption that waned rapidly to a sustained period of vulcanian eruption triggered dome collapse events producing voluminous block and ash flow baf deposits the earliest baf deposits accumulated rapidly enough immediately downslope of the vent to retain heat and weld using the deposit as a paleoviscometer determines an effective viscosity of 109 1010 pa s during welding this prolific production of hot lava and block and ash flows in a steep mountainous terrain created a 110 m high largely impermeable dam capped by permeable non welded baf deposits and unconsolidated avalanche deposits that blocked the flow of the lillooet river and created a temporary lake the welded pyroclastic dam was compromised and overtopped at least once before the peak dam height was reached renewed eruption caused buildup of the dam to a maximum of 780 m above sea level asl and grew the temporary lake to an elevation of 740 m asl and a minimum volume of 0 55 km3 the rise of lake level led to catastrophic failure of the top of the dam generating an outburst flood that carved a canyon through most of the dam and resulted in a voluminous lahar that is traced at least 65 km downstream based on current flow rates of the lillooet river the lake would have overtopped the final dam at a minimum of 39 65 days after its formation the peak deluge lasted approximately 8 h and eroded a 2 5 km long canyon into the still hot dam core before returning to background flow rates

constriction structures related to viscous collision southern prince charles mountains antarctica macroscopic structures are investigated in a zone of highly contorted migmatites from the southern prince charles mountains antarctica here l tectonite fabrics rods mullions boudin pods elongate enclaves and fold hinges are persistent linear features all plunging gently to the northeast in contrast amoeboid folds ptygmatic folds and folded boudins with different orientations are the characteristic structures in transverse sections perpendicular to the lineation no consistent shear sense is recognised in any dimension together with strain and shape analysis these observations strongly suggest that the deformation pattern is one of folding and stretching by constriction previous timing constraints indicate that this deformation overlapped with the waning stages of anatexis during decompression at approximately 510 ma up to 30 million years after initial orogeny at 540 ma the zone affected by constriction is several kilometres wide and has a contorted flower like shape confined between two broad domal antiforms in this context the constricted zone is interpreted as a relatively late tectonic feature that could have formed via deep seated viscous collision in response to orogenic collapse and doming

anisotropic viscosity of a dispersion of aligned elliptical cylindrical clasts in viscous matrix abstract self consistent averaging using an auxiliary solution for an elliptical anisotropic viscous inclusion in an anisotropic viscous host provides estimates of the principal bulk viscosities of a dispersion of aligned elliptical viscous clasts in an isotropic or anisotropic viscous matrix analysis and results are for a two dimensional analog of a composite rock with clasts that are cylinders with axes normal to the plane of flow the ratio of principal viscosities n in clast parallel extension or shortening s in clast parallel shear m n s is smaller than that obtained using an auxiliary solution in which the host is isotropic results for the limiting case of rigid clasts indicates that the latter procedure overestimates the stress concentration in axis parallel extension or shortening at intermediate clast volume fraction f if the matrix is anisotropic bulk anisotropy derives from both the shape anisotropy and the intrinsic anisotropy of the matrix and unsymmetrical relations for the principal viscosities and m f result the results suggest that rheological anisotropy in rocks with a planar fabric is greatly reduced if the components are lenticular in form rather than continuous layers a general solution is given for an elliptical inclusion for the case that the principal axes of anisotropy in both the host and the inclusion are oblique to the axes of the elliptical section and the host is subjected to homogeneous stress far from the inclusion

rare earth element geochemistry of garnet lherzolite and megacrystalline nodules from minette of the colorado plateau province abstract mineral and whole rock ree abundances in garnet lherzolite and megacrystalline nodules from the thumb display broad correlations with major element compositions lherzolites with 12 modal clinopyroxene plus garnet high ca al lherzolites have relatively flat chondrite normalized whole rock ree patterns lherzolites poor in clinopyroxene and garnet low ca al lherzolites have lower hree in clinopyroxenes and garnets and higher whole rock lree hree it is concluded that the low ca al lherzolites may have undergone lree metasomatism after depletion of the major element compositions by partial melting and that much of the garnet now present was originally dissolved in aluminous orthopyroxene the high ca al lherzolites may be interpreted either as primordial mantle samples or as products of equilibration with very lree enriched liquids the megacrystalline nodules are medium to ultracoarse grained intergrowths and megacrysts with mineral compositions similar to discrete nodule suites in kimberlites the ree abundances of the megacrystalline minerals are consistent with an origin as cumulates from magma with extremely fractionated ree similar to minette or kimberlite the patterns of correlation of ree and major elements in this inclusion suite are similar to the patterns observed in the garnet lherzolite and discrete nodule suites of southern african kimberlites both of the subcontinental mantle provinces represented by these suites contain three distinct petrogenetic components refractory garnet lherzolite enriched in lree and depleted in hree fertile garnet lherzolite with generally chondritic ree abundances and a suite of ultracoarse minerals precipitated from magma with extremely fractionated ree generally similar to the host magmas

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

viscous anisotropy of two phase composites and applications to rocks and structures abstract anisotropy of viscosity in rocks has a significant effect on the development of geological structures as demonstrated in several theoretical and analogue model studies this paper develops the results of a self consistent mechanics approach to modelling the bulk viscous properties of two phase composites to reveal the strength of the viscous anisotropy anisotropy factor is defined as the ratio of normal to shear viscosity n s the value of is determined for different types of composites of two viscous phases with varying viscosity ratio m 5 10 100 ranging from bilaminates to those with elliptical shape fabrics given by axial ratio r bilaminates with equal layer thickness are the most highly anisotropic systems where simply depends on m these results are applied to rocks modelled as two phase composites of competent and incompetent lithologies with the aim of quantifying the viscous anisotropy that is associated with natural rock deformation and the formation of geological structures it is deduced that m 5 or 10 is an appropriate maximum viscosity contrast for many metasedimentary rocks e g psammite pelite composites but that m 100 might be effectively reached in limestone marl pairs or in rocks containing surfaces of slip or zones of recrystallisation or grain boundary sliding these m values give rise to maximum values of 3 5 5 and 25 5 respectively an anisotropy factor of 25 is effectively a composite of a rigid and a viscous phase and may only be achieved in solid melt mixtures anisotropic rocks whether layered or containing a strong shape fabric will develop folds more easily than isotropic rocks characteristic structures are similar folds chevron folds kink bands and shear bands geometric features of folds especially fold angularity may provide methods of quantifying the viscous anisotropy of rocks and composites

when do folds unfold during progressive shear folded layers in rocks can be stretched again potentially unfolding the folds back to straight layers little is known however about how to recognize partly or even entirely unfolded layers when folded layers can unfold what determines their mechanical behavior and how can we recognize them in the field in order to address these questions we present a series of numerical simulations of the stretching of previously folded single layers and multilayers in simple shear layers do not completely unfold when they undergo softening before or during the stretching process or when adjacent competent layers prevent them from unfolding intrafolial folds and cusp like folds adjacent to straight layers as well as variations in fold amplitudes and limb lengths of irregular folds are indicative of stretching of a fold train

the role of pre existing tectonic structures and magma chamber shape on the geometry of resurgent blocks analogue models abstract a set of analogue models has been carried out to understand the role of an asymmetric magma chamber on the resurgence related deformation of a previously deformed crustal sector the results are then compared with those of similar experiments previously performed using a symmetric magma chamber two lines of experiments were performed to simulate resurgence in an area with a simple graben like structure and resurgence in a caldera that collapsed within the previously generated graben like structure on the basis of commonly accepted scaling laws we used dry quartz sand to simulate the brittle behaviour of the crust and newtonian silicone to simulate the ductile behaviour of the intruding magma an asymmetric shape of the magma chamber was simulated by moulding the upper surface of the silicone the resulting empty space was then filled with sand the results of the asymmetric resurgence experiments are similar to those obtained with symmetrically shaped silicone in the sample with a simple graben like structure resurgence occurs through the formation of a discrete number of differentially displaced blocks the most uplifted portion of the deformed depression floor is affected by newly formed high angle inward dipping reverse ring faults the least uplifted portion of the caldera is affected by normal faults with similar orientation either newly formed or resulting from reactivation of the pre existing graben faults this asymmetric block resurgence is also observed in experiments performed with a previous caldera collapse in this case the caldera collapse related reverse ring fault is completely erased along the shortened side and enhances the effect of the extensional faults on the opposite side so facilitating the intrusion of the silicone the most uplifted sector due to an asymmetrically shaped intrusion is always in correspondence of the thickest overburden these results suggest that the stress field induced by resurgence is likely dictated by the geometry of the intruding magma body and the related deformation is partially controlled by pre existing tectonic and or volcano tectonic structures

modeling dislocation creep as a near field material transfer process during spherical pluton expansion implications for strain rates and their preservation in pluton aureoles abstract in this paper we model coupled spatial and temporal changes in stress and temperature to calculate dislocation creep strain rates in host rock associated with spherical magma chamber expansion with and without material removal by stoping and or assimilation given a constant magma chamber pressure of 100 mpa we show that stress and temperatures in the host rock range from 100 to 10 mpa and 800 to 300 c respectively using a flow law for dislocation creep of wet quartzite with recently calibrated parameters maximum dislocation creep strain rates on the order of ca 10 10 s 1 occur only close to the pluton margin despite the simple geometry relative to real plutons the models yield several thought provoking predictions if the outer portions of the pluton are allowed to cool a wide thermal and structural aureole forms maintaining the liquidus temperature in the entire pluton allows for rapid aureole deformation resulting in a narrow thermal and structural aureole because the deformation outpaces heat conduction dislocation creep strain rates in the inner portions of aureoles are extremely sensitive to pressure changes only potentially resulting in large transient variations in strain rate under isothermal conditions evidence for elevated strain rates in pluton aureoles may not be preserved in host rock aureoles if solidified pluton participates in the deformation or it may be completely removed

the role of extensional structures on experimental calderas and resurgence the structure and shape of collapses and resurgences is often controlled by pre existing discontinuities such as normal faults in rift zones in order to study the role of extensional structures on collapse and resurgence we used analogue models dry sand simulated the brittle crust silicone located at the base of the sand pack simulated magma in the experiments regional extension pre dated collapse or resurgence forming normal faults in a graben like structure the graben was filled with additional sand simulating post rift deposits a piston then moved the silicone downward or upward inducing collapse or resurgence within the previously deformed sand the collapses showed an ellipticity length of minor axis length of major axis between 0 8 and 0 9 with the major axis parallel to the extension direction the partial reactivation of the pre existing normal faults was observed during the development of the caldera reverse faults which conversely to what was expected from experiments without pre existing extension became partly inward dipping resurgence showed an elongation of the uplifted part with the main axis perpendicular to the extension direction at depth pre existing normal faults were partly reactivated by the reverse faults formed during resurgence these locally became outward dipping normal faults a total reactivation of pre existing faults was also observed during resurgence the experiments suggest that the observed elongation of calderas and resurgences is the result of the reactivation of pre existing structures during differential uplift such a reactivation is mainly related to the loss in the coefficient of friction of the sand the results suggest that elliptic calderas and resurgences in nature may develop even from circular magma chambers

rheology of fine grained siliciclastic rocks in the middle crust evidence from structural and numerical analysis constitutive models to describe the rheology of the crust are based on a combination of indirect methods and many aspects are still unknown and controversial here we present a new method to quantify the rheology of fine grained siliciclastic rocks which are common in the middle crust and deform by solution precipitation processes we use a combination of structural analysis at different scales and a geomechanical model to develop a parameter estimation scheme to calculate rheological parameters at 350 400 8c and geologic strain rates the results of this study demonstrate that fine grained siliciclastic rocks in the middle crust have a newtonian viscous rheology approximately 10 times weaker than wet quartz this is in agreement with observed microstructures our results imply that the strength of polyphase quartz rich rocks located in the middle crust is much lower than predicted by conventional models based on flow laws for dislocation creep because fine grained siliciclastic rocks control the rheology of the middle crust in many sedimentary basins our results provide new quantitative parameters for geodynamic modelling in settings where dissolution precipitation creep is important d 2005 elsevier b v all rights reserved

the role of extensional tectonics at different crustal levels on granite ascent and emplacement an example from tuscany italy abstract the role of regional extension on the rise and emplacement of granites in the crust is still debated pluton ascent and emplacement widely occurred in tuscany italy since late miocene during the post orogenic collapse of the inner apennines and are presently occurring in the geothermal areas of amiata and larderello tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels ductile extension enhanced the segregation and ascent of granitic melts in the lower crust controlling pluton emplacement in correspondence with the brittle ductile transition in the brittle crust magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction pluton emplacement mainly occurred by means of roof lifting the case of tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle extending crust the estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates this suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes roof lifting accommodated the space required for pluton emplacement

a field method to determine unsaturated hydraulic conductivity using flow nets a practical new field method is proposed to evaluate hydraulic conductivity in partially saturated media near a water impoundment the new method is an inverse procedure which utilizes a flow net constructed from the steady state total hydraulic head distribution around the water source in the vicinity of constant head sources and downstream along any stream tube wetness and conductivity generally decrease knowing the stream tube geometry and hydraulic gradient from the flow net darcy s law is used to determine unsaturated hydraulic conductivities within the stream tube relative to some segment of the stream tube where conductivity is known this approach was used successfully to predict the unsaturated hydraulic conductivity which was input to two variably saturated numerical models utilizing total hydraulic head fields generated by the models this procedure is also applied to pressure head and water content data collected in the field surrounding a constant head borehole infiltration test originally designed to determine only saturated hydraulic conductivity above the water table for practical purposes the new procedure compares very favorably with 1 results of a field experiment to obtain unsaturated hydraulic conductivity in situ using the instantaneous profile method and 2 values of unsaturated conductivity calculated from field and laboratory measurements of water content and pressure head

sheath folds formed by drag induced by rotation of rigid inclusions in viscous simple shear flow nature and experiment abstract natural examples of fold patterns associated with rotated boudins occur in the ossa morena zone s portugal and triggered the present study in ductile marbles with embedded competent mafic boudins the folds seem to have originated by rotation of these bodies and show differential development from gentle deflections to sheath folds this suggests a dependence of the structure type on the distance of the metamorphic layering to the rigid body the experimental work presented here was performed with rotating inclusions in bulk simple shear and introduces a new variable the distance of the marker layer from the rigid inclusion d expressed in terms of e the ratio between the spacing between marker layers d and the greater principal dimension of the inclusion a contrary to previous experimental work in the literature we used planar marker layers without concomitant boudinage our results show new processes of development of sheath folds from planar layers and also that some non cylindrical layer deflections are transient and thus that sheath folds do not originate from them when d is small and the inclusion starts with its longest axis normal to the shear plane sheath folds develop by drag pull processes on originally planar marker layers transient non cylindrical deflections develop on originally planar marker layers when the distance between the marker layers is smaller than the longest principal dimension of the rigid particle the monoclinic symmetry of the drag sheath fold patterns associated with rotating rigid inclusions indicate non coaxial flow and can be used as a shear sense criterion quarter structure

garnet peridotite from colorado plateau ultramafic diatremes hydrates carbonates and comparative geothermometry crystal fragments of pyrope from diatremes of ultramafic microbreccia in the navajo province of the colorado plateau contain inclusions of olivine pyroxene spinel chlorite amphibole chlorapatite and dolomite the included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the plateau lithosphere garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15 20 kb no evidence was found for inclusions from greater depths temperature estimates are 800 900 c for garnet clinopyroxene equilibration but only 500 700 c for garnetolivine equilibration inherent differences between geothermometry methods account for only part of the discrepancy pronounced fe mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling the garnet olivine temperature estimates may be the best approximations to mantle temperatures before eruption our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large scale horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

the importance of incompetence metabasaltic dikes that are known to be competent and to deform more slowly than granitoid country rocks in the upper crust are likely to be incompetent and deform faster than granitoid hosts in the higher temperatures of the lower crust layers that deform as competent strain markers develop inward pointing cusps these deformation structures are called folds in shortened layers and necks in extended layers and are reasonably well understood by contrast layers that deform as incompetent markers develop strong internal foliations and outward pointing cusps these structures are currently known as mullions in shortened contacts and inverse folds in extended contacts but are comparatively poorly understood here we emphasize the importance of the deformation structures in the incompetent hydrated metabasaltic intrusions and enclaves that must be among the structures that seismic reflection profiles attempt to image deep in the continental crust

evidence for low viscosity garnet rich layers in the upper mantle abstract the rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from western norway that garnet rich rocks can have lower solid state viscosities than olivine rich rocks modeling of melt free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet rich and olivine rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine grained garnet deforms by diffusion creep while the coarse grained olivine deforms by dislocation creep and or diffusion creep in general modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine grained garnets in a dry mantle environment or at conditions where the garnet pyroxene layer is partially molten i e close to solidus liquidus conditions in the upper mantle alternatively in cold plate tectonic settings e g in subduction zones some water weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine

influence of lithological heterogeneity mechanical anisotropy and magmatism on the rheology of an arc north cascades washington abstract many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere several of the controlling factors for rheology are difficult to study experimentally particularly lithological heterogeneity mechanical anisotropy and magmatism and we focus on these in a study of the deformation patterns in a thick crustal section 5 to 40 km paleodepth through the cretaceous cascades core of the nw cordillera this magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section and the remainder of the section is characterized by heterogeneous fold dominated ductile deformation early tight to isoclinal recumbent folds and associated axial planar fabrics are refolded by one or more cycles of nearly coaxial open to isoclinal upright to overturned folds layering played a mechanically active role during folding at all levels as indicated by cleavage refraction boudinaged layers and kinematic indicators that record fold related shear ductile deformation intensifies in the narrow structural aureoles of plutons and sw directed reverse shear was partitioned into some of the aureoles the poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus as commonly predicted all of the plutons have magmatic foliations that at least in part reflect regional strains and these foliations are strong in the deeper plutons the thinner sheets acted as competent bodies during folding and boudinage after they reached the solidus but generally did not cause marked strain gradients in their hosts a relative strength profile constructed for the cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles however in more detail relative strengths are markedly variable units were able to accumulate large ductile strains but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid to deep crustal part of the profile this profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale and should be applicable to many other magmatic arcs

a nontectonic origin for the present day stress field in the paris basin france the large scale stress patterns observed in intra plate areas are often considered to result from far field boundary forces that drive plate tectonics however no present day deformation has been detected in the french paris basin yet significant deviatoric stresses are measured in limestone formations observed above soft argillite layers encountered in this region further the pore pressure measured in the argillite is larger than that measured in the surrounding permeable zones these observations suggest a presently active source of stress in this sedimentary system we propose that this stress field is not related to tectonics but only to gravity acting on the series of visco elastic orthotropic geomaterials that fill up the basin this visco elastic response is linked to pressure solution effects activated by pore pressure transients related to climatic variations these pressure transients develop in the fracture system that affects some of the geomaterials and imply a time dependant deformation field with time constants related to those of climatic variations this model outlines the influence of time dependent material properties on the present day stress and pore pressure fields that prevail at various depths it may be considered as a possible loading mechanism for the analysis of intra plate seismicity

estimating the viscosity and prandtl number of the tso morari crystalline gneiss dome indian western himalaya the tso morari crystalline tmc gneiss dome in the indian himalaya extruded from a depth of 120 km through an inclined subduction channel of sub elliptical cross section at the leading edge of the indian plate the velocity profile of this gneiss dome is derived after 1 presuming its incompressible newtonian rheology 2 finding the best fit of the outcrop of the gneiss dome to an ellipse 3 taking into account different lithologies to have existed at the top of the extruding gneiss body 4 considering the extrusion to have been driven by the buoyant push of the denser mantle beneath the lighter gneiss and 5 assigning a range of plausible densities for different litho units fitting the known rates of extrusion from a few centimetres up to about one hundredth of a millimetre per year from 53 ma onwards of this gneiss dome to its velocity profile constrains its maximum possible viscosity to 7 5 1022 pa s this magnitude is 102 104 times higher than previous estimates for gneisses and granites alternative explanations of our data are the following 1 there was a fall in extrusion rates of the tmc gneiss from 53 to 30 ma because of an increase in the estimated maximum viscosity from 6 2 1020 to 7 5 1022 pa s possibly indicating a fall in temperature and or compositional change of the tmc gneiss 2 lower the extrusion rates higher are the estimated viscosities 3 the tmc gneiss was more viscous probably due to its eclogite content 4 the estimated maximum viscosity is 102 times higher than that in collision zones and 102 104 times than that in the tibetan lower crust but broadly conforms to that for the crustal channel and average lithospheric and asthenospheric values the high magnitude of maximum possible prandtl number of 1028 of the tmc gneiss might be related to isothermal decompression of the gneiss during its extrusion

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

relatively rapid emplacement of dome forming magma inferred from strain analyses the case of the acid latian dome complexes central italy abstract we have analysed the relationship between the volcanic substratum and magma body emplacement for the acid dome complexes of latium central italy our study shows that the volcanic edifices which are mainly pleistocene cryptodomes and related explosive products were derived from mantle magmas contaminated by crustal materials the cimini tolfa and cerite manziate dome complexes of latium show the following characteristics a shallow laccolith origin emplacement in basins that have identical tectonic evolution and geological structure the same magmatic composition and density contrast between magma and host rock and geochronological data that are inconsistent with field evidence in the cimini and tolfa dome complexes the deformation induced by shallow intrusions was accompanied by 200 m uplift of the sedimentary cover the estimated pluton infilling time for the cimini and tolfa domes is 10 2 years while the strain rate required to uplift their pliocene overburden by 200 m is e m 10 9 s 1 the rapid evolution of the dome complexes is consistent with field data that show no relevant interruptions in the volcanic activity and no significant compositional changes in the volcanic products related to the extrusion of the domes for the cerite manziate dome complex the minimal input rate of magma favoured a monogenetic style of volcanism independent of the regional stress conditions

are viscosity ratios of rocks measurable from cleavage refraction abstract in this questions contribution i consider some of the accepted methods of measuring viscosity ratios in rocks and pose a new one for a wide range of strain and orientations it can be shown that the bedding normal is immeasurably close to the xy plane and this has applications to the relationship between cleavage and strain i therefore propose that with some limitations cleavage refraction can provide a measure of effective viscosity ratios in layered rocks examples show that cleavage refraction across competence contrasts yields surprisingly small viscosity ratios this method might also provide a way of distinguishing newtonian from non newtonian behaviour of rocks over time and space

strain analysis and rheology contrasts in polymictic conglomerates an example from the seine metaconglomerates superior province canada deformed conglomerates typically display contrasting strains between different lithological clast populations and matrix which can be exploited to provide information about effective viscosity contrasts we analyzed three different clast populations granitoid mafic volcanic and felsic volcanic across a strain gradient in the seine metaconglomerates of ontario canada we calculated finite strain using the rf 4 method and bootstrap statistics granitoid clasts exhibited the least amount of strain while the mafic and felsic clasts had similar greater strains we assumed that the bulk finite strain was recorded by the mafic clasts given their large proportion of the rock and their similar composition to the matrix we calculated effective viscosity contrasts using the finite strain information assuming newtonian viscous rheology the two volcanic clast populations have similar effective viscosities regardless of strain magnitude with ratios of felsic mafic ranging from 0 27 to 2 12 and average ratio of 1 19 the granitoid clasts are significantly more competent than the mafic clasts with effective viscosity ratios ranging from 2 35 to 12 39 and average ratio of 5 61 the results are consistent with a qualitative

softening the lower crust modes of syn transport transposition around and adjacent to a deep crustal granulite nappe parry sound domain grenville province ontario canada 1 the parry sound domain is a granulite nappe stack transported cratonward during reactivation of the ductile lower and middle crust in the late convergence of the mesoproterozoic grenville orogeny field observations suggest the following with respect to the ductile sheath 1 formation of a carapace of transposed amphibolite facies gneiss derived from and enveloping the western extremity of the parry sound domain and separating it from high strain gneiss of adjacent allochthons this ductile sheath formed dynamically around the moving granulite nappe through the development of systems of progressively linked shear zones 2 transposition initiated by hydration amphibolization of granulite facies gneiss by introduction of fluid along cracks accompanying pegmatite emplacement shear zones nucleated along pegmatite margins and subsequently linked and rotated the source of the pegmatites was most likely subjacent migmatitic and pegmatite rich units or units over which parry sound domain was transported comparison of gneisses of the ductile sheath with high strain layered gneiss of adjacent allochthons show the mode of transposition of penetratively layered gneiss depended on whether or not the gneiss protoliths were amphibolite or granulite facies tectonites before initiation of transposition resulting in e g folding before shearing no folding before shearing respectively meter scale truncation along high strain gradients at the margins of both types of transposition related shear zones observed within and marginal to parry sound domain mimic features at kilometer scales implying that apparent truncation by transposition originating in a manner similar to the ductile sheath may be a common feature of deep crustal ductile reworking

peridotite xenoliths in silica rich potassic latite from the transition zone of the colorado plateau in north central arizona potassic latite in the transition zone of the colorado plateau near chino valley arizona contains abundant eclogite and amphibolite xenoliths and minor websterite and pyroxenite xenoliths one unit contains peridotite xenoliths analyzed samples have mg ratios of 68 and 71 58 and 63 wt sio2 and are enriched in potassium and other large ion lithophile lil elements rare earth element ree patterns are light ree enriched with la greater than 100 times chondritic abundance the peridotite xenoliths are partly to totally altered but contain remnant olivine orthopyroxene and clinopyroxene one harzburgite nodule also contains spinel mineral compositions from the xenoliths are relatively refractory and similar to those in other spinel peridotite xenoliths from the colorado plateau geothermometry on olivine spinel and two pyroxene pairs indicates equilibration temperatures of less than 800 c for the peridotite nodules the relatively low temperatures calculated from mineral equilibria are consistent with temperature estimates for other mantle nodules from under the colorado plateau

quantitative estimation of exponents of power law flow with confidence intervals in ductile shear zones abstract talbot journal of structural geology 21 1535 1551 1999b introduced a new model for ductile shear zones as counterflow boundaries in pseudoplastic power law fluids and demonstrated that natural shear zones empirically fit theoretical curves generated by his model thus estimates of the power law exponent relating stress and strain are deduced it is shown here how application of standard non linear statistical techniques to shear zone displacement data allows quantitative estimation of the power law exponent together with confidence intervals of the calculated value application to natural shear zones verifies the method and also validates the applicability of the model it is hoped that this will encourage field geologists to apply the model of talbot to natural examples thereby a database of power law exponents for a wide variety of lithologies and metamorphic grades may be assembled

analogue models of collapse calderas and resurgent domes collapse calderas and resurgent domes are a common association related to inflation deflation processes in volcanic systems the structure of calderas and domes depends upon the permitted relative movements of crustal volumes at depth the so called space problem in order to study the structures of collapse calderas and resurgent domes and to take the space problem into account several analogue models were made dry quartz sand was used to simulate the rheology of the brittle crust while newtonian silicone putty located at the base of the sand pack simulated the ductile behaviour of the magma a piston moved the silicone putty downward or upward inducing collapse and doming within the sand three separate sets of experiments simulated 1 caldera collapse 2 resurgence and 3 superimposition of resurgence on collapse and vice versa collapse experiments are characterized by the development of two concentric depressions the first formed depression is bordered by outward dipping reverse ring faults the subsequent outer concentric depression is bordered by inward dipping normal ring faults the deformation pattern during resurgence is a function of the overburden thickness t and the dome diameter d for higher t d ratios a dome forms bordered by inward dipping high angle reverse ring faults outward dipping normal ring faults develop at late stages for lower t d ratios the dome shows at late stages a crestal depression accompanied by radial fractures subsequently an apical extrusion of silicone occurs the superimposition of resurgence over collapse and vice versa is characterized by the complete reactivation with opposite kinematics of all the pre existing ring faults during inversion both in caldera and resurgence reverse ring faults form in the early stages due to differential uplift extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults the experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences

shape of pinch and swell structures as a viscosity indicator application to lower crustal polyphase rocks abstract pinch and swell structures occur where a more competent layer in a weaker matrix is subjected to layer parallel extension in this contribution we use numerical models to explore the use of pinch and swell structure shape symmetry and asymmetry as a determinant of relative viscosity between layers maximum asymmetry is attained when the matrix viscosity on one side is subtly weaker than the competent layer while the other side is significantly weaker our numerical results are directly applied to asymmetrically developed pinch and swell structures in exposed lower continental crust here shape geometries observed in a shear zone comprised of plagioclase dominated garnet dominated and mixed amphibole plagioclase dominated bands reveals that the plagioclase dominated band is the most competent band and is marginally stronger 2 and significantly stronger 10 40 than the fine grained garnet dominated and mixed amphibole plagioclase dominated band respectively based on the experimentally determined viscosity of a plagioclase dominated material and quantitative microstructural analysis the viscosity range of the natural rock bands is 2 8 1015 to 1 1 1017 pa s consequently the assumption that the experimentally derived plagioclase flow law is an appropriate proxy for the middle to lower continental crust may lead to a viscosity over estimation by up to forty times