the tertiary structural and thermal evolution of the central alps compressional and extensional structures in an orogenic belt the western alpine arc has been created during the cretaceous and the tertiary orogenies the interference patterns of the tertiary structures suggest their formation during continental collision of the european and the adriatic plates with an accompanying anticlockwise rotation of the adriatic indenter extensional structures are mainly related to ductile deformation by simple shear these structures developed at a deep tectonic level in granitic crustal rocks at depths in excess of 10 km in the early palaeogene period of the tertiary orogeny the main tertiary nappe emplacement resulted from a nw thrusting of the austroalpine penninic and helvetic nappes heating of the deep zone of the upper cretaceous and tertiary nappe stack by geothermal heat flow is responsible for the tertiary regional metamorphism reaching amphibolite facies conditions in the lepontine gneiss dome geothermal gradient 25 c km the tertiary thrusting occurred mainly during prograde metamorphic conditions with creation of a penetrative nw se oriented stretching lineation xi finite extension parallel to the direction of simple shear earliest cooling after the culmination of the tertiary metamorphism some 38 ma ago is recorded by the cooling curves of the monte rosa and mischabel nappes to the west and the suretta nappe to the east of the lepontine gneiss dome the onset of dextral transpression with a strong extension parallel to the mountain belt and the oldest s vergent backfolding took place some 35 to 30 ma ago during retrograde amphibolite facies conditions and before the intrusion of the oligocene dikes north of the periadriatic line the main updoming of the lepontine gneiss dome started some 32 30 ma ago with the intrusion of the bergell tonalites and granodiorites concomitant with s vergent backfolding and backthrusting and dextral strike slip movements along the tonale and canavese lines argand s insubric phase subsequently the center of main updoming migrated slowly to the west reaching the simplon region some 20 ma ago this was contemporaneous with the westward migration of the adriatic indenter between 20 ma and the present the western aar massif toce culmination was the center of strong uplift the youngest s vergent backfolds the glishorn anticline and the berisal syncline fold the 12 ma rb sr biotite isochron and are cut by the 11 ma old rhone simplon line the discrete rhone simplon line represents a late retrograde manifestation in the preexisting ductile simplon shear zone this fault zone is still active today the oligocene neogene dextral transpression and extension in the simplon area were concurrent with thrusting to the northwest of the helvetic nappes the prealpes 35 15 ma and with the jura thin skinned thrust 11 3 ma it was also contemporaneous with thrusting to the south of the bergamasc 35 5 ma and milan thrusts 16 5 ma the oligocene neogene dextral transpression in the central alps is of the same age as the thrusting to the west and southwest in the external french part and to the east in the internal italian part of the alpine arc of the western alps

relating collisional kinematics to exhumation processes in the eastern alps abstract based on a review of the surface and deep structure of the eastern alps we link the timing and the inferred displacement fields to exhumation of upper and lower crustal units of the orogenic nappe stack during collision the discussion focuses mainly on the tauern window and its country rocks the only area of the eastern alps where the orogenic wedge from its uppermost austroalpine nappes down to its deepest european basement nappes is continuously exposed we summarize and discuss the long standing controversy on the mechanisms of exhumation of this nappe stack on the base of a synthesis of structural and geochronological data and restorations of collisional displacements both in cross sections and map views we conclude that the large amounts of exhumation assessed for the western eastern alps resulted from large amounts of thickening and erosion not observed in the eastern part of the eastern alps extensional faults laterally bounding the area of major thickening and exhumation are inferred to nucleate in order to accommodate displacement around the indenter corner in the west and in order to reduce a large gradient of crustal thickness and surface elevation in the east restorations to the pre indentation stage document an amount of northward increasing orogen parallel extension varying 45 km to 85 km corresponding to 15 of extension that is partly accommodated along normal faults n s shortening between the northern calcareous alps and the dolomites indenter attained 75 km in the west and decreased to 30 km in the east 55 km out of these 75 were accommodated in the area of the tauern window our kinematic model shows that lateral extrusion accommodated along conjugate strike slip faults requires large amounts of north south shortening in the western part of the eastern alps such shortening is consistent with the reconstructed upright folding and erosion of the tauern window thus explaining the largest amount of its exhumation in contrast the eastern termination of the eastern alps represents an area where collisional convergence was barely accommodated by crustal thickening this transition from a highly shortened thickened and exhumed wedge in the west mainly affected by orogen perpendicular displacements to a barely shortened and exhumed wedge in the east mainly characterised by orogen parallel displacements spatially coincides with a change in the deep structure of the european slab indeed the inferred continental european slab imaged in the west disappears into a low velocity anomaly where no slab is detected in the east an inherited step in the geometry in map view of the european passive margin causing its crust to enter the subduction zone earlier than the area east of the tauern window may explain the rapid decrease of shortening of thickening the different syn collisional p t gradients and the disappearance of the continental slab east of the katschberg fault

metamorphic field gradients in the central alps metamorphic field gradients were determined across the entire amphibolite grade central alps c 50 100 km p t were calculated from 116 samples acquired from our own field work from samples provided to us by others and from rocks with mineral compositions described in the literature only fluid conserved equilibria were used to determine p t the use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors the results are presented in contour maps temperature increases from 500 to 550 c along the limit of amphibolite grade metamorphism in the north and west to c 675 c toward the south at the insubric line near the town of bellinzona maximum recorded pressures of c 7 kbar are in a central region c 20 km north of the insubric line and decrease both to the north 5 5 kbar and south 4 5 kbar the p t results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep 40 c of the kyanite sillimanite phase boundary comparison of p t conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source examination of the two dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses aected the central alps during the tertiary in the second heat aected only the southern parts of the area and overprinted the previously established p t gradients

correlations between fluid composition and deep seated structural style in the footwall of the simplon low angle normal fault switzerland structural and fluid inclusion analyses reveal a correlated change in structural style and fluid composition along strike in footwall mylonites of the simplon low angle normal fault of switzerland in the south early postmylonitic structures are semibrittle shears and brittle faults synkinematic fluid inclusions within the faults contain a carbonic component whereas those in the shears lack co 2 to the north early postmylonitic structures are extension veins and kinkbands and synkinematic fluid inclusions are aqueous entrapment conditions for all fluid inclusions are estimated to be 500 c and 5 kbar we speculate that different wetting characteristics of carbonic versus aqueous fluids influenced the mechanical behavior of the rocks nonwetting carbonic fluids locally increased fluid pressure and induced brittle failure resulting in development of abundant closely spaced brittle faults in areas lacking carbonic fluids ductile deformation continued to lower pressures and temperatures

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

synthetic fluid inclusions vii re equilibration of fluid inclusions in quartz during laboratory simulated metamorphic burial and uplift p t conditions inferred from fluid inclusions in metamorphic rocks often disagree with the values predicted from mineral equilibria calculations these observations suggest that inclusions formed during early stages of regional metamorphism continue to re equilibrate during burial and subsequent uplift in response to differential pressure p t conditions accompanying burial and uplift were experimentally simulated by initially forming pure h2o inclusions in quartz at elevated temperatures and pressures and then re equilibrating the inclusions in the presence of a 20 wt nacl solution such that final confining pressures ranged from 5 kbar above to 4 kbar below the initial internal pressure of the inclusions at the temperature of re equilibration in all samples re equilibrated at confining pressures below the internal pressure some inclusions were formed that had compositions of 20 wt nacl and densities in accord with the final p t conditions additionally some inclusions were observed to contain fluids of intermediate salinities between 0 and 20 wt nacl densities of these inclusions were also consistent with formation at the re equilibration p t conditions the remainder of the fluid inclusions observed in these samples contained pure h2o and their homogenization temperatures corresponded to densities intermediate between the initial and final p t conditions in short term experiments 7 days where the initial internal overpressure exceeded 1 kbar no inclusions were found that contained the original density and none were found to have totally re equilibrated instead most h2o inclusions re equilibrated until their internal pressures were between 750 and 1500 bars above the confining pressure regardless of the initial pressure differential in a long term experiment 52 days inclusions re equilibrated at a lower confining pressure than the initial internal pressure displayed homogenization temperatures corresponding to a range in final internal pressures between 0 kbar i e total re equilibration and 1 2 kbar above the confining pressure in experiments where the confining pressure during re equilibration exceeded the initial internal pressure densities of pure h2o inclusions increased to values intermediate between the initial and final p t conditions additionally these inclusions were generally surrounded by a three dimensional halo of smaller inclusions also of intermediate density resulting in a texture similar to that previously ascribed to decrepitation from internal overpressure in extreme cases where confining pressures were 4 5 kbar above the initial pressure the parent inclusion almost completely closed leaving only the three dimensional array of small 5 m inclusions the outline of which may be several times the volume of the original inclusion groups of such inclusions closely resemble textures commonly observed in medium to high grade metamorphic rocks inclusions containing 10 and 42 wt nacl solutions trapped at 600 c and 3 kbar were re equilibrated at 600 c and 1 kbar for 5 days in dry argon to evaluate the importance of h2o diffusion as a mechanism of lowering the inclusion bulk density salinities of re equilibrated inclusions obtained from freezing point depressions and halite dissolution temperatures indicate that original compositions were preserved density changes similar to those previously described were noted in these experiments in inclusions showing no visible microfractures therefore density variations observed in inclusions in this study re equilibrated under rapid deformation conditions are considered to result from a change in the inclusion volume without significant loss of contents by diffusion or leakage

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

synthetic fluid inclusions in natural quartz iii determination of phase equilibrium properties in the system h2o nacl to 1000 c and 1500 bars phase equilibria in the system h2o nacl have been determined to 1000 c and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion free brazilian quartz in the presence of the two coexisting immiscible h2o nacl fluids at various temperatures and pressures petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present or mixtures of the two salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single homogeneous fluid phase results of the present study are consistent with previously published data on the h2o nacl system at lower temperatures and pressures and indicate that the two phase field extends well into the p t range of most shallow magmatic hydrothermal activity as a consequence chloride brines exsolved from many epizonal plutons during the process of second boiling should immediately separate into a high salinity liquid phase and a lower salinity vapor phase and produce coexisting halite bearing and vapor rich fluid inclusions this observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions particularly the porphyry copper deposits in which halite bearing and coexisting vapor rich inclusions are commonly associated with the earliest stages of magmatic hydrothermal activity

pore pressure stress increase and fault weakening in low angle normal faulting low angle dip 30 normal faults accommodate much extension of the continental crust they apparently move under low resolved shear stress and are anomalously weak characteristics that they share with the san andreas fault structural textural and geochemical arguments suggest that low angle normal faults are weak in both the ductile and brittle regimes partly or totally due to elevated pore fluid pressure high pore pressure in detachment zones may be contained by upper plate strata mineral precipitation in their hanging walls formation of low permeability microbreccia layers threshold pressure gradients and low permeability mylonites below chlorite breccia mechanical analysis shows that fault weakening may preclude equality of the regional and fault zone stress tensors and predicts reorientation and increase of principal stresses in weak fault zones these changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks fault weakening focuses extension in the upper crust onto low angle normal ductile brittle shear zones in the midcrust promoting propagation of low angle brittle normal faults into the upper crust

eastern alpine tectono metamorphic evolution constraints from two dimensional p t t modeling we use two dimensional 2 d p t t modeling to constrain the thermal and rheological aspects of different scenarios for the late mesozoic and cenozoic tectonic evolution of the eastern alps inferred from excellent data sets from the tauern window tw models invoking subduction of the south penninic sp oceanic lithosphere during thrusting and subsequent erosion of the austro alpine aa upper plate nappe stack are inconsistent with the observed thermal evolution within the aa and penninic units in these models predictions for the aa peak thermal conditions are lower than observed after exhumation and cooling to midcrustal levels and subduction of the continental middle penninic mp block the aa undergoes a phase of renewed heating to almost the previous peak temperatures simultaneously the penninic units experience a phase of heating upon subduction followed by cooling after onset of subduction of the north penninic np basin the model predictions are inconsistent with the observed nearly isothermal uplift path of the sp after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper aa amphibolite facies down to the higher penninic unit greenschist facies a model with the beginning of subduction of the sp occurring after crustal thickening of the aa and subsequent return to normal crustal thicknesses is compatible with the p t t data in this model peak temperature conditions are higher in the aa followed by a phase of strong cooling in the aa upper plate with the onset of underthrusting this model also explains successfully nearly isothermal exhumation of the mp and inverted metamorphic peak conditions in the deeper aa material accreted to the hanging wall from the oceanic crust sp experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heat up to its thermal climax after the subduction of trailing continental lithosphere the subsequent pt path of the penninic units strongly depends on the timing and rates of underthrusting by the foreland observed pt paths in the mp within the tw require continuous subduction of the np and the trailing european foreland under the exhuming mp block documented rapid cooling in the final uplift phase of the penninic units in the tw requires exhumation rates up to approximately 4 mm yr predictions of slightly elevated present day geothermal gradients in the tw area are consistent with available heat flow data as a result of mesozoic rifting followed by late mesozoic crustal thickening of the aa paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the penninic units can explain observed late cretaceous crustal extension in the aa units in terms of gravitational spreading ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric mantle beneath the central orogen further enhanced by rapid late stage uplift predictions for the present day rheological structure of the eastern alps support the existence of a strong upper crustal layer two wedge shaped strong upper mantle layers to the north and the south of the orogen and a weak upper mantle underlying the central orogen

seismic velocity reduction and accelerated recovery due to earthquakes on the longmenshan fault various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults although there are growing numbers of observations on coseismic velocity reductions generally there is a lack of detailed observations of the healing phases here we report on a pronounced coseismic reduction of velocities around two locked sections asperities of the longmenshan fault with a large slip during the 2008 mw 7 9 wenchuan earthquake and subsequent healing of the velocities the healing phase accelerated significantly at the southern asperity right after the nearby 2013 mw 6 6 lushan earthquake the results were obtained by joint inversions of travel time data at four different periods across the wenchuan and lushan earthquakes the rapid acceleration of healing in response to the lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes we suggest that stress redistribution plays an important role in rebuilding fault strength after a fault ruptures the recovery of its strength may be accelerated by earthquakes that redistribute stress according to an analysis of temporal variation in crustal seismic velocity on the longmenshan fault in china

titanium in quartz thermometry on synkinematic quartz veins in a retrograde crustal scale normal fault zone abstract previous studies have suggested that estimation of deformation temperatures in quartz mylonites by titanium in quartz geothermometry is only possible at temperatures 500 c above which efficient ti exchange is achieved via grain boundary migration recrystallization based on quartz mylonite samples collected across the simplon fault zone sfz we demonstrate that deformation temperatures of dynamic recrystallization can be obtained down to 350 c a prerequisite for such temperature estimates at the low temperature end of ductile deformation of quartz is the formation of synkinematic quartz veins and their immediate overprint either by subgrain rotation sgr or bulging recrystallization blg it is the slow growth of the synkinematically precipitating vein quartz that allows for equilibration of ti in the vein quartz this ti concentration may only slightly be modified during sgr hence ti in qtz thermometry provides a close approach to the vein formation temperature ti concentrations are partially reset during blg and resulting temperatures are thus maximum temperatures of quartz recrystallization importantly undeformed vein quartz always yield vein formation temperatures investigation of the dynamic recrystallization processes overprinting synkinematic quartz veins thus allows for a critical independent evaluation of the ti in quartz temperatures obtained for the sfz there is a decrease in recrystallized grain sizes towards the fault plane and a change in the dominant recrystallization process associated with a narrowing of the shear zone as indicated by the ti in quartz temperature estimates this strain localization correlates with cooling from 560 c in the oldest microstructures at the periphery of the shear zone down to 350 c in the youngest microstructures of the footwall near the hanging wall contact a great benefit of the approach presented here is that intermediate to low temperature plastic deformation in quartz can now also be assessed such novel temperature constraints on quartz crystallization are essential for better constraining deformation and rheology in the upper earth s crust

exhumation in a convergent orogen the western tauern window unroofing of the western tauern window involved both low angle detachment faulting brenner fault and enhanced footwall erosion contemporaneous with upright antiformal folding this combination reflects orogen parallel e w extension during continued n s alpine convergence new fission track ages establish the relative chronology of folding and faulting and demonstrate that displacement was not always accommodated on the same surface during exhumation some units migrated from the footwall to the hanging wall of the main detachment fault due to the interplay between folding and faulting the region can effectively be divided into 3 distinct domains 1 the penninic units of the western tauern window were always in the footwall to the fault with maximum exhumation in the core of the dome due to folding and erosion 2 the lower austroalpine unit north of the tauern window was first part of the hanging wall to the brenner fault at a later stage this unit was exhumed by a further 4 5 km as part of the footwall to a more discrete through going fault the silltal fault 3 the middle and upper austroalpine units west of the tauern window were always within the hanging wall exhumation of the footwall from an initial depth of 25 km led to a transition in mechanical behaviour the curviplanar folded ductile shear zone marking the boundary to the tauern window was eventually transected by a more planar discrete brittle fault silltal fault with unit 2 now in the footwall along which the pre existing mylonites were passively exhumed to the surface

estimating displacement along the brenner fault and orogen parallel extension in the eastern alps the major structure accommodating orogen parallel extension in the eastern alps is inferred to be the brenner fault which forms the western boundary of the tauern window the estimated amount of extension along this fault varies from a minimum of 10 20 km to a maximum of 70 km all investigations that have attempted to constrain this amount of extension have calculated the fault plane parallel displacement required to restore the difference in structural level between footwall and hanging wall as constrained by geobarometry however these calculations neglected the component of exhumation of the footwall resulting from folding and erosion therefore the total amount of extensional displacement was systematically overestimated in the present study we project a tectonic marker surface from the footwall and hanging wall of the brenner fault onto a n s striking cross section this marker surface which is the base of the patscherkofel unit in the footwall and the base of the otztal basement in the hanging wall is inferred to have occupied the same structural level in the hanging wall and footwall of the brenner fault before its activity therefore the difference in height between the marker projected from the footwall and from the hanging wall is a measure of the vertical offset across the brenner fault this construction shows that the vertical offset of the marker horizon on both sides of the brenner fault varies strongly and continuously along strike of the brenner fault attaining a maximum value of 15 km at the hinge of the folded footwall tauern dome the along strike change of vertical offset is explained by large scale upright folding of the footwall that did not affect the hanging wall of the brenner fault therefore the difference in vertical offset of 10 km between the area of the brenner pass and the area immediately south of innsbruck corresponds to the shortening upright folding component of exhumation of the footwall the remaining 5 km of vertical offset must be attributed to extensional deformation the brenner fault itself is barely folded its dip varies between 20 and 70 and it crosscuts the upright folds of the western tauern window given the offset of 5 km the dip of the fault constrains the extensional displacement to be between 2 and 14 km we conclude that the tauern window was exhumed primarily by folding and erosion not by extensional unroofing

late stages of exhumation constrained by structural fluid inclusion and fission track analyses sesia lanzo unit western european alps in metamorphic areas paleogeothermal gradients are difficult to properly estimate leading to great uncertainty in exhumation rate calculations based on fission track analysis in this work a new multidisciplinary approach based on fluid inclusion analysis structural analysis and fission track dating has proven to be able to constrain the late stages of exhumation of metamorphic rocks the study area is located in the sesia lanzo unit of the western european alps along the hone shear zone kinematic analysis at different scales unraveled a polyphase postmetamorphic deformation history constrained in the p t space thanks to laboratory investigation of kinematically referenced fluid inclusions coupled with the analysis of the rheological behavior of quartz and feldspar inside syn kinematic veins the resulting p t path points to an increasing paleogeothermal gradient from 18 to 30 c km 1 after greenschist facies metamorphic conditions and provides a reference framework for a quantitative interpretation of fission track data in terms of exhumation rates and fault throws according to fission track data throws accommodated by the hone shear zone between 33 and 30 ma are on the order of 4 5 km and would be underestimated by 25 if a normal steady gradient was assumed for the whole post metamorphic history of the area the deepest portions of this structure were exhumed thanks to the activity of the e w fault system that accommodated a throw of 4 km between 28 and 20 ma and a throw of 0 8 km during the last 20 myr

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

postmylonitic deformation in the raft river metamorphic core complex northwestern utah evidence of a rolling hinge much of the recent debate over low angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model the model predicts tilting of seismogenic high angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation this shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex the predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms two end members have been proposed subvertical simple shear and flexural failure each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction subvertical simple shear svss should generate subvertical faults and kink bands with a shear sense antithetic to the detachment for an svss hinge the hinge related strain magnitude should depend only on initial fault dip rolling hinge structures should shorten the mylonitic foliation by 13 for an initial fault dip of 30 in flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation extensional sets could predominate as a result of superposition of far field and flexural stresses strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam postmylonitic structures were examined in the footwall of the raft river metamorphic core complex in northwestern utah to test these predictions observed structures strike perpendicular to the regional extension direction and include joints normal faults tension gash arrays and both extensional and contractional kink bands aside from the subvertical joints the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets range wide the extensional structures accomplish about 4 elongation of the mylonitic foliation contractional structures dip steeply mainly record shear antithetic to the detachment and accomplish 1 contraction of the foliation these observations are consistent with the presence of a rolling hinge in the raft river mountains but a rolling hinge that reoriented a high angle normal fault by svss is excluded the pattern and magnitudes of strain favor hinge related deformation mainly by flexural failure with a subordinate component of svss

isostatic rebound due to tectonic denudation a viscous flow model of a layered lithosphere a four layer model of the upper 150 km of the earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation the model comprises a strong upper crustal layer a weak lower crustal layer a very strong mantle lithosphere layer and a weak mantle asthenosphere layer which is in accord with experimental constraints on strength depth profiles for continental lithosphere the strength of each layer is represented by its effective viscosity flow in the crust and mantle is driven by buoyancy forces which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements earthquakes or rapid creep events numerical solutions obtained by using a finite element technique predict footwall uplift moho deflection and surface topography that are consistent with observations from the basin and range province of the western united states the calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault such bending need not be elastically controlled hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust the upward deflection of the moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities by matching observations from the basin and range province which indicate no moho deflection and low magnitude of surface topography 3 5 km we estimate the upper crustal lower crustal and mantle lithospheric viscosities in the ranges 1021 1023 pa s 1019 1021 pa s and 1021 1023 pa s respectively

geology and thermochronology of tertiary cordilleran style metamorphic core complexes in the saghand region of central iran an 100 km long north south belt of metamorphic core complexes is localized along the boundary between the yazd and tabas tectonic blocks of the central iranian micro continent between the towns of saghand and posht e badam amphibolite facies mylonitic gneisses are structurally overlain by east tilted supracrustal rocks including thick 1 km steeply dipping nonmarine siliciclastic and volcanic strata near the detachment the neybaz chatak fault the gneisses are generally overprinted by chlorite brecciation crosscutting relationships along with u pb zircon and 40 ar 39 ar age data indicate that migmatization mylonitic deformation volcanism and sedimentation all occurred in the middle eocene between ca 49 and 41 ma the westernmost portion of the tabas block immediately east of the complexes is an east tilted crustal section of neoproterozoic cambrian crystalline rocks and metasedi mentary strata 10 km thick the 40 ar 39 ar biotite ages of 150 160 ma from structurally deep parts of the section contrast with ages of 218 295 ma from shallower parts and suggest late jurassic tilting of the crustal section these results define three events 1 a late jurassic period of upper crustal cooling of the western tabas block that corresponds to regional jurassic cretaceous tectonism and erosion recorded by a strong angular unconformity below mid cretaceous strata throughout central iran 2 profound approximately east west middle eocene crustal extension plutonism and volcanism ca 44 40 ma and 3 2 3 km of early miocene ca 20 ma erosional exhumation of both core complex and tabas block assemblages at uppermost crustal levels resulting from significant north south shortening the discovery of these and other complexes within the mid tertiary magmatic arcs of iran demonstrates that cordilleran style core complexes are an important tectonic element in all major segments of the alpine himalayan orogenic system

fluid inclusion constraints on the kinematics of footwall uplift beneath the brenner line normal fault eastern alps dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear each class of model predicts a different incremental strain history that should be recorded in the footwall in the tauern window eastern alps postmylonitic structures in the footwall of the brenner line normal shear zone predominantly consist of closely spaced steep west down and east down microfaults formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear in contrast formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes the field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed however both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion lined microfaults with the macroscopic west down and east down faults obtaining density data for the inclusions and correlating these data with previously obtained geochronologic data the results indicate that the west down structures formed at depths of 10 20 km and temperatures 450 c in the mid to late oligocene and that the east down structures formed at 2 to 10 km depth and temperatures of 300 50 c in the mid miocene these data support the hypothesis that a rolling hinge was present in the footwall of the brenner line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear the depths at which west down and east down faulting occurred coupled with the angle of dip of the brenner line yield a minimum lateral displacement on the fault of 15 26 km approximately coeval ductile shearing and brittle faulting at depths of 15 20 km and temperatures in excess of 400 c may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

geomorphic expression of midcrustal extension in convergent orogens 1 channel orientation gradient drainage basin spacing drainage patterns and alluvial fan sedimentology define a set of geomorphic observables for fingerprinting modes of extensional tectonism in convergent settings anomalous finely textured trellis drainage patterns in the alps are spatially coincident with the low angle simplon and brenner detachments the detachment footwalls are characterized by numerous high angle brittle faults which are etched out by low order stream gullies and channels we quantify the relationship between these faults and watershed scale geomorphology using high resolution digital topography to extract channel and basin metrics topographic data are complemented by a field data set of channel gradients and the integrated erosional response of the watersheds preserved in valley bottom alluvial fans in footwall regions adjacent to the detachments channels are steeper more closely spaced have statistically coherent orientations and feed coarse steep alluvial fans drainage spacing increases in the footwall away from the detachment and the trellis pattern diminishes in the detachment hanging walls which typically lack high angle brittle faults there is a poor correlation between channel steepness density or orientation and hanging wall alluvial fans tend to be relatively larger gentler and debris flow dominated comparisons are made to the ticino dome region of the alps as well as to the pioneer core complex of central idaho in both cases our analysis suggests high angle brittle faulting of the footwall is particularly well preserved in the region adjacent to a detachment these results are consistent with a subvertical simple shear model for footwall extrusion as well as annealing or a lack of reactivation of brittle structures in the footwall distal to the rolling hinge

deformation at depth associated with the 12 may 2008 mw 7 9 wenchuan earthquake from seismic ambient noise monitoring 1 we track the temporal evolution of seismic wave speed to detect changes in material properties at depth driven by deformation associated with the 2008 mw 7 9 wenchuan earthquake we analyze ambient noise correlation functions to monitor seismic wave speed variations the data were continuously recorded during 2 years by 114 broadband stations in a region that covers the southern two thirds of the ruptured fault we perform the analysis in the 12 to 20 s period band by comparison with measurements in the band 1 to 3 s we show that the seismic velocity changes cannot be explained by a shallow perturbation but are related to deformation at depth in the crust the spatial and temporal characteristics of these variations can thus be viewed as reflecting the middle crust behavior in particular the results suggest that the deformation in the middle crust is different beneath tibet and the sichuan basin

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

lateral displacement during neogene convergence in the western and central alps although major deformation in the western alps is clearly the result of north northwest directed thrust tectonics there is evidence suggesting that orogen parallel deformation may have been important in the late tectonic history of the western alps the authors propose that this northeast trending deformation includes 1 southwest directed normal fault movement along the simplon line 2 a diffuse zone of northeast striking dextral strike slip deformation along the rhone valley in switzerland between the mont blanc and aiguilles rouges massifs and through the belledonne massif and 3 southwest directed thrusting in the embrunais ubaye and digne nappe systems of southeastern france the correlation of these three broad regions of deformation is based on similar amounts of minimum displacement consistent kinematics and timing of deformation

two dimensional thermal modelling of normal faulting the simplon fault zone central alps switzerland abstract two dimensional finite difference thermal modelling of the major extensional simplon fault zone in the central alps establishes that the shape of cooling curves for samples from the footwall and hanging wall at various distances from the detachment are very sensitive to the actual displacement history if the displacement had occurred within a very limited time span 18 15 ma samples from the hanging wall close to the detachment should have significantly increased in temperature with a peak around 14 ma this is not discernible in the cooling curves interpreted from the mineral ages if the rate of normal fault displacement had been lower but constant from 18 ma to present the cooling curves for the footwall should show a correspondingly steady decrease in temperature the interpreted cooling curves from mineral ages are steep after 18 ma becoming shallower in more recent times this shape can be reproduced by the thermal modelling for a variable rate of fault displacement initially rapid total relative displacement 10 mm a across the fault zone between 18 and 15 ma but reducing to a more steady 0 4 mm a between 15 ma and 3 ma the corresponding exhumation rate of the footwall was 4 6 mm a from 18 to 15 ma and 0 6 mm a from 15 to 3 ma whereas the hanging wall was exhumed at a constant rate of 0 4 mm a the difference in cooling curves for samples with increasing horizontal distance into the footwall reflects a distributed ductile shear with the sample at 20 km distance initially at 25 km depth at 25 ma whereas that at 70 m distance was only at 19 km depth at the same time samples in the hanging wall were at 10 km depth total relative displacement parallel to the fault plane was around 36 km the thermal modelling demonstrates that during rapid fault displacement the apparent exhumation rate calculated assuming a constant geothermal gradient is too low whereas the apparent rate calculated for the subsequent period of slower displacement is too high

active tectonism in the central alps contrasting stress regimes north and south of the rhone valley an integrated interpretation of seismicity fault plane solutions and deep seismic reflection data suggests that the ne sw to nw se trending rhone simplon fault zone and the gently s dipping basal penninic thrust separate fundamentally different stress regimes in the western swiss alps north of the rhone simplon fault zone strike slip earthquakes on steep dipping faults within the helvetic nappes are a consequence of regional nw se compression and ne sw extension to the south vertical maximum stress and n s extension are responsible for normal mechanism earthquakes that occur entirely within the penninic nappes above the basal penninic thrust such normal faulting likely results from extension associated with southward movements collapse of the penninic nappes and or continued uplift and relative northward displacements of the underlying alpine massifs geological mapping and fission track dating suggest that the two distinct stress regimes have controlled tectonism in the western swiss alps since at least the neogene

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

synthetic fluid inclusions vi quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure h2o synthetic inclusions formed by healing fractures in natural quartz three different modes of non elastic deformation referred to as stretching leakage or partial decrepitation and total decrepitation have been observed the internal pressure required to initiate non elastic deformation is inversely related to inclusion size according to the equation internal pressure kbar 4 26 d 0 423 where d is the inclusion diameter in microns regularly shaped inclusions require a higher internal pressure to initiate non elastic deformation than do irregularly shaped inclusions of similar size heating inclusions through the quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion long term heating experiments 2 years suggest that the internal pressure required to initiate non elastic deformation does not decrease significantly with time and indicates that short lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature inclusions that do exhibit decreased density higher homogenization temperature are however always accompanied by a change in shape from irregular to that of a negative crystal observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals these results indicate that an inclusion will not be influenced by a neighbouring inclusion or other defect in the host phase as long as the distance between the two is 2 4 diameters of the larger of the two inclusions

miocene lateral extrusion in the inner western alps revealed by dynamic fault analysis 1 from early miocene to the present day the core parts of the western european alps experienced brittle extensional deformations mostly in a strike parallel direction here we present new data constraining the brittle deformation of the vanoise area french alps and a synthesis of 312 paleostress tensors in the whole arc of the internal western alps the data show a continuous change in the direction of extension from n065 simplon area to n s vanoise area and to nnw sse briancon area the abundance of orogen perpendicular 3 axes increases from the north to the south in the brianconnais area an extensional reactivation of the basal penninic thrust seems to be the origin of the e w to ne sw oriented 3 in light of these new data and the regional paleostress synthesis we propose a predominant orogen parallel extension in the internal zone as a whole this orogen parallel extension is related to the indentation rotation of the apulian microplate and to the opening of the ligurian sea during the lower middle miocene the locally observed orogen perpendicular extension is interpreted as an effect of the exhumation of the internal crystalline massifs the uplift of the external crystalline massifs and or the present day geodynamics postorogenic gravitational collapse some transcurrent tectonics older than the extension in the valais area and younger than the extension farther south is observed in the entire inner western alps strike slip movements are correlated with the apulian rotation and local permutation of stress axes

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

extensional neotectonics around the bend of the western central alps an overview the western alps active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive compressive tectonics along its borders we examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics numerical modeling gps morphotectonics fieldwork and brittle deformation analysis extension appears to be the dominant process in the present day tectonic activity in the western alps affecting its internal areas all along the arc shortening in contrast is limited to small areas located along at the outer borders of the chain strike slip is observed throughout the alpine realm and in the foreland the stress orientation pattern is radial for 3 in the inner extensional zones and for 1 in the outer transcurrent tranpressional ones extensional areas can be correlated with the parts of the belt with the thickest crust quantification of seismic strain in tectonically homogeneous areas shows that only 10 20 of the geodesy documented deformation can be explained by the alpine seismicity we propose that alpine active tectonics are ruled by isostasy buoyancy forces rather than the ongoing shortening along the alpine europe adria collision zone this interpretation is corroborated by numerical modeling the neogene extensional structures in the alps formed under increasingly brittle conditions a synthesis of paleostress tensors for the internal parts of the west alpine arc documents major orogen parallel extension with a continuous change in 3 directions from ene wsw in the simplon area to n s in the vanoise area and to nnw sse in the briancon area minor orogen perpendicular extension increases from n to s this second signal correlates with the present day geodynamics as revealed by focal plane mechanisms analysis the orogen parallel extension could be related to the opening of the ligurian sea during the early middle miocene and to compression rotation of the adriatic indenter inducing lateral extrusion

coseismic fault slip of the 2008 mw 7 9 wenchuan earthquake estimated from insar and gps measurements 1 we infer co seismic fault slip during the 2008 mw 7 9 wenchuan earthquake from interferometric synthetic aperture radar insar and gps observations of ground deformation we use alos palsar data from ascending orbits on six tracks and we do not use data that are strongly affected by ionospheric perturbations we use a fault model composed of three planar fault segments of the beichuan fault and one planar segment representing the parallel pengguan fault maximum thrust slip is up to 6 7 m near the surface and occurs in two locations near yingxiu in the south and beichuan in the center of the rupture maximum strike slip is over 4 m and occurs near pingtong and nanba along the northern end of the rupture we find that the ratio of coseismic thrust to strike slip on the beichuan fault decreases from 1 5 to 0 7 from the sw to the ne citation feng g e a hetland x ding z li and l zhang 2010 coseismic fault slip of the 2008 mw 7 9 wenchuan earthquake estimated from insar and gps measurements geophys res lett 37 l01302 doi 10 1029 2009gl041213

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

fluid inclusion evidence for basement decompression during permo triassic extension se new england usa co2 bearing fluid inclusions in strongly lineated but weakly foliated late precambrian gneisses within the hope valley shear zone of connecticut and rhode island are of mixed composition xco2 0 1 7 wt nacl equivalent and variable density 0 59 0 86 g ml and occur mainly as isolated inclusions also present are dilute 3 wt nacl equivalent aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism isochores for dense isolated co2 bearing inclusions indicate pressures of 7 5 9 kbar at 500 600 c the estimated temperature conditions of peak metamorphism published 40ar 39ar hornblende plateau age spectra indicate cooling through about 500 c at 265 5 ma isochores for low density co2 bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism 325 400 c and indicate pressures of 3 4 kbar published 40ar 39ar biotite plateau ages indicate cooling through about 300 c at 250 5 ma these data define a p t uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0 4 and 3 3 mm year in permian time exhumation of basement gneisses was coeval with normal west down motion along the regional basement cover contact honey hill lake char willimantic fault system and is interpreted as due to post orogenic extensional collapse of the alleghanian orogeny

fluid inclusions tectonic indicators abstract during the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters density of the mineralising fluids apart from a few pioneering works in the middle of the century fluid inclusions were not used as structural markers before the late 1980s during the same time experiments as well as observations of natural samples have shown that the volume shape and density of fluid inclusions may change when submitted to pressure temperature conditions different from their trapping isochores in view of these experiments and based on natural examples from the higher himalaya metamorphic rocks it is evident how fluid inclusions can be used as thermobarometric structural and tectonic markers points of interest for further studies on fluid inclusions in nature and experiments are also stressed

hydrothermal systems and tertiary low angle normal faulting in the southwestern united states many tertiary normal faults in the southwestern united states dip gently 30 some of these normal faults apparently were initiated with gentle dips in conflict with the predictions of elementary dynamic analysis close correspondence between low angle normal faulting and intense hydrothermal alteration in the mojave desert western arizona and the rio grande rift is consistent with the hypothesis that hydrothermal systems play an important role in the genesis of these faults we present a mechanical model to demonstrate that low angle normal faults could form in response to stress trajectories that become reoriented in a periodically sealed geothermal system beneath a sloping surface

structural expression of a rolling hinge in the footwall of the brenner line normal fault eastern alps the kinematic and temporal sequence of structures observed to overprint mylonites along the brenner line low angle normal fault may record passage of the footwall through two rolling hinges at the top and bottom of a ramp in the shear zone the structures comprise west down brittle and brittle ductile structures and east down brittle structures pt conditions of formation 250 to 400 c and 2 23 km depth obtained from analysis of oriented fluid inclusion planes indicate that west down structures were formed at greater depths and temperatures and therefore earlier than the east down structures these data suggest that the brittle structures formed under conditions that permit crystal plastic deformation at long term geologic strain rates and therefore probably reflect transient rapid strain rates and or high fluid pressure structures inferred to have formed at a lower hinge are consistent with viscous flow models of rolling hinge deformation and support the concept of a crustal asthenosphere such high temperatures at shallow crustal depth also suggest significant upward advection of heat by extensional unroofing of warm rocks which may have reduced the flexural rigidity of the footwall and thus affected mechanical behavior at the upper rolling hinge exposed mylonitic foliation within a few hundred meters of the brenner line and on top of the east west trending anticlines in the footwall dips 15 west our data favor a ramp dip of 25 but permit a dip as great as 45 fluid inclusion data suggest that structures related to the hinge at the base of the ramp formed at depths of 12 25 km if the average dip of the brenner shear zone to those depths was 20 intermediate between the favored ramp dip and the dip of exposed foliation then the horizontal component of slip could be as high as 33 63 km the two discrete sets of structures with opposite shear senses formed in the temporal sequence indicated by pt data are consistent with subvertical simple shear models of rolling hinge strain this kinematic pattern is not predicted by the flexural failure model for rolling hinges however the predominance of normal slip at the upper hinge which extends rather than shortens the mylonitic foliation fails to match the subvertical simple shear model which predicts shortening of the foliation there one possible solution is that superposition of regional extension upon hinge related stresses modified the rolling hinge kinematics such a modified subvertical shear model can account for the observed small foliation parallel extensional strains if the foliation was bent 5 10 passing through the upper hinge if more bending than that occurred the data suggest rolling hinge kinematics in which deformation is achieved by uniform sense simple shear across the shear zone as in the subvertical simple shear model but in which material lines parallel to the shear zone foliation and the detachment fault undergo very small length changes presumably indicating that footwall rocks retained significant resistance to shear and underwent minimal permanent strain the mechanics that would generate such a rolling hinge are uncertain but may incorporate aspects of both subvertical simple shear and flexural failure an important kinematic consequence of such a rolling hinge is that all of the net slip across a normal fault not only its horizontal component is converted into horizontal extension this implies a significantly larger magnitude of crustal extension across dipping normal faults whose footwalls passed through a rolling hinge than for those that did not develop along with a hinge

microfracturing and regional stress field a study of the preferred orientations of fluid inclusion planes in a granite from the massif central france abstract a study of the deformation of a granitic massif indicates a relationship between palaeostress fields and the geometry of microfractures as defined by fluid inclusion trails that is healed microfractures a statistical study of the distribution of the trails leads to the following conclusions trails exhibit several distinct preferred orientations on the scale of a grain which may be observed in many samples the orientations of the trails are similar to those of micro and mesoscale fractures in the granite and the dominant trail direction is parallel to the main direction of regional shortening thus fluid inclusion trails are mode i cracks which can be used as excellent markers of palaeostress fields

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

thrust regime and transpression regime tectonics in the tauern window eastern alps below the penninic nappes of the tauern window the deepest level of the alpine thrust system is exposed in the parautochthonous sedimentary sequence of helvetic facies

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

stress field rotation and rooted detachment faults a coulomb failure analysis several well known mechanical models have shown that unusual boundary or loading conditions can alter principal stress orientations into configurations consistent with low angle normal faulting such models however have not demonstrated whether magnitudes of reoriented stresses are sufficient to initiate and promote slip on low angle surfaces we present the results of simple coulomb failure analyses to determine whether and where such models predict frictional slip assuming geologically plausible boundary stresses pore pressures and rock strengths models that invoke a sizable shear traction at the base of the upper crust or spatially varying loads on the upper crust reorient principal stresses and failure planes but do not produce frictional failure on crustal scale detachments either in the absence of pore fluids or at hydrostatic pore fluid pressures models that reorient stresses by midcrustal dike intrusion produce slip on low angle surfaces at relatively deep crustal levels but only in the area of the dike tip the low angle failure surfaces curve into a high angle orientation a short distance from the dike all of these models also imply unsustainably high absolute tensile stresses in the upper 5 km of the crust and suggest that in any system in which stresses are allowed to evolve over time failure and stress release will occur on high angle faults before low angle ones have developed these assertions are true even when near lithostatic pore pressures are assumed unless there is an inhomogeneous extraordinarily fortuitous distribution of pore pressures and rock strengths at the time of initiation of a new detachment fault one model we tested for example required pore pressures exceeding 0 96 times lithostatic in the area of the hypothesized low angle normal fault with lower pore pressures both above and below the detachment to prevent slip and stress release on high angle normal faults in the upper part of the modeled region and on low angle thrust faults in the lower part

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

far field effects of late miocene subduction in the eastern carpathians e w compression and inversion of structures in the alpine carpathian pannonian region kinematic and paleostress data constrain a late miocene e w compressional event that affected the entire alpine carpathian pannonian system after 9 ma and prior to 5 3 ma the maximum horizontal compression directions obtained from 110 stations show a mean 1 orientation of 083 deformation is mainly strike slip e w directed compression followed early to middle miocene upper plate extension in the pannonian basin which was caused by the retreating subduction boundary in the outer carpathians this compressional event terminated early to middle miocene eastward lateral extrusion of the eastern alps and reverted strike slip faults which bounded earlier extruded wedges such as the salzachtal ennstal fault the periadriatic fault the mur murz vienna basin fault system and strike slip faults in the western carpathians e w compression caused the reorientation of the extension direction of the alpine crustal stack east directed tectonic unroofing of the metamorphic domes in the central eastern alps terminated between 9 and 6 ma and early to middle miocene orogen parallel e w extension switched to late miocene n s extension which parallels modern topographic slopes in the pannonian basin the change from synrift extension to late miocene compression during the postrift phase caused the pronounced postrift subsidence by stress induced downward flexure of the loaded lithosphere we relate late miocene e w compression to coeval late stage west directed subduction of the european plate below the eastern carpathians slab pull of the subducted plate caused subduction roll back and upper plate extension of the pannonian area up to the middle miocene subduction slowed down and ceased when up to 60 km thick buoyant continental crust entered the subduction zone during the short period of continued convergence e w directed compressive stress was transmitted across the subduction boundary into the upper plate compressive stress was transferred through the uppermost brittle crust of the earlier extended pannonian region into the eastern alps up to 1400 km behind the subduction zone e w compression terminated during the pliocene when the pliocene to recent nnw sse compressive stress field was established in central europe

linked fluid and tectonic evolution in the high himalaya mountains nepal fluid inclusions were studied in a quartz lens from the structurally highest unit of the himalaya mountains in nepal from a textural geometrical chemical and isotopic point of view six types of fluid inclusions were distinguished one of these types consists of annular inclusions this shape is attributed to a confining pressure increase in a non isotropic stress field two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity the bulk composition of the fluid was dominated by co2 84 mol and h2o the composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and or that one single fluid was reworked in situ by decrepitation stable isotope of fluids and minerals indicate 1 that fluids were buffered by surrounding rocks for o and c and 2 that at least two types of water metamorphic and meteoric were involved finally a p t t e path is proposed for the sample taking into account the southward thrusting along the main central thrust the northward tectonic denudation of the himalaya mountains inducing tectonic burying below the annapurna range and lastly rapid uplift

paleostress directions from the preferred orientation of closed microfractures fluid inclusion planes in sandstone east texas basin u s a abstract closed extensional microfractures are useful indicators of paleostress directions in shallowly buried quartz cemented quartz arenites of the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 670 m thick that was deposited in the gradually subsiding northern gulf of mexico basin the closed microfractures are syndiagenetic fluid inclusion rich planes that have an ene strike that is subparallel to contemporary quartz filled extension veins and regional normal faults indicating that closed microfractures contain the maximum paleostress direction these microstructures are potentially useful for mapping paleostress trajectories in passive margin and platform settings particularly where samples are obtained from core previous experimental studies of microfracture closure suggest that microfractures evolved into fluid inclusion planes by dendritic quartz precipitation in east texas microfracture closure may have occurred at temperatures as low as 85 c and depths as shallow as 1500 m

two and three dimensional thermal modeling of a low angle detachment exhumation history of the simplon fault zone central alps 1 two alternative models have been proposed to explain footwall exhumation along major low angle detachments 1 crustal scale exhumation along a detachment fault that maintained a low dip angle or 2 exhumation along a high angle fault passively rotated by isostatic rebound rolling hinge model these proposed models were tested against a well documented example of a low angle detachment fault in the european central alps the simplon fault zone sfz an extensive thermochronological data set provides the basis for 2 and 3 d thermokinematic models pecube coupled with a stochastic inversion algorithm the neighbourhood algorithm model results establish that the thermochronological pattern is better reproduced by a low angle detachment that maintained a 30 dip rather than by a rolling hinge model although a range of histories involving either steady state or variable exhumation rates is possible the preferred model of highest probability is for a variable rate with the fault zone initiated at 18 5 2 5 ma and active until the present day footwall exhumation was relatively fast until 14 5 1 5 ma 1 4 mm yr 1 this enhanced sfz footwall exhumation is similar in timing and kinematics to orogen parallel extension reported throughout the alpine orogen after 14 5 ma sfz footwall exhumation continued at a reduced rate 0 7 mm yr 1 until 4 ma the subsequent increase to 1 mm yr 1 reflects enhanced regional erosion rates across both footwall and hanging wall after circa 4 ma from 0 35 0 15 mm yr 1 to 0 70 0 15 mm yr 1 probably in response to climate changes during the pliocene

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

kinematics of the northern part of the simplon line central alps abstract structural studies were carried out along the northern part of the simplon line in order to constrain the kinematic history of the line during tertiary times the ductile strain accumulated within both basement units and mesozoic cover reveals the upper units sheared towards the west southwest the simplon line can be described as a major shear zone along which the main movement occurred perpendicular to the line this shearing developed during retrograde metamorphic conditions and postdates earlier deformations associated with the high temperature lepontine episode finally it was followed by the formation of the berisal syncline back folding episode which generated some brittle movements towards the south localized at the contact between the lower and upper pennine units i e on the simplon line

comment on tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california by john m bartley john m fletcher and allen f glazner

fault slip analysis and late exhumation of the tauern window eastern alps exhumation of the eastern alps from the early tertiary to the late miocene was localized mainly in the tauern window a thermal and structural dome located in front of the dolomites indenter stress inversions based on new structural investigations over the entire tauern window indicate a regional zoning of the paleostress field with a predominance of strike slip states of stress in the core of the investigated area and dominant extensional regimes in the eastern and western borders of the dome few inverse fault structures have been highlighted we propose a two stage deformation history in order to explain the different types of structures that characterise the ductile and the brittle domains during the first stage of exhumation of the tauern window corresponding to the folding event the brittle crust was probably dominated by n s shortening and compression the second stage of exhumation was marked by normal faulting at the borders of the dome and strike slip faulting in the core during the second stage the brittle part of the crust that was previously affected by compressive structures belonging to the first stage was eroded normal faulting associated to e w extension along the eastern and western borders of the tauern window was accommodated by strike slip faulting located in the core of the tauern window yielding e w extension and n s shortening the orientation of the extensional axes and the nature of the stress tensors are similar to the ones inferred for the late stage brittle overprint of the internal basement massifs of the central alps pointing to a possible common large scale state of stress acting well beyond the area of the eastern alps

discussion comparison of slope instability screening tools following a large storm event and application to forest management and policy abstract this discussion is in response to the article entitled comparison of slope stability screening tools following a large storm event and application to forest management and policy by kara whittaker and dan mcshane geomorphology 145 146 2012 115 122 the discussion is coauthored by several geologists at the washington department of natural resources wdnr including those from the research and policy sections of the state agency

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

styles of footwall uplift along the simplon and brenner normal fault systems central and eastern alps the simplon and brenner extensional shear zones of the central and eastern alps respectively are low angle detachments that accommodated orogen parallel extension and unroofed midcrustal rocks in their footwalls an integrated field and fluid inclusion study of the simplon low angle shear zone reveals several postmylonitic structure types that are consistent with passage of a complex rolling hinge through the simplon footwall including synthetic west down and antithetic east down semibrittle and brittle structures and strike slip extensional and contractional structures all of which are consistent with southwest directed extension along the main detachment zone fluid inclusion analysis of kinematically referenced inclusion arrays indicates that west down semibrittle and brittle shears were active at conditions above 300 450 c and 14 26 km antithetic deformation along brittle faults occurred at much shallower conditions of 300 c and 3 7 km deformation along these structures accommodated footwall uplift by a subvertical simple shear mechanism in the southern part of the simplon region these results are strikingly similar to those from the brenner shear zone in contrast with the brenner region however the northern simplon footwall shows strike slip dominated postmylonitic deformation related to slip along the rhone line the footwalls of these detachment systems define a unique class of core complex characterized by the development of a midcrustal synthetic lower hinge that acts to steepen the detachment system and accommodate footwall uplift by a subvertical simple shear mechanism both systems have poorly developed upper hinges that flatten the middle to upper crustal ramp by only a few degrees of dip which is in contrast to detachment fault systems of the western united states where evidence of a deep hinge is rarely observed and the upper hinge acts to deactivate the flattened updip part of the detachment our results emphasize the role of tectonic setting buoyancy forces and synkinematic fluids in controlling the observed style and mechanisms of large magnitude extension during collision between continental masses

experimental decrepitation and re equilibration of fluid inclusions in synthetic quartz abstract strong discrepancies between densities of the fluids trapped in fluid inclusions f i and p tp couples deduced from other geological considerations are common so it was decided to test experimentally the possible f i re equilibrations when bringing the host crystal in p tp conditions different from trapping conditions runs were conducted on synthetic quartz in a gas apparatus and in a griggs solid medium apparatus the following modifications of the f i were observed 1 1 drastic shape changes of the primary f i and appearance of clusters or trails of small secondary inclusions sometimes related to a crack 2 2 lowering of the fluid densities such a re equilibration is a decrepitation phenomenon first hydraulic fracturing then sealing of the cracks with appearance of secondary inclusions when using the gas apparatus the new measured densities are in very good accordance with the nominal p tp conditions when using the solid medium apparatus the new densities are surprisingly low relative to the nominal p t conditions probably due to a strongly anisotropic stress state in the apparatus

the origin of large local uplift in extensional regions large localized uplift is commonly observed in continental regions undergoing extension these observations can be modelled by planar high angle normal faulting of an elastic upper crust overlying an inviscid lower crust isostasy provides the necessary driving force the model quantifies the role of flexural rigidity density variations in the crust and erosion and deposition of sediment