anisotropic behavior of grip ices and flow in central greenland abstract mechanical tests have been performed on strongly textured ice samples coming from a wide range of depths from 1328 down to 2868 m of the greenland ice core project grip for different sample orientations with respect to the prescribed stress in this way two directional viscosities corresponding to the easy glide and to the hard glide orientations were determined along the core the viscoplastic anisotropy gradually increases down to a depth of 2600 m and slightly decreases below revealing a clear correlation between rheology and texture the experimental mechanical response compares well with that predicted by the viscoplastic self consistent vpsc model the vpsc model is also applied to ice samples that exhibit an axisymmetric texture to show in more detail the sensitivity of the rheology to specific texture parameters this leads to a number of recommendations for future mechanical tests on anisotropic samples a large scale ice flow model is finally used to estimate the influence of ice anisotropy on the flow along the grip gisp2 flow line the particular mechanical behavior of deep grip ices in the stress regime corresponding to an ice divide leads to deformation rates that are highly sensitive to bedrock topography and texture pattern this feature is likely to favour the formation of stratigraphic disturbances in deep ice layers as observed in the last 300 m of both grip and gisp2 cores

influence of hegenshan heihe suture on evolution of late mesozoic extensional structures in wunite depression erlian basin inner mongolia china we integrate previous work on the hegenshan heihe suture with our interpretation of geomagnetic anomaly and seismic reflection data to investigate the role of the hegenshan heihe suture in the evolution of late mesozoic extensional structures in the wunite depression of the erlian basin sags in the wunite depression present as ne50 trending in the western sector n0 ne30 trending in the central sector and ne45 trending in the eastern sector our results highlight the importance of the pre existing hegenshan heihe suture in the evolution of the late mesozoic rift system and reveal the following details 1 the ne50 extent sags in the western sector are controlled by the c ne50 trending suture moreover the extensional deformation of the reactivated suture during early cretaceous time resulted in a further vertical and horizontal extension of major border faults 2 the n0 ne30 extent sags in the central sector are influenced by the c ne75 trending suture the sinistral strike slip component of the reactivated suture during early cretaceous time resulted in a strike rotation of major border faults from nee trending following the suture to nne trending 3 because of strike slip deformation which resulted from the deformation of the reactivated suture accrued in major border faults light dip slip deformation led to less vertical offset 4 the ne45 trending sags in the eastern sector are controlled by the c ne45 trending suture moreover the extensional deformation of the reactivated suture during early cretaceous time facilitated a further vertical and horizontal prolongation of major border faults

recumbent folding of divide arches in response to unsteady ice divide migration arches in stratigraphic layers directly under a flow divide raymond bumps are predicted by models of steady ice sheet flow and have been observed in several ice domes here we model the evolution of these layers when a formerly stationary divide migrates rapidly to a new position then again becomes stationary leaving the arched layers in a flank position as they are then carried downstream with the flow these abandoned arches can develop into recumbent folds these folds can occur over a wide range of divide migration speeds the shearing flow that produces these recumbent folds also distributes the folded layers over a wide distance downstream from the original divide location if the divide offset is abrupt pre cores or material lines comprising core relative isochrones can be used to quickly identify portions of an abandoned raymond bump that would be overturned at any future ice core site downstream if as appears to be the case in greenland the divide is never stable long enough to produce a mature arch folds of this type would not occur the most likely place to find such folds might be the flank of an ice ridge bounded by unsteady ice streams

what sets topographic relief in extensional footwalls we use three large normal fault arrays in the northeastern basin and range province western united states to document catchment development and relief production during fault growth fault slip and slip rates increase systematically along strike from zero at the fault tips catchment relief and across strike range width both increase as slip accumulates but reach maximum values at a distance of 15 km from the fault tips and remain uniform along strike over much of the footwalls catchment outlet spacing also increases away from the fault tips but does not reach a uniform value and may vary by a factor of 5 6 along strike we infer that catchments first elongate in the across strike direction as slip accumulates and the range half width increases once the half width reaches its maximum value continued catchment growth is possible only by along strike capture which increases outlet spacing but not relief the close correspondence between catchment relief and range half width suggests that geomorphically limited hillslope and channel gradients are achieved within the 15 km tip zone thus the limiting factor in footwall development is the width of the range which is controlled by two external agents the geometry and spacing of the major faults and the elevations of base level on both flanks

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

strain migration during multiphase extension observations from the northern north sea many rifts develop through multiphase extension it can be difficult however to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried using 2 d and 3 d seismic reflection and borehole data from the northern north sea we examine the style magnitude and timing of reactivation of a preexisting permian triassic rift phase 1 fault array during a subsequent period of middle jurassic to early cretaceous rift phase 2 extension we show that rift phase 2 led to the formation of new n s striking faults close to the north viking graben but did not initially reactivate preexisting rift phase 1 structures on the horda platform we suggest that at the beginning of rift phase 2 strain was focused in a zone of thermally weakened lithosphere associated with the middle jurassic north sea thermal dome rather than reactivating extant faults diachronous reactivation of the permian triassic fault network eventually occurred with those faults located closer to the middle jurassic to early cretaceous rift axis reactivating earlier than those toward the eastern margin this diachroneity may have been related to flexural down bending as strain became focused within the north viking graben and or the shifting of the locus of rifting from the north sea to the proto north atlantic our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field

flow properties of the ice from the greenland ice core project ice core the reason for folds long term deformation tests on ice from the greenland ice core project grip deep ice core show that ice from the different climate zones in the ice core has flow properties correlated with the concentrations of impurities in the sample the deformation tests are performed by uniaxial unconfined compression at 16 degrees c with an octahedral compression stress of 3 bars the ice samples are compressed for 1 2 to 3 years until the tertiary strain rate is reached it is believed that by the end all downhole flow conditions are forgotten and that the ice sample has settled in a state determined by the applied stress and temperature conditions all samples are tested under the same stress and temperature conditions so the resulting deformation rates and final ice crystal size and fabrics can only differ due to varying impurity concentrations the results show that ice from cold climatic periods with high concentrations of impurities deforms more slowly than ice from warm climatic periods in compression when tertiary creep is reached the crystal size is smaller in the cold ice than in the warm the ice from warmer climatic periods with lower concentrations of impurities deforms at a factor of 2 3 times more rapidly in compression the tertiary steady state crystal size is increased by 50 and the ice crystals have oriented more favorably for the applied compression in the warm ice which is believed to be the reason why the strain rates are greater here than in the cold ice in the bottom 200 m of the grip ice core zones are observed with folds on the scale of 1 8 cm an investigation of the ice layers in and around the folds shows that the layers are composed of ice from different climatic zones the folding is believed to result from the different flow and rheological properties of the layers involved in the folding structures

comments on the interpretation of deformation textures in rocks abstract in rocks that undergo ductile deformation preferred orientation develops as a result of intracrystalline slip and mechanical twinning the orientation distribution is a consequence of the microscopic mechanisms and of the strain path it can be used to get some insight into the deformation history however it is never unique the interpretation relies largely on polycrystal plasticity theory the concepts of stress equilibrium and strain compatibility which are two extreme assumptions made to model deformation are discussed new approaches such as the viscoplastic self consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations all these theories defy the popular notion which is becoming entrenched in the geological literature that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction an orientation referred to by geologists as easy glide it is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition in accordance with an old recommendation of walter schmidt

strain rate enhancement at dye 3 greenland ice at depth in ice sheets can be softer in bed parallel shear than glen s flow law predicts for example at dye 3 greenland enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting previous authors have identified crystal fabric as the dominant contributor but the role of impurities and crystal size is still incompletely resolved here we use two formulations of anisotropic flow laws for ice azuma s and sachs models to account for the effects of anisotropy and show that the measured anisotropy of the ice at dye 3 cannot explain all the detailed variations in the measured strain rates the jump in enhancement across the holocene wisconsin boundary is larger than expected from the measured fabrics alone dust and soluble ion concentration divided by crystal size correlates well with the residual enhancement indicating that most of the excess deformation may be due to impurities or crystal size while the major features of the deformation at dye 3 are explained by anisotropy and temperature results also suggest that further research into the role of impurities and crystal size is warranted

potential for stratigraphic folding near ice sheet centers lack of agreement between the deep portions of the greenland icecore project grip and greenland ice sheet project ii gisp2 ice cores from central greenland suggests that folds may disrupt annual layering even near ice divides we use a simple kinematic flow model to delineate regions where slope disturbances wrinkles introduced into the layering could overturn into recumbent folds and where they would flatten leaving the stratigraphic record intact wrinkles are likely to originate from flow disturbances caused internally by inhomogeneities and anisotropy in the ice rheological properties rather than from residual surface structures sastrugi or from open folds associated with transient flow over bed topography if wrinkles are preferentially created in anisotropic ice under divides where the resolved shear stress in the easy glide direction can be weak and variable then the deep intact climate record at dye 3 may result from its greater distance from the divide alternatively the larger simple shear at dye 3 may rapidly overturn wrinkles so that they are not recognizable as folds the ice core record from siple dome may be intact over a greater fraction of its depth compared to the central greenland records if its flat bedrock precludes fluctuations in the stress orientation near the divide

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

sonic methods for measuring crystal orientation fabric in ice and results from the west antarctic ice sheet wais divide we describe methods for measuring crystal orientation fabric with sonic waves in an ice core borehole with special attention paid to vertical girdle fabrics that are prevalent at the wais divide the speed of vertically propagating compressional waves in ice is influenced by vertical clustering of the ice crystal c axes shear wave speeds particularly the speed separation between fast and slow shear polarizations are sensitive to azimuthal anisotropy sonic data from the wais divide complement thin section measurements of fabric thin sections show a steady transition to strong girdle fabrics in the upper 2000 m of ice followed by a transition to vertical pole fabrics below 2500 m depth compressional wave sonic data are inconclusive in the upper ice due to noise as well as the method s inherent insensitivity to girdle fabrics compared with available thin sections sonic data provide better resolution of the transition to pole fabrics below 2500 m notably including an abrupt increase in vertical clustering near 3000 m our compressional wave measurements resolve fabric changes occurring over depth ranges of a few meters that cannot be inferred from available thin sections but are sensitive only to zenithal anisotropy future logging tools should be designed to measure shear waves in addition to compressional waves especially for logging in regions where ice flow patterns favor the development of girdle fabrics

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

mechanics of continental rift architecture continental rifts are long narrow features bounded on one or both sides by normal faults along which several kilometers of displacement has occurred resulting in deep rift valleys that form sedimentary basins continental rifts all share the same geometrical features they have widths that are characteristic of the region in which they form they are predominantly asymmetric and they are segmented along axes at a length scale that is also characteristic of the region we describe a very simple mechanical model to explain these three features the rift initiates with the formation of a conjugate set of normal faults that intersect at the center of the seismogenic layer one of these becomes the boundary fault system the other the failed conjugate fault being offset by the former becomes locked early in the rifting history hence an asymmetric graben is formed which is the basic building block of rifts the failed conjugate fault has distinctive properties that allow it to be easily distinguished from other faults examples of these faults from the tanganyika and malawi rifts show excellent agreement with predictions about their positions with respect to the boundary fault system and their net displacement and slip history the width of the asymmetric graben is a simple function of seismogenic thickness the net displacement on the boundary fault system is limited by the resistive friction and the flexural restoring force we calculate this limiting boundary fault system displacement and using the displacement length scaling law for normal faults suggest that this controls the along axis segmentation length both maximum fault slip and segmentation length are functions of the seismogenic thickness and the effective elastic thickness of the crust these calculations agree very well with observations of segmentation from the african rifts and the maximum length of range fronting normal faults in nevada

structural and geomorphological constraints on active normal faulting and landscape evolution in calabria italy calabria is one of the fastest uplifting and most seismically active regions in the mediterranean yet the time averaged c 10 6 years rates of normal faulting remain poorly constrained here we use digital elevation model analysis geological cross sections and a compilation of published data to quantify systematically along strike the fault throws and time averaged throw rates of the serre cittanova armo east crati and west crati faults we show that regional uplift has uplifted both their hanging wall and footwall blocks by c 200 400 m and the aspromonte massif by up to c 1150 1300 m we find that these faults have throws between c 640 and 1430 m and throw rates between c 0 6 and 1 4 mm a 1 their footwall ranges have variable proportions of inherited relief up to c 300 800 m the channels draining these ranges reflect the active normal faulting and relief in their steepness indices becoming c 5 8 m 0 9 steeper for each 0 1 mm a 1 throw rate increment and each 100 m relief increase finally the presence of knickpoints suggests that these channels are transiently responding to changes in relative base level which could be due to fault linkage or regional strain rate increases for the southern calabrian faults and to regional uplift acceleration in the case of the crati faults supplementary material topographic profiles of the marine terraces and long profiles of rivers are available at https doi org 10 6084 m9 figshare c 3689464

interaction of an antecedent fluvial system with early normal fault growth implications for syn rift stratigraphy western corinth rift greece continental overfilled conditions during rift initiation are conventionally explained as due to low creation of accommodation compared with sediment supply alternatively sediment supply can be relatively high from the onset of rifting due to an antecedent drainage system the alluvial lower group of the western plio pleistocene corinth rift is used to investigate the interac tion of fluvial sedimentation with early rifting this rift was obliquely super imposed on the hellenide mountain belt from which it inherited a significant palaeorelief detailed sedimentary logging and mapping of the well exposed syn rift succession document the facies distributions palaeocurrents and stratigraphic architecture magnetostratigraphy and bios tratigraphy are used to date and correlate the alluvial succession across and between fault blocks from 3 2 to 1 8 ma a transverse low sinuosity braided river system flowed north north east to east across east west striking active fault blocks 4 to 7 km in width deposits evolved downstream from coarse alluvial conglomerates to fine grained lacustrine deposits over 15 to 30 km the length scale of facies belts is much greater than and thus not directly controlled by the width of the fault blocks at its termination the distribu tive river system built small stacked deltas into a shallow lake margin the presence of a major antecedent drainage system is supported by i a single major sediment entry point ii persistence of a main channel belt axis iii downstream fining at the scale of the rift basin the zones of maximum sub sidence on individual faults are aligned with the persistent fluvial axis sug gesting that sediment supply influenced normal fault growth instead of low accommodation rate during the early rift phase this study proposes that facies progradation can be controlled by continuous and high sediment sup ply from antecedent rivers

structures and fabrics in glacial ice a review abstract glaciers ice sheets and ice caps represent tectonic systems driven by gravity their movement can be studied in real time and the rheological properties and strength of ice determined from laboratory experiments and field measurements all glacial ice has primary stratification exhibited by variations in grain size bubble content and debris content as it deforms with deformation dominated by plastic flow and recrystallization accompanied locally by fracture under tension a suite of structures develops that reflects the primary fabric of the ice and the anisotropy that develops as a result of cumulative deformation initial variations in solid impurity content and strain dependent anisotropy as a result of a crystallographic fabric give rise to effective viscosity increases or decreases compared to isotropic polycrystalline ice of about a factor of ten foliation develops from inherited mostly stratification or introduced mostly ice veins or fracture traces fabric elements and from dynamic recrystallization it is largely dependent on the accumulated strain which is highest at the base and near the margins of glaciers ice sheets and ice streams folds develop largely passively due to initial amplification of irregularities in the primary stratification to variations in flow with time or to inhomogeneous flow associated with shear zones and ductile accommodation around open fractures buckle folds and boudinage mostly on a small scale occur where viscosity contrast is large mostly in basal ice thrusting and wrench faulting are documented in surging glaciers but theoretically most unlikely and rare or absent elsewhere many structures interpreted as faults are not due to shear failure but rather result from shear displacements during opening and closing of tensile fractures

the growth of geological structures by repeated earthquakes 2 field examples of continental dip slip faults a strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault bounded geological structures such as basins and ranges which are built by repeated cycles of deformation along strike slip faults the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign resulting in little permanent deformation except for the fault offset for dip slip faults portions of the crust are lifted and dropped and so buoyancy forces are exerted the seismic and interseismic deformations do not balance and structures grow and become subject to erosion and deposition we consider three examples for which the structure and fault geometry are well known the white wolf reverse fault in california site of the 1952 kern county m 7 3 earthquake the lost river normal fault in idaho site of the 1983 borah peak m 7 0 earthquake and the cricket mountain normal fault in utah site of quaternary slip events basin stratigraphy and seismic reflection records are used to profile the structure and coseismic deformation measured by leveling surveys is used to estimate the fault geometry to reproduce these structures we add the deformation associated with the earthquake cycle the coseismic slip and postseismic relaxation to the flexure caused by the observed sediment load treating the crust as a thin elastic plate overlying a fluid substrate the cumulative deformation is principally dependent on the elastic plate thickness modestly sensitive to the sediment substrate density difference and insensitive to the fluid viscosity for the 4 to 8 ma structures we deduce a longterm flexural rigidity of 2 15 1019 nm this is equivalent to an elastic plate thickness of 2 4 km for a young s modulus of 2 5 1010 nm 2 this value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km but where coseismic fault slip extends to a depth of 10 15 km thus much of the seismogenic crust must weaken substantially during the life of active faults causing the fault bounded basins to narrow over time

unexpected earthquake hazard revealed by holocene rupture on the kenchreai fault central greece implications for weak sub fault shear zones this work forms part of the nercand esrc funded project earthquakes without frontiers and was partly funded by the nerc grant looking inside the continents from space

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

evidence for different scaling of earthquake source parameters for large earthquakes depending on faulting mechanism scaling relationships between seismic moment rupture length and rupture width have been examined for this purpose the data from several previous studies have been merged into a database containing more than 550 events for large earthquakes a dependence of scaling on faulting mechanism has been found whereas small and large dip slip earthquakes scale in the same way the self similarity of earthquakes breaks down for large strike slip events furthermore no significant differences in scaling could be found between normal and reverse earthquakes and between earthquakes from different regions since the thickness of the seismogenic layer limits fault widths most strike slip earthquakes are limited to rupture widths of between 15 and 30 km while the rupture length is not limited the aspect ratio of dip slip earthquakes is similar for all earthquake sizes hence the limitation in rupture width seems to control the maximum possible rupture length for these events the different behaviour of strike slip and dip slip earthquakes can be explained by rupture dynamics and geological fault growth if faults are segmented with the thickness of the seismogenic layer controlling the length of each segment strike slip earthquakes might rupture connected segments more easily than dip slip events and thus could produce longer ruptures than dip slip events of the same width

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

locating distributed faulting contributions from insar imaging to probabilistic fault displacement hazard analysis pfdha abstract prediction of location and amount of slip for distributed faulting associated to strong earthquakes is a recently explored issue with major implications in terms of hazard assessment currently the best practice involves the applications of probabilistic fault displacement hazard analysis pfdha whose results fit sufficiently well real data in the near range of the primary fault but show considerable inaccuracies in the far range we believe that this inaccuracy descends from the biased earthquake databases used for regression analysis whose data relative to old earthquakes were largely collected only by field surveys focused close to the primary fault i e near range remotely sensed data i e interferometric radar imaging insar offer the opportunity to precisely measure the surface deformation induced by strong earthquakes and thus to explore its possible relation with distributed faulting we analyze the l aquila earthquake case study 29th april 2009 mw 6 3 and explore the correlation between location and slip on distributed faulting and insar derived deformation field we find a significant correlation between occurrence of distributed faulting and profile curvature of the dislocation field in spite of the distance from the primary fault moreover distributed faults tend to occur within the area deformed by the earthquake as imaged by insar data and whose extent is directly proportional to the earthquake magnitude mw according to a dataset of 30 recent earthquakes we then propose that these observations have to be incorporated into the present pfdha practice as limit boundaries to possible scenarios of probabilistic analysis and that an integrated use of field based and remotely data collection have to be implemented following strong earthquakes

recumbent folding in ice sheets a core referential study to better understand apparent stratigraphic disturbances in ice cores such as greenland ice sheet project 2 gisp2 we examine how ice sheet flow can transform gentle open folds into order disturbing recumbent folds the initial disturbances in the stratigraphy have their roots in transient dynamic processes and local rheological inhomogeneities but the kinematics of even a simple ice flow model can deform these disturbances enough to alter paleoclimatic interpretation of an ice core the local vorticity number suggests which structures can be passively overturned but analyzing the finite strain along particle paths gives a more complete picture especially when taken relative to a hypothetical core location core relative isochrones or pre cores predict which stratigraphic disturbances will appear as obviously overturned layers in a core the deformation gradient tensor along particle paths allows us to calculate the rotation of segments of various reference slopes these calculations suggest that observed 20 dips in the gisp2 core are rotating on a time scale of a few hundred years and could result from distortions with much smaller slopes produced upstream the time during which they can be recognized to be overturning is short because the rotation rate is high once overturned they are flattened further and may be hard to recognize especially in the small cross section of a core

extension of continental crust at the margin of the eastern grand banks newfoundland seismic and gravity observations from the rifted margin of the eastern grand banks newfoundland support a new model for extension of the continental crust from the shelf edge to odp site 1277 where mantle rocks are exhumed we find that the largest decrease in crustal thickness from about 28 km to 6 km occurs beneath the continental slope of the grand banks over a distance of just 20 km this rapid decrease in crustal thickness coincides with anomalously high seismic velocities 7 0 7 2 km s 1 in the lower crust of the shelf edge the thin crust of the continent ocean transition cot in this area has a smooth basement surface void of upper crustal blocks and prerift sediments we compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust the geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins beneath the continental shelf of the grand banks the moho and the strong lower crust rotated upwards toward to a 50 dip without visible internal deformation the presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward with ongoing extension some high angle normal faults may have rotated to a sub horizontal orientation which would explain the lack of brittle deformation visible in the seismic reflection data the two geodynamic models produce different amounts of extension of continental crust in the distal margins the hot rifting model localizes strain much more rapidly leaving narrow zones of extended continental crust and it produces a relatively large amount of melt 30 in the final stages of rifting continental breakup may occur rapidly in hot lithosphere 5 myr on the other hand a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin similar to what we inferred at the eastern grand banks the cold lithospheric model requires about 23 myr of extension before continental breakup and it predicts much less melting in the mantle 13 the long rift duration wide zones of thinned continental crust and small amount of magmatism make the cold rifting model the most applicable to newfoundland iberia rift

spatio temporal evolution of strain accumulation derived from multi scale observations of late jurassic rifting in the northern north sea a critical test of models for lithospheric extension abstract we integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern north sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire viking graben and its flanking platform areas these data show how syn rift accommodation within the basin varied in space and time with sub kilometer scale spatial resolution and a temporal resolution of 2 3 myr regional interpretations of 2d seismic reflection refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust these image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio temporal evolution of strain accumulation throughout the lithosphere during the 40 myr 170 130 ma period of late jurassic extension across this basin for the first 25 30 myr strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined as strain rates across the whole basin were consistently very low 16 s l thermally induced strength loss cannot explain this phenomenon the strain localisation is manifest in the near surface by a systematic migration of fault activity the pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control especially when estimates of flexural rigidity for this area are considered the best explanation for what is observed in this time period is a coupling between near surface strain localisation driven by brittle or plastic failure and the evolving thermal structure of the lithosphere we demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate dependence of frictional strength observed in laboratory studies during the final 10 myr of basin formation strain accumulation was focused within the axis and strain rates declined rapidly replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline

subaerial salt extrusions in iran as analogues of ice sheets streams and glaciers abstract ice h20 and salt halite nacl share many physical properties and resemble each other in hand specimens and subaerial gravity driven flows however while most significant bodies of ice accumulate in cold highlands and gravity spread where and soon after they form most significant bodies of salt accumulate in tropical marine basins and have to be buried by 1 km of other rocks before they flow buried salt is driven by differential loading into various categories of piercing structures known as diapirs many diapirs extrude onto the surface as sheets of allochthonous out of place salt thousands of sheets of allochthonous salt have been interpreted in over 35 basins worldwide in the last 25 years mainly in the toes of passive continental margins and in orogenic belts where some are 103 km2 in area most former salt sheets are now submarine or subsurface but several active examples are beautifully exposed in iran these were compared to ice glaciers soon after they were introduced to western science a comparison that has been neglected since here we update this analogy and use modern understanding of flowing ice and salt to examine the similarities and differences that might be mutually beneficial to both fields of study as well as to extraterrestrial scientists the profiles internal structures and fabrics in flowing bodies of ice and salt are sensitive gauges of the histories of their budgets of supply and loss however whereas snow merely compacts where it accumulates salt sheets are fed from below by already deformed salt when salt diapirs first emerge on land they extrude domes that mature to the profiles of viscous fountains that often feed glacier like flows known as namakiers after locally exhausting their deep source layers salt fountains spread to the profiles of viscous droplets normal for ice caps ice typically deforms at 80 usually 90 of its absolute melting temperature while most salt deforms at

graviquakes in italy abstract we discuss the mechanics of crustal normal fault related earthquakes and show that they represent dissipation of gravitational potential energy graviquakes and their magnitude increases with the involved volume delimited by the seismogenic fault and an antithetic dilated wedge in its hangingwall and the fault dip the magnitude increases with the deepening of the brittle ductile transition bdt which in turn enlarges the involved volume the fault dip seems rather controlled by the static friction of the involved crustal layers we apply the model to the extensional area of the italian peninsula whose geodynamics is controlled by the alpine and apennines subduction zones the latter has a well developed backarc basin and a large part of the accretionary prism is affected by on going extensional tectonics which is responsible for most of peninsular italy seismicity analyzing the seismic record of the apennines the length of seismogenic normal faults tends to be at most about 3 times the hypocenter depth we compile a map of the brittle ductile transition depth and assuming a fixed 45 or 60 fault dip and a dilated wedge developed during the interseismic period almost perpendicular to the fault plane we compute the maximum volume of the hangingwall collapsing at the coseismic stage and estimate the maximum expected magnitude lower magnitude values are obtained in areas with thinner brittle layer and higher heat flow moreover lower magnitude relative to those theoretically expected may occur in areas of higher strain rate where faults may creep faster due to lower frictional values

variations in the lithosphere thickness and vertical neotectonic movements abstract thermal changes and mechanical deformation of the lithospheric thickness are presumed to cause vertical neotectonic movements it is established that heterochronous fragments of the continental lithosphere respond differently to cenozoic thermotectonic activation lithospheric segments which had consolidated in the archean were less mobile in the cenozoic those of riphean ma and late phanerozoic age were mobile the archean fragments comprise the stable blocks of ancient and young continental platforms the riphean and late phanerozoic fragments comprise orogenic belts these regional differences may be due to different degrees of cratonization of the continental lithosphere

grain scale processes folding and stratigraphic disturbance in the gisp2 ice core flow disturbances have been shown to alter stratigraphic order in the lower part of the ice sheet in central greenland vertical thin sections of the greenland ice sheet project 2 ice core show that in the lower 30 the expected c axis vertical fabric is interrupted by planes of grains stripes with c axes oriented approximately in the dip directions of the planes stripe parallel shear produces small scale folds the stripes can be explained qualitatively by a simple nucleation and growth model based on the strong anisotropy of ice crystal deformation nucleation probably is sensitive to spatially variable rates of polygonization producing spatially variable stripe densities stripes are modeled to affect the ice viscosity so variations in stripe density may contribute to viscosity contrasts that might produce larger deformational features and loss of stratigraphic order

a model of rift basin evolution constrained by first order stratigraphic observations the stratigraphy of many nonmarine rift basins records a common sedimentological history consisting of a relatively abrupt increase of water depth followed by a slower shoaling upward trend we explore in this paper the origin of these sequences through a three dimensional physical model that incorporates the affects of fault growth flexure erosion sedimentation and isostasy fault growth is assumed to occur in a self similar manner and is treated as dislocations inducing flexure in a thin elastic plate topography generated by faulting and flexure is allowed to undergo diffusion to simulate erosive and sedimentary processes isostatic response to the redistribution of mass results in additional flexure of the thin plate the model is tested using two models of fault growth and we compare the numerical results with the stratigraphy observed in nonmarine rifts basins generated using a constant lengthening rate mode of fault growth fail to reproduce the stratigraphy of continental rift basins we find that the commonly observed stratigraphic succession of lacustrine to fluvial conditions requires the displacement rate of the bounding fault to decrease with time consistent with opening at a relatively constant areal extension rate

an analytical approach to deformation of anisotropic ice crystal aggregates deformation rates of single hexagonal crystals deforming by glide on the basal plane are described as a function of stress state and crystal orientation these results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric this type of fabric is approximated by a uniform distribution of c axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis for simple shear stress acting on a single maximum fabric there is a slight de enhancement for cone angles of 60 90 in uniaxial compression a maximum enhancement of 1 7 occurs at a cone angle of 57 a pure shear stress state has similar features with the additional complication that it causes a non zero transverse strain rate except for perfect vertical alignment of crystals and isotropic fabric in combined states of stress the contribution of each stress component to the strain rate depends on fabric a single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states

fault strain and seismic coupling on mid ocean ridges the contribution of extensional faulting to seafloor spreading along the east pacific rise epr axis near 3 s and between 13 n and 15 n is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data it is found that faulting may account for of the order of 5 10 of the total spreading rate which is comparable to a previous estimate from the epr near 19 s given the paucity of normal faulting earthquakes on the epr axis a maximum estimate of the seismic moment release shows that seismicity can account for only 1 of the strain due to faulting this result leads us to conclude that most of the slip on active faults must be occurring by stable sliding laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding while increasing temperature promotes stable sliding by applying a simple frictional model to mid ocean ridge faults it is shown that at fast spreading ridges 90 mm yr the seismic portion of a fault ws is a small proportion of the total downdip width of the fault w the ratio ws w interpreted as the seismic coupling coefficient x and in this case x 0 in contrast at slow spreading rates 40 mm yr ws w and therefore x 1 which is consistent with the occurrence of large magnitude earthquakes mb 5 0 to 6 0 occurring for example along the mid atlantic ridge axis

flow at ice divide triple junctions 1 three dimensional full stokes modeling 1 ice domes are either axisymmetric high points along ridges or ridge triple junctions we model time dependent isothermal flow near triple junctions solving the full set of mechanical equations with a nonlinear power law rheology forcing is applied through the boundary conditions which affect flow patterns at outlets where such forcing is purely axisymmetric an axisymmetric dome is formed if a threefold symmetry in the forcing is applied the axisymmetric dome breaks up into three ridges subtending angles of 120 sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state here in steady state the ridges join the triple junction at nearly 120 but one ridge curves to satisfy the boundary forcing the slope pattern in the immediate dome vicinity depends only on a dimensionless parameter which is a function of the ice consistency the accumulation and the rheological power law index attempts to replicate the topography around summit greenland obtained a good fit with n 3 at a triple junction the dome is really distinct from the surrounding ridges contrary to the highest point of a single ridge divide as a consequence the raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center along the ridges leaving the dome the raymond effect is still present and decreases with the ratio of the flow across and along the ridge in the vicinity of the dome horizontal strain rates vary strongly from uniaxial to biaxial large scale effects represented in our model as fluxes at boundaries seem to be the primary controls on dome position and shape

an anisotropic flow law for ice sheet ice and its implications abstract a new flow law or anisotropic polycrystalline ice is presented the strain rate tensor is related by a geometrical actor tensor g to the stress tensor the g factor tensor can be obtained from the c axis fabric data and stress condition this new flow law describes well the direction dependent mechanical properties of anisotropic ice which cannot be demonstrated by glen s flow law for example the new flow law can explain the fact that a strong single maximum fabric ice such as dye 3 vvisconsin ice can deform several times aster than isotropic ice under horizontal shear but can hardly deform under vertical or horizontal normal stress i ve also show that at a deeper part of an ice sheet where a single maximum fabric develops a positi e vertical strain rate can be produced with only a horizontal shear stress 1 introduction ice sheet modelling and studies on ice sheet dynamics carried out so far have been based on glen s flow law