diapiric transfer of melt in kilauea iki lava lake hawaii a quick efficient process of igneous differentiation kilauea iki lava lake formed in 1959 is a large pond of picritic basalt average mgo content 15 34 by weight which has cooled and crystallized as a small self roofed magma chamber repeated drilling of the upper crust of the lake down to its molten core and more recent 1981 drilling through the thermal maximum and part way through the lower crust have made it possible to monitor the differentiation processes in the lake in detail differentiation processes recognized as active in the lake include rather inefficient settling of the larger 2 10 mm olivine phenocrysts formation of segregation veins and formation of diapir like vertical olivine rich bodies all processes which occur in one or more of the other kilauean lava lakes as well in addition most of the central part of kilauea iki has been affected by diapiric melt transfer in this process relatively low density liquid present at 1145 1160 c rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt passing through the hotter but denser melt in the core of the lake without mixing the source volume from which the low density liquid was extracted is depleted in tio 2 and other incompatible elements and enriched in feo and cao the upper part of the lake shows the opposite effects the crystalline assemblage present was olivine augite minor plagioclase the crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m the process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others calculations suggest that between 21 and 42 wt liquid has been extracted from the depleted zone at 56 78 m in the center of the lake making this a very efficient process of chemical differentiation

differentiation of mesozoic basalts of the hartford basin connecticut the talcott oldest of the three basalts in the mesozoic hartford basin of connecticut contains abundant euhedral olivine and plagioclase phenocrysts and less common rounded augite plagioclase aggregates and orthopyroxene phenocrysts rimmed by augite and olivine the orthopyroxene which has a remarkably constant composition en 84 and is cr rich is believed to be a refractory residue from an upper mantle source region the holyoke basalt the middle unit is essentially aphyric with only minor plagioclase and olivine phenocrysts the hampden youngest of the three basalts contains abundant plagioclase and minor olivine and augite phenocrysts materials balance calculations using phenocryst compositions from the talcott basalt indicate that the holyoke can be derived from a talcott magma by addition of 7 8 orthopyroxene and removal of 7 9 olivine 15 clinopyroxene and 13 3 plagioclase the hampden basalt cannot be derived from a holyoke magma by any reasonable fractionation scheme but it can be derived from a talcott magma by removal of 4 4 olivine 12 0 clinopyroxene and 14 3 plagioclase one atmosphere melting experiments under controlled oxygen fugacities indicate successively lower liquidus temperatures for talcott holyoke and hampden basalts both the talcott and the holyoke have olivine and plagioclase on the liquidus simultaneously with augite and pigeonite appearing 15 c below the liquidus the hampden has plagioclase alone on the liquidus olivine and augite appear only 5 c below the liquidus orthopyroxene is not present in the low pressure experiments but is thought to have been stable on the liquidus at depths corresponding to pressures exceeding 8 kbar assimilation of orthopyroxene and crystallization of other phases necessary to produce the holyoke basalt from a talcott magma must have taken place at depth however because the low pressure phases obtained in the experiments are those required to produce the hampden from the talcott magma this fractionation likely occurred in a near surface magma chamber

phenocryst crystallization during ascent of alkali basalt magma at rishiri volcano northern japan abstract phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers a fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions on the basis of their large sizes however this assumption is not always true as demonstrated here for a porphyritic alkali basalt kutsugata lava from rishiri volcano northern japan all phenocryst phases in the kutsugata lava plagioclase olivine and augite have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals rarely a core region with distinct composition is present in all phenocryst phases phenocrysts excluding this core are occasionally in direct contact with each other forming crystal aggregates the equilibrium liquidus temperature of plagioclase the dominant phase 35 vol in the kutsugata lava can never exceed the estimated magmatic temperature unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent this suggests that most phenocrysts in the kutsugata lava were formed by decompression of the magma during ascent in a conduit rather than by cooling during residence in a magma reservoir in the magma chamber before eruption probably located at depth of more than 7 km only cores of the phenocrysts were present and the magma was nearly aphyric 4 wt in the magma chamber and liquidus temperatures of phenocryst phases were significantly suppressed large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution low ascent rate of the kutsugata magma which is suggested by pahoehoe lava morphology and no association of pyroclastics gave sufficient time for crystallization furthermore the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size which characterizes the porphyritic nature of the kutsugata lava alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems

fault reactivation control on normal fault growth an experimental study field studies frequently emphasize how fault reactivation is involved in the deformation of the upper crust however this phenomenon is generally neglected except in inversion models in analogue and numerical models performed to study fault network growth using sand silicon analogue models we show how pre existing discontinuities can control the geometry and evolution of a younger fault network the models show that the reactivation of pre existing discontinuities and their orientation control i the evolution of the main fault orientation distribution through time ii the geometry of relay fault zones iii the geometry of small scale faulting and iv the geometry and location of fault controlled basins and depocenters these results are in good agreement with natural fault networks observed in both the gulf of suez and lake tanganyika they demonstrate that heterogeneities such as pre existing faults should be included in models designed to understand the behavior and the tectonic evolution of sedimentary basins

samples from the crystallising boundary layer of a zoned magma chamber the spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr b p tephra deposit of laacher see volcano the melt showed vertical chemical zonation immediately prior to eruption and throughout most of the magma volume only a small fraction of crystals were present the eruption also ejected crystal rich nodules ranging from mafic to felsic in composition which are samples of the materials crystallising at the boundaries of the chamber new data on nodule petrography and chemical compositions of whole rocks minerals and interstitial glasses are presented volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol i e the crystals interlock one exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt felsic nodules show much less pronounced differences with their interstitial melt felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor modal mineralogy glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber a broader range of mineral compositions are often found mineral assemblages were especially diverse in the upper part of the chamber it is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and or time

modelling hanging wall accommodation above rigid thrust ramps abstract experimental models are used to study the role of material rheology in hanging wall accommodation above rigid flat ramp flat thrust footwalls the deformation in the hanging wall was accomplished by forwards sliding along a rigid basal staircase trajectory with a variable ramp angle ranging from 15 to 60 we model different ramp angles to examine hanging wall accommodation styles above thrust ramps of overthrust faults ranging from 15 to 30 as well as above pre existing normal faults ranging from 45 to 60 for the hanging walls we used stratified frictional sand and viscous silicone putty materials in this paper we study three types of models type 1 models represent purely frictional hanging walls where accommodation above thrust ramps was by layer parallel thickening and by generating a series of back thrusts type 2 and 3 models represent stratified frictional viscous hanging walls in these models accommodation was by a complex association of reverse and normal faults mainly controlled by the rheological anisotropy as well as by the ramp inclination angle in type 2 models the silicone covered only the lower flat while in type 3 models it also covered the rigid ramp for 30 in type 2 models and 45 in type 3 models the viscous layer inhibited the development of back thrusts in the frictional hanging wall instead the silicone thickened to develop a ductile ramp for values higher than 30 in type 2 models and 45 in type 3 models back thrusts develop in response to the bulk compression the experiments simulate many structures observed above natural thrust ramps with 30 and pre existing normal faults with 45 the models emphasise the importance of a basal ductile layer which allows the hanging wall to step up over the rigid ramp by building up its own ductile ramp the models also emphasise that foreland directed normal faulting can develop at a thrust front in the case that the vertical stress due to gravity exceeds the horizontal stress due to end loading within a thrust wedge

the influence of depth on reactivation in normal faulting abstract the study examines the influence of depth on the development of steep normal faults above a gently dipping fault which is reactivated in its deeper part and abandoned in its upper part the isotropic and anisotropic mohr coulomb anderson theory is used to determine the ability of any part of a fault to be reactivated by calculating the difference between the stress deviator necessary to induce reactivation and that required to induce fracturing when linear depth dependent properties are assumed it appears that a new normal fault can branch off a gentle pre existing fault only if the friction coefficient of the latter is much less than that used by byerlee and if the ratio cohesion of fault cohesion of intact rock is non negligible for a 30 dip fault the friction coefficient has to be less than 0 35 friction angle less than 20 neither pore fluid pressure nor constant excess fluid pressure affects the above conditions when the cohesion of the surrounding rocks increases with depth steep normal faults can branch off byerlee faults with dips as gentle as 30 a low shear strength of the lowest part of the preexisting fault also increases the chances of reactivation of the deeper part and abandonment of the upper part

convection and crystallization in magma cooled from above calculations are presented for the cooling from above of melts in the di an system in which the kinetics of crystallization are incorporated and play a dominant role the results indicate that even with no initial superheat whatsoever convection plays an important role in the cooling of magma chambers and allows substantial internal cooling crystallization and differentiation the calculations show in agreement with observations that in magma bodies hundreds of meters thick crystallization occurs predominantly in the interior or at the floor even though heat is lost only from the roof the ratio of the final thickness of the layer formed at the floor to that formed at the roof increases as the overall size of the chamber increases owing to the effects of convection

the transition from single layer to foliation boudinage a dynamic modelling approach abstract foliation boudinage is a deflection of foliation in the vicinity of a central discontinuity in foliated rocks mostly filled with vein material it shows evidence for brittle deformation and void opening during ductile flow we used a two dimensional visco elastic spring model based on a discrete element approach to study the dynamic development of foliation boudinage and the behaviour of anisotropic visco elastic material deformed under pure shear conditions the anisotropies are set by defining rheological heterogeneities in the models with 1 a single layer in a weaker matrix 2 multi layers with different elastic properties and 3 random distributed micas rows of horizontally aligned elements with the same elastic properties the models show the nucleation and propagation of fractures during ductile deformation and flow perturbation and localization in the vicinity of fractures voids localize along extension fractures at intersections of conjugate shear fractures and in small pull apart structures along shear fractures which look identical to boudin necks in isolated stiff layers results show that strain localization is strong in models with lower elastic constants and anisotropy whereas it is distributed in the models with higher elastic constants and anisotropy in which asymmetric and complex geometries develop the models successfully show the development of and transitions between a single layer multi layer and foliation boudinage that are common structures in high strain zones

boundary layer fractionation constrained by differential information from the kutsugata lava flow rishiri volcano japan detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber kutsugata lava a suitable target of such study is a quaternary alkali basalt 51 3 53 2 wt in sio2 from rishiri volcano northern japan despite the narrow range in the whole rock compositional variation chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition in the north lava 51 3 51 9 wt in sio2 the less differentiated portion of the kutsugata lava most crystals which include low ni olivine and plagioclase were derived from the mushy boundary layer the main part of the magma body was principally aphyric 30 vol by such mechanisms as compaction and compositional convection this fractionated melt was mixed with the overlying main magma causing differentiation of the kutsugata magma the average temperature of the extracted melt is 1010 c significantly lower than 1100 c estimated for the main magma a quantitative model of magmatic differentiation which includes thermal and compositional evolution of a mushy boundary layer can successfully reproduce the observed compositional trends of the north lava

extensional reactivation of an abandoned thrust a bound on shallowing in the brittle regime abstract shallow dip angles 45 suggested by field observations of continental extensional faults are not predicted by classical isotropic mohr coulomb anderson theory earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30 some structural evidence suggests brittle normal faulting with dip angles as shallow as 10 one explanation of the apparent conflict between theory and structural seismic observations is that intrinsically weak shallow dipping pre existing faults are preferentially reactivated any reduction in frictional strength of the pre existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated enhanced fluid pore pressure on the pre existing fault reduces the effective strength and can further enhance shallow fault reactivation an analytical treatment clarifies the roles of geometry intact pre existing fault strengths and fluid pore pressures frictional strength ratios of 3 or greater could account for extremely shallow normal faults dips 10 20 without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector moderate reduction in friction 3 4 with respect to wall rock can reduce the dip to 30 and can account for shallow normal slip earthquakes

modelling the influence of pre existing brittle fabrics on the development and architecture pull apart basins abstract we use new analogue modelling experiments to analyse the development of pull apart basins in an upper crust characterised by the presence of pre existing discrete fabrics as in previous models lateral movement of rigid basal plates induced strike slip deformation of a sand pack local extension allowing the formation of a pull apart basin was produced by the step over geometry of the master faults in this area a basal silicone layer was introduced to distribute deformation and reproduced a weaker crust in the basin conditions of neutral overlapping and underlapping interacting master faults were reproduced the model upper crust modelled by a sand mixture was characterised by the presence of pre existing structures the orientation of these inherited heterogeneities was systematically varied in different experiments model results indicate that depending on their orientation with respect to the strike slip displacement pre existing structures can be reactivated both within and at the margins of the pull apart basins inside the basin reactivation occurs when the pre existing structures are orthogonal or sub orthogonal to the strike slip displacement in this case the pre existing fabrics delay the development and linkage of cross basin faults and increase the complexity of the deformation pattern giving rise to a new set of faults characterised by an atypical trend pre existing fabrics oblique to the local extension direction may be partly reactivated in the central part of the basin as segments of cross basin faults at the margins of the pull apart reactivation occurs if the fabrics spatially coincide with the lateral boundaries of the silicone layer in these conditions reactivation allows a faster development of the border faults which are less segmented than in the homogenous models this also results in a more regular final geometry of the pull apart

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

a 2 d differential stress based analysis on the tendency of mining induced fault reactivation fault reactivation related geological disasters such as mine seismicity ground subsidence water inrush and gas outburst seriously affect mine production and miner s safety developing a practical analytical method for the risk assessment of mining induced fault reactivation is of great significance for underground mining safety in this study a 2 d systematic procedure for a rapid tendency analysis of mining induced fault reactivation is developed it is an analytical procedure developed on the basis of the mining induced differential stress and of the relationship between the differential stress and the corresponding critical reactivation angles in this work we proposed to use the following three conditions to assess the tendency of mining induced fault reactivation 1 whether the fault angle closes to the optimum angle of reactivation 2 whether the mining induced differential stress reached the minimum triggering differential stress required for fault reactivation 3 if the second condition is satisfied then evaluating whether the fault angle is within the range of the two corresponding critical reactivation angles the assessment procedure proposed in this study is practical and it can provide a significant reference for mining engineers to better understand the mining induced fault reactivation and to better assess the risk of fault reactivation related geological disasters

velocity dispersion a tool for characterizing reservoir rocks apparent discrepancies between velocity measurements made with different frequencies in a formation at the gypsy test site are explained in terms of elastic scattering and intrinsic attenuation the elastic scattering component of the dispersion 38 in a marine interval above the gypsy sandstone is estimated via simple models constructed from well log information any dispersion above the predicted value for elastic scattering in this interval is assigned to intrinsic attenuation 62 using the vertical measurements in the well the marine interval directly above the gypsy sandstone has an estimated intrinsic q sub i 51 and an effective q because of the scattering of q sub sc 85 the total q of the combined mechanisms is 32 the dispersion of the vertical measurements through the heterogeneous sands and shales of the gypsy formation can be explained using an intrinsic q sub i 30 and neglecting the effects of scattering the horizontal observations require a more detailed modeling effort to unravel the relative roles of path and volume effects elastic scattering attenuation and intrinsic anisotropy thin layers barely resolvable on the sonic logs play a significant role in modifying the crosswell response potentially the dispersion can be a key to mapping reservoirmore properties using crosswell and surface seismic data less

role of faults and layer interfaces on the spatial variation of stress regimes in basins inferences from numerical modelling abstract the spatial variation of the computed state of stress is studied in a series of numerically solved boundary value problems of basin scale deformation these numerical experiments illustrate simple situations of decollement tectonism under compression the study focuses on the local variation of the state of stress in a sedimentary overburden where major discontinuities such as large scale faults and layer interfaces are present the states of stress are computed using either the distinct element method or the finite element method classic elasto plasticity laws are used for the behaviour of the continuum and the discontinuities are modelled as coulomb contact surfaces a geologic interpretation of the numerical experiments is proposed to highlight the role played by major structural discontinuities in the local variations of the stress regime the models are two dimensional in space and assume plane strain but the full three dimensional stress strain constitutive relation is used therefore the stress regime variations can be represented by constructing the contour map of the wallace bott stress ratio this simple technique together with the construction of diagrams of principal stress magnitude across the model enables us to rapidly examine the stress field pattern and further discuss the deformation modes of faulting and fracturing which may take place at a small scale we have found that the local variations of the stress tensor are in good agreement with common geologic interpretation of brittle deformations in foreland and thrust systems we demonstrate how the presence of layer interface and the difference in mechanical properties of formation lithology may strongly affect the variation of stress in space this modelling approach has important issues when interpreting well tests to gain access to in situ stresses

control on thrust tectonics in the himalayan foothills a view from a numerical model abstract the evolution of an intracontinental accretionary wedge is controlled by a the displacement of several thrust sheets along faults and an underlying decollement b the vertical motion of the rigid basement beneath the accretionary wedge and c superficial processes erosion sedimentation a numerical model is proposed based on the coulomb wedge theory a 2 5 d forward kinematic system progressive tilting of the underthrust basement and an erosion sedimentation model in order to characterise the sequence of fault activation in a thrust system and the syn orogenic sedimentation in such a system the siwalik belt the frontal thrust system of the himalayas represents a structure on which the model can be applied it presents mainly simple structures made of one fault system branched on an underlying decollement conglomerate deposits of the plio pleistocene upper siwalik are overthrust by miocene formations lower siwalik at the front of the wedge and along inner faults structural maps show that thrust sheets are laterally relayed and simultaneously emplaced the comparison between the deformation pattern in the himalayan foothills and the modelling results supports the concept that the himalayan thrust wedge has a steady state type evolution controlled by horizontal convergence and superficial mass transport and characterised by an irregular spatial and temporal distribution of shortening within the whole wedge during this evolution piggy back basins or strongly displaced internal sheets hide some thrust sheets the exhumation path of these sheets is then discontinuous though the shortening of the wedge is regular

pre existing normal faults have limited control on the rift geometry of the northern north sea abstract many rifts develop in response to multiphase extension with numerical and physical models suggesting that reactivation of first phase normal faults and rift related variations in bulk crustal rheology control the evolution and final geometry of subsequent rifts however many natural multiphase rifts are deeply buried and thus poorly exposed in the field and poorly imaged in seismic reflection data making it difficult to test these models here we integrate recent 3d seismic reflection and borehole data across the entire east shetland basin northern north sea to constrain the long term regional development of this multiphase rift we document the following key stages of basin development i pre triassic to earliest triassic development of multiple sub basins controlled by widely distributed nnw to ne trending east and west dipping faults ii triassic activity on a single major ne trending west dipping fault located near the basins western margin and formation of a large half graben and iii jurassic development of a large e dipping n to ne trending half graben near the eastern margin of the basin which was associated with rift narrowing and strain focusing in the viking graben in contrast to previous studies which argue for two discrete periods of rifting during the permian triassic and late jurassic early cretaceous we find that rifting in the east shetland basin was protracted from pre triassic to cretaceous we find that during the jurassic most pre jurassic normal faults were buried and in some cases cross cut by newly formed faults with only a few being reactivated previously developed faults thus had only a limited control on the evolution and geometry of the later rift we instead argue that strain migration and rift narrowing was linked to the evolving thermal state of the lithosphere an interpretation supporting the predictions of lithosphere scale numerical models our study indicates that additional regional studies of natural rifts are required to test and refine the predictions of physical and numerical models more specifically our study suggests models not explicitly recognising or including thermal or rheological effects might over emphasise the role of discrete pre existing rift structures such as normal faults

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

influence of early strike slip deformation on subsequent perpendicular shortening an experimental approach we used sandbox analogue models to study the influence of previous pure strike slip and transpressional structures on subsequent perpendicular compression we performed also a brittle viscous system in order to analyse the presence of a viscous basal level experimental results show that a previous pattern generated under pure strike slip and transpressive regime causes the reactivation of structures with favourable orientation and the nucleation of oblique thrusts and non rectilinear deformation fronts under compression perpendicular to the first fault system the presence of a viscous basal level with the imposed strain rate which does not favour the coupling between the viscous layer and cover inhibits the reactivation of previous structures comparison with a sector of northern chile margin 24 25 s located in the forearc trench parallel region supports the experimental results

crystal mush compaction and the origin of pegmatitic segregation sheets in a thick flood basalt flow in the mesozoic hartford basin connecticut

detachment folding related miocene submarine slope instability in the romagna apennines italy 1 the relationships between slope failure and growing thrust related structures have been explored in the miocene marnoso arenacea foredeep basin romagna apennines impressive fossil slump layers are distributed along the footwall of the regional mount nero thrust which evolved from detachment fold to breakthrough thrust during the detachment fold stage development of growth strata on the rotating forelimb was preceded by the emplacement of slump layers that are currently part of the mount nero thrust footwall in this process sliding is assumed to nucleate along the contacts between different lithologies in the semiconsolidated rock mass the conditions of sliding specifically the critical angle of sliding crit are quantified by adding the influence of cohesion of sediments to the equilibrium analysis reported in hubbert and rubey s 1959 classical work the resulting angles crit are discussed with respect to the depth of the basal surface of sliding and pore fluid pressure states on the basis of such an analysis and field work results an evolutionary model is proposed in which slide sheets come from the crest and upper forelimb of growing detachment folds and attain a deeper detachment depth as the forelimb rotates and steepens pore fluid pressure is found to play a major role by radically decreasing the critical angle of sliding likewise localization of overpressures may also control the depth of sliding during progressive forelimb rotation to allow sliding along gently dipping bedding surfaces slope failure is interpreted as being related to the reactivation of anisotropies represented by lithological discontinuities

recent movements along the main boundary thrust of the himalayas normal faulting in an over critical thrust wedge abstract the main boundary thrust mbt is one of the major himalayan thrusts occurring during the cainozoic and it is presently incorporated within the himalayan thrust wedge lesser and outer himalayas displaced above the indian lithosphere nonetheless the mbt shows recent normal displacement along most of its length we suggest that the orientation of the major principal stress within the himalayan thrust wedge deviates significantly from the horizontal and when this deviation exceeds the dip of the vectors normal to back tilted thrusts the normal component of displacement may act along these faults steep north dipping segments of the mbt therefore show a normal component of displacement if a geometrical definition is used but they are faults in a compressional regime where the major principal stress axis has deviated from the horizontal micro structural data recorded along the surkhet ghorahi segment of the mbt are consistent with a strong deviation of the state of stress the presence of such peculiar normal faulting along the mbt is used to calibrate the mechanical characteristics of the belt considered as a coulomb wedge the following characteristics are suggested a very poor strength contrast between basal decollement and rocks in the wedge body b a high pore fluid pressure ratio probably close to 0 8 0 9 and a higher fluid pressure ratio close to 1 0 along the active normal faults if a high internal friction angle close to the byerlee value is considered the strong deviation in principal stress direction may have recently increased due to a taper of the himalayan wedge exceeding the stability boundary and may be controlled by erosion and isostatic uplift rebound of the himalayan range

networking of shear zones at the brittle to viscous transition cap de creus ne spain a crustal scale shear zone network at the fossil brittle to viscous transition exposed at cap de creus ne spain evolved by coeval fracturing and viscous mylonitic overprinting of an existing foliation initial fracturing led to mylonitic shearing as rock softened in ductilely deformed zones surrounding the fractures mylonitic shear zones widened by lateral branching of fractures from these shear zones and by synthetic rotation of the existing foliation between the fractures and shear zones shear zones lengthened by a combination of fracturing and mylonitic shearing in front of the shear zone tips shear zones interconnected along and across their shearing planes separating rhomb shaped lozenges of less deformed rock lozenges were subsequently incorporated into the mylonitic shear zones by widening in the manner described above in this way deformation became homogeneous on the scale of initial fracturing metre to decametre scale in contrast the shear zone network represents localisation of strain on a decametre length scale the strength of the continental crust at the time of coeval fracturing and viscous shearing is inferred to have decreased with time and strain as fracturing evolved to mylonitic shearing and as the shear zones coalesced to form a through going network subparallel to the shearing plane crustal strength must therefore be considered as strain and scale dependent

mozambique earthquake sequence of 2006 high angle normal faulting in southern africa 1 we report source mechanisms for the six largest shocks of the mozambique earthquake sequence of 22 february 2006 the main shock of this sequence is one of the largest mw 7 0 to occur in africa over the past 100 years and its p waveforms alone are sufficient to show that north south trending normal faulting near the surface continues to depths of more than 15 km along an exceptionally steep dip of 76 4 this new result shows that globally seismogenic normal faulting spans a wide range of dips from about 30 to 75 s waveforms when combined with a minor component of left lateral slip observed in the field indicate the rake to be between 80 and 89 which in turn places a new constraint on relative plate motions complications from a nonspecific rovuma microplate notwithstanding our results favor the euler pole between the somalia and nubia plates to lie southward of the epicenters

effects of lateral variations of crustal rheology on the occurrence of post orogenic normal faults the alto tiberina fault northern apennines central italy abstract the northern apennines na are characterized by formerly compressive structures partly overprinted by subsequent extensional structures the area of extensional tectonics migrated eastward since the miocene the youngest and easternmost major expression of extension is the alto tiberina fault atf we estimate 2d rheological profiles across the na and conclude that lateral rheological crustal variations have played an important role in the formation of the atf and similar previously active faults to the west lithospheric delamination and mantle degassing resulted in an easterly migrating extension compression boundary coinciding at present with the atf where i the thickness of the upper crust brittle layer reaches a maximum ii the critical stress difference required to initiate faulting at the base of the brittle layer is at a minimum and iii the total strengths of both the brittle layer and the whole lithosphere are at a minimum although the location of the fault is correlated with lithospheric rheological properties the rheology by itself does not account for the low dip 20 of the atf two hypotheses are considered a the low dip of the atf is related to a rotation of the stress tensor at the time of initiation of the fault caused by a basal shear stress 100 mpa possibly related to corner flow associated with delamination or b the low dip is associated to low values of the friction coefficient 0 5 coupled with high pore pressures related to mantle degassing our results establishing the correlation between crustal rheology and the location of the atf are relatively robust as we have examined various possible compositions and rheological parameters they also provide possible general indications on the mechanisms of localized extension in post orogenic extensional setting the hypotheses to account for the low dip of the atf on the other hand are intended simply to suggest possible solutions worthy of further study

critical stress difference fault orientation and slip direction in anisotropic rocks under non andersonian stress systems abstract the coulomb navier failure criterion is applied to geological faulting in the general three dimensional case of rocks containing arbitrarily oriented strength anisotropies and subject to non andersonian stress systems i e with none of the principal stresses acting in a vertical direction general expressions for the critical stress difference necessary to cause failure as a function of depth are given in terms of material parameters pore fluid pressure orientation of the stress field and orientation of the strength anisotropy the range of angles between a plane of anisotropy and the maximum principal stress direction for which slip occurs along the pre existing anisotropy rather than along a new fault is calculated as a function of depth for different stress regimes when the stress field is non andersonian and or strength anisotropies not containing the intermediate stress direction occur in the rock faulting generally is oblique slip a kinematic classification of faulting is given on the basis of the angle between the strike direction and the slip direction on the fault plane triangular diagrams analogous to those used in petrology are introduced to describe i faulting in isotropic rock subject to arbitrarily oriented stress fields and ii faulting in anisotropic rock when one principal stress direction is vertical the type of faulting as a function of stress field and anisotropy orientation can be read off directly from these diagrams

possible focal mechanism change during reactivation of a previously ruptured subduction zone and stress tensor implications summary body waveform inversion methods usually assume a constant double couple source or a combination of such subevents previous studies suggest that the source rupture of mature sources in terms of coupling along smooth subduction zones are usually resolved on this assumption however for events where the source area overlaps previously ruptured zones the conventional inversion methods fail to resolve the source process for such events the subduction coupling may vary between the mature source area and the previously ruptured zone causing a variation in the focal mechanism too we study the implications on the stress tensor of the coexistence of two such focal mechanisms using a major aftershock of the 1985 chilean earthquake m 6 0 the 1978 kurile m 7 6 and 1980 santa cruz islands m 7 7 earthquakes for the first event we use the two different sets of focal parameters obtained by a study which describes a focal mechanism change during the process the epicentres of the latter two large events are located in the spots which had remained unbroken in the preceding large events e g the 1963 and 1969 kurile earthquakes and the 1966 santa cruz is earthquake respectively for each event one of the focal mechanism solutions shows a substantial strike slip component the stress tensors are examined under the constraints that the shear stress is parallel to the slip vector and a friction law is satisfied two particular families of stress tensors examined are 1 the tensors that are optimally orientated with respect to the mature source area and 2 the tensors that have a vertical principal stress direction the first type of tensors can fully account for the observed slip orientation only for the chile aftershock but it comes close to the observed one in the case of the kurile island event and would indeed reactivate the decoupled area at a reasonably low stress level the second type of tensors can be compatible with the two observed slip orientations in all cases however for the santa cruz is and kurile is earthquakes the friction law which should be satisfied on each plane requires too high a stress deviation

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

thermal and compositional evolution of a cooling magma chamber by boundary layer fractionation model and its application for primary magma estimation a boundary layer fractionation model for the thermal and compositional evolution of a basaltic magma chamber is presented the model utilizes a multicomponent thermodynamic approach for relating the compositional structure with the thermal structure of a cooling magma body the magma composition in the main magma body evolves by transportation of a fractionated interstitial melt from the mushy boundary layer the consideration of both the thermal and compositional evolution of the magma body enables a realistic simulation of magmatic differentiation as a function of time and space the model is used to evaluate a primary magma estimation from volcanic rock series it is shown that the common procedure of olivine addition gives an estimation which is significantly different from the true primary composition if the magma actually differentiated by boundary layer fractionation this can cause significant errors in estimating the conditions at which the primary magma was generated

stress variability and brittle tectonic structures the problem of stress variability both in space and time during brittle deformation of rocks is faced in this note several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed a genetic partitioning of the stress tensor is proposed showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific genetic components the genetic components discussed in this note are the gravitational the tectonic the fluid the thermal and the diagenetic one each genetic component is a function of several parameters but for all of them the possible important role played by the variable time is emphasised according to numerous researches based on theoretical and experimental grounds brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small and large scale spatial variations as well as about short and long term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event the relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches

stress orientation and tectonic regime in the northwestern valencia trough from borehole data the orientation of the maximum horizontal stress sh is obtained from the analysis of borehole breakouts covering a depth range from 300 to 3415 m below sea level in twelve offshore exploration wells in the northwestern valencia trough the orientation of sh is roughly coincident with the strike of major extensional structures from n to s it changes counterclockwise from a ne sw orientation to a n s orientation estimates of the tectonic regimes indicate that the area is characterised predominantly by normal faulting with a strike slip component both the stress orientations and the tectonic regimes are consistent with neotectonic studies in the nearby catalan coastal ranges an established method of estimating the tectonic regime by moos and zoback 1990 was modified by the inclusion of a nontrivial cohesion but this changes the results insignificantly

mechanical decoupling of high pressure crustal units during continental subduction the mechanics of the transition from continental subduction toward upper crustal nappe stacking is still poorly understood and is studied here through a 2d thermal and strength numerical modelling of a subducted passive margin geological observations in the core of most mountain belts show the piling up of several hp lt upper crustal units that are most likely related to the detachment of upper crustal units from the subducted continental margin and to the subsequent stacking of the detached units at depths the adula unit lepontine dome central alps switzerland is a long and thin upper crustal unit and is used here as a natural case study as it provides a well documented example of these units 2d thermal modeling shows that two steps successive in time characterized the burial history of the passive margin undergoing continental subduction 1 an increase in the margin strength due to an increase in the confining pressure during the first million years of the margin subduction and 2 the progressive heating of the subducted margin from the overlying lithosphere leads to a decrease in the margin strength due to thermal weakening which progressively counter balances the increase in confining pressure two strength gradients develop within the subducted lithosphere 1 along the slab the strength decreases with increasing burial depth and 2 perpendicular to the slab the strength increases with depth due to an inverse temperature distribution the detachment of hp lt continental units from the subducted margin could occur when the slab strength becomes lower than the applied net stress this allows the detachment of ductile weakened thin and long upper crustal units the thickness and length of the detached crustal units are controlled by the following parameters in order of their importance subduction dip angle crustal rheology mantle heat flux and subduction velocity the comparison of our model results with the geometry and pt conditions of the adula unit yields an estimate of the alpine subduction dip angle at the time of deformation and metamorphism

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

on the mechanism and source parameters of the deep crustal jabalpur earthquake india of 1997 may 21 constraints from aftershocks and changes in static stress summary the 1997 may 21 jabalpur earthquake mb 6 0 occurred in the lower crust of the ene wswtrending narmada son palaeorift zone in central india known as the narmada son lineament nsl we correlate the aftershock distribution of this earthquake with the computed changes in static stress caused by slip on the earthquake rupture in order to constrain some of the source parameters of the main event we estimate that the downdip edge of the earthquake rupture extended up to a depth of about 38 km and that the majority of the aftershocks occurred due to unclamping in the hangingwall near the downdip edge of the rupture considering the intermediate to moderately high heat flow in the region we suggest that 1 high pore pressure in the lower crust which developed due to the dehydration of the serpentinite body during previous episodes of thermo tectonic processes 2 a lower frictional coefficient possibly arising due to fractured rocks at depth 3 a higher strain rate and 4 the favourable influence of n s compression due to the motion of the indian plate are responsible for the infrequent earthquakes in the lower crust of the nsl zone

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

basin inversion and contractional reactivation of inherited normal faults a review based on previous and new experimental models abstract compressional inversion of former extensional sedimentary basins is a particularly common phenomenon given that passive margins share the same destiny of being subsequently incorporated into fold and thrust belts during basin inversion pre existing faults may concentrate stress and localise future thrust ramps or may be reactivated recording reversal of movement from extension to reverse this process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects and seismic hazard assessment since more than two decades analogue modelling has been used to indentify and or test a number of parameters relevant for the inversion process this paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject coupling such previous results with those of a new experimental series inspired to the northern apennines stratigraphy and evolution past models have explored the role of main factors governing fault reactivation susceptibility such as fluid pressure fault weakness fault steepness angle of shortening and sediment loading the new models have addressed the role played by geometry and strength of a basal ductile layer during inversion two sets of sand silicone models were first extended orthogonally at different velocities to produce dissimilar pre inversion internal geometries and then shortened coaxially at different velocities to vary the brittle ductile coupling the experimental results confirm the selectiveness of fault inversion and reveal strong similarities with structural styles of tectonic inversion reported from the northern apennines and various areas worldwide the preferred geological model inferred from this modelling involves contribution of 1 syn inversion rotation of pre existing faults to shallower dip and 2 rotation of principal stress axes which could explain the invariable reactivation of only one of the two oppositely dipping graben bounding faults these factors would thus interact with the other well established parameters controlling the fault reactivation process

effects of layering and anisotropy on fault geometry the geometry and orientation of faults are examined using several field examples of small scale 1 with dihedral angles usually of about 50 as predicted by the coulomb theory of failure in layered rocks the geometry of faults varies with the orientation of layering with respect to the stress field where 1 is approximately normal to layering or anisotropy conjugate faults also develop symmetrically about 1 where rocks have interbedded layers with different mechanical behaviours however faults tend to initiate orthogonal to the more brittle layers i e originate as extension fractures sub parallel to 1 but oblique to the less brittle layers as the fault steps through the layering pull aparts are developed which may reduce the dihedral angle where 1 is oblique c 25 75 to anisotropy one set of faults is developed at a high angle to layering with another at a low angle usually showing a ramp flat geometry large dihedral angles up to 90 may result and 1 does not bisect this angle where 1 is approximately parallel to layering two cases can be recognized where 3 is approximately normal to layering faults with layer parallel flats and contractional ramps develop where 2 is approximately normal to layering conjugate faults develop which are symmetrical about 1 the geometry resembling that in isotropic rocks these observations are in agreement with rock deformation experiments which show the strong effect of anisotropy on fault orientation but the observations incorporate the effects of layering of materials with different deformation characteristics

critical stress difference and orientation of faults in rocks with strength anisotropies the two dimensional case abstract equations are derived for the critical stress difference on thrust normal and strike slip faults with finite cohesive strength both in homogeneous isotropic rock and along pre existing strength anisotropies with different cohesion and coefficient of friction subject to the limitation that the plane of anisotropy contains the intermediate axis of stress the range of orientations for which sliding occurs along pre existing planes of weakness rather than along a new fault is given as a function of material parameters and critical stress difference for the intact rock given the principal stress directions the equations allow the direct calculation of the stress and orientation conditions for the three faulting regimes

basin inversion and fault reactivation in laboratory experiments abstract a series of analogue models have been performed to get insights into the relationships between the reactivation of pre existing normal faults and the orientation of a successive compressive stress field the models were deformed in two phases 1 a first extension phase h 3 generating a graben and 2 a successive compressive phase with the orientation of the shortening direction h 1 trending between 0 and 90 obliquity angle to the faults bounding the graben the deepening of this graben was accompanied by syn tectonic sedimentation that included a basal ductile layer simulating salt in nature overlain by a brittle sand pack the experimental results suggest that the development of compressive structures is strongly controlled by the pre existing normal faults and by the obliquity angle the experiments have invariably shown that the extensional faults were reactivated during shortening whatever the value of the angle whereas strike slip reactivation of major normal faults is favoured by low obliquity angles mostly dip slip reactivation of pre existing structures is instead promoted for high obliquity angles the analysis of model results highlights the important role played by a ductile layer at the base of the basin fill and the obliquity angle in controlling both strain partitioning and brittle ductile decoupling

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

origin of pegmatitic segregation veins within flood basalts subhorizontal veins and lenses of coarse grained rock composed of plagioclase augite and fe ti oxides in a glassy and vesicular mesostasis occur within several thick subaerial basalt flows in the columbia river basalt province of washington and in the eastern north american mesozoic basalt province the veins and lenses referred to as pegmatitic segregation veins are typically 1 10 cm thick and are enriched in fe ti k p cu zr ba and ree by a factor of about 1 1 to 3 over the host basalt al ca sr mg ni and cr are lower and si and na are typically unchanged we interpret the pegmatitic segregation veins as the product of residual melt carried in vapor bubbles from the lower crystallization front of partially crystallized flows to the crystal mush within the upper crystallization front movement through the elevated temperatures of flow interiors fused any crystal nuclei in the segregation melt diminished nucleation density during crystallization may partially account for coarse grain sizes in addition diffusion of ions to large rapidly growing skeletal crystals was probably enhanced by the viscosity reduction effect of water enrichment the water content of accumulated segregation melt may have increased whenever the escape of volatiles from partiallymore crystallized flows was temporarily sealed off under a rigid barrier of solidified basalt rupture of the barrier by downward propagation of columnar joints would allow the resumption of effervescence and any remaining partially crystallized segregation melt phase would be degassed and quenched into a glassy and vesicular mesostasis 59 refs 8 figs 2 tabs less

partition coefficients for iron between plagioclase and basalt as a function of oxygen fugacity implications for archean and lunar anorthosites abstract experimental studies were undertaken to determine d for feot between plagioclase and melt as a function of oxygen fugacity for a basaltic composition that occurs as matrices for plagioclase megacrysts that are similar to those in calcic anorthosites at liquidus conditions the d for feot between calcic plagioclase and tholeiitic basalt changes very little from 0 030 to 0 044 between the very low o 2 of the iron wustite buffer and that of the quartz fayalite magnetite qfm buffer at fugacities above qfm the value for d increases rapidly to 0 14 at the magnetite hematite buffer and 0 36 in air the increase in d results from the fact that nearly all of the fe is in the fe2 state at o 2 below qfm and the fe 3 fe 2 ratio in the melt increases rapidly above qfm causing more fe to enter the plagioclase which accepts fe3 more readily than fe2 there is essentially no change in values of d over cooling rates that vary by two orders of magnitude the d for feot of 0 030 at iron wustite is in good agreement with earlier studies at low o2 terrestrial calcic plagioclase matrix pairs display ds of about 0 043 in excellent agreement with the experimental value of 0 044 at qfm which is the accepted fugacity for terrestrial tholeiitic basalts application of the ds for feot and other components to terrestrial calcic anorthosites indicates that the anorthosites were in equilibrium with nearby coeval tholeiitic basalts at qfm as was expected lunar anorthosites however have anomalously low contents of feot as well as mgo and would be in equilibrium with melts that are several times lower in these components than expected the low values of feot and mgo in the plagioclase of lunar anorthosites are a result of subsolidus reequilibration with pyroxene olivine and other fe mg bearing phases during high temperature metamorphism as is the case in terrestrial anorthosites that have undergone granulite grade metamorphism the ds for k2o mgo and tio2 were determined as 0 21 0 044 and 0 033 respectively and show no variation with o 2 the value for k2o is in good agreement with natural assemblages the values for mgo and tio2 however are lower in natural assemblages but the high degree of mobility of mg and the very low contents of tio2 in plagioclase makes the reliability of the natural values rather low also the d for tio2 may be affected by rate of cooling attempts to determine the d for cr2o3 as a function of o 2 were made difficult by the low levels of cr2o3 300 to 500 ppm in the melt when saturation with a cr spinel occurred because the cr concentrations in plagioclase are nearly 100 times less than those in the melt it was not possible to obtain reliable analyses for plagioclase these uncertainties constrain the d for cr to be less than 0 07 at the qfm buffer

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

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

graben inversion in nature and experiments the inversion of crustal scale basement grabens is studied here through laboratory experiments on small scale models and available oil industry seismic lines from the southern north sea two basic configurations are considered first both the basement and the sedimentary cover are brittle and inversion does not involve any potential decollement between them second the basement and sedimentary cover are separated by a weak ductile layer e g salt which can allow decollement of the cover during both extension and later compression and inversion the second configuration is more complicated and can lead to a large variety of geological structures laboratory experiments were carried out on brittle ductile models built with sand to represent brittle layers basement and sedimentary cover and silicone putty to simulate the decollement layer between basement and cover a mechanically based classification of inversion structures is proposed the effects of some crucial parameters are investigated including obliquity between the direction of shortening and normal faults as well as strength profiles and the presence or absence of salt diapirs the experimental investigation leads to the following conclusions 1 the inversion of the graben by reactivation of normal faults implies that the angle between the direction of compression and the graben is less than 45 2 if there is a superficial decollement e g basement cover interface inversion initiates low dipping thrust faults in the cover localized at graben borders 3 salt diapirs or salt walls localized along the graben borders in the cover are preferential sites for the development of thrust faults and 4 when the cover is decoupled from the basement by a decollement layer inversion induced deformation in the cover which is partitioned between thrust faults along the graben borders and strike slip faults within the graben trending oblique to the graben borders experimental results are compared with field examples in particular from the southern north sea

the role of pre existing faults in basin evolution constraints from 2d finite element and 3d flexure models the lithospheric driving forces which cause intraplate basin deformation are relatively constant over large areas consequently lateral variations in deformation and stress and strain concentrations seem to be primarily caused by pre existing heterogenities in the rheological signature of the continental lithosphere underlying the sedimentary basins in this paper we explore the weak character of upper crustal faults and their control on basin shape for a number of case studies on intraplate phanerozoic basin settings using a 2d finite element and a 3d flexure model of key importance is the integration of seismic data and field observations with the tectonic modelling allowing the investigation of deformation processes and their expressions on different scales operating on different levels of the lithosphere finite element models for sub basin scales incorporating weak upper crustal faults predict strong control of these weak zones on local stress distributions and subsequent deformation in agreement with observed deformation patterns slip along upper crustal faults control stress distribution and subsequent faulting in overlying sedimentary rocks the effect of weak upper crustal fault movements on basin wide regional upper crustal flexure is looked at in two case studies on 1 extensional tectonics in the lake tanganyika rift zone east africa and 2 compressional tectonics in the central system and tajo basin central spain both settings indicate that basement warpings are controlled by large amounts of slip along so called weak crustal scale border faults which are mostly planar adopting border fault displacements in the 3d flexure model the results indicate low effective elastic thickness eet values in a range of 3 7 km for induced basement deflection patterns in accordance with observations the low eet values most likely reflect a partly decoupling of upper crustal and subcrustal deformation facilitated by the weak lower crust and in agreement with standard rheological assumptions for phanerozoic lithosphere in contrast the inferred weakness of upper crustal faults relative to surrounding rock is not evident from uniform rheological assumptions however observations of reactivations of faults which are not preferably aligned with the stress field and reactivations of basin deformation on long time scales are in support of this feature

modelling of structural complexity in sedimentary basins the role of pre existing faults in thrust tectonics abstract analogue and numerical models have been used to study the role of pre existing faults in compressive regimes from a theoretical point of view reactivation is mainly controlled by fault attitude stress regime and frictional properties of fault planes in scaled down sandbox experiments precut faults are introduced in the homogeneous granular media with a nylon wire which is forced through the sand cake producing a thin planar disturbed zone systematic experiments of thrust inversion with various dips and strikes of such planar discontinuities have been modelled comparison of experimental results with theoretical diagrams indicate that disturbed zones have a friction angle which is 10 20 lower than the homogeneous sand and that the compressive regime in the sandbox has a shape factor close to 0 4 the static analysis of fault reactivation is in accordance with the experimental observations except for pre existing faults dipping at very low angle however numerical modelling using the udec code shows that low angle faults can be reactivated as a result of stress concentration in the lower part of the fault in addition sandbox experiments indicate that in thrust systems reactivation of pre existing faults is not only dependent on their attitude but also on their spacing and location relative to the thrust system

solidification front fractionation in phenocryst free sheet like magma bodies basal cumulate zones are common in sills formed of magma carrying phenocrysts upon emplacement and the process leading to these cumulates and the corresponding roofward depleted zones are well understood many large sills which seem to have been emplaced with phenocryst free magma are nearly uniform in composition and show little tendency to differentiate by crystal settling we examine whether for an initially crystal free sill solidifying inwardly from top and bottom crystals nucleated and grown along the roof can escape incorporation survive resorption in settling through the hotter central region and reach the basal solidification front to form a cumulate we call this process solidification front fractionation and build a relatively simple model to examine the rheologic dynamic and crystal growth rate conditions leading to the downward escape of newly grown single crystals from the roof of a sheet like body of mafic magma cooling by conduction our results suggest that whereas a thin micro