chloritisation along the thanewasna shear zone western bastar craton central india its genetic linkage to cu au mineralisation abstract the cu au mineralisation of thanewasna bastar craton central india is hosted in quartz chlorite veins traversing granitoid rocks mineralisation and associated alteration are controlled by a nw se trending brittle ductile shear zone large scale alkali magmatism in the form of k rich granite emplacement is found in the vicinity of the shear zone the emplacement of this granite 1587 14 ma along the northern shoulder of the godavari rift produced a high geothermal gradient resulting in intense chloritisation and mineralisation field and petrographic studies show that the chlorite formed in two possible modes viz a replacement of ferromagnesian minerals of the host rock and or b precipitated directly from hydrothermal solutions mineral chemistry epma coupled with sem and xrd studies reveals that chamosite with minor clinochlore varieties of chlorite is commonly associated with cu and au mineralisation chlorite shows variation in fe fe mg ratios from 0 54 to 0 60 0 55 to 0 64 and 0 49 to 0 55 and al iv from 1 88 to 2 40 1 91 to 2 50 and 1 80 to 1 94 apfu respectively at the surface and in the sub surface mineralisation and host rock empirical and thermodynamic geothermometers have been used to determine the temperature of chlorite formation based on chemical composition six empirical methods based on al iv al vi and fe fe mg ratio showed a temperature range of 187 c to 381 c temperatures of chlorite formation have also been determined by the thermodynamic solid solution method the thermodynamic equations of three intra crystalline equilibria at 2 kbar pressure show that temperature ranges from 215 to 454 c for low si contents 2 8 apfu the thermodynamic method of vidal et al 2001 and 2005 was not so compatible for an estimation of temperature of hydrothermal fe rich chlorites associated with cu au mineralisation at the thanewasna copper deposit tcd the temperature of chlorite formation increases from the host rock mean 201 c to proximal mineralisation zone mean 234 c to distal k na alteration zone mean 251 c integrating field and laboratory results suggested that tcd chlorite is part of a structurally controlled high sulphidation epithermal system this characterization would help in the regional exploration for base and precious metals and the results are applicable for the entire western bastar craton wbc of central india and similar deposit elsewhere

seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

weak faults rotten cores earthquakes are triggered when crustal deformation produces displacements along fault zones the rock in these zones is found to be weaker than adjacent material but what are the mechanisms of this loss of strength in his perspective holdsworth 1 discusses how recent studies of exposed ancient fault zones combined with laboratory analyses are revealing that rock properties at the grain scale can determine the large scale behavior of crustal deformation 1 http www sciencemag org cgi content full 303 5655 181

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

friction of foliated fault gouge with a biotite interlayer at hydrothermal conditions abstract to investigate the influence of foliation and structure within fault zones synthetic fault gouges of quartz containing various thin layers of biotite were sheared in a triaxial testing system at hydrothermal conditions to acquire the faulting behaviour along with quartz biotite mixtures tested for comparison first in series 1 experiments gouges were sheared at a temperature of 100 c with effective confining pressure of 200 mpa pore pressure of 30 mpa and loading rate of 1 22 m s the results revealed that strength of quartz biotite mixture weakened linearly with increasing biotite proportion whereas the gouges with a biotite interlayer weakened much more significantly even for biotite content as little as 5 wt series 2 experiments were performed on simulated gouge with 15 wt biotite interlayer at temperatures of 25 600 c effective normal stress of 200 230 mpa and pore pressure of 30 mpa with stepped shear rates of 0 0488 to 1 22 m s at effective normal stress of 200 mpa the simulated foliate gouge showed a transition from velocity strengthening to velocity weakening around 200 c which persisted up to 600 c stick slips above 400 c were associated with minor a b averagely of 0 0006 and submicron dc values at effective normal stress of 230 mpa and temperature of 600 c stick slip motions evolved with slip distance to viscous like behaviour at a displacement of 3 mm microstructural observation on deformed samples revealed that shear deformation mainly occurred within the biotite interlayer in most cases with surrounding quartz gouge only slightly deformed but increasingly compacted as temperature increased viscous like behaviour corresponded to migration of dominant shear deformation to the quartz gouge as indicated by the development of riedel shears therein our results may help constrain fault strength and depth range of seismogenesis within fault zones associated with biotite in form of foliations and or interconnected networks

representation of faults in reservoir scale geomechanical finite element models a comparison of different modelling approaches abstract different approaches exist to incorporate faults in reservoir scale geomechanical models challenges are the proper representation of the fault geometry as well as the small scale variations in the internal architecture and the mechanical properties of the fault zone regarding the typical size of such models i e kilometres to tens of kilometres the present study utilizes a simple generic fault zone model to compare three different possibilities commonly used to represent faults in finite element reservoir models two differ in the basic grid geometry and the arrangement of mechanically weak fault zone elements respectively the third uses a discontinuous grid and contact elements to represent the fault modelling results show remarkable differences in the calculated stress and strain patterns the relatively strongest perturbations result for a continuous curvilinear grid adapted to the fault geometry in contrast the fault implementation has the least impact on local stresses and strains if it is represented as a stair step structure contained in a rectangular grid the use of contact elements has an intermediate effect modelling results are used to infer some general recommendations concerning the appropriate approach of representing faults in a numerical geomechanical reservoir models depending on fault geometry model scale and scope of interest

fault zone structure and weakening processes in basin scale reverse faults the moonlight fault zone south island new zealand abstract the 200 km long moonlight fault zone mfz in southern new zealand was an oligocene basin bounding normal fault zone that reactivated in the miocene as a high angle reverse fault present dip angle 65 75 regional exhumation in the last c 5 ma has resulted in deep exposures of the mfz that present an opportunity to study the structure and deformation processes that were active in a basin scale reverse fault at basement depths syn rift sediments are preserved only as thin fault bound slivers the hanging wall and footwall of the mfz are mainly greenschist facies quartzofeldspathic schists that have a steeply dipping 55 75 foliation subparallel to the main fault trace in more fissile lithologies e g greyschists hanging wall deformation occurred by the development of foliation parallel breccia layers up to a few centimetres thick greyschists in the footwall deformed mainly by folding and formation of tabular foliation parallel breccias up to 1 m wide where the hanging wall contains more competent lithologies e g greenschist facies metabasite it is laced with networks of pseudotachylyte that formed parallel to the host rock foliation in a damage zone extending up to 500 m from the main fault trace the fault core contains an up to 20 m thick sequence of breccias cataclasites and foliated cataclasites preserving evidence for the progressive development of interconnected networks of partly authigenic chlorite and muscovite deformation in the fault core occurred by cataclasis of quartz and albite frictional sliding of chlorite and muscovite grains and dissolution precipitation combined with published friction and permeability data our observations suggest that 1 host rock lithology and anisotropy were the primary controls on the structure of the mfz at basement depths and 2 high angle reverse slip was facilitated by the low frictional strength of fault core materials restriction of pseudotachylyte networks to the hanging wall of the mfz further suggests that the wide phyllosilicate rich fault core acted as an efficient hydrological barrier resulting in a relatively hydrous footwall and fault core but a relatively dry hanging wall

shrimp geochronology for the 1450 ma lakhna dyke swarm its implication for the presence of eoarchaean crust in the bastar craton and 1450 517 ma depositional age for purana basin khariar eastern indian peninsula comment abstract as critical comments to the recent paper by ratre et al 2010 journal of asian earth sciences 39 565 577 we cite u pb shrimp and chime ages of magmatic and detrital zircon and monazite from the chhattisgarh and the khariar basins in the bastar craton to argue that these basins closed ca 1000 ma we further argue that geochronologic data geological evidence and geological logic strongly indicate that sedimentation in the khariar basin did not continue up to or beyond 517 ma as stated by ratre et al op cit

strain refraction viscosity ratio and multi layer deformation a mechanical approach a solution for the deformation of viscous layers during a two dimensional general deformation is derived layer deformation is found to consist of components of pure shear simple shear and pure rotation when considered with respect to the layer orientation it is shown that for external bulk pure shear the layer deformation is typically vastly different to that outside the layer ranging from pure to simple to super shear depending on layer orientation and viscosity contrast treagus method for determining viscosity ratio from cleavage refraction is re evaluated and is found to applicable under bulk pure shear however the presence of earlier lps layer parallel shortening can lead to erroneous viscosity ratio estimates a new approach is developed relying on the variation of cleavage orientation with layer orientation that is found to be robust with respect to lps the method is applied to examples from the irish variscides and the new approach has implications for determining whether or not lps occurred in this foreland setting

sedimentology of a proterozoic erg the venkatpur sandstone pranhita godavari valley south india reappraisal of the late proterozoic venkatpur sandstone indicates that the bulk of the sandstone is aeolian in origin aeolian stratification types namely i inverse graded translatent strata ii adhesion laminae iii grainflow strata and iv grainfall strata are present throughout the outcrop belt nine facies have been identified that represent both aeolian and related aqueous environments within a well developed erg cosets of large cross beds at the bellampalli section in the nw of the study area record dune fields in the interior of the sand sea to the se at the godavari river and ramgundam sections a progressive increase in the relative proportion of the flat bedded to cross bedded facies and intercalated non aeolian facies delineates the transition from the dune field to sand sheet environment an alternating sequence of aeolian and marine sediments at laknavaram in the extreme se marks the termination of the sand sea palaeocurrent data suggest that the nw se trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction abundant horizontally stratified units in the vankatpur sandstone do not always represent the interdune sediments on the basis of the thickness and geometry of the units nature of bounding surfaces and associated facies sequence the facies is variously interpreted to represent interdune inland sabkha sand sheet and coastal sand flat deposits

the widespread occurrence of low angle normal faults in a rift setting review of examples from thailand and implications for their origin and evolution abstract at least 31 low angle 1 km cenozoic normal faults are identified on seismic reflection data onshore and offshore thailand although some faults have been moderately rotated to lower angles pre rotation dips still indicate formation at low angles about 25 and 35 the dominant east dip of low initial fault dips suggest that pre existing fabrics controlled the fault dip direction in the mergui basin patches of gently dipping basement reflections on seismic reflection data support this observation well data points to a pre rift basement with a widespread component of slaty phyllitic and chlorite schist lithologies that are associated with significant strength anisotropy the subduction zone setting of se asia with a high fluid flux into the crust associated with high levels of co2 has parallels with the setting for lanfs in italy where it is suggested high pore fluid pressures played an important role in lanf reactivation in cross section most lanfs curve from a high angle in sedimentary section to planar or stepped in basement until they reach the brittle ductile transition zone in a few cases lanfs have a convex up geometry some long segmented faults are composed of mixed regions of high and low angle segments the local mix of high and low angle faults indicates that the principal stresses probably were vertical and horizontal during fault formation any rotation of stresses would have to be local not regional lanfs in the mergui basin have very high displacement gradients as low as 2 1 indicating the same fault segments some 20 30 km long have repeatedly slipped while displacements can decline dramatically at key discontinuities the lanf segments are inferred to be very weak and probably slipped mostly aseismically while adjacent lower displacement high angle segments were probably seismically active hence in areas of mixed high and low angle fault segments seismicity may erroneously suggest that high angle normal faults are the dominant fault type the presence of such a high concentration of low and high displacement lanfs in a non metamorphic core complex setting demonstrates that lanfs can form initially at a low angle that isostatic rotation of initially high angle faults to a low angle is not necessary or appropriate to explain the faults and that low angle normal faults are not just features associated with orogenic collapse

lead isotope behaviour in a polyphased pb zn ore deposit les malines cevennes france les malines mine cevennes france provides an example of pb zn ore bodies that underwent a polyphased evolution on the pb pb diagram experimental points of the les malines ore types always plot in the central part of the field defined for the cevennes metallogenic province this homogeneity the similarities with the pb isotopic compositions of the surrounding rocks and the comparison with pb isotopes evolution of whole rocks and minerals of the neighbouring continental lands which emerged during triassic and liassic times rule out the participation of extraneous pb during the successive concentration stages and agree with an initial metal stock transformed and mobilized in a pb closed system hercynian k feldspars are the main source of the pb preconcentrated during triassic times most of the cevennes pb zn ore bodies could derive from this triassic metal stock nevertheless local pb isotopic anomalies suggest hydrothermal sources for some pb zn ore bodies located along the cevennes fault

prelithification and synlithification tectonic foliation development in a clastic sedimentary sequence the current view regarding the timing of regionally developed penetrative tectonic fabrics in sedimentary rocks is that their development postdates lithification of those rocks in this case fabric development is achieved by a number of deformation mechanisms including grain rigid body rotation crystal plastic deformation and pressure solution the latter is believed to be the primary mechanism responsible for the domainal structure of cleavage in low grade metamorphic rocks in this study we combine field observations with strain studies to characterize considerable 50 acadian crustal shortening in a devonian clastic sedimentary sequence from southwest ireland despite these high levels of shortening there is a marked absence of the domainal cleavage structure and intraclast deformation that are expected with this level of deformation fabrics in these rocks are predominantly a product of rigid body rotation and repacking of extraformational clasts during deformation of a clastic sedimentary sequence before lithification was complete

a chronology of foreland deformation ultra violet laser 40ar 39ar dating of syn late orogenic intrusions from the variscides of southwest ireland abstract the upper palaeozoic sedimentary rocks of the munster and south munster basins southern ireland lie within the rhenohercynian zone of the european variscan orogeny this foreland region accommodated shortening during the asturian phase of variscan deformation at the end of the carboniferous by the development of kilometre scale and lower order folding high angle reverse faulting and regional fabric development at the southwestern extremity of the belt lies the black ball white ball heads area of the beara peninsula where high level igneous intrusions locally exhibit a close relationship with both the ductile and late brittle phases of variscan deformation 40 ar 39 ar ultra violet laser analysis of phlogopite crystals from these intrusions principally trachytic dykes has yielded ages that when combined with structural field relationships help to constrain the timing of variscan deformation in southern ireland these ages include 314 44 1 00 ma for a penetratively deformed lamprophric pipe on black ball head 301 98 1 47 298 08 0 61 ma for dyke material associated with later stage brittle deformation and a date of 296 88 0 60 ma for an undeformed post orogenic dyke on white ball head

existence de mineralisations plombo zinciferes syngenetiques du substratum cambrien du district des malines gard france l essentiel du minerai actuellement exploite dans le district minier des malines gard france provient d amas plombo zinciferes apparamment constitues par le remplissage de cavites paleo karstiques post stephaniennes developpees dans la dolomie cambrienne a proximite de la surface de discordance qui la separe de sa couverture mesozoique l etude detaillee des mineralisations du substrat carbonate revele l existence de concentrations egalement plombo zinciferes mais stratoides et syngenetiques de la dolomie qui les supporte en effet ces mineralisations ont enregistre tous les phenomenes ayant affecte la dolomie encaissante figures de sedimentation et de deformation synmetamorphe orogenie varisque fracturation post schisteuse anterieure a la discordance permo triasique il est propose de considerer que ce type de mineralisation alimenta en metaux de base voire en soufre les pieges karstiques post hercyniens on en trouve notamment remanie a l etat d elements brechiques dans les remplissages paleo karstiques

sub greenschist facies metamorphism from the variscides of sw ireland an early syn extensional peak thermal event illite crystallinity studies on metaclastics from the variscides of sw ireland range in value from 0 18 to 0 25 2 indicating a metamorphic grade in the upper anchizone lower epizone c 275 325 c chlorite geothermometry yields a similar metamorphic temperature range 280 315 c combining these data with known overburden estimates for the area c 5km implies late palaeozoic geothermal gradients in excess of 60 ckm 1 fluid inclusion studies on quartz veins in these metaclastics reveal two vein types characterized by fluid densities of 0 7 0 93 g cm 3 group 1 and 0 9 1 0gcm 3 group 2 respectively this variation in fluid density is thought to be dominantly controlled by temperature with group 1 veins linked to late palaeozoic extension and high geothermal gradients and group 2 veins associated with subsequent variscan deformation and lower temperature conditions

development of interconnected talc networks and weakening of continental low angle normal faults fault zones that slip when oriented at large angles to the maximum compressive stress i e weak faults represent a significant mechanical problem here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite abundant talc along a low angle normal fault within the fault core talc forms an interconnected foliated network that deforms by frictional sliding along 50 200 nm thick talc lamellae the low frictional strength of talc combined with dissolution precipitation creep can explain slip on low angle normal faults in addition the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures the development of phyllosilicates such as talc by fluid assisted processes within fault zones cutting mg rich carbonate sequences may be widespread leading to profound and long term fault weakness

mesoproterozoic ophiolitic melange from the se periphery of the indian plate u pb zircon ages and tectonic implications abstract the kanigiri melange within the proterozoic nellore khammam schist belt in southern peninsular india includes ophiolitic fragments that represent the remnants of an oceanic plate the ophiolitic units were accreted along a ne trending suture that juxtaposes the proterozoic eastern ghats granulite belt eggb against the archean nellore schist belt of the dharwar craton the ophiolite components in the kanigiri melange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature we report laser ablation icp ms age data and ree geochemistry of zircons from the gabbro and granite the zircons from both gabbro and granite show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source zircon ree abundances and normalized patterns show little intersample and intrasample variations u pb dating of the zircons reveals prominent mesoproterozoic ages for the plagiogranite with the ca 1 33 ga age of the kanigiri ophiolitic melange offering important clues for arc continent collision during the final stages of amalgamation of the columbia derived fragments within the neoproterozoic supercontinent assembly

late archaean 2550 2520 ma juvenile magmatism in the eastern dharwar craton southern india constraints from geochronology nd sr isotopes and whole rock geochemistry the results of field geochronologic geochemical and isotopic studies are presented for the granitoids that occur east of the closepet batholith up to the kolar schist belt ksb field data such as common foliation strong shear deformation occasionally leading to mylonitization together with petrographic data including reduction in grain size with corroded borders show characteristics of the syn kinematic emplacement of the granitoids single zircon evaporation ages define a minimum age of 3127 ma for the tonalitic trondhjemitic granodioritic ttg basement and 2552 2534 ma plateau ages for the emplacement of the granitoids which slightly predate 20 30 ma the emplacement of the 2518 ma closepet batholith major and trace element data together with isotopic data suggest at least four magmatic suites from closepet batholith to the east which have independent magmatic evolution histories the observed data are compatible with magma mixing for the closepet batholith melting of ttg and assimilation fractional crystallization processes for bangalore granites either melting of heterogeneous source or different degree of melting of the same source for the granitoids of hoskote kolar and fractional crystallization for the western margin of the ksb isotopic nd sr and geochemical data lree and lil elements suggest highly enriched mantle and ancient ttg crust for the closepet batholith enriched mantle and ttg crust for the bangalore granites c a chondritic mantle source for the granitoids of hoskote kolar and the quartz monzonites of the western margin of the ksb and slightly depleted mantle for granodiorites of the eastern margin of the ksb we interpret all these geochronologic geochemical and isotopic characteristics of granitoids from the closepet batholith to the east up to the ksb in terms of a plume model the centre of the plume would be an enriched hot spot in the mantle that lies below the present exposure level of the closepet batholith melting of such an enriched mantle hot spot produces high temperature magmas closepet that penetrate overlying ancient crust where they strongly interact and induce partial melting of the surrounding crust these magmas cool very slowly as the hot spot maintains high temperatures for a long time thus they appear younger 2518 ma on the contrary to the east the plume induces melting of c a chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older 2552 2534 ma this plume model can also account for late archaean geodynamic evolution including juvenile magmatism heat source for reworking inverse diapirism and granulite metamorphism in the dharwar craton

reaction localization and softening of texturally hardened mylonites in a reactivated fault zone central argentina the tres arboles ductile fault zone in the eastern sierras pampeanas central argentina experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes much of the fault zone is characterized by a d2 ultramylonite composed of fine grained biotite plagioclase that lacks a well defined preferred orientation the d2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep these ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase sillimanite and garnet inherited from the gneissic and migmatitic protolith d1 of the hangingwall the assemblage of garnet sillimanite biotite suggests that d1 related fabrics developed under upper amphibolite facies conditions and the persistence of biotite garnet sillimanite plagioclase suggests that the ultramylonite of d2 developed under middle amphibolite facies conditions greenschist facies mylonitic shear bands d3 locally overprint d2 ultramylonites fine grained folia of muscovite chlorite biotite truncate earlier biotite plagioclase textures and coarser grained muscovite partially replaces relic sillimanite grains anorthite content of shear band d3 plagioclase is c an30 distinct from d1 and d2 plagioclase c an35 the anorthite content of d3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite biotite is partially replaced by muscovite and or chlorite particularly in areas of inferred high strain quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric all d3 reactions require the introduction of h and or h2o indicating an open system and typically result in a volume decrease syntectonic d3 muscovite quartz chlorite preferentially grew in an orientation favourable for strain localization which produced a strong textural softening strain localization occurred only where reactions progressed with the infiltration of aqueous fluids on a scale of hundreds of micrometre local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids however the predominant greenschist facies deformation d3 along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system

the formation and growth of pressure solution seams in clastic rocks a field and analytical study abstract pressure solution seams psss are closing mode structures of localised dissolution resulting in a volume reduction we present new observations constraining the processes by which psss initiate and grow in low porosity clastic rocks from southern ireland psss initiate as inter granular psss at quartz grain to grain contacts as quartz dissolves clay remains as a residue along the contacts as well as filling the adjacent pores to form incipient psss involving multiple grain boundaries further lengthening and thickening occurs by lateral linkage and transverse coalescence of neighbouring segments of incipient psss multiple pss segments are observed to concentrate in thin tabular zones that appear as single macroscopic psss visible in hand samples published numerical modelling results show the ability of both lateral and transverse growth of psss due to the presence of asperities on their flanks which confirm our interpretations based on our laboratory and field observations

ramgundam sandstone a middle proterozoic shoal bar sequence abstract much of the abundant precambrian sedimentary record of india appears to have been deposited in shallow coastal environments one example of this the ramgundam sandstone is a moderately thick 120 m unit composed of lens shaped bodies of arkosic sandstone and interbedded sandstone and shale outcrops are mostly limited to small quarry pits but sufficient exposures are present to show major bedding characteristics principal components are wave built bars and interbar fillings the bars 1 to 5 m thick and tens of meters wide contain laterally accreting sets of trough cross bedded sandstone interbar deposits are composed of horizontal to subhorizontal beds of ripple laminated sandstones and interlaminated shales siltstone and sandstone containing wavy lenticular and flaser laminae shallow water subaerial exposure and probable tidal fluctuations are recorded on the bar surfaces by flat topped ripples and interference ripples water recession marks and desiccation cracks

continental breakup and uhp rock exhumation in action gps results from the woodlark rift papua new guinea we show results from a network of campaign global positioning system gps sites in the woodlark rift southeastern papua new guinea in a transition from seafloor spreading to continental rifting gps velocities indicate anticlockwise rotation at 2 2 7 myr relative to australia of crustal blocks north of the rift producing 10 15 mm yr of extension in the continental rift increasing to 20 40 mm yr of seafloor spreading at the woodlark spreading center extension in the continental rift is distributed among multiple structures these data demonstrate that low angle normal faults in the continents such as the mai iu fault can slip at high rates nearing 10 mm yr extensional deformation observed in the d entrecasteaux islands the site of the world s only actively exhuming ultra high pressure uhp rock terrane supports the idea that extensional processes play a critical role in uhp rock exhumation gps data do not require significant interseismic coupling on faults in the region suggesting that much of the deformation may be aseismic westward transfer of deformation from the woodlark spreading center to the main plate boundary fault in the continental rift the mai iu fault is accommodated by clockwise rotation of a tectonic block beneath goodenough bay and by dextral strike slip on transfer faults within and surrounding normanby island contemporary extension rates in the woodlark spreading center are 30 50 slower than those from seafloor spreading derived magnetic anomalies the 0 5 ma to present seafloor spreading estimates for the woodlark basin may be overestimated and a reevaluation of these data in the context of the gps rates is warranted

the rayner complex of east antarctica complex isotopic systematics within a proterozoic mobile belt new isotopic rb sr u pb zircon and sm nd and petrological data are presented for part of an extensive proterozoic mobile belt locally known as the rayner complex in east antarctica much of the belt is the product of mid proterozoic 1800 2000 ma juvenile crustal formation melting of this crust at about 1500 ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived deformation and transitional granulite amphibolite facies conditions which peaked at 750 50 c and 7 8 kbar 0 7 0 8 gpa produced open to tight folding about e w axes and syn tectonic granitoids about 960 ma ago subsequent felsic magmatism occurred at about 770 ma and not as has been widely advocated at 500 550 ma which appears to have been a time of widespread upper greenschist facies 400 500 c metamorphism localized shearing and faulting sm nd model ages of 1 65 2 18 ga disprove a previously favoured hypothesis that the rayner complex mostly represents reworked archaean rocks from the neighbouring craton napier complex models that involve rehydration of the napier complex are no longer required since the rayner complex was its own source of water two episodes of proterozoic crustal growth are identified the later of which occurred between about 1200 ma and 1000 ma and was relatively minor sedimentation took place only shortly before late proterozoic orogenesis the multiphase history of the rayner complex has resulted in complex isotopic behaviour three temporally discrete episodes of pb loss from zircon have been identified the earliest two of which are responses to the c 960 ma and 540 ma tectonothermal events fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth pb loss during the high grade event was probably governed by the same process or by lattice annealing some zircon suites also document recent pb loss most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete pb loss during two or more secondary events whereas all zircon separates from the orthogneisses produce u pb isotopic alignments zircons from the only analysed paragneiss produce scattered data in part reflecting a range of provenance the 960 ma event was also associated with the growth of a characteristically low u zircon 300 g g in rocks of inferred high zr content there is ubiquitous evidence for the resetting of rb sr total rock isochrons even samples separated by up to 10 km fail to produce igneous crystallization ages minor mineralogical changes produced by the 540 ma upper greenschist facies metamorphism were sufficient to almost completely reset some rb sr isochrons and to produce open system conditions on outcrop scale at least in one location

low angle normal fault earthquakes and triggering geologic evidence for low angle normal faulting is overwhelming and recent seismic geodetic and neotectonic studies have identified probable active examples thus apparent low levels of historical seismicity on such faults is enigmatic however one large mainshock was a low angle normal event and three others may have been one earthquake sequence had significant moment release from a triggered low angle normal fault and three earthquakes may have had normal subevents on low angle planes such sub events are hard to recognize may be underrepresented in catalogs helping to explain the apparent seismic enigma and pose increased seismic hazard due to delayed strong motions triggered sub events may be along strike or down dip from their triggering shock in cases where the triggered slip is downdip of the steep fault and under its hanging wall elevated pore fluid pressure or fault weakness may be required

the structure and rheological evolution of reactivated continental fault zones a review and case study abstract repeated reactivation of structures and reworking of crustal volumes are characteristic though not ubiquitous features of continental deformation reactivated faults and shear zones exposed in the deeply exhumed parts of ancient orogenic belts present opportunities to study processes that influence the mechanical properties of long lived fault zones at different palaeo depths ancient basement fault systems typically comprise heterogeneous superimposed assemblages of fault rocks formed at different times and depths for which down temperature thermal histories are most common several lithological and environmental factors influence the evolution of fault rock fabrics and rheology but most fault shear zone arrays appear to develop as self organized deformation systems once mature the kinematic and mechanical evolution of the system is strongly influenced by the rheological behaviour of the interconnected fault shear zone network a case study from the crustal scale great glen fault zone ggfz scotland reveals a complex evolution of mid to upper crustal deformation textures formed adjacent to the frictional viscous transition fluid influx in the mid crust has led to reaction softening of the rock aggregate as strong pre existing phases such as feldspar are replaced by fine grained strongly aligned aggregates of weak phyllosilicates in addition a grainsize controlled switch to fluid assisted diffusional creep occurs in the highest strain regions of the fault zone it is proposed that this led to a shallowing and narrowing of the frictional viscous transition and to long term overall weakening of the fault zone relative to the surrounding wall rocks cataclasis is particularly important in the deeper part of the frictional regime as it helps to promote retrograde metamorphism and changes in deformation regime by both reducing grainsize and promoting pervasive fluid influx along fault strands due to grain scale dilatancy equivalent processes are likely to occur along many other long lived crustal scale fault zones

experimental investigation into the microstructural and mechanical evolution of phyllosilicate bearing fault rock under conditions favouring pressure solution abstract mature crustal fault zones are known to be zones of persistent weakness this weakness is believed to result from microstructural modifications during deformation such as grain size reduction and foliation development around the brittle ductile transition phyllosilicates are expected to have a significant effect on fault strength in particular under conditions favouring pressure solution to study such effects we performed rotary shear experiments on brine saturated halite kaolinite mixtures aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate the results show significant strain weakening and a transition with progressive strain towards more rate sensitive and less normal stress sensitive behaviour this was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate asymmetric clasts in a fine grained foliated matrix the results demonstrate that strain weakening and the development of a typical mylonitic microstructure can occur as a consequence of grain size reduction by cataclasis and a transition to pressure solution accommodated deformation even in the absence of dislocation creep the data raise questions regarding the reliability of microstructures as rheology indicators as well as on the use of low strain monomineralic flow laws for modelling crustal dynamics

shear folding in low grade metasedimentary rocks reverse shear along cleavage at a high angle to the maximum compressive stress shear folding which is also referred to as slip folding involves shear along planes that are oriented approximately parallel to the axial plane of the fold structure these planes which are typically axial planar cleavage planes facilitate high angle reverse slip leading to fold limb rotation and amplification this study builds on recent advances in our understanding of the role of weak fault zones in facilitating slip on misoriented faults i e faults at a high angle to the maximum principal tectonic stress 1 analysis of folded marine sedimentary rocks from the variscan of southern ireland provides unambiguous microstructural evidence for reverse shear on chemically weakened cleavage domains significant silica loss in these cleavage domains and as a consequence marked mechanical weakening is seen as the primary cause for the reverse slip associated with the shear folding of these sedimentary rocks

distribution of pre folding linear indicators of movement direction around the spring hill synform vermont significance for mechanism of folding in this portion of the appalachians abstract three distinctly oriented sets of pre folding and one set of syn folding axes of curved inclusion trails are preserved in garnet porphyroblasts in 50 samples around the doubly plunging spring hill synform in southeast vermont over one third of the samples contain consistent changes in the trend of these axes from the core to rim since the core grew before the rim this enabled the relative timing of each set of axes to be determined from the oldest to the youngest as ne sw e w nnw sse and nne ssw the youngest trend is parallel to the axial plane of the regional folds only those samples with the latter trend have their inclusion trails connected continuously to the matrix foliation the three pre folding sets of axes have the same orientation on both limbs this consistency in orientation has significant implications for the processes operating during folding and three mechanisms are presented that could potentially explain it these involve the classic card deck model of shear folding de sitter s model of clay bricks shortening as they shear past one another and the progressive bulk inhomogeneous shortening model the relative merits of each of these models are discussed

proterozoic platform sequences of peninsular india implications towards basin evolution and supercontinent assembly abstract a group of less disturbed and unmetamorphosed proterozoic platformal sediment package lying over deformed and metamorphosed archean to paleoproterozoic basement in several parts of peninsular india is informally termed as purana basins in indian stratigraphy the trend shift in the understanding of these basin fills in last two decades often supported by international collaborations has made significant advancements in respect to unconformity and sequence development relationship with accretion and break up of the supercontinents initiation and evolution of the basins in different tectonic set ups nature of sediment provenance paleoclimate and paleo weathering patterns and basin water chemistry geophysical backups encompassing the basins occasionally extending up to the craton and adjacent mobile belts helped in refining the understanding on the basement structure and tectonic forcings the present paper is an attempt towards the collation of the database generated in last two decades in order to present the status of current understanding on different purana basins with regard to interactions of physical chemical and biological parameters of the proterozoic earth

proterozoic events in the eastern ghats granulite belt india evidence from rb sr sm nd systematics and shrimp dating metamorphic and protolith ages of five rock types mafic granulite orthopyroxene granulite leptynite sillimanite granite and metapelite from rayagada in the north central part of the eastern ghats granulite belt eggb india were determined from rb sr and sm nd whole rock and mineral isochrons in combination with shrimp u pb zircon data most of the whole rock isochron ages in both sm nd and rb sr systems point to either 1450 or 1000 ma and the mineral isochron ages are 1000 800 and 550 ma shrimp u pb zircon ages of 940 ma were obtained from metapelite which are in close agreement with the sm nd and rb sr isochron ages from all these data four age clusters 1450 1000 800 and 550 ma have been noted the 1450 ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite the 1000 ma age cluster is regarded as the intrusion ages of sillimanite granite and charnockite and associated granulite facies metamorphism two other age clusters 800 a

scale dependence strain compatibility and heterogeneity of three dimensional deformation during mountain building a discussion plate motion at convergent margins causes crustal shortening and orogenic thickening relative motion that is oblique to the plate margins is an inevitable consequence of plate kinematics on a sphere and results in non coaxial three dimensional deformation that cannot be approximated to simple shear models of mountain building that include a smoothly varying component of pure shear shortening allow strain compatibility to be maintained by deforming the upper free surface of the earth without disrupting the material continuum however such models do not reflect accurately the nature of deformation in many areas of high strain in the upper crust which are characterized by interconnected arrays of kinematically linked faults that can be active on several scales of magnitude simultaneously as brittle deformation increases the coherence of the material continuum is highly reduced in such situations strain compatibility is maintained by partitioning the deformation amongst structures of varying kinematic significance over a wide range of scales and not by smooth variations in strain magnitude acting on a single scale across a material continuum there is a marked tendency for such partitioned domains to be oriented parallel or sub parallel to the orogenic grain alignment of domains in this way represents a strong structural anisotropy which acts as a highly significant boundary condition that controls deformation at subordinate scales finite strain observed within an individual domain at a given scale need not therefore display the same magnitude or orientation as bulk finite strain at the plate scale and consequently data must be collected from as large an area as possible to relate outcrop scale structures to global scale tectonics

the neoproterozoic cratonic successions of peninsular india abstract the peninsular india hosts extensive record of mesoproterozoic and neoproterozoic successions in several mobile belts and cratonic basins the successions provide excellent opportunities for chronostratigraphic classification in tune with the chronometric classification adopted by iugs for inter regional correlation on a global scale major tectono thermal events at 1000 950 ma in the mobile belts correlatable with the grenville orogeny may be considered as the datum for meso neoproterozoic classification in india principles of chronostratigraphic classification however can not be applied yet to the cratonic successions of india because of inadequate radiometric data paucity of biostratigraphic studies and lack of regionally correlatable stratigraphic or palaeoclimatic datum the kimberlite magmatism which affected the peninsular india on a continental scale at about 1100 ma holds the key to the identification of neoproterozoic successions of the cratonic basins thus the stratigraphically confined diamond bearing conglomerates and or the tuffs associated with kimberlites may be considered as the datum to define the base of the neoproterozoic fixed at about 1000 ma accordingly the rewa and bhander groups in the vindhyan basin the kurnool group in the cuddapah basin the jagdalpur formation in the indravati basin and the sullavai group in the pranhita godavari basin are taken to represent the neoproterozoic successions in the peninsular india the chattisgarh group in the central india the lower part of the marwar supergroup in western rajasthan the badami group in the kaladgi basin and the bhima group are the other possible neoproterozoics in the peninsula the closing phase of the mesoproterozoic in all these basins are characterised by stable shelf lithologic associations attesting to high crustal stability the neoproterozoic basins by contrast mark a new phase of rifting and extension and the basin fills exhibit signatures of initial instability which evolved with time into a more stable platformal condition a major episode of sea level rise has been recorded in most of the basins the riftogenic origin and evolution of the basins are comparable with the history of neoproterozoic basins of australia though there is no unequivocal record of glaciation in the indian formations

geochemistry of the m passa pb zn deposit niari syncline people s republic of the congo arguments in favor of a hydrothermal origin the pb zn deposits of the m passa mine are located on faults running in a combian i e n50 e direction the interpretation preferred by most authors over the last decade is that synsedimentary ore concentrations have been redistributed in the vicinity of fault zones this paper presents the results of a detailed study of the behavior of 11 metallic elements v mn fe co cu zn mo ag cd sb pb systematically measured along a well exposed section 94 samples in 28 of which as was also measured principal component analysis of the data shows a regular decrease in the total metal content with increasing distance from the fault zones it also brings to light groupings of metallic elements into a group of mobile elements pb zn cd and a group of less mobile elements ag cu mo fe co the latter are concentrated in and near the fault zone this distribution corroborates a normal thermal zoning of elements issued from the faults rather than a lateral leaching of a preexisting ore concentration down the fault zone a gradient in fe content in sphalerite observed over both ore trends could also indicate regularly decreasing thermal conditions during ore formation in addition exploratory measurements of sulfur isotopes clearly indicate an ore concentration process different from an euxinic process other synsedimentary metal concentrations related to organic matter evolution seem difficult to reconcile with the total residual organic carbon content and characteristic ratios regarding organic carbon do not show at the present time any relation with organic matter evolution rather than assuming that all organic indicators were erased during the geological evolution of the m passa deposit a model is preferred in which local hydrothermal deposition issued from the fault and diffused through specific dolomitic strata paleokarstic features may be regarded as passive ore reservoirs or could be the result of alteration associated with hydrothermal fluids

shear veins observed within anisotropic fabric at high angles to the maximum compressive stress some faults seem to slip at unusually high angles 45 relative to the orientation of the greatest principal compressive stress1 2 3 4 5 this implies that these faults are extremely weak compared with the surrounding rock6 laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre existing frictionally weak surface7 8 9 weakening from chemical reactions10 elevated fluid pressure11 12 13 or dissolution precipitation creep14 15 here we describe shear veins within the chrystalls beach accretionary melange new zealand the melange is a highly sheared assemblage of relatively competent rock within a cleaved anisotropic mudstone matrix the orientation of the shear veins compared with the direction of hydrothermal extension veins that formed contemporaneously indicates that they were active at an angle of 80 5 to the greatest principal compressive stress we show that the shear veins developed incrementally along the cleavage planes of the matrix thus we suggest that episodic slip was facilitated by the anisotropic internal fabric in a fluid overpressured heterogeneous shear zone a similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings we therefore conclude that incremental slip along a pre existing planar fabric coupled to high fluid pressure and dissolution precipitation creep may explain active slip on severely misoriented faults

the malines cambrian carbonate shale hosted pb zn deposit france thermometric and isotopic h o evidence for pulsating hydrothermal mineralization microthermometric stable isotope d h 18o 16o and raman spectroscopic data are given for four different mineralization events located beneath a triassic unconformity these events include the pb zn bearing kii mineralization which fills geodes and fractures in the main karstic mineralization ki and the f mineralization which fills fractures in the cambrian carbonates and shales post ki pre kii and f mineralization fluids were hot 150 c moderately saline 10 equiv wt nacl and precipitated red dolomite subsequently equally hot but more saline 20 equiv wt nacl fluids deposited white dolomite and then sphalerite of the f mineralization they were followed by another generation of dolomite at about 70 c from fluids with about 15 equiv wt nacl and even later barites 165 c 10 equiv wt nacl variation in the homogenization temperature with salinity for the different dolomite generations suggests that the hot saline fluids were repeatedly diluted by a cooler less saline fluid the fluids are interpreted to be of formation water origin having possibly developed in the deep levels of the rhone basins to the southeast which was at least 3 km thick at the time of mineralization post hetangian the inferred metal bearing hot brines were probably episodically expulsed during the dewatering of the basin depositing their mineralization in the more permeable fault and karstic zones associated with the uplift of the st bresson horst cold surface waters probably invaded the mineralized zones between the pulses of hydrothermal solutions this hydrothermal model with the introduction of sulfides can account for the precipitation and dissolution textures associated with the hydrothermal mineralizations

shrimp u pb zircon provenance of the sullavai group of pranhita godavari basin and bairenkonda quartzite of cuddapah basin with implications for the southern indian proterozoic tectonic architecture abstract proterozoic basins in cratonic india referred to as purana basins cover about 20 of the archean basement of the subcontinent although the stratigraphy of most of these basins has been well established it is only recently that radiometric age constraints have been obtained for some of the sedimentary sequences this study provides new data for two of the purana basins in southern india using shrimp u pb analysis of detrital zircons for the pranhita godavari basin an age limit of 709 ma is provided for the age of deposition of the sullavai group this prolongs the duration of proterozoic sedimentation to approximately one billion years ca 1700 709 ma and establishes the venkatpur sandstones of sullavai group as the youngest purana basin succession identified in india so far a major zircon provenance age peak at about 1000 ma and zircon ages between 800 and 750 ma are correlated with major tectonothermal events in the eastern ghats mobile belt egmb and intrusion of granites and associated pegmatites at that time the main provenance area of the venkatpur sandstones is interpreted to be the egmb with subordinate supply of sediments from the eastern dharwar craton it is suggested that portions of the egmb must have been an integral part of peninsular india at the time of deposition of these sediments for the cuddapah basin new u pb detrital zircon age constraints are provided for the nallamalai group sediments bairenkonda quartzites within the nallamalai fold belt the youngest population of which yielded an age of ca 1550 ma this age is similar to the emplacement age of the vinukonda granite 1589 4 4 ma in the nellore schist belt this suggests that the nellore schist belt was a source region for the bairenkonda quartzites and that deposition continued until at least 1550 ma a high precision archean age ca 3366 ma for a detrital zircon grain indicates the presence of paleoarchean components within the sediment source region provided that paleoarchean rocks are as yet unknown from the largely juvenile neoarchean eastern dharwar craton there exists a possibility that the nallamalai group was sourced from the ongole domain of the egmb

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

proterozoic rifting in the pranhita godavari valley implication on india antarctica linkage abstract the pranhita godavari pg valley a major lineament within the south indian cratonic province that preserves sediment dominated deposits spanning from mesoproterozoic to mesozoic appears to be a key element in supercontinent reconstruction the sedimentary basins of the valley include a thick succession of early mesoproterozoic to late neoproterozoic rocks the godavari supergroup which is unconformably overlain by the late palaeozoic mesozoic gondwana sequence the godavari supergroup is internally punctuated by several regional and interregional unconformities into a number of unconformity bound sequences having group level and subgroup level status the lithostratigraphic attributes of the succession indicate multiple events of fault controlled sedimentation marked by transgression and regression as well as uneven rates of uplift and subsidence of the basin floor in an extensional tectonic regime the amplitude of translation of the unconformity surfaces across the base level attests to collective role of tectonic movement and sea level changes in building the stratigraphic framework of the valley the stratigraphic framework and depositional systems such as fan and fan deltas together with local outburst of felsic volcanism further indicate repeated rifting of the craton geochronologic data indicate that the rift basin started to open in early mesoproterozoic concomitantly with the breakup of the mesoproterozoic supercontinent during which the india and east gondwana fragments were separated the spatial variation in the declivity of the unconformity surfaces and the trend of thickness variation of the unconformity bound sequences point that the basin deepened and opened towards southeast to join an ocean that developed between the south indian craton and east antarctica the contractional deformation structures preserved in several lithounits were produced under ne sw directed regional compression during late neoproterozoic basin inversion

the purana basins of southern cratonic province of india a case for mesoproterozoic fossil rifts abstract since its cratonization in the palaeoproterozoic southern peninsular india witnessed the development of a number of large intracratonic sedimentary basins traditionally referred to as purana basins spanning in age from late palaeoproterozoic through neoproterozoic the localization of these intracratonic basins namely pranhita godavari pg chattisgarh and cuddapah basins is apparently controlled by pre existing sutures and or weak zones the pg basin and the cuddapah basin host unconformity bound thick sediment dominated successions attesting to several cycles of fluvial shallow marine to shelf slope basin sedimentation deposition was punctuated by block uplifts resulting in local hiatuses and or volcanic upheavals leading to intercalation of thin but persistent basaltic flows and acid tuffs and ignimbrites basin margin deep faults apparently played a role in the facies distribution in these basins based on these features we propose that these basins initiated as continental rifts which however never opened up into a full fledged ocean basin but links with open seaway are evident from frequent occurrence of deposits representing tidal and storm influence particularly in upper part of the chattisgarh succession spatial distribution of facies and sediment thickness in the cuddapah and pg basins suggest that an open seaway existed to the east of the south indian cratonic province during the mesoproterozoic while similar criteria point to the existence of an open seaway north of the chattisgarh basin development history including nature of inversion suggest that the southern cratonic province of india existed as a single large continental mass since the mesoproterozoic in spite of episodes of supercontinent build up and fragmentation involving india and east gondwana during the proterozoic

proterozoic evolution of eastern dharwar and bastar cratons india an overview of the intracratonic basins craton margins and mobile belts abstract based on critical analysis of stratigraphic succesions in the proterozoic intracratonic basins geochronological data and possible connections or disconnections with adjacent fold belts and craton margin mobile belts an overview of the proterozoic evolution of the eastern dharwar and bastar cratons in india is presented the chattisgarh and cuddapah basins in these cratons with archean nuclei host sedimentary successions ranging in age from the paleoproterozoic to the ediacaran period and representing several silicilastic and carbonate cycles the proterozoic successions of the pranhita godavari valley pgv basin along the dharwar bastar join record multiple rifting since late neoarchean suturing of the two cratons we postulate a first order stratigraphic correlation across the cratons and their join and discuss how major sea level fluctuations coupled with regional tectonic events shaped the development of unconformity bound successions adjoining fold belts and the mobile belts with supposed global tectonic connections the oldest paleoproterozoic intracratonic sedimentation is preserved in the cuddapah and pgv basins while sedimentation in the bastar craton is largely mesoproterozoic the fluviodeltaic to shallow marine neoproterozoic sedimentation followed emplacement of end mesoproterozoic kimberlites and lamproites indicating thermal rejuvenation of the indian continental crust the allochthonus nallamalai fold belt in eastern cuddapah basin and the early mesoproterozoic fold belt in pgv basin point to craton margin crustal convergences which punctuated the paleoproterozoic sedimentation widespread development of carbonate bearing sequences in the mesoproterozoic to neoproterozoic successions point to peneplanation of the cratons and stable platformal sedimentation the neoproterozoic sedimentation was terminated following the docking of the eastern ghats belt to the eastern margin of the dharwar bastar ensemble and possible final tectonic hiccups in the central indian tectonic zone bordering the bastar craton the available data and their analysis rule out any foreland basin type connection between the eastern ghats belt and cratonic sedimentation in bastar and eastern dharwar

active aseismic creep on the alto tiberina low angle normal fault italy the existence of active low angle normal faults has been questioned because the standard theory of fault mechanics precludes normal faults from slipping at low

alkaline magmatism along the southeastern margin of the indian shield implications for regional geodynamics and constraints on craton eastern ghats belt suturing abstract deformed alkaline complexes at the contact between the eastern ghats belt egb and the archaean cratons in southeastern india mark the location of a collisional suture superimposing an earlier mesoproterozoic 1 5 1 3 ga rift this article offers a plausible geodynamic model explaining the alkaline magmatism and its relationship to regional crustal evolution the mesoproterozoic rift can be correlated to the breakup of the supercontinent columbia and may have opened an ocean between eastern india and east antarctica where the sedimentary sequences of the eastern ghats province egp were deposited between 1 4 and 1 2 ga inversion of the rift basin during the late mesoproterozoic 1 0 ga led to the collision of eastern india with east antarctica during rodinia assembly the collision formed the grenvillian egp rayner complex orogen where the egp sediments and the rocks of the rayner complex were deformed metamorphosed and migmatized at granulite facies condition the mesoproterozoic rifting and grenvillian basin closure may thus represent a wilson cycle related to the breakup of columbia and the assembly of rodinia the craton egb suture was subsequently modified significantly during pan african tectonism 0 5 0 6 ga when the egb granulites were thrust westward over the cratonic foreland along a number of amphibolite facies shear zones at the contact the shearing deformed and metamorphosed the cratonic margin and the alkaline complexes it hosts the thrust stacking in many places has the craton boundary and the alkaline rocks in the footwall and the egb granulites in the hanging wall the continuity of the pan african shear fabric across the craton granulite contact and the lack of grenvillian ages from the alkaline and other craton margin rocks in such regions give the apparent impression that the two crustal segments amalgamated during pan african tectonism and not during the earlier grenvillian collision geodynamic constraints however indicate that this was unlikely and the inconsistency arises because the present crustal geometry exposes only the eroded pan african thrust contact between the craton and the egb and not the original grenvillian suture which may be lying unexposed under the granulite thrust sheets

fabric induced weakness of tectonic faults 1 mature fault zones appear to be weaker than predicted by both theory and experiment one explanation involves the presence of weak minerals such as talc however talc is only a minor constituent of most fault zones and thus the question arises what proportion of a weak mineral is needed to satisfy weak fault models existing studies of fault gouges indicate that 30 of the weak phase is necessary to weaken faults a proportion not supported by observations here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt talc provided the talc forms a critically aligned through going layer observations of foliated fault rocks in mature large offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults

on palaeozoic mesozoic brittle normal faults along the sw barents sea margin fault processes and implications for basement permeability and margin evolution palaeozoic mesozoic brittle normal faults onshore along the sw barents sea passive margin off northern norway give valuable insight into fault and fluid flow processes from the lower brittle crust microstructural evidence suggests that late permian early triassic faulting took place during multiple phases with initial fault movement at minimum p t conditions of c 300 c and c 240 mpa c 10 km depth followed by later fault movement at minimum p t conditions of c 275 c and c 220 mpa c 8 5 km depth the study shows that pore pressures locally reached lithostatic levels 240 mpa during faulting and that faulting came to a halt during early deep stages of rifting along the margin fault permeability has been controlled by healing and precipitation processes through time which have sealed off the core zone and eventually the damage zones after faulting a minimum average exhumation rate of c 40 m ma 1 since the late permian is estimated it implies that the debated late cenozoic uplift of the margin may be explained by increased erosion rates in the coastal regions owing to climate detoriation which caused subsequent isostatic recalibration and uplift of the marginal crust the studied faults may be used as analogues of basement involved fault complexes offshore revealing details about the offshore nature of faulting including past and present basement and fault zone permeability

a new correlation of sapphirine granulites in the indo antarctic metamorphic terrain late proterozoic dates from the eastern ghats province of india abstract uranium lead and thorium lead ages were obtained from perrierite and zircons in two samples from the eastern ghats province of south india a sapphirine bearing granulite from anakapalle and a charnockite from visakhapatnam the isotopic data are concordant or close to concordant at 1 ga which is interpreted to be the age of granulite facies metamorphism and charnockite plutonism in the vicinity of anakapalle and visakhapatnam and provides the first report of a late proterozoic granulite facies event in the eastern ghats province in reassemblies of gondwanaland based on dutoit s original proposal the eastern ghats province is juxtaposed against a high grade terrain in east antarctica affected by a granulite facies event and charnockitic plutonic activity 800 1100 ma ago the ages reported here suggest that a portion of the eastern ghats province represents an extension of the late proterozoic terrain from antarctica into india the radiometric results on the anakapalle rocks are the first evidence in south india for sapphirine formation during the late proterozoic other south indian sapphirine localities appear to be archaean in age a review of sapphirine localities in the indo antarctic metamorphic terrain indicates that sapphirine developed under a range of pressure temperature conditions during the archaean and proterozoic evidence for a systematic change in peak metamorphic conditions with time is obscured by these regional variations characterization of the entire pressure temperature path in addition to estimates for the peak conditions is needed to distinguish the archaean and proterozoic metamorphic regimes

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault