pattern of opening rates along the axis of the mid atlantic ridge vine and wilson1 using the vine and matthews2 hypothesis were the first to try to relate the magnetic pattern over the crests of the mid ocean ridges to the known geomagnetic time scale to determine the spreading rate over the past few million years many determinations of spreading rate at the axis of the ridge during plio pleistocene times have been published le pichon3 and morgan4 for example used spreading rates computed for the axial magnetic pattern 10 myr anomaly 5 of heirtzler et al 5 or for the past 5 myr respectively even when trying to assess the properties of instantaneous plate kinematics likewise in the systematic inversion of minster et al 6 anomalies 3 and 5 were generally used thus averaging plate speeds over the past 5 10 myr minster and jordan7 sought to improve this and redetermined the spreading rates using magnetic anomalies 2 and 2 yielding a mean averaging interval of 3 myr here we examine plate behaviour over a shorter time scale in the reasonably well surveyed axial region of the mid atlantic ridge south of the azores the average opening rates we obtained for this region for magnetic anomalies 1 brunhes 2 and 2 using two different geomagnetic time scales indicate that 1 there was a relative slowing down of the opening between anomalies 1 and 2 which may be of global significance 2 the opening is compatible with a rigid plate model when it is averaged over 2 5 myr although for shorter averaging times 1 8 and 0 7 myr there is some evidence of non rigidity of the plates thus different segments of the mid atlantic ridge may display an independent behaviour and with respect to relatively short time intervals may show departures from characteristics typically associated with slow accretion

formation and maintenance of shear zones the processes that initiate and maintain prograde ductile shear zones are not well understood we have performed shear experiments 1 to 3 on a fine grained 100 150 m gneiss 59 quartz 28 plagioclase 13 aligned but not interconnected biotite to determine the evolution of deformation mechanisms that produce dramatic strain weakening and localization at conditions 1 5 gpa 800 c dot gamma 2 10 5 s where pure quartz aggregates deform homogeneously by dislocation creep initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase allowing local biotite interconnection by slip on 001 and initiating strain weakening after yield the interconnected biotites kink and thus strengthen but the highly strained parts of grains react to a fine grained mixed phase assemblage which deforms by grain size sensitive creep allowing further strain weakening and localization at 3 3 the strain and strain rate in the 100 m thick shear zone are 100 times that of the enclosing host rock similar to the localization observed in natural shear zones thus the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone and once formed a shear zone may be permanently weakened

crustal structure of the sheared southwestern barents sea continental margin abstract eighteen expanding spread profiles from the southwestern barents sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data the velocity depth profiles from the slope intercept herglotz wiechert and sum methods were compared and contrasted to the velocity depth curve calculated from the iterative forward modelling of the amplitudes the velocity depth profiles derived from the sum and forward modelling of the amplitudes were used to produce synthetics in both the x t and p domains to choose the technique that most accurately represented the data the results showed the oceanic crust at the senja fracture zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5 7 6 4 km s 1 and slightly smaller crystalline crustal thickness the continent ocean transition at crustal depths was shown to be narrow in the range of 10 20 km our results were consistent with two different crustal provinces east of the senja fracture zone the tertiary marginal basin and the cretaceous basin region the velocity depth curves enabled us in a gross sense to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles depths down to 20 km were inferred for the base of the sedimentary section in the southwestern barents sea the crust thinned gently towards the continent ocean boundary at the senja fracture zone whereas the crustal thickness changed abruptly below the hornsund fault zone at the margin farther north we associated the change in crustal structure with cenozoic in the thick crust beneath the svalbard platform in the north the depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area

matar a forgotten but promising pleistocene locality in east java abstract matar is a locality near ngandong located on the east bank of the solo river archaeological fieldwork in this locality revealed the existence of several ancient alluvial terraces fossils and lithic artifacts found embedded on these terraces were in poorly consolidated conglomerate comprising sand and gravels faunal remains from matar consisted of several extinct taxa such as stegodon trigonocephalus bubalus paleokarabau bibos paleosondaicus and hexaprotodon sivalensis these fauna represented typical dry and open woodland environment in the middle and upper pleistocene in java in addition lithic artifacts were found in the same layer and consisted of flake tools and a small number of crude core tools flake tools from matar were quite similar to the description of flake tools found in ngandong see movius jr 1948 van heekeren 1972 however the raw material used was not limited to chalcedony but included chert and jasper our study has revealed that matar and its surrounding environment is a promising locality although it has been forgotten for more than two decades since g j bartstra visited this locality for the first time in 1986

petrogenetic significance of orthopyroxene free garnet clinopyroxene plagioclase quartz bearing metabasites with respect to the amphibolite and granulite facies orthopyroxene free garnet clinopyroxene plagioclase quartz bearing mineral assemblages represent the paragenetic link between plagioclase free eclogite facies metabasites and orthopyroxene bearing granulite facies metabasites although these assemblages are most commonly developed under p t conditions consistent with high pressure granulite facies they sometimes occur at lower grade in the amphibolite facies thus these assemblages are characteristic but not definitive of high pressure granulite facies compositional factors favouring their development at amphibolite grade include fe rich mineral compositions ca rich garnet and plagioclase and ti poor hornblende the generalized reaction that accounts for the prograde development of garnet clinopyroxene plagio clase quartz from a hornblende plagioclase quartz bearing amphibolite precursor is hbl pl qtz grt cpx liquid or vapour depending on whether the reaction occurs above or below the solidus there are significant discrepancies between experimental and natural constraints on the p t conditions of orthopyroxene free garnet clinopyroxene plagioclase quartz bearing mineral assemblages and therefore on the p t position of this reaction semi quantitative thermo dynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data

control of the geometry of precursor brittle structures on the type of ductile shear zone in the adamello tonalites southern alps italy abstract amphibolite facies ductile shear zones developed during the immediate post intrusive cooling history of the adamello tonalites southern alps italy shear zones include i thin a few mm s thick fault like shear zones that accommodate shear strain values up to several 100 s the dominant type ii mylonitic horizons dm s thick in sharp contact with the undeformed wall rock iii continuous shear zones with sigmoidal shaped s or composite s c foliations a transition between the different types occurs along strike over short distances discontinuous shear zones are also localized on pegmatite dykes and quartz veins and both i and ii are discontinuously outlined by quartz layers detailed surface mapping reveals that most fault like shear zones are arranged en echelon mainly forming contractional steps markers crosscut by shear zones allow the displacement to be measured at several positions along shear zones and this reveals very steep displacement gradients close to the shear zone tips differential displacement is mainly accommodated at contractional steps by the development of foliated domains geochemical analyses of major and trace elements show that there is no compositional change along strain gradients the overall features are consistent with nucleation of shear zones on former sets of en echelon joints in many cases intruded by pegmatite dykes or filled with quartz reactivation of joints produced strongly localized shear zones whereas broader foliated zones evolved from the contractional jogs between adjacent stepped joint terminations during progressive shearing these jogs were progressively involved in the accommodation of shear displacement and overall shear zone development

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

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

evidence for proterozoic mylonitization in the northwestern wyoming province the 3 km thick madison mylonite zone encompasses the contact between the cherry creek metamorphic suite and the pre cherry creek metamorphic complex in the southern madison range of southwest montana in the shear zone greenschist and epidote amphibolite facies assemblages overprint earlier amphibolite and granulite facies assemblages large scale metasomatic conversion of granitic gneiss to mylonite gneiss increased siderophile fe mn mg p and ca elements and decreased k al and si in the mylonite gneiss suggesting extensive fluid flux in the madison mylonite zone southeast dipping foliations adjacent to the madison mylonite zone are rotated about their northeastern strike through the vertical to northwest dipping orientations indicating southeast directed thrusting of the pre cherry creek metamorphic complex over the cherry creek metamorphic suite complete stratigraphic continuity of cherry creek units within the mylonite zone shows the predominance of ductile shearing ductile thrusting is also indicated by asymmetric fabrics stretching lineations finite strain and metamorphic gradients within the shear zone if the foliations were passively rotated during simple shear a minimum of 10 km of distributed ductile slip occurred in the madison mylonite zone 40ar 39ar thermochronology of hornblende and muscovite adjacent to the madison mylonite zone records a late archean 2 5 ga cooling event within the shear zone a discordant 40ar 39ar muscovite spectrum is consistent with initial cooling to argon closure at about 1 9 ga with subsequent disturbance at about 1 6 ga hornblendes from the madison mylonite zone show highly discordant argon loss spectra with temperature steps starting at 1 8 ga and ending at 2 5 ga these argon age spectra indicate early proterozoic heating and argon loss in the madison mylonite zone resetting and subsequent retrogressing of muscovite but only partial resetting of hornblende due to its higher argon closure temperature this study strongly suggests early proterozoic 1 80 1 90 ga and possibly later thrusting in the madison mylonite zone which may be correlative with retrograde metamorphism and early proterozoic isotopic resetting northwest of the madison range the madison mylonite zone is probably a foreland thrust zone on the margin of a major compressional orogen of early proterozoic age that reworked the archean basement of the northwestern wyoming province

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

geochronology and geochemistry of precambrian gneisses metabasites and pegmatite from the tobacco root mountains northwestern wyoming craton montanathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh t e krogh deceased april 2008 the middle mountain metamorphic domain of the montana metasedimentary terrane northwestern wyoming craton within the northwestern tobacco root mountains mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic hornblende gneiss both of which are cut by metamorphosed mafic rocks together these gneisses are defined as middle mountain gneiss archean tonalitic gneiss from west of and amphibolitic gneiss from east of the bismark fault give from chemically and air abraded zircon grains u pb id tims ages of 3325 5 1 7 and 3340 ma respectively these results reflect primary magmatic ages and show that the middle mountain gneiss extends into the northern area of the central fault block between the bismark and mammoth faults older crustal processes in the tonalitic gneiss are evidenced by inherited grains the oldest of which is 3460 ma a metabasite hosted in tonalitic gneiss in the bismark fault selvage zone yields a zircon age of 2468 ma which is interpreted as the time of

development of lattice preferred orientation in clinoamphiboles deformed under low pressure metamorphic conditions a sem ebsd study of metabasites from the aracena metamorphic belt sw spain the mechanical properties of the lower continental crust are intimately related to the rheology of calcic clinoamphibole naturally deformed clinoamphibole typically shows brittle behaviour under a wide range of deformation conditions whereas crystal plasticity is rarely observed in this study the microfabric of clinoamphiboles from metabasites deformed under low pressure medium to high temperature metamorphic conditions is presented amphiboles from all samples have developed lpo that can be attributed to different deformation mechanisms depending on deformation temperature fluid content structure and phase strength contrasts rigid body rotation of amphibole prisms within a weaker plagioclase matrix corresponds to metabasites deformed under medium deformation temperatures that show a weak layer structure and a high phase strength contrast between plagioclase and clinoamphibole lower temperatures and the presence of fluids mean that one of the studied rocks was affected also by dissolution precipitation creep and cataclastic flow in the final sample dislocation creep was accommodated by recovery and subgrain rotation dynamic recrystallization in monominerallic hornblende layers at higher temperatures which could be representative of the rheology of amphiboles in a lower continental crust at high temperature conditions

lateral and vertical variability in crustal velocity structure in a small area of the north atlantic abstract the detailed velocity structure of a 70 by 35 km area of 6 10 ma old crust on the flank of the mid atlantic ridge at 24 n was studied using 358 explosive charges and several hundred 16 4 1 airgun shots fired into an array of eight ocean bottom hydrophones inversion of the first arrival refracted travel times shows that the crust comprises a normal oceanic section about 5 km thick with a steep velocity gradient in the upper crust increasing from about 3 5 km sec at the seafloor overlying a typical oceanic layer 3 and a probably anisotropic mantle delay time function mapping using two datasets containing arrivals from layer 3 and from the mantle show that lateral variability is generally low over most of the survey area with a small region of high delay times in the northwest corner caused by the presence of abnormal crust probably associated with a minor fracture zone we find that the topography of the base of layer 2 is similar to that of the top indicating that the normal faulting which occurs along the margins of the median valley extends down at least into layer 3 our observations from mantle arrivals are consistent with a much flatter moho which constrains possible models of crustal formation at the spreading centre

seismic anisotropy observed in upper oceanic crust seismic anisotropy in the upper 150om of oceanic basement has been observed by polarization analysis of three component bore hole seismometer records the most convincing evidence for the anisotropy is shear wave splitting for explosive sources at four azimuths compressional wave particle motion deviations suggestive of anisotropy are also observed but they may be caused by lateral inhomogeneities the anisotropy was not resolved by travel time analysis the observed velocities and particle motions in the horizontal plane can be modelled to within a standard deviation by assuming a perfectly elastic homogeneous anisotropic layer 2 with hexagonal symmetry and a horizontal symmetry axis the most probable cause of the anisotropy is preferred crack orientation

the structure and origin of the azores biscay rise north east atlantic ocean summary the azores biscay rise is a roughly linear north east south west trending feature rising 1500 3000 m above its surroundings which extends from about 45 n 15 30 w towards the azores its south western termination is near 40 30 n 21 30 w about halfway along its length the rise intersects the wnw trending king s trough in 1978 a set of bathymetric magnetic gravity gloria and seismic reflection and refraction data were obtained in the vicinity of the rise together with earlier data these observations suggest that 1 there has been no substantial post emplacement tectonic activity with the possible exception of the construction of some volcanic seamounts at the south western end of the rise and 2 the rise is underlain by a low velocity low density lower crust and is in isostatic equilibrium the rise can be convincingly shown to be the eastern half of a pair of ridges formed by abnormal crustal generation at the mid atlantic ridge crest between the times of anomalies 33 and 24 76 56ma ago the western counterpart of the rise includes gauss and milne seamounts in the newfoundland basin magnetic anomaly 3 1 passes uninterruptedly across the rise and therefore hypotheses that the northern part of the rise was the site of a cenozoic transform fault or subduction zone are not supported by our data it is speculated that king s trough was linked to the north spanish trough by an early cenozoic east west transform fault across the northern iberia abyssal plain this plate boundary became inactive about the middle of the oligocene epoch

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

coupled trace element mobilisation and strain softening in quartz during retrograde fluid infiltration in dry granulite protoliths this study focuses on the retrograde rheological and chemical evolution of quartz and the behaviour of quartzites during retrograde metamorphism following dry high grade metamorphism at 750 c 7 kbar sem cl and la hr icp ms are applied to document quartz texture and chemistry respectively four generations of quartz were distinguished by sem cl qz1 qz2 qz3 and qz4 qz1 brecciated and partly dissolved old grains is enriched in b al and ti when compared with the other types qz2 formed during brecciation and partial dissolution of qz1 has low al contents 50 ppm but due to rutile inclusions variable ti contents when occurring in amphibolite 210 10 ppm but more consistent values when occurring in quartzites peak value 32 ppm qz3 dark grey luminescent quartz forming fluid migration channels fluid pathways has ti 5 ppm and al contents below 10 ppm and b 1 ppm qz4 comprises are group of quartz later than qz3 filling micron thick cracks and pods with very low luminescent quartz i e darker than qz3 the textural and chemical evolution of quartz in our study is explained by two major influxes of aqueous fluids during regional uplift and retrogression they facilitated rehydration and recrystallisation in the otherwise dry high grade quartzites the first introduction of aqueous fluids was associated with brecciation of the high grade quartz qz1 and dissolution precipitation of quartz qz2 ti in quartz geothermometry wark and watson contrib mineral petrol 152 6 637 652 gives 626 c in agreement with the retrograde pt path deduced from phase diagrams later fluid influx associated with scapolitisation of amphibolite caused localised recrystallisation qz3 and alteration of biotite to muscovite along mm wide fluid migration channels during subsequent deformation qz3 deformed plastically and recovered by subgrain rotation recrystallisation sgr resulting in a reduction of grain size whereas qz1 quartz formed micro faults qz2 was plastic but did not experience sgr to the same degree as qz3 quartz increased plasticity and recovery rates most likely relate to an increased h2o fugacity and the depletion in trace elements of the quartz lattice by promoting strain softening processes dislocation climb and recovery

quartz shielding of sub 10 μm zircons from radiation damage enhanced pb loss an example from a metamorphosed mafic dike northwestern wyoming craton abstract the coupling of two new approaches enabled acquisition of in situ u pb data from zircons as small as 5 m in a metamorphosed mafic dike from the northern madison range montana despite negligible zircon yield by mineral separation automated mineralogy rapidly identified 375 sub 20 m zircons in a single thin section subsequently zircon crystals were dated using modifications to the conventional sims technique to preferentially collect secondary ions emitted from a domain a few microns in size within the 20 m diameter analysis pit this approach allowed analysis of zircons too small to be dated by standard sims and tims methods u pb data define a discordia array with upper and lower intercepts of 1753 9 ma and 63 8 ma respectively 1 error mswd 1 5 we interpret the upper intercept to reflect zircon growth during high temperature and high pressure metamorphism 800 c 1 2 gpa based on textural relationships between the dated zircons and the peak metamorphic assemblage the lower intercept is attributable to the thermal pulse associated with emplacement of the nearby ca 75 ma tobacco root batholith percent discordance is linked with both textural setting and u concentration zircons located along grain boundaries or within fractured host grains display a positive correlation between u 1049 2817 ppm and percent discordance 12 82 that is consistent with radiation damage enhanced pb loss in contrast three zircons housed completely within unfractured quartz yield the most concordant analyses of the dataset 7 discordant despite u concentrations comparable to highly discordant matrix grains this relationship suggests that the included zircons were shielded from pb loss by the encapsulating quartz crystals the results imply that targeting zircons located completely within unfractured host phases may aid in isolating earlier portions of geologic histories the primary geological implication of this dataset is to increase the documented extent of paleoproterozoic high grade metamorphism in the northwestern wyoming craton

the effects of porphyroblast growth on the effective viscosity of metapelitic rocks implications for the strength of the middle crust numerical models are used to examine the effects of porphyroblast growth on the rheology of compositionally layered rocks metapelites and metapsammites and by extension the middle crust during prograde metamorphism as porphyroblast abundance increases during prograde metamorphism metapelitic layers will strengthen relative to porphyroblast free metapelitic units and potentially relative to quartzofeldspathic metapsammitic units as metapelitic layers become stronger the integrated strength of compositionally layered successions increases potentially causing large volumes of mid crustal rock to strengthen altering the strain rate distribution in the middle crust and affecting the geodynamic evolution of an orogenic belt the growth of effectively rigid porphyroblasts creates strength heterogeneities in the layer undergoing porphyroblast growth which leads to complex strain rate distributions within the layer at the orogen scale the strengthening of large crustal volumes on the order of thousands of cubic kilometres changes the strain rate distribution which may change exhumation rates of high grade metamorphic rocks the geothermal structure and the topography of the orogen the presence of a strong zone in the middle crust causes strain rate partitioning around the zone suppressed uplift rates within and above the zone and leads to the development of a basin on the surface

retrogression of eclogite facies shear zones by short lived fluid infiltration during the caledonian orogeny lofoten islands norway norwegian eclogite facies shear zones in the lofoten islands served as major pathways for short lived pulses of both 40 ar rich and 40 ar poor hot fluids during eclogitization and retrogression events related to the caledonian orogeny the open system for ar affected most of the minerals leading to old 40 ar 39 ar ages particularly in trioctahedral micas 40 ar concentration in the fluids appears to decrease during the amphibolite facies retrogression and ca rich amphiboles yield 40 ar 39 ar ages of c 415 ma scandian event late and post caledonian 40 ar 39 ar muscovite and k feldspar ages and rb sr biotite ages coincide with multiple extensional events fluid infiltration and thermal activity during the final exhumation of the crustal rocks potentially reflecting major tectonic episodes such as rifting of pangaea and seafloor spreading between europe and north america

water loss and the origin of thick ultramylonites hydrolytic weakening has been suggested as a major process facilitating strain localization in line with many studies that found a positive correlation between water content and intensity of deformation we examine the role of water in the unusually thick ultramylonite of the el pichao shear zone northwestern argentina we used fourier transform infrared spectroscopy to measure water content in quartz and feldspar comparing ultramylonitic rocks to mylonites and weakly deformed rocks quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks contrary to findings in previous studies we propose that the kilometer thick ultramylonite formed in three stages 1 localized deformation and recrystallization caused release of intracrystalline water to grain boundaries which promoted grain boundary sliding forming the ultramylonite 2 high pressure in the shear zone continuously expelled intercrystalline water to the surroundings drying the boundaries and leading to strain hardening and 3 water migrated to less deformed rocks causing hydrolytic weakening repeating the cycle and widening the ultramylonite

paleoproterozoic metamorphism in the northern wyoming province implications for the assembly of laurentia abstract u pb ages measured on zircons from the tobacco root mountains and monazite from the highland mountains indicate that the northwestern wyoming province experienced an episode of high grade metamorphism at 1 77 ga leucosome emplaced in archean gneisses from the tobacco root mountains contains a distinctive population of zircons with an age of 1 77 ga but also contains zircons to 3 5 ga it is interpreted to have been derived primarily by anatexis of nearby archean schist a granulite facies mafic dike that cuts across archean gneissic banding in the tobacco root mountains contains two distinct populations of zircons a group of small 50 m nonprismatic grains is interpreted to be metamorphic and yields an age of 1 76 ga a group of slightly larger prismatic grains yields an age of 2 06 ga which is interpreted to be the time of crystallization of the dike monazite from a leucogranite from the highland mountains yields a well defined age of 1 77 ga which is interpreted as the time of partial

deformation enhanced metamorphic reactions and the rheology of high pressure shear zones western gneiss region norway microstructural and petrological analysis of samples with increasing strain in high pressure hp shear zones from the haram garnet corona gabbro give insights into the deformation mechanisms of minerals rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions scanning electron microscopy with electron backscattering diffraction sem ebsd compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization plagioclase with a shape preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the 0 0 1 1 0 0 slip system in omphacite crystallographic preferred orientations indicate slip on 1 0 0 0 0 1 and 1 1 0 0 0 1 during deformation fine grained garnet deformed by diffusion creep and grain boundary sliding ilmenite deformed by dislocation glide on the basal and at higher strains prism planes in the a direction relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500 650 c and 0 5 1 4 gpa and continued during prograde metamorphism to ultra high pressure uhp conditions both products and reactants show evidence of syn and post kinematic growth indicating that prograde reactions continued after strain was partitioned away the restriction of post kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away demonstrating that deformation played an important role in enhancing metamorphic reactions two important consequences of deformation observed in these shear zones are i the homogenization of chemical composition gradients occurred by mixing and grain boundary migration and ii composition changes in zoned metamorphic garnet by lengthening diffusion distances the application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100 150 mpa and lower limits for strain rates of 10 12 to 10 13 s 1 as deformation conditions for the shear zones in the haram gabbro that were produced during subduction of the baltica craton and resulted in the production of hp and uhp metamorphic rocks

the oceanographer fracture zone i crustal structure from seismic refraction studies summary this paper presents the results of four lookm long seismic refraction lines carried out in and around the active transform of the oceanographer fracture zone which offsets the mid atlantic ridge rightlaterally by 130 km near 35 n the seismic data are interpreted using standard travel time inversions the time term method and two dimensional ray tracing techniques the results show that beneath the oceanographer fracture zone the crust is typically only 4 5 km thick it consists of a thin upper crustal layer in which the p wave velocity increases rapidly with depth and a main crustal layer with velocity increasing from 6 4 km s l at the top to 6 8 km s at the base of the crust some degree of crustal thinning is found to extend for distances of 30 40 km on either side of the fracture zone axis in addition to this a small area 10 15 km long of crust less than 2 5 km thick is found beneath the centre of the transform valley crust less than 4 km thick is also observed on a smaller offset fracture zone 85 km to the south of the oceanographer but no evidence is found for significantly thin crust beneath another small offset fracture zone 65 km to the north of it we conclude that the moderate thnning of the crust at the fracture zone is due to a progressive reduction in the crustal production rates of the oceanic spreading centres as they approach the transform fault however we attribute the occurrence locally of extremely thin crust to a different process in which a thin lid of mafic and ultramafic rocks overlying unaltered upper mantle material is accreted at the junction of a spreading centre and a transform fault this may happen if the accretion rate at the end of the spreading centre becomes so slow that the usual processes which give rise to normal oceanic crust can no longer operate it therefore seems that there are two distinct types of crust either of which may be found at different points along a fracture zone this might account for the extreme variability in crustal thickness found in different fracture zones by previous studies

review of oceanic transform activity and development oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on euler poles of relative plate motions it is proposed that the special properties of transforms arise because they are inherently leaky i e new plate area forms along them as transforms have finite widths the adjacent ridge tips are at different distances from the euler poles and the plate boundaries along transforms are slightly oblique to the direction of plate motions such that new plate area forms along them however along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process transforms are underlain by crust consisting of variable proportions of a material emplaced in the transform domains and b stretched ridge generated crust the transform structure is further modified by shearing deformation related to irregularities of lateral faults and because of propagation of adjacent ridges transform leakiness is not considered to result merely from changes of plate motions though such changes will influence transform structure but it reflects the segmented structure of mid oceanic ridges the persistence of transforms and their migration in the absolute reference frame with a component perpendicular to their strikes shows that the ridge structure is decoupled from flow in deeper levels thermal anomalies along transforms especially enhanced heat losses stabilize the segmented ridge structure

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

interactions between serpentinite devolatilization metasomatism and strike slip strain localization during deep crustal shearing in the eastern alps the greiner shear zone in the tauern window eastern alps changes from a zone of distributed dominantly sinistral shear in supracrustal rocks to a series of narrow gully forming dextral splays where it enters basement gneisses within these splays granodiorite is transformed into quartz poor biotite and or chlorite schists reflecting hydration removal of si ca and na and concentration of fe mg and al stable isotope analyses show a prominent increase in d and a decrease in 18o from granodiorite into the shear zones these changes indicate significant channelized flow of an externally derived low 18o high d fluid through the shear zones the shear zone schists are chemically similar to blackwall zones developed around serpentinite bodies elsewhere in the greiner zone and the stable isotope data support alteration via serpentinite derived fluid monazite in schist from one shear zone yields spot dates of 29 20 ma indicating that the fluid influx and switch from sinistral to dextral shear occurred at or shortly after the thermal peak of the alpine orogeny c 30 ma we suggest that alpine metamorphism of serpentinites released large amounts of high d low 18o si undersaturated fe mg saturated fluids that became channelized along prior zones of weakness in the granodiorite infiltration of this fluid facilitated growth of chlorite and biotite which in turn localized later dextral strain in the narrow splays via cleavage parallel slip this dextral strain event can be linked to other structures that accommodated tectonic escape of major crustal blocks during dextral transpression in the eastern alps this study shows that serpentinite devolatilization can play an important role in modifying both the chemistry and rheology of surrounding rocks during orogenesis

permeability underpressures and convection in the oceanic crust near the costa rica rift eastern equatorial pacific in situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at dsdp site 504b these measurements have relevance for the transition from convective to conductive heat flow on the south flank of the costa rica rift conventional slug and constant rate injection tests were made below a hydraulic packer set at various depths in the hole the packer was first set in a massive flow unit 37 m below the sediment basement interface the bulk permeability of the 172 5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys 4 10 10 cm2 measurements over 3 and 15 m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2 4 millidarcys these values are thought to be accurate to 30 formation pore pressures were found to be approximately 8 bars 2 below hydrostatic interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment furthermore the agreement between observed and modeled underpressures implies that hole 504b penetrated an active ocean crustal convection system the thick sedimentary layer layers of basal chert and massive flow basalts above the layer 2a pillow flows apparently form an impermeable lid effectively isolating the convection system from the ocean

microstructural tectonometamorphic processes and the development of gneissic layering a mechanism for metamorphic segregation the mary granite in the east athabasca mylonite triangle northern saskatchewan provides an example and a model for the development of non migmatitic gneissic texture gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase orthopyroxene and matrix in the igneous rocks plagioclase phenocrysts were progressively deformed and recrystallized first into core and mantle structures and ultimately into plagioclase rich layers or ribbons garnet preferentially developed in the outer portions of recrystallized mantles and with further deformation produced garnet rich sub layers within the plagioclase rich gneissic domains orthopyroxene was replaced by clinopyroxene and garnet and hornblende if sufficient water was present which were in turn drawn into layers with new garnet growth along the boundaries the igneous matrix evolved through a number of transient fabric stages involving s c fabrics s c c fabrics and ultramylonitic domains in addition quartz veins were emplaced and subsequently deformed into quartz rich gneissic layers moderate to highly strained samples display extreme mineralogical compositional segregation yet most domains can be directly related to the original igneous precursors the mary granite was emplaced at approximately 900 c and 1 0 gpa and was metamorphosed at approximately 750 c and 1 0 gpa the igneous rocks crystallized in the medium pressure granulite field opx pl but were metamorphosed on cooling into the high pressure grt cpx pl granulite field the compositional segregation resulted from a dynamic mutually reinforcing interaction between deformation metamorphic and igneous processes in the deep crust the production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust

serpentinization faults and their role in the tectonics of slow spreading ridges two major families of faults dominate the tectonics of slow spreading ridges the transform faults paralleling the spreading direction and the gloria faults observed by large scale side scan sonar normal to it the observation of microearthquakes to 8 km depth beneath the mid atlantic ridge axis implies that not only the transform faults but the gloria faults as well penetrate into the mantle since large horizontal temperature gradients are often associated with them both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle under these conditions serpentinization of upper mantle rocks will occur in a predictable way its influence on the tectonic development of slow spreading ridges is discussed

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

physical properties of upper oceanic crust ocean drilling program hole 801c and the waning of hydrothermal circulation 1 the hydrologic evolution of oceanic crust from vigorous hydrothermal circulation in young permeable volcanic crust to reduced circulation in old cooler crust causes a corresponding evolution of geophysical properties ocean drilling program odp hole 801c which obtained the world s oldest section of in situ normal oceanic crust provides the opportunity to examine relationships among hydrologic properties porosity permeability fluid flow crustal alteration and geophysical properties at both core plug and downhole log scales within these upper crustal basalts fluid flux in zones with high porosity and associated high permeability fosters alteration particularly hydration consequently porosity is correlated with both permeability and a variety of hydration indicators porosity dependent alteration is also seen at the log scale potassium enrichment is strongly proportional to porosity we extend the crustal alteration patterns observed at hole 801c to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of deep sea drilling project and odp core physical properties and downhole logs increasing crustal age entails macroporosity reduction and large scale velocity increase despite intergranular velocity decrease with little microporosity change the changes in macroporosity and velocity are significant for pillows but minor for flows matrix densities provide the strongest demonstration of systematic age dependent alteration on the basis of observed decreases in matrix density that are proportional to the logarithm of age approximately half of all intergranular scale crustal alteration occurs after the first 10 15 myr apparently crustal alteration continues at a decreasing rate throughout the lifetime of oceanic crust

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

reaction induced weakening of plagioclase olivine composites abstract the localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions in order to study the effects of solid solid mineral reactions on plastic deformation of rocks we have investigated the shear deformation of plagioclase olivine composites during the reaction plagioclase olivine orthopyroxene clinopyroxene spinel garnet microstructures of plagioclase olivine composites were studied after shear deformation experiments in a griggs apparatus experiments were performed on anorthite forsterite an fo and labradorite forsterite lab fo composites at 900 c confining pressures between 1000 1600 mpa and with constant shear strain rates of 5 10 5 s 1 in absence of reaction lab fo composites are stronger than pure olivine and labradorite end members that deform with a high temperature plasticity mechanism lab fo composites strain harden due to the inhibition of extensive recrystallisation by interphase boundaries in an fo composites the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine grained size

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

an automated strategy for calculation of phase diagram sections and retrieval of rock properties as a function of physical conditions we formulate an algorithm for the calculation of stable phase relations of a system with constrained bulk composition as a function of its environmental variables the basis of this algorithm is the approximate representation of the free energy composition surfaces of solution phases by inscribed polyhedra this representation leads to discretization of high variance phase fields into a continuous mesh of smaller polygonal fields within which the composition and physical properties of the phases are uniquely determined the resulting phase diagram sections are useful for understanding the phase relations of complex metamorphic systems and for applications in which it is necessary to establish the variations in rock properties such as density seismic velocities and volatile content through a metamorphic cycle the algorithm has been implemented within a computer program that is general with respect to both the choice of variables and the number of components and phases possible in a system and is independent of the structure of the equations of state used to describe the phases of the system

crustal velocity structure on the flanks of the mid atlantic ridge at 24 n summary a detailed seismic refraction survey was made over a 70 35 km area of young 9 ma crust at 24 n 45 w to investigate the detailed velocity structure of a portion of typical oceanic crust an array of eight ocean bottom hydrophones was used to monitor 358 explosive charges and over 750 airgun shots the crust exhibits normal oceanic structure with a steep vertical velocity gradient through layer 2 and almost 3 km of layer 3 material with a mean velocity of 6 65 km s 1 overlying probably anisotropic mantle ray tracing shows that the major amplitude variations observed in the upper crustal returns are governed more strongly by the seafloor topography than by the seismic velocity structure within the crust a delay time function method was used to invert the 1613 well determined first arrival travel times from the explosives we find that the delay time vanes by only 0 05 0 10s over most of the survey area but that there are rather larger delay times in the north west corner which may be associated with crust formed in the vicinity of a ridge jump we also find a systematic correlation between variations in delay time and in the seafloor depth at the ray entry point with smallest delay times where the seafloor is deepest this is consistent with a model of the crust in which the topography of the base of layer 2 and the moho is a subdued copy of the upper basement surface

fracturing fluid flow and shear zone development relationships between chemical and mechanical processes in proterozoic mafic dykes from southwestern montana usa fluids can play an important role in the localization of deformation in the deep crust yet the specific mechanisms active during the complex interactions between metasomatism metamorphism and deformation remain elusive precambrian metagabbronorite dykes in southwest montana contain fractures filled with hbl grt and discrete cm scale shear zones with well preserved strain gradients this system offers an ideal opportunity to constrain the chemical and mechanical processes that facilitated strain localization an early m1 assemblage of grt1 cpx1 pl1 qz developed at conditions of 0 51 0 85 gpa and 500 700 c and is preserved largely as a static replacement of relict igneous phases opx pgt pl in coronitic textures an m2 assemblage characterized by grt2 pl2 cpx2 hbl scp qz developed at 0 86 1 00 gpa and 660 730 c coincided with fluid flow and deformation associated with shear zone development microstructural observations in marginal protomylonite mylonite and laminated ultramylonite suggest a shear zone evolution that involved 1 nucleation from preexisting fractures that were sites for major fluid infiltration 2 initial widening coincident with grain size reduction by microfracturing dislocation creep and synkinematic metamorphic reaction by solution transfer and 3 a switch in the dominant deformation mechanisms active in the ultramylonite from grain size insensitive mechanisms to grain size sensitive granular flow accommodated by fluid assisted diffusion throughout this evolution the effective bulk compositions of the rock volumes responding to metamorphism changed through a combination of mechanical and metasomatic processes this article is protected by copyright all rights reserved

mid crustal shear zone formation in granitic rocks constraints from quantitative textural and crystallographic preferred orientations analyses abstract this paper presents quantitative microstructural and crystallographic preferred orientation cpo analyses of an alpine amphibolite facies shear zone developed in the fibbia metagranite gotthard massif central alps the weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse grained monomineralic aggregates derived from quartz and k feldspar porphyroclasts and softer fine grained plagioclase bearing shear bands a characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates cpos and microtextures suggest that quartz and k feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid assisted diffusive mass transfer and grain boundary sliding in the mylonite and ultramylonite shear zone core the porphyroclasts derived quartz and k feldspar layers are broken down to produce a polyphased matrix that is characterized by a homogeneous micron scale grain size and regular random distribution here the deformation of the whole aggregate occurs via a fluid assisted dissolution precipitation creep and grain boundary sliding referred as a fluid assisted granular flow we propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain the lateral broadening of the shear zone is driven by 1 the increase in fluid pressure in permeable albite oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates and 2 the thermodynamic re equilibration via metasomatic reactions of the shear zone walls

seismic constraints on the crustal structure within the vema fracture zone results from recent studies of the in well surveyed areas such as the famous area in kane and oceanographer fracture zones suggest that the central north atlantic ramberg et al 1977 significant variations in crustal thickness are transform faults are found every 50 to 100 km associated with parts of these fracture zones making them one of the most common tectonic with total crustal thicknesses in places less than features in the ocean basins morphologically half the thickness of normal oceanic crust the fracture zones on slowly spreading ridges display extent of this anomalous crust and its origin are many similar characteristics deep closed con still very poorly constrained by available geo tour depressions are usually present at ridge physical data here we report results from a 75 axis transform intersections sleep and biehler km long refraction line shot in late 1977 over 1970 the axis of the transform is typically three hawaii institute of geophysics ocean bottom defined by a deep linear trough bounded by escarpments rising eimom uuyuu withe ve valley near its western intersection with the mid transform valley floor a diverse suite of atlantic ridge 10o53 n 43o39 w we show that the apparent crustal thickness beneath this por tion of the vema transform valley is about 5 km not substantially different from crustal thick nesses typically associated with oceanic crust however the vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the kane and oceanographer fracture zones it is characterized by low compressional wave velocities a relatively uniform velocity gradient throughout most of the crust and the absence of a typical layer 3 refractor this anomalous crust is interpreted as intensely fractured hydrothermally altered and serpenti nized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls the large variation in moho depths found in these large offset slowly slipping transforms is attributed to differences in the depth and extent of serpen tnnization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer one important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor

seismic evidence for hydrothermally altered mantle beneath old crust in the tydeman fracture zone abstract to study the structure of old atlantic fracture zones seismic refraction profiles were obtained using ocean bottom seismometers and two ship multichannel expanding spread experiments on two inactive sections of the tydeman fracture zone the results indicate that a crustal seismic velocities are low compared with normal oceanic crust b there is a marked absence of an identifiable layer 3 c the depth to the moho is shallow though not exceptionally so and d anomalously low seismic velocities of 7 2 7 5 km s 1 are observed over ranges at which upper mantle velocities are normally detected s wave arrivals corresponding to both crustal and mantle refractions are also observed which enable the first estimates of poisson s ratio within a fracture zone to be made using these results and those from other surveys we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age these findings suggest that hydrothermal circulation which can lower the upper mantle velocity by serpentinization persists under fracture zones for at least 70 ma and occurs at greater depths than for normal oceanic crust the serpentinization process is not sufficiently rapid to obscure the location of the original crust mantle boundary

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

the tydeman morphology and seismic structure of old fracture zone crust the tydeman fracture zone was identified 160 km se of the azores in 1977 from a detailed bathymetry and magnetic survey between 22 309 and 23 409w twigt et al 1979 the fracture zone forms a linear trough here with a width of 20 km water depth of 4 7 km and orientation of 085 this study extends the path of this fracture zone between 18 and 30 w though not yet to the ridge crest using a larger dataset of bathymetric seismic reflection and magnetic profiles a discontinuity in the fracture zone valley indicates an interruption in its formation possibly during the formation of the azores platform two separate wide angle seismic experiments carried out over old crust in the active section of the tydeman fracture zone show low crustal velocities compared with normal oceanic crust of comparable age an absence of an identifiable layer 3 and a shallower than normal moho overlying a region of anomalously low upper mantle velocities estimates of poisson9s ratio suggest that hydrothermal circulation may continue for as long as 70 ma after crustal formation

late cretaceous remagnetization of proterozoic mafic dikes southern highland mountains southwestern montana a paleomagnetic and 40ar 39ar study paleomagnetic results from early proterozoic metabasite sills and middle proterozoic diabase dikes from the southern highland mountains of southwestern montana give well defined dual polarity magnetizations that are statistically identical to those from a small late cretaceous pluton that cuts the dikes the concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the proterozoic rocks were remagnetized probably during late cretaceous time paleomagnetic rock magnetic and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low ti magnetite combining virtual geomagnetic poles from metabasite sills diabase dikes and the late cretaceous pluton we obtain a paleomagnetic pole at 85 5 n 310 7 e k 19 9 a 95 9 1 n 14 sites that is similar to a reference pole from the 74 ma adel mountain volcanics of western montana biotite and hornblende 40 ar 39 ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca 1800 ma these dates probably record cooling of the southern highland mountains following high grade metamorphism at 1 9 1 8 ga the gneiss and metabasite age spectra show virtually no evidence of disturbance indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems thus the overprint magnetization of the highland mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby late cretaceous plutons remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of late cretaceous hydrothermal fluids related to epithermal mineralization the absence of significant disturbance to the 40 ar 39 ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated even if 40 ar 39 ar dating is applied

tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts paleomagnetic samples of many deep sea drilling project dsdp basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid ocean ridges rather than arising from peculiarities of the geomagnetic field the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes several lines of evidence support this conclusion observations of the basin and range province an extensional tectonic regime occurring in continental crust observations of the troodos complex cyprus an oceanic extensional tectonic regime now exposed on land and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid oceanic ridges the tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30 to 50 although rotations as large as 70 to 90 are not uncommon the paleomagnetic consequences of these rotations can be calculated for dsdp basalts if the original strike and latitude of a ridge are known at four dsdp sites for which adequate data are available namely sites 319 332 410 and 417 rotations ranging from 50 to 70 are required to explain the paleomagnetic results if the original strike and location of the ridge are not known as is the case for most of the older parts of the pacific ocean the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations this information combined with a similar but independent relationship derived from the marine magnetic anomalies puts severe constraints on the original strike and location of the ridge

amphibole fabric formation during diffusion creep and the rheology of shear zones we conducted experiments to examine the effect of amphibole forming hydration reactions on mineral fabric development and the strength of mafic rocks both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions 800 c 1 gpa amphibole that formed under hydrostatic conditions exhibits a random lattice preferred orientation lpo in contrast amphibole formed during deformation exhibits both a strong shape preferred orientation spo and lpo with a 001 maximum aligned subparallel to the shear direction plagioclase in both hydrostatic and deformed samples shows a very weak to random lpo at low effective strain rates 10 5 s 1 to 10 6 s 1 the stress exponent is 1 1 5 consistent with deformation accommodated by diffusion creep the correlation of the spo and lpo coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation the experimentally produced fabrics are strikingly similar to those observed in amphibolite grade natural shear zones supporting interpretations that such rocks deform by diffusion creep in addition the rheology of the fine grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite thus both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust

rates of fe mg mn and ca diffusion in garnet extraction of the thermal and chemical histories preserved by modified compositional zoning in garnet is compromised by large uncertainties in experimentally determined diffusion coefficients for principal divalent cations some of this uncertainty derives from long down temperature extrapolations but much of it arises from an inability to account accurately for large variations in diffusivity as a function of garnet composition numerical simulation of stranded diffusion profiles in partially resorbed natural garnets constrains diffusivities at low temperature and reveals a systematic dependence of diffusion rates on host garnet composition a comprehensive model incorporating a simple compositional dependence allows accurate prediction of diffusion rates for fe mg mn and ca for nearly all garnet compositions across the full range of geologically relevant temperatures pressures and oxygen fugacities

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

details of the gabbro to eclogite transition determined from microtextures and calculated chemical potential relationships permian aged metagabbros from the eclogite type locality in the eastern european alps were partially to completely transformed to eclogite during eoalpine intracontinental subduction microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro to eclogite transition allowing the details of the eclogitization process to be investigated original anorthite rich igneous plagioclase is pervasively replaced by fine grained intergrowths of clinozoisite kyanite and na rich plagioclase where plagioclase was in contact with igneous orthopyroxene 100 200 m thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains the rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet igneous clinopyroxene does not show any breakdown textures however jadeite content gradually increases towards the rims in addition exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds given that the igneous plagioclase is pervasively replaced by clinozoisite kyanite and na rich plagioclase whereas kyanite diopside symplectites are confined to narrow rim zones we suggest that the development of these textures was controlled by the im mobility of different elements on different length scales the presence of hydrous minerals in the core of anhydrous plagioclase indicates that h2o diffusivity occurred on a mm scale by contrast the size of the anhydrous diopside kyanite and diopside garnet symplectites indicate that fe mg ca na diffusivity was limited to a 10s of m scale chemical potential relations calculated in the idealized ncash chemical system show that the clinozoisite kyanite albite intergrowths formed due to an increase of h2o to plagioclase whereas all other elements remained effectively immobile on the scale of this texture fluid conditions indicated by this texture span from virtually dry conditions ah2o 0 15 to h2o saturation and therefore does not imply that the rocks were ever fluid saturated calculations in the cmas and ncfmas systems show that the gabbro to eclogite transition is characterized by the growth of garnet diopsidic clinopyroxene and kyanite due to diffusion of ca na and mg fe along a cao na2o mgo feo chemical potential gradient developed between orthopyroxene and plagioclase compositional domains the anhydrous nature of the textures indicate that the gabbro to eclogite transition is not driven by hydration however increased h2o acts as a catalyst that increases diffusivity of all elements and rates of dissolution precipitation allowing the overstepped metamorphic reactions to occur our results show that crustal eclogite formation requires low h2o content confirming that true eclogites are dry rocks