regional variations in deformation mechanisms and structural styles in the central appalachian orogenic belt orogenic belts such as the central appalachian fold and thrust belt consist of distinct tectonic provinces characterized by different structural styles as a result of deformation under different physical conditions a quantitative study of deformation mechanisms was undertaken for quartzites and sandstones of the central appalachian orogenic belt in order to understand the controls of different physical parameters on the relative importance of these mechanisms and on the geometry and mechanics of formation of structures in the different provinces in the blue ridge province the dominant deformation mechanisms are pressure solution dislocation creep and extension fracturing with some accompanying grain boundary sliding and recrystallization the proportion of dislocation creep to pressure solution strain increases both with temperature and deviatoric stress an axial plane slaty cleavage which contains the maximum and intermediate principal directions of the strain ellipsoid maintains a uniform orientation in both the blue ridge and the great valley provinces folds in the blue ridge province have a similar geometry formed by progressive flattening of parallel folds due to internal deformation ductile deformation zones on the overturned limbs of folds acted as precursors for the propagation of thrust faults in the late stages of folding in the valley and ridge and massanutten synclinorium important deformation mechanisms in the sandstone units include pressure solution plastic deformation cataclasis brecciation extension fracturing and slip along minor faults and bedding planes major folds are strongly disharmonic and show variations between concentric and angular geometries they are associated with thrust ramps that propagated through progressively developing folds in the late stages of folding brittle deformation zones characterized by high intensities of extension fractures and cataclastic bands formed on the steep forelimbs these features are interpreted to be the result of layer parallel extension and the propagation of splay faults through these limbs cleavage developed during layer parallel shortening prior to the onset of folding and was rotated to its present fanned geometry during folding on the basis of the results of this study the central appalachian orogenic belt can be subdivided into two deformational provinces dominated by different deformation mechanisms and characterized by different geometries and mechanics of formation of the structures for orthoquartzites the tectonite front which separates the tectonites of the hinterland from the non tectonites of the foreland is located along the blue ridge front for carbonates and argillaceous sandstones the tectonite front is located along the north mountain fault

relations between thrust belt evolution grain scale deformation and metamorphic processes cape smith belt northern canada abstract an important step towards understanding how continental thrust belts evolve is to study the relationship between the map scale structures and the associated grain scale deformation mechanisms and metamorphic processes this relationship is examined in the early proterozoic cape smith thrust belt in the context of the belt s p t t history and fluid rock interactions overprinting deformation metamorphism relationships allow the thrust belt s evolution to be described in terms of four events 1 1 piggyback thrusting above a basal decollement localized at the basement cover contact 2 2 greenschist to amphibolite grade metamorphism and development of a ductile basal shear zone 3 3 re imbrication out of sequence faulting of the internal zone at pre to syn thermal peak conditions 4 4 post thermal peak out of sequence thrusting and retrograde metamorphism out of sequence thrusting was accommodated by the development of ductile shear zones in general relatively broad ductile shear zones developed adjacent to thrusts during prograde metamorphism while relatively narrow shear zones developed adjacent to thrusts during retrograde metamorphism the width of the shear zones was controlled by factors such as the competition between softening and hardening processes prograde shear zones and the extent of fluid influx retrograde shear zones four processes were primarily responsible for accomplishing penetrative deformation and accommodating thrust faulting during the belt s history 1 1 partitioning of strain into pre existing weak zones or into relatively weak layers in a compositionally stratified sequence 2 2 growth and or rotation of metamorphic products into orientations of maximum resolved shear stress 3 3 transitions in dominant deformation mechanism from brittle or semi brittle to crystal plastic 4 4 retrograde metamorphism involving fluid influx and hydration reactions producing weaker products than reactants

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

effluent from diffuse hydrothermal venting 2 measurement of plumes from diffuse hydrothermal vents at the southern juan de fuca ridge an array of temperature and velocity sensors was deployed for two weeks in the bottom boundary layer at the southern juan de fuca ridge during the national oceanic and atmospheric administration vents 90 field experiment the instruments were placed close to the seafloor to detect effluent from diffuse hydrothermal sources located 1 2 km distant over the duration of the field deployment we detected effluent from two vent sources the plume from the larger of the two sources was sensed at all of the instrument locations permitting estimates of dilution based on the field measurements our data also permitted estimation of energy flux from diffuse vents approximately 125 75 mw from a source to the north of the instrument array and 12 8 mw from a small source to the east the largest source of error in the data was estimation of the plume width we have described a sensor array that would reduce this error in future work such an array would be suitable for a long term vent activity monitor

strain softening induced ductile flow in the sarv thrust sheet scandinavian caledonides abstract the external zone of the scandinavian caledonides is characterized by thrust sheets composed of sedimentary rocks derived from the continent baltica the sarv thrust sheet is unique amongst them because the 4 5 6 km thick flat lying tossasfjallet group clastics were intruded by a sub vertical tholeiitic dike swarm prior to thrusting the sarv thrust sheet is a large horse 100 km long and 80 km wide with a classical ramp flat geometry the original intrusive relationships of the dikes are preserved throughout most of the thrust sheet and deformation is concentrated at the base in a mylonite zone several tens of meters thick in this zone meter thick dikes rotate into parallelism with the basal thrust and concordant mylonites of both lithologies are intercalated on a centimeter to millimeter scale the overall rigid block geometry of the sarv thrust sheet and the intense strain gradient recorded by the rotation of the dike swarm and the concurrent development of mylonites in the thrust zone suggest that thrust emplacement took place by ductile yield and on going pseudoplastic flow continual strain softening localized ductile flow in this zone by means of grain size reduction reaction enhanced ductility and grain boundary sliding processes inferred from the microstructures

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

reaction enhanced ductility the role of solid solid univariant reactions in deformation of the crust and mantle abstract in collision belts and subduction zones penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks reaction enhanced ductility may result if a reaction produces a significant decrease in grain size thus allowing the rock to deform superplastically the product grain size of a reaction is a function of the ratio of nucleation and growth rates this ratio varies along any pressure temperature time ptt path and only attains a significant value at a certain distance from an equilibrium boundary a comparison of rates of change of p and t along typical segments of ptt paths for crustal rocks leads to the conclusion that a high nucleation rate a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening or during uplift an example of reaction enhanced ductility occurs in eclogite facies metagranitoids in the sesia lanzo zone western alps where medium grained plagioclase reacted to fine grained jadeite quartz during early alpine underthrusting microstructural evidence from shear zones shows that the jadeite quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation further evidence suggests that this process operated during regional deformation of metagranitoids rubie 1982 the reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate ringwood 1975 may result in fine grained products and thus may be analogous to the albite jadeite quartz reaction the consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions

microbial ammonia oxidation and enhanced nitrogen cycling in the endeavour hydrothermal plume abstract ammonium was injected from the subseafloor hydrothermal system at the endeavour segment juan de fuca ridge into the deep sea water column resulting in an nh 4 rich 177 nm neutrally buoyant hydrothermal plume this nh 4 was quickly removed by both autotrophic ammonia oxidation and assimilation the former accounted for at least 93 of total net nh 4 removal with its maximum rate in the neutrally buoyant plume 53 nm d 1 up to 10 fold that in background deep water ammonia oxidation in this plume potentially added 26 130 mg no 3 m 2 d 1 into the deep sea water column this oxidation process was heavily influenced by the presence of organic rich particles with which ammonia oxidizing bacteria aob were often associated 40 68 aob contributed up to 10 8 of the total microbial communities within the plume and might constitute a novel lineage of proteobacterial aob based on 16s rrna and amoa phylogenetic analyses meanwhile nh 4 assimilation rates were also substantially enhanced within the neutrally buoyant plume 26 4 nm d 1 and accounted for at least 47 of total net nh 4 removal rates the combined nh 4 oxidation and assimilation rates always exceeded total net removal rates suggesting active in situ nh 4 regeneration rates of at least an order of magnitude greater than the particulate nitrogen flux from the euphotic zone ammonia oxidation is responsible for nh 4 turnover of 0 7 13 days and is probably the predominant in situ organic carbon production process 0 6 13 mg c m 2 d 1 at early stages of endeavour neutrally buoyant plumes

tidal current effects on temperature in diffuse hydrothermal flow guaymas basin a twelve day record of temperature collected from the diffuse flow area of a guaymas basin hydrothermal site exhibits variations from a minimum of 3 05 c to a maximum of 4 87 c whose periodicity is correlated with tides measured at the nearby town of guaymas a simple model based on the hypothesis that temperature variations result as changes in tidal bottom currents induce changes in the height of the thermal boundary layer is in good quantitative agreement with observed temperatures for most of the record the success of this model illustrates that the effects of tidal currents can be strong enough to dominate the time variability of a temperature signal at a fixed point in hydrothermal flow therefore tidal currents must be taken into account when using temperature measurements to estimate time varying heat fluxes from hydrothermal diffuse flow regions

the subsolidus segregation of layer parallel quartz feldspar veins in greenschist to upper amphibolite facies metasediments layer parallel i e parallel to foliation or bedding vein formation in the graywackes and pelites of the quetico metasedi mentary belt occurred during synchronous prograde metamorphism and regional dj com pression in a traverse across metasediments which change in metamorphic grade from green schist to upper amphibolite migmatite facies layer parallel veins show the following trends 1 an increase in thickness and internal corn plexity the latter due to successive boudinage 2 low grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks but at higher grades other sites are also used and 3 a systematic increase in plagioclase quartz ratio in the veins towards higher grade adjacent mafic selvedges first exhibit quartz depletion then in the amphibolite facies plagioclase depletion mineralogical zoning is often preserved in a single vein older parts are more quartz rich than younger mass balance calculations and whole rock geochemistry based on veins mafic selvedges and country rock are consistent with a closed system subsolidus segregation origin the layer parallel veins are syntectonic and migration of the mobile components required to form their mineralogy is a stress induced mass transfer the source of these components appears to be dominantly pressure solution of the same minerals in the host rocks although metamor phic reactions may also have contributed veins nucleated first at those sites where initial sedimentary heterogeneites such as fine scale graded bedding provided gradients of normal stress across grain boundaries and hence of chemical potential necessary to drive the sub solidus segregation process the earliest veins are thus parallel to bedding later nucleation of the veins could also occur along more randomly distributed sites within the metasediments and these veins grew parallel to the schistosity rather than bedding if the two were distinct once formed the veins themselves which are more competent than the surrounding rock provide the stress heterogeneity required for their further growth the increasing plagioclase quartz ratio in the veins may be due to a tem perature dependent increase in plagioclase com ponent mobility relative to quartz alterna tively the increasing transfer distances for silica resulting from prior quartz depletion in the inner parts of the mafic selvedge may increase the relative mobility of plagioclase component

crustal fissuring and its relationship to magmatic and hydrothermal processes on the east pacific rise crest 9 12 to 54 n the deep towed argo i optical acoustical vehicle and a geographic information system gis have been used to establish the abundance widths and spatial distribution of fissures as well as the relative age distribution of lavas along the narrow 500 m wide axial zone of the east pacific rise epr from 9o12 to 9o54 n on a second order scale 78 km long wider but less numerous fissures are found in the northern portion of the survey area this changes to narrower more abundant fissures in the south a profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike near 9o49 9o35 and 9o15 n where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle filling and covering older fissures on a fourth order scale 5 15 km long the mean density of fissuring on a given segment is greater where relative axial lava age is greater fissure density also correlates with hydrothermal vent abundance and type increased cracking toward segment tips is observed at the second order scale whereas fourth order segments tend to be more cracked in the middle cracking on a fourth order scale may be driven by the propagation of dikes rather than by the far field plate stresses the above relations constrain the model of haymon et al 1991 in which individual fourth order segments are in different phases of a volcanic hydrothermal tectonic cycle

structural evolution of the cape smith thrust belt and the role of out of sequence faulting in the thickening of mountain belts out of sequence thrusting represents a mechanism capable of heterogeneous internal deformation in thrust belts and was largely responsible for internal thickening in the early proterozoic cape smith belt the south verging thrust belt is marked by a complex history of accretion internal deformation thickening induced metamorphism uplift and erosion an overall chronology for thrust belt evolution is built around the timing of thermal peak metamorphism prethermal peak deformation includes 1 initial piggyback stacking of basalt dominated thrust sheets above a basal decollement localized at the basement cover contact 2 greenschist to amphibolite grade metamorphism accompanied by development of a ductile shear zone at the base of the thrust belt and 3 out of sequence thrusting precipitating the metamorphic thermal peak both above and below these faults synthermal peak out of sequence thrusting was responsible for accretion of autochthonous crystalline basement slices and eventual abandonment of the external basal decollement postthermal peak deformation is characterized by the continued development of out of sequence thrusts which appear to young toward the hinterland north internal thickening by development of flattening cleavages or folding appears to be regionally insignificant when compared to that accomplished by the out of sequence faults this suggests that 1 an eroding and accreting thrust wedge may be able to maintain its critical shape by out of sequence thrusting and 2 this style of internal deformation is favored in belts dominated by relatively strong material e g basalt as in the cape smith belt

strain geometry microstructure and mineral chemistry in metagabbro shear zones a study of softening mechanisms during progressive mylonitization abstract the strain geometry microstructure and metamorphism is described from two minor shear zones from the chatelaudren metagabbro n brittany a serially slabbed shear zone reveals a strain geometry consistent with simple shear deformation strain calculations based on x trajectory angles coincide with those obtained from elliptical mineral clusters at high values of strain only strain profiles typically show a broad low strain region with a narrow high strain peak at the centre of the zones microstructures typically show distinct grain size reduction in both amphibole and feldspar towards the high strain region of shear zones and this is discussed in terms of deformation mechanisms related to strain softening a palaeostress estimate based on recrystallized feldspar grain sizes gives a differential stress of 32 mpa for the low strain region and 119 mpa in the shear zone centre electron probe analyses reveal chemical and mineralogical changes accompanying metamorphism within the shear zones this suggests local conditions favourable for ionic diffusion and the activity of fluids is implied

the role of transiently fine grained reaction products in syntectonic metamorphism natural and experimental examples it is proposed that one of the most important mechanisms of interrelationship between deformation and metamorphism is the facilitation of one of several grain size sensitive deformation mechanisms resulting from the formation of fine grained products of a metamorphic reaction during prograde metamorphism such effects are likely to be transient because grain coarsening and textural equilibration are likely in response to rising temperature conditions thus deformation mechanisms are often difficult to infer from such naturally deformed rocks in localized shear zones exhibiting retrogressive metamorphism evidence of enhanced deformability by such mechanisms is most likely to be preserved because cooling conditions inhibit grain growth and both deformed and undeformed rocks are likely to be preserved an experimental study has been made of the effects of deformation on serpentinite under conditions of progressive dehydration but with controlled pore pressure a marked weakening near linear viscous rheol

footwall involvement during arc continent collision ungava orogen northern canada geological evidence and theoretical models are presented to illustrate deformation paths followed by continental crust during underthrusting subduction and collision fieldwork in an early proterozoic collisional orogen indicates that a thrust belt and its autochthonous footwall basement are obliquely exposed in a crustal cross section with significant structural relief the collisional orogen records an evolution in the style and extent of involvement of the archaean footwall from 1 underthrusting without evidence of internal deformation 2 limited thrusting and ductile deformation of footwall rocks to 3 folding of the entire thrust belt and its footwall into cuspate lobate long wavelength structures the structural evolution occurred during hydration of basement rocks coeval with collision related metamorphism lithospheric strength profiles constrained by simple numerical heat conduction models suggest that the observed deformation path for the footwall crust was controlled by weakening due to heating of basement rocks and by changes in the relative strengths of different crustal layers the profiles predict a very weak lower crust and upper mantle at the time of footwall involved folding suggesting that the lower crust and upper mantle deformed by ductile flow concomitant with buckling of the stronger upper crust

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

syn tectonic magmatism and the development of compositional layering ungava orogen northern quebec canada abstract layer and foliation parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now exhumed middle crustal sections of both archean and paleoproterozoic age in the northern ungava peninsula quebec canada in the archean superior province diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite facies conditions coeval with voluminous granitic magmatism the paleoproterozoic narsajuaq arc contains evidence for contemporaneous magmatism transpressional deformation and granulite facies metamorphism prior to its collision with the superior province basement in both plutonic domains regional compositional layering is defined by 1 metre to kilometre scale alternation of generally well foliated tonalite and quartz diorite bodies and 2 centimetre to kilometre scale variably deformed granitic veins and sheets that lie parallel to layering tectonic foliation in the host rocks syn tectonic intrusion of a substantial portion of the veins along extension fractures sub parallel to layering foliation i e at high angle to the regional shortening direction is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures compositional layering generated and or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust

nucleation of ductile shear zones on dilatant fractures small shear zones in granitic rocks of the sierra nevada california and near roses rosas northeast spain display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation many zones have sharp nearly planar boundaries between their highly deformed interiors and the undeformed wall rock the roses shear zones narrow continuously to hairline fractures at their ends mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones we suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes thereby promoting chemical alteration and or hydrolytic weakening

the mineral chemistry of new titanates from the jagersfontein kimberlite south africa implications for metasomatism in the upper mantle new members of the crichtonite mineral series are described in which k ba ca and ree are in significant concentrations 5 wt oxides filling the a formula position in am21o38 these phases are chromium 16 wt cr2o3 titanates 58 wt tio2 enriched in zro2 5 wt and constitute a mineral repository for refractory and large ion lithophile elements in the upper mantle the mineral senes coexists with mg cr ilmenite nb cr rutile and ca cr nbzr armalcolite that have equally unusual chemistries kimberlitic crichtonites are depleted in the intermediate lanthanides but highly enriched in lree and hree with chondrite normalized abundances of 103 to 105 crichtonite armalcolite and nb cr rutile occupy a compositional range in tio2 contents bridging the gap between ilmenite and rutile two minerals having a widespread distribution in kimberlites and mantle derived nodule suites in common with other associations and based on similarities in mineral chemistry it is concluded that these minerals formed at p 20 30 kb 900 1100 c by reaction of peridotite with metasomatizing fluids kimberlitic crichtonite may be expressed as spinel cr ferropseudobrookite and armalcolite is equivalent to cr geikielite rutile in the system femg tio2 cr2o3 this system contains a number of cr ti compounds not found as minerals but it is proposed that the ubiquitous occurrence of ilmenite intergrowths in kimberlitic rutile results from decomposition of high pressure pbo2 type crystallographic shear structures the new minerals have exotic chemistries and the high k affinities broaden the scope for the origin of alkalic rocks the generation of highly potassic magmas in the upper mantle and suggest that alkali metasomatism may be pervasive

experimental syntectonic hydration of basalt abstract this paper outlines in general terms the role of metamorphic transformations during rock deformation long duration triaxial stress relaxation tests at 600 c 160 mpa confining pressure on wet crushed basalt sheared in sawcuts made in intact basalt cylinders are reported mechanical data are interpreted in terms of a material which shows a small strain rate sensitivity to large variations in applied stress but which work hardens dramatically as a result of marked microstructural changes which occur during straining permanent distortions of c 20 were produced in the samples tests at 100 mpa pore water pressure resulted in the formation of oriented overgrowths of amphibole preferentially developed on less stressed interfaces between original grains and in pore spaces substantial redistribution of feldspar also occurred at low pore pressures no amphibole formed and only feldspar was recrystallized much of the original porosity of the rock was eliminated in all cases the mechanical and microstructural data are interpreted in terms of flow by some combination of grain sliding and diffusional accommodation with the reprecipitated diffused material being the product of a chemical reaction involving all original solid phases plus water from a consideration of the effects of the reaction on the chemical potential gradients driving diffusion it is concluded that although the syntectonic hydration exerts a profound effect on the microstructural and mineralogical development of the rock during deformation there is no substantial enhancement of the rate of strain similar microstructural developments also occur in natural rocks deformed during retrogressive metamorphic events and examples are described for comparison

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

extreme ductility of feldspars from a mylonite parry sound canada abstract single feldspar crystals in mylonites from the grenville structural province of canada have accumulated extreme strains by ductile mechanisms the samples studied are from a deep crustal shear zone and were collected at parry sound ontario the mylonites were derived from granite and leucogabbro and the feldspar crystals originated in late syntectonic pegmatite dykes optical and transmission electron microstructures of microperthitic alkali feldspars show evidence of syntectonic and synchronous dislocation climb leading to recovery si al ordering and na k interdiffusion evidence for the operation of these processes is respectively ubiquitous subgrains development and coarsening of tweed texture and transition of monoclinic to triclinic k feldspar and exsolution domain reorganization these processes occurred concurrently with extreme straining of the host crystals by dislocation and diffusion mechanisms the effects of ductile deformation preserved in these crystals are quite different from those usually observed in feldspars the unusual microstructures are thought to be due to the conditions of high temperature and confining pressure which existed during deformation in concert with a deformation enhancing point defect chemistry possibly associated with incorporation into the crystals of a species of water

long term monitoring of hydrothermal heat flux using moored temperature sensors cleft segment juan de fuca ridge heat flux from submarine vent fields can vary gradually on interannual scales or nearly instantaneously in response to volcano tectonic events in the underlying crust neither case is well documented because measurements of vent field scale heat flux are scarce we report here a new approach to hydrothermal plume monitoring sensitive to both progressive changes and hydrothermal events from june 1991 to may 1992 we moored 35 self contained temperature sensors and six current meters on seven moorings located in and around the plume from a vent field on the cleft segment juan de fuca ridge the hydrothermal plume was identified by a local temperature anomaly of 0 01 to 0 03 c in the lowermost 200 m of the water column plume heat flux defined as the net advection of this temperature anomaly averaged about 250 mw during the deployment this flux is less than previous estimates and thus supports speculation that the heat flux is declining after a sudden reinvigoration triggered by a seafloor rifting event in 1986

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

gradients in the composition of hydrothermal fluids from the endeavour segment vent field phase separation and brine loss hydrothermal fluid samples collected in 1984 1987 and 1988 from a large vent field near 47 57 n on the endeavour segment of the juan de fuca ridge jfr have been analyzed for major and minor elements and gases there are of the order of 100 individual smoker vents on 10 large sulfide structures which are localized along faults and fault intersections across the vent field each sulfide structure has a characteristic fluid composition which varies very little from one vent orifice to the next or from year to year on a given structure however there are large gradients in fluid composition across the vent field with endmember chlorinity increasing from 255 mmol kg in the sw to 505 mmol kg in the ne end member concentrations of major elements are well correlated with chlorinity and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids the gradients in composition across the vent field and measured vent fluid temperatures 400 c are consistent with supercritical phase separation and loss of brine phase below the seafloor the factor of 2 variation in co2 and h2s is larger than expected for loss of a very high chlorinity brine concentrations of iron and manganese are not positively correlated with chlorinity suggesting that temperature and ph are more important in controlling metal solubility elevated ammonia and bromide chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter and high boron concentrations point to a sedimentary source

large scale fluid infiltration metasomatism and re equilibration of archaean basement granulites during palaeoproterozoic thrust belt construction ungava orogen canada abstract the metamorphic history of the archaean superior province crystalline basement in the palaeoproterozoic ungava orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite granulite transition two distinct metamorphic suites separated in age by nearly one billion years are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses the older suite comprises c 2 7 ga granulite facies assemblages orthopyroxene clinopyroxene hornblende plagioclase ilmenite biotite quartz that record moderate pressures 5 kbar and high temperatures 800 c a younger c 1 8 ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal decollement of the overlying proterozoic cape smith thrust belt deformation mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re equilibration of the archaean granulites occurred during thrust belt deformation the transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet hornblende quartz cummingtonite coronas between plagioclase and orthopyroxene clinopyroxene as well as titanite coronas on ilmenite multi equilibrium thermobarometry on the coronitic assemblages documents re equilibration of the granulitic gneiss to 7 7 kbar at 644 c in the south and 9 8 kbar at 700 c in the north the variably deformed amphibolite facies domain sandwiched between the coronitic garnet zone and the basal decollement is marked by significant metasomatic changes in major element concentrations within tonalite these changes are compatible with equilibrium flow of an aqueous chloride fluid down a temperature gradient the source of fluids for basement hydration metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt with fluid flow probably focused along the basal decollement

characteristics of 4 day oscillations trapped by the juan de fuca ridge current meter observations from different studies along the crest of the juan de fuca ridge have been combined to show new features of the relatively large broad spectral peak centered near a period of about 4 days the coherence range of the 4 day oscillations has been extended to the extreme ends of the ridge and the trapped waves appear to propagate northward along the entire length of the ridge at about 1 m s spectral amplitudes are about double on the south end compared to the north end and a recent model for bottom trapped subinertial waves over topography shows this may be due to variations in ridge width allen and thomson 1993 new observations show coherence between temperatures and currents near the south cleft vent field and suggest 4 day oscillations can play a major role in the local advection of hydrothermal plumes these observations combined with earlier results demonstrate that 4 day oscillations are a significant ridge trapped feature and they suggest that other ridges may be important in transmitting energy over much longer distances

seismic structure of the endeavour segment juan de fuca ridge correlations with seismicity and hydrothermal activity 1 multichannel seismic reflection data collected in july 2002 at the endeavour segment juan de fuca ridge show a midcrustal reflector underlying all of the known high temperature hydrothermal vent fields in this area on the basis of the character and geometry of this reflection its similarity to events at other spreading centers and its polarity we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface the endeavour magma chamber reflector is found under the central topographically shallow section of the segment at two way traveltime twtt values of 0 9 1 4 s 2 1 3 3 km below the seafloor it extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends on cross axis lines the axial magma chamber amc reflector is only 0 4 1 2 km wide and appears to dip 8 36 to the east while a magma chamber underlies all known endeavour high temperature hydrothermal vent fields amc depth is not a dominant factor in determining vent fluid properties the stacked and migrated seismic lines also show a strong layer 2a event at twtt values of 0 30 0 09 s 380 120 m below the seafloor on the along axis line and 0 38 0 09 s 500 110 m on the cross axis lines a weak moho reflection is observed in a few locations at twtt values of 1 9 2 4 s below the seafloor by projecting hypocenters of well located microseismicity in this region onto the seismic sections we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone indicating thermal related cracking the presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system morphological and hydrothermal differences between the intermediate spreading endeavour and fast spreading ridges are attributable to the greater depth of the endeavour amc and the corresponding possibility of axial faulting

large massive sulfide deposits in a newly discovered active hydrothermal system the high rise field endeavour segment juan de fuca ridge submersible observations and side scan sonar imagery delineate ten large venting sulfide sulfate silica structures located on an uplifted block of the axial valley floor on the central portion of the endeavour segment juan de fuca ridge this newly discovered vent field lies 1 8 km north of the main endeavour vent field the axial valley a horst and faults controlling sulfide deposition trend parallel to the ridge axis the horst rises 10 to 15 m above the valley floor and offsets a lava lake which extended 500 m in width across the valley the largest sulfide structures are perched along the major horst bounding faults one structure is the tallest single edifice 45 m reported in the pacific ocean smaller less active and inactive sulfide edifices are associated with faulted remnants of the lava lake surface fissured pressure ridges and at the edge of collapse pits based on the relationship between faults and sulfide structures the morphology of sulfide edifices and their venting characteristics we infer that sulfide structures grow vertically by chimney accumulation and laterally by flange development late stage deposition of amorphous silica toughens the large deposits and may be responsible for the unusual size and aspect ratio of endeavour edifices

review of crustal architecture and evolution in the ungava peninsula baffin island area connection to the lithoprobe ecsoot transect within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks of the superior province margin and two assemblages of accreted paleoproterozoic units ungava and meta incognita crustal terranes define a basic tripartite crustal architecture which is characterized by polyphase deformation and metamorphic histories early structures and mineral assemblages are recognized in all three crustal components and shown to be temporally distinct the earlier structures and assemblages are overprinted by younger elements and mineral assemblages that are related to at least three regional contraction episodes and one thermal event that are common to all crustal components of the orogen close correlation of regional aeromagnetic data in northern quebec and southern baffin island with the three crustal components offers a first order approximation of the distribution of basement lithological units across hudson strait continuation of the aeromagnetic domains beneath ungava bay to northeastern quebec and northern labrador allows for the southward extrapolation of the tripartite trans hudson orogen crustal architecture into the lithoprobe eastern canadian shield onshore offshore transect area

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

is the cape smith belt northern quebec a klippe the cape smith belt is one of the most interesting and controversial of the proposed geosutures in the canadian shield a new tectonic model is presented in which the mafic ultramafic thrust sheets of the belt constitute a klippe 20 000 km2 in area separated from underlying basement of superior province and its thin autochthonous cover by a continuous decollement exposed along the north margin and the plunging eastern end of the belt thrusting is directed southward and the entire stack is folded into a regional antiform north of the belt and synform the belt itself it is proposed that the decollement is rooted 30 90 km north of the belt in a zone paralleling sugluk inlet across which there is a major positive deflection in the bouguer gravity field an abrupt switch from broad north south to narrow east west magnetic anomalies and a change in metamorphic grade from amphibolite to granulite facies from south to north in the model this zone is a north dipping ductile shear zone juxtaposing cru

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

structure of two hydrothermal megaplumes the dynamic signatures of two megaplumes above the juan de fuca ridge are analyzed the chemical properties of these two lenslike masses of water were described by baker et al 1989 and clearly indicate that they were generated by massive and rapid ventings of hot hydrothermal fluid from the ridge both are nearly circular with radii of about 6 5 km the isopycnals bow upward around these cores of anomalous water leading to an anticyclonic circulation a cyclogeostrophic balance gives maximum currents at the edge of the core of 0 11 m s 1 for the first megaplume mp1 and 0 07 m s 1 for the second megaphone mp2 currents extend beyond the core to a radius of 12 15 km the centers of the cores are in nearly solid body rotation with relative voracities of 0 5 mp1 and 0 3 mp2 and potential vorticity anomalies expressed in units of equivalent relative vorticity of 0 8 mp1 and 0 6 mp2 where is the coriolis frequency the aspect ratio of each megaplume gives a burger number of 0 22 in terms of these nondimensional numbers the megaplumes are very similar to eddies of mediterranean water found in the eastern atlantic meddies despite their very different origin

magnitude and variance of near bottom horizontal heat flux at the main endeavour hydrothermal vent field 1 we characterize hydrothermal plumes and hydrography within the axial valley of the endeavour segment juan de fuca ridge northeast pacific using data from an autonomous underwater vehicle a lowered ctd and two current meter moorings our survey of the main endeavour hydrothermal vent field mef shows hydrographic variability on scales as short as 20 m and 10 min within the 100 m deep axial valley the mean flow is 1 5 cm s to the north and tidal oscillations have 5 cm s amplitude combining the mean flow with an estimate of the difference in average temperature observed north and south of mef we find that the net along axis horizontal heat flux has a mean magnitude of 8 42 mw an advection diffusion model forced by local current records explains the observed temporal hydrographic variability and shows that mean horizontal flux variance is correspondingly high standard deviation 63 mw a steady state mef heat budget constrained by the observed horizontal flux and measurements of vertical fluxes from other studies has two implications the mef heat flux is partitioned about equally between diffuse and focused vents and about 85 of the flux from diffuse plumes is entrained by focused plumes

near axis heat flow measurements on the northern juan de fuca ridge implications for fluid circulation in oceanic crust present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process in axial regions abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid in older crust where the insulating sediment cover is thick previous studies have shown that basement topography is the dominant forcing factor for within crust fluid circulation in the intermediate region where young crust is lightly sedimented heat flow data are difficult to obtain with traditional techniques to determine whether topography or permeability is the dominant process controlling fluid circulation in the near axis region we conducted a profile of heat flow measurements using the submersible alvin on the endeavour segment of the juan de fuca ridge our data indicate that topographic forcing is responsible for the long wavelength variations with high heat flow at the ridge summits and low values in the inter ridge valleys the locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge valley topographic pair comprises a single circulation cell this model predicts that the source area for the high temperature axial vents may be in the flanking inter ridge valleys

unraveling the boundary between turbidites of the kisseynew belt and volcano plutonic rocks of the flin flon belt trans hudson orogen canada the flin flon kisseynew boundary in the eastern trans hudson orogen is interpreted here as an early thrust fault that places 1 86 1 84 ga kisseynew belt turbidites over previously deformed 1 91 1 88 ga arc and ocean floor assemblages of the flin flon belt the basin in which sedimentary rocks of the kisseynew belt were deposited has been interpreted to have formed partly within the flin flon belt the fault that juxtaposes the two belts is interpreted to have been localized near the ancestral basin margin resulting in development of a major ramp zone during basin closure this interpreted ramp zone provides an explanation for the steep to shallow structural transition that corresponds to increasing metamorphic grade collapse of the kisseynew sedimentary basin and juxtaposition of the two belts are attributed to southwest verging folding and thrusting that initiated prior to emplacement of 1 83 ga plutons this magmatism was followed by regional greenschist to upper amphibolite grade metamorphism 1 8

hydrothermal venting and geothermal heating in cascadia basin observations in cascadia basin on the eastern flank of the northern juan de fuca ridge reveal significant bottom water modification as a result of regional conductive heating and local hydrothermal venting seafloor conductive heating occurs throughout the sedimented basin while hydrothermal fluid discharge is confined to small 1 km2 isolated igneous basement outcrops in the northern sector of the plateaulike basin the vertical fluxes of heat and mass associated with these seafloor processes lead to the formation of a 250 to 350 m thick geothermal boundary layer characterized by anomalously high temperature reduced vertical stability and high dissolved silicate concentration using a basinwide average lithospheric heat flux of 0 3 w m 2 and the observed thermal anomaly structure of the water column we obtain a mean residence time of 1 to 2 years for the deep water over cascadia basin detailed water property data collected in 1992 and 1993 within the immediate vicinity of three isolated igneous basement outcrops in the north central sector of the basin indicate that local bottom water heating arises from low temperature venting through the summit and flanks of the outcrops near the smallest edifice especially well defined layers of anomalously warm particle laden water were found within 20 m of the outcrop summit depth of 2610 m maximum anomalies of temperature light attenuation coefficient and dissolved silicate concentration in the layers were 0 040 c 0 015 m 1 and 5 mol l 1 respectively we estimate the local heat flux fo from the smallest outcrop to be 2 4 0 8 u 109 w where u m s 1 is the mean horizontal current at the venting depth for reasonable mean currents in the range 10 3 to 10 2 m s 1 we find fo 0 2 to 2 4 107 w assuming that the depressed conductive heat flow of 0 05 w m 2 observed through the sedimented seafloor surrounding the smallest outcrop reflects the advective loss of heat through the outcrop the radial distance over which crustal fluids must collect heat and converge on the outcrop is about 10 km

physical characteristics of the endeavour ridge hydrothermal plume during july 1988 abstract we conducted ctd transmissometer tows from 8 to 26 july 1988 within 15 km of the central hydrothermal vent site 47 57 n 129 06 w on the endeavour segment of juan de fuca ridge anomalies of temperature salinity and light attenuation reveal possible new vent sites 4 and 8 km northeast and 6 km south of the central vent site as a result of widespread plume dispersion background values of potential temperature salinity and light attenuation below the 1900 m depth exceeded those for pristine ambient waters by 0 05 c 0 05 psu and 0 03 m 1 respectively maximum plume anomalies relative to the background waters were of the order of 0 10 c 0 010 psu and 0 10 m 1 at core depths of 2000 2050 m heat and salt anomalies were detectable more than 5 km from the central vent site whereas light attenuation particle anomalies were confined to within 2 5 km of the vent site based on the background water property anomalies and moored current meter records the mean time averaged heat fluxes for the survey region were 2 3 1 5 10 8 w in the along ridge direction 20 t and 7 7 4 7 10 8 w in the cross ridge direction 110 t mean along and cross ridge salt fluxes were 7 5 and 25 15 kg s 1 mean particle fluxes were 0 09 0 06 and 0 29 0 18 kg s 1 estimates of the instantaneous fluxes derived from coincident current and plume measurements indicate that heat fluxes from the central vent field may have been as high as1 2 0 6 10 10 w and corresponding particulate fluxes as high as6 3 kg s 1

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

upper and lower plate juxtaposition deformation and metamorphism during crustal convergence trans hudson orogen quebec baffin segment canada abstract the lower plate of the quebec baffin segment of paleoproterozoic trans hudson orogen comprises parautochthonous archean crystalline basement of the superior province 3 22 2 74 ga and imbricated paleoproterozoic rift margin strata 2 04 1 92 ga the collisional upper plate contains 1 allochthonous ophiolitic units 2 00 ga 2 a continental arc terrane narsajuaq arc 1 86 1 82 ga 3 shelf sequence strata ca 1 86 ga and basement orthogneisses ca 1 95 ga 4 foreland basin deposits 1 86 ga and 5 a batholith which intrudes the shelf and foreland basin rocks cumberland batholith 1 86 1 85 ga lower and upper plate assemblages were juxtaposed during a 1 82 1 79 ga accretion collision event and deformed into regional scale folds during two post collisional folding episodes ca 1 76 1 74 ga as a result of the structural relief generated by the post collisional folding and subsequent exhumation all orogenic elements can be directly examined along a 400 km across strike transect in the external zone northern quebec lower plate rift margin units were internally imbricated cape smith belt and translated southward across the superior province basement paleoproterozoic amphibolite facies mineral growth is syn to post thrusting tectonic imbrication has resulted in a structural inversion of the accreted i e upper plate ophiolite stratigraphy north of the cape smith belt mid crustal plutonic units of the narsajuaq arc form an upper plate crystalline wedge whose sole thrust is localized near the thrust belt s basal decollement within narsajuaq arc syn magmatic granulite facies assemblages are retrogressed to amphibolite facies assemblages in proximity to major accretion collision faults in the internal zone southern baffin island a tripartite orogen scale tectonostratigraphy is exposed on the flanks of the post collisional fold structures at the lowest structural level lower plate superior province orthogneisses are imbricated with paleoproterozoic rift margin units and mineral growth is syn thrusting at intermediate structural level upper plate orthogneisses are correlated with the narsajuaq arc in northern quebec at the highest structural level a north facing shelf sequence is structurally overlain by an assemblage of clastic rocks interpreted as a foreland basin sequence dominantly monzogranite plutons of the cumberland batholith are restricted to the upper structural level and indicate pre emplacement imbrication of tectonostratigraphic units within both upper and lower plates granulite facies assemblages are retrogressed to upper amphibolite assemblages in proximity to 1 82 1 79 ga accretion collision structures exposures of superior province basement across the orogen document a progressive increase in the extent of reworking during accretion of upper plate units from a narrow shear zone in the external zone to km scale imbrication and more pervasive shear deformation in the internal zone a minimum of 160 km of basement underthrusting is constrained by the structural overlap of upper plate allochthons on lower plate units basement involvement during post collision shortening is manifest as thick skinned folding indicating coupling across the collisional basal decollement

earthquakes and rock deformation in crustal fault zones the physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress although the possibility now exists that the nucleation regions for large earthquakes at depths of 10 km may shortly become directly accessible by deep drilling our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources these include seismological studies surface studies of fault zones and earthquake ruptures geodetic information on modes of fault slip geophysical constraints on fault zone structure and rheology and information garnered from materials science and experi mental rock deformation studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones although descriptions of fault rocks are widespread in the geological literature e g spry 1969 higgins 1971 it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths sibs on 1977 watts williams 1979 anderson et a11983 wise et ai 1984 such interpretations are still at an early stage but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels power dissipation and seismic efficiency during earthquake faulting on fluid pressure levels and episodic fluid flow accompanying

role of shear in development of the helvetic fold thrust belt of switzerland the geometric features of the helvetic nappes of the alps show intense internal deformation set up predominantly by simple shear during nappe transport the nappes are bounded by thrusts that parallel incompetent layers and ramp upward stratigraphically across competent units in the direction of tectonic transport stratal layering is oblique to nappe boundaries thrust surfaces formed after ductile shear zones developed in underlying pre triassic gneissic basement the broad zone of shearing narrowed down as deformation increased to form zones of intense shear and ultimately to form thrust faults contraction along competent layers obliquely inclined to shear surfaces produced buckle folds the geometry of these folds was modified by further shearing as their normal limbs became aligned in the extension field of the strain ellipse for simple shear producing normal faults

partitioning of deformation within several external thrust zones of the appalachian orogen mesoscopic contractional and extensional faults dominate many external thrust zones the fault arrays occur in the hanging walls of some thrusts and in the footwalls of others in a thrust zone from the little mountains fold thrust belt of new york state mesoscopic faults cut and offset a major thrust contact and involved rocks from the thrust s hanging wall and its footwall simultaneously the little mountains exposure indicates that minor faults were active during thrust slip and are not early formed features which were passively translated with the remainder of a sheet when both contractional and extensional mesoscopic faults occur in thrust zones foreland dipping extensional faults overprint contractional faults contractional and extensional microfaults with r 1 and r 2 riedel shear geometries are common in external thrust zone cataclasites and resemble the mesoscopic fault patterns though the evolution of cataclasite layers differs from that of mesoscopic fault arrays contraction and ext

discrete and diffuse heat transfer atashes vent field axial volcano juan de fuca ridge abstract heat fluxes of discrete and diffuse components of hydrothermal discharge are estimated from submersible measurement in the ashes vent field located in the caldera of axial volcano on the central juan de fuca ridge the discrete component comprises discharge from individual vents and the diffuse component comprises seepage from large areas of the seafloor the investigation centered on the high temperature portion of the field a 100 100 m area of fractured lobate and sheet lava flows encompassing discrete discharge 326 c from seven individual vents and diffuse flow from intervening areas a total of 4 4 2 10 6 w was estimated for the discrete heat flux based on measurements of effluent temperature flow rate and orifice diameter at all the known individual vents by d s v alvin in september 1987 a total of 15 75 10 6 w was calculated with a standard plume model for the diffuse flux based on a grid of temperature measurements at altitudes of 1 m and 20 m above the study area using a 1 m long vertical array of temperature sensors mounted at the front of alvin s instrument platform the study indicates that 1 the diffuse component rises separately from the discrete component of hydrothermal discharge and is laterally advected by a prevailing current below the discrete component and that 2 the diffuse component of convective heat flux is approximately an order of magnitude greater than the discrete component in the high temperature portion of the ashes vent field

chemical reaction rates and entrainment within the endeavour ridge hydrothermal plume the aging of the hydrothermal plume over the endeavour segment of the juan de fuca ridge was estimated by measuring the222rn3he ratio in the plume as it dispersed despite uncertainties in the source function of hydrothermal input it wa determined that the relative sequence of removal from the plume ish2 c 222rn ch4 mn where c is a measure of particle concentration and the mean life of222rn is 5 5 days h2 is removed from the plume within hours of input while mn is not removed within the two week timescale of the radon helium clock entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone this is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient like profile in the deep ocean thus we found that significant fractions of the si and226ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height

limited basement involvement in caledonian deformation hinnoy nnorth norway and tectonic implications abstract structural and petrographic relationships on east hinnoy north norway indicate that the caledonian nappes which are exposed directly to the east were emplaced over an older precambrian crystalline terrane lofoten terrane while at amphibolite facies metamorphic conditions caledonian structural and metamorphic effects disappear structurally downward away from the basal thrust of the nappe stack the limited involvement of the basement in caledonian deformation is explained by the limited availability of water which was introduced from external sources the external water source is hypothetically the autochthonous and allochthonous cover and the precambrian austerfjord group metasedimentary rocks within the basement which underwent prograde metamorphic dehydration during caledonian orogenesis the mechanisms by which addition of water concentrated strain in the basement probably include reaction induced ductility and hydrolytic weakening of the constituent silicate minerals there are two main tectonic implications 1 in collisional metamorphic belts the lithosphere of the underthrusted plate may remain rigid below 15 25 km depth and 2 the common phenomenon of detachment of crystalline thrust sheets at mid crustal levels may be controlled in some cases by the limited access of water to pre existing crystalline rocks rather than by thermal structure

the relationship between flow and permeability field in seafloor hydrothermal systems the permeability of oceanic crust is spatially variable and probably anisotropic as well using realistic permeability fields for young oceanic crust we have performed numerical simulations of finite amplitude steady and unsteady convective fluid flow in layered and or anisotropic porous media heated from below to investigate particular patterns of fluid flow and temperature in mid ocean ridge hydrothermal systems on the flanks of mid ocean ridges permeability measurements in deep sea boreholes suggest that only the top few hundred meters of oceanic crust is permeable given this permeability structure and assuming some minimum permeability and layer thickness our models predict that convection occurs in the form of numerous cells with aspect ratios of order unity within this permeable layer such convection results in fluid flux and diagenetic reactions within the permeable layer with a negligible effect on heat flow at the seafloor in agreement with field observations estimates of oceanic crust permeability based on hydrothermal veins in ophiolites and measurements in deep sea boreholes suggest that pillow basalts lava flows are much more permeable than underlying sheeted dikes this is particularly true at the ridge crest where voids have not yet collapsed and filled given this permeability structure our study suggest that a small percentage of the fluid entering the ridge crest circulates through the sheeted dikes before exiting the system at high temperatures in very focused discharge zones in our models these discharge zones narrow considerably at the interface between the sheeted dikes and the pillow basalts lava flows much of the fluid entering the system however never circulates below the pillow basalts lava flows and exits the seafloor at low temperatures in general discharge zones are more focused than recharge zones these results are consistent with observations of narrow and focused discharge zones in ophiolites and localized high temperature venting amid widespread low temperature flow on the ridge crest the spacing of upflow zones at the surface is strongly controlled by convective flow in the bottom permeable layer in our models near field effects dominate over far field effects in systems with lateral variations in permeability and no large scale flow develops between widely spaced areas of contrasting permeability the time to steady state and evolution of convective flow in porous media vary considerably with initial conditions our simulations suggest that the time to steady state is relatively long and that it is possible that hydrothermal convection at the ridge axis never reaches a steady state flow pattern in the sense that the variations in the system boundary conditions such as basal heat flux may occur on a time scale less than the response time of the hydrothermal system it is possible however that these systems may be quasi steady for significant periods of time

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

the deformational behaviour of bimineralic limestone halite aggregates abstract a compositional foliation banding develops within synthetic limestone halite aggregates experimentally deformed in axially symmetric compression and in simple shear the strength of the bulk aggregate decreases as the foliation becomes stronger foliation weakening in calcite rich specimens the development of foliation starts with the formation of cataclastic shear systems in calcite domains between isolated halite domains with increasing deformation halite is sheared into these systems the halite domains coalesce into halite rich shear systems and accommodate an increasing amount of the bulk deformation the bulk strength of bimineralic aggregates at a given amount of strain increases with decreasing halite content the influence of halite on the bulk strength is much greater than would be expected from its volume fraction and becomes dominant with increasing deformation due to the development of the foliation it is postulated that such processes also occur during natural deformation of polyphase rocks competent clasts within a weaker ductile matrix showing no recrystallization differ in shape and rotational behaviour from recrystallizing clasts such as feldspar augen these competent clasts denoted as type clasts may cause problems in deducing the sense and direction of shear when formed as almost exact mirror images of type recrystallizing clasts

hydrothermal particle plumes over the southern juan de fuca ridge hydrothermal particles originate in buoyant plumes emanating from seafloor thermal vents1 4 and accumulate as metalliferous sediments found along oceanic spreading centres5 7 observational evidence of the transport pathway of hydrothermal particles is scarce however and the structure extent and particle concentrations of hydrothermal plumes remain largely conjectural to evaluate the potential of hydrothermal plumes as particle transport agents we mapped their distribution along and across the 80 100 m deep axial valley of the southern juan de fuca ridge in the north east pacific ocean 44 30 n 130 w an area of known hydrothermal activity8 11 continuous light scattering measurements reported here defined numerous particle plumes centred 30 120 m above bottom m a b elemental analysis of the plume particles and the theta potential temperature s salinity signatures of the plume water verified their hydrothermal origin concentrations of hydrothermal particles remained elevated at ridge crest depths at least 100 km from the ridge indicating distant off axis dispersal of hydrothermal emissions

armalcolite bearing fe ti oxide assemblages in graphite equilibrated salic volcanic rocks with native iron from disko central west greenland native iron bearing strongly sediment contaminated andesitic to dacitic lavas from the malig t formation on disko contain an early phenocryst assemblage of plagioclase low ca pyroxene s and ilmenite the phenocrystic ilmenite has reacted to form complex fe ti oxide metal sulphide aggregates which contain one or more of the oxides ilmenite armalcolite and rutile the armalcolite is very similar to the lunar type 1 armalcolite of haggerty 1973 and approximate compositionally the ternary system feti2o5 mgti2o5 ti3o5 92 to 97 mol when evidence from several salic rocks is combined the fe ti oxide metal sulphide aggregates display reactions which may represent one isobaric invariant assemblage ilmenite armalcolite rutile iron and the 3 boundary univariant reactions in the system fe ti o the compositional and textural features of ferro magnesian silicates oxides and metals show that most rocks were affected by a rapidly declining fo2 during magma ascent and cooling as displayed in the fe ti oxide metal sulphide aggregates by the cross cutting of one or several t fo2 buffer curves in the system fe ti o prominent sulphidation reactions are observed in the oxide aggregates and are always of the type where feo in oxide is replaced by fes while the liberated oxygen is consumed in a reduction process carbon bound as graphite or cohenite occurs throughout the rocks and is mostly enclosed in phenocrysts and xenocrysts when the salic magmas ascended from pre eruption reservoirs at 1 to 1 5 kb the reduction was largely controlled by strongly pressure dependent carbon oxygen equilibria resulting in rapidly declining t fo2 paths recorded by the oxide assemblages in the simplified c o gas system carbon barometry sato 1979 applied to the selected rocks would indicate final equilibrium pressures of between 10 and 100 bars the scarcity of preserved graphite in the lavas would suggest that the carbon controlled reductions were terminated when available carbon was exhausted during the magma ascent and solidification

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

characteristics of hydrothermal plumes from two vent fields on the juan de fuca ridge northeast pacific ocean deep ctd transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally buoyant plumes emanating from each of two major vent fields on the southern symmetrical segment sss and endeavour segment es of the juan de fuca ridge at both vent fields emissions from point and diffuse hydrothermal sources coalesced into a single 200 m thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0 02 0 05 c and light attenuation anomalies of 0 01 0 08 m 1 10 80 g l above background temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped 10 15 km downcurrent light attenuation trends were non uniform and differed between plumes apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates vent fluids from the sss are metal dominated and mostly precipitate very fine grained hydrous fe oxides that remain suspended in the plume vent fluids from the es are sulfur dominated and precipitate a high proportion of coarser grained fe sulfides that rapidly settle from the plume the integrated flux of each vent field was estimated from measurements of the advective transport of each plume heat flux was 1700 1100 mw from the es and 580 351 mw from the sss particle flux varied from 546 312 g s to 204 116 g s at the es depending on distance from the vent field and was 92 48 g s from the sss

hydrothermal heat enhances abyssal mixing in the antarctic circumpolar current upwelling in the world s strongest current the antarctic circumpolar current is thought to be driven by wind stress surface buoyancy flux and mixing generated from the interaction between bottom currents and rough topography however the impact of localized injection of heat by hydrothermal vents where the antarctic circumpolar current interacts with mid ocean ridges remains poorly understood here a circumpolar compilation of helium and physical measurements are used to show that while geothermal heat is transferred to the ocean over a broad area by conduction heat transfer by convection dominates near hydrothermal vents buoyant hydrothermal plumes decrease stratification above the vent source and increase stratification to the south altering the local vertical diffusivity and diapycnal upwelling within 500 m of the sea floor by an order of magnitude both the helium tracer and stratification signals induced by hydrothermal input are advected by the flow and influence properties downstream

evolution of regional metamorphism in the cape smith thrust belt northern quebec canada interaction of tectonic and thermal processes syn metamorphic re imbrication of the internal part of thrust belts can result in distinct pressure temperature time deformation p t t d pathways for different structural metamorphic domains in the early proterozoic cape smith thrust belt canada an external piggyback sequence thrusting domain is characterized by thermal peak metamorphism occurring after deformation in contrast thermal peak metamorphism in an internal domain occurred during re imbrication by out of sequence thrusting the interactions of tectonic and thermal processes have been studied using three methods i qualitative evaluation of the timing between mineral growth and deformation ii analytical p t paths from growth zoned garnet porphyroblasts and iii numerical modelling of vertical heat conduction derived p t t d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading in contrast paths for the internal domain indicate that the out of sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain this is attributed to thickening by out of sequence thrusting and possibly to extensional faulting at now eroded higher structural levels observations on the timing of metamorphism coupled with numerical modelling suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out of sequence thrusts stack in the internal portion of the thrust belt

terrane boundaries within trans hudson orogen quebec baffin segment canada changing structural and metamorphic character from foreland to hinterland within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks and two assemblages of accreted paleoproterozoic units crustal terranes are separated by major tectonic boundaries and associated deformation zones field petrological and isotopic data document an increase in the width of the deformation zones and a concomitant increase in p t conditions of metamorphism from orogenic foreland to hinterland tectonostratigraphic and geochronological criteria allow footwall and hangingwall units to be consistently distinguished in spite of the changing structural and metamorphic aspect of the terrane boundaries across the width of the orogen

tracing southwest pacific bottom water using potential vorticity and helium 3 abstractthis study uses potential vorticity and other tracers to identify the pathways of the densest form of circumpolar deep water in the south pacific termed southwest pacific bottom water spbw along the 28 2 kg m 3 surface this study focuses on the potential vorticity signals associated with three major dynamical processes occurring in the vicinity of the pacific antarctic ridge 1 the strong flow of the antarctic circumpolar current acc 2 lateral eddy stirring and 3 heat and stratification changes in bottom waters induced by hydrothermal vents these processes result in southward and downstream advection of low potential vorticity along rising isopycnal surfaces using 3he released from the hydrothermal vents the influence of volcanic activity on the spbw may be traced across the south pacific along the path of the acc to drake passage spbw also flows within the southern limb of the ross gyre reaching the antarctic slope in places and contributes via entrainment to the formation of

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

fluid flow metasomatism and amphibole deformation in an imbricated ophiolite north cascades washington abstract metasomatic tremolite rich mylonites are widespread in imbricate thrust slices of ultramafic rocks of the ophiolitic ingalls complex in washington state protoliths for these amphibolite facies mylonites were peridotite and serpentinite abundant syntectonic tremolite veins in the ultramafites record narrowly channelized flow of infiltrating fluids whereas metasomatic mylonite zones record more pervasive flow fluids were probably released mainly by prograde devolatization reactions within serpentinite and mafic ophiolitic rocks that experienced earlier hydrothermal metamorphism olivine apparently deformed by dislocation creep in the mylonites in the tremolite rich rocks locally preserved amphibole porphyroclasts deformed mainly by microfracturing acicular tremolites which dominate the mylonites form syntectonic overgrowths on porphyroclasts and probably record diffusive mass transfer which may have accompanied cataclasis acicular tremolites subsequently were folded and define both post crystalline crenulations and polygonal arcs fluid flow deformation and metamorphism were apparently complexly interrelated in the imbricate zone thrusts juxtaposed contrasting rock types that were sources and sinks for fluids and shear zones focused fluid flow metamorphism probably facilitated deformation through the release of fluids during dehydration reactions high fluid pressure may have led to hydraulic fracturing and may have controlled strain softening in the tremolitic mylonite zones as it favored microcracking and diffusive mass transfer over dislocation creep infiltrating metasomatic fluids probably play an important role in the evolution of shear zones in many ultramafic bodies during medium grade metamorphism

locating hydrothermal vents by detecting buoyant advected plumes an improved method of detecting buoyant hydrothermal plumes and locating their source vents is introduced plumes are detected by computing fluid stability from conductivity temperature and depth measurements acquired during navigated towed vertically oscillating casts over the endeavour segment of the juan de fuca ridge for each instability detected the maximum range to its hydrothermal source is estimated by multiplying a theoretical plume equilibration time by a measured current velocity using an estimate of current direction the method reliably locates plume sources where they are known to exist in all of the focused vent fields mapped by submersible and in several isolated diffuse flow sites the method generates a distribution of hydrothermal sources that is more consistent with variations in surface permeability than with circulation cells spaced evenly along a uniformly permeable axis axial instabilities are nearly continuous along the heavily fissured and fractured western wall of the axial valley beyond the axial valley instabilities evidence that hydrothermal upflow penetrates the outer slopes of the ridge crest probably along a boundary between sheet and pillow flows