fluid inclusions and hydrothermal alteration on the dixie valley fault nevada footwall rocks of the 1954 rupture segment of the dixie valley fault show extensive hydrothermal alteration related to fluids that were present on the fault during tectonic events hydrothermal alteration of granitic host rocks consists of temporally and spatially overlapping mineral assemblages an early biotite feldspar assemblage is followed by later fe chlorite and epidote both chlorite and epidote are replaced by hydrothermal sericite and cross cut by calcite hematite and quartz calcite veins biotite is partially replaced by prehnite the latest hydrothermal minerals are stilbite laumontite kaolinite alunite smectite illite and pervasive replacement of rock units with fine grained quartz chalcedony and opal secondary fluid inclusions trapped in healed microfractures in igneous quartz include type i inclusions that contain a moderate salinity aqueous liquid and vapor type ii inclusions that contain a moderate salinity aqueous liquid and co2 type iii inclusions that show eutectic melting temperatures below the nacl h2o eutectic and contain substantial cacl2 and type iv inclusions containing halite and other daughter minerals microthermometric measurements on these inclusions yield variable compositions and homogenization temperatures salinities of type i inclusions vary from 0 1 to 12 9 wt nacl with the mode in the interval 0 to 1 salinities of type ii co2 bearing inclusions range from 0 62 to 6 81 wt nacl relative to h2o and salinities of type iii inclusions with low eutectic melting temperatures are 12 9 to 25 3 nacl equivalent wt salinities of halite bearing inclusions are 30 1 to 39 2 wt nacl homogenization temperatures span the range 120 to 400 c the processes of isochemical cooling with upward displacement of the footwall mixing of cool low salinity water with hotter components and mixing of cool evaporite brine with hotter components could be responsible for variable fluid inclusion compositions homogenization temperatures and densities the p t path of the fault fluids is established by mineral equilibria and fluid inclusion characteristics the path includes a lithostatic fluid pressure at 305 c and 1570 bars along with cooling and escape of co2 from fluids the fluid p t path probably approaches hydrostatic pressure conditions at lower temperatures hydrothermal alteration product minerals fluid temperatures pressures and compositions in the footwall of the dixie valley fault constrain minimum fault age to 20 to 25 ma displacement to 6 km with about 3 km of pre 10 to 13 ma and 3 km of post 10 ma uplift fluid compositions and p t data suggest the following mechanism for rupture initiation and arrest ruptures may be initiated as a result of high fluid pressures then opening of dilatant fractures causes drastic decrease in fluid pressure separation of steam and co2 the drastic reduction in fluid bulk modulus that accompanies volatile phase separation permits propagation of the ruptures even though fluid pressure is reduced in areas where fluid pressure reduction is not accompanied by phase separation fractures are arrested by dilatant hardening

discussion on mississippi valley type lead zinc deposits through geological time implications from recent age dating research by d l leach d bradley m t lewchuk d t a symons g de marsily and j brannon 2001 mineralium deposita 36 711 740

fluid inclusion evidence for minimum 11 km vertical offset on the wasatch fault utah the footwall of the wasatch fault in the corner creek area near salt lake city is hydrothermally altered and deformed quartz monzonite of the oligocene little cottonwood stock secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation thermometric measurements of fluid inclusion characteristics indicate entrapment of a co2 h2o nacl fluid at minimum temperatures of 223 353 c and minimum fluid pressures of 900 2800 bar the 2800 bar fluid pressure is near lithostatic pressure at a depth of 11 km the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates the large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall and by geologic reconstruction of the eroded footwall

eruptive history and structural development of the toquima caldera complex central nevada the toquima caldera complex located in the toquima range of central nevada consists of three overlapping to nested calderas the moores creek caldera is the largest 30 by 20 km it formed 27 2 ma in response to eruption of the high silica rhyolite tuff of moores creek because of recurrent volcanic activity and subsequent basin range faulting only the northern segment of the moores creek caldera is preserved its eastern and western margins are downfaulted below valley fill and its southern part was obscured by collapse of the mount jefferson caldera eruption of the tuff of mount jefferson resulted in collapse of the 18 by 20 km mount jefferson caldera 26 5 ma the ash flow tuff exposed at round mountain is a silicic outflow facies equivalent of the compositionally zoned 76 67 wt sio 2 intracaldera tuff of mount jefferson pyroclastic eruptive activity in the complex concluded 23 6 ma with formation of the comparatively small 8 by 10 km trail canyon caldera with time caldera size diminished and the focus of volcanic activity shifted progressively southeastward the southeastward migration of volcanism in the complex approximately parallels regional northwest striking faults suggesting fundamental structural control in the rise and eruption of magma arcuate faults and small aphyric to porphyritic plugs outline the structural margins of the calderas some plugs on the basis of gradational textural changes from vitroclastic in the tuff to flow layered and porphyro aphanitic in the plug appear to represent lava choked ash flow tuff feeder vents caldera collapse breccias are locally well exposed in all three calderas caldera resurgence is not strongly developed in the toquima caldera complex the mount jefferson massif which dominates the complex at an elevation of 3 640 m is an artifact of young basin range block faulting the limited development of resurgence in the mount jefferson caldera is attributed to ponding of the bulk of erupted material within the caldera intermediate to mafic volcanic rocks premonitory or coeval to silicic ash flow tuff are volumetrically minor over central nevada and are lacking in the toquima caldera complex the less silicic magma apparently became lodged in the middle to lower crust below an extensive zone of more silicic less dense magma

plutonism in three orogenic pulses eastern blue ridge province southern appalachians department of geological sciences university of north carolina mitchell hall chapel hill nc 27599 3315

geology and k ar geochronology of the paradise peak mine and the relationship of pre basin and range extension to early miocene precious metal mineralization in west central nevada the paradise peak mine is a major gold silver mercury deposit located in the southwestern part of the paradise range near the eastern edge of the walker lane in the western great basin nevada the tertiary volcanic rocks of the area can be divided into a sequence of 26 to 24 ma intermediate lavas a sequence of 24 to 22 ma silicic ash flow tuffs and a sequence of 20 to 15 ma intermediate lavas we classify these rocks as the older middle and younger sequences sedimentary rocks and latites and basalts 12 ma old or younger locally overlie the younger intermediate lavas south of the mine silicified rhyolite tuff in the lower part of the middle tuff sequence is the principal host of the precious metal ore most ore occurs under an acid leached zone of alunitic alteration and is contained in hydrothermal breccias that crosscut early quartz pyrite and alunite alteration field relations and k ar ages of hypogene alunite indicate that precious metal mineralization and alunitic and silicic alteration formed at about 19 to 18 ma several million years after eruption of the main host rock mineralization formed during high angle faulting related to crustal extension low angle normal faults are present 5 km northeast of the paradise peak mine but they formed more than 2 ma after formation of the paradise peak deposit silicic and alunitic alteration and precious metal mineralization are generally absent in areas of low angle faulting regional stratigraphic relations and k ar ages indicate that volcanism changed from silicic ash flow tuffs to intermediate lavas at about 20 to 19 ma regionally extensive angular unconformities indicate that a period of pre basin and range crustal extension occurred between about 22 to 19 ma this extension was penecontemporaneous with the shift in the style of volcanism and with gold silver mineralization in the paradise peak mine and in the goldfield and tonopah districts of western nevada the close temporal and spatial relationships of precious metal mineralization with pre basin and range extension suggest that extension was a major factor in the genesis of early miocene precious metal deposits in the western great basin

fault valve action and vein development during strike slip faulting an example from the ribeira shear zone southeastern brazil abstract fluid inclusion microthermometry and structural data are presented for quartz vein systems of a major dextral transcurrent shear zone of neoproterozoic cambrian age in the ribeira river valley area southeastern brazil geometric and microstructural constraints indicate that foliation parallel and extensional veins were formed during dextral strike slip faulting both vein systems are formed essentially by quartz and lesser contents of sulfides and carbonates and were crystallized in the presence of co2 ch4 and h2o co2 ch4 nacl immiscible fluids following unmixing from a homogeneous parental fluid contrasting fluid entrapment conditions indicate that the two vein systems were formed in different structural levels foliation parallel veins were precipitated beneath the seismogenic zone under pressure fluctuating from moderately sublithostatic to moderately subhydrostatic values 319 397 c and 47 215 mpa which is compatible with predicted fluid pressure cycle curves derived from fault valve action growth of extensional veins occurred in shallower structural levels under pressure fluctuating from near hydrostatic to moderately subhydrostatic values 207 218 c and 18 74 mpa which indicate that precipitation occurred within the near surface hydrostatically pressured seismogenic zone fluid immiscibility and precipitation of quartz in foliation parallel veins resulted from fluid pressure drop immediately after earthquake rupture fluid immiscibility following a local pressure drop during extensional veining occurred in pre seismic stages in response to the development of fracture porosity in the dilatant zone late stages of fluid circulation within the fault zone are represented dominantly by low to high salinity 0 2 to 44 wt equivalent nacl h2o nacl cacl2 fluid inclusions trapped in healed fractures mainly in foliation parallel veins which also exhibit subordinate h2o nacl cacl2 co2 ch4 and h2o co2 ch4 nacl fluid inclusions trapped under subsolvus conditions in single healed microcracks recurrent circulation of aqueous carbonic fluids and aqueous fluids of highly contrasting salinities during veining and post veining stages suggests that fluids of different reservoirs were pumped to the ruptured fault zone during faulting episodes a fluid evolution trending toward ch4 depletion for co2 ch4 bearing fluids and salinity depletion and dilution approximation of the system h2o nacl for aqueous saline fluids occurred concomitantly with decrease in temperature and pressure related to fluid entrapment in progressively shallower structural levels reflecting the shear zone exhumation history

implications of selected subsurface data on the structural form and evolution of some basins in the northern basin and range province nevada and utah

quartz veining in slates and variscan deformation insights from the luarca sector nw spain abstract a structural and geochemical analysis of quartz veins is made in order to determine their evolution and the physical chemical conditions that enabled their development in this sector of the variscan belt westasturian leonese zone three phases of deformation have been described however only the first and third phases are represented in the study sections the first phase d 1 resulted in tight or closed folds f 1 verging towards the foreland and associated slaty cleavage s 1 the third phase d 3 resulted in approximately upright asymmetric folds f 3 with associated crenulation cleavage s 3 the veins are hosted in slates and approximately follow the slaty cleavage s 1 the veins started their development at the beginning of the deformation phase d 3 as a result of s 1 near parallel shortening through a process of progressive deformation this compression gave rise to the folding of the cleavage and eventually of the veins the f 3 folds have associated crenulation cleavage parallel to the axial planes in some cases the location of the veins was controlled by irregularities in the bedding due to sedimentary structures the quartz of the veins underwent notable intracrystalline plastic deformation and the contact zone between the veins and the host rock was affected by pressure solution the microstructures produced by the latter mechanism indicate the greatest compressive stress forming a high angle with the vein walls at a time post dating vein generation quartz precipitated from an aqueous carbonic fluid at temperatures between 350 c and 375 c under fluid pressure fluctuations of up to 140 mpa at constant depth maximum values of fluid pressure of 220 mpa and minimum values of 75 mpa were recorded by fluid inclusion assemblages in quartz of the veins under study pressure fluctuation from lithostatic to infralithostatic at constant depth was caused by the opening and sealing of the dilatant fractures

feldspar influenced rock rheologies progressive fracture subsequent cataclasis and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz mica phyllonite during early tertiary deformation in a 20 30 m wide shear zone in southeastern arizona the ductile deformation proceeded at temperatures and pressures approximating middle to upper crustal levels most of the deformation was taken up by the feldspar and its alteration products quartz deformed by both brittle fracture and crystal plastic mechanisms these results show that under certain temperature pressure strain rate conditions feldspars are weaker than quartz the observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature activated plastic deformation in quartz nor of the frictional behavior of rocks as inferred in the familiar strength vs depth curves it is more likely that the transition is complex and depends on a variety of parameters one such factor delineated in this study is the fracture and fracture facilitated syntectonic alteration of feldspar under hydrated conditions

petrogenesis and structure of the buck creek mafic ultramafic suite southern appalachians constraints on ophiolite evolution and emplacement in collisional orogens integration of detailed field petrological structural and geochemical observations of the buck creek mafic ultramafic suite which is among the larger southern appalachian ultramafic exposures points to an igneous origin as a mid ocean ridge cumulate massif with subsequent emplacement in a deep subduction zone setting these results help to clarify the unresolved role of alpine type ultramafic bodies in the southern appalachians a problem complicated by their generally small and fragmented nature and by significant tectonic overprinting as an ophiolite fragment the buck creek rocks are unusual in their preservation of relatively high pressure anhydrous conditions and thus they provide constraints on mechanisms of ophiolite emplacement field support for a cumulate origin for the buck creek suite includes centimeter to meter scale interlayering of dunite troctolite anorthosite and spinel rich layers and local gradation between buck creek troctolitic rocks and surrounding amphibolites relict anorthitic plagioclase forsteritic olivine and augitic clinopyroxene define a cumulate triangle on an mgo versus al 2 o 3 diagram meta troctolite and meta dunite compositions define a linear array between olivine and plagioclase compositions and many amphibolites fall within this triangle reflecting gabbroic protoliths the relatively abundant troctolite cumulates scarce pyroxene in the ultramafic rocks and the high al 2 o 3 tio 2 and low cr numbers in spinel best match lot or l type lherzolite ophiolites consistent with crystallization in a slow spreading mid ocean ridge setting sapphirine bearing spinel symplectites in the metatroctolites suggest that buck creek rocks remained anhydrous to 800 c and 0 9 1 1 gpa representing conditions atypical of ophiolite emplacement comparisons to the zermatt saas ophiolite and bergen eclogite suggest rapid subduction of buck creek rocks to depths of 30 km where partial hydration perhaps facilitated by pro grade dehydration reactions in surrounding rocks caused strain softening that permitted ductile disaggregation and aided in their emplacement into the overlying accretionary complex alternatively emplacement might have resulted from a switch in subduction polarity with hydration following emplacement uplift shortening and hydration within the accretionary sequences are responsible for the dominant structural features

patterns and timing of late quaternary faulting in the great basin province and relation to some regional tectonic features large scale surface faulting events each tens of kilometers long and involving more than a meter of displacement of late quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the great basin province furthermore faulting apparently has not been uniformly distributed over time the average recurrence interval of large scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years in contrast to such long recurrence intervals in the central nevada and eastern california seismic belts large scale faulting events have occurred during the last 111 years in an apparently coherent sequential belt filling pulse of activity in which events were separated by only a few years or a few decades the questions are where are other pulses of large scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the black rock carson sink zone of extension and the central nevada downwarp the central nevada seismic belt lies along the dividing line between these two subprovinces and the northwest margin of the black rock carson sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern nevada

geometry of cenozoic extensional faulting dixie valley nevada precise definition of geometric relationships between individual basins and ranges may help to reveal the mechanical processes of basin and range cenozoic extensional faulting at depth previous studies have attempted to identify simple horsts and grabens tilted crustal blocks with planar faulting or tilted crustal blocks with listric faulting in the shallow crust normal faults defining these crustal blocks may root 1 individually in the ductile lower crust 2 in regional or local low angle detachment faults or 3 in igneous intrusions or decoupling surfaces produced by the intrusions the present study in dixie valley west central nevada makes use of a seismic reflection survey gravity models seismograms from earthquakes occurring on december 16 1954 and geometrical block models these data show a structurally asymmetric basin bounded by a single zone of faulting on the northwest and by a downbowed and step faulted floor to the southeast the northwest bounding fault is moderately dipping 50 and planar to a depth of 3 km the southeast boundary is step faulted and altogether the faults indicate an extension of 20 across the valley at the rate of 0 38 mm y for the last 8 my synthetic earthquake seismograms confirm a focal depth of 15 km and fault dip of 62 for the fairview peak earthquake and suggest that the focal depth of the dixie valley earthquake was also 15 km instead of the previously reported 40 km local microearthquakes cluster around 10 15 km the geometrical block models indicate that crustal horst graben faulting and planar high angle normal faults rooted in a low angle detachment surface do not readily account for development of the subsidiary step faults found in dixie valley extension of the crust by intrusion may develop high angle faults and with further intrusion may develop the subsidiary faults and produce a complex sagged asymmetric graben like dixie valley

chronology of paleozoic metamorphism and deformation in the blue ridge thrust complex north carolina and tennessee the blue ridge province in northwestern north carolina and northeastern tennessee records a multiphase collisional and accretionary history from the mesoproterozoic through the paleozoic to constrain the tectonothermal evolution in this region radiometric ages have been determined for 23 regionally metamorphosed amphibolites granitic gneisses and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone the garnet ages from the pumpkin patch a thrust sheet 458 455 and 451 ma are similar to those from the structurally overlying spruce pine thrust sheet 460 456 455 and 450 ma both thrust sheets exhibit similar upper amphibolite facies conditions because of the high closure temperature for garnet the garnet ages are interpreted to date growth at or near the peak of taconic metamorphism devonian metamorphic ages are recognized in the spruce pine thrust sheet where sm nd and rb sr garnet ages of 386 and 393 ma and mineral isochron ages of 397 14 and 375 27 ma are preserved hornblendes record similar sup 40 ar sup 39 ar sm nd and rb sr ages of 398 to 379 ma devonian sup 40 ar sup 39 ar hornblende ages are also recorded in the structurally lower pumpkin patch thrust sheet the devonian mineralmore ages are interpreted to date a discrete tectonothermal event as opposed to uplift and slow cooling from an ordovician metamorphic event the mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the alleghanian orogeny the composite thrust stack of the blue ridge complex was subsequently thrust northwestward along the linville falls fault during middle alleghanian orogeny about 300 ma less

formation of monazite via prograde metamorphic reactions among common silicates implications for age determinations three lines of evidence from schists of the great smoky mountains nc indicate that isogradic monazite growth occurred at the staurolite in isograd at 600 c 1 monazite is virtually absent below the staurolite in isograd but is ubiquitous several hundred grains per thin section in staurolite and kyanite grade rocks 2 many monazite grains are spatially associated with biotite coronas around garnets formed via the reaction garnet chlorite muscovite biotite plagioclase staurolite h2o 3 garnets contain high y annuli that result from prograde dissolution of garnet via the staurolite in reaction followed by regrowth and rare monazite inclusions occur immediately outside the annulus and in the matrix but not in the garnet core larger monazite grains also exhibit quasi continuous th zoning with high th cores and low th rims consistent with monazite growth via a single reaction and fractional crystallization during prograde growth common silicates may host sufficient p and lrees that reactions among them can produce observable lree phosphate specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section lrees may be more limiting but sheet silicates and plagioclase can contain tens to 100 ppm lree so recrystallization of these silicates to lower lree contents could produce hundreds of grains of monazite per thin section monazite ages determined via electron and ion microprobes are 400 ma directly linking prograde barrovian metamorphism of the western blue ridge with the acadian orogeny in contrast to previous interpretations that metamorphism was taconian 450 ma interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite forming reactions including reactions previously viewed as involving solely common silicates

geophysical confirmation of low angle normal slip on the historically active dixie valley fault nevada the december 16 1954 dixie valley earthquake ms 6 8 followed the nearby fairview peak earthquake ms 7 2 by 4 min 20 s waveforms from the fairview peak event contaminate those from the dixie valley event making accurate fault plane solutions impossible a recent geologic study of surface rupture characteristics in southern dixie valley suggests that the dixie valley fault is low angle 30 along a significant portion of the 1954 rupture to extend these observations into the subsurface we conducted a seismic reflection and gravity experiment our results show that a portion of the dixie valley ruptures occurred along a fault dipping 25 to 30 as such the dixie valley event may represent the first large low angle normal earthquake on land recorded historically our high resolution seismic reflection profile images the rupture plane from 5 to 50 m depth medium resolution reflections as well as refraction velocities show a smoothly dipping fault plane from 50 to 500 m depth stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth gravity profiles conservatively constrain maximum basin depth and define overall geometry extension along the low angle section may have occurred in two phases during the cenozoic current fault motion postdates a 13 to 15 ma basalt imaged in the hanging wall and inherits from a fault formed during an earlier extensional pulse concentrated at 24 2 to 24 4 ma the earlier extension suggests extraordinary slip rates as high as 18 mm yr resulting in the formation of the low angle fault break sections of the dixie valley fault where there is no evidence for current low angle slip correlate well with areas where no pre 15 ma slip has been documented

evolution of metamorphic fluids at the variscan fold and thrust belt in eastern belgium abstract quartz veins in the southern greenschist facies metamorphic border of the stavelot venn massif belgium have been examined a detailed petrographical study of these quartz veins indicates the presence of eleven quartz stages a microthermometric analysis shows the expulsion of three main fluid types during the palaeozoic in the first quartz stage i of caledonian age mixed aqueous gaseous h 2 o co 2 nacl fluid inclusions are present the post caledonian and pre variscan second stage ii is characterised by gaseous co 2 n 2 inclusions occurring along inter crystal grain boundaries and by aqueous h 2 o nacl intra crystal inclusions the quartz stages iii vi which formed during the variscan period contain primary mixed aqueous gaseous h 2 o co 2 nacl inclusions all with an equal bulk chemical composition post variscan quartz stages vii xi could not be investigated microthermometrically the combination of microthermometry chlorite geothermometry and geothermal gradients reveals minimum trapping conditions of 200 mpa and 200 c for stage i quartz and 175 275 mpa and 385 435 c for quartz stages iii vi the trapping conditions of stage ii quartz could not be determined due to the strong recrystallisation of the quartz veins at the lower metamorphic variscan thrust front distinct temperature anomalies are associated with thrust faults as indicated by conodont colour alteration index cai vitrinite reflectance data chlorite geothermometry and fluid inclusion microthermometry the expulsion of metamorphic fluids generated at the southern part of the stavelot venn massif could have caused the distinct temperature anomalies along variscan thrust faults alternatively some of these anomalies could have been induced by deeply circulating meteoric fluids

geochronological evolution of the moyar bhavani and palghat shear zones of southern india implications for east gondwana correlations abstract the amphibolite facies moyar bhavani and palghat shear zones separate the south indian dharwar craton dc from the nilgiri and madurai high grade crustal provinces we present sm nd and rb sr isotopic whole rock mineral and small slab data for remnant granulites mylonitic gneisses and granitoids that reveal a neoproterozoic tectonothermal imprint in these shear zones sm nd garnet crystallisation ages and rb sr mica cooling ages from reworked rocks suggest that pan african structurally controlled retrogression and successive cooling in the moyar shear zone msz garnet 624 591 ma muscovite 594 ma biotite 604 540 ma predates tectonometamorphism in the bhavani shear zone bsz garnet 552 ma biotite 521 491 ma and palghat shear zone psz garnet 521 ma biotite 488 485 ma a rb sr small slab isochron age of 624 ma indicates that pan african retrogression in the msz also led to isotopic reequilibration of the rb sr system on a cm scale a sm nd garnet crystallisation age of 513 5 ma reflects emplacement of a postdeformative pegmatite in the bsz during the late cambrian a sm nd garnet age of 2355 18 ma obtained from a granulite remnant in the msz records early palaeoproterozoic high grade metamorphism which may be related to the amalgamation of the nilgiri block and dc along the palaeo moyar zone a relict charnockite from the southern bsz yielded a reproducible sm nd garnet age of 1705 11 ma presumably recording late palaeoproterozoic metamorphism neoarchaean and palaeoproterozoic nd model ages 3 0 2 2 ga suggest that the shear zone rocks are mainly reworked material from the adjacent nilgiri and madurai crustal domains nd model ages of granites and tonalites which intruded the msz syntectonically provide evidence for 1 7 ga crust generation unsheared metasediments exposed between the bhavani and palghat high strain zones were subjected to pan african high grade metamorphism as constrained by sm nd age data of garnets grown during coaxial deformation at 610 560 ma sm nd whole rock data from these supracrustal rocks indicate average crustal residence ages of 1 9 ga which implies contributions from an unknown proterozoic crustal source and disproves their previously suggested correlation to the archaean sargur group these model ages provide an upper limit for the juxtaposition of dc and madurai block along the bsz and psz

synthetic fluid inclusions in natural quartz i compositional types synthesized and applications to experimental geochemistry synthetic fluid inclusions having a wide range of geologically applicable compositions in the c o h s na k ca cl cu fe system have been formed by healing fractures in inclusion free natural quartz and by precipitating new quartz overgrowths on an original seed crystal inclusion types synthesized include 1 1 liquid rich pure h2o inclusions of primary and secondary origin 2 2 pure h2o inclusions with the critical density 3 3 liquid rich inclusions containing undersaturated aqueous solutions of nacl or kcl or cacl2 or mixtures of the three salts 4 4 h2o nacl inclusions containing halite daughter minerals 5 5 h2o nacl kcl inclusions containing halite and sylvite daughter minerals 6 6 h2o co2 inclusions of various compositions containing liquid h2o and either co2 vapor or co2 liquid or both at 25 c 7 7 h2o co2 nacl inclusions containing an aqueous phase liquid and vapor co2 and halite at 25 c 8 8 c o h inclusions containing liquid h2o a co2 ch4 liquid phase and graphite daughter minerals at 25 c 9 9 h2o nacl inclusions that contain a chalcopyrite daughter mineral and 10 10 inclusions representing trapping of the coexisting immiscible fluids in the h2o nacl h2o co2 nacl and na c o h systems the inclusions exhibit uniform phase ratios at room temperature and the temperatures of various phase changes within individual inclusions agree with those predicted from experimental and theoretical data indicating that the inclusion fluid has the same composition and density as the parent solution these miniature autoclaves thus may be used to study various problems related to fluid inclusion research to calibrate analytical equipment used to study natural inclusions and to study phase equilibria solubility and pvt relations of a variety of chemical systems

fault rocks and past to recent fluid characteristics from the borehole survey of the nojima fault ruptured in the 1995 kobe earthquake southwest japan the mineralogy fluid inclusions and distribution of fault rocks of the nojima fault were examined in the core recovered from a borehole drilled by the geological survey of japan gsj 12 months after the 1995 kobe hyogo ken nanbu earthquake mjma 7 2 in southwest japan the borehole was drilled across a slipped portion of the fault to a depth of 746 7 m nearly continuous coring between 152 2 and 746 7 m recovered granodiorite protolith porphyry dikes and fault related rocks the fault zone was intersected at 426 2 m and is characterized by a greater intensity of brittle deformation and or hydrothermal alteration than typical host granodiorite the fault core consists of three types of fault gouge and occurs at the depth range of 623 1 to 625 3 m the fault normal thicknesses of the fault core and the fault zone are 0 3 m and 46 5 m respectively three types of hydrothermal alteration are recorded by mineral assemblages and fluid inclusions the first type is characterized by chloritization of mafic minerals at 200 c and occurred prior to the fault activity during the intrusion and cooling of the granodiorite the second type occurred during faulting and is recorded by zeolite mineralization at 200 c the third type is recorded by carbonate mineralization related to recent fluid flow although most of the second type of alteration occurred prior to the third type repeated mineralization is recorded by mutually crosscutting relationships between zeolite and carbonate veins and between zeolite vein and carbonate precipitated fault gouge this may record repeated changes in fluid chemistry within the fault zone in connection with the seismic cycle although the nojima fault slipped in the 1995 earthquake ancient fault related textures and mineral alteration are well preserved in the fault rocks

ion probe and fluid inclusion evidence for co seismic fluid infiltration in a crustal detachment we have investigated the geochemical pattern of fluid infiltration in the extensional detachment of tinos island cyclades greece ion microprobe o isotope analyses and fluid inclusion studies have been conducted in strain fringes developing around pyrite blasts in the mylonite of the shear zone micro scale traverses in quartz and calcite fibres show that 18o increases from 17 18 to 20 21 in 1 mm towards the blast drops of 3 in 200 m then rises again in the direction of growth 18o variations are interpreted as transient influxes of exotic fluids into the shear zone between periods of closed system buffering by the host rock fluid inclusions trapped in the fibres show fluctuating salinities 0 4 wt nacl eq and densities that reflect drops of the pore pressure from lithostatic 1 to hydrostatic 0 4 values during fringe growth isotopic and microthermometric data are consistent with models of seismic pumping developed for compressive shear zones we therefore suggest that co seismic pore pressure variations developed suction forces sufficient to drive large scale fluid migration in the tinos detachment as in convergent tectonic settings

timing and deformation conditions of the tallulah falls dome ne georgia implications for the alleghanian orogeny structural domes composed of crystalline rocks metamorphosed to midcrustal conditions are present in every major orogen and record deformation of the deep interior of mountain ranges the tallulah falls dome in northeast georgia is a shear zone bounded body of metasedimentary rocks that is defined by a foliation pattern that crosscuts the regional fabric of the eastern blue ridge crystalline nappe in the southern appalachian mountains previously the tallulah falls dome was interpreted as the expression of duplexed underlying slices of laurentian margin sedimentary rocks imbricated during alleghanian emplacement of the allochthonous blue ridge thrust sheet under greenschist facies conditions we provide new observations of quartz and feldspar recrystallization fabrics and new 40 ar 39 ar dates from muscovite that complement existing thermochronometric ages to determine the timing and conditions of doming within the tallulah falls dome our microstructural observations are consistent with deformation mechanisms active dominantly at amphibolite facies conditions with some samples recording temperatures in excess of 650 c thermochronometric ages define a period of relatively rapid cooling from 550 c to 340 c within and around the tallulah falls dome between 321 and 317 ma with differential cooling across the northwestern dome bounding shear zone lasting until at least 313 ma our observations reveal a period of early alleghanian cooling from amphibolite to greenschist facies conditions associated with displacement along a shear zone bounding the tallulah falls dome across the blue ridge the alleghanian orogeny has long been characterized only by thrusting at greenschist facies conditions similarly dome formation has been interpreted as the result of duplexing within the thin skinned blue ridge crystalline thrust sheet as it was emplaced upon the laurentian margin both of these interpretations are inconsistent with the new data presented here which suggest more complex alleghanian deformation over a broader range in temperatures and perhaps a diverse suite of structural styles our results are best explained by doming of the footwall of a normal displacement shear zone at amphibolite facies conditions during the earliest alleghanian period

fracturing and hydrothermal alteration in normal fault zones large normal fault zones are characterized by intense fracturing and hydrothermal alteration displacement is localized in a slip zone of cataclasite breccia and phyllonite surrounding corrugated and striated fault surfaces slip zone rock grades into fractured but less comminuted and hydrothermally altered rock in the transition zone which in turn grades abruptly into the wall rock fracturing and fluid flow is episodic because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes

paleomagnetic evidence for counterclockwise block rotation in the north nevada rift region paleomagnetic data from mid miocene dikes and flows at three localities within the north nevada rift indicate that some crustal blocks have rotated approximately 19 degree plus minus 7 degree counterclockwise relative to stable north america as one possible consequence a revised mid miocene extension direction for the area is about n89 degree e s89 degree w indicating an approximately 25 degree difference between mid miocene and modern least principal stress directions the rotation may be accommodated by a right lateral component of slip on northwest trending oblique slip faults these as well as data from other parts of the basin and range province indicate that strike slip faulting and associated block rotation contributed to cenozoic extension

reaction weakening and emplacement of crystalline thrusts diffusion control on reaction rate and strain rate abstract in the southern appalachians the blue ridge piedmont crystalline thrust sheet was emplaced onto low grade late precambrian and paleozoic sedimentary rocks in the footwall along a basal detachment consisting of phyllosilicate rich mylonites phyllonites the phyllonites developed first by mechanical breakdown of feldspar followed by chemical breakdown to white mica in the presence of a pore fluid diffusion of solute in the pore fluid is the rate limiting step in controlling reaction rate and also the strain rate assuming solute diffusion follows the stokes einstein equation the shear strain rate is given by t 2 n k t 5 r x 2 for shear stress 20 mpa where n is a constant is a geometric factor k is boltzmann s constant t is absolute temperature is water viscosity r is the atomic radius of the diffusing species and x is the diffusion distance a bulk diffusion coefficient in the range of 10 10 to 10 12 m 2 s over distances of 10 100 m results in strain rates of 10 14 to 10 13 s 1 in the temperature range 200 400 c it is concluded that greenschist grade crystalline thrust sheets develop on pre existing basement faults that become weak during reaction softening and localize into high strain phyllonite zones in which pore fluid diffusion controls reaction rate and strain rate

geodetic evidence for continuing tectonic activity of the carboneras fault se spain abstract the carboneras fault zone cfz is a prominent onshore offshore strike slip fault that forms part of the eastern betic shear zone ebsz located in se spain in this work we show for the first time the continuing tectonic activity of the cfz and quantify its geodetic slip rates using continuous and campaign gps observations conducted during the last decade we find that the left lateral motion dominates the kinematics of the cfz with a strike slip rate of 1 3 0 2 mm yr along the n48 direction the shortening component is significantly lower and poorly constrained recent onshore and offshore paleoseismic and geomorphic results across the cfz suggest a minimum late pleistocene to present day strike slip rate of 1 1 mm yr considering the similarity of the geologic and geodetic slip rates measured at different points along the fault the northern segment of the cfz must have been slipping approximately at a constant rate during the quaternary regarding the eastern alpujarras fault zone corridor afz located to the north of the cfz our gps measurements corroborate that this zone is active and exhibits a right lateral motion these opposite type strike slip motion across the afz and cfz is a result of a push type force due to nubia and eurasia plate convergence which in turn causes the westward escape of the block bounded by these two fault zones

paleozoic age of the walden creek group ocoee supergroup in the western blue ridge southern appalachians implications for evolution of the appalachian margin of laurentia in the foothills of the great smoky mountains in tennessee low grade metamorphic rocks of the walden creek group ocoee supergroup contain a fossil assemblage composed of trilobites bryozoans ostracodes pelmatozoans algae and agglutinated foraminifers indicating a silurian or younger age the new paleontological data contradict older ideas on the neoproterozoic age of the walden creek group the walden creek group was not deposited in a continental rift basin preceding the opening of the iapetus ocean but possibly in a pull apart basin formed in a transtensional event during the acadian orogeny there is no field evidence indicating that the walden creek group is separated from the lower units of the ocoee supergroup by a basin wide unconformity and hiatus a tentative geodynamic model is presented for the ocoee supergroup basin the model is dependent on paleontological data indicating a paleozoic age for the walden creek group and concentrates on lithologic petrographic and basin characteristics the model allows for the possibility that the entire ocoee supergroup was deposited in a paleozoic transtensional basin

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

late quaternary history of the stillwater seismic gap nevada abstract fault scarps that formed during the 1915 and 1954 earthquakes in central nevada are roughly aligned but are separated by a gap of 40 km the gap contains no historical scarps and has been termed the stillwater seismic gap the abundance size and distribution of prehistorical fault scarps is similar in the stillwater seismic gap and the 1915 and 1954 earthquake areas and overall height and long term uplift rates of range blocks are similar throughout the area studied thus the faults in the gap do not appear to be locked or appreciably more or less active on a long term basis than those in areas of the 1915 and 1954 earthquakes none of the scarps in the gap is likely younger than about 300 yr so that no major earthquake occurred in that period the stillwater seismic gap is part of the central nevada eastern california seismic belt where large scale surface faulting events have occurred at intervals of a few years to a few decades in a pattern that suggests incremental filling of the belt if that pattern of belt filling continues the stillwater gap is a likely site for future major faulting but the low level of seismicity in the gap area suggests that the next major earthquake there is not imminent

surface faulting of the 1954 fairview peak ms 7 2 and dixie valley ms 6 8 earthquakes central nevada abstract the 1954 fairview peak earthquake was followed 4 min and 20 sec later by the dixie valley earthquake surface ruptures were distributed among six different faults that define a complex north trending zone 100 km long and over 15 km wide the rupture sequence progressed from south to north the initial event produced a 32 km rupture of right oblique slip along the east dipping fairview fault where the components of right lateral offset and vertical separation reached 2 9 and 3 8 m respectively the northernmost ruptures occurred during the subsequent event along a 46 km portion of the east dipping dixie valley fault where the sense of slip was normal and reached a maximum vertical separation of 2 8 m average surface offset for both events is about 1 2 m the west dipping west gate louderback mountains and gold king faults also ruptured during the earthquake sequence these faults form a complex structural linkage within a 15 km left step that separates the surface traces of the fairview and dixie valley faults the west dipping faults likely played an important role in the redistribution of static stress from the fairview peak earthquake and subsequent triggering of the dixie valley earthquake right lateral offsets of more than a meter occurred along both the west gate and louder back mountains faults slip was limited to normal motion of 1 m or less along the gold king fault the change from right oblique motion along the fairview fault to pure normal motion on the dixie valley fault may reflect the transition from principally northwest extension in the south to more westerly extension in the north alternatively slip at the latitude of the dixie valley fault may be partitioned between pure normal slip along the dixie valley fault and strike slip motion on faults immediately to the west observations of fault dip provide insight to the complex three dimensional geometry of the rupture sequence of particular note a significant section of the dixie valley fault appears to be characterized by dip angles as low as 25 at the surface slip variation or earthquake endpoints do not generally appear to have been influenced by changes in bedrock type we do not observe systematic relations between slip variation and aspects of fault trace geometry such as salients and reentrants yet the relative competence of footwall bedrock appears to have influenced the long term development of large salients and reentrants along both the dixie valley and the fairview faults as well as the style of rupture along the southern portion of the dixie valley fault

feasibility of estimating cementation rates in a brittle fault zone using sr ca partition coefficients for sedimentary diagenesis abstract cement phases such as calcite or quartz often incorporate trace elements from the parent fluids as they crystallize experimental sedimentary diagenesis indicates that trace element partition coefficients reflect rates of cementation the applicability of these findings to fault zone cementation is examined as we make a preliminary attempt to estimate calcite cementation rate in a brittle fault zone directly from the fault rock composition data samples for this study were collected from the knoxville outcrop of the saltville fault in tennessee the cementation rates for the fault rock samples range from 1 10 12 to 3 10 13 m 3 h per m in agreement with some experimental rates and the rates reported for samples from the dsdp sites when applied to a non responsive pore system model these rates result in rapid precipitation sealing indicating the influence exerted by the surface area volume ratio of the pore network we find it feasible to obtain a reasonable range of values for the cementation rate using the trace element partition method however the study also indicates the need for relatively accurate values for the trace carrier element ratio in the fault zone syntectonic pore fluid and exhumed cement

paleomagnetic and structural evidence for middle tertiary counterclockwise block rotation in the dixie valley region west central nevada paleomagnetic data from late oligocene to early miocene ash flow tuffs at four localities in the northern dixie valley region west central nevada indicate that parts of the crust have rotated counterclockwise by at least 25 sup 0 and perhaps significantly more in late cenozoic time field relations in white rock canyon stillwater range suggest that rotation 1 was accommodated by right lateral slip on northwest trending faults 2 spanned ash flow tuff emplacement and 3 probably ceased before eruption of overlying middle miocene basalts accurate estimates of cenozoic extension as well as evaluation of earlier mesozoic structures must include the strain partitioned into rotation in the area

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

structural geology and kinematic history of rocks formed along low angle normal faults death valley california several late cenozoic low angle normal or detachment faults on the western flank of the black mountains are each characterized by the following in descending structural order 1 a hanging wall of upper tertiary to quaternary sedimentary and volcanic strata that displays little evidence for fault related damage other than widely spaced planar or listric normal faults 2 a sharp planar and locally striated principal slip plane forming the lower boundary of the hanging wall 3 an upper zone zone i of very fine grained fault generated rocks composed dominantly of gouge 4 a lower zone zone ii of coarser grained fault rocks consisting chiefly of foliated breccia and 5 a variably damaged footwall consisting of partly mylonitic precambrian units or tertiary plutonic rocks that has been exhumed from depths of 10 12 km since late miocene time the fault rocks which were mostly derived from the footwalls preserve evidence for cataclastic and particulate flow fault rocks contain authigenic minerals but lack the cyclically deformed mineral filled syntectonic veins that are abundant in some other late cenozoic high angle and strike slip faults meso and microscale fabrics in zones i and ii indicate that finite shear strains increase progressively upward toward the principal slip plane in a conceptual kinematic model of a shear box displacements of the hanging wall produced a shearing flow in the fault rocks below some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones i and ii since 770 ka during the latest stages of incremental deformation in the brittle shear zones distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes in this respect the detachment faults differ from inactive segments of the san andreas system along which displacements were progressively and irreversibly localized onto a single principal slip surface the strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry strain gradients in the black mountains are notably asymmetric they are present only below the principal slip planes not above

timing of late quaternary faulting in the 1954 dixie valley earthquake area central nevada the 1954 dixie valley earthquake m 6 9 in central nevada produced about 3 m of total vertical displacement distributed across two principal fault zones along the east flank of the stillwater range most of the 1954 displacement was along the range front fault with minor amounts on the piedmont fault zone in contrast to an earlier holocene displacement that was restricted to the piedmont fault detailed chronostratigraphic exploratory drilling and trenching studies indicate that faulting events have migrated back and forth between the range front and piedmont fault zones in the late quaternary prior to the 1954 earthquake the range front fault last ruptured in the late pleistocene during a large magnitude event here called the ixl event the northern half of the piedmont fault zone last ruptured between 1 5 and 6 8 ka during a large magnitude event here called the bend event on the basis of 6 m total slip since the deposition of shoreline gravels at approximately 12 ka the estimated holocene vertical slip rate is 0 5 mm yr for the dixie valley rupture zone overlapping and migratory patterns of late quaternary faulting indicate that the dixie valley zone does not fit a simple segmentation model

lithosphere dynamics and continental deformation the unifying theme in this section is the remarkable weakness of major faults i will consider the diverse new evidence for weakness and the evidence for high pore pressure localized in faults as a fundamental cause with this background one can better understand why faults remain active even after large rotations with respect to stress i will look at large neogene 23 7 million year old rotations about horizontal axes in the basin and range province and about vertical axes along the pacific margin recent developments will be summarized from studies of neogene tectonics neotectonics in california alaska and the mississippi embayment in the context of a weak north american stress field that results mainly from topographic forces to close i will present new geophysical studies relevant to the continuing controversy over whether the basic structure of the north american mantle lithosphere was altered by an early tertiary episode of flat subduction

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the soapstone ridge complex southern appalachians petrographic mineral compositional and oxygen isotope investigation samples of the soapstone ridge complex ssr in the southern appalachians examined for this study are composed of megacrystic orthopyroxenites amphibole schists and talc chlorite amphibole schists the mineralogy of the ssr samples studied consists of varying proportions of amphibole chlorite talc orthopyroxene and fe ti oxides amphibole is the domi nant mineral in almost all ssr samples studied the amphiboles display zoning whereby cores that are composed of magne siohornblende and edenite consistent with amphibolite facies conditions are rimmed by tremolite actinolite consistent with greenschist facies conditions oxygen isotope compositions of ssr amphiboles investigated show a wide range in 18o values from a low value of 1 8 to a high value of 6 1 such wide ranges in 18oamphibole values occur in ophiolite complexes and oceanic crustal rocks relict orthopyroxene from the ssr has a 18o value of 3 4 a value lower than those from layered intrusions but comparable to some pyroxene 18o values from ophiolites equilibration temperatures calculated for actinolite chlorite yield a temperature of 227 c consistent with greenschist facies conditions it is proposed that the ssr amphibole cores were metamorphosed probably during terrane terrane collision under amphibolite facies conditions with the actinolite rims possibly being formed during thrusting of the ssr rocks onto the continental margin

fluid inclusions in epidote minerals and fluid development in epidote bearing rocks the widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies apart from high to very high temperature and low to intermediate pressure conditions epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust e g poli and schmidt 1998 yet fluid inclusion studies on epidote minerals are surprisingly scarce even in fluid saturated environments like certain vein type deposits or hydrothermal volcanic vugs and druses for example epidote minerals are not mentioned in the subject index of roedder s 1984 outstanding summary and review of fluid inclusion studies and occurrences which lists more than sixty different host minerals for fluid inclusions nonetheless more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems ore deposits and high pressure to ultra high pressure rocks the aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes which are based on temperature t pressure p molar volume v and composition x data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments the review starts with a chapter on some typical mixed volatile solid fluid equilibria involving epidote minerals which are relevant for the here discussed environments this is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals the next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low grade metamorphic rocks and constraints on the p t x properties of the fluids present in these systems this is followed by a short introduction

mineral precipitation as a mechanism of fault core growth abstract faults vary in structural style from simple planes to complex systems composed of fault cores and damage zones increased fault complexity results from the interaction of mechanical and chemical processes including fracture growth shear and linkage and mineral dissolution and precipitation although water rock interaction is traditionally associated with fault rock weakening and shear localization we investigate processes of fault core widening by water rock interactions that resulted in quartz precipitation we combine field and petrographic observations with prior mechanical characterization to assess the impact of alteration and cementation on fault architecture at the dixie comstock epithermal gold deposit nevada usa mineralized portions of the fault contain strong thick silicified fault cores and wide weak damage zones with evidence for widening of the core through entrainment of damage zone material and repeated cycles of embrittlement dilation and cementation we present a model of fault zone evolution in which the hydrothermal regimes favoring either alteration weakening or precipitation strengthening result in distinct fault zone architecture and mechanical and flow properties of fault systems alteration weakening favors localization of the fault into thinner clay rich low permeability fault cores precipitation strengthening promotes thick strong and low permeability fault cores with mineralization embrittlement enhancing transient permeability following coseismic failure

resolving the timing of orogenesis in the western blue ridge southern appalachians via in situ id tims monazite geochronology both taconian ca 450 ma and acadian ca 380 ma ages of high grade deformation and metamorphism have been advocated for the western blue ridge southern appalachians united states to resolve this debate metamorphic monazite from the great smoky mountains was dated via in situ microsampling plus isotope dilution thermal ionization mass spectrometry id tims u pb ages are 450 5 ma 2 error and mean 207 pb 206 pb ages are 453 10 ma these data indicate that peak metamorphism and high grade deformation occurred during the taconian orogeny although they do allow for a low grade sub amphibolite facies acadian overprint unresolved issues remain regarding the comparability of u pb tims ages with those obtained via other methods

pore fluid and seismogenic characteristics of fault rock at depth on the wasatch fault utah fluid inclusions and structural fabric in fault rock in the exhumed footwall of the wasatch fault allow measurement of fluid pressure temperature and composition and estimation of rheological characteristics of fault behavior hydrothermally altered and deformed oligocene quartz monzonite of the little cottonwood stock forms a partially preserved footwall carapace at the southern end of the salt lake segment of the wasatch normal fault zone cataclasite and phyllonite contain two syndeformational alteration mineral assemblages that formed during progressive faulting and displacement of the footwall an early epidote chlorite sericite magnetite alteration assemblage developed in phyllonite and cataclasite and a later laumontite prehnite hematite alteration assemblage developed during cataclasis fluid inclusions associated with the formation of alteration minerals are preserved in healed fractures in quartz grains fluid inclusions that record the transition temperature from quasi plastic flow to frictional deformation are associated with hydrothermal alteration to chlorite epidote and sericite and were trapped at minimum temperatures of 223 c 353 c and minimum fluid pressures of 1150 2800 bars the fluids average 13 mol co2 and 8 2 wt nacl a radiometric age of hydrothermal sericite suggests that the age of the alteration is 17 6 0 7 m y if maximum fluid pressure is near lithostatic the minimum depth of formation of the alteration was 11 km at 337 c consistent with a thermal gradient of 30 c km the ratio of pore fluid pressure to lithostatic pressure varied from 0 52 to nearly 1 0 and indicates that effective normal stress on the fault at these depths may have approached zero structural fabrics and fluid inclusion characteristics are used to estimate seismogenic characteristics of the fault cleavage lineation and s c bands in the phyllonite indicate that deformation at temperatures above 350 c and depths greater than 11 km was dominated by aseismic creep unstable fracture propagation in cataclasite developed during seismic slip at shallower levels the downdip seismogenic length of the salt lake segment is estimated as 16 km using a fault zone dip of 45 and a minimum depth of 11 km for the quasi plastic frictional transition the segment strike length is 35 km and the seismogenic area is about 600 km2 compatible with generation of m 7 0 earthquakes

deformation and metamorphic history of the western blue ridge eastern blue ridge terrane boundary southern appalachian orogen 1 the western eastern blue ridge boundary involved formation by premetamorphic thrusting of an oceanic fragment accretionary wedge island arc upon laurentian basement and metasedimentary cover detailed outcrop scale mapping structural and petrographic analysis reveal both terranes experienced the same deformation at the same metamorphic grade early f1 and f2 isoclinal folds are overprinted by f3 open to tight folds f3 transposes s2 to an axial planar cleavage s3 the youngest penetrative feature prekinematic to synkinematic relative to s2 barrovian metamorphism attained kyanite grade in the western blue ridge wbr upper amphibolite facies in the eastern blue ridge ebr and anatexis in both terranes postkinematic m2b is characterized by appearance of fibrolite throughout the kyanite zone coarsening to granular sillimanite and identification of a kyanite sillimanite isograd in the wbr d3 strain was accommodated by dynamic recrystallization of quartz neocrystallization folding and basal slip of micas pressure solution of plagioclase folding of kyanite and fracturing of garnet scattered lower greenschist facies m4 static retrograde metamorphism resulted in replacement of kyanite and garnet by sericite and chlorite respectively although the juxtaposition of two lithologically distinct terranes implies a fault relationship polymetamorphism and deformation obliterated any microfabrics mineral assemblages or structures that might have developed during premetamorphic thrusting the boundary cannot be an extension of a continent scale dextral strike slip fault system inferred to have developed along laurentian margin in the devonian

fluid regimes fault valve behavior and formation of gold quartz veins the morro do ouro mine ribeira belt brazil abstract fluid inclusion studies petrographic analyses and structural analyses for neoproterozoic auriferous veins from the morro do ouro mine ribeira belt southeastern brazil are presented the deposit is classified as an orogenic gold deposit the vein type au mineralization at the mine is restricted to structurally controlled domains in a low grade calymmian ca 1500 1450 ma metasedimentary sequence that also hosts syngenetic au mineralization two main auriferous quartz vein systems are present i nw trending subvertical extensional veins and ii ne trending subvertical veins parallel to the axial surface of tight folds the mineralized veins are adjacent to a major dextral transcurrent fault zone and their structural relationships indicate that this fault is severely misoriented for frictional reactivation the nw trending veins present a fluid inclusion assemblage dominated by co 2 n 2 ch 4 inclusions h 2 o co 2 n 2 ch 4 nacl cacl 2 inclusions and h 2 o nacl cacl 2 inclusions both with highly variable salinities 0 5 to 52 wt nacl equivalent while the ne trending veins are dominated by h 2 o co 2 n 2 ch 4 nacl cacl 2 and h 2 o nacl cacl 2 inclusions also of highly variable salinities 1 to 45 wt nacl equivalent the entrapment of fluids occurred at temperatures between 225 and 240 c in the nw trending veins and at approximately 208 c in the ne trending veins involving processes of fluid immiscibility and mixing between fluids of contrasting compositions the nw trending extensional veins were formed predominantly under pressure fluctuating between near lithostatic and strongly supralithostatic conditions 100 250 mpa during pre seismic failure stages the ne trending veins precipitated predominantly under subhydrostatic to lithostatic pressure conditions 10 150 mpa following the discharge of fluids from different reservoirs along the ruptured zone after earthquake rupture stages the hydrothermal fluids were probably enriched with gold through interactions with the host rocks and its precipitation is attributed to changes in physicochemical properties due to fluid immiscibility and mixing between fluids of highly contrasting salinities as a consequence of cyclic fluctuations in the values of fluid pressure and tectonic stresses accompanying episodes of seismogenic fault activity

chemical interaction of aqueous solutions with epidote feldspar mineral assemblages in geologic systems ii equilibrium constraints in metamorphic geothermal processes thermodynamic analysis of the system na sub 2 o k sub 2 o cao feo fe sub 2 o sub 3 al sub 2 o sub 3 sio sub 2 h sub 2 o hcl co sub 2 at pressures and temperatures to 5 kb and 600 sup 0 c affords quantitative description and interpretation of phase relations among epidote garnet plagioclase and alkali feldspar solid solutions in hydrothermal systems comparison of computed and observed compositions of these minerals and coexisting fluids suggests that the calculations afford close approximation of equilibrium and mass transfer in metamorphic geothermal processes the standard molal enthalpies of decomposition reactions for plagioclase are of the order of 4 to 50 kcal mole sup 1 which requires the activity of the anorthite component of plagioclase coexisting with epidote solid solutions to increase with increasing temperature at constant pressure in contrast because the standard molal volumes of these reactions are also negative a sub caal sub 2 si sub 2 o sub 8 decreases with increasing pressure at constant temperature either stoichiometric epidote or clinozoisite is compatible with plagioclase quartz calcite and an aqueous phase but only at x sub co sub 2 less than or equal to 0 2 the complex zoning commonly exhibited by both plagioclase and epidote solid solutions in geologic systems can be attributed to minor isothermal isobaric changes in the composition of coexisting aqueous solutions logarithmic activity and fugacitymore diagrams constructed with provision for solid solution permit documentation of such changes and facilitate thermodynamic interpretation of mineral and fluid compositions in metamorphic geothermal systems less

petrology and isotopic composition of a grenvillian basement fragment in the northern appalachian orogen blair river inlier nova scotia canada mesoproterozoic metaplutonic rocks in northern cape breton island introduction nova scotia occur in a tectonic inlier within the appalachian in eastern north america mesoproterozoic laurentian orogen although they have been multiply metamorphosed and basement rocks and their neoproterozoic to mid paleovariably deformed the petrology and geochemistry of these rocks zoic volcanic and sedimentary cover preserve a record provide insight into the tectonomagmatic evolution of easternmost of the proterozoic to mid paleozoic geologic history of laurentia anorthosite syenite and granitoid plutons eastern laurentia the record includes mesoproterozoic 1100 980 ma intruded the sailor brook and polletts cove river orogeny on the ancient north american craton breakgneisses new nd isotopic data are presented from a biotite rich up of a neoproterozoic supercontinent development of part of the sailor brook gneiss ndi 0 7 two anorthosite a passive continental margin and mid paleozoic orogeny samples ndi 2 1 and 2 8 and a charnockite unit exposures of mesoproterozoic laurentian basement in ndi 0 4 new u pb zircon data from the anorthosite yield easternmost canada and the usa occur in a dispb pb ages between 975 and 1095 ma disrupted u pb continuous band of inliers in the appalachian orogen systematics preclude a unique age interpretation but the 1095 ma from newfoundland to alabama fig 1 they consist single grain date is a minimum age for the anorthosite field relations mostly of rocks formed or deformed during major and trace element geochemistry and isotopic characteristics 1190 980 ma grenvillian tectonic and magmatic indicate that the anorthosite and charnockite were probably coexisting events and are distinct from neoproterozoic and melts but not differentiates of a single parent magma the lithological paleozoic units of accreted appalachian terranes in that and chemical characteristics of the lowland brook syenite are they include granulite facies metamorphic rocks and similar to those of a contemporaneous belt of ultrapotassic plutons in many cases anorthosite charnockite mangerite suites in the southern grenville province and both have the chemical bartholomew 1984 and papers therein pettingill et al characteristics of certain modern continental margin arc magmas 1984 rankin et al 1989 1993 owen erdmer 1990 these data indicate that the mesoproterozoic units in the blair hughes et al 1997 river inlier were juvenile crustal additions to eastern laurentia the sinuous trend of the band of basement inliers during grenville related orogenic events broadly reflects promontories and re entrants that formed

retrograde metamorphism of eclogite in the southern appalachian mountains u s a a case involving seamount subduction this work supports a growing body of evidence that the ashe metamorphic suite ams of the eastern blue ridge province in north carolina has an ensimatic origin and is part of a subduction related accretionary melange marking the taconic suture between the north american craton and the inner piedmont in a palinspastic reconstruction the thrust fault at the base of the ams appears to have intercepted the greatest depths i e highest p metamorphic rocks beneath parts of the ams now exposed adjacent to the grandfather mountain window the greatest volume of mafic rock is found in these same areas we suggest that the nascent subduction related basal thrust fault was deflected downward by an obstacle in the form of an isolated mafic volcanic edifice on the oceanic crust a sea mount pelitic and mafic rocks dominate the ams north of the grandfather mountain window retrograded eclogite occurs in the amphibolite near the base of the ams textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses a eclogite i facies omphacite garnet quartz b eclogite ii facies omphacite garnet epidote quartz c symplectic diopside plagioclase garnet epidote quartz d amphibolite facies diopside plagioclase garnet epidote hornblende quartz e amphibolite facies plagioclase garnet epidote hornblende quartz p t conditions estimated from geothermobarometry applied to relevant mineral compositions are c 720 c and c 16 kbar for b eclogite ii facies c 655 c and c 8 5 kbar for e amphibolite facies

origin and migration of palaeofluids in the upper visean of the campine basin northern belgium upper visean limestones in the campine basin of northern belgium are intensively fractured the largest and most common fractures are cemented by non ferroan dull brown orange luminescent blocky calcite first melting temperatures of fluid inclusions in these calcites are around 57 c suggesting that precipitation of the cements occurred from nacl cacl2 mgcl2 fluids the final melting temperatures tmice are between 5 and 33 c the broad range in the tmice data can be explained by the mixing of high salinity fluids with meteoric waters but other hypotheses may also be valid homogenization temperatures from blocky calcite cements in the shelf limestones are interpreted to have formed between 45 and 75 c in carbonates which were deposited close to and at the shelf margin precipitation temperatures were possibly in the range 70 85 c and 72 93 c respectively on the shelf the calcites have a 18o around 9 3 pdb and they are interpreted to have grown in a fluid with a 18o between 3 5 and 1 0 smow at the shelf margin blocky calcites 18o 13 5 pdb could have precipitated from a fluid with a 18o betweenn 4 0 and 1 1 smow the highest oxygen isotopic compositions are comparable to those of late carboniferous marine fluids 18o 1 smow the lowest values are more positive than a previously reported composition for carboniferous meteoric waters 18o 7 smow precipitation is likely to have occurred in marine derived fluids which mixed with meteoric waters sourced from near the brabant massif fluids with a similar negative oxygen isotopic composition and high salinity are actually present in palaeozoic formations the higher temperature range in the limestones near the shelf margin is explained by the upward migration of fluids from the basinal area along fractures and faults into the shelf

fluid mineral relations in the comstock lode the comstock lode a world class precious metal vein deposit in the comstock district 15 mi 24 km southeast of reno nevada formed 13 7 ma in miocene eruptive rocks of the alta and kate peak formations and in pre tertiary metavolcanic and intrusive rocks production of 8 25 million oz 311 850 kg in gold and 192 million oz 7 26 million kg in silver came largely from bonanzas or vein segments containing hundreds of thousands to millions of tons and grading or 0 4 oz ton approximately 14 g ton gold and or 10 oz ton approximately 340 g ton silver within the 8 mi 12 9 km long comstock fault bonanza ores are distributed 3 mi 4 8 km along strike and persist from the present surface to 3 300 ft 1 007 m below the surface during the middle miocene andesitic to rhyodacitic alta and kate peak rocks formed a stratovolcano that exceeded 7 000 ft 2 134 m in elevation bonanza ores were deposited within this volcano from a 200 degrees c hydrothermal system that affected more than 37 mi 3 150 km 3 of rock at least one cretaceous and several miocene hydrothermal events were imposed on comstock district rocks the comstock lode vein assemblage which resulted from the youngest and only economically important event consists of quartz electrum silver minerals mainly acanthite pyrite base metal sulfides and minor amounts of calcite sericite chlorite k feldspar and albite andesite adjacent to veins is altered to quartz sericite and pyrite at vein contacts and quartz sericite montmorillonite chlorite and pyrite distally kaolinite is present in wall rocks adjacent to vein segments in which boiling occurred addition of the external components water co 2 and sulfur gases sufficed to produce all comstock vein alteration assemblages based on paragenetic and fluid inclusion studies multiple generations of hydrothermal fluid formed comstock district veins but the earliest and highest temperature fluid deposited ore minerals it ranged in temperature from 300 degrees c at the north end of the lode cedar hill to 6 wt percent nacl and was composed of both meteoric and magmatic water according to fluid isotopic analyses younger cospatial fluid s contained up to 2 1 wt percent co 2 the thermal center of the comstock lode hydrothermal system cedar hill is also the site of the highest measured ore fluid salinities ore fluid isotherms and to an extent salinity isopleths display reversals and convolutions that are spatially related to bonanzas these sharp temperature gradients coupled with mixing of ore fluid and at least one other high temperature fluid indicated by enthalpy salinity relationships and deuterium enrichment most likely caused precipitation of ore minerals sphalerite and electrum compositions indicate that bonanzas were deposited over a narrow range of f sub s 2 13 5 to 15 2 while sphalerites from low grade and barren vein segments indicate more widely variable f sub s 2 gold silver ratios both in electrum and bulk ore appear to be temperature controlled as higher temperatures generally correlate with high au ag ratios local boiling of 250 degrees to 285 degrees c fluid provides a paleosurface datum but vein temperatures 300 degrees c require up to 3 800 ft 1 152 m of cover over the tops of bonanzas during mineralization the distribution of both ore stage and late vein calcite partially reflects hydrothermal fluid circulation patterns in veins at district margins the abundance of late calcite marks the demise of hot water circulation and the influx of large amounts of cold miocene ground water into vein interstices kate peak intrusions between cedar hill and mount davidson share a common structural control with vein segments that comprise the comstock lode and their ages span vein ages the coeval kate peak magmas that provided heat for hydrothermal circulation formed the core of and uplifted the miocene stratovolcano which was centered over cedar hill partial symmetry of thermal compositional and kinetic gradients about cedar hill suggests that additional bonanzas await discovery along unexplored comstock fault segments within declining palcothermal gradients to the north

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

sedimentary fluids fault interaction during syn rift burial of the lodeve permian basin herault france an example of seismic valve mechanism in active extensional faults abstract during basin burial interstitial fluids initially trapped within the sedimentary pile easily move under thermal and pressure gradients as the main mechanism is linked to fluid overpressure such fluids play a significant role on frictional mechanics for fault reactivation and sediment deformation the lodeve permian basin herault france is an exhumed half graben with exceptional outcrop conditions providing access to barite sulfide mineralized systems and hydrocarbon trapped into syn rift roll over faults architectural studies show a cyclic infilling of fault zone and associated bedding parallel veins according to three main fluid events during dextral normal faulting contrasting fluid entrapment conditions are deduced from textural analysis fluid inclusion microthermometry and sulfur isotope geothermometer we conclude that a polyphase history of trapping occurred during permian syn rift formation of the basin the first stage is characterized by an implosion breccia cemented by silicifications and barite during an abrupt pressure drop within fault zone this mechanism is linked to the dextral strike slip motion on faults and leads to a first sealing of the fault zone by basinal fluid mineralization the second stage consists of a succession of barite ribbons precipitated under overpressure fluctuations derived from fault valve action this corresponds to periodic reactivations of fault planes and bedding controlled opening localized at sulphide rich micro shearing structures showing a normal movement this process formed the main mineralized ore bodies by the single action of fluid overpressure fluctuations undergoing changes in local stress distribution the last stage is associated with the formation of dextral strike slip pull apart infilled by large barite and contemporaneous hydrocarbons under suprahydrostatic pressure values this final tectonic activation of fault is linked to late basinal fluids and hydrocarbon migration during which shear stress restoration on the fault plane is faster than fluid pressure build up this integrated study shows the interplay action between tectonic stress and fluid overpressure in fault reactivation during basin burial that clearly impact potential economic reservoirs

crustal scale transcurrent fault development in a weak layered crust from an integrated geophysical research carboneras fault zone eastern betic cordillera spain new magnetotelluric and receiver transfer function studies provide insights from the upper to the lower crust of the eastern betic cordillera which is deformed by large folds normal faults and a major transcurrent left lateral fault the carboneras fault zone cfz receiver function analysis determines a nnw dipping moho reaching 20 that increases in depth from 20 km south of the cfz up to 34 km in the sierra de los filabres in addition seismic discontinuities determined in the upper crust are interpreted as major contacts between metamorphic complexes that are detached and folded the mt inversion model reveals a conductive zone also representing a crustal seismic discontinuity associated with the alpujarride nevado filabride contact and fitting the n vergent geometry of the sierra alhamilla antiform a small flexure at moho coincides with the cfz as revealed by the bouguer anomaly trend in agreement with the receiver function results moreover the bahr strike and tipper angle at the stations placed closest to the cfz clearly reveal the continuity of the cfz at least down to approximately 15 km in depth crossing all the detected crustal discontinuities up to the moho the lack of a clear moho offset associated with the carboneras fault supports the idea that some large strike slip faults tend to accommodate the deformation by a broadening fault zone at lower crustal levels its nucleation could occur at the base of a thin crust where melting processes critically reduced the lithospheric strength during the late miocene to then propagate upward reaching the topographic surface northward the lithosphere comprised moderately larger strength and the crustal discontinuities favored the development of larger folds with kilometric amplitude instead of strike slip faults since the late miocene

contrasting origin of palaeofluids in a strike slip fault system calcite cements in a dextral strike slip fault system in the variscan front complex belgium have been investigated to determine the origin of the fluids migrating along the fault system generation of non ferroan and ferroan calcites occurs along the fault plane a right stepover between two faults is occupied by a breccia it formed due to repeated hydraulic implosions within a dilational jog ferroan calcites cement the breccia the general geological setting and the microthermometric data tt 50 c indicate that calcite precipitation along the faults took place at shallow depth however the geochemistry and origin of the fluids from which the calcites along the fault plane and in the breccia precipitated were different the non ferroan calcites along the fault plane precipitated from a highly saline fluid 18 1 to 25 3 eq wt cacl2 at a temperature around 50 c secondary inclusions in trails in these calcites demonstrate that similar highly saline fluids reached temperatures of at least 130 c the latter migrated from a depth of over 4 km implying the fluid was tapped from the lower palaeozic siliciclastic basement the ferroan calcites along the strike slip fault plane also formed from highly saline fluids but which interacted with upper palaeozoic strata before migration to shallow depth in contrast primary fluid inclusions in the ferroan calcites in the breccia indicate cementation occurred from a low salinity fluid 1 7 to 8 6 eq wt nacl at a temperature of 50 c or less this fluid likely originated from shallow depth the contrasting origin of the fluids indicate different flow regimes have been intersected by the faults during their strike slip reactivation

fluid pressure transients on seismogenic normal faults abstract fluid inclusions in hydrothermally altered footwall rocks of the dixie valley fault nevada and the wasatch fault utah indicate that pore fluid pressure fluctuated minimum entrapment pressures for fluid inclusions consisting of h2o co2 nacl ranged from 295 mpa to 60 mpa in the temperature range 350 to 170 c on the wasatch fault and from 158 to 35 mpa in the temperature range 350 to 200 c on the dixie valley fault scatter in the pressure estimates at constant temperature is interpreted as paleo fluid pressure transients at depths of up to 11 km on the wasatch fault and 3 to 5 km on the dixie valley fault observed pressure transients range from 5 mpa within the limits of error in pressure determination to 120 mpa on the wasatch fault and 7 to 126 mpa on the dixie valley fault the pressure transients are greatest on both faults in the temperature range 270 to 310 c the fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures high fluid pressures may initiate rupture then dilatancy pore pressure reduction and dilatant hardening may arrest the rupture however decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90 which reduces the dilatant hardening effect and may permit ruptures to propagate

fluid pressure and salinity variations in shear zone related veins central pyrenees france implications for the fault valve model abstract fluid inclusion microthermometry data is presented from quartz veins associated with high angle reverse shear zones cutting the neouvielle massif in the central french pyrenees a complex series of veins were formed during both normal and reverse movement on brittle faults within the shear zone brittle structures developed contemporaneously with quasi plastic deformation in the shear zones fluid inclusion data are presented from a variety of vein types and shows that a wide variation in fluid density and salinity is present in the fault system these variations occur both within individual veins and between different vein types temperatures at the time of vein filling are constrained to be in the range 310 360 c by chlorite microthermometry the variation in homogenisation temperature is too large to be accounted for by variations in the fluid temperature and is interpreted to reflect variations in fluid pressure between lithostatic 500 mpa and hydrostatic 200 mpa values at the time of quartz growth in the veins the shear zones carry modest displacements and may not themselves have been seismogenic a model is presented in which fluid pressure variations and stress cycling within the steeply dipping shear zones is controlled by the seismogenic cycle on large underlying thrust structures the model predicts that the highest maximum trapping pressures should occur in fluid inclusions from low angle veins formed in a high pfluid compressional regime immediately before earthquake rupture lower maximum pressures are expected in steep veins related to extension and normal fault movement across the shear zones immediately after rupture the fluid inclusion data support this model when maximum trapping pressures are considered but the majority of inclusions in all vein types formed at pressures well below lithostatic values this suggests that the dominant factor causing quartz precipitation was the sudden pressure drop immediately after earthquake rupture regardless of vein type

fault gouge dating in the southern appalachians usa illite age analysis iaa the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite has been successfully applied to several rock types including fault gouge shales and argillaceous limestones iaa results are presented for fi ve fault rocks including four clay gouges and one cataclasite from the exhumed southern appalachian foreland fold thrust belt eastern united states determining detrital versus authigenic illite is now an established procedure utilizing x ray analysis to quantify illite polytypes less established however is how to interpret 40 ar 39 ar ages as a function of diagenetic grade both a total gas age incorporating the recoiled argon fraction after irradiation and a retention age omitting the recoiled fraction are obtained for a sample we relate their respective use to diagenetic grade and specifi cally the crystallite thickness of illite when measured crystallite sizes are equal to or smaller than that calculated from ar recoil we use total gas ages here 5 nm while for crystallites greater than 10 nm we use retention ages the four clay gouge ages are all the same within error and much younger than that of the cataclasite we conclude that a major period of frontal appalachian fault activity occurred during the early permian cisuralian 276 280 ma and that these foreland faults were active simultaneously as part of an internally deforming regional thrust wedge

the burnsville fault evidence for the timing and kinematics of southern appalachian acadian dextral transform tectonics there has been a conspicuous absence of documented acadian structures in the blue ridge province of the southern appalachians even though a growing body of evidence suggests that the middle paleozoic acadian orogeny affected the rocks of this region new mapping along with structural petrologic and geochronologic data from western north carolina show that the contact between the metasedimentary and metavolcanic rocks of the ashe metamorphic suite and the grenville basement rocks of the western blue ridge corresponds to a devonian high grade dextral strike slip fault zone the burnsville fault timing of motion on the burnsville fault is constrained by field relationships metamorphic fabrics and assemblages and radiometric ages a u pb zircon crystallization age for a pegmatite sheared by the burnsville fault shows that the last movement on the fault must he younger than 377 ma previously published 4 0 ar 3 9 ar cooling ages show that the burnsville fault must be older than ca 360 ma we have mapped the burnsville fault for 100 km in northwest ern north carolina the gossan lead fault of northwesternmost north carolina and southwestern virginia is the likely continuation of the burnsville fault to the north east southwest of asheville north carolina the burnsville fault may connect to the devonian dahlonega shear zone or may be cut by post devonian thrust faults associated with the alleghanian orogeny diachroneity of acadian elastic wedge the presence of silurian devonian pull apart basins the presence of devonian high grade dextral shearing in the inner piedmont and recent plate reconstructions for the middle paleozoic support the interpretation that the late phase of the acadian orogeny in the southern appalachians was primarily a dextral transpressional event the burnsville fault and the inner piedmont formed the boundaries of a dextral transform margin that may have extended the length of the appalachian orogen this interpretation requires that models for the paleozoic tectonics of the appalachians incorporate major pre alleghanian dextral displacement

borax lake a thermal lake confined within a biomorphic mound pueblo valley oregon usa abstract relations among geothermal system dynamics evolution of an 8 m thick biomorphic mound and requirements to protect critical habitat of gila boraxobius a fish protected under the endangered species act of 1973 were explored at borax lake a shallow thermal lake in the tectonically active northwestern basin and range province neutral ph thermal water at 105 c discharges within a carrot shaped vent at a depth of 29 m and rapidly mixes within the vent and lake which is mostly 1 and proximity of seismic sources indicates the mound may be susceptible to liquefaction and slope failure during dynamic earthquake loading borax lake is the unique product of the geothermal system and the biological communities it supports portions of the mound may have failed in the past during seismic loading eroded and been rebuilt by processes similar to those active today