paleostress magnitudes determination by using fault slip and fluid inclusions planes data paleostress magnitudes can be estimated by using fault slip data rupture and friction laws angelier 1989 for dry conditions however their estimation is difficult if fluids are present during deformation the lithostatic load and the fluid pressure being generally unknown this paper shows that the quantitative estimation of the lithostatic load and the fluid pressure during a tectonic event can be derived from paleofluid analysis in fluid inclusion planes fip as fip are healed mode i cracks oriented in a consistant manner relative to regional or local structures stress and fluid features may be obtained for a given deformation event this approach has been applied to a fault system affecting an hercynian granite of the nw french massif central a nw se compression has been defined from a population of 51 faults characterized by orientations around n60 e to n110 e for dextral strike slips and n135 e to n175 e for sinistral movements the stress ratio has been determined with fault slip data around 0 52 0 08 and by using angelier s method the ratio 3 1 was estimated around 0 27 0 03 and the friction coefficient was estimated around 0 58 0 1 fip orientation were measured in a sample collected at 1 meter from the fault the dominant trend is nw se vertical or dipping toward the sw inclusion fluids from fip are characterized by homogenization temperatures in the 260 380 c range with a mode around 300 c and melting temperatures from 0 c to 1 5 c 0 5 equivalent nacl with a mode around 1 0 c 1 7 equivalent nacl these data yield assuming a geothermal gradient in the 60 80 c km range and hydrostatic conditions a fluid pressure estimate of about 50 10 mpa and a v around 132 10 mpa thus a major stress ratio 1 3 can be estimated in the 70 105 mpa range

early miocene change in direction of least principal stress southwestern united states conflicting inferences from dikes and metamorphic core detachment fault terranes previous investigations have suggested that the direction of least principal stress in the western united states has shifted from northerly to easterly between 50 to 30 ma an effort is made to more tightly constrain the time of this shift using igneous dike host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 ma in such systems the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock these systems indicate that a northerly least principal stress orientation persisted more recently than previously believed until probably after 30 ma possibly about 26 ma and conceivably as recently as 18 ma in some areas the data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift however the change could be diachronous earlier in the southeast and later in the northwest major easterly trending magmatic and tectonic elements of tertiary age in the great basin of utah and nevada are compatible with the dike data however numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes this inconsistency is not clearly resolvable but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by mesozoic compression

geologic and kinematic constraints on late cretaceous to mid eocene plate boundaries in the southwest pacific abstract starkly contrasting tectonic reconstructions have been proposed for the late cretaceous to mid eocene 85 45 ma evolution of the southwest pacific reflecting sparse and ambiguous data furthermore uncertainty in the timing of and motion at plate boundaries in the region has led to controversy around how to implement a robust southwest pacific plate circuit it is agreed that the southwest pacific comprised three spreading ridges during this time in the southeast indian ocean tasman sea and amundsen sea however one and possibly two other plate boundaries also accommodated relative plate motions in the west antarctic rift system wars and between the lord howe rise lhr and pacific relevant geologic and kinematic data from the region are reviewed to better constrain its plate motion history during this period and determine the time dependent evolution of the southwest pacific regional plate circuit a model of 1 west dipping subduction and basin opening to the east of the lhr from 85 55 ma and 2 initiation of northeast dipping subduction and basin closure east of new caledonia at 55 ma is supported west dipping subduction and basin opening were not driven by convergence as has previously been proposed our plate circuit analysis suggests that between at least 74 ma and subduction initiation at 55 ma there was little net relative motion between the pacific plate and lhr

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

emplacement and growth of alkaline dikes insights from the shonkinite dikes elchuru alkaline complex se india abstract the mesoproterozoic elchuru alkaline complex in the prakasam alkaline province pap eastern ghats belt se india is intruded by coeval shonkinite dikes magmas parental to both the host nepheline syenite and shonkinite dikes formed during ca 1350 ma continental rift related magmatism the shonkinite dikes show fine to medium grained equigranular to foliated textures with clinopyroxene biotite amphibole k feldspar perthite and nepheline as major mineral phases the dikes grew by inflation and coalescence of dike segments along intrusive steps anastomosis and subsequent coalescence and thermal mechanical erosion of the host rocks a few dikes resulted by filling the fractures created by shearing inflation of intrusive steps between magma segments by mode i opening is the major mechanism of shonkinite dike emplacement and growth however the shonkinite dikes also demonstrate the application of remote shear stress on the dike walls resulting in anastomosis entrapment and segmentation of tabular host rock xenoliths and in extreme cases brecciation of the host rocks under volatile rich and low viscous melt conditions fractal analysis of xenolith bearing dike further illustrates that thermal erosion was facilitated and accentuated by mechanical breakdown of host rock xenoliths due to interactions of tensile and shear forces cross cutting relationships between the shonkinite dikes suggest local deviation in the least compressive stress direction during dike emplacement the shonkinite dikes of elchuru demonstrate that spatially and temporally restricted high volatile low viscous alkaline magmas may have distinctly different styles of emplacement as controlled by interaction of near field magmatic tensile stress and far field tectonic shear stress in an evolving continental rift setting

relations between fluid inclusion trails and regional stress field a tool for fluid chronology an example of an intragranitic uranium ore deposit northwest massif central france abstract episyenites within the uranium ore bearing le bernardan granite northwest french massif central were formed through the dissolution of quartz by hydrothermal solutions these solutions may now be found fossilized as fluid inclusions f i the f i form rectilinear trails along microcracks which trapped the percolating fluids simultaneous study of the orientations of the trails and the chemistry of the inclusions shows that 1 1 the trails may be grouped according to direction each related to successive regional stress fields 2 2 the fluids of each group are compositionally distinct marking different stages of the hydrothermal event this example shows that the geometric relations between secondary f i trails and regional stress fields may be used to establish the chronology of both during a region s geological evolution

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

a geometric technique for relocating hotspots and refining absolute plate motions an age independent geometric relationship is presented that links hotspots to the seamounts which they produce and so permits the use of undated seamounts to refine the motion of tectonic plates this technique has the potential to rigorously assess hotspot fixity and to locate extinct hotspots the present application of this method points to a recent change in pacific plate motion and suggests a relocation of the louisville hotspot to the hollister ridge south of the eltanin fracture zone

observations of geometry and ages constrain relative motion of hawaii and louisville plumes abstract the classic view of linear island chains as volcanic expressions of interactions between changing plate tectonic motions and fixed mantle plumes has come under renewed scrutiny in particular observed paleolatitudes from the emperor seamounts imply that the hawaii hotspot was 5 15 further north during formation of these seamounts and that rapid retardation of its southward migration was the primary agent forming the angular hawaii emperor bend supporting this view are predictions from fluid dynamic experiments that suggest the general mantle circulation may displace narrow mantle plumes consequently the surface locations of hotspots are not fixed and may have varied considerably in the past however the locations and ages of available rock samples place fundamental limits on the relative motion between the hawaii and louisville hotspots here we use such data to estimate empirical age progression curves for separate chains and calculate the continuous variations in hotspot separations through time while the data are sparse the inferred inter hotspot motion for ages 55 myr appears significant but the observed relative motion is only about half of what is predicted by mantle dynamics models to reconcile the observed paleolatitudes with our observed relative motion requires either a larger contemporaneous southward motion of the louisville hotspot than previously suggested or a moderate component of true polar wander

the influence of rock fabric on excavation damage in the lac du bonnett granite this paper documents the contribution of rock fabric on excavation damage development in the granite of the lac du bonnet batholith ldbb it is concluded that the ldbb contains structural elements and distinct textural varieties which differ significantly in their response to rock properties testing and to excavation data collected in this study demonstrate the highly heterogeneous internal structure characteristic of apparently homogenous batholiths and the influence of this heterogeneity on rock mass response evidence for anisotropy of the rock mass is demonstrated to have occurred on the geological time scale and its influence is still present in recent excavations this anisotropy exists at a wide range of scales from the localization of fault zones along low dipping layering at the kilometre scale to the localization of failure around tunnels by separation along this and other weakness directions

late stages of exhumation constrained by structural fluid inclusion and fission track analyses sesia lanzo unit western european alps in metamorphic areas paleogeothermal gradients are difficult to properly estimate leading to great uncertainty in exhumation rate calculations based on fission track analysis in this work a new multidisciplinary approach based on fluid inclusion analysis structural analysis and fission track dating has proven to be able to constrain the late stages of exhumation of metamorphic rocks the study area is located in the sesia lanzo unit of the western european alps along the hone shear zone kinematic analysis at different scales unraveled a polyphase postmetamorphic deformation history constrained in the p t space thanks to laboratory investigation of kinematically referenced fluid inclusions coupled with the analysis of the rheological behavior of quartz and feldspar inside syn kinematic veins the resulting p t path points to an increasing paleogeothermal gradient from 18 to 30 c km 1 after greenschist facies metamorphic conditions and provides a reference framework for a quantitative interpretation of fission track data in terms of exhumation rates and fault throws according to fission track data throws accommodated by the hone shear zone between 33 and 30 ma are on the order of 4 5 km and would be underestimated by 25 if a normal steady gradient was assumed for the whole post metamorphic history of the area the deepest portions of this structure were exhumed thanks to the activity of the e w fault system that accommodated a throw of 4 km between 28 and 20 ma and a throw of 0 8 km during the last 20 myr

oceanic microplate formation records the onset of india eurasia collision abstract mapping of seafloor tectonic fabric in the indian ocean using high resolution satellite derived vertical gravity gradient data reveals an extinct pacific style oceanic microplate mammerickx microplate west of the ninetyeast ridge it is one of the first pacific style microplates to be mapped outside the pacific basin suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern pacific ridges the microplate formed at the indian antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south whose conjugate is located north of the kerguelen plateau independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric also seen in multibeam data magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o 47 3 ma plate reorganizations can trigger ridge propagation and microplate development and we propose that mammerickx microplate formation is linked with the india eurasia collision initial soft collision the collision altered the stress regime at the indian antarctic ridge leading to a change in segmentation and ridge propagation from an establishing transform fast indian antarctic spreading that preceded microplate formation and kerguelen plume activity may have facilitated ridge propagation via the production of thin and weak lithosphere however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event prior to the collision the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and w shaped lineations that record back and forth ridge propagation microplate formation provides a precise means of dating the onset of the india eurasia collision and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories

age constraints on jerritt canyon and other carlin type gold deposits in the western united states relationship to mid tertiary extension and magmatism carlin type gold deposits are difficult to date and a wide range of ages has been reported for individual deposits therefore several methods were employed to constrain the age of the gold deposits in the jerritt canyon district dated igneous rocks with well documented crosscutting relationships to ore provided the most reliable constraints k ar and 40 ar 39 ar dates on igneous rocks are as follows andesite dikes 324 ma sericitic alteration in andesite dikes 118 ma basalt dikes 40 8 ma quartz monzonite dikes 39 2 ma and calc alkaline ignimbrites 43 1 to 40 1 ma of these only the andesite and basalt dikes are clearly altered and mineralized the gold deposits are therefore younger than the 40 8 ma basalt dikes the sericitic alteration in the andesite dikes is unrelated to the gold deposits a number of dating techniques did not work k ar and 40 ar 39 ar dates on mica from mineralized ordovician to devonian sedimentary rocks gave misleading results the youngest date of 149 ma from the smallest 187 os 188 os initial 0 9 3 6 but did not yield a meaningful isochron due to very low re and os concentrations and large analytical uncertainties paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the host rocks leaving no remnant magnetism when published isotopic dates from other carlin type deposits in nevada and utah are subject to the rigorous evaluation developed for the jerritt canyon study most deposits can be shown to have formed between 42 and 30 ma k ar and 40 ar 39 ar dates on the youngest preore igneous rocks range from 41 to 32 ma whereas the oldest postore igneous rocks range from 35 to 33 ma hydrothermal adularia from the twin creeks deposit yields similar 40 ar 39 ar dates of 42 ma k ar dates on supergene alunite range from 4 to 30 ma k ar and 40 ar 39 ar dates on micas separated from sedimentary 395 43 ma and igneous 145 38 ma rocks are usually much older than the gold deposits and most are suspect because they are from incompletely reset preore micas or from mixtures of preore and ore stage mica fission track dates on zircons are also generally older than the deposits 169 35 ma and are not completely reset by mineralization apatites are likley to be reset by the hydrothermal systems and by younger thermal events and yield dates 83 22 ma that are younger than those from zircon independent support of a mid tertiary age is provided by the low delta d sub h 2 o values of hydrothermal fluids from 10 widely separated carlin type deposits 134 or 23ppm the low delta d sub h 2 o values are consistent with a mid tertiary age when the climate was cool but are inconsistent with the cretaceous and jurassic ages favored by some workers when the climate was warm the age and distribution of carlin type deposits coincides with areas that underwent extension and calc alkaline magmatism over the same time period despite this relationship the deposits lack clear spatial or genetic relationships to mid tertiary epizonal plutons rather many deposits are localized along preexisting crustal fault zones in the getchell trend structures tapped deep sourced metamorphic or magmatic fluids and subsequently variably exchanged meteoric water in the other trends and districts only variably exchanged meteoric water has been detected these relationships suggest either that all of the deposits formed from deep sourced fluids and were subsequently flooded by meteoric water or that fluids from different sources evolved to produce deposits with similar charateristics

are fluid inclusion planes useful in structural geology abstract quantification of deformation or stresses in structural geology is often difficult due to the uncertainity of the paleodepth or fluid pressure the best records of fluid percolation are paleofluids trapped as fluid inclusions in healed microcracks of the rock forming minerals fluid inclusion planes fip are mode i cracks which form in sets with a predominant orientation perpendicular to the least principle stress 3 however the repeated microfracturing and healing of the rock forming minerals yield complex superimposed patterns of healed microcracks such patterns are often difficult to interpret due to the lack of suitable chronological criteria these problems have been recently documented and solved by coupling deformation studies detailed examination at all scales of the relationships between trapped fluids and their host structures and studies of fluid inclusions this paper summarizes recent advancements in fip analysis and suggests further ways for research

pacific absolute plate motion since 145 ma an assessment of the fixed hot spot hypothesis 1 using the geometry and ages from 12 pacific seamount chains we have determined two new absolute plate motion models that now extend our self consistent and high resolution models with covariance estimates back to 145 ma the wk08 a model maps the full uncertainty in the age progressions into uncertainties in rotation opening angles yielding a relatively smooth plate motion model the wk08 g model relaxes the mapping of age uncertainties in order to better isolate secondary geometry changes seen along many coregistered chains both models have been used to assess the viability of the fixed hot spot hypothesis in the pacific in determining the models we found that only a small group of age samples had to be discarded on the grounds that they were discordant with the dominant trends we were able to connect plate motions for pre and post emperor age intervals by including the ratak gilbert ellice and musicians trails in our analysis however as no active hot spot locations exist for the older chains their inclusion adds additional model parameters both age and geometry misfits increase with age reflecting the observed increase in age uncertainties and the general widening of trails secondary and short lived changes in absolute plate motion mapped in wk08 g appear to correlate with the timing and sense of motion of known pacific rim tectonic events analysis of interchain distances between coeval samples from the hawaii and louisville chains suggests possible discrepancies during the older emperor stage that are compatible with predictions of hot spot drift we computed a new apparent polar wander path for the pacific and found a high degree of correspondence with paleomagnetically derived paths as long as solutions allowing for anomalous skewness were included in the latter our polar wander path suggests that there might have been some true polar wander during the emperor stage complemented by a smaller amount of hot spot drift than otherwise required we show that chain geometries and ages combined with future paleolatitude determinations from additional sites and chains could enable an observation based description of both hot spot and plate motions without relying on predictions of hot spot drift derived from mantle flow calculations

aseismic fracturing and cataclasis involving reaction softening within core material from the cajon pass drill hole cataclastic deformation features in crystalline rocks from the cajon pass drill hole located some 4 km ne of the san andreas fault trace in southern california appear to have developed at slow strain rates there is no clear evidence of seismic deformation most of the observed structures and microstructures are inferred to have formed during pre pliocene distributed deformation before the san andreas fault became active in this area extension fractures are filled by laumontite which increases in grain size from a fine amorphous habit to coarser prismatic crystals unidirectionally across fractures and does not generally display crack seal textures fragments within fractures were derived from adjacent grains without rotation or shear fracture opening and cementation were therefore synchronous and slow fluids were present at all times during fracture opening but there was no repeated hydrofracturing after fracture formation however pore fluid pressures probably stayed close to the least principal stress 3 during subsequent fracture growth plagioclase compositions in all fractured areas change from oligoclase to albite or anorthite as a consequence of albitization and laumontization particle size distributions in dilational areas and in extension and in shear fracture fillings show that alteration of the major phase plagioclase is the fundamental process of grain size reduction in a variety of rock types cataclastic stresses and strain rates were controlled by alteration reaction rates in a coupled process that is a form of transformation modified deformation fracturing leading to weakening was assisted by stresses due to the 60 volume increase accompanying in situ laumontization of plagioclase and by stress corrosion and subcritical crack growth facilitated by alteration deformation was also enhanced by replacement of plagioclase by weaker laumontite such dilatant cataclasis is consistent with fluid pressure levels having remained high during deformation with effective least principal compressive stress close to zero this transformation modified deformation at low temperatures 90 t 250 c may be a common process in feldspar rich rocks the structures and microstructures described here could be used to distinguish the products of slow and fast cataclastic deformation

on the motion of hawaii and other mantle plumes abstract the hawaiian hotspot and its prominent bend at 47 ma have figured prominently in the development of ideas on the nature of mantle plumes and plate motion here i review paleomagnetic analyses associated with ocean drilling program odp leg 197 which together with studies of plate circuits and geodynamic modeling results indicate southward motion of the hawaiian hotspot during formation of the emperor seamounts these analyses confirm the long posited idea that mantle plumes should be influenced by mantle flow moreover these results indicate that late cretaceous early paleogene hotspot motion was the dominant factor in forming the emperor track and thus the famous bend morphology an important corollary of this finding on hotspot motion is that long term polar wander of earth which has been inaccurately assessed by viewing paleomagnetic data in a fixed hotspot reference frame has been far less than previously thought salient points for future research raised by the odp leg 197 paleomagnetic results include the scale of mantle flow recorded by hotspot track segments and the underlying processes that cause track segments to be dominated by mantle flow rather than plate motion for given time intervals

organization of the tectonic plates in the last 200 myr the present tessellation of the earth s surface into tectonic plates displays a remarkably regular plate size distribution described by either one sornette and pisarenko 2003 or two bird 2003 statistically distinct groups characterised by large and small plate size a unique distribution implies a hierarchical structure from the largest to the smallest plate alternatively two distributions indicate distinct evolutionary laws for large and small plates the first tied to mantle flow the second determined by a hierarchical fragmentation process we analyse detailed reconstructions of plate boundaries during the last 200 myr and find that i large and small plates display distinct statistical distributions ii the small plates display little organisational change since 60 ma and iii the large plates oscillate between heterogeneous 200 170 myr and 65 50 ma and homogeneous 120 100 ma plate tessellations on a timescale of about 100 myr heterogeneous states are reached more rapidly while the plate configuration decays into homogeneous states following a slower asymptotic curve suggesting that heterogeneous configurations are excited states while homogeneous tessellations are equilibrium states we explain this evolution by proposing a model that alternates between bottom and top driven earth dynamics physically described by fluid dynamic analogies the rayleigh benard and benard marangoni convection respectively we discuss the implications for true polar wander tpw global kinematic reorganisations 50 and 100 ma and the earth s magnetic field inversion frequency

fluid percolations in a fault zone a study of fluid inclusion planes in the st sylvestre granite northwest massif central france abstract a method for relating fluid flow evolution direction of fluid percolation to the bulk brittle network microfracture system at each state in a sequence of deformational events is presented fluids percolating through cracks are generally trapped as fluid inclusion planes alignment of fluid inclusions the method involves the use of fluid inclusions not only as a tool for the establishment of the physico chemical conditions under which fluids were entrapped but also as a microstructural marker of the geometry of permeability the study shows that through the determination of the paleopressures for each stage of fluid migration the chronology of the cracks and p t conditions prevailing during each crack formation can be established this methodology has focussed on a granitic example from the french massif central but may be applied to other geological environments

early fluid migration in a deep part of the larderello geothermal field a fluid inclusion study of the granite sill from well monteverdi 7 abstract metamorphic magmatic to hydrothermal fluids trapped in fluid inclusion planes fip were studied in a microfractured granite sill from a deep level 3483 3486 m b g l of the well monteverdi 7 larderello geothermal field this granite has recorded a complex succession of fluid migration through a dense set of healed microcracks nine generations of fluids trapped within specific microcrack networks are distinguished and include vapors and liquids of the c h o n s system aqueous liquids and brines with distinctive sets of cation chlorides licl nacl nacl cacl 2 and nacl detailed studies of the geometrical and chronological relationships between the fluid inclusion planes allow the establishment of a clear relative chronology with the c h o n s and licl fluids being earlier than the nacl cacl 2 and nacl aqueous liquids and brines in particular it can be shown that the c h o n s and licl fluids are strongly associated in space and time since the corresponding fluid inclusion planes display evidence of mutual contamination of inclusions within the planes significant changes in the physical chemical parameters occurred in the conditions of fluid trapping during the evolution of the larderello field all observed paleofluids have compositions differing from that encountered in the present day fluids and record a complex p t t x evolution from an early stage dominated by magmatic and metamorphic derived fluids towards a later stage dominated by more surficial waters at the early stage hydrothermal circulation occurred under lithostatic pressure at a depth of ca 4 5 km at temperatures as high as 450 500 c such conditions are very reminiscent of contact metamorphism and were probably created by the deep intrusive body detected by geophysical studies batini et al 1983 1985 the involved fluids are c h o n s vapors and liquids of metamorphic derivation and aqueous li bearing brines of probable magmatic derivation they are consistent with the hypothesis of the intrusive being a granite at the later stage physical conditions had evolved towards hydrostatic pressure at a depth close to the present 3 5 km one and temperatures of 400 350 c still higher than the present day ones early fluids were no longer present the observed fluids record the involvement of more surficial waters both meteoric and connate transient boiling events were probably occurring heralding the present day conditions vapor dominated geothermal system nevertheless no trapped fluids display similar composition and density with the present day geothermal fluids at larderello in old geothermal fields such as larderello it must be emphasized that the use of fluid inclusions in order to reconstruct past hydrothermal evolution is strongly dependent onto pressure estimation since significant temperature corrections may be needed in the larderello case failing to take in account the pressure effects would have led to seriously misleading conclusions

the bent hawaiian emperor hotspot track inheriting the mantle wind bends in volcanic hotspot lineaments best represented by the large elbow in the hawaiian emperor chain were thought to directly record changes in plate motion several lines of geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by mantle dynamics causes bends in hotspot tracks inverse modeling suggests that although deep flow near the core mantle boundary may have played a role in the hawaiian emperor bend capture of a plume by a ridge followed by changes in sub pacific mantle flow can better explain the observations thus hotspot tracks can reveal patterns of past mantle circulation

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

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 analysis of the brigham city weber segment boundary zone wasatch normal fault utah implications for fault growth and structure we investigate the history kinematics principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the wasatch fault zone utah three fault types developed in archean crystalline rocks reflect progressive uplift of fault related footwall rocks chlorite breccias and phyllonites reflect deep level reaction assisted plastic deformation along the north striking part of the segment planar fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment youngest faults are hematitecoated extremely narrow polished surfaces slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north striking portion of the segment slip vectors and fault orientation along the northwest striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace yet slip is confined to a broad fault parallel zone small faults at the southernmost tip of the segment indicate a strong influence of the north striking weber segment to the south inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that 3 may have reoriented approximately 60 over the life of the segment subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest plunging bedrock high which is reflected by a sharp bend in the brigham city segment the southern end of the brigham city segment may have started as a straight north striking fault which has bent due to changes in stress orientations and or interaction with the adjacent weber segment

ridge spotting a new test for pacific absolute plate motion models relative plate motions provide high resolution descriptions of motions of plates relative to other plates yet geodynamically motions of plates relative to the mantle are required since such motions can be attributed to forces e g slab pull and ridge push acting upon the plates various reference frames have been proposed such as the hot spot reference frame to link plate motions to a mantle framework unfortunately both accuracy and precision of absolute plate motion models lag behind those of relative plate motion models consequently it is paramount to use relative plate motions in improving our understanding of absolute plate motions a new technique called ridge spotting combines absolute and relative plate motions and examines the viability of proposed absolute plate motion models we test the method on six published pacific absolute plate motions models including fixed and moving hot spot models as well as a geodynamically derived model ridge spotting reconstructs the pacific farallon and pacific antarctica ridge systems over the last 80 myr all six absolute plate motion models predict large amounts of northward migration and monotonic clockwise rotation for the pacific farallon ridge a geodynamic implication of our ridge migration predictions is that the suggestion that the pacific farallon ridge may have been pinned by a large mantle upwelling is not supported unexpected or erratic ridge behaviors may be tied to limitations in the models themselves or for indo atlantic models discrepancies in the plate circuits used to project models into the pacific realm ridge spotting is promising and will be extended to include more plates and other ocean basins

micromechanical modeling of healed crack orientations as a paleostress indicator application to precambrian granite from illinois and wisconsin we have used a micromechanical four grain model to simulate intragranular cracks produced during primary cooling of a granitic pluton in a horizontally anisotropic stress field the orientations of vertical intragranular cracks were obtained using the criterion that they are perpendicular to the maximum principal tensile stress when it exceeds the local tensile strength the preferred orientation was always parallel to the maximum regional compressive stress direction the sharpness of the distribution peak increases with the ratio of the maximum to minimum horizontal stress we applied the model results to new and previous data on precambrian granites from wisconsin and illinois our new data are healed crack orientations and fluid inclusion properties in unoriented granite core from illinois borehole uph 2 the fluid inclusion measurements and isotopic age dates indicate that most healed cracks were formed during the primary cooling of the pluton the fluid inclusion characteristics and shape of the crack orientation distribution were almost identical to those previously determined for uph 3 1 km away kowallis et al 1987 suggesting that the preferred orientation of healed cracks in granite from uph 2 is the same as for uph 3 n25 w 5 the orientation distribution from the micromechanical model for a stress ratio of 4 is very similar to the healed crack orientation distribution measured in several precambrian granites from the midcontinent region we conclude that healed crack orientations are useful indicators of the paleostress field and that they may be used as a tool for orienting core in stable structurally simple regions

fluid inclusions in crack seal veins at dugald river mount isa inlier implications for palaeostress states and deformation conditions during orogenesis abstract an integrated geometrical investigation and microthermometric analysis of fluid inclusion trails in fibrous crack seal veins at dugald river mount isa inlier allowed distinction of two general types of fluid inclusions a co 2 ch 4 rich variety and a h 2 o rich variety at least three stages of fluid percolation are identified to be related to the vein formation trails of pseudosecondary inclusions have a constant orientation relative to the direction of the regional maximum principal shortening direction 3 this relationship allows the trails to be regarded as potential structural markers however trails of secondary inclusions generally show a deviation from the established bulk shortening direction and possibly reflect the overprinting of a later deformation event microthermometric data combined with knowledge of metamorphic reactions suggest that the p t path of crackseal deformation started at about 450 c and 2 8 kbar then crossed 340 c and 1 2 kbar and possibly terminated around 130 c and at near surface pressure corresponding to the regional crustal uplift this study shows the importance of combined geometrical and microthermometric studies of fluid inclusions for understanding the physical conditions of crack seal deformation a common phenomenon in low grade metamorphic terrains

deep mantle structure as a reference frame for movements in and on the earth earth s residual geoid is dominated by a degree 2 mode with elevated regions above large low shear wave velocity provinces on the core mantle boundary beneath africa and the pacific the edges of these deep mantle bodies when projected radially to the earth s surface correlate with the reconstructed positions of large igneous provinces and kimberlites since pangea formed about 320 million years ago using this surface to core mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander we have developed a model for absolute plate motion back to earliest paleozoic time 540 ma for the paleozoic we have identified six phases of slow oscillatory true polar wander during which the earth s axis of minimum moment of inertia was similar to that of mesozoic times the rates of paleozoic true polar wander 1 my are compatible with those in the mesozoic but absolute plate velocities are on average twice as high our reconstructions generate geologically plausible scenarios with large igneous provinces and kimberlites sourced from the margins of the large low shear wave velocity provinces as in mesozoic and cenozoic times this absolute kinematic model suggests that a degree 2 convection mode within the earth s mantle may have operated throughout the entire phanerozoic

coupling of south american and african plate motion and plate deformation although the african plate s northeastward absolute motion slowed abruptly 30 million years ago the south atlantic s spreading velocity has remained roughly constant over the past 80 million years thus requiring a simultaneous westward acceleration of the south american plate this plate velocity correlation occurs because the two plates are coupled to general mantle circulation the deceleration of the african plate due to its collision with the eurasian plate diverts mantle flow westward increasing the net basal driving torque and westward velocity of the south american plate one result of south america s higher plate velocity is the increased cordilleran activity along its western edge beginning at about 30 million years ago

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

identification of fluid inclusions in relation to their host microstructural domains in quartz by cathodoluminescence abstract the geometry of fluid trapping in relation to microfracture healing or filling was investigated using massive quartz vein samples characterized by multistage deformation and fluid trapping the multidisciplinary approach used in this study includes 1 mapping of oriented thin sections of quartz using the cathodoluminescence cl mode of a scanning electron microscope sem sem cl images display areas of different intensities from dark to white which correspond to the intensity of the cl and are caused by trace elements present in the quartz microfracture healing by newly formed quartz crystallization may occur under physical chemical conditions different from those which characterized the early quartz matrix formation thus evidences of linear cathodoluminescent markers have been searched and identified by paralleling sem cl images and transmitted light microphotographs data show that cathodoluminescence makes clearly visible the healed crack networks which are only detected in part by transmitted light microscopy the technique is essential to establish the microcrack chronology and to estimate the width affected by healing along a fluid inclusion trail such data are useful for the estimation of the microfracture permeability changes throughout the history of fluid migration 2 systematic and statistical analysis of the microstructural markers in horizontal and oriented planes using an interactive videographic analyzer which gives the geometry of the paleofluid pathways orientation dip and width of the veinlets fluid inclusion trails and cathodoluminescent microdomains 3 microthermometric and raman analyses of fluid inclusions in selected fluid inclusions for the determination of the p v t x conditions of fluid migration in a specific microcrack network studied quartz samples come from au mineralized quartz veins from the french massif central and some quartz samples from the cassiar mountains in british columbia have been observed for comparison in all samples the quartz matrix exhibits intense microfracturing suggesting strong brittle deformation which postdates the main stage of quartz lens formation the quartz veins are characterized by multistage quartz crystallization healed microfractures clear quartz bands as well as quartz comb veinlets were studied as functions of the physicochemical conditions in relation to stages of deformation and ore deposition this study shows that systematic measurements of microstructural marker orientations together with detailed fluid inclusion and mineral petrography and microthermometry may lead to a precise determination of ore fluid pathways and chronology

seeing red in cyclic stratigraphy spectral noise estimation for astrochronology 1 fundamental to the development of astronomical time scales is the recognition of oscillatory variability within stratigraphic data and its evaluation relative to a null noise hypothesis in this study monte carlo simulations are used to investigate the suitability of two commonly used noise hypotheses the conventional and robust ar1 approaches and the results highlight important limitations in both for cyclostratigraphic application perhaps most problematic the robust ar1 method can result in inflated confidence level estimates and excessive clumping of false positives within the low frequency portion of the spectrum especially when the underlying noise process has a high lag 1 autocorrelation given typical cyclostratigraphic records this technique will often impose significant eccentricity band variability even in the case of pure ar1 noise an alternative spectral noise estimation method is proposed to overcome these problems which simultaneously allows for departures from the ar1 assumption and obtains high statistical power that is the ability to accurately identify astronomical signals when they are present in the data we apply the method to un tuned 18o data from miocene sediments of the ceara rise indicating statistically significant spectral power at frequencies that are consistent with the published orbital interpretation of weedon et al 1997 furthermore evaluation of the frequency arrangement of the significant spatial bedding periods using the average spectral misfit method for astrochronologic testing reveals that the null hypothesis of no orbital influence can be rejected with a high degree of confidence the 99 8 confidence level

uplift unbuckling and collapse flexural history and isostasy of the wind river range and granite mountains wyoming two dimensional gravity and flexural models constrain the structure and isostatic compensation of the wind river range and granite mountains wyoming these ranges are asymmetric basement cored laramide uplifts their large positive isostatic gravity anomalies imply that the topography of the uplifts is not locally compensated for the wind river range gravity data are consistent with thrusting along a listric fault which flattens and soles out at 30 km depth with no detectable offset on the moho we model the large bouguer gravity high over the range with high density lower crustal rocks emplaced over lower density middle crustal rocks along the wind river thrust the high positive gravity anomaly over the granite mountains can be modeled as an extension of the south pass greenstone belt from the southern wind river range eastward beneath the emigrant trail thrust a crustal root as required in local airy compensation for either range requires unreasonably high density material in the upper crust to obtain the observed anomaly combined gravity and flexural models show that a flexural rigidity at least 2 55 1022 n m maximum deflection of 2 km of the moho is required to satisfy the gravity data our results are consistent with models of uplift due to horizontal compression and support of the resulting topography by lithospheric strength an otherwise puzzling episode of oligocene uplift of the wind river range can be explained as unbuckling decrease of deflection of lithosphere accompanying release of laramide compressional stresses flexurally supported uplifts will collapse if the lithosphere is weakened a number of laramide uplifts have collapsed primarily east west trending structures such as the granite mountains the southern end of the wind river range and the uinta mountains we propose that collapse has occurred along zones of established weakness in the crust in response to north south extension

fluid inclusions tectonic indicators abstract during the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters density of the mineralising fluids apart from a few pioneering works in the middle of the century fluid inclusions were not used as structural markers before the late 1980s during the same time experiments as well as observations of natural samples have shown that the volume shape and density of fluid inclusions may change when submitted to pressure temperature conditions different from their trapping isochores in view of these experiments and based on natural examples from the higher himalaya metamorphic rocks it is evident how fluid inclusions can be used as thermobarometric structural and tectonic markers points of interest for further studies on fluid inclusions in nature and experiments are also stressed

mantle updrafts and mechanisms of oceanic volcanism convection in an isolated planet is characterized by narrow downwellings and broad updrafts consequences of archimedes principle the cooling required by the second law of thermodynamics and the effect of compression on material properties a mature cooling planet with a conductive low viscosity core develops a thick insulating surface boundary layer with a thermal maximum a subadiabatic interior and a cooling highly conductive but thin boundary layer above the core parts of the surface layer sink into the interior displacing older colder material which is entrained by spreading ridges magma characteristics of intraplate volcanoes are derived from within the upper boundary layer upper mantle features revealed by seismic tomography and that are apparently related to surface volcanoes are intrinsically broad and are not due to unresolved narrow jets their morphology aspect ratio inferred ascent rate and temperature show that they are passively responding to downward fluxes as appropriate for a cooling planet that is losing more heat through its surface than is being provided from its core or from radioactive heating response to doward flux is the inverse of the heat pipe mantle plume mode of planetary cooling shear driven melt extraction from the surface boundary layer explains volcanic provinces such as yellowstone hawaii and samoa passive upwellings from deeper in the upper mantle feed ridges and near ridge hotspots and others interact with the sheared and metasomatized surface layer normal plate tectonic processes are responsible both for plate boundary and intraplate swells and volcanism

microfracturing and regional stress field a study of the preferred orientations of fluid inclusion planes in a granite from the massif central france abstract a study of the deformation of a granitic massif indicates a relationship between palaeostress fields and the geometry of microfractures as defined by fluid inclusion trails that is healed microfractures a statistical study of the distribution of the trails leads to the following conclusions trails exhibit several distinct preferred orientations on the scale of a grain which may be observed in many samples the orientations of the trails are similar to those of micro and mesoscale fractures in the granite and the dominant trail direction is parallel to the main direction of regional shortening thus fluid inclusion trails are mode i cracks which can be used as excellent markers of palaeostress fields

tertiary minette and melanephelinite dikes wasatch plateau utah records of mantle heterogeneities and changing tectonics a swarm of minette and melanephelinite dikes is exposed over 2500 km2 in and near the wasatch plateau central utah along the western margin of the colorado plateaus in the transition zone with the basin and range province to date 110 vertical dikes in 25 dike sets have been recognized strikes shift from about n80 w for 24 ma dikes to about n60 w for 18 ma to due north for 8 7 m y these orientations are consistent with a shift from east west oligocene compression associated with subduction to east west late miocene crustal extension minettes are the most common rock type mica rich minette and mica bearing melanephelinite occurs in 24 ma dikes whereas more ordinary minette is found in 8 7 ma dikes one melanephelinite dike is 18 ma these mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions they appear to have crystallized from primitive magmas major trace element and nd sr isotopic data indicate that melanephelinite which has similarities to ocean island basalt was derived from small degree melts of mantle with a chondritic sm nd ratio probably located in the asthenosphere but it is difficult to rule out a lithospheric source in contrast mica bearing rocks mica melanephelinite and both types of minette are more potassic and have trace element patterns with strong nb ta depletions and sr nd isotopic compositions caused by involvement with a component from heterogeneously enriched lithospheric mantle with long term enrichment of rb or light rare earth elements ree epsilon nd as low as 15 in minette light ree enrichment must have occurred anciently in the mid proterozoic when the lithosphere was formed and is not a result of cenozoic subduction processes after about 25 ma foundering of the subducting farallon plate may have triggered upwelling of warm asthenospheric mantle to the base of the lithosphere melanephelinite magma may have separated from the asthenosphere and while rising through the lithosphere provided heat for lithospheric magma generation varying degrees of interaction between melanephelinite and small potassic melt fractions derived from the lithospheric mantle can explain the gradational character of the melanephelinite to minette suite

petrogenesis and geological history of a uranium source rock a case study in northeastern washington u s a abstract a small 4 km 2 drainage basin in northeastern washington contains highly uraniferous groundwater and highly uraniferous peaty sediments of holocene age the u is derived from granitic bedrock that underlies the entire drainage basin and that contains 9 16 ppm u this local bedrock was studied by petrographic chemical and isotopic methods to determine conditions of its petrogenesis and post emplacement history that may have contributed to its present high u content and source rock capability the original magma was derived by anatexis of precambrian continental crust of probable mixed metaigneous and metasedimentary character mineral melt partitioning controlled the enrichment of u in chemically evolved phases of the crystallizing melt following emplacement in the upper crust at 100ma the pluton interacted with meteoric hydrothermal water at ambient temperatures 300 c locally intense fracturing promoted alteration and fracturing and alteration probably continued during later regional uplift in the eocene regional uplift was followed by low temperature alteration and weathering in the middle to late tertiary the combined result of hydrothermal alteration and low temperature alteration and weathering was the redistribution of u from primary mineral hosts such as allanite to new sites on fracture surfaces and in secondary minerals such as hematite zones of highly fractured and altered rock show the most obvious evidence of this process a model is proposed in which high angle fractures beneath the drainage basin were the sites of tertiary supergene enrichments of u recent glacio isostatic uplift has elevated these older enriched zones to shallow levels where they are now being leached by oxidizing groundwater the chemistry mineralogy texture and geological history of this u source rock suggest criteria for locating other granitic terrane that may contain uraniferous waters and associated young surficial u deposits the details of u distribution and mobility at this site also apply to the general topic of u mobility in granitic rocks

stress and fracture evolution in a cooling pluton an example from the diamond joe stock western arizona u s a abstract an analytical model of fracturing in a crystallizing cylindrical pluton is applied to the natural fracture distribution in the diamond joe stock western arizona u s a the model accounts for three inputs to the stress during cooling thermal contraction magma pressure and regional tectonics fracture type shear or tensile orientation and density were determined at over 300 locations throughout the 8 km diameter stock a distinct pattern of fractures radiates outward from the center of the stock a deflection of these fracture trends toward the regional maximum principal stress direction provides a constraint on the magnitude and orientation of regional stress at the time of crystallization and cooling a calculated model of the fracture pattern suggests that regional maximum and minimum principal stress were 75 and 15 mpa with the orientation controlled by local tectonism the distribution of fractures appears to be insensitive to variations in magma pressure except near the wall of the chamber hence little constraint on magma pressure could be derived from the model thermal gradients during cooling and regional stress appeared to have a greater effect than magma pressure on the overall fracture distribution larger thermal gradients during the early stages of crystallization led to abundant early tensile fractures assuming that the crystalline portions of the pluton behaved elastically during cooling and that all strain was accommodated by fractures the variation of theoretical strain is similar to the distribution of fracture densities within the stock

paleostress history of the basin and range province in western utah and eastern nevada from healed microfracture orientations in granites healed microfractures in quartz grains in 14 to 169 ma granites from the basin and range province in western utah and eastern nevada record a history of paleostress the microfracture data are in good agreement with the orientations of paleostresses determined for this same region by using orientations of dikes emplaced into only slightly older igneous host rocks samples generally have two or three steeply dipping microfracture sets one set is related to the tectonic stress field at the time of magma emplacement it strikes northeast southwest in 31 169 ma samples and east northeast west southwest in 25 30 ma samples rotates to east west by about 22 ma and strikes northwest southeast in the youngest samples about 14 ma plutons younger than about 14 ma are not exposed at the surface in western utah and eastern nevada other crack sets in the samples could have formed during cooling of the pluton during a post cooling regional thermal or tectonic event or during some local stress event a strong north south trend in these other crack sets may suggest a formation during basin and range extension

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

factors influencing the locations of hot spots determined by the hot spotting technique seafloor beneath hot spot produced seamounts have crustal flow lines that intersect at the hot spot locations provided there is more than one stage rotation and that hot spots are stationary convolving seamount shapes with their associated flow lines yields an image of cumulative volcano amplitude cva we call the technique of correlating local cva maxima with hot spot locations hot spotting this technique is geometrical and requires no age information for seamounts or seafloor we analyze synthetic and actual seamount data and demonstrate that factors such as across trail scatter in seamount locations inaccurate stage poles and migration of hot spots all exert strong influences on the locations and appearances of cva maxima the interpretation of raw cva images is therefore not straightforward

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

toward a generalized plate motion reference frame an absolute plate motion apm model is required to address issues such as the thermochemical evolution of earth s mantle all apm models have to rely on indirect inferences including those based on hot spots and seismic anisotropy each with their own set of uncertainties here we explore a seafloor spreading aligned reference frame we show that this reference frame fits azimuthal seismic anisotropy in the uppermost mantle very well the corresponding euler pole is close to those of hot spot reference frames ridge motion minimizing models and geodynamic estimates of net rotation and predicts clear trench motion patterns we conclude that a net rotation pole guided by the spreading aligned model at 64 e 61 s with moderate rotation of 0 2 0 3 myr could indeed represent a standard comprehensive reference frame for present day plate motions with respect to the deep mantle

tests of fixity of the indo atlantic hot spots relative to pacific hot spots rates of inter hot spot motion have been debated for decades herein we present updated predictions for the tracks of the tristan da cunha reunion and iceland hot spots assuming them to be fixed relative to pacific hot spots uncertainties in pacific hot spot rotations which include uncertainties in the current locations of hot spots of 100 200 km are combined with uncertainties in relative plate motions accumulated through the plate circuit to obtain the final uncertainty in the predicted positions including uncertainties of 150 200 km in the current locations of the indo atlantic hot spots improvements to reconstruction methods to relative plate reconstructions to age dates along the tracks and to the geomagnetic reversal timescale lead to significant changes from prior results when compared with the observed tracks the predicted tracks indicate nominal rates of motion of only 2 6 mm a 1 of these indo atlantic hot spots relative to pacific hot spots over the past 48 ma within the uncertainties the rates range from no motion to rates as high as 8 13 mm a 1 for reconstructions prior to 48 ma b p however the apparent rates of inter hot spot motion are much larger 46 55 20 mm a 1 if the motion occurred entirely between 68 ma b p and 48 ma b p either hot spots moved rapidly before 48 ma b p and slowed drastically at 48 ma b p or global plate circuits through antarctica become less reliable as one goes increasingly further into the past most paleomagnetic data favor the latter explanation

indian and african plate motions driven by the push force of the reunion plume head mantle plumes are thought to play an important part in the earth s tectonics yet it has been difficult to isolate the effect that plumes have on plate motions here we analyse the plate motions involved in two apparently disparate events the unusually rapid motion of india between 67 and 52 million years ago and a contemporaneous transitory slowing of africa s motion and show that the events are coupled with the common element being the position of the indian and african plates relative to the location of the reunion plume head the synchroneity of these events suggests that they were both driven by the force of the reunion plume head the recognition of this plume force has substantial tectonic implications the speed up and slowdown of india the possible cessation of convergence between africa and eurasia in the palaeocene epoch and the enigmatic bends of the fracture zones on the southwest indian ridge can all be attributed to the reunion plume an analysis of the plate motions involved in two apparently disparate tectonic events the unusually rapid motion of india between 67 million and 52 million years ago and a transitory slowing of africa s motion at around the same time has uncovered a possible common cause for the two the reunion volcanic hotspot beneath the indian ocean has been active for about 67 million years and the close coupling of the motion of india and africa strongly suggests that it is the waxing and waning of the reunion plume head that is driving these motions

constraints on past plate and mantle motion from new ages for the hawaiian emperor seamount chain 1 estimates of the relative motion between the hawaiian and louisville hot spots have consequences for understanding the role and character of deep pacific mantle return flow the relative motion between these primary hot spots can be inferred by comparing the age records for their seamount trails we report 40ar 39ar ages for 18 lavas from 10 seamounts along the hawaiian emperor seamount chain hesc showing that volcanism started in the sharp portion of the hawaiian emperor bend heb at 47 5 ma and continued for 5 myr the slope of the along track distance from the currently active hawaiian hot spot plotted versus age is constant 57 2 km myr between 57 and 25 ma in the central 1900 km of the seamount chain including the heb this model predicts an age for the oldest emperor seamounts that matches published ages implying that a linear age distance relationship might extend back to at least 82 ma in contrast hawaiian age progression was much faster since at least 15 ma and possibly as early as 27 ma linear age distance relations for the hawaii emperor and louisville seamount chains predict 300 km overall hot spot relative motion between 80 and 47 5 ma in broad agreement with numerical models of plumes in a convecting mantle and paleomagnetic data we show that a change in hot spot relative motion may also have occurred between 55 ma and 50 ma we interpret this change in hot spot motion as evidence that the heb reflects a combination of hot spot and plate motion changes driven by the same plate mantle reorganization

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated

multistage deformation of au quartz veins laurieras french massif central evidence for late gold introduction from microstructural isotopic and fluid inclusion studies abstract the relative chronology of fluid migration quartz and au deposition in a silicified fault from the main au district laurieras st yrieix district from northern french massif central has been determined from microstructural fluid inclusion isotopic and ore mineral evidences three main stages of fluid circulation microfracturing and quartz crystallization and ore deposition were distinguished on the basis of textural relationships and the pressure temperature and composition of the palaeo fluids 1 a series of early fluid events was responsible for the localized drainage of retrograde metamorphic fluids along the main fault and the subsequent sealing by milky and microcrystalline quartz preceeded the main au ore stages early fluids were aqueous carbonic trapped under lithostatic to sublithostatic pressures at temperatures in the range 350 500 c subsequently several types of microstructures were developed in the early quartz matrix 2 ns microfractures filled by clear quartz arsenopyrite and boulangerite i contain significant refractory gold concentrations clear quartz formed from aqueous carbonic fluids of lower densities than those of the earlier fluids significant pressure drops down to pressures around 55 mpa were responsible for a local immiscibility of the aqueous carbonic fluids at temperatures of 340 20 c 3 the main ore stage is characterized by the formation of dense sets of sub vertical ew microfractures healed fluid inclusion planes in quartz and filled by ore minerals native gold galena and boulangerite ii when they crosscut earlier sulfides the fluids are aqueous with low and decreasing salinity and probable trapping temperatures around 230 c isotopic data obtained on microfissured quartz indicate these dilute aqueous fluids may be considered as meteoric waters that deeply infiltrated the crust late microfissuring of a mesothermal quartz vein originally barren only with pyrite and arsenopyrite appears to be the main factor controlling gold enrichment it can be related to late hercynian deformational stages disconnected from the early fault formation and silicification these late stages which affected the hercynian basement during its uplift are of critical importance for the formation of au ores we concluded that this type of au ore formed under rather shallow conditions is distinct from those generally described in most mesothermal au veins