distribution and mechanisms of strain within rocks on the northwest ramp of pine mountain block southern appalachian foreland a field test of theory fractures faults stylolites and strain determined from an analysis of twinning in calcite in the mississippian newman limestone at the base of the pine mountain block show that deformation was accomplished primarily by a combination of bedding plane slip and layer parallel shortening poles to joints are either parallel to strike of the thrust sheet or are in the transport plane small slip surfaces with syntaxial calcite fibers lie either parallel to bedding or parallel to the strike of the pine mountain block and dip steeply movement along the former is in the direction of transport of the block whereas movement on the latter is approximately along strike tectonic stylolites are rare but appear to be controlled by subtle changes in lithology wackestones contain more stylolites than do packstones strains in calcite from closely spaced samples show the converse relationship they are highest in units without stylolites maximum shortening strain e 1 10 4 and lowest in units with stylolites or small faults e 1 2 0 the maximum principal shortening strain directions however are identical within the error and are parallel to both bedding and the transport direction a sequence of events in this portion of the pine mountain block is 1 layer parallel shortening producing tectonic stylolites and a strong preferred orientation of compression axes 2 development of fractures parallel to the strike of the block on the lower hinge of the ramp to accommodate stretching parallel to the transport direction these strike parallel fractures can also be interpreted as having occurred quite late 3 during 2 above and continuing the start of bedding plane slip marked by syntaxial calcite fibers which trend in the direction of transport and 4 complications such as shear parallel to the strike of the pine mountain fault as a result of motion of the entire block between bounding tear faults events 2 through 4 most likely overlapped in time these data suggest that the acquisition of dip of the rocks on the ramp was primarily accomplished by bedding plane slip on the slip surfaces which experienced a shear stress of

crosshole ip imaging for engineering and environmental applications induced polarization ip imaging is a promising tool in engineering and environmental studies application of this technique for near surface investigations has previously been limited by incomplete understanding of the physicochemical controls on the ip response together with a lack of appropriate methods for data inversion as laboratory studies have shown description of ip in terms of complex electrical conductivity enables access to various structural characteristics pertinent to practical issues such as subsurface lithology definition hydraulic permeability estimation or hydrocarbon contaminant mapping in particular analysis in terms of real and imaginary conductivity components offers improved lithological characterization since surface polarization effects are separated from electrolytic and surface conduction effects an occam type ip inversion algorithm based on complex algebra is described which accounts for these advances in ip interpretation by directly solving for complex conductivity results from crosshole applications at two case study sites demonstrate the suitability of the ip imaging approach for subsurface characterization in the first case study the imaging results correlate with the observed complex sequence of quaternary sediments at a waste disposal site characterization of the polarizability of these sediments offers significant value in lithological differentiation in the second case study the results of ip imaging at a hydrocarbon contaminated site illustrate the potential of the method in environmental studies the hydrocarbon location is clearly evident from the ip image and a markedly different response is observed at an uncontaminated region of the site by adopting empirical structural electrical relationships images of textural and hydraulic properties are estimated as a step toward improved quantitative characterization the success of the method for these contrasting applications supports further investigation into understanding the physical and chemical processes that control observed ip

timing of carbonate mineral precipitation and fluid flow in sea floor basalts northwest indian ocean the strontium isotope ratios of authigenic carbonates from indian ocean sea floor basalts have been used to determine the timing of carbonate mineral precipitation and fluid flow the samples include calcites from 57 2 ma crust from ocean drilling project odp site 715 and calcites aragonites and siderites from 63 7 ma crust from odp site 707 at site 715 calcite precipitation may have begun at any time after the basalts cooled and it continued until approximately 31 ma or 26 m y after basalt eruption at site 707 aragonite and siderite did not begin to precipitate until about 36 ma almost 30 m y after basalt eruption and continued to precipitate until at least 30 and 28 ma respectively calcite precipitation began at approximately 32 ma and continued until 22 ma these ages suggest that vein mineral deposition and low temperature fluid circulation in the ocean crust may continue for much longer periods of time than previously observed

spectral induced polarization characteristics of rock types from the skarn deposit in gagok mine taebaeksan basin south korea spectral induced polarization sip which measures the responses to alternative current of multi frequencies below 1 khz is an excellent technique for exploring the economic minerals in the ore deposits however the studies on the sip characteristics of rocks from ore deposits have rarely been conducted due to the decline of mining activities for effective exploration the sip responses of in situ rocks should be recognized first therefore this study was conducted to understand the characteristics of the sip responses depending on the rock type using the laboratory measurements rock samples were obtained from both host rocks and skarn rocks in the gagok mine which is a typical skarn deposit in south korea the responses of the host rocks which include dielectric minerals were found to depend on the induced polarization ip effect of the compact layer in contrast the responses of the skarn rocks which were mixtures of dielectric and metallic minerals depended on the ip effect of the diffuse layer this study clearly demonstrates that sip responses are determined by the electrochemical characteristics of the composing minerals

calcite strains kinematic indicators and magnetic flow fabric of a proterozoic pseudotachylyte swarm minnesota river valley usa abstract near granite falls minnesota sub parallel pseudotachylyte mafic dikes and calcite veins crosscut archean granulite facies rocks in the minnesota river valley adjacent to the north dipping yellow medicine shear zone ymsz n80 e that separates the montevideo and morton tectonic terranes the docking of these two archean terranes occurred prior to intrusion of the 2 067 ga kenora kabetogama dike swarm as demonstrated by aeromagnetic anomalies correlated with field exposures that cross the ymsz without offset tectonic adjustments along the ymsz associated with the penokean orogeny 1 8 ga are likely responsible for pseudotachylyte formation pseudotachylyte is exposed in 22 sub parallel veins n80 e 90 each less than 2 cm wide across an outcrop width of 45 m the pseudotachylyte matrix is commonly banded and contains crystal fragments quartz plagioclase amphibole rutile apatite ilmenite ulvospinel magnetite microlites flow banding swirls amygdules filled with calcite ankerite and siderite collapsed vesicles and abundant lithic clasts pseudotachylyte formed in a number of phases kinematic reconstruction is complex utilizing winged porphyroclasts s c structures in the country rock and fault drag indicators along the pseudotachylyte zones dextral motion along the ymsz is the most common observation mechanically twinned calcite within amygdules in the pseudotachylyte preserves horizontal shortening normal to the pseudotachylyte strike calcite veins are apparently contemporaneous with the pseudotachylyte one set preserves twinning strains identical to the calcite amygdule strains and the second set contains a horizontal vein parallel n70 e shortening strain the pseudotachylyte contains a flow fabric as determined by ams techniques that is a proxy for vertical flow kmax is vertical the kenora kabetogama dikes identified geochemically are locally parallel to the pseudotachylyte and the adjacent ymsz tectonic suture and preserve a vertical to horizontal dike parallel ams fabric from east franklin to west granite falls hornblende andesite dikes 055 1 8 ga are not found south of the suture are not associated with pseudotachylyte and have a different paleopole and ams fabric

spectral ip studies of finnish ore prospects spectral induced polarization ip field measurements both in the frequency domain and in the time domain were made at six early proterozoic sulfide oxide and graphitic gneiss deposits in finland core samples were also measured the textures of the mineralizations were studied from thin and polished sections deposits with large differences in texture such as graphitic gneiss and coarse grained disseminated sulfide can be separated on the basis of their phasespectra time constants a good correlation was found between the observed grain size in thin sections and the grain size calculated from the apparent field survey phase spectra in the case of homogeneous disseminated textures the measured frequency domain phase spectra and phase spectra calculated from the time domain cole cole parameters are very similar in a comparable frequency band for both techniques the phase maximum lies beyond the lowest measured frequency the possibilities to expand the frequency band to levels low enough to rea

orientation of in situ stresses in the oceanic crust two in situ measurements of principal stress directions have been made in dsdp holes 504b south of the costa rica rift on the nazca plate and 597c west of the east pacific rise on the pacific plate in both cases the orientations of in situ principal stresses determined from borehole breakouts are consistent with the stress directions inferred from intraplate earthquakes located near the sites

regional shortening fabrics in eastern north america far field stress transmission from the appalachian ouachita orogenic belt paleozoic carbonates of cratonic eastern north america comprise the footwall of the appalachian ouachita fold and thrust belt and contain a layer parallel shortening lps fabric that is preserved by mechanically twinned calcite shortening directions are generally parallel to the appalachian ouachita thrust transport direction in carbonates of the thrust belt proper restored width 400 km and within carbonates up to 1700 km into the foreland giving a pre thrusting sedimentary prism 2100 km wide through which compressive orogenic stresses were transmitted the shortening strain magnitudes 6 and the inferred calcite twinning differential stress magnitudes 90 mpa decrease exponentially away from the orogenic front calcite twinning strain patterns in other adjacent tectonic provinces such as the grenville laramide keweenawan rift and newark basin are distinct from the twinning strains preserved in the cratonic paleozoic carbonates appalachian orogen far field stresses calcite microstructures

experimental evidence for high temperature felsic melts formed during basaltic intrusion of the deep crust basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high alumina olivine tholeiite haot and metapelite at 11 kbar rapid diffusion of cao and na 2 o into the metapelite and h 2 o k 2 o sio 2 and al 2 o 3 into the basalt resulted in spectacular changes in the phase assemblage garnet megacrysts crystallized profusely across the basalt layer cumulate assemblages closely resembled rocks found in underplating localities and glass compositions graded from dacite to rhyolite even at temperatures as high as 1150 c silicic melts can be generated during interaction of basalt with metapelite at temperatures well above the h 2 o saturated granite minimum calculations based on these results and comparison with natural occurrences suggest that high temperature felsic melts with 2 o can form and segregate at rates of 1 cm yr

zinc lead skarns of the yeonhwa ulchin district south korea the yeonhwa i yeonhwa ii and ulchin mines producing 13 5 million metric tons of ore in the period 1962 to 1981 account for 80 percent of south korea s zn and pb production these skarn deposits occur in basal cambrian limestone locally dolomitic overlying slate quartzite and precambrian granitic basement and they are the result of hydrothermal activity associated with felsic calc alkaline plutonism of late cretaceous to early tertiary age skarn at yeonhwa i consists of pipes of pyroxene or garnet at depth branching to veins of rhodocrosite pyrite near the surface in an area of sparse quartz porphyry dikes in contrast skarns at yeonhwa ii and ulchin display higher garnet pyroxene ratios and occur as stratabound lenses and irregular masses at or near contacts with larger bodies of quartz monzonite exoskarns are zoned in the sequence garnet pyroxene pyroxenoid marble relative to slate limestone contacts or fissures in limestone the lack of zoning relative to igneous contacts and the presence of porphyry limestone contacts without skarn indicate that skarn forming fluids did not originate from magma at the present level of exposure electron microprobe analyses reveal that calc silicates are notably enriched in mn and fe garnets are intermediate grandite andradite with 1 to 5 mole percent spessartine calcic rhodonite is characteristic of endoskarns and garnet zones whereas a more calcic pyroxenoid manganoan bustamite occurs in pyroxene skarn pyroxenes are manganoan salite ferrosalite average fe mn ratio and mole hedenbergite in pyroxene decreases from 5 7 65 at ulchin to 3 5 59 at yeonhwa ii to 1 6 30 at yeonhwa i this ratio also decreases with distance from garnet zones and correlates with an increase in mg suggesting a progressive depletion of fe in solution due to precipitation of iron rich garnet near channelways and a progressive enrichment in mg in the fluid as it approached equilibrium with dolomitic wall rocks based on this and the geologic setting ulchin is the most proximal and yeonhwa ii the most distal skarn in terms of distance from their sources of fluids pyrrhotite accompanied by sphalerite lesser galena and minor chalcopyrite are concentrated in pyroxene zones variably altered to chlorite mn rich clays calcite quartz and fluorite pyroxene compositions indicate that the maximum oxidation sulfidation state that could have been imposed on the ore fluid by early skarn was located near the pyrrhotite pyrite magnetite buffer only the hedenbergitic skarn at ulchin lay below this buffer and could have controlled the precipitation of pyrrhotite rather than pyrite ulchin also contains the district s highest grades of pb zn in skarn suggesting that reduction of aqueous sulfate by ferrous iron was an important factor in ore deposition later ore fluids and fluids that reached beyond skarn in the most distal deposit envolved to higher sulfidation states as demonstrated by pyrite rhodochrosite veins that cut skarn and pyrrhotite ore and by the near surface pyrite sphalerite galena rhodocrosite veins in limestone overlying skarn pipes at yeonhwa i

fold axis parallel rotation within the laramide derby dome fold wind river basin wyoming usa abstract derby dome a doubly plunging anticline 7 3 km on the eastern flank of the wind river range wyoming trends nw se in response to the regional ne sw directed shortening of the cretaceous eocene laramide orogeny mesozoic sediments are exposed around the fold hinge above an east dipping thrust fault that offsets archean crystalline rocks at depth stress and strain ellipsoidal data were determined through the measurement of mechanically twinned calcite in limestones triassic alcova through j k morrison formation rocks 13 samples calcite cements 5 samples and synfolding calcite veins 16 samples around the northern half of the fold on the outer limbs of the fold the maximum shortening strain axis 3 5 15 nevs in the limestones and cements is sub horizontal layer parallel and normal ne sw to the fold hinge reflecting regional sevier laramide shortening this regional layer parallel strain fabric is rotated into a fold axis parallel orientation nw se near the fold hinge indicating that significant rotations occurred during folding synfolding calcite veins of varying orientations also preserve a local sub horizontal hinge parallel shortening strain 4 0 17 nevs suggesting that the regional laramide stress and strain field was locally rotated into parallelism with the fold during shortening and displacement on the underlying thrust fault in both the country rock cement and vein data sets the strain overprint noise nevs increases toward the fold hinge inferred differential stress magnitudes are also higher for the vein calcite than for the country rock limestones or cements and there is no interpretable pattern around the fold avg 560 bars range of 240 2000 bars fracture measurements n 74 in different lithologies have different orientations on each side of the adjacent dallas dome fold suggesting layer parallel rotation during folding or active fracturing occurred uniquely on each fold limb

calcite twinning constraints on stress strain fields along the mid atlantic ridge iceland calcite veins and amygdule fillings within basalts older than 0 7 ma are mechanically twinned and preserve a subhorizontal shortening strain that resulted from compression and shortening normal to the mid atlantic ridge on both sides of the plate boundary our sample suite includes 19 specimens 7 from the north american plate 4 veins 3 amygdule fillings and 12 from the european plate 9 veins 3 amygdule fillings 18 of which record ridge normal subhorizontal shortening five of the strain analyses two from the north american plate and three from the european plate have a high percentage of negative expected values and these secondary strain results record a ridge parallel shortening strain with plunges that vary parallel to the ridge axis averaged shortening strain magnitudes for the twinned calcite 2 5 european plate 6 02 north american plate and inferred differential stresses 48 mpa that caused the twinning are modest and are thought to represent regional tectonic conditions i e ridge push not local e g hotspot or glacial loading phenomena

the early history of the induced polarization method this paper traces the early development of the induced polarization method starting with field observations by conrad schlumberger in a mining region in france about 1913 starting about 1929 he introduced this technique into hydrocarbon borehole logging in the ussr and this resulted in further development in eastern and western europe

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

deformation mechanisms in the southeastern ramp region of the pine mountain block tennessee a field and petrofabric study of the southwestern part of the pine mountain thrust sheet shows that most deformation features may be attributed to one of two processes the first layer parallel compression is responsible for a a layer parallel maximum shortening direction in the mechanically twinned fabric of the limestone and dolomite country rock b transport parallel extension fractures and joints c transport normal stylolite surfaces and d some small thrusts the second bending at thrust ramp hinges is responsible for a non bedding parallel maximum shortening directions in mechanically twinned calcite from fracture fillings b transport normal extension fractures and joints and c small normal faults possible explanations for the failure of calcite in the country rock to twin in response to bending at the ramp include both 1 an early imposed pre thrusting calcite twin strain hardening and 2 deformation of the country rock by competing mechanisms such as pressure solution small normal faults are parallel to the strike of the thrust sheet and seem to have evolved from strike parallel extension fractures in response to continued extension both small thrusts and stylolite surfaces are rare in outcrop there seems to be no major change in deformation of rocks which have traveled through different fractions of the total ramp length so long as the rocks have passed onto the ramp

calcite twin analysis in syntectonic calcite cape fold belt south africa implications for fold and cleavage formation within a shallow thrust front abstract lithologies of two formations in the frontal ranges of the cape fold belt south africa host synorogenic calcite precipitated at low metamorphic grade during the end permian deformation of this gondwana wide orogen the calcite crystals are mechanically twinned and this deformation is linked to both folding and the formation of an axial planar cleavage with associated extensional veins twinning strain analysis reveals a complex rotational synfolding history in the upper thin layered prince albert formation twinned calcite within the fold preserves two unique strain events the primary shortening strain fabric is layer parallel and transport parallel north south the shortening strain overprint 5 82 is layer normal and plunges steeply to the northeast these results are inconsistent with a flexural slip folding mechanism by contrast a homogeneous syn cleavage stress strain field is recorded in the underlying massive thick bedded dwyka tillite analyses of calcite twins in clast hosted syn cleavage fibrous calcite and rare limestone clasts define a sub horizontal n s shortening and sub horizontal e w extension the intermediate axis e 2 is vertical and preserves shortening 5 3 the extension axis e 3 is horizontal and parallel to the clast tension gash fracture plane these deformed syn cleavage calcite materials provide an independent constraint to the debate about contemporaneous stress strain fields associated with folding and formation of an axial planar cleavage

laboratory technique for measurement of spectral induced polarization response of soil sampies1 abstract laboratory measurements of soil samples are necessary to assess the effect of mineralogy grain size distribution moisture content and electrolyte composition on the resistivity spectrum of soil material laboratory results are also required for the interpretation of field data induced polarization phenomena in glacial soils are poorly understood and so far no convenient laboratory techniques are available for its measurement coarse grain size and the need to measure unsaturated samples and to monitor the homogeneity of the sample require a sample holder electrode construction that differs from those presented in clay mineral studies this study presents a spectral induced polarization laboratory system that is suitable for measuring fine and coarse grained and both saturated and unsaturated soil samples the noise caused by the electrode electrolyte interface is studied in detail it is shown that easy to use platinum or acid free steel potential electrodes are convenient over a broad frequency band ranging from 0 016hz up to more than 1000 hz the laboratory experiments and comparisons between laboratory and field results also indicate that sampling and sample packing procedures have only a minor influence on the phase spectrum of glacial soils

comparing induced polarization responses from airborne inductive and galvanic ground systems tasmania abstractwe have first analyzed the ability of polarizable and superparamagnetic thin sheets in the near surface to fit airborne electromagnetic aem data using data from western tasmania then we analyzed the results of such fitting in the context of geologic mapping and available ground induced polarization ip data small to large ip effects were found to considerably improve the fit to the observed aem data and the overall fitted ip parameters were spatially consistent however the locations of anomalous ip parameters were quite distinct from anomalies in other geophysical data the airborne chargeability highs were adjacent to or surrounded the ground chargeability highs in the five cases analyzed from tasmanian data modeling using the established cole cole physical property values for sulfides predicts that an inductive airborne system is insensitive to many conventional ip targets unless the mineral grain size is substantially less than 1 mm in the cases in which airborne ip responses were ad

north american plate dynamics deformation within the north american plate in response to various tectonic processes is modeled using an elastic finite element analysis a coarse grid contains 328 elements and 190 nodes while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2 3 abundant information about the present day state of intraplate stress constrains the modeling the dominant pattern that must be fit by all acceptable models is an ene trend for the maximum compressive stress for most of the plate east of the rocky mountains the tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere shear and normal stresses transmitted across transforms normal stresses transmitted across convergent boundaries stresses due to horizontal density contrasts within the continent and shear tractions applied along the base of the plate model stresses are calculated with respect to a lithostatic reference stress state distributed ridge forces of magnitude 2 1012 n m predict deviatoric stresses of the order of 20 40 mpa that are capable of accounting for the dominant observed ene stress trend driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate assuming that ridge forces account for the dominant ene stress trend bounds may be placed on other possible forces for example shear stresses transmitted across transform boundaries along the san andreas and caribbean are small of the order of 5 10 mpa also compressive stresses of the order of 5 10 mpa transmitted across the major transforms improve the fit to the data compressive stresses across convergent margins along the aleutians and the middle america trench are important

divariant and trivariant reaction line slopes in fmas and cfmas theory and applications the theory of divariant reactions is investigated in some detail and approaches involving simultaneously operating reactions and x mg isopleths are combined using the gibbs method spear et al 1982 a general equation is derived for x mg isopleth slopes in fmas and cfmas which may be applied to any divariant assemblage in these systems isopleth slopes are calculated for a number of divariant fmas and cfmas reactions inferred to have operated in sri lankan granulites giving more rigorous constraints on the local p t path isopleth slopes are also calculated for the trivariant cfmas reaction orthopyroxene plagioclase garnet quartz the resulting dp dt range from 8 to 18 bar k application to sri lankan granulites confirms an earlier hypothesis faulhaber and raith 1991 that a retrograde path with a dp dt of 10 15 bar k may cause garnet growth in charnockites and garnet breakdown in metabasites it is shown here that dp dt depends primarily on the ca contents of garnet and plagioclase large variations in isopleth and reaction line slopes are possible due to the thermodynamic effect of additional components in solid solutions e g albite in plagioclase this has a good potential for the assessment of the exact form of p t paths equations to calculate isopleth slopes could be included in software packages aimed at defining p t estimates enabling geologists to derive realistic p t paths

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

titanium substitution in biotite an empirical model with applications to thermometry o2 and h2o barometries and consequences for biotite stability abstract an empirical model for solution of ti in biotite is developed and calibrated on the basis of various equilibria among biotite and ti saturating assemblages sources of data used for the calibration include both experimental products and published analyses of natural assemblages ti solution in biotite can be satisfactorily modelled by means of the exchange component tife 2 whose non ideal behavior can be approximated empirically by means of a linear function of the relative concentrations of ti and octahedral al in biotite within assemblages which fix the chemical potential of the exchange component tife 2 the concentration of this component in biotite is very sensitive to temperature and oxygen fugacity making it possible to use biotite oxide assemblages to obtain reliable t and f o 2 estimates in igneous and metamorphic rocks in assemblages which also contain alkali feldspar it is possible to use breakdown equilibria of ti additive components in biotite to estimate h 2 o fugacity such uses are likely to find important applications in rocks in which other assemblages sensitive to temperature f o 2 and or f h 2 o are absent in general rocks containing a single oxide phase plus biotite as the only mafic silicate phase alkali feldspar the empirical model for solution of ti in biotite also allows to calculate the loci of various terminal reactions for the assemblage titanian biotite quartz which by comparison with equivalent equilibria in ti free systems provide insight into the effect of ti on the stability of biotite solubility of ti in biotite increases with temperature in a strongly non linear fashion as a consequence calculations predict that the effect of ti on the stability of biotite is very small up to conditions typical of the beginning of the granulite facies of metamorphism but beyond those conditions ti concentration increases markedly and biotite is rendered very refractory at mid to deep crustal pressures 10 kbar the vapor absent solidus of biotite quartz in rocks containing ti saturating phases is likely to exceed 900 c even in bulk compositions with subequal concentrations of fe and mg

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

precision and accuracy of the calcite strain gage technique the twinned calcite strain gage technique allows the determination of strain magnitude and direction in mildly deformed sedimentary rocks in that the measurements required for the calculation are rather time consuming it is important to know the minimum number required to obtain a precision that is comparable with the accuracy of the result this paper documents a series of tests on three different limestones having different textures and porosities in order to determine the precision and accuracy of the technique as a function of the number of measurements and the number of perpendicular thin sections the tests show that for most purposes the best measurement strategy will be to make 25 measurements on each of two perpendicular thin sections the best determined principal strain axis is the one having a magnitude that is opposite in sign from the other two over the range of zero to 11 shortening the magnitude of the odd signed strain axis will than have a precision and accuracy of about 11 in magnitude and a precision and accuracy of about 6 to 8 in orientation differences in texture and porosity do not appear to have significant effects upon precision and accuracy

sevier laramide deformation of the continental interior from calcite twinning analysis west central north america abstract paleozoic mesozoic carbonates that cover cratonic western north america contain a regional layer parallel shortening lps fabric that is preserved by mechanically twinned calcite shortening directions are generally parallel to the sevier thrust transport direction e w in carbonates of the idaho wyoming portion of the thrust belt and within carbonates as far as 2000 km into the plate interior the inferred calcite twinning differential stress magnitudes generally decrease across the thrust belt and decrease exponentially away from the orogenic front into the craton synorogenic calcite cements and veins preserve a distinct twinning deformation history in the thrust belt twinning strains commonly record local out of transport piggyback strain events with high differential stresses

anatexis of interlayered amphibolite and pelite at 10 kbar effect of diffusion of major components on phase relations and melt fraction we have conducted h2o undersaturated melting experiments on charges consisting of a layer of a sillimanite bearing metapelite and a layer of garnet bearing amphibolite with the goal of studying how layered metamorphic rocks interact during anatexis experiments with the layered charges were performed at 10 kbar 1 gpa at 900 c 334 h 925 c 341 h and 950 c 50 161 332 h dehydration melting of the amphibolite alone and of the pelite alone were studied at the same p t conditions when run alone at 900 c the pelite and the amphibolite yielded ca 40 vol and 15 vol melt respectively corresponding melt fractions at 950 c were ca 50 vol and 30 vol when run side by side melt abundance in the pelite is fairly uniform and ca 20 vol higher than in the solo runs at all temperatures studied in contrast complex zoning patterns parallel to the interface develop in the amphibolite layer far from the interface the melt fraction is 5 10 vol lower than in the amphibolite solo runs closer to the interface the melt fraction in the amphibolite layers are comparable to or somewhat higher than in the amphibolite solo runs and both clinopyroxene and plagioclase react out and are replaced by orthopyroxene and quartz melt fraction closest to the interface reaches 65 70 vol and the only crystalline phase present is garnet with hollow cores the increase in melt fraction in the pelite layers of the layered runs is caused by transfer of na from the amphibolite to the pelite which forces biotite to be eliminated at lower temperature along the biotite garnet reaction boundary k is transferred from the pelite to the amphibolite and the lower melt fraction in the distal end of the amphibolite layers is caused by faster diffusion of na compared to k which results in a net depletion of alkalies ca is transferred from the amphibolite to the pelite and the resulting changes in cao concentrations affect the stabilities of plagioclase pyroxenes garnet and quartz and cause the observed zoning in the amphibolite layer ca entering the pelite layer induces crystallization of large plagioclase neoblasts radiating from the interface into the pelite layer these results show that diffusion of components between contiguous lithologies undergoing anatexis can cause profound changes in phase relations melt fractions and restite compositions

petrology of rocks formed by interaction of mafic magmas and high grade metamorphic rocks at suya taco area northern sierra de comechingones argentina abstract interaction between mafic magmas and crustal metamorphic rocks at mid crustal levels causes modification of mafic magma composition by assimilation or reaction with crustal materials this petrologic process is studied at the suya taco complex where a suite of mafic magmas interacted with metamorphic rocks at 6 6 1 kbar the suya taco complex consists of close spaced bodies of biotite bearing gabbronorites these magmas intruded into a supracrustal sequence consisting of amphibolite to granulite facies gneisses and migmatites the peak metamorphic mineral assemblage bt qtz pl an 30 grt kfs crd sil of supracrustal rocks crystallised before igneous mafic magmatism interaction between mafic magmas and metamorphic rocks produces two major hybrid lithologies diorites made up of opx bt pl an 38 50 ilm cpx qtz and garnet and orthopyroxene bearing rocks characterised by the coexistence of bt pl an 30 45 grt opx qtz ilm spl kfs the latter lithology may locally appear as a quartz absent and high aluminous mineral assemblage the two hybrid rocks occur interlayered at the contact zone between igneous mafic rocks and high grade gneisses individual layers range from centimeter to meter scales moreover dioritic and orthopyroxene and garnet bearing layers present a spectrum of mineral assemblages and mineral chemistry that spread among those that appear in igneous mafic and metamorphic rocks the work shows that the changes of mineral assemblages and variations of mineral chemistry in the hybrid diorites and orthopyroxene and garnet bearing rocks are the result of interaction processes between mafic magmas and regional metamorphic rocks field relationships petrographic features mineral compositional changes and qualitative analyses of phase relations suggest that rocks formed by interaction between mafic magmas and metamorphic rocks results from 1 assimilation of metamorphic rocks by mafic magmas at variable degrees and 2 ion diffusion among mineral assemblages of contrasting composition the implication of these processes is that ca is transferred from mafic magmas to metamorphic rocks which forces the breakdown of garnet and quartz and induces the crystallisation of plagioclase and orthopyroxene in the resulting hybrid rocks

amphibolite to granulite transition in aluminous greywackes from the sierra de comechingones cordoba argentina we discuss upper amphibolite to granulite facies early palaeozoic metamorphism and partial melting of aluminous greywackes from the sierra de comechingones se sierras pampeanas of central argentina consistent p t estimates obtained from equilibria involving al and ti exchange components in biotite and from more traditional thermobarometric equilibria suggest that peak metamorphism of the exposed section took place at an essentially constant pressure of 7 8 kbar and at temperatures ranging from 650 to 950 c mineral compositions record an initial decompression after peak metamorphism of c 1 5 kbar which was accompanied by a cooling of c 100 c upper amphibolite facies gneisses consist of the assemblage qtz pl bt grt rt ilm the transition to the granulite facies is marked by the simultaneous appearance of the assemblage kfs sil and of migmatitic structures suggesting that the amphibolite to granulite transition in the sierra de comechingones corresponds to the beginning of melting rocks with structural and or chemical manifestations of partial melting range from metatexites to diatexites to melt depleted granulites consisting of the assemblage grt crd pl qtz ilm ath the melting stage overlapped at least partially with decompression as suggested by the occurrence of cordierite in both the migmatites and the residual granulites of two distinct textural types idiomorphic porphyroblasts probably representing peritectic cordierite and garnet rimming coronas metapelitic rocks are unknown in the sierra de comechingones therefore it appears most likely that the al rich residual assemblages found in the migmatites and residual granulites were formed by partial melting of muscovite and sillimanite undersaturated metagreywackes we propose a mechanism for this that relies on the sub solidus stabilization of garnet and the ensuing changes in the octahedral al content of biotite with pressure and temperature

non coaxial horizontal shortening strains preserved in twinned amygdule calcite dsdp hole 433 suiko seamount northwest pacific plate abstract core sections from the lower basaltic portions of dsdp hole 433 total depth 550 5 m in suiko seamount contain amygdule and vein fillings of calcite which contain mechanical twins analysis of the calcite twins reveals the presence of two horizontal shortening strains that are nearly orthogonal to one another the azimuthal orientation of these strains is only known with respect to stratigraphic top and bottom as the cores are not oriented in any other manner maximum shortening strain magnitudes for the best developed positive expected value pev twin lamellae set is 1 7 for the lesser developed negative expected value nev split the preserved horizontal strain magnitude is 4 3 inferred compressive paleostress magnitudes were on the order of 26 mpa horizontal differential stresses of this magnitude could be 1 associated with hotspot plumes and or 2 are transmitted to the central portions of thin oceanic plates from distant plate boundaries suiko seamount is composed of paleocene basalts overlain by lithified middle paleocene limestones and is part of the emperor seamount chain on the pacific plate the absence of secondary vein calcite in the overlying sediments suggests that the underlying amygdule calcite is paleocene in age which indicates that the twinned calcite preserves a stress field reorganization the nev split after the middle paleocene that is oriented 64 from the earlier paleocene stress field the pev split

evolution of the sierras de cordoba argentina abstract located in the southeastern part of the sierras pampeanas argentina the pampean ranges of cordoba show the evolution at the western margin of the rio de la plata craton from extensional stages during neoproterozoic times to compressive stages during the pampean orogeny the cumbre de gaspar group represents distal deep sea clastic sedimentation deposited in a passive margin environment very possibly during a time of incipient oceanization the santa sabina group representing shelf sedimentation rests conformably on the cumbre de gaspar group stocks of granites and granodiorites of calc alkaline affinity intruded the cumbre de gaspar group during the early pampean or earlier the polyphase pampean orogeny involves several distinct metamorphic and orogenic events metamorphism is discussed using the chemical composition of minerals the major effect of the pampean orogeny was the formation of a pile of westward thrust blocks the rio de la plata craton thrust above its marginal basin forms the uppermost unit the intrusion of the el pilon granite dated at 520 5 ma post dates these tectonic and metamorphic events after this main orogenic event the domain remained permanently located on the cratonic side of younger famatinian and andean orogenic events the mermela group probably corresponds to the molasse of the pampean orogeny and to the base of a new sedimentary cycle it is difficult to distinguish famatinian tectonics in the sierras de cordoba because the main orogeny occurred further west however this orogenic cycle is marked by the intrusion of the large achala batholith during the late palaeozoic and mesozoic the sierras de cordoba remained a stable platform the whole region was rejuvenated by the andean orogeny which induced tilted blocks in its foreland and the pocho volcanics of shoshonitic affinities

thermoelastic stress in oceanic lithosphere due to hotspot reheating we investigate the effect of hotspot reheating on the intraplate stress field by modeling the three dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere a thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements the stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series wherein each term is a source or an image modified from a closed form half space solution we apply the thermal stress solution to midplate swells in oceanic lithosphere with various thermal structures and plate velocities our results predict a stress field with a maximum deviatoric stress on the order of 100 mpa 1 kbar covering a broad area around the hotspot plume the predicted principal stress orientations show a complicated geographical pattern with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere although stress data near oceanic swells are limited the source parameters of intraplate earthquakes near several hotspots are consistent with the thermal stress model these results indicate that thermal stress due to reheating may be an important contributor to stress fields near hotspots in old oceanic lithosphere

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

tectonic analysis and paleo stress determination of the upper lava section at odp iodp site 1256 east pacific ocean abstract research on the deep sea is of great importance for a better understanding of the mechanism of magma emplacement and the tectonic evolution of oceanic crust however details of the internal structure in the upper levels of the oceanic crust are much less complete than that of the more fully studied sub aerial areas for the first time this study proposes a dynamic analysis using the inversion method on core data derived from the drilled basement of the present day intact oceanic crust at odp iodp site 1256 in the cocos plate the research is based on an innovative core reorientation process and combines different stress hypothesis approaches for the analysis of heterogeneous failure slip data via exploitation of two distinct techniques from the analysis of the failure slip data both techniques produce 5 distinct subsystem datasets all calculated subsystems are mechanically and geometrically admissible interpretation of the results allows the researchers to note a complex local and regional tectonic evolution deriving from the interplay of 1 the ridge push and rotation of both the east pacific rise and the cocos nazca spreading center 2 the effect of the slab pull of the middle america trench 3 the influence of lava emplacement mechanisms and 4 intra plate deformation

high grade metamorphism dehydration and crustal melting a reinvestigation based on new experiments in the silica saturated portion of the system kalo 2 mgo sio 2 h 2 o co 2 at p 1 5 gpa the system kalo2 mgo sio2 h2o co2 has long been used as a model for the processes of granulite facies metamorphism and the development of orthopyroxene bearing mineral assemblages through the breakdown of biotite bearing assemblages there has been considerable controversy regarding the role of carbon dioxide in metamorphism and partial melting we performed new experiments in this system at pressures of 342 to 1500 mpa with t between 710 and 1045 c and xflh2o between 0 05 and 1 00 accurately locating most of the dehydration and melting equilibria in p t xflh2o space the most important primary result is that the univariant reaction phl qtz fl en sa melt must be almost coincident with the fluid absent reaction phl qtz en sa melt in the co2 free subsystem in conjunction with the results of previous measurements of co2 solubility in silicate melts and phase equilibrium experiments our theoretical analysis and experiments suggest that co2 cannot act as a flux for partial melting crustal melting in the presence of h2o co2 mixed fluids will always occur at temperatures higher than with pure h2o fluid present magmas produced by such melting will be granitic s l in composition with relatively high sio2 and low mgo contents irrespective of the h2o co2 ratio in any coexisting fluid phase we find no evidence that lamprophyric magmas could be generated by partial fusion of quartz saturated crustal rocks the granitic melts formed will not contain appreciable dissolved co2 the channelled passage of hot co2 rich fluids can cause local dehydration of the rocks through which they pass in rock dominated as opposed to fluid dominated systems minor partial melting can also occur in veins initially filled with co2 rich fluid as dehydration and local disequilibrium drive the fluid towards h2o rich compositions however co2 is unlikely to be a significant agent in promoting regional granulite grade metamorphism melting magma generation metasomatism or long range silicate mass transfer in earth s crust the most viable model for the development of granulite facies rocks involves the processes of fluid absent partial melting and withdrawal of the melt phase to higher crustal levels

structure timing and kinematics of the early eocene south fork slide northwest wyoming usa abstractthe south fork slide sfs is exposed over an area of 800 km2 in northwest wyoming and its evolution is connected in space and time to the adjacent and overlying 48 87 ma heart mountain slide hms the youngest rocks deformed as part of the sfs are the eocene willwood formation sands and mudstones which have a u pb detrital zircon youngest depositional age tuffzirc calculation of 51 80 1 04 1 08 ma as well as a spectrum of older zircons that were eroded from the sevier highlands to the west and the archean beartooth uplift to the north n 379 u pb zircon ages the sfs is overlain by allochthonous paleozoic carbonate and eocene volcanic rocks of the hms the sfs deformation involves shallowly plunging thin skinned folds that trend northeast southwest where the slip surface could have been the top of the jurassic gypsum springs formation detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin skinned belt as well as the fac

calcite twinning strains from syn faulting calcite gouge small offset strike slip normal and thrust faults we have evaluated the stress strain behavior of calcite precipitated and mechanically twinned in small offset strike slip normal and thrust faults of a variety of ages and from a variety of tectonic settings n 3001 twin measurements 63 strain analyses from 18 field sites five strike slip faults with syn faulting horizontally striated calcite rake 0 were studied and we report the orientations of the contemporaneous stress strain field associated with each fault intrusion of the marathon large igneous province mafic dikes 2 1 ga in archean crust minnesota usa post keweenaw rift 1 1 ga faulting island lake fault central ontario canada subduction associated with metamorphic core complex formation cretaceous china subduction cretaceous to miocene italy and continental extension recently active furnace creek fault death valley california usa seven normal faults with synfaulting dip slip striated calcite were studied and are from the following tectonic settings a normal fault slip surface in an ordovician piedmont fold appalachian s paleo subduction associated with cretaceous metamorphic core complex formation china 3 sites the paleo extensional atlantic margin 55 ma ireland 1 site with a u pb calcite age continental extension 1 active site mojave desert a transcurrent margin jamaica 1 active fault site and subduction 2 active faults along the eur african margin in italy with calcite u th disequilibria ages and crete respectively six thrust fault examples are all from convergent orogenic settings the basal thrust of the penokean 1850 ma fold and thrust belt the penokean orogen foreland in mesabi range banded iron formation folds an offset breccia body in the permian gondwanide belt ellsworth mountains antarctica the frontal thrust of the gondwanide cape belt south africa the paleocene frontal prospect thrust sevier belt wyoming and an alpine foreland back thrust lulworth cove u k for each strike slip fault system the twinning shortening strain is horizontal and at an angle of 0 60 to the respective fault plane dextral or sinistral although in the majority of cases the shortening axis is parallel to fault strike 13 of 23 results in each normal fault example dip slip kinematic striations dominate the faulted surface yet the orientation of the maximum principal compressive stress 1 and shortening strain axis e1 are not 45 to the fault plane as predicted but are sub horizontal and either strike parallel 25 of 35 results or strike normal 10 of 35 results thrust faults preserve shortening strain axes parallel to the dip slip kinematic direction within the fault plane plane strain and not at 45 to the principal plane 5 of 5 results none of the fault stress strain field results reported here support the andersonian or mohr coulomb criteria for stress strain relations predicted along faults

stress magnitude estimates from earthquakes in oceanic plate interiors we propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms using a depth dependent rheology we show it is possible to estimate the differential stress 1 3 averaged over some reference lithospheric thickness the resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis we apply the method to the central indian ocean where intraplate seismicity is high from well studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals this result is consistent with the high level of stress that was found from numerical model calculations by cloetingh and wortel 1985 1986 from the few intraplate events in the pacific plate we also estimate differential stresses in this area

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation