conodont color and textural alteration an index to regional metamorphism contact metamorphism and hydrothermal alteration experimental and field data are used to extend the utility of conodonts as semi quantitative thermal indices into the regimes of regional and contact metamorphism as well as hydrothermal alteration an arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color alteration indices cai above 300 c that is for cai values of 5 through 8 such cai values occur in very low to medium grade regionally metamorphosed contact metamorphosed and hydrothermally altered rocks the uniformity or variability of cai values within a sample together with conodont texture can help to distinguish grades and environments of metamorphism particularly in metacarbonate sequences induced cai by pyrolysis in a water methane mixture at kbar results in retardation of cai and in a disparate mixture of both low and high cai values within each experimental sample in this system color alteration processes above a cai of 2 to 3 seem to change from predominantly carbonization to predominantly loss of organic matter presumably by oxidation and volatilization of oxides these experiments approximate the type of cai mixture characteristically found in conodonts recovered from hydrothermally altered rocks these data indicate that cai values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization

pure shear and simple shear calcite textures comparison of experimental theoretical and natural data abstract the pattern of lattice preferred orientation texture in deformed rocks is an expression of the strain path and the acting deformation mechanisms a first indication about the strain path is given by the symmetry of pole figures coaxial deformation produces orthorhombic pole figures while non coaxial deformation yields monoclinic or triclinic pole figures more quantitative information about the strain history can be obtained by comparing natural textures with experimental ones and with theoretical models for this comparison a representation in the sensitive three dimensional orientation distribution space is extremely important and efforts are made to explain this concept we have been investigating differences between pure shear and simple shear deformation incarbonate rocks and have found considerable agreement between textures produced in plane strain experiments and predictions based on the taylor model we were able to simulate the observed changes with strain history coaxial vs non coaxial and the profound texture transition which occurs with increasing temperature two natural calcite textures were then selected which we interpreted by comparing them with the experimental and theoretical results a marble from the santa rosa mylonite zone in southern california displays orthorhombic pole figures with patterns consistent with low temperature deformation in pure shear a limestone from the tanque verde detachment fault in arizona has a monoclinic fabric from which we can interpret that 60 of the deformation occurred by simple shear

late cretaceous extensional unroofing in the funeral mountains metamorphic core complex california new field and geochronologic data document the existence of late cretaceous extensional structures in the death valley region california nevada we have mapped two major low angle ductile shear zones that omit stratigraphy in the footwall of the funeral mountains metamorphic core complex intervening strata have been strongly attenuated although stratigraphic offset across the shear zones is only 1 5 km the presence of a large metamorphic discontinuity suggests that the amount of unroofing must be much greater the timing of shear zone formation attenuation and subsequent northwest vergent folding is constrained by u pb geochronology on 1 prekinematic or synkinematic and 2 postkinematic pegmatites deformation was taking place by 72 ma and had ended by 70 ma these results support earlier petrologic and geochronologic data that suggested substantial unroofing of the funeral mountains in late cretaceous time and add to a growing body of evidence for widespread mesozoic extension in the hinterland of the sevier thrust belt

deformation partitioning shear zone development and the role of undeformable objects abstract the formation of a mylonite zone by homogeneous progressive simple shear is highly unlikely because heterogeneous strain of grains with different compositions causes deformation partitioning and results in progressive inhomogeneous simple shear or non coaxial progressive bulk inhomogeneous shortening however the presence of rigid bodies in a zone of rock undergoing bulk simple shear spreads and homogenizes the stress field such that the instantaneous displacement field is essentially homogeneous at a scale up to at least four times the diameter of the rigid object that is the portion of rock surrounding the rigid body deforms by homogeneous progressive simple shear and the rigid object is forced to rotate this has considerable implications for the rotation or lack thereof of unstrained porphyroclasts and porphyroblasts and hence the information such grains preserve about the sense of shear on their margins or through inclusion trails geometric relationships between porphyroclasts or porphyroblasts and the surrounding matrix that are superficially similar can indicate exactly the opposite sense of shear depending on whether the porphyroclasts or porphyroblasts have or have not rotated leading to considerable problems in interpretation of the bulk movement in a shear zone resolution of these relationships enables ready distinction of whether the deformation history involved progressive bulk inhomogeneous shortening or progressive simple shear in particular these results have considerable implications for the origin of blueschist minerals preserved in garnet porphyroblasts via the roles of strain softening and hardening during mylonitization due to the destruction of feldspar porphyroclasts and growth of garnet porphyroblasts respectively

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

spectral properties and geologic processes on eros from combined near nis and msi data sets from april 24 to may 14 2000 the near earth asteroid rendezvous near shoemaker mission s near infrared spectrometer nis obtained its highest resolution data of 433 eros high signal to noise ratio nis reflectance spectra cover a wavelength range of 8002400 nm with footprint sizes from 213 x 427 m to 394 x 788 m this paper describes improvement in instrument calibration by remediation of internally scattered light derivation of a pseudo channel for nis at 754 nm using multispectral imager msi eros approach maps at 951 and 754 nm synthesis of a 3127 spectrum high resolution data set with the improved calibration and expanded wavelength coverage and investigation of global and localized spectral variation with respect to mineralogy composition and space weathering of eros comparing the findings with previous analyses scattered light removal reduces the red slope of eros spectra though not to the level seen by telescopic observations the pseudo channel completes sampling of eros 1 micron band i absorption feature enabling direct comparison of nis data with other asteroid and meteorite spectra without additional scaling or correction following scattered light removal and wavelength range extension the spectral parameters of average eros plot well inside the s iv field of gaffey et al 1993 and are consistent with the l6 chondrite meteorite fields of gaffey and gilbert 1998 although eros shows no evidence of mineralogical heterogeneity modest spectral variations correlate with morphologically and geographically distinct areas of the asteroid eros bright to dark spectral ratios are largely consistent with laboratory space weathering experiment results and modeling of space weathering effects eros brightness variation unaccompanied by significant spectral variation departs from lunar type where band depths slopes and albedoes all correlate and ida type where significant spectral variation is unaccompanied by corresponding brightness variation the brightest areas on eros steep crater walls have lesser spectral slope and deeper band i consistent with exposure of fresher less space weathered materials bright crater slope materials have opx opx olv of 0 240 29 and may be more representative of the subsurface mineralogy than average eros which is probably affected by space weathering the floors of the large craters psyche and himeros have lower albedo and contain the most degraded or altered looking materials nis spectra retain a red spectral slope at greater than 2 microns the recalibrated and expanded nis spectra show better agreements with mixing models based on space weathering of chondritic mixtures

uniqueness of geological correlations an example from the death valley extended terrain the northern death valley furnace creek fault zone is a major northwest trending fault system in the death valley region but the magnitude of offset and its significance to extensional tectonism in the region are controversial stratigraphic data have provided first order constraints on offset across the fault zone but offer limited resolution previous correlations of mesozoic thrust faults across the fault zone have relied on regional stratigraphic trends to fingerprint structurally similar thrust plates we show that a northeast trending mesozoic backfold within the predominantly east directed fold and thrust belt was probably continuous from the southern panamint range california to the belted range nevada the relative position of this antithetic structure allows uniquely compatible correlations of three mesozoic structures between two range blocks across the northern death valley furnace creek fault zone independent of stratigraphic arguments the offset structures have geometric properties of size vergence order and spacing that indicate a probability less than 10 4 of the mesozoic orogen identically repeating this structural suite in two unrelated places reconstruction of these correlative structures indicates 68 4 km n48 w 6 w of apparent dextral offset between the cottonwood and funeral mountains in general agreement with previous estimates based on stratigraphic observations but of substantially higher precision this reconstruction and the restoration of a 30 km separation between the funeral and grapevine mountains indicated by offset segments of the backfold suggest a simple pre extensional geometry for the mesozoic orogen in the death valley region consistent with observations from unextended blocks and analogous to that of the cordilleran orogen at the latitude of calgary alberta

rise and tilt of metamorphic rocks in the lower plate of a detachment fault in the funeral mountains death valley california the funeral mountains in eastern california preserve a record of early cretaceous metamorphism followed by ductile deformation uplift and low angle normal detachment faulting 40ar 39ar age spectra indicate that cooling and uplift of the lower plate began in cretaceous time uplift was accommodated by normal sense movement along a wide northwest dipping shear zone mylonitic fabrics some of which have been dated as late cretaceous deformed older high temperature metamorphic textures analyses of shear bands mica fish and porphyroclasts grain shape fabrics and folds indicate that the upper surfaces moved toward 299 12 top to the northwest relative to lower surfaces uplift continued until the near present the youngest phase being accommodated by top to the northwest movement along the detachment fault which formed subparallel to lower plate mylonitic fabrics fission track apatite data indicate that exposure of the lower plate to the surface occurred sometime after 6 ma reconstruction along the movement vector places the grapevine mountains over the funeral mountains having been displaced at least 40 km isograds and thermobarometry in pelitic schist from the lower plate indicate increasing pressures and temperatures of equilibration toward the northwest the maximum temperature and pressure was determined on a sample from monarch canyon using thermobarometry 700 c at a depth of 32 km at chloride cliff 5 km southeast of monarch canyon 4 samples yielded 575 600 c at depths of 19 27 km at indian pass 17 km southeast of monarch canyon a temperature of 490 c was determined in the southern funeral mountains about 50 km southeast of monarch canyon conodont color alteration indexes indicate temperatures of 325 425 c these data indicate that the lower plate is presently tilted strongly to the southeast from the orientation it maintained at the peak of metamorphism thermochronologic data k ar on muscovite biotite and hornblende 40ar 39ar on hornblende and fission track on apatite titanite and zircon indicate that both tilting and the transition from ductile to brittle styles of quartz deformation are confined to the interval 21 6 ma during the latter part of this interval 11 6 ma rapid uplift and movement along the detachment fault are documented the findings support current theories of detachment fault evolution in which a dipping fault surface undergoes rotation to a subhorizontal orientation while the lower plate undergoes a comparable tilt

fission track evidence for quaternary uplift of the nanga parbat region pakistan the north striking nanga parbat haramosh massif protrudes into the northwestern himalaya along the axis of a great syntaxis1 2 fig 1 where the hindu kush karakorum and himalayan ranges converge as the indus suture zone3 enters this region from the east it bifurcates into two branches encircling what may be a docked island arc terrane4 the southern branch the main mantle thrust crops out on both flanks of the nanga parbat massif forming a tight structural loop5 this massif and the adjacent terrane contain some of the highest peaks in the himalaya nanga parbat and the indus river located just 20km away define the world s greatest continental relief 6 930 m we report here the discovery of unexpectedly young sphene zircon and apatite fission track dates from the nanga parbat haramosh massif these dates as low as 1 3 myr for zircon and 0 4 myr for apatite imply that during the pleistocene the nanga parbat region was uplifted and eroded at nearly 1 cm yr 1

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

internally consistent geothermometry and h 2 o barometry in metamorphic rocks the example garnet chlorite quartz temperature and h2o activity can be determined with high precision using metamorphic mineral assemblages that define both a dehydration equilibrium and a temperature sensitive cation exchange equilibrium such determinations are obtained by applying the gibbs method and then integrating two resulting differential equations as illustrated here for the assemblage garnet chlorite quartz the first equation a geothermometer that monitors temperature based upon fe mg exchange between garnet and chlorite was calibrated using rocks at pecos baldy new mexico 0 0 05 p bars 19 02 t k 4607 ln kd 24 156 with errors of 8 c based upon analytical precision the second equation monitors differences in the activity of water between specimens 1 and 2 0 0 1 xmg chl 1 2 05 p2 p1 33 02 5 96 ln xfe chl 1 xalm 1 t2 t1 2 67 rt1ln a h2o 2 a h2o 1 5 96 t1ln xfe chl 2xalm 1 xfe chl 1xalm 2

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

permian triassic plutonism and tectonics death valley region california and nevada significant contractional structures that deform permian rocks but predate an early triassic overlap sequence are recognized within the cordilleran orogen western united states thrusting in the death valley region of the orogen however has been regarded as middle triassic or younger and thus kinematically distinct we present new isotopic age limits on two posttectonic stocks that intrude major structures of the death valley thrust belt the stocks are no younger than middle triassic but are likely late permian in age consistent with stratigraphic and structural data suggesting that thrusting predates the overlap sequence we hypothesize that permian shortening may have affected more than 700 km of the cordilleran orogen at the same time arc activity began within cratonic north america but prior to early triassic emplacement of the structurally higher sonomian arc terrane

on the inter relationship between grain size sensitive creep and dynamic recrystallization of olivine abstract the recent theoretical results of twiss 1976 and goetze 1978 suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain boundary sliding or nonlinear coble creep mechanisms this would result following the production of a fine grain size by dynamic recrystallization for the high temperature experimental work performed by karato et al 1982 dry single crystals of olivine were almost totally recrystallized during creep and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates however the activation energy for creep determined by karato et al 1982 was far higher than that predicted by the models of twiss 1976 or goetze 1978 although the conditions required for operation of at least the model of twiss 1976 apparently were satisfied the data for the highly recrystallized specimens from the higher stress lower temperature experiments of zeuch and green 1979 and zeuch 1980 are in good agreement with the results of karato et al 1982 these latter experiments were conducted under conditions where either the model of twiss 1976 or goetze 1978 should have been applicable i tentatively conclude that although fine grain sizes were produced in the experiments of karato et al 1982 zeuch and green 1979 and zeuch 1980 by dynamic recrystallization there is no evidence for a transition to grain boundary diffusional creep mechanisms at high or low stresses despite the predictions of twiss 1976 or goetze 1978 instead deformation is dominated by dislocation movement with recovery by dynamic recrystallization

thermobarometry in granulites pitfalls and new approaches abstract computer models of cooling granulite facies rocks with the assemblage garnet biotite sillimanite k feldspar plagioclase quartz are presented that incorporate simultaneous reaction net transfer and exchange and multicomponent diffusion in garnet in models where cooling is accompanied by exchange reactions plus diffusion only the calculated garnet core matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate the size of the garnet and the volumetric ratio of garnet to biotite maximum calculated core temperatures of approximately 700 750 c are to be expected from typical granulite facies garnet biotite assemblages and calculated core temperatures can easily be as low as 600 c in models where both net transfer and exchange reactions are possible calculated garnet core matrix biotite temperatures are typically higher than the metamorphic peak temperatures by more than 150 c for large garnets r 2 mm but may be lower than the metamorphic peak for small garnets r the composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence calculated p t trajectories are very sensitive to the petrologic interpretation of this recrystallization history for models in which all the modal plagioclase reacts the core and rim pressures may be faithfully recovered provided that the temperature estimate for the core is reliable for models in which only a small percentage of the modal plagioclase reacts very small modal amounts of plagioclase may be in equilibrium with garnet rim which if incorrectly interpreted may lead to grossly erroneous p t path interpretations a method to test an inferred p t path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion an example from western new hampshire usa is presented that closely matches the measured profile

jurassic barrovian metamorphism in a western u s cordilleran metamorphic core complex funeral mountains california large magnitude retroarc shortening of cretaceous age is well documented in the sevier orogenic belt of the western united states and has been associated with eastward franciscan subduction that began in the middle late jurassic but evidence for major late jurassic retroarc shortening has been lacking here we report new lu hf garnet geochronology pressure temperature p t paths and 40 ar 39 ar thermochronology data that document tectonic burial of late jurassic age in a barrovian metamorphic terrain the funeral mountains metamorphic core complex california located in the hinterland of the sevier orogenic belt the p t paths determined from growth zoned garnets in upper greenschist facies pelitic schist show steep p t trajectories consistent with metamorphism during thrust loading the age of thrust loading is constrained by a five point lu hf garnet isochron to be 158 2 2 6 ma 2 partial exhumation recorded in 146 153 ma 40 ar 39 ar muscovite cooling ages closely followed garnet growth late jurassic barrovian metamorphism has not been previously recognized in cordilleran metamorphic core complexes possibly due to being obscured by late cretaceous to tertiary deformation magmatism and metamorphism this study finds that the period of large magnitude crustal shortening in the retroarc extended into the late jurassic and may have closely followed the formation of the coherent orogenic system associated with east dipping franciscan subduction

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

grain growth kinetics of quartz and calcite aggregates grain growth rates for novaculite flint and jasper have been determined between 1000 and 800 c and 15 to 2 kb water pressure a pre treatment established an equilibrium microstructure and a nearly uniform mean grain diameter bar d 0 at 1000 c and 15 kb the grain growth data are linear on a log bar d bar d 0 vs log time plot and there is no significant difference between the three samples although the jasper contained 5 10 vol iron oxide at lower pressures and temperatures the growth curves are approximately parallel to the 1000 c and 15 kb data but growth is slower and that for jasper is slower than for the other two the growth rate is strongly dependent on the solubility of quartz in the intergranular fluid and the activation energy is about 11 kcal mole the data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant the growth rate of calcite grains s

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

major late miocene cooling of the middle crust associated with extensional orogenesis in the funeral mountains california the funeral mountains of the death valley region consist of a metamorphic core complex that underlies the miocene boundary canyon detachment fault fissiontrack age determinations on sphene zircon and apatite from the highest grade portions of the funeral mountains collected in monarch canyon indicate rapid cooling from above 285 c at 9 10 ma the steep dt dt slope implied by the cooling path envelope as well as its synchroneity with development of the boundary canyon detachment suggests tectonic unroofing of between 9 5 and 17 km since 10 ma assuming typical geothermal gradients prior to detachment development of between 20 30 c km previous studies report muscovite 40ar 39ar plateau ages from the same area indicating that the funeral mountains core may have cooled below 350 c between 110 55 ma these data when combined with the fission track ages suggest residence of the metamorphic terrain at 285 350 c from late cretaceous to late miocene time recent studies using petrologic constraints have suggested that much of the unroofing of the funeral mountains as well as development of tectonite fabrics occurred during mesozoic extension recognition of temperatures in the 300 c range during miocene time suggests some of the ductile extensional fabrics may be late miocene in age

petrologic constraints on the unroofing history of the funeral mountain metamorphic core complex california this is the published version copyright 1990 american geophysical union all rights reserved

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

equilibria within the mineral assemblage quartz muscovite biotite garnet plagioclase and implications for the mixing properties of octahedrally coordinated cations in muscovite and biotite interaction parameters derived using empirical calibration methods indicate strong non ideality in the mixing of octahedrally coordinated cations in muscovite and biotite the data set used for calibration comprises mineral compositions from 49 samples containing quartz muscovite biotite garnet plagioclase and al2sio5 kyanite or sillimanite pressures and temperatures in the data set were determined through the simultaneous application of geothermometry based on the garnet biotite femg1 exchange equilibrium and geobarometry based on the anorthite breakdown equilibrium two equilibria yielded simple expressions from which binary interaction parameters for octahedrally coordinated cations in biotite could be directly determined a four component fe2 mg al ti regular symmetric mixing model was assumed for biotite one equilibrium yielded a simple expression from which an interaction parameter for the mixing of the mgal celadonite component in muscovite could be directly determined two sets of calculations were performed utilizing different calibrations of the garnet biotite geothermometer and the anothite breakdown geobarometer and different garnet activity models both placed samples within or near the stability field of the al2sio5 phase present in each sample and both yielded similar values for the interaction parameters within narrow uncertainties indicating that the values are insensitive to differences in the underlying methods using the derived interaction parameters activity models were formulated for the annite phlogopite eastonite and siderophyllite components of biotite and for the mgal celadonite component of muscovite these were utilized for the empirical calibration of 45 fluid independent equilibria involving unique combinations of phase components from the mineral assemblage garnet plagioclase biotite muscovite quartz forty three of the equilibria may be applied as geobarometers to equilibrium assemblages of quartz muscovite biotite garnet plagioclase when care is taken to insure that applications are restricted to valid compositional ranges for these the calibrations yielded multiple correlation coefficients ranging from 0 953 to 0 998 and standard deviations of the residuals ranging from 597 to 118 bars