evaluation of paleomagnetism as a correlation criterion for mogollon datil ignimbrites southwestern new mexico paleomagnetic and rock magnetic data show that 36 24 ma ignimbrite outflow sheets in the mogollon datil volcanic field carry thermoremanent magnetizations trms that provide useful reliable correlation criteria over 90 of 404 ignimbrite sites exhibit well grouped 95 500 m intracaldera ignimbrite sequences agree in polarity with associated outflow sheets but show less consistent trm directions probably reflecting syncooling secular variation and discrepancies between eutaxitic foliations and the syncooling paleohorizontal mean virtual geomagnetic pole positions vgps of normal and reverse polarity ignimbrites are nearly antipodal 175 6 apart showing no evidence of long term differences in the zonal harmonics of the normal and reverse polarity states less antipodal mean vgps 161 2 apart from normal and reverse polarity lavas are interpreted as defects in the data set probably including incompletely removed normal polarity viscous overprints the mean paleomagnetic pole position for mogollon datil ignimbrites n 68 independent unit mean vgps is located at 78 8 n 166 6 e k 19 8 a95 4 0 18 2

e i corrected paleolatitudes for the sedimentary rocks of the baja british columbia hypothesis paleomagnetic inclinations from sediments of the western terranes of canada are consistently too shallow for their reconstructed paleogeographic positions two contradicting explanations for these discrepancies are 1 terranes have been displaced northward with respect to the stable american craton by several thousands of kilometres between the late cretaceous 75 ma and the eocene 50 ma and 2 sedimentary inclination error has caused a shallow bias in the paleomagnetic directions here we apply the elongation inclination e i method to paleomagnetic data sets from sedimentary rocks of supposedly allochtonous terranes of western north america to correct for inclination flattening our results indicate that the paleomagnetic directions from the continental silverquick sediments 95 92 ma of southern british colombia are not seriously affected by inclination error because the magnetic signal most likely concerns a chemical remanent magnetisation crm in contrast the marine sediments of the nanaimo group 84 72 ma of vancouver island region appear seriously affected by inclination flattening f 0 7 and the e i corrected mean inclinations are about 98 steeper than the original data we arrive at corrected inclinations paleolatitudes of i 578 k 388n for the silverquick and i 558 k 368n for the nanaimo sediments our corrected paleolatitudes indicate that the canadian terranes were indeed located adjacent to the baja californian margin during the late cretaceous thus supporting the baja bc hypothesis d 2005 elsevier b v all rights reserved

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

magnetic properties in an ash flow tuff with continuous grain size variation a natural reference for magnetic particle granulometry the tiva canyon tuff contains dispersed nanoscale fe ti oxide grains with a narrow magnetic grain size distribution making it an ideal material in which to identify and study grain size sensitive magnetic behavior in rocks a detailed magnetic characterization was performed on samples from the basal 5 m of the tuff the magnetic materials in this basal section consist primarily of low impurity magnetite in the form of elongated submicron grains exsolved from volcanic glass magnetic properties studied include bulk magnetic susceptibility frequency dependent and temperature dependent magnetic susceptibility anhysteretic remanence acquisition and hysteresis properties the combined data constitute a distinct magnetic signature at each stratigraphic level in the section corresponding to different grain size distributions the inferred magnetic domain state changes progressively upward from superparamagnetic grains near the base to particles with pseudo single domain or metastable single domain characteristics near the top of the sampled section direct observations of magnetic grain size confirm that distinct transitions in room temperature magnetic susceptibility and remanence probably denote the limits of stable single domain behavior in the section these results provide a unique example of grain size dependent magnetic properties in noninteracting particle assemblages over three decades of grain size including close approximations of ideal stoner wohlfarth assemblages and may be considered a useful reference for future rock magnetic studies involving grain size sensitive properties

paleomagnetism magnetic fabric and 40ar 39ar dating of pliocene and quaternary ignimbrites in the arequipa area southern peru 40ar 39ar ages and paleomagnetic correlations using characteristic remanent magnetizations chrm show that two main ignimbrite sheets were deposited at 4 86 0 07 ma la joya ignimbrite lji and at 1 63 0 07 ma arequipa airport ignimbrite aai in the arequipa area southern peru the aai is a 20 100 m thick ignimbrite that fills in the arequipa depression to the west of the city of arequipa the aai is made up of two cooling units an underlying white unit and an overlying weakly consolidated pink unit radiometric data provide the same age for the two units as both units record exactly the same well defined paleomagnetic direction 16 sites in the white unit of aai dec 173 7 inc 31 2 95 0 7 k 2749 and 10 sites in the pink unit of aai dec 173 6 inc 30 3 95 1 2 k 1634 showing no evidence of secular variation the time gap between emplacement of the two units is unlikely to exceed a few years the 50 m thick well consolidated white underlying unit of the arequipa airport ignimbrite provides a very specific magnetic zonation with low magnetic susceptibilities high coercivities and unblocking temperatures of nrm above 580 c indicating a ti poor titanohematite signature the anisotropy of magnetic susceptibility ams is strongly enhanced in this layer with anisotropy values up to 1 25 the fabric delineated by ams was not recognized neither in the field nor in thin sections because most of the aai consists in a massive and isotrope deposit with no visible textural fabric pumices deformation due to welding is only observed at the base of the thickest sections ams within the aai ignimbrite show a very well defined pattern of apparent imbrications correlated to the paleotopography with planes of foliation and lineation dipping often at more than 20 toward the expected vent buried beneath the nevado chachani volcanic complex in contrast with the relatively small extent of the thick aai the la joya ignimbrite covers large areas from the altipano down the piedmont ti poor titanomagnetites are the dominant magnetic carriers and ams values are generally lower than 1 05 magnetic foliations are sub horizontal and lineations directions are scattered in the lji the ams fabrics are probably controlled by post depositional compaction and welding of the deposit rather than transport dynamics

a magma loading model for barrovian metamorphism in the southeast coast plutonic complex british columbia and washington barrovian metamorphism linked to crustal thickening in the southeast coast plutonic complex of british columbia and washington is proposed in this paper to be the product of magmatic loading relevant observations and arguments are the following 1 isotopic ages coupled with structural and fabric relations document that throughout the region barrovian metamorphism and plutonism are broadly coeval 2 baric patterns in country rock indicate steep sided crustal loads originating within the high grade parts of the orogen 3 patterns of subsidence and uplift are localized and diachronous precluding a single event regional thickening mechanism 4 country rock fabrics are dominated by steep orogen parallel foliations and shallow strike parallel lineations features not easily reconciled with tectonic thickening mechanisms 5 abroad zone of schists flanking the large 25 x 100 km scuzzy batholith in british columbia bears metamorphic isograds and foliations that track the border of the pluton across regional structural trends implicating pluton emplacement as the cause of metamorphism 6 the crustal loading event in the vicinity of the scuzzy pluton is bracketed by mineral and textural features as being coeval with emplacement of the pluton which occurred over a period of 7 m y 7 the regionally developed chiwaukum schist in washington has petrologic and structural features and a metamorphic age that suggest it was formed by and represents the floor of an eroded extension of the scuzzy pluton the proposed magma loading model invokes the diapir and ballooning concepts of previous workers in which rising plutons displace country rock downward this mechanism results in a type of convective crustal overturn and at least partially solves the space problem related to batholithic emplacement as a result of this process relatively broad tracts of country rock acquire barrovian mineralogy following plutonism magmatically thickened crust is eroded and isostatically up lifted to expose the plutonic roots and high grade country rock basement

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

active deformation in the bolivian andes south america a method is described for determining the horizontal field of velocity and velocity gradients in the bolivian andes in the vicinity of the bolivian orocline using the following data 1 space geodetic measurements of crustal motions 2 the style distribution and rate of late miocene quaternary folding and faulting and 3 paleomagnetic measurements of rigid body rotations about a vertical axis these data were analyzed in a network of 26 triangles which spanned the bolivian andes by solving the velocity gradient compatibility equations between adjacent triangles subject to the input data constraints this yielded 120 140 linear equations to constrain 108 unknowns making the problem moderately overdetermined a series of experiments were carried out with different combinations of the input data to determine the best fit in the least squares sense field of quaternary horizontal velocities and velocity gradients the azimuths of velocity vectors in the high andes are subparallel to both the relative plate convergence vector and the symmetry axis of the bolivian orocline with magnitudes relative to stable south america in the range 7 17 mm yr 1 accommodating up to 30 of the relative plate convergence however the azimuths of velocities along the range front are nearly orthogonal to the range with a component of distributed range parallel dextral or sinistral shear within the eastern cordillera and altiplano the horizontal velocity field requires bending about a vertical axis of both limbs of the bolivian orocline the field of horizontal dilatation assuming homogeneous deformation of the entire crustal thickness requires vertical velocities of material points up to 2 5 mm yr in the sub andean zone decreasing to 0 1 mm yr farther east in the eastern cordillera and altiplano the direction of maximum gradients of buoyancy stress determined from the field of gradients of appropriate combinations of velocity gradients using a thin sheet model of viscous flow are broadly parallel to the direction of maximum topographic gradients in the eastern cordillera providing support for the idea that the bulk behaviour of the bolivian lithosphere is fluid like a comparison of these gradients with buoyancy stress gradients calculated from crustal thickness contrasts suggest that the bulk lithospheric viscosity is in the range 0 5 1 0 1022 pa s gradients of velocity gradients along the eastern margin of the bolivian andes can be interpreted in terms of marked gradients of viscosity with viscosity increasing toward the brazilan shield in the cratonic core of south america buoyancy stresses calculated from crustal thickness contrasts calibrated with the crustal thicknesses at the changeovers from thrust to strike slip or normal faulting suggest that the average lithosphericscale deviatoric stresses are in the range 7 15 mpa

the nature of magnetic single domains in volcanic glasses of the kbs tuff magnetic and optical variations in volcanic glasses of the east african rift have been investigated by means of experimental and analytical study the results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature origin and significance of fine grained magnetic minerals and on the magnetic petrology of glassy rocks the samples that we have studied in detail glass shards from the kbs tuff of northern kenya contain a superparamagnetic precipitate of microcrystals 2 to 10 nm in size the precipitate is likely to have formed at elevated temperature during or subsequent to eruption about 1 9 m y ago we have studied these microcrystals by means of transmission electron microscopy tem ac and dc magnetic methods electron spin resonance esr mossbauer spectroscopy and optical spectroscopy our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism s responsible for the optical and magnetic behavior of these glasses the particle size determination for the larger of these microcrystals has been achieved using tem imaging and mossbauer spectroscopy the mineralogy is nonstoichiometric magnetite based on mossbauer x ray diffraction and electron diffraction results the ratio of saturation remanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single domain theory for a uniaxial anisotropy the origin of which may lie with stress or surface effects we interpret magnetic and esr results to infer the possible existence of microcrystals or amorphous clusters of fe atoms 1 nm or smaller in kbs glasses that appear homogeneous in high resolution tem images results from optical absorption experiments show that the color variations in kbs shards arise due to optical absorption by the precipitate a result consistent with classical electromagnetic theory

magnetic properties and emplacement of the bishop tuff california anisotropy of magnetic susceptibility ams and characteristic remanence were measured for 45 sites in the 0 76 ma bishop tuff eastern california thirty three sites were sampled in three stratigraphic sections two in owens gorge south of long valley caldera and the third in the adobe lobe north of long valley the remaining 12 sites are widely distributed but of limited stratigraphic extent weakly indurated highly porous to dense welded ash flow tuffs were sampled saturation magnetization vs temperature experiments indicate two principal iron oxide phases low ti magnetites with 525 570 c curie temperatures and maghemite with 610 640 c curie temperatures af demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite maghemite predominantly in the multidomain to pseudo single domain size ranges remeasurement of ams after application of saturating direct fields indicates that randomly oriented single domain grains are also present the degree of anisotropy is only a few percent typical of tuffs the ams ellipsoids are oblate with kmin axes normal to subhorizontal foliation and kmax axes regionally aligned with published source vents for 12 of 16 locality means kmax axes plunge sourceward confirming previous observations regarding flow sense topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from kmax axes deposition east of the benton range occurred by flow around the south end of the range and through two gaps benton notch and chidago gap flow down mammoth pass of the sierra nevada is also evident at least some of the adobe lobe in the northeast flowed around the west end of glass mountain eastward flow directions in the upper owens gorge and southeast directions in the lower owens gorge are parallel to the present canyon suggesting that the present drainage has been established along the pre bishop paleodrainage characteristic remanence directions from 45 sites 267 samples yield an overall mean of d 348 i 53 for the bishop tuff a correlation is found in two of the three profiles between density and remanence inclination a mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash fall tuff at lake tecopa is d 353 i 54 k 172 95 2 9 n 15

passive roof thrust model for the emplacement of the pelona orocopia schist in southern california united states the early tertiary orocopia chocolate fault in southern california was originally interpreted as a southwest directed thrust assisting tectonic underplating of the orocopia schist beneath north america during subduction however the predicted thrusting direction is opposite to that observed for the fault although this problem appeared to be resolved by interpreting the orocopia chocolate fault as a normal fault this alternative is inconsistent with the fact that neither an early tertiary breakaway nor early tertiary upper plate extension has been observed i suggest that the fault was a northeast directed passive roof thrust that linked with a southwest directed basal decollement the vincent thrust during early tertiary flat subduction of the farallon plate beneath north america this model may also explain a similar structural relationship between the main central thrust and the south tibet detachment fault in the himalaya

numerical simulation of stress induced piezomagnetic fields at etna volcano summary 3 d magneto elastic models based on finite element method fem have been developed to compute piezomagnetic fields caused by magmatic overpressure sources we solved separately i the elastostatic equation for the stress field and ii the coupled poisson s equation for magnetic potential field the numerical computations were focused on a more realistic modelling of etna volcano where remarkable magnetic changes have been observed during eruptive events we evaluated the effects of topography and medium heterogeneities considering different multilayered crustal structures constrained by seismic tomography and geological evidences when topography and magneto elastic heterogeneities are included in the numerical models the piezomagnetic field changes show significant deviations from the homogeneous half space solution generally it was found that the analytical model overestimates the expected magnetic changes the numerical models of the piezomagnetic field showed significant differences in presence of strong magnetization contrasts and in proximity of steep cliffs that is at the volcano summit the fem approach presented here allows considering a picture of a fully 3 d model of etna volcano which could advance the reliability of model based assessments of magnetic observations

u pb zircon geochronology of rocks in the salinas valley region of california a reevaluation of the crustal structure and origin of the salinian block the salinian block in the salinas valley region of central california consists of arc granitic and metasedimentary rocks schist of sierra de salinas sandwiched between coeval high pressure low temperature melange belts u pb zircon ages of three granitic plutons from this region range from 88 to 82 ma and coexisting biotite yielded 4 0 ar 3 9 ar cooling ages of 76 75 ma the u pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117 81 ma igneous provenance muscovite and biotite 4 0 ar 3 9 ar cooling ages of 72 68 ma from the nearby schist are distinctly younger than those from the granitic plutons these data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks contradicting the prevailing view that the schist comprises the local framework for the salinian arc we propose that the schist of sierra de salinas was thrust beneath the salinian magmatic arc along a campanian thrust fault that has not been recognized this hypothesis implies that the salinian arc originated as a klippe of basement rocks derived from the vicinity of the western mojave desert thrusting initiated southeastward migrating laramide tectonism of a style similar to that which formed the vincent thrust and the latest cretaceous and paleocene pelona and orocopia schists of southern california and southwestern arizona

discrete single domain and pseudo single domain titanomagnetite particles in silicic glass of an ash flow tuff single domain and pseudo single domain titanium poor magnetite grains are present in silicic vitrophyre glass of an ash flow tuff from western nevada conventional transmission electron microscopy and analytical electron microscopy corroborate inferences made solely from rock magnetic and remanence data that the thermoremanent magnetization in the vitrophyre is carried by such grains the magnetite grains vary in size shape and distribution throughout the glass they probably crystal lized during violent eruption and welding of the ash flow at temperatures well in excess of 600 c

aeromagnetic mapping of the structure of pine canyon caldera and chisos mountains intrusion big bend national park texas analysis of aeromagnetic and gravity data reveals new details of the structure igneous geology and temporal evolution of the prominent enigmatic ca 32 ma pine canyon caldera and the chisos mountains big bend national park texas the main caldera fi lling pine canyon rhyolite the oldest member of the south rim formation is reversely magnetized allowing it to be used as a key marker bed for determining caldera fi ll thickness modeling of gravity and magnetic anomalies indicates that the pine canyon rhyolite is probably thicker in the northeastern part of the caldera lineaments in the magnetic data suggest the presence of buried faults beneath the caldera that may have led to increased downdrop in the northeast versus the southwest allowing a thicker section of caldera fi ll to accumulate there the pine canyon caldera has been interpreted as a downsag caldera because it lacks surfi cial faulting so these inferred faults are the fi rst mapped features there that could be responsible for caldera collapse the caldera boundary correlates well with the margins of a gravity low general features of the caldera match well with basic models of downsag calderas meaning that the pine canyon caldera may be a classic example of downsagging of which few well described examples exist in terms of a geophysical signature the source of a long wavelength magnetic high over the chisos mountains is interpreted as a previously unknown broad intrusion the long axis of which trends parallel to a major crustal boundary related to the ouachita orogeny or an even earlier precambrian margin this feature represents the largest intrusion 28 34 km diameter 1 4 km thick 700 3000 km 3 in volume in an area where relatively small laccoliths are ubiquitous the intrusion most likely represents a long lived 1 m y reservoir replenished by small batches of magma of varying composition as refl ected in the variation of eruptive products from the pine canyon and sierra quemada calderas the intrusion may represent the easternmost occurrence of voluminous tertiary magmatism in the southwestern united states

magnetic grain size variations through an ash flow sheet influence on magnetic properties and implications for cooling history rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks provide a basis for interpretation of aeromagnetic data over volcanic terranes and yield insights into the depositional and cooling histories of ash flow sheets a rhyolitic ash flow sheet the miocene aged tiva canyon member of the paintbrush tuff contains both titanomagnetite phenocrysts present in the magma prior to eruption and cubic fe oxide microcrystals that grew after emplacement systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the nevada test site natural remanent magnetization varies from less than 1 10 4 to more than 8 10 4 a m3 kg 1 and in phase magnetic susceptibility varies from less than 1 10 6 to more than 10 10 6 m3 kg 1 the microcrystals which include both magnetite and maghemite have curie points and maximum unblocking temperatures between 580 c and 640 c rock magnetic data including in phase and quadrature magnetic susceptibilities as well as hysteresis parameters demonstrate that these microcrystals are of superparamagnetic and single domain sizes titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15 m thick zones at the top and bottom within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet the decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break possibly represented by a concentration of pumice laboratory heating produces changes in magnetic properties which vary complexly from sample to sample such changes include 1 decrease in saturation magnetization 2 both increase and decrease in in phase and quadrature magnetic susceptibilities and 3 shifts in coercivity spectra the complex sample to sample variation in response to heating is attributed to differences in initial grain size distributions

protolith age of the swakane gneiss north cascades washington evidence of rapid underthrusting of sediments beneath an arc 1 the metamorphic core of the north cascades largely comprises island arc and oceanic terranes juxtaposed prior to circa 96 ma magmatism however the tectonic affinity of the structurally deepest terrane the 9 12 kbar swakane gneiss is distinctly different from other terranes in the core it is not intruded by arc related plutons and contains abundant precambrian zircons new u pb analyses of detrital zircons from the swakane gneiss yield dates from 73 ma to 1610 ma with a dominant late cretaceous population these data indicate that the swakane protolith was deposited as late as 72 5 0 6 ma the 206pb 238u date of the youngest detrital grain following deposition the gneiss was intruded by peraluminous leucogranite sheets that may represent partial melt derived from the gneiss at near to postpeak p t conditions one sheet yielded a u pb crystallization age of 68 36 0 07 ma which indicates that the gneiss was deeply buried within 5 myr of deposition two possible mechanisms considered for this rapid burial 7 mm yr are overthrusting of a forearc or back arc basin by older crystalline rock or underthrusting of trench sediments during low angle subduction the model involving overthrusting of a forearc or back arc basin is most consistent with thermobarometric and isotopic data and the regional geologic setting of the cascades core rapid burial of the swakane protolith is coincident with burial of sediments that formed the pelona orocopia and rand schists of southern california however differences in peak metamorphic temperatures indicate that conditions of burial must have varied along the plate margin

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

geologic origin of magnetic volcanic glasses in the kbs tuff volcanic glass shards and banded pumice from the kbs tuff of northern kenya exhibit marked variations in magnetic susceptibility and color colorless to dark brown the darker glass shards exhibit superparamagnetism which they now know to be carried by a population of tiny microcrystals of fe rich cubic oxide approx 20 to approx 100 a in size thought to be magnetite a theory for their origin is one of nucleation and growth precipitation in quenched homogeneous glass colorless at temperatures of approx 700 1000 sup 0 c during and immediately subsequent to eruption results from high temperature heating experiments on kbs shards support this idea the precipitate appears in the kbs shards as a consequence of their cooling history and is the origin of their coloring the origin cannot lie in negligible compositional differences between the colorless and darkest shards on the other hand banded pumice from the kbs tuff has both dark and colorless glasses of differing compositions the pumice appears banded because precipitation occurred preferentially in the fe rich glass although magma mixing may have played a role in the eruption of these materials on the basis of their survey of distal eruptive products it would appear that the volumetric amount of the maficmore end member dark pumice was insubstantial less

intracrustal subduction and gravity currents in the deep crust sm nd ar ar and thermobarometric constraints from the skagit gneiss complex washington isotopic and thermobarometric pressure temperature p t data on a gneissic quartz diorite combined with previous u pb ages and p t data on wall rock metapelites define a precise p t time series for a portion of the skagit gneiss complex a deep crustal crystalline complex within a zone of intracontinental collision concordant u pb and sm nd ages on zircon indicate crystallization at 68 ma which preceded metamorphism of nearby pelitic wall rocks garnet core assemblages and the pluton plagioclase hornblende coronas on garnet at 800 900 mpa 700 800 c following nearly isothermal decompression and sillimanite cordierite metamorphism of wall rocks garnet rims plus matrix at 300 500 mpa 650 700 c a sm nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 ma slow cooling 10 c m y extends from approximately 60 ma to 50 ma hornblende and biotite ar ar ages of 47 ma and 45 ma respectively define a second unroofing event cooling rates during this period were approximately 100 c m y late cretaceous deep burial followed by two distinct unroofing episodes is also observed in some cordilleran metamorphic core complexes and may suggest a common origin we propose that regional shortening in the cordilleran interior is accommodated by a process of intracrustal subduction i e the upper crust is forced downward into a crustal asthenosphere resulting in a cold buoyant root heating and weakening of the root cause the formation of gravity currents effective viscosity being in the range of 10 16 10 19 pa s spreading outward below nonfluid upper crust spreading of the gravity current which does not necessarily contribute to crustal thinning is driven by the small density contrast between the root and its mid crustal surroundings hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle the root then cools slowly until final unroofing to near surface levels which may occur via either extension or shortening with consequent erosion

growth stabilization and reactivation of proterozoic lithosphere in the southwestern united states growth of proterozoic continental lithosphere in the southwestern united states involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca 1 74 1 70 and 1 65 1 60 ga prograde metamorphism accompanied orogenic assembly and peak metamorphic conditions outlasted deformation regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m y after assembly regions of low metamorphic grade were not uplifted substantially after assembly we suggest that 1 relatively thin lithospheric fragments were assembled into isostatically stable normal thickness continental lithosphere 2 assembly did not erase lithospheric scale heterogeneities 3 the present juxtaposition of different crustal levels reflects differential uplift related to 1 4 1 1 ga tectonomagmatic activity and 4 the boundaries between different lithospheric blocks were repeatedly reactivated from precambrian through tertiary time

paleomagnetic constraints on the geometry and timing of deformation at yucca mountain nevada paleomagnetic data from miocene ash flow sheets indicate that southern yucca mountain has undergone about 30 of clockwise vertical axis rotation since emplacement of the tiva canyon member of the paintbrush tuff at about 13 ma declinations of remanent magnetization from 32 sites in the reversely magnetized tiva canyon member display a systematic southward increase over the 25 km north south extent of yucca mountain probable errors in estimating structural attitudes are a major source of uncertainty in determining the amount of rotation between any two sites analysis of tilt corrected directions of remanent magnetization from the tiva canyon sites indicates that structural attitudes determined from outcrop patterns on geologic maps are generally more accurate than field measurements of attitudes of compaction foliation or contacts attitudes determined from map patterns apparently minimize effects of 1 initial dips 2 poorly defined compaction foliation and 3 tilting that occurred after compaction but before blocking of remanence however such attitudes cannot accurately represent tilting related to unmapped local structures rotations implied by data from three older ash flow sheets the topopah spring member of the paintbrush tuff seven sites and the prow pass four sites and bullfrog three sites members of the crater flat tuff are generally consistent with the amount and sense of rotation documented by data from the tiva canyon member uncertainties in the declinations of tilt corrected site mean directions from these older units preclude evaluating whether older units have undergone more rotation than the tiva canyon member

metamorphism of the deepest exposed arc rocks in the cretaceous to paleogene cascades belt washington evidence for large scale vertical motion in a continental arc the swakane gneiss and the overlying napeequa complex in the north cascade range washington were metamorphosed and deformed during development of a cretaceous paleogene continental arc and are among the structurally deepest exposed rocks within the cordilleran arcs of north america peak metamorphic conditions in both the swakane gneiss and napeequa complex were c 640 750 c 9 12 kbar clockwise paths and widespread evidence for high p metamorphism in meta supracrustal rocks burial to 40 km document major vertical tectonic motion during arc construction and unroofing these and other moderately high pressure rocks in the north cascades coast mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other cordilleran arcs the exhumed arc complexes of the sierra nevada and peninsular ranges are dominated by relatively low p metamorphic and plutonic rocks typically 6 kbar there is no evidence that the northern cordillera was thickened to a greater degree than these other belts suggesting that the greater magnitude of vertical motion in the cascades may have been related to exhumation mechanisms eocene extension in the northern cordillera vs erosional unroofing in the central and southern cordillera

late cretaceous and early tertiary plutonism and deformation in the skagit gneiss complex north cascade range washington and british columbia the skagit gneiss complex forms a more or less continuous terrane within the northern more deeply eroded part of the north cascade range the complex comprises abundant plutons intruded at mid crustal depths into a variety of metamorphosed supracrustal rocks of both oceanic and volcanic arc origin a plethora of syntectonic pegmatite small plutons and granitic dikes gives the complex a migmatitic aspect u pb zircon ages from gneissic plutons within and near the skagit gneiss complex indicate magmatic crystallization between 75 and 60 ma deformation recrystallization and migmatization in part postdate intrusion of the 75 60 ma plutons this latest cretaceous and earliest tertiary plutonism and migmatization may reflect thermal relaxation following early late cretaceous orogeny documented else where in the north cascades the complex was ductilely extended northwest southeast shortly after intrusion of granite dikes at 45 ma but before emplacement of the earliest 34 ma plutons of the cascade arc outcrops of late cretaceous and earliest tertiary plutons migmatites of the skagit gneiss complex and rocks with young ductile deformation are roughly coextensive all apparently marking a region of greater middle eocene unroofing unroofing was apparently contemporaneous with east west extension in the okanogan region to the east and north south and northwest southeast strike slip faulting within the north cascades

is the southeast coast plutonic complex the consequence of accretion of the insular superterrane evidence from u pb zircon geochronometry in the northern washington cascades zircon u pb geochronometry of orthogneisses and plutons in the southwestern crystalline core of the north cascades coupled with fabric and textural studies of the orthogneisses plutons and their metamorphic host rocks indicates extensive synmetamorphic plutonism at 89 96 ma metamorphic mineral assemblages define a culmination composed of an axial kyanite sillimanite zone rimmed by lower grade zones high grade index minerals are typically syntectonic to posttectonic metamorphic fabrics are characterized by an orogen parallel northwest striking steep foliation that contains a subhorizontal stretching and mineral lineation interpreted to be the product of ductile strike slip deformation this fabric is crosscut by 96 92 ma plutons yet is imprinted on 92 89 ma orthogneisses suggesting spatially diachronous fabric development during orogeny documentation of the spatial and temporal coincidence of magmatism with the peak of orogeny together with the kinematic significance of the metamorphic fabric precludes generation of the metamorphic fabric and plutons in response to thrust loading the authors suggest that this part of the coast plutonic complex evolved as a transpressional magmatic arc

characterizing the superparamagnetic grain distribution f v hk by thermal fluctuation tomography 1 in 1965 d j dunlop showed that the joint distribution of particle volumes and microcoercivities f v hk0 can be determined for magnetically monomineralic thermally stable single domain ssd ensembles by taking advantage of the joint temperature and field dependence of relaxation time we have developed a procedure that follows dunlop s strategy to obtain f v hk0 for ensembles containing both superparamagnetic and ssd grains based on backfield remanence curves measured over a range of temperatures each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding blocking contour on the neel plot a suitable set of such line integral samples can be used to reconstruct the f v hk0 distribution using the methods of tomographic imaging samples of the basal tiva canyon tuff have narrow size distributions of elongate ti poor titanomagnetite tomographic inversion of the low temperature backfield spectra yield sharply peaked f v hk0 distributions from which we calculate modal grain dimensions in good agreement with those observed by transmission electron microscopy analysis of synthetic samples containing bimodal populations clearly distinguishes the two modes because our simplified forward calculations incompletely account for the effects of orientation distribution the width of the coercivity distribution at each temperature is underestimated and consequently the inverse calculations yield grain distributions that are overly broad frequency and temperature dependent susceptibilities calculated for the inverted f v hk0 distributions accord fairly well with measured susceptibilities for the weakly interacting tiva canyon samples less well for a moderately interacting paleosol specimen and poorly for a strongly interacting ferrofluid

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

reconciliation of two sided thrusting burial metamorphism and diachronous uplift in the cascades of washington and british columbia several recently published data sets have shed new light on many of the fundamental kinematic aspects of mid cretaceous orogeny in the cascades of northwestern washington and southwestern british columbia specifically a it is now clear that contractional deformation associated with this orogeny affected rocks east of the crystalline core of the range to a greater degree than had previously been suspected and b geobarometric data from the crystalline core of the range now permit a quantitative reconstruction of the crustal thickening event responsible for core metamorphism to be attempted i propose a new kinematic model for mid cretaceous orogeny that reconciles these data with previously collected data sets that focused on deformation in the northwest cascades san juan islands thrust system the model derived from this analysis suggests that the cascades underwent a minimum of between 400 and 500 km of east west shortening between about 115 and 85 ma owing to the collision of the greater insular terrane which probably included rocks of the chilliwack nooksack terrane with the western edge of the greater intermontane terrane which included the methow basin and bridge river hozameen terrane on the basis of their relative and absolute times of uplift i assign the deepest part of the metamorphic core the skagit gneiss to the greater insular terrane and the structurally higher carapace of the core the settler nason metapelites and the cogburn creek mad river metabasalt metachert to the westernmost intermontane terrane the contact between the skagit gneiss and the overlying assemblages must represent a fault or suture across which at least 120 km of displacement took place this fault is also the root zone from which many of the nappes in the northwestern cascades and san juan islands were derived i concur with previous workers who suggested that the collisional event was not precipitated by the closure of a several thousand kilometer wide ocean but was rather due to a short lived margin normal convergence event that both followed and was followed by periods during which the continental margin was highly influenced by oblique subduction and strike slip faulting

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

paleomagnetic data bearing on the origin of arcuate structures in the french peak massachusetts mountain area of southern nevada the eastern margin of the yucca flat basin in southern nevada is bounded by north northwest striking tilted fault blocks of the halfpint range whose strikes curve as much as 90 clockwise into east northeast strikes in the french peak massachusetts mountain fpmm area this pattern of arcuate structures has been attributed to clockwise drag along a postulated northwest trending right lateral shear zone the flexure model implies that rocks within the fpmm area were rotated strongly clockwise about a vertical axis directions of remanent magnetization of the middle miocene ammonia tanks and rainier mesa members of the timber mountain tuff and of the topopah spring member of the paintbrush tuff indicate no systematic vertical axis rotation in the fpmm area and disprove the flexure model after tilt correction declinations of 29 site means obtained from the three ash flow sheets in the fpmm area are not systematically different than declinations of 17 site means from the halfpint range or declinations of 16 site means from little deformed mesa areas to the west the paleomagnetic data thus indicate that structures in the fpmm area initiated with arcuate trends and were not originally straight elements that were rotated by right lateral drag the structures probably formed under the influence of spatially variable stress fields the fpmm area lies in an accommodation zone between domains of oppositely tilted extensional fault blocks interaction between stress fields associated with propagating normal fault zones may have been responsible for the arcuate structures in the fpmm area

magnetic detection and characterization of biogenic magnetic minerals a comparison of ferromagnetic resonance and first order reversal curve diagrams australian research council grant number dp120103952 australia new zealand iodp consortium anzic grant number le0882854

driving mechanisms for 40 km of exhumation during contraction and extension in a continental arc cascades core washington 1 in the nw north american cordillera the cascades core region of the coast plutonic complex underwent late cretaceous 96 ma to locally 73 ma sw ne contraction and crustal thickening followed by dextral transpression 73 to 55 ma then transtension 3 mm yr by local thrusting in regions undergoing crustal thickening in the central part of the core chelan block 40 km of exhumation occurred between 91 and 45 ma about half of which occurred during early contraction driven by thrusting and half during top to north arc oblique shear during reactivation of a midcrustal cretaceous thrust the dinkelman decollement the footwall of this thrust consists of the swakane biotite gneiss a cretaceous metaclastic assemblage with recorded pressures of 10 12 kbar no arc related magmatism and structures dominated by pervasive top to north shearing the hanging wall consists of the napeequa complex an oceanic assemblage with recorded pressures of 6 12 kbar voluminous arc related magmatism and complex structures indicating early top to wsw shearing younger top to north shearing and widespread folding in the napeequa top to north shearing started by 73 ma during melt present conditions at pressures 6 kbar top to north shearing in both hanging wall and footwall continued during exhumation 1 6 mm yr and cooling to greenschist facies conditions during which slip became increasingly localized eventually resulting in formation of pseudotachylite on discrete slip surfaces we suggest that arc oblique extension was driven by along arc heterogeneity in displacements erosion initially during transpression and underplating of continental sediments and later during transtension

cretaceous high p granulites at milford sound new zealand metamorphic history and emplacement in a convergent margin setting granulite facies orthogneiss of the arthur river complex arc at milford sound western fiordland records a complex early cretaceous magmatic and orogenic history for the pacific gondwana margin that culminated in the emplacement and burial of a dioritic batholith the western fiordland orthogneiss wfo enstatite bearing mafic to intermediate protoliths of the arc and wfo intruded the middle to upper crust the early deformation history of the arc is preserved in the pembroke granulite where two pyroxene s1 assemblages that reflect p 750 c were only patchily recrystallized during later deformation s1 is cut by garnet bearing leucogabbroic to dioritic veins which are cut by distinctive d2 fractures involving anorthositic veins and garnet diopside plagioclase bearing reaction zones these zones are widespread in the arc and wfo and record conditions of p 14 kbar and t 750 c garnet clinopyroxene bearing corona reaction textures that mantle enstatite in both the arc and wfo reflect early cretaceous burial by approximately 25 km of continental crust most of the arc is formed from the milford and harrison gneisses which contain steeply dipping s4 assemblages that envelop the pembroke granulite and involve garnet hornblende diopside clinozoisite rutile and plagio clase with or without kyanite the p t history of rocks in western fiordland reflects pronounced early cretaceous convergence related tectonism and burial possibly related to the collision and accretion of island arc material onto the pacific gondwana margin

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

internal structure of ustica volcanic complex italy based on a 3d inversion of magnetic data a ground magnetic study of ustica island was performed to provide new insights into subsurface tectonic and volcanic structures the total intensity anomaly field obtained after a data reduction procedure shows the presence of a w e striking magnetic anomaly in the middle of the island and another two intense anomalies which seem to continue offshore in the southwestern and the northeastern sides respectively the detected anomalies were analyzed by a quadratic programming qp algorithm to obtain a 3d subsurface magnetization distribution the volcano magnetization model reveals the presence of intensely magnetized volumes interpreted as the feeding systems of the main eruptive centers of the island which roughly follow the trend of the main regional structural lineaments these findings highlight how regional tectonics has strongly affected the structural and magmatic evolution of the ustica volcanic complex producing preferential ways for magma ascent

implications of magmatic epidote bearing plutons on crystal evolution in the accreted terranes of northwestern north america magmatic epidote in a plutonic rock signifies that the rock probably crystallized at pressures of at least 8 kbar 25 30 km depth in western north america undeformed late cretaceous plutons containing magmatic epidote occur within a mobile belt extending from northern california to southeastern alaska this belt has been interpreted as consisting of accreted terranes because the present crustal thickness within this belt is about 30 km the plutons imply a crustal thickness at the time of emplacement of 55 60 km comparable to that in the central andes the plutons further imply paleogeothermal gradients in the host country rock at the time of intrusion of not more than 20 c km and may be closer to 12 c km such cool thick crust suggests that subduction and piling of thrust sheets were probably the principal processes of crustal accretion in western north america the implications regarding the depth of crystallization of the plutons can be used to estimate the minimum rate of regional uplift it is on the order of 1 mm yr the rapid uplift of the mobile belt by and large did not affect the craton

paleomagnetic evidence for displacement from the south of the coast plutonic complex british columbia the mid cretaceous spuzzum and porteau plutons of the coast plutonic complex of british columbia have two magnetizations a and b the a magnetization eight sites 83 specimens d 30 3 i 56 7 95 4 9 paleolatitude 37 5 n paleopole 65 0 n 14 9 w a95 6 2 is considered to have been acquired in the age range 105 90 ma this result differs from the field established for cratonic north america in this time range the difference could be caused either by previously undetected tilting about a horizontal axis of the plutons or by their rotation about a vertical axis and lateral displacement relative to the craton previously observed mid cretaceous magnetizations from other rock units from the western canadian cordillera and the cascades of washington united states are similarly discordant with respect to the craton this similarity over such a large area indicates that although local undetected tilting could be partly responsible it is unlikely to be the prime cause and we argue ther

curie temperatures of titanomagnetite in ignimbrites effects of emplacement temperatures cooling rates exsolution and cation ordering pumices ashes and tuffs from mt st helens and from novarupta contain two principal forms of titanomagnetite homogeneous grains with curie temperatures in the range 350 500 c and oxyexsolved grains with similar bulk composition containing ilmenite lamellae and having curie temperatures above 500 c thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the curie temperature of the homogeneous phase the exsolved grains are most common where emplacement temperatures exceeded 600 c and in laboratory experiments heating to over 600 c in air causes the homogeneous titanomagnetites to oxyexsolve rapidly where emplacement temperatures were lower curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales and thermomagnetic curves are generally irreversible with lower curie temperatures measured during cooling but little or no change is observed in room temperature susceptibility we interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites although we cannot conclusively rule out an alternative interpretation involving fine scale subsolvus unmixing short range ordering within the octahedral sites may play a key role in the observed phenomena changes in the curie temperature have important implications for the acquisition stabilization and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites

magnetism and magnetic mineralogy of ash flow tuffs from yucca mountain nevada the magnetic susceptibility and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement spatial variations in these properties therefore provide a potentially useful tool for geological studies maxima and minima in measured along profiles at yucca mountain nevada identify persistent magnetic marker horizons within vitric portions of the tiva canyon and underlying topopah spring members of the paintbrush tuff the correlatable variations of with vertical position in these units exist over the 8 km of strike examined the observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of fe ti oxide phenocrysts and the presence shape size and mineralogy of magnetic fe oxide microcrystals that precipitated at high temperature after emplacement of each sheet vitrophyre near the top of the topopah spring member has a distinctive maximum in due to titanomagnetite phenocrysts susceptibility in the lower portion of the overlying tiva canyon member reaches maximum values of about 1 3 10 2 si near the base of the member beneath the columnarly jointed vitrophyre due to the presence of abundant superparamagnetic cubic fe oxide these elongate microcrystals are several tens of nm long and are typically deformed and tapered up section in the tiva canyon member the susceptibility decreases going to a minimum within the columnar vitrophyre at this stratigraphic level the precipitated fe oxides are larger and carry intense nrm of up to 30 a m is only 3 10 3 the whisker like microcrystals of fe oxide which typically nucleated on what appears to be chain silicate grew as needles up to 25 m in length the size variations of the precipitated fe oxides which were established using transmission electron microscopy tem and petrographic observation are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization several interpretations of the shape anisotropy of the precipitated feoxide are possible including growth by a dislocation mechanism additionally the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field susceptibility variations as established at the outcrop as well as in the borehole offer a potentially useful tool for stratigraphic correlation of ash flow sheets

sediment volume and mass beneath the bay of bengal rates of sediment accumulation and the amount of sedimentary fill in depocenters lying downstream of erosion in the himalayas and tibet can provide some insight into tectonics and geological history the objective of this paper is to put on record the best estimates which are possible with existing data of the volume and mass of sediments sedimentary rock and metasedimentary rock beneath the sea floor of the bay of bengal the sedimentary section in the bay of bengal is divided into two parts 1 eocene through holocene sediments and sedimentary rocks which post date the initial india asia collision volume 12 5 106 km3 mass 2 88 1016 t this is most of the bengal fan including its eastern lobe the nicobar fan plus some of the outer bengal delta 2 early cretaceous through paleocene pre collision sedimentary and metasedimentary rocks volume 4 36 106 km3 mass 1 13 to 1 18 1016 t these are interpreted as continental rise and pelagic deposits

paleomagnetic evidence of localized vertical axis rotation during neogene extension sierra san fermin northeastern baja california mexico paleomagnetic data from sierra san fermin in the gulf of california extensional province indicate that localized clockwise rotations about vertical axes occurred during pliocene through recent extension and dextral shear relative declination discordances in upper miocene and pliocene ash flow tuffs indicate a net clockwise rotation of 30 16 clockwise rotation between 12 5 and 6 ma is statistically insignificant 11 17 structural observations and geochronological data suggest that rotations in this area began post 6 ma comprising uniform sense block rotations oblique divergence associated with extension and dextral slip in the northwest striking boundary between the pacific and north american plates northeast striking sinistral slip faults and north striking normal faults accommodate distributed dextral shear in this area allowing fault blocks to rotate in a clockwise sense a model for oblique divergence predicts 21 km of shear in the direction of relative plate motion and 20 7 km ene directed extension perpendicular to the main gulf escarpment a broad region of northeastern baja california may have undergone similar distributed shear two possible dynamic models may explain this shear in one model rotation accumulates above a deep subhorizontal basal shear zone rotating blocks may extend downward to a detachment beneath the extensional province either a low angle eastward continuation of the san pedro martir fault or to a basal shear surface on top of a subducted remnant of the farallon plate alternatively distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults in either case shear may have transferred northward onto faults west of the san andreas fault contributing to late miocene to recent clockwise rotation of the western transverse ranges this shear is not accounted for in the 300 km of dextral slip computed from cross gulf geologic tie points

ferromagnetic resonance characterization of greigite fe3s4 monoclinic pyrrhotite fe7s8 and non interacting titanomagnetite fe3 xtixo4 ferromagnetic resonance fmr spectroscopy has become an increasingly useful tool for studying the magnetic properties of natural samples magnetite fe3o4 is the only magnetic mineral that has been well characterized using fmr this limits the wider use of fmr in rock magnetism and paleomagnetism in this study we applied fmr analysis to a range of magnetic minerals including greigite fe3s4 monoclinic pyrrhotite fe7s8 magnetically non interacting titanomagnetite fe3 xtixo4 and synthetic magnetite chains to constrain interpretation of fmr analysis of natural samples and to explore applications of fmr spectroscopy we measured the fmr signatures of a wide range of well characterized samples at the x and q bands fmr spectra were also simulated numerically to compare with experimental results the effects of magnetic anisotropy mineralogy domain state and magnetostatic interactions on the fmr spectra are discussed for all studied minerals our experimental and theoretical analyses of magnetically non interacting tuff samples and magnetically interacting chains enable quantitative assessment of contributions of magnetostatic interactions and magnetic anisotropy to the fmr spectra our results also indicate that intact magnetosomes are a unique system with distinct fmr signatures while fmr analysis is useful for characterizing magnetic properties of natural samples care is needed when making interpretations because of overlaps in a range of fmr signatures of different magnetic minerals with different magnetic properties our analyses will help to constrain such interpretations in rock magnetic studies

magnetic petrofabric of igneous rocks lessons from pyroclastic density current deposits and obsidians abstract measurement of the anisotropy of magnetic susceptibility ams of igneous rocks can provide clues concerning their mechanism of formation and in particular are very helpful as flow direction indicators unlike other igneous rocks however pyroclastic density current deposits pdcds present a challenge in the interpretation of ams measurements due to the complexity of their mechanism of emplacement in this paper we review the most common assumptions made in the interpretation of the ams of pdcd taking advantage of key lessons obtained from obsidians despite the complexities on the mechanism of formation of pdcds it is shown that a key element for the fruitful interpretation of ams is to give proper attention to the various components likely to be involved in controlling their general petrofabric the anisotropies of ferromagnetic crystals whether as free phases or embedded within clasts or shards and those of paramagnetic minerals mainly ferrosilicates need to be taken into consideration when interpreting the ams measurements of pdcds variations of the deposition regime both as a function of position and of time also need to be considered on the interpretations nevertheless if a suitable sampling strategy is adopted the potential of the ams method as a petrofabric indicator is maximized

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

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

significance of seismic reflections beneath a tilted exposure of deep continental crust tehachapi mountains california the focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface with the tehachapi mountains in california being the case in point as a prime example of the lower crust these mountains expose cretaceous gneisses that formed 25 30 km down in the sierra nevada batholith and appear to be underlain by the ensimatic rand schist integrated geophysical and geological studies by the calcrust program have produced a cross section through this post mid cretaceous structure and suggest a general model for its development seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the san joaquin basin s tejon embayment two south dipping reverse faults on the rim of the tejon embayment were discovered in the reflection data and verified in the field the faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the rand schist the upper and lower crusts are separated by a zone of laterally discontinuous reflectors reflections from the lower crust form a wedge the base of which is a nearly flat moho at 33 km regional geological relations and gravity models both suggest that the reflective zone corresponds to the rand schist and the newly recognized faults account for its neogene exposure alternatively the reflective zone maybe part of the gneiss complex suggesting that the schist either lies deeper or is not present under the gneisses if the rand schist underlies the tehachapi mountains and mojave region to their south a model for their evolution can be constructed from regional geological relations it seems that during late cretaceous laramide subduction the protolith of the schist was thrust eastward beneath the mojave along this portion of the cordilleran batholithic belt the subduction was evidently at very low angles the bottom of the batholith was removed and replaced by a thick section of schist fluids from which weakened the overlying batholith this thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat lying slabs when emplacement of the schist ended in latest cretaceous earliest paleocene the underlying mantle rose compensating for the extension and providing material for magmatic underplating in the neogene transpression and rotation of the upper crust along the san andreas and garlock faults resulted in the exposure of the schist

qualitative thermobarometry of inverted metamorphism in the pelona and rand schists southern california using calciferous amphibole in mafic schist abstract the rand and pelona schists consist of eugeoclinal rock types overlain by continental basement along the vincent chocolate mountains vcm faults both schists display inverted metamorphic zonation defined in part by a systematic variation in composition of calcic to sodic calcic amphibole in mafic schist structurally upward the compositional progressions include increase of total a1 a1iv and ti but decrease in the ratios of na na ca to a1 a1 si and nam4 to a1vi fe3 ti these variations imply that structurally high rocks belong to a lower pressure metamorphic fades series than those at depth this result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences amphibole composition is dependent not only on structural position i e p t but also upon bulk rock composition the important controls are whole rock mg mg fe2 mn and fe3 fe2 the greatest impact of these factors however is on the absolute values of na and al rather than their ratio thus interpretation of facies series is not seriously hindered by compositional variability sodic amphibole in epidote blueschists from the rand schist is extensively replaced by sodic calcic amphibole sodic calcic amphibole in the rand schist and pelona schist is itself rimmed by actinolitic amphibole similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation in the rand and pelona schists the sequence probably formed during burial

palaeomagnetism and chronology of the central taupo volcanic zone new zealand summary the central taupo volcanic zone tvz of new zealand is a region of intense quaternary silicic volcanism active since 1 6 ma we report palaeomagnetic measurements from 59 distinct volcanic units sampled at 98 sites in the tvz these are mainly rhyolitic ignimbrites and lava domes with a few basaltic andesitic and dacitic lavas most have new k ar or 4oar 39ar ages the remanent magnetizations are generally stable to both thermal and alternating field demagnetization and well determined mean palaeodirections were obtained for all sites our findings suggest that the taupo whakamaru maroa reporoa rotorua and okataina volcanic centres were magnetized during the brunhes normal chron kapenga is an older volcanic centre where activity commenced around 0 89 ma and extended into the brunhes mangakino volcanic centre is significantly older and was active from 1 6 to 0 95 ma transitional or intermediate palaeodirections were obtained from ahuroa ignimbrite 1 18 ko 02 ma and mamaku ignimbrite 0 22k0 01 ma the former almost certainly corresponds to the cobb mountain event the latter is significantly older than the blake event and probably corresponds to the recently reported pringle falls summer lake magnetic episode multiple sites from the whakamaru ignimbrite have indistinguishable 4oar 39ar ages 0 33 f 0 01 ma and glass composition but divergent palaeomagnetic directions this contrast suggests that either 1 the different sites were formed during a phase of extremely violent activity lasting up to a few hundred years during which geomagnetic secular variation was recorded or 2 that they were formed in a single eruption and rotation during subsequent extensional tectonism has caused divergence of the palaeodirections 40ar 39ar ages of 0 77 0 03 ma for the reversely magnetized rahopeka ignimbrite and 0 71 k 0 06 ma for the overlying normally magnetized waiotapu ignimbrite bracket and constrain the age of the matuyama brunhes transition

p t t evidence for mechanisms of vertical tectonic motion in a contractional orogen north western us and canadian cordillera the 1800 km long coast mountains north cascades orogen of the canadian cordillera and north western us developed as a continental magmatic arc metamorphic rocks in the orogen contain widespread evidence for burial of supracrustal rocks to depths of c 40 km followed by nearly isothermal decompression to depths of 10 km near many shallowly emplaced mid cretaceous plutons low pressure contact metamorphic effects were overprinted by high pressure regional metamorphic minerals and textures as evidenced by kyanite staurolite pseudomorphs after andalusite in metapelitic rocks therefore near pluton rocks record the loading history of the orogen metapelitic rocks not associated with plutons only preserve evidence for high pressure conditions and or high temperature decompression as indicated for example by sillimanite and cordierite after kyanite and garnet respectively petrological evidence for burial and decompression is therefore recorded in different rocks various regions of the orogen differ in timing of metamorphism the overall shape of p t paths and the relative timing and regional extent of the high pressure event but most of these data and observations are consistent with thrusting and or pure shear thickening as primary loading mechanisms throughout the orogen as opposed to magma dominated loading this interpretation is further supported by comparison with thermal models which demonstrate that the p t paths are consistent with simultaneous thrusting and folding at a high initial geothermal gradient 35 40 c km 1 in much of the orogen a high geothermal gradient supports tectonic models invoking intra arc contraction and suggests that magmatism played an important role in regional temperature time paths this tectonic thermal history may be typical of other contractional orogens and illustrates the importance of large vertical displacement of crust in magmatic arcs

structure and chronology of the oval peak batholith and adjacent rocks implications for the ross lake fault zone north cascades washington the 200 km long ross lake fault zone rlfz forms the northeastern border of the crystalline core of the north cascades this late cretaceous and paleogene structure is an 15 km wide system of faults and fault zones several of which deform the paleocene 65 ma u pb zircon oval peak batholith the southern segment of the gabriel peak tectonic belt of the rlfz is defined by strongly deformed rocks of the foliated southwestern margin of the oval peak batholith structurally underlying pre late cretaceous twisp valley schist and structurally highest part of the subjacent skagit gneiss which here consists of late cretaceous 87 ma and paleocene 60 ma orthogneiss kinematic indicators in moderately steeply northeast dipping mylonitic orthogneiss record reverse slip the strong solid state flattening fabric in the foliated margin of the oval peak batholith formed during or shortly after emplacement and overprinted a magmatic fabric observed in the core of the batholith foliation and associated stretching lineation within the gabriel peak tectonic belt are oblique to the regional structural grain but concordant with the contacts of the batholith these patterns in part record emplacement of the batholith as an expanding diapir into the tectonic belt reverse slip in the tectonic belt occurred before during and shortly after emplacement but ended by 55 58 ma the core of the oval peak batholith intruded a large tongue of twisp valley schist on the north porphyroblast microstructures and ar ar and k ar geochronology indicate that amphibolite facies dynamothermal metamorphism of the schist occurred during intrusion the foggy dew and twisp river fault zones separate the batholith and its wall rocks on the northeast from weakly metamorphosed strata of the methow basin the foggy dew fault zone contains mylonitic gneiss derived from the oval peak batholith skagit gneiss and tonalitic rock that displays u pb zircon systematics suggestive of an eocene age sillimanite schist and amphibolite are also present mylonites record dextral strike slip with a component of normal slip down to the northeast movement in the fault zone continued subsequent to deformation in the gabriel peak tectonic belt and ended by 48 ma mylonites in the twisp river fault zone also reveal dextral strike slip but apparently with less normal slip the rlfz experienced a diachronous history from at least 65 to 45 ma this history was dominated by dextral strike slip but there were significant components of normal slip and reverse slip on some structures during the latter part of this interval 57 45 ma the rlfz probably recorded a transtensional tectonic regime transitional between extension to the east in omineca metamorphic core complexes and dextral strike slip to the west related to oblique convergence along the continental margin early cenozoic plutonism metamorphism and ductile deformation have strongly overprinted any of the major cretaceous deformation postulated in previous tectonic models for the rlfz

indo burma range a belt of accreted microcontinents ophiolites and mesozoic paleogene flyschoid sediments this study provides an insight into the lithotectonic evolution of the n s trending indo burma range ibr constituting the southern flank of the himalayan syntaxis paleogene flyschoid sediments disang barail that represent a shallow marine to deltaic environment mainly comprise the west central sector of ibr possibly resting upon a continental base on the east these sequences are tectonically flanked by the eocene olistostromal facies of the disang which developed through accretion of trench sediments during the subduction the shelf and trench facies sequences of the disang underwent overthrusting from the east giving rise to two ophiolite suites naga hills lower ophiolite nhlo andvictoria hills upper ophiolite vhuo but with different accretion history the ophiolite and ophiolite cover rock package were subsequently overthrusted by the proterozoic metamorphic sequence originated from the burmese continent the nhlo suite of late jurassic to early eocene age is unconformably overlain by mid eocene shallow marine ophiolite derived clastics on the south the vhuo of mesozoic age is structurally underlain by continental metamorphic rocks the entire package in victoria hills is unconformably overlain by shallow marine late albian sediments both the ophiolite suites and the sandwiched continental metamorphic rocks are thrust westward over the paleogene shelf sediments these dismembered ophiolites and continental metamorphic rocks suggest thin skinned tectonic detachment processes in ibr as reflected from the presence of klippe of continental metamorphic rocks over the nhlo and the flyschoid disang floor sediments and half windows exposing the disang beneath the nhlo

mid cretaceous thrusting in the southern coast belt british columbia and washington after strike slip fault reconstruction a major thrust system of mid cretaceous age is present along much of the coast belt of northwestern north america thrusting was concurrent and spatially coincided with emplacement of a great volume of arc intrusives and minor local strike slip faulting in the southern coast belt 52 to 47 n thrusting was followed by major dextral slip faulting which resulted in significant translational shuffling of the thrust system in this paper we restore the displacements on major dextral slip faults of the southern coast belt and then analyze the mid cretaceous thrust system two reconstructions were made that use dextral faulting on the yalakom fault 115 km castle pass and ross lake faults 10 km and fraser fault 100 km the reconstructions differ in the amount of dextral offset on the straight creek fault 160 and 100 km and how much the ne part of the cascades crystalline core expanded 30 km and 0 km during eocene extension reconstruction a produces the best match of lithotectonic units and thrust systems our synthesis shows that the southern coast belt thrust system was 250 180 km wide after thrusting the thrust system was mainly southwest vergent but had a belt of northeast vergent back thrusts on the northeast side associated with the tyaughton methow basin which may indicate large scale tectonic wedging thrust faults are commonly low to moderate angle but high angle faults also occur especially as late stage out of sequence structures involving plutons the amount of thrust displacement across the system is unknown but must be at least 100 km and may be many hundreds of kilometers most thrusting occurred from 100 to 80 ma and did not migrate systematically until after 90 ma when thrusting and magmatism shifted to the northeast for a few million years widespread thrusting occurred both near plutons and where there are no or small plutons which strongly suggests that thrust faulting was caused by regional to plate scale forces such as rapid plate convergence and or arc continent collision

linking zircon u pb and garnet sm nd ages to date loading and metamorphism in the lower crust of a cretaceous magmatic arc swakane gneiss wa usa abstract synorogenic basin development and subsequent collapse and loading provide important constraints on the late cretaceous crustal structure of the north america cordillera high grade metasedimentary rocks of the swakane gneiss in the north cascades have been interpreted to include ca 73 ma detrital zircon that requires a protolith age considerably younger than metamorphic ages of adjacent terranes this detrital interpretation requires rapid loading rates of 7 8 mm yr heating to 750 c in garnet preserves compositional evidence for growth during high pressure amphibolite facies metamorphism and little subsequent modification new garnet rock sm nd isochrons of 73 5 1 2 71 3 2 8 and 65 8 0 7 ma tightly constrain the timing of garnet growth swakane gneiss zircons preserve complex magmatic detrital and metamorphic histories most zircons have distinct cores and homogeneous overgrowths detrital zircon cores include significant proterozoic and mesozoic components with low u th ratios homogeneous metamorphic zircon rims have high u th 5 and define an array of concordant u pb dates from 75 to 63 ma the new zircon u pb and garnet sm nd ages establish that the youngest regional metamorphism was diachronous and occurred between 75 and 63 ma our dataset precludes post 73 ma deposition and instead suggests that sedimentation occurred after ca 91 ma and ended before ca 75 ma using these age constraints for deposition and metamorphism we propose new loading rates of 1 3 mm yr and conclude that the swakane gneiss protolith was likely deposited in a small short lived intra arc basin that subsided filled in and collapsed quickly due to overthrusting of arc crust

paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff southern nevada implications for welding temperatures two voluminous rhyolitic to quartz latitic ash flow sheets the tiva canyon and topopah spring members of the paintbrush tuff were erupted from the miocene claim canyon cauldron in southern nevada although lithologically similar these units differ greatly in their recording of the ancient geomagnetic field the reversely magnetized tiva canyon member yields remanent directions that are well grouped both within and between sites the normally magnetized topopah spring member however yields remanent directions that vary both laterally and vertically remanent directions from a 100 m thick section of nonwelded to partially welded topopah spring are well grouped precision parameter k 200 and give a mean direction d 293 i 69 that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling in three 300 m thick sections of more densely welded tuff inclinations from the upper parts of the topopah spring are steep 70 but flatten with depth to about 20 and then steepen abruptly near the base of the unit in general the directional dispersion increases with the thickness of the section k 30 for sections that are about 300 m thick much of the directional variation in the topopah spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding the possibility of subblocking temperature deformation in an ash flow sheet is enhanced by 1 high blocking temperature spectra of the primary magnetic phases 2 relatively low emplacement temperatures and 3 great stratigraphic thickness thermal demagnetization results from the topopah spring member indicate that welding may have continued to temperatures as low as 475 c in the basal vitrophyre of the ash flow sheet remanent directions from the tiva canyon member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the topopah spring

interpretation of anisotropy of magnetic susceptibility fabric of ignimbrites in terms of kinematic and sedimentological mechanisms an anatolian case study abstract a study of the neogene low aspect ratio kizilkaya ignimbrite in the calc alkaline central anatolian volcanic province turkey has been carried out using the anisotropy of magnetic susceptibility ams technique on 46 sites irm hysteresis loops and curie temperature analysis indicate that magnetite is the main carrier of the magnetic signal in the ignimbrite the shape of the ams ellipsoids varies widely from site to site and the anisotropy is commonly low we review the data processing used in the literature on the ams of ignimbrites to calculate principal directions or to evaluate the quality of the results and we develop various simple techniques data filtering and contouring procedures in order to more accurately define the axis or symmetry of the ams fabric there is no simple relationship between the shape of the ams ellipsoids and the shape of the petrofabric ellipsoids similarly there is no direct relationship between magnetic and kinematic axes our results show that the source of the measured magnetic signal in these rocks is a complex result of various contributions including shape anisotropy of free and fixed magnetic crystals shape ams crystallization of magnetic grains on all surfaces and discontinuities during cooling of the ignimbrite distribution anisotropy and alteration by hydrothermal and meteoric water particular attention is paid to the interpretation of the ams stereoplots departing from the standard fabric of sedimentary rocks formed under unidirectional flow in order to explain these non standard ams fabrics we discuss their possible origin 1 in analytical and mineralogical terms instrumental artifact pollution by xenoclasts mineralogical inversion of the susceptibility axes secondary mimetic fabric and 2 in terms of sedimentological mechanisms rheology depositional processes in the present state of knowledge we conclude that due to the physico chemical complexity of the magnetic source the ams signal should be used and interpreted with care when attempting to decipher the sedimentological or rheological processes occurring in moving pyroclastic flows on the other hand ams has it greatest potential when used in conjunction with other techniques and when a large number of sampling sites and specimens is used

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

subduction and exhumation of the pelona orocopia rand schists southern california the pelona orocopia and rand schists of southern california and southwestern arizona are thought to have formed in either the same east dipping subduction zone as the franciscan complex or in a southwest dipping subduction zone related to collision of an outboard continental fragment with north america the principal justification for the collision model has been the observation that continental rocks overlying the schists locally show transport to the northeast field and petrographic studies now confirm that the northeast movement occurred during exhumation of the schist not during subduction combined with regional geologic relations this evidence makes the collision model unlikely

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

the use of magnetic susceptibility and its frequency dependence for delineation of a magnetic stratigraphy in ash flow tuffs variations in the grain size amount and mineralogy of magnetic phases in layered volcanic rocks define a magnetic stratigraphy that can be identified by means of magnetic susceptibility measurements in ash flow tuffs grain size and other variables that control susceptibility are to a large extent a function of the cooling and alteration history the phases responsible for the susceptibility of ash flow tuffs consist of post emplacement high temperature precipitates of fe oxides in volcanic glass and the phenocrystic fe ti oxides at yucca mountain nevada laterally continuous high susceptibility approximately 10 sup minus 2 si horizons exist in the paintbrush tuff due to the presence of either precipitates or phenocrystic fe ti oxides the frequency dependence of magnetic susceptibility chi omega can be used to discriminate between horizons with multi domain phenocrystic material the chi omega exhibits a 26 decrease per decade of increasing frequency for precipitate grain sizes where the superparamagnetic single domain state gives way to stable single domain behavior and thus chi omega offers an indirect method for rapid estimation of magnetic grain sizes the interpretation of variations established by field and laboratory susceptibility data has been constrained by petrography and transmission electron microscopy tem

paleomagnetism and rotation constraints for the middle miocene southwestern nevada volcanic field middle miocene rocks of the southwestern nevada volcanic field swnvf lie across the projection of the walker lane belt within the basin and range province and thus provide an interesting opportunity to test for late cenozoic vertical axis rotation paleomagnetic data from individual ash flow sheets document no significant relative vertical axis rotation among localities within central swnvf an area of relatively low stratal tilts and widely spaced faults a time averaged mean paleomagnetic direction d 351 4 i 52 7 95 4 5 calculated from data from numerous separate rock units suggests that the central swnvf underwent minimal counterclockwise vertical axis rotation r 7 1 6 6 with respect to the north american craton no clockwise vertical axis rotation is found to support projection of dextral faults of the walker lane beneath the central swnvf clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field these clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central swnvf block and more strongly extended areas to the south and southwest of the field negligible rotation of 11 45 ma to 13 25 ma tuffs relative to the central swnvf was found at the southeast margin of the field where 90 clockwise rotation at the northwest termination of the las vegas valley shear zone had been postulated any clockwise rotation in this area must predate 13 25 ma and thus dextral shear within this part of the walker lane belt was not synchronous or connected across the southern margin of the field small counterclockwise vertical axis rotation relative to the craton as found for the central swnvf block might be a regional feature in the western great basin

intra arc crustal loading and its tectonic implications north cascades crystalline core washington and british columbia widespread high pressure cretaceous metamorphism in the coast plutonic complex of southeast alaska and british columbia and its southeast extension the cascades crystalline core is commonly attributed to thrust loading during superterrane collision however terrane boundaries within the cascades core are intruded by relatively shallow mid cretaceous plutons and crustal loading of 2 5 kbar postdates these plutons these observations are not consistent with proposed collisional models and we suggest that loading occurred by structural and or plutonic processes operating within a magmatic arc such loading may be an important process in arc tectonics although it has not been widely reported

four cordilleran paleorivers that connected sevier thrust zones in idaho to depocenters in california washington wyoming and indirectly alaska upper cretaceous sandstones from 17 localities from california to southeastern alaska united states contain unexpectedly large populations of detrital zircons with proterozoic u pb ages with age peaks at 1800 1650 and 1380 ma these peaks are indicative of a sediment source region in the southern part of the proterozoic belt supergoup basin in central idaho which hosts 1800 1650 ma detrital zircons and which was intruded by rift related 1380 ma bimodal plutons and sills belt rocks were strongly uplifted and eroded during late cretaceous sevier shortening and fed four paleoriver systems the lemhi pass hawley creek river system flowed east and sourced the beaverhead harebell pinyon nonmarine megafan in the cordilleran foreland basin the kione river flowed southwest to northern california where it sourced a very large ca 82 80 ma 600 m thick delta and submarine fan complex within the northern great valley forearc basin considerable kione detritus also transited the forearc basin to reach the franciscan trench sourcing a pulse of deposition and subduction accretion in central california and even part of southern california the swakane river flowed northwest out of idaho into washington sourcing the protolith for the high grade swakane gneiss more speculatively a yakutat river may have flowed northwest and deposited yakutat strata in a trench off washington or british columbia before those rocks were translated north to southeastern alaska recognition of a major source area in central idaho for zircons with an uncommon age of 1380 ma helps constrain the ca 85 65 ma paleogeography and paleotectonics of major sectors of the north american convergent margin orogen

rates of processes in magmatic arcs implications for the timing and nature of pluton emplacement and wall rock deformation abstract the construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions it is critical therefore to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton wall rock systems our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following 1 crystal growth rates in magma of 10 4 cm year 1 2 growth rates of metamorphic porphyroblasts between 10 5 and 10 2 cm year 1 3 long term magma supply rates of 10 1 km 3 year 1 and short term rates of up to 350 km 3 year 1 4 diapiric ascent rates for mafic plutons of 1 3 m year 1 5 cooling of plutons to ambient wall rock temperatures in 10 5 10 6 years 6 final crystallization of plutons in a small fraction of the time needed for complete cooling 7 fault displacements of 3 cm year 1 8 development of cleavages in fault zones in less than 10 6 years at strain rates of 10 13 s 1 or higher and 9 the development of regional cleavages in 10 6 years at strain rates of 10 14 s 1 these rates indicate that processes operating in magmatic arcs are relatively rapid pluton emplacement cleavage development etc occur over time spans of tens of thousands to no more than a few million years however the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed at deeper levels or in zones undergoing faster strain rates the time spans of the various processes approach one another thus plutons with otherwise similar characteristics emplaced in regions undergoing fast strain rates or at deeper crustal levels may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration finally we argue that the structural and other characteristics of pluton wall rock systems will depend on rates of various processes involved and that these rates at the very least influence and sometimes invalidate the timing criteria previously published by us and others

regionally extensive mid cretaceous west vergent thrust system in the northwestern cordillera implications for continent margin tectonism the intermontane insular superterrane boundary zone represents a fundamental crustal boundary separating the two largest allochthonous crustal fragments in the north american cordillera structural stratigraphic and geochronologic relations along this boundary indicate that substantial west vergent compression and concomitant crustal thickening occurred there in mid cretaceous time this orogenic zone extends for more than 1200 km along strike length between southern southeast alaska and northern washington in southern southeast alaska and northwest british columbia rocks of the insular superterrane were imbricated along a series of west to southwest vergent thrust faults in northern washington and southwestern british columbia a wide zone encompassing the margins of the two superterranes as well as numerous intervening smaller fragments was shortened principally along west vergent thrusts known geologic relations do not discriminate among existing tectonic models that explain the origin of the mid cretaceous thrust system

fe oxide microcrystals in welded tuff from southern nevada origin of remanence carriers by precipitation in volcanic glass although it is widely recognized that remanent magnetism in ash flow tuffs is carried by fine grained fe oxides the origin mineralogy and significance of such magnetic carriers are not well understood the authors have obtained transmission electron microscope images of distinctive fe oxide microcrystals in rhyolitic samples located 3 8 7 6 and 18 5 m above the base of a 110 m thick section of the miocene tiva canyon member of the paintbrush tuff the fe oxide microcrystals are lath shaped and increase in size from 20 x 140 nm in the lowermost sample near base of the member to 120 x 800 nm in the uppermost sample within the flow interior microcrystals in this size range are within or close to the range of single domain grain size for magnetite electron diffraction and analytical x ray data indicate that the microcrystals in the lower two samples are cubic fe oxides magnetite maghemite with less than 10 mol ti end member and that those in the uppermost sample are manganiferous hematite systematic variations in magnetic properties are consistent with the observed variations in size and mineralogy of the microcrystals these microcrystals are morphologically distinct from grains that the authors interpret to be fragments of phenocrysts the morphology and spatial distribution ofmore the microcrystals as well as their increase in grain size from the rapidly cooled base of the ash flow sheet into the flow interior are consistent with an origin by nucleation and growth from volcanic glass at elevated temperature subsequent to emplacement less

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

continental influence on canadian cordilleran terranes from nd isotopic study and significance for crustal growth processes nd isotopic data are presented for rocks of the terrane assembly that lies inboard of the stikine terrane in the canadian cordillera of british columbia and yukon these are from most inboard outward cassiar kootenay slide mountain quesnel and cache creek terranes they are regarded as documenting a transition from terranes whose evolution was closely tied to that of the north american continental margin out to far traveled oceanic terranes the results emphasize sedimentary rocks as indicators of tectonic position of the crustal fragments sedimentary rocks of the cassiar and kootenay terranes show a strong connection to miogeoclinal sediment sources argillites of pennsylvanian permian age from a paleontologically controlled section in the slide mountain terrane are also consistent with sediment sources in the north american miogeocline igneous rocks of the slide mountain quesnel and cache creek terranes show juvenile oceanic or arc origins based on nd values between 3 and 10 and are essent