near end member magnesiochloritoid in prograde zoned pyrope dora maira massif western alps in the ultrahigh pressure metamorphic terrane of the dora maira massif western alps magnesiochloritoid up to 97 mol end member occurs along with ellenbergerite talc chlorite or kyanite as prograde inclusions in a few pyrope megablasts whereas most pyrope megablasts in this terrane are relatively homogeneous prp90 prp98 the magnesiochloritoid bearing megablasts commonly preserve a clear prograde fe mg zonation pattern from prp70 to prp98 the nearly homogeneous chloritoid inclusions also record this zonation pattern their xmg increasing from 0 70 to 0 96 from garnet core to garnet rim these zonations are shown to be prograde and to record increasing p and t conditions up to about 700 c 30 kbar the occurrence of magnesiochloritoid in some garnets simply reflects either a slightly more fe rich system general case or a more al rich and originally corundum bearing system one unzoned near end member pyrope megablast fe mg partitioning between chloritoid and zoned host garnet changes from normal to reverse i e xmgcld xmggrt from core to rim or from rim to core in other megablasts of yet similar composition in case of reverse partitioning a diffusion zone of a few tens of mu m wide exists in garnet around the inclusions toward which xmggrt decreases and reaches a normal partitioning value at the contact these features are interpreted in terms of disequilibrium and tentatively ascribed to slow diffusion in chloritoid with respect to garnet growth rate or to two competing garnet forming reactions with the metastable one which produces a more mg rich garnet being kinetically favoured the prograde inclusions of magnesiostaurolite show a consistent fe mg reversal with garnet i e xmgst xmggrt which is interpreted as an equilibrium feature at high pressure and high xmg magnesiochloritoid locally breaks down into symplectites of chlorite and an extremely low si magnesiostaurolite down to 7 0 si per formula unit which is probably metastable with respect to magnesiostaurolite corundum

comment on depositional history of the eocene chumstick formation implications of tectonic partitioning for the history of the leavenworth and entiat eagle creek fault systems washington by james e evans

correction for the static shift in magnetotellurics using transient electromagnetic soundings shallow inhomogeneities can lead to severe problems in the interpretation of magnetotelluric mt data by shifting the mt apparent resistivity sounding curve by a scale factor which is independent of frequency on the standard log apparent resistivity versus log frequency display the amount of parallel shift commonly referred to as the mt static shift can not be determined directly from conventionally recorded mt data at a single site one method for measuring the static shift is a controlled source measurement of the magnetic field unlike the electric field the magnetic field is relatively unaffected by surface inhomogeneities the controlled source sounding which may be a relatively shallow sounding made with lightweight equipment can be combined with a deep mt sounding to obtain a complete undistorted model of the earth inversions of the static shift corrected mt data provide a much closer match to well log resistivities than do inversions of the uncorrected data the particular controlled source magnetic field sounding which we used was a central induction transient electromagnetic tem sounding correction for the static shift in the mt data was made by jointly inverting the mt data and the tem data a parameter which allowed vertical shifts in the mt apparent resistivity curves was included in the computer inversion to account for static shifts a simple graphical comparison between the mt apparent resistivities and the tem apparent resistivities produced essentially the same estimate of the static shift within 0 1 decade as the joint computer inversion central induction tem measurements were made adjacent to over 100 mt sites in central oregon the complete data base of over 100 sites showed an average static shift between 0 and 0 2 decade however in the rougher topography and more complex structure of the cascade mountain range the majority of the sites had static shifts of the order of 0 3 to 0 4 decade the static shifts in this area are probably due to a combination of topography and surficial inhomogeneities the tem apparent resistivity which is used to estimate the unshifted mt apparent resistivity does not necessarily agree with either the transverse electric te or the transverse magnetic tm mt polarization tem apparent resistivity may occur between the two or may agree with one of the two polarizations or may lie outside the mt polarizations

western eisila regio venus radar properties of volcanic deposits 1988 arecibo observatory dual polarization radar images are presented for western eisila regio venus the polarization information and pioneer venus orbiter reflectivity and altimetry data are analyzed for volcanic deposits on two 400 500 km radius constructs sif and gula montes many of the large effusive deposits studied appear to require superposed flows or multiple vents to explain the observed progression of roughness along their length high fresnel reflectivity material may be present along the summit region of gula mons and in an embayed tessera like region to the n radar dark units on the flanks of sif mons are inferred to be pyroclastic deposits but radar dark features near the summits of both edifices are more consistent with very smooth lava flows higher spatial resolution magellan data will be useful in testing these predictions

stratigraphy age and paleogeography of the eocene chuckanut formation northwest washington the eocene chuckanut formation of washington s north cascades comprises as much as 6000 m of alluvial strata and is one of the thickest nonmarine sequences in north america it is exposed in several disconnected outcrop belts that are remnants of what was probably an extensive fluvial system in western washington in this study seven stratigraphic members are defined in the main outcrop belt of the chuckanut formation near the town of bellingham their ages based on fission track zircon dates palynologic studies and correlation by mapping extend from the early to the late eocene three types of fluvial systems contributed to the chuckanut formation and are distinguished on the basis of their lithology sedimentology and petrology the first comprises fine load meandering river deposits of the lower eocene bellingham bay member and the middle eocene slide member sandstones are arkosic and had their main source in rapidly uplifted high grade metamorphic terranes in eastern washington the second type

synthesis of staurolite in melting experiments of a natural metapelite consequences for the phase relations in low temperature pelitic migmatites we document experiments on a natural metapelite in the range 650 775 c 6 14 kbar 10 wt of added water and 700 850 c 4 10 kbar no added water staurolite systematically formed in the fluid present melting experiments above 675 c but formed only sporadically in the fluid absent melting experiments the analysis of textures phase assemblages and variation of phase composition and fe mg partitioning with p and t suggests that supersolidus staurolite formed at near equilibrium during fluid present melting reactions the experimental results are used to work out the phase relations in the system k2o na2o feo mgo al2o3 sio2 h2o appropriate for initial melting of metapelites at the upper amphibolite facies the p t grid developed predicts the existence of a stable p t field for supersolidus staurolite that should be encountered by aluminous fe rich metapelites during fluidpresent melting at relatively low temperature and intermediate pressures 675 700 c 6 10 kbar for xh2o 1 in the knfmash system but not during fluid absent melting the implications of these findings for the scarcity of staurolite in migmatites are discussed

petrological constraints and pt path of devonian collapse tectonics within the scandian mountain belt western gneiss region norway in norway post caledonian extension controlled the formation and sedimentation of middle devonian sedimentary basins these collapse basins were deposited along a low angle extensional detachment fault on top of caledonian high grade metamorphic rocks of the western gneiss region this paper concerns the petrological analysis of eclogite and high pressure micaschist collected within the extensional shear zone study of gneisses and micaschists surrounding these high pressure rocks allows description of the complete pt path which characterized this area application of appropriate geothermometers and geobarometers confirms that the retrograde evolution following high pressure conditions ll 15kbar at 500 600 c was characterized by nearly isothermal decompression followed by amphibolite facies metamorphism the shape of the pt path is compared with theoretical models of thermal evolution of continental orogenic crust during uplift we can observe an increase in temperature during the first stages of decompression smaller than predicted by the models considering the evidence for extensional tectonic in the study area we propose to explain such a small increase in temperature during the decompression by the intervention of extension which accelerates the collapse process and reduces the late orogenic heating of the caledonian thickened crust

conodonts as biostratigraphic tools for redefinition and correlation of the cambrian ordovician boundary the cambrian ordovician boundary is recognized at different horizons on various continents by utilizing several fossil groups conodonts are abundant diverse and less provincial and facies controlled than other fossils in this interval many species are widespread and some are cosmopolitan strata representing the cambrian ordovician boundary interval from asia australia and north america can be correlated easily when those strata are from cratonal or shelf environments strata from slope facies have fewer conodonts fewer taxa and taxa may have different ranges compared with cratonal and shelf facies it may be easier to correlate intercontinentally within cratonal shelf facies than to correlate intracontinentally from cratonal shelf facies to slope facies a new hierarchical arrangement of conodont interval zones and subzones is proposed for western north america these include the proconodontus tenuiserratus zone the p posterocostatus zone new the p muelleri zone new the eoconodontus zone new with eoconodontus notchpeakensis and cambrooistodus minutus subzones the cordylodus proavus zone emended with hirsutodontus hirsutus fryxellodontus inornatus and clavohamulus elongatus subzones the cordylodus intermedius zone new with hirsutodontus simplex and clavohamulus hintzei subzones the cordylodus lindstromi zone new and cordylodus angulatus zone new the boundary point for the base of the ordovician system will be chosen so as to coincide with a correlatable conodont zonal boundary other fossil groups will support correlation of this boundary point three alternative horizons are being considered the base of the cordylodus proavus zone is the most distinctive but is regarded by some as older than is appropriate for the base of the ordovician the base of the cordylodus intermedius zone is recognizable by faunal changes in several evolutionary lineages at a level that is slightly younger than the presently recognized boundary in australia north america and parts of asia but slightly older than the base of the tremadoc series of europe the base of the c lindstromi zone can be recognized in most areas by the lowest occurrence of the nominate species which is of questioned taxonomic validity and uncertain biostratigraphic utility this horizon is closest to the base of the tremadoc series of europe choice of a stratotype section has been narrowed to the cow head group in newfoundland eastern canada and to the fengshan and yehli formations in jilin province northeastern china the cow head group was deposited on and near the base of the continental slope erosion by debrisslide breccias resulted in a significant hiatus near the boundary interval in the broom point sections a condition contrary to guidelines for choice of boundary sections this may be less of a problem in other sections such as green point the dayangcha section in china was deposited on the outer part of a continental shelf and has acritarchs conodonts graptolites and trilobites additional study of the china section is needed to document fully the ranges of critical conodont taxa

progressive metamorphism in a crustal scale shear zone an example from the leon region north west brittany france this paper describes the progressive metamorphism and deformation of a series of metasediments le conquet schists and their higher grade equivalents which occur as tectonically emplaced screens within a sequence of foliated gneisses the gneiss de brest and gneiss de lesneven the sequence exhibits a steep south to north increase in metamorphic grade from garnet staurolite schist to sillimanite gneiss and sillimanite k feldspar migmatite the relationship of mineral growth to foliation development has been established for individual screens at least five phases of deformation d1 d5 are preserved analysis of porphyroblast inclusion trails is used to demonstrate sequential mineral growth during the successive development of orthogonal foliations s1 s4 porphyroblasts continued to grow during the subsequent development of c s mylonite fabrics and extensional crenulation cleavages which are genetically related to a series of high strain zones d5 mineral assemblages phase relations and mineral chemistry are consistent with porphyroblast growth being the result of continuous reactions microstructure porphyroblast relations are used to show that although mineral growth proceeded during continuous reactions these only operated episodically phase relations mineral chemistry and p t estimates are used to constrain p t trajectories and these are linked to the deformation histories within individual screens a comparison between the resulting pressure temperature deformation paths is used to demonstrate that the metamorphic peak occurred progressively later and at successively lower pressures with increasing metamorphic grade it is suggested that the early evolution of the belt is the result of crustal thickening by overthrusting the subsequent history is one of progressive heating and unroofing of the higher grade rocks in a dextral strike slip transtensional shear zone

superzoned garnets in the coesite bearing brossasco isasca unit dora maira massif western alps and the origin of the whiteschists abstract rare centimeter sized superzoned garnets szgs were discovered in two coesite bearing whiteschists of the brossasco isasca unit biu southern dora maira massif dmm western alps the superzoned garnet consists of a reddish brown almandine core crowded with inclusions of staurolite chloritoid kyanite chlorite and paragonite and of a pinkish pyrope rim with sporadic inclusions of kyanite and magnesian chlorite the core rim contact is relatively sharp and marks the termination of the inclusion rich portion the core composition of the superzoned garnet is almost identical to or slightly richer in mg than that of the rim of porphyroblastic garnet in metapelites from the same unit in the rim of the superzoned garnet mg fe ratio increases abruptly towards the outermost rim whose composition is identical to that of the common pyrope in the whiteschist at the core rim boundary there is no chemical gap chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite the staurolite composition si 2 00 and total r 2 grt cld chl the resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages such as st cld chl vs cld chl ky new and literature data and results of experiments in the kfash system suggest that 1 the superzoned garnet was formed under a single prograde high pressure ultra high pressure hp uhp alpine metamorphism 2 the almandine inclusion rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600 c and less than 16 kbar probably from a former metapelite xenolith included in a variscan granitoid and 3 the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth i e at a stage close to the uhp climax

early palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms abstract stylophorans cornutes mitrates represent one of the most diverse classes of cambro ordovician echinoderms they were free living benthic non radiate forms closely related to asterozoans and crinoids taphonomic sedimentological and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology such a combined approach suggests that the rarity of stylophorans in proximal environments above storm wave base was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test conversely stylophorans were relatively abundant in deeper settings below storm wave base where they frequently occur in low diversity dense beds several tens to hundreds of individuals per square metre such assemblages were either largely dominated by stylophorans only middle cambrian darriwilian or by the association of both stylophorans and ophiuroids latest darriwilian silurian comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well oxygenated oligotrophic waters with low predatory pressure and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans seven main stylophoran biofacies are identified and described the associated environments and tropical distribution of primitive heavily plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings earliest middle cambrian late cambrian however from this primitive stock stylophorans radiated several times towards deeper and or colder environments possibly because of the lower predation pressure in such settings cothurnocystid cornutes in the middle cambrian mitrates in the late cambrian hanusiids in the tremadocian as a consequence the fact that most cambro ordovician cornutes and mitrates were psychrospheric i e cold water adapted probably explains their relatively wide palaeobiogeographic distribution in high palaeolatitudes cambro ordovician stylophorans were abundant and diverse in shallow to deep environments but they were generally restricted to deep settings in low palaeolatitudes the great caradocian transgression favoured the extension of stylophorans from circum polar regions e g western margin of gondwana towards external settings at lower palaeolatitudes avalonia baltica and eastern margin of laurentia where one group of mitrates underwent a major radiation anomalocystitids and extended towards shallower and warmer palaeoenvironments in contrast all silurian stylophoran assemblages known so far are rare poorly diversified and restricted to shallow warm to temperate environments in tropical regions this distribution partly results from 1 the probable eradication of psychrospheric forms e g cornutes because of the prolonged anoxia of deep environments in the late hirnantian early silurian of west gondwana 2 the radiation of some mitrates anomalocystitids in shallow tropical settings in the late ordovician and 3 of strong taphonomic and or sampling biases numerous ghost lineages

neodymium isotopic composition of cambrian ordovician biogenic apatite in the baltoscandian basin implications for palaeogeographical evolution and patterns of biodiversity biogenic apatite preserved in 148 samples of conodonts and organophosphatic shelled brachiopods from cambrian through ordovician successions of the baltoscandian basin baltica plate preserves a sensitive record of early palaeozoic sea water chemistry interpreted via neodymium isotope ratios consistent e nd t values of 9 6 to 8 3 for lower to middle cambrian samples suggest no significant lateral or temporal variation across the region average upper cambrian values are 7 2 to 7 7 sedimentary analysis suggests that the influence of continental weathering from baltica as a major source of radiogenic nd was negligible ordovician samples show a rise to 5 to 6 in the early arenig early mid llanvirn and late caradoc sea water mixing from the southeast iapetus ocean was a constant factor throughout cambrian ordovician times the rise reflects erosion of obducted volcanic arc complexes along the caledonian margin and probably also relates to pollution of the baltica sector of iapetus from the approaching avalonia plate patterns of evolutionary biodiversity and palaeobiogeographical linkages support the geochemical signatures in interpreting the tectonic history of the region extinction of lingulate brachiopod faunas in the tremadoc followed by subsequent recovery and emergence of benthic assemblages typical of the ordovician evolutionary fauna in the billingen early volkhov regional stages coincide with significant changes in geochemical characteristics of water masses across the baltoscandian basin the early and mid ordovician arenig to llandeilo brachiopod faunas of the north estonian confacies belt are characterized by high endemism and low turnover rates whereas increased immigration resulted in the extinction of a number of local lineages in the late llanvirn from the mid caradoc to mid ashgill when baltica was drifting on a course to collide eventually with avalonia and gradually approach laurentia brachiopod assemblages were characterized by higher turnover rates at the same time they gradually became more cosmopolitan and less influenced by the invasion of new faunas

central washington seismicity evidence for a reactivated buried continental rift and northwest trending structural zones analysis of central washington seismicity of the past two decades reveals some interesting features shallow seismicity and deep seismicity occur as different geographic distributions concentration of seismicity along north to northwest oriented trends appears to be related to a buried continental rift and possible associated fault zones hypothesized extensions of the chiwaukum graben and straight creek fault systems are plausible structural controls on the seismicity

integrated 3d geophysical and geological modelling of the hercynian suture zone in the champtoceaux area south brittany france this paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3d geometry of a segment of the hercynian suture zone of western europe in the champtoceaux area brittany france the champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists with eclogite bearing units the exhumation of the complex during early carboniferous times was accompanied by deformation during regional dextral strike slip associated with a major hercynian shear zone the south armorican shear zone sasz dextral shearing produced a km scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components based on the seismic constraint direct 2d crustal scale modelling was performed throughout the champtoceaux fold on seven radial gravity profiles also using geological data and density measurements from field and drill hole samples the 3d integration of the cross sections the digitised geological map and the structural information foliation dips insure the geometrical and topological consistency of all sources of data the 2d information is interpolated to the whole 3d space using a geostatistical analysis finally the 3d gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the bouguer anomaly the final 3d model is thus compatible with the seismic and gravity data as well as with geological data main geological results derived from the modelling are i the overall 3d geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the champtoceaux complex which was coeval with strike slip along the south armorican shear zone ii the gravity modelling suggests the presence of a relatively dense body below the champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the e w trending wrench thrust system iii the northern limb of the champtoceaux anticline is a relatively shallow feature and iv vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the sasz and spreads away from the strike slip zone within weak country rocks d 2004 elsevier b v all rights reserved

cambrian ordovician trilobite family missisquoiidae hupe 1955 systematic revision and palaeogeographical considerations based on cladistic analysis abstract the missisquoiids are among the most important trilobites in the cambrian ordovician boundary interval of laurentia and gondwana this study deals with a systematic review of the family missisquoiidae based on a cladistic analysis and explores their palaeobiogeographical history a total of 22 missisquoiid species were selected for cladistic analysis the cladistic results demonstrate that the family missisquoiidae includes parakoldinioidia pseudokoldinioidia tangshanaspis and tasmanocephalus lunacrania and hardyia are included with reservation the well known genus missisquoia is treated as a junior synonym of parakoldinioidia based on the distribution of the missisquoiids six palaeogeographical areas are recognised sino korea yangtze australia southern laurentia northwestern laurentia and northeastern laurentia palaeogeographical analyses show that the missisquoiids originated in gondwana and continued to expand their geographical range within the continent and eventually expanded into the laurentia optimisation results of geographical area transitions demonstrate that there were transitions between the two continents and evolution of the laurentian missisquoiids appears to have been strongly influenced by the gondwanan counterparts we suggest that dispersal of non fossilised egg stage probably transported by oceanic currents may have been responsible for these inter continental transitions

petrology of fe mg carpholite bearing metasediments from ne oman abstract fe mg carpholite occurs in metasediments of tectonically disrupted basement shelf and foreland basin units that structurally underlie the semail ophiolite in ne oman in the lower grade structurally higher units fe rich carpholite coexists with paragonite quartz illite kaolinite and chlorite whereas in deeper units fe mg carpholite occurs with pyrophyllite sudoite phengite and or chloritoid mineral compositions in these units indicate that chlorite is more magnesian than coexisting fe mg carpholite at low temperatures and pressures but at higher metamorphic grades xmg decreases in the order sudoite carpholite chlorite chloritoid this suggests a reversal in fe mg partitioning between fe mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite quartz to pyrophyllite and at xmg 0 35 phase relations and mineral equilibria indicate that the p t conditions of formation of the fe mg carpholite bearing rocks of ne oman range from 280 315 c 3 6 kbar for the structurally highest units to 325 440 c 6 9 5 kbar for the deepest units indicating a systematic down section increase in metamorphic grade textural relations in these rocks interpreted in the context of pertinent equilibria are consistent with the clockwise p t paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

ages and stratigraphy of lower and middle tertiary sedimentary and volcanic rocks of the central cascades washington application to the tectonic history of the straight creek fault in the central cascade range of washington three structural blocks of early tertiary sedimentary and volcanic rocks help to define the position and history of the southern segment of the straight creek fault east of the fault in the teanaway river block the early eocene fluviatile feld spathic sandstone of the swauk formation is interbedded with largely dacitic volcanic rocks of the silver pass volcanic member zircon fission track ages on the silver pass are about 52 m y overlying the tightly folded swauk and silver pass is the teanaway formation a gently dipping accumulation of basalt andesite rare dacite and rhyolite that yields middle eocene about 47 m y whole rock k ar ages conformably overlying the teanaway is the roslyn formation a coal bearing fluviatile feld spathic sandstone of probable middle to late eocene age the late eocene and oligocene naches formation exposed west of the straight creek fault in the cabin creek block is rich in fluviatile feldspathic sandstone and rhyolite flows and tuffs in its lower part but grades upward into a sequence dominated by basalt but with some andesite the basal guye sedimentary member of the naches formation underlies and is interbedded with the mount catherine rhyolite member in the snoqualmie pass area on the basis of zircon fission track ages on silicic tuffs and whole rock k ar ages on basalt the basal part of the naches is about 40 to 44 m y old the formation is tightly folded and complexly faulted along the straight creek fault the straight creek fault intersects the olympic wallowa lineament in the strongly deformed manastash river block in this block fluviatile coal bearing feldspathic sandstone of the manastash formation is overlain by principally dacitic volcanic rocks of the taneum formation overlying the taneum is the basalt of frost mountain we correlate this threefold sequence feldspathic sandstone dacite basalt with the sequence in the teanaway river block small patches of rhyolitic ash flow tuff dated at 33 m y late oligocene and interbedded feldspathic sandstone overlie the roslyn and teanaway formations in the teanaway river block and are probably correlative with the late oligocene wenatchee formation exposed in the chiwaukum graben east of the teanaway river block as well as with the late oligocene volcanic rocks 30 m y along the cascade crest to the southwest these latter rocks are little deformed and unconformably overlie the naches formation the ages and depositional record in the three blocks indicate a tertiary history of dominantly vertical movement along the straight creek fault and its southeasterly splays that merge with the olympic wallowa lineament the horst and graben structure of the blocks as well as enechelon fold axes in the swauk formation suggests some post early eocene right lateral shear along the fault although there is no direct evidence of lateral offset vertical movement significant since early eocene swauk deposition followed late eocene naches deposition but tapered off by late oligocene time and ceased by miocene time when the fault was intruded by the 25 m y old snoqualmie batholith and other plutons however structures with olympic wallowa lineament trends appear to have influenced folding of the miocene yakima basalt subgroup

the mesozoic rimrock lake inlier southern washington cascades implications for the basement to the columbia embayment the mesozoic rimrock lake inlier lies between the north cascades blue and klamath mountains and thus has significant implications for the basement to the columbia embayment the inlier consists of the late jurassic indian creek complex and the voluminous late jurassic early cretaceous russell ranch complex the indian creek complex consists of trondhjemitic to gabbroic rocks some of which underwent dynamo thermal medium grade metamorphism that was probably broadly synchronous with plutonism lithologies and geochemical data indicate that the complex represents the roots of an arc the russell ranch complex is a tectonic melange consisting mainly of arkose and mudstone with subordinate chert conglomerate and pillowed greenstone and minor green tuff deposition may have occurred on an inner fan or the inner part of a mid fan sandstones were derived from a plutonic terrane presumably a dissected are the geochemistry of greenstones apparently is most compatible with eruption as within plate tholeiites or morb the complex displays a block in matrix mesoscopic fabric greenstone and the foliated twin sisters lakes silicic metavolcanic unit occur as kilometer scale blocks predominantly steep faults riddle the complex overlapping time relations between normal and thrust faults several generations of folds and cleavages record protracted deformation steep faults separate the units and are bracketed between early cretaceous and earliest eocene as is deformation of the russell ranch complex these are mostly brittle structures but one mylonite zone has a gentle stretching lineation suggestive of strike slip large displacements probably occurred between the indian creek and russell ranch complexes but an original basement and cover relationship cannot be precluded the complexes are broadly correlative with rocks to the north the late jurassic are represented by the indian creek complex may include rocks in other inliers in central washington the melange belts of north western washington and the san juan islands the russell ranch complex most closely resembles parts of the western melange belt and the constitution formation late jurassic arc type rocks also occur in the klamath and blue mountains and the russell ranch complex may correlate with the yolla bolly terrane of the franciscan complex these correlations suggest that lithological belts are more continuous between washington and california than implied by models which propose major tectonic dispersal the mesozoic arc type crust and continentally derived clastic rocks in the inlier demonstrate that the basement to the columbia embayment includes a variety of tectonic slices or terranes and that the embayment is not simply underlain by tertiary or mesozoic oceanic crust as is commonly proposed

paleomagnetism of an east west transect across the cascade arc in southern washington implications for regional tectonism paleomagnetic data from a transect across the cascade arc in southern washington were collected to test models of vertical axis rotations for this part of the pacific northwest and to provide information on the tectonic history of the st helens seismic zone the 75 site mean directions are divided into three groups based on isotopic fission track and stratigraphic ages group 1 consists of samples from rocks deposited between 39 and 30 ma and shows a mean clockwise vertical axis rotation of 34 13 group 2 consists of samples from rocks deposited between 30 and 24 ma and shows a mean clockwise rotation of 17 11 and group 3 consists of samples from rocks between 24 and 16 ma and shows a mean clockwise rotation of 20 12 although these three values of rotation are statistically indistinguishable at the 95 confidence level we interpret them in combination with other data to indicate differential rotation across the st helens seismic zone shz the available paleomagnetic data for eocene oligocene rocks west of the seismic zone show clockwise vertical axis rotations of 30 8 and 35 9 consistent with the value for group 1 the rotational values for groups 2 and 3 and the intrusive suite of kidd creek 13 ma despite their different ages have similar values east of the shz comparing these groups of data east and west of the seismic zone indicates a differential rotation of 10 3 across it in addition the rates of rotation in southern washington are similar to those for rocks in the western cascades of oregon and indicate that rotational deformation of miocene age extends northward into southern washington and eastward into the axis of the arc

high pressure micro inclusions and development of an inverted metamorphic gradient in the santiago schists ordenes complex nw iberian massif spain evidence of subduction and syncollisional decompression the santiago schists are located in the basal unit of the ordenes complex one of the allochthonous complexes outcropping in the inner part of the hercynian belt in the north west of the iberian peninsula their tectonothermal evolution is characterized by the development of an eo hercynian metamorphic episode c 374 ma of high p low to intermediate t the mineral assemblage of the high p episode is preserved as a very thin si s1 foliation included in albite porphyroblasts being composed of albite garnet i white mica 1 chlorite 1 epidote quartz rutile ilmenite the equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 10 c and 14 7 0 7 kbar probably minimum pressure the later evolution syn d2 of the schists defines a decompressive and slightly prograde p t path which reached its thermal peak at c 525 10 c and 7 kbar decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient so that the zones of garnet ii biotite with an upper subzone with chloritoid and staurolite developed from bottom to top of the formation the estimated p t path for the santiago schists suggests that the basal unit probably a fragment of the gondwana continental margin was uplifted immediately after its subduction at the beginning of the hercynian orogeny it also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension probably related to the continued subduction of the gondwana continental margin and the contemporaneous development of compensatory extension above it the inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin when it came into contact with the upper parts of the schists along a detachment probably of extensional character the general metamorphic evolution of the santiago schists with the development of high p assemblages with garnet prior to decompressive and prograde parageneses with biotite is unusual in the context of the european hercynian belt and shows a close similarity to the tectonothermal evolution of several high p low to intermediate t circum pacific belts

cambrian origin of all skeletalized metazoan phyla discovery of earth s oldest bryozoans upper cambrian southern mexico exquisite pywackia baileyi landing n gen and sp specimens from the lower tinu formation southern mexico extend the bryozoan record into the upper cambrian they are 8 m y older than the purported oldest bryozoans from south china and show that all skeletalized metazoan phyla appeared in the cambrian the new form differs from similar twig like cryptostomes by its shallow autozooecia and an elongate axial zooid which may be homologous to the stolon in nonmineralized ctenostomes it may morphologically resemble mineralized stem group bryozoans that retained a stolon like individual although an ability to bud was acquired by the feeding individuals autozooids the latest cambrian origin of bryozoans several mollusk classes polyplacophorans cephalopods and euconodonts was a major evolutionary development and can be considered the onset of the ordovician radiation of more complex marine communities

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

tectonics and seismicity of the southern washington cascade range abstract geophysical geological and seismicity data are combined to develop a transpressional strain model for the southern washington cascades region we use this model to explain oblique fold and fault systems transverse faults and a linear seismic zone just west of mt rainier known as the western rainier zone we also attempt to explain a concentration of earthquakes that connects the northwest trending mount st helens seismic zone to the north trending western rainier zone our tectonic model illustrates the pervasive effects of accretionary processes combined with subsequent transpressive forces generated by oblique subduction on eocene to present crustal processes such as seismicity and volcanism

comparison of geoelectrical tectonic models for suture zones in the western u s a and eastern europe are black shales a possible source of high conductivities abstract large scale geoelectrical anomalies have been mapped with geomagnetic depth sounding gds and magnetotelluric mt surveys in the carpathian mountains region these anomalies are associated with the zone of closure between stable europe and a complex of microplates in front of the converging african plate the zone of closure or suture zone is largely occupied by an extensive deformed flysch belt the models derived to fit the observed geoelectrical data are useful in the study of other suture zones and carpathian structures have been compared with areas currently being studied in the western cordillera of the u s a models derived for a smaller scale suture zone mapped in western washington state have features that are similar to the carpathian models the geoelectrical models for both the carpathian and washington anomalies require dipping conductive slabs of 1 5 m material that extends to depths 20 km in both instances there is evidence that these materials may merge with lower crustal mantle conductors along the down dip margins of the slab the main conductive units are interpreted to be sedimentary rocks that have been partially subducted due to collisional processes heat flow is low in both regions and it is difficult to explain fully the deep conduction mechanisms however evidence suggests that the conduction at depth may include electronic conduction in sulfide mineral or carbon films as well as ionic conduction in fluids or partial melt

the chiwaukum structural low cenozoic shortening of the central cascade range washington state usa the chiwaukum area on the eastern flank of the cascade range of washington state is a type locality for a clastic basin in what is broadly speaking a continental arc province evans 1994 1 concluded that the eocene chumstick formation was deposited syntectonically in the chiwaukum graben in a

paleoclimatology and paleobotany of the eocene chumstick formation cascade range washington usa a rapidly subsiding alluvial basin abstract the middle to late eocene chumstick formation comprises 12 km of fluvial and lacustrine strata deposited in a strikeslip basin in washington state the chumstick paleoflora is a mixed suite of broad leaved evergreen and broad leaved deciduous taxa with physiognomic characteristics intermediate between paratropical and subtropical rainforest types the chumstick paleoflora probably occupied an intermediate paleogeographic position between lowland and montane rainforest in a setting about 150 km inland from the pacific coast at elevations up to about 750 m most plant macrofossils are found in proximal overbank sequences or on the preserved tops of emergent mid channel bars certain taxa are more typical of floodplain versus channel margin sites key differences were also noted between upland proximal to fault scarps and lowland taxa which may be related to soil drainage chumstick paleosols include entisols poor horizontal development inceptisols incipient cca or k horizon development and histosols histic epipedons containing up to 6 total organic carbon paleovegetation had a significant effect on fluvial deposition in the chumstick formation even in coarse grained fault proximal facies there is evidence for bank cohesion and stability bank undercutting and failure soil erosion and overbank deposits the dominance of vertical accretion over lateral channel migration in more distal deposits can be attributed to rapid subsidence coupled with bank stability imparted by vegetation

hydrocarbon exploration in western oregon and washington continuing development of the mist gas field and stepout discovery wells affirm the hydrocarbon prospectivity of western oregon and washington understanding the factors controlling the mist gas field accumulation helps define an exploration strategy for the pacific northwest reservoir sandstones in the mist area are in the cowlitz formation of middle to late eocene age reservoir quality sandstones have average porosities of 25 and average permeabilities of 200 md the reservoir sands are well sorted feldspathic quartzose sandstones and are less susceptible to diagenetically formed pore filling authigenic minerals than are the more lithic sandstones of other horizons and less well sorted depositional environments potential hydrocarbon source rocks consist of marine shale to coaly facies organic matter is predominately terrestrially derived mist gas field pools are small and have variable gas types suggesting to some workers that the gas is generated from rocks immediately adjacent to the reservoir gas wetness and dgr13c values indicate that gas from the bruer flora and newton pools is probably thermally generated shales encasing the mist gas field sandstone reservoirs are thermally immature having vitrinite reflectance values less than 0 4 thermal gas most likely would have been generated downdip within nearby depocenters and migrated into the reservoir the integration of paleogeographic models for mineralogic provenance well sorted sand accumulation and thermal maturation within cenozoic depocenters provides an exploration strategy for defining areas of highest hydrocarbon potential in western oregon and washington each of the many eocene formations in oregon and washington with relatively lithic free feldspathic quartzose mineralogy should be evaluated as a potential target

experimentally determined femg exchange between synthetic staurolite and garnet in the system mgofeoal2o3sio2h2o the fe mg exchange between synthetic staurolite and garnet was determined experimentally as a function of pressure temperature and bulk composition in the range of 5 25 kbar 600 750 c on the basis of the reaction12 pyrope 14 fe staurolite 13 almandine 14 mg staurolite eq 1 the results of the exchange runs clearly indicate that the fe mg fractionation between staurolite and garnet does not depend much on pressure but on temperature and bulk composition the compositional data on coexisting grtss and stst as determined by microprobe were processed by a nonlinear least squares procedure using different mixing models for staurolite solid solutions and the asymmetric solution model for garnet hackler and wood 1989 in all the models staurolite solid solutions show large negative deviation from ideality the experimental results are best described by a temperature dependent symmetric mixing model with wsst 79 j k one site model such a large negative ws value however is rather unrealistic especially considering that only a relatively small temperature interval has been investigated therefore a symmetric mixing model with wgst 11539 830 j one site model is recommended for use in thermodynamic calculations the expression for the derived geothermometer is based on the temperature dependent mixing parameters because these describe the fe mg fractionation between staurolite and garnet very well the geothermometer is valid for staurolites with xfe 0 6 and is t 18 02 0 044 p rt in fegrt rt in mggrt 87 78 0 057 p xmgst2 xmgst 2 12 55 xcagrt xmngrt r 10 3 in kd 0 021 0 079 xfest 2 xmgst 2 where pressure p is in kbar and temperature in c

regional tectonic and earthquake hazard implications of a crustal fault zone in southwestern washington the st helens seismic zone shz is a crustal earthquake zone in southwestern washington that is defined by small to moderate magnitude 2 5 to 5 5 earthquakes and focal mechanisms this zone seismically active along a distance of 90 km is interpreted as a fault zone capable of generating a moderate to large magnitude shallow earthquake at the northern end of this seismic zone about 15 km north of the cowlitz river a large area of seismic quiescence for earthquakes above magnitude 3 0 has predominated since 1977 this quiescence leaves open the question whether the shz continues northward into southern puget sound earthquake focal mechanisms along the shz indicating nearly vertical strike slip faulting with the preferred fault planes striking north south from these focal mechanisms we infer that the direction of maximum compression is northeast approximately parallel with the direction of plate convergence between the north american and juan de fuca plates we interpret the focal mechanism data as evidence for locked subduction this tectonic model raises the possibility of a large magnitude subduction earthquake neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment for washington

reply to comment on depositional history of the eocene chumstick formation implications of tectonic partitioning for the history of the leavenworth and entiat eagle creek fault systems washington

a crustal mineral in a mantle diamond recent studies of diamonds inclusions in diamonds and rock fragments from the earth s mantle have shown the presence of minerals and rocks with a chemical composition that suggests a previous origin in the crust presumably this crustal material was transported into the mantle at a subduction zone the chemical evidence of previous residence of mantle materials in the crust has come principally from determinations of the proportions of the isotopes of oxygen1 2 sulphur3 4 lead4 5 and carbon6 which typically show a much wider range of composition in the crust than in material of purely mantle origin here we report a less subtle crustal signature in a mantle derived diamond an inclusion of the mineral staurolite staurolite has never previously been found in mantle diamonds or rocks but is a common mineral in metamorphic rocks in the crust the characteristics of the staurolite suggest not only the probability that it formed in rocks with a crustal bulk composition but also the possibility that it formed by metamorphism in the crust or in a subduction zone before being incorporated in the mantle and eventually in a growing diamond

glaucophane chloritoid bearing assemblages from ne oman petrologic significance and a petrogenetic grid for high p metapelites pelitic layers and lenses interbedded with blueschists and eclogites in saih hatat ne oman contain chloritoid and sodic amphibole bearing mineral assemblages that are useful for reconstructing the p t history of the area textural and mineral chemical relations suggest that coexisting glaucophane gln and chloritoid ctd formed at the expense of chlorite chl paragonite pg and later broke down to garnet gt pg during prograde metamorphism according to the reaction gln ctd qz gt pg h2o during retrogression gln and chl first formed at the expense of gt and pg followed by the breakdown of ctd and gt to chl the final stages of retrogression are marked by the breakdown of gln to an aggregate of chl albite ab a projection from quartz qz h2o and phengite ph on the al2o3 fe2o3 feo mgo na2o plane in the system nfmash is best suited for the representation of the phase relations in high p metapelites petrogenetic grids for the model systems nmash and nfash were calculated using program geo calc berman et al 1987 and its database berman 1988 after the retrieval of s i o and h f o for gln and ctd by mathematical programming and calculating all possible reactions among gln ctd chl jadeite jd ab gt pg talc tc pyrophyllite prl and kyanite ky the calculated petrogenetic grid for the system nfash shows that fe ctd and ferroglaucophane coexist at p 6 5 kbar and t 6 kbar this grid is consistent with the p t estimates for high p metapelites from oman new caledonia seward peninsula ile de groix sifnos and peloponnese where gln ctd bearing units are interbedded with cofacial mafic blueschists and eclogites the grid also explains the observed textural relations in the metapelites of oman and is consistent with the clockwise p t path proposed for this area but differs from the grids of guiraud et al 1990 in showing a larger stability field for gln ctd that overlaps with the stability fields of jd qz as well as ab the petrogenetic grid calculated for the system nmash shows that mg ctd gln bearing assemblages require unusually low geothermal gradients to form in metapelites it also shows that mg rich ctd tc coexist at high pressures over a wider p t range than predicted by guiraud et al 1990 this grid can therefore be applied to high p metamorphic assemblages from the eastern and western alps

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

terminal cambrian and lowest ordovician succession of mexican west gondwana biotas and sequence stratigraphy of the tinu formation the tinu formation of oaxaca state is the only fossiliferous lower palaeozoic unit between the laurentian platform in northwest mexico and gondwanan successions in andean south america the tinu traditionally has been referred to the lower ordovician tremadoc and regarded as having a provincially mixed fauna with laurentian avalonian and gondwanan elements bio and lithostratigraphic re evaluation demonstrates that the tinu is a gondwanan passive margin succession it includes a lower thin to 16 m condensed uppermost cambrian yudachica member new the yudachica nonconformably overlies middle proterozoic basement as a result of very high late late cambrian eustatic levels the yudachica changes from storm dominated but slightly dysoxic shelf facies fossil hash limestone and shale in the south to an upper slope facies with debris flows 50 km to the north three biostratigraphically distinct depositional sequences comprise the yudachica the yudachica has gondwanan aspect trilobites with low diversity conodonts characteristic of unrestricted marine temperate facies the upper tinu or rio salinas member new is a lower ordovician tremadoc depositional sequence that records strong early but not earliest tremadoc eustatic rise marked by graptolite and olenid bearing dysoxic mudstones higher strata shoal upward into shell hash limestones and proximal tempestite sandstones with upper lower tremadocian unrestricted marine temperate conodonts new taxa include orminskia rexroadae landing gen et sp nov from the cordylodus andresi zone this euconodont is related to hyaline coniform genera best known from ordovician tropical platform successions cornuodus clarkei landing sp nov resembles the coeval upper lower tremadoc tropical species scalpellodus longipinnatus ji barnes

singular equilibria in paragonite blueschists amphibolites and eclogites chemographic constraints from high pressure mafic rocks of the introduction nevado filabride complex betic rif mountain belts se spain the relevance of singular equilibria has been largely and from worldwide high pressure terrains have been used to unravel recognized in chemical systems of interest for the earth phase relationships between amphibole garnet clinopyroxene and sciences after the early experiments of konovalov on albite in the system na2o cao feo mgo al2o3 sio2 h2o azeotropic or congruent transformations ricci 1951 ncfmash at quartz paragonite clinozoisiteand waterexhaustively discussed the fundamentals of singularities saturated conditions extensive analysis of element partitioning two types of singularities were recognized the first between amphibole and clinopyroxene suggests that the fe mg called type m and most relevant is associated with azeopartition coefficient is low at low temperature and high pressure tropic points and the second called type k is related to conditions high temperature blueschists low temperature eclogites the intersection of solvi with compositional loops as and a progressive increase is achieved by an increase in temperature occurs in the alkali feldspar system at low pressures or by a decrease in pressure or by a decrease in the sodium content morse 1970 examples and discussion of both type m of the clinopyroxene mass balance analysis of paragonite eclogites and type k singular points in igneous systems have been and amphibolites suggests that at least two singular reactions are given by morse 1980 e g see chapters 16 and 18 the possible in ncfmash clinopyroxene paragonite quartz widespread occurrence of singularities in igneous systems amphibole albite clinozoisite water garnet absent and is obviously related to the extremely large compositional amphibole clinozoisite garnet clinopyroxene paragonite range of silicate liquids the well known concept of quartz water albite absent albite absent singular equithe thermal barrier or thermal divide implies the librium is relevant for the blueschist eclogite transition whereas occurrence of singular equilibria and their importance garnet absent equilibrium may control the eclogite to amphibolite in the petrogenesis of granites e g modelled in the binary transition a complex arrangement of divariant reactions singular system kalsilite silica lindsley 1966 or basalts e g in reactions and conjugate non degenerate reactions arises in topologies the binary system forsterite silica or in the ternary system at the blueschist eclogite amphibolite transitions schreinemakers forsterite silica diopside or forsterite silica anorthite analysis in p t diagrams and mixed variable t xm xc and presnall et al 1979 has been widely discussed p xm xc diagrams suggest that complex pressure temperature in contrast the significance and implications of singular paths are not mandatory if a recurrent appearance and disappearance point nets at subsolidus conditions have been largely of similar phase assemblages is observed in paragonite bearing mafic undervalued although korzhinskii 1963 analysed azeorocks the p t location of the singular curve net is discussed in tropic transformations in subsolidus open systems only the context of nevado filabride complex mafic rocks recently have applications to metamorphic assemblages

sedimentology and stratigraphy of the cretaceous virginian ridge formation methow basin washington the cretaceous methow basin of north central washington is the southernmost of a series of mesozoic successor basins in the cordillera of western north america the albian campanian virginian ri

review of trilobite biofacies distributions at the cambrian ordovician boundary detailed analysis of trilobite assemblages at the cambrian tremadoc transition reveals the presence of four possibly five biogeographic provinces baltic north china southeast china north american and possibly siberian one or more discrete biofacies characterize the transition in each province when plotted palaeogeographically these appear to be largely temperature controlled interdigitation of provincialized trilobite biofacies is recognized by the representation of dominant types of trilobite olenaceans ceratopygaceans remopleuridaceans dikelocephalaceans and leio stegiaceans in particular on key boundary sections these sections at nochixtlan mexico batyrbay southern kazakhstan broken skull river district of mackenzie canada and jiangshan chang shan western zhejiang china permit the correlation of biogeographic provinces globally as a result of the application of trilobite biostratigraphy relatively few problems prevent the maintenance of a cambrian ordovician boundary stratotype section in its traditional baltic biogeographic setting

the october 17 1989 loma prieta california earthquake and its aftershocks geometry of the sequence from high resolution locations hypocenters of the loma prieta sequence form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane most aftershocks cluster around the perimeter of the zone surrounding a relatively aseismic center which approximates the region of mainshock rupture at its southeastern end the dipping aftershock zone warps into a vertical surface that corresponds to the san andreas fault in the central and northwestern parts of the zone at depths above 10 km the aftershocks define numerous disjoint fault structures the large component of reverse slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend we do not believe that the loma prieta earthquake occurred on the sargent fault however we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active fault

mapping of a buried basement combining aeromagnetic gravity and petrophysical data the substratum of southwest paris basin france aeromagnetic and gravity data have proven to be among the most effective methods for mapping deeply buried basin basement interfaces however the data interpretation generally suffers from ambiguities due to the non uniqueness of the gravity and magnetic signatures here we tie the gravity and magnetic signatures with a petrophysical characterization of the lithologies outcropping around the french paris basin our methodology investigates the lithology and structure of its hidden variscan substratum at the junction between the armorican massif and massif central our approach is based on the combination of potential field data magnetic susceptibilities measured in the field density values of sample rocks and information documented in boreholes in order to propose a new interpretative geological map of the buried substratum of the paris basin the petrophysical description is combined with geophysical patterns of the substratum mapped through statistical unsupervised classification of suitably selected magnetic and gravity maps the first step of interpretation consists in extending the outcropping major structures below the meso cenozoic sedimentary cover of the paris basin the litho structural units in between these major structures are then interpreted separately the second step consists in assigning lithologies within each unit with respect to its magnetization and density as derived from the petrophysical compilation and mapping its extension under cover integrating punctual borehole information overall with a special emphasis on relating geophysical signatures and petrophysical characteristics of litho structural units this methodology permits a precise structural and lithological cartography of a segment of the buried variscan substratum in the southwestern part of the paris basin this approach reveals i the limited eastward extension of central brittany ii the eastward extension of the major cholet fault iii the emphasis on n150e n160e striking fault and their n30e conjugates controlling the opening of permo carboniferous basins and iv the eastward extension of the eo variscan suture

early palaeozoic brachiopods and associated shelly faunas from western gondwana their bearing on the geodynamic history of the pre andean margin

crustal structure along the west flank of the cascades western washington knowledge of the crustal structure of the washington cascades and adjacent puget lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region we present a model for crustal velocity structure derived from analysis of seismic refraction wide angle reflection data collected in 1991 in western washington the 280 km long north south transect skirts the west flank of the cascades as it crosses three tectonic provinces including the northwest cascades thrust system nwcs the puget lowland and the volcanic arc of the southern cascades within the nwcs upper crustal velocities range from 4 2 to 5 7 km s 1 and are consistent with the presence of a diverse suite of mesozoic and paleozoic metasediments and metavolcanics in the upper 2 3 km of the puget lowland velocities drop to 1 7 3 5 km s 1 and reflect the occurrence of oligocene to recent sediments within the basin in the southern washington cascades upper crustal velocities range from 4 0 to 5 5 km s 1 and are consistent with a large volume of tertiary sediments and volcanics a sharp change in velocity gradient at 5 10 km marks the division between the upper and middle crust from approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from 6 0 to 7 2 km s 1 these high velocities do not support the presence of marine sedimentary rocks at depths of 10 20 km beneath the cascades as previously proposed on the basis of magnetotelluric data crustal thickness ranges from 42 to 47 km along the profile the lowermost crust consists of a 2 to 8 km thick transitional layer with velocities of 7 3 7 4 km s 1 the upper mantle velocity appears to be an unusually low 7 6 7 8 km s 1 when compared to velocity models from other regions this model most closely resembles those found in active continental arcs distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect in the nwcs and puget lowland most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5 10 km depth and continues to 20 30 km depth the region of transition between the nwcs and the puget lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two earthquakes within the cascades are generally shallower 0 20 km and are dominated by events associated with the rainier seismic zone

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

protaspides of uppermost cambrian trilobite missisquoia with implications for suprafamilial level classification of the genus the protaspides of missisquoia depressa missisquoiidae trilobita from the uppermost cambrian part of the rabbitkettle formation mackenzie mountains northwestern canada are described three metaprotaspid stages are recognized using size as well as features of the anterior border of the cranidium and the shape of the glabella the morphology of the protaspides is not typical of the order corynexochida suggesting that missisquoia is not a member of the order to which the genus has been previously assigned this further indicates that its affinity to the stratigraphically younger styginids and illaenids is questionable

time scales of metamorphism deformation and crustal melting in a continental arc north cascades usa the record of metamorphism deformation and melting in the north cascades continental arc provides insights into the timing and mechanisms of extensional unroofing that followed crustal thickening the skagit gneiss north cascades is composed of variably deformed 90 45 ma tonalitic to granitic intrusive rocks these lithologies and metasedimentary rocks are migmatitic zircon and monazite from different textural varieties of migmatite from three outcrops along an east west transect were analyzed using the isotope dilution thermal ionization mass spectrometry id tims technique the data reveal two main migmatization pulses 1 68 63 ma and 2 53 47 ma in the westernmost locality leucosome zircon yields group 1 dates and mesosome sillimanite garnet biotite gneiss zircon is as young as 61 ma leucosome zircon in the easternmost outcrop yields only group 2 dates and biotite gneiss contains 47 ma zircon the third outcrop has leucosome dates from both groups 65 ma and 53 ma monazite from leucosomes and mesosomes likely records the timing of prograde metamorphism ca 69 ma and later synkinematic fluid mediated growth 49 46 ma these results indicate that partial melting occurred during 71 61 ma metamorphism and lasted through 46 ma deformation associated with exhumation the results and field observations suggest the presence of ductilely deforming crust from 68 to 46 ma the similarity between the youngest u pb dates in the migmatites and cooling and basin filling ages suggests a link between ductile and brittle processes over a range of structural levels in the arc from a zone of crustal melting flow and migmatite crystallization to earth9s surface

a new metamorphic constraint for the eburnean orogeny from paleoproterozoic formations of the man shield aribinda and tampelga countries burkina faso abstract progressive metamorphic zones developed in pelites around the aribinda granite burkina faso indicate contact metamorphism at 5 6 kbar biotite chloritoid staurolite kyanite zones are distributed concentrically around the aribinda pluton on its northern margin but are displaced by later thrust faults in the south of the aribinda area and in the nearly tampelga area these metamorphic and structural characteristics provide new constraints for the geodynamical evolution of the eburnean orogeny in the north part of the baoule mossi domain they show the leading part of the granitoid emplacement in the thermic structuration of the greenstone belts and the post metamorphic character of the tangential tectonic in burkina faso the eburnean orogeny underwent 1 initial greenschist facies metamorphism 2 emplacement of granitoids at mid deep crustal levels leading to unusual medium pressure metamorphism 3 later thrusting

new paleomagnetic data from the aleutian islands implications for terrane migration and deposition of the zodiac fan sedimentary rocks of late eocene to early oligocene age from the aleutian islands have been paleomagnetically studied to determine the paleolatitude of this island arc samples were collected from gently dipping thin bedded laminated siltstone mudstone and very fine grained sandstone of marine origin after demagnetization samples from each locality showed both polarities of a remanent magnetization that thermal isothermal remanent magnetization and js t experiments suggest is carried by magnetite after tectonic correction the 95 decreases in both localities suggesting magnetization was acquired before folding of the beds the characteristic magnetization shows that no significant poleward motion of the aleutian arc has occurred with respect to the north american plate since the early oligocene amlia f 4 f 5 umnak f 0 8 f 3 5 but the declinations are rotated significantly clockwise amlia r 70 r 23 umnak r 36 7 r 11 2 using these paleomagnetic data relevant euler poles and hotspot locations we propose a new model for the formation of the zodiac fan in this model the start of zodiac fan deposition corresponds to accretionary igneous and erosional events in the pacific northwest and fragmentation of the farallon plate along the margin of the north american plate into small plates fan deposition on the pacific plate was controlled by the geometry of the bowie and cobb seamount chains which prevented sediment from being dispersed over the bathymetrically lower oceanic crust to the west and by the bathymetry of the pacific plate which focused sedimentation in the magnetic bight region of the plate our model suggests that the present day zodiac fan is only a remnant of a much larger fan system

construction of mid crustal sheeted plutons examples from the north cascades washington large parts of many orogenic belts consist of highly elongate plutons constructed by the injection of multiple sheets of magma we describe in detail two such intrusions the deep 20 25 km mid crustal cardinal peak and entiat plutons and emphasize the dynamic nature of these magma chambers these ca 72 73 ma plutons consist of heterogeneous mafic complexes in their margins and tips that give way inward to thicker but still abundant sheets of tonalite intrusive and petrologic relationships provide evidence for injection of early mafic mantle and crustal melts development of heated pathways utilized by larger batches of more felsic magma and potentially a filtering mechanism in which conduits represented by the plutons fed larger more homogenized magma bodies at shallower crustal levels well developed gently to moderately dipping magmatic and subsolidus foliations in these plutons are commonly discordant to sheet contacts internal layering and pluton contacts but are continuous with host rock foliation they define small to map scale magmatic folds which have orientations and geometries similar to those of regional host rock folds these magmatic structures thus primarily record regional contractional tectonism during chamber construction host rocks also record complex processes during chamber evolution with narrow structural aureoles displaying remarkable lateral variability deflection of gently to moderately dipping host rock markers toward parallelism with steep pluton contacts indicates vertical and largely downward ductile material transfer along sheet margins during emplacement several sharply discordant segments of pluton contacts imply late stoping regional ductile flow and folding also played a role but regional faulting did not further evidence for chamber construction is provided by host rock rafts which are abundant along sheet contacts in the cardinal peak pluton and particularly between marginal sheets of the entiat pluton but are sparse elsewhere in the latter pluton we interpret these relationships to indicate a temporal sequence in which magmatic sheets initially intruded host rock then wedged aside this rock and coalesced preserving pieces of the host along margins and eventually formed large relatively inclusion free chambers as host rock was detached and transported vertically downward through the chamber

possible regional tectonic controls on mammalian evolution in western north america abstract previous work has suggested that tectonically active regions act as speciation pumps for mammals and plant species but little is known about how fast or widespread tectonism must be in order to directly influence evolution here we use oxygen and hydrogen isotopic data from miocene sedimentary deposits to characterize the topographic evolution of the southern columbia plateau snake river plain and northern rocky mountain regions during the yellowstone hotspot passage with the ultimate goal of understanding whether topographic changes caused by the hotspot influenced mammalian evolution within those regions we conducted oxygen isotope analyses of 130 samples of lacustrine and paleosol carbonate from miocene stratigraphic sections that span much of the northern rocky mountain region and combined these data with previously published isotopic records collectively these isotopic data show that caldera formation associated with the yellowstone hotspot has modified regional topography and rearranged drainages along the track of the hotspot and that the hotspot has left a topographic depression in its wake we explore the extent to which these topographic changes influenced or are decoupled from diversity changes exhibited by the local mammal faunas and conclude that the passage of the hotspot and consequent surface uplift created rainshadows in the lee of high elevation calderas and or generated large volumes of volcanic materials influencing soils and vegetation collectively that may explain a possible rise in mammal diversity in the cp srp region at 14 ma coincident with a drop in diversity in the nrm it is still unclear however how different taphonomic pathways and sample standardization problems are influencing apparent diversity peaks at this temporal and geographic resolution

petrological and structural evolution of high grade metamorphic rocks from the floor of the alboran sea basin western mediterranean the metamorphic basement beneath the alboran sea a neogene introduction extensional basin drilled during ocean drilling program odp late orogenic extension of previously thickened conleg 161 site 976 consists of high grade pelitic schist overlying tinental lithosphere is a widely documented phenomenon migmatitic pelitic gneiss inferred assemblages in the high grade e g dewey 1988 malavieille 1993 ruppel 1995 schist evolved from garnet staurolite biotite muscovite but it is unclear whether the thermal evolution of the plagioclase assemblage 1 to biotite sillimanite k feldspar lithosphere in this situation differs significantly from that plagioclase garnet assemblage 2 to andalusite biotite associated with extension in non orogenic settings if k feldspar plagioclase assemblage 3 in the gneiss which extension is triggered by removal of part of the lower shows abundant migmatitic textures the present assemblage is biotite lithosphere e g england houseman 1989 there may andalusite sillimanite muscovite cordierite kbe a detectable rise in temperature during exhumation feldspar plagioclase garnet phase relations and thermoof rocks platt england 1994 whereas extension barometric calculations suggest that the high grade schist experienced caused by far field forces is likely to produce isothermal decompression accompanied by heating from 500 c at 10 5 kbar decompression paths followed by cooling ruppel et al through 600 30 c at 6 7 kbar assemblage 1 and thence 1988 platt et al 1999 decompression accompanied by to 650 700 c at 3 4 kbar assemblage 2 followed by cooling heating in extensional settings has been demonstrated in through 500 600 c at 2 kbar or less assemblage 3 peak the seward peninsula in alaska patrick lieberman temperature and melting occurred under low pressure conditions 1988 the cordillera darwin in the southern andes the widespread disequilibrium and overstepping of metamorphic kohn et al 1993 the tauern window in the alps reactions suggests that decompression was rapid the p t evolution selverstone et al 1984 the aegean buick holland is consistent with metamorphism in a late orogenic extensional 1989 the high himalaya inger harris 1992 and basin and suggests the existence of an external source of heat the betic cordillera van der wal vissers 1993 an probably indicating the complete removal of lithospheric mantle interesting related question is the significance of crustal beneath the extending region melting in extensional settings in principle partial melt

paleoseismic evidence for late holocene tectonic deformation along the saddle mountain fault zone southeastern olympic peninsula washington abstract trench and wetland coring studies show that northeast striking strands of the saddle mountain fault zone ruptured the ground about 1000 years ago generating prominent scarps three conspicuous subparallel fault scarps can be traced for 15 km on light detection and ranging lidar imagery traversing the foothills of the southeast olympic mountains the saddle mountain east fault the saddle mountain west fault and the newly identified sund creek fault uplift of the saddle mountain east fault scarp impounded stream flow forming price lake and submerging an existing forest thereby leaving drowned stumps still rooted in place stratigraphy mapped in two trenches one across the saddle mountain east fault and the other across the sund creek fault records one and two earthquakes respectively as faulting juxtaposed miocene age bedrock against glacial and postglacial deposits although the stratigraphy demonstrates that reverse motion generated the scarps slip indicators measured on fault surfaces suggest a component of left lateral slip from trench exposures we estimate the postglacial slip rate to be 0 2 mm yr and between 0 7 and 3 2 mm yr during the past 3000 years integrating radiocarbon data from this study with earlier saddle mountain fault studies into an oxcal bayesian statistical chronology model constrains the most recent paleoearthquake age of rupture across all three saddle mountain faults to 1170 970 calibrated years cal b p which overlaps with the nearby m w 7 5 1050 1020 cal b p seattle fault earthquake an earlier earthquake recorded in the sund creek trench exposure dates to around 3500 cal b p the geometry of the saddle mountain faults and their near synchronous rupture to nearby faults 1000 years ago suggest that the saddle mountain fault zone forms a western boundary fault along which the fore arc blocks migrate northward in response to margin parallel shortening across the puget lowland

from opening to subduction of an oceanic domain constrained by la icp ms u pb zircon dating variscan belt southern armorican massif france in the variscan belt of western europe the lifetime and evolution of the oceanic domain is poorly constrained by sparse outdated and unreliable multigrain id tims u pb zircon dating in this article we present a complete in situ la icp ms dataset of about 300 u pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of southern brittany comprising new dating of previously published zircon populations and newly discovered rock samples in situ dating and cathodo luminescence imaging of each zircon grain yields new absolute time constraints on the evolution of the galicia moldanubian ocean the new results confirm that the opening of this oceanic domain is well defined at about 490 ma in contrast the generally quoted 400 410 ma age for the high pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites in light of these new data we propose that the obduction of oceanic rocks occurred at about 370 380 ma while the high pressure event is recorded at 355 ma in only a few zircon grains of some eclogite samples additionally this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their u pb systems do not record that metamorphism systematically these zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith another example of an eclogite sample from the french massif central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

evidence for a margin truncating transcurrent fault pre late eocene in western washington i postulate that a major dextral transcurrent fault truncated the pre tertiary continental framework of western washington and southern vancouver island during the late cretaceous and early tertiary the fault trace trends north and is covered in the puget lowland it corresponds to the west trending san juan and northwest trending survey mountain faults on southern vancouver island the geometry of the structure has probably been greatly modified by subsequent deformation sedimentary basins that formed marginal to the fault are characterized by rapid fades changes and sediment accumulation rates abrupt stratigraphic thinning and thickening irregular basin margins petrographic mismatches and other features consistent with origin in a zone of strike slip faulting the structure was probably part of a network of intracontinental late cretaceous early tertiary strike slip faults in the northern north american cordillera as an outboard member of this fault network it likely served as a major avenue of northward translation along the northeast margin of the pacific basin

the southern washington cascades conductor a previously unrecognized thick sedimentary sequence geophysical studies in the southern washington cascades have outlined a possible previously unrecognized sequence of sedimentary rocks these postulated sedimentary units are interpreted to correspond to at least the upper section of a low resistivity high conductivity assemblage of rocks at depths of 1 10 km and with thicknesses up to 15 km that we call the southern washington cascades conductor swcc structure on the upper surface of this conductive assemblage correlates in some places with anticlines that bring tertiary marine rocks near the surface these anticlines have recently been imaged in detail with seismic reflection surveys the conductive rocks have also been traced to the surface west of morton washington where they correlate with eocene marine sands ones and shales the geometry of the conductive rocks consists of a east dipping low angle wedge that thickens to the north and with an undulating upper surface corresponding to the anticlines we suspect that marine sedimentary rocks are the primary constituent of the swcc but recognize that parts of the conductive section may be composed of nonmarine and marginal marine sedimentary rocks like those in the upper part of the puget group geothermal fluids may be a contributing factor to low resistivities in the deeper parts of the conductive section greater than 6 7 km other possible candidates for the conductive units are thick sections of highly altered volcanic flows and or graphitic conductive pre tertiary metamorphic rocks

reversal of fe mg partitioning between garnet and staurolite in eclogite facies metapelites from the champtoceaux nappe brittany france this paper concentrates on the petrology of eclogite facies metapelites and particularly the significance of staurolite in these rocks a natural example of staurolite bearing eclogitic micaschists from the champtoceaux nappe brittany france is first described the champtoceaux metapelites present in addition to quartz phengite and rutile two successive parageneses 1 chloritoid staurolite i garnet cores and 2 garnet rims hkyanite chloritoid detailed microprobe analyses show that garnet and chloritoid evolve towards more magnesian compositions and that staurolite is more fe rich than coexisting garnet a comparison of the studied rocks with other known occurrences of eclogitic metapelites shows that whereas staurolite is always more fe rich than garnet in high pressure eclogites the reverse is true in low to medium pressure micaschists phase relations between garnet staurolite chloritoid biotite and chlorite are analysed in the kfmash system with excess quartz phengite rutile and h20 the topology of univariant reactions is depicted for a normal and a reverse fe m g partitioning between garnet and staurolite mineral compositional changes are also predicted for varying bulk rock chemistries in the studied micaschists the zonal arrangement of garnet inclusions and the progressive compositional changes of ferromagnesian phases record part of the prograde p t path before the attainment o f peak metamorphic conditions at about 650 700 c 18 20 kb the retrograde path which records the uplift of the champtoceaux nappe occurs under decreasing temperatures

spatial and temporal relations between early tertiary shortening and extension in nw washington based on geology of the pipestone canyon formation and surrounding rocks detailed mapping of the deformed paleocene pipestone canyon formation and surrounding cretaceous rocks in nw washington suggests that uplift erosion and unroofing of adjacent rocks was influenced by shortening at high angle to the continental margin during mid cretaceous circa 88 110 ma and early tertiary 47 65 ma time although significant shortening is mid cretaceous in age additional shortening expressed as reverse faults and major folds is early tertiary in the map area and regions to the west and northwest the skagit methow western okanogan area the shortening is essentially unmodified by extensional deformation whereas in regions east and northeast of the map area the omineca and eastern okanogan regions widespread eocene extension has obscured the shortening and caused additional unroofing as well the conglomeratic pipestone canyon formation lies unconformably on intruded early cretaceous rocks of the lower methow sequence approximately 3 5 km of mid cretaceous methow sequence strata are missing at the unconformity yet are present 15 km to the west and northwest these observations indicate late cretaceous erosion and unroofing in the southeastern methow area prior to deposition of the pipestone canyon formation furthermore conglomerate clasts in the pipestone canyon formation record sources resulting from relative uplift of the crystalline okanogan complex east of the basin and slightly later from relative uplift of the volcano sedimentary methow sequence and structurally underlying migmatitic skagit complex west to northwest of the basin following deposition which ended sometime between 55 and 65 ma the pipestone canyon formation and underlying methow sequence were tightly folded and faulted and juxtaposed with the okanogan complex by reverse faulting the map pattern of these folds and faults suggests ene wsw shortening and concomitant uplift crosscutting relations north of the map area constrain this deformation to be pre 47 ma similarity of pipestone canyon fold geometry to folds elsewhere in the methow and skagit areas supports the interpretation that a significant fold belt of circa 47 65 ma age developed in these areas and overprinted the mid cretaceous folding and thrusting regional data suggest the circa 47 65 ma folding formed a synclinorium in the methow area and an anticlinorium in the skagit area roughly concurrent with cooling ages for deep 8 9 kbar skagit rocks for the skagit methow western okanogan region as a whole the 47 65 ma shortening and unroofing were accompanied by uncertain amounts of fold belt parallel strike slip faulting timing estimates for shortening in the skagit methow western okanogan area are generally older but overlap with those established for extension farther east and northeast in the okanogan complex and omineca belt supporting interpretations that the belt of extension began to develop during or very soon after regional shortening existing thermal rheological models of extension immediately after shortening point to the need for heat sources other than mantle conduction and crustal radioactivity to facilitate the extension apparently the skagit methow fold belt failed to extend because it lacked the necessary heat

depositional history of the eocene chumstick formation implications of tectonic partitioning for the history of the leavenworth and entiat eagle creek fault systems washington the chumstick basin opened as an extensional half graben prior to 51 ma and was subsequently modified by two episodes of tectonic partitioning of drainage prior to basin deformation initially westward flowing fluvial systems formed a unified depositional system with the swauk basin tectonic partitioning of drainage at 51 49 ma and at 44 42 ma was controlled by localized uplift on the leavenworth lfz eagle creek ecfz and entiat efz fault zones and led in each instance to the truncation of regional depositional systems modification and reversal of paleoflow and internal drainage relief on the lfz at 51 49 ma may be the result of isostatic uplift of the extensional footwall producing the swauk and chumstick basins as a pair of west facing half grabens the earliest convincing evidence for the onset of oblique slip in the region is at about 48 ma folding in the swauk basin or about 44 42 ma probable transpressive uplift at left stepping bends of the lfz development of a transtensional step over basin between the ecfz and efz horsetail splays in the ecfz and possible flower structures in the lfz and ecfz in the chumstick basin each episode of tectonic partitioning was followed by proximal onlap and overtopping of fault zones to reestablish regional flow systems the chumstick formation was deformed by dextral transpression between 37 34 ma and is unconformably overlain by the oligocene wenatchee formation the chumstick basin is an example of an extensional basin modified by subsequent strike slip tectonics thus caution should be used in applying idealized basin models