the cowichan fold and thrust system vancouver island southwestern british columbia the cowichan fold and thrust system is a southwest verging linked thrust system formed by northeast southwest large scale crustal contraction involving the upper cretaceous nanaimo group and its wrangellian basement the geometry of the thrust system from plan and profile perspectives illustrates its thick skinned structural style the thrust system is interpreted to be a leading imbricate fan thrusting was most likely in sequence creating an estimated minimum 20 30 shortening at the basement cover interface both fault propagation and fault bend folding are evident the former being more common kinematic indicators and the geometry of the thrust system show no evidence of significant transpressional or transtensional displacement fields contraction postdates deposition of the nanaimo group and is indirectly dated as late eocene the sole fault of the thrust system is interpreted to rise from northwest to southeast on a series of deep lateral to oblique ramps and merge with the san juan fault system this explains the rapidly westward thickening wedge of thrusted wrangellian basement in the study area in the east the sole fault is interpreted to have locally permitted the nanaimo group to overthrust the san juan terrane investigation of the cowichan fold and thrust system reveals how shortening during westward progressing terrane accretion is accommodated in a forearc region dominated by a large slab of rigid semicontinental crust

viscous flow and deformation of regional metamorphic belts at convergent plate boundaries 1 viscous flow and deformation of a newtonian fluid between rigid boundaries have been modeled for investigating deformation of a forearc wedge and a regional metamorphic belt as a weak deforming zone between the plates at convergent boundaries three types of analytic formulations together with numerical formulations for a newtonian fluid with a constant viscosity are used for investigating various flows and deformation 1 flows induced by squeezing between two parallel boundaries 2 flows induced by squeezing and heterogeneous mass influx in the wedge between two oblique boundaries e g accretionary wedge over the subducting plate with the continental plate as a backstop and 3 flows induced by dragging along the boundaries and by homogeneous mass influx in the wedge as in type 2 all cases include strike slip movements of the boundaries which allows us to investigate three dimensional 3 d deformation e g 3 d corner flow the corresponding flow deformation of infinitesimal and finite elements and the timescale of flow are calculated for each configuration with various mechanical boundary conditions the model results show that configuration and mechanism of the flow can be inferred from the spatial variation of finite deformation e g deformed radiolarians as a strain marker and geometry of folding as finite deformation of a large block in particular prolate strain nearly parallel to the strike of subduction zone together with a large scale folding can be a good indicator for deformation in the forearc wedge associated with 3 d corner flow induced by oblique subduction deformation observed in the cretaceous regional metamorphic belts in southwest japan can be explained by this mechanism

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

linked thrust and strike slip faulting during late cretaceous terrane accretion in the san juan thrust system northwest cascades orogen washington the san juan thrust system makes up the western elements of the late cretaceous northwest cascades orogen of washington a stack of metamorphic nappes with terranes ranging in age from early paleozoic to middle cretaceous the tectonic development of the nappes is an unresolved issue important to understanding plate convergent processes operating in the north american cordillera in the creta ceous as well as at other times two terrane bounding fault zones were studied in this project the lopez and the rosario fault zones the fault zone rocks were affected by two cataclastic plastic late cretaceous deformational events d1 and d2 that included accretion and lateral motion during blueschist facies metamorphic conditions d1 produced upright folds and a subvertical axial planar s1 foliation fabric in the lopez fault zone and southwest vergent folds and gently northeast dipping axial planar s1 foliation in the rosario fault zone in the lopez fault zone d2 produced northwest striking and subvertical strike slip shear zones s2 with dominant sinistral displacements whereas in the rosario fault zone d2 generated low angle s2 shear zones and various slip crenulation fabrics formed by northwest directed thrust motion the d1 fabrics are inferred to be related to initiation of subduction related accretion of the nappes by orogen normal southwest northeast shortening and accretionary tectonics the subsequent d2 fabrics developed as temporally overlapping subduction related low angle thrusts in the rosario fault zone and high angle sinistral strike slip faults in the lopez fault zone in an overall transpressive setting due to oblique convergence this model suggests that the high angle lopez fault zone d2 was located above a gently northeast dipping thrust detachment s2 expressed by the rosario fault zone that allowed the accreted nappes to move parallel to the orogen the kinematic model cannot validate regional scale dextral terrane translation of the san juan thrust system as inferred by paleomagnetic data elsewhere in the western united states but addresses important tectonic processes caused by oblique subduction and changing plate conditions

continental subduction intracrustal shortening and coeval upper crustal extension p t evolution of subducted south iberian paleomargin metapelites betics se spain abstract the nevado filabride complex represents the basement and permo triassic cover of the south iberian passive margin metamorphosed during miocene continental subduction under the alboran crustal domain it forms a nappe stack with the thick calar alto unit 6 km thick sandwiched between the ragua unit below and the bedar macael unit above the nappe stacking occurred by ductile flow along basal syn metamorphic shear zones structural analysis of the calar alto unit together with thermobarometric data of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of intracrustal shortening in the context of a strongly extended orogen multiequilibrium thermobarometry shows that the calar alto schist underwent a prograde p t path at mantle depths from 1 6 gpa at 450 500 c to 1 0 gpa at 550 570 c during the growth of the schistosity s1 parallel to the compositional layering after detachment from the downgoing slab and underplating the unit underwent nearly isobaric cooling by up to 250 c from peak metamorphic conditions the main crenulation cleavage s2 and the mylonitic foliation sm developed within the crustal nappe stack during decompression and heating sm at the base of the unit grew between 0 8 gpa at 400 c and 0 4 gpa at 500 c while s2 at the top of the unit developed between 0 6 gpa at 300 c and 0 2 gpa at 400 c these shortening structures developed simultaneously to the activity of overlying middle miocene extensional detachments intracrustal shortening and coeval upper crustal extension resulted in a mode of exhumation where the material rise entailed an important horizontal component of displacement at rates of approximately 15 mm year being exhumed along an s shaped p t path after peak metamorphic conditions and its detachment from the subducting slab

kinematic analysis of the san juan thrust system washington shear related fabric in the middle to late cretaceous san juan thrust system washington is observed in imbricated blocks and in fault zones a few millimeters to tens of meters thick foliation and lineation are defined in tectonic blocks by alignment of 1 newly grown muscovite and chlorite and 2 flattened and stretched radiolaria mineral porphyroclasts and rock fragments thin argillaceous shear zones bear a pronounced lineation defined by lithologic streaks and slickenside corrugations the slickenside and stretching lineations show a well defined preferred orientation in a horizontal northwest southeast direction widespread s c fabrics asymmetric pressure shadows fault drag features and asymmetric folds in the x z plane yield a sense of tectonic transport at most localities of upper plate to the northwest aragonite prehnite lawsonite pumpellyite and rare na amphibolite and na pyroxene characterize the metamorphism during the thrusting the findings of this study indicate north west orogen parallel transport of the san juan thrust system we suggest that this structure is a large step over zone transferring dextral strike slip from the western cascades to the oceanward side of wrangellia

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

strain and flow in the metamorphic core complex of ios island cyclades greece we have analysed strain and flow kinematics in the footwall of the south cyclades shear zone scsz an important tectonic boundary within the attic cycladic crystalline complex exposed on ios island cyclades aegean sea coarse grained augen gneisses in the basement unit flooring the scsz and forming a metamorphic core complex are excellently suited to measure finite strain using the fry method and estimate the vorticity number wk of flow with the blocked object method the results show that oligo miocene exhumation of the basement unit during extension brought approximately 70 n s crustal stretching and up to 40 subvertical shortening in a plane strain environment k 0 99 linear down section strain decrease constrains a zone of contact deformation of the scsz of about 1 5 km thick kinematic vorticity number estimates suggest little deviation from pure shear wk 0 26 finite strain and wk are not correlated indicating that the ios basement and the overlying cover units were stretched compatibly while the scsz is a localized zone of high strain net displacement however may be restricted to about ten kilometres this has important repercussions on large scale tectonic models for extension in the aegean

strain variations in an ancient accretionary complex implications for forearc evolution strain magnitudes and orientations and incremental strain histories are characterized throughout the kodiak formation a portion of a late cretaceous early tertiary accretionary prism the results of these analyses are used to evaluate the history of underplating penetrative strain and unroofing in the forearc of the southwest alaskan convergent margin five major deformation events are recognized in the kodiak formation 1 layer parallel extension and layer parallel shear d1 related to underthrusting 2 seaward verging fold and thrust deformation d2 with development of both a slaty cleavage s2 and a downdip stretching lineation l2 related to underplating and early penetrative strain within the wedge 3 development of a regional scale antiform d3 4 development of a shear zone along the northwest limb of the d3 antiform d4 and 5 right lateral strike slip faulting and penetrative right lateral simple shear in the seaward half of the kodiak formation d5 based on sharp differences in structural style and the magnitude and orientation of strain associated with d2 the kodiak formation has been subdivided into three structural units informally called the landward central and seaward belts the structural variations observed in these three belts correspond to differences in structural relief of 1 5 km associated with the d3 antiform the lowest d2 structural level is in the core of the antiform central belt and progressively higher structural levels are exposed on both limbs landward and seaward belts the regional distribution of incremental strain histories associated with d2 d3 d4 and d5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids from the highest to the lowest structural level d2 strain is characterized by 1 an increase in the magnitude of finite strain 2 a change in the orientation of finite extension from nearly vertical to gently northwest plunging 3 a change in the incremental extension direction from nearly vertical to moderately northwest plunging and finally 4 a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting we interpret these d2 strain patterns to reflect a strain field that was fixed relative to the basal decollement with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain probably simple shear near the base subsequent variations in the orientation of incremental extension associated with d3 are consistent with relatively rapid vertical translation of the accreted sequences relative to the d2 strain field we specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain these two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the late cretaceous and middle tertiary

petrologic evolution and paleogeography of the late cretaceous nanaimo basin washington and british columbia implications for cretaceous tectonics systematic variations in sandstone petrofacies of the upper cretaceous nanaimo group are related to basin evolution and uplift of surrounding tectonic provinces a subaerially exposed horst partially separated southern and northeastern portions of the nanaimo basin volcaniclastic and arkosic sediment was derived from this horst the horst subsided below wave base during very early evolution of the nanaimo basin but it controlled sediment dispersal throughout basin development lower units of the nanaimo group in the southern nanaimo basin are chert rich and were derived largely from terranes of the san juan islands and north cascades basaltic and plutonic debris from the insular belt was deposited into the northwestern nanaimo basin during early basin development minor deposition of arkosic debris from the coastal plutonic belt also occurred upper units of the nanaimo group contain abundant plagioclase rich sediment derived from the coastal plutonic belt rocks along the southeastern margin of the basin however reflect continued sedimentation from terranes of the san juan islands and north cascades arkosic and dacitic debris from insular belt sources is recognized in the northwestern nanaimo basin the nanaimo basin may have developed as a pull apart basin within a broad zone of normal and transcurrent faulting in response to oblique convergence or transform faulting between the kula plate and the pacific margin of washington and british columbia

deep seated flow as a mechanism for the uplift of broad forearc ridges and its role in the exposure of high p t metamorphic terranes forearc systems evolve through the interplay of numerous processes but aside from lateral movements much of the observed variability in morphology can be related to evolution of ductility in the deeper levels of the subduction complex in the eastern aleutian lesser antilles makran and cascade arc trench systems a 50 to 200 km wide forearc ridge is developed along the inboard edge of the subduction complex these forearc ridges did not form until subduction accretion had expanded the arc trench gap to 275 300 km and the near vertical rise of the ridge occurred 50 km or more from the trench slope break the forearc high apparently maintained by accretion at the trench thus these broad forearc ridges are not emergent trench slope breaks the process responsible for uplift is occurring within the subduction complex and the size of the accreted mass is apparently a major factor rheological models assuming frictional sliding laws at shallow levels and power law rheologies at depth predict different behaviors for various parts of a typical large subduction complex the older forearc basement beneath which the inboard part of the subduction complex is emplaced represents a rigid brittle massif that extends 100 150 km from the volcanic arc a prediction supported by seismicity studies in modern convergent margins because of the wet quartz rich lithologies of the subduction complexes however the brittle to ductile transition apparently occurs at depths of 10 20 km within the accreted mass thus in large subduction complexes where the downgoing slab is 40 50 km beneath the inboard edge a significant region at the deeper levels of the subduction complex is apparently capable of flow the predicted zone of ductile deformation is directly beneath the area in which a forearc ridge is generally developed hence we conclude that the broad structural highs of large subduction complexes are dynamically maintained by deep seated flow the character of this flow is uncertain yet it is likely that the stripping of low viscosity partially subducted sediments plays a role and the volume of sediments added by underplating may be the controlling factor in the dynamic uplift of broad forearc ridges kinematically the flow could be a pure shear deformation induced by subhorizontal loading but we prefer the familiar corner flow mechanism because it provides the simplest explanation for the observations the interaction between offscraping at the trench and underplating affects the distribution of mass on the downgoing slab and probably controls uplift rates in broad forearc ridges because of the complexity of the interaction and the observation that modern forearc ridges appear to be rising slowly we conclude that high pressure rocks are only exposed in unusual forearc systems e g systems with either a prolonged period of uplift or large influxes of deep underplated sediment the evolution of ductility plays a major role in the development of forearc morphologies when the subduction complex is small brittle deformation and processes related to accretion at the trench dominate and the classic growth models are applicable however as continued growth of the subduction complex depresses the downgoing lithosphere and increases the arc trench gap the subduction complex becomes ductile at depth and ultimately deep seated flow leads to abrupt uplift along the inboard edge of the subduction complex

the extensional messaria shear zone and associated brittle detachment faults aegean sea greece structural thermochronological and metamorphic data are used to elucidate the tectonic nature and evolution of the ductile extensional messaria shear zone and the associated brittle messaria and fanari detachment faults which exhumed their footwall from mid crustal depths on the island of ikaria in the aegean thermobarometric data indicate that the messaria shear zone formed at 350 400 c and 3 4 kbar i e at a depth of c 15 km normal faulting was accompanied by the intrusion of two granites which together with the thermobarometric data indicate a relatively high thermal field gradient of 25 35 c km 1 zircon and apatite fission track and apatite u th he ages demonstrate rapid cooling in the footwall of the messaria detachment from c 400 c to c 40 c between 11 and 3 ma age distance relationships of the data suggest that the messaria shear zone and the messaria detachment slipped at apparent rates of c 6 9 km ma 1 kinematic indicators show a consistent top to the nne shear sense for the extensional faults however at the southern part of the messaria detachment some late stage shear sense indicators are top to the ssw and are assumed to be associated with updoming of the footwall numerous deformed pegmatite veins in the messaria shear zone allow the reconstruction of deformation and flow parameters the mean kinematic vorticity number ranges from 0 13 to 0 80 indicating that shearing deviated significantly from simple shear that is extensional shearing was associated with vertical ductile thinning which contributed to tectonic exhumation finite strain shows oblate geometries and axial ratios of the finite strain ellipse in sections parallel to tectonic transport and normal to the mylonitic foliation range from 1 8 to 19 9 we calculate using a 1d numerical model that vertical ductile thinning contributed c 20 to exhumation during extensional shearing normal faulting was the major agent exhuming the footwall from c 15 km depth

primary fabric ellipsoids in sandstones implications for depositional processes and strain analysis abstract when measuring strains in deformed clastic rocks geologists usually assume one or more of the following 1 objects were initially circular 2 objects were initially non circular but had uniform orientations 3 object populations initially had a fabric but that this fabric had a symmetrical relationship to bedding or that 4 initial fabrics are recognizable even after straining to evaluate these assumptions we measured 43 fabric ellipsoids in non strained poorly sorted sandstones from four depositional settings dsdp core 174 fan and abyssal plain deposits off the coast of oregon the cretaceous great valley sequence california fore arc deposits the cretaceous pigeon point formation california accreted and slumped turbidites and crossbedded sandstones from intracontinental basins in california and australia our results indicate the following 1 in two dimensional cuts individual grains have variable but usually small axial ratios these data and microstructures in grains and matrix cements also argue against significant compaction during burial in these clast supported sand rich units and instead suggest that the primary fabrics largely reflect grain packing processes during deposition and or slumping and packing in the case of the pigeon point samples

strain patterns within part of the willard thrust sheet idaho utah wyoming thrust belt abstract the willard thrust sheet located in northern utah usa contains a 10 15 km thick sequence of strata that were internally deformed and translated about 50 km ese upper levels of the sheet are relatively little deformed lower levels have minor folds and cleavage that increase in intensity downward and rotate into parallelism with the thrust and the basal fault zone contains mylonite and ultracataclasite finite strain displays systematic changes with structural position and lithology basal levels have strain ratios r r xz of 3 5 and angles between stretch directions and the thrust of r of 1 5 3 and of 10 35 with overall correlations between increasing r decreasing and greater mica content overlying micaceous quartzite has relatively low strain with r of 1 3 2 and 50 principal stretching x directions trend perpendicular to oblique to minor fold axes which are locally oblique to the overall transport direction reflecting partial decoupling of internal deformation from thrust slip principal axial ratios have r yz r xy but microfabrics indicate only limited extension in the intermediate stretching direction y corresponding to volume losses of about 10 40 strain is factored into components of thrust perpendicular thinning and thrust parallel shear that increase downward and in mica rich layers minor thrust parallel shortening to extension minor longitudinal extension and minor longitudinal wrench shear these patterns reflect litho mechanical layering of the thrust sheet which includes a very weak basal fault zone that formed at elevated fluid pressures a weak mica rich lower level with widespread crystal plastic and mass transfer deformation and a stronger upper part with limited internal deformation

structural geology and accretionary history of the northwest cascades system washington and british columbia pre tertiary rock units on the western flank of the cascades in washington and southern british columbia constitute the northwest cascades system nwcs this litho tectonic package is characterized by predominantly oceanic lithologies subjected to high p t metamorphism and structurally mixed to form a regional melange although the focus of this report is on rocks of the nwcs as originally defined survey of the regional geology suggests broadening of the limits of this system to include the san juan islands the pacific rim complex and pre tertiary rocks on the western flank of the central and southern washington cascades the shuksan metamorphic suite a relatively coherent and broadly exposed unit of the nwcs contains a regionally consistent stretching lineation which provides evidence of east northeast directed underthrusting subduction during the early cretaceous blue schist metamorphism metamorphic f2 folds are approximately normal to the stretching lineation and appear to have developed late in the same deformational event the melange structure postdates the high pressure regional metamorphism it is characterized by a tectonic assemblage of diverse and unrelated rock units occurring as fragments as much as 10 or more kilometres in breadth and separated by a network of low to high angle north northwest striking faults pre fault restoration is impossible argillaceous or serpentinitic material commonly occurs in deformation zones between more competent lithologies mylonite zones as much as several metres thick are widely developed and display stretched clast lineations indicating a north northwest south southeast transport direction for both low and high angle faults the high angle faults are therefore strike slip in nature shear sense indicators are mixed in thrust fault zones but many indicate dextral movement for the strike slip faults k ar whole rock ages of mylonites range from 20 127 ma but two late cretaceous ages of 87 and 91 3 ma are considered as best representing the time of faulting the inferred shift in direction of tectonic transport from east northeast west southwest during early cretaceous metamorphism to north northwest south southeast during late cretaceous faulting and melange formation is correlated with a mid cretaceous change in plate interaction between the north american and farallon kula plates from high to low angle convergence the nwcs melange is interpreted to have formed during large dextral transform motion along the continental margin and to have reached final emplacement by thrusting against the southern end of wrangellia and the coast plutonic complex

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

deformed conglomerates and textural zones in the otago schists south island new zealand abstract the prehnite pumpellyite to greenschist facies rocks of the classic low grade otago schist terrane are subdivided into a number of textural zones based on the development of slaty cleavage and the appearance of segregation lamination in metagreywackes in this study conglomerates from fifteen localities have been studied in order to investigate the relationship between ductile strain and the sequence of textural zones strain measurements on lithic pebbles show a clear correlation between strain and textural zone ranging from around 40 shortening in tz iia with the development of slaty cleavage to more than 70 in tz iiia corresponding to the appearence of segregation lamination the much lower and more prolate strains of the quartzose pebbles compared to the corresponding lithic clasts may be explained by differential strain partitioning between clast and matrix microstructural evidence including well developed strain shadows and solution seams indicates that solution transfer was the dominant deformation mechanism the development of quartz segregation laminae in tz iii is due to three related processes the development of mica rich solution seams demarcating quartzo feldspathic microlithons the development of amalgamated and recrystallised strain shadow overgrowths on clastic grains and the formation rotation and disruption of early formed veins the large increase in strain from tz ii to tz iii is responsible both for increased segregation and for the rotation and flattening of segregation features into the foliation thus producing a segregation lamination large strains developed by solution transfer during the low grade metamorphism of greywackes will lead inevitably to segregated schists

contribution of ductile flow to exhumation of low temperature high pressure metamorphic rocks san juan cascade nappes nw washington state the san juan cascade sjc nappes were subducted to a depth of 18 km metamorphosed under low temperature high pressure conditions and then exhumed all within 16 m y during exhumation penetrative deformation by solution mass transfer smt resulted in a widespread spaced cleavage strain directions were determined for 27 sandstone samples and absolute strain measurements for a subset of 19 samples z directions generally plunge moderately to the ne and x and y directions are scattered in the plane perpendicular to z smt deformation is constrictional at the local scale but the tensor average indicates plane strain uniaxial shortening at the regional scale with sx sy and sz equaling 1 01 0 91 and 0 58 respectively the average flattening plane xy dips 30 to the ne and the average x direction plunges 20 to the north the large shortening in z was compensated by a mass loss volume strain of 47 we present a simple one dimensional model that illustrates the relationship between finite strain and ductile exhumation for a steady state convergent wedge assuming depth dependent ductile flow and no reversal of principal strain rates with depth this model indicates that ductile thinning of the sjc nappes accomplished only 13 of the total exhumation despite a vertical shortening strain of 36 there is no evidence that normal faulting contributed significantly to exhumation we conclude that erosion operating at an average rate of 1 1 km m y 1 was the dominant exhumation process

millennial and centennial scale droughts at the northern margin of the east asian summer monsoon during the last deglaciation sedimentological evidence from dali lake abstract the semi arid areas at the northern margin of the east asian summer monsoon easm have experienced an increased frequency of drought in recent decades however it is unclear whether past monsoonal precipitation in the region were determined mainly by el nino southern oscillation enso variability or high latitude cooling in this study we use a high resolution grain size dataset combined with a new bayesian age depth model of a sediment core from dali lake to analyze the occurrence of millennial and centennial scale droughts at the easm margin during the interval of climatic warming during the last deglaciation the polymodal grain size distributions are partitioned using a log normal distribution function fitting method high percentages of the coarse components c4 c5 c6 are recognized as proxy indicators of strong aeolian activities and low lake levels and thus droughts in the region the drought events indicated thereby are generally accompanied by decreases in regional temperature catchment surface runoff and bio productivity on both millennial and centennial timescales these results imply that the droughts at the easm margin were caused by a significantly weakened easm intensity in addition the droughts may be linked within the age uncertainties to the heinrich 1 h1 and younger dryas yd events on millennial timescales and to the older dryas od cold event on centennial timescales implying strong high latitude forcing given that these cold reversals over northern high latitudes were induced by the rapid input of glacial melt water to the north atlantic during the last deglaciation it is possible that ongoing climatic warming and the resulting high northern latitude ice sheet melting may cause an increased incidence of drought in northern china

revised ages of blueschist metamorphism and the youngest pre thrusting rocks in the san juan islands washington new ages of rocks in the san juan islands northwest washington significantly change our understanding of the evolution of the san juan islands thrust system re examination of foraminifera bearing mudstones at richardson on lopez island indicates a late aptian 112 115 ma not late albian 100 ma age as currently presented in the literature the age brackets of thrusting marked by these pre thrusting mudstones and 84 ma post thrusting sedimentary rocks span a much longer period than previously thought diminishing controls on rates of displacement in the thrust system and the timing of regional deformation in western washington new 40 ar 39 ar plateau ages of phengite in blueschist facies meta volcanic rock also at richardson are 124 0 7 ma 2 late barremian these blueschist facies volcanic rocks are in fault contact with the fossiliferous mudstones therefore the blueschist facies metamorphism at richardson previously inferred to be associated with the thrusting now appears to have occurred prior to thrusting further the ar ages demonstrate that blueschist facies fabric formed earlier than the thrust event and is therefore not directly useful in analyzing the thrusting kinematics the richardson 40 ar 39 ar age is similar to isotopic ages found in the eastern san juan islands and in the shuksan blueschist terrane in the northwest cascades and thus fits into an emerging regional age pattern of blueschist facies metamorphism during late jurassic early cretaceous up to barremian but not late albian cenomanian if this pattern is more broadly confirmed for the san juan islands all the blueschist facies metamorphism can be regarded as having formed in subduction zones elsewhere along the continental margin rather than in the anomalous setting of an on land thrust system as in the san juan islands resume de nouveaux ges pour les roches des iles san juan dans le nord ouest de l etat de washington changent profondement notre comprehension de l evolution du systeme de chevauchement san juan un reexamen des mudstones foraminiferes a richardson sur l ile lopez indique un ge aptien tardif 112 115 ma et non albien tardif 100 ma tel qu indique actuellement dans la litterature les plages d ges pour le chevauchement marquees par ces mudstones precedant le chevauchement et des roches sedimentaires 84 ma posterieures au chevauchement couvrent une periode beaucoup plus grande que stipulee anterieurement diminuant les controles sur les taux de deplacement dans le systeme de chevauchement et le moment de la deformation regionale dans l ouest de l etat de washington de nouveaux ges plateaux 40 ar 39 ar de la phengite dans des roches volcaniques au facies des schistes bleus aussi a richardson datent

correlation of early cretaceous blueschists in washington oregon and northern california the protolith and metamorphic histories of early cretaceous blueschists that occur in washington oregon and california are remarkably similar these blueschists are the shuksan metamorphic suite of northwestern washington the condrey mountain schist of northern california and southern oregon and the pickett peak terrane of northern california and southwestern oregon we suggest that these bodies were coextensive during their formation dispersion of the blueschists occurred in late cretaceous time by lateral translation of up to 900 km along the continental margin in response to oblique plate convergence

solution mass transfer deformation adjacent to the glarus thrust with implications for the tectonic evolution of the alpine wedge in eastern switzerland abstract we have studied aspects of absolute finite strain of sandstones and the deformation history above and below the glarus thrust in eastern switzerland the dominant deformation mechanism is solution mass transfer smt which resulted in the formation of a semi penetrative cleavage our analysis indicates that the verrucano and melser sandstones which lie above the thrust were deformed coaxially with pronounced contraction in a subvertical z direction and minor extension in a subhorizontal x direction trending at 200 most of the contraction in z was balanced by mass loss volume strains averaging 36 below the glarus thrust sandstones of the north helvetic flysch have smaller principal strains but similar volume strains deformation there was also approximately coaxial the x direction is horizontal and trends 160 which is different by 40 from the x direction in the hanging wall the hanging wall of the glarus thrust verrucano and melser sandstones was deformed first after it was accreted deep beneath the alpine wedge continued northward advance of the wedge accomplished in part by motion on the glarus thrust allowed the wedge to override and accrete the north helvetic flysch which then started to form an smt cleavage the difference in x directions may reflect a change in transport direction but this conclusion is difficult to accept since extension was minor and was accommodated by coaxial flattening and not simple shear our work indicates that mass loss volume strains were important in sandstones of the helvetic nappes the missing mass cannot be accounted for at the local scale and appears to have been transported beyond the helvetic zone

deformational geometry and tectonic significance of a portion of the chilliwack group northwestern cascades washington the northwestern cascades structural province can be interpreted as an accretionary complex comprising fault bounded blocks of pre tertiary metamorphic rocks of diverse age and lithologic type this paper documents the deformation in a portion of the chilliwack group a unit in this complex the chilliwack group is a thick sequence of volcaniclastic sedimentary rocks calc alkaline volcanic rocks and limestone that is metamorphosed to low grade blueschist facies the rocks underwent ductile deformation during a late cretaceous orogenic event producing a subhorizontal foliation and in appropriate lithologies subhorizontal stretching lineations that trend n20 w finite strain sustained by coarse clastic rocks produced rxz values averaging 3 5 the deformation at least partially postdates the high pressure metamorphic event based on the presence of bent and broken high pressure mineral grains although early studies postulated west vergent thrust imbrication of units in the northwest cascades the n20 w

vorticity analysis in a metachert from the sanbagawa belt sw japan abstract common tectonic features of deformed metachert include both sets of extended and shortened veins and quartz c axis preferred orientation patterns in such rocks information is also commonly available on the orientation and magnitude of the finite strain these features can be used to derive two independent estimates of the degree of non coaxiality during deformation in an example from the sanbagawa belt sw japan the two methods give consistent results and show that deformation was intermediate between simple and pure shear such types of progressive deformation distributed over a broad region could cause substantial subvertical thinning and be a significant factor in the exhumation of high p t rocks in the region

pressure solution in siliciclastic hp lt metamorphic rocks constraints on the state of stress in deep levels of accretionary complexes abstract the magnitude of differential stress in deep levels of accretionary complexes along convergent plate margins is poorly constrained by theoretical models and cannot be directly measured the only direct evidence to infer rheology and state of stress for such crustal settings is the microstructural memory of high pressure low temperature metamorphic rocks recorded during their crustal evolution microfabrics in hp lt metamorphic t 400 50 c p 10 2 kbar phyllites and quartzites of the phyllite quartzite unit on the island of crete southern aegean reveal 1 that progressive deformation was by pressure solution dissolution precipitation creep 2 that clastic quartz grains in the phyllites show no evidence for crystal plastic deformation during burial and exhumation 3 that the unilaterally rational 001 mica quartz phase boundaries were sites of strongly enhanced dissolution and 4 dislocation creep was restricted to a minor role in quartzites poor in mica consequently the magnitude of differential stress in relation to temperature in the phyllites has remained below the level required to drive dislocation creep throughout the history of burial to a depth of more than 30 km and subsequent exhumation currently available flow laws for quartzite indicate that the differential stress in the phyllites remained well below 15 mpa at temperatures around 400 c at a depth of 30 km this implies that the effective viscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocation creep

stratigraphy depositional setting and tectonic significance of the clastic cover to the fidalgo ophiolite san juan islands washington the san juan islands of northwest washington state comprise a diverse assemblage of paleozoic and mesozoic terranes amalgamated during a regional cretaceous orogenic event detailed tectono stratigraphy of the sedimentary cover to the fidalgo complex indicates the presence of several stratigraphically distinct units which are described and formalized in this paper the fidalgo complex and its sedimentary cover are the structurally highest rocks in the san juan thrust system the fidalgo complex is a highly disrupted middle to upper jurassic ophiolite with arc related intrusives volcanics and sediments the trump unit is an informally named sequence of siliceous sediments volcanic graywacke and minor volcanics that occurs at the stratigraphically highest portion of the fidalgo complex complex facies lithologies and provenance indicate that deposition of this oxfordian to upper tithonian unit occurred in an arc proximal setting the upper tithonian and younger lummi group elevated here lies depos

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

kinematics of crystal growth in syntectonic fibrous veins abstract detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth this calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis based on surface roughness characteristics of the vein wall we develop a simple model for shape growth of crystals in a crack seal environment which can account for both tracking and non tracking behaviours and propose a set of tracking criteria for antitaxial veins finally we discuss ways by which the model can be tested against natural examples

brittle frictional mountain building 1 deformation and mechanical energy budget an active fold and thrust belt is analogous to a wedge of soil or snow that forms in front of a moving bulldozer such wedges exhibit a critical taper and a regional state of stress that is everywhere on the verge of coulomb failure the width of such a critically tapered fold and thrust belt does not depend on its brittle strength or frictional properties but rather on the accretionary influx rate of fresh material at its toe and on the rate of erosion a steady state fold and thrust belt is one in which the accretionary influx is balanced by the erosive efflux rocks are accreted at the toe and then horizontally shortened as they are transported toward the rear those that enter lower in the accreted section are more deeply buried before being uplifted by erosion mass balance and isotropy constrain the kinematics of this large scale deformation enabling us to infer the trajectories residence times and stress strain histories of rocks incorporated into eroding fold and thrust belts a typical rock resides in the steady state taiwan wedge for 2 3 m y before it is uplifted and eroded during its motion through the wedge it experiences strain rates in the range 10 13 to 10 14 s 1 the mechanical energy budget of brittle frictional mountain building is described by the equation where is the rate of work performed on the base and front of the fold and thrust belt by the subducting plate is the rate at which energy is dissipated against friction on the decollement fault is the rate at which energy is dissipated by internal frictional processes within the deforming brittle wedge and is the rate of work performed against gravitational body forces in a reference frame attached to the overriding plate the total mechanical power being supplied by the subducting eurasian plate to the active fold and thrust belt in taiwan is slightly over 3 gw approximately 60 of this work of steady state mountain building is being dissipated against friction on the decollement fault and about another 25 is being dissipated against internal friction this leaves only 15 or roughly half a gigawatt available to do useful work against gravity in general fold and thrust belts with moderate pore fluid pressures are dominated by work done against friction on the decollement fault however those with nearly lithostatic pore fluid pressures may be dominated by work done against gravity internal frictional dissipation is always less than basal frictional dissipation as it is in taiwan an alternative and equivalent description of the mechanical energy balance of a steady state fold and thrust belt is provided by the equation in this version the quantity on the left is the rate at which work is performed on the back of the wedge by the overriding plate and is the rate of work performed against gravity in a reference frame attached to the subducting plate the latter quantity is always positive for any critically tapered fold and thrust belt whose decollement fault dips toward its rear in contradiction to the central premise of the gravity spreading theory of fold and thrust tectonics

recurring global sea level changes recorded in shelf deposits near the g m polarity transition wanganui basin new zealand implications for redefining the pliocene pleistocene boundary abstract a well exposed 1 km thick association of late pliocene and early pleistocene ca 2 6 1 7 ma strata in wanganui basin new zealand contains a progradational stack of twenty 5th order 41 ka duration depositional sequences that accumulated in shelf to shoreface paleoenvironments in response to global glacioeustatic sea level fluctuations the sequences are correlated with 18o isotope stages 100 58 and each 41 ka glacial interglacial stage couplet is represented by an individual sequence comprising transgressive tst highstand hst and regressive rst systems tracts that can be readily related to generic sequence stratigraphic models in general deposition of each of the sequences occurred during odd numbered interglacial isotope stages and the sequence boundaries developed during the maxima of even numbered glacial isotope stages the key palaeomagnetic tiepoints used for pinning the isotope record to the section are the gauss matuyama boundary 2 60 ma in stage 104 and the top of olduvai subchron 1 77 ma in stage 63 a deeply incised sequence boundary with up to 30 m of relief at the base of te rimu sand represents the first widespread subaerial exposure in wanganui basin during the matuyama chron it is correlated with the high amplitude positive glacial 18o shift in isotope stage 100 which is considered to mark the initiation of major northern hemisphere continental glaciations at ca 2 52 ma the overlying hautawa shellbed containing the subantarctic bivalve chlamys delicatula defines the base of the nukumaruan stage and represents the earliest faunal evidence of climatic cooling in new zealand pliocene pleistocene marine sequences the sequence boundary at the base of hautawa shellbed is correlated with isotope stage 98 ca 2 48 ma traditionally the pliocene pleistocene boundary in new zealand was located at the base of hautawa shellbed but beu and edwards 1984 showed that the boundary as defined in the vrica stratotype astronomically calibrated age ca 1 81 ma actually lies in the upper part of the nukumaruan stage the hautawa shellbed which lies closer to the gauss matuyama palaeomagnetic transition 2 60 ma is therefore much older than the currently defined pliocene pleistocene boundary from a new zealand perspective a more logical position for the pliocene pleistocene boundary would be close to or just above the gauss matuyama polarity transition such a position would take advantage of the boundaries proximity to a readily identifiable polarity reversal and lie close to the time of initiation and development of major northern hemisphere ice sheets

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

deformation temperatures vorticity of flow and strain in the moine thrust zone and moine nappe reassessing the tectonic evolution of the scandian foreland hinterland transition zone abstract examination of deformation temperature 3 d strain and flow vorticity wm in mylonites from the assynt loch more region of the moine thrust zone mtz allows quantitative kinematic and thermal characterization of shearing at the base of the scandian 435 425 ma orogenic wedge quartz microstructures and c axis fabric opening angles from mylonites in the immediate hangingwall and footwall to the moine thrust suggest that deformation temperatures are highest in the eastern part of the assynt region including mylonites close to alkaline intrusive complexes and decrease along strike both to the north stack of glencoul loch more and to the south knockan quartz c and a axis fabrics together with limited 3 d strain data indicate that deformation in both the footwall and hangingwall mylonites dominantly involved plane strain to general flattening although domains of more constrictional flow are identified adjacent to thrust transport parallel lineaments in the overlying moine nappe rigid grain analyses indicate a remarkably constant flow vorticity for tens of kilometers along orogenic strike 40 60 pure shear in both the hangingwall and footwall mylonites integration with previously reported strain and vorticity estimates from the assynt region indicates a 50 75 sub vertical shortening perpendicular to gently dipping foliation that must have been accommodated by either volume loss or extrusion of material toward the synorogenic topographic surface extrusion implies a causal link between upper and lower crustal processes with significant implications for the kinematic and geometric evolution of the scandian wedge

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

metamorphic pressure estimates and sediment volumes for the alpine orogeny an independent control on geobarometers theories which regard the deep erosion 20 30 km of orogenic belts as an important factor in the thermal development of the continents rely on pressure estimates from metamorphic geobarometers to give an estimate of the depth scale for this process the imprecision of geobarometers and the apparent lack of sediments that would result from this amount of erosion are frequently made arguments for much smaller erosion depths the alps are a well studied young mountain belt and afford the opportunity to compare overburden masses inferred from geobarometry with sediment masses in the surrounding sedimentary basins this comparison suggests that metamorphic geobarometry has not significantly overestimated burial depths and that the average erosion to date has been over 15 km with a maximum of 25 km or more it is probable that the total erosion in the belt will eventually average 20 25 km with a maximum of around 40 km a significant feature of the sediment distribution is that over 50 of the volume is outside the neighbouring along strike sedimentary basins this fact which accounts for previous low estimates of alpine sediment volumes follows from the response of the lithosphere to loading by continental thickening and may be a common feature of post orogenic sedimentation