shrimp zircon u pb dating of quartz porphyry from zhongjia tin polymetallic deposit in longyan area fujian province and its geological significance mineralizations of the zhongjia tin polymetallic deposit in longyan area of fujian province are composed of vein type ore bodies in the external contact zone and veinlet disseminated sn wo mineralization in the inner contact zone belonging to post magmatic hydrothermal type related to quartz porphyry of the area shrimp u pb dating of twenty zircon grains from the quartz porphyry gived the weighted mean age of 98 5 1 7 ma n 29 mswd 1 8 suggesting that the quartz porphyry in the deposit was produced by regional tectono magmatic activities during the late stage of early cretaceous tin polymetallic mineralization of the zhongjia deposit occurred in late cretaceous instead of in early jurassic and constituted a part of late yanshanian metallogenic activity on the margin of the south china tungsten tin and polymetallic province as evidenced by regional metallogenic correlation the authors hold that the prospecting targets of tin polymetallic deposits in southwestern fujian province should be late yanshanian granitic bodies and their external contact zones

identification of multivariate geochemical anomalies using spatial autocorrelation analysis and robust statistics abstract multivariate geochemical anomaly identification is one of the most important tasks in exploration geochemical data analysis this paper proposes a new approach to identify multivariate geochemical anomalies which accounts for the degree of spatial autocorrelation among observations this method is a combination of robust mahalanobis distance rmd spatial autocorrelation analysis and robust statistics the minimum covariance determinant mcd estimator based rmds were first used to reduce the dimension of geochemical data sets spatial correlogram and local moran s i statistics were then applied to determine the spatial variability spatial scales and to measure the degree of spatial autocorrelation at global and different local scales respectively negative and positive anomalies were finally separated from a set of local moran s i values using robust statistics the median 1 5 iqr iqr interquartile range rule we illustrated the proposed method by using a data set of 1842 stream sediment samples associated with cu au mo and sn mineralizations in the jiurui ore district jiangxi province southeastern china the results showed that i the maximum spatial variability of rmds were 12 km in which strong positive spatial autocorrelation was detected at the first three spatial scales ranging from 0 to 6 km ii both of negative and positive multivariate geochemical anomalies were strongly correlated with 13 known cu au mo and cu mo deposits especially at spatial scales where high degrees of positive spatial autocorrelation were measured iii weak anomalies associated with a magmatic hydrothermal related sn and polymetallic mineralization were successfully identified in the zengiialong and jianfengpo sn deposits which were not detected in previous studies we demonstrate the combination of spatial autocorrelation analysis and robust statistics allows for improved identification of significant multivariate geochemical anomalies of the deposit type sought from stream sediment geochemical data

provenance of albian cenomanian rocks of the methow and tyaughton basins southern british columbia a mid cretaceous link between north america and the insular terrane basin analysis of albian cenomanian rocks of the tyaughton and methow basins suggests that the two basins were filled by three principle petrofacies during a regionally significant contractional ev

structural geometry and kinematic evolution of the eastern cascades foldbelt washington and british columbia the eastern cascades foldbelt is one of three structural domains lying within the complex collision zone between the insular and intermontane composite terranes in northern washington and southern british columbia the foldbelt resides between the high grade metamorphic backbone of the cascade orogen on the west and rocks of the composite intermontane terrane to the east it encompasses the stratigraphically coherent basalt floored jura cretaceous methow basin as well as more chaotically disposed permian jurassic oceanic rocks of the hozameen terrane methow basin rocks are thought to have been sutured above the oceanic rocks prior to the middle cretaceous contractional episode described in this report based on the analysis presented herein between ca 100 and 88 ma the rocks in the foldbelt underwent shortening in an east northeast west southwest direction by 50 km or more largely by displacement on the east vergent jack mountain chuwanten thrust system the early stages of contraction occurred by

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

strain and displacement across the pinaleno mountains shear zone arizona u s a abstract the 27 km long 0 5 km thick mylonite zone of the pinaleno mountains metamorphic core complex is interpreted as a normal displacement simple shear zone on the basis of major minor and microscopic scale structures shear strains calculated as a function of the angle between the foliation and the lower boundary of the shear zone range from 2 to 19 and have a mean value of 3 5 corresponding to a mean angular shear of 74 these values are the same as those calculated independently from rotations of sheared lithologic contacts integration of the calculated shear strains yields a minimum translation estimate of 2 9 km

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

northeast trending folds in the western skeena fold belt northern canadian cordillera a record of early cretaceous sinistral plate convergence abstract the western portion of the skeena fold belt northern canadian cordillera contains northeast trending folds that are highly oblique to northwest trending folds in the eastern portion of the fold belt and to most mesozoic contractional structures in the northern cordillera the northeast trending folds locally interfere with the northwest trending folds and one region includes transected folds geometric relationships within and between the two fold sets are not easily reconciled by notions of the northeast trending folds resulting from vertical axis rotation of blocks influence of basement features or lateral variations in magnitude of shortening the northeast trending folds are inferred to result from sinistral plate convergence early in the history of the fold belt early cretaceous northeast trending folds in the skeena fold belt are the most conspicuous elements of a seldom studied group of similarly oriented contractional structures which collectively define a belt at least 1700 km long within and bordering the coast belt the extent of early cretaceous structures potentially related to sinistral convergence supports them having originated in response to the relative plate motion rather than local controls e g indentors this agrees with relative plate motion studies based on ocean floor reconstructions which suggest a mid cretaceous change from sinistral to dextral convergence

mesozoic cenozoic deformation in the canadian cordillera the record of a continental bulldozer abstract a possible mesozoic cenozoic trajectory for the north american craton is outlined from latitude changes of the craton derived from a revised apparent polar wander path and from westward movement of the craton based on the assumption that africa has been the least mobile continent geographically since the latest paleozoic during each of five time intervals that span 220 million years the craton trajectory had a different vector each vector appears to be reflected in the canadian cordillera by the dominant style and orientation of structures formed during that interval for much of the past 220 million years the meridionally oriented western margin of the pangea laurasia north america plate has been the site of arc magmatism where weak arc back arc lithosphere sandwiched between strong ocean floor and craton lithospheres at times coupled across convergent or transform plate boundaries focused and recorded strain when the craton apparently moved due westward between 180 160 ma and 120 60 ma the dominant structures formed then record orogen normal compression and were accompanied by orogeny structures formed during the earlier episode are mainly in the eastern and interior cordillera and their formation shortly followed or coincided with accretion of most terranes to the craton margin in the later episode compressional structures span the entire cordillera which emerged as a tectonic and physiographic entity before and between these intervals when the craton apparently moved mostly northwestward mainly geological and paleomagnetic considerations indicate some terranes moved southward sinistrally relative to the craton after 60 ma southwestward movement of the craton was concurrent with large northward dextral strike slip faults that disrupted the newly established ancestral cordillera the coincidence between the age of structures that record dominant orogen normal compression at times when the craton apparently moved due westward and orogen parallel displacements when the craton had either northward or southward components of motion suggests the craton acting as a continental bulldozer was the primary driver of cordilleran deformation and orogenesis

tectonic history of the eastern edge of the alexander terrane southeast alaska rocks exposed west of the coast plutonic complex in southern southeast alaska form an imbricate thrust belt that overprints the tectonic boundary between two of the largest allochthonous crustal fragments in the north american cordillera the insular and intermontane composite terranes in the alexander terrane insular composite terrane lower paleozoic metavolcanic and metasedimentary rocks descon formation and dioritic plutons are unconformably overlain by lower devonian clastic strata karheen formation these rocks are overlain locally by upper triassic basalt rhyolite and marine clastic strata hyd group upper jurassic and lower cretaceous metavolcanic and metasedimentary strata of the gravina sequence unconformably overlie the alexander terrane the gravina sequence forms a structural package over 15 km thick and records intermittent arc volcanism along the eastern edge of the alexander terrane the gravina sequence is structurally overlain by upper paleozoic and lower mesozoic metamorphosed basaltic strata marble and argillite alava sequence and locally by lower paleozoic supracrustal rocks and orthogneiss kah shakes sequence together these constitute the taku terrane which we correlate with the intermontane composite terrane local unconformity of gravina sequence strata over the alava sequence demonstrates that the gravina sequence overlapped an earlier structural boundary between the intermontane and insular composite terranes the rocks were deformed in the mid cretaceous by west vergent thrusting that was was broadly coeval with arc magmatism deformation involved emplacement of west directed thrust nappes over the structurally intact and relatively unmetamorphosed alexander terrane basement mid cretaceous tonalite granodiorite and quartz diorite intrude rocks of the thrust belt and are locally affected by the deformation mid cretaceous deformation occurred during two episodes that were contemporaneous with the emplacement of large sill like plutons older structures record ductile southwest vergent folding and faulting regional metamorphism and development of axial planar foliation the second generation structures developed during the later stages of southwest directed thrust faulting which juxtaposed rocks of contrasting metamorphic pressures and temperatures structural stratigraphic and geochronologic data indicate that the two phases of regional thrusting in southeast alaska occurred between 113 ma and 89 ma rocks in the western part of the thrust belt were uplifted regionally by 70 ma deformation involved the collapse of a marginal basin s and a magmatic arc and overprinted the older tectonic boundary between the insular composite terrane and the late mesozoic western margin of north america at that time the intermontane composite terrane contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism and thus records intra arc tectonism late paleocene to early eocene igneous activity and extensional deformation subsequently affected the thrust belt

crustal structure of the active margin south central alaska an interpretation of seismic refraction data from the trans alaska crustal transect seismic refraction and wide angle reflection data from the u s geological survey s trans alaska crustal transect are used to investigate the upper crustal structure of southcentral alaska and to develop a model using two dimensional asymptotic ray theory the data considered here are from the n s 126 km cordova peak refraction profile which was positioned to cross the chugach and prince william terranes these two tectonostratigraphic terranes form part of a large accretionary complex related to the alaska subduction zone interpretation of data from the cordova peak profile indicates systematically higher velocities in chugach terrane rocks as compared with prince william terrane rocks at comparable depths unconsolidated sediment and glacial ice overlie chugach terrane rocks of unusually high compressional velocities 4 7 7 0 km s in the upper 10 km of crust a northward thickening metasedimentary and metavolcanic flysch sequence having an average velocity of 6 0 km s is underlain by metavolcanic mafic basement rocks having an average velocity of 6 6 km s the upper 10 km of the prince william terrane have compressional velocities ranging from 3 7 to 6 2 km s and are correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks and underlain by mafic to ultramafic prince william terrane basement rocks the upper crustal velocity structures of the chugach and prince william terranes are distinct to at least 10 km depth which suggests that the contact fault zone is a boundary which extends to at least 10 km midcrustal layers beneath the chugach terrane contain two velocity reversals which may indicate the presence of off scraped oceanic sediment overpressurized zones shear zones or some combination of these possibilities data from energy sampling midcrustal layers beneath the prince william terrane do not provide similar evidence for low high velocity zones in that area

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

the hozameen fault system and related coquihalla serpentine belt of southwestern british columbia the hozameen fault of southwestern british columbia is associated with the coquihalla serpentine belt and separates two distinct crustal units northeast of the fault are greenstones of the early t

u pb zircon and monazite ages for the okanogan range batholith washington implications for the magmatic and tectonic evolution of the southern canadian and northern united states cordillera new u pb zircon and monazite dates for the okanogan range batholith a suite of trondhjemitic plutonic rocks in north central washington are interpreted to reflect minimum crystallization ages of 114 111 ma these new data suggest that the okanogan range batholith may represent an older southerly segment of the late albian spences bridge group which crops out in southern british columbia this would constitute a 300 km long north northwest trending belt of 130 100 ma volcanic and plutonic rocks along the western margin of the intermontane belt herein named the okanogan spences bridge arc the okanogan spences bridge arc and the gambier arc in the coast belt of british columbia were coeval widely separated early cretaceous magmatic belts in the southern canadian northern united states cordillera the cbronologic data require reevaluation of models proposing one versus two east dipping subduction zones to explain the origin of these two parallel magmatic belts neither model adequately reconciles all geologic observations

effects of misclassification costs on mapping mineral prospectivity abstract mineral prospectivity mapping is a classification process because in a given study area a specific region is classified as either a prospective or non prospective area the cost of false negative errors differs from the cost of false positive errors because false positive errors lead to wasting much more financial and material resources whereas false negative errors result in the loss of mineral deposits traditional machine learning algorithms using for mapping mineral prospectivity are aimed to minimize classification errors and ignore the cost sensitive effects in this study the effects of misclassification costs on mapping mineral prospectivity are explored the cost sensitive neural network csnn for minimizing misclassification costs is applied to map fe polymetallic prospectivity in china s southwestern fujian metalorganic belt sfmb a csnn with a different cost ratio ranging from 1 10 to 10 1 was used to represent various misclassification costs the cross validation results indicated a lower misclassification cost compared to traditional neural networks through a threshold moving based csnn the csnn s predictive results were compared to those of a traditional neural network and the results demonstrate that the csnn method is useful for mapping mineral prospectivity the targets can be used to further explore undiscovered deposits in the study area

coast plutonic complex a mid cretaceous contractional orogen early late cretaceous east vergent thrusts deform much of the eastern margin of the coast plutonic complex in western british columbia similarities in timing and style suggest that the faults represent a system of backthrusts to a west vergent thrust belt on the west side of the complex this geometry and the rarity of mid cretaceous strike slip faults indicate that the coast plutonic complex was a strongly contractional orogen in mid cretaceous time

nature of the contact zone between the north cascades crystalline core and the methow sequence in the ross lake area washington implications for cordilleran tectonics previous analyses of the accretionary history of nw washington sw british columbia have suggested the possibility of large scale dextral slip faulting events to explain the present day regional outcrop pattern the contact zone between the crystalline skagit complex and the volcano sedimentary methow sequence has been considered to be a major fault named the ross lake fault by misch 1966 and has been considered to be a possible large scale strike slip fault by later workers detailed mapping of this contact zone in the ross lake area shows that the brittle faults and mylonites thought by misch to express dextral shearing along the contact are not continuous throughgoing structures and that the contact between skagit orthogneisses and methow sequence strata is primarily intrusive furthermore the mylonites show both left and right lateral shear the skagit methow contact is therefore not a major terrane boundary but rather a tectonized intrusive contact that has not accommodated significant motion between the two regions based on these observations and regional synthesis of timing relations deformation in the contact zone is interpreted to express regional ene wsw shortening not major dextral slip in the early tertiary since no terrane bounding fault exists the cretaceous tertiary history of the ross lake area can be interpreted as follows 1 jurassic early cretaceous forearc rift basin sedimentation deposition of the lower methow sequence 2 110 95 ma imbrication of the lower methow sequence and its basement of oceanic crustal rocks by east vergent thrusting with concomitant development of a west derived foredeep in the upper methow sequence 3 95 85 ma arc magmatism with genesis of the skagit migmatite from methow and older basement protoliths and 4 paleocene early eocene folding faulting uplift and exposure of a relatively intact 25 km crustal section termed the skagit methow crustal section by kriens and wernicke 1987

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

modeling of cu au prospectivity in the carajas mineral province brazil through machine learning dealing with imbalanced training data abstract machine learning ml is becoming an appealing tool in various fields of earth sciences especially in mineral prospectivity mapping mpm to support mineral exploration ml algorithms are designed to assume a relatively balanced amount of training data for the estimation of the decision boundaries between the classes of interest i e in mpm mineralized and non mineralized locations however in mpm the numbers of mineralized and non mineralized locations are naturally imbalanced as the number of known mineral deposit occurrences as a proxy of mineralized or positive class are naturally much smaller than the number of non mineralized locations the negative class the use of imbalanced data leads to difficulties in the training of ml models for mpm due to the learning bias towards the features of the predominant i e negative class in the present study using support vector machine for cu au prospectivity modeling in the carajas mineral province brazil we evaluated the effects of synthetic minority over sampling technique smote which addresses the issue of imbalanced training data on the performance of mpm the original training data for the positive i e minority class was modified by over sampling the mineralized locations using smote and by randomly under sampling the non mineralized locations at different proportions producing 400 training datasets with proportions of mineralized to non mineralized samples ranging from 600 30 to 30 600 each of these individual training datasets was used to evaluate the performance of mpm under different proportions of mineralized to non mineralized samples the performance of each prospectivity model was objectively evaluated using the f1 score and the success rate curve the results show that smote can significantly increase the performance and the spatial efficiency of mpm the main differences between the performances of the derived prospectivity models illustrate the sensitivity of mpm to the number of samples and distribution of classes in the training data according to the results better performance is achieved using smote when the prospectivity models are trained with an equal number of mineralized and non mineralized samples the best prospectivity model trained with a modified dataset with 600 600 proportion of mineralized to non mineralized samples resulted in 100 classification of the training mineralized locations and almost 80 of the testing mineralized locations and outlined only 7 of the study area as prospective

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

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

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

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

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

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

tectonic implications of syn and post emplacement deformation of the mount stuart batholith for mid cretaceous orogenesis in the north cascades the 93 96 ma mount stuart batholith intruded across the boundary between the northwest cascades thrust system and the crystalline core of the north cascades although previously considered posttectonic the northeast margin of the mount stuart batholith and its wall rocks have been involved in syn to post emplacement southwest directed thrusting and folding and west northwest stretching contraction ended shortly after emplacement as indicated by high temperature recrystallization in thrust related mylonites of the pluton and by geochronological data whereas west northwest stretching continued for an unknown period of time this is the best documented mid cretaceous contractional belt in the main part of the crystalline core the shortening direction and timing are identical to that of southwest vergent thrusts in the offset continuation of the core in british columbia the contractional belt provides a link between thrusting in the northwest cascades thrust system and deformation in the crystalline

a middle jurassic to early tertiary andean sierran arc model for the coast belt of british columbia regional tectonic features of the western canadian cordillera can be interpreted in terms of middle jurassic accretion of a single composite superterrane stikinia wrangellia alexander to ancestral north america closure of the intervening cache creek bridge river ocean moved the continental edge to a new position west of the accreted superterrane the coast belt is primarily a succession of superimposed middle jurassic to early tertiary magmatic arcs related to prolonged subduction of pacific ocean lithosphere beneath the new north american margin discrete magmatic pulses separated by lulls or periods of relative quiescence successively overprinted the new margin the insular and intermontane superterranes previously viewed as widely separate entities prior to mid cretaceous time were already amalgamated before an initial middle jurassic overprint produced an ancestral coast belt late jurassic to early cretaceous rift related marine basins mid to late cretaceous compressional structures and early tertiary extensional features coincident with the coast belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary subduction related andean magmatic arc

mapping mineral prospectivity through big data analytics and a deep learning algorithm abstract identification of anomalies related to mineralization and integration of multi source geoscience data are essential for mapping mineral prospectivity in this study we applied big data analytics and a deep learning algorithm to process geoscience data to identify and integrate anomalies related to skarn type iron mineralization in the southwestern fujian metallogenic zone of china based on the geological setting and environment for the formation of skarn type iron mineralization 42 relevant variables including two geological one geophysical and 39 geochemical variables were analyzed and integrated for detecting anomalies related to mineralization using a deep autoencoder network the results indicate that the mapped prospectivity areas have a strong spatial relationship with the locations of known mineralization and demonstrate that big data analytics supported by deep learning methods is a potential technique to be considered for use in mineral prospectivity mapping

application of lu hf garnet dating to unravel the relationships between deformation metamorphism and plutonism an example from the prince rupert area british columbia abstract the tectonic history of the prince rupert area is marked by profound crustal thickening from thrusting that produced inverted metamorphic field gradients and transpression in crustal scale shear zones syn tectonic garnet in the prince rupert area has lu hf ages of 102 6 3 7 ma and 108 3 4 1 ma 2 porphyroblast matrix relationships in these samples and samples from the same outcrops indicate syn tectonic garnet growth these relationships imply that the garnet ages directly date the development of the metamorphic foliations a third sample of migmatitic garnet amphibolite from the contact aureole of the 94 90 ma ecstall pluton had complex isotope systematics interpreted to indicate a garnet growth episode 105 ma similar to the ages obtained from the other samples and a growth or equilibration event at 94 ma during pluton emplacement the data show that the older lu hf garnet ages date prograde metamorphism during foliation development and modification during pluton emplacement the ecstall pluton was emplaced 10 to 15 m y after regional metamorphism and thrust stacking in the prince rupert area in order to place our samples in a regional tectonic context we compare our results to patterns of regional deformation metamorphism and plutonism throughout the north american cordillera in the time period between 110 and 85 ma contractional and transpressional deformation occurred throughout much of the north american cordillera at this time from southeastern alaska to the baja peninsula in mexico left lateral transpression dominated the canadian cordillera whereas right lateral transpression affected areas south of the idaho salmon river suture zone including the sierra nevada batholith this reversal in kinematics in the northern and southern cordillera within coeval magmatic belts appears to be a first order feature of the geology of the north american cordillera during the cretaceous

structural and geochronologic relations along the western flank of the coast mountains batholith stikine river to cape fanshaw central southeastern alaska abstract geologic and u pb geochronologic studies in central southeastern alaska provide constraints on the mid cretaceous to mid tertiary deformation west of the coast mountains batholith the ne dipping sumdum and fanshaw faults record the w directed emplacement of the ruth assemblage yukon tanana terrane and taku terrane over the subjacent gravina belt and alexander terrane ductile fabrics of the sumdum fanshaw fault system truncate early formed foliation and thrust faults in albian and older gravina belt strata u pb age data from syn and post tectonic plutons suggest that deformation was ongoing at 92 9 3 0 ma but had mostly ceased by 90 ma the sumdum fanshaw fault system marks the tectonic boundary between the alexander wrangellia terrane and inboard yukon tanana and stikine terranes and lies within a thrust belt that extends from southern alaska to northern washington mid cretaceous structures are truncated to the east by ne dipping ductile fabrics of the coast shear zone sheet like paleocene tonalites were emplaced into and deformed within the shear zone undeformed mid eocene dikes cross cut the tonalites kinematic relations in the coast shear zone suggest a complex displacement history that includes both east side up reverse and west side up normal shear the shear zone probably accommodated the collapse of overthickened crust developed during mid cretaceous shortening

on determining paleohorizontal and latitudinal shifts paleomagnetism of spences bridge group british columbia the spences bridge group of late albian age palynology and u pb zircon date of 104 5 ma is situated in the intermontane belt of the southern canadian cordillera zeolite grade andesite lavas and interbedded volcaniclastic rocks have been sampled through a thickness of about 2900 m sampling has been confined to localities whose bedding attitudes are well defined and to rocks that have not undergone internal deformation of the 27 sites studied 25 gave single component magnetizations with well grouped directions high coercivities and high unblocking temperatures applying corrections for geological tilt to all 25 sites yielded minimum dispersion after 65 untilting analysis indicates that this is a consequence of the interaction of two modes of behavior one exhibited by y magnetizations at 8 sites in which minimum dispersion occurs after 35 untilting and one x magnetizations at 17 sites in which minimum dispersion occurs after 100 correction actually the dispersion is essentially constant in the range 88 to 100 correction the y magnetizations are uninterpretable in terms of the paleofield because no estimate of paleohorizontal can be made because of their positive response to the tilt test the x magnetizations were acquired before tilting and it is argued that geologically determined bedding planes are good estimators of paleohorizontal at the time of remanence aquisition there may be errors because of paleoslope but arguments are presented for believing that these are small on this basis the best estimate of the mean direction of the paleofield in spences bridge time is estimated to be d i declination inclination 38 7 63 9 17 sites k 44 compared with k 11 in situ 95 5 4 with a paleopole at 64 4 n 321 0 e k 24 compared with k 5 in situ a95 7 5 when compared with the cratonic reference field for mid cretaceous time this pretilting magnetization indicates a clockwise rotation of the study area of 66 3 11 8 p 0 05 and an apparent displacement from the south of 15 5 7 1 1725 790 km when these and other data previously obtained from cretaceous rocks of the pacific northwest are compiled they show an increase in apparent displacement from east to west across the cordillera this result is discussed in terms of three hypotheses 1 no translation but systematic tilts of about 30 to the southwest 2 translation of a substantial part of the pacific northwest referred to as baja british columbia and is defined more exactly in the text by variable amounts up to more than 2000 km from the south together with clockwise rotations and 3 lesser translation of baja british columbia by about 1500 km combined with variable tilting predominantly to the south it is concluded that paleomagnetic data as they are presently known are consistent with hypotheses 2 and 3 but unless serious sources of error are present are not consistent with hypothesis 1 the most likely candidate for such errors is the uncertainty in estimating initial dip of beds paleoslope in volcanic rock units

geology of the western flank of the coast mountains between cape fanshaw and taku inlet southeastern alaska the western flank of the coast mountains batholith between cape fanshaw and taku inlet is underlain primarily by jura cretaceous strata of the gravina belt pre permian permian and triassic strata of the taku terrane and mid proterozoic to upper paleozoic rocks of continental margin affinity the continental margin rocks include mid proterozoic to lower paleozoic quartzite and marble of the tracy arm assemblage mid paleozoic metavolcanic and subordinate metasedimentary rocks of the endicott arm assemblage and upper paleozoic quartz rich metaturbidites and metaconglomerate of the port houghton assemblage we suggest that these three assemblages are correlative with components of the yukon tanana terrane which underlies a large region of yukon and eastern alaska rocks of the gravina belt taku terrane and yukon tanana terrane are juxtaposed along west vergent thrust faults of mid cretaceous age and are internally deformed and disrupted along latest cretaceous to early eocene dip slip and possibly strike slip shear zones these rocks and structures together with mid cretaceous to eocene plutons of the coast mountains batholith separate rocks of the alexander wrangellia terrane to the west from rocks of the stikine terrane to the east mid cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the alexander wrangellia terrane from central alaska to northwestern washington the continental margin rocks in the northern coast mountains may be part of the in situ north american margin that has been overthrust by the stikine and adjacent terranes alternatively these rocks may have 1 rifted from and then returned to the north american margin 2 moved 800 km along left lateral or right lateral faults from elsewhere along the north american margin or 3 originated near a continent other than north america

tilt and northward offset of cordilleran batholiths resolved using igneous barometry considerable controversy surrounds the suggestion based on palaeomagnetic evidence that large segments of the north american cordillera travelled long distances parallel to the coast during the latest cretaceous and early tertiary periods well after the amalgamation of exotic terranes discordant palaeomagnetic data from mid cretaceous plutonic rocks of the peninsular ranges batholith of coastal southern california and baja and the mount stuart batholith of the cascade range in washington play a pivotal role in the controversy the discordant data were originally interpreted to reflect northward transport of 1 000 km relative to cratonal north america after the batholiths cooled through their magnetic blocking temperatures1 5 more recently it has been argued that the discordances arise from local tilting of batholiths rather than northward offset6 7 here we present and implement new methods based on hornblende barometry8 for determining the palaeohorizontal in granitic batholiths and correcting palaeomagnetic data for tilting our results indicate that the peninsular ranges and mount stuart batholiths have undergone northward offsets of 1 000 450 and 2 900 700 km respectively and also significant tilting

high pressure metamorphism in the western cordillera of north america the skagit gneiss a major component of the crystalline core of the north cascades was metamorphosed during a mid cretaceous to early tertiary high p event driven by the collision of the insular and intermontane superterranes maximum pressures recorded by metapelitic rocks are 8 10 kbar at 650 725 c high pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the skagit gneiss and adjacent cascade river schist mineral reactions continued during nearly isothermal decompression from 8 10 kbar to c 3 5 kbar early high p minerals e g kyanite are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass skeletal relics of kyanite are also present in the groundmass of lower grade staurolite bearing schists that contain texturally later cordierite this matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature these results represent a revision of the metamorphic history of the skagit gneiss which was formerly thought to have experienced only relatively low p barrovian metamorphism qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression geobarometric results for the skagit gneiss are interpreted as evidence for major crustal thickening in the north cascades recognition that pressures of c 9 kbar were attained supports a contractional model for north cascades orogenesis and requires that tectonic syntheses account for the burial of the skagit gneiss protoliths to a depth of c 25 30 km

mid cretaceous strike slip and contractional fault zones in the western intermontane terrane washington and their relation to the north cascades southeastern coast belt orogen structural and geochronologic data from the 111 114 ma okanogan range batholith in north central washington are used to characterize the timing and style of deformation during the early stages of the mid to late cretaceous north cascades southeastern coast belt nc secb orogen the pasayten fault zone bounds jurassic cretaceous sedimentary and volcanic rocks of the methow basin to the west against predominantly mesozoic igneous rocks of the intermontane terrane to the east the pasayten fault zone accommodated intrusion of the western units of the okanogan range batholith during high angle slip then approximately 20 km of left lateral strike slip at 109 95 ma south of the pasayten fault zone the red shirt and methow river thrust zones accommodated 1 west vergent contraction at amphibolite grade that began by 113 ma and ended by 112 ma based on new u pb zircon dates for deformed and undeformed granitoids and 2 northwest vergent contraction in the methow river thrust zone followed by west vergent contraction in the red shirt thrust zone both at greenschist grade prior to 104 ma these contractional displacements thrust the methow basin 10 20 km southeastward beneath the western okanogan range batholith deformation in the nc secb thus began by earliest albian time earlier than previously thought and was characterized by coeval sinistral transcurrent and west vergent contractional faulting sinistral strike slip in the pasayten fault zone may have reflected the pre 100 ma sense of oblique plate convergence or relatively minor southward tectonic escape of the adjacent methow basin during contractional deformation the slip history of the pasayten fault zone is undocumented between approximately 80 ma and 60 ma when on the basis of paleomagnetic data some workers have proposed that the methow basin moved 1700 km northward to its present position relative to the intermontane terrane however significant late cretaceous to early eocene dextral slip on the pasayten fault zone is unlikely because the western intrusive units of the okanogan range batholith continue without disruption across its southeastward trend

gis based mineral prospectivity mapping using machine learning methods a case study from tongling ore district eastern china abstract predictive modelling of mineral prospectivity using gis is a valid and progressively more accepted tool for delineating reproducible mineral exploration targets in this study machine learning methods including support vector machine svm artificial neural networks ann and random forest rf were employed to conduct gis based mineral prospectivity mapping of the tongling ore district eastern china the mineral systems approach was used to translate our understanding of the skarn cu mineral system into mappable exploration criteria resulting in 12 predictor maps that represent source transport physical trap and chemical deposition processes critical for ore formation predictive svm ann and rf models were trained by way of predictor maps and corroborated using a 10 fold cross validation the overall performance of the resulting predictive models was assessed in both training and test datasets using a confusion matrix set of statistical measurements receiver operating characteristic curve and success rate curve the assessment results indicate that the three machine learning models presented in this study achieved satisfactory performance levels characterized by high predictive accuracy in addition all models exhibited well interpretability that provided consistent ranking information about the relative importance of the evidential features contributing to the final predictions in comparison the rf model outperformed the svm and ann models having achieved greater consistency with respect to variations in the model parameters and better predictive accuracy importantly the rf model exhibited the highest predictive efficiency capturing most of the known deposits within the smallest prospective tracts the above results suggest that the rf model is the most appropriate model for cu potential mapping in the tongling ore district and therefore was used to generate a prospectivity map containing very high high moderate and low potential areas in support of follow up exploration the prospective areas delineated in this map occupy 13 97 of the study area and capture 80 95 of the known deposits the fact that two newly discovered deposits occur within the prospective areas predicted by the prospectivity model indicates that the model is robust and effective regarding exploration target generation

geometry evolution and tectonic framework of the skeena fold belt north central british columbia the intermontane belt of the canadian cordillera has long been viewed as a passive relatively rigid block between two metamorphic plutonic belts the coast plutonic complex and the omineca belt however the skeena fold belt which spans most of the width of the northern intermontane belt exhibits shortening comparable with that in the rocky mountain fold and thrust belt the skeena fold belt has many features of thin skinned fold and thrust belts such as thrust faults which sole into a detachment a wide variety of structural styles which depend on rock type a foreland basin which was cannibalized by continued deformation a frontal triangle zone and a hinterland of metamorphic and plutonic rocks coast plutonic complex the skeena fold belt thus is comparable with the rocky mountain fold and thrust belt but rather than deforming a continental terrace wedge it developed in a terrane stikinia which had accreted to north america in the early mesozoic and in jurassic and cretaceous clastic successions bowser lake and sustut groups which overlie stikinia structural and stratigraphic relationships show that the earliest deformation occurred between oxfordian and albian time and that the last folds developed in latest cretaceous or early tertiary time as much as 160 km of northeastward shortening in the skeena fold belt was broadly contemporaneous with crustal thickening in the coast plutonic complex and omineca belt with dextral strike slip faulting east of the skeena fold belt and with shortening in the rocky mountain fold and thrust belt therefore between latest jurassic and early tertiary times horizontal shortening occured across most of the width of the northern canadian cordillera concurrent shortening across the cordillera suggests that a common detachment or detachments fed all of these zones as far east as the rocky mountain fold and thrust belt

mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group the age relations geochemistry and sedimentology of the rocky ridge formation of the skeena group are used to test competing tectonic reconstructions for the mid cretaceous canadian cordillera as well as the timing and location of the accretion of the insular superterrane pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the bowser basin in the early albian to early cenomanian single crystal fusion and step heating 40ar 39ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded middle cenomanian ages of 94 3 0 4 95 6 1 6 and 95 0 1 6 ma vesicular basalt flows interbedded with crystal rich tuff breccias contain evidence for hot emplacement as pyroclastic flows individual eruptive centers are identified by their proximal facies paleoflow indicators within the lava flows paleoflow indicators within interbedded volcaniclastic fluvial deposits geochemical differences and geographic isolation of volcanic deposits major and trace element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics the basalts of the northern rocky ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative nb ta anomalies whereas the basalts of the southern tahtsa lake volcanic center show depletion to slight enrichment of light rare earth elements slight enrichment of large ion lithophile elements with minimal negative nb ta anomalies the geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension the presence of deeper marine facies to the west and the lack of a western sediment source in the skeena group indicate that the technically active insular superterrane was not west of the study area during mid cretaceous time thus we reconsider the omineca belt as the main axis of a mid cretaceous continental arc placing the intermontane superterrane in the intraarc to forearc position with the rocky ridge volcanics erupted along the forearc side of the omineca arc coeval regional strike slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for rocky ridge intraarc extension an electronic supplement of tables a1 a2 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 95tc03496 mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group kari n bassett and karen l kleinspehn diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

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

comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward

mineral potential mapping with a restricted boltzmann machine abstract a restricted boltzmann machine can be trained to encode and reconstruct training samples from a training sample population with an unknown complex probability distribution small probability samples can be differentiated from the training sample population due to their comparatively larger encoded and reconstructed errors in mineral potential mapping mineral potential areas usually take up only a small proportion of the whole mineral exploration region and have specific evidence map patterns assume that the whole mineral exploration region is divided into uniform cells and a restricted boltzmann machine is then trained on all the cells as a small proportion of the total training cells mineral potential cells have much larger encoded and reconstructed errors compared to non mineral potential cells based on the encoded and reconstructed errors of the training cells asc and ase are thus defined as two mineral potential indicators for mineral potential mapping with the trained restricted boltzmann machine the altay district in northern xinjiang in china which is linked with a complex geological setting is chosen as a case study area restricted boltzmann machines with 12 visible units and differing hidden units are constructed and trained on 9582 training cells in the study area the asc ase posterior probability and deposit bearing probability are used to predict the mineral potential of each grid cell in the study area the auc area under the curve a measure of aggregated classification performance in machine learning is applied to measure the mineral potential mapping performance of these four mineral potential indicators the results show that the four mineral potential indicators perform similarly well therefore a restricted boltzmann machine can be trained to map mineral potentials and the asc and ase are two feasible indicators for the mineral potential mapping with the trained restricted boltzmann machine

metamorphism and plutonism in the central part of the okanogan range washington this report is concerned with the bedrock geology of a 1 000 km 2 area in the central part of the okanogan range in north central washington most of the rocks underlying this area were formed under conditions of high grade amphibolite facies metamorphism and or plutonism the metamorphism accompanied by metasomatic granitization and anatexis occurred between late triassic and late early cretaceous time plutonic rocks in the area are grouped into three temporal suites 1 an early premetamorphic suite represented by inclusions of mafic to granitic basement rock in gneisses of a trondhjemitic batholith which also is believed to have served as basement to supracrustal rocks deposited in a late paleozoic early mesozoic geosyncline 2 a synmetamorphic suite of gneisses including both concordant and discordant intrusions and 3 a post metamorphic suite of massive rocks which generally are discordant the igneous rocks range in composition from peridotitic to granitic trondhjemites and granodiorites are most abundant many of these units are somewhat leucocratic for their respective compositions younger intrusive rocks are more felsic than older ones after the plutonic rocks had been deroofed probably in early tertiary time the eastern part of the area was covered by quartz keratophyre flows these flows widely scattered diatremes and other hypabyssal intrusives are the youngest bedrock elements structures generated during metamorphism and plutonism generally trend northwest and indicate lateral compression some younger middle and late tertiary structures followed the established northwest grain but most formed athwart it these structures represented a basin and range type block faulting episode in the region a period of presumed lateral extension the strike of these fault block structures seldom varies more than a few degrees from north

kinematic inversion for the 2 d horizontal and vertical qp wave velocities and depths to interfaces applied to the tact seismic profile southern alaska summary we propose a quasi compressional wave qp wave traveltime inversion technique for determination of the 2 d horizontal and vertical qp wave velocities and depths to interfaces from seismic reflectiodrefraction data sets synthetic test model studies demonstrate that the method is effective if ray path coverage within the model is adequate and indicate that horizontal qp wave velocities and depths to interfaces are better resolved than vertical qp wave velocities we applied our inversion technique to the trans alaska crustal transect tact profile southern alaska between sp19 and sp11 the results of the inversion suggest that the upper 5 km of the crust is anisotropic the range of anisotropy for the derived model is between 10 and 25 per cent consistent with the range determined in previous studies and from laboratory measurements the cause of anisotropy is thought to be preferred mineral orientations in the foliation planes of the metasedimentary rocks based on the test model studies and the applications to actual seismic data sets it is suggested that future traveltime inversion studies will require particularly high quality data with dense ray path coverage in which multiple sources and receivers and both reflection and refraction data are utilized

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

artificial neural networks applied to mineral potential mapping for copper gold mineralizations in the carajas mineral province brazil motivated by recent successful results of artificial neural network modelling in a variety of problems related to the geosciences we have applied a radial basis functional link net to a regional scale mapping of the potential for cu au mineralizations in the carajas mineral province northern brazil to derive the input feature vectors we have used geological and both radiometric and magnetic geophysical data a k fold cross validation method was employed in order to tune the parameters of the network and to select the best radial basis functional link net model amongst several others subsets of the available data set were used for training and validation and the estimated overall accuracy of the selected model is 91 7 the plotting of a cumulative area versus favourability curve allowed us to define favourability zones of occurrences of cu au mineralizations and to assess the efficiency and the predictive power of the model a binary map showing high and low favourability sectors was produced for the study area as an end product that can be used to guide and support more detailed exploration efforts our results show that 4 18 of the study area has an extremely high potential to contain cu au mineralizations especially those of iron oxide cu au type which are related to volcanic rocks and hydrothermal alteration

jurassic and cretaceous plutonic and structural styles of the eagle plutonic complex southwestern british columbia and their regional significance the eagle plutonic complex is an elongate north northwest trending body of deformed middle to late jurassic and middle cretaceous rocks which underlies the southwestern margin of the intermontane terrane new mapping of the complex and its country rocks in concert with geochronometry has defined episodes of contractional ductile deformation in the middle to late jurassic and middle cretaceous as well as brittle deformation in tertiary time synkinematic middle to late jurassic eagle tonalite at the eastern margin of the eagle complex intrudes mylonitic nicola group rocks and structurally overlies them along a southwest dipping belt of high strain eagle shear zone with a structural thickness of 1 km and a strike length of 100 km in the central and western eagle complex eagle tonalite grades into tonalite orthogneiss eagle gneiss and both are crosscut by mid cretaceous muscovite bearing plutons of the fallslake plutonic suite fallslake suite rocks are themselves ductilely deformed along the

orogen parallel extension in the north cascades crystalline core washington metamorphic stretching lineations throughout the crystalline core of the north cascades washington show a strong preferred orientation in a horizontal nw se direction foliations strike nw and dip steeply preliminary observations of shear sense features indicate noncoaxial right lateral motion we interpret these observations to indicate that the crystalline core was a broad right lateral shear zone the ages of the fabrics as recorded by metamorphic minerals and syntectonic plutons range from pre 85 ma in the south to post 45 ma in the north thus the shear regime was active during an extended interval of metamorphism and plutonism in the orogen a similar kinematic pattern is suspected for at least part of the coast plutonic complex british columbia which lies on strike with the crystalline core to the northwest

evidence for mid cretaceous wrench faulting in the methow basin washington tectonostratigraphic setting of the virginian ridge formation the cretaceous methow basin of north central washington is the southernmost of a series of mesozoic successor basins in the canadian cordillera the albian campanian sedimentary sequence there documents a restricted asymmetric basin filled by a west derived laterally amalgamated fan delta system the virginian ridge formation and an east derived stable fluvial deltaic system the winthrop formation followed by a locally derived volcanic sequence the midnight peak formation this study focuses on the depositional and tectonic setting of the albian cenomanian virginian ridge formation the mid cretaceous methow basin is interpreted as a wrench fault basin generated in response to transpressive dextral offset and uplift of a highland to the west stratigraphy and sedimentology of the albian cenomanian methow sequence indicates that 1 the basin was asymmetric normal to its axis in that the western margin was a mobile zone generating considerable relief while the eastern margin was passive stable and of relatively low relief 2 the basin was asymmetric along its axis documenting transpression and dextral offset along the western margin 3 the relatively short and steep fan delta system deposited along the western margin was laterally amalgamated and possibly time transgressive to the north this basin and similar coeval successor basins to the north may record extensive transtensile transpressive mobility along the cordillera in middle to late cretaceous time

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

reply to comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look by e irving et al

evaluation of uncertainty in mineral prospectivity mapping due to missing evidence a case study with skarn type fe deposits in southwestern fujian province china in this paper the southwestern fujian metallogenic belt one of the important fe polymetallic belts in southern china was chosen as a case study area to evaluate the uncertainty due to missing evidence in mineral prospectivity mapping four geological features were considered important for mineral prospectivity mapping based on the geological model for skarn deposits in the belt 1 the yanshanian granitic intrusions because they provided energy fluids and part of the metal budget 2 faults because they served as pathways for hydrothermal fluids 3 the carboniferous permian carbonate sequences because they supplied part of the metal budget and served as host rocks for ore deposition and 4 geochemical anomalies because they indicate presence of skarn type alteration and mineralization four separate predictive evidence layers depicting each the above geological features were extracted from relevant geoscience data and were integrated using a modified fuzzy weights of evidence model which retains the advantages of both fuzzy weights of evidence and logistic regression modeling the results show that most of the known fe deposits occur in or near and thus have strong correlation with areas of high posterior probabilities as indicated by the delineated prospective areas that occupy only 4 6 of the studied region but contain 88 5 of the total number of known fe deposits different combinations of predictive evidence layers were used to examine prediction uncertainty related to missing evidence plots of accumulative number of fe deposits versus accumulative prospective areas show that the order of the most to the least important evidence of mineral prospectivity in the studied region is favorable rock types geochemical anomalies intrusive rocks faults results of blind testing the prospectivity models indicate that the geological model of fe mineralization in the belt is suitable for predicting exploration target areas for skarn type fe deposits and suggest that the highest risk in follow up exploration is related to sequences of carbonates in the carboniferous permian formations that were likely either not mapped or not represented at the present regional scale of mapping

geochronometry of the eagle plutonic complex and the coquihalla area southwestern british columbia new u pb k ar and rb sr dates from the eagle plutonic complex and adjacent map units place timing constraints on intrusive and deformational events along the southwestern margin of the intermontane belt u pb zircon minimum dates for eagle tonalite and gneiss 148 6 156 4 and 157 4 ma document previously unrecognized middle to late jurassic magmatism and syn intrusive deformation along the eastern margin of the eagle plutonic complex and the southwestern margin of the intermontane terrane widespread mid cretaceous albian cenomanian resetting of k ar and rb sr isotopic systematics in jurassic and older rocks is coeval and cogenetic with emplacement of plutons of the fallslake plutonic suite 110 5 2 ma u pb which crosscut jurassic plutons and structures but were themselves ductilely deformed along the pasayten fault during sinistral east side up reverse displacement k ar and rb sr cooling dates for the fallslake suite of ca 100 ma including dates from mylonites along the pasayten fau

cretaceous sedimentation and tectonics tyaughton methow basin southwestern british columbia the mesozoic tyaughton methow basin straddles the fraser yalakom pasayten straight creek fypsc strike slip fault zone between six tectono stratigraphic terranes in southwestern british columbia data from hauterivian cenomanian basin fill provide constraints for reconstruction of fault displacement and paleogeography the early cretaceous eastern margin of the basin was a region of uplifted jurassic plutons and active intermediate volcanism detritus shed southwestward from that margin was deposited as the marine jackass mountain group albian inner to mid fan facies of the jackass mountain group can be correlated across the yalakom fault suggesting 150 25 km of post albian dextral offset deposits of the jackass mountain group overlap the major strike slip zone fypsc if that zone represents the eastern boundary of the tectono stratigraphic terrane wrangellia then accretion of wrangellia to terranes to the east occurred before late early cretaceous time the western margin of the basin first b

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

possible pre cenozoic left lateral slip on the yalakom fault southwestern british columbia the yalakom fault strikes northwest for nearly 250 km in southern british columbia immediately north of the town of lillooet between the yalakom and fraser faults structures within jurassic and cretaceous clastic rocks suggest that the yalakom fault had a history of left lateral followed by high angle contractional motion north and northeast trending folds offset by west northwest trending left lateral and normal faults plus numerous small conjugate left and right lateral faults with associated extension fractures fit a strain ellipse for left lateral slip on the yalakom fault younger right lateral bedding plane faults a large vertically plunging open fold and a thrust fault adjacent to the yalakom fault zone suggest later high angle contractional motion left lateral motion preceded whereas contractional motion probably coincided with 50 ma fraser fault deformation to the northwest along the yalakom fault and on the pasayten fault in washington left lateral slip possibly occurred between about 100 and 95 ma left lateral motion may prove an important element in the pre tertiary history of southern british columbia

the mineralization age of the makeng fe deposit south china implications from u pb and sm nd geochronology the makeng fe deposit is located in the southwestern fujian district south china the sm nd isochron ages of seven samples of pure garnet and five of pure magnetite separates from the makeng ores yielded an isochron age of 157 15 ma laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating of the nearby exposed the dayang juzhou dj porphyritic biotite granite and fine grained syenogranite yielded 206pb 238u ages of 140 2 1 1 and 140 1 1 0 ma respectively these results suggest that the intrusion of the dj granite and the makeng skarn alterations and fe mineralization are contemporaneous the dj granite exhibits geochemical characteristics of a type granites including high values of na2o k2o 8 13 8 92 wt feot mgo 3 4 21 5 and ga al 2 64 3 45 10 4 and low al2o3 10 71 13 29 wt value chondrite normalized rare earth element patterns are characterized by obviously negative eu anomalies eu 0 02 0 28 and primitive mantle normalized spidergrams show the enrichment in high field strength element and depleting in sr ti ba and eu the geochemical characteristics of dj granite suggest that the granite was derived from partial melting of the paleoproterozoic metasedimentary rocks of the cathaysia basement and some underplating of mafic magma in the lower tholeiitic crust and or depleted mantle might be involved and provide the heat source for the partial melting the dj granite also fits the spatiotemporal distribution of the jurassic cretaceous coastward migration of both extensional and arc related magmatism and fills the a type granites gap in the early stage of the early cretaceous 145 125 ma therefore it is suggested that the late jurassic and early cretaceous magmatism in southwestern fujian district were generated in an extensional environment responding to the slab rollback and concomitant retreating arc system of the paleo pacific plate within the south china block and the fe metallogeny in southwestern fujian district is genetically linked with the magmatism during this period

large scale transpressive shear zone patterns and displacements within magmatic arcs the coast plutonic complex british columbia the coast plutonic complex is the largest magmatic arc of the north american cordillera extending from northwestern washington state to eastern alaska it forms the transition between two tectonic domains that are suspected to have undergone several phases of large several thousands of kilometers orogen parallel displacement during the mesozoic and early cenozoic a compilation of fabric data published isotopic ages and new structural observations shows that the western coast plutonic complex was affected by subvertical orogen parallel crustal scale shear zones these shear zones mainly reflect sinistral transpression and were sequentially active from 110 to 87 ma during the intrusion of voluminous batholiths sinistral shearing was roughly coeval with the development of the thrust belts flanking the coast plutonic complex between 101 and 85 ma suggesting plate scale transpression was a first order process in the construction of the coast mountains orogen these shear zones separate panels with distinct plutonic and cooling histories suggesting the sinistral displacements between crustal blocks were large greater than tens to hundreds of kilometers this transpressive shear system likely reflects the jurassic to early late cretaceous migration of outboard cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models this example from the coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold and thrust belts our analysis further shows that the 2000 km long late cretaceous to early tertiary coast shear zone has a minimum extent toward the south to at least 51 30 n

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