relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

structural characteristics of the lake van basin eastern turkey from high resolution seismic reflection profiles and multibeam echosounder data geologic and tectonic implications the structural evolution of lake van basin eastern turkey was reconstructed based on seismic reflection profiles through the sedimentary fill as well as from newly acquired multibeam echosounder data the major sub basins tatvan basin and northern basin of lake van bound by ne trending faults with normal components formed during the past 600 ka probably due to extensional tectonics resulting from lithospheric thinning and mantle upwelling related to the westward escape of anatolia rapid extension and subsidence during early lake formation led to the opening of the two sub basins two major still active volcanoes nemrut and suphan grew close to the lake basins approximately synchronously their explosive deposits making up 20 of the drilled upper 220 m of the ca 550 m thick sedimentary fill during basin development extension and subsidence alternated with compressional periods particularly between 340 and 290 ka and sometime before 14 ka when normal fault movements reversed and gentle anticlines formed as a result of inversion the 14 ka event was accompanied by widespread uplift and erosion along the northeastern margin of the lake and substantial erosion took place on the crests of the folds a series of closely spaced eruptions of suphan volcano occurred synchronously suggesting a causal relationship compression is still prevalent inside and around lake van as evidenced by recent faults offsetting the lake floor and by recent devastating earthquakes along their onshore continuations new high resolution bathymetry data from lake van reveal the morphology of the northern ridge and provide strong evidence for ongoing transpression on a dextral strike slip fault as documented by the occurrence of several pop up structures along the ridge

new u pb and ar ar isotopic age constraints on the timing of eocene magmatism fort fraser and nechako river map areas central british columbia twenty three new precise eocene u pb and 40 ar 39 ar age determinations for calc alkaline volcanic rocks of the ootsa lake group and associated intrusive rocks widespread in the nechako plateau in central british columbia constrain the timing of the eocene magmatism to 53 2 47 6 ma with a local duration of as little as 2 3 million years the new dates show that magmatism in the study area is partly coeval with that responsible for the babine igneous suite newman volcanic suite 53 50 ma to the north and for the endako group 51 45 ma which overlies the ootsa lake group however locally the three magmatic suites are distinct in age and or the strata record a magmatic hiatus of as much as 7 5 million years the ages generally young from north to south 52 47 ma along the western portion of the study area the babine igneous suite newman volcanic suite represents the oldest member of this series however in the east the ootsa lake group volcanic rocks are generally older 53 51 ma the anomalously older ages may be related to the interaction of magmatism and formation of a nearby and coeval core complex which ongoing studies show was uplifted at about the same time during the eocene felsic plutonism associated with the ootsa lake group occurred between 50 5 and 47 3 ma these plutons were emplaced in an extensional setting along north northeast trending faults the new dates stratigraphic relationships and suggested correlations of eocene strata in the study area with that to the west and north require a revision of the stratigraphic nomenclature for the ootsa lake and endako groups 696

eocene melting of precambrian lithospheric mantle analcime bearing volcanic rocks from the challis kamloops belt of south central british columbia abstract potassic silica undersaturated mafic volcanic rocks form a minor portion of the predominantly calc alkaline eocene challis kamloops volcanic belt which extends from the northwestern united states into central british columbia canada their major occurrence is in the penticton group in south central british columbia where they reach a thickness of up to 500 m and form the northwestern edge of the montana alkaline province these analcime bearing rocks 53 52 ma old are typically rhomb porphyries of ternary feldspar an28ab52or20 additional phenocryst phases include clinopyroxene analcime phlogopite and rare olivine the rocks are characterized by high total alkalis particularly k2o 4 5 wt as well as by a distinct enrichment of large ion lithophile elements versus heavy rare earth elements and high field strength elements they have unusual isotopic compositions compared to most other rocks of the challis kamloops belt particularly high negative nd values and elevated but relatively uniform initial 87sr 86sr ratios 0 7065 the potassic silica undersaturated rocks overlie precambrian crust and lithosphere and were at least in part derived from ancient metasomatized subcontinental mantle lithosphere which was modified in a precambrian subduction setting the alkaline rocks of the challis kamloops belt are related to a slab window environment in particular they were formed above the southern edge of the kula plate adjacent to the kula farallon slab window whereas the montana alkaline province situated well to the southeast was formed directly above the kula farallon slab window upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle

tertiary calc alkaline magmatism associated with lithospheric extension in the pacific northwest the long standing assumption that calc alkaline magmatism requires contemporaneous subduction is critically reviewed two of many widely spread fields of tertiary calc alkaline magmatism in the pacific northwest one eocene and one miocene provide clear examples of the association between calc alkaline magmatism and lithospheric extension we suggest that the calc alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle enriched by previous subduction events than by contemporaneous eocene subduction this model proposes that extension will cause calc alkaline dominated magmatism in areas of continental crust with a history of previous subduction as along the western margin of north america currently popular models for the tectonic magmatic evolution of the basin and range province in which subduction driven calc alkaline magmatism is followed by extension related bimodal magmatism may need modification much more of this magmatism may result from extension in the pacific northwest we suggest that subduction beneath the idaho batholith ceased at the end of the cretaceous blocked by the accreted terranes to be reactivated in its present position beneath the cascades during the eocene the various isolated eocene volcanic fields spread across the pacific northwest were due to lithospheric extension not to the presence of a subducting plate immediately below

three dimensional crustal structure of the eastern cordillera southwestern canada and northwestern united states lithoprobe and cocorp crustal seismic reflection profiles across the southwestern foreland and omineca belts of southern british columbia and the adjacent united states provide regional three dimensional coverage of orogen parallel structural variations in this area north american cratonic basement and cover rocks are deformed into east verging folds and thrusts which increase in scale and depth to the west this simple pattern may be complicated by out of sequence faulting in the upper and middle crust of the kootenay arc and purcell anticlinorium in southern british columbia the southern omineca belt is the structural and metamorphic hinterland of the foreland belt and contains the monashee priest river crustal scale tectonic ramp which records a protracted history of east directed contraction it is overlain by the eagle river vernon colville crustal scale antiform of probable late cretaceous to paleocene age which locally may have formed above blind thrusts an embayment in the autochthonous craton between the monashee and priest river ramps corresponds spatially with the kootenay arc structural salient and eastward deflection of the lower paleozoic shelf to basin facies transition the south flank of the monashee and valhalla complexes and the north flank of the purcell anticlinorium near the international border correspond with the lateral margins of this east trending embayment on an even larger scale the increasing northward tectonic overlap of accreted terranes onto thick cratonic crust from the northern united states into british columbia and associated basement thickening coincide spatially with the northward increased width and depth of the early tertiary foreland basin in this part of the cordillera eocene crustal extension did not completely mask older contractional fabrics and appears to have been accomplished by two mechanisms 1 in the east by crust penetrating shear zones that separate the omineca and foreland belts and 2 in the west by crustal scale boudinage within and west of the omineca belt

the origin of mount st helens andesites abstract mount st helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants the mafic lavas include a transitional olivine plagioclase basalts with low k 2 o and incompatible trace element abundances b calc alkaline olivine plagioclase clinopyroxene basalts enriched in k 2 o tio 2 and incompatible trace elements and c calc alkaline olivine plagioclase basaltic andesites with incompatible trace element contents transitional between the two basalt types intermediate lavas include a tholeiitic two pyroxene andesites b calc alkaline plagioclase two pyroxenes olivine amphibole mafic andesites 56 59 wt sio 2 and c calc alkaline plagioclase two pyroxenes amphibole high silica andesites 61 62 wt sio 2 eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches it is unlikely that any of the andesites or dacites are derived by fractional crystallization of the recognized basaltic variants formation of the andesites simply by contamination or assimilation fractional crystallization of basaltic magma is also improbable more plausibly the andesites represent mixing between basaltic and dacitic end member magmas each of which may be somewhat heterogeneous or vary in composition with time in this model efficient mixing must occur in some parts of the magma plumbing system while some conduits or storage reservoirs must be effectively isolated

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

dusty and honeycomb plagioclase indicators of processes in the uchino stratified magma chamber izu peninsula japan abstract two types of melt inclusion bearing plagioclase coexist in the uchino basalt izu peninsula japan 1 dusty plagioclase with an ab rich core mantled by a dusty zone consisting of more an rich plagioclase and fine melt inclusions and 2 honeycomb plagioclase with a central core containing larger melt inclusions the dusty plagioclase probably forms through partial dissolution of ab rich plagioclase as dusty zones truncate the concentric zonal structure of the ab rich inner core the honeycomb plagioclase is formed by skeletal growth under supercooling conditions this is indicated by the euhedral geometry of the boundary between the inner inclusion free and the outer inclusion bearing zones of the honeycomb plagioclase which has a concentric inner core bimodal distribution of the chemical compositions of phenocryst cores and assemblages in crystal clots suggest that hybridization of basaltic and andesitic magmas took place before crystallization of phenocryst rims the core rim boundaries of phenocrysts derived from the andesitic magma and the basaltic magma have a rounded and a euhedral geometry respectively this suggests the andesitic magma was heated and the basaltic magma was cooled prior to hybridization these features are consistent with the development of a thermal diffusive interface between overlying andesitic magma and underlying basaltic magma in a stratified magma chamber

eocene sedimentation and volcanism in the fig lake graben southwestern british columbia the fig lake graben is a narrow complex eocene basin that developed along part of the coldwater fault system in southwestern british columbia its origin as a pull apart basin is probably related to dextral wrench faulting along the fraser fault and low angle normal faulting of the okanagan shear zone within the graben are kamloops group volcanic and sedimentary rocks the thickness of which implies that one fault block has been downthrown at least 4 5 km geochemical interpretation of previously published analyses of kamloops group volcanic rocks indicates that magma production was genetically related to both extension and subduction

middle eocene dike swarms and their relation to contemporaneous plutonism volcanism core complex mylonitization and graben subsidence okanogan highlands washington sheeted porphyry dike swarms are concentrated along and parallel to the margins of the republic and keller grabens in northeast washington they were emplaced during graben subsidence and in part during mylonitization of the kettle and okanogan domes core complexes above the transition into the brittle deformation zone during eocene regional extension the swarms consist of older high k biotite rhyolite and younger high k hornblende dacite and andesite dikes the biotite rhyolite is correlative with the basal o brien creek formation now exposed in the grabens but has no documented plutonic equivalent in the adjacent colville batholith the hornblende dacite and andesite dikes scatter creek formation are divided into 1 a low zr low k sub 2 o type that fed the lower part of the sanpoil volcanics and 2 a high zr hig k sub 2 o type that fed the upper part of the sanpoil volcanics respectively the scatter creek formation dikes are the hypabyssal equivalents of both the devils elbow intrusive suite and the earlier phases of the herron creek intrusive suite in the colville batholith the occurrencd of the dike swarms both within and adjacent to grabens filled with comagmatic volcanic rocks demonstrates that the graben fill was deposited in situ during regional extension and ismore not an allochthonous fault block transported to its present location during regional compression less

geology geochemistry and isotopic character of the colville igneous complex northeastern washington the colville igneous complex in northeastern washington comprises paleocene orthogneisses and leucocratic granitic rocks and eocene volcanic plutonic suites which were emplaced during early tertiary extension the colville igneous complex is divided into three components 1 65 4 ma orthogneisses of the okanogan dome or metamorphic complex and similar gneisses in the kettle and lincoln complexes 2 61 51 ma leucocratic granitic rocks and 3 53 47 ma intermediate to felsic volcanic plutonic rocks the leucocratic granites and younger volcanic plutonic rocks are divided on the basis of age geochemical and isotopic characteristics and emplacement mechanism the older leucocratic granites are silica rich corundum normative and sr and light rare earth element ree enriched and they form several large discrete compositionally zoned plutons and intrusive suites the younger eocene volcanic plutonic rocks can be divided into two suites an older compositionally restricted 68 72wt sio2 sr enriched medium k suite and a younger intermediate to felsic high k calcalkalic suite sr isotopic measurements of volcanic and plutonic rock in the colville igneous complex yield variable initial 87sr 86sr ratios sri ranging from about 0 704 to values greater than 0 710 in general the volcanic rocks are especially variable with little or no correlation between sri and geographic location the leucocratic granites however show a systematic west to east increase across the area which probably reflects an increased involvement with precambrian crust the variable nature of sri in the colville igneous complex may also be controlled by crustal thickness and or depth of melting within individual volcanic and plutonic suites there is a crude correlation of mean sri with age there is a systematic decrease in sri with increasing age which may reflect either a migration of the site s of melting or a decreased upper crustal residence time possibly related to the onset of extension the leucocratic granites may have formed by small degrees of partial melting in the lower crust perhaps facilitated by thermal blanketing of an initially overthickened crust the ascent of large bodies of leucocratic magmas may have increased the geothermal gradient which allowed for upper more radiogenic crust to be melted

the volcaniclastic series from the luang prabang basin laos a witness of a triassic magmatic arc abstract the paleogeographic evolution of south east asia sea during the early mesozoic is still poorly understood and a number of models have recently been put forward to account for the geodynamic evolution of sea the luang prabang basin north laos located in the core of a paleogeographic jigsaw in sea recorded a long lasting volcanism that spanned for 35 my from the earliest triassic up to late triassic as evidenced by combined stratigraphic and geochronological u pb zircon analyses performed on both volcanic and volcaniclastic series the volcanic rocks are arc tholeiites and calk alkaline andesites to dacites the volcaniclastic rocks contain in part volcaniclasts produced contemporaneously with sedimentation both the volcanic and volcaniclastic series display geochemical features characteristic of a subduction related volcanism therefore the luang prabang basin documents a magmatic arc in a good agreement with the recent recognition of neighboring ophiolitic rocks in the luang prabang area following a passive margin setting that prevailed from the late carboniferous to the late permian an active margin then initiated along the western margin of the indochina block this active magmatic arc developed as the result of an east dipping subduction below the indochina block during most of the triassic at least from ca 250 to 215 ma subsequently this oceanic subduction episode must have been followed by a continental collision of the indochina block with the eastern simao block at a period that remains to be defined

a new hydrothermal scenario for the 2006 lusi eruption indonesia insights from gas geochemistry abstract the 29th of may 2006 gas and mud eruptions suddenly appeared along the watukosek fault in the north east of java indonesia within a few weeks several villages were submerged by boiling mud the most prominent eruption site was named lusi to date november 2011 lusi is still active and a 7 km2 area is covered by the burst mud breccia the mechanisms responsible for this devastating eruption remain elusive while there is consensus about the origin of the erupted mud the source of water is uncertain the origin of the gas is unknown and the trigger of the eruption is still debated in order to shed light on these unknowns we acquired a wide set of data of molecular and isotopic composition of gas sampled in several lusi vents in the surrounding mud volcanoes in the closest natural gas field wunut and in the hydrothermal vents at the neighbouring volcanic complex in the period 2006 2011 the boiling fluids erupted in the crater zone are apparently co2 dominated while colder ch4 dominated and c2 c3 bearing fluids are identified at several sites around the crater zone gas genetic diagrams maturity plots and gas generation modelling suggest that the hydrocarbons are thermogenic 13c1 up to 35 13c2 up to 20 deriving from marine kerogen with maturity of at least 1 5 ro for instance in the 4400 m deep ngimbang source rocks co2 released from the crater and surrounding seeps is also thermogenic 13c from 15 to 24 related to kerogen decarboxylation or thermal ch4 oxidation in deep rocks although three vents just outside the crater showed an apparent inorganic signature 7 5 4000 m magmatic intrusions and hydrothermal fluids responsible for the enhanced heat that altered source rocks and or gas reservoirs the neighbouring magmatic arjuno complex and its fluid pressure system combined with high seismic activity could have played a key role in the lusi genesis and evolution within this new model framework lusi is better understood as a sediment hosted hydrothermal system rather than a mud volcano

magmagenesis at the eocene electric peak sepulcher mountain complex absaroka volcanic province usa abstract igneous rocks exposed on electric peak and sepulcher mountain represent the preserved intrusive and extrusive components of an early eocene calc alkaline eruptive center in the absaroka volcanic province avp of montana and wyoming usa the electric peak stock has an outcrop area of 1 km2 and consists of six discrete phases ranging in composition from quartz diorite to granite representing multiple small intrusions of compositionally distinct magmas lava flows and dikes on sepulcher mountain are basaltic andesitic to dacitic in composition with silicic rocks generally peripheral to mafic rocks important geochemical characteristics of the rocks from both localities include high ni and cr concentrations in andesitic and dacitic rocks lower rare earth element ree concentrations in evolved rocks relative to more mafic rocks and variable sr and nd isotopic ratios although petrographic evidence permissive of magma mixing is limited compositional data suggest that andesitic rocks are hybrids formed by mixing variably fractionated and contaminated mantle derived mafic magmas with diverse composition silicic crustal melts derived from partial melting of amphibolitic lower to mid crustal rocks isotopic and trace element systematics of mafic rocks such as high ratios of large ion lithophile to high field strength elements also suggest that parental magmas were derived from melting of subduction modified lithospheric mantle the significance of these results is that the calc alkaline nature of the center was inherited from crustal processes whereas the arc like trace element signature reflects generation of the magmas in a tectonic environment related to geometrically complex subduction of the farallon plate under the north american continent

subduction style magmatism in a non subduction setting the colville igneous complex ne washington state usa of the klondike mountain formation which show no decoupling the colville igneous complex is located within the eocene magof the lile and hfse i e no subduction signature precluding matic belt of the north american cordilleran interior it straddles the presence of a subduction slab beneath this part of the north the us canadian border in northeast washington and southern american cordillera during the eocene we propose the alternative british columbia the complex consists of three intrusive and two model that the colville igneous complex formed as a result of extrusive phases the first extrusive phase being contemporaneous decompression melting of crust and mantle during post laramide with the latter two intrusive phases as a consequence of suborogenic collapse of an overthickened crust solidus re equilibration in the plutonic rocks this study concentrates on the two extrusive phases the sanpoil volcanic formation and the klondike mountain formation the sanpoil volcanic formation consists of andesites dacites and rare trachyandesites sio2

the colville batholith tertiary plutonism in northeast washington associated with graben and core complex gneiss dome formation the colville batholith in the central okanogan highlands of washington is composed of numerous individual granitoid intrusions plutons most of which may be grouped on the basis of field evidence modal analysis and major element chemical composition into three successive intrusive suites the keller butte suite kbs the devils elbow suite des and the herron creek suite hcs the kbs is distinguished from the younger intrusive suites by 1 modes restricted to the fields of granite and granodiorite 2 lack of horn blende 3 a relatively fe rich afm variation trend 4 an alkali lime index classification of calcic as opposed to calc alkalic and alkali calcic for the younger suites 5 compositions 64 sio2 and 6 low mgo relative to sio2 the des is distinguished from the hcs by 1 its lower modal alkali feldspar and correspondingly low k2o relative to sio2 and 2 a slightly more fe rich afm variation trend biotite and hornblende k ar ages the most believable and geologically significant of which are concordant ages from plutons of the des indicate the batholith is early tertiary in age norms of highly evolved residual aplitic granite suggest that the magmas completed crystallization at 2 to 3 km depth the structural relations of the plutonic rocks suggest that associated core complex and graben structures were active both before and after emplacement of kbs and des plutons and that the graben structures continued to develop until after emplacement of the hcs the structural relations and radiometric ages taken together lead to the conclusion that the batholith core complexes and graben structures are all broadly contemporaneous and chiefly of early tertiary age

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

volatility of se and te during subduction related distillation and the geochemistry of epithermal ores of the western united states subduction partial melting of mantle and magma generation and ultimately magma degassing can lead to the preferential mobilization of volatile elements and compounds research indicates that devolatilization reactions in a subducted slab can cause the overlying mantle to be enriched in volatile ore forming constituents water co2 sulfur metal loid s this process of mantle enrichment plays an important role in subsequent magma generation and initiation of ore genesis furthermore the relative volatility of metal loid s appears to play a role in the ultimate geochemistry of shallow epithermal epizonal hydrothermal ores in the western united states for example there is a general west to east geographic zonation of tertiary ores from se rich to te rich it is proposed that this geographic geochemical zonation is the direct result of the higher geochemical volatility of se as compared to te previously au ag te se hg as sb and tl have been referred to as the epithermal suite of elements it is suggested here that their geochemical cycles are linked by their similar volatile behavior through subduction partial melting in the mantle and subsequent magma degassing processes essentially specific tectonomagmatic events that affected the western united states e g oligocene eocene magmatism in the great basin and southern rocky mountains mid miocene magmatism associated with the inception of the yellowstone hotspot in the pacific northwest were distillation events that drove devolatilization of fertile mantle after the laramide shallow slab event furthermore the metal loid volatility behavior likely occurs in other regions affected by subduction and may play a fundamental role in the metal endowment of these regions

toroda creek half graben northeast washington late stage sedimentary infilling of a synextensional basin the toroda creek half graben is one of several north northeast trending eocene extensional basins associated in time and space with the okanogan and kettle metamorphic core complexes in the southern omineca crystalline belt of washington and british columbia sedimentary rocks of the middle eocene klondike mountain formation are the youngest stratified nonvolcanic rocks in the toroda creek half graben these sedimentary rocks preserve details of the character of toroda creek half graben basin development in a humid temperate setting help constrain the timing of brittle faulting in the basin and provide insight into approximate rates of okanogan core complex uplift facies distributions and provenance determinations indicate that the klondike mountain formation accumulated in a localized basin there is no evidence that the klondike mountain formation strata in the toroda creek half graben were once part of a more widespread deposit that later was segmented by faulting into its present configuration klondike mountain sedimentary strata consist of sedimentary gravity dominated rock avalanche slide and debris flow deposits that accumulated on alluvial fans that delivered their sediments into either a single elongate lake or a series of possibly interconnected lakes provenance determinations and facies distribution patterns suggest that klondike mountain detritus primarily was derived from steep unstable highlands exposed in the footwall of the bodie mountain fault on the east side of the basin sediments at the southern end of the basin were shed from an upland created by a west northwest trending transfer fault granite creek fault rates of footwall uplift and or basin subsidence estimated from unroofing of plutonic and presumed mylonitic rocks suggest rapid and significant basin margin uplift

petrogenesis of the colville igneous complex northeast washington implications for eocene tectonics in the northern u s cordillera eocene igneous complexes of the u s and canadian cordillera have long been thought to represent a broad and somewhat discontinuous subduction related magmatic arc extending from northern british columbia to wyoming this model has become increasingly untenable as knowledge of the structural and tectonic settings of these rocks has expanded new major trace and rare earth element and isotope data presented here virtually preclude the interpretation of a contemporaneous subduction related source for mantle derived magmas in the colville igneous complex of northeast washington state we suggest an alternative model whereby post laramide orogenic collapse resulted in partial melting of a mid crustal source followed by partial melting of a distinctly different lower crustal source the magma from which mixed with magma derived from a lithospheric mantle source the subduction signature and the calc alkalic nature of the magmas was inherited from previous proterozoic subduction events