lithospheric processes and products in the southern canadian cordillera a lithoprobe perspective analyses of lithoprobe and other data from southwestern canada provide new insights on how this portion of the cordillera formed during plate convergence along the western margin of north america crustal rocks are detached from their mantle lithosphere which must have been consumed during subduction detachment occurred at or near the base of the crust beneath the intermontane and or omineca belts probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool mechanically rigid north american craton during the late cretaceous and early tertiary rotation of detached rocks caught between the north american craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western cordillera and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern cordillera during the same time interval transpression assoc

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

last 1000 years of environmental history in southern bucovina romania a high resolution multi proxy lacustrine archive geo ecological and landscape change studies at a local scale are scarce in central eastern europe and particularly in romania however this focus is directly relevant to ecosystems and humans as both are dependent on local environmental changes we perform a high resolution multi proxy analysis of physical and geochemical sediment properties coupled to plant macrofossil pollen and charcoal analysis at iezerul sadovei lake and present the first palaeoenvironmental reconstruction from bucovina mountains the north eastern romania the sedimentary record spanning the last 950 years is laminated throughout and reveals high intensity catchment responses to changes in hydrological conditions and human impact results show major landscape disturbances 950 years ago and during the last 200 years the enhanced catchment instability from the onset of sedimentation to 1400 ce was characterised by landsliding and subsequent erosion and related mainly to natural factors however the most recent since c 1800 ce environmental changes i e deforestation enhanced channel and slope erosion and lake in filling are a consequence of direct human interventions changes in the composition and extent of forest cover were also driven by the combination of natural disturbances climate variability and anthropogenic activities from 1400 ce and more clearly over the last 200 years early successional and disturbed habitat taxa appeared to respond primarily to anthropogenic drivers including forest management the higher proportion of abies alba 30 and fagus sylvatica 20 in the local forest and their dramatic decline was recorded earlier i e 1150 ce and 1400 ce respectively than reported in other parts of the romanian carpathians using this palaeoenvironmental evidence as an analogue projected climate changes i e increased seasonal heavy precipitation and more frequent extreme events superimposed on increasing human pressure are likely to lead to the destabilisation of the catchment through erosion and possible reactivation of landslides

magmatism as a response to exhumation of the priest river complex northern idaho constraints from zircon u pb geochronology and hf isotopes abstract zircon and monazite u pb geochronology and zircon hf isotopes place constraints on the temporal and source relationships between crustal anatexis magmatism and exhumation of the priest river metamorphic core complex northern idaho granitoids that intruded the migmatitic pelitic hauser lake gneiss include the

the newport fault eocene listric normal faulting mylonitization and crustal extension in northeast washington and northwest idaho the newport fault is a spoon shaped shallowly dipping fault zone across which a proterozoic to tertiary sedimentary suprastructure is juxtaposed above an infrastructure of basement gneiss and granitic batholiths as a result of eocene normal faulting and crustal attenuation chloritic microbreccia occurs at the top of the newport fault zone below which a zone of mylonitization as much as 500 m thick is developed in footwall rocks including the eocene silver point quartz monzonite detailed kinematic analysis of fabric and structures in the mylonite and microbreccia establishes that displacement was normal in sense on both sides of the u shaped fault trace the direction of extension was 74 254 as shown by consistently oriented mylonitic lineation crustal attenuation across the newport and adjacent purcell trench faults may have reached as much as 68 km 120 and in any case it exceeded 35 km 40 during faulting the footwall moved up and out to the east and west relative to the hanging wall forming two flanking footwall culminations or crustal scale boudins tectonic denudation of the footwall infrastructure is shown by eocene k ar cooling ages for granitic rocks within it chrontours that increase in age outward from the newport fault record abrupt cooling of the infrastructure as it was drawn away from the hanging wall the silver point quartz monzonite was emplaced into the dilatant zone between the footwall infrastructural culminations during displacement hanging wall strata were rotated down the listric fault surface the footwall rotated upward and the fault surface flattened probably as a consequence of isostatic adjustment to mass redistribution caused by normal faulting a roll over anticline developed in the hanging wall as it moved into the zone of attenuation between the footwall culminations syntectonic eocene extrusive rocks and coarse clastic deposits accumulated in a growth fault basin on the west flank of the hanging wall anticline crustal extension and normal displacement on the newport fault are compatible with a regional eocene strain regime that crossed northern idaho washington and southern british columbia and produced dextral transcurrent faulting core complex development and clockwise rotation of crustal blocks throughout that area

geology and u pb geochronology of archean basement and proterozoic cover in the priest river complex northwestern united states and their implications for cordilleran structure and precambrian continent reconstructions precambrian basement rocks exposed within tectonic windows in the north american cordillera help to define the precambrian crustal structure of western north america and possible reconstructions of the late proterozoic supercontinent rodinia new geologic mapping and u pb dating in the infrastructure of the priest river metamorphic complex northern idaho documents the first archean basement 2651 20 ma north of the snake river plain in the north american cordillera the archean rocks are exposed in the core of an antiform and mantled by a metaquartzite that may represent the nonconformity between basement and the overlying hauser lake gneiss which is correlated with the prichard formation of the belt supergroup a structurally higher sheet of augen gneiss interleaved with the hauser lake gneiss yields a u pb zircon crystallization age somewhat greater than 1577 ma the slivers of augen gneiss were tectonically interleaved with the surrounding hauser lake gneiss near the base of the spokane dome mylo

structure and tectonic development of the southern rocky mountain trench the rocky mountain trench is one of the youngest most prominent and most enigmatic structures of the canadian cordillera approximately 650 km of seismic reflection data providing regional three dimensional coverage over an area of 10 000 km include six crossings of the rocky mountain trench between 49 n and 50 15 n prominent reflections from mid proterozoic moyie sills outline thrust and fold structures of a late jurassic to early cretaceous fault system that was cut by the rocky mountain trench fault in the tertiary the near basement reflections outline a 10 km high west facing basement ramp the hinge line of which spatially coincides with the rocky mountain trench in this area this ramp is part of a mid proterozoic margin upon which the belt purcell supergroup was deposited and is preserved beneath the trench during mesozoic contraction the basal detachment of the foreland belt closely followed the craton cover contact across the basement ramp thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp in the eocene miocene the basement ramp and the culmination above it focused stress reactivating the basal detachment and causing extensional faulting in the southern rocky mountain trench the rocky mountain trench fault may be linked via the basal detachment to the flathead fault on the east and the eocene extensional faults that flank the omineca belt on the west

strain analysis of the ninemile fault zone western montana insights into multiply deformed regions abstract by means of detailed field mapping and a systematic strain analysis we investigated the development of the ninemile fault in western montana the fault has been considered to be either a mesozoic or mid tertiary strike slip fault or a late eocene early oligocene normal fault the rock of the study area experienced a mesozoic to early tertiary ne sw shortening expressed by thrusts folds and pervasive cleavage we estimated total strain by measuring preferred orientation of chlorite grains in samples from within and near the ninemile fault zone and by calculating the strain according to the march theory our goal was to explain how strain accumulated by successive superposition of strains during the two deformation events onto an early compaction strain in all samples the minimum elongation direction maximum shortening is nearly vertical the maximum elongation directions of all the samples outside the ninemile shear zone are subperpendicular to the strike of the shear zone whereas in the shear zone they are either parallel to the strike of the shear zone or form an angle of about 45 the systematic variation in the principal strain directions is the result of two deformational events expressed by local structures created by early ne sw shortening and later ne sw extension the observed strain requires that the ninemile fault a normal fault during the tertiary had earlier been either a thrust or a zone of contractional shear deformation along which there was fault perpendicular shortening and fault parallel extension although the average value of maximum elongation is greater within the ninemile shear zone than outside it varies significantly inside the fault zone suggesting a highly inhomogeneous strain distribution created by a complex deformation history

tectonic domains and exhumation history of the omineca belt in southeastern british columbia from 40ar 39ar thermochronology a large geochronological data set comprising 40ar 39ar and k ar hornblende muscovite biotite and k feldspar rb sr muscovite fission track zircon and apatite and u pb zircon and monazite

timing of extension in the pioneer metamorphic core complex with implications for the spatial temporal pattern of cenozoic extension and exhumation in the northern u s cordillera 1 the pioneer core complex pcc in central idaho lies along a transition between early eocene and ca 40 ma core complexes to the north and south respectively thus the age of extensional development of the pcc is important in understanding the spatial temporal patterns of core complex development in the north american cordillera new results including structural observations and u pb zircon shrimp and icpms geochronology constrain the early extensional history of the footwall for the first time high temperature strain with a top wnw shear sense is pervasive throughout metamorphic rocks of the northwestern footwall an isoclinally folded dike yields a crystallization age of 48 47 ma whereas a crosscutting dike yielded an age of 46 ma metamorphic rocks are also intruded by the 50 48 ma pioneer intrusive suite pis a w dipping granodiorite sheet displaying a magmatic fabric northwest trending lineations are locally visible and also defined by anisotropy of magnetic susceptibility indicating that during emplacement the pis was undergoing similarly oriented extensional strain as the enclosing metamorphic rocks therefore wnw directed extension spanning this structural section occurred between 50 and 46 ma following emplacement of crosscutting 46 ma dikes deformation was partitioned into the wnw directed wildhorse detachment motion on the detachment occurred between 38 and 33 ma as documented by previous 40ar 39ar thermochronology it is not clear however whether extension was continuous through the interval between these two time periods although early eocene extension in the pcc was synchronous with extension in core complexes to the north rates of footwall exhumation in central idaho were much lower this southward slowing is compatible with n s differences in inferred subduction zone geometry kinematics and in the internal character of the orogenic wedge

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

rapid extension recorded by cooling age patterns and brittle deformation naxos greece the metamorphic core complex exposed as the island of naxos in the aegean sea records an unusually complete sequence of structures developed as a result of continental extension the structures formed during miocene rise and cooling from ductile upper amphibolite facies and anatectic conditions to brittle near surface conditions beneath the naxos detachment fault top to the north ductile fabrics in the footwall which initially developed during amphibolite facies prograde metamorphism were overprinted by a succession of north directed normal sense lower temperature brittle structures as the footwall was tectonically unloaded and unroofed pseudotachylite and cataclasite formation brittle faulting alteration and erosion of the footwall occurred during continued slip and tectonic denudation neogene conglomerate and megabreccia in part derived from exhumation of this footwall lie structurally above the peripheral naxos fault published k ar and 40ar 39ar ages for hornblende white mica and biotite in the footwall decrease northwestward apparent ages 17 50 ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained published ages 16 10 ma in higher grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the nne direction of extension assuming this is the direction of unroofing we interpret the rate of this younging as the fault slip rate as the footwall rocks moved 20 km ssw relative to their hanging wall along the base of the naxos detachment fault the calculated rates of slip average 5 8 mm yr comparable to maximum rates reported in the basin and range province

low angle normal fault lanf zone architecture and permeability features in bedded carbonate from inner northern apennines rapolano terme central italy abstract fault zones have the capacity to be hydraulic conduits within upper crustal levels allowing migration of large volume of fluids through shallow and deeper geological environments low angle normal faults lanfs crosscutting carbonate rocks produce damaged volumes that may have a relevant role in channelling or hosting geothermal fluids therefore deserving of investigation to better predict mining targets deformation along lanfs zones dissecting carbonate successions produces permeable volumes presently exploited in the larderello and monte amiata geothermal areas italy in this paper the architectural and permeability features of an exhumed lanf zone exposed in the northern apennines rapolano terme central italy affecting cretaceous bedded limestone are presented such a fault was not affected by circulation of geothermal fluids but its features could reveal much on the potential impact on fluids migration in the active geothermal areas therefore resulting an intriguing analogue the study lanf zone consists of faults which enucleated at depth 4 km during its earlier stage of evolution dissolution seams often arranged in s c fabric characterised the whole damage zone dissolution seams developed under very low grade metamorphism t 100 150 c as indicated by illite crystallinty analyses fault zone architecture and permeability features changed during the fault growth and exhumation permeability heterogeneity and anisotropy characterised the lanf zone during its development if geofluids circulated within the fault zone it could be an effective barrier during its earlier evolution being accompanied by dissolution seams on the contrary it could play as combined barriers conduits during its later evolution progressively at shallower levels being characterised by intersecting fault planes which define pipe like conduits parallel to the direction of the tectonic transport such a configuration could have the capacity to impact on fluids migration for long paths across the upper crustal levels this adds significant information for predicting deeper geothermal circuits and the location of potential exploitable reservoirs in active tectonic environments

identification and interpretation of azimuthally varying crustal reflectivity with an example from the southern canadian cordillera a new approach to seismic interpretation seismic skeletonization is used to analyze crustal reflection data by identifying numbers of reflection events their lengths their energies and their dips with respect to azimuths of recorded profiles application of the method to crustal reflection data in the southern canadian cordillera demonstrates strong directional dependence of crustal reflectivity with persistent trends at 025 040 205 220 085 095 265 275 115 125 295 305 and 140 160 320 340 these trends are visible in both the upper and lower crust and may be related to regional structural trends further applications of the technique particularly in different tectonic regions may allow categorization of seismic patterns and cataloguing of signatures for complex reflection data in a manner similar to seismic stratigraphy and structural patterns in stratified rocks

breaking up the hanging wall of a rift border fault the 2009 karonga earthquakes malawi 1 the southern east african rift has an unusually large seismogenic thickness 35 40 km which is responsible for wide tilted basins and extremely long faults with the potential for m7 8 normal faulting earthquakes from 6 19 december 2009 a shallow earthquake sequence four of mw 5 5 hit the karonga region of northern lake malawi the location is 50 km west of the rift bounding livingstone fault within the hanging wall we used seismology and insar to obtain source parameters and combined this with information on rift structure from geomorphology and seismic profiles the deformation is consistent with rupture of a shallow west dipping fault with no evidence for the involvement of magmatic fluids although the livingstone fault dominates local geomorphology the karonga earthquakes demonstrate that the hanging wall block is actively breaking up reflecting temporal and spatial migration of activity or the release of stresses within it

extension in high grade terranes of the southern omineca belt british columbia evidence from paleomagnetism paleomagnetic observations are used to construct a map of structural tilts in metamorphic and plutonic rocks of the southern omineca belt in southeastern british columbia during eocene extension midcrustal rocks were exhumed in footwalls of low to moderate angle normal faults that dip east and west in the eastern and western parts of the study area respectively tilts averaging 30 to 40 are consistent over large areas and generally antithetic to the major faults in the central part of the study area however the eastern half of the coryell pluton is untilted and its western half in the hanging wall of the east dipping kettle river fault is tilted down to the east in the opposite sense to that expected north of the pluton these eastward tilted rocks are juxtaposed against westward tilted rocks to explain the anomalous tilts a major west dipping eocene normal fault arrow lake fault is inferred and a revised kinematic history for the area proposed observations of magnetic polarity 86 normal laboratory unblocking temperatures and isotopic age data suggest that magnetizations in the west were acquired during chron 23n and in the east in chron 23n and or chron 24n unless extension postdated the acquisition of magnetization tilts would not be observed thus it is argued that extension in the west occurred after 50 6 ma and in the east between 51 6 possibly 53 3 and 50 6 ma these time intervals do not correspond to any known global plate reorganization event indicating that the last phase of eocene extension in the southern omineca belt was governed by regional rather than global processes

late paleogene extensional collapse of the cordilleran foreland fold and thrust belt the cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle eocene to early miocene ca 49 20 ma episode of crustal extension the sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern canada to central utah extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy structural relief and sole faults of preexisting thrust fold structures the sole faults dip 3 6 west above an undeformed precambrian crystalline basement and accommodated tectonic transport of a thick up to 20 km eastward tapering hanging wall during regimes of crustal shortening and extension the chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces dated movement on two extensionally reactivated thrusts the lewis thrust of northwest montana and southeast british columbia and the medicine butte thrust of southwest wyoming and northeast utah suggests that the hiatus between the end of crustal shortening in the early or early middle eocene and the start of extension in the early middle eocene was brief lateral spreading and extensional basin formation in the cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late paleogene cordilleran orogenic wedge is accomplished by applying the extensional wedge coulomb critical taper model the rapid drop in north america pacific plate convergence rate and or steepening of the subducted oceanic slab at ca 50 ma resulted in a large reduction in east west horizontal compressive stress in the cordillera as a result the cordilleran orogenic wedge was left unsupported and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca 20 ma subsequently crustal extension and magmatism during the basin and range event ca 17 0 ma overprinted much of this earlier phase of extension

middle eocene sedimentary and volcanic infilling of an evolving supradetachment basin white lake basin south central british columbia the middle eocene white lake and skaha formations in the white lake basin british columbia record the sedimentary and volcanic infilling of a supradetachment basin that developed during the latter stages of shuswap metamorphic core complex exhumation the 1 1 km thick white lake formation is characterized by volcanogenic sediment gravity flow fluvial and sheetflood facies interbedded with volcanic deposits facies relations suggest white lake strata accumulated on coalesced west sloping alluvial fans that drained an active volcanic center the overlying 0 3 km thick skaha formation records increased tectonism and mass wasting pervasively shattered skaha avalanche slide and sheetflood deposits accumulated on alluvial fans shed from hanging wall and footwall sources exposed along the okanagan valley fault clast compositions of the white lake and skaha formations record alluvial and tectonic stripping that locally eliminated hanging wall blocks mylonite clasts in upper skaha beds imply significant okanagan valley fault footwall uplift during the middle eocene and syntectonic erosion of the shuswap metamorphic core complex the syntectonic sedimentary record preserved within the white lake basin elucidates the relations and timing between core complex exhumation and extensional tectonism in this region the white lake and skaha formations are the apparent age equivalent of the klondike mountain formation of northern washington usa white lake basin strata however are more complexly interstratified post depositionally disrupted and contain a more complete record of core complex unroofing variations in the spatial distributions and textural and compositional character of middle eocene strata in this area underscore the need to exercise care when developing regional scale sedimentary tectonic volcanic models

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