mega meander paleochannels of the southeastern atlantic coastal plain usa abstract paleodischarge estimates based on the slope area method and channel boundaries determined from stratigraphic cross sections indicate that large terminal pleistocene meandering paleochannels mega meanders in river valleys of the southeastern atlantic coastal plain of the united states represent bankfull flows that were at least double the magnitude of those on modern rivers correlation of radiocarbon and luminescence dated paleomeanders with previously reported pollen and eolian sedimentary records suggests that greater discharge was driven by seasonally wetter conditions resulting from dynamic changes to regional precipitation and runoff that occurred in association with global warming at the end of the pleistocene while reflecting larger channel forming flows the exceptionally large widths and radii of curvature of mega meanders were nonetheless maintained by a relatively modest discharge magnitude that was between two and four times larger than modern bankfull flow and within the size range of the present two to five year flood despite their large planform the paleochannels conveyed relatively modest bankfull discharges because a wide shallow shape limited their cross sectional area and hydraulic radius within the late quaternary evolution of fluvial systems in the region scrolled mega meanders constitute a transitional meandering planform that remained influenced by large volumes of sandy bedload following the sand bed braided channels of the late wisconsin interval circa 30 17 ka in addition to greater discharge the large planimetric dimensions of paleochannels reflect a lack of cohesive vertical accretion facies on paleomeander floodplains a sediment regime that transported large quantities of bedload sand and the influence of these factors on channel boundary composition bank stability and channel shape findings underscore the importance of reconstructing channel cross sectional dimensions and slope when estimating discharge for infilled paleomeanders this approach reduces uncertainties surrounding channel cross sectional area gradient and boundary composition inherent to studies lacking subsurface data that rely upon meander geometry to retrodict discharge

earthquake ground motion estimation using strong motion records a review of equations for the estimation of peak ground acceleration and response spectral ordinates abstract engineering seismology is the link between earth sciences and engineering the main input of engineering seismology in engineering design are loading conditions which must satisfy certain conditions regarding their level and frequency of occurrence during the lifetime of a structure one method for estimating these loading conditions is through equations based on strong ground motion recorded during previous earthquakes these equations have a handful of independent parameters such as magnitude and source to site distance and a dependent parameter such as peak ground acceleration pga or spectral acceleration and the coefficients in the equation are usually found by regression analysis this review examines such equations in terms of data selection accelerogram processing techniques of the strong motion records used to construct the equations the characterisation of earthquake source travel path and local site used and regression techniques employed to find the final equations it is found that little agreement has been reached in the past 30 years of ground motion estimation relation studies workers have chosen their techniques based on the available data which varies greatly with geographical region also it is noted that there is a need to include more independent parameters into ground motion estimation equations if the large uncertainties associated with such equations are to be significantly reduced the data required to do this is unfortunately scarce

definition of subfossil meandering palaeochannels the determination of the bottom of a subfossil meandering palaeochannel is important for investigations of the palaeohydrology of valley bottoms and crucial for the quantitative estimation of past river discharges the authors present a procedure on the basis of which the bottom of a palaeochannel can be precisely determined it is based on an analysis of the stratigraphic variability of such features of deposits as mean diameter standard deviation of the grain size composition the percentage of sand and ignition loss

fission track ages of late cenozoic distal tephra beds in the yukon territory and alaska six distal tephra beds from the yukon territory and alaska have been dated by the fission track method zircon and glass ages were determined for the fort selkirk and lost chicken tephra beds but only glass ages for the others assuming that no track fading has occurred in the glass old crow and dawson tephra beds are younger than 120 000 and 52 000 years bp respectively mosquito gulch tephra is 1 22 ma old fort selkirk tephra is about 1 ma old the ester ash bed is 0 45 ma old and the best estimate of the age of lost chicken tephra is the range 1 7 2 6 ma it is evident from these results and from the known abundance of tephra beds within late cenozoic deposits of the yukon territory and alaska that application of the fission track method to distal tephra in conjunction with detailed characterization studies offers great potential for elucidation of the late cenozoic geologic history of alaska and the yukon territory

petrologic variations in quaternary volcanic rocks british columbia and the nature of the underlying upper mantle volcanic activity has produced late tertiary and quaternary cinder cones and flows between the snake river plain u s a and the yukon territory canada the rock types include basanites alkali olivine basalts high iron basalts hawaiites ankaramites nephelinites and olivine tholeiites the alkali olivine basalts basanites and hawaiites sampled are chemically similar to rocks from the mid atlantic islands associated with the volcanic rocks are xenoliths of ultramafic rocks gabbros granites and granulites

northern cordilleran volcanic province a northern basin and range introductionthe western margin of north america hosts adiverse assemblage of large cenozoic igneousprovinces fig 1 inset the temporal spatial andpetrological characteristics of these provinces areattributed to a variety of magmatic processes andtectonic environments including lithosphericextension e g basin and range migration ofmantle plumes e g snake river plain and sub duction e g cascade magmatic arc however neogene alkaline volcanic rocks in the cordilleraof northwestern canada between 55 and 64 n fig 1 currently lack a clear linkage to tectonicprocesses these volcanic centers constitute thenorthern cordilleran volcanic province and repre sent a significant gap in our understanding of thecenozoic tectonomagmatic activity in westernnorth america fig 1 inset our objective is to demonstrate that an exten sional tectonic model adequately explains thepetrologic radiometric and tectonic data forneogene quaternary volcanic rocks in the north ern cordillera furthermore the model we pro pose is consistent with models for neogene quaternary interactions between the pacific andnorth american plates northern cordilleran volcanicprovincethe northern cordilleran volcanic provinceencompasses a broad area of neogene to qua ternary volcanism approximately 1200 km longand 400 km wide in the northern part of west ern canada fig 1 the majority of the vol canoes within this region formed east of thedenali fault system and west of the tintinatrench fig 1 volcanic expressions of magma tism in the province range from large basalticplateaus and stratovolcanoes to individual cin der cones lava flows and eroded vents petrologyneogene to quaternary volcanic rocks withinthe northern cordilleran volcanic province arealkaline and chemically bimodal at least 70 ofthe volcanic rocks comprise alkali olivine basaltand hawaiite fig 2a basaltic lavas are gener ally olivine and plagioclase phyric with plagio clase olivine titanaugite and magnetite in thegroundmass the province also hosts a variety of other alka line and peralkaline rock types that are uncom mon in the north american cordillera such asnephelinite basanite phonolite trachyte and

seismic structure of the upper mantle beneath the northern canadian cordillera from teleseismic travel time inversion abstract teleseismic tomography is used to image upper mantle structure beneath the northern canadian cordillera with the objective of determining the physical state of the upper mantle along the western half of the snorcle lithoprobe transect the resulting three dimensional p wave velocity model resolves structure beneath the southern yukon and northernmost british columbia between 100 and 600 km depth two significant anomalies are identified the first a relatively shallow high velocity feature located at the western edge of the model is interpreted as being the edge of the pacific slab from the southern alaska subduction zone the second is a large tabular low velocity anomaly centred at 60 n by 136 w elongate northwest southeast dipping southwest and reaching a depth of 450 500 km this low velocity anomaly is judged to reflect a thermal anomaly of 100 200 c with a possible compositional component its northeastern boundary is particularly sharp and is interpreted to represent the boundary between thin cordilleran lithosphere and a colder cratonic mantle root our preferred explanation for the low velocity feature is a thermal anomaly resulting from the advective upflow produced by the opening of a slab window beneath the northern cordillera a possible alternative is dense downward percolating partial melt generated by sub orogenic heating as past work e g stolper et al j geophys res 86 1981 6261 6271 suggests that basic partial melts may be denser than peridotite below 200 km depth

estimation of ground motion in mexico city from a repeat of the m 7 0 acambay earthquake of 1912 abstract the acambay earthquake of 1912 m 7 0 which occurred in the central trans mexican volcanic belt ctmvb about 100 km west northwest of mexico city has been thought to be a critical scenario event in the estimation of seismic hazard of the city we use seismograms of two small earthquakes located near actopan 15 december 2003 18 may 2010 and recorded at station cuig a hill zone site in mexico city as empirical green s functions egfs for the acambay region because actopan like acambay is situated in the ctmvb and both are located at about the same distance from cuig the use of the recordings of the actopan earthquakes as egfs is partly justified we first analyze data of the two small earthquakes at a local station dhig to estimate their seismic moment and stress drop as there is considerable uncertainty in the estimated stress drops of the two events we choose a reasonable range of values for them and apply a technique of random summation of egfs to simulate ground motion at cuig from a postulated m w 7 earthquake the estimated geometric mean horizontal peak ground acceleration pga and peak ground velocity pgv at cuig range from 2 7 to 9 7 cm s 2 and from 1 1 to 3 0 cm s respectively ground motion maps for the entire city are presented using a simulated trace at cuig and the known transfer functions of many sites within the city the results are consistent with reported seismic intensities in mexico city and pga and peak ground displacement pgd at a 2 s period at the seismic station of tacubaya during the acambay earthquake estimated ground motions suggest that a repeat of the event does not present significant hazard to mexico city

petrology of recent lava flows volcano mountain yukon territory canada abstract volcano mountain yukon territory a quaternary edifice located near the junction of the pelly and yukon rivers erupted two recent nephelinite lava flows between 3000 and 7300 years bp and a third recent lava flow that is probably younger the porphyritic nephelinites have olivine phenocrysts fo 91 fo 79 and ti rich diopside microphenocrysts the groundmass consists of olivine fo 83 fo 80 ti rich diopside ulvospinel magnetite solid solutions nepheline leucite patches of residual glass and traces of calcite the nephelinites also carry spinel lherzolite xenoliths and xenocrystic olivine fo 87 fo 90 the xenocrysts have lower cao contents than the phenocrysts samples from the two older flows have similar thompson space representations whereas the youngest flow has a distinctly different representation the thompson space representations of the volcano mountain nephelinites are different from the representations of nephelinites from other centers in the cordillera thompson space representations and pearce element ratio diagrams suggest the two older flows are comagmatic whereas the youngest flow is from a separate magma batch thermodynamic modeling suggests the magmas that supplied the lava flows originated at pressures near 2 5 gpa left behind a residue containing olivine and rose along an isenthalpic path after separating from the source rocks

interaction between climate volcanism and isostatic rebound in southeast alaska during the last deglaciation abstract observations of enhanced volcanic frequency during the last deglaciation have led to the hypothesis that ice unloading in glaciated volcanic terrains can promote volcanism through decompression melting in the shallow mantle or a reduction in crustal magma storage time however a direct link between regional climate change isostatic adjustment and the initiation of volcanism remains to be demonstrated due to the difficulty of obtaining high resolution well dated records that capture short term climate and volcanic variability traced to a particular source region here we present an exceptionally resolved record of 19 tephra layers paired with foraminiferal oxygen isotopes and alkenone paleotemperatures from marine sediment cores along the southeast alaska margin spanning the last deglacial transition major element compositions of the tephras indicate a predominant source from the nearby mt edgecumbe volcanic field mevf we constrain the timing of this regional eruptive sequence to 14 6 13 1 ka the sudden increase in volcanic activity from the mevf coincides with the onset of bolling allerod interstadial warmth the disappearance of ice rafted detritus and rapid vertical land motion associated with modeled regional isostatic rebound in response to glacier retreat these data support the hypothesis that regional deglaciation can rapidly trigger volcanic activity rapid sea surface temperature fluctuations and an increase in local salinity i e 18osw variability are associated with the interval of intense volcanic activity consistent with a two way interaction between climate and volcanism in which rapid volcanic response to ice unloading may in turn enhance short term melting of the glaciers plausibly via albedo effects on glacier ablation zones

a new locality for primary xenolith bearing nephelinites in northwestern british columbia high ni abundances 420 500 ppm and mg values 100 mg mg fe2 69 71 and the presence of mantle derived xenoliths indicate that a subvolcanic nephelinite intrusion in northwestern british columbia represents an unmodified primary magma a separate closely associated nephelinite intrusion shows evidence of minor olivine fractionation from a similar composition only three other occurrences of primary nephelinite have been described this new occurrence suggests that these magmas may not be so rare as previously supposed the trace element abundances closely resemble those of primary nephelinites of similar la content from freemans cove canada such compositions are usually taken as evidence of intraplate rifting and doming therefore these rocks are further evidence of late tertiary or quaternary rifting in the stikine volcanic belt

queen charlotte area cenozoic tectonics and volcanism and their association with relative plate motions along the northeastern pacific margin cenozoic relative plate interactions along the queen charlotte margin based on published models for the north america and northeast pacific plates are related to the tectonic and igneous history of the region plate motion models indicate convergence and subduction prior to the eocene since that time there has been transcurrent motion with varying small amounts of oblique extension or compression assuming that the pacific america farallon triple junction was to the south of the region a late miocene or early pliocene about 4 ma to present period of oblique convergence is well resolved the present queen charlotte fault zone along the west coast of the islands may have initiated more recently within the cenozoic transcurrent regime a period of oblique extension in the mid tertiary 36 20 ma in one model is supported by a variety of volcanic and tectonic evidence a small plate motion change at about 20 ma may have resulted in the transfer of the yakutat terrane from the north america plate to the pacific plate and the resulting motion of the terrane northwestward to its present position along the margin of alaska the onset of tertiary masset volcanism that is extensive on the queen charlotte islands corresponds within a few million years to the time of major plate reorganization at 43 ma that has tectonic expression around the entire pacific basin the period of most extensive masset volcanism and plutonism appears to correlate with the model oblique extension in the mid tertiary for one model the geochemistry and physical volcanology of the masset volcanic s are indicative of an extension regime the main syntectonic deposition in the queen charlotte basin skonun formation as inferred from seismic reflection and well data also appears temporally correlated with this time interval of oblique extension a variety of data including plate models dikes normal faults basin subsidence crustal thickness from seismic refraction and present and paleo heat flow indicate mid tertiary crustal extension of at least 20 in the queen charlotte region with up to 150 up to 2 5 in the main queen charlotte basin posttectonic basin subsidence and deposition may correlate with the model time interval of general transcurrent or oblique convergence motion from 20 to 4 ma more recent shortening deformation observed in outcrop and seismic sections of the northern parts of the basin may correlate with plate model oblique convergence from 4 ma to the present the latter convergence is associated with underthrusting that formed a trough or trench and an accretionary sedimentary prism off the west coast of the queen charlotte islands and with inferred uplift and erosion of the western part of the islands

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

subglacial volcanism in north central british columbia and iceland the subglacial genesis of five volcanoes in british columbia has been confirmed by field and laboratory studies and by comparison with the stapar table mountains of iceland two of the canadian tuyas are flat topped piles of hyaloclastite overlain by basalt flows these mountains are almost identical in form to their icelandic counterparts three other tuyas are large conical piles of unconsolidated lapilli two of which overlie vitric tuff and pillow basalt layers and may be analogous to relatively rare icelandic subglacial scoria cones all five volcanoes can be accounted for by eruption into englacial meltwater lakes the differences in gross morphology and physical composition can be accounted for by minor variations in the relative importance of magmatic versus phreatomagmatic eruption

spectral analysis and earthquake scaling of the petatlan earthquake m s 7 6 aftershocks recorded at stations along the pacific coast and between the coast and mexico city spectral parameters have been estimated for 214 petatlan aftershocks recorded at stations between petatlan and mexico city and between petatlan and acapulco the spectral parameters were used to obtain empirical relations for the estimation of seismic moment from coda length and fromm l stress drops using brune s model were calculated for these aftershocks six events with large stress drop are located within a previously suggested asperity and seven more suggest a boundary zone at the intersection of the petatlan and zihuatanejo aftershock rupture volumes stress drops increase with increasing seismic moment up to 1020 dyne cm but appear to be constant at greater moment values the peak horizontal velocity times distance of aftershocks recorded near the coast and between the coast and mexico city 30 to 270 km away scales linearly with seismic moment and predicts well the peak horizontal values of large m s 7 0 coastal thrust events recorded on rock sites at mexico city peak horizontal velocity is a straightforward measurement thus this relation allows us to evaluate expected ground motion between the pacific coast and mexico city from the seismic moment of subduction related earthquakes along the coast

queen charlotte basin and coast mountains paired belts of subsidence and uplift caused by a low angle normal fault the queen charlotte basin and adjacent coast mountains are paired belts of synchronous subsidence and uplift that formed inboard of the queen charlotte fault in neogene time accompanied by regional basaltic volcanism we propose that a combination of pure and simple shear on a lithosphere scale low angle normal fault could have been responsible for the observed vertical motions extensive crustal thinning in the basin decreases toward the coast mountains and has resulted in net subsidence of as much as 6 km since 20 ma east of the basin in the coast mountains more than 3 5 km of surface uplift has taken place since 14 ma probably because the upper mantle lithosphere has been locally thinned and replaced with less dense asthenosphere magmatic activity in the basin and mountain belt could have been caused by decompression melting in the deformed lithosphere and upwelling mantle

episodic north america and pacific plate motions the north america plate motion for the period 20 0 m y is modeled by two absolute angular velocity vectors with a change occurring at about 9 m y the model incorporates new geologic constraints based on 1 the recognition of a discontinuity in north america plate motion at 9 m y 2 a reexamination of the iceland and yellowstone hotspot traces 3 improved delineation of the present north america pacific relative motion and 4 a revised history of pacific absolute motion the two stage model is consistent with several features of the late cenozoic geologic history of north america including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of north america this plate model combined with a previously established pacific plate model makes specific predictions concerning the 15 0 m y history of the north america pacific plate boundary which compare favorably with the known history these models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates the implications of the north america plate model are the greatest for those events related to the 9 m y change in its motion among the geologic consequences of this change are rifting in the western united states intense compressional deformation along the northern caribbean plate boundary a hiatus in aleutian arc volcanic activity and a change in the orientation of the lesser antilles arc this picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances the predictive power of the model may help in interpreting the history of the north america plate boundary in those areas where clear observations are lacking an extra force needs to be applied to a plate to cause a change in plate motion by treating the plate as a rigid spherical shell overlying a uniformly viscous fluid we derive a simple class of forces sufficient to cause a given change in plate motion such force balance considerations indicate that an additional force applied in the northwest pacific region toward the northeast would result in the observed change in north america plate motion this force is surprisingly similar to that which apparently caused a large change in pacific plate motion about 5 m y later we propose that a common mantle process is responsible for both changes and is related to the ability of the northwest pacific subducted lithosphere to enter into the lower mantle

non synchronous response of adjacent floodplain systems to holocene environmental change abstract two adjacent upland floodplains are compared to establish the local response of floodplains to environmental change radiocarbon dating the analysis of sedimentary exposures terrace mapping aerial photography and archaeological evidence are used to examine late quaternary valley fill sediments on the afon tanat and the afon vymwy in the upper severn basin wales uk the alluvial stratigraphy of the two floodplain systems consists of late devensian last glacial fluvio glacial sediments at the valley margins deposited under a braided outwash river regime holocene age terraces with floodplain and palaeochannel deposits composed of gravels overlain by silty sands are inset into this older unit and were formed by meandering fluvial channel systems a combination of field and laboratory data demonstrates that from the mid late holocene the two floodplain systems had divergent development the afon vyrnwy has remained vertically stable for the last ca 4000 yrs whereas the adjacent afon tanat continued to be vertically and laterally active both floodplain systems have been affected by late quaternary climatic fluctuations and anthropogenic activity from the bronze age to the roman period but local geomorphic gradient controls combined with a possibly greater focus of anthropogenic activity in the tanat catchment may explain the differential evolution of the two systems the data demonstrate that multiple reach scale studies are essential for revealing significant stages in the chronology and historical development of fluvial systems

potassium argon geochronology of a basalt andesite dacite arc system the mount adams volcanic field cascade range of southern washington high precision k ar dating and detailed mapping have established an eruptive chronology for a cascade stratovolcano and its surrounding array of coeval basaltic centers mount adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 quaternary volcanic field that contains 60 discrete vents k ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping within analytical error there are no discrepancies between k ar ages and stratigraphic sequence major activity began in the area ca 940 ka and inception of the central stratovolcano took place at ca 520 ka a plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 k y and 80 of the products were basaltic andesites are volumetrically dominant and were emplaced in three main cone building episodes centered at 500 450 and 30 ka at eruptive rates of 1 6 5 km 3 k y at a lower rate of 0 05 0 1 km 3 k y the magmatic system remained almost continuously active between the main pulses although breaks in activity as long as 30 k y are permitted by the k ar data andesitic dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k y and their products are interstratified the last main episode of cone construction occurred ca 40 10 ka the oldest andesite identifiably derived from within the present day edifice having an age of 33 14 ka andesites forming the south summit rim and the true summit have ages of 13 8 ka and 15 8 ka respectively the time volume composition data bear upon several fundamental questions concerning the long term behavior of arc volcanoes stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity large stratocone systems may remain active for half a million years subdivision of complex stratovolcanoes into eruptive or constructional stages without detailed mapping geochronology and compositional data should be treated skeptically discussion of volumetric eruptive rates can be misleading without an adequate time scale stratovolcanoes need never develop large upper crustal magma chambers basalt erupts peripherally but its ascent is suppressed beneath stratovolcanoes by deep crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting

tholeiitic alkalic transition at subglacial volcanoes tuya region british columbia canada ash mountain south tuya and tuya butte are three small basaltic volcanoes in the stikine volcanic belt of northern british columbia the volcanoes rise 700 500 and 400 m above their bases and are about 3 2 1 6 and 2 6 km3 in volume respectively they began eruptive activity under several hundred meters of overlying glacial ice or water in an ice impounded lake and undegassed pillow lava was erupted and forms the bases of all three later as the vents grew into shallow water explosive phreatomagmatic activity erupted partly degassed glassy tuffs finally when the volcano emerged through the surface of the ice or water or the water was drained degassed subaerial lava flows were erupted and were converted to assemblages of foreset bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes the undegassed subglacial pillow base of ash mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes at south tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava at tuya butte the pillow base is overlain by foreset bedded pillow lava pillow breccias and hyaloclastite tuffs which in turn are overlain by subaerial lava flows composing a small shield volcano the undegassed basal subglacial pillow lava of the three volcanoes contain 0 10 0 01 wt sulfur and 0 5 wt h2o the overlying partly degassed assemblages contain 0 06 0 02 sulfur and 0 2 h2o at ash mountain 0 07 0 01 sulfur at south tuya and 0 03 0 01 sulfur at tuya butte the differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava when the volcanoes switched from subglacial to shallow water or subaerial eruptions as shown by change to more explosive activity and then to subaerial lava flows and by a marked reduction of sulfur in volcanic glass the magma shifted from tholeiitic to alkalic composition this transition occurs at each of the three volcanoes the tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were therefore the first to erupt under subglacial conditions as the volcano grew through the ice or ice impounded water the volcanic conduit vented to the atmosphere producing a partial depressurization of the conduit and the subsurface chamber this sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts particularly of the more volatile rich alkalic melts causing them to rise to the top of the chamber and erupt

holocene tephras in lake cores from northern british columbia canada sediment cores recovered from alpine and subalpine lakes up to 250 km apart in northern british columbia contain five previously unrecognized tephras two black phonolitic tephras each 5 10 mm thick occur within 2 4 cm of each other in basal sediments from seven lakes in the finlay river dease lake area the upper and lower finlay tephras are slightly older than 10 220 10 560 cal year b p and likely originate from two closely spaced eruptions of one or two large volcanoes in the northern cordilleran volcanic province the finlay tephras occur at the transition between deglacial sediments and organic rich postglacial mud in the lake cores and therefore closely delimit the termination of the fraser glaciation in northern british columbia sediments in bob quinn lake which lies on the east edge of the northern coast mountains contain two black tephras that differ in age and composition from the finlay tephras the lower bob quinn tephra is 3 4 mm thick basaltic in composition and is derived from

late neogene motion of the pacific plate oceanic crust between 38 s and 65 s along the pacific antarctic ridge records a complete history of neogene pacific antarctic motion we have modeled magnetic stages corresponding to portions of isochrons 1 2 2a 3 3a 4 and 5 from 17 high quality marine magnetic traverses of the ridge to determine the neogene finite rotation poles for this plate pair we chose to model velocities over time intervals of about 0 5 m y because this greatly increased the number of estimates of pacific antarctic plate velocity our analysis shows that a change in pacific antarctic relative motion occurred between chron 2a 3 40 ma and the beginning of chron 3 time 3 86 ma we used 203 estimates of pacific antarctic spreading velocity to calculate a 0 3 4 ma and 3 86 10 3 ma euler pole combined with tracks of seamounts that record the passage of the pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the pacific plate with respect to the hotspot frame of reference using these euler poles to calculate pacific antarctic africa north america circuit euler poles show that this recent change in pacific plate absolute motion corresponds to a change from strike slip to transpressive motion along the coastal california pacific north america plate boundary interestingly this age of increased tectonism agrees in time with deformation in new zealand and northern japan

evolution of an englacial volcanic ridge pillow ridge tindar mount edziza volcanic complex ncvp british columbia canada abstract glaciovolcanic deposits are critical for documenting the presence and thickness of terrestrial ice sheets and for testing hypotheses about inferred terrestrial ice volumes based on the marine record deposits formed by the coincidence of volcanism and ice at the mount edziza volcanic complex mevc in northern british columbia canada preserve an important record for documenting local and possibly regional ice dynamics pillow ridge located at the northwestern end of the mevc formed by ice confined fissure fed eruptions it comprises predominantly pillow lavas and volcanic breccias of alkaline basalt composition with subordinate finer grained volcaniclastic deposits and dykes the ridge is presently 4 km long 1000 m in maximum width and 600 m high fifteen syn and post eruptive lithofacies are recognized in excellent exposures along the glacially dissected western side of the ridge we recognize five lithofacies associations 1 poorly sorted tuff breccia and dykes 2 proximal pillow lava dykes and tuff breccia 3 distal pillow lava poorly sorted conglomerate and well sorted volcanic sandstone 4 interbedded tuff lapilli tuff and tuff breccia units and 5 heterolithic volcanogenic conglomerate and sandstone given the abundance of pillow lavas and the lack of surrounding topographic barriers capable of impounding water we agree with souther souther j g 1992 the late cenozoic mount edziza volcanic complex geol soc can mem vol 420 320 pp that the bulk of the edifice formed while confined by ice but have found evidence for a more complex and variable eruption history than that which he proposed preliminary estimates of water ice depths derived from ftir analyses of h2o give ranges of 300 to 680 m assuming 0 ppm co2 and 857 to 1297 m assuming 25 ppm co2 variations in depth estimates among samples may indicate that water ice depths changed during the evolution of the ridge which is consistent with our interpretations for the origins of different lithofacies associations given that the age of the units are likely to be ca 0 9 ma souther j g 1992 the late cenozoic mount edziza volcanic complex geol soc can mem vol 420 320 pp pillow ridge may be the best documentation of a regional high stand of the cordilleran ice sheet cis in the middle pleistocene and an excellent example of the lithofacies and stratigraphic complexities produced by variations in water levels during a prolonged glaciovolcanic eruption

crystal fractionation of the basalt comendite series of the mount edziza volcanic complex british columbia major and trace elements abstract the mount edziza volcanic complex in north central british columbia includes a group of overlapping basaltic shields salic composite volcanoes domes and small calderas that range in age from 7 5 ma to less than 2000 years b p the volcanic assemblage is chemically bimodal comprising voluminous alkali olivine basalt and hawaiite a salic suite of mainly peralkaline trachyte and comendite plus a relatively small volume of intermediate rocks trachybasalt tristanite mugearite benmoreite the complex is the product of five cycles of magmatic activity each of which began with alkali olivine basalt and culminated with the eruption of salic magma the regular chemical variation shown by almost 100 major and trace element analyses suggests a genetic lineage between the basic and salic members of each cycle least squares mathematical modelling indicates that the salic rocks trachyte and comendite have formed by fractionation of observed phenocryst and cumulate nodule mineral phases from a common alkali olivine basalt parent magma hawaiite is thought to be a cumulate rock formed by partial fractionation and feldspar accumulation within rising columns of primary alkali olivine basalt fractionation leading from alkali olivine basalt through trachybasalt and trachyte to comendite is believed to have taken place where primary basalt became trapped in large crustal reservoirs the early removal of olivine clinopyroxene and plagioclase leading to a trachytic residuum and subsequent fractionation of mainly alkali feldspar leading to the peralkaline end members is consistent with major and trace element variation and with isotopic and ree data the chemical diversity of the complex is attributed to its location over a zone of crustal extension where mantle derived basalt trapped in large high level reservoirs underwent prolonged fractionation

crustal structure of nw british columbia and se alaska from seismic wide angle studies coast plutonic complex to stikinia crustal structure beneath the transition from the coast belt to the intermontane superterrane of the northern canadian cordillera is interpreted from the inversion of refraction and wide angle reflection seismic data the profile traverses an accretionary suture zone coast plutonic complex to continental crust deformed by the transpressive collision stikine terrane using data acquired by the accrete onshore offshore experiment and by a partially overlapping lithoprobe onshore experiment p wave travel time inversion and forward amplitude modeling are employed to determine crustal velocity structure the model exhibits a well defined transition throughout the crust that distinguishes the coast plutonic complex cpc from stikinia average crustal velocities beneath the cpc 6 45 km s are considerably faster than those beneath stikinia 6 25 km s crustal thickness also changes across the transition thin crust beneath the coast belt 30 32 km thickens beneath stikinia 35 37 km the observations within the coast belt are consistent with a tectonic history most recently dominated by extensional deformation primary structural control could be associated with either neogene extension and or the processes that are responsible for exhuming the coast belt during the early paleogene and have been inferred from geological studies slow mantle velocities 7 8 7 9 km s beneath the entire profile are indicative of high upper mantle temperatures comparison with the southern cordilleran coast belt reveals similar velocity structure within the massive plutonic complexes however substantial differences between the northern and southern coast belts emphasize along strike variations in terranes orogen geometry and postorogenic tectonics

concepts for diamond exploration in on off craton areas british columbia canada abstract the tectonic setting of british columbia bc differs from classic diamond bearing intracratonic regions such as the northwest territories and south africa nevertheless several diamond occurrences have been reported in bc it is also known that parts of the province are underlain by proterozoic and possibly archean basement because the continents of today are composites of fragments of ancient continents it is possible that some of the regions underlain by old crystalline basement in eastern british columbia were associated with a deep crustal keel the keel may have predated the break up of the early neoproterozoic supercontinent called rodinia and was preserved possibly until the triassic some of these old continental fragments may have been displaced relative to their position of origin and dissociated from their keel or the keel may have since been destroyed such fragments represent favourable exploration grounds in terms of the diamondiferous mantle root model dmr model if they were intersected by kimberlites or lamproites prior to displacement or destruction of their underlying deep keel therefore extrapolation of fragments of the diamond bearing precambrian basement from the northwest territories or alberta to bc provides a sufficient reason for initiating reconnaissance indicator mineral surveys the eclogite subduction zone model es model predicts formation of diamonds at lower pressure i e depth than required by the dmr model in convergent tectonic settings although not proven this model is supported by thermal modeling of cold subduction zones and recent discoveries of diamonds in areas characterized by convergent tectonic settings if the es model is correct then the parts of bc with a geological history similar to today s cold subduction zones such as honshu japan or to continental collision zones such as kokchetav massif kazakhstan and the dabie sulu terrane east central china may be diamondiferous the terranes where geological evidences suggest an ultrahigh pressure uhp metamorphic event followed by rapid tectonic exhumation which could have prevented complete resorption of diamonds on their journey to the surface are worth investigating if uhp rocks were intercepted at depth by syn or post subduction diamond elevators such as kimberlites lamproites lamprophyres nephelinites or other alkali volcanic rocks of deep seated origin the diamond potential of the area would be even higher

the seismic alert system for mexico city an evaluation of its performance and a strategy for its improvement the seismic alert system sas for mexico city has now been in operation for about 15 years the sas takes advantage of the fact that the city is located more than 300 km from the foci of many of the potentially damaging earthquakes the system consists of 15 accelerometers located along the coast of the state of guerrero above a segment of subduction plate boundary that is a mature seismic gap an algorithm estimates the magnitudes of earthquakes from the near source accelerograms and issues public and restricted alerts for earthquakes with m 6 and 5 m 6 respectively an evaluation of the sas s performance during 1991 2004 reveals a surprisingly high rate of failure and false alerts this poor performance results from an inadequate detection algorithm and a limited areal coverage by the sas this renders the alert system of limited use in this article we propose an alternative strategy for detecting earthquakes potentially damaging to mexico city that differs substantially from the one presently implemented by the sas although our analysis is based on close to source accelerograms of 45 mexican earthquakes and the corresponding peak accelerations recorded at a reference site in mexico city cu there is no restriction on the distance to the field station except that its location should provide sufficient alert time to mexico city based on these data an attenuation relation is derived to compute expected peak acceleration at cu a red from root mean square acceleration a rms at a field station the relation permits specification of an a red threshold given the peak acceleration at cu a cu for which an alert is desired along with prescribed probabilities of failure and false alert we find that the use of bandpass filtered 0 2 1 0 hz accelerograms leads to an improved performance of the sas the choice of the filter is guided by the frequency band of amplification of seismic waves in the lake bed zone of mexico city we think that a single level of general public alert may be the best option a good choice appears to be an alert for a cu 2 gal for 0 2 1 0 hz bandpass filtered accelerograms with 1 probability of failure to accomplish this we must set a red 0 8 gal the data since 1985 suggest that such an alert would occur about once or twice a year and the event will be felt by most persons in the lake bed zone the proposed algorithm along with an array of sensors located 30 to 40 km apart and distributed in a roughly semicircular arc of 310 km radius centered at mexico city should considerably improve the areal coverage and performance of the sas and potentially save thousands of lives

the oxbow sedimentary subenvironment its value in palaeogeographical studies as illustrated by selected fluvial systems in the upper odra catchment southern poland research was conducted in valleys of sinuous rivers that drain medium high mountain foreland basin and upland areas where loose quaternary deposits predominate the cyclic pattern determined for deposits of the oxbow sedimentary subenvironment in the upper odra basin can be expressed as a few alternative sequences the best developed cycle occurring on a relatively high level of probability is represented by the unilateral transition from channel to overbank deposits and upwards bilateral oscillations between overbank and biochemical as well as biochemical and slope deposits in the first stage oxbow filling processes are usually determined by autocyclic factors resulting from spatial relations between abandoned channels and the active channel as a consequence overbank and or biochemical sediments usually overlie channel deposits later allocyclic processes could easily dominate autocyclic processes in these cases overbank slope and late holocene alluvial fan deposits were delivered to oxbow

palaeohydrology of the seim river basin mid russian upland based on palaeochannel morphology and palynological data abstract morphological geological geochronological and palynological analyses show that the landscape climate and hydrological history of the seim river basin includes 1 a period of sandy terrace 1 formation by shallow flows with the surface transformation by cryogenic and aeolian processes close to the last glaciation maximum 2 a cold and dry period about 16 14 k radiocarbon years b p with very high surface runoff and extremely large meandering channel formation 3 a period between the oldest dryas and the preboreal of non steady surface runoff decrease large meander abandonment and formation of smaller channels against the background of climate warming 4 a period of relative stability of hydrological regime and small variations of water runoff as in modern rivers throughout the holocene

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

primary alkaline magmas associated with the quaternary alligator lake volcanic complex yukon territory canada the alligator lake complex is a quaternary alkaline volcanic center located in the southern yukon territory of canada it comprises two cinder cones which cap a shield consisting of five distinct lava units of basaltic composition units 2 and 3 of this shield are primitive olivine phyric lavas 13 5 19 5 cation mg which host abundant spinel lherzolite xenoliths megacrysts and granitoid fragments although the two lava types have erupted coevally from adjacent vents and are petrographically similar they are chemically distinct unit 2 lavas have considerably higher abundances of lree lile and fe but lower hree y ca si and al relative to unit 3 lavas the 87sr 86sr and 143nd 144nd isotopic ratios of these two units are however indistinguishable the differences between these two lava types cannot be explained in terms of low pressure olivine fractionation and the low concentrations of sr nb p and ti in the granitoid xenoliths relative to the primitive lavas discounts differential crustal contamination the abundance of spinel lherzolite xenoliths and the high mg contents in the lavas of both units indicates that their compositional differences originated in the upper mantle the al and si systematics of these lavas suggests that compared to unit 3 magmas the unit 2 magmas may have segregated at greater depths from a garnet lherzolite mantle the identical isotopic composition and similar ratios of highly incompatible elements in these two lava units argues against their differences being a consequence of random metasomatism or mantle heterogeneity the lower y and hree contents but higher concentrations of incompatible elements in the unit 2 lavas relative to unit 3 can be most simply explained by differential partial melting of similar garnet bearing sources the unit 2 magmas thus appear to have been generated by smaller degrees of melting at a greater depth than the unit 3 magmas the contemporaneous eruption of two distinct but volumetrically restricted primary magmas from adjacent vents at the alligator lake volcanic complex suggests that volcanism in this region of the canadian cordillera is controlled by localized small batch processes

early pleistocene volcanism and glaciation in central yukon a new chronology from field studies and paleomagnetism the paleomagnetism of the selkirk volcanics and nearby stratified pleistocene sediments was investigated to resolve the chronology of early pleistocene glaciations in central yukon radiometric dates on these low k basalts have proven to be erroneously old most sampled sediments and all basalts accurately record the paleofield and true reversals the valley filling phase of the selkirk volcanics was in part coeval with the younger pre reid glaciation it was erupted during the matuyarna chron either post cobb mountain subchron or post jararnillo subchron over a period too brief to average secular variation the older pre reid glaciation occurred after ca 1 60 ma and prior to the eruption of the fort selkirk tephra pre jaramillo or pre cobb mountain sediments investigated at revenue creek and braden s canyon are normally magnetized the assigned brunhes age is compatible with their occurrence in valleys that were cut or deepened sometime after the pre reid glaciations

distribution nature and origin of neogene quaternary magmatism in the northern cordilleran volcanic province canada the northern cordilleran volcanic province encompasses a broad area of neogene to quaternary volcanism in northwestern british columbia the yukon territory and adjacent eastern alaska volcanic rocks of the northern cordilleran volcanic province range in age from 20 ma to ca 200 yr b p and are dominantly alkali olivine basalt and hawaiite a variety of more strongly alkaline rock types not commonly found in the north american cordillera are locally abundant in the northern cordilleran volcanic province these include nephelinite basanite and peralkaline phonolite trachyte and comendite the most mgo rich nephelinites basanites and alkaline basalts from throughout the northern cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post 5 ma alkaline basalts from the basin and range our petrologic observations help constrain the origin of northern cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath stikinia which underlies the southern part of the northern cordilleran volcanic province than beneath the cache creek and yukon tanana terranes which underlie the northern part of the northern cordilleran volcanic province our results support a model for initiation of northern cordilleran volcanic province magmatism due to incipient rifting of the northern cordillera driven by changes in relative plate motion between the pacific and north american plates ca 15 10 ma

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

the age of the tseax volcanic eruption british columbia canada a recent volcanic eruption occurred at tseax volcano that formed a series of tephra cones in northwestern british columbia canada the explosive to effusive eruption also formed a 32 km long seque

geochronology of neogene alkaline volcanic rocks miles canyon basalt southern yukon territory canada the relative effectiveness of laser 40ar 39ar and k ar geochronology miles canyon basalt is an informal term used to describe numerous exposures of young alkaline olivine basalt flows in southern yukon the volcanic rocks are part of the northern cordilleran volcani

volcanology and petrology of mathews tuya northern british columbia canada glaciovolcanic constraints on interpretations of the 0 730 ma cordilleran paleoclimate petrological volcanological and geochronological data collected at mathews tuya together provide constraints on paleoclimate conditions during formation of the edifice the basaltic tuya was produced via pleistocene glaciovolcanism in northern british columbia canada and is located within the tuya volcanic field 59 195 n 130 434 w which is part of the northern cordilleran volcanic province ncvp the edifice comprises a variety of lithofacies including columnar jointed lava pillow lava massive dikes and volcaniclastic rocks collectively these deposits record the transition from an explosive subaqueous to an effusive subaerial eruption environment dominated by pleistocene ice as is typical for tuyas the volcaniclastic facies record multiple fragmentation processes including explosive quench and mechanical fragmentation all samples from mathews tuya are olivine plagioclase porphyritic alkali olivine basalts they are mineralogically and geochemically similar to nearby glaciovolcanic centers from the southeastern part of the tuya volcanic field e g ash mountain south tuya tuya butte as well as the dominant ncvp rock type crystallization scenarios calculated with melts account for variations between whole rock and glass compositions via low pressure fractionation the presence of olivine microphenocrysts and the absence of pyroxene phenocrysts constrain initial crystallization pressures to less than 0 6 gpa the eruption of mathews tuya occurred between 0 718 0 054 ma and 0 742 0 081 ma based on 40ar 39ar geochronology weighted mean age of 0 730 ma the age determinations provide the first firm documentation for large 700 m thick pre fraser wisconsin glaciers in north central british columbia 0 730 ma and correlate in age with glaciovolcanic deposits in russia e g komatsu et al geomorph 88 352 366 2007 and with marine isotopic evidence for large global ice volumes 0 730 ma

the nature and rheology of the cordilleran upper mantle of british columbia inferences from peridotite xenoliths abstract peridotite xenoliths in alkalic basaltic rocks from eight localities within the british columbian cordillera include metamorphic tectonites metacumulates and cumulates chemical analyses of these show that the xenoliths encompass the range of both depleted and primitive undepleted mantle application of two pyroxene thermobarometric techniques yield geotherms characteristic of normal oceanic mantle and diapiric mantle wherein upper mantle stratigraphic profiles are either homogeneously stratified or heterogeneously mixed respectively stress estimates as a function of depth are obtained for peridotite xenoliths from each of the localities by applying recrystallised grain size paleopiezometry and pyroxene thermobarometry stress and temperatures obtained are combined with a representative creep flow law to calculate strain rate and viscosity profiles that appear to be physically reasonable these profiles are juxtaposed to present a dynamic model of a mantle diapir convecting through a stratified mantle section and these two different types of mantle stratigraphic profile are related to current large scale tectonic theories of the cordillera

the 1987 whittier narrows california earthquake a metropolitan shock just 3 hours after the whittier narrows earthquake struck it became clear that a heretofore unseen geological structure was seismically active beneath metropolitan los angeles contrary to initial expectations of strike slip or oblique slip motion on the whittier fault whose north end abuts the aftershock zone the focal mechanism of the mainshock showed pure thrust faulting on a deep gently inclined surface hauksson et al 1988 this collection of nine research reports spans the spectrum of seismological geodetic and geological investigations carried out as a result of the whittier narrows earthquake although unseen the structure was not unforeseen namson 1987 had published a retrodeformable geologic cross section meaning that the sedimentary strata could be restored to their original depositional position 100 km to the west of the future earthquake epicenter in which blind or subsurface thrust faults were interpreted to be active beneath the folded southern transverse ranges working 25 km to the west hauksson 1987 had also found a surprising number of microearthquakes with thrust focal mechanisms south of the santa monica mountains another clue to a subsurface system of thrust faults finally davis 1987 had presented a preliminary cross section only 18 km to the west of whittier narrows that identified as fault b the thrust that would rupture later that year not only was the earthquake focus and its orientation compatible with the 10 15 km depth and north dipping orientation of davis proposed thrust but fault b appears to continue beneath the northern flank of the los angeles basin skirting within 5 km of downtown los angeles an area of dense commercial high rise building development these results are refined and extended by davis et al this issue

chronology of the peralkaline late cenozoic mount edziza volcanic complex northern british columbia canada the late cenozoic mount edziza volcanic complex covers an area of about 1 000 km 2 in north central british columbia 300 km east of the transcurrent boundary between the north american and pacific plates it is made up of a group of overlapping basaltic shields and intermediate to salic peralkaline composite domes flows and central volcanoes that are associated with extensional structures in the underlying basement new k ar and fission track dates 45 and rb sr and sr isotope analyses 12 from the mount edziza volcanic complex are reported the age dates are for the most part consistent with the stratigraphy and indicate that frequent eruptive activity occurred during the past 8 m y five major magmatic cycles each began with the eruption of basalt and culminated with the eruption of oversaturated peralkaline magma low 87 sr 86 sr initial ratios 0 7028 0 0001 indicate a mantle source for the basalts low sr contents and high 87 sr 86 sr ratios in the salic end members suggest that the oversaturated rocks were derived from the basaltic magma by crystal fractionation in crustal reservoirs rb sr isochrons suggest that residence times for the fractionating magma were about 0 7 to 1 m y early removal of large amounts of plagioclase followed by fractionation of potash feldspar can account for most of the observed petrological and isotopic relationships a few individual compositions and one suite of mainly intermediate samples contain anomalously large amounts of both 87 sr and radiogenic argon this indicates that contamination with crustal material and possibly mixing of parental basalt with partly fractionated magma from previous events may have produced the relatively small volume of intermediate rocks