geometry of subducted plates and island arcs viewed as a buckling problem during subduction a plate9s leading edge is forced to squeeze into a smaller total width than it occupied at the surface yet the plate resists change of dimension in its own plane a possible outcome is that the plate buckles as it descends at a depth of 500 km the amplitude of buckling could be as much as one tenth the wavelength and dip angles for a plate descending at 45 on average could range from 35 to 55 observed plate geometries though not ideally regular are compatible with these suggestions before being subducted a lithosphere plate is convex upward at all points whereas to buckle it must become convex downward in a series of plunging synclines the pop through is most likely to occur after the leading edge has descended about 100 km at the moment of pop through the subduction hinge line changes from being smooth to being a series of arcs and cusps back arc spreading associated with the change can be 150 km or more in the arc embayments but should be zero opposite the cusps where a compressive regime is sustained in the pop through event elastic strain energy is released and the associated back arc spreading should be a comparatively sudden event on a geologic time scale

long term fore arc basin evolution in response to changing subduction styles in southern alaska detrital zircon u pb and fission track double dating and hf isotopes from the mesozoic and cenozoic strata in the southern alaska forearc basin system reveal the effects of two different modes of flat slab subduction on the evolution of the overriding plate the southern margin of alaska has experienced subduction of a spreading ridge ca 62 50 ma and an oceanic plateau ca 40 0 ma when a subducting spreading ridge drives slab flattening our data suggest that after the ridge has moved along strike retro arc sediment sources to the forearc become more predominant over more proximal arc sources spreading ridge subduction also results in thermal resetting of rocks in the upper plate that is revealed by thermochronologic data that record the presence of young age peaks found in subsequent thin sedimentary strata in the forearc basin when a subducting oceanic plateau drives slab flattening our data suggest that basin catchments get smaller and local sediment sources become more predominant crustal thickening due to plateau subduction drives widespread surface uplift and significant vertical uplift in rheologically weak zones that combined create topography and increase rock exhumation rates consequently the thermochronologic signature of plateau subduction has generally young age peaks that generate short lag times indicating rapid exhumation the cessation of volcanism associated with plateau subduction limits the number of syndepositional volcanic grains that produce identical geochronologic and thermochronologic ages this study demonstrates the merit of double dating techniques integrated with stratigraphic studies to expose exhumational age signatures diagnostic of large scale tectonic processes in magmatic regions

depositional environment and mechanism of preservation of microfossils upper proterozoic bitter springs formation australia microfossils preserved in black chert nodules from the upper proterozoic bitter springs formation of australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds groundwaters of halite salinity pickled the microorganisms and thus inhibited their early bacterial degradation these same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions nodules and bipyramidal quartz crystals where the chert nodules formed in areas of buried cyanobacterial mats further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved

results of a magnetotelluric traverse across western oregon crustal resistivity structure and the subduction of the juan de fuca plate abstract as part of project emslab we have collected and analysed wideband magnetotelluric data along an east west transect in western oregon preliminary modelling of the data using one dimensional inversions based upon rotationally invariant earth response functions was followed by finite element two dimensional modelling the models produced indicate the presence of an electrical conductor beneath the oregon coast range dipping eastward at 12 18 from a depth of 23 32 km we believe that this conductor includes the thrust surface of the subducting juan de fuca plate and or adjacent water saturated rocks its high conductance about 200 s is thought to be due to one or more of the following mechanisms 1 sediments subducted atop and with the juan de fuca plate 2 saline fluids produced by dehydration of the former or 3 seawater contained within subducted oceanic basalts there is a distinct possibility that the high conductivity is due primarily to the presence of subducted sediments in contrast with the notion that the subduction of young buoyant lithosphere retards sediment subduction at this convergent margin the conductive layer is overlain by relatively resistive rocks presumed to be accreted oceanic lithosphere model determined resistivities for the upper part of the coast range section are in good agreement with deep well log data a strong electrical contrast appears in the determinant phase pseudosection between the coast range and the willamette valley suggesting a structural boundary between the two provinces a surficial conductor is present in the valley to depths of 1 2 km and is due to alluvial fill induction arrow data show the geomagnetic coast effect and a smaller effect by the willamette valley alluvial fill

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

hypocentral trend surface analysis probing the geometry of benioff zones a hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the middle of a benioff zone in this paper we take a relatively simple approach to hypocentral trend surface analysis the hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface the vertical deviation of a hypocenter from the hypocentral trend surface is called its residual consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution any analysis of benioff zone geometry trades off presumed thickness of the zone with its presumed shape this trade off can be investigated by examining suites of candidate hypocentral trend surfaces together with their associated residual distributions generated by varying the number of degrees of freedom available to the trend surface hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in honshu japan cook inlet alaska and south peru hypocenters beneath honshu generate a bimodal distribution of residuals about their trend surface the vertical separation of the upper and lower sheets of this double benioff zone averages about 33 km no double benioff zone configuration is discovered below cook inlet or south peru regional trend surfaces are established for intermediate depth teleseismic data from south and middle america in south america 0 40 s teleseismic data suggest that along strike transitions between flat and moderately steeply dipping sections of the benioff zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear bound flaps or separate tongues of lithosphere hypocentral trend surface analysis of teleseismic data from south peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of hasegawa and sacks an eastward shallowing benioff zone beneath southwest mexico is closely associated with the geometry of the trans mexican volcanic arc the benioff zone beneath central america is more steeply dipping the flat slab steep slab transition near 96 w may relate to the difference in age of oceanic lithosphere on either side of the tehuantepec ridge while the strong flexure of the subducted nazca plate beneath ecuador is closely associated with the carnegie ridge the nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath south peru the flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding south american plate

2 2 ga age of zircons in three occurrences of upper proterozoic clastic rocks of the northern cassiar terrane yukon and british columbia analyses of detrital zircons from three occurrences of upper proterozoic clastic rocks of the northern cassiar terrane in yukon and northern british columbia yield a u pb age of 2224 22 ma the zircons apparently belong to a single population similar in age to zircons in stratigraphically equivalent rocks of the southern cassiar terrane and to zircons in rocks in the yukon tanana terrane of alaska a source terrane or area of the required age and extent has not yet been identified

erosional denudation of the british columbia coast ranges as determined from fission track ages of detrital zircon from the tofino basin olympic peninsula washington in this study we use up section changes in fission track ft ages of detrital zircons to infer the tectonic evolution of the source region that fed sediments to the tofino basin a tertiary forearc basin in northwestern washington state and southwestern british columbia we have dated 50 grains from each of eight samples from a 6 km thick section that ranges in age from 40 to 19 ma the detrital zircons have not been reset and thus preserve information about the thermal and denudation history of the source region from which the sediments were derived each of our eight samples contains a distinct set of grain age populations or peaks indicating that the source region contained several ft source terrains the number of peaks and their lag time peak age minus depositional age contain thermochronologic information about these source terrains in this study there is little evidence of active volcanism in the source region thus the ft ages reflect cooling during denudation of the source regions the oldest sample in our suite from the upper eocene lyre formation shows a dominant peak at ca 40 ma coincident with cooling ages of denuded metamorphic rocks of the leech river schist on southern vancouver island this interpretation is supported by paleocurrent directions indicating a source in the vicinity of vancouver island the development of a late eocene unconformity across much of southern vancouver island and western washington and the predominance of lithic detritus in this part of the section samples from strata above the lyre formation are feldspathic and they presumably reflect erosional denudation of a plutonic source terrain peaks in samples from the lower part of this feldspathic interval have widely scattered jurassic to mid cretaceous ft ages young zircons are notably absent up section peak ages in samples from the upper part of the feldspathic section form a dominant peak that gets progressively younger with time while the age of the dominant peak becomes younger in younger samples the lag time for this peak remains constant at 40 m y this peak can be recognized in a compilation of zircon ft ages from modern exposures of the canadian coast mountains we infer that zircons in our samples were derived from progressive erosional denudation of the coast plutonic complex which had based on our results a constant rate of denudation of 250 m m y the scatter of ages in the older samples from the feldspathic interval may indicate the unroofing of a high level sedimentary basin containing the cretaceous nanaimo and burrard groups that used to overlie parts of the coast plutonic complex our study along with others shows that up section studies of ft ages of detrital zircons can be effective in resolving the long term denudation history of continental source regions

seismic evidence for widespread serpentinized forearc upper mantle along the cascadia margin petrologic models suggest that dehydration and metamorphism of subducting slabs release water that serpentinizes the overlying forearc mantle to test these models we use the results of controlled source seismic surveys and earthquake tomography to map the upper mantle along the cascadia margin forearc we find anomalously low upper mantle velocities and or weak wide angle reflections from the top of the upper mantle in a narrow region along the margin compatible with recent teleseismic studies and indicative of a serpentinized upper mantle the existence of a hydrated forearc upper mantle wedge in cascadia has important geological and geophysical implications for example shearing within the upper mantle inferred from seismic reflectivity and consistent with its serpentinite rheology may occur during aseismic slow slip events on the megathrust in addition progressive dehydration of the hydrated mantle wedge south of the mendocino triple junction may enhance the effects of a slab gap during the evolution of the california margin

subduction zone anisotropy beneath corvallis oregon a serpentinite skid mark of trench parallel terrane migration 1 we studied the back azimuth dependence of ps converted phases at gsn station cor corvallis oregon using broadband p receiver functions computed from 602 teleseismic earthquakes the amplitudes and polarities of the transverse ps phases are largely two lobed which indicates anisotropy with a tilted symmetry axis a double peaked ps conversion at 4 5 6 5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the ps converted phase by the dipping interface a polarity flip on the transverse receiver function near north south back azimuth indicates an anisotropic symmetry axis aligned north south far from the convergence direction n68 e the ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed vp 6 0 6 5 km s near 40 km depth at the slab interface with the overriding north american plate one dimensional modeling suggests 10 anisotropy in the supraslab layer with a slow symmetry axis oriented n5 w at a 60 tilt from the vertical adjustments for the effect of slab interface dip on ps amplitude suggest a somewhat lower 7 anisotropy we infer a thin 7 km anisotropic detachment zone for the northward slippage of the siletz forearc terrane along the top of the descending slab a motion consistent with gps measurements and models of regional lithospheric dynamics serpentinite is a likely constituent for the deep anisotropic layer owing to its reduced vp and high poisson ratio the ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone

additional paleomagnetic results from the sierra nevada further constraints on basin and range extension and northward displacement in the western united states modal analysis of new paleomagnetic results that have a skewed distribution from four plutons in the kings canyon national park area central sierra nevada produces a pole position concordant with the results of frei and others 1984 from the yosemite area these studies suggest a that stable magnetization resides in the more mafic plutons and inclusions in the sierra nevada and b that the amount of internal deformation within the central sierra nevada does not exceed a few degrees at the 95 confidence level the mean pole of both studies limits the relative displacement between the sierra nevada and cratonal north america to 6 8 of clockwise rotation and 6 6 of poleward displacement the clockwise rotation agrees well with the model of frei and others 1984 for basin and range extension which predicts 2 of clockwise rotation for the sierra nevada the poleward displacement suggests right lateral shear of 600 km within the basin and range between 90 and 35 ma

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position

isotopic evidence for the sources of cretaceous and tertiary granitic rocks east central alaska implications for the tectonic evolution of the yukon tanana terrane magnetotelluric traverses across the southern yukon tanana terrane ytt reveal the presence of a thick conductive layer or layers beneath paleozoic crystalline rocks these rocks have been interpreted to be flysch of probable mesozoic age on the basis of the occurrence of jurassic cretaceous flysch in the kahiltna assemblage and gravina nutzotin belt flanking the ytt to the southwest and southeast respectively the pb nd sr and o isotopes in cretaceous and tertiary granitic rocks that crop out throughout the ytt were measured to determine if these rocks do in fact contain a component of flysch previous limited analyses indicated that the pb isotopes of the granitic rocks could be a mixture of radiogenic pb derived from paleozoic crystalline rocks of the ytt with an increasing component of relatively nonradiogenic pb with decreasing age our nd sr and o data along with additional pb isotope data eliminate flysch as a likely source and strongly suggest that the nonradiogenic end member was deri

geophysical constraints on washington convergent margin structure gravity and magnetic maps of western washington reveal the lateral structure and fabric of the washington coast range puget basin and southern washington cascade range the magnetic and gravity maps show large amplitude positive anomalies associated with the shallow but largely buried section of washington coast range mafic rocks which are separated by negative anomalies over deep sedimentary basins the positive anomalies indicate that the coast range mafic basement extends farther east than previously thought at least as far east as the longitude of seattle linear and steep gravity and magnetic gradients indicate many unmapped often buried faults in the washington coast range province magnetic highs are also associated with mapped batholiths in the cascade arc several magnetic highs observed east of the coast range rocks and west of these batholiths may be associated with buried tertiary plutons or ophiolites two dimensional gravity and magnetic modeling constrained with geological and other geophysical data indicate that the coast range province rocks are about 1 km thick at the coast thickening to as much as 30 km near their postulated eastern edge a maximum boundary on the average density of the upper 15 20 km of the rocks that compose the coast range province of 2920 kg m3 was established by the modeling suggesting a composition largely of basalt and gabbro with little interbedded sediments under these rocks may be mantle or a subduction complex composed of dense mafic ultramafic and sedimentary rocks like that proposed to underlie vancouver island previous gravity models of the washington margin include lower densities for the proposed subduction complex than for vancouver island suggesting a lower component of mafic and ultramafic rocks than the rocks underlying vancouver island however my washington model requires that the proposed subduction complex be more dense than the trench sediments and therefore that material denser than sediments be incorporated within it the absence of continental mantle and the modeled wedge shape of the coast range province upper crust suggest that erosion of the bottom of the overriding plate by subduction processes may have occurred

cenozoic active margin and shallow cascades structure cocorp results from western oregon two deep seismic reflection profiles totaling 98 km were recorded by the consortium for continental reflection profiling cocorp in western oregon in 1984 providing some of the first onshore deep seismic reflection data from the forearc and arc of an active convergent margin these data are of lower quality than most cocorp data but when combined with other geological and geophysical data they provide some useful insights into the subduction zone beneath oregon and into extensional structures in the cascades line 1 crossed the eocene sediments and underlying pillow basalts of the coast range and western willamette valley reflections from the willamette valley clearly define the lower and western boundary of flat lying rhythmically bedded eocene sediments these sediments appear to be underlain by as much as 8 km of seismically transparent eocene pillow basalts layered reflections at depths of 8 16 km may indicate a remnant crust upon which the eocene pillow basalts were erupted east dipping reflections at depths of 35 40 km may represent the decollement above the subducting juan de fuca plate oregon line 2 crosses the cenozoic volcanic arc terrains of the western and high cascades interpretation of cocorp seismic data suggests modification of existing models for normal faulting that postulate a symmetric graben or grabens in the cascades of oregon in the high cascades along cocorp oregon line 2 reflections suggest a large gently west dipping half graben with major offset only on the west side of the range the seismic reflection data along with other geological and geophysical data suggest that the high cascades were built on a series of blocks rather than a single graben the blocks were differentially faulted during the pliocene seismic data from the western cascades indicate several normal faults with down to the east offset including a major previously unidentified fault of probable miocene age

seismic tomography to obtain velocity gradients and three dimensional structure and its application to reflection data on vancouver island a seismic reflection survey was carried out on vancouver island as part of lithoprobe phase i to study the crustal structure and tectonics of this convergent margin four seismic profiles which

geochronology of augen gneiss and related rocks yukon tanana terrane east central alaska using several isotopic techniques we have determined the ages of selected metamorphic rocks in the yukon tanana terrane ytt of east central alaska u pb zircon data from an augen gneiss body in the big delta quadrangle indicate that the granitoid protolith of the gneiss was intruded 341 3 m y ago lower intercept age an upper intercept age of 2 136 31 m y indicates an inherited early proterozoic component in these zircons this inheritance age is substantiated by a sm nd whole rock model age of 2 09 0 08 b y from the big delta augen gneiss body detrital zircons from quartzitic wall rocks to this body were also derived from an early proterozoic 2 1 to 2 3 b y old crustal source s zircons from three other augen gneisses occurring in an east west belt which extends into the southern yukon territory canada have similar mississippian and early proterozoic intercept ages a rb sr whole rock isochron from widely separated bodies of augen gneiss has an age of 333 26 m y and an initial 87 sr 86 sr ratio of 0 728 0 002 confirming the mississippian intrusive age for the protolith the high initial 87 sr 86 sr ratio further indicates an old crustal component in these rocks a rb sr mineral isochron 115 4 m y k ar data from hornblende and micas 128 to 107 m y and u pb data from sphene 134 m y from augen gneiss and related rocks are similar to many k ar ages in this region and confirm the occurrence of an early cretaceous thermal event u pb ages of zircons from three metavolcanic units in the ytt suggest that extrusion of the protoliths of these rocks occurred 360 380 m y ago scatter in the data is caused by ubiquitous inheritance and multiple lead loss events significant devonian mississippian igneous activity 380 340 m y took place in the ytt followed by one or possibly two metamorphic episodes mississippian and cretaceous as yet no early proterozoic source rocks for the paleozoic magmas have been identified in the ytt but rocks of similar early proterozoic ages occur to the east and south in the northwest territories and british columbia canada

nd isotopic characterization of metamorphic rocks in the coast mountains alaskan and canadian cordillera ancient crust bounded by juvenile terranes nd isotopic data are reported for 52 samples from the crustal region between the alexander wrangellia terrane and the stikine terrane of the alaskan and canadian cordillera this region is composed of the gravina belt a jurassic cretaceous assemblage of volcanic and clastic sedimentary rocks the taku terrane a terrane of probable early permian to late triassic age and four assemblages of metamorphic rocks that occur to the west of and within the coast mountains batholith the gravina belt has end t values that range from 1 1 to 8 3 similar to values of the underlying alexander terrane and consistent with the interpretation that it is a juvenile belt that formed in a back arc or intra arc basin within the alexander terrane mid cretaceous plutons that were emplaced into the gravina belt have end t values of 4 4 to 5 7 and were probably produced by mantle derived melts that incorporated some alexander terrane crust the taku terrane has end 0 values that range from 5 5 to 3 3 with corresponding depleted mantle model tdm ages of 440 to 1430 ma a mid cretaceous pluton intruding the taku terrane has an end t value of 5 1 a value indistinguishable from those determined for cretaceous plutons intruding the gravina belt metamorphic rocks east of and structurally overlying the taku terrane are divided into the tracy arm assemblage end 0 26 to 0 tdm 800 2450 ma the endicott arm assemblage end 0 10 to 1 3 tdm 950 1500 ma the port houghton assemblage end 0 9 4 to 1 1 tdm 550 1500 ma and the ruth assemblage end 0 9 4 to 2 0 tdm 650 1300 ma these isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from precambrian continental crust the metamorphic rocks from these assemblages are lithologically very similar to rocks of the yukon tanana ytt terrane of eastern alaska and yukon territory and have such similar u pb detrital zircon ages and nd isotopic compositions to ytt rocks that they are considered part of that terrane possible tectonic scenarios that can explain the present geometry of the ytt with respect to the alexander wrangellia and stikine terranes include 1 the ytt is the upturned stratigraphic basement of the stikine terrane 2 part of the ytt was structurally emplaced beside the stikine terrane in a transpressive tectonic regime 3 the stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the ytt before the final accretion of the alexander wrangellia terrane

silica replaced oolites bedded shelf cherts and paleozoic changes in the silica cycle abstract oolites that were replaced by early diagenetic silica and were not reworked from their original depositional setting can serve as a useful tool to evaluate a previously suggested retreat of biosiliceous facies from peritidal environments to deeper water settings in the early paleozoic the silica replaced oolites suggest that the retreat was largely over by the middle ordovician which is consistent with emerging compilations of the spatial and temporal distribution ordovician cherts whether the retreat was a function of declining concentration of dissolved silica in shallow marine waters or of radiation of faunas from nearshore into offshore settings or some combination of both is an issue that may be clarified with improved resolution of the timing of the retreat a small upper carboniferous peak in silica replaced oolite abundance coincides with a minor rebound in shelf chert accumulation and the common presence of coastal flint deposits in upper carboniferous cyclothems suggesting that cherty facies returned to shallow water environments at this time

a magnetotelluric transect in the oregon coast range as part of emslab broadband magnetotelluric data have been collected and analyzed at 11 sites along an east west transect in the oregon coast range using one and two dimensional modelling the coast range was found to consist of marine sediments about 4 km deep near the coast and volcanic rocks about 8 km deep farther inland willamette valley consists of surficial conductive sediments to a depth of 1 km which overlie an eastern extension of the rocks that compose the coast range an east dipping conductor at a depth of approximately 20 km is associated with the upper surface of the subducting juan de fuca plate possible mechanisms for the high conductance are water saturated fractured basalts forming the surface of the plate subducted sediments and or the dehydration of the subducting slab trapped by metamorphic layering within the lower crust this conductor correlates with an east dipping reflector 35 40 km below the eastern flank of the coast range defined by cocorp profiling and thought to represent the surface of the subducting juan de fuca plate

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

detrital zircon record of neogene exhumation of the central alaska range a far field upper plate response to flat slab subduction far field deformation in the upper plate of many ancient and modern convergent margins is commonly attributed to flat slab subduction processes recent studies have suggested that the high topography of the central alaska range alaska usa for example is the far field deformational response to neogene shallow subduction processes in southern alaska in this study we analyze six stratigraphically controlled detrital zircon samples from miocene pliocene sandstone of the tanana foreland basin to evaluate the temporal and spatial links between exhumation of the central alaska range and shallow subduction the tanana basin located north of the central alaska range contains 2 km of neogene strata lower miocene strata from the basin have detrital zircon ages with dominant precambrian populations these grain ages are attributed to sediment derivation from the yukon composite terrane source area located at the northern edge and north of the alaska range during the early middle miocene the provenance of the tanana basin switched to a distinctly mesozoic source signal these mesozoic age grains are attributed to sediment derived from source areas within and south of the alaska range the early middle miocene change in provenance from predominantly precambrian to mesozoic sediment sources in the context of previously published basin analysis studies is interpreted to represent the onset of neogene uplift of the central alaska range the timing of exhumation of the central alaska range is important not only for understanding the tectonics of the highest mountain range in north america but additionally because recent modeling studies predict that this range is one of the first expected products of neogene flat slab subduction along the southern alaska convergent plate boundary results of our study provide a needed chronologic framework to fully evaluate the temporal and spatial upper plate responses to shallow subduction processes

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

electrical structure of the central cascadia subduction zone the emslab lincoln line revisited abstract the emslab experiment was an ambitious onshore offshore magnetotelluric mt transect of the cascadia subduction zone when completed 1985 1988 it was the largest experiment of its kind modeling and inversion capabilities at the time were however not sufficiently sophisticated to handle a fully regularized inversion of the data including the seafloor data and bathymetric constraints with the main final model presented based on trial and error forward modeling of the responses moreover new data collected as part of the earthscope usarray program are of higher quality due to improvements in instrument technology and augment the original emslab data set presenting an opportunity to revisit the structure in this part of the subduction system we have integrated the original wide band mt data as well as several long period stations from the original emslab data set and invert these in conjunction with emslab seafloor responses and new earthscope data on land this new composite data set has been analyzed in several ways within a two dimensional geometry in which conductivity is assumed to be invariant along a strike direction roughly coincident with that of the subduction zone we have solved for fully smooth regularized models as well as solutions that allow discontinuities in conductivity along the top surface of the descending slab finally we have tested specific features in the emslab model notably a moderately shallow 30 km depth forearc conductor a feature similar to this shallow conductor is a consistent and required feature in our new inversion models but the new models highlight the connection between the slab and what is interpreted to be an accumulation of aqueous fluids in the deep crust the depth 40 km at which the conductor intersects the slab suggests that the fluids are released by the transition of hydrous basalt to eclogite at upper greenschist facies and higher metamorphic grade the nose of the mantle wedge has a conductivity consistent with a dry peridotite composition and thermal models of the system at a depth of around 80 km the mantle intersecting the slab shows a slight increase in conductivity this increase is not sufficient to require the presence of melt but a conductor indicative of melt can be inserted into the model at this depth without compromising the fit

cache creek terrane entrapment oroclinal paradox within the canadian cordillera exotic and far traveled oceanic crustal rocks of the cache creek terrane cc are bordered by less exotic quesnel qn and stikine st arc terranes to the east north and west all of these terranes are enveloped by an arcuate belt of displaced continental margin rocks the kootenay ko nisling ns and parts of the yukon tanana ytt terranes that have indirect ties to ancestral north america na initial 87sr 86sr isopleths conform to this arcuate pattern such a pattern of concentric belts presents a geological conundrum how did the qn st and cc come to be virtually enveloped by terranes with ties to na past and current models that explain assembly of the canadian cordillera are deficient in their treatment of this problem we propose that early mesozoic qn and st were joined through their northern ends as two adjacent arc festoons that faced south toward the cache creek ocean panthalassa oceanic plateau remnants within the cc today were transported from the tethyan realm and collided with these arcs during subduction of the cache creek ocean counterclockwise oroclinal rotation of st and ns terranes in the late triassic to early jurassic caused enclosure of the cc rotation continued until these terranes collided with qn in the middle jurassic paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for permian to early jurassic sites in st but moderate clockwise rotations for sites in qn specific modern analogues for the cordilleran orocline include the yap trench where the caroline rise is colliding end on with the mariana arc and the banda arc located on the southeastern tail of the asian plate which is being deformed into a tight loop by interactions with the australian and pacific plates

listric normal faulting on the cascadia continental margin analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern oregon and washington continental shelf and upper slope suggesting e w extension in this region fault activity began in the late miocene and in some cases has continued into the holocene most listric faults sole out into a subhorizontal d6collement coincident with the upper contact of an eocene to middle miocene m61ange and broken formation mbf known as the hoh rock assemblage onshore whereas other faults penetrate and offset the top of the mbf the areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the mbf evidence onshore and on the continental shelf suggests that the mbf is overpressured and mobile for listric faults which become subhorizontal at depth these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low angle 0 1o 2 5 o detachment coincident with the upper mbf contact mobilization extension and unconstrained westward movement of the mbf may also contribute to brittle extension of the overlying sediments no pliocene or quaternary extensional faults have been identified off the central oregon or northernmost washington coast where the shelf is underlain by the rigid basaltic basement of the siletzia terrane quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope suggesting that the shelf and upper slope are decoupled from subduction related compression

paleomagnetism of the flores volcanics vancouver island in place by eocene time the eocene flores volcanics of the insular belt of southwestern vancouver island have u pb zircon ages of 51 50 ma and a mean direction of magnetization d i of 349 8 69 6 12 collecting sites

geophysical data reveal the crustal structure of the alaska range orogen within the aftershock zone of the mw 7 9 denali fault earthquake geophysical information including deep crustal seismic reflection magnetotelluric mt gravity and magnetic data cross the aftershock zone of the 3 november 2002 mw 7 9 denali fault earthquake these data and aftershock seis micity jointly interpreted reveal the crustal structure of the right lateral slip denali fault and the eastern alaska range orogen as well as the relationship between this structure and seismicity north of the denali fault strong seismic reflections from within the alaska range orogen show features that dip as steeply as 25 north and extend downward to depths between 20 and 25 km these reflections reveal crustal structures probably ductile shear zones that most likely formed during the late cretaceous but these structures appear to be inactive having produced little seis micity during the past 20 years furthermore seismic reflections mainly dip north whereas alignments in aftershock hypocenters dip south the denali fault is nonre flective but modeling of mt gravity and magnetic data suggests that the denali fault dips steeply to vertically however in an alternative structural model the denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the alaska range orogen modeling of mt data indicates a rock body having low electrical resistivity 10 xm that lies mainly at depths greater than 10 km directly beneath aftershocks of the denali fault earthquake the maxi mum depth of aftershocks along the denali fault is 10 km this shallow depth may arise from a higher than normal geothermal gradient alternatively the low electrical resistivity of deep rocks along the denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after shock zone

results of the dekorp 1 belcorp dekorp deep seismic reflection studies in the western part of the rhenish massif summary within the dekorp project dekorp deutsches kontinentales reflexionsseismisches programm a joint deep seismic reflection venture with the belcorp belgian continental reflection seismic programme group of the belgian geological survey was carried out in 1987 across the rhenish massif a part of the mid european variscides this orogenic belt developed in the upper devonian carboniferous mostly devonian rocks crop out at the surface the rhenish massif is bordered by two sedimentary troughs the sub variscan foredeep in the north and the permo carboniferous saar nahe basin in the south in the east west direction it is subdivided by the axial depression of the eifel nord sud zone the aim of the survey which totals almost 220 km of seismic profiling was to investigate the crustal structure of the western part of the rhenish massif and to compare it with the line dekorp 2 n which crosses the eastern portion of the massif the results indicate the presence of nw vergent tectonics of various styles that can often be traced down to deep parts of the crust horizontal variscan compression plays a dominant role in the northern part while post variscan extension seems to dominate in the saar nahe basin although even there traces of variscan compression seem to be preserved in the middle crust common characteristics of the pre palaeozoic basement differences between the western and eastern parts of the rhenish massif and the deep extension of the aachen thrust faille du midi have been clearly observed this prominent thrust in the north with its characteristic ramp and flat structure has been followed over 100 km length down

uplift shortening and steady state topography in active mountain belts we present a tectonic surface process model used to investigate the role of horizontal shortening in convergent orogens and the effects on steady state topography the tectonic model consists of a specified velocity field for the earth9s surface and includes a constant uplift rate and a constant horizontal strain rate which varies to reflect the relative importance of frontal accretion and underplating in an orogenic wedge the surface process model includes incision of a network of rivers formed by collection of applied precipitation and diffusive hillslope mass transfer three non dimensional parameters describe this model a ratio of the maximum horizontal velocity to the vertical velocity a peclet number expressing the efficiency of the hillslope diffusion relative to the uplift rate and a fluvial erosion number reflecting the fluvial incision efficiency relative to the uplift rate a series of models are presented demonstrating the resultant steady state landforms parameterized by these three numbers a finite velocity ratio results in an asymmetric form to the model mountain range although the magnitude of the asymmetry also depends on the peclet number topographic steady state is achieved faster for models with no horizontal component to the velocity field with finite horizontal velocity topographic steady state is achieved only at the scale of the entire mountain range even the first order drainage basins are unstable with time in the presence of horizontal shortening we compare our model results to topographic profiles from active mountain ranges in taiwan new zealand and the olympic mountains of washington state all these examples exhibit asymmetric topographic form with the asymmetry consistent with the polarity of subduction suggesting that horizontal tectonic motion is affecting the macro geomorphic form of these ranges

chertification histories of some late mesozoic and middle palaeozoic platform carbonates a consistent pattern for the silica sources depositional environments and timing of chertification was observed in a diverse suite of five late mesozoic and middle palaeozoic carbonate sequences the 1 upper greensand cretaceous and 2 portland limestone jurassic of southern england 3 the ramp creek formation mississippian of southern indiana and the 4 lower helderberg group devonian and 5 onondaga limestone devonian of new york state nodular chert formation in all five limestone sequences occurred in sediments that were largely uncemented ghosts of pre chertification carbonate cements are present in some chert nodules but are volumetrically minor in every limestone sequence except the upper greensand chertification occurred after burial to a depth sufficient for intergranular pressure solution and mechanical grain deformation of carbonate sand nodular chert is most abundant in subtidal normal marine wackestones and mudstones that were deposited at or below fair weather wave base and is absent or rare in supratidal intertidal and high energy subtidal limestones and dolomites an intraformational sponge spicule silica source for chert nodules is suggested by direct evidence such as calcitized sponge spicules in the host limestone and circumstantial evidence such as ghosts of sponge spicules in chert nodules and a correlation of chert abundance with depositional environment most molds of siliceous sponge spicules were apparently obliterated by post chertification intergranular compaction we propose that these general trends for the depositional environments silica sources and timing of chertification are representative of most mesozoic to middle palaeozoic platform limestones

efficient three dimensional data inversion soil characterization and moisture monitoring from cross well ground penetrating radar at a vermont test site we extend our methodology for three dimensional parameter structure and value estimation and apply it to a vermont test site ground penetrating radar gpr cross well travel times are inverted for estimation of heterogeneous gpr soil velocities before and after a controlled release of salt water in the unsaturated zone the method which is based on an approximation of the extended kaiman filter in conjunction with data driven zonation automatically estimates not only distributed zone values but also the number of zones zone geometry and zone covariance resultant gpr velocity estimates are shown to reduce travel time estimation errors and to be consistent with independent cone penetrometer measurements at all five walls at the site comparison of velocity estimates before and after forced injection of salt water is used to detect and visualize soil moisture patterns in three dimensions by varying the cluster tolerance criterion in the data driven zonation process the user can obtain a desired resolution of heterogeneity number of zones used in the resultant model

lateral variation in slab orientation beneath toba caldera northern sumatra the investigator fracture zone ifz subducts beneath toba caldera the earth s largest quaternary caldera in northern sumatra suggesting a possible relationship between them locations of sub crustal earthquakes based on arrival times of p and s waves at a seismograph network surrounding toba reveal the geometry of the subducted slab and the ifz beneath toba a vertical tear of less than 20 km in the slab across the ifz as previously suggested cannot be ruled out but the large scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench the slab shape is probably a response to the trench curvature can explain the change in trend of the volcanic arc near toba and may cause shallowing of the forearc basin near nias island the decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle possibly resulting in lateral compressive stress in the slab the high rate of seismicity along the subducted investigator fracture zone that intersects the slab obliquely to its plunge direction is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

protolith relations of the gravina belt and yukon tanana terrane in central southeastern alaska metamorphic rocks west of the coast mountains batholith in central southeastern alaska are divided into the gravina belt taku terrane and newly defined ruth assemblage the ruth assemblage comprises metapelite quartzose metaclastic strata quartzite marble felsic metatuff mafic metavolcanic rocks and orthogneiss depositional and emplacement ages of 367 pm 10 ma and 345 pm 13 ma inferred from discordant u pb zircon analyses on felsic metatuff and granodioritic orthogneiss respectively require that at least portions of the ruth assemblage be late devonian and early mississippian in age the assemblage is similar in age and protolith to and thus correlated with the yukon tanana terrane the gravina belt is characterized by upper jurassic and lower cretaceous mafic volcanic rocks and tuffaceous turbiditic clastic strata that unconformably overlie the alexander terrane metamorphic rocks that structurally underlie the taku terrane and ruth assemblage are included in this assemblage trace

microfossils from the early proterozoic hornby bay group district of mackenzie northwest territories canada spheroidal and filamentous organic walled microfossils have been detected in ca 1 7 ga old cherts of the hornby bay group northwest territories canada the majority of the spheroidal forms range from 1 to 4 m in diameter are referable to the genus sphaerophycus and probably represent the preserved sheaths of chroococcacean cyanophytes a single robust walled 27 m diameter spheroidal microfossil of undetermined affinity is also present the filamentous forms are tubular unbranched and range from 1 to 8 m in diameter they appear to represent the preserved sheaths of nostocalean cyanophytes or filamentous bacteria the filaments illustrate the relationship between matrix mineralogy and the fidelity of preservation of organic walled microfossils where they occur in silica the filaments are preserved as three dimensional tubular micro structures which are readily recognized as microfossils in contrast where they extend from silica into adjacent dolomite they are highly compressed and not read

application of the principal parameters method to the 1983 coalinga california aftershock sequence abstract a new technique called the method of the principal parameters ebblin and michelini in press to infer the orientations of active fault planes of an aftershock sequence has been applied to the sequence that followed the coalinga california earthquake of 2 may 1983 m l 6 7 the method is based on observed clustering in time and space clustering is a characteristic feature of aftershock sequences and it suggests interdependence of the events it follows that the spatial locations of time successive foci may provide additional information about the geometries of the rupturing fault system the method involves sliding of a temporal window of a fixed number of foci along the sequence and estimating the eigenvalues and eigenvectors of a spatial matrix for each window set this matrix can be interpreted as an ellipsoid which is fitted through the foci during the earthquake sequence the different trends of the seismicity pattern can be isolated by selecting and averaging the greatly flattened ellipsoids in the case of the coalinga sequence the trends are generally consistent with one of the focal planes obtained from the fault plane solutions given in earlier published studies the method appears to offer a simple way to infer average active fault geometries in complex areas from hypocentral locations only

topographic form of the coast ranges of the cascadia margin in relation to coastal uplift rates and plate subduction the coast ranges of the cascadia margin are overriding the subducted juan de fuca gorda plate we investigate the extent to which the latitudinal trend in average topography of the coast ranges is a function of the latitudinal change in attributes related to the subduction process these attributes include the variable age of the subducted slab that underlies the coast ranges and average vertical crustal velocities of the western margin of the coast ranges for two markedly different time periods the last 45 years and the last 100 kyr these vertical crustal velocities are computed from the resurveying of highway bench marks and from the present elevation of shore platforms that have been uplifted in the late quaternary respectively topography of the coast ranges is in part a function of the age and buoyancy of the underlying subducted plate this is evident in the fact that the two highest topographic elements of the coast ranges the klamath mountains and the olympic mountains are underlain by youngest subducted oceanic crust the subducted blanco fracture zone in southernmost oregon is responsible for an age discontinuity of subducted crust under the klamath mountains the northern terminus of the topographically higher klamaths is offset to the north relative to the position of the underlying blanco fracture zone the offset being in the direction of migration of the fracture zone as dictated by relative plate motions vertical crustal velocities at the coast derived from bench mark surveys are as much as an order of magnitude greater than vertical crustal velocities derived from uplifted shore platforms this uplift rate discrepancy indicates that strain is accumulating on the plate margin to be released during the next interplate earthquake in a latitudinal sense average coast range topography is relatively high where bench mark derived short term vertical crustal velocites are highest because the shore platform vertical crustal velocites reflect longer term permanent uplift we infer that a small percentage of the interseismic strain that accumulates as rapid short term uplift is not recovered by subduction earthquakes but rather contributes to rock uplift of the coast ranges the conjecture that permanent rock uplift is related to interseismic uplift is consistent with the observation that those segments of the subduction zone subject to greater interseismic uplift rates are at approximately the same latitudes as those segments of the coast ranges that have higher magnitudes of rock uplift over the long term

structure of the benioff zone under southern peru to central chile abstract an attempt is made to define the shape of the benioff zone between southern peru and central chile approximately 708 earthquake events with mb 3 5 and depths of 35 300 km located within a zone bounded by 15 s 30 s and 60 w 75 w were selected two aseismic zones are observed one starts at about 150 km depth and is located near 17 s latitude the second is located near 25 s latitude beginning at a depth of 70 km the subducting plate appears to dip between 12 and 25 eastward as evidenced from vertical cross sections through the hypocenter locations trend analysis of the benioff zone is made by fitting polynomial surfaces through the hypocenter locations the first degree trend surface approximates the subducting plate as a planar surface dipping 12 towards n75 e the second through fourth degree trend surfaces accentuate the curved nature of the benioff zone the overall shape of the benioff zone appears to be a continuous but undulating slab rather than separate tongues or a segmented slab of the subducting lithosphere

discussion of origins of hinterland basins to the rif betic cordillera and carpathians abstract the hinterland basins of the rif betic cordillera western alboran sea and the carpathians pannonian basin are in part composed of a thick in excess of 3 4 km thermal sag basin sequence the underlying crust has been thinned to 25 km in parts of the pannonian basin and to 15 km in parts of the alboran sea modern tectonic models for these areas envision tectonic collision and thickening of the crust followed by crustal thinning either by gravitational collapse back arc extension or a combination of the two contemporaneous with thrusting basin and range type extension has been envisioned for parts of these extensional collapse systems extensional features are common and they interact in a complex way with strike slip systems flexural cantilever modelling agrees with earlier mckenzie type pure shear modelling of the extensional systems in suggesting that extension alone cannot generate sufficient thermal decay to explain the great thickness of the sag basins associated with the alboran sea and pannonian basin consequently it is suggested that uplift of the asthenosphere accompanying subduction roll back is an important factor in the evolution of these basins roll back of the mantle lithosphere permits hot asthenospheric mantle to upwell and lie close to the base of the crust initially uplift and erosion of the crust overlying the asthenosphere occurs as the thermal perturbation cools the crust thermally subsides to produce a large sag basin

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

p waveform analysis for local subduction geometry south of puget sound washington the local subduction geometry at a site south of puget sound in western washington is investigated using teleseismicp waveforms recorded on a three component event triggered seismograph the data are processed using source equalization deconvolution in order to isolate locally convertedp to s arrivals and stacked to improve the signal to noise ratio stable arrivals in the radial component indicate an oceanic moho within the subducted slab at a depth of about 53 km beneath the station observed amplitude variations with azimuth in the radial data as well as qualitative aspects of the tangential data are used to establish a slab dip of 16 to the southeast our results are compatible with previous results from a site 60 km to the west and further confirm a substantial warp in the regional geometry of the subducted juan de fuca plate

u pb zircon and geochemical evidence for bimodal mid paleozoic magmatism and syngenetic base metal mineralization in the yukon tanana terrane alaska new shrimp sensitive high resolution ion microprobe u pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic yukon tanana terrane in east central alaska help define the tectonic setting of mid paleo zoic magmatism and syngenetic hydrothermal zn pb ag mineralization along the ancient pacific margin of north america we compare data from similar greenschist facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks in the wood river area of the alaska range and the salcha river area of the yukon tanana upland and from amphibolite facies augen gneiss and mafic gneiss amphibolite in the goodpaster river area of the upland allowing for analytical uncertainties igneous crystallization age ranges of 376 353 ma 378 346 ma and 374 358 ma are indicated by 13 new shrimp u pb dates for the wood river salcha river and goodpaster river areas respectively bimodal magmatism is indicated by late devonian crystallization ages for both augen gneiss 371 3 and 362 4 ma and associated orthoamphibolite 369 3 ma in the upland and by stratigraphic interleaving of mafic and felsic rocks in the alaska range metabasites in all three study areas have elevated hfse high field strength element and ree rare earth element contents indicative of generation in a within plate extensional tectonic setting within plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the bonnifield district in the wood river area and for metarhyolite tuff interlayered with the carbonaceous nasina assemblage which hosts sedimentary exhalative sulfide occurrences in the salcha river area most of the other felsic metaigneous samples from the alaska range and the yukon tanana upland have geochemical signatures that are similar to those of both average upper continental crust and continental margin arc rocks generated in thick continental crust given the absence in our study areas of intermediate composition magmatic products generally found in most arcs and the presence of bimodal magmatism the alkalic within plate chemistry of the mafic rocks and some of the felsic rocks and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin we consider it most likely that this prolonged late devonian to early mississippian magmatic episode resulted from attenuation of the ancient continental margin of western north america rather than development of an arc as proposed by many others

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

slab geometry of the cascadia subduction zone beneath washington from earthquake hypocenters and teleseismic converted waves using earthquake hypocenters obtained from the washington regional seismograph network and analysis of ps converted phases from teleseisms at special temporary stations located south of the olympic peninsula we have developed a preliminary model of the structure of the subducted slab beneath western washington and southern british columbia the slab is smoothly arched beneath puget sound subducting at a dip of about 10 12 at the top of the arch north and south of the arch crest the slab dips 15 20 subcrustal earthquakes are localized near the top of the arch this deformation from planar slab geometry is caused by the change in azimuth of the trench or hinge line of the slab from almost ns south of 47 north latitude to approximately n30 w north of 48 north the slab appears to be easily flexed probably due to small elastic thickness but its geometry reflects conservation of surface area and minimization of in plane strain crustal seismicity in the puget sound basin may result indirectly from the presence of the arch in the underlying juan de fuca plate

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

gravity anomalies crustal structure and seismicity at subduction zones 2 interrelationships between fore arc structure and seismogenic behavior an ensemble averaging technique is used to remove the long wavelength topography and gravity field associated with subduction zones short wavelength residual anomalies are attributed to the tectonic structure of subducting and overthrusting plates a paired positive negative fore arc anomaly is observed consisting of a long 1000 km linear trench parallel ridge landward of the deep sea terrace basin ridges have amplitudes of 1500 3000 m and 160 240 mgal wavelengths of 150 200 km and high gravity anomaly to topography ratios 50 75 mgal km 1 the ridge crests correlate with the downdip limit of coseismic slip and strong interplate coupling and in cascadia the updip limit of tremor epicenters the ridge crest may be interpreted as defining the boundary between the velocity weakening and seismogenic region of the subduction interface and the downdip frictional transition zone in tonga kermadec the kuril islands and chile landward ridges are associated with extinct volcanic arcs paired anomalies are attributed to the preferential subduction erosion of the outer fore arc and a spatially varying combination of a lower crustal underplating beneath the inner fore arc b the transformation of interseismic strain into permanent geologic strain via faulting folding or buckling of the inner fore arc and c the relative trenchward migration of extinct volcanic arcs in regions operating with a net crustal deficit along strike transitions in fore arc morphology and seismogenic behavior are related to preexisting crustal structure of subducting and overthrusting plates fore arcs have the added potential of recording the time integrated response of the upper plate to subduction processes and fore arc structure should be considered in tandem with seismological observations

exceptional preservation of fossils in an upper proterozoic shale late proterozoic organisms must have been diverse and widely distributed but in general their fossil record is both taxonomically and environmentally limited exceptional preservation of proterozoic fossils is not unknown but it is usually associated with silicified carbonates from restricted peritidal or playa lake environments1 2 we report here an exceptionally well preserved and distinctive assemblage of late proterozoic fossils from subtidal marine shales in addition to the sphaeromorphic acritarchs and cyanobacterial sheaths routinely preserved in proterozoic rocks this assemblage includes multicellular algae seaweeds a diverse assortment of morphologically complex protistan vesicles and probable heterotrophic bacteria thus it provides one of the clearest and most taxonomically varied views of proterozoic life yet reported

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

lead isotopic fingerprinting of tectono stratigraphic terranes east central alaska common lead isotopic compositions have been determined on feldspars from meta igneous rocks from nine tectono stratigraphic terranes or subterranes in east central alaska most of the terranes have distinct and well defined signatures in terms of isotopic composition thus most can be distinguished on conventional lead isotopic diagrams lead isotopic ratios provide evidence for 1 possible sources for the igneous rocks 2 time of metamorphism 3 correlation of terrane fragments and 4 delineation of juxtaposed terranes determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono stratigraphic terranes as to mantle or crustal origin and in correlation particularly where terrane relationships are enigmatic

three dimensional model of the cascadia subduction zone using earthquake hypocenters western washington abstract the identification of seismically active fault planes is important for assessment of earthquake potential as well as development of neotectonic models and analysis of stress systems for a region an automated set of computer programs seispln has been developed for the identification of three dimensional 3 d alignments of seismic hypocenters the seispln technique is based on rotating a 3 d elongate grid system through the hypocenter distribution the number of events in each cell is tabulated and those cells which contain statistically significant numbers of events are considered to be potential fault surfaces a multiple linear regression is applied to the set of hypocenters in each delineated cell to model the 3 d geometry of each of the inferred fault surfaces we applied the technique to earthquakes located by the washington regional seismic network to separate the seismic events associated with the subducted juan de fuca jdf plate from events in the overlying crust a total of 259 jdf associated events were delineated and the resulting 3 d models of the jdf slab beneath washington reveal a complex combination of arching and flattening also a steep orientation for the slab is observed beneath the southern portion of the washington cascades while a much shallower orientation is inferred beneath the northern cascades

modeling atmospheric o2 over phanerozoic time a carbon and sulfur isotope mass balance model has been constructed for calculating the variation of atmospheric o2 over phanerozoic time in order to obtain realistic o2 levels rapid sediment recycling and o2 dependent isotope fractionation have been employed by the modelling the dependence of isotope fractionation on o2 is based for carbon on the results of laboratory photosynthesis experiments and for sulfur on the observed relation between oxidation reduction recycling and s isotope fractionation during early diagenetic pyrite formation the range of fractionations used in the modeling agree with measurements of phanerozoic sediments by others results derived from extensive sensitivity analysis suggest that there was a positive excursion of o2 to levels as high as 35 during the permo carboniferous high o2 at this time agrees with independent modeling based on the abundances of organic matter and pyrite in sediments and with the occurrence of giant insects during this period the cause of the excursion is believed to be the rise of vascular land plants and the consequent increased production of o2 by the burial in sediments of lignin rich organic matter that was resistant to biological decomposition

three dimensional dislocation model for great earthquakes of the cascadia subduction zone there have been no historical cascadia great subduction thrust earthquakes but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event geodetic measurements in the coastal region from northern california to southern british columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault the segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two dimensional 2 d elastic dislocation modeling of interseismic deformation geodetic data in this study a general 3 d dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture the model is based on die surface deformation due to shear faulting in an elastic half space the 3 d model of the cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin the bend in the margin trend and subducting slab end effects are included there is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement an initial 3 d model based upon 2 d dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data the best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km the two zones lie mainly offshore beneath the continental shelf and slope the locked and transition zone widths vary smoothly along the margin being greater off northern washington where the thrust dip is shallow and narrower off central oregon assuming that the locked plus transition zones approximate the maximum coseismic rupture area these widths permit amw 9 earthquake

deep crustal fracture zones control fluid escape and the seismic cycle in the cascadia subduction zone abstract seismic activity and non volcanic tremors are often associated with fluid circulation resulting from the dehydration of subducting plates tremors in the overriding continental crust of several subduction zones suggest fluid circulation at shallower depths but potential fluid pathways are still poorly documented using receiver function analysis in the cascadia subduction zone we provide evidence for a seismic discontinuity near 15 km depth in the crust of the overriding north american plate this interface is segmented and its interruptions are spatially correlated with conductive regions of the forearc and shallow swarms of seismicity and non volcanic tremors these observations suggest that fluid circulation in the overriding plate is controlled by fault zones separating blocks of accreted terranes these zones constitute fluid escape routes that may influence the seismic cycle by releasing fluid pressure from the megathrust

ordovician aleman ribbon cherts an example of silicification prior to carbonate lithification abstract the ordovician aleman formation southwestern new mexico is primarily composed of dolomite with ribbons of chert the presence of stromatolites relict evaporite nodules and other features indicates an intertidal to shallow subtidal depositional environment the stromatolitic layers are preferentially silicified with dissolution of opaline sponge spicules providing the main if not sole source of silica silicification occurred in both the vadose and phreatic realms essentially penecontemporaneous with accumulation of the sediment and concomitant with dolomitization this is indicated by the preservation in silicified form of original anhydrite lath morphology central cavities in chert nodules partially filled with geopetal matrix dolomite and matrix dolomite immediately adjacent to conchoidal fractures of silicified carbonate grainstones the latter relationship indicates that the chert was rigid and fractured before the carbonate host sediments were lithified

trends in oolite dolomitization across the neoproterozoic cambrian boundary a case study from death valley california abstract changes in dolomite fabric within the neoproterozoic cambrian stratigraphic succession in death valley california preserve evidence for a first order biogeochemical change in early diagenetic environments through the neoproterozoic cambrian transition oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time two key generalizations can be made 1 late neoproterozoic dolomitization in death valley occurred very early in the paragenetic sequence was volumetrically significant and preserved fine detail of the original aragonite or calcite precursor and 2 early cambrian dolomitization in death valley occurred late in the paragenetic sequence with burial was commonly incomplete and obliterated details of the calcite or aragonite precursor early diagenetic dolomitization of any kind was absent the dolomitization fabrics in death valley parallel the presumed pattern of seawater mg ca with respect to mimetic dolomitization in the paleozoic low mg ca little no mimetic dolomitization and cenozoic higher mg ca more common mimetic dolomitization however the volume of neoproterozoic mimetic dolomitization is immense compared to the cenozoic examples we speculate that a fundamental shift in the early diagenetic environment occurred in late neoproterozoic time perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment capping microbial mats an oscillatory trend in the fabric and volume of dolomite through the phanerozoic has been suggested but the termination of volumetrically significant mimetic dolomitization across the neoproterozoic cambrian transition a critical time in earth history for many reasons dwarfs the oscillatory trends noted in the phanerozoic

a survey of paleozoic microbial fossils in chert abstract non acritarch microbial fossils from paleozoic marine cherts exhibit generally poor preservation with rare exceptional preservation microbial microfossils were discovered in 11 out of 17 stromatolitic and non stromatolitic paleozoic units containing replacement cherts from the continental united states many microfossils likely have a cyanobacterial affinity having both coccoid and filamentous morphotypes examined units range in age from middle cambrian to late permian and were deposited in peritidal open shelf and basinal settings all of which have preserved microbial assemblages cherts from the examined units generally formed during early diagenesis and the quality of microbial preservation does not seem to have changed during the paleozoic which is a period that experienced significant secular changes in the mode of marine silica formation

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

current deformation and the width of the seismogenic zone of the northern cascadia subduction thrust evidence has been obtained for the accumulation of elastic strain across the northern cascadia subduction zone that may be released in a future very large subduction thrust earthquake vertical and horizontal strain rates across the southern vancouver island region have been determined through 1 long term trends in tide gauge data 2 changes in repeated accurate leveling surveys 3 changes in repeated high accuracy gravity profiles and 4 horizontal shortening observed in repeated precise positioning surveys the outer coast is uplifting at a rate of a few millimeters per year decreasing landward and shortening is occurring across the 100 km wide coastal region at a rate of about 0 1 microstrain per year mm km 1yr 1 the results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault the location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern vancouver island and northern washington provided the locked portion is restricted to a 60 km wide band offshore beneath the continental shelf and slope this conclusion also results from modeling the coseismic subsidence on the outer coast of vancouver island about 300 years ago deduced from paleoseismicity data the unusually narrow downdip extent of the subduction thrust seismogenic zone that extends little if at all beneath the coast is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments the high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km the distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of vancouver and victoria from this source the narrow width may also limit the earthquake size however events of magnitude well over 8 are possible

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

high pressure metamorphism and uplift of the olympic subduction complex the discovery of the critical assemblage lawsonite quartz calcite indicates that a significant part of the cenozoic olympic subduction complex of northwestern washington state formed by underplating at a depth of about 11 km the deep structural level exposed in this area is attributed to the presence of a 10 km high arch in the underlying juan de fuca plate we postulate that this arch was formed when the southern cordilleran coastline swung westward as a result of middle miocene to recent extension in the basin and range province

crustal structure beneath the strait of juan de fuca and southern vancouver island from seismic and gravity analyses 1 wide angle and vertical incidence seismic data from seismic hazards investigations in puget sound ships gravity modeling and seismicity are used to derive two dimensional crustal models beneath the strait of juan de fuca the eocene volcanic crescent siletz terrane is significantly thicker than previously recognized and extends from near the surface to depths of 22 km or greater for the northern strait a weak midcrustal reflector dipping east from 12 to 22 km depth is inferred from wide angle reflections a stronger deeper reflector dipping eastward from 23 to 36 km depth is associated with the top of reflector band e a zone of high reflectivity on coincident multichannel seismic mcs data interpreted as a shear zone a high velocity zone 7 60 0 2 km s 1 between these reflectors is interpreted as a localized slice of mantle accreted with the overlying crescent siletz terrane for the southern strait no deep high velocity layer is observed and the e band reflectivity is weaker than to the north a strong deep reflector interpreted as the oceanic moho dips eastward from 35 to 42 km seismicity within the subducting slab occurs mainly above the inferred oceanic moho gravity modeling constrained by the wide angle seismic models and seismicity is consistent with the inferred large thickness of crescent siletz and high density rocks 3030 kg m 3 in the lower crust

nodular chert from the arbuckle group slick hills sw oklahoma a combined field petrographic and isotopic study nodular chert from the middle and upper arbuckle group early ordovician in the slick hills sw oklahoma was formed by selective replacement of grainstones burrow fillings algal structures and evaporite nodules chert nodules are dominantly microquartz with minor fibrous quartz both quartzine and chalcedony megaquartz and microflamboyant quartz lepisphere textures of an opal ct precursor are preserved in many especially in finely crystalline chert nodules the 18o values of microquartz chert range from 23 4 to 28 80 00 smow significantly lower than those of cenozoic and mesozoic microquartz chert formed both in the deep sea and from near surface sea water the 18o values of chert decrease with increasing quartz crystal size silicification in the arbuckle group occurred during early diagenesis with the timing constrained by the relative temporal relationships among silicification burial compaction and early dolomite stabilization silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica enriched sea water chert nucleation appears to have been controlled by the porosity permeability and organic matter content of precursor sediments this conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter growth of chert probably occurred by a maturation process from opal a to opal ct to quartz as indicated by the presence of opal ct precursor textures in many chert nodules although field and petrographic evidence argues for an early marine origin for chert in the arbuckle group the light 18o values are inconsistent with this origin meteoric resetting of the 18o values of the chert during exposure of the carbonate platform best explains the light 18o values because i the 18o values of chert nodules decrease with decreasing 18o values of host limestones and ii chert nodules from early dolomite which underwent more extensive meteoric modification than associated limestones have lighter 18o values than chert nodules from limestones increasing recrystallization of chert nodules by meteoric water resulted in progressive 18o depletion and quartz crystal enlargement

tectonic framework of the upper paleozoic and lower mesozoic alava sequence a revised view of the polygenetic taku terrane in southern southeast alaska fragments of upper paleozoic and lower mesozoic metavolcanic and metasedimentary sequences of the taku terrane are exposed discontinuously along a narrow belt in southeast alaska and form a distinct lithostratigraphic package in the ketchikan area called the alava sequence crinoidal and argillaceous marble carbonaceous phyllite argillite mafic flows pillow breccia pyroclastic tuff and quartzite characterize the sequence these strata are unconformably overlain by upper jurassic to lower cretaceous fine to coarse grained epiclastic rocks of the gravina sequence the upper paleozoic part of the alava sequence may be correlative with the yukon tanana terrane whereas the middle and upper triassic portion of the alava sequence may represent a metamorphic vestige of the stikine terrane both parts are now exposed on the western flank of the coast plutonic complex in contrast with their correlatives to the east these relations suggest that the stikine and alexander terranes were juxtaposed prior to deposition of the gravina sequence the western boundary between rocks of north american affinity and allochthonous ensimatic crustal fragments of the alexander and wrangellian terranes lies west of the coast plutonic complex

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

a model for terrane accretion yukon tanana and slide mountain terranes northwest north america to date many north american cordilleran geologists employing the concept of suspect terranes have emphasized differences in lithology metamorphism styles of deformation and timing of these distinct tectonic units arguing that each evolved separately from its neighbors this practice of focusing on dissimilarity and nomenclature used in terrane analysis may be detrimental to our understanding of the evolutionary relationships between terranes i present a model in which the yukon tanana yt and slide mountain sm terranes which do exhibit certain lithotectonic disparities are correlated within a unifying tectonic framework in which each terrane represents a different crustal level of the same late paleozoic early mesozoic magmatic arc and west dipping subduction complex formed during plate convergence outboard of western north america the sm represents the upper crustal level of the arc and accretionary complex and the heterogeneously metamorphosed and ductilely deformed yt represents the deeper crustal level the teslin suture zone tsz the high p t portion of the yt formed the decollement between the yt sm composite terrane hanging wall and the descending north american plate tsz structural relations thus provide insight into the kinematic interaction of this composite terrane with north america prior to its late stage emplacement onto north america the present day collage of suspect terranes clearly comprises a multitude of structurally dismembered and dissimilar lithotectonic assemblages nonetheless it is possible to see order in the chaos to reassemble the pieces in a tectonically meaningful fashion the yt tsz sm association in yukon may serve as a model for tectonically coherent continental margin expansion

structural and kinematic evolution of the teslin suture zone yukon record of an ancient transpressional margin abstract the teslin suture zone tsz yukon which forms the fundamental boundary between rocks deposited along the ancient margin of north america and allochthonous terranes to the west preserves a complex history of pre mid jurassic convergence and dextral strike slip translation which is overprinted locally by late cretaceous dextral strike slip shear the tsz comprised of a 15 20 km thick structural sequence of ductilely deformed sedimentary and volcanic strata basalt peridotite and granitoids is divisible into distinct elongate structural domains based on the distribution of two well defined but differently oriented stretching lineations lds and lss lds and lss both formed during non coaxial ductile deformation lds trends e w and plunges down dip whereas lss trends nnw sse and plunges shallowly two 1 2 km wide nnw trending anastomosing shear zones of lss cross cut lds structures indicating lss is younger other field relations indicate further that lds began forming earlier than lss followed by a period of coeval lds and lss formation latest motion was dominantly parallel to lss synkinematic mineral assemblages associated with both lds and lss related structures record greenschist to albite epidote amphibolite facies conditions lds tectonites record a complex movement history to the west textural asymmetries indicate west side down or normal shear whereas in eastern domains top to the east thrust style asymmetries dominate lss tectonites consistently record dextral or top to the north shear parallel to lss tectonic motion began as chiefly penetrative dip slip shear lds at a high angle to the present trend of the tsz and evolved to dominantly dextral strike slip shear lss parallel to the trend of the zone the geometry kinematics and sequence of deformation in tsz tectonites evoke a general model of terrane accretion during oblique plate convergence involving initial shortening at a high angle to the convergent margin and progression to margin parallel translation

seismic amplification within the seattle basin washington state insights from ships seismic tomography experiments recent observations indicate that the seattle sedimentary basin under lying seattle and other urban centers in the puget lowland washington amplifies long period 1 5 sec weak ground motions by factors of 10 or more we computed east trending p and s wave velocity models across the seattle basin from seismic hazard investigations of puget sound ships experiments to better characterize the seismic hazard the basin poses the 3d tomographic models which resolve features to a depth of 10 km for the first time define the p and s wave velocity structure of the eastern end of the basin the basin which contains sedimentary rocks of eocene to holocene is broadly symmetric in east west section and reaches a maximum thickness of 6 km along our profile beneath north seattle a comparison of our velocity model with coincident amplification curves for weak ground motions pro duced by the 1999 chi chi earthquake suggests that the distribution of quaternary deposits and reduced velocity gradients in the upper part of the basement east of seattle have significance in forecasting variations in seismic wave amplification across the basin specifically eastward increases in the amplification of 0 2 to 5 hz energy correlate with locally thicker unconsolidated deposits and a change from crescent formation basement to pre tertiary cascadia basement these models de fine the extent of the seattle basin the seattle fault and the geometry of the basement contact giving insight into the tectonic evolution of the seattle basin and its influence on ground shaking

trace element geochemistry of metabasaltic rocks from the yukon tanana upland and implications for the origin of tectonic assemblages in east central alaska we present major and trace element geochemical data for 27 amphibolites and six greenstones from three structural packages in the yukon tanana upland of east central alaska the lake george assem

detrital zircon record of a mesozoic collisional forearc basin in south central alaska the tectonic transition from an oceanic to continental arc

secular distribution of biogenic silica through the phanerozoic comparison of silica replaced fossils and bedded cherts at the series level abstract the modern marine silica cycle is dominated by silica secreting phytoplankton principally diatoms but this cycle has evolved considerably during the phanerozoic we analyzed the temporal distribution of silica replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies trends in silica replacement of fossils match faunal radiations during the ordovician and siluro devonian with peaks in silica replacement in the late ordovician and middle devonian corresponding to peaks in the abundance and diversity of siliceous sponges sharp drops in the abundance of silica replaced fossils and or bedded cherts coincide with four of the five major mass extinctions no discernible decrease marks the extinction at the end of the triassic we expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling orogenic episodes which may trigger continental glaciati

late quaternary stream incision and uplift in the forearc of the cascadia subduction zone western oregon documentation of a latest pleistocene earliest holocene episode of strath formation and fluvial aggradation in the oregon coast range provides a datum from which long term bedrock stream incision rates are determined variations in long term incision rates probably reflect cumulative differential uplift in the forearc of the cascadia subduction zone although factors such as bedrock and climatic controls and isostatic adjustments to erosion obscure the precise relationship between surface uplift and stream incision patterns of differential incision are most striking near the latitude of newport where a steep gradient divides a region of higher rates 0 6 0 9 mm yr in the northern coast range from a region of lower rates 0 1 0 3 mm yr in the central coast range the steep incision gradient is nearly coincident with abrupt changes in marine terrace 80 125 kyr uplift rates the locations of quaternary faults and the southern flank of a saddle of low historic 40 70 years uplift the exact causes of these variable patterns of incision uplift are unknown analogies with uplift patterns in other subduction zones and comparisons with other neotectonic data in the region indicate that patterns of differential incision probably are caused by variations in permanent strain accumulation along the cascadia subduction zone such variations may be related to differences in seismic moment release during individual earthquakes to changes in plate geometry or rates of wedge accretion to segmentation of earthquake ruptures and or to deformation on active structures in the north american plate and accretionary wedge

ten kilometer vertical moho offset and shallow velocity contrast along the denali fault zone from double difference tomography receiver functions and fault zone head waves we examine the structure of the denali fault system in the crust and upper mantle using double difference tomography p wave receiver functions and analysis spatial distribution and moveout of fault zone head waves the three methods have complementary sensitivity tomography is sensitive to 3d seismic velocity structure but smooths sharp boundaries receiver functions are sensitive to quasi horizontal interfaces and fault zone head waves are sensitive to quasi vertical interfaces the results indicate that the mohorovicic discontinuity is vertically offset by 10 to 15 km along the central 600 km of the denali fault in the imaged region with the northern side having shallower moho depths around 30 km an automated phase picker algorithm is used to identify 1400 events that generate fault zone head waves only at near fault stations at shorter hypocentral distances head waves are observed at stations on the northern side of the fault while longer propagation distances and deeper events produce head waves on the southern side these results suggest a reversal of the velocity contrast polarity with depth which we confirm by computing average 1d velocity models separately north and south of the fault using teleseismic events with m 5 1 we obtain 31 400 p receiver functions and apply common conversion point stacking the results are migrated to depth using the derived 3d tomography model the imaged interfaces agree with the tomography model showing a moho offset along the central denali fault and also the sub parallel hines creek fault a suture zone boundary 30 km to the north to the east this offset follows the totschunda fault which ruptured during the m7 9 2002 earthquake rather than the denali fault itself the combined results suggest that the denali fault zone separates two distinct crustal blocks and that the totschunda and hines creeks segments are important components of the fault and cretaceous aged suture zone structure

seismicity and focal mechanisms associated with the subduction of the juan de fuca plate beneath the olympic peninsula washington abstract two and a half years of data from an expanded telemetered seismic network on the olympic peninsula reinforce the conclusion that the juan de fuca plate is subducting beneath washington however the rate of seismicity is presently much lower than typical rates at other convergent margins the located earthquakes can be divided into two groups on the basis of depth at the coast a shallow zone extends to a depth of approximately 20 km while a deep zone begins at 25 km on the eastern edge of the peninsula the shallow zone has thickened to 30 km and the depth to the top of the deep zone has increased to 45 km the rate of crustal seismicity is much lower on the peninsula than in the adjacent puget sound region the deep zone is 5 to 10 km thick and dips to the east at an angle of about 11 the deep events appear to be occurring in the upper part of the subducting oceanic lithosphere focal mechanisms were constructed for 16 events in the suite of deep events with magnitudes ranging from 1 6 to 4 3 the solutions show primarily normal faulting with the p axes normal to the slab and the t axes displaying a range of angles within the slab with the average t axis in the downdip direction small events with down dip tension in the downgoing slab are consistent with the slab sinking under its own weight several strike slip events may be caused by a shearing within the slab these features are consistent with ongoing subduction of the juan de fuca plate beneath north america