synoptic settings of westerly wind bursts the morphology of westerly wind bursts wwbs during the 1980s is investigated using 1000 mbar winds analyzed by the european center for medium range weather forecasts ecmwf the variety of synoptic situations in which wwbs occur are explored so that the dynamics of wwb generation and maintenance can be studied and so that those interested in short and long term forecasts of conditions over the equatorial pacific will have an increased knowledge of those elements associated with wwbs wwbs are identified by searching for episodes in which the zonal wind was at least 5 m s 1 over at least 10 longitude for two or more days this definition based on surface winds rather than elevated winds or wind anomalies reflects the impact wwbs have on air sea interaction processes the spatial and temporal criteria provide a synoptic scale envelope for such interactions and mitigate against erroneous analyses according to these criteria 131 distinct wwbs occurred during the 1980s a new two dimensional classification scheme has been developed the nine patterns in it describe the near surface flow during 90 of these wwbs a single cyclone or a series of cyclones in one or both hemispheres and several different types of cross equatorial flow are the major components of the patterns only 8 of the bursts were associated with concurrent cyclones in both the northern and southern hemispheres four bursts involving cross equatorial flow are subjected to further study the flow is decomposed into barotropic and baroclinic parts and into rotational and divergent baroclinic components one of the bursts was predominantly barotropic and another predominantly baroclinic the cross equatorial flow in all four cases was predominantly divergent in nature the information from these cases coupled with the typical vertical structures and small meridional length scales associated with the various synoptic patterns suggests that simple baroclinic models of circulations driven by near equatorial heating are not adequate to describe the dynamics during wwbs

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

experiments on tropical circulation with a simple moist model a simple model of the circulation of the tropical atmosphere is constructed where the circulation is determined entirely by the sea surface temperature s s t pattern or rather by a forcing field which is closely related to s s t it is an equatorial beta plane model with an assumed simple baroclinic vertical structure the moisture and precipitation fields are allowed to evolve freely so there are feedbacks between the latent heating which contributes to driving the motion and the low level velocity field which produces the moisture convergence required to generate precipitation experiments with idealized pacific s s t distributions produce qualitatively realistic precipitation patterns for january a patch in the west pacific over the equator and july a zonal strip north of the equator analytic solutions for zonally symmetric flow are given to illustrate the basic dynamics and to study the parameter dependence of the precipitation an analysis of zonal variations shows that a good simplifying approximation for this component is to neglect rayleigh friction while retaining newtonian cooling the effect of changing the zonal contrast in the prescribed forcing is to gradually alter the circulation from hadley like to walker like the model suggests that along the equator a relatively small contrast is needed to break a zonal band of precipitation

evolution of the cloud pattern during the formation of tropical cyclone twins symmetrical with respect to the equator abstract the evolution of the pattern of the deep convective cloud is presented for three selected cases of tropical cyclone twins symmetrical with respect to the equator in each case the pattern evolution is similar and can be separated into four distinct stages

synoptic scale disturbances near the equator abstract a spectral analysis of winds analyzed and initialized at the european centre for medium range weather forecasts reveals an abundance of power in the 850 mb meridional wind along the equator with periods near four days the power is mostly in the westward propagating component using high pass filtered data it is shown that the waves have westward phase and eastward group propagation relative to the mean wind the longest wavelengths are found over the pacific ocean while the shortest are found over the convectively variable regions of indonesia south america and africa mean phase speeds at 850 mb are positively correlated with the mean wind on the equator at 500 mb and below and negatively correlated with the mean wind above that level the effective advecting zonal wind of the disturbances seems to be the density weighted average of the lower troposphere the structure of the disturbances bears resemblance to the expected structure of an equatorially trapped mixed rossby gravity wave over t

structure of large scale convective anomalies over tropical oceans abstract geographical variations in the variance and cross correlation of monthly mean sea surface temperature sst outgoing longwave radiation olr a proxy for deep convection and vertical motion and convergence of winds at the surface and at 850 mb across the tropical indian and pacific oceans are examined within about 10 of the equator at most longitudes the variance of these quantities associated with the seasonal cycle is less than the variance associated with anomalies from the seasonal cycle largest variances in the sst and surface convergence data occur across the eastern near equatorial pacific whereas olr and 850 mb convergence variances are largest across the western pacific olr anomalies are significantly correlated with collocated sst and surface convergence anomalies from the date line eastward to the south american coast but are uncorrelated west of the date line the olr and 850 mb convergence anomalies are significantly correlated from about 120 w westward but are uncorrelated

crustal structure of the colorado plateau arizona application of new long offset seismic data analysis techniques the colorado plateau is a large crustal block in the southwestern united states that has been raised intact nearly 2 km above sea level since cretaceous marine sediments were deposited on its surface controversy exists concerning the thickness of the plateau crust and the source of its buoyancy interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick a thick crust could support the observed topography of the colorado plateau isostatically while a thinner crust would indicate the presence of an underlying low density mantle this paper reports results on long offset seismic data collected during the 1989 segment of the u s geological survey pacific to arizona crustal experiment that extended from the transition zone into the colorado plateau in northwest arizona we apply two new methods to analyze long offset data that employ finite difference travel time calculations 1 a first arrival time inverter to find upper crustal velocity structure and 2 a forward modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals we find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern basin and range province to about 42 km beneath the northern transition zone and southern colorado plateau margin we observe some crustal thinning to 37 km thick and slightly higher lower crustal velocities farther inboard beneath the kaibab uplift on the north rim of the grand canyon the crust thickens to a maximum of 48 km we observe a nonuniform crustal thickness beneath the colorado plateau that varies by 15 and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations crustal compositions as inferred from seismic velocities appear to be the same beneath the colorado plateau as those in the basin and range province to the southwest implying that the plateau crust represents an unextended version of the basin and range some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the proterozoic era

vs and density structure beneath the colorado plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data 1 the colorado plateau is a physiographic province in the western us with an average elevation of 1 9 km where in contrast to neighboring provinces there is little evidence of large scale tectonic deformation or magmatism recent availability of earthscope usarray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau s uplift and relative stability using phase velocities for fundamental mode rayleigh waves and p receiver functions we perform over 800 joint inversions for 1 d shear wave velocity vs profiles sampling the plateau and surrounding regions down to 150 km depth we image a sharp change in crustal thickness at the western edge of the colorado plateau with a more gradual increase eastward moving into the rocky mountains a relatively thick 100 km lithosphere beneath the plateau extends into the rocky mountains to the north we use empirical scaling relations to estimate densities from our vs results and predict the associated gravity anomalies which are inconsistent with the observed distribution of the bouguer gravity anomalies we somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant while there is some evidence for small scale convection and lithospheric removal at its edges the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the farallon slab we conclude that the lithospheric root is key to the long term stability of the colorado plateau leading to a colder stronger crust

inferences for the socorro magma body from teleseismic receiver functions receiver functions constructed from teleseisms arriving from the northwest and recorded at station wtx in socorro nm show strong p to s conversions we interpret as coming from a mid crustal sill like magma body analysis of these conversions suggest that both the upper and lower surfaces of the magma body are distinct and provide sharp velocity contrasts with the surrounding host rock models that provide acceptable fits to these conversions all have magma body thicknesses less than 200 m the absence of similar phases in data from the south and southeast suggests that the magma body is discontinuous near its edges

support of high elevation in the southern basin and range based on the composition and architecture of the crust in the basin and range and colorado plateau toexplorethenatureofhowthestructureandphysicalpropertiesofthecrustvaryfromextendedtorelativelyunextendeddomains we present teleseismic receiver functions which measure crustal thickness shear wavespeed structure and the vp vs ratio at 12 seismic stations in eastern arizona the crustal thickness is 28 km increases slightly eastward and remains nearly uniform beneath the varying elevations in the southern basin and range the observed vp vs ratio in the basin and range 1 78 exceeds the global average the southern colorado plateau exhibits thicker crust 40 km and a slightly greater observed vp vs 1 81 a discrete region in the colorado plateau generates an unusually high vp vs ratio 1 90 and contains low wavespeed zones which serve as evidence of partial melt related to quaternary volcanism the metamorphic core complexes in the southern basin and range likewise exhibit anomalously high vp vs values 1 79 1 87 and lack locally compensating crustal roots density models show that 85 kg m 3 lighter crust or 35 kg m 3 lighter mantle than that of the surrounding basin and range helps these metamorphic core complexes maintain their high elevation compositional modeling of intrusive bodies exposed throughout the catalina rincon metamorphic core complex indicates that the observed high vp vs ratio and modeled low density could result from substantial amounts of a plagioclase rich quartz poor rock these vp vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant tertiary extension may have been preconditioned for orogenic collapse

exploratory analysis of surface winds in the equatorial western pacific and el nino abstract exploratory data analysis is used to examine several key characteristics of surface wind along a ship track in the equatorial western pacific a month by month examination is undertaken based on daily ship data from a recent 30 year record 1958 87 the characteristics considered here include the expected frequency of occurrence the intensity the range of variability and the extreme value of the wind from the eight point compass directions a hodograph and constancy of monthly mean wind vectors are also presented results from the 30 year climatology suggest that the equatorial western pacific is affected by monsoonal and trade flows from each winter hemisphere and by the eastern pacific subtropical highs during the transition season i e may equatorial westerlies peak in november 20 and december 18 composite analysis reveals the further influence of the trade flows from the northwest pacific from january to may and the southern hemisphere influence from june to september during

cocorp deep reflections moho at 50 km 16 s beneath the colorado plateau cocorp deep reflection data on the colorado plateau reveal complex reflections to about 16 s two way travel time 50 km beyond which there is an abrupt decrease of reflectivity this boundary is interpreted to represent the moho an inference consistent with the existing but limited refraction data sets when they are reconciled with the recent independent determination of an 8 10 8 15 km s pn velocity beneath the colorado plateau this 50 km depth to moho and 8 10 8 15 km s pn velocity for the colorado plateau closely resembles that of the high plains just east of the rocky mountains the absence of continuous strong reflections at the moho suggests that the moho beneath the colorado plateau is a transition not a velocity step function on the scale of the reflection experiment the reflection boundary may represent the base of complex and discontinuous reflectors of the crust and crust mantle transition below which the mantle may be largely peridotite with a velocity of 8 1 km s these results strongly reinforce the suggestion that the moho on deep reflection data in relatively stable continental regions is represented by an abrupt decrease in reflectivity below which the mantle is relatively seismically transparent

observing long period fluctuations of surface winds in the tropical pacific initial results from island data abstract the utility of studying low frequency surface weather phenomena with long time series of meteorological data from tropical pacific islands is demonstrated the wind stress changes associated with el nino events in the period 1950 78 are examined at seven locations zonal wind stress anomalies at the equator near the date line often exhibit strengthening and subsequent weakening of the trade winds prior to each el nino as originally suggested by wyrtki an exception is the weak 1963 el nino which is preceded by meridional wind stress anomalies at the equator the strongest zonal and meridional wind stress anomalies however occur well after the first occurrence of anomalously warm water off the coast of peru for each el nino in agreement with prior analyses of merchant marine data away from the equator variability of the wind stress anomalies from one el nino to the next is strong leading to numerous discrepancies with published profiles of the mean el nino wind changes power spectra of

the association of the evolution of intraseasonal oscillations to enso phase abstract the pacific ocean intraseasonal kelvin wave is a leading oceanic mode that links intraseasonal tropical atmospheric variations with interannual variations in the coupled ocean atmosphere system this study considers the premise that these waves may evolve differently with their associated weather patterns during different phases of el nino southern oscillation enso if atmospheric and oceanic intraseasonal modes interact and evolve differently during various stages of enso this result may provide useful information with regard to the role of these intraseasonal processes in enso evolution this work utilizes signals of the oceanic kelvin wave as a statistical basis for a simple composite averaging technique that is applied during different phases of enso to objectively analyze the evolution of oceanic and the associated portions of atmospheric intraseasonal oscillations results confirm the above premise and suggest that coupling between kelvin waves and atmospheric convection evolves differen

crustal structure and evolution beneath the colorado plateau and the southern basin and range province results from receiver function and gravity studies over the past several decades contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long term stability of the colorado plateau cp and crustal thinning beneath the basin and range province brp in the southwestern united states here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15 500 p to s receiver functions recorded at 72 usarray and other broadband seismic stations on the southwestern cp and the southern brp our results reveal that the brp is characterized by a thin crust 28 2 0 5 km a mean vp vs of 1 761 0 014 and a mean amplitude r of p to s converted wave relative to that of the direct p wave of 0 181 0 014 that are similar to a typical continental crust consistent with the model that the thin crust was the consequence of lithospheric stretching during the cenozoic the cp is characterized by the thickest crust 42 3 0 8 km largest vp vs 1 825 0 009 and smallest r 0 105 0 007 values in the study area in addition many stations on the cp exhibit a clear arrival before the p to s converted phase from the moho corresponding to a lower crustal layer of about 12 km thick with a mafic composition we hypothesize that the lower crustal layer which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines contributed to the long term stability and preuplift low elevation of the colorado plateau

a review of the regional geophysics of the arizona transition zone a review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the basin and range and colorado plateau occur in or directly adjacent to the region defined as the arizona transition zone although this region was designated on a physiographic basis studies indicate that it is also the geophysical transition between adjoining provinces the transition zone displays anomalous crustal and upper mantle seismic properties shallow curie isotherms high heat flow and steep down to the plateau bouguer gravity gradients seismic and gravity studies suggest that the change in crustal thickness from thin crust in the basin and range to thick crust in the colorado plateau may occur as a series of steps rather than a planar surface anomalous p wave velocities high heat flow shallow curie isotherms and results of gravity modeling suggest that the upper mantle is heterogeneous in this region a relatively shallow asthenosphere beneath the basin and range and transition zone contrasted with a thick lithosphere beneath the colorado plateau would be one explanation that would satisfy these geophysical observations

subseasonal atmospheric variability and el nino waveguide warming observed effects of the madden julian oscillation and westerly wind events abstractwesterly wind events wwes have previously been shown to initiate equatorial pacific waveguide warming the relationship between wwes and madden julian oscillation mjo activity as well as the role of mjo events in initiating waveguide warming is reconsidered here over the 1986 2010 period wwes are identified in observations of near surface zonal winds using an objective scheme mjo events are defined using a widely used index and 64 are identified that occur when the el nino southern oscillation enso is in its neutral state of these mjo events 43 have one or more embedded wwes and 21 do not the evolution of sea surface temperature anomaly over the equatorial pacific waveguide following the westerly surface wind phase of the mjo over the western equatorial pacific is examined waveguide warming is found for the mjo with wwe events in similar magnitudes as following the wwes not embedded in an mjo there is very little statistically significant waveguide warming following mjo events that

episodes of surface westerly winds as observed from islands in the western tropical pacific we describe the characteristics of surface westerly wind episodes in the tropical pacific between mid 1957 and 1980 as they can be determined from a collection of daily average wind records from islands several types of frequency of occurrence and duration statistics are presented events in which the maximum westerly anomalies exceed 7 m s 1 are not common but are prominent in the records there is strong seasonal and geographical variation in frequency of occurrence four types of westerly episodes are found we call them types n c s or fs according to whether the maximum westerly wind is found north of centered on south of or far south of the equator near the international date line sixteen 45 47 and 47 events of type n c s and fs are identified respectively the strongest westerly winds are associated with type c and s events maximum daily average values can exceed 20 m s 1 only type c and s events are found to have substantial equatorial zonal wind anomalies several examples of type c and s events are presented composite events of each type have been computed and their characteristics are described in order to explore relationships between equatorial westerly events and cyclones all of the named tropical cyclones within a region encompassing the near date line islands have been identified various statistics are presented but no absolute relationships have been identified the similarity between the composite type c and s anomaly patterns and patterns of the el nino southern oscillation monthly mean zonal wind anomaly previously documented in the literature is striking evidently the westerly events often dominate in the monthly average anomaly

characterization of a proposed palaeoproterozoic suture in the crust beneath the curnamona province australia abstract we use constrained forward and inverse modeling of bouguer gravity data to investigate a regional positive gravity anomaly that defines but is not confined to the eastern margin of the curnamona province best fit results imply that the positive gravity values are sourced from a relatively dense terrane 0 04 mg m 3 with respect to surrounding crust located at middle crustal levels 16 27 km depth which is likely dismembered and offset into discrete slivers by later tectonism consideration of pre 1500 ma plate reconstructions of the australian continent correlates this terrane with an interpreted buried suture in the mount isa inlier of northern australia where allochthonous crust was amalgamated to the australian margin at ca 2 20 1 85 ga bouguer gravity forward and inverse modeling also delineates a shallow west dipping crustal scale structure that possibly represents a primary boundary between precambrian basement of australia and the eastern fold belts accreted to the precambrian margin during the palaeozoic

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

preservation of proterozoic terrane boundaries within the colorado plateau and implications for its tectonic evolution abstract during the proterozoic the north american continent grew by the accretion of arcs onto the archean craton subsequent tectonic events affected much of the lithosphere of western north american resulting in the destruction of many of the features that reflect continental growth the colorado plateau which is centrally located within the tectonically active western portion of north america experienced only minor amounts of volcanism during subsequent magmatic events and has undergone comparatively little internal deformation during the remainder of the precambrian and phanerozic despite the occurrence of some significant tectonic events because the lithosphere of the plateau has not been significantly reworked by tectonic events it may have retained structures since accretion that give insight into processes of continental growth the crust of the southeastern portion of the colorado plateau revealed here through seismic data recorded by the temporary spe and la ristra arrays is greater than 40 km thick and possesses distinct changes in intercrustal layering and the character of the moho the moho appears quite dim towards the center of the plateau and much brighter along its southeastern margins connecting changes in crustal features that are common between the two arrays indicates a northeastern structural trend that matches the pattern of gravity anomalies in the region differences in crustal structures between terranes extend across much of the crust suggesting that entire distinct crustal columns align with terrane boundaries it is unlikely that extensive crustal flow as required by some models of plateau evolution could have traversed across the entire colorado plateau without producing features that cross cut these boundaries

the crustal structure of the arizona transition zone and southern colorado plateau from multiobservable probabilistic inversion the arizona transition zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western us namely the southern colorado plateau and the southern basin and range provinces as such the internal crustal structure and physical state of this transitional zone hold clues for understanding i the amalgamation of these provinces ii the partitioning of deformation due to both past and present day stress fields and iii the role of thermal versus compositional effects in controlling surface observables here we employ and expand a novel multiobservable probabilistic inversion method and jointly invert fundamental mode rayleigh phase velocities receiver functions surface heat flow geoid height and absolute elevation to obtain an internally consistent 3 d model of the temperature density vs and vp of the arizona transition zone and the southern portions of the colorado plateau and basin and range our results confirm a significant crustal thickening from 28 km in the sw of the arizona transition zone and southern basin and range to 48 km beneath the southern colorado plateau inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere asthenosphere boundary is not deeper than 70 km in most of the region we find that major pre cambrian surface structures and or shear zones separate crustal domains with distinct bulk properties suggesting that the juxtaposed crustal blocks still retain at least in part their original characteristics however widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths locally overprinting their original compositions and creating highly heterogeneous crustal sections a dominant and large scale internal crustal pattern of sw dipping planes structures is evident in our models coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms while we cannot categorically corroborate the presence of melt or aqueous fluids within the crust our results are compatible with these scenarios beneath some parts of the basin and range the mogollon datil and springerville volcanic fields

evidence for a local crustal root beneath the santa catalina metamorphic core complex arizona we used data from the tucson arizona broad band seismic station to analyze crustal and mantle structure under and adjacent to the catalina metamorphic core complex the data can be modeled with a simple two layer crust having an average moho depth of 30 km and seismic velocities consistent with a felsic composition the lower crust thickens under the catalina mountains depressing the moho by 4 2 km the crustal root can isostatically compensate the average topography of the catalina mountains indicating that the depth of isostatic compensation is at or below the moho and that the catatina mountains act as a coherent block bounded by high angle normal faults the presence of the crustal root is not consistent with models that predict upward bowing of the moho under the catalina metamorphic core complex the presence of the root cannot rule out models of lower crustal flow during the formation of the core complex however the presence of a root favors a model of metamorphic core complex development described by crustal thickening in shortening episodes crustal welt model followed by collapse of the crustal welt through extension

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

migration of volcanism in the san francisco volcanic field arizona the remanent magnetization of volcanic rocks has been determined at 650 sites in the san francisco volcanic field in the southern part of the colorado plateau the polarity of remanent magnetization combined with k ar age determinations spatial and petrographic associations stratigraphic relations and state of preservation of the cinder cones provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and 100 intermediate to silicic flows flow sequences and vents the age assignments for basaltic rocks include 243 brunhes 0 73 ma vents 220 matuyama 0 73 to 2 48 ma vents and 147 pre matuyama 2 48 to about 5 0 ma vents basaltic volcanism migrated northeastward before matuyama time at a rate of 1 2 cm yr and eastward s87 5 e over the past 2 5 m y at a rate of 2 9 0 3 cm yr concomitant acceleration in total magma production from 75 to 1 400 10 6 km3 yr and frequency of basaltic eruptions from 1 per 17 000 yr to 1 per 3 000 yr occurred between 5 and 0 25 ma for the past 0 25 m y magma production 180 10 6 km3 yr and perhaps eruption frequency have decreased this evolutionary sequence coupled with the lead and strontium isotopic composition of the rocks can be explained by magmatism caused by shear heating at the base of the lithosphere we propose that this eastward drift of volcanic activity represents absolute westward motion of the north american plate our model is in agreement with a model in which the african plate is fixed to the deep mantle

some simple analytical solutions to the problem of forced equatorial long waves this paper describes some simple time dependent analytical solutions to the problem of a sudden switch on of heating localized about the equator the equations used are the shallow water equations on the plane under the long wave approximation it is shown that with a reasonable amount of dissipation it can take several days for a steady state response to be established in the early stages the circulation differs considerably from that of the steady state the long time scale for the adjustment process has implications in numerical weather prediction since it implies that if the initial data are not in balance with the model s forcing it may take several days for a balance to be established by which time the quasi stationary tropical flow may have been irrevocably changed

variability of monthly averaged surface and 850 mb winds at tropical pacific islands abstract we examine the variability of monthly mean winds at 850 mb and the surface from five island stations in the tropical western pacific ocean climatological winds and 850 mb surface wind shear are evaluated and used to construct time series of monthly mean wind and shear anomalies wind variance at 850 mb tends to be substantially greater than at the surface and large temporal variations in shear are found prominent anomalies are associated with el nino southern oscillation periods composite el nino event anomalies are examined it is found that the westerly wind anomalies associated with warm central pacific sea surface temperatures are stronger at 850 mb than at the surface and that the anomalous 850 mb surface shears are as large as the surface wind anomalies themselves several simple techniques are described to investigate the feasibility of estimating surface wind anomalies from 850 mb wind anomalies because strong correlations exist between the zonal winds at these levels zonal est

the use of velocity spectrum for stacking receiver functions and imaging upper mantle discontinuities summary in order to improve the signal to noise ratio of receiver function data it is typical to stack receiver functions calculated from events at similar distances and back azimuths we have adapted the velocity spectrum stacking vss technique used extensively in reflection seismology to the receiver function method in order to stack data with different ray parameters thereby improving further the signal to noise ratio perhaps more importantly by producing the velocity spectrum stacks we take advantage of the differences in the shapes of the moveout curves of converted phases and reverberations to identify and separate the various phases and to infer velocity structure by comparison of velocity spectrum stacks produced from the observed data at the iris ida broad band station at obninsk russia obn with those produced from prem synthetics we have identified ps phases from the 400 and 670 km discontinuities the p to s ps converted phase from the 400 km discontinuity observed at obn is much larger compared with that of the 670 km discontinuity than is predicted by prem this suggests a higher velocity contrast at 400 km than in prem by adapting a bootstrap method to assess the depth and velocity estimates determined by vss we find that the ps phase from the 670 km discontinuity is best imaged with higher than prem upper mantle p and s velocities and at a depth of 663 3 km we find no evidence of a 220 km discontinuity beneath obn in these data

large scale circulation associated with westerly wind bursts and deep convection over the western equatorial pacific the relationship between deep equatorial convection over the western pacific and atmospheric circulation during november to february 1986 1992 is studied using cross correlations between outgoing longwave radiation olr and national meteorological center global analyses we focus on intraseasonal convective events on the 6 to 30 day timescale over two regions just east of borneo and over the tropical ocean and global atmosphere toga coupled ocean atmosphere response experiment coare domain to the northeast of new guinea although comparatively little work has been done on this timescale we show that the bulk of the intraseasonal variability in both convection and low level circulation over the tropical western pacific occurs in this period range as a comparison with these higher frequency events the evolution of coare convection on the 30 to 70 day timescale is also considered the results of the historical data analysis are compared to a case study of deep convection associated with strong surface westerly winds during november 1989 this analysis is meant to serve as a benchmark for future case studies from the tropical ocean global atmosphere coare experiment and other periods in general 6 to 30 day equatorial convection in the regions studied occurs in conjunction with low level equatorial westerly wind anomalies paired anomalous cyclonic circulations straddling the equator and a strengthening of the sea level pressure gradient along the equator the increase in the pressure gradient appears most often tied to the simultaneous equatorward movement of high pressure systems originating over southeastern asia and australia a surge of trades over the central pacific also accompanies the development of convection over both study regions following the convective peak the low level westerlies slowly decay as the northern cyclone moves northwestward away from the equator in a track characteristic of a tropical depression it is also observed that low level westerly anomalies occur frequently without deep convection the vertical structure during convective events along the equator on the 30 to 70 day timescale and over indonesia in the 6 to 30 day band is baroclinic with easterly anomalies at upper levels overlying low level westerlies at 200 mbar a strengthened meridional outflow from the olr signal into a wave train on the equatorward side of the asian jet is observed to develop following the convective peak suggesting forcing of this circulation by the convection over the coare domain there is evidence that 6 to 30 day convection often occurs in conjunction with deep westerly flow it is suggested that these types of events are frequently characterized by vertical propagation of westerlies from the surface to the upper troposphere as is shown for a case study from november 1989 this westerly burst episode is shown to have many features in common with the typical case as defined from historical data

the role of cross equatorial tropical cyclone pairs in the southern oscillation abstract the locations and frequencies of cross equatorial tropical cyclone pairs over the pacific ocean are compared between years of contrasting equatorial pacific sea surface temperature sst and atmospheric circulation anomalies i e high versus low southern oscillation so indices during the period 1971 79 weak southeast trade winds associated with low so indices allow warm 29 c surface water in the equatorial 0 5 s zone to extend eastward across the dateline encouraging formation of cyclone pairs near the dateline the cyclone pairs in turn provide the linkage between warm water and equatorial convection as indicated by canton island rainfall during southern summer in general the occurrences of cyclone pairs vary in longitude with the eastward excursions of warm equatorial water furthermore cyclone pairs appear to provide a positive feedback by advancing the eastward warm water extent in 1974 75 when no cyclone pairs developed near the dateline in spite of sufficiently high sst

eastern equatorial pacific response to three composite westerly wind types it has been suggested recently that episodes of westerly wind near the international date line in the western pacific tend to fall into one of four types three of the types have enough equatorial zonal wind variation to be able to induce an eastern pacific response by forcing equatorial waves in this note we examine the central and eastern pacific waveguide response to idealized representations of these three types of westerly wind episodes using a combination of linear theory and results from an ocean general circulation model the idealized westerly wind types are the c type maximum anomalies centered on the equator the n type maximum anomalies just north of the equator and the s type maximum anomalies just south of the equator the composite c type event excites equatorially trapped kelvin pulses that alter sea surface temperature in the central and eastern pacific warm anomalies caused by the c type episode have approximately equal contributions from a zonal advection of heat by the kelvin pulses and a meridional advection of heat by an altered instability wave field because the center of forcing of the s type event is displaced off of the equator the kelvin pulses excited by the s type episode have smaller amplitude than those of the c type episode even though the maximum wind anomaly is the same in the two cases the s type anomaly also excites weak rossby gravity waves that can propagate into the eastern pacific because the rossby gravity waves are dispersive their amplitude diminishes as the waves propagate into the eastern pacific the n type episode as observed in winds from the western tropical pacific is much weaker and of shorter duration than the c and s types and therefore excites only a very weak eastern pacific response although all types of westerly wind excite a local western pacific response only the c and s type events excite a significant eastern pacific response

images of crustal variations in the intermountain west 1 we develop a map of crustal thickness variations across the great basin colorado plateau rocky mountain and great plains provinces of the western united states using common conversion point stacking of teleseismic receiver functions below the rocky mountains and high plains in colorado we find the thickest crust in the region at 45 50 km thick beneath the basin and range thinner between 30 and 40 km crust is found thin 30 km thick crust is present in the northern portion of nevada and utah despite elevations similar to those farther south crustal thickness across the colorado plateau can be characterized as a broad transitional region between the thin crust of basin and range to the thicker crust of the rocky mountains the impedance contrast across the mohorovicic discontinuity decreases below the colorado plateau as converted arrivals recorded in this region appear weak compared to surrounding areas variations in vp vs across the region indicate higher values along the western boundary of the basin and range in the rocky mountains and in the western great plains we are not able to characterize vp vs in the colorado plateau we find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western united states

upper mantle structure beneath the eastern colorado plateau and rio grande rift revealed by bouguer gravity seismic velocities and xenolith data 1 the lithospheric architecture of the colorado plateau rio grande rift transition zone reflects a narrow corridor of active tectonism and magmatism between the tectonically stable colorado plateau and the stable interior of north america in this paper we analyze bouguer gravity data crustal thickness variations xenolith data and seismic wave speed structure to argue that the rio grande rift and southeastern colorado plateau is underlain by a low density upper mantle province that does not trend along upper crustal tectonic boundaries but rather is correlated with regions of middle to late tertiary magmatism we model uppermost mantle density contrasts using bouguer gravity along five parallel profiles across the colorado plateau and validate our models by converting modeled density contrasts to seismic velocity variations for comparison with regional seismic data as well as comparing modeled densities to calculated densities from xenoliths we find that upper crustal features and crustal thickness variations do not explain long wavelength 200 km variations in bouguer gravity across the colorado plateau rio grande rift a large 200 440 km wide region of low uppermost mantle density is modeled beneath the eastern colorado plateau margin jemez lineament rio grande rift and parts of the westernmost great plains this region has a density contrast of 90 to 30 kg m3 relative to the adjacent colorado plateau and great plains provinces and coincides with previously imaged regions of low upper mantle seismic velocities in situ densities inferred from xenolith data in the colorado plateau support the modeled upper mantle density variations and support the idea that the eastern and western margins of the plateau have been significantly modified by metasomatism we suggest that the region of low upper mantle density likely reflects some combination of temperature variations partial melt contents and compositional variations related to tertiary magmatic activity and or thinned lithosphere the relationship of this zone of modified upper mantle to extension in the rio grande rift remains enigmatic however particularly given the lack of spatial correlation between the trends of upper mantle structures and surficial extensional structures

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

a crustal seismic refraction study in west central arizona a reversed seismic refraction profile was recorded in the southern basin and range province between parker and globe arizona interpretation of both refracted and reflected phases indicates a two layered crust 23 25 km thick and a low pn velocity 7 67 km s pmp pn amplitude analysis on individual traces and synthetic seismogram modeling indicate that a positive velocity gradient may exist in the upper mantle the low pn velocity is indicative of partial melting in the upper mantle and implies that the crust is identical with the lithosphere the heat flow data suggest that the lithosphere was about 40 km thick approximately 5 m y ago on the basis of this and a negative free air gravity anomaly we propose that the lithosphere has been thinned from about 40 to 24 km over the past 5 m y and will probably thin and uplift further in the southern basin and range in arizona

loca1 and remote forcing of enso ocean waveguide response abstract several experiments using an ocean general circulation model have been carried out in order to explore the degree to which the oceanic waveguide response during the 1982 83 enso event was 1ocally and remotely forced experiments in which the chosen monthly mean surface stress field was imposed only within three degrees of the equator 3 n s and within seven degrees of the equator 7 n s reveal that the 7 n s winds reproduce the equatorial results of the full winds case to within differences small compared to the variability of interest the 3 n s winds case reproduce equatorial dynamic height acceptably but introduces errors in sst and upper ocean currents that approach the enso signal a 7 n s experiment in which the meridional stress is set to zero noyst snows that meridional stress plays a nontrivial but not dominant role in the 1982 83 model behavior errors generally are comparable to those of the 3 n s case a final experiment in which the 1982 83 winds were imposed west of the datel

the march 1997 westerly wind event and the onset of the 1997 98 el nino understanding the role of the atmospheric response in a previous study the effect of the march 1997 westerly wind event wwe on the evolution of the tropical pacific ocean was studied using an ocean general circulation model gcm the response was characterized by i a cooling of the far western pacific 0 88c ii a rapid eastward displacement of the warm pool 2000 km in a month and iii a weak warming of the central eastern pacific along the path of the oceanic kelvin wave excited by the wwe 0 58c in this study the atmospheric response to these aspects of the sea surface temperature sst response are investigated using an atmospheric gcm forced with the sst anomalies from the ocean only experiments the results have demonstrated that the three aspects of the sst anomaly field generated by the wwe

seismic evidence for partial lithospheric delamination model of colorado plateau uplifts the lithospheric structure beneath the colorado plateau and great basin of the western united states is investigated using pn and surface wave data from the colorado plateau great basin broadband seismic experiment average pn velocities of 8 0 km s in the colorado plateau and 7 8 km s in the eastern great basin are found teleseismic rayleigh waves are also measured and eastern great basin phase velocities are found to be 3 4 higher than colorado plateau phase velocities in the 20 35 s range but indistinguishable at periods greater than 40 s the apparent pn surface wave discrepancy is resolved with models that include a thicker crust and faster mantle lid for the colorado plateau than the eastern great basin models for both regions require a mantle low velocity zone the slow mantle deep beneath the colorado plateau may suggest a thermal origin of plateau uplift which has not yet penetrated the mantle lid

assessing the relationship between mjo and equatorial pacific wwbs in observations and cmip5 models abstractthis study evaluates the relationship between the madden julian oscillation mjo and the occurrence of equatorial pacific westerly wind bursts wwbs during the convective mjo phase anom

data fusion and porphyry copper prospectivity models southeastern arizona abstract mineral exploration programs commonly use a combination of geological geophysical and remotely sensed data to detect sets of optimal conditions for potential ore deposits prospectivity mapping techniques can integrate and analyse these digital geological data sets to produce maps that identify where optimal conditions converge three prospectivity mapping techniques weights of evidence fuzzy logic and a combination of these two methods were applied to a 32 000 km 2 study area within the southeastern arizona porphyry cu district and then assessed based on their ability to identify new and existing areas of high mineral prospectivity validity testing revealed that the fuzzy logic method using membership values based on an exploration model identified known cu deposits considerably better than those that relied solely on weights of evidence and slightly better than those that used a combination of weights of evidence and fuzzy logic this led to the selection of the prospectivity map created using the fuzzy logic method with membership values based on an exploration model three case study areas were identified that comprise many critical geological and geophysical characteristics favourable to hosting porphyry cu mineralisation but not associated with known mining or exploration activity detailed analysis of each case study has been performed to promote these areas as potential targets and to demonstrate the ability of prospectivity modelling techniques as useful tools in mineral exploration programs

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

cross equatorial influences of submonthly scale southerly surges over the eastern indian ocean during southern hemisphere winter 1 lower tropospheric southerly surges occurring at submonthly timescales over the eastern indian ocean eio during southern hemisphere sh winter are extratropical tropical interaction phenomena involving various processes this study examined the physical mechanism driving equatorial wave disturbances associated with surge induced cross equatorial flow over the eio japanese 25 year reanalysis jra25 and tropical rainfall measuring mission satellite trmm 3b42 data products from june to august for the 7 years from 1998 to 2004 were analyzed composite analyses of low level wind and vorticity fields reveal that an equatorial clockwise gyre develops just after cross equatorial penetration of southerly surge winds this gyre is interpreted as an equatorially trapped rossby wave that develops asymmetrically around the equator in response to surge induced cross equatorial flow eddy vorticity budget analysis demonstrates the dynamic role of the southerly surge in the genesis of the equatorial clockwise gyre the result indicates that an injection of negative absolute vorticity into the equatorial tropics due to both advective and convergent processes associated with the surge winds is primarily responsible for the gyre genesis cross equatorial advection of basic state absolute vorticity by the surge eddy winds and vortex stretching by surge wind convergence are most effective in spinning up the equatorial clockwise gyre wave activity flux analysis further emphasizes that rossby wave energy propagation from the sh extratropics into the equatorial eio region is caused by the southerly surge these dynamic diagnostics suggest that the southerly surge plays a lateral forcing role in exciting the equatorial wave disturbance over the eio composite analyses of moisture flux and precipitation anomalies characterize the equatorial clockwise gyre as a convectively coupled equatorial rossby wave enhanced cross equatorial moisture transport due to the southerly surge appears to be linked to precipitation along the west coast of the southern indo china peninsula a case study of the southerly surge event during august 2000 illustrates the formation of the equatorial clockwise gyre and its convectively coupled equatorial rossby wave signature

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

mantle compensation of active metamorphic core complexes at woodlark rift in papua new guinea in many highly extended rifts on the earth tectonic removal of the upper crust exhumes mid crustal rocks producing metamorphic core complexes these structures allow the upper continental crust to accommodate tens of kilometres of extension but it is not clear how the lower crust and underlying mantle respond also despite removal of the upper crust such core complexes remain both topographically high and in isostatic equilibrium because many core complexes in the western united states are underlain by a flat moho discontinuity it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating these processes should decouple upper crust extension from that in the mantle in contrast here we present seismic observations of metamorphic core complexes of the western woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension these core complexes are actively being exhumed at a rate of 5 10 km myr 1 and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density as indicated by low seismic velocities we conclude that at least in this case the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon but must involve mantle extension

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

the central andean altiplano puna magma body receiver function analysis of 14 teleseismic events recorded by 6 temporary passcal broadband stations within the altiplano puna volcanic complex apvc shows a consistent 2 s negative polarity p to s conversion for all stations for all available azimuths forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of 19 km with a vs 0 5 km s and a thickness of 750 810 m we interpret the characteristics of the low velocity zone low vs areal extent and flatness to be consistent with a sill like magma body on the basis of additional data from the german ancorp experiment the altiplano puna magma body appears to underlie much of the apvc and it may therefore be the largest known active continental crustal magma body

crustal structure from three dimensional gravity modeling of a metamorphic core complex a model for uplift santa catalina rincon mountains arizona gravity modeling shows that the observed fluctuations in gravity over the catalina rincon metamorphic core complex of southeastern arizona are caused primarily by shallow crustal density contrasts the wilderness suite granites the probable source of a 20 mgal low over the central part of the core complex may extend to depths of 7 12 km below the surface seismic refraction information is consistent with the existence of a deep crustal root under the catalina rincon core complex we hypothesize that the voluminous wilderness suite granites are related to a crustal thickening event that built the root in early tertiary time modeling of flexural isostatic uplift resulting from an excess crustal root predicts two episodes of uplift for the catalina rincon metamorphic core complex 1 low relief flexural uplift in the middle tertiary and 2 complete uplift precipitated by high angle basin and range faulting at 10 15 ma the uplift and exposure of core complexes therefore can be an isostatically driven process that occurs in regions of locally thickened crust

the extreme el nino of 2015 2016 the role of westerly and easterly wind bursts and preconditioning by the failed 2014 event at the beginning of 2015 as one year earlier in 2014 the scientific community anticipated that el nino conditions could develop in the tropical pacific by year end such projections were related to the occurrence of westerly wind bursts during winter spring of each year that generated strong downwelling kelvin waves indicative of an emerging el nino however the event s progression quickly stalled in 2014 but actively continued in 2015 leading to an extreme warm event comparable to 1997 or 1982 here we compare climate evolution during these two years using satellite observations and numerical simulations we show that during 2014 el nino development was interrupted mid year by an exceptionally strong easterly wind burst whereas during the second year it continued through the summer further we show that the failed 2014 event created favorable conditions for el nino development during the next year as it kept ocean heat content recharged and the western pacific warm pool extended eastward subsequently the winter spring westerly wind bursts in 2015 were followed by a series of state dependent westerly bursts as part of a strong positive bjerknes feedback analogue simulations with a coupled gcm wherein we superimpose the observed sequences of westerly and easterly wind bursts support these conclusions stressing the role of the failed 2014 event in preconditioning the ocean atmosphere system for the development of an extreme el nino in our simulations the probability of an extreme event following early year westerly wind bursts increases from 14 to nearly 60 due to this preconditioning thus the interplay between westerly and easterly wind bursts shapes el nino development and diversity

static and dynamic support of western united states topography article history accepted 7 october 2013 available online 31 october 2013 editor p shearer isostatic and dynamic models of earth s surface topography can provide important insights into the driving processes of tectonic deformation we analyze these two estimates for the tectonically active western united states using refined structural models derived from earthscope usarray for the crust use of recent moho depth measurements and crustal density anomalies inferred from passive source seismology improve isostatic models however seismically determined lithospheric thickness variations from lithosphere asthenosphere boundary lab maps and lithospheric and mantle density anomalies derived from heat flow or uppermost mantle tomography do not improve isostatic models substantially perhaps this is a consequence of compositional heterogeneity a mismatch between thermal and seismological lab and structural complexity caused by smaller scale dynamics the remaining non isostatic dynamic component of topography is large topography anomalies include negative residuals likely due to active subduction of the juan de fuca plate and perhaps remnants of formerly active convergence further south along the margin our finding of broad scale positive residual topography in the basin and range substantiates previous results implying the presence of anomalous buoyancy there which we cannot fully explain the colorado plateau does not appear dynamically anomalous at present except at its edges many of the residual topography features are consistent with predictions from mantle flow computations this suggests a convective origin and important interactions between vigorous upper mantle convection and intraplate deformation

the southern oscillation part vii meteorological anomalies over the indian and pacific sectors associated with the extremes of the oscillation abstract composite surface pressure temperature and precipitation anomalies are mapped over the indian and pacific sectors during the various stages of warm and cold events in the southern oscillation in the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial pacific year 1 of a warm event a strong south pacific high is associated with below normal surface pressure over australia and the indian ocean this occurs concurrently with a poleward displacement of the pacific convergence zones with above normal air temperature and precipitation over the subtropical pacific and opposite conditions along the equator by the next year year 0 of the warm event thew anomalies have the opposite sign the sequence of anomalies during a cold event is inverse to that during a warm event but otherwise the anomaly patterns are remarkably similar it appears that enhanced convection and low surface pressure within the pacific convergence zones contribute