lateral variations of the guerrero oaxaca subduction zone mexico derived from weak seismicity mb3 5 detected on a single array at teleseismic distance

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

geochronology and tectonic implications of magmatism and metamorphism southern kootenay arc and neighbouring regions southeastern british columbia part i jurassic to mid cretaceous k ar dates and u pb zircon dates define three periods of igneous activity in the southern kootenay arc 1 emplacement of late synkinematic to post kinematic granodioritic plutons in mid jurassic time 170 165 ma accompanying amphibolite facies regional metamorphism 2 emplacement of post kinematic granitic plutons in mid cretaceous time 100 ma and 3 emplacement of small bodies of syenite in eocene time 50 ma in the western part of the area micas from mid jurassic plutons that yield the oldest k ar dates 158 166 ma also yield plateau shaped 40ar 39ar age spectra age spectra for biotites younger than these but older than 125 ma reflect thermal overprinting in southeastern british columbia the kootenay arc marks the transition from the north american rocks of the cordilleran miogeocline to the tectonic collage of allochthonous terranes that have been accreted to it deformation metamorphism and plutonism recorded in rocks of the southern kootenay arc commenced in mid jurassic time as a co

automatic analysis of the gorkha earthquake aftershock sequence evidences of structurally segmented seismicity we present the first 3 months of aftershock activity following the 2015 april 25 gorkha earthquake m w 7 8 recorded on the nepalese seismic network we deployed an automatic procedure composed of three main stages 1 coarse determination of the p and s onsets 2 phase association to declare events and 3 iterative addition and refinement of onsets using the kurtosis characteristic function in total 9188 events could be located in the kathmandu region with the majority having small location errors 4 5 9 and 10 km in the x y and z directions respectively additionally we propose a new attenuation law to estimate local magnitudes in the region this new seismic catalogue reveals a detailed insight into the gorkha aftershock sequence and its relation to the main shock rupture models and tectonic structures in the region most aftershocks fall within the main himalayan thrust mht shear zone or in its hangingwall significant temporal and lateral variations of aftershocks location are observed among them 1 three distinct stages highlighting subsequent jump offs at the easternmost termination 2 the existence of a seismic gap north of kathmandu which matches with a low slip zone in the rupture area of the main shock 3 the confinement of seismic activity in the trace of the may 12 m w 7 3 earthquake within the mht and its hangingwall through a 30 30 km 2 region and 4 a shallow westward dipping structure east of the kathmandu klippe these new observations with the inferred tectonic structures at depth suggest a tectonic control of part of the aftershock activity by the lateral breaks along the mht and by the geometry of the duplex above the thrust

multiobjective optimization of regional and teleseismic data to constrain the source of the 12 september 2016 mw 5 4 earthquake in south korea

bowser basin northern british columbia constraints on the timing of initial subsidence and stikinia north america terrane interactions clastic strata composing the northern bowser basin record the accretion of stikinia to the composite western edge of the north american plate cache creek quesnellia slide mountain kootenay north america in early middle jurassic time and the concomitant demise of the intervening cache creek ocean initial flexural subsidence of the northern bowser basin resulting from thrust loading of cache creek terrane on stikinia is represented by an organic rich shale of aalenian age the abou formation of the spatsizi group coarse grained sediment first appeared in early bajocian time following uplift and subaerial exposure of cache creek rocks in the upper plate thus the inception of the bowser basin was aalenian rather than bajocian as believed by earlier workers aalenian southwest vergent thrusting at the composite western edge of north america is also known from southern british columbia a coincidence that implicates collision with stikinia in the south as a cause of that deformation

jura cretaceous oxfordian to cenomanian stratigraphy of the north central bowser basin northern british columbia three new formations of late jurassic and early to mid cretaceous age are defined for a 2000 m thick section of jura cretaceous rocks exposed in the north central bowser basin the currier formation oxfordian to kimmeridgian or tithonian consists of 350 600 m of interbedded shales siltstones sandstones coals and carbonates the mcevoy formation barremian to as young as albian consists of 400 800 m of siltstones and shales with minor sandstones thin coals limestones and conglomerates the devils claw formation in part mid albian to cenomanian consists of 300 600 m of strata characterized by thick pebble and cobble conglomerates with associated coarse sandstones and minor siltstones and shales two successive coarsening upward sequences are identified in the study area the first begins with middle jurassic marine shales of the jackson unit grading upwards to coarser upper jurassic facies of the currier formation the currier formation is conformably or unconformably overlain by siltstones and

drainage of a deep magma reservoir near mayotte inferred from seismicity and deformation the dynamics of magma deep in the earth s crust are difficult to capture by geophysical monitoring since may 2018 a seismically quiet area offshore of mayotte in the western indian ocean has been affected by complex seismic activity including long duration very long period signals detected globally global navigation satellite system stations on mayotte have also recorded a large surface deflation offshore here we analyse regional and global seismic and deformation data to provide a one year long detailed picture of a deep rare magmatic process we identify about 7 000 volcano tectonic earthquakes and 407 very long period seismic signals early earthquakes migrated upward in response to a magmatic dyke propagating from moho depth to the surface whereas later events marked the progressive failure of the roof of a magma reservoir triggering its resonance an analysis of the very long period seismicity and deformation suggests that at least 1 3 km3 of magma drained from a reservoir of 10 to 15 km diameter at 25 to 35 km depth we demonstrate that such deep offshore magmatic activity can be captured without any on site monitoring recent seismicity near mayotte in the indian ocean is due to dyke propagation from and drainage of a 25 35 km deep magma reservoir according to an analysis of earthquake and deformation data

climatic evolution across oceanic anoxic event 1a derived from terrestrial palynology and clay minerals maestrat basin spain studies dealing with the response of the continental biosphere to the environmental perturbations associated with cretaceous oceanic anoxic events oaes are comparatively rare here a quantitative spore pollen record combined with clay mineral data is presented which covers the entire early aptian oae 1a interval forcall formation maestrat basin east spain the well expressed oae 1a carbon isotope anomaly is paralleled by changes in the clay mineral assemblage and by a stepwise decline in the normalized frequency of classopollis pollen produced by xerophytic cheirolepidiaceae with lowest contents occurring during the positive 13 c shift in contrast araucariacites and inaperturopollenites pollen show a pronounced increase in relative abundance from low background values to become a significant component of the palynological assemblage during the classopollis minimum the observed changes in clay minerals and pollen distribution patterns are interpreted to reflect a major change in the composition of the hinterland vegetation of the maestrat basin most probably due to short lived but pronounced climatic cooling and changes in humidity temperature anomalies driven by organic carbon burial and associated co 2 decline have been postulated for all major mesozoic oaes the palynomorph record from the iberian maestrat basins indicates that the climax of this cooling episode was significantly delayed in comparison to the end of organic carbon rich deposition in the world oceans

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

stratigraphy geochronology and geochemistry of the georgie river area northwest british columbia and implications for mineral exploration georgie river area is underlain by greenschist facies volcanic clastic and plutonic rock high precision u pb isotopic ages on zircon enable regional stratigraphic correlation by constraining the lower to lower middle jurassic depositional history a sample from pyroxene phyric tholeiitic basalt is upper triassic and or lower jurassic in age its upper age limit is constrained by a crosscutting intrusion dated at 189 8 0 3 ma the bulldog creek pluton at 193 0 0 3 ma is significantly older than the previously reported k ar age hazelton group strata include a coarse grained volcaniclastic and volcanic unit overlain by a fine grained clastic and bimodal volcanic flow unit the coarse volaniclastic unit is coeval and probably cogenetic with the outram lake porphyry intrusion 189 7 0 3 ma and near the top includes andesitic tuff 189 8 0 6 ma these data constrain the age of deposition of the unit and support correlation with the lower jurassic hazelton group and members of the texas creek plutonic suite age of the fine clastic and bimodal volcanic unit is no older than the youngest detrital zircons ca 185 ma near its base that age and the age of a rhyolite unit 176 6 1 1 ma at the top support correlation with the middle jurassic salmon river formation of the hazelton group resolution of the litho and chronostratigraphy of the area reveal close similarities in age rock types succession and composition with strata that host the base and precious metal rich stratiform eskay creek deposit revealing the potential for eskay type volcanogenic massive sulphide vms deposits in the georgie river area

the 11 october 2010 novaya zemlya earthquake implications for velocity models and regional event location characterizing the seismicity of novaya zemlya and the surrounding arctic seas requires accurate event location estimates low magnitude events in this region are currently observed only by a small number of stations in the european arctic with a large azimuthal gap making the accuracy of regional velocity models all the more important regional travel time calibration is difficult given the scarcity of sufficiently well constrained events on 11 october 2010 a magnitude 4 5 event occurred close to the northern tip of novaya zemlya this event is significant in that it is the first event in this region to have been recorded both on the relatively recent regional networks and arrays and also teleseismically with good azimuthal coverage we examine how well we can constrain the location and origin time using only teleseismic phases using only first teleseismic p arrivals we constrain the epicenter to approximately 76 25 n and 64 75 e but with no depth resolution clear depth phases notably on stations in the southern united states indicate a depth between 9 and 15 km this independent hypocenter and origin time estimate allow evaluation of regional phase travel time prediction using different models the predicted sn travel time appears to cause the greatest variability in regional location estimates the 3d regional seismic travel times models provide excellent pn travel time estimates for barents sea paths but may slightly overestimate sn travel times from this source region a modified regional 1d velocity model is defined which best predicts pn and sn observations at multiple stations up to 15 the significance of the regional travel time models for estimating location is demonstrated for a low magnitude event on or close to the northern island of novaya zemlya in march 2014 recorded with a satisfactory signal to noise ratio at only four stations

jurassic cretaceous basins along the canadian coast belt their bearing on pre mid cretaceous sinistral displacements the dominantly plutonic canadian coast belt is flanked on the northwest by a late jurassic early cretaceous intra or back arc basin in the south it contains strata of the same age deposited in an accretionary fore arc complex east of a coeval arc this distribution is ascribed to pre mid cretaceous sinistral movement of part of the jurassic cretaceous north american plate margin arc to a position west of the accretionary fore arc complex

epicentre accuracy based on seismic network criteria summary we establish reliable and conservative estimates for epicentre location accuracy using data that are readily available in published seismic bulletins a large variety of seismic studies rely on catalogues of event locations making proper assessment of location uncertainty critical event location and uncertainty parameters in most global regional and national earthquake catalogues are obtained from traditional linearized inversion methods using a 1 d earth model to predict traveltimes reported catalogue uncertainties are based on the assumption that error processes are gaussian zero mean and uncorrelated unfortunately these assumptions are commonly violated leading to the underestimation of true location uncertainty especially at high confidence levels we find that catalogue location accuracy is most reliably estimated by station geometry we make use of two explosions with exactly known epicentres to develop local network location 0 2 5 accuracy criteria using monte carlo simulations of network geometry we find that local network locations are accurate to within 5 km with a 95 per cent confidence level when the network meets the following criteria 1 there are 10 or more stations all within 250 km 2 an azimuthal gap of less than 110 3 a secondary azimuthal gap of less than 160 and 4 at least one station within 30 km to derive location accuracy criteria for near regional 2 5 10 regional 2 5 20 and teleseismic 28 91 networks we use a large data set of exceptionally well located earthquakes and nuclear explosions beyond local distances we find that the secondary azimuthal gap is sufficient to constrain epicentre accuracy and location error increases when the secondary azimuthal gap exceeds 120 when station coverage meets the criterion of a secondary azimuth gap of less than 120 near regional networks provide 20 km accuracy at the 90 per cent confidence level while regional and teleseismic networks provide 25 km accuracy at the 90 per cent confidence level

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

coast plutonic complex a mid cretaceous contractional orogen early late cretaceous east vergent thrusts deform much of the eastern margin of the coast plutonic complex in western british columbia similarities in timing and style suggest that the faults represent a system of backthrusts to a west vergent thrust belt on the west side of the complex this geometry and the rarity of mid cretaceous strike slip faults indicate that the coast plutonic complex was a strongly contractional orogen in mid cretaceous time

late paleozoic and mesozoic terrane interactions in north central british columbia five clearly defined terranes comprising from northeast to southwest ancestral north america slide mountain quesnellia cache creek and stikinia are the dominant tectonic elements of north central british columbia stratigraphic sedimentological plutonic metamorphic and structural data show that the slide mountain terrane evolved as a subduction accretion and island arc complex during permian time sedimentological data hint at the demise of the slide mountain and cache creek oceanic environments in the permian or early triassic and late triassic respectively subduction led to the development of volcanic plutonic island arcs on stikinia quesnellia and locally on the cache creek terrane in late triassic to middle jurassic time marked inter and intra terrane contraction in the middle jurassic resulted in the south westward thrusting of the cache creek terrane onto stikinia the subsequent development of the bowser basin on stikinia and possible coeval culmination of the emplacement of que

distribution of paleogene and cretaceous rocks around the nazko river belt of central british columbia from 3 d long offset first arrival seismic tomography across the nechako chilcotin plateau of british columbia the distribution of cretaceous sedimentary rocks which are considered prospective for hydrocarbon exploration is poorly known due to the surface cover of glacial deposits and tertiary volcanic rocks to constrain the subsurface distribution of these cretaceous rocks in 2008 geoscience bc acquired seven long up to 14 4 km offset vibroseis seismic reflection lines across a north northwest trending belt of exhumed sedimentary rocks inferred to be part of the taylor creek group p wave velocity models which are consistent with sonic logs from nearby wells have been estimated using three dimensional first arrival tomography to depths ranging from 1 to 4 km igneous basement can be identified on most lines using the 5 5 km s isovelocity contour which locates the top of the basement to an accuracy of 400 m where its depth is known in exploration wells there is no general distinction on the basis of seismic velocity between cretaceous sedimentary

geometry and seismic properties of the subducting cocos plate in central mexico the geometry and properties of the interface of the cocos plate beneath central mexico are determined from the receiver functions rfs utilizing data from the meso america subduction experiment mase the rf image shows that the subducting oceanic crust is shallowly dipping to the north at 15 for 80 km from acapulco and then horizontally underplates the continental crust for approximately 200 km to the trans mexican volcanic belt tmvb the crustal image also shows that there is no continental root associated with the tmvb the migrated image of the rfs shows that the slab is steeply dipping into the mantle at about 75 beneath the tmvb both the continental and oceanic moho are clearly seen in both images and modeling of the rf conversion amplitudes and timings of the underplated features reveals a thin low velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab by inverting rf amplitudes of the converted phases and their time separations we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting cocos plate and its thickness high poisson s and vp vs ratios due to anomalously low s wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure the evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates this may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust

application of a cepstral f statistic for improved depth estimation the depth of a seismic event can be determined using peaks corresponding to depth phase delay times found in the cepstrum of the seismic signal until now however there has been no method available to determine the significance of various peaks in the cepstrum we have formulated a cepstral f statistic by using a classic approach to detect a signal in a number of stationarily correlated time series the method attaches a statistical significance level to peaks in the cepstrum of seismic data caused by echoes in the signal the method is particularly well suited for detecting depth phase echoes pp and sp recorded on regional seismic arrays detections determined from the peaks in the cepstral f statistic are then stacked as a function of pp p and sp p travel times predicted from the iaspei global 1 d model kennett and engdahl 1991 using a modification of the method of depth phase beamforming woodgold 1998 murphy et al 1999 tests on synthetic data show the method is most successful when the p wave arrival has a signal to noise ratio snr greater than 4 7 and the depth phase exhibits an snr greater than 2 tests on complex regional data suggest that the depth phase snr must be between 4 and 8 for consistent successful application of the method we tested the method by using events from the hindu kush region of afghanistan with well determined depths as recorded on arrays at teleseismic distances we determined the correct depths for 14 of the 19 events in this dataset 74 success rate and we determined incorrect depths for two events for a false alarm rate of 10 5 the difference between the depths calculated by the cepstral method and the prototype international data center pidc decrease with increasing magnitude for events with m b 3 8 the average difference between the cepstral and pidc depths was 7 9 km with no false alarms to test the operational capabilities of this method as a tool for data center use we first analyzed 32 events that occurred on 12 february 2000 as located by the pidc additionally we analyzed 29 events located in central and south america between 05 may and 15 june 2000 with event characteristics published by the international data center idc most of these events were also located by the national earthquake information center our method determined statistically significant depths for 40 of these 61 events with 11 having a low snr at three or more recording arrays while another 10 were either too shallow for analysis or did not exhibit depth phases only seven of the 61 events had depth phases listed in the reviewed event bulletin produced by the pidc and idc for these events the average difference between the reported and cepstral f statistic depths was 4 0 km the pidc idc fixed the depth of 26 of 61 events to 0 0 the cepstral f statistic method determined depths ranging from 16 to 92 km for 15 of the events that had been constrained as surface foci overall we believe the method would be most valuable when used by analysts to detect possible depth phases that could then verify or improve network calculated focal depths manuscript received 25 february 2001

regional depth controlled hydrothermal metamorphism in the zymoetz river area british columbia an estimated 6 km of basic to silicic volcanic flows and clastic rocks of the early jurassic telkwa formation is exposed in moderately east dipping fault blocks along the zymoetz river british columbia extensive wholesale zeolitic replacement of porous tuff beds suggests widespread hydrothermal activity metamorphic grade increases regionally from laumontite albite facies to prehnite pumpellyite facies with increasing stratigraphic depth telkwa formation strata and the imparted metamorphic zonation are cut and tilted by rotational block faulting and are repeated in each of the upturned blocks late mesozoic to tertiary plutonism locally thermally overprinted the regional facies particularly in the western part of the area fluid inclusion isochores combined with calculated mineral equilibria suggest that metamorphism took place at fluid pressures of 2 kbar 1 kbar 100 mpa or less consistent with estimates of stratigraphic burial metamorphic fluids were h2o rich and low in dissolved salts maxi

lithostratigraphic and tectonic framework of jurassic and cretaceous intermontane sedimentary basins of south central british columbia1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia the south central intermontane belt of british columbia has a complex architecture comprising late paleozoic to mesozoic volcanic and plutonic arc magmatic suites marine and nonmarine clastic basins high grade metamorphic complexes and accretionary rocks jurassic and cretaceous clastic basins within this framework contain stratigraphy with hydrocarbon potential the geology is complicated by cretaceous to eocene deformation dismemberment and dislocation the eocene to neogene history of the southern intermontane belt is dominated by non arc volcanism followed by pleistocene to recent glaciation the volcanic and glacial cover makes this a difficult region to explore for resources much recent work has involved re evaluating the challenges that the overlying volcanic cover has historically presented to geophysical imaging of the sedimentary rocks in this region in light of technological advances in geophysical data collection and analysis this paper summarizes the lithological and stratigraphic fra

major dextral transcurrent displacements along the northern rocky mountain trench and related lineaments in north central british columbia the northern rocky mountain trench and a number of other prominent lineaments along and east of the eastern margin of the intermontane belt mark faults along which dextral transcurrent movements have been dominant offsets of shelf to off shelf facies boundaries in lower paleozoic rocks indicate a cumulative displacement of at least 750 km and probably 900 km within the system of faults related to those in the northern rocky mountain and the tintina trenches farther west another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the intermontane belt by as much as 300 km these faults including the kutcho and the pinchi connect in part with the teslin suture zone in yukon territory and probably with the fraser river straight creek fault zone in southern british columbia although dextral transcurrent faulting may have taken place between the middle jurassic and early cenozoic the most convincing evidence points to middle cretaceous and particularly to early cenozoic eocene displacements the eocene movements were temporally related to plutonism volcanism lamprophyre dike emplacement high heat flow sedimentation in grabens and rapid uplift of northwesterly trending elongate ranges climactic episodes of granite emplacement particularly in and near the northern omineca crystalline belt at 100 m y 70 m y and 50 m y ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain

lower to middle jurassic pliensbachian to bajocian stratigraphy of the northern spatsizi area north central british columbia the lower to middle jurassic pliensbachian to lower bajocian spatsizi group in the northern spatsizi area of north central british columbia is formally defined and subdivided into the joan wolf den melisson abou and quock formations each formation reflects deposition in a different dominantly fine clastic environment with a varying input of volcanic epiclastic or pyroclastic detritus the spatsizi group represents the basinward sedimentary equivalent of the coeval cold fish volcanics a group of calc alkaline flows and breccias that accumulated in a volcanic arc along the southern flank of the stikine arch arc to basin facies trends are best developed in the joan and wolf den formations and are characterized by a decrease in the volcaniclastic component of the sediments an overall reduction in grain size and a progressively deeper water environment of deposition as inferred from both sedimentological and faunal evidence in the study area the spatsizi group underlies with a slight angular di

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

comparison of airborne and marine magnetic data in donegal basin ireland exploration of ireland s donegal basin in the last 10 years concentrated on structural plays within the triassic sherwood sandstone and culminated in the corrib gas discovery in 1998 despite this economic success the structural history and hence petroleum geology of the basin remained poorly understood due in large measure to igneous intrusives and salt which degrade seismic data quality these problems caused operators to consider alternative methods and given the success of potential fields in adjacent areas faroes irish sea new gravity and magnetic surveys were undertaken in 1999 before 1999 the only speculative magnetic data set over donegal basin was a 5 km grid of various vintages compiled by the geological survey of canada this was useful for identifying regional structural trends but little else as exploration intensified in 1999 a 2d seismic survey was acquired along with marine gravity and magnetics data the new data enabled better structural definition within the basin but still left many unanswered questions about the detailed character and extent of igneous intrusions to address this a high resolution aeromagnetic hram survey was flown in late 1999 the dense data coverage and high sampling rate provided much greater detail than previous surveys table 1 view this table table 1 comparison of acquisition parameters for various surveys acquired in the donegal basin this paper discusses the advantages and disadvantages of the various data types acquired in donegal basin each has an optimum application determined by technical objectives and budget but to choose the most appropriate it is important to understand the limitations that each has the underlying aim of this paper is to help explorationists make better informed decisions when planning acquisition of potential fields data the 2d marine gravity and magnetics data acquired in conjunction with the 1999 seismic survey had line orientations that were north south and east west respectively and line spacing

slow slip events and strain accumulation in the guerrero gap mexico 1 global positioning system gps time series in guerrero mexico reveal the existence of large slow slip events sses at the boundary between the cocos and north american plates in this study we examined the last three sses that occurred in 2001 2002 2006 and 2009 2010 and their impact on the strain accumulation along the guerrero subduction margin gps displacements were inverted to retrieve the slip distribution during each sse and the inter sse coupling of the subduction interface the three analyzed sses have equivalent moment magnitudes of between 7 50 and 7 65 their lateral extents are variable and they all show significant slip in the guerrero seismic gap during the inter sse epochs the interplate coupling is high in the area where slow slip subsequently occurs in the guerrero gap the shallow portion of the plate interface from the trench to the coast is weakly coupled the average slip deficit accumulated in the guerrero gap over a period of 12 years which corresponds to three cycles of sse is only 1 4 of the slip deficit accumulated on both sides of the gap moreover the regions of large slip deficit coincide with the rupture areas of recent large earthquakes we conclude that the sses in the guerrero gap release a significant part of the strain accumulated during the inter sse period if large subduction thrust earthquakes occur in the guerrero gap their recurrence time is probably increased compared to adjacent regions

source pulse enhancement by deconvolution of an empirical green s function observations of the earthquake source time function are enhanced if path recording site and instrument complexities can be removed from seismograms assuming that a small earthquake has a simple source its seismogram can be treated as an empirical green s function and deconvolved from the seismogram of a larger and or more complex earthquake by spectral division when the deconvolution is well posed the quotient spectrum represents the apparent source time function of the larger event this study shows that with high quality locally recorded earthquake data it is feasible to fourier transform the quotient and obtain a useful result in the time domain in practice the deconvolution can be stabilized by one of several simple techniques in this paper the method is implemented and tested on high quality digital recordings of aftershocks of the jan 9 1982 miramichi new brunswick earthquake in particular seismograms from a jan 17 aftershock 017 13 33 gmt local mag 3 5 exhibit path or site effects which complicate the determination of source parameters after deconvolution the apparent far field source of this event is a simple pulse in displacement with duration 0 07 second for both p and s

geochronology and tectonic implications of magmatism and metamorphism southern kootenay arc and neighbouring regions southeastern british columbia part ii mid cretaceous to eocene a north south trending belt of amphibolite facies regional metamorphism parallels the purcell trench transects the kootenay arc and is in part fault bounded towards the axis of this belt progressively higher pressure metamorphic mineral assemblages are exposed in the contact aureoles of post kinematic mid cretaceous 100 ma plutons and in metapelites contours of k ar biotite dates for plutonic rocks 55 95 ma are regular are broadly conformable with metamorphic isograds appear to cross internal intrusive contacts of post kinematic plutons and young towards the highest pressure and temperature regional metamorphic zones within the sillimanite zone most micas yield k ar dates between 40 and 55 ma rb sr muscovite dates for deformed and undeformed pegmatites and for muscovite bearing monzogranite and granodiorite fall between 53 and 84 ma u pb zircon dates for the kaniksu batholith and nearby gneiss of uncertain origin yield a lower concordia intercept of 94 ma micas from mid jurassic and mid

early to middle cretaceous paleogeography of north central british columbia stratigraphy and basin analysis of the skeena group the lower middle cretaceous skeena group records the early cretaceous evolution of the southern margin of the jura cretaceous bowser basin in north central british columbia we formalize skeena group nomenclature and present interpretations of three distinct paleogeographic and tectonic phases during the first phase neocomian aptian skeena deposition was limited to a restricted tidal basin represented by laventie formation black shale deposits surrounded by coal swamp deltas of the lower bulkley canyon formation the lower skeena group correlated to the mcevoy formation bowser lake group in the northern basin represents final filling of the bowser foredeep produced by jurassic accretion of the intermontane superterrane to north america in the second phase early albian early cenomanian marine deposition transgressed eastward and southward accompanied by intrabasinal rocky ridge volcanism shedding volcanic detritus into the kitsuns creek member of the bulkley canyon formation the rocky ridge

aeromagnetic data in the uk a study of the information content of baseline and modern surveys across anglesey north wales a number of modern multiparameter high resolution airborne geophysical surveys termed hires have been conducted over the past decade across onshore uk these were undertaken in part as a response to the limited resolution of the existing uk national baseline magnetic survey data set acquired in the late 1950s and early 1960s modern magnetic survey data obtained with higher precision and reduced line spacing and elevation provide an improved data set however the distinctions between the two available resources existing and new are rarely quantified in this contribution we demonstrate and quantify the improvements that can be anticipated using the new data the information content of the data sets is examined using a series of modern processing and modelling procedures that provide a full assessment of their resolution capabilities the framework for the study involves two components the first relates to the definition of the shallow magnetic structure in relation to an ongoing 1 10 k and 1 50 k geological map revision the second component relates to the performance of the datasets in defining maps of magnetic basement and assisting with larger scale geological and structural interpretation one of the smaller hires survey areas the island of anglesey ynys mon off the coast of nw wales is used to provide a series of comparative studies the geological setting here is both complex and debated and cultural interference is prevalent in the low altitude modern survey data it is demonstrated that successful processing and interpretation can be carried out on data that have not been systematically corrected decultured for non geological perturbations across the survey area a wide number of near surface magnetic features are evident and are dominated by a reversely magnetized palaeogene dyke swarm that extends offshore the average depth to the upper surfaces of the dykes is found to be 44 m the existing baseline data are necessarily limited in resolving features 3 km to upper surface assessments of magnetic basement this further assessment is successfully provided by the larger scale baseline data which locates and defines a mid crustal magnetic basement feature centred beneath the snowdon massif and illustrates that basement of similar characteristic extends beneath much of anglesey

the 13 january 2001 el salvador earthquake a multidata analysis 1 on 13 january 2001 a large normal faulting intermediate depth event mw 7 7 occurred 40 km off the el salvadorian coast central america we analyze this earthquake using teleseismic regional and local data we first build a kinematic source model by simultaneously inverting p and sh displacement waveforms and source time functions derived from surface waves using an empirical green s function analysis in an attempt to discriminate between the two nodal planes 30 trenchward dipping and 60 landward dipping we perform identical inversions using both possible fault planes after relocating the hypocentral depth at 54 km we retrieve the kinematic features of the rupture using a combination of the neighborhood algorithm of sambridge 1999 and the simplex method allowing for variable rupture velocity and slip we find updip rupture propagation yielding a centroid depth around 47 km for both assumed fault planes with a larger variance reduction obtained using the 60 landward dipping nodal plane we test the two possible fault models using regional broadband data and near field accelerograms provided by universidad centro americana 2001 near field data confirm that the steeper landward dipping nodal plane is preferred rupture propagated mostly updip and to the northwest resulting in a main moment release zone of approximately 25 km 50 km with an average slip of 3 5 m the large slip occurs near the interplate interface at a location where the slab steepens dip significantly the occurrence of this event is well explained by bending of the subducting plate

105 million years of igneous activity wrangell alaska to prince rupert british columbia the coast mountains orogen which lies along coastal british columbia and southeastern alaska records extensive mesozoic and cenozoic igneous activity here we focus on the two youngest periods 105 50 ma and 30 ma present the history of subduction orogen uplift and finally extension associated with this magmatic activity can be related to plate motion since the mid cretaceous the 105 50 ma period is related to terrane accretion and subduction the northwest southeast trending coast shear zone divides the resulting coast mountains calc alkaline continental margin batholith into a western 105 90 ma and an eastern 80 50 ma arc melting of hydrous mantle overlying a dehydrating slab generated the plutons of the western arc the plutons of the eastern arc show a wider range of compositions their origin consists of mantle melts modified by melts from lower crustal rocks of continental affinity and possibly amphibolitic hydrated basalt in both parts of the arc igneous bodies also resulted from crustal melting these are very abundant in the eastern arc during the younger period bi modal igneous suites consisting of mantle derived mafic magmas and coeval crustal melts are associated with crustal extension and block faulting

submarine fan deposystems and tectonics of a late cretaceous forearc basin along an accretionary convergent plate boundary maccoll ridge formation wrangell mountains alaska analysis of upper cretaceous sedimentary and volcanic strata in the wrangell mountains of south central alaska provides an opportunity to study the tectonics depositional systems and provenance of a forearc basin that developed along an accretionary convergent plate boundary new data from the 1150 m thick maccoll ridge formation indicate that deposition occurred during the campanian on a coarse grained submarine fan that was derived from an uplifted allochthonous terrane exposed in the hanging wall of a fault system that separated the forearc basin from the subduction complex new age controls include palynoflora indicative of a late middle to late campanian age and compatible radiometric age determinations of volcanic vitric crystal tuffs near the top of the formation which have 40ar 39ar isochron ages of 79 4 0 7 and 77 9 2 1 ma sedimentological and paleontological data show that sedimentation occurred on the inner portions of a sand and gravel rich submarine fan system evidence for this inte

geology of the hazelton volcanic belt in british columbia implications for the early to middle jurassic evolution of stikinia the hazelton group is an early to middle jurassic volcanic and sedimentary succession that was deposited on the stikine terrane stikinia of the canadian cordillera the group is distributed across the entire width of stikinia a distance of about 300 km prior to post hazelton contractional deformation the width across which volcanism had occurred was at least 450 km a review of chronologic and stratigraphic information corroborates an earlier suggestion that the hazelton group formed as a pair of coeval partly subaerial volcanic chains separated by a subsiding mainly sedimentary marine basin hazelton trough the trough developed in response to extension of stikinia during the latest triassic and or earliest jurassic and remained the locus of moderate extension during deposition of the hazelton group volcanic rocks of the group are diverse comprising subaerially and subaqueously deposited lavas and volcaniclastic rocks of mafic to felsic composition calc alkaline to tholeiitic characteristics and depletion of high field strength elements relative to alkali and alkaline earth elements indicate a subduction related origin the degree of alkalinity medium to high k is fairly consistent throughout the group the original width across which volcanism occurred is two to five times greater than widths of modern volcanic arcs the anomalous width cannot be accounted for by time transgression of a single volcanic arc because the two volcanic chains were coeval extremely low angle subduction of a single oceanic plate is also rejected as an explanation because alkalinity does not differ significantly between the two volcanic chains genesis of the hazelton group above a single subduction zone is therefore considered unlikely we propose a model in which a pair of oceanic plates were subducted beneath stikinia from opposite sides in a manner analagous to the tectonic regime of the philippine archipelago subduction angles of the two downgoing plates are postulated to be 30 to a depth of 150 km a configuration consistent with a postulated forearc width of about 175 km on each side stikinia is thereby considered to have been an 800 km wide microplate on which two island arcs developed the hazelton trough was a back arc region common to both volcanic arcs during deposition of the hazelton group stikinia was not part of the intermontane superterrane an amalgam composed of more inboard terranes rather it belonged to an oceanic microplate that accreted to the inboard terranes in late early jurassic to early middle jurassic time during which hazelton volcanism waned and ended

paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia this study contributes the first paleomagnetic data from dated cretaceous units of the coast belt 51 3 n 123 8 w in which paleohorizontal bedding surfaces can be identified with certainty we sampled 38 sites through a 1 5 km stratigraphic interval of cenomanian silverquick sedimentary strata and cenomanian to campanian powell creek volcanics in the tatlow syncline in the tyaughton methow basin magnetizations at 21 sites have high unblocking temperatures tub and pass the tilt test precision k 5 before untilting maximum k 48 after 100 untilting magnetic minerals of volcanics show original textures mean declinations in volcanic and sedimentary rocks are statistically identical and polarity is consistently normal the mean inclination 55 4 3 4 probability p 0 05 of this pretilting magnetization is 20 1 4 0 shallower than that expected if the rocks were deposited in their present position relative to cratonic north america field and laboratory observations rule out other causes for these anomalous shallow inclinations these rocks therefore have been displaced northward by 3000 500 km relative to cratonic north america since mid cretaceous time this displacement is almost 3 times greater than that estimated from mid cretaceous volcanics in the now adjacent intermontane belt evidently the two belts were not amalgamated until latest cretaceous or early tertiary time these results are inconsistent with all previous geologically based tectonic reconstructions of the cordillera magnetizations at eight sites in silverquick and powell creek beds have low tub and directional characteristics similar to those in three younger diorite intrusions reversed and normal polarities occur the rocks are commonly highly altered and maximum precision occurs after 40 untilting the magnetizations therefore are interpreted as overprints of latest cretaceous age acquired during folding

application of lu hf garnet dating to unravel the relationships between deformation metamorphism and plutonism an example from the prince rupert area british columbia abstract the tectonic history of the prince rupert area is marked by profound crustal thickening from thrusting that produced inverted metamorphic field gradients and transpression in crustal scale shear zones syn tectonic garnet in the prince rupert area has lu hf ages of 102 6 3 7 ma and 108 3 4 1 ma 2 porphyroblast matrix relationships in these samples and samples from the same outcrops indicate syn tectonic garnet growth these relationships imply that the garnet ages directly date the development of the metamorphic foliations a third sample of migmatitic garnet amphibolite from the contact aureole of the 94 90 ma ecstall pluton had complex isotope systematics interpreted to indicate a garnet growth episode 105 ma similar to the ages obtained from the other samples and a growth or equilibration event at 94 ma during pluton emplacement the data show that the older lu hf garnet ages date prograde metamorphism during foliation development and modification during pluton emplacement the ecstall pluton was emplaced 10 to 15 m y after regional metamorphism and thrust stacking in the prince rupert area in order to place our samples in a regional tectonic context we compare our results to patterns of regional deformation metamorphism and plutonism throughout the north american cordillera in the time period between 110 and 85 ma contractional and transpressional deformation occurred throughout much of the north american cordillera at this time from southeastern alaska to the baja peninsula in mexico left lateral transpression dominated the canadian cordillera whereas right lateral transpression affected areas south of the idaho salmon river suture zone including the sierra nevada batholith this reversal in kinematics in the northern and southern cordillera within coeval magmatic belts appears to be a first order feature of the geology of the north american cordillera during the cretaceous

interpretation of structures in the southeastern nechako basin british columbia from seismic reflection well log and potential field data1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia 2geological survey of canada contribution 20100002 the structure and stratigraphy of the southeast nechako basin which are poorly understood primarily because of substantial volcanic cover are investigated in an analysis of seismic reflection well and potential field data formation and development of the se nechako basin resulted in sub basins containing cretaceous and eocene rocks interpretation reveals that dextral transtension in the early to middle eocene created nnw trending en echelon strike slip faults linked by pull apart basins which locally contain a thickness of eocene volcaniclastic rocks of 3 km this structural pattern is consistent with regional observations that suggest the transfer of slip from the yalakom fault to the north via a series of en echelon strike slip faults in the middle to late eocene faults associated with a change in the direction of stress echoed by the north trending right lateral fraser fault reactivated and cut earlier structures a simple model agrees with local observations that northeast directed comp

conformable late jurassic oxfordian to early cretaceous strata northern bowser basin british columbia a sedimentological and paleontological model reply the upper jackson unit in the northern bowser basin includes marine strata of late jurassic middle oxfordian to tithonian age on the basis of the presence of vaugonia doroschini eichwald buchia concentrica sowerby buchia mosquensis von buch buchia piochii gabb and buchia fischeriana d orbigny herzogina chavan and plant macrofossils in the overlying currier formation indicate an early cretaceous age macrofloras of the overlying mcevoy and devils claw formations are most similar to early cretaceous pre albian floras of the western interior basin of north america a model of continuous sedimentation from the jurassic to early cretaceous time is proposed with a progression from marine conditions of the jackson unit to marginal marine conditions with lower delta plant deposition in the currier formation to nonmarine fluvial conditions with upper delta plain deposition in the mcevoy formation this stratigraphic sequence culminated in alluvial deposits of the devils claw formation the pres

geometry evolution and tectonic framework of the skeena fold belt north central british columbia the intermontane belt of the canadian cordillera has long been viewed as a passive relatively rigid block between two metamorphic plutonic belts the coast plutonic complex and the omineca belt however the skeena fold belt which spans most of the width of the northern intermontane belt exhibits shortening comparable with that in the rocky mountain fold and thrust belt the skeena fold belt has many features of thin skinned fold and thrust belts such as thrust faults which sole into a detachment a wide variety of structural styles which depend on rock type a foreland basin which was cannibalized by continued deformation a frontal triangle zone and a hinterland of metamorphic and plutonic rocks coast plutonic complex the skeena fold belt thus is comparable with the rocky mountain fold and thrust belt but rather than deforming a continental terrace wedge it developed in a terrane stikinia which had accreted to north america in the early mesozoic and in jurassic and cretaceous clastic successions bowser lake and sustut groups which overlie stikinia structural and stratigraphic relationships show that the earliest deformation occurred between oxfordian and albian time and that the last folds developed in latest cretaceous or early tertiary time as much as 160 km of northeastward shortening in the skeena fold belt was broadly contemporaneous with crustal thickening in the coast plutonic complex and omineca belt with dextral strike slip faulting east of the skeena fold belt and with shortening in the rocky mountain fold and thrust belt therefore between latest jurassic and early tertiary times horizontal shortening occured across most of the width of the northern canadian cordillera concurrent shortening across the cordillera suggests that a common detachment or detachments fed all of these zones as far east as the rocky mountain fold and thrust belt

mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group the age relations geochemistry and sedimentology of the rocky ridge formation of the skeena group are used to test competing tectonic reconstructions for the mid cretaceous canadian cordillera as well as the timing and location of the accretion of the insular superterrane pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the bowser basin in the early albian to early cenomanian single crystal fusion and step heating 40ar 39ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded middle cenomanian ages of 94 3 0 4 95 6 1 6 and 95 0 1 6 ma vesicular basalt flows interbedded with crystal rich tuff breccias contain evidence for hot emplacement as pyroclastic flows individual eruptive centers are identified by their proximal facies paleoflow indicators within the lava flows paleoflow indicators within interbedded volcaniclastic fluvial deposits geochemical differences and geographic isolation of volcanic deposits major and trace element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics the basalts of the northern rocky ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative nb ta anomalies whereas the basalts of the southern tahtsa lake volcanic center show depletion to slight enrichment of light rare earth elements slight enrichment of large ion lithophile elements with minimal negative nb ta anomalies the geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension the presence of deeper marine facies to the west and the lack of a western sediment source in the skeena group indicate that the technically active insular superterrane was not west of the study area during mid cretaceous time thus we reconsider the omineca belt as the main axis of a mid cretaceous continental arc placing the intermontane superterrane in the intraarc to forearc position with the rocky ridge volcanics erupted along the forearc side of the omineca arc coeval regional strike slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for rocky ridge intraarc extension an electronic supplement of tables a1 a2 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 95tc03496 mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group kari n bassett and karen l kleinspehn diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

moderate earthquake teleseismic depth estimations new methods and use of the comprehensive nuclear test ban treaty organization network data abstract we investigate the possibility to estimate the depths of earthquakes of moderate magnitude 3 5 m m b 3 6 ardeche earthquake france 3 august 2011 in the case of the po plain earthquake italy 27 january 2012 this provides another example of the great interest in teleseismic analysis where the detection of the moho reflected phase pmp shows without doubt an upper mantle event finally the murcia event spain 11 may 2011 shows the limits of these methods for very shallow events

model based separation filtering of magnetic data separating the fields produced by sources at different depths is a common requirement in the interpretation of potential field data approaches to this problem are generally data or model based data based methods require clear linear segments in the logarithmic power spectrum of the data corresponding to different components of the field various types of filters can then be designed to carry out the separation when the logarithmic power spectrum shows no identifiable linear spectral segments other approaches are necessary we outline a model based method that does not depend on power spectral information but requires independent estimates of the average depths of the source distributions e g from seismic interpretations an ensemble of models using fractal source distributions is computed based on these known values and filter parameters are determined that produce the closest fit in a least squares sense to the theoretical fields that each source distribution generates this approach is used to separate basement effects from intrasedimentary sources in magnetic data collected over the colville hills northwest territories canada seismic data interpretation places crystalline basement at 10 km depth and an intrasedimentary basaltic layer at 2 km our approach results in an optimal separation filter with a cutoff wavelength of 12 km that appears to provide an effective separation of the two source effects

u pb rb sr and k ar isotopic constraints for ductile deformation and related metamorphism in the teslin suture zone yukon tanana terrane south central yukon the teslin suture zone tsz comprises a portion of the yukon tanana terrane yt in the yukon formed by steeply dipping layering and l s tectonite foliation the tsz forms the fundamental tectonic boundary between rocks deposited along the ancient margin of north america and allochthonous terranes to the west tsz tectonites evolved during initial penetrative dip slip deformation dds and later dextral strike slip shear dss along steep 1 3 km wide shear zones several workers have speculated that the tsz and related yt heterogeneous ductile deformation and associated metamorphism are devonian to mississippian in age and related to the intrusion of a similar age orthogneiss throughout the yukon and alaska however recent structural and metamorphic studies of the tsz provide evidence contradicting this view new isotopic evidence presented herein indicates that tsz dynamothermal metamorphism was cooled by early jurassic time that it cannot be related to devonian mississippian and permian granitic

a new improved and fully automatic method for teleseismic depth estimation of moderate earthquakes 4 5 m 5 5 application to the guerrero subduction zone mexico

two stage exhumation of midcrustal arc rocks coast mountains british columbia 1 new geologic data from the central gneiss complex along douglas channel help delineate the burial and exhumation of the late cretaceous to eocene coast mountains magmatic arc arc plutonism was on going between 90 and 60 ma replaced by formation of dikes to 52 ma supracrustal rocks were buried to midcrustal levels by 90 ma and garnet and kyanite grew from 90 through 70 ma subsequent exhumation was nearly isothermal and took place in two stages the first occurred from 70 to 59 ma as the arc contracted obliquely and sillimanite replaced kyanite exhumation was slow 0 5 mm yr probably accomplished by erosion aided by coaxial crustal thinning exhumation rates about doubled in the second stage of exhumation after 59 ma and before 52 ma penetrative deformation ended prior to intrusion of the quottoon pluton at 59 ma and 6 5 kbar production of cordierite rims on garnet at 4 5 kbar and 700 c signaled the end of near isothermal decompression prior to 52 ma rapid cooling 100 c 106 years of the central gneiss complex followed cordierite growth temperatures dropped to 250 by 48 ma the increase in exhumation rate and the subsequent rapid cooling are attributed to excision of 6 km of crust on a detachment system on the northeastern side of the central gneiss complex comparison to other parts of the coast mountains arc the sierra nevada and fiordland new zealand shows that the amount and tempo of exhumation vary greatly within and between arcs suggesting that the processes accommodating exhumation vary significantly

reply to comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look by e irving et al

structural relationships of the skeena fold belt west of the bowser basin northwest british columbia the skeena fold belt is a regional fold and thrust belt that extends across most of the width of the northern intermontane belt of the canadian cordillera structural and stratigraphic relationships at its northeast margin show that it developed between latest jurassic and early tertiary time that it involved strata at least as low as lower and middle jurassic hazelton group and that it is characterized by northeast verging folds and thrust faults the structures accommodated at least 44 shortening and appear to root to the west most of the fold belt is distinguished by folds in thinly layered jurassic and cretaceous clastic rocks of the bowser and sustut basins its boundary is difficult to establish west of the bowser basin in poorly layered middle jurassic and older strata however map relationships show that hazelton group strata are folded with bowser lake group it is suggested here that the fold belt continues westward to the east margin of the coast plutonic complex where the increase in m

improved location procedures at the international seismological centre summary the international seismological centre isc is a non governmental non profit organization with the primary mission of producing the definitive account of the earth s seismicity the isc bulletin covers some 50 yr 1960 2011 of seismicity the recent years have seen a dramatic increase both in the number of reported events and especially in the number of reported phases owing to the ever increasing number of stations worldwide similar ray paths will produce correlated traveltime prediction errors due to unmodelled heterogeneities in the earth resulting in underestimated location uncertainties and for unfavourable network geometries location bias hence the denser and more unbalanced the global seismic station coverage becomes the less defensible is the assumption that is the observations are independent which is made by most location algorithms to address this challenge we have developed a new location algorithm for the isc that accounts for correlated error structure and uses all iaspei standard phases with a valid ak135 traveltime prediction to obtain more accurate event locations in this paper we describe the new isc locator and present validation tests by relocating the ground truth events in the iaspei reference event list as well as by relocating the entire isc bulletin we show that the new isc location algorithm provides small but consistent location improvements considerable improvements in depth determination and significantly more accurate formal uncertainty estimates we demonstrate that the new algorithm through the use of later phases and testing for depth resolution considerably clusters event locations more tightly thus providing an improved view of the seismicity of the earth