microseismic measurement of wave energy delivery to a rocky coast rocky coasts are attacked by waves that drive sea cliff retreat and etch promontories and embayments into the coastline understanding the evolution of such coastlines requires knowledge of the energy supplied by waves which should depend upon both the deep water waves and the coastal bathymetry they cross we employ microseismic measurements of the wave induced shaking of sea cliffs near santa cruz california as a proxy for the temporal pattern of wave energy delivery to the coast during much of the winter 2001 storm season visual inspection of the time series suggests that both deep water wave heights and tide levels exert considerable control on the energy delivered we test this concept quantitatively with two models in which synthetic time series of wave power at the coast are compared with the shaking data in the first model deep water wave power is linearly scaled by a fitting parameter because this model fails to account for the strong tidal signal it fits poorly in the second model the wave transformation associated with shoaling and refraction diminishes the nearshore wave power and dissipation associated with bottom drag and wave breaking is parameterized by exponential dependencies on two length scales this model reduces the variance by 32 45 and captures the essence of the full signal shoaling and refraction greatly modulate the wave power delivered to the coast energy dissipated by bottom drag across the shelf is relatively small the dissipation length scale is many times the path length across the shelf in contrast much energy is dissipated in the surf zone the tidal dissipation depth scale is of the same order as the tidal range 1 2 m which accounts for the strong dependence of the cliff shaking on the tide

identifying and removing noise from the monterey ocean bottom broadband seismic station mobb data when compared to quiet land stations the very broadband monterey ocean bottom seismic station mobb shows increased long period background as well as signal generated noise both sources of noise are unavoidable in shallow ocean bottom installations and postprocessing is required to remove them from seismic observations the long period background noise observed for periods longer than 20 s is mainly due to infragravity waves and ocean currents the shorter period signal generated noise on the other hand is due to reverberations of the seismic waves in the shallow sedimentary layers as well as in the water layer we first present the steps that were taken prior to and during the instrument deployment to minimize instrument generated noise as well as to avoid noise due to water flow around the instrument we then present results from two postprocessing methods that can be used to remove the long period background noise which both utilize the ocean bottom pressure signal locally recorded on a differential pressure gauge dpg one consists of subtracting the locally recorded ocean bottom pressure from the vertical seismic acceleration signal in this case the frequency independent scale factor is linearly estimated from the data the other one makes use of the transfer function between the vertical seismic and pressure signal to predict the vertical component deformation signal the predicted signal is then removed from the vertical seismic data in either frequency or time domain we also present two methods that can be used to remove the signal generated noise one employs the empirical transfer function constructed from mobb data and nearby land station data that do not show the signal generated noise the other one uses a synthetic transfer function computed by modeling the response of shallow layers at the mobb location using either of the two transfer functions most of the signal generated noise can be removed from the mobb data by deconvolution

environmental significance of bedrock jointing in southern ontario canada the bedrock surface of many glaciated areas is obscured by thick drift deposits in southern ontario canada the buried bedrock surface is dissected by channels infilled with glacial deposits as much as 150 m thick that are part of a wider mid continent preglacial fluvial system that predates formation of the modern great lake basins the infills of bedrock channels form major groundwater aquifers influence regional groundwater flows and contaminant migration to lake ontario and may localize the release of thermogenic methane and radon within heavily urbanized surface environments a quantitative comparison of the regional pattern of bedrock joints and the orientation pattern of buried bedrock channels and modern river valleys shows that all these orientation patterns are virtually coincident buried bedrock channels in south central ontario are not part of a simple antecedent drainage system but were likely predesigned by bedrock joint patterns that have subsequently been propagated upward into overlying pleistocene sediments joints in sediments are of considerable environmental significance for example subsurface contaminant and gas migration in fine grained clayey sediments and of many origins stress release desiccation etc but are widely assumed to be a predominantly surface related phenomena the existence of deeper joints has been noted by some authors but their origin is obscure data presented herein from south central ontario confirm that in addition to surface related joints a second population of bedrock related joints reflecting the upward propagation of bedrock fractures is present in pleistocene sediments of south central ontario

three dimensional numerical simulation of the subduction dynamics in the sunda arc region southeast asia abstract the sunda arc has been modelled as a test subduction zone with geometrical complexities and varying dynamics we simulated spatially the structural heterogeneities of the subduction zone and carried out our computations by a three dimensional finite element approach the aim is to estimate the lateral variation in the effective horizontal compressive stress in the interplate region which can produce the present day observed seismogenic stress field in our simplified approach the stress field has been modelled by varying the traction ridge push while keeping the age dependent gravitational body force slab pull constant our result suggests that there is a considerable amount of stress dissipation in the decoupled segment of java at sumatra the required traction is much greater than that expected from the age dependency relations in our modelling the transition from the dominance of the horizontal compressive stress to that of the gravitational pull in the regional stress field occurs around a depth range of 150 200 km lateral variation in the subducted slab volume emerges crucial to the seismogenic stress field and the derived pattern of displacement subduction of the lighter oceanic rise material should enhance the stress level the effect of abrupt lateral variation in structure e g sudden changes in the slab dip or slab depth may also be significant on the regional stress field viscoelastic modelling attempted for estimating the temporal variation in subduction dynamics suggests that the thrust level will increase southward of the present day thrust zone in java while strike slip fault type stress field will be developed south of sumatra

transoceanic wave propagation links iceberg calving margins of antarctica with storms in tropics and northern hemisphere 1 we deployed seismometers on the ross ice shelf and on various icebergs adrift in the ross sea including b15a a large 100 km by 30 km fragment of b15 which calved from the ross ice shelf in march 2000 the data reveal that the dominant energy of these floating ice masses is in the 0 01 to 0 1 hz band and is associated with sea swell generated in the tropical and extra tropical pacific ocean in one example a strong storm in the gulf of alaska on 21 october 2005 approximately 13 500 km from the ross sea generated swell that arrived at b15a immediately prior to and during its break up off cape adare on 27 october 2005 if sea swell influences iceberg calving and break up a teleconnection exists between the antarctic ice sheet mass balance and weather systems worldwide

seismic observations of sea swell on the floating ross ice shelf antarctica 1 a seismometer operating on the floating ross ice shelf near its seaward ice front nascent iceberg for 340 days out of 730 days during the 2004 2005 and 2006 antarctic field seasons recorded the arrival of 93 distantly sourced ocean swell events displaying frequency dispersion characteristic of surface gravity waves propagating on deep water comparison of swell event dispersion with the noaa wave watch iii nww3 ocean wave model analysis reveals that 83 of these events were linked to specific storms located in the pacific southern and indian oceans nearly all major storms in the nww3 analysis of the pacific ocean were linked to signals observed at the nascent site during the period of seismometer operation swell induced motion of the ross ice shelf is found to increase by several orders of magnitude over the time period that sea ice surrounding antarctica decreases from its maximum extent october to its minimum extent february the amplitude of vertical vibration of the ice shelf in the frequency band between 0 025 and 0 14 hz varies between tens of micrometers to millimeters as sea ice decays to its minimum seasonal extent this suggests that climate influence on sea ice extent may indirectly modulate swell energy incident on the calving margins of the antarctic ice sheet the largest swell signals observed on the ross ice shelf come from storms in the tropical pacific and gulf of alaska these remote events emphasize how the iceberg calving margin of antarctica is connected to environmental conditions well beyond antarctica

tilts in strong ground motion most instruments used in seismological practice to record ground mo tion are pendulum seismographs velocigraphs or accelerographs in most cases it is assumed that seismic instruments are only sensitive to the translational motion of the instrument s base in this study the full equation of pendulum motion including the inputs of rotations and tilts is considered it is shown that tilting the accelerograph s base can severely impact its response to the ground motion the method of tilt evaluation using uncorrected strong motion accelerograms was first suggested by graizer 1989 and later tested in several laboratory experiments with different strong motion instruments the method is based on the difference in the tilt sensi tivity of the horizontal and vertical pendulums the method was applied to many of the strongest records of the mw 6 7 northridge earthquake of 1994 examples are shown when relatively large tilts of up to a few degrees occurred during strong earthquake ground motion residual tilt extracted from the strong motion record at the pacoima dam upper left abutment reached 3 1 in n45e direction and was a result of local earthquake induced tilting due to high amplitude shaking this value is in agreement with the residual tilt measured by using electronic level a few days after the earthquake the method was applied to the building records from the north ridge earthquake according to the estimates residual tilt reached 2 6 on the ground floor of the 12 story hotel in ventura processing of most of the strongest records of the northridge earthquake shows that tilts if happened were within the error of the method or less than about 0 5

assimilating coastal wave observations in regional swell predictions part i inverse methods abstract inverse methods are used to assimilate wave observations into numerical predictions of ocean swell 0 04 0 12 hz surface waves propagating over complex continental shelf bathymetry model predictions of swell on the shelf can be degraded by the limited accuracy and resolution of the initializing offshore unsheltered deep water frequency directional spectra so usually derived from buoy measurements or meteorological hindcasts the authors use a spectral refraction wave propagation model to find improved estimates of so that are consistent with both unsheltered and sheltered altered by coastal bathymetry observations and vary smoothly in direction and time in part i linear and nonlinear inverse assimilation methods are developed their potential and limitations are illustrated qualitatively using a scenario where no a priori knowledge of so is used in the inverse estimates inverse estimates of so based solely on sheltered wave data routinely collected in the southern california bight are

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

seismic precursory patterns before a cliff collapse and critical point phenomena 1 we analyse the statistical pattern of seismicity before a 1 2 103 m3 chalk cliff collapse on the normandie ocean shore western france we show that a power law acceleration of seismicity rate and energy in both 40 hz 1 5 khz and 2 hz 10khz frequency range is defined on 3 orders of magnitude within 2 hours from the collapse time simultaneously the average size of the seismic events increases toward the time to failure these in situ results are derived from the only station located within one rupture length distance from the rock fall rupture plane they mimic the critical point like behavior recovered from physical and numerical experiments before brittle failures and tertiary creep failures our analysis of this first seismic monitoring data of a cliff collapse suggests that the thermodynamic phase transition models for failure may apply for cliff collapse

topographic effects on the seismic response of steep slopes a frequency domain parametric study using generalized consistent transmitting boundaries has been performed to evaluate the significance of topographic effects on the seismic response of steep slopes the results show that the peak amplification of motion at the crest of a slope occurs at a normalized frequency h 0 2 where h is the slope height and is the wavelength of the motion the importance of the natural site frequency is illustrated by the analysis of a stepped layer over a half space it was found that the natural frequency of the region behind the crest can dominate the response relative to the topographic effect for the conditions studied moreover the effect of topography can be handled separately from the amplification due to the natural frequency of the deposit behind the crest of the slope this concept of separating the amplification caused by topography from that caused by the natural frequency is advantageous to the development of a simplified method to estimate topographic effects

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

k ar ages of basic rocks in the patia valley southwest colombia abstract the patia valley situated between the western and central cordilleras of the southwest colombian andes contains two areas in which mesozoic basic and ultrabasic rocks crop out in abundance late cretaceous diabase group pillow basalts which make up much of the western cordillera are at least 81 5 ma in the e1 tambo e1 penol area 105 97 ma hornblende and whole rock dates from the ophiolitic los azules complex indicate an albian age of formation although many dates are lower 65 62 ma owing the low grade ocean floor metamorphism the metamorphic age distribution here supports an end of cretaceous emplacement for the complex rather than the early mid cretaceous emplacement suggested for north colombian ophiolites similarly aligned along the romeral fault system tertiary dacites intruding the low azules complex are 36 15 ma

coastal cliff ground motions from local ocean swell and infragravity waves in southern california 1 ground motions atop a southern california usa coastal cliff are compared with water level fluctuations observed at the cliff base and with ground motions observed 10 km inland at high tide cliff top ground motions in three frequency bands were generated locally by ocean waves at the cliff base 1 high frequency 0 3 hz shaking caused by waves impacting the cliff and 2 gravitational loading induced swaying at the frequency of the incident sea swell waves 0 05 0 1 hz and 3 slow swaying at infragravity frequencies 0 006 0 05 hz at high tide at infragravity and incident sea swell wave frequencies cliff top vertical ground displacement and cliff base water level fluctuations are coherent and oscillate in phase with occasional deviation at sea swell frequencies and spectral levels at the cliff top are much higher than at the inland seismometer in contrast at double frequencies 0 1 0 3 hz spectral levels of vertical motions are nearly identical inland and at the cliff top consistent with a common distant or spatially distributed source at low tide when ocean waves did not reach the cliff base power levels of vertical ground motions at the cliff top decreased to inland levels at incident wave frequencies and higher and only infragravity band motions were noticeably forced by local ocean waves

orthogonal joint systems in the bergen area southwest norway and their regional significance abstract joint orientation measurements are reported from the bergen district southwest norway the prevailing joint systems form two near vertical near orthogonal sets striking preferentially nw se and ne sw the orientation of these fissure systems is consistent with fissure systems dissecting the surface formations precambrian to quaternary throughout britain and northwest france they also show general agreement with lineaments in southern norway as revealed by landsat imagery it is suggested that the orthogonal joint network represents a fundamental tectonic structure originally implanted into the mobile outer layer of the earth in precambrian time the network has been repeatedly reactivated throughout the earth s dynamic tectonic history it was probably formed in a shear regime caused by torsional deformation associated with rotation of large scale lithospheric units which is still operating today

the early cretaceous san juan plutonic suite ecuador a magma chamber in an oceanic plateau sections through an oceanic plateau are preserved in tectonic slices in the western cordillera of ecuador south america the san juan section is a sequence of mafic ultramafic cumulates to establish that these plutonic rocks formed in an oceanic plateau setting we have developed criteria that discriminate intrusions of oceanic plateaus from those of other tectonic settings the mineralogy and crystallization sequence of the cumulates are similar to those of intra plate magmas clinopyroxene predominates throughout and orthopyroxene is only a minor component rocks of intermediate composition are absent and hornblende is restricted to the uppermost massive gabbros within the sequence the ultramafic cumulates are very depleted in light rare earth elements lree whereas the gabbros have flat or slightly enriched lree patterns the composition of the basaltic liquid in equilibrium with the peridotite calculated using olivine compositions and ree contents of clinopyroxene contains between 16 and 8

bedrock jointing and geomorphology in southwestern ontario canada an example of tectonic predesign in southwestern ontario canada palaeozoic sedimentary strata are covered by pleistocene glacial sediments the topography of the buried bedrock surface has been mapped over an area of some 45 000 km2 using new computerised depth to bedrock data from more than 200 000 waterwells this information shows that arching of the underlying proterozoic basement canadian shield and differential erosion of carbonate and shale strata are first order regional controls on bedrock geomorphology these data also define a network of buried bedrock channels quantitative analysis of the trends of these channels reveals a statistical correspondence between channel orientation and regional bedrock joint sets formed in response to the mid continent stress field the orientations of bedrock channels and joints correspond regionally with mean trends at 153 and 164 respectively in addition two dominant orientations of postglacial river valleys and bedrock joints correspond with mean trends at 104 vs 104 and 54 vs 52 respectively our findings show that the erosional channelled topography cut on bedrock together with the postglacial drainage pattern cut into younger glacial sediments are related to the trend of bedrock joints and consequently can be described as tectonically predesigned in addition this study yields new insights into the form of the preglacial tertiary drainage system in southern ontario prior to pleistocene glaciation regardless of the precise age of this preglacial system and degree of subsequent glacial modification an important factor in its geomorphic development has been the mid continent stress field

ground motion amplitude across ridges abstract local amplification and wave diffraction on an elongated ridge near sourpi in central greece were studied by the analysis of seismic records of local and regional earthquakes data were obtained during field work especially designed for this purpose these data were analyzed in the frequency and time domains in the frequency domain spectral ratios show amplifications of 1 5 to 3 at the ridge top relative to the base of the ridge the horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations theoretical spectral ratios calculated by the indirect boundary element method are dependent on earthquake location but are in general agreement with the observed spectral ratios another dataset from mont st eynard in the french alps showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank these relative amplifications are within the range predicted by numerical simulations the numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge the use of a seven station array on the ridge at sourpi made it possible to identify such waves the analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography induced amplification on ridge tops our results also show that such amplification is moderate for the ridges under study

joints as neotectonic plate signatures abstract it is the contention of the present author that surface joints are signatures of the plate tectonic stress field according to the writer s view joints are shear features caused by plate tectonic stresses with the maximum compression axis lying in the smaller quadrant formed by the regional sets these contentions are substantiated by new joint measurements geotectonic lines in europe are reflected in the joint orientations and the subduction direction in the philippines is indicated by them other instances of joints being plate tectonic signatures which have been published previously are referred to

tobago west indies a fragment of a mesozoic oceanic island arc petrochemical evidence the pre cenozoic rocks of tobago west indies represent an exceptional cross section through a portion of a composite mesozoic arc system other parts of which may be found in the villa de cura klippe the aruba blanquilla island chain and perhaps in the western cordillera of colombia and ecuador the geochemical data from tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted mesozoic arc of the southern caribbean mountain system can be deduced the mesozoic rocks of tobago can be divided into four main groups the north coast schist the ultramafic to mafic plutonic complex the tobago volcanic group and late mafic dykes rocks of the north coast schist served as wallrock for the plutonic volcanic dyke complex which evolved during the mid cretaceous geochemical and nd and sr isotopic data presented demonstrate the oceanic island arc character of all the pre cenozoic rocks of the island all suites span the calc alkaline and tholeiitic regions on discrimination diagrams rare earth element ree patterns for the north coast schist tobago volcanic group and plutonic suite are essentially identical and are moderately lree enriched la yb 1 1 2 7 the dykes exhibit the most unevolved ree patterns and are interpreted as most closely representative of parental liquid compositions initial nd 6 6 to 9 4 and 87 sr 86 sr 0 70303 0 70364 for all samples except north coast schist and tobago volcanic group argillites are similar in value and overall range to other oceanic island arcs limited isotopic data from argillite samples from tobago suggest an important change in the continental derived sediment contribution during mesozoic arc growth nd isotopic data from north coast schist siliceous argillite indicate virtually no continental detritus in this neocomian sediment while nd isotopic data from the tobago volcanic group argillite suggest a definite continental contribution during albian arc growth

nloadf a program for computing ocean tide loading the loading of the earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique in order to compute these most accurately a combination of global and local models of the ocean tides may be needed the program nloadf convolves the green functions for loading with ocean tide models using a station centered grid with fixed dimensions making it easy to combine different ocean models without overlap in the convolution the program computes all the quantities of interest gravity displacement tilt and strain and includes the case where measurements are made beneath the surface of the ocean

loading and gravitational effects of the 2004 indian ocean tsunami at syowa station antarctica the 2004 indian ocean tsunami reached syowa station antarctica approximately 12 5 hr after the december sumatra andaman earthquake we have analyzed the tsunami signals recorded on ocean bottom pressure gauges broadband seismometers sts 1 and a superconducting gravimeter sg we calculated the sea level variation tilt and gravity changes induced by the tsunami and compared these results to observations from this comparison we confirmed the loading and gravity effects of the tsunamis on the sts 1 horizontal components and the sg records at syowa station the magnitudes of these effects given as root mean square amplitudes are as follows for the tilt effects obtained from 20 hr long sts 1 records at frequencies in the range 0 3 0 6 mhz 5 and 8 gal 10 8 m sec 2 in the east west and north south directions respectively and for the gravity effect obtained from the sg records for the same time period of 20 hr at frequencies in the range 0 1 0 2 mhz 0 2 gal by using detailed bathymetry around syowa station the synthetic amplitudes similar to the observed were obtained although the waveforms of synthetic and observation are not always consistent

nearshore wave induced cyclical flexing of sea cliffs 1 evolution of a tectonically active coast is driven by geomorphically destructive energy supplied by ocean waves wave energy is episodic and concentrated sea cliffs are battered by the geomorphic wrecking ball every 4 25 s we measure the response of sea cliffs to wave assault by sensing the ground motion using near coastal seismometers sea cliffs respond to waves in two distinct styles high frequency motion 20 hz reflects the natural frequency of the sea cliff as it rings in response to direct wave impact low frequency motion in the 0 1 0 05 hz 10 20 s band consistently agrees with the dominant nearshore wave period integrating microseismic velocities suggests 50 m and 10 m displacements in horizontal and vertical directions respectively displacement ellipsoids exhibit simultaneous downward and seaward sea cliff motion with each wave video footage corroborates the downward sea cliff flex in response to the imposed water load on the wave cut platform gradients in displacement amplitudes documented using multiple seismometers suggest longitudinal and shear strain of the flexing sea cliff on the order of 0 5 4 strains during each wave loading cycle as this sea cliff flexure occurs approximately 3 million times annually it has the potential to fatigue the rock through cyclical loading local sea cliff retreat rates of 10 cm yr imply that a given parcel of rock is flexed through roughly 109 cycles of increasing amplitude before exposure to direct wave attack at the cliff face

high mg extrusive rocks from the romeral zone ophiolites in the southewestern colombian andes abstract in the southwestern colombian andes high mg extrusive rocks occur within the mesozoic late cretaceous ophiolitic fragments from the romeral fault zone a major suture between the continental domain of the central cordillera and the oceanic domain of the western cordillera the high mg rocks include picrites and picritic basalts and are associated in places with peridotites showing transitional to extrusive textures plagioclase free picrites characterized by quench textures have been found in two localities and occur as pillow lava rio boloblanco or as clasts of monogenic and polygenic volcanic breccias el encenillo they show similar olivine phyric micro spinifex textures with groundmass consisting of hopper and plate olivine or ca poor pyroxene at el encenillo skeletal clinopyroxene and glass hyaloporphyritic texture occurs in the rio boloblanco picrites at pillow rims chemically these picrites are characterized by mgo contents of 20 and are distinguished by the content of tio 2 and incompatible elements in a high ti type rio boloblanco and a very high ti type el encenillo the latter particularly remarkable for the high contents of ti zr and nb which give evidence of an alkaline tendency phase chemistry is consistently different between the two picrite types but comparably high al contents up to 14 of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions olivine phyric picritic rocks characterized by microlitic non quenched groundmasses occur as clasts of volcanic breccias at el encenillo picritic basalts and are present with comparatively high frequency within major los azules and minor upper chapungo valley ophiolitic fragments from the romeral zone either as lavas and volcanigenic breccias these rocks are mostly richer in olivine than the quench textured picrites contain modal plagioclase and relatively low al pyroxenes as regards chemistry a high ti and very low al group los azules upper chapungo valley and a very high ti group el encenillo can also be distinguished within these picrites a comparison with the ultramafic volcanic rocks from gorgona island in the nearby pacific domain off colombia among which the unique phanerozoic komatiites were discovered indicates close similarities between the rio boloblanco picrites and some gorgona island komatiites olivine phyric komatiites with micro spinifex groundmass regarding their petrochemical features it is suggested that they record a unique event related to the same tectonic environment and consequently that they belong to the same structural unit developed in the late cretaceous and extending from continental colombia to the pacific domain this is in contrast with previous subdivisions of the ophiolitic complexes from southwestern colombia into distinct age groups progressively younger from the central cordillera toward the pacific ocean

alluvial lithofacies recognition in a humid tropical setting abstract cobble gravel deposits in the antigua formation accumulated on a large alluvial fan or braid plain west of the cordillera occidental in southwest colombia this formation was probably deposited during the pleistocene in a very wet tropical climate 500 cm yr rainfall fining upwards sequences of clast supported imbricated boulders and cobbles dominate with maximum clast sizes between 30 and 300 cm the sand matrix in the antigua gravels and the minor 10 sand facies are weathered to clay at depths of up to 20 m the sand facies contains abundant drift logs and leaf mats except for the absence of debris flows and the very coarse nature of the gravel the antigua gravels have lithofacies similar to the glacial outwash braid plain in the proximal area of the scott type model gravels and sands of the younger panambi formation were deposited by a braided stream that was smaller confined by valley walls and flowing at a lower gradient than the river that deposited the antigua gravels we recognize no sedimentologic characteristics of these deposits as diagnostic of a humid tropical environment except for textural and compositional changes in matrix sediments caused by deep and rapid chemical weathering

the andean ophiolitic megastructures on the buga buenaventura transverse western cordillera valle colombia abstract a structural study of the buga buenaventura transverse in the central part of the western cordillera of colombia has shown the presence of three structural units which from bottom to top and from west to east are the rio dagua unit the rio calima unit and the loboguerrero window unit all three units comprise strata between 120 and 80 m y old overlying a basement of green rocks showing the characteristics of submarine flows the bolivar ultrabasic and basic massif is geographically linked to the rio calima unit in which green rocks predominate and is separated from it by a tectonic contact the upper part of the massif on the other hand shows high temperature metamorphic rocks formed during its emplacement the rio dagua unit shows two tectonic phases with isoclinal folding and development of schistosity the first phase is contemporaneous with low grade metamorphism equivalent to lower greenschist facies conditions the loboguerrero window unit shows a large recumbent fold oriented towards the southeast deformation in the rio calima unit is weaker and appears to correspond to a higher structural level than in the two other units as the attitudes of the s1 schistosity in the rio dagua unit and of the shear zones located at the green rocks sediments contact in this same unit are similar to that of the overthrust at the base of the rio calima unit we are led to postulate that the overthrust belongs to the first phase of deformation as also does the recumbent fold in the loboguerrero window unit we are thus led to propose a southeastward direction of emplacement for the nappes in the western cordillera of colombia in conclusion the authors extend their observations and propose a new structural interpretation of the occidente colombiano

seismic observations of glaciogenic ocean waves micro tsunamis on icebergs and ice shelves seismometers deployed over a 3 year period on icebergs in the ross sea and on the ross ice shelf antarctica reveal that impulsive sources of ocean surface waves are frequent e g 200 events per year in the ross sea in the ice shelf and iceberg covered environment of coastal antarctica the 368 events recorded by our field deployment suggest that these impulsive events are generated by glaciological mechanisms such as 1 small scale calving and edge wasting of icebergs and ice shelf fronts 2 edge on edge closing and opening associated with iceberg collisions and 3 possibly the impulsive opening of void space associated with ice shelf rifting and basal crevasse formation the observations described here provide a background of glaciogenic ocean wave phenomena relevant to the ross sea and suggest that these phenomena may be exploited in the future using more purposefully designed observation schemes to understand iceberg calving and ice shelf disintegration processes

ophiolite complex from la tetilla southwestern colombia south america a disrupted ophiolitic complex of cretaceous age from the romeral fault zone in the southwestern colombian andes occurs in the la tetilla area near popayan it represents a peculiar association of low ti and high ti tholeiitic basalts among the ophiolitic complexes from the romeral zone it includes 1 a main sequence with incipient metamorphism under prehnite pumpellyite to greenschist facies conditions consisting in ascending order of cumulus wehrlite and gabbro massive gabbro and basaltic and doleritic lavas and breccias the basalts are characterized by the crystallization of clinopyroxene prior to plagioclase cpx phyric basalt group and have a primitive low ti tholeiite composition mg numbers between 70 6 and 65 4 the clinopyroxene chemistry is consistent with this magmatic affinity the plutonic rocks can be related to a similar low ti tholeiitic magma 2 a sequence of pervasively metamorphosed basalts with greenschist facies metamorphism of oceanic type structurally underlies the mai

cretaceous basaltic terranes in western colombia elemental chronological and sr nd isotopic constraints on petrogenesis accreted terranes comprising mid to late cretaceous picrites basalts and dolerites occur in three north south trending belts in western colombia in the central cordillera western cordillera and along the pacific coast the geochemistry of these rocks is consistent with an oceanic plateau plume related origin and they most probably formed in the pacific as part of the caribbean oceanic plateau these igneous rocks display small but significant inter cordillera variations being younger and more depleted in incompatible trace element ratios and with more positive end values to the west the igneous rocks of the pacific coast serrania de baudo are dated at 73 78 ma 40ar 39ar and those of the western cordillera at 90 ma whereas the volcanics of the central cordillera are believed to be older than 100 ma most of the igneous rocks are basaltic and it is suggested that they have fractionated from picritic primary magmas generated by partial melting within a hot mantle plume variable and positive end values reveal that the plume must have been heterogeneous originating from a mantle source with a long term history of depletion partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of 20 mixing of melts from different depths either in the mantle melting column or during fractionation in lithospheric magma chambers can explain the relative homogeneity of basaltic lavas erupted to form this and other oceanic plateaux the caribbean colombian oceanic plateau may have formed at an oceanic spreading centre and valuable comparisons can be made between iceland and the caribbean colombian plateau

seismic monitoring of the indian ocean tsunami the 26 december 2004 sumatra andaman earthquake of mw 9 3 triggered a massive tsunami in the indian ocean we here report on observations of the indian ocean tsunami at broadband seismic stations located on islands in the area the tsunami induces long period 1000 s signals on the horizontal components of the sensor frequency time analysis shows that the long period signals cannot be due to seismic surface waves but that it arrives at the expected time of the tsunami the waveforms are well correlated to tide gauge observations at a location where both observations are available to explain the signals we favour tilt due to coastal loading but we cannot at the present stage exclude gravitational effects the density of broadband stations is expected to increase rapidly in the effort of building an earthquake monitoring system they may unexpectedly become useful tsunami detectors as well

a critical review of tectonic processes at continental margin orogens abstract a cordilleran type orogen has previously been defined as contractional deformation of a continental margin due to subcontinental subduction the plate tectonic setting of cordilleran type orogenesis is fundamentally different from that of collisional orogenesis intracontinental orogens such as the himalayas and alps form by collision between a continental margin and a continental margin subduction zone orogens at continental margins may form due to subcontinental subduction cordilleran type orogeny or due to collision of a continental margin with an intra oceanic subduction zone which is typically followed by a reversal of subduction polarity in the latter case collisional orogenesis may proceed along with the reversed subcontinental subduction modern arcs are mostly extensional or neutral so subduction hangingwall contraction is not a necessary consequence of subduction crustal contraction in known collisional orogens lasts for tens of millions of years so collisional deformation at a continental margin may last for tens of millions of years after arc continent collision and subduction reversal collisional orogenesis is a predictable consequence of subduction and collisional deformation is kinematically linked to the kinematics of subduction noncollisional cordilleran type orogenesis is not predicted by plate theory a geologic history that would demonstrate noncollisional orogenesis is a magmatic arc built into a passive margin sequence with strong deformation but without an earlier arc or ophiolite accretion event this history has not been recognized in the geologic record the classic examples of cordilleran type orogens including the cretaceous cenozoic of the andes and the jurassic early tertiary of the western united states may be interpreted as arc continent collisions however the youngest increments of deformation in these two orogens occurred 100 million years after the youngest collisions and are thus difficult to ascribe to collisional processes notwithstanding the long time scales these deformations occurred at the end of space time continua of probable collisional orogenies alternatively these deformations may represent lithosphere scale fault bend folding duplex folding backthrusting and or out of sequence forethrusting during shallow subduction