crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

cryogenic infrared spectrometers and telescopes for the atmosphere crista experiment and middle atmosphere variability the cryogenic infrared spectrometers and telescopes for the atmosphere crista instrument was built to determine whether and to what extent small scale structures in global trace gas distributions and in dynamics are present in the middle atmosphere to achieve this trace gases were measured in the middle infrared by the limb scan technique at the highest possible horizontal and vertical resolution crista uses three telescopes i e three view directions simultaneously and has three grating spectrometers for the middle ir 4 14 m and one spectrometer for the far ir 15 71 m the optics and detectors are cooled to cryogenic temperatures by supercritical helium or subcooled helium respectively in a double cryostat an instrument overview is given and the design guidelines are sketched the crista experiment was flown on the space shuttle sts 66 as part of nasa mission atlas 3 on november 3 14 1994 orbit altitude was 300 km and inclination was 57 a campaign of ground based balloon and rocket validation and complementary measurements was performed simultaneously the crista instrument performed flawlessly a horizontal resolution of 200 km 650 km was achieved at the equator with higher horizontal resolution at higher latitudes a vertical resolution of 2 5 km or better was obtained the middle atmosphere was found to be highly variable at scales of 1000 km in the stratosphere three streamers of tropic subtropic air extending to higher latitudes are described their meridional scale is 1000 km while the zonal scale is of the order of 10 000 km and more the streamers appear to be typical of specific winter conditions and to play a role in meridional transport at mesospheric heights a strong tidal temperature oscillation was observed which extended well into the lower thermosphere

extension discrepancy at north atlantic nonvolcanic rifted margins depth dependent stretching or unrecognized faulting rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide angle data or subsidence this extension discrepancy has been interpreted in terms of depth dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere here i show that at those nonvolcanic margins where the velocity structure is well known the upper and lower crust appear to thin equally toward the margin implying that the discrepancy is not due to depth dependent stretching but more likely due to unrecognized polyphase and top basement faulting

modeling ozone laminae in ground based arctic wintertime observations using trajectory calculations and satellite data reverse trajectory calculations initialized with ozone observed by the upper atmosphere research satellite microwave limb sounder mls provide high resolution ozone profiles for comparison with lidar and ozonesonde observations from the arctic stratospheric observatory facility near eureka in the canadian arctic 80 n 86 w by statistical measures calculated profiles show a small average improvement over mls profiles in the agreement of small scale structure with that in ground based observations throughout the stratosphere and a larger although still modest and more consistent improvement in the lower stratosphere nearly all of the calculated profiles initialized with daily gridded mls data show some improvement in the lower stratosphere even in cases where overall agreement between profiles is mediocre there are frequently one or more individual features in the calculated profiles that strongly resemble laminae in the ground based observations differential advection of ozone by the large scale winds leads to lamination in three distinct ways filamentation results in lamination throughout the stratosphere with comparable features arising from initializations with gridded mls data and with potential vorticity space reconstructions of mls data reconstructed rc fields laminae also form in the middle and lower stratosphere in conjunction with intrusions into the vortex while calculations initialized with rc fields produce laminae the agreement of structure calculated using gridded mls initialization data with ground based observations is distinctly better inside the lower stratospheric vortex laminae form by advection of local features in the mls initialization fields rc initialized calculations fail to produce any significant features since these local ozone variations are not strongly correlated with potential vorticity that local features observed by mls are needed to produce laminae resembling those in independent ground based observations at eureka indicates that both datasets are capturing real atmospheric features

a new chemical lagrangian model of the stratosphere clams 1 formulation of advection and mixing 1 recent satellite observations and dynamical studies have demonstrated the existence of filamentary structures in chemical tracer fields in the stratosphere it is also evident that such features are often below the spatial resolution of the highest resolution eulerian models that have been used up to the present time these observations have motivated the development of a novel chemical lagranigan model of the stratosphere clams that is based on a lagrangian transport of tracers the description of clams is divided into two parts part 1 this paper concentrates on the lagrangian dynamics i e on the calculation of trajectories and on a completely new mixing algorithm based on a dynamically adaptive grid while part 2 describes the chemical integration and initialization procedure the mixing of different air masses in clams is driven by the large scale horizontal flow deformation and takes into account the mass exchange between the nearest neighbors determined by delaunay triangulation here we formulate an isentropic i e two dimensional version of the model and verify the mixing algorithm using tracer distributions measured during the space shuttle crista 1 experiment where highly resolved stratospheric structures were observed in early november 1994 a comparison of the measured southern hemispheric n2o distribution with clams results allows the intensity of simulated mixing to be optimized the long term robustness of the transport scheme is investigated in a case study of the 1996 1997 northern hemisphere polar vortex this study further provides a dynamical framework for investigations of chemical arctic ozone destruction discussed in part 2

stratospheric transport by planetary wave mixing as observed during crista 2 1 planetary waves drive the mean meridional circulation of the stratosphere and at the same time facilitate quasi horizontal mixing of trace gases this paper presents significant day to day variability of stratospheric trace gas fields associated with large planetary wave activity observed during the second mission of the cryogenic infrared spectrometers and telescopes for the atmosphere crista experiment geopotential height data of the uk met office show that the crista 2 observations in the southern hemisphere winter were made during a period of extremely large amplitudes of both wave 1 and wave 2 the planetary wave 1 usually a quasi stationary feature moved eastward with the traveling planetary wave 2 the large amplitudes of both wave 1 and wave 2 led to a significant displacement of the edge of the polar vortex toward the tropics down to 30 s as a result of the large wave amplitudes and favorable phase alignment the anticyclone drawing up tropical air was unusually strong and thus considerable wave induced trace gas flux from the tropics toward midlatitudes was observed mainly in the form of a pronounced planetary scale tongue advected out of the tropics around the vortex and into the anticyclone quantitative transport calculations based on a sequential data assimilation system highlight the importance of such transport events for trace gas eddy flux in the southern hemisphere winter stratosphere

crustal structure of the ocean continent transition at flemish cap seismic refraction results 1 we conducted a seismic refraction experiment across flemish cap and into the deep basin east of newfoundland canada and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots the continental crust at flemish cap is 30 km thick and is divided into three layers with p wave velocities of 6 0 6 7 km s across the southeast flemish cap margin the continental crust thins over a 90 km wide zone to only 1 2 km the ocean continent boundary is near the base of flemish cap and is marked by a fault between thinned continental crust and 3 km thick crust with velocities of 4 7 7 0 km s interpreted as crust from magma starved oceanic accretion this thin crust continues seaward for 55 km and thins locally to 1 5 km below a sediment cover 1 9 3 1 km s oceanic layer 2 4 7 4 9 km s is 1 5 km thick while layer 3 6 9 km s seems to disappear in the thinnest segment of the oceanic crust at the seawardmost end of the line the crust thickens to 6 km mantle with velocities of 7 6 8 0 km s underlies both the thin continental and thin oceanic crust in an 80 km wide zone a gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth normal mantle velocities of 8 0 km s are observed 6 km below basement there are major differences compared to the conjugate galicia bank margin which has a wide zone of extended continental crust more faulting and prominent detachment faults crust formed by seafloor spreading appears symmetric however with 30 km wide zones of oceanic crust accreted on both margins beginning about 4 5 m y before formation of magnetic anomaly m0 118 ma

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

a transect across australia s southern margin in the otway basin region crustal architecture and the nature of rifting from wide angle seismic profiling abstract the otway basin in southeastern australia formed on a triangular shaped area of extended continental lithosphere during two extensional episodes in cretaceous to miocene times which ultimately led to the separation of australia and antarctica the velocity structure and crustal architecture of the otway continental margin has been interpreted from offshore onshore wide angle seismic profiling data along a transect extending from near the northern otway basin margin with palaeozoic outcrop to the deep ocean basin under the southern ocean along this transect the otway continental margin ocm transect the onshore half graben geometry of early cretaceous deposition gives way to a 5 km thick basin sequence p wave velocity 2 2 4 6 km s extending down the continental slope offshore to at least 60 km from the shoreline at 120 km from the nearest shore sonobuoy data indicate a 4 5 km sedimentary sequence overlying 7 km of crustal basement rocks above the moho at 15 km depth water depth 4220 m conspicuous strong moho reflections are evident under the continental slope at about 10 2 s twt basement is interpreted to be attenuated faulted palaeozoic rocks of the delamerian and lachlan orogens intruded with jurassic volcanics that thin from 16 km onshore to about 3 5 km at 120 km from the nearest shore these rocks comprise a 3 km section that has a velocity of 5 5 5 7 km s overlying deeper basement with a velocity of 6 15 6 35 km s over the same distance the moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore and then to about 12 km in the deep ocean at the limits of the profile water depth 5200 m the continent ocean boundary cob is interpreted to be at a prominent topographic inflection point at the bottom of the continental slope in 4800 m of water p wave velocities in the lower crust are 6 4 6 8 km s above a transition to the moho with an upper mantle velocity of 8 05 km s there is no evidence of massive high velocity 7 km s intrusives underplate material in the lower crust nor any syn rift or early post rift subaerial volcanics indicating that the otway continental margin can be considered a non volcanic margin similar in many respects to some parts of the atlantic ocean margins e g the nova scotia newfoundland margin off canada and the galicia bank off the iberian peninsula

stratigraphy and geochronology of the comondu group near loreto baja california sur mexico abstract upper oligocene to middle miocene volcanic and sedimentary rocks in the loreto region baja california sur are widely exposed and make up the comondu group as redefined here following mcfall the comondu group is part of a volcanic arc and forearc basin that formed along the northwestern margin of mexico regional to detailed scale mapping stratigraphic analysis and geochronology in a 10 20 km wide and 70 km long belt from the gulf escarpment to the coast near loreto reveal three main units in the comondu group and a composite thickness of 1 5 2 km 1 the lower clastic unit 30 19 ma contains 200 300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin 2 the middle breccia and lava flow unit 19 15 ma is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows 3 the upper lava flow and breccia unit 15 12 ma consists almost entirely of up to 600 meters of andesite lava flows a few kilometers west of loreto that are part of a composite volcano north and south of loreto for up to 25 km the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers the three units of the comondu group in the loreto area correlated with the salto pelones and ricason formations of the bahia concepcion area the lower clastic unit of this study is time equivalent to the marine san gregorio el cien and isidro formations on the western and southern baja california peninsula the middle and upper units of the loreto area are the proximal equivalent to the type section of the comondu group near the village of comondu during deposition of the lower part of the comondu group the arc lay to the east on mainland mexico in the sierra madre occidental and then migrated to the position of the gulf of california at about 25 ma at 19 ma the proximal part of the arc migrated abruptly 50 km westward to approximately the position of carmen island and at 15 ma the arc once again stepped west forming eruptive centers along the coast near loreto until 12 ma also at 15 ma the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment

thermal evolution of monte blanco dome low angle normal faulting during gulf of california rifting and late eocene denudation of the eastern peninsular ranges footwall rocks of the canada david detachment fault northeastern baja california record late eocene earliest oligocene and late neogene cooling events previously unrecognized in the region biotite 40 ar 39 ar ages of 65 ma reflect slow cooling through 350 c 5 to 10 m y later than is typical in the region multiple diffusion domain modeling of k feldspar 40 ar 39 ar release spectra shows very slow cooling 1 c m y from 65 to 45 ma accelerated cooling from 315 c to 215 c between 45 and 33 ma records at least 3 4 km of denudation that we relate to east side up reactivation of late cretaceous structures that generally follow the oceanic continental suture previously established rivers flowing west from mainland mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast ultimately surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m y before the cooling event ended footwall rocks remained nearly isothermal from 30 to 15 10 ma when renewed rapid cooling 33 17 c m y began in response to footwall exhumation by top to the west low angle normal faulting that accommodated rift related extension in what finally became the gulf of california apatite fission track and u th he ages of 5 ma and 4 ma respectively record final detachment related cooling through 110 c to 70 c thermal kinematic modeling suggests that 5 7 km of late neogene tectonic denudation and 10 12 km of horizontal extension were necessary in order to unroof the samples by 2 ma additional extension of unknown magnitude has probably occurred subsequently geodetically measured horizontal extension rates are considerably higher than the long term extension rate that can be attributed to detachment faulting

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

active continental rifting in southern baja california mexico implications for plate motion partitioning and the transition to seafloor spreading in the gulf of california three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern gulf of california focal mechanism solutions for earthquakes of the april 1969 and june 1995 series indicate nearly pure normal displacement across nnw striking planes this seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the gulf extensional province the dominant normal faults control distribution of neogene basins and have been active since middle to late miocene time epicenters of the august 1969 earthquake series lie south of the baja california peninsula and focal mechanism solutions indicate strike slip displacement this seismic activity is likely related to active wnw striking faults in the continental borderland which isolate baja california as a microplate within the pacific north american plate boundary therefore modern pacific north america plate motion is partitioned into at least three distinct fault systems oceanic spreading centers and transforms in the gulf of california normal faults along the western margin of the gulf of california and faults along the pacific margin of the baja california peninsula kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting continental rifting and seafloor spreading have overlapped in time for the past 3 6 myr which also is likely attributed to the highly oblique kinematics of rifting

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

oceanic ridge subduction vs slab break off plate tectonic evolution along the baja california sur continental margin since 15 ma the interaction of the pacific farallon spreading centers with the north american convergent margin off baja california mexico supposedly ceased at 12 ma when plate convergence and seafloor spreading stopped we propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23 n to 27 n famex cruise of the r v l9atalante april 2002 the data unveil a major clockwise rotation of the pacific farallon spreading direction starting ca 14 ma that formed a series of short spreading centers that became extinct ca 8 7 ma we suggest that the transcurrent motion between the pacific and north america along baja california was accommodated by seafloor spreading and oblique convergence along the trench this change in spreading direction was followed by a concomitant progressive demise of both pacific farallon seafloor spreading and farallon north america subduction that are attributed to the break off of the farallon slab this also resulted in the opening of a trench parallel slab window beneath baja california

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

mechanisms of extension at nonvolcanic margins evidence from the galicia interior basin west of iberia 1 we have studied a nonvolcanic margin the west iberia margin to understand how the mechanisms of thinning evolve with increasing extension we present a coincident prestack depth migrated seismic section and a wide angle profile across a mesozoic abandoned rift the galicia interior basin gib the data show that the basin is asymmetric with major faults dipping to the east the velocity structure at both basin flanks is different suggesting that the basin formed along a paleozoic terrain boundary the ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension at the rift flanks stretching factor 2 the ratio is fairly constant indicating that stretching of upper and lower crust was uniform toward the center of the basin 3 5 5 5 fault block size decreases as the crust thins and faults reach progressively deeper crustal levels indicating a switch from ductile to brittle behavior of the lower crust at 3 5 faults exhume lower crustal rocks to shallow levels creating an excess of lower crust within their footwalls we infer that initially extension occurred by large scale uniform pure shear but as extension increased it switched to simple shear along deep penetrating faults as most of the crust was brittle the predominant brittle deformation might have driven small scale flow 40 km of the deepest crust to accommodate fault offsets resulting in a smooth moho topography the gib might provide a type example of nonvolcanic rifting of cold and thin crust

in situ evidence for the nature of the seismic layer 2 3 boundary in oceanic crust the igneous oceanic crust is typically thought of as comprising two layers an upper crust seismic layer 2 characterized by a rapid increase in seismic velocity with depth and a thicker lower crust seismic layer 3 which is distinguished from layer 2 by both a higher p wave velocity 6 69 0 26 km s l and a much smaller vertical velocity gradient 1 8 km continuous hole drilled into the oceanic crust11 13 and find that at this location the seismic layer 2 3 boundary lies within the sheeted dyke complex where it is associated with gradual downhole changes in crustal porosity and alteration not a litho logical transition from sheeted dykes to gabbro

meteorological conditions of the stratosphere for the crista 2 campaign august 1997 1 during the crista 2 campaign in august 1997 the antarctic polar vortex according to uk met office ukmo data was strong but temporarily disturbed by planetary waves this winter was in general warmer than the five previous winters nevertheless temperatures cold enough to allow the formation of nitric acid trihydrate nat and ice particles occurred throughout the period from mid june to the end of august the cold vortex core developed in the potential temperature range from 400 to 700 k at the beginning of the winter initially centered at 600 k but slowly progressing downward to 450 k in the subsequent months the vortex disturbances showed a clear zonal wave number 1 and 2 pattern this planetary wave activity caused a strong day to day fluctuation of the vortex shape during august when the second cryogenic infrared spectrometers and telescopes for the atmosphere crista 2 mission took place the vortex was elongated toward the southern atlantic ocean processing slowly eastward the perturbation of the vortex was associated with air masses transported from middle and low latitudes into the polar region in small scale streamers similarly tongues of vortex air peeled off the vortex edge and moved equatorward enhancing the exchange between polar regions and the tropics in addition the disturbance of the vortex led to the generation of filaments at the inner side of the vortex edge thus covering the vortex interior with small scale structures a comparison between crista observations and potential vorticity derived from ukmo data shows discrepancies concerning small scale structures high resolution potential vorticity fields calculated from reverse domain filling trajectory simulations on isentropic surfaces based on horizontal winds from ukmo analyses and vertical motion from diabatic calculations are much more consistent with the measurements both observation and reverse domain filling rdf simulations show the strong atmospheric variability of this period of unusual enhanced planetary wave activity in the southern hemisphere

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

tectonics of the pliocene loreto basin baja california sur mexico and evolution of the gulf of california the pliocene loreto basin is a transtensional basin exposed just north of loreto baja california sur that consists of nonmarine to marine sedimentary rocks and interbedded tuffs on the basis of stratigraphic study and 40ar 39ar dating of tuffs the southern loreto basin began to form at 3 4 ma as an alluvial basin with moderate rates of subsidence 0 4 mm yr between 2 46 and 2 36 ma extremely rapid subsidence 5 10 mm yr of the basin was accompanied by deposition of vertically stacked gilbert type fan deltas during this period the southernmost basin began to be cut by an array of dextral normal faults the basin returned to a moderate rate of subsidence from 2 36 to 2 0 ma when volcanism increased within the northern part of the basin the pacific north america boundary was first fully located within the gulf of california beginning at 3 5 ma at which time the zone of transform related deformation widened to initiate formation of the loreto basin the major change in the loreto basin at 2 46 ma may be coeval with the beginning of faulting in southern california on the elsinore and san jacinto faults these widespread events may indicate a minor change in the plate boundary at 2 5 ma

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

a reappraisal of seafloor spreading lineations in the gulf of california implications for the transfer of baja california to the pacific plate and estimates of pacific north america motion an analysis of seafloor spreading lineations in the southern gulf of california demonstrates that divergence of the baja peninsula from north america since 3 6 ma has been significantly slower than pacific north america pa na motion estimated solely from closures of global plate circuits this discrepancy remains unchanged or increases with modifications to the plate circuits and kinematic data used to predict pa na motion suggesting that it arises from the probably incorrect assumption that baja california has been rigidly coupled to the pacific plate since 3 6 ma a 15 increase in seafloor spreading rates in the southern gulf after 1 ma could have been caused by a transfer of motion along faults west of baja california to the modern day plate boundary or alternatively could indicate a recent speedup in pa na motion both interpretations imply that pa na motion is now 4 2 mm yr 1 2 faster than predicted by the 3 16 myr average nuvel 1a model

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

tectonic topography and late pleistocene activity of the west qinling fault northeastern tibetan plateau abstract a precise measurement of the slip rate of active faults within and around the tibetan plateau will provide definite and explicit knowledge of continental dynamics and present day active tectonic processes the wnw trending west qinling fault wqlf the target of this study is one of the major active strike slip faults developed in the northeastern tibetan plateau the interpretation of three dimension 3d perspective topographic maps field investigations and 14c dating results reveal that i the drainage systems mountain ridges alluvial fans and terrace risers have been systematically left lateral deflected or offset by 6 m up to hundreds of meters during the late pleistocene holocene along the wqlf in the northeastern marginal zone of the tibetan plateau ii the left lateral strike slip rate is estimated to be 2 5 2 9 mm yr with an average value of 2 7 mm yr since the late pleistocene iii compared to the well studied strike slip active faults developed in the northern and northwestern tibetan plateau such as the altyn tagh fault and the kunlun fault with high slip rate of 10 20 mm yr the wqlf displays a characteristically low strike slip rate of

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

a revised estimate of pacific north america motion and implications for western north america plate boundary zone tectonics for the past 20 years much effort has been directed to determining the present day relative motion of the pacific and north american plates using two independent approaches one uses geologic observations and geodetic measurements along the san andreas fault and other faults in the plate boundary zone the other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies transform azimuths and earthquake slip vectors geologic and geodetic studies find two principal elements of deformation slip along the san andreas of 34 mm yr directed n41 w and extension across the basin and range province of about 10 mm yr directed n56 w in contrast plate motion studies find 56 60 mm yr directed n35 w the discrepancy between these estimates a vector of about 15 mm yr oriented nearly due north is often attributed to a combination of slip along faults parallel to the san andreas and shortening normal to it here we revise the estimate of pacific north america motion by analyzing marine magnetic profiles from the gulf of california since 3 ma spreading has averaged 48 mm yr 10 mm yr slower than estimated before and consistent with the 49 mm yr spreading predicted by a new global plate motion model derived without any data along the pacific north america boundary the discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm yr parallel to the san andreas 60 less than estimated before and 7 mm yr of shortening across the san andreas similar to prior estimates these results suggest that strike slip motion on faults west of the san andreas is less than thought before a conclusion consistent with geodetic seismological and other geologic observations

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

patterns of continental extension along the conjugate margins of the central and north atlantic oceans and labrador sea the anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading we estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and north atlantic oceans and labrador sea from the total tectonic subsidence of the margins in the central atlantic basin the integrated continental extension varies from 225 km for the break between the southern appalachian segment of the north american margin and africa to over 600 km for the break between the blake plateau region of north america and the cape verde terrace region of africa the original width of the extended zone varies from 240 to 770 km in the north atlantic and labrador basins the integrated extension varies from 60 km for the break between northern labrador and greenland to over 600 km for the break between newfoundland and europe the original width of the extended zones in these basins varies from 100 km to over 1460 km in general continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain as a result seafloor spreading tends to begin first along segments which follow the structural grain extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain failure parallel to the structural grain exhibits greater variability from margin to margin than failure across the grain this phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain we conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain

the behavior of wave 2 in the southern hemisphere stratosphere during late winter and early spring abstract a detailed analysis is presented of the behavior of the zonal wavenumber 2 component of the flow wave 2 for july through october in the southern hemisphere stratosphere wave 2 in the stratosphere is characterized by a broad meridional structure peaking between 55 and 65 s and regular eastward propagation with periods ranging from 5 to 40 days maximum geopotential height amplitudes for a year range from approximately 600 to 1000 m examination of vertical structure suggests that during episodes of largest growth wave 2 propagates upward from the upper troposphere regular eastward propagation is however evident only within the stratosphere there are also episodes of wave 2 growth that do not appear connected with the troposphere in general the wave 2 amplitude is not as large in these cases there are several years when wave 1 and wave 2 amplitudes are strongly anticorrelated in time during september and october there are also years with strong positive correlation during august and

seismic structure of the southern gulf of california from los cabos block to the east pacific rise 1 multichannel reflection and coincident wide angle seismic data collected during the 2002 premier experiment sea of cortez addressing the development of oblique rifting pescador experiment provide the most detailed seismic structure to date of the southern gulf of california multichannel seismic mcs data were recorded with a 6 km long streamer 480 channel aboard the r v maurice ewing and wide angle data was recorded by 19 instruments spaced every 12 km along the transect the mcs and wide angle data reveal the seismic structure across the continent ocean transition of the rifted margin typical continental and oceanic crust are separated by a 75 km wide zone of extended continental crust dominated by block faulted basement little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust suggesting a constant rate of magmatic productivity since seafloor spreading began oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature 1300 c and passive mantle upwelling at the early stages of seafloor spreading the crustal thickness width of extended continental crust and predicted temperature conditions all indicate a narrow rift mode of extension on the basis of upper and lower crust stretching factors an excess of lower crust was found in the extended continental crust total extension along transect 5w is estimated to be 35 km following crustal extension new oceanic crust 6 4 km thick was formed at a rate of 48 mm a 1 to accommodate plate separation

computations of diabatic descent in the stratospheric polar vortex a radiation model together with national meteorological center temperature observations was used to compute daily net heating rates in the northern hemisphere nh for the arctic late fall and winter periods of both 1988 1989 and 1991 1992 and in the southern hemisphere sh for the antarctic fall and winters of 1987 and 1992 the heating rates were interpolated to potential temperature surfaces between 400 k and 2000 k and averaged within the polar vortex the boundary of which was determined by the maximum gradient in potential vorticity the averaged heating rates were used in a one dimensional vortex interior descent model to compute the change in potential temperature with time of air parcels initialized at various values as well as to compute the descent in log pressure coordinates in the nh vortex air parcels which were initialized at 18 km on november 1 descended about 6 km by march 21 while air initially at 25 km descended 9 km in the same time period this represents an average descent rate in the lower stratosphere of 1 3 to 2 km per month air initialized at 50 km descended 27 km between november 1 and march 21 in the sh vortex parcels initialized at 18 km on march 1 descended 3 km while air at 25 km descended 5 7 km by the end of october this is equivalent to an average descent in the lower stratosphere of 0 4 to 0 9 km per month during this 8 month period air initialized at 52 km descended 26 29 km between march 1 and october 31 in both the nh and the sh computed descent rates increased markedly with height the descent for the nh winter of 1992 1993 and the sh winter of 1992 computed with a three dimensional trajectory model using the same radiation code was within 1 to 2 km of that calculated by the one dimensional model thus validating the vortex averaging procedure the computed descent rates generally agree well with observations of long lived tracers thus validating the radiative transfer model