timing and tectonic setting of grenvillian metamorphism constraints from a transect along georgian bay ontario systematic mapping of a transect along the well exposed shores of georgian bay ontario combined with the preliminary results of structural analysis geochronology and metamorphic petrology places some constraints on the geological setting of high grade metamorphism in this part of the central gneiss belt correlations within and between map units gneiss associations have allowed us to recognize five tectonic units that differ in various aspects of their lithology metamorphic and plutonic history and structural style the lowest unit which forms the footwall to a regional decollement locally preserves relic pre grenvillian granulite facies assemblages reworked under amphibolite facies conditions during the grenvillian orogeny tectonic units above the decollement apparently lack the early granulite facies metamorphism out of sequence thrusting in the south produced a duplex like structure two distinct stages of grenvillian metamorphism are apparent the earlier stage c 1160 1120 ma produced granulite facies assemblages in the parry sound domain and upper amphibolite facies assemblages in the parry island thrust sheet the later stage c 1040 1020 ma involved widespread dominantly upper amphibolite facies metamorphism within and beneath the duplex deformation and metamorphism recently reported from south and east of the parry sound domain at c 1100 1040 ma have not yet been documented along the georgian bay transect the data suggest that early convergence was followed by a period of crustal thickening in the orogenic core south east of the transect area with further advance to the north west during and after the waning stages of this deformation

gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

dynamic recrystallization and associated exsolution in perthites evidence of deep crustal thrusting mylonitic gneisses from parry sound ontario are intensely deformed and occur within a major deep crustal shear zone developed during thrusting of granulite facies rocks 800 c 900 1100 mpa northward onto an amphibolite facies terrane we have examined by optical and electron microscopy microstructures in perthites from these mylonites that preserve the deformation recrystallization cycle which operated during high ductility deformation of the crystals the exsolution and recrystallization microstructures are intimately associated exsolution in the perthites occurred in two stages a calcic plagioclase oligoclase exsolved first and subsequently coarsened and deformed during the single crystal ductility phase of shear zone deformation a progressive increase in size and boundary incoherence of oligoclase domains during deformation led to recrystallization of the host perthite crystals as discrete orthoclase and oligoclase grains the principal mode of recrystallization was subgrain rotation with a component of grain growth that followed the formation of mobile high angle boundaries misorientation of subgrains to form grains at the recrystallization front is abrupt and occurred over one or two subgrain diameters the second exsolution stage postdated perthite deformation and saw formation of fine albite lamellae in both host and recrystallized orthoclase temperatures inferred from the exsolution textures for independently determined confining pressures indicate that this deformation occurred under at least granulite facies conditions the perthites thus record deformation from crustal depths of the order of 34 42 km continental megathrusts such as the parry sound shear zone provide mechanisms for both the generation of large strains observed in otherwise nonductile minerals and the preservation of ensuing microstructures

using ambient noise seismic interferometry and local and teleseismic earthquakes to determine crustal thickness and moho structure of the northwestern gulf of mexico margin

hyperextension serpentinization and weakening a new paradigm for rifted margin compressional deformation the early eocene magma rich northeast atlantic rifted margins contain a large number of pre breakup and post breakup compressional structures located in abandoned early cretaceous hyperextended basins with crustal stretching factors of 3 4 or more the deformation both predates and postdates the magma rich breakup the hyperextended basins are often underlain by high velocity lower crustal bodies which we argue represent partially serpentinized upper mantle long lived lithospheric weakening and proneness to deformation is proposed to relate to crustal hyperextension probably enhanced by mantle hydration

constraints on the tectonic development of the eastern gulf of mexico provided by magnetic anomaly data boundaries between oceanic and transitional crust in the eastern gulf of mexico have been delineated using detailed accurately navigated high quality aeromagnetic data these data indicate that considerably more oceanic crust may be present than previously suggested by other geophysical data especially seismic reflection data the n s extent of oceanic crust substantially increases westward from approximately 280 km near 87 w to roughly 390 km near 90 w indicating the proximity 10 of the pole of rotation variations in the amount of oceanic crust together with discontinuities in the magnetic anomaly patterns interpreted as the traces of fossil transform faults or fracture zones have been used to obtain a rough estimate of the location of the pole of rotation 24 n 81 5 w for the seafloor spreading phase of opening the magnetic data have been used to develop a model for the opening of the gulf and to identify some geometrical relationships between its conjugate continental margins based upon the estimated pole position the eastern gulf is interpreted to have opened in an approximately nne ssw direction with the counterclockwise rotation of yucatan away from north america the seafloor spreading phase of movement accounts for about 25 of the counterclockwise motion of yucatan rotating the yucatan block clockwise by this amount to restore it to its predrift position places the campeche bank adjacent to the mississippi trough louisiana shelf further clockwise rotation of yucatan by an additional 30 35 about this pole restores extended crust to its original thickness and is consistent with rotations of yucatan predicted from paleomagnetic data from chiapas

crustal structure beneath the high lava plains of eastern oregon and surrounding regions from receiver function analysis 1 we analyze teleseismic p to s receiver functions to image crustal structure beneath the high lava plains hlp of eastern oregon and surrounding regions coverage from 206 broadband seismic stations provides the first opportunity to resolve variations in crustal composition thickness and heterogeneity on scales of a few km in depth and tens of km laterally across the hlp region we utilize both h stacking and a new gaussian weighted common conversion point stacking technique we find crust that is 40 km thick beneath the cascades idaho batholith and owyhee plateau and thinner 31 km crust beneath the hlp and northern great basin low poisson s ratios of 0 240 characterize the granitic crust beneath the idaho batholith while the owyhee plateau exhibits values of 0 270 typical of average continental crust the owyhee plateau is a thick simple crustal block with distinct edges at depth the western hlp exhibits high average values of 0 304 typical for regions of widespread basaltic volcanism combined with other geological and geophysical observations the areas of abnormally high poisson s ratios 0 320 and low velocity zones in the crust beneath north central and southern oregon are consistent with the presence of partial melt on either side of the hlp trend suggesting a central zone where crustal melts have drained to the surface perhaps enabled by the brothers fault zone thicker crust and an anomalous n s band of low poisson s ratios 0 252 skirting the steens mountain escarpment is consistent with residuum from a midcrustal magma source of the massive flood basalts supporting the view of extensive mafic underplating and intraplating of the crust from cenozoic volcanism

lithospheric structure of the texas gulf of mexico passive margin from surface wave dispersion and migrated ps receiver functions the seismic velocity structure beneath texas gulf coastal plain gcp is imaged by migrating ps receiver functions with a seismic velocity model found by fitting surface wave dispersion we use seismic data from a linear array of 22 broadband stations spaced 16 20 km apart a common conversion point ccp stacking technique is applied to earthquake data to improve the s n ratios of receiver functions using an incorrect velocity model for depth migration of a stacked ccp image may produce an inaccurate image of the subsurface to find sufficiently accurate p and s velocity models we first apply a nonlinear modeling technique to fit rayleigh wave group velocity dispersion via very fast simulated annealing vs ranges from 1 5 km s in shallow layers of the gcp to 4 5 km s beneath the llano uplift and just outboard of the balcones fault zone bfz the bfz is characterized by slow velocities that persist to nearly 100 km depth in the stacked image the largest amplitude positive polarity event ranges from the surface at the llano uplift to a maximum depth of 16 km beneath matagorda island we attribute this event to the sediment basement contact which is expected to produce a large impedance contrast another large amplitude and positive polarity event at 35 km depth which likely marks the moho disappears outboard of the luling fault zone the disappearance of the moho beneath the gcp may be due to serpentinization of the upper mantle which would reduce the impedance contrast between the lower crust and upper mantle dramatically

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

potential field evidence for a volcanic rifted margin along the texas gulf coast potential fi eld data along the texas portion of the gulf of mexico indicate a large amplitude coast parallel magnetic maximum and a smaller bouguer gravity high models constrained by seismic refraction data indicate that these maxima manifest a deeply buried volcanic rifted passive margin or other magnetic high in the outer transitional crust buried 12 15 km the source is 220 km wide similar to the voring plateau in norway and the u s east coast this margin which formed during the opening of the gulf of mexico differs in origin from the transform boundary of the northeast mexico margin tehuantepec transform and we infer a jurassic triple junction related to the borderland rift system which is traceable as far as southeast california

seismic vp vs tomography of texas oklahoma with a focus on the gulf coast margin abstract the northwestern gulf of mexico passive margin contains an extensive record of continental collision and rifting as well as deformation associated with orogenic events and heavy sedimentation seismic traveltime tomography that incorporates new data from 328 broadband seismic stations deployed throughout the region reveals features that correlate well with expected mantle structures as well as features that have no obvious expression at the surface among the former are a large fast anomaly that corresponds to the southern extent of the laurentia craton and a large slow anomaly associated with the southern oklahoma aulacogen among the latter are a slow layer that we interpret to be a shear zone at the base of the cratonic and transitional continental lithosphere a zone that is bounded at its top and bottom by discontinuities and high levels of seismic anisotropy identified in companion receiver function and shear wave splitting studies respectively a high velocity body underlying the gulf coastal plain may mark delaminating lower crust if this is true it could provide indirect evidence for an elevated geotherm during the rifting process that created the gulf of mexico

crustal thickness and bulk poisson ratios in the dominican republic from receiver function analysis abstract the dominican republic dr comprises the eastern two thirds of the island of hispaniola which is located on the caribbean north american plate boundary previous studies have described the shallow structure and stratigraphy of the island but due to a dearth of broadband seismic data little is known about the lower crust and mantle since august 2014 thirteen broadband three component seismic stations have been deployed temporarily in the dr they are combined with four permanent broadband stations in the dr and three in neighboring haiti to enhance studies of hispaniola s lower crust and upper mantle we report results of a receiver function study using teleseismic data recorded by these stations specifically we compute receiver functions and determine crustal properties via h stacking we propose additional intracrustal and upper mantle layers based on identifications of prominent and coherent p to s converted phases observed in receiver function gathers our results show that crustal thickness varies beneath the island of hispaniola over the range 13 47 km with a median of 24 km and bulk vp vs ratios vary over 1 58 1 99 with median 1 79 receiver function modeling reveals intracrustal discontinuities at station paph port au prince haiti at a depth of 8 0 km station jidr jimani dr 8 8 km depth and sddr sabaneta dr 15 8 km depth additional discontinuities appear in the upper mantle beneath paph 60 1 km depth pcdr punta cana 60 km sdd santo domingo 70 km and sc01 santiago 81 km two north south cross sections show variations in bulk vp vs ratios that are consistent with expectations based upon surface geology but variations in crustal thickness are more scattered and do not conform to expectations from simple tectonic models of hispaniola s formation our stacking results reveal consistency between vp vs ratios in four distinct regions which are consistent with the compositional domains identified by previous studies

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

shear wave velocity structure in the northern basin and range province from the combined analysis of receiver functions and surface waves a new method based on the joint inversion of receiver functions and surface wave phase velocities results in well determined shear velocity structures that are consistent with the compressional wave structure gravity heat flow and elevation data in the northern basin and range this new inversion method takes advantage of average velocity information present in the surface wave method and differential velocity information contained in the receiver function method thus min imizing the nonuniqueness problem that results from the velocity depth trade off an unusually thick 38 kin and relatively faster crust and upper mantle are found in central and eastern nevada compared to the thin 28 to 34 km and slower crust and upper mantle of the western basin and range we interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less cenozoic ex tension relative to the surrounding regions to the west and north the thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern nevada simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface indicating a near classical airy type of compensation in the province we analyze in detail the shear wave s wave velocity model derived from the receiver functions at station bmn and compressional wave p wave velocity models derived from the 1986 passcal experiment in northwestern nevada the most interesting feature of these models is the presence of negative velocity gradients in the s wave model with no corresponding velocity decrease in the p wave models between depths of 10 and 24 km this combined velocity model may be explained by high pore fluid pressures at these depths this model favors a layered fluid porosity model proposed in the literature to explain extensive middle to lower crust conti nental seismic reflections and high electrical conductivity an upper mantle grada tional low velocity zone is present between 32 and 38 km in the s wave model this upper mantle shear wave low velocity zone is consistent with partial melt which may be the source material for magmatic underplating in this region

green s functions source signatures and the normalization of teleseismic wave fields 1 we examine the canonical source green s function separation problem in the context of teleseismic p wave scattering from receiver side crust and upper mantle structure conventional receiver function analysis affords a leading order approximation to the s component of the green s function but provides no information on p to p scattering we demonstrate that an improved estimate of the three dimensional earth s green s function including scattered p contributions can be achieved through consideration of its theoretical spectral properties under conditions typical of the real earth the p component of the green s function is shown to be minimum phase this behavior is responsible for the success of receiver functions in mantle studies the minimum phase property is used here to normalize the source signature on p wave seismograms thereby facilitating implementation of multichannel multicomponent deconvolution of both green s function and source signature within the log spectral domain examples using both synthetic simulations and seismograms recorded on the canadian national seismograph network illustrate the recovery of accurate and reproducible estimates of the p wave green s function our approach can be adapted to a range of source receiver configurations in particular it may prove useful in the recovery of compressional properties beneath portable field arrays where calibration is provided by nearby permanent installations

lithospheric structure of the arabian shield from the joint inversion of receiver functions and surface wave group velocities abstract we estimate lithospheric velocity structure for the arabian shield by jointly modeling receiver functions and fundamental mode group velocities from events recorded by the 1995 1997 saudi arabian portable broadband deployment receiver functions are primarily sensitive to shear wave velocity contrasts and vertical travel times and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages so that their combination bridge resolution gaps associated with each individual data set our resulting models correlate well with the observed surface geology the asir terrane to the west consists of a 10 km thick upper crust of 3 3 km s overlying a lower crust of 3 7 3 8 km s in the afif terrane to the east the upper crust is 20 km thick and has an average velocity of 3 6 km s and the lower crust is about 3 8 km s separating the terranes the nabitah mobile belt is made of a gradational upper crust up to 3 6 km s at 15 km overlying an also gradational lower crust up to 4 0 km s the crust mantle transition is found to be sharp in terranes of continental affinity east and gradual in terranes of oceanic affinity west the upper mantle shear velocities range from 4 3 to 4 6 km s temperatures around 1000 c are obtained from our velocity models for a thin upper mantle lid observed beneath station taif and suggest that the lithosphere could be as thin as 50 60 km under this station

a detailed receiver function image of the upper mantle discontinuities in the japan subduction zone abstract we have imaged the upper mantle discontinuities in a 30 20 large region at the active continental margin of the japan subduction zone and neighboring areas using p to s converted phases from teleseismic records of permanent broadband stations the 410 km discontinuity is detected within 10 km of its global average position an interesting exception in its observation is a gap near 135 e very close to the slab penetration of the 410 km discontinuity and in an area where we have rather high data density the 660 km discontinuity reaches 700 km depth at two places where it is hit directly by the slab these data generally show good agreement with tomographic results both the 660 km discontinuity depression and the p velocity anomalies suggest about 400 500 k temperature deficit within the slab however no downward bending of the 660 km discontinuity is observed in eastern china where the flat lying slab is imaged by tomography this suggests that the slab does not cool the 660 km discontinuity in this region therefore the positive buoyancy required to keep the slab lying flat cannot have been provided by the negative clapeyron slope of the spinel perovskite phase transition another mechanism is needed which could possibly be metastable olivine in the cold core of the slab we have also imaged the shallower portion of the slab down to about 150 km underneath some seismic stations likely because metastable gabbro is still existing to this depth providing a sufficient velocity contrast a strong negative converted phase is observed at 150 200 km depth underneath a volcanic region in japan if real observed at one station only the zone which produces this conversion and which may begin exactly above the slab would require fluid fed melting

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

evolution of nonvolcanic rifted margins new results from the conjugate margins of the labrador sea the crust across each margin of the labrador sea a conjugate rift margin pair consists of three well defined zones thinned continental crust 70 80 km wide transitional crust and oceanic crust the transition zone is characterized by a low velocity upper crust 4 5 km s underlain by a 6 4 7 7 km s layer we propose that the lower layer is serpentinized upper mantle it is less likely to be gabbroic igneous lower crust the low velocity upper crust may be either an oceanic basaltic layer or very thin 2 km continental crust results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting 1 mm yr on average resulting in an asymmetric crustal thinning profile with breakup occurring against the west greenland shelf this asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small scale convection in the lower lithosphere and asthenosphere at the sides of the rift the thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible consistent with slow spreading or rifting the transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth signaling an increase to normal oceanic crustal thickness

late cretaceous volcanism in south and central texas stratigraphic structural and seismic models abstract abstract since their discovery in 1915 hydrocarbon traps in and around serpentine plugs have produced about 47 million barrels of oil and have significant potential for additional small discoveries production is from isolated reservoirs within mounds of altered volcanic tuff and associated shoal water carbonates a review of the more than 200 volcanic centers and intrusive bodies of south and central texas has led to development of stratigraphic and seismic models useful in exploration and production the so called serpentine plugs are largely tuff mounds formed by accumulation of volcanic ash altered to palagonite on the seafloor around a submarine volcanic vent volcanic activity peaked during deposition of the chalk and marl of the upper austin and lower taylor groups about 80 million years ago after their eruption the tuff mounds localized the deposition of shoal water carbonates with good porosity and permeability low permeability organic rich marine shale and marl of the taylor group capped the carbonates serving as both a hydrocarbon source and a stratigraphic seal compactional draping of overlying san miguel and olmos sands with minor offset faulting created important additional traps in south texas central texas volcanic centers are highly aligned along strike oriented regional faults and fractures of the balcones and luling systems the magmas in both central and south texas were ultramafic and alkaline suggesting that partial melting occurred at depths of about 40 miles 60 kilometers the magma rose rapidly to the surface probably in an extensional stress regime controlled by pre tertiary balcones luling faults the palagonite tuff of a typical productive volcanic center has low seismic velocity and is encased in high velocity carbonates the strong velocity contrast coupled with the distinctive shape of the tuff mass yields a characteristic seismic pattern modern seismic techniques together with stratigraphic data allow accurate delineation of buried tuff mounds and prediction of productive carbonate facies

some comparisons of the structure and evolution of the southern appalachian ouachita orogen and portions of the trans european suture zone region abstract many aspects of the tectonic history of the appalachian ouachita orogen and the trans european suture region are similar in a tectonic sense they are geographically linked because at the end of the paleozoic one could travel from texas to poland following a continuous orogenic belt much of each of these orogenic belts is however buried by younger strata and thus geophysical studies are a key element of efforts to better understand them many more geophysical data are needed to provide a comprehensive picture of the deep structure but in several areas new models of the lithospheric structure are emerging in this paper we review some of these models and introduce two new ones for the southern appalachians in addition we explore the implications of these models for the evolution of these orogenic belts drawing attention to similarities and differences in their structure and evolution the appalachian and caledonian orogens represent temporally parallel development along irregular late proterozoic continental margins but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures for example in the appalachian ouachita orogen the thrust related transport distance for major features such as the blue ridge piedmont sheet and the benton uplift is at least 200 km on a crustal scale deformation in the appalachian ouachita orogen varies greatly in style and intensity in the ouachita orogen the late proterozoic continental margin seems to be preserved while in the northern appalachians crustal scale deformation is intense along the us appalachians the major crustal structure variation is the thin dense crust of avalonia abutting the relatively thick crust of laurentia along the ouachita orogen several large crustal blocks can be identified that are unlike anything seen along the appalachians

improved green s functions for passive source structural studies over the past two decades teleseismic receiver functions have proved to be a useful tool to investigate crustal structure because they represent a first order approximation to the s wave component of the teleseismic p green s function receiver functions provide valuable information on physical properties related to shear modulus however the implicit use of the p component seismogram as a proxy for the source precludes the recovery of information on discontinuous structure involving contrasts in compressional modulus by deconvolving improved estimates of complex source time functions generated by earthquakes one may move beyond the conventional receiver function paradigm to a more accurate approximation of the earth s green s function using a new deconvolution method we present estimates of the p component of the teleseismic p green s functions at several stations of the canadian national seismic network cnsn that clearly show the receiver side pure p wave crustal multiple the identificat

sp receiver function imaging of a passive margin transect across texas s gulf coastal plain abstract the gulf coast of texas has been the subject of intensive geological and geophysical investigation in pursuit of hydrocarbons but studies that penetrate beyond the upper crust are limited to a few refraction profiles and regional surface wave investigations the passing of earthscope s transportable array has facilitated regional investigations of the lithosphere but its 70 km station spacing does not allow many important tectonic features to be imaged a broadband seismic transect across the texas gulf coastal plain was therefore performed in order to image deep structure beneath this passive margin and the transition to the neighboring craton a 2d sp receiver function common conversion point ccp stacked image produced for this transect reveals several discontinuities in the sub crustal lithosphere the region nearest the shoreline is underlain by an anomalous 18 km thick low velocity layer that produces a strong negative pulse in the sp receiver functions the drop in velocity is too large to be due to any reasonable change in fe or mg content but could be produced by partial melt or mantle hydration it is unlikely that partial melt would still be found in a 160 180 year old passive margin such as the gulf coast but hydration possibly introduced by a through going balcones fault system and resulting serpentinization could produce the observed anomaly an event with negative polarity appears at a depth of 110 km which we interpret to be the lithosphere asthenosphere boundary lab thermal variations alone would not produce a sufficiently sharp discontinuity to be imaged by sp converted phases recent shear wave splitting studies revealed unusually large delay times in this region along with fast polarization directions that differ from measurements on the laurentian craton large delay times may imply significant flow which could also produce frictional heating due to shearing and partial melt which would steepen the velocity gradients an additional layer is bounded by discontinuities at 145 165 km depth with negative polarity and 185 205 km depth with positive polarity the appearance of these discontinuities suggests that the mantle beneath the lab is more complicated here than elsewhere and that the transition from lithosphere to asthenosphere occurs over a broad 85 100 km thick depth interval

microstructure c axis pattern microstrain and kinematics of some s c mylonites in grenville gneiss abstract in a major tectonic zone late extension related s c mylonites locally overprint the predominant coarser quartz microstructures which are related to earlier thrusting some of the s c mylonites display a microstructural evolution which began with the formation of deformation bands in the coarser pre existing microstructure and continued with the formation of asymmetric quartz microfoliations either by continued formation of deformation bands or fine new grains oblique to the deformation band boundaries the orientation of boudinaged and passively reoriented rutile needles show that i the formation of deformation bands was preceded and accompanied by the accumulation of strain ii that the deformation bands and oblique microfoliation which formed directly from them lie close to the finite stretching direction whereas iii other microfoliations form oblique to deformation bands and extended rutile needles near the probable instantaneous stretching direction the latter are therefore interpreted to be strain insensitive steady state foliations the crystallographic preferred orientation of the original deformation bands appears to determine that of the microfoliations the two types of microfoliations showing distinct but related patterns the element common to both types is the presence of two maxima near y in a yz girdle a feature inherited from the deformation bands which were formed in the initial stages of shortening of the aggregate favourably disposing it for rhomb slip and providing nucleation sites for subsequent recrystallization the data confirm that despite the fact that a variety of microstructures and crystallographic microfabrics result from recrystallization processes kinematic information is usually recoverable from the crystallographic microfabrics owing to the primacy of intracrystalline slip processes

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

lithospheric structure of the south central united states recent seismic data in the ouachita mountains area and the gulf of mexico make it possible to construct a lithospheric scale cross section transect from the midcontinent region to the gulf the authors constructed a transect in the form of a gravity model but it incorporates all available seismic drill hole and geologic data as constraints the thrust sheets of the ouachita orogenic belt appear as a thin veneer covering the southern part of the arkoma basin and the preserved paleozoic continental margin mesozoic rifting is evident in three areas 1 southern arkansas and northern louisiana where extension was minor 2 the vicinity of the texas louisiana coastline where modification of the lithosphere and subsidence were considerable and 3 the deep gulf of mexico where rifting was successful a significant variation in the average density of the mantle which could delineate the north american craton as a lithospheric feature was detected near the paleozoic continental margin

crustal and uppermost mantle structure in the middle east assessing constraints provided by jointly modelling ps and sp receiver functions and rayleigh wave group velocity dispersion curves

constraining crustal and uppermost mantle structure beneath station tbz trabzon turkey by receiver function and dispersion analyses summary we jointly invert teleseismic radial component receiver functions and regional rayleigh and love surface wave group velocities for 1 d shear wave velocity structure beneath station tbz located on the northern side of the eastern pontides an influence factor is employed to control the relative influence of receiver function and surface wave dispersion on the resultant velocity depth profile radial and transverse component receiver functions at station tbz exhibit an azimuthal amplitude and polarity pattern consistent with 2 d receiver structure that has a general dip direction towards approximately south the radial component receiver functions are least affected by the dipping structures along the strike direction and thereby we prefer teleseismic events sampling along strike structures to alleviate the deflecting effect of dipping interfaces on the 1 d solution the 1 d inversion effectively reveals the two layer nature of the crust which is perturbed by high and low velocity layers and serves as a provisional model for the 2 d forward modelling minor to moderate changes to the 1 d model such as changing depth to and velocity contrast across an interface are needed to achieve the results with the 2 d modelling dipping interfaces and seismic anisotropy are included in the 2 d modelling to fit both radial and transverse component receiver functions the upper crust is characterized by a shear velocity of 3 5 km s 1 and cut through by a 4 km thick high velocity i e 3 8 km s 1 layer overlying the upper crust the sedimentary cover i e the top 5 km has velocities within the range 2 0 3 5 km s 1 a mid crustal velocity discontinuity between the upper granitic crust and the lower basaltic crust is identified at 16 km depth this boundary is analogous to the mid crustal discontinuity found under the black sea basin across which the shear velocity jumps from 3 5 to 4 1 km s 1 a relatively thick i e 12 km low velocity layer in the lower crust with a velocity reversal from 4 1 to 3 7 km s 1 is needed to better explain reverberations off this depth range we infer a 2 d moho discontinuity placed at 35 km depth beneath the station the proposed dip angle for the moho is rather steep i e 25 although coincident with regional gravity studies the associated sn velocity i e 4 4 km s 1 is rather low indicating disturbed upper mantle structure beneath the region initial amplitudes of transverse component receiver functions are rather energetic for which we propose substantial p and s velocity anisotropy 12 per cent for the topmost depths 5 km

lithospheric velocity model of texas and implications for the ouachita orogeny and the opening of the gulf of mexico a 3 d shear wave velocity model of texas has been developed from rayleigh wave phase velocities by using ambient noise data recorded at the usarray stations in the upper crust the ouachita front separates high velocity in the laurentia to its west from low velocity in the east and south texas basins the ouachita belt is characterized as a high velocity zone with local maximums coinciding with known uplifts which we interpret as accreted island arcs during the ouachita orogeny our model evidences a strong ouachita lithosphere that helped to buffer crust thinning from the mesozoic rifting a significantly low velocity anomaly is present in southeast texas in the lower crust and upper mantle we associate this anomaly with a past asthenosphere upwelling that likely originated from the edge of the subducted slab during the ouachita collision and was potentially responsible for the opening of the gulf of mexico

combined receiver function and surface wave phase velocity inversion using a niching genetic algorithm application to patagonia a combined inversion of body wave receiver functions and rayleigh wave phase velocities using a niching genetic algorithm nga increases the unique ness of the solution over separate inversions and also facilitates explicit parameteri zation of layer thickness in the model space this parameterization requires fewer layers and a priori constraints for modeling more complex structures than traditional linearized inversion ngas solve for a suite of locally optimal solutions in addition to isolating the globally optimal solution by using an evolutionary paradigm this nonlinear examination of the error space provides an opportunity to examine trade off within the model space the method when applied to synthetic data locates interfaces within 2 km of the test structure and identifies appropriate structure and velocity applying this method to data from a deployment of five broadband stations in chilean patagonia yields a regional model of crustal structure that is consistent with the geological history of the region sediment thickness is inversely proportional to crustal thickness with sediment thicknesses reaching more than 4 km where the crustal thickness thins to 28 km this is consistent with previous geological studies which suggests that the rocas verdes basin in western patagonia formed by crustal thinning isostatic compensation and subsequent sedimentation

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

implications of continental crust extension for plate reconstruction an example from the gulf of mexico the prerift reconstruction of continental plates bounded by rifted margins requires the closure of relative motion accomplished both by seafloor spreading and by the extension of continental crust during the rift phase of continental breakup continental extension is not expected to be pervasive throughout the plate but to be confined to a zone of up to several hundred kilometers in width the direction of particle motion in this zone is expected to be parallel to flow lines followed by the rigid portions of the plates the best fit between rifted continental margins is then described by the rotation angle about an euler pole which best closes both the oceanic crust and the extension within the continental crust along small circles about the euler pole as an example application of these concepts the pattern of late mesozoic crust extension within the gulf of mexico basin is used to constrain the location of the euler pole and angle of rotation of the yucatan block with respect to north america both crust type and the degree of extension within the transitional crust surrounding the basin are estimated on a point by point basis from bathymetry and basement depth the root mean square rms misfit in the apparent total closure of both oceanic crust and extension within the continental crust is computed for a range of possible poles of opening a contour map of misfit versus pole location reveals a global minimum rms misfit of 47 km for a 45 counterclockwise opening of the gulf of mexico basin about an euler pole at 25 n 79 w an elongate trend of poles for which the misfit is less than 100 km extends from the bahama islands eastward across the atlantic and through north africa the reconstruction of the gulf of mexico for the best fit pole places the palinspastically restored yucatan block in contact with north america along the texas and louisiana shelf the resulting restored southern margin of north america fits against the the northern margin of south america in its prerift position to within the uncertainty in the north america africa south america plate circuit we conclude that the prerift placement of the yucatan block between north and south america along the texas louisiana shelf is consistent with both the pattern of extension within the gulf of mexico basin and seafloor spreading history of the central atlantic basin

the lithosphere below the rockall trough wide angle seismic evidence for extensive serpentinisation abstract the longer range data from wide angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the rockall trough in the north atlantic two subcrustal p and s wave reflections are observed the first defines a layer 3 10 km thick below the moho where p wave velocities vary from 7 5 to 7 8 km s v p v s ratios increase within this layer from 1 80 in the north to 1 83 in the south of the basin along the axial profile the second reflection from approximately 34 km depth identifies a layer 15 20 km thick with a vp velocity of approximately 8 1 km s and a v p v s ratio of 1 73 these values are typical for normal mantle peridotites both p and s wave velocities and v p v s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the moho about 15 volume alteration of the parent mantle peridotite is required to produce the observed seismic properties this degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin syntectonic cooling occurred during differential lithospheric stretching as the upper to mid crust became more extended over a narrower region than the mantle lithosphere this served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle ductile transition zone in the crust migrated downwards into the mantle lithosphere the resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle

waveform modelling of teleseismic s sp sspmp and shear coupled pl waves for crust and upper mantle velocity structure beneath africa summary we describe a waveform modelling technique and demonstrate its application to determine the crust and upper mantle velocity structure beneath africa our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a very fast simulated annealing vfsa we match the s sp sspmp and shear coupled pl phases in seismograms of deep 200 800 km moderate to large magnitude 5 5 7 0 earthquakes recorded teleseismically at permanent broad band seismic stations in africa using our technique we produce p and s wave velocity models of crust and upper mantle beneath africa additionally our use of the shear coupled pl phase wherever observed improves the constraints for lower crust and upper mantle velocity structure beneath the corresponding seismic stations our technique retains the advantages of receiver function methods uses a different part of the seismogram is sensitive to both p and s wave velocities directly and obtains helpful constraints in model parameters in the vicinity of the moho the resulting range of crustal thicknesses beneath africa 21 46 km indicates that the crust is thicker in south africa thinner in east africa and intermediate in north and west africa crustal p 4 7 8 km s 1 and s wave velocities 2 5 4 7 km s 1 obtained in this study show that in some parts of the models these are slower in east africa and faster in north west and south africa anomalous crustal low velocity zones are also observed in the models for seismic stations in the cratonic regions of north west and south africa overall the results of our study are consistent with earlier models and regional tectonics of africa

growth and retrograde zoning in garnets from high grade metapelites implications for pressure temperature paths a previously unrecognized pattern of garnet zoning has been preserved in garnets from the upper amphibolite facies britt domain ontario grenville province some large garnets contain discrete regions of high grossular content as well as variations in the mg mg fe and mn nearly parallel to grain boundaries the latter are superimposed on and cut across regions of high grossular content such crosscutting zoning patterns have not been previously recognized in garnets the ca zoning involves changes of 4 to 10 mol grossular and is interpreted to represent growth zoning whereas the decreasing mg mg fe and increasing mn from core to rim are believed to be a diffusional retrograde effect the ca zoning indicates a pressure drop of ca 5 kbar at 700 50 c

deep structure of the texas gulf passive margin and its ouachita precambrian basement results of the cocorp san marcos arch survey this cocorp deep seismic survey provides a comprehensive image of the southeast texas part of the gulf passive margin and its accreted ouachita arc foundation beneath the updip limit of the cenozoic sediment wedge a prominent antiformal structure is imaged within the interior zone of the buried late paleozoic ouachita orogen the structure appears to involve precambrian grenville basement the crest of the antiform is coincident with the cretaceous tertiary luling mexia talco fault zone some of these faults dip to the northwest counter to the general regional pattern of down to the basin faulting and appear to sole into the top of the antiform suggesting that the ouachita structure has been reactivated as a hingeline to the subsiding passive margin the antiform may be tied via this fault system and the ouachita gravity gradient to the similar devils river waco and benton uplifts interpreted as precambrian basement cored massifs above the paleozoic sequence a possible rift related graben is imaged near the updip limit of jurassic salt paleoshelf edges of the major tertiary depositional sequences are marked by expanded sections disrupted by growth faults and shale diapirs within the wilcox formation the transect crosses the mouth of the 900 m deep yoakum canyon a principal pathway of sediment delivery from the laramide belt to the gulf beneath the wilcox the comanchean lower cretaceous shelf edge capped by the stuart city reef is imaged as a pronounced topographic break onlapped by several moundy sediment packages because this segment of the line parallels strike the topographic break may be interpreted as a 2000 m deep embayment in the cretaceous shelf edge and possibly a major submarine canyon older and deeper than the yoakum canyon

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

joint analysis of teleseismic receiver functions and surface wave dispersion using the genetic algorithm teleseismic p wave receiver function data and rayleigh wave phase velocity measurements are combined using the genetic algorithm a global optimi zation technique to model crustal structure in southern korea the two datasets complement each other because receiver functions are sensitive to shear wave ve locity contrasts in layered structures while surface wave dispersion is sensitive to averages of shear wave velocities the genetic algorithm is more useful than line arized inversion in regions where there is little a priori information about local velocity structure because it is not sensitive to the initial model the stability and variability of resulting crustal model parameters are quantified by using a monte carlo technique in specifying a suite of initial models depths to the moho discon tinuity in southern korea were estimated to be 29 30 km for stations near the western coast and 33 36 km for inland stations a well resolved crustal low velocity zone was inferred for some stations

structure and evolution of the central gulf of mexico continental margin and coastal plain southeast united states as in many continental margins mesozoic rifting in the gulf of mexico trended subparallel to older tectonic structures created by opening and closure of a prior ocean basin unlike the situation in most other margins however extension in the gulf of mexico did not involve widespread reactivation of these older tectonic features this difference may be related to the unusual facts that 1 the antecedent passive continental margin was a transform rather than a rift margin and 2 the prior orogeny did not substantially thicken the crust in order to characterize the interaction between mesozoic rifting and prior tectonic events two geophysical transects were constructed across the central gulf of mexico coastal plain and continental shelf the transects show that the early paleozoic passive continental margin buried beneath the late paleozoic ouachita orogen was little affected by compressional tectonism or mesozoic extension during opening of the gulf of mexico the thickness of the crust on this ancient margin decreases from 35 km to 10 km over a distance of only 50 km consistent with previous interpretations that this was a transform margin the ouachita fold and thrust belt is shown to be a thin skinned doubly vergent orogen that formed above a well preserved southward dipping subduction system relict cambrian oceanic crust may exist beneath the southern part of the orogen but its presence is not required by the data the thin crust and shallow mantle in the ouachita suture created a zone of relatively high strength that acted as a barrier to mesozoic extension which is restricted to regions south of the orogen extension was initially distributed over a broad area that included the mississippi interior salt basin and regions south of the wiggins arch extension became progressively more focused south of the wiggins arch with time culminating in the onset of seafloor spreading beneath the modern continental rise mesozoic extension factors are 1 4 1 9 beneath the mississippi interior salt basin 2 9 immediately south of the wiggins arch and 4 adjacent to the oldest oceanic crust in the central gulf of mexico

geologic implications of seawater circulation through peridotite exposed at slow spreading mid ocean ridges peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow spreading centers hosts hydrothermal circulation of seawater the reaction of seawater with peridotite causes serpentinization which generates a high ph strongly reducing fluid rich in methane and hydrogen and is accompanied by as much as 40 volume expansion complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk like limestone this may be the main mechanism of deposition of ophicarbonates common in ophiolites the degree of induration is a function of the fluid flux through the sediment column calcareous ooze infiltrates faults and fractures and can be deformed following lithification focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite aragonite and brucite such as those in the recently documented lost city vent field 30 n mid atlantic ridge geophysical implications of serpentinization include 1 creation of magnetic anomalies due to growth of magnetite in serpentinite and 2 lowered seismic velocity integrated studies of geologic and geophysical effects of serpentinization may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor

lunar subsurface investigated from correlation of seismic noise by correlating seismic noise recorded by four sensors placed on the moon during the apollo 17 mission we have retrieved a well defined dispersed rayleigh wave pulse inversion of its group velocity provides new constraints on the lunar subsurface structure the estimated signal to noise ratio snr of the retrieved rayleigh wavetrain is strongly dependent on solar illumination effectively making solar heating a source of seismic noise on the moon this result suggests that in future planetary missions it is feasible to extract information on the internal structure of extraterrestrial objects by correlating seismic noise even when natural quakes are absent

paleomagnetism of the chiapas massif southern mexico evidence for rotation of the maya block and implications for the opening of the gulf of mexico we have analyzed 180 oriented samples of the batholithic complex and overlying red beds of the chiapas massif in southern mexico for a paleomagnetic study both stepwise alternating field and thermal demagnetization procedures revealed dual polarity characteristic magnetizations above 10 30 mt or 200 400 c in about 60 of the samples demagnetized three distinct and ancient paleomagnetic directions have been identified late permian intrusions 256 10 ma yield nearly equatorial southern hemisphere paleolatitudes and a paleopole 13 4 s 176 1 e 12 sites k 15 4 a95 11 4 rotated 75 counterclockwise with respect to the north american reference pole a selected set of late triassic jurassic red beds of the todos santos formation yields a paleopole at 22 6 n 170 6 e 4 sites k 56 8 a95 12 3 which implies 60 counterclockwise rotation and a near equatorial northern hemisphere paleolatitude middle late jurassic dikes and granites yield a paleopole at 77 4 n 97 4 e 6 sites k 269 2 a95 4 1 near the mid jurassic segment of the north american apparent polar wander path these data are consistent with a pre rift configuration in which the maya block is rotated to a position off the coast of texas and louisiana in the present day area of the gulf of mexico a further implication is that by oxfordian times chiapas and by inference the maya block was essentially in its current position with respect to cratonic north america

petrology of types 4 6 carbonaceous chondrites we have studied the petrology of three type 4 carbonaceous chondrites coolidge karoonda and pecora escarpment 82500 which was previously classified as an ll6 chondrite and compared them to published descriptions of the other known metamorphosed c chondrites yamato 6903 and mulga west all except coolidge have abundant matrix typically 80 vol composed largely of homogeneous olivine fa 29 33 and surprisingly heterogeneous plagioclase mostly an 20 75 their opaque mineralogy indicates high oxygen fugacities during metamorphism metallic fe ni and troilite are rare and magnetite and pentlandite are abundant with accessory pyrrhotite chalcopyrite and pyrite possibly because of a lower matrix abundance coolidge has more reduced minerals metallic fe ni and olivine with fa 14 the properties of all c4 6 chondrites appear to be consistent with an origin by metamorphism of c3 like precursors by processes analogous to those operating in ordinary chondrite parent bodies or planetesimals however type 4 6 material was not well mixed with type 3 material in the cv3 parent body as it was in the ordinary chondrite parent bodies conceivably c4 6 chondrites come from one or more separate parent bodies

application of very fast simulated annealing to the determination of the crustal structure beneath tibet summary the simulated annealing method has recently been applied to several multiparameter optimization problems including those of geophysical inversion a new variant of simulated annealing called very fast simulated annealing vfsa overcomes some of the drawbacks of a conventional simulated annealing it has been found to be a practical tool for geophysical inversion sen stoffa 1995 the method is particularly useful for non linear problems with multiple parameters where a grid search method is prohibitively expensive here we have applied vfsa to retrieve the crustal structure beneath seismic stations in tibet using teleseismic body waveform data our approach is to compare the radial component records with generalized ray synthetics directly in the time domain for any given crustal structure we have formulated a direct relationship between the radial and the vertical component signal so that we can generate synthetic radial component data from recorded vertical component seismograms zhao frohlich 1996 we have tested the vfsa inversion algorithm using synthetics and confirmed that it works very well i e it finds solutions very close to the true solution using the algorithm we have determined the crustal structures beneath 11 stations in tibet from south to north the total crustal thickness is quite uniform our tectonic interpretation of these crustal models is that they may represent crust from the eurasian plate injected beneath the crust of the converging indian plate certain features of the models are consistent with the presence of a small convection cell or plume beneath north central tibet as suggested by molnar 1990

nacr14 a 3d model for the crustal structure of the north american continent abstract based on the large number of crustal seismic experiments carried out in the last decades we create nacr14 a 3d crustal model of the north american continent at a resolution of 1 1 we present maps of thickness and average velocities of the main layers that comprise the north american crystalline crust obtained from the most recent seismic crustal models within the usgs crustal structure database however the crustal data are unevenly distributed and in some cases discrepancies exist between published models in order to construct a consistent 3d crustal model with three layers in the crystalline crust we refrained from a direct interpolation of the crustal seismic parameters in the database instead we implemented the following sequence of steps 1 definition of the geometry of the main tectonic provinces of north america 2 selection and evaluation of the reliability of seismic crustal models in the database 3 estimation of the p wave seismic velocity and thickness of the upper middle and lower crust for each tectonic province 4 estimation of the interpolated pn velocity distribution the resulting average velocity of the crystalline crust is mostly consistent with that of the seismic points the main variations of the structure of the crystalline crust of north america displayed in the model can be related to its tectonic evolution the model available in a digital form can be used in various geophysical applications such as the correction for the crustal effects in gravity and seismic tomography and models of dynamic topography in order to detect heterogeneities characterizing the underlying upper mantle

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

lithospheric seismic structure beneath two broadband station sites of the eastern part of chhotanagpur plateau new constraints from receiver functions and dispersion curves abstract the chhotanagpur plateau in eastern india was formed due to the severe upheaval of tectonic forces deep inside the earth the deep geological processes associated with its subsurface along with geodynamical rationales are never understood properly due to the dearth of good quality geophysical data we use high quality teleseismic earthquakes waveforms 5 5 m 8 0 recorded at two seismic stations ism and bokr located in dhanbad and bokaro respectively these data are used to jointly model discrete but complementary datasets such as ps and sp receiver functions and rayleigh wave group velocity dispersion curves via very fast simulated annealing vfsa a two step optimization procedure is carried out to find the best fitting models and associated uncertainties in terms of posterior probability density ppd functions our results reveal that on an average the eastern part of chhotanagpur plateau which is mostly composed of granite gneiss complex with high heat flow is characterized by the thick crust for ism station up to 150 km depth vp and vs vary between 6 1 km s 8 4 km s and 3 3 km s 4 8 km s respectively we observe that vp and vs ranges between 6 2 km s to 8 3 km s and 3 2 km s to 4 6 km s respectively for seismic station bokr we find that the crustal thickness beneath seismic stations ism and bokr is 43 and 44 km respectively the higher values of poisson s ratio 0 25 for station ism and bokr are 0 30 and 0 29 respectively which may indicate the presence of partial melt present in the lower crust

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

southern louisiana salt dome xenoliths first glimpse of jurassic ca 160 ma gulf of mexico crust no direct information about the age and composition of rift related igneous activity associated with the late jurassic opening of the gulf of mexico exists because the igneous rocks are deeply buried beneath sediments three salt diapirs from southern louisiana exhume samples of alkalic igneous rocks these salt domes rise from the base of the sedimentary pile and overlie an isolated magnetic high which may mark the position of an ancient volcano three samples from two domes were studied they are altered but preserve relict igneous minerals including strongly zoned clinopyroxene diopside to ti augite and cr rich spinel rimmed with titanite 40 ar 39 ar ages of 158 6 0 2 ma and 160 1 0 7 ma for ti rich biotite and kaersutite from two different salt domes are interpreted to represent the time the igneous rock solidifi ed trace element compositions are strongly enriched in incompatible trace elements indicating that the igneous rocks are low degree melts of metasomatized upper mantle isotopic compositions of nd and hf indicate derivation from depleted mantle this information supports the idea that crust beneath southern louisiana formed as a magma starved rifted margin on the northern fl ank of the gulf of mexico ca 160 ma these results also confi rm that some magnetic highs mark accumulations of mafi c igneous rocks buried beneath thick sediments around the gulf of mexico margins

apparent isobaric cooling paths from granulites two counterexamples from british columbia and new hampshire textural evidence for the reaction histories of pelitic granulites from british columbia and new hampshire shows that pressure temperature paths obtained from thermobarometry on these samples are an artifact of the calculation technique samples from both localities show breakdown of garnet to form cordierite a reaction that occurs during decompression additional reaction textures and fluid inclusion data also imply decompression at high temperatures for both localities however core rim thermobarometry yields nearly isobaric cooling paths similar cooling paths have been used by other authors to argue for a magmatic underplating model for regional mafic granulites we show that mineral homogenization in pelitic granulites can lead to erroneous isobaric cooling paths and hence to incorrect conclusions concerning tectonic similarities between different types of granulites

mesoproterozoic plate tectonics a collisional model for the grenville aged orogenic belt in the llano uplift central texas the llano uplift of central texas united states exposes the core of a mesoproterozoic orogenic belt that formed along the southern margin of laurentia during grenville time a new collisional model is proposed that reconciles differences in structural stacking apparent tectonic transport and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high pressure eclogitic rocks emplacement of ophiolitic rocks and subsequent late stage to postcollisional plutonism our model proposes that subduction with southward polarity resulted in collision of an exotic arc with laurentia emplacement of ophiolitic rocks and telescoping of the intervening basinal sediments followed by overriding of the arc and margin of laurentia by a southern continent with transport toward laurentia the model further proposes that convergence led to subduction of the laurentian margin resulting in high pressure metamorphism but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from laurentia in a manner similar to that proposed for the alpine orogeny slab breakoff resulted in upwelling of the asthenosphere leading to intrusion of juvenile granitic plutons subduction along strike caused continued contraction that waned with time the eastern uplift records continent arc continent collision whereas the western uplift records continent continent collision the two regions also expose different crustal levels in the orogen the striking similarity with phanerozoic orogens including emplacement of ophiolites and formation of high pressure rocks implies that plate tectonic processes including subduction were active prior to the neoproterozoic

receiver function imaging of upper mantle complexity beneath the pacific northwest united states abstract small scale topographic variations on the upper mantle seismic discontinuities provide important constraints on the thermal influences of upwellings and downwellings in geodynamically complex regions subduction of the juan de fuca plate and other tectonic processes dominating the pacific northwest united states in the cenozoic involve massive thermal flux that likely result in an upper mantle that has strong 3 d temperature variations we address the interaction of such processes in the region using receiver functions to image the upper mantle seismic discontinuities at 410 and 660 km we utilized over 15 000 high quality receiver functions gathered from 294 teleseismic earthquakes recorded at 277 regional broadband seismic stations primarily those of the earthscope usarray transportable array we find the average depths of the discontinuities to be 412 km and 658 km respectively with no obvious 520 km discontinuity detected the peak to peak range is greater on the 410 than the 660 suggesting the possibility of more significant regional dynamic processes at upper mantle depths our results are not consistent with a mantle plume below central oregon in the high lava plains region our observation of a thinner transition zone beneath the western snake river plain region however is consistent with a regional increase in mantle temperatures perhaps due to either asthenospheric flow from beneath and around the southern edge of the juan de fuca plate or to vertical flow in the form of regional mantle upwelling related to the snake river plain yellowstone hotspot track further our results are not consistent with a simple subducting juan de fuca slab morphology but rather suggest similar levels of significant complexity in slab structure found by recent regional tomographic studies we find evidence for a thickened and therefore cooler mantle transition zone beneath the wallowa idaho batholith region consistent with tomographic models which suggest broad scale increased seismic velocities to transition zone depths we speculate that this region may be the location of mantle downwelling at larger scales than previously proposed

an integrated gravity model for europe s crust and upper mantle we present an integrated gravity model of the european lithosphere based on an analysis of a number of new data sets leading to a much higher resolution than provided by previous models first of all a recent crustal model eucrust 07 is used to quantify the crustal contribution to the observed gravity field and to identify the effect of mantle heterogeneity the new gravity field model is based on a combination of satellite champ and grace and terrestrial data we also use these data sets to estimate residual mantle gravity anomalies and residual topography reflecting the effect of mantle density variations induced by temperature and compositional heterogeneity the separation of these effects is vital for a proper assessment of mantle structure and evolution in addition we utilize a new tomographic model for p and s velocity anomalies beneath europe which is a priori corrected for crustal structure using eucrust 07 the seismic velocity anomalies were subsequently converted into temperature anomalies using a mineral physics approach we estimate the effect of temperature variations on the gravity field and subtract it from the total mantle field the residual fields point to an important role of compositional density anomalies in the upper mantle a number of key features of the compositional density distribution so far invisible in seismic tomography data are detected for the first time

a multistep approach for joint modeling of surface wave dispersion and teleseismic receiver functions implications for lithospheric structure of the arabian peninsula 1 we present a multiple step procedure for joint modeling of surface wave group velocity dispersion curves and teleseismic receiver functions for lithospheric velocity structure the method relies on an initial grid search for a simple crustal structure followed by a formal iterative inversion an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve love rayleigh surface wave dispersion discrepancy it considers longer period surface wave group velocity swgv dispersion allowing for the resolution of deeper structure compared to previous joint inversions the grid search for simple crustal structure is facilitated using a library of precomputed receiver functions and swgv dispersion curves the iterative inversion improves fit to the data by increasing the number of layers in the crust when necessary in order to fit the swgv for periods greater than about 50 s we perform a grid search over mantle velocities including the mantle lid and low velocity zone keeping the crustal structure fixed to the values from the previous step in some cases a clear love rayleigh discrepancy prevents a simultaneous fit of the group velocities with an isotropic model the love rayleigh discrepancy can be resolved by allowing shear wave transverse isotropy with a vertical symmetry axis vsh vsv differences in the uppermost mantle the method is applied to 10 stations in the arabian peninsula sampling various tectonic environments including active continental rifting and stable regions the resulting shear velocity models confirm rapid crustal thinning of the arabian shield toward the red sea however we do not find strong evidence for crustal thickening toward the arabian platform our results suggest that the mantle lithosphere thickness varies regionally but that the mantle shear velocities beneath the arabian shield and red sea coast are generally anomalously low furthermore our results indicate the presence of strong polarization anisotropy up to about 10 in the lithospheric upper mantle in the vicinity of as well as farther away from the red sea our modeling yields vsv vsh in the southwestern part of the arabian peninsula consistent with vertical flow and vsh vsv in the northwestern part of the arabian peninsula and the continental interior consistent with horizontal flow indicating that the mantle flow pattern is not uniform along the axis of the red sea