seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

crossing the several scales of strain accomplishing mechanisms in the hinterland of the central andean fold thrust belt bolivia depictions of structures at outcrop regional and tectonic scales enforce horizontal shortening and vertical thickening as the predominant style of deformation at all scales within the hinterland of the central andean fold thrust belt outcrop scale structures document a progression of strain that created 1 flexural slip folds 2 fold flattening via axial planar cleavage 3 vertical stretching via boudinage and late stage faulting and finally 4 kink folding these examples of intraformational deformation are generally concentrated just beyond the tip lines of thrust faults where fault propagation folds and related structures are well developed fault propagation folding accommodated the accrual of strain indicated by outcrop scale structures while the structures themselves indicate how deformation developed within each individual fold fault propagation fold geometries at a regional scale emerge from the construction of regional balanced cross sections the sections were drawn with careful attention to 1 known map relationships 2 field inspection of key contacts 3 bedding orientation data 4 local seismic control and 5 principles of balance the pervasive ese wnw shortening and vertical elongation seen at the outcrop and regional scales developed during the formation of the central andean backthrust belt in the hinterland of the andes the central andean backthrust belt is a large scale west vergent thrust system along the western side of the eastern cordillera in the generally east vergent andean fold thrust belt of bolivia strain accrual within this west verging zone of deformation is proposed to be a taper building mechanism that allowed the fold thrust belt to continue propagating eastward

reconstructing the kinematic evolution of curved mountain belts internal strain patterns in the wyoming salient sevier thrust belt u s a analyses of mesoscopic structural and strain patterns in red beds of the triassic ankareh formation and limestones of the jurassic twin creek formation in concert with complementary paleomagnetic studies constrain the three dimensional kinematic evolution of curved fold and thrust systems in the wyoming salient of the sevier thrust belt spaced cleavage fracture and vein networks minor folds and minor faults in limestones and red beds accommodated early layer parallel shortening lps concentrated in front of the growing thrust wedge along with minor strike parallel extension and wrench shear related to the development of orogenic curvature strain estimated using mass balance relations for cleavage seams crinoid ossicles in bioclastic limestone and reduction spots in red beds displays systematic regional patterns principal shortening directions are subperpendicular to structural trend around the salient reflecting a combination of primary curvature and secondary rotation of early lps fabrics lps magnitudes vary from 5 in central parts of the frontal hogsback thrust system where cleavage is absent to 10 30 in the more interior crawford thrust system where cleavage intensity is moderate to strong strain also increases toward the salient ends internal strain is a significant component of total deformation and should be considered when restoring cross sections strain patterns are consistent with a kinematic model involving curved fault slip and differential shortening that produced progressive secondary curvature during thrusting

three dimensional tomographic model of the crust beneath the hong kong region we present the first three dimensional seismic velocity model of the hong kong region including the dangan island fault zone difz the crust beneath hong kong is predominantly igneous and is characterized by relatively high seismic velocity further south we observe an elongated velocity anomaly beneath part of the difz where significant seismicity has been recorded this anomaly demarcates the contrasting seismic velocity structures on the opposite sides of the fault zone from the surface to a depth of at least 20 km suggesting that the difz extends to the lower crust and dips subvertically our model also indicates that there is an abrupt change in the along strike crustal structure of the fault zone with a significantly higher seismic velocity in the region west of the seismically active area we anticipate that this first three dimensional subsurface tectonic model of hong kong and the difz will help assess the influence of crustal heterogeneities on the spatial pattern of intraplate earthquakes in this densely populated region of south china

plate boundary geometry to moho depths within the 9 03 n and 12 54 n overlapping spreading centers of the east pacific rise abstract multichannel seismic reflection data from the east pacific rise provide a three dimensional view of overlapping spreading centers oscs at 9 03 n and 12 54 n previous studies of the 9 03 n osc based on position of the axial magma chamber top are extended to the base of the crust using the observation that the moho reflection is disrupted in the vicinity of the ridge axis the smaller 12 54 n osc is investigated for comparison like the shallower features the moho disruption is offset within the oscs at the larger 9 03 n osc misalignment between the morphologic ridge axis and the axial magma body extends to the base of the crust where the axial gap in the reflection moho does not consistently underlay the shallower structures spatial relationships within the oscs between axial topography axial magma chamber and axial gap in reflection moho suggest that ridge tips propagate first by brittle fracturing of the upper crust followed by intracrustal magma residence followed finally by establishment or capture of magma pathways from the mantle interpreting all three features as demarcating the axial zone of weakness along which new magma enters the crust the axial plane within the 9 03 n osc appears to dip and to rotate with depth the sense of overlap in the shallow crust is dextral while the sense of offset in the deep crust is sinistral spatial relationships within the 9 03 n osc suggest that the northern propagating ridge tip has already captured a portion of the deep crustal magma conduit connected with the southern ridge

deepening of the axial magma chamber on the southern east pacific rise toward the garrett fracture zone a wide aperture profile along the ridge axis from 14 29 s to 13 39 s 120 km to 30 km south of the garrett fracture zone is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector five areas along the 90 km line are examined in detail with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area a genetic algorithm code is used to find a best fit model from a comparison of the data and wkbj synthetic seismograms one hundred starting models are generated using a predefined set of velocity nodes with a fixed window of allowable depth variations between nodes an evolutionary process favors the better fitting models in each generation and a satisfactory misfit is usually obtained within 40 generations within individual areas the models were in good agreement with the depth of a given velocity node generally varying by not more than 20 m the depth discretization interval for the models a consistent deepening trend of the axial magma chamber amc is observed across the five areas as the garrett fracture zone is approached the depth varies from 0 99 km at area 1 which is approximately 100 km south of the garrett to 1 23 km at area 5 which is approximately 40 km south of the garrett the depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age for the areas examined here layer 2a is observed to be relatively constant in thickness along the axis although it is about 40 m thicker over area 5 where the axial magma chamber is deepest this variation is within the scatter of previously detailed layer 2a measurements at 13 n on the east pacific rise where an effectively constant thickness is observed this implies that layer 2a thickening is not a significant factor along this profile and that the amc deepening is real rather than apparent theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the garrett fracture zone which marks a major interruption of hundreds of kilometers of continuous ridge axis the deepening may also be interpreted as a downward limb from a central injection point however there is no indication of a similar downward trend in the other southern direction furthermore there is no accompanying systematic variation in axial depth or axial volume both of which are proposed to be indicators of central injection and along axis flow

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

three dimensional shallow crustal emplacement at the 9 03 n overlapping spreading center on the east pacific rise correlations between magnetization and tomographic images we report a three dimensional 3 d seismic reflection and tomographic survey conducted at the 9 03 n overlapping spreading center osc on the east pacific rise to understand crustal accretion at this feature inversions of travel time data from 19 ocean bottom hydrophones provide a 3 d image of the shallow velocity structure beneath the nontransform offset and associated discordance zone seismic analysis indicates that layer 2a thickness varies between 100 and 900 m and averages 430 m throughout the study area the heterogeneous upper crustal structure at the osc region contrasts with the simpler symmetric structure flanking the midsegments of the east pacific rise the crust affected by the osc migration carries evidence for the complex accretion at the axial discontinuity where the overlap basin may act as a lava pond an area of thick layer 2a covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magnetic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks

development of slaty cleavage and degree of very low grade metamorphism a review illite crystallinity ic and other indicators of the grade of very low grade metamorphism associated with the appearance of various stages of slaty cleavage in phyllosilicate rich rocks have been compiled from a wide variety of terranes ic values have been converted to a kubler equivalent standard scale but the diverse characterizations of the cleavage fabrics in published descriptions do not always allow an unequivocal identification of equivalent stages of cleavage development nevertheless there exists a distinct relationship between grade and the appearance of various stages of cleavage development 1 indications of incipient slaty cleavage such as s0 s1 pencil structure appearance of primary s1 crenulation cleavage and of closely spaced cleavage without parallel fabric in the microlithons is associated with a wide range of mostly medium and high grade diagenetic ic values 2 the appearance of smooth cleavage with a strong parallel fabric in the microlithons and or quartz mica beards and the chlorite mica stacks shortened at a high angle to 001 and of irregular cleavage in sandy beds is associated with a much narrower range of predominantly low and medium grade anchimetamorphic grades rarely high grade diagenetic the first appearance of these stages of cleavage development with higher grades can often be related to post kinematic magmatic heating polymetamorphism pre cleavage metamorphism or static recrystallization without cleavage formation for example in low strain zones there exists a relationship between finite strain fabric and metamorphic grade in mudstones and slates in coarser clastic rocks the same finite strain fabric relationship occurs at appreciably higher grades a relationship between finite strain in carbonate rocks and ic in the nearby rocks has been reported from the helvetic zone of the swiss alps the earlier stages of cleavage formation are associated with little improvement in ic the narrow range of ic associated with smooth cleavage is concluded to represent recrystallization and grain growth concurrent with cleavage formation

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

3 d sediment basement tomography of the northern marmara trough by a dense obs network at the nodes of a grid of controlled source profiles along the north anatolian fault a 3 d tomographic inversion of first arrival times of shot profiles recorded by a dense 2 d obs network provides an unprecedented constraint on the p wave velocities heterogeneity of the upper crustal part of the north marmara trough nmt over a region of 180 km long by 50 km wide one of the specific aims of this controlled source tomography is to provide a 3 d initial model for the local earthquake tomography let hence in an original way the controlled source inversion has been performed by using a code dedicated to let after several tests to check the results trade off with the inversion parameters we build up a 3 d a priori velocity model in which the sea bottom topography the acoustic and the crystalline basements and the moho interfaces have been considered the reliability of the obtained features has been checked by checkerboard tests and also by their comparison with the deep penetration multichannel seismic profiles and with the wide angle reflection and refraction modelled profiles this study provides the first 3 d view of the basement topography along the active north anatolian fault beneath the marmara sea even beneath the deepest part of three sedimentary basins of nmt clear basement depressions reaching down 6 km depth below the sea level bsl have been found beneath these basins the north imrali basin located on the southern continental shelf is observed with a similar sedimentary thickness as its northern neighbours between central and cinarcik basins the central high rises up to 3 km depth below bsl its crest position is offset by 10 km northwestward relatively to the bathymetric crest on the contrary tekirdag and central basins appear linked forming a 60 km long basement depression beneath the bathymetric relief of western high low velocities are observed down to 6 km depth bsl and no basement high have been found the obtained 3 d vp heterogeneity model allows the consideration of the 3 d supracrustal heterogeneity into the future earthquake relocations in this region the topographic map of the pre kinematic basement offers the possibility to take into account the locking depth variations in future geohazard estimations by geomechanical modelling in this region

influence of enhanced melt supply on upper crustal structure at a mid ocean ridge discontinuity a three dimensional seismic tomographic study of 9 n east pacific rise we present a three dimensional upper crustal model of the 9degrees03 n overlapping spreading center osc on the east pacific rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels our p wave velocity model obtained from tomographic inversion of similar to 70 000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs extrusive volcanic rocks above the western melt sill are inferred to be thin similar to 250 m more extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill the extrusive layer inferred above the eastern melt sill thickens from similar to 350 at the neovolcanic axis to 550 m to the west of the melt sill volcanic construction is likely to be significant in the formation of ridge crest morphology at the osc particularly at the tip of the eastern limb on the basis of our interpretation of the velocity model we propose that enhanced magma supply at crustal levels at the osc may provide an effective mechanism for the migration of ridge discontinuities this dynamic magma supply model may explain the commonly observed nonsteady migration pattern of ridge discontinuities by attributing this to the temporal fluctuations in melt availability to the overlapping spreading limbs

significance of widespread low effusion rate eruptions over the past two million years for delivery of magma to the overlapping spreading centers at 9 n east pacific rise abstract ever since the discovery that the mid ocean ridge axis deepens at most axial discontinuities there has been vigorous debate about whether ridge axis depth and shape reflect magma supply here we report evidence from one of the most thoroughly studied ridge discontinuities on a fast spreading mid ocean ridge that volcanic eruption effusion rates have been low relative to nearby segment centers throughout its two million year history this pattern of low volcanic effusion rates is inferred from the fields of constructional volcanic mounds on the seafloor which are much more abundant in the off axis discordant zone of the east pacific rise epr 9 n overlapping spreading centers osc than on typical epr seafloor detailed seafloor mapping in 2007 shows that these mound fields consist of concentrations of large pillow mounds that formed within the current osc basin and ridge tips rather than off axis pillow lava erupted at approximately an order of magnitude lower effusion rate than lobate or sheet flows primarily builds mounds on the seafloor crustal thickness a proxy for long term magma supply does not decrease in spite of the decreased lava effusion rates implied by widespread pillow eruptions to resolve the problem of achieving the same magma emplacement rate with lower effusion rates without violating our observations by requiring drastically increased eruption frequency or unreasonably long eruption durations we propose that eruptions occur across the entire osc basin the current configuration of mid crustal melt sills within the osc as resolved by multichannel seismic reflection surveys have neither the correct location nor appropriate size to deliver magma to these eruptive vents in the osc basin via vertically propagating dikes instead we propose that magma delivery to eruptions occurs via laterally propagating dikes from robust mid crustal melt sills outside of the overlap zone

the case for dynamic subsidence of the u s east coast since the eocene the dynamic subsidence of the united states east coast is addressed using the discrepancy between regional and global estimates of sea level elevation of paleoshorelines and adjoint models of mantle convection that assimilate plate motions and seismic tomography the positions of eocene and miocene paleoshorelines are lower than predicted by global sea levels suggesting at least 50 m and possibly as much as 200 m of subsidence since the end of the eocene dynamic models predict subsidence of the east coast since the end of eocene although the exact magnitude is uncertain this subsidence has been occurring during an overall global sea level fall with the eustatic change being larger than the dynamic subsidence this results in a regional sea level fall in the absence of land subsidence dynamic subsidence is consistent with the difference between eustasy and regional sea level at the new jersey coastal plain

constraining crustal emplacement processes from the variation in seismic layer 2a thickness at the east pacific rise abstract a stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic crustal structure similar to that observed in seismic data from the east pacific rise epr in ocean drill holes and in ophiolites in this model the location of successive dike intrusion events relative to the ridge axis is determined by a gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a gaussian shaped lava pile we interpret wide angle seismic reflections from the steep velocity gradient at the base of seismic layer 2a to be the extrusive sheeted dike contact seismic data from the northern and southern epr place constraints on the on axis extrusive layer thickness 230 50 m the distance over which the thickening of the extrusive layer occurs width of the accretion zone 1 3 km and its off axis thickness 300 800 m ophiolites and ocean drill holes dsdp hole 504b provide additional estimates of the thickness of the extrusive layer and constrain the thickness of the transition region from extrusives to sheeted dikes 100 200 m a simple stochastic emplacement model where the lavas are described by one mean flow length fits the thickening of the extrusive layer off axis inferred from the deepening of seismic layer 2a but the predicted transition from sheeted dikes to extrusives is too thick in order to match the dimensions and flat topped shape of the seismic layer 2a boundary as well as the thickness of the extrusive sheeted dike transition a bimodal distribution of lava flows is used short flows confined within the axial summit caldera asc build up approximately half the extrusive volume occasional voluminous flows spill out of the asc or erupt outside of the asc and pond at a considerable distance off axis to build up the remainder of the extrusive section the upper part of the final extrusive section will be dominated by the off axis flows while the lower portions will be primarily composed of short flows erupted within the asc magnetic transition widths predicted from the overlap of lavas 2 km in this model are similar to those measured in deep tow studies assuming a smoothing function which acts over one seismic wavelength the upper crustal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the epr the ages of seafloor lavas in this model are younger than the tectonic spreading model ages by 30 70 kyr in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the epr

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

three dimensional tomographic inversion of combined reflection and refraction seismic traveltime data summary a tomographic inversion method is presented for the determination of 3 d velocity and interface structure from a wide range of body wave seismic traveltime data types it is applicable to refraction wide angle reflection normal incidence and multichannel seismic data and is best suited to a combination of these that provides good independent constraints on seismic velocities and interface depths the inversion process seeks a layer interface minimum structure model that is able to explain the given data satisfactorily by inverting to minimize data misfit and model roughness norms simultaneously this regularized inversion and the use of smooth functions to describe velocities and depths allows the highly non linear tomographic problem to be approximated as a series of linear steps the inversion process begins by optimizing the fit to the data of a highly smoothed initial model in each subsequent step structure is allowed to develop in the model with successively greater detail evolving until a satisfactory fit to the data is obtained parameter uncertainties for the final model are then estimated using an a posteriori covariance matrix analysis smooth layer interface models are parametrized using regular grids of velocity and depth nodes from which spline interpolated interface surfaces and velocity fields are defined forward modelling is achieved using ray perturbation theory and a two point ray tracing method that is optimized for a large number of closely spaced shot or receiver points the method may be used to generate 1 and 2 d models from for example vertical seismic profile data or 2 d surveys in which the 3 d geometry of a survey is correctly accounted for the ability of the method to resolve typical target structures is tested in a synthetic salt dome inversion from a set of noisy traveltime data the model converges quickly to a well resolved final model from different starting models the application of this method to real data is demonstrated with a combined 3 d inversion of refraction and reflection data which provide p wave velocity constraints on the methane hydrate stability zone in the cascadia margin offshore vancouver island

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

thermochronology of lower cretaceous source rocks in the idaho wyoming thrust belt lower cretaceous organic rich source rocks that are thermally mature to postmature crop out on the absaroka darby and prospect plates in linear belts that run parallel to the trace of the thrusts in the idaho wyoming portion of the idaho wyoming utah thrust belt although the common assumption is that burial by thrust plates and the synorogenic sediments derived from them have been responsible for thermal maturation of the organic rich strata commercial amounts of hydrocarbons have not been found in structural traps in this portion of the thrust belt in a companion paper burtner and nigrini demonstrated that gravity driven fluid flow in the idaho wyoming portion of the thrust belt was responsible for moving large amounts of heat from the depths of the early cretaceous foreland basin eastward toward the stable platform in this paper we demonstrate through the application of organic maturation indicators and a new refinement of the apatite fission track technique that this process heated lower cretaceous organic rich source rocks to temperatures sufficient to generate hydrocarbons hydrocarbon generation and migration occurred prior to the development of the thrusts that are often assumed to have played a major role in the generation and entrapment of hydrocarbons in this portion of the thrust belt

foreland basin stratigraphic control on thrust belt evolution the link between orogenic activity and foreland basin stratigraphy is well established however potential controls by foreland basin stratigraphy on thrust belt architecture have not been fully evaluated mechanical properties of typical foreland basin stratigraphic successions influence the structural development of fold thrust belts in predictable ways fundamental features of foreland basins include the onset of rapid subsidence and deposition of a coarsening upward sedimentary succession in the lower part of this succession are fine grained distal foreland basin deposits enlargement of the orogenic wedge through frontal accretion incorporates the foreland basin strata into the thrust belt and distal foreland basin depositional units may be preferentially exploited as a thrust detachment zone resulting in multiple detachment levels we propose that foreland basin stratigraphic architecture has significant influence on the structural development of thrust belts and that by extension processes that influence foreland basin sedimentation may ultimately influence orogenic evolution far removed in time and space

relationship of spaced cleavage to folds and thrusts in the idaho utah wyoming thrust belt the idaho utah wyoming thrust belt of the rocky mountain cordillera preserves structures that developed at very shallow levels of the crust t 200 c the geometry of major structures is well documented and synorogenic conglomerates record the timing of movement on major thrusts spaced cleavage and pencil fracturing are developed within micrites of the jurassic twin creek formation the cleavage developed dominantly by pressure solution early in the deformation history of each thrust sheet aided by elevated temperatures and pressures in areas where increased overburden resulted from emplacement of earlier overriding sheets cleavage within any sheet is well developed in the area covered by the pre erosional extent of the previous sheet forelandward from this area cleavage intensity decreases drastically thermal modelling of the sheets agrees well with temperature estimates from illite crystallinity studies on material from the cleavage planes cleavage is axial planar to small folds and formed concurrently with small scale buckle folds and contraction faults during the early history of movement on each thrust however cleavage strongly fans about large scale folds formed later during movement suggesting passive rotation of cleavage with bedding during the later stages of thrust history the rotated cleavage may also be folded and overprinted by a second weaker cleavage during a second ramping or during movement on a later lower thrust cross folding may also develop as a result of lateral ramping thus complex patterns of interfering folds multiple cleavages and associated fiber filled fractures may be produced during motion on a single thrust fault and so magnify the problem of correlating cleavages and deciphering multiple deformations

3 d deformation pattern analysis and evolution of the anisclo anticline southern pyrenees understanding the genetic relationships of folding related deformation structures is of importance for both academic and industrial purposes we analysed the three dimensional deformation pattern of the anisclo anticline in the southern pyrenees the specifically designed statistical method includes sequential steps of statistical and spatial data analysis and classification our results indicate that pressure solution cleavage frequency exhibits a spatial distribution that depends on the position within the fold conversely such a dependence does not occur in joints and veins consequently our data suggest that pressure solution cleavage is the most appropriate deformation structure for unravelling fault fold kinematics in carbonate multilayer in particular we found that the spacing of solution cleavages s is related to the corresponding bed thickness h this supports the use of h s value for deformation intensity quantification instead of spacing in the anisclo anticline analysis of h s indicates the occurrence of three main deformation panels whose restoration suggests that fold evolution progressed from layer parallel shortening to decollement folding up to thrust breakthrough and fault propagation folding comparison of h s values with finite strain data indicates that line length was not preserved during fold evolution this evidence has important implications for the kinematic interpretation of layer parallel shortening and cross section balancing

the axial summit graben and cross sectional shape of the east pacific rise as indicators of axial magma chambers and recent volcanic eruptions abstract the axis of the east pacific rise epr undulates up and down hundreds of meters over distances of 30 200 km along strike the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers oscs we have suggested that systematic variations in depth and cross sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment 1 a comparison of recently collected multichannel seismic mcs data 2 with our sea beam and seamarc ii data have allowed us to test and advance this hypothesis along the epr from 9 to 13 n there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment the cross sectional shape of the rise the presence or absence of an axial summit graben and the presence or absence of a shallow axial magma chamber as interpreted from mcs data where the axial magma chamber is present the cross sectional shape of the ridge is broad and an axial summit graben is recognized along the axis in contrast where the cross sectional shape of the rise is narrow and triangular an axial magma chamber is not detected and an axial summit graben is absent these ridge axis characteristics tend to occur along deeper portions of a given ridge segment often near ridge axis discontinuities we suggest that these systematic variations in ridge axis morphology cross sectional shape and structure presence or absence of an axial graben reflect spatial and temporal variations in the magmatic budget of the ridge axis where the magmatic budget is waxing shallow level magma reservoirs in the crust and underlying upper mantle swell creating a broad axial bulge with a summit graben where the magmatic budget is diminished the crustal magma chamber is small this proposed correlation of shape structure and magmatic parameters fails along only two short portions of the ridge in these areas there is evidence for an axial magma chamber and the rise has a broad cross sectional shape but there is no summit graben bottom photographs and submersible results however show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting suggesting that the summit graben has been recently filled in by lava flows and the development of a summit graben or a linear caldera by volcano tectonic collapse has not yet occurred

contrast in crustal structure across the clipperton transform fault from travel time tomography a three dimensional 3 d seismic refraction study of the clipperton transform fault northern east pacific rise reveals anomalously low compressional velocities from the seafloor to the moho we attribute this low velocity anomaly to intensive brittle deformation caused by transpression across this active strike slip plate boundary the seismic velocity structure south of the clipperton transform appears unaffected by these tectonic forces but to the north seismic velocities are reduced over 10 km outside the zone of sheared seafloor this contrast in seismic velocity structure corresponds well with the differences in mid ocean ridge morphology across the clipperton transform we conclude that the amount of fracturing of the upper crust which largely controls seismic velocity variations is strongly dependent on the shallow temperature structure at the ridge axis intermittent supply of magma to the shallow crust north of the clipperton transform allows seawater to penetrate deeper and the cooler crust is brittle to a greater depth than south of the transform where a steady state magma lens is known to exist the crustal thickness averages 5 7 km only slightly thinner than normal for oceanic crust and variations in moho depth in excess of 0 3 km are not required by our data the absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal east pacific rise type crust perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies

three dimensional crustal structure and magma chamber geometry at the intermediate spreading back arc valu fa ridge lau basin results of a wide angle seismic tomographic inversion in this paper the results of a tomographic analysis of a 3 d wide angle seismic refraction data set acquired at the valu fa ridge vfr in 1995 are presented the vfr is an intermediate spreading ridge located in the southern lau back arc basin in the southwest pacific the ridge comprises three morphological segments the southern central and northern valu fa ridges separated by overlapping spreading centres oscs previous seismic experiments have identified a robust axial magmatic system beneath the central segment cvfr and the osc with the northern segment nvfr the experiment described in this paper aimed to resolve details of the structure of this magma chamber and the adjacent post rift crust a regularized inversion scheme that minimizes model roughness was applied to the first arrival traveltime picks made from the wide angle data a quantitative approach for determining data uncertainties is described based on the signal to noise ratio of the arrivals several initial model assumptions were tested including one with a thin melt lens representing a seismic reflector identified in previous studies explicitly included in the initial model the inversion results suggest that crustal layer 2 exhibits northward thickening which mirrors a similar northward thickening of the whole crust in addition local thinning of layer 2 is identified in the vicinity of the boundary between pre and post rift crust which is thought to represent thinning of the crust prior to the onset of rifting axial low velocity anomalies are identified in layer 2b c and layer 3 the models are consistent with a continuous 6 km wide negative velocity anomaly in layer 3 with an amplitude of 0 7 0 9 km s 1 relative to off axis post rift crust this anomaly is consistent with the presence of an axial mush zone comprising a small percentage 1 per cent of partial melt the negative velocity anomaly in layer 2b c is modelled with its largest amplitude 0 5 km s 1 beneath the northern osc possible origins for this anomaly include locally thicker crust or locally higher porosity near the osc or a high temperature anomaly associated with the axial magmatic system

seismic evidence of a complex multi lens melt reservoir beneath the 9 n overlapping spreading center at the east pacific rise the crustal structure at fast spreading ridges is characterized by the presence of an axial melt lens along a significant part of the ridge axis over a partially molten lower crust using three dimensional seismic reflection data and waveform inversion here we show the existence of two to three stacked melt lenses beneath both limbs of the 9 n overlapping spreading center on the east pacific rise which we suggest are the source of the very evolved lavas locally observed on the seafloor the melt lenses are 2 4 km wide and lie in a 900 m depth range we suggest that the presence of a complex multi lens melt reservoir in the crust is due to the reduced ambient stress regime hence cracking resulting from the overlap of the two segments of the ridge allowing melt to stay longer in the crust at different depths

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

strain fabric associated with the meade thrust sheet implications for cross section balancing abstract emplacement of the meade thrust sheet southeastern idaho u s a was accompanied by the development of a strain fabric and significant footwall deformation characteristics of the strain fabric are based on strain estimates using pressure solution cleavage and deformed fossils within the jurassic twin creek formation the strain fabric formed during early layer parallel shortening at the thrust front and accommodates from 10 to 35 shortening within the incipient thrust sheet footwall strata were later folded above a detachment at the base of the twin creek formation and the early strain fabric was passively rotated the structural development of this area consisting of 1 strain fabric development 2 folding of the footwall and 3 the truncation of footwall structures by imbricates of the meade thrust are placed in a relative time frame to aid in section reconstruction the passive reorientation of the strain fabric and the truncation and incorporation of footwall structures into the thrust sheet are used as kinematic and geometric constraints for cross section balancing we present a sequential retrodeformation of the meade thrust sheet in which strain is removed from the cross section at the appropriate geometric stages in the deformation these sections illustrate some methods by which strain data can be incorporated into cross sections and also indicate the difficulty involved in balancing cross sections of internally deformed thrust sheets interpretation of these sections suggests that a significant amount of footwall deformation accompanied the emplacement of the meade thrust sheet and that footwall deformation is strongly controlled by the local stratigraphic package the sections also illustrate that the earliest deformation within a thrust sheet may be unrelated to motion on the underlying incipient thrust and that the trace of a thrust fault which cuts previously deformed strata will not restore to a typical ramp and flat geometry

recent eruptions on the coaxial segment of the juan de fuca ridge implications for mid ocean ridge accretion processes the 1993 seismic swarm and volcanic eruption on the coaxial segment of the juan de fuca ridge was the first verified mid ocean ridge accretion event monitored by the national oceanic and atmospheric administration u s navy sound surveillance system sosus ambiguity in the location of the initial seismicity resulted in an uncertainty as to whether the dike intruded from the summit and north rift zone of the adjacent axial volcano or whether it arose locally within the coaxial segment however analyses of multibeam side scan sonar towed camera submersible and geochemical data show that the coaxial segment is morphologically structurally and geochemically distinct from the north rift zone of axial volcano there is no geologic or geochemical evidence that dike injections from axial volcano have extended north of 46 18 n in the past whereas the 1993 eruption site is at 46 31 n furthermore all seafloor manifestations of the 1993 dike injection lie along the central neovolcanic zone of the coaxial segment analyses of repeat seabeam surveys combined with seafloor observations show that two other eruptions of approximately the same volume as the 1993 eruption occurred along the coaxial segment in the 1981 1991 interval these three diking events may have relieved decades of accumulated stress over 35 km or more of this segment the spatial pattern and hydrothermal history of the 1993 event is consistent with a dike with a significant lateral component of injection

strike slip fault propagation cleavage in carbonate rocks the mattinata fault zone southern apennines italy disjunctive spaced solution cleavage in carbonate rocks is genetically associated with the propagation of the left lateral strike slip mattinata fault in the gargano promontory italy typical cleavage development is restricted within the 200 300 mwide fault zone which is bounded by virtually unfractured wall rocks the cleavage consists of sub parallel solution surfaces which are often reactivated as sheared solution planes geometrical and kinematic relationships exist between the fault and the associated cleavage planes thus 1 cleavage fault intersection lines lie parallel to the fault and the sheared cleavage rotational axes and 2 the cleavage fault angle is almost constantly equal to 408 a model for the development of the mattinata fault is proposed in which the cleavage surfaces are interpreted as fault propagation deformations cleavage nucleates as solution planes at the front of the advancing fault as the result of stress concentration in this region two distinct time sequential processes are shown to operate during the fault propagation 1 typical millimetre to centimetre spaced solution surfaces form in the distal tip zone of the advancing fault plane 2 as the tip advances the fault plane breaks through the cleavage as minor shear displacements reactivate some of these nascent surfaces these observations may prove useful in understanding mechanisms for fault controlled enhanced reduced permeability and fluid pathways 1999 elsevier science ltd all rights reserved

multistage strain localisation and fluid assisted cataclasis in carbonate rocks during the seismic cycle insights from the belluno thrust eastern southern alps italy abstract strain localisation in carbonate rocks impacts upon the nucleation and development of structures within the shallow seismogenic crust studying deformed carbonate rocks is thus a key to the refined understanding of the mechanical behaviour of rock successions with high potential to fail coseismically in response to stress accumulation we studied deformation structures and mechanisms of the miocene belluno thrust from the seismically active eastern southern alps italy the thrust contains several structural facies and features pressure soluted and veined lithons cataclasites s c fabrics principal slip surface which make it possible to define a sequence of deformation events in space and through time during thrusting image analysis of cataclasite particle size distribution and morphology supported by cathodoluminescence imaging indicates corrosive wearing as the main comminuting mechanism during cataclasis and repeated ingress of overpressured aqueous fluids promoting carbonate clast dissolution we propose an evolutionary scenario of multiple episodes of fabric formation and rejuvenation that localised and accommodated increasing shear strain into a progressively narrowing fault core we propose that strain localisation in carbonate rocks can be greatly facilitated by cyclic fluctuations between strain hardening and softening conditions punctuated by transient spikes of coseismic failure accommodated by cataclasis during the ingress of overpressured chemically reactive fluids that control carbonate precipitation and dissolution

deep tow studies of the overlapping spreading centers at 9 03 n on the east pacific rise the deeply towed instrument package of the scripps institution of oceanography was used to study for the first time the fine scale structure of an overlapping spreading center osc system the 9 03 n osc on the east pacific rise epr the eastern and western limbs of this osc system which are 8 km apart and overlap by 27 km show marked differences away from the tips the spreading centers are highly tectonized and differ greatly from the ridge tips the western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area in both cases the ridge tip fissures are 10 15 oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively the eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through going trace of the east pacific rise in contrast the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis some of the very few tectonic features observed in the overlap basin may reflect the existence of a short lived shear couple due to the interaction of the two overlapping spreading centers sediment cover over the whole area and especially in the overlap basin is relatively thick supporting the idea of a time averaged deficit in the magmatic budget at this osc we suggest that the 9 03 n osc developed where two magmatic pulses independent in space and time and propagating along the strike of the east pacific rise away from the loci of melt emplacement failed to meet misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing osc geometry independent lines of evidence derived from the three dimensional inversion of the magnetic field at the 9 03 n osc sempere et al 1984 from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field sempere and macdonald 1986 and from geochemical data langmuir et al 1986 support our interpretation of the deep tow data we suggest that deviations from axial linearity of the spreading centers devals langmuir et al 1986 and small nonoverlapping offsets snoos batiza and margolis 1986 are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on oscs and some saddle points i e devals snoos are all small rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the east pacific rise

solution cleavage and its relationship to major structures in the idaho utah wyoming thrust belt micrites of the jurassic twin creek formation in the idaho utah wyoming thrust belt show spaced cleavage developed during folding and thrusting cleavage in each thrust sheet forms mainly by pressure solution aided by elevated temperatures and pressures caused by increased overburden as a result of emplacement of earlier overriding thrust sheets within each thrust sheet cleavage develops early and is contemporaneous with microscopic to mesoscopic buckle folds and faults large scale ramp folds formed during major motion on the thrust postdate the cleavage and passively rotate it into major fans

vertical and fold axis parallel extension within a slate belt in a transpressive setting northern appalachians abstract an incremental and finite strain study of a mid paleozoic slate belt in a transpressive setting in gaspe canadian appalachians has been undertaken using syntectonic fibers from pyrite type pressure shadows regionally strain estimates are quite consistent and reflect only slightly higher strain in the central part of the anticlinorial structure defining the slate belt extensions of up to 160 vertically and 110 parallel to the fold axis were recorded in the slate belt combined with an estimated overall regional shortening of 80 the fibers first rotated toward the final position of the cleavage within a vertical section normal to the cleavage and then rotated from a vertical to a horizontal orientation this change in fiber orientation observed in cleavage parallel sections indicates an abrupt change in the incremental stretching direction from sub vertical to sub horizontal fold axis parallel the vertical extension is related to folding and cleavage development during coaxial flattening the horizontal extension reflects the strain imposed on the rocks during regional simple shear which produced strike slip faulting and further tightening of the folds folding cleavage development fold axis parallel extension and faulting are all related to oblique convergence during the middle devonian acadian orogeny in this part of the canadian appalachians and clearly indicate an overall transition from a pure shortening deformation to a simple shear dominant deformation in an overall transpressive setting

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

variation in cross sectional area of the axial ridge along the east pacific rise evidence for the magmatic budget of a fast spreading center along the fast and ultrafast spreading east pacific rise the cross sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation using an automated method we measure the ridge area volume per kilometer along axis where there is complete bathymetric coverage along 3500 km of the northern and southern east pacific rise our two study areas encompass numerous transform faults large overlapping spreading centers small overlapping spreading centers and smaller discontinuities first second third and fourth order discontinuities the cross sectional area variation mimics the undulation of the ridge crest depth local area maxima occur toward the middle of segments and the axial area decreases by 40 or more at first and second order discontinuities third order discontinuities are generally marked by smaller disruptions in the ridge area and fourth order offsets do not systematically correspond with features of the cross sectional area profile the correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross sectional area represents a longer term average of the ridge s magmatic state than axial depth a correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low density crust and mantle also a correlation between larger area and higher mgo content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone the cross sectional area of the axial ridge appears to correlate with the width of the axis centered low velocity zone in the crust these observations as well as the absence of large relict axial ridges off axis indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis portions of the northern east pacific rise underlain by a magma chamber reflector generally occur where the ridge cross sectional area is greatest this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt thus while the axial high on fast spreading centers resembles a constructional volcano in cross section it is more like a long narrow balloon whose cross sectional area is a sensitive indicator of magma supply using this relationship we predict with 75 confidence that at least 45 of the unsurveyed northern east pacific rise 18 n to 13 n and 9 n to 5 n and at least 60 of the southern east pacific rise 4 s to 23 s is underlain by a magma chamber reflector

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

contribution of volcanism and tectonism to axial and flank morphology of the southern east pacific rise 17 10 17 40 s from a study of layer 2a geometry multichannel seismic images from the east pacific rise 17 10 17 40 s are used to study the geometry of seismic layer 2a in the near axis region wave equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2a event processed stacks are compared with seafloor morphology and tectonic fabric imaged in side scan sonar data assuming layer 2a corresponds with the extrusive crust these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge crest and ridge flank morphology we find variations in axial thickening of layer 2a which imply twofold variation in the width of the extrusive layer accumulation zone as well as systematic changes in the pattern of accumulation of this layer on the ridge flanks the layer 2a horizon mimics abyssal hill relief consistent with a horst and graben origin for this topography short wavelength variations in the thickness of layer 2a are superimposed on this relief which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone the integrated off axis thickness of layer 2a is correlated with ridge cross sectional area and indicates persistent 100 000 years spatial gradients in extrusive layer thickness systematic changes in the cross axis shape of the ridge observed over distances of 5 10 km cannot be attributed to the extrusive layer and it appears that axial structure beneath the volcanics governs the cross axis morphology of the ridge

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

seafloor photo geology and sonar terrain modeling at the 9 n overlapping spreading center east pacific rise 1 a fundamental goal in the study of mid ocean ridges is to understand the relationship between the distribution of melt at depth and seafloor features building on geophysical information on subsurface melt at the 9 n overlapping spreading center on the east pacific rise we use terrain modeling dsl 120a side scan and bathymetry photo geology jason ii and whoi towcam and geochemical data to explore this relationship terrain modeling identified four distinct geomorphic provinces with common seafloor characteristics that correspond well to changes in subsurface melt distribution visual observations were used to interpret terrain modeling results and to establish a relative seafloor age scale calibrated with radiometric age dates to identify areas of recent volcanism on the east limb recent eruptions in the north are localized over the margins of the 4 km wide asymmetric melt sill forming a prominent off axis pillow ridge along the southern east limb recent eruptions occur along a neovolcanic ridge that hugs the overlap basin and lies several kilometers west of the plunging melt sill our results suggest that long term southward migration of the east limb occurs through a series of diking events with a net southward propagation direction examining sites of recent eruptions in the context of geophysical data on melt distribution in the crust and upper mantle suggests melt may follow complex paths from depth to the surface overall our findings emphasize the value of integrating information obtained from photo geology terrain modeling lava geochemistry and petrography and geophysics to constrain the nature of melt delivery at mid ocean ridges

the use of finite strain data in constructing a retrodeformable cross section of the meade thrust sheet southeastern idaho u s a abstract a method for drawing a balanced cross section is presented which uses the constraint of both the interpretation of the geometry and the analysis of the internal deformation within the meade thrust sheet southeastern idaho u s a earliest deformation associated with the meade thrust sheet is layer parallel shortening which is indicated by a spaced solution cleavage and deformed fossils in the jurassic twin creek formation this early layer parallel shortening fabric is rotated by folding in front of the main meade thrust sheet these folds are then truncated by the meade thrust locally finite strain markers indicate fault parallel shear

mechanical decoupling and thermal structure at the east pacific rise axis 9 n constraints from axial magma chamber geometry and seafloor structures abstract we study the relationships between the seafloor structures and the axial magma chamber geometry in the 9 n overlapping spreading center osc area on the fast spreading east pacific rise epr our observations are based on a new high resolution bathymetric map of the 9 n osc area derived from picks of the seafloor arrival on 3d seismic data and on previously published data that constrain the presence and distribution of melt below the 9 n osc differences in the orientation of structures between the seafloor and the magma chamber indicate a sharp change in principal stress directions with depth suggesting that the brittle crust above the melt sill is decoupled from the melt sill itself and the ductile crust underlying it the stress field within the brittle upper crust results from a local interaction of the two overlapping spreading centers whereas the stress field in the crust below the melt sill corresponds to the regional stress field imposed by plate separation given this mechanical structure of the crust the melt sill shape and location appear to be controlled by the following factors the location of the deep melt source below the melt sill the ambient stress field at the depth of the melt sill and the stress field in the brittle upper crust above the melt sill which thermally shapes the roof of the melt sill through repeated eruptions

three dimensional seismic structure of the dragon flag oceanic core complex at the ultraslow spreading southwest indian ridge 49 39 e the southwest indian ridge swir is an ultraslow spreading end member of mid ocean ridge system we use air gun shooting data recorded by ocean bottom seismometers obs and multibeam bathymetry to obtain a detailed three dimensional 3 d p wave tomographic model centered at 49 degrees 39 e near the active hydrothermal dragon flag vent results are presented in the form of a 3 d seismic traveltime inversion over the center and both ends of a ridge segment we show that the crustal thickness defined as the depth to the 7 km s isovelocity contour decreases systematically from the center similar to 7 0 8 0 km toward the segment ends similar to 3 0 4 0 km this variation is dominantly controlled by thickness changes in the lower crustal layer we interpret this variation as due to focusing of the magmatic activity at the segment center the across axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting a locally corrugated oceanic core complex dragon flag occ on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient we infer that this occ may be predominantly made of gabbros we suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections hydrothermal activity at the dragon flag vents is located next to the detachment fault termination we infer that the detachment fault system provides a pathway for hydrothermal convection

solution slip and separations on strike slip fault zones theory and application to the mattinata fault italy we present a set of relationships to determine the component of slip and separations generated by the cleavage controlled volume contraction in strike slip fault zones the fault walls can translate toward each other along the cleavage normal axis of maximum shortening as rock is dissolved by pressure solution along patterned cleavage surfaces within strike slip fault zones the fault zone shortening produces an apparent slip and possible separations of reference stratigraphic surfaces across the fault zone solution related slip and separations can differ in magnitude and have either the same or the opposite sense these discrepancies depend upon the amount of fault zone shortening and upon the angles between the fault and the shortening axis and between the fault and the reference stratigraphic surface separations can be considerable at any scales even for very low amounts of fault zone thinning apparent slip can be appreciable for large amounts of fault zone thinning and or high fault to cleavage incidence angles with the proper geometrical conversions the relationships here presented can apply to any fault type the application of this technique to the left lateral mattinata fault italy demonstrated that both left and right lateral strike separations can occur along the fault even for low amounts of fault zone contraction by rock dissolution q 2002 elsevier science ltd all rights reserved

stratigraphic uncertainty and errors in shortening from balanced sections in the north american cordillera shortening from balanced cross sections is typically cited as a minimum estimate because of the uncertainty in the position of eroded hanging wall cutoffs we show here using area balancing that the single most significant source of error is that associated with the shape and thickness of the initial stratigraphic wedge involved in the deformation new methods and software are introduced that in combination with freely available scans of virtually all geologic maps published by the u s geological survey usgs and geological survey of canada gsc allow one to make hundreds of map thickness measurements in just a few hours these methods are applied to geologic maps and cross sections from the western north american cordillera to determine the uncertainty of shortening in iconic cross sections by r a price and p r fermor and by f royse one sigma stratigraphic thickness uncertainty measured in homoclinal dip packages of prominent paleozoic units within single thrust plates and single quadrangles ranges from 20 to 30 of average thickness in the costigan and sulfur mountain plates in southern canada and can be as high as 37 in the darby and absaroka plates of the idaho wyoming united states thrust belt assuming that all individual paleozoic units in the cross sections have similar errors the entire paleozoic section used in the area balancing would have 8 11 uncertainty assuming a normal or gaussian distribution because the stratigraphic wedges in southern canada and idaho wyoming do not have a uniform original taper our basic area balance underestimates shortening by balancing the non uniform taper back to a uniform taper we can account for this discrepancy total shortening determined in this way is 104 17 km which is identical to that obtained by price and fermor the idaho wyoming section of royse with maps by the usgs yields similarly large errors in shortening magnitude 56 10 km for the three frontal thrust faults of those errors uncertainty in the shape and thickness of the initial stratigraphic wedge accounts for 56 and 70 of the total in southern canada and western wyoming respectively eroded hanging wall cutoffs account for just 15 in both areas with the remainder coming from uncertainty in the subsurface geometry including the position of the decollement

strain variation in thrust sheets across the sevier fold and thrust belt idaho utah wyoming implications for section restoration and wedge taper evolution abstract although strain in fold and thrust belts is typically inhomogeneous on the largest scale strain within thrust sheets shows regular patterns the strain which is a quantitative measure of the deformation of any thrust sheet is independent of the translation of the sheet the internal deformation can be represented by a deformation profile the change in shape during thrusting of a fault perpendicular line which can then be used to unstrain the sheet during restoration in cross section balancing deformation profiles in well studied thrust sheets show regular geometric patterns and compatibility constraints can be used to minimize the data needed to define the profiles in other sheets from the geometry of deformation profiles and direct field observations the most important component of internal deformation of thrust sheets in many fold and thrust belts is early layer parallel shortening lps this can be removed in a straightforward manner during restoration and significantly affects the geometry i e taper of the restored section original basin wedge taper may have important implications for the mechanical evolution of fold and thrust belts thus changes in original basin taper resulting from strain removal may affect significantly the way in which we interpret the evolution of a fold and thrust belt in an area

magmatic and tectonic segmentation of the intermediate spreading costa rica rift a fine balance between magma supply rate faulting and hydrothermal circulation 3 d tomographic modelling of wide angle seismic data recorded at the intermediate spreading costa rica rift has revealed a p wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non transform discontinuity at the ridge axis this low velocity zone displays a maximum velocity anomaly relative to the background ridge axis crustal structure of 0 5 km s 1 has lateral dimensions of 10 5 km and extends to depths 2 5 km below the seabed placing it within layer 2 of the oceanic crust we interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre that results in higher upper crustal bulk porosity and permeability evidence for ongoing magmatic accretion at the costa rica rift ridge axis takes the form of an axial magma lens beneath the western ridge segment and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system we propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate these fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present the costa rica rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading the characteristics inherited from each spreading mode have been preserved in the crustal morphology off axis for the past 7 myr using potential field data we contextualize our seismic observations of the axial ridge structure at the whole segment scale and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off axis may also be apparent along axis and that the current larger scale magma supply system at the costa rica rift may be relatively weak based on all available geophysical observations we suggest a model for the inter relationships between magmatism faulting and fluid circulation at the costa rica rift across a range of scales which may also be influenced by large lithosphere scale structural and or thermal heterogeneity

spatial distribution and temporal evolution of crustal melt distribution beneath the east pacific rise at 9 10 n inferred from 3 d seafloor compliance modeling determining the melt distribution in oceanic crust at mid ocean ridges is critical to understanding how magma is transported and emplaced in the crust seafloor compliance deformation under ocean wave forcing is primarily sensitive to regions of low shear velocity in the crust making it a useful tool to probe melt distribution analysis of compliance data collected at east pacific rise between 9 and 10 n through 3 d numerical modeling reveals strong along axis variations in the lower crustal shear velocities as well as temporal variation of crustal shear velocity near 9 48 n between measurements spanning 8 years compliance measured across the rise axis at 9 48 n and 9 33 n suggest a deep crustal low velocity zone beneath the ridge axis with a low vs vp ratio consistent with melt in low aspect ratio cracks or sills changes in compliance measured at 9 48 n between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval perhaps following the 2005 2006 seafloor eruption this temporal variability provides direct evidence for short term variations of the magmatic system at a fast spreading ridge

asymmetric melt sills and upper crustal construction beneath overlapping ridge segments implications for the development of melt sills and ridge crests a new three dimensional tomographic velocity model and depth converted reflection images of the melt sills beneath the 9 03 n overlapping spreading center on the east pacific rise show that the upper crustal construction at this ridge discontinuity is highly asymmetric with reference to the bathymetric ridge crests of the overlapping limbs despite the similarly curved ridge crests the asymmetries are markedly different under the two limbs and appear to be related to the contrasting evolutionary history of the limbs the overlap basin is closely related to the propagating eastern limb in terms of its seismic structure by contrast the western limb forms a distinct morphologic region that displays little structural relationship to the adjacent overlap basin and other relict basins as the overlapping spreading center is migrating southward the differential development of melt sills and ridge crests may be inferred from the results of this study ridge propagation appears to involve two major processes the advancement of the melt sill at the ridge tip and the development of ridge crest morphology and the neovolcanic axis to the north of the overlap basin region near the existing propagating limb the latter process may result in the abandonment of the current neovolcanic axis leading to the self decapitation of the propagating limb by contrast the self decapitation of the western limb is related to the receding melt sill which lags behind the anticlockwise rotational motion of the ridge crest

conditions during syntectonic vein formation in the footwall of the absaroka thrust fault idaho wyoming utah fold and thrust belt abstract the twin creek limestone in the footwall of the absaroka thrust sheet contains three sets of bed normal syntectonic calcite veins vein formation occurred during cretaceous motion along the absaroka thrust fault as indicated by 1 crosscutting relationships among these vein sets 2 a previously dated solution cleavage and 3 calcite twin analysis fluid inclusions in the veins and overburden estimates constrain inclusion entrapment temperatures to be between 175 c and 328 c results from stable oxygen isotopes indicate that the host and vein fluid compositions were in near isotopic equilibrium applying both reasonable geothermal gradients and constraints on overburden temperature yields fluid pressures during vein precipitation that are near hydrostatic all data taken together suggest both that vein formation within the twin creek formation occurred in a relatively closed system and that the veins filled near hydrostatic fluid pressure because the veins fill precursory cracks vein filling might not reflect the maximum fluid pressure that existed during the complete vein forming process

crustal structure accretionary processes and rift propagation a gravity study of the intermediate spreading valu fa ridge lau basin summary the valu fa ridge is an intermediate spreading full rate of 60 mm yr 1 ridge located in the lau basin in 1995 this ridge was surveyed using a multidisciplinary geophysical approach to image crust and upper mantle structure with the aim of investigating the processes of oceanic crustal accretion in a back arc tectonic environment as part of this experiment a network of gravity profiles was acquired together with seismic magnetic swath bathymetry and controlled source electromagnetic data presented in this paper are the results of forward modelling of a subset of the acquired gravity profiles two oriented ridge perpendicular and one ridge parallel using the preferred seismic models of turner et al 1999 as a basis of initial model construction in addition the gravity data set in its entirety has been used to calculate the mantle and residual mantle bouguer anomalies with the aim of investigating variability in crustal structure both density and layer thickness and the nature of the underlying upper mantle of particular interest are the overlapping spreading centre between the central and northern valu fa ridges where seismic modelling implies a generally thickened crust and a magma chamber located beneath the overlap basin rather than separate chambers supplying each ridge and the propagating rift tip and associated basin bounding pseudo fault modelling results suggest that the pre and post rift crusts have different compositional origins with lower densities required 12 km off axis to fit the observed free air gravity anomaly the locations of the transitions into regions of lower density correspond with those of turner et al 1999 derived from seismic modelling which in turn correspond in location to the rift related pseudo fault identified by wiedicke collier 1993 calculation and interpretation of the mantle and residual mantle bouguer anomalies also confirms the lower off axis densities and indicates a general increase in crustal thickness northwards towards the overlapping spreading centre between the central and northern valu fa ridge segments the presence of thicker crust corresponding to a region of negative mantle and residual mantle bouguer anomaly implies that the region surrounding the overlapping spreading centre has been in the recent geological past or is presently the site of an increased magma supply the residual mantle bouguer anomaly also reveals features related directly to off axis lateral variation in density and layer thickness associated with the southward propagation of the valu fa ridge into the havre trough modelling of the north south variation in crustal thickness along axis shows that the anomaly trend can be explained simply by thicker crust beneath each of the valu fa ridge segments and the overlapping spreading centre between the northern and central valu fa ridges the observation of segment centred crustal thickening is consistent with models of ridge segmentation that suggest that melt influx is located towards segment centres and that segment length is controlled by the maximum lateral extent to which this melt can flow macdonald et al 1988 tolstoy et al 1993 however the overlapping spreading centre remains anomalous from these models in that it also appears to be associated with thicker crust and an increased melt supply

evaluation of morphological indicators of magma supply and segmentation from a seismic reflection study of the east pacific rise 15 30 17 n hydrosweep bathymetry and multichannel seismic reflection data from two contrasting segments of the east pacific rise 15 30 17 n are used to assess the relationship between crustal structure morphological indicators of magma supply and ridge segmentation from stacked and migrated seismic profiles we evaluate the width and depth of the axial magma lens and the geometry of the seismically inferred extrusive crust layer 2a in contrast to other studies correlation between axial morphology and magma lens characteristics is found with a lens that on average is twice as wide and slightly shallower 100 200 m beneath the unusually shallow and broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment however large local scale variations in lens depth and width are also observed that are not related to morphology we conclude that magma supply contributes to the regional characteristics of the magma lens beneath a ridge segment with large variations within a segment due to local processes of magma eruption and delivery a negative correlation between zero age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment in this innermost axial region above the magma lens factors such as magma pressure not magma volume in the lens may govern the thickness of extrusives that accumulate the width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment 5 8 km and a narrower zone 2 5 km of minor thickening less than double along the northern segment and toward ridge segment ends this relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply we find a close correspondence between discontinuities in the narrow axial summit trough and changes in magma lens presence and geometry indicating a genetic link between the finest scale tectonic segmentation of the ridge and segmentation of the magma lens in several locations discrete magma lenses at different levels within the crust offset by several hundred meters are imaged beneath adjacent fourth order ridge segments

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

miocene marine nonmarine correlations atlantic and gulf coastal plains north america abstract a survey of the records of the remains of miocene land mammals from nearshore marine deposits on the atlantic and gulf coastal plains of the united states reveals a number of occurrences where the relationship between mammalian and invertebrate biochronologies can be directly compared vertebrate paleontologists have constructed and calibrated a time scale of geochrons north american mammal ages and informal subages based on the occurrence of certain mammalian genera this scale can be compared directly with neritic molluscan diatom and foraminiferal biostratigraphies that in turn can be calibrated with the neogene planktonic foraminiferal zones in the warm water provinces surrounding southeastern north america such cross correlations allow the tracing of geochrons from the land to sea as well as across facies boundaries within these major environments the demonstration of the wider utility of the mammalian time scale in correlation and the testing of correlations proposed by other disciplines is an important conclusion from this study

distribution of magma beneath the east pacific rise near the 9 03 n overlapping spreading center from forward modeling of common depth point data we have reprocessed six cross axis and three along axis common depth point cdp profiles near the 9 03 n overlapping spreading center osc to understand the relationship between axial magma chamber amc width and seafloor morphology travel time modeling of the amc reflector reveals an asymmetric distribution of melt across the 9 03 n osc the variation of modeled amc width beneath either osc limb is minimal but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4 15 km near the 9 17 n ridge axis discontinuity additionally melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis neovolcanic zone but is displaced toward the western rise flank depth migration based on a continuum velocity model consistent with postcritical reflections from the base of layer 2a places the skewed amc reflector beneath a nearly constant thickness sheeted dike section which dips 10 away from the rise axis to confirm amc continuity beneath the western rise flank we use the maslov synthetic seismogram method to show that amplitude enhancement of the amc reflector is consistent with a continuous melt body underlying a thickening extrusive layer analysis of along strike cdp profiles indicates an amc which is neither overlapping nor discontinuous when projected onto the along strike plane identifying intracrustal events on along axis cdp lines however requires extreme caution we have modeled out of plane scattering using a kirchhoff formulation and we show that a coherent event identified beneath the overlap basin results from diffraction off the amc which lies nearly 3 km to the west of the profile we attribute the asymmetric pattern of melt to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust in this model melt ascends upward buoyancy forces until deflected by the impermeable sheeted dike complex melt then migrates updip beneath the base of the sheeted dikes toward the neovplcanic zone where fissuring produces a temporary conduit for emplacement discrete jumps in modeled amc width toward the overlap basin represent a further displacement defocusing of melt supply western amc edge relative to the neovolcanic zone eastern amc edge the asymmetric pattern of melt therefore represents a gradual en echelon accommodation of melt supply across the 9 km of ridge axis offset at 9 03 n thus for asymmetric configurations amc width may not correlate solely with magmatic robustness but may signify the amount of decoupling which exists between melt supply and extrusive emplacement within the neovolcanic zone here we present a new model for osc development which invokes a significant component of cross axis melt migration moreover abrupt changes in amc width near ridge axis discontinuities e g 9 17 n deviation in axial linearity suggest that any along axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length migration of melt proposed in some current models the transition of melt supply beneath the overlap basin might favor a continuous low velocity zone underlying this feature if true basin development may be related to the subsidence of a mechanically weak crustal lid the proposed model for osc development therefore views ridge axis discontinuities as the surficial response of misalignment and or defocusing of melt supply in the uppermost mantle

the axial topographic high at intermediate and fast spreading ridges abstract an axial topographic high is commonly observed at both fast spreading ridges and some segments of intermediate spreading ridges at fast rates the axial high is primarily created by the buoyancy of hot rock and magma beneath the rise as newly formed crust is transported off axis little vestige of an axial high is observed on the ridge flanks in contrast at intermediate rates a significant component of the positive topography may be a volcanic construction preserved on the ridge flanks as abyssal hills which are split axial volcanoes we suggest this difference in the nature of the axial high reflects a lithosphere strong enough to support construction of a volcanic crestal ridge at intermediate spreading rates but only rarely at fast rates relict overlap ridges found within the discordant zones left by overlapping spreading centers is one class of ridge flank topography which appears to have a significant volcanic constructional component even at fast spreading ridges unlike topography away from these discontinuities the relief and shape of overlapping spreading centers is preserved as relict ridge tips are rafted onto the ridge flanks reduced magma supply at these discontinuities may give rise to an axial lithosphere strong enough to support volcanic construction of overlap ridges low axial lithospheric strength may also account for the lack of normal faults within the innermost 1 2 km of fast and some intermediate spreading ridges with a thin weak brittle layer at the ridge crest tensile failure will predominate and few normal faults will form depths to the axial magma chamber reflector observed in multi channel seismic data limit the thickness of the brittle layer on axis to less than 1 2 km for much of the epr this depth is comparable to depths over which tensile failure within the oceanic crust will predominate estimated from the griffith criteria for fracture initiation 0 5 1 5 km as the brittle layer thickens strengthens away from the ridge shear failure begins and the large scale normal faults associated with abyssal hill relief develop

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

strain studies of cleavage development in the chelmsford formation sudbury basin ontario abstract cleavage formation in the precambrian chelmsford formation of the sudbury basin ontario is directly related to the finite strain and deformation path finite strain was measured at the outcrop scale from the shapes and orientations of deformed concretions within sandstones at the microscopic scale from the relative positions of quartz grain centers and via quantitative correlations between the anisotropy of magnetic susceptibility and strain two dimensional strains measured in different sections at each site were combined into three dimensional strain ellipsoids at 16 sites distributed throughout the chelmsford formation outcrop using the magnetic susceptibility correlation a further 527 individual strain ellipsoids were obtained throughout the sites a prominent strain gradient is mirrored by the progressive development of cleavage in the chelmsford formation with strain and fabric development increasing in intensity from nw to se at the least deformed sites cleavage is weak or absent the finite strain reflects a combination of vertical compaction followed by coaxial tectonic layer parallel shortening acting in a subhorizontal nw se direction pressure solution is the dominant deformation mechanism responsible for the formation of cleavage material removed by pressure solution is redeposited as pressure fringes on quartz grains an observation which is compatible with the form of the deformation path which indicates little effect due to volume loss in cleavage formation in these sandstones folds within the chelmsford formation indicate formation by buckling but have very low limb dips decomposition of the finite strain tensor into a compactional part and a layer parallel shortening part suggests that fold initiation and amplification was preceded by about 38 layer parallel shortening the large number of strain ellipsoids obtained from the chelmsford formation provide a significant data base from which a number of important tectonic problems can be examined including the predeformational shape of the sudbury structure

the foreland deformation in the south western basque cantabrian belt spain abstract in the south western portion of the cenozoic basque cantabrian belt structural data collected in pre orogenic upper cretaceous limestones were combined with aerial images interpretation in flat lying areas observations about fracture strike type orientation and overprinting relationships both at the macro and mesoscales allow to constrain the pre thrusting and pre folding deformational sequence that affected the area when it was forming part of the foreland basin wnw ese striking extensional structures represent the oldest elements developed during the very late stages of the mesozoic rifting associated with the opening of the bay of biscay the onset of convergence is marked by the development of extensional structures striking roughly nnw sse and wsw ene which are associated with belt parallel stretching in the foredeep and outer arc extension in the peripheral bulge respectively and developed in a non fluid assisted environment these elements are postdated by a pre folding layer parallel shortening pattern which includes wsw ene striking pressure solution cleavages and reverse faults both of them frequently reactivating inherited early orogenic wsw ene striking extensional elements and right lateral reactivation of wnw ese striking pre orogenic extensional structures this stage was followed by a syn to post folding e w oriented belt parallel compression wnw ese striking left lateral faults being the most outstanding elements associated with this stage the presence of such elements in upper cretaceous sediments provides support to the idea that left lateral shear zones of the area related with a cenozoic intrabelt lateral extrusion tectonics our results indicate coherently with previously published works in the pyrenean orogen that most of the mesoscale deformation has been acquired before layer s tilting

fluid rock interactions associated with regional tectonics and basin evolution an integrated approach consisting of fracture analysis petrography carbon oxygen and strontium isotope analyses as well as fluid inclusion micro thermometry led to a better understanding of the evolution of fluid rock interactions and diagenesis of the upper permian to upper triassic carbonates of the united arab emirates the deposited carbonates were first marked by extensive early dolomitization during progressive burial the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture filling dolomite quartz and calcite cements after considerable burial during the middle cretaceous sub vertical north south oriented fractures f1 were cemented by dolomite derived from mesosaline to hypersaline fluids upon the late cretaceous maximum burial and ophiolite obduction sub vertical east west fractures f2 were cemented by dolomite dc2 and saddle dolomite ds derived from hot highly saline fluids then minor quartz cement has precipitated in fractures from hydrothermal brines fluid inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot 200 degrees c saline brines 20 to 23 nacl eq through one dimensional burial history numerical modelling the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200 degrees c tectonically driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the late cretaceous maximum burial and lasted until the oligocene miocene compressional tectonics and related uplift the circulation of such hydrothermal brines led to partial dissolution of dolomites dc2 and ds and to precipitation of hydrothermal calcite c1 in new mainly oriented north south f3 and pre existing reactivated fractures the integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history hence this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes which may subsequently affect reservoir properties

soils and landscape evolution of eolian plains the southern high plains of texas and new mexico holliday v t 1990 soils and landscape evolution of eolian plains the southern high plains of texas and new mexico in p l k knuepfer and l d mcfadden editors soils and landscape evolution geomorphology 3 489 515 sheets of eolian sediment cover many areas of the earth s surface sand seas dune fields and loess sheets being the best known examples of such features less well known are deposits of sandy eolian sediment forming extensive plains an excellent example of such a region is the semi arid southern high plains northwest texas and eastern new mexico the level landscape of the area was created by depositon of multiple extensive 80 000 km 2 sheets of eolian sediment biackwater draw formation over the past 1 4 ma this deposit grades from sandy southwest to silty and clayey northeast and is up to 27 m thick surface soils at least 30 000 and possibly 120 000 years old are well developed 5yr hues agrillic horizons 1 2 m thick with prismatic structure stage ii iii calcic horizons and are generally paleustolls and paleustalfs with some paleargids and haplargids morphologic variation is due mainly to textural variation of the eolian parent material although locally thickness of the parent material and wind erosion and cumulization are important factors and locally slight variation in effective precipitation may be significant the blackwater draw formation contains as many as six well developed buried soils each formed in individual layers of eolian sediment similar to or more strongly expressed 2 syr hues higher illuvial clay content than the regional surface soils the presence of the buried soils indicates that sedimentation was episodic and separated by long periods of relative landscape stability eolian processes also appear to have been important during the periods of stability and pedogenesis by providing clayey calcareous dust that was added to the soil promoting formation of the argillic and calcic horizons the sedimentologic and pedologic uniformity of the deposit suggests that the regional environment has not varied significantly during the quaternary except for periods of increased sedimentation or wind deflation underlying the blackwater draw formation is an upper tertiary deposit up to 36 m thick of eolian sand silt and clay ogallala formation this deposit contains buried soils very similar to those in the blackwater draw formation suggesting that the geomorphic processes that created the quaternary landscape of the southern high plains began to operate in the late tertiary perhaps as much as 11 million years ago

ductile to brittle transition in shear during thrust sheet emplacement southern appalachian thrust belt abstract mesoscopic structures in anchimetamorphic t 200 300 c strata of the pulaski thrust sheet southern appalachian thrust belt developed in progressive heterogeneous simple shear near the ductile to brittle transition shear 3 was localized in weak anisotropic pelitic rocks rome formation along the base of this 5 11 km thick thrust sheet folds which vary from upright and open to isoclinal and nw facing developed during ductile shearing and display a correlation between tightness and axial surface dip movement along brecciated thrust zones which evolved progressively from zones of greatest ductile strain resulted in low angle truncation of fold axis trends coaxial refolding of earlier structures and imbrication of the thrust sheet transient variations in fluid pressure pf controlled the mechanical behavior of the thrust sheet systematic veins imply pf 3 t t tensile strength during ductile deformation whereas later non systematic vein arrays in high strain zones record periods of nearly hydrostatic stress elevated pf which led to fracturing dilation and fault initiation appears confined to pelitic zones within the rome formation this coupled with decreasing temperature resulted in the transition from ductile folding to brittle thrusting changing physical conditions probably reflect erosional unroofing during uplift and late paleozoic thrust sheet emplacement

recent dueling propagation history at the fastest spreading center the east pacific rise 26 32 s the east pacific rise epr 26 32 s located between the easter microplate and the juan fernandez microplate has the world s fastest spreading rate and tectonics characterized by dueling ridge propagation a glori b and sea beam 2000 side scan bathymetry survey together with other geophysical data collection was conducted aboard the r v melville covering an approximately 5 by 5 area centered on this part of epr the side scan and bathymetric data reveal the complete geometry of the propagator system the western and eastern ridges both curve inwardly and are overlapped 120 km with an offset to overlap ratio of nearly one several abandoned ridges and rifts are clearly mapped on the east flank and a few failed rifts are also mapped near the western ridge tip a fracture zone discovered south of easter island indicates a stage of stable ridge transform intersection in the past an inversion with magnetic anomaly vectors was carried out for the area a resultant magnetic boundary strike map was interpreted with the downward component profiles to construct a magnetic isochron map the spreading and propagation rates were estimated from the magnetic isochrons the net southward propagation rate of the western ridge is 120 km m y for the last 1 9 m y the spreading rate of the eastern ridge has been 150 km m y and its current propagation rate is estimated as 500 km m y toward the north intense asymmetric spreading as high as 30 is observed at both ridges together with the side scan and bathymetric data the magnetic lineations provide significant constraints on the evolution of the propagator system and a propagation model with cyclic rift failure was applied to model the tectonic evolution during the last 2 m y a normal ridge transform intersection evolved into a nontransform overlapping offset 1 95 ma and the eastern and the western ridges have propagated alternately the dueling propagation history is characterized by the dominant southward propagation of the western ridge and the transitory propagation events of the eastern ridge with a variety of overlap lengths widths and ridge curvatures in the transition zone

uplift induced residual strain release and late thrusting extension in the anaran mountain front anticline zagros iran abstract in this work we present and discuss the pattern of extensional faults of the anaran anticline lurestan province of the zagros iran one of the most aesthetically appealing mountain front anticlines of the zaros this 100 km long structure is affected by extensional faults occurring in different along strike sectors of the anticline running parallel and oblique to the local trend of the fold hinge line and having displacements of hundreds of metres relationships between fold growth and extensional deformation are investigated by quantitatively analysing the attributes i e curvature elevation and throw across faults of a 3d surface representing the top of the ilam formation results indicate that the main stage of extensional deformation occurred during folding in particular we deduce that extensional faulting was associated with the erosional uplift of the crestal sector of the anticline where exhumation of previously shortened strata implied release of residual compressive strain with consequent annihilation of the horizontal stress component paralleling the tectonic transport direction this determined the onset of a large scale extensional regime at the shallower structural levels of the growing anticline

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

migration of deformation fronts during progressive deformation evidence from detailed structural studies in the pennsylvania anthracite region u s a abstract detailed study of selected outcrops in the pennsylvania anthracite region has resulted in identification of five at least partly continuous stages of alleghanian progressive deformation these structural stages are in sequence a layer parallel shortening and top to the foreland layer parallel shear b a second episode of layer parallel shortening c flexural slip and flexural flow folding d fold modification and late thrust faulting and e late veining and fracturing at each outcrop an average shortening direction was determined for each observed structural stage the mean regional shortening direction for each structural stage was calculated and maps showing the spatial distribution of the shortening directions for each stage were constructed the mean regional shortening directions for the five structural stages successively rotated 25 30 clockwise during the evolution of the fold and thrust belt shortening directions for structural stages b and c over the region show a systematic clockwise change in orientation from hinterland to foreland indicating that these structural stages of progressive deformation were time transgressive on a regional scale

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

source of anomalous magnetization in area of hydrocarbon potential petrologic evidence from jurassic preuss sandstone wyoming idaho thrust belt the jurassic preuss sandstone which crops out in the central part of the wyoming idaho thrust belt on trend with a hydrocarbon producing region to the south has been previously identified as the source of anomalous magnetization in the area elsewhere anomalous magnetization in sedimentary rocks near hydrocarbon accumulations has been attributed to hydrocarbon engendered magnetic minerals but magnetization of the preuss is controlled by detrital magnetite evidence of a detrital origin for magnetite includes 1 concentration of magnetite grains along laminations containing other heavy minerals 2 the presence of exsolved ilmenite hematite and spinel in the magnetite grains and 3 titanium contents typical of igneous derived magnetite that detrital magnetite is responsible for the anomalous magnetization in the preuss is further indicated by the systematic eastward decrease in magnetite abundance corresponding to a similar eastward decrease in magnetic susceptibility and remanent magnetization of the unit petrologic and vitrinite reflectance studies indicate a complex low temperature 150 c or 302 f diagenetic history for the preuss nevertheless preservation of detrital magnetite the presence of diagenetically early ferric oxide minerals and the absence of sulfide minerals all indicate that the preuss has not experienced sulfidic reducing conditions common in areas of hydrocarbon seepage the marine carbon isotopic composition of calcite that cements most preuss sandstones dgr13c values ranging from 2 47 to 1 48 pmil is evidence that carbonate diagenesis also was not influenced by hydrocarbons the results of this multidisciplinary study of the preuss underscore the importance of similar studies when evaluating the sources of aeromagnetic anomalies in areas of hydrocarbon potential

continuous mantle melt supply beneath an overlapping spreading center on the east pacific rise tomographic images of upper mantle velocity structure beneath an overlapping spreading center osc on the east pacific rise indicate that this ridge axis discontinuity is underlain by a continuous region of low p wave velocities the anomalous structure can be explained by an approximately 16 kilometer wide region of high temperatures and melt fractions of a few percent by volume our results show that oscs are not necessarily associated with a discontinuity in melt supply and that both osc limbs are supplied with melt from a mantle source located beneath the osc we conclude that tectonic segmentation of the ridge by oscs is not the direct result of magmatic segmentation at mantle depths

late cenozoic marine deposition in the united states atlantic coastal plain related to tectonism and global climate abstract major hiatuses in upper cenozoic marine deposits in the united states atlantic coastal plain are recognized on the basis of molluscan faunal changes at erosional unconformities these hiatuses generally coincided with periods of global cooling and ice sheet formation such hiatuses provide information to supplement global climatic data major hiatuses are recognized within the early miocene 23 20 m y ago at the end of the middle miocene 11 10 m y ago at the end of the late miocene 6 5 5 m y ago at the end of the early pliocene 4 0 2 5 m y ago at the end of the late pliocene 1 9 or 1 8 m y ago within the pleistocene 1 1 0 5 m y ago and several times within the last 0 4 m y estimates of the amount of water contained in ice sheets at different times in the pliocene and pleistocene facilitate calculation of probable minimum sea levels on the coastal plain during different high stands of the sea the altitudes of dated shoreline deposits in the atlantic coastal plain show that the amount of uplift in the cape fear area has averaged at least 1 3 cm per 1000 years since the beginning of pliocene time the coastal plain of georgia has apparently experienced relatively little vertical deformation during this same time

layer parallel shortening across the sevier fold thrust belt and laramide foreland of wyoming spatial and temporal evolution of a complex geodynamic system abstract varying patterns of layer parallel shortening lps and vertical axis rotations from the thin skin sevier fold thrust belt to the thick skin laramide foreland of wyoming are quantified from integrated structural anisotropy of magnetic susceptibility ams and paleomagnetic analyses within the sevier belt widespread early lps was accommodated by spaced cleavage fracture sets minor folds and minor faults lps directions are subperpendicular to structural trends of systematically curved thrust sheets of the wyoming salient reflecting a combination of primary dispersion and secondary rotation during thrusting within the laramide foreland limited lps was accommodated mostly by minor faults with conjugate wedge and strike slip geometries lps directions in gentler fold limbs vary from perpendicular to acute with structural trends of variably oriented anastomosing basement cored arches steep forelimbs display more complex relations including younger fault sets that developed during evolving stress states and localized vertical axis rotations although internal strain is limited weak ams lineations defined by kinked and rotated phyllosilicates are widely developed and consistently oriented perpendicular to measured lps directions palinspastically restored lps directions corrected for paleomagnetically determined vertical axis rotations vary on average from w e in the sevier belt to wsw ene in the laramide foreland in detail lps directions display deflections related to primary sedimentary wedge geometry and basement fabrics lps in the sevier belt is interpreted to partly reflect stress transmitted from the hinterland through the growing orogenic wedge and topographic stress along the front of the wedge lps in the laramide foreland is interpreted to partly reflect basal traction during flat slab subduction beneath thick cratonic lithosphere with spatial temporal variations in stress trajectories related to basement heterogeneities and evolving fault systems evidence for significant late n s laramide shortening as proposed by multi stage shortening models is not observed

variability in oceanic crustal thickness and structure multichannel seismic reflection results from the northern east pacific rise multichannel seismic reflection data acquired between 8 50 and 9 50 n and between 12 30 and 13 30 n along the east pacific rise provide a three dimensional view of the young oceanic crust seafloor to moho reflection travel times vary by up to 0 9 s within our study areas the total range of crustal travel times in the 9 n area is 1 55 to 2 45 s the total range in the 13 n area is 1 60 to 2 05 s the variation is systematic indicating thinner crust locally associated with overlapping spreading centers oscs and in the 9 n area segment scale variation along crustal isochrons crustal travel time is found to be a valid proxy for oceanic crustal thickness outside of the axial low velocity volume thickness can be calculated from time to 500 m even in the axial region thickness can be calculated to 1 km if low velocity zone position is known crustal thicknesses calculated from travel times vary by 2 6 km in the 9 n area and by 1 5 km in the 13 n area the majority of this variation is attributed to seismic layer 3 the lower crust segment scale variation of 1 8 km 5 5 to 7 3 km thickness is observed in the 9 n area with thinnest crust formed between 9 40 and 9 50 n and thickest formed between 9 15 and 9 20 n results imply a three dimensional pattern of magma supply to the 9 n segment the osc at 9 03 n is associated with major disruptions of the segment scale pattern in the form of local thin areas within the discordant zone the smaller osc at 12 54 n is not associated with dramatic changes in thickness of the surrounding crust in the absence of oscs the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment thicker crust does not always correlate with shallower ridge bathymetry broader axial cross section or more negative mantle bouguer or subcrustal gravity anomaly variable thickness of the crust mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result we hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick moho transition zone and a subcrustal gravity low such a diapiric upwelling center appears to be now located beneath the east pacific rise near 9 40 to 9 50 n