crustal structure of a transform plate boundary san francisco bay and the central california continental margin wide angle seismic data collected during the bay area seismic imaging experiment provide new glimpses of the deep structure of the san francisco bay area block and across the offshore continental margin san francisco bay is underlain by a veneer 300 m of sediments beneath which p wave velocities increase rapidly from 5 2 km s to 6 0 km s at 7 km depth consistent with rocks of the franciscan subduction assemblage the base of the franciscan at 15 18 km depth is marked by a strong wide angle reflector beneath which lies an 8 to 10 km thick lower crust with an average velocity of 6 75 0 15 km s the lower crust of the bay area block may be oceanic in origin but its structure and reflectivity indicate that it has been modified by shearing and or magmatic intrusion wide angle reflections define two layers within the lower crust with velocities of 6 4 6 6 km s and 6 9 7 3 km s prominent subhorizontal reflectivity observed at near vertical incidence resides principally in the lowermost layer the top of which corresponds to the 6 s reflector of brocher et al 1994 rheological modeling suggests that the lower crust beneath the 6 s reflector is the weakest part of the lithosphere the horizontal shear zone suggested by furlong et al 1989 to link the san andreas and hayward calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust a transect across the continental margin from the paleotrench to the hayward fault shows a deep crustal structure that is more complex than previously realized strong lateral variability in seismic velocity and wide angle reflectivity suggests that crustal composition changes across major transcurrent fault systems pacific oceanic crust extends 40 50 km landward of the paleotrench but contrary to prior models probably does not continue beneath the salinian block a cretaceous arc complex that lies west of the san andreas fault in the bay area the thickness 10 km and high lower crustal velocity of pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby pioneer seamount the salinian block consists of a 15 km thick layer of velocity 6 0 6 2 km s overlying a 5 km thick high velocity 7 0 km s lower crust that may be oceanic crust cretaceous arc derived lower crust or a magmatically underplated layer the strong structural variability across the margin attests to the activity of strike slip faulting prior to and during development of the transcurrent pacific north american plate boundary around 29 ma

seismic images of the transition fault and the unstable yakutat pacific north american triple junction in southern alaska the pacific plate and yakutat terrane subduct beneath the north american plate along the aleutian trench and pamplona zone respectively and are sliding past each other at minimal rates along the transition fault as the deformation front of the pamplona zone stepped eastward during the pliocene pleistocene the pacific north american yakutat triple junction became unstable four recent seismic images reveal that the transition fault changes from a single strike slip boundary east of the deformation front to three strands that step increasingly seaward between the deformation front and the aleutian trench the southern two strands deform the pacific crust and the outermost of these became increasingly convergent sometime since 1 ma as demonstrated by young growth strata we propose that this internal deformation of the pacific plate is an attempt to re attain stability which can only be reached by creating a tectonic boundary collinear with the pamplona zone the plate reorganization will result in initiation of subduction such that a portion of former pacific crust will become accreted to the north american plate such accretion events caused by triple junction instability may be an important mechanism for transferring oceanic crust to continental margins

patterns rates and possible causes of saltmarsh erosion in the greater thames area uk the estuary dominated coast of the greater thames in england has experienced rapid lateral erosion and internal dissection of saltmarshes this paper provides an overview of saltmarsh development in this area and re examines the role of environmental and human forcing factors it draws on documentary evidence including historical maps survey data and time series data of forcing factors lateral marsh retreat began in the 19th century in the medway and blackwater estuaries followed by other estuaries in the greater thames region at the beginning of the 20th century the outer estuaries and the wider parts of the inner estuaries especially have experienced erosion erosion has been modest in wave sheltered areas e g the colne and the inner crouch in the 1960s and more widely the 1970s a phase of rapid erosion took place with erosion rates of up to ca 16 ha year 1 per site notably along the open coast of dengie and foulness and in the blackwater and thames estuaries at all sites vertical sediment accretion was well able to keep up with sea level rise over the past century evidence indicates that there may have been several causes for the erosion of saltmarshes these are notably land claim and embankment construction increasing the tidal range and current velocities and a continuous rise of especially high and extreme water levels the latest episode of rapid erosion in the 1970s is largely attributed to changes in the wind wave climate for example erosion at wave exposed sites coincided with a peak in high magnitude waves combined with a high incidence of southeasterly waves the study shows that many factors including natural forcing factors and human activities have to be taken into account when explaining saltmarsh development

seismically active fold and thrust belt in the san joaquin valley central california structural analysis of late cenozoic folds along the western and southern margins of the san joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics the folds have formed by the processes of fault bend folding and fault propagation folding which commonly occur in fold and thrust belts our structural interpretation attributes the seismically active coalinga and kettleman hills north dome anticlines to fault bend folding above a thrust fault s that steps up from a detachment within the franciscan assemblage to a detachment at the base of the great valley group this thrust does not reach the surface blind thrust and its slip is consumed in backthrusting and formation of subsurface folds under the san joaquin valley movement on the postulated thrust s would explain the cause of the may 2 1983 coalinga earthquake and the august 15 1985 avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes well documented examples of fault bend and fault propagation folding also occur at wheeler ridge in the san emigdio mountains and at kettleman hills south dome deformation associated with fold and thrust belts can be extremely complicated considerable fault movement can occur without accompanying surface rupture or shallow level faulting conversely surface rupture can occur that has no direct relationship to fault slip at depth an example would be flexural slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth consequently traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts our interpretation of the structural style that developed in central california during the late cenozoic requires that the strain associated with the transpressive motion between the pacific and north american plates be resolved into normal and tangential components thrust faulting and folding normal to the plate boundary and strike slip faulting parallel to the plate boundary san andreas fault the thrust faults root in a deecollement at the brittle ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction

tectonic and magmatic significance of strontium isotopic variations in cenozoic volcanic rocks from the western united states more than 800 sr isotopic analyses of cenozoic volcanic rocks have been compiled for the western united states and parts of northern mexico these data indicate significant variations in 87 sr 86 sr that reflect changes in tectonic evolution and differences in magmatic style that is origin and evolution of igneous rocks for this region as a function of time and space late cenozic volcanic rocks mainly basalts represent magmatism associated with essentially extensional tectonism over much of the western united states in general 87 sr 86 sr ratios in these basalts are lower than in early cenozic rocks but display systematic variations in space for the most part isotopic compositions of these rocks are considered representative of their respective mantle source regions if this is the case the mantle source regions are isotopically heterogeneous and seemingly reflect a compositional provinciality that is correlated with the age of crustal rocks in this region specifically ancient subcontinental lithospheric mantle may be characterized by higher 87 sr 86 sr ratios than the mantle beneath younger crustal terranes early cenozoic volcanic rocks mainly intermediate to silicic calc alkalic in composition formed while the western united states was undergoing regional compression associated with subduction along the pacific margin sr isotopic ratios in these rocks are generally higher and more variable than in the younger extensional regime magmas and they are highest in areas of the eastern cordillera where old sialic crust is present the isotopic data are in accord with the involvement of crustal material in the genesis and evolution of these calc alkalic magmas crustal contamination of primitive subduction related magmas and crustal anatexis due to advective heat transfer associated with their ascent are considered as dominant factors in early cenozic magmatism variables in the subduction process itself in the nature and age of crust and mantle wall rocks and in the amount and provenance of subducted sediments likely contribute to the diversity of subduction related magmatism

geology of the point arguello discovery chevron as operator for its partners phillips champlin and impkemix discovered the point arguello oil field in 1981 the discovery well the chevron et al p 0316 1 was drilled in federal waters 8 5 mi 13 7 km south of point arguello california delineation drilling has confirmed the discovery of a giant oil field with estimated recoverable reserves in excess of 300 million bbl of oil the oil field is located within a small depocenter at the southern edge of the offshore santa maria basin this local depocenter may contain over 15 000 ft 4 600 m of neogene rocks the point arguello accumulation is trapped in a large north northwest trending anticlinal complex the primary reservoir is the middle and upper miocene monterey formation composed of fractured cherts porcellanites siliceous mudstones and dolostones calculated fracture permeabilities range up to 3 darcys crestal wells have indicated productivities after acid of approximately 6 000 bopd

crustal structure along the strike of the offshore santa maria basin california abstract the geology of the central california offshore region contains the record of the early miocene transition of the western north american continental margin from a site of subduction to one of transform motion the fundamental elements of crustal structure and composition within the margin are defined by the seismic reflection image on the marine profile pge b 10 12 km of franciscan basement rocks c a 7 10 km thickness of oceanic crust seismic sequence analysis provides a foundation for interpreting the evolution of the offshore santa maria basin results indicate that the basin fill is predominantly miocene to pliocene in age and that subsidence began in early miocene times a late miocene to early pliocene shortening event led to faulting and folding of basin strata and to uplift of basement blocks the franciscan complex an accretionary wedge assemblage which floors the basin reveals little internal seismic structure however reflections from the franciscan oceanic crust boundary at 12 15 km depth supply evidence for the existence of an antiform with 2 3 km relief within the oceanic crustal layer this antiform probably formed during early miocene shortening of the oceanic crust alternatively the antiform could represent primary oceanic crustal topography such as seamounts or a fracture zone

precipitation generated oscillations in open cellular cloud fields cloud simulation is one of the most challenging tasks in regional to global scale climate modelling in many cases the physical mechanisms responsible for observed cloud dynamics are unknown making it difficult to simulate their structure and behaviour realistically feingold et al use satellite imagery and numerical models to show how downdrafts induced by precipitating clouds produce an open cellular cloud pattern characterized by low reflectivity that oscillates between different metastable states the resulting cloud structure forms an oscillating and self organizing cloud field as new cloud patterns emerge to replace the old ones cloud simulation is one of the most challenging tasks in regional to global scale modelling in many cases the physical mechanisms responsible for observed cloud dynamics are unknown making it difficult to realistically simulate their structure and behaviour these authors show that open cellular clouds characterized by low albedo can be created by precipitation driven downdrafts and that the resulting cloud structure forms an oscillating self organizing cloud field cloud fields adopt many different patterns that can have a profound effect on the amount of sunlight reflected back to space with important implications for the earth s climate these cloud patterns can be observed in satellite images of the earth and often exhibit distinct cell like structures associated with organized convection at scales of tens of kilometres1 2 3 recent evidence has shown that atmospheric aerosol particles through their influence on precipitation formation help to determine whether cloud fields take on closed more reflective or open less reflective cellular patterns4 5 the physical mechanisms controlling the formation and evolution of these cells however are still poorly understood6 limiting our ability to simulate realistically the effects of clouds on global reflectance here we use satellite imagery and numerical models to show how precipitating clouds produce an open cellular cloud pattern that oscillates between different weakly stable states the oscillations are a result of precipitation causing downward motion and outflow from clouds that were previously positively buoyant the evaporating precipitation drives air down to the earth s surface where it diverges and collides with the outflows of neighbouring precipitating cells these colliding outflows form surface convergence zones and new cloud formation in turn the newly formed clouds produce precipitation and new colliding outflow patterns that are displaced from the previous ones as successive cycles of this kind unfold convergence zones alternate with divergence zones and new cloud patterns emerge to replace old ones the result is an oscillating self organized system with a characteristic cell size and precipitation frequency

the effect of decollement dip on geometry and kinematics of model accretionary wedges we conducted a series of sand box models shortened asymmetrically above a frictional plastic decollement to study the influence of amount and sense of the decollement dip on the geometry and kinematics of accretionary wedges model results illustrate that the amount and direction of decollement dip strongly influence the geometry and mode of deformation of the resulting wedge in general for models having similar decollement frictional parameters the resulting wedge is steeper grows higher and is shorter when shortened above a decollement that dips toward the hinterland at 42 bulk shortening the length height ratio of wedges formed above a 5 dipping decollement was equal to 2 4 whereas this ratio was equal to 3 for wedges shortened above a horizontal decollement moreover models with a hinterland dipping decollement undergo larger amounts of layer parallel compaction lpc and area loss than models shortened above a non dipping decollement the effect of decollement dip on wedge deformation is most pronounced when basal friction is relatively high b 0 55 whereas its effect is less significant in models where the basal decollement has a lower friction b 0 37 model results also show that increasing basal slope has a similar effect to that of increasing basal friction the wedge grows taller and its critical taper steepens

tracing the subducted oceanic crust beneath the central california continental margin results from ocean bottom seismometers deployed during the 1986 pacific gas and electric edge experiment large aperture seismic data were collected on several ocean bottom seismometers obs deployed along a deep crustal seismic profile that was shot across the central california continental margin the line of shots extends from the oceanic crust seaward of the santa lucia escarpment to the california coast near morro bay and crosses the santa lucia basin santa lucia bank santa maria basin and hosgri fault zone the obs data permit us to trace the subducted oceanic crust from seaward of the santa lucia escarpment to beneath the central part of the santa maria basin a complementary study using onshore recordings of the same shots extends the crustal model onshore just seaward of the santa lucia escarpment the oceanic crust is subhorizontal and covered by a thin layer of low velocity sediment the velocity 4 5 km s and gradient 1 20 1 25 km s km of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern pacific beneath the santa lucia escarpment and santa lucia basin the oceanic crust dips approximately 16 to the east it is overlain by material with a velocity that increases from 4 8 to 6 4 km s at a depth of 1 7 5 5 km below the seafloor beneath the santa lucia basin a low velocity zone may be sandwiched between the subducted crust and this shallow high velocity material which we interpret to represent obducted oceanic crustal material beneath the eastern edge of santa lucia basin the dip of the subducted oceanic crust decreases to less than 2 the configuration of the subducted crust in this region is consistent with imbrication of the subducted crust as suggested by others based on common midpoint reflection data beneath the central santa maria basin the top of the subducted oceanic crust is at a depth of about 14 16 km and the moho is at 19 21 km

model for late mesozoic early tertiary tectonics of coastal california and western mexico and speculations on the origin of the san andreas fault cretaceous and early tertiary arc and forearc rocks found along the coast in southern california and baja california have been shown by paleomagnetic measurements to have originated many hundreds or even several thousands of kilometers south of their current locations northward transport also is found in cretaceous batholithic rocks near the edge of the continent in washington and british columbia the consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of north america have been translated along the coast by strike slip faulting strike slip faults are shown to be an expected response to north oblique subduction of the farallon and or kula plates faulting and subduction probably were concurrent activities i e the arc and forearc grew and were displaced simultaneously because displacement was dominantly parallel to subduction related lithic belts arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts subduction related strike slip faulting is shown to be favored by high angles of oblique convergence shallow dip of the subducting slab relatively easy slip on potential faults within the arc or forearc and the existence of a place for the moving sliver to go it is suggested that these conditions are not difficult to satisfy and that subduction related strike slip faulting may thus be a common feature of convergent orogens the location of the san andreas fault inboard from the continental margin may be attributable to localization of the pacific north american transform by a system of precollision subduction related faults

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

lithospheric evolution in the wake of the mendocino triple junction structure of the san andreas fault system at 2 ma as the mendocino triple junction mtj moves northwards up the north american margin the tectonic regime changes from subduction to strike slip for the first few million years following triple junction migration the san andreas fault system consists of several strike slip faults distributing deformation over a region 150 km wide this same region is expected to be affected by a slab gap beneath north america created by the northward removal of the subducting gorda plate and into which asthenospheric mantle is thought to rise to crustal depths the onshore and offshore mendocino triple junction seismic experiment mtjse provides a continuous seismic velocity reflectivity cross section across the deforming zone from the pacific ocean basin to the eastern edge of the california coast ranges the accretionary complex rocks that make up most of the crustal thickness are underlain by a 5 10 km thick high velocity 6 4 7 2 km s 1 layer at the base of the crust that extends from the pacific to at least 50 km and probably 90 km east of the san andreas fault the top of the lower crustal layer deepens from 7 km beneath the pacific ocean basin at the west end of the profile to 23 km at the east end by a gentle 5 10 eastward dip punctuated by abrupt offsets at the san andreas and maacama fault zones at each fault the top of the lower crust is offset by up to 4 km down to the east the moho is similarly deformed beneath the faults although by only 2 km such localized deformation of the moho implies that these two strike slip faults penetrate through the entire crust to the upper mantle good agreement between seismic velocity and seismic reflectivity in the vicinity of the faults gives confidence in these results although details of the offset beneath the san andreas fault are better resolved than those under the maacama fault seismic velocities in the upper mantle show only a small change along the profile from 8 1 km s 1 beneath the pacific to about 7 9 km s 1 beneath the coast ranges we infer that upwelling of asthenosphere into the slab gap is limited laterally or a lithospheric lid is present in the slab gap by 2 ma gravity data and crustal density structure show that most of the margin width is in local airy isostasy with the changes in crustal thickness near the strike slip faults corresponding closely to changes in surface topography the crustal blocks defined by the strike slip faults appear to be independently in isostatic equilibrium provided that the mantle beneath the coast ranges has a somewhat lower density than that beneath the pacific plate the densities in the coast range upper mantle are consistent with limited temperature elevation suggesting that the asthenospheric mantle is present beneath the depth of seismic energy penetration from our survey

marine deep seismic reflection profiles off central california a strong reflection horizon at two way travel time of approximately 6 s is observed in a deep seismic profile across the outer continental shelf of central california it is interpreted as the seismic image of subducted oceanic crust emplaced prior to the change from principally convergent to principally transcurrent motion between the pacific and north american plates during the late paleogene the reflector dips landward at a very shallow angle and is at a depth of 14 15 km under santa lucia bank the reflection is not observed or at best is very discontinuous under the inner shelf santa maria basin this suggests that tectonic or other processes have produced significantly different structural styles or compositions on the two sides of the santa lucia bank fault under the outer shelf a prominent apparently deeper later arrival time horizon dips more steeply and diverges from the 6 s reflector the deep horizon is at least partially composed of diffracted energy but is nearly linear after migration possible interpretations are that the horizon indicates crustal imbrication or out of plane diffractions alternatively it is a relict feature imparted to the crust at the now inactive pacific farallon spreading ridge reflective zones at intermediate depths are observed in apparently accreted sediments in parts of the santa lucia and santa maria basins these features could represent technically induced fabrics within the accretionary complex or they could be coherent depositional sequences

the kinematic paradox of the san andreas fault the pacific north america plate boundary along the san andreas fault system is notoriously a right lateral transpressive margin where both almost pure thrust and strike slip tectonics take place the pacific plate travels wnw forming an angle of about 25 with the boundary since the pacific is moving wnw faster than north america right lateral transtension should result along the san andreas system north america in turn travels westward obliquely to the boundary and a left lateral transpressive component would be expected along the same margin therefore the right lateral transpression of the san andreas system can be partitioned into i a sinistral transpression along the southwestern margin of the north america plate obliquely overriding ii a faster right lateral transtension occurring along the transfer margin of the pacific plate between the east pacific rise in the california gulf and the gorda ridge to the north west this is due to the oblique trend of the pacific and north america plate margins with respect to their motion in a absolute reference frame the geodynamics of california is marked by a unique setting in which there is a special subduction where in contrast with classic subduction zones the footwall of the subduction plane is obliquely diverging from the hanging wall in an e w section while it is converging at slower rates in a ne sw direction the extensional e w component is absorbed into the basin and range rifting whereas the compressive ne sw component is mainly expressed in the coast ranges and california offshore the compression perpendicular to the san andreas is then not intrinsic in the strike slip movement but it is rather an independent tectonic factor therefore the san andreas system cannot be considered as an archetype of a pure strike slip fault

the effect of calcium ions in pore water on the residual shear strength of kaolinite and sodium montmorillonite this paper reviews the literature concerning the effects of pore water chemistry on residual shear strength it presents the results of index limit and residual shear strength testing undertaken on kaolinite and sodium montmorillonite using concentrations of calcium chloride similar to the calcium levels measured in groundwaters obtained from landslipped calcareous mudrock of the fuller s earth formation the residual shear strength of the sodium montmorillonite increased markedly for the sample treated and tested with 80 mg 1 cat compared with that tested with deionized water greater concentra tions of ca2 resulted in only a marginal further increase in the residual shear strength these changes were also reflected in the interpreted parameters or and c the liquid limit of the montmorillonite decreased dramatically when the samples were treated with 80 mg 1 ca2 and this effect persisted and or was slightly enhanced when the concentration of ca2 was increased the residual shear strength

shortening within underplated oceanic crust beneath the central california margin structures within a tectonically underplated layer of early miocene age oceanic crust indicate that the lower crust of offshore central california has been deformed under compression prior to the inception of the san andreas transform system along the continental margin shortening in the lower crust is inferred from the geometry of two structures the santa lucia monocline and the santa maria antiform delineated by an integrated interpretation of multichannel and wide angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions the 700 km of multichannel seismic data and 300 km of wide angle seismic data from the 1986 pacific gas and electric edge seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the wnw trend of the antiform in cross sectional view the santa lucia monocline is a 30 kink in the oceanic crust while the santa maria antiform is characterized by local thickening of the oceanic crust these structures are probably the result of convergence between the monterey and arguello microplates that took place as subduction along the margin waned the multichannel seismic reflection data image two miocene age shelf basins the santa lucia and offshore santa maria basins that formed within a transtensional stress regime the offshore santa maria basin directly overlies the santa maria antiform suggesting that either the upper and lower crustal stress regimes were completely decoupled in miocene time or that the two features formed in different places differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic continental transform zone

evolutionary aspects of lithosphere discontinuity structure in the western u s 1 we have produced common conversion point ccp stacked ps and sp receiver function image volumes of the moho and lithosphere asthenosphere boundary lab beneath the western united states using transportable array data the large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities the moho is a nearly continuous topographic surface whereas the lab is not and the seismic images show a more complex expression the first order change in lab depth in the western u s occurs along the cordilleran hingeline the former laurasian passive margin along the southwestern precambrian north american terranes the lab is about 50 deeper to the east of the hingeline than to the west with most of the increase in lab thickness being in the mantle lithosphere we infer that the moho and the lab are late mesozoic or cenozoic everywhere west of the hingeline modified during farallon subduction and its aftermath between the hingeline and the rocky mountain front the lab and to a lesser extent the moho have been partially reset during the cenozoic by processes that continue today seismicity and recent volcanism in the interior of the western u s are concentrated along gradients in crustal and or lithospheric thickness for example the hingeline and the eastern edge of the coastal volcanic magmatic terranes to us this suggests that lateral gradients in integrated lithospheric strength focus deformation similarly areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness

coherent noise in marine seismic data three mechanisms that might produce noise patterns on stacked sections are examined direct and trapped waves that propagate outward from the seismic source cable motion caused by the tugging action of the boat and tail buoy and scattered energy from irregularities in the water bottom and sub bottom different noise patterns can be observed on shot profiles and common midpoint cmp gathers these patterns can be diagnostic of the dominant mechanism in a set of data field data suggest that the dominant noise is from waves scattered within the shallow sub bottom this type of noise is enhanced by cmp stacking moreover it can be as strong as surface wave noise on stacked land seismic data as well moveout filtering is best for suppressing the noise while preserving signal since the scattered noise does not exhibit a linear moveout pattern on cmp sorted gathers moveout filtering must be applied either to traces within shot records and common receiver gathers or to stacked traces our data example demonstrates that although it is more costly moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data dependent processing steps of predictive deconvolution and velocity analysis modified journal abstract

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

from mesozoic rifting to apennine orogeny the gran sasso range italy the apennines are a low temperature accretionary prism generated by the west directed subduction of the adriatic ionian plate whose structural origin is still to be fully understood the highest and best exposed seg ment of the apennines the gran sasso range is here documented to unravel the tectonic history of the northern tip of gondwana it is located along a ne verging salient of thrust sheets decoupling the sedimentary cover of the subducting adriatic lithosphere field mapping and structural analysis along the e w trending left lateral transpressive segment of the salient highlight the interplay of the inherited mesozoic passive margin stratigraph ic and tectonic framework with the neogene contraction the rheological differences between the massive carbonate platform and the well bedded turbiditic and pelagic limestones determined along strike undulations of the thrusts geometries and fold styles during shortening heterogeneities are due to inherited syn and post rift mesozoic tectonics the gran sasso overturned anticline shows a backlimb anomalously tilted toward the foreland and we infer this dip as being related to a deeper back thrust of a triangle zone the pinching out of the foredeep sequence on the growth anticline forelimb dates the contractional phases of the region to the late messinian from the late pliocene to present the area has been uplifted and extended about 2 km by oblique normal faults cross cutting the accretionary prism some of them are seismically active as shown by the 2009 mw 6 3 l aquila earthquake

a monolith sampler for saltmarsh sediments a method is described for the collection of unconsolidated salt marsh sediment resulting in no compaction and minimal disturbance to the sample the monolith sampler consists of an acrylic casing of optional length with a wide flat base and short right angled sides that is inserted into a cleaned vertical face use of this inexpensive technique allows observation of any structures present through the sediment record and provides sufficient material for various high resolution geochemical and sedimentological analyses

the dip of the foreland monocline in the alps and apennines the foreland monocline dips underneath thrust belts and accretionary wedges both in oceanic and continental subduction zones we present new data on the dip of the monocline in the frontal part of two orogens the alps and the apennines there is an overall difference between the dip of the relative monoclines and there is also a strong lateral variation along both arcs in the alps the regional dip varies between 0 in the remote foreland to an average of 2 3 at the front of the thrust belt below the foredeep to about 5 beneath the external thrust sheets within 40 km from the leading edge of the accretionary wedge the regional dip of the monocline in the apennines has an average of 4 5 at the front of the thrust belt below the foredeep to about 10 beneath the external thrust sheets within 40 km from the leading edge of the accretionary wedge there are areas where the dip exceeds 20 the apennines though topographically lower than the alps present higher monocline dips and a deeper foredeep moreover there are variations in the dip of the monocline moving along the strike of the two belts the low values coincide with permian mesozoic inherited horsts whereas the steeper values correspond to basinal areas and they usually match the salients of the thrust belt front within the salients the distance between thrust ramps increases therefore there are two orders of mean values of the dip of the foreland monocline the first at the orogen scale more than 1000 km wavelength the second at the regional scale 100 200 km wavelength within the single orogen lateral variations in the lithospheric buoyancy due to the inherited mesozoic stretching may explain the second order variations in foreland dip but not the first order mean values which seem to be more sensitive to the geographic polarity of the subduction rather than to the lithospheric composition which is rather similar in the alpine and in the central northern apennines slabs

migration of wide aperture onshore offshore seismic data central california seismic images of late stage subduction we conducted a high shot and receiver density onshore offshore seismic experiment in 1986 across the central california transform margin as part of the pacific gas and electric edge deep crustal geophysical survey the data set consists of land recordings of 406 marine air gun shots fired along a 70 km track across the santa maria basin the seismic vessel s track was collinear with 154 land seismic recorders deployed over 50 km across the sur obispo terrane and part of the salinian block the source receiver offsets range from 2 5 km to 120 km the unique recording geometry and high shot and receiver density permit imaging the deeper crustal structure of the california transform margin but pose serious challenges for conventional seismic imaging methods consequently we developed a unified approach to wide angle imaging which incorporates a tomography technique for obtaining initial migration velocity models and a prestack kirchhoff depth migration with depth focusing of the main reflectors for velocity model refinement and imaging this approach retains the velocity and dip information in the data and allows user control over the imaging process we applied this processing to subsets of the onshore offshore data to obtain an upper crustal velocity model and a 32 km long wide angle depth migrated image of the lower crust in the coastal region under the santa maria basin and the sur obispo terrane the migration image shows a part of an underplated oceanic crustal layer interpreted as the monterey plate meltzer and levander 1991 miller et al 1992 which dips coastward at depths of 18 to 28 km the plate appears to be kinked or imbricated beneath the coastline the image correlates well with existing wide angle seismic interpretations based on ray tracing walter and sharpless 1987 putzig 1988 howie et al 1993 the migrated lower crustal image has more structural information than is apparent in conventional ray based interpretations along the same crossing and adjacent profiles the crustal image demonstrates that automated imaging techniques applied to wide angle data can provide images of crustal structure comparable in fidelity to conventionally processed and migrated near vertical incidence data

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

decollement depth versus accretionary prism dimension in the apennines and the barbados 1 along representative cross sections of the apennines and the northern barbados accretionary prisms we measured the area the decollement depth the angle a of the upper envelope and the angle b of the dip of the regional monocline the continental sections of the apennines accretionary prism have a deeper decollement than the oceanic sections of the northern barbados 6 10 km depth and 1 km depth respectively because the sediment pile is thinner on the incoming barbados plate and its denser oceanic structure is more easily subducted considering the frontal 50 km the apennines have an average cross sectional area of 500 km 2 and the northern barbados ridge of 100 km 2 the total area is a function of the depth of the decollement plane therefore at a given amount of subduction the deeper the decollement depth is the bigger the area of the wedge will result assuming negligible compaction and erosion as a consequence the larger area volume and higher elevation of the apennines with respect to the barbados is determined by the apenninic deeper decollement despite these differences the geometry of both decollements is in some cases comparable in particular close to the boundary between the crystalline crust and the sediment pile where the main density and strength contrasts are concentrated variations in depth of the decollement occur moving along strike in both accretionary prisms the geometry of the prisms is further controlled by the different values of a and b their sum and the distance of the accretionary prism relative to the subduction hinge index terms 8099 structural geology general or miscellaneous 8150 tectonophysics evolution of the earth plate boundary general 3040 8122 tectonophysics dynamics gravity and tectonics keywords decollement depth accretionary prism apennines barbados citation bigi s f lenci c doglioni j c moore e carminati and d scrocca decollement depth versus

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

oligocene tectonics and sedimentation california abstract during the oligocene epoch california was marked by extensive nonmarine sedimentation in contrast to its pre oligocene and post oligocene depositional history the oligocene continental deposits are especially widespread in southern california and fill a number of small and generally partly restricted basins fluvial facies in many basins prograded over previously deposited lower tertiary turbidites volcanism from widespread centers was associated with the nonmarine sedimentation however some basins remained marine and a few contain oligocene turbidites and pelagic sediments deposited at bathyal depths the oligocene redbeds of california do not form a post orogenic molasse sequence comparable to the old red sandstone or alpine molasse they are synorogenic and record local uplift of basins and surrounding source areas late cretaceous to contemporary orogenesis in california has been generally characterized by the formation of small restricted basins of variable depth adjacent to small upland areas in response to strike slip faulting deposition of oligocene redbeds was associated with climatic change from warm and humid to cold and semiarid and a global lowering of sea level oligocene tectonism occurred during the transition from subduction of the farallon plate to initiation of the modern san andreas transform system however the major influence that caused uplift formation of fault bounded basins and extensive redbed deposition especially in southern california was the approach of the pacific farallon spreading ridge to the western margin of california

layer stripping reverse time migration1 we present a layer stripping method of migration for irregularly layered media in which first order velocity discontinuities separate regions of constant or smoothly varying velocity we use the reverse time method to migrate seismic data layer by layer from the surface downwards as part of the migration of a given layer the bottom boundary of the layer is defined based on power in the migrated signal and a seismic section is collected along it this new section serves as the boundary condition for migration in the next layer this procedure is repeated for each layer with the final image formed from the individual layer images layer stripping migration consists of three steps 1 layer definition 2 wavefield extrapolation and imaging and 3 boundary determination the migration scheme when used with reverse time extrapolation is similar to datuming with an imaging condition the reverse time method uses an explicit fourth order time tenth order space finite difference approximation to the scalar wave equation the advantages of layer stripping reverse time migration are 1 it preserves the benefits of the reverse time method by handling strong velocity contrasts between layers and steeply dipping structures 2 it reduces computer memory and saves computation time in high velocity layers and 3 it allows interpretational control of the image post stack layer stripping reverse time migration is illustrated with a synthetic cmp data example prestack migration is illustrated with a synthetic data set and with a marine seismic reflection profile across the santa maria basin and the hosgri fault in central california

crustal structure of the northwestern vizcaino block and gorda escarpment offshore northern california and implications for postsubduction deformation of a paleoaccretionary margin the vizcaino block is an anomalously shallow region of the western u s continental margin located southwest of the mendocino triple junction it originated as part of the accretionary prism of the north america plate and was transferred to the pacific plate in the miocene as the pacific north america plate boundary migrated 130 km eastward forming the gorda escarpment at its northern boundary we present hybrid crustal models for the northwestern part of the vizcaino block derived from marine seismic and gravity data the velocity and density structure of the northwestern vizcaino block are compatible with paleoaccretionary complex material similar to san simeon patton terrane overlying oceanic crust or a mafic layer the most significant result of our modeling is an abrupt increase in moho dip from 5 to 20 30 beneath the western edge of the oconostota ridge along the northwestern margin of the vizcaino block this moho dip is steeper than observed anywhere along the cascadia subduction zone indicating postsubduction deformation we suggest that the paleotrench was deformed by compression which reactivated preexisting thrust faults in the upper crust and thickened the crust within this apparent weak zone at least part of the deformation predates late pliocene pacific north america plate convergence and may result mainly from north south compression between the pacific juan de fuca plates across the mendocino transform fault north south compression continues today and may dynamically support the uplifted northern margin of the vizcaino block although the primary locus of deformation shifted to the relatively weak gorda plate sometime prior to 3 ma

autocompaction of shallow silty salt marsh clay abstract empirically derived algorithms are suggested by means of which it is possible to evaluate relations between physical properties in silty salt marsh clay a relation between loss on ignition sand content and bulk dry density in the topmost 5 cm was found to explain 84 of the variation of the latter the bulk dry density in the topmost 0 5 m was found to vary as a logarithmic function of depth below the surface controlled by two constants directly dependent on the bulk dry density in the topmost 5 cm integration of this function illustrates that mass depth msd z down core can be directly related to the bulk dry density of the surface layer for sediments with a surface bulk dry density between 400 kg m 3 and 800 kg m 3 the porosity was found to vary 10 with msd z as another logarithmic function the results allow for an evaluation of the use of marker horizons in the topmost layers right after such a layer has been spread on the surface sedimentation measured in length per time accretion will be significantly larger than for subsequent periods using marker horizons it is therefore not trivial to know the level of the marker as deeper located markers will indicate successively smaller accretion rates with the same sediment input

episodic north america and pacific plate motions the north america plate motion for the period 20 0 m y is modeled by two absolute angular velocity vectors with a change occurring at about 9 m y the model incorporates new geologic constraints based on 1 the recognition of a discontinuity in north america plate motion at 9 m y 2 a reexamination of the iceland and yellowstone hotspot traces 3 improved delineation of the present north america pacific relative motion and 4 a revised history of pacific absolute motion the two stage model is consistent with several features of the late cenozoic geologic history of north america including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of north america this plate model combined with a previously established pacific plate model makes specific predictions concerning the 15 0 m y history of the north america pacific plate boundary which compare favorably with the known history these models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates the implications of the north america plate model are the greatest for those events related to the 9 m y change in its motion among the geologic consequences of this change are rifting in the western united states intense compressional deformation along the northern caribbean plate boundary a hiatus in aleutian arc volcanic activity and a change in the orientation of the lesser antilles arc this picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances the predictive power of the model may help in interpreting the history of the north america plate boundary in those areas where clear observations are lacking an extra force needs to be applied to a plate to cause a change in plate motion by treating the plate as a rigid spherical shell overlying a uniformly viscous fluid we derive a simple class of forces sufficient to cause a given change in plate motion such force balance considerations indicate that an additional force applied in the northwest pacific region toward the northeast would result in the observed change in north america plate motion this force is surprisingly similar to that which apparently caused a large change in pacific plate motion about 5 m y later we propose that a common mantle process is responsible for both changes and is related to the ability of the northwest pacific subducted lithosphere to enter into the lower mantle

timing of late cenozoic volcanic and tectonic events along the western margin of the north american plate a number of well to relatively well dated significant volcanic and tectonic events along the western margin of the north american plate began ended or increased intensity at 16 1 m y b p 10 2 m y b p and 5 1 m y b p continental events at 16 1 m y b p were related to the upwelling of an elongated mantle diapir which created a widespread thermal disturbance east of the subducting farallon plate oceanic events at 16 1 m y b p were largely related to the approach of the east pacific rise toward the north american plate continental and oceanic events at 10 2 m y b p were related to the clockwise rotation of the pacific plate spreading direction and to the increased rate of relative motion between the pacific and north american plates most events at 5 1 m y b p were related to the opening of the gulf of california and to the oceanic plate reorganizations of which the gulf opening was a part different styles of response reflect differences between intraplate volcanic and tectonic subdomains pulses of volcanic activity do not seem to be directly or solely related to changes of plate motion increased volcanic activity at arcs and some intraplate volcanic settings may be indicators or precursors of significant tectonic transition intervals if so we may presently be in the midst of such a transition interval

clay mineralogy grain size distribution and their correlations with trace metals in the salt marsh sediments of the skallingen barrier spit danish wadden sea to understand the behavior of trace metals in the salt marsh at skallingen danish wadden sea we investigated a profile from surface to 25 cm depth of the salt marsh sediment focusing primarily on clay mineralogy and grain size distribution of the sediments and their relationship with trace metals the clay assembly of the sediment consists of illite kaolinite and much less chlorite and smectite the major clay minerals of illite kaolinite as well as chlorite correlate very poorly with all the trace metals investigated due probably to the weak competing strength of these clays compared with the other adsorbents and to low availability of the mobile trace metals in the system correlation between trace metals and clay minerals may therefore be used as an indicator in environmental assessment fine grain fractions of the sediment increased markedly after salt marsh invasion in about 1931 but decreased after 1964 this increase was contributed mainly by the grains finer than 20 m while the fraction of 40 63 m actually decreased soon after the invasion the relation between most of the trace metals and grain size distributions is close in general but far from linear the drastic turning point is observed at about 30 m around where the correlation coefficient r drops from 0 8 to 0 1 adsorption is the controlling mechanism for the behavior of trace metals in the salt marsh fe mn hydr oxides and organic matter play the key role

triple junction instability as cause for late cenozoic extension and fragmentation of the western united states regional extension in the western united states began soon after 30 m y b p the time of first interaction between the pacific and north american plates previous explanations for extension include lithospheric active processes and asthenospheric active processes the former are favored because of coincidence of timing with lithospheric interactions relative timing of volcanism and deformation and coincidence with pre existing structures although extension has occurred in part in a back arc setting it cannot be attributed to age of subducted crust or azimuth of subduction because both attributes favor back arc contraction characteristic behavior before 40 m y b p the mendocino triple junction has been unstable since its inception the continental margin was relatively straight before 30 m y b p and it has become more convex westward as the triple junction has migrated northward the continental margin and arc have been anchored to the subducting slab farallon juan de fuca whereas the triple junction must move parallel to the san andreas transform the combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the san andreas transform relative to the coast these effects result from the unstable geometry of the mendocino triple junction lack of a subducted slab slab window beneath the extended lithosphere enables asthenospheric rise but does not cause extension

deep crustal reflection profiling offshore southern central california deep crustal reflection profiling offshore southern central california shows that the offshore santa maria basin and adjacent shelf and slope are underlain by oceanic crust which underthrusts the margin on common midpoint cmp sections the top of the oceanic crust appears as a strong coherent reflection occurring at 6 s two way time twt the reflection can be traced continuously more than 50 km from the deep ocean basin eastward beneath the continental shelf depth conversion places the top of oceanic crust at 6 16 km depth with a dip of 8 toward the coast beneath the santa lucia high the crustal reflection breaks up into several east dipping segments above the oceanic crust an accretionary wedge increases in thickness from 3 km at the base of the slope to over 12 km beneath the santa lucia high the neogene sedimentary section above the accretionary prism exhibits relatively little deformation implying that most of the accretion occurred during pre neogene convergence between the north american plate and oceanic plates located to the west the offshore santa maria basin exhibits at least two periods of neogene deformation an early extensional phase accompanied by strike slip faulting occurred in the lower to middle miocene followed by compression and basin inversion in the upper miocene lower pliocene the development of structures in the basin began in the northwest and is progressively younger to the southeast faults identified in the offshore sedimentary column offset acoustic basement but are not imaged in the franciscan basement material below 2 3 s twt and do not extend to or offset the deep reflections we suggest that the oceanic crust served as a detachment surface above which the shallow deformation occurred neogene sediments in the offshore basins show only 1 3 km of shortening this implies that shortening due to neogene oblique plate motion between the pacific and north american plates extends east of the offshore santa maria basin and is distributed across the entire transform margin

stratigraphic significance of siliceous microfossils collected during nautiperc dives off peru 5 6 s the geological evolution of the northern peru convergent margin can be traced using samples collected during deep sea dives of the submersible nuutile in the paita area 5 6 s the sedimentary sequence was intensively sampled along the main scarp of the middle slope area it consists of upper miocene 7 9 ma to pleistocene siltstone sandstone and rare dolostone the age distribution of these samples is the basis for a new geologic interpretation of the multichannel seismic line cdp3 siliceous microfossils both diatoms and radiolarians show influence of both cold and temperate waters local species mixed with upwelling ones diatom assemblages studied from the npl 13 and npl 15 dives bear a strong resemblance to assemblages from the pisco formation of southern peru micropaleontological data from siliceous microfossils provide evidence for two main unconformities one is at the base of the quatemary sequence and the other corresponds to a hiatus of 1 myr separating the upper miocene 7 8 ma sediments from uppermost miocene 5 6 ma sediments during the past 400 kyr a wide rollover fold developed in the middle slope area associated with a major seaward dipping detachment fault a catastrophic debris avalanche occurred as the result of an oversteepening of the landward flank of the rollover fold the gravity failure of the slope recognized by seabeam and hydrosweep mapping displaced enough material to produce a destructive tsunami which occurred 13 8 f 2 7 kyr ago

effect of the northward migrating mendocino triple junction on the eel river forearc basin california structural evolution offshore northern california the gorda plate is subducting obliquely beneath north america the resulting complicated tectonic setting forms the southern end of the cascadia subduction zone the southern cascadia subduction zone and overlying eel river forearc basin lie just north of the unstable mendocino triple junction the neogene strata of the eel river basin record structural deformation caused by the underthrusting of the gorda plate as well as deformation generated by northward migration and encroachment of the mendocino triple junction three distinct deformation regimes are present in the eel river forearc basin 1 along the western margin of the forearc basin and within the foreslope of the accretionary prism thrust faults and anticlines record pliocene pleistocene shortening caused by subduction of the gorda plate 2 the southern part of the basin rotated counterclockwise in the late pleistocene resulting in modern transpressional deformation offshore humboldt bay the rotation and deformation are caused by north south convergence across the boundary between the pacific plate and the southernmost part of the forearc basin at the triple junction 3 the northeastern margin of the eel river basin is deformed by high angle faults with a component of strike slip motion that may represent the incipient northward propagation of the pacific north american transform system north of the triple junction

deep crustal seismology of continental margins this review discusses results from recent seismic experiments at continental margins conducted by u s investigators as well as those by non u s investigators necessary to place the results of u s workers into context continental margins may be characterized as either rifted transform or convergent rifted or passive margins form where continents are torn apart during continental drift transform margins accommodate transform plate motions along strike slip faults such as the san andreas fault at convergent margins oceanic crust is being thrust beneath or subducted under the continental crust all three types of margins are found in north america

saltmarsh soil evolution after land reclamation in atlantic estuaries bay of biscay north coast of spain abstract tidal wetland soils exhibit particular physical chemical properties governed mainly by tidal dynamics on the atlantic seaboard of the iberian peninsula a great swath of tidal wetlands which correspond to semi confined estuaries represents land reclaimed from the 18th century to the first half of the 20th century and used for urban development or agricultural production these interventions led to the end of tidal flooding in extensive areas of salt marshes causing a drop in the water table that triggered pyrite oxidization processes leaching of cations and salts and soil acidification in many cases these transformations have given rise to environmental problems such as the development of acid sulphate soils a resulting decrease in ph and the leaching of carbonates the organic carbon storage capacity has also been altered and the release of metal cations from sediments has caused toxic waters to appear in order to investigate the alterations caused by the historic reclamation of estuarine soils on the north coast of spain natural and reclaimed lands were mapped in two estuaries villaviciosa and san vicente de la barquera and zoned in accordance with flood frequency the different evolutionary paths of the two studied estuarine sites after reclamation can be assessed through the description and analysis of their soils despite reclamation some parts i e lower areas of the reclaimed lands do not remain fully isolated from tidal dynamics whose influence varies throughout the soil profiles that have developed since the strength of the morphological e g genetic horizons chemical ph metal cation concentration and functional balance of the carbon budget changes suffered by the original soils seem to depend strongly on the remaining tidal influence as well as the land use of the area

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

rayleigh and s wave tomography constraints on subduction termination and lithospheric foundering in central california the crust and upper mantle structure of central california have been modified by subduction termination growth of the san andreas plate boundary fault system and small scale upper mantle convection since the early miocene here we investigate the contributions of these processes to the creation of the isabella anomaly which is a high seismic velocity volume in the upper mantle there are two types of hypotheses for its origin one is that it is the foundered mafic lower crust and mantle lithosphere of the southern sierra nevada batholith the alternative suggests that it is a fossil slab connected to the monterey microplate a dense broadband seismic transect was deployed from the coast to the western sierra nevada to fill in the least sampled areas above the isabella anomaly and regional scale rayleigh and s wave tomography are used to evaluate the two hypotheses new shear velocity vs tomography images a high velocity anomaly beneath coastal california that is sub horizontal at depths of 40 80 km east of the san andreas fault a continuous extension of the high velocity anomaly dips east and is located beneath the sierra nevada at 150 200 km depth the western position of the isabella anomaly in the uppermost mantle is inconsistent with earlier interpretations that the isabella anomaly is connected to actively foundering foothills lower crust based on the new vs images we interpret that the isabella anomaly is not the dense destabilized root of the sierra nevada but rather a remnant of miocene subduction termination that is translating north beneath the central san andreas fault our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern sierra nevada where we find a lower crustal low vs layer consistent with a small amount of partial melt the high elevations relative to crust thickness and lower crustal low vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a 3 4 ma pulse of basaltic magmatism

estuarine fluvial floodplain formation in the holocene lower tagus valley central portugal and implications for quaternary fluvial system evolution abstract we present a brief synthesis of the quaternary fluvial record in the lower tagus basin central portugal concentrating on factors controlling infill and incision the holocene part of the record forms the focus of this paper and guides the questioning of the basic assumptions of the established quaternary fluvial evolution model in particular the link between sea level change and fluvial incision deposition we suggest that several incision aggradation phases may have occurred during glacial periods major aggradation events may overlap with cold episodes while incision appears to concentrate on the warming limb of climate transitions the complex stratigraphy of the quaternary record in the lower tagus valley is influenced by repeated base level and climate changes this paper submits the first chronostratigraphic framework for valley fill deposits in the lower tagus area sea level rise forced aggradation and controlled deposition of the fine grained sedimentary wedge underlying the low gradient lower tagus floodplain investigations have focused on the lower muge tributary where rapidly aggrading estuarine and fluvial environments were abruptly established 8150 cal bp as sea level rose base level at the valley mouth controlled the upstream extent of the fine grained backfill tidal environments disappeared abruptly 5800 cal bp when the open estuary at the muge confluence was infilled by the tagus river the decrease and final still stand of sea level rise led to floodplain stabilisation with peat 6400 5200 cal bp and soil formation 5200 2200 cal bp localised renewed sedimentation 2200 200 cal bp is linked to human activity

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

integrated crustal structure across the south central california margin santa lucia escarpment to the san andreas fault the continental margin of south central california has undergone different styles of deformation over the past 30 m y as the north america pacific plate boundary has evolved from subduction to transform tectonics results presented here identify strike slip and compressional structures in the lower crust that probably developed during this period of tectonic transition an integrated model of crustal structure along deep seismic line pg e 3 crosses the entire continental margin west of the san andreas fault near the latitude of san luis obispo a compressional wave velocity model was developed using coincident marine multi channel seismic and onshore offshore wide angle reflection refraction seismic data and was tested independently with gravity modeling a high velocity layer 6 8 7 0 km s about 6 km thick in the lower crust that extends east possibly as far as the san andreas fault is interpreted as tectonically underplated oceanic crust the upper and middle crust of the patton and sur obispo franciscan terranes extends to depths of 14 to 20 km the velocity and density modeling indicate lower average velocities and densities for the patton terrane supporting earlier interpretations that it is a distinct terrane within the franciscan complex abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the santa lucia escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 ma following the subduction of the pacific arguello ridge an imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust convergence between the monterey and arguello microplates during the final stages of farallon subduction about 24 to 16 ma probably caused the imbrication

seismicity of south central coastal california october 1987 through january 1997 earthquakes recorded along south central coastal california from october 1987 through january 1997 by the pacific gas and electric company9s 20 station central coast seismic network and the u s geological survey9s northern and southern california networks are used to study their relation to previous seismicity mapped quaternary faults and regional tectonic features we developed p and s wave velocity models station corrections and duration magnitude calibrations to process 1184 reliable earthquake locations and 212 well constrained focal mechanisms seismicity generally coincides with mapped quaternary faults and areas of tectonic uplift including the southern offshore santa maria basin and areas to the west and the uplifted blocks within the los osos tectonic domain comparison of our data to previous microearthquake locations 1969 1987 and larger m 5 historical earthquakes shows that these patterns have been persistent seismically quiescent areas have few or no mapped quaternary faults and typically are structural lows the base of the seismogenic zone is about 12 km in depth above the relic oceanic crust microearthquake locations and strike slip focal mechanisms document the strike slip nature of the hosgri fault zone and its step over to the san simeon fault along the western boundary of the los osos domain uplifting blocks within the los osos domain are most seismically active toward their western ends along the hosgri fault zone and they exhibit reverse and oblique faulting consistent with block uplift and interaction with the hosgri fault zone the observed patterns and character of microseismicity and macroseismicity clearly show transpressional deformation associated with the hosgri fault zone overprinted by rotation and westward migration of the western transverse ranges within this tectonic setting clockwise rotation of the western transverse ranges causes internal deformation of the los osos domain and westward migration of the ranges appears to deflect the hosgri fault zone westward and causes east west directed crustal shortening in the southern offshore santa maria basin

geological evidence for a global tectonic polarity inferences about plate motions over the past 40 ma based on structural and geodetic data suggest that there is a coherent movement direction on a global scale moreover a westward drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot spot reference frame independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative eastward or northeastward mantle flow also support this global tectonic polarity unfortunately most hot spot reference frames are based on assumptions which strongly limit their validity for example many combine magmatic sources originating from different depths in the mantle including hot spots located along oceanic ridges which are not fixed relative to one another using a hot spot reference frame filtered to exclude shallow hot spots and anomalous wet spots we would predict a much higher relative westward drift of the lithosphere plate motions are not random rather that they are controlled by the relative motion between lithosphere and asthenosphere and by gradients in lithospheric thickness and composition triangular shaped plates may experience rotations resulting from differential drag at the base of the lithosphere with respect to the asthenosphere e g the clockwise rotation of south america plates appear to follow an undulatory pattern of motion on a global scale and their poles of rotation fall into a main cone located near the earth s rotation poles plate rotations induced by triangular shape differential basal drag or lithospheric anisotropies along subduction zones may generate the scattered distribution of euler poles this might have inhibited the recognition of a coherent global tectonic pattern

deep structure and structural inversion along the central california continental margin from edge seismic profile ru 3 deep penetration seismic reflection profile ru 3 crosses the central california continental margin revealing a subducted oceanic plate a modified accretionary prism and complex structures of the overlying sedimentary basins this structural framework was established by subduction processes during paleogene and earlier time and subsequently was modified by neogene transform motion combined with apparent components of extension and compression subducted rocks are indicated by deep gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km we interpret the subducted crust as either a part of the pacific plate or more likely a subducted fragment derived from the farallon plate a set of more steeply dipping deep events may indicate faulting within the subducted plate or its boundary with a no slab zone the overlying largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast the santa lucia basin landward of the steep continental slope originated as a slope basin during paleogene subduction the lower strata of this basin were deposited onto and partially incorporated into the accretionary complex the offshore santa maria basin exhibits a variety of compressional structures that formed in the last 3 5 m y and whose locations correspond to an earlier framework of extensional faults structural inversion has occurred in miocene depocenters adjacent to the santa lucia bank fault and at the queenie structure miocene and lower pliocene strata also thicken toward the hosgri fault zone where subsequent compression is characterized by low angle thrusts and folding

precipitation onset as the temporal reference in convective self organization in a dry convective boundary layer convective patterns of typical scales spontaneously develop qualitatively similar to those in a fluid which is placed between two horizontal plates and sufficiently heated from below as soon as precipitating cumulus clouds form this pattern is disturbed and a transition to a different state occurs here we use idealized large eddy simulations to explore how the horizontal scale of convection is modified during this transition in the course of a diurnal cycle before onset of precipitation cells with relatively constant diameter self organize with diameters roughly on the scale of the atmospheric boundary layer height we find that the onset of precipitation then signals an approximately linear increase in horizontal scale with time for our transient simulations this scale increase progresses at a speed which is relatively insensitive to modifications in mean surface temperature modifications in the rate at which surface temperature changes or the initial lapse rate when exploring the strength of the spatial correlations we find that precipitation onset causes a sudden disruption of order and a subsequent decline of organization until precipitation eventually ceases we discuss possible implications for the development of extreme precipitation events

paleoconvergence rates determined from k2o sio2 ratios in magmatic rocks and their application to cretaceous and tertiary tectonic patterns in southwestern north america spatial temporal and chemical data may be used to calculate paleodip angles for paleosubduction zones assuming the position of the paleotrench is reasonably well known in turn paleoconvergence rates may be calculated according to the equation developed by luyendyk 1970 that relates dip angle to convergence rate data from cretaceous tertiary magmatic rocks indicate paleoconvergence rates of 7 to 8 cm yr for the period 130 to 85 m y ago convergence rates markedly increased to an average of about 14 cm yr for the period 85 to 45 m y decreased convergence rates are indicated from 45 to 20 m y paleoconvergence rates determined from magmatic rocks are generally in agreement with those determined independently by other workers from paleomagnetic data on ocean floor rocks the anomalously high rates of convergence determined from the igneous rock data may be due to subduction of younger more buoyant oceanc crust after 65 m y b p although recent observations from independent data suggest that eocene convergence may have been a high 20 to 26 cm yr paleomagnetically the period of constant convergence constant dip subduction from 130 to 85 m y ago correlates almost exactly with the long cretaceous normal polarity interval as suggested on the new lowrie alvarez 1981 geomagnetic time scale in contrast the post 85 m y period of variable convergence and variable dip subduction correlates with polarity events of short duration and frequent reversal tectonically the period of constant convergence rates from 130 to 85 m y ago correlates with sevier orogeny in nevada and utah the period of increased convergence from 85 to 45 m y ago correlates with classic laramide orogeny in wyoming and colorado the period of decreased convergence from 45 to about 20 m y ago correlates with a poorly understood and incompletely defined mid tertiary orogenic event

offset along the san andreas fault of eocene strata from the san juan bautista area and western san emigdio mountains california analyses of eocene strata that rest on gabbroic basement rocks in the western san emigdio mountains and northernmost gabilan range indicate that the 2 sequences have been offset right laterally 305 320 km along the san andreas fault in both areas basal shallow marine conglomerate of penutian age grades upward into shale of ulatisian and narizian age that was deposited at bathyal water depths in a basin with access to open ocean additional similarities in the upper eocene and oligocene stratigraphic units suggest original contiguity of the depositional basins the inferred offset indicates that the salinian block was attached to western california by at least early eocene time the longer distances of northward transport of the salinian block that are suggested by preliminary paleomagnetic data from upper mesozoic granitic rocks and upper cretaceous and paleocene strata must have occurred during paleocene or earlier time

geophysical evidence for miocene extension and mafic magmatic addition in the california continental borderland the california continental borderland is a key region for understanding the effect on crustal structure of tectonic processes associated with the initiation and elongation of the san andreas transform system because the borderland lies near the point where the pacific farallon spreading system first encountered the north american plate margin whereas the central and northern california margins have been extensively investigated by geophysical surveys in recent years the crustal structure of the borderland particularly the outer borderland has received relatively little attention this paper presents three gravity models that stretch from the pacific ocean floor to shore they provide new evidence that the crust of the entire california continental borderland has been extended and infiltrated by mafic melt since early miocene time the gravity models were constructed from interpretations of nearly 750 km of seismic reflection data seismic refraction studies and geologic maps thus minimizing the nonuniqueness of the method the evidence for extension in the models is that the crust of the accretionary complex and forearc basin that make up the borderland is only 22 km thick 35 thinner than comparable crustal sections in central and northern california extension could well have been accommodated by eastward flow of accretionary complex material during middle miocene formation of a metamorphic core complex in the inner borderland the gravity models also show that the accretionary complex is 100 200 kg m 3 more dense than commonly observed to the north this density difference may result from influx of basaltic melts due to mantle upwelling at the rivera triple junction and beneath the highly extended inner borderland the borderland models permit the occurrence of a discrete mafic layer that forms the base of the crust as is seen along the central and northern california coast but do not require such a layer9s presence anywhere except immediately inland of the shelf slope break

three dimensional thermal modeling of the california upper mantle a slab window vs stalled slab in order to gain a better understanding of the behavior of microplates after their subduction we studied two end member scenarios for the post subduction history of two offshore california microplates in the first scenario monterey and arguello microplate remnants are present today below the north america plate while in the second scenario subducted microplate remnants are absent 3 d numerical modeling of the thermal evolution implied by these scenarios results in two different present day thermal structures of the central and southern california upper mantle by comparing the model predicted surface heat flow values and seismic velocities to heat flow data and tomography we find that we cannot discriminate between the two scenarios as they both are consistent with the data this result means that the present day upper mantle temperature field is relatively insensitive to the assumed microplate scenarios a slabless window is not needed for the generation of partial melt either which is consistent with earlier 2 d studies for this region

evaluation of recurrence intervals characteristic earthquakes and slip rates associated with thrusting along the coast range central valley geomorphic boundary california abstract a seismically active blind fold and thrust belt underlies the coast range central valley geomorphic boundary crcv california earthquakes associated with this fault system include the 1983 m w 6 5 coalinga and 1985 m w 6 1 kettleman hills events published estimates of the crcv slip rate derived from published studies range from 1 to 10 mm yr and estimated recurrence intervals of coalinga type events range from 200 to 2000 yr relative consistency of geomorphic expression and regional geologic relationships suggests that deformation rates are similar along the 500 km length of the crcv we evaluated historical seismicity to further constrain recurrence interval and slip rate estimates based on structural geology geomorphology and historical seismicity the crcv may comprise 18 to 25 segments from 12 to 57 km in length with most segments between 20 and 30 km the characteristic earthquake for the average length segment may be m w 6 3 to 6 4 or slightly smaller than the coalinga earthquake and the longest segments may yield earthquakes as large as m w 6 8 to 7 1 during the approximately 150 yr period of historical record 11 earthquakes of m w 5 8 to 6 8 of which eight may be characteristic events have taken place on the crcv the characteristic earthquakes appear to have ruptured single segments using the estimates of co seismic rupture length for the 1983 coalinga and 1985 kettleman hills earthquakes to calibrate earlier events the historical earthquake record corresponds to rupture of 32 to 39 of the length of the crcv in 142 yr a recurrence interval of 360 to 440 yr for the average segment combining the recurrence estimates with estimates for co seismic displacement yields slip rates of 1 8 to 8 0 mm yr because recent vlbi studies suggest that the total convergence rate normal to the pacific north american plate boundary at the latitude of the crcv is 3 mm yr the lower end of the slip rate range 1 8 to 3 mm yr is favored for the crcv if these estimates are correct the crcv accommodates most of the plate convergence between the north american and pacific plates indicating that the historical earthquakes efficiently released strain accumulated on the crcv and suggesting that aseismic deformation along the crcv is minor

organic matter in the franciscan and cedros subduction complexes the problems of instantaneous maturation and missing petroleum in accretionary prisms abstract most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock past studies of accretionary complexes have concluded that they have minimal petroleum potential although they do contain potential source rocks owing to their great bulk of sedimentary rock large amounts of petroleum must be generated in accretionary prisms and these hydrocarbons must be trapped in the complexes or vented into the ocean or on land to understand the nature and abundance of organic material in accretionary complexes organic matter in the shales limestones and melange matrix of the franciscan and cedros subduction complexes was studied mostly using rock eval pyrolysis the largest amounts of total organic carbon toc were found in the calera limestone from the san francisco area 1 6 toc terrigenous sedimentary rocks of probable trench or trench slope origin contain from

some remarks on the origin of foredeeps abstract geological observations show that there are two main types of foredeep basins on the base of their subsidence rate and their relationship with the associated orogen or accretionary wedge high rates of subsidence up to 1600 m myr and a ratio lower than 1 between the area of the elevated belt and the area of the basin characterize the foredeeps associated with w dipping subduction low rates of subsidence up to 300 m myr and a ratio higher than 1 between the area of the orogen and the area of the basin characterize the foredeeps associated with e or ne dipping subductions this observation enables us to interpret the slow filling of foredeeps of the first type and the much faster filling of foredeeps of the second type moreover w dipping subductions have a steeper and faster monocline bounding the base of the deep foredeep in contrast to e or ne dipping subductions which have a shallower monocline this differentiation supports the notion of the eastward mantle flow with respect to the lithosphere detected in the hot spot reference frame in this view the foredeep depth in w dipping subductions is mainly controlled by the roll back of the subduction hinge pushed by the relative eastward mantle flow while foredeep depth in e or ne dipping subductions is instead mainly generated by the load of the thrust sheets and by the roll back of the subduction hinge due to the advancing upper plate contrasting the upward push of the mantle the shape of the foredeeps is regularly arcuate in case of w dipping subduction while is linear or following the shape of the inherited continental margin in case of e or ne dipping subduction fold development in the two opposite foredeeps is significantly different in w dipping subduction the folds are transported down in subduction while they are forming and consequently they are poorly eroded syntectonic sediments drape a preserved fold in the e ne dipping subduction folds and thrust sheets are instead uplifted and deeply eroded this is also predicted by the envelope of the crests in the two distinct accretionary wedges in the w dipping case the envelope may dip toward the subduction while it is rising toward the hinterland in the other cases this approach may explain the differences in terms of geometries amount and rates of subsidence between the foredeeps around the adriatic plate for instance the foredeep of the southern alps back thrust belt foredeep is deformed in the western part by the later apenninic foredeep development associated with the sinistral oblique ramp of the w dipping adriatic plate subduction the relationship in the frontal parts of a thrust belt between accretion and eustacy has to be analyzed in terms of when where and how fast thrusts propagation took place with respect to the randomly distributed sea level third order fluctuations and by the type of subduction which is connected to the foredeep and thrust belt or accretionary wedge therefore the stratigraphic patterns in foredeep basins are peculiar and casual successions that record the combination of regional tectonic evolution with independent wider scale sea level changes

seismic images of modern convergent margin tectonic structure in 1980 a w bally assembled and edited an innovative three volume picture and work atlas of seismic reflection record sections this compilation of more than 130 excellent seismic section reproductions provided much new data from the earth s subsurface for petroleum geologists academic scientists and students it is the field geologist s counterpart of outcrops and type sections the smaller collection of seismic sections presented in this studies 26 publication was inspired by bally s atlas and follows his general format and philosophy the objective is to provide a reference series of sections for the earth sciences and a vehicle for continuing scientific dialogue relating to modern convergent margins this publication s assembled sections represent a single facet of that dialogue by bringing together a series of exceptional seismic sections from the fronts of presently active convergent margins around the pacific

on the mechanism of tectonic transport in zones of oblique subduction abstract many cases of oblique subduction are known throughout the world today and many more must have existed in the past in at least one modern case sumatra the over riding plate has responded to oblique subduction by forming a strike slip fault that traverses the magmatic arc and displaces its leading edge in the direction of the transverse component of oblique convergence conditions favoring this sort of tectonic response are shown to be 1 1 a high angle of oblique convergence 2 2 a low angle of subduction and 3 3 relative thermal softening of the magmatic arc this mechanism may have been responsible in part for northward transport of western cordilleran magmatic arc and subduction zone rocks in the late cretaceous and earliest tertiary it also may help to explain evidence of southward transport and left lateral shear in northern mexico and the caribbean region in pre jurassic time

crustal structure of the offshore southern santa maria basin and transverse ranges southern california from deep seismic reflection data seismic reflection data from the 1986 edge pacific gas and electric experiment together with refraction and gravity data are used to construct an integrated north south geophysical cross section along the offshore santa maria basin and across the offshore extension of the transverse ranges thrust faults offsetting acoustic basement and folding miocene sediments root in a midcrustal detachment at 12 13 km depth the detachment coincides with an increase in reflectivity seen on the seismic reflection section and the maximum depth of earthquakes a high velocity oceanic crustal layer at the base of the crust 17 22 km and moho 25 km is offset by a low angle thrust fault with an apparent dip 10 to the north offshore point arguello gravity models indicate crustal thickening and a change in the depth to moho from 25 to 30 km in this region however the contribution of elastic flexure from loading of the transverse ranges is considered to be minor in this part of the borderland the cross section constrains neogene shortening to less than 3 based on faulting and folding of the sedimentary section shortening of the upper crust is balanced by a combination of ductile flow and subduction of the oceanic crust beneath the midcrustal detachment

mesoscale shallow convection in the atmosphere this paper is a review of the observational experimental theoretical and numerical studies of mesoscale shallow convection msc in the atmosphere typically msc is 1 to 2 km deep has a horizontal length scale of a few to a few tens of kilometers and takes distinctive planforms linear and hexagonal the former is called a cloud street roll or band while the latter is called mesoscale cellular convection mcc comprising three dimensional cells msc is characterized by its shape horizontal extent convective depth and aspect ratio the latter is the ratio of the horizontal extent to that in the vertical for cells the horizontal extent is their diameter whereas for rolls it is their spacing rolls usually align along or at angles of up to 10 from the mean horizontal wind of the convective layer with lengths from 20 to 200 km widths from 2 to 10 km and convective depths from 2 to 3 km the typical value of aspect ratio ranges from 2 to 20 rolls may occur over both water surface and land surfaces mesoscale convective cells may be divided into two types open and closed open cell circulation has downward motion and clear sky in the cell center surrounded by cloud associated with upward motion closed cells have the opposite circulation both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50 and both occur in a convective layer with a depth of about 1 to 3 km both the magnitude and direction of horizontal wind in the convective layer change little with height msc results from a complex and incompletely understood mix of processes these processes are outlined and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of msc