lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

evidence of a collision between the yucatan block and mexico in the miocene we present the evidence for an anomalous southwest dipping slab in southern mexico the main evidence comes from a clear receiver function image along a seismic line across the isthmus of tehuantepec and is also supported by a previous global tomographic model the slab dips at 35 is approximately 250 km in length and appears to truncate the cocos slab at about 120 km depth we hypothesize that the slab was created by subduction of oceanic lithosphere prior to the collision of the yucatan block with mexico at approximately 12 ma this scenario would explain the chiapas fold and thrust belt as the product of this collision and its age constrains the date of the event to be in the miocene

the differential response of kelp to swell and infragravity wave motion we present field measurements of the movement of the giant kelp macrocystis pyrifera under wave forcing we resolve the depth and frequency dependent responses along the stipe and find different and counterintuitive patterns of response at the infragravity and swell wave forcing frequencies at swell frequencies tilting of the stipe is largest toward the holdfast whereas at infragravity frequencies the stipe tilting is largest closer to the water surface it is postulated that the stretching of blades and subsequent pull on the stipe is in part responsible for these patterns this conclusion is supported by results of manipulative experiments which show a more along stipe uniform response after removal of blades from the kelp the length of the kelp also exerts a strong control on the relative magnitudes of movements in the different frequency bands with the swell band becoming more important relative to the infragravity band for shorter length kelp these results indicate that kelp will differentially dissipate energy over both frequencies and varying depths within the water column the variety of movement responses over differing wave forcing frequencies may also imply that there exist differing rates of breakage for kelp exposed to hydrodynamics stressors of multiple frequencies

integrated model of the crustal structure in the gulf of california extensional province abstract a method of constructing 3d earth models from a suite of 1d profiles is demonstrated using delaunay triangulation accurate crustal models are required to correct for crustal contamination in seismic tomography of the mantle and can be used as starting points for higher resolution tomography of the lithosphere and asthenosphere using a simple input data set 1d vertical profiles provides for rapid prototyping model building and modification any structural length scale in the final 3d model is possible using this method through precise placement of 1d profiles and triangulation collections of layers within the 1d profiles are grouped and identified by keys called superlayers allowing for 3d discontinuities and variations along these important interfaces the method was applied to the gulf extensional province and baja california regions to account for the laterally discontinuous crustal variations during a tomographic inversion for mantle structure use of the 3d model and appropriate regional crustal wave speeds minimizes wave speed variations in the tomographic model and assists interpretation of the final model

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

raft model of crustal extension evidence from seismic reflection data in southeast arizona an archlike zone of seismic reflectivity interpreted as an uplifted zone of ductilely deformed middle and lower crust is imaged below the pinaleno mountains core complex in southeast arizona the top of the reflective zone coincides with the base of an inferred mid tertiary detachment fault beneath the safford basin but diverges from the detachment fault as an apparent mylonite front to form a culmination at 1 9 s 4 km beneath the pinalenlo mountains from this culmination the zone of reflectivity dips to the southwest below the eagle pass detachment fault and flattens at 4 8 s 13 5 km beneath the relatively unextended upper crust of the galiuro mountains most of the reflective fabric probably formed during mid tertiary extension although some of it may be older these data suggest that mylonite zones form not only as the continuation of detachment faults into the brittle ductile transition but also along a regional zone of decoupling between the middle and upper crust highly extended and relatively unextended domains in the basin and range may be separated by zones of discrete simple shear in the upper crust but both are rafted above regional bulk pure shear in the middle and lower crust

lithospheric extension near lake mead nevada a model for ductile flow in the lower crust small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions in this study we examine the boundary between the unextended colorado plateau and a strongly extended domain in the basin and range province in the lake mead nevada region bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required finite element modeling of newtonian flow and power law creep shows that flow over the length scale of the eastern basin and range 500 km or more corresponding to upper crustal extension by a factor of 1 4 3 over 10 m y requires effective viscosities less than 10 18 10 20 pa s for ductile channels 10 25 km thick flow over shorter length scales 150 km may be accommodated with effective viscosities as high as 10 21 pa s modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory derived power law creep rates the longer scale flow may require elevated crustal temperatures more than 700 c depending on the composition and material properties assumed under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep this work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions and thereby explaining the relative uniformity in gravity and topography between such regions

an optimized scheme of lettered marine isotope substages for the last 1 0 million years and the climatostratigraphic nature of isotope stages and substages a complete and optimized scheme of lettered marine isotope substages spanning the last 1 0 million years is proposed lettered substages for marine isotope stage mis 5 were explicitly defined by shackleton 1969 but analogous substages before or after mis 5 have not been coherently defined short term discrete events in the isotopic record were defined in the 1980s and given decimal style numbers rather than letters but unlike substages they were neither intended nor suited to identify contiguous intervals of time substages for time outside mis 5 have been lettered or in some cases numbered piecemeal and with conflicting designations we therefore propose a system of lettered substages that is complete without missing substages and optimized to match previous published usage to the maximum extent possible our goal is to provide order and unity to a taxonomy and nomenclature that has developed ad hoc and somewhat chaotically over the decades our system is defined relative to the lr04 stack of marine benthic oxygen isotope records and thus it is grounded in a continuous record responsive largely to changes in ice volume that are inherently global this system is intended specifically for marine oxygen isotope stages but it has relevance also for oxygen isotope stages recognized in time series of non marine oxygen isotope data and more generally for climatic stages which are recognized in time series of non isotopic as well as isotopic data the terms stage and substage in this context are best considered to represent climatostratigraphic units and thus climatic stages and climatic substages because they are recognized from geochemical and sedimentary responses to climate change that may not have been synchronous at global scale

solutions in chronostratigraphy the paleocene eocene boundary debate and aubry vs hedberg on chronostratigraphic principles abstract in several recent papers m p aubry et al have argued that hedbergian principles of chronostratigraphy are being violated by the international commission on stratigraphy ics when selecting global stratotype sections and points gssps for the formal divisions of the geological time scale the current debate over the definition of the paleocene eocene p e boundary has been a major focus of their arguments unfortunately aubry et al have obscured matters by misusing the term unit stratotype and by equivocally using the term stage for the very different concepts of synthem and global chronostratigraphic stage the p e boundary option most repugnant to aubry et al carbon isotope excursion cie p e thanetian ypresian boundary is perfectly compatible with h d hedberg s views in contrast another option preferred by aubry et al recognition of new 1 m y duration age stage between thanetian and ypresian is inconsistent with hedberg s views additional problems with the p e boundary arguments of aubry et al include the fact that a ypresian unit stratotype does not exist the fact that the base of the ypresian synthem is not immutable and the fact that the nannofossil tribrachiatus digitalis is of dubious relevance to the boundary debate as for chronostratigraphy in general aubry et al have misrepresented hedberg s views by 1 falsely claiming that the content of a stage is what determines its boundaries 2 misunderstanding the general concept of the boundary stratotype 3 distorting the base defines boundary principle 4 falsely claiming that traditional pre gssp chronostratigraphic boundaries cannot be changed 5 falsely implying that traditional stage unit stratotype boundaries can be adjusted by no more than 300 000 years when defining formal age stage boundaries with gssps 6 falsely claiming that the definition of a geochronologic chronostratigraphic boundary should precede its correlation 7 claiming that traditional unconformable stage boundaries may be suitable horizons for gssps 8 distorting the meaning of arbitrariness in regard to the definition of geochronologic chronostratigraphic boundaries and 9 claiming that gssps are inherently unstable in that they are subject to redefinition whenever a more powerful element of correlation is discovered if taken seriously the unit stratotype sanctifying chronostratigraphic philosophy of aubry et al would require the creation of perhaps dozens of new phanerozoic ages stages of relatively very short duration wherever there was a significant gap between two successive historical stage unit stratotypes for all of these reasons the arguments of aubry et al have no merit

velocity and turbulence affected by submerged rigid vegetation under waves currents and combined wave current flows abstract vegetation can change flow structure and turbulence characteristics however few studies have focused on combined wave current flows in canopy with vertically varying density therefore laboratory experiments were carried out to investigate hydrodynamics affected by submerged rigid vegetation with different densities i e sparse dense and vertically varying density under waves currents and combined wave current flows in submerged canopy with vertically uniform density large shear stress occurs around the canopy top thus in the vertical distributions the greatest values of maximum wave velocity under pure waves and the greatest values of turbulent kinetic energy tke under unidirectional currents and combined wave current flows appear around the canopy top in canopy with vertically varying density the shear stress at each canopy top is relatively small resulting in the vertically smooth distributions of velocity and tke under pure waves while under unidirectional currents and combined wave current flows canopy scale turbulence throughout the entire water column occurs owing to the influence of multiple canopy tops an improved formula considering drag coefficient vegetation density and diameter was proposed to predict stem scale turbulence using characteristic velocity the formula is appropriate for vegetated flow in which stem scale turbulence is dominant and it can be further used as a simple method to assess tke for sediment or substances transport affected by vegetation

the real southern basin and range mid to late cenozoic extension in mexico much of northern and central mexico underwent east northeast extension in the mid to late cenozoic this area constitutes a major but little recognized part of the basin and range province the extended region is bounded on the east by the laramide thrust front of the sierra madre oriental on the west the relatively unfaulted sierra madre occidental separates the extended area in central mexico from that around the gulf of california extension occurred as far south as what is now the trans mexican volcanic belt and in oaxaca south of the belt the basin and range province in mexico constitutes approximately half of the 19 x 10 5 km 2 of western north america that underwent mid to late cenozoic extension north northwest orientations of numerous epithermal vein systems indicate that east northeast extension began as early as 30 ma in areas north of the volcanic belt major episodes of faulting began at about 23 to 24 ma and 12 to 13 ma both in mexico and in the southwestern united states faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting widespread quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the trans mexican volcanic belt in contrast the tectonics in and south of the belt are now probably related to subduction of the rivera plate contemporaneity of the 12 ma episode with early extension around the gulf of california attributed to pacific north american plate boundary reorganization suggests that extension is related predominantly to plate boundary effects the beginning of extension at 30 ma may be related to initial encroachment of the east pacific rise upon the trench that lay off western north america

folding of mylonitic zones in cordilleran metamorphic core complexes evidence from near the mylonitic front metamorphic core complexes of north america are asymmetric when viewed perpendicular to the direction of tectonic transport on one side mylonitic fabrics and the overlying detachment fault dip gently in the direction of upper plate transport and the mylonitic rocks have a down dip sense of shear on the opposite side the mylonitic zone rolls over to a dip opposed to the direction of upper plate transport diverges downward from the detachment fault and dies out upward across a mylonitic front new kinematic data show that the backdipping mylonitic zone in most complexes is a downfolded continuation of the main mylonitic zone folding of the mylonitic zone was accompanied by the formation of antithetic shear zones and occurred during the late stages of mylonitization

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

stratigraphic record of basin development within the san andreas fault system late cenozoic fish creek vallecito basin southern california the fish creek vallecito basin contains a 5 5 km thick section of late miocene to early pleistocene sedimentary rocks exposed in the hanging wall of the west salton detachment fault these deposits preserve a high fidelity record of late cenozoic subsidence and basin filling that resulted from deformation in the san andreas fault system of southern california existing and new paleomagnetic data combined with new u pb zircon ages of two tuffs high in the section show that the section ranges in age from ca 8 0 0 4 ma at the base to ca 0 95 ma at the top geohistory analysis reveals 1 moderate subsidence 0 46 mm yr from ca 8 0 to 4 5 ma 2 rapid subsidence 2 1 mm yr from 4 5 to 3 1 ma 3 moderate subsidence 0 40 mm yr from 3 1 to 0 95 ma and 4 rapid uplift and erosion that has exhumed the section since ca 1 ma onset of sedimentation at ca 8 0 0 4 ma records earliest extension or transtension in the area possibly related to localization of the pacific north america plate boundary in the salton trough and gulf of california alternatively marine incursion at 6 3 ma may be the earliest record of plate boundary deformation in the gulf of california salton trough region a thick interval higher in the section records progradation of the colorado river delta into and across the basin starting ca 4 9 ma progradation continued during an abrupt increase in subsidence rate at 4 5 ma and fluvial deltaic conditions persisted for 1 4 m y during the rapid subsidence phase indicating that delta progradation was driven by a large increase in rate of sediment input from the colorado river uplift and inversion of the basin starting ca 1 0 ma record initiation of strike slip faults that define the modern phase of dextral wrench tectonics in the western salton trough

improved parameterization of seagrass blade dynamics and wave attenuation based on numerical and laboratory experiments seagrass blade dynamics were explored through numerical and laboratory experiments in order to improve parameterization of wave attenuation by submerged aquatic vegetation in the presence of a background current in the numerical model a single blade was modeled as a series of rigid plates attached by torsion springs for the laboratory model strips of low density polyethylene were placed in a recirculating wave flume a new form of the keulegan carpenter number based on the horizontal excursion of the blade tip was found to be an excellent predictor of drag coefficient an algebraic model for predicting wave attenuation was developed based on the following observations during the portion of the wave period when the fluid velocities are highest the blade motion is almost completely arrested and the vast majority of the turbulence production occurs during this time turbulence production when the blade is pronated is accurately predicted by the maximum fluid velocity over the wave period the relative contribution to the total turbulence production over the wave period is determined by the relative strength of the waves and the current therefore using a simple algebraic fit the total depth integrated time averaged turbulence production can be accurately predicted by two flow parameters the maximum fluid velocity over the wave period and a non dimensional number that compares the wave and current velocities by fitting the algebraic model to data from a particular site it can be used to efficiently estimate wave attenuation and drag coefficient in seagrass exposed to waves with a background current

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

experimental study on vegetation flexibility as control parameter for wave damping and velocity structure abstract vegetation can contribute to coastal defence by damping incoming waves however prior studies have shown that attenuation varies greatly among plant species plant flexibility is a mechanical property that is commonly omitted but varies considerably between shrubs and grasses on salt marshes therefore we present an experimental study in a laboratory wave flume with artificial vegetation that differs in flexibility only we measured wave attenuation and water particle velocities around rigid and flexible salt marsh vegetation waves were measured using a series of gauges and particle image velocimetry piv was used to measure spatio temporal variations of water particle velocities in the x z plane around the vegetation our results show that flexible vegetation attenuates waves up to 70 less than rigid vegetation due to swaying of flexible plants furthermore we find that rigid vegetation modifies the velocity structure whereas flexible vegetation does not specifically a mean current in the direction of wave propagation develops around the canopy and the horizontal particle velocities are amplified directly above the canopy these results indicate that plant flexibility is a key parameter in the wave vegetation interaction that controls wave damping and velocity structure

wave dissipation by flexible vegetation 1 dissipation of waves propagating through natural salt marsh vegetation was about half the dissipation expected for rigid vegetation this low dissipation was predicted by a theoretical model that accounts for bending of vegetation motions a transect of 3 pulse coherent acoustic doppler profilers recorded water velocity and pressure at 8 hz within the dense 650 stems m2 canopy of semi flexible single stem vegetation schoenoplectus americanus most wave energy 56 81 was dissipated within 19 m of the marsh edge two dissipation models the first assuming rigid vegetation and the second simulating wave forced vegetation motion using the theory for bending of linearly elastic beams were tested after choosing optimal drag coefficients both models yielded a good fit to the observed dissipation skill score 0 96 0 99 however fitted drag coefficients for the rigid model 0 58 0 78 were below the range 0 98 2 2 expected for the observed reynolds numbers 13 450 and canopy densities accounting for interactions between stem wakes whereas drag coefficients for the flexible model 0 97 1 6 were nearer the expected range indicating that prediction of wave dissipation was improved by simulating vegetation motion

extension of the exmouth plateau offshore northwestern australia deep seismic reflection refraction evidence for simple and pure shear mechanisms the response to extensional stresses that deformed and finally ruptured the northwest australian passive margin was achieved by an interplay of simple and pure shear mechanisms the results of a two ship seismic reflection and refraction experiment show that the outer exmouth plateau is dominated by large rotated blocks bounded by deeply penetrating normal faults some of which are heavily intruded magmatic underplating attended this extension but is absent in the central region where a set of prominent subhorizontal mid crustal arrivals are interpreted to be detachment surfaces extensive exploration seismic coverage drilling and sampling suggest that deformation on the outer plateau postdates that in the central plateau we propose that initial deformation of the region was dominated by a simple shear detachment system along low angle fault planes this passed laterally and temporally into a region of pure shear deformation characterized by high angle normal faults and magmatism and this became the locus of final rupture

the composition and emplacement of basaltic magmas produced during the development of continental margin basins the gulf of california mexico the gulf of california is a region of active ensialic basin development and transform faulting legs 64 and 65 of the deep sea drilling project have demonstrated the diversity of basement stratigraphy drilling at sites 474 482 483 and 485 at the mouth of the gulf recovered massive basaltic flows and occasional pillow lavas and sills whereas drilling in the central part of the gulf in the guaymas basin recovered a sequence of sediments and massive doleritic sills very high sedimentation rates over 1000 m ma are characteristic of the guaymas basin and prevent extrusion of the basaltic magma onto the seafloor thus resulting in at least partial intercalation of oceanic layers 1 and 2 such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development when sedimentation rates may be very high over 200 samples of basalts and dolerites from the gulf of california have been analysed by x ray fluorescence and neutron activation techniques basalts from the mouth of the gulf are tholeiitic and contain very low abundances of the more hygromagmatophile more hyg elements k rb ba th ta nb and light ree relative to the hyg elements y zr ti and heavy ree thus resembling depleted n type morb from elsewhere along the e pacific rise epr dolerites from the guaymas basin also have a tholeiitic chemistry and some epr basalt characteristics e g la n sm n

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

the hydrodynamics of surface tidal flow exchange in saltmarshes abstract modeling studies of estuary circulation show great sensitivity to the water exchange into and out of adjacent marshes yet there is significant uncertainty in resolving the processes governing marsh surface flow the objective of this study is to measure the estuary channel to saltmarsh pressure gradient and to guide parameterization for how it affects the surface flow in the high marsh current meters and high resolution pressure transducers were deployed along a transect perpendicular to the nearby little ogeechee river in a saltmarsh adjacent to rose dhu island near savannah georgia usa the vertical elevations of the transducers were surveyed with static gps to yield high accuracy water surface elevation data it is found that water level differences between the little ogeechee river and neighboring saltmarsh are up to 15 cm and pressure gradients are up to 0 0017 m of water surface elevation change per m of linear distance during rising and falling tides the resulting little ogeechee river to saltmarsh pressure gradient substantially affects tidal velocities at all current meter locations at the velocity measurement station located closest to the little ogeechee river bank the tidal velocity is nearly perpendicular to the bank at this location surface flow is effectively modeled as a balance between the pressure gradient force and the drag force due to marsh vegetation and bottom stress using the darcy weisbach lindner s equations developed for flow through vegetation analysis in open channel flow the study thus provides a direct connection between the pressure gradient and surface flow velocity in the high marsh thereby overcoming a long standing barrier in directly relating flow through saltmarsh studies to flow through vegetation studies in the open channel flow literature

a transform continental margin rich in hydrocarbons gulf of california conventional and near bottom geophysical surveys mapped shallow deformational structures and sediment accumulation patterns along part of the actively shearing continental margin of sonora including the intersection with a guaymas basin spreading center the principal strike slip faults occupy a zone 1 2 km 0 6 1 2 mi wide along the lower slope with a ridge of tightly folded sediments raised by uplift of sheared basement at the abrupt boundary between continental and oceanic crust this transform ridge and a continental rise on its seaward slope grows as the fault zone ages away from the spreading center intersection shoaling at a net rate of 500 m m y 1 600 ft m y despite erosion of its crest a clathrate horizon in the upper 80 m 260 ft of sediment is inferre from bottom simulating reflectors in the rise and lower slope hydrocarbon seeps occur at 1 600 m 1 mi below sea level along the crest of the transform ridge and patchy gas accumulation and seepage are recognized on a young marginal plateau

evolution of rift basins and low angle detachments in multilayer analog models multilayer analog models comprising a brittle upper layer sand and clay a strong ductile middle layer high viscosity silicone putty and a weak ductile basal layer low viscosity silicone putty are used to demonstrate the importance of lithospheric strength maxima in the development of rift basins and low angle detachments although a wide range of rift geometries formed a general evolution can be derived from the experimental results an initial period of distributed extension is followed by localized deformation into rift basins due to boudinage of the strong layer delamination at the interface between the strong putty and weak putty layers results in the formation of subhorizontal ductile shear zones below the strong ductile layer this basal shear zone alters the symmetry of the boudins and sets up a stress regime mechanically suitable for the growth of low angle normal faults in the neck lines of developing rift basins continued boudinage eventually tears the strong ductile layer the brittle upper layer is subsequently dragged over the basal ductile layer along a series of listric detachment faults

sedimentation and boundary changes of virginia salt marshes abstract sediment collections on a mainland fringing marsh a lagoon marsh and a barrier island fringing marsh were conducted in conjunction with geographic information systems gis analysis of aerial photographs to relate changes in marsh area to sedimentary processes the island marsh lost 7 2 of its area in 8 years by overwash the lagoon marsh lost 10 6 of its area over 41 years by recession of marsh edges the mainland marsh area increased by 8 2 over 50 years primarily by upland encroachment surface sediment was collected monthly at the mainland and lagoon marshes for 1 year to identify changes in the mean grain size organic content and mass of sediment deposited on sampling plates short term variability in these characteristics obscured seasonal differences grain size and monthly sediment accumulation decreased towards the interior of both marshes while the organic matter content increased on the lagoon marsh coarse sediment at creekside stations as well as grain size contrasts between surface and subsurface sediment are consistent with erosion evident in gis analysis on the mainland marsh sediment does not vary as much with location or depth and topography appears stable tidal currents appear to be competent to resuspend most particle sizes represented in surface sediment samples including aggregates except at the edge of the lagoon site decreasing rates of marsh edge change reflect moderation of oceanic processes while the interior of both lagoon and mainland marshes remained stable throughout the study both lagoon marsh losses to recession and mainland marsh gains from upland reflect the submergence which this shoreline experiences

backscattering and geophysical features of volcanic ridges offshore santa rosalia baja california sur gulf of california mexico abstract volcanic ridges formed by series of volcanic edifices are identified in the central part of the gulf of california between isla tortuga and la reforma caldera santa rosalia region isla tortuga is part of the 40 km long tortuga volcanic ridge tvr that trends almost perpendicular to the spreading center of the guaymas basin the rosalia volcanic ridge rvr older than tvr is characterized by volcanic structures oriented towards 310 following a fracture zone extension and the peninsular slope it is interpreted that most of the aligned submarine volcanic edifices are developed on continental crust while isla tortuga lies on oceanic like crust of the guaymas basin from a complete bouguer anomaly map it is observed that the alignments of gravity highs trending 310 and 290 support the volcanic and subvolcanic origin of the bathymetric highs volcanic curvilinear structures lava flows and mounds were identified from backscattering images around isla tortuga and over a 400 m high virgenes high where the tvr and the rvr intersect a refraction wide angle seismic profile crossing perpendicular to the virgenes high together with gravity and magnetic data indicate the presence of shallow intrusive bodies presumably of basaltic or andesitic composition it is inferred that most volcanic edifices along the ridges have similar internal structures we suggest that the growth of different segments of the ridges have a volcano tectonic origin the older rvr lies along the extension of a fracture zone and it probably is associated with pliocene ne sw extension

conditions for flow in the continental crust 1 earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields the lower continental crust can be stronger than the upper mantle and imply that the upper mantle can flow even when the lower crust does not however in other places substantial flow in the lower crust does occur particularly near extensional metamorphic core complexes we suggest that the lower crust only flows when its viscosity is significantly reduced either by heating from igneous intrusions or by the addition of water in both cases the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs the timescale is controlled either by conductive cooling of intrusions or by the separation of melt from its matrix which removes the water

long term variations in backbarrier salt marsh deposition on the skallingen peninsula the danish wadden sea abstract accretion on a natural backbarrier salt marsh was determined and modeled as a function of high tide level initial salt marsh level and distance from the salt marsh edge accretion measurements were based on up to 67 year old marker horizons supplemented by 210pb 137cs datings the salt marsh is situated on the backbarrier of the skallingen peninsula in the northern part of the danish wadden sea the tidal range mean 1 5 m is strongly affected by wind tide which occasionally adds up to about 3 m to the astronomical high tide level accretion is restricted to a narrow vertical band from about 0 1 m below to about 0 7 m above the mean high water level in the outer part of the backbarrier close to the tidal flat mean accretion is about 4 mm yr 1 and in the inner part it is about 2 mm yr 1 the decrease takes place in a similar manner as the decrease across a flood plain the major part of salt marsh accretion is associated with high water levels corresponding to weather conditions characterized by gales the long term variation of salt marsh accretion correlates with variations in the north atlantic oscillation winter index the number of over marsh high tides decreases exponentially with high tide level the function constants controlling this distribution vary in direct ratio to the mean sea level plausible near future scenarios of tidal development were obtained by extrapolating this relation three sea level scenarios were tested i sea level rise continues at a constant long term rate of 2 3 mm yr 1 ii sea level rise continues at a constant short term rate of 4 2 mm yr 1 and iii sea level rise accelerates to a constant rate of 6 4 mm yr 1 in the first case accretion on the salt marsh will keep pace with the high water level rise in the second case deposition in the inner part of the salt marsh will lag behind while that of the outer part of the salt marsh will keep pace with the rising high water level in the third case corresponding to a worst case scenario for the 21th century the salt marsh will gradually drown

is the sevier desert reflection of west central utah a normal fault a prominent west dipping reflection that can be traced in seismic reflection profiles over an area of 7000 km 2 beneath the sevier desert basin of west central utah is generally referred to as the sevier desert detacbment and is widely regarded as one of tbe best examples of an upper crustal low angle normal fault the absence of evidence for fault related deformation in drill cuttings and core from two industry boreholes that intersect this feature casts doubt on the fault interpretation the existing interpretation is based mainly on the observation that high angle normal faulting is restricted largely to tertiary sedimentary and volcanic rocks above the reflection an alternative explanation is tbat the bigh angle faults are related to the withdrawal of early deposited lacustrine salt which even today is as much as 2 km thick reevaluation of the seismic data suggests that the sevier desert reflection consists of two spatially and genetically distinct segments a shallow segment here interpreted as an unconformity between paleozoic and tertiary strata and a fortuitously aligned deeper segment that is traceable to mid and lower crustal levels and that appears to represent a thrust fault related to the cretaceous sevier orogeny

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

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

wind waves and moveable bed bottom friction abstract effects of moveable bed bottom friction for wave observations and wave modeling are investigated using a state of the art bottom friction model this model combines the hydrodynamic friction model of madsen et al with a moveable bed roughness model based on grant and madsen analyzing the present model for idealized swell cases it is shown that swell might result in wave generated sand ripples the large change of roughness corresponding to initial ripple formation results in a preferred wave height for swell related to bathymetric scales as generally occur in shelf seas away from the coast the corresponding wave generated bottom roughness is not defined by the local wave conditions but is related to the overall energy balance of the wave field sediment data thus is imperative for the interpretation of observed decay rates and friction factors for swell for idealized depth limited wind seas near bottom wave motion is expected to generate partially washed out ripples and moderate sheet flo

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

3d shear velocity structure beneath the gulf of california from rayleigh wave dispersion active extension in the gulf of california is characterized by the transition from continental rifting to seafloor spreading puzzling variations in the patterns of both tectonics and magmatism are observed along the length of the gulf and are likely to be related to mantle heterogeneity regional scale mantle structure however has been difficult to constrain due to the lack of broadband seismic stations in the region in this study we utilized new data from the deployment of the nars baja array and other networks and computed a three dimensional shear speed model of the upper mantle beneath the region applying a combination of cross correlation analysis and multimode waveform inversion we measured interstation rayleigh wave dispersion for 450 pairs of stations in a broad period range of 9 250 s we computed phase velocity maps and then inverted the phase velocity data for shear speed structure our results suggest that the location of the transition from seafloor spreading south to continental rifting north in the gulf of california as well as differences in volcanism across baja california and the gulf can be explained by heterogeneity in the upper mantle in particular by the presence of a slab remnant beneath the south central part and an absence of such a slab remnant beneath the northern part of the gulf

reflection characteristics of a mylonite zone based on compressional wave velocities of rock samples summary a combined field and laboratory study of the seismic reflection response and seismic properties of the mylonite zone capping the kettle dome metamorphic core complex provides important constraints on reflectivity of mylonites because of its structural and petrologic characteristics this mylonite zone can be regarded as a model for mid to deep crustal mylonite zones cdp reflection profiling revealed that the mylonite zone is reflective laboratory measurements of compressional wave velocities v at high confining pressures in samples collected directly up dip from the cdp reflection line revealed a wide variation of v in common mid crustal lithologies mafic and carbonate rock mean v values range from 6 67 to 7 31 km s at 600 mpa and quartzite and quartzo feldspathic values range from 6 16 to 6 52 km s seismic anisotropy at low pressures can be related to open microcracks and preferred mineral orientation whereas the latter effect controls anisotropy at high pressures the laboratory data provide the basis for a seismic model which indicates that the kettle dome mylonite zone can generate reflections from zones where high and low velocity lithologies are interlayered these reflections correlate with those observed on the cdp data thus demonstrating that the reflectivity of the mylonite zone is a direct result of its lithologic diversity and layered nature in this case it is not necessary to explain the reflections by the effects of anisotropy retrogression high pore pressure or constructive interference

stratigraphic regulation and guidance a critique of current tendencies in stratigraphic codes and guides a distinction in stratigraphy is made between conventions that need to be stable and therefore regulated by codes for formal use and those that evolve with developing science some stratigraphic codes and guides in adding categories to the essential minimum have both weakened the code and given a spurious authority to matters beyond regulation moreover the tendency to multiply categories has confused the logic of what is being done the essential stratigraphic code is the procedure to establish all rock bodies internationally as formal lithic units in a neutral nongenetic category for basic reference common to all disciplines the procedure is also available to consolidate rules which are already virtually agreed upon and for expressing age in terms of both chronostratic and numerical time divisions beyond these space time framework categories guidance as to conventions may be more suitable than regulation with respect to categories involving contingent elements of new observations and opinion for example biostratigraphy magnetostratigraphy tectonostratigraphy climatostratigraphy and sequence stratigraphy

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

evidence of upper mantle processes related to continental rifting versus oceanic crust in the gulf of california summary receiver functions from teleseismic events recorded by stations around the gulf of california are used to map the upper mantle seismic discontinuities we observe a mean transition zone thickness comparable to the global average for most of the region a low velocity layer is detected above the 410 discontinuity that varies in thickness along the gulf of california the 660 discontinuity shows complex waveforms south of latitude 30 n as a result of the phase change of garnet to perovskite within the transition zone a complex behaviour of the receiver functions is observed mainly at the southern end of the gulf the north south variations of this zone are likely associated with a slab window at the northern gulf resulting from the cessation of subduction of the farallon plate 12 ma and the subduction of the guadalupe and magdalena microplates at the southern end resulting in a hydrated upper mantle our results suggest that change in rifting styles occurring along the gulf of california mirrors deeper processes in the upper mantle

cocorp arizona transect strong crustal reflections and offset moho beneath the transition zone the consortium for continental reflection profiling cocorp transect across the southwest margin of the colorado plateau in arizona reveals prominent subhorizontal zones of reflections here termed the bagdad reflection sequence brs which dominate the upper and middle crust in the transition zone tz between the colorado plateau and the basin and range province a cenozoic origin for the brs is considered most likely it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions possibly emplaced along preexisting zones of weakness moho reflections observed beneath the tz and the basin and range province contrast with the nonreflective moho beneath the colorado plateau suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the moho the moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place this is the best example of offset moho yet found by cocorp and contrasts sharply with cocorp observations of a smooth moho elsewhere this offset is probably a late tectonic expression of crustal extension and thinning across the tz

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

reflections from a mylonitized zone in central sweden dipping reflections imaged on a deep seismic reflection profile acquired within the baltic shield can be indirectly linked to strain induced acoustic impedance contrasts within a regional brittle ductile shear zone the singo shear zone measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis velocity measurements include some made on cored field samples at confining pressures of 0 4 gpa in a laboratory 10 mhz sensor frequencies and some made in situ using ultrasonic 54 khz sensors on exposures within the shear zone laboratory measurements show a 10 change in seismic anisotropy and a contrast of 0 5 km s 1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite the in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes including zones where strain and associated volume loss of silica remain elevated without mylonitization synthetic seismograms generated using a one dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6 25 hz frequencies ray trace two dimensional forward modeling of a high density wide angle reflection section demonstrates that deep dipping reflections could result from strain induced acoustic impedance contrasts within the singo or similar nearby shear zones

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

notes on geochronologic and chronostratigraphic units there is no need to regard the term geochronology as a redundant synonym of either geologic time scale or radiometric dating this term may reasonably be used in reference to all principles and methods employed in geology to determine the relative and numerical ages of all kinds of rocks the term chronostratigraphy would best be restricted to the process of defining the formal subdivisions of the geologic time scale geochronologic unit is best used as a general category term it includes aurichronologic units spans of time defined operationally by golden spikes geochronometric units spans of time defined theoretically in terms of absolute numerical age before present and biochronologic units spans of time defined theoretically by paleobiological events a chronostratigraphic unit is a set of material existing stratified rock that was formed during a given span of time geochronologic unit therefore a geochronologic unit must be defined before the corresponding chronostratigraphic unit can even be conceptualized chronostratigraphic units are generally not isochronous bodies of rock because owing to the unconformities that exist over any appreciable geographic area their upper and lower material boundaries are not everywhere synchronous thus particular systems series and stages are material stratigraphic units of geographically variable time value within the maximum temporal limits of the corresponding periods epochs and ages geochronologic boundaries that have been formally ratified serve a conventional function and will rarely if ever need to be redefined we can continue to discover and debate important events in earth history without insisting that such events must be used to constantly redefine the boundaries of our global temporal pigeonholes

problems in chronostratigraphy stages series unit and boundary stratotypes global stratotype section and point and tarnished golden spikes abstract stratigraphy is a magnificent subject magnificent in its breadth and scope magnificent in its importance to so many lines of human endeavor it is essentially that branch of geology which deals with the arrangement the distribution and the chronological succession of rock strata and other associated rock bodies with respect to any or all of the various characters properties and attributes which rocks may possess hedberg 1958 p 1881 chronostratigraphy the temporal ordering of geologic strata is at the heart of earth history in reviewing chronostratigraphic theory from d orbigny to hedberg we see that concepts and practices have evolved under pressure of constant improvements in time control and data quality such stimuli are naturally felt more strongly in the youngest part of the stratigraphic record where dating is most precise and the biostratigraphic data are most abundant and well preserved it is not surprising therefore to find that chronostratigraphy is conceptually more advanced in the cenozoic erathem than in the mesozoic and paleozoic erathems where biostratigraphy and chronostratigraphy are widely confused evidently out of necessity taking the five inter series boundaries in the cenozoic erathem as case histories we review their current status in relation to the lower boundary of the lowest standard stage above the boundary in light of the hierarchical principle recommended in the international stratigraphic guide hedberg 1976 salvador 1994 in each instance what should be a simple nested relationship turns out to be complex and difficult and in the cases of the proposed global stratotype section and point gssps for the eocene oligocene series boundary and the paleocene eocene series boundary there are discrepancies so large with regard to the base of the rupelian and ypresian standard stages respectively that some essential relaxation of the hierarchic rule seems to be required if the historic value of these long known stages is not to be violated since stage boundaries should not be defined a posteriori to fit series boundaries we propose here to reconsider the role of the standard stage as the obligate elemental subdivision in a rigidly hierarchical chronostratigraphic scale and suggest that series and subseries should be formalized and their boundaries defined by gssps we do not suggest that stages be abandoned on the contrary we propose to integrate them in a two step hierarchical chronostratigraphic framework so as to use them in complementary fashion with series and subseries however the boundary concept is more appropriate for stage than the gssp we recognize the need for precise and globally valid chronostratigraphic correlations based on gssps in attempts to understand the earth system while at the same time we emphasize the need for conceptual continuity with regard to the central place that the stage has played in chronostratigraphy over several decades

thermomechanical implications of sediment transport for the architecture and evolution of continental rifts and margins erosion and deposition redistribute mass as a continental rift evolves which modifies crustal loads and influences subsequent deformation surface processes therefore impact both the architecture and the evolution of passive margins here we use coupled numerical models to explore the interactions between the surface crust and lithosphere this interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink if transport is efficient there are two possible outcomes 1 faulting within the zone of extension is longer lived and has larger offsets this implies a reduction of the number of faults and the width of the proximal domain 2 efficient transport of sediment leads to significant deposition and hence thermal blanketing this will induce a switch from brittle to ductile deformation of the upper crust in the distal domains the feedbacks between these two outcomes depend on the extension history the underlying lithospheric rheology and the influence of submarine deposition on sediment transport high erosion sedimentation during early faulting leads to abrupt crustal necking while intermediate syntectonic sedimentation rates over distal deep submarine hotter crust leads to unstructured wide distal domains in models where rheological conditions favor the formation of asymmetric conjugate margins only subaerial transport of sediments into the distal domains can increase conjugate symmetry by plastic localization these models suggest that passive margin architecture can be strongly shaped by the solid earth structure sea level and climatic conditions during breakup

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

magnitude of crustal extension in the southern great basin strike slip faults in the southern great basin separate areas of cenozoic upper crustal extension from relatively stable tectonic blocks linear geologic features offset along the garlock fault las vegas valley shear zone and lake mead fault system allow reconstruction of the southern great basin to a pre extension configuration the sierra nevada mojave desert spring mountains and colorado plateau are treated as stable unextended blocks that have moved relative to each other in response to crustal extension with the spring mountains held fixed to the mojave block our reconstruction indicates a minimum of 65 extension 140 km between the southern sierra nevada and colorado plateau

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

effects of wave steepness and relative water depth on wave attenuation by emergent vegetation abstract a physical model experiment was conducted to investigate the effects of wave nonlinearity on the attenuation of irregular waves propagating through a stand of uniform emergent vegetation in constant water depth the experimental conditions spanned relative water depths from near shallow to near deep water waves 0 45

structural controls on the continent ocean transition in the northern gulf of california in the gulf of california the pacific north america plate boundary changes character from an oceanic type spreading center and transform fault system to the south to a region of diffuse continental deformation to the north the presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric heat flow gravity or seismic data which indicate significant differences north and south of latitude 30 n we suggest instead that north of 30 n a continent ocean transition begins which we name the wagner transition zone wtz diffuse deformation characterizes the wtz where slip occurs along reactivated north to nnw striking normal faults developed during late miocene or pliocene ene directed extension transtensional deformation varies from ene directed extension along dip slip faults in the west to dextral shear along the coast to dextral oblique slip along inferred north to nnw striking faults submerged in the northern gulf by accounting for rotational and extensional plate motion deformation in northeastern baja california vector constraints require that submerged structures accommodate 30 mm yr of slip in a direction slightly clockwise of the relative plate motion direction the juxtaposition of the discrete spreading center system in the central gulf with the diffuse wtz appears to have been a stable configuration since 4 6 ma perhaps controlling the evolution of spreading center jumps between upper and lower tiburon and delfin basins due to the juxtaposition of kinematically partitioned structural domains different histories of prerift extension and subduction related arc magmatism along the length of the gulf partly related to the migration of the rivera triple junction may explain the location of the continent ocean transition

spectral wave flow attenuation within submerged canopies implications for wave energy dissipation 1 communities of benthic organisms can form very rough surfaces canopies on the seafloor previous studies have shown that an oscillatory flow induced by monochromatic surface waves will drive more flow inside a canopy than a comparable unidirectional current this paper builds on these previous studies by investigating how wave energy is attenuated within canopies under spectral wave conditions or random wave fields defined by many frequencies a theoretical model is first developed to predict how flow attenuation within a canopy varies among the different wave components and predicts that shorter period components will generally be more effective at driving flow within a canopy than longer period components to investigate the model performance a field experiment was conducted on a shallow reef flat in which flow was measured both inside and above a model canopy array results confirm that longer period components in the spectrum are significantly more attenuated than shorter period components in good agreement with the model prediction this paper concludes by showing that the rate at which wave energy is dissipated by a canopy is closely linked to the flow structure within the canopy under spectral wave conditions wave energy within a model canopy array is dissipated at a greater rate among the shorter period wave components these observations are consistent with previous observations of how wave energy is dissipated by the bottom roughness of a coral reef

nature based flood protection the efficiency of vegetated foreshores for reducing wave loads on coastal dikes this paper analyses the effect of vegetation on wave damping under severe storm conditions based on a combination of field measurements and numerical modelling the field measurements of wave attenuation by vegetation were performed on two salt marshes with two representative but contrasting coastal wetland vegetation types cordgrass spartina anglica and grassweed scirpus maritimus the former is found in salty environments whereas the latter is found in brackish environments the measurements have added to the range with the highest water depths and wave heights presented in the literature so far a numerical wave model swan has been calibrated and validated using the new field data it appeared that the model was well capable of reproducing the observed decay in wave height over the salt marsh the model has been applied to compute the reduction of the incident wave height on a dike for various realistic foreshore configurations and hydraulic loading conditions additionally the efficiency of vegetated foreshores in reducing wave loads on the dike has been investigated where wave loads were quantified using a computed wave run up height and wave overtopping discharge the outcomes show that vegetated foreshores reduce wave loads on coastal dikes significantly also for the large inundation depths that occur during storms and with the vegetation being in winter state the effect of the foreshore on the wave loads varies with wave height to water depth ratio on the foreshore the presence of vegetation on the foreshore extends the range of water depths for which a foreshore can be applied for effective reduction of wave loads and prevents intense wave breaking on the foreshore to occur this research demonstrates that vegetated foreshores can be considered as a promising supplement to conventional engineering methods for dike reinforcement

hydrodynamics induced by wind waves in a vegetation field the wave induced kinematics and dynamics of a submerged or emerged vegetation field is analyzed using potential flow and an eigenfunction expansion the problem is solved considering regular as well as irregular incident waves the model takes into account the vegetation motion and solves for the complete wave system on the vegetation field and in its vicinity the model is validated against experimental laboratory data obtained by other authors showing a much better agreement than previous theoretical models in this paper the model is used to evaluate wave height evolution damping vegetation and fluid motion and forces and moments on the vegetation furthermore the inclusion of irregular waves provides force and moment distributions on the vegetation field depending on the wave climate statistics

efficient three dimensional reconstruction of aquatic vegetation geometry estimating morphological parameters influencing hydrodynamic drag abstract aquatic vegetation can shelter coastlines from energetic waves and tidal currents sometimes enabling accretion of fine sediments simulation of flow and sediment transport within submerged canopies requires quantification of vegetation geometry however field surveys used to determine vegetation geometry can be limited by the time required to obtain conventional caliper and ruler measurements building on recent progress in photogrammetry and computer vision we present a method for reconstructing three dimensional canopy geometry the method was used to survey a dense canopy of aerial mangrove roots called pneumatophores in vietnam s mekong river delta photogrammetric estimation of geometry required 1 taking numerous photographs at low tide from multiple viewpoints around 1 m2 quadrats 2 computing relative camera locations and orientations by triangulation of key features present in multiple images and reconstructing a dense 3d point cloud and 3 extracting pneumatophore locations and diameters from the point cloud data step 3 was accomplished by a new sector slice algorithm yielding geometric parameters every 5 mm along a vertical profile photogrammetric analysis was compared with manual caliper measurements in all 5 quadrats considered agreement was found between manual and photogrammetric estimates of stem number and of number mean diameter which is a key parameter appearing in hydrodynamic models in two quadrats pneumatophores were encrusted with numerous barnacles generating a complex geometry not resolved by hand measurements in remaining cases moderate agreement between manual and photogrammetric estimates of stem diameter and solid volume fraction was found by substantially reducing measurement time in the field while capturing in greater detail the 3d structure photogrammetry has potential to improve input to hydrodynamic models particularly for simulations of flow through large scale heterogenous canopies

the 4 january 2006 mw 6 6 san pedro martir earthquake example of an earthquake for calibrating excitation and attenuation studiesthe mw 6 6 san pedro martir earthquake for calibrating excitation and attenuation studies

large magnitude oligo miocene extension in southern sonora implications for the tectonic evolution of northwest mexico new structural and 40ar 39ar data from the santa rosa area southeastern sonora shed light on the magnitude and timing of extensional tectonism episodic magmatic activity from the late cretaceous to the miocene is recorded by a 3 4 km thick structural stratigraphic section that includes a composite laramide granodiorite batholith and its andesitic wall rocks early to middle tertiary rhyolite ignimbrites and silicic domes and late oligocene to middle miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate the older units are complexly faulted and steeply tilted by several generations of normal faults palinspastic reconstructions indicate cumulative extension of 90 early gently dipping nw striking normal faults document major extension oriented n50 e angular unconformities and growth fault relations within oligo miocene sequences bracket this extension between 26 and 20 ma younger widely spaced ns to nnw trending high angle normal faults cut the previously faulted and tilted sections and produced modest 10 15 e w extension that is largely bracketed between 20 and 17 ma the magnitude of neogene extension in this region is greater and the timing is older than previously recognized reconnaissance of other areas between the sierra madre occidental and the coast at this latitude suggests that most extension in sonora occurred between 27 and 12 ma while remnants of the farallon plate were still being subducted plate tectonic models that predict substantial extension in sonora during proto gulf 10 5 ma transtensional deformation may need to be reevaluated an alternative model presented here is that baja california began moving with pacific plate motion shortly after the 11 ma termination of subduction and that northwest motion of baja relative to mainland mexico might total 500 km distributed across the previously extended oligo miocene magmatic arc

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

pleistocene stratigraphy in the southern lower mississippi valley abstract scientific inquiry into pleistocene stratigraphy of the lower mississippi valley lmv dates to early writings of european naturalists in the late 19th century by the early 20th century landscape evolution concepts stratigraphic models and regional syntheses had developed for most areas the 1944 monograph of h n fisk marks the advent of a predictive stratigraphic and landscape evolution model that links form and process to a predominantly glacioeustatic mechanism the fiskian model gained widespread acceptance and decades passed before significant alternate models began to emerge revised stratigraphic and geomorphic concepts are presently developing from newly acquired environmental and engineering data present scenarios classify pleistocene outcrop areas into erosional and constructional landscapes and veneers of eolian colluvial fluvial coastal and marine origin can drape both types of surfaces the southern lmv and adjacent gulf coastal plain gcp experienced significant landscape change during the pleistocene late tertiary pliocene to early pleistocene deposition of the upland complex was by streams with a high sand and gravel load relative to its mud load the regional drainage network and fluvial system behavior was probably significantly different from the modern braided stream alluvial fan complexes received sediment from highland source areas adjacent to the lmv and the glaciated mid continent it is plausible that part of upland complex deposition predates initial glacial advances from early to middle pleistocene an erosional landscape formed during a dissection period that chiefly postdates soil formation on stable landscape positions of the upland complex slope evolution truncated a regionally extensive geosol in multiple phases and parts of the erosion surface complex are graded to the oldest preserved constructional alluvial plains in present valleys toe and foot slope positions of the erosion surface complex and its correlative alluvial plains are presently delineated as the intermediate complex constructional landscapes formed at this time are sparsely preserved fisk s montgomery terrace in the lower red river valley lrrv is the best preserved example influences on the development of erosion surfaces in the lmv are not well understood however interactions of relative sea level fall climate change and epirogenic crustal movement are plausible factors from the latter part of middle pleistocene to the holocene there was widespread evolution of modern constructional landscapes constructional alluviation preserved lithofacies of mixed load laterally accreting meandering streams that developed over large areas of the southern lmv to form parts of the prairie complex lateral planation in valleys and stable rates of upland sediment generation were dominant processes during prairie complex deposition pleistocene stratigraphic examples considered important by fisk are still considered relevant to modern stratigraphic investigators presently pleistocene units of the southern lmv the adjacent lrrv and central gcp can be correlated only by relative stratigraphic relationships refined chronostratigraphic and paleoenvironmental models for these areas would help improve the understanding of the geomorphic influences on quaternary landscape evolution in the region