geometry and physical properties of the socorro new mexico magma bodies analysis of multichannel seismic reflection profiles gathered by the consortium for continental reflection profiling cocorp constrains the geometry and material properties of a proposed magma body located within the rio grande rift a new spectral ratio method is used to find the reflectivities of the proposed midcrustal intrusion large and spatially variable reflectivities support the interpretation of the reflector as an intrusive molten magma complex modeling of the reflectivity suggests that in one location the intrusion took the form of several thin 30 to 40 m thick layers seismic wave attenuation within the rift s midcrust is low

nature of the eastern boundary of the rio grande rift from cocorp surveys in the albuquerque basin new mexico reprocessing and synthetic seismic modeling of cocorp profiles across the eastern margin of the rio grande rift at abo pass new mexico suggest that synthetic and antithetic cenozoic normal faults sole into or are truncated by a northwest dipping listric master fault which bounds the eastern side of the basin and reaches a depth of at least 10 km beneath the southern albuquerque basin this fault appears to project toward the surface position of the los pinos cenozoic normal fault which lies basinward of and trends parallel to a mapped laramide duplex thrust zone the los pinos fault may sole into the older thrust reactivating this preexisting compressional structure at depth a lateral change in reflection amplitude and frequency is conspicuous across the eastern rift margin analysis demonstrates that source coupling is the main cause of a much deteriorated signal below the rift basin although complex ray paths and attenuation by the graben fill may contribute

geochemistry and geochronology of the sierra de gomez limestone hosted u deposit chihuahua implications for distribution of rio grande rift mineral deposits in northern mexico abstract uranium deposits form in a variety of settings they are partially controlled by the secular evolution of earth processes including deposits in extension related settings such as the intra cratonic rio grande rift plio quaternary volcanism mineral deposits and hydrothermal spots occur along the chihuahua central graben the age of the sierra de gomez u deposit is 1 8 ma based on la mc icp ms dating on a uranophane monocrystal which is contemporaneous with the late mineralization event of the pena blanca u deposit as well as rio grande rift rgr type deposits in chihuahua and intraplate volcanism studies of fluid inclusions in fluorite and late calcite indicate the presence of hydrocarbons and ch4 rich brine homogenization temperatures range from 87 to 112 c and the mean composition 2 0 mol nacl and 0 3 mol cacl with ch4 is comparable to mineralizing brines in mvt deposits and carbonated hydrocarbon reservoirs evolution of c and o stable isotopic values for the calcite cement in the sierra de gomez limestone hosted u deposit illustrates that two separate calcite precipitation events occurred 1 travertine filling karst structures in the presence of meteoric water and 2 u mineralization during deep hydrothermal fluid circulation that included interactions with a heat source and basement leaching in a regional context a metallogenic model suggests that the chihuahua trough area is deep enough to generate fluid migration by hydrothermal and or compaction processes through rgr extensional faults until a favorable trapping horizon is reached this causes uranium precipitation because water rock interaction processes generate a local redox barrier

a high resolution seismic reflection refraction study of the chugach peninsular terrane boundary southern alaska we present results from a high resolution seismic refraction analysis of the shallow approximately 2 km crustal structure along the 107 km long trans alaska crustal transect chugach reflection line in southern alaska and a comparison with laboratory measurements of field samples the refraction analysis includes the two dimensional interpretation of several thousand first and secondary arrival travel times digitized from 1024 channel split spread common shot gathers the velocity model derived from this analysis better defines the location and geometry of terrane boundaries than does the normal incidence reflection section and agrees well with surface mapping of lithologies furthermore the model predicts travel times within 100 ms of the reflection times recorded from the base of the quaternary on the chugach reflection section thicknesses of quaternary deposits with velocities between 1 1 and 2 0 km s correlate inversely with the quantity of observed lower crustal reflections on the chugach section suggesting that the presence or absence of these sediments in sufficient thickness exerted primary control on the quality of the deeper portion of the section there is a significant velocity contrast between crystalline rocks across the border ranges fault 5 0 versus 5 6 km s the major contact between the chugach and peninsular terranes in agreement with laboratory measurements of field specimens in the peninsular terrane the modeling indicates that an unnamed fault delimiting the southern flank of the copper river basin dips steeply northward at 50 and has about 1300 m of vertical offset laboratory measurements document a maximum velocity anisotropy of 20 for phyllitic schists of the valdez group in the chugach terrane in agreement with the observed e w strike and near vertical dip of the valdez group we determined a significant 14 velocity anisotropy for ray paths oriented n s versus ne sw

underwater bleaching of signals from sediment grains new experimental data abstract the results of an underwater bleaching experiment are reported the purpose of the experiment was to evaluate the degree to which thermoluminescence tl and optically stimulated luminescence osl signals are reduced by a three hour exposure to light at different water depths samples of quartz and feldspar separates were annealed in the laboratory and then given fixed doses of laboratory beta radiation before exposure the results indicate that a three hour light exposure is sufficient to zero the optical signals down to water depths of 10 m for feldspar infra red stimulated luminescence and 12 14 m for quartz osl the tl results for the same quartz and feldspar samples exhibit some bleaching effects with tl signals at 14 m water depth being reduced to 50 65 of the initial value scatter of the tl results limits their interpretation

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

optically stimulated luminescence dating of sediments over the past 200 000 years optical dating of sediment using optically stimulated luminescence has become important for studying earth surface processes and this technique continues to develop rapidly a group of closely linked luminescence methods can be used to estimate the time since grains of quartz and feldspar were last exposed to daylight by detecting their subsequent response to environmental ionizing radiation exposure the technique is well suited to the dating of deposits as young as one year to several hundred thousand years recent technical developments have established a dating protocol with improved precision a high degree of reliability and an in built means to detect incomplete signal removal during deposition this approach has been extended to age estimation for single grains opening up a wider range of potential environments and new possibilities for understanding postdepositional grain movement ongoing research offers the possibility of significant age range extension and novel applications including low t

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

depth migration of imaged time sections none of the leading approaches to the migration of seismic sections the kirchhoff summation method the finite difference method or the frequency domain method readily migrates seismic reflections to their proper positions when overburden velocities vary laterally for inhomogeneous media the diffraction curve for a localized buried scatterer is no longer hyperbolic and its apex is displaced laterally from the position directly above the scatterer hubral observed that the kirchhoff summation method images seismic data at emergent image ray locations rather than at the desired positions vertically above scatterers in addition distortions in diffraction shapes lead to incorrect imaging i e incomplete diffraction collapse and hence to further displacement errors for dipping reflections the finite difference method has been believed to continue waves downward correctly through inhomogeneous media in conventional implementations however both the finite difference method and frequency domain a

luminescence dating sediment analysis and flood dynamics on the sabie river south africa abstract floods on sediment rich semiarid rivers often lead to significant geomorphic change but luminescence dating of sediments deposited by past flood events has hitherto been unable to meaningfully inform on past flood dynamics this is important because luminescence dating has commonly been applied to flood sediments but the calculated sediment ages are in almost all cases older than the ages of known floods responsible for sediment deposition correctly interpreting these older ages in the context of flood dynamics is therefore a key problem this study addresses this problem by discussing relationships between luminescence dating sediments and reach scale geomorphology along the semiarid sabie river northeastern south africa twelve luminescence ages were obtained from near surface sediments

a 3 d seismic modeling study of the ladron horst near socorrro new mexico three dimensional seismic modeling demonstrates that the intragraben horst like feature imaged on cocorp s socorro abo pass survey in the albuquerque basin new mexico likely corresponds to the northeast plunging continuation of the ladron uplift this accords with the original interpretation which was expounded with reservations that the feature might be a result of side reflections off a nearby exposed basement uplift this domed feature on socorro line 1a referred to hereafter as the ladron horst cannot be a consequence of side reflections from the nearby ladron mountains three dimensional modeling defines a minimum northeastern extent of the ladron uplift in the subsurface confirms some aspects of previous interpretations and offers a new interpretation of the apparent western boundary of the horst second order discontinuities in the reflections off the horst are modeled as normal faults within the subsurface basement uplift

shallow velocity structure of the rio grande rift north of socorro new mexico a reinterpretation shallow upper 1 2 km compressional wave velocity models from line 2a of the socorro new mexico consortium for continental reflection profiling cocorp survey of the rio grande rift supplement a previous refraction study of socorro cocorp lines 1 and 1a first arrival refraction analysis and slant stacks of the refraction profiles derived from the reflection data are used to determine velocity models of socorro cocorp line 2a exposure of paleozoic and mesozoic sedimentary formations in the vicinity of socorro line 2a allow direct correlation of a unit s velocity to its geological era weathered mesozoic and paleozoic formations within the rift are inferred to have significantly lower velocities than previously believed reinterpretation of the rio grande rift structure in the survey area results in a model that is consistent with available gravity data and provenance studies the 13 km of cenozoic spreading determined from the model is at least 5 km higher than that found 80 km to the north there is evidence of minor paleozoic deformation or nondeposition

seismic signal penetration beneath postrift sills on the newfoundland rifted margin analysis of averaged smoothed instantaneous frequency and amplitude of seismic reflection data collected on the newfoundland magma starved rifted margin along the ocean drilling program odp leg 210 transect demonstrate that the transparency of transitional crust arises from poor signal penetration in principle the high frequency spectral content and amplitude of seismic reflection data should decrease with increasing traveltime as a result of absorption geometric spreading and scattering so long as seismic energy continues to return from deeper levels of the subsurface as a result if average frequency and amplitude cease to decrease with depth background noise rather than returning seismic energy likely dominates the record average frequency increases and average amplitude remains comparatively constant below bright reflections overlying transitional crust on the newfoundland margin similar patterns are not observed at the sediment basement contact farther seaward in oceanic crust where in

dip moveout by fourier transform the conventional normal moveout nmo and common midpoint cmp stacking process enhances reflections having a particular moveout velocity while attenuating events such as multiple reflections having different moveout velocities unfortunately this process also acts as a dip filter applied to the cmp stack in other words nmo and stacking enhances reflections having a particular slope in the cmp stack while attenuating reflections having different slopes nmo and stacking like any dip filter degrades lateral resolution fortunately this dip filtering action can be suppressed by applying in addition to nmo a prestack process known variously as devilish prestack partial migration and dip moveout as the latter term implies this process is a dip dependent moveout correction that enables reflections from both horizontal and dipping reflectors to be stacked with the same nmo velocity stated another way nmo velocities estimated from dip moveout corrected seismograms are independent of the dips o

reconstruction of floodplain sedimentation rates a combination of methods to optimize estimates reconstruction of overbank sedimentation rates over the past decades gives insight into floodplain dynamics and thereby provides a basis for efficient and sustainable floodplain management we compared the results of four independent reconstruction methods optically stimulated luminescence osl dating caesium 137 cs 137 dating heavy metal analysis and flood bed interpretation applied at three embanked floodplain sites along lower rhine river distributaries in the netherlands all methods indicate significant sedimentation rates on the floodplains varying between 2 7 mm a in the distal zones and 3 9 mm a in the proximal zones on a rapidly developing sand bar along a natural levee sedimentation rates of 9 to 25 mm a were found except for some minor inconsistencies in cs 137 dating results all methods show decreasing sedimentation rates with increasing distance from the river channel intercomparison of the results of the different dating methods revealed the potential errors associated with each method particularly where disagreement among the results were found uncertainties may arise due to 1 grain size dependent downward migration of cs 137 2 smoothing of the vertical heavy metal and cs 137 profiles 3 delayed sediment associated input of cs 137 in addition to direct atmospheric fall out 4 overestimation of the burial age in os l dating due to incomplete resetting of the osl signal or 5 non linear relationships between sediment deposition and flood magnitude in the count from the top correlation between sediment lamination and past observed flood records still taking the uncertainties associated with each method into account the results are generally in good agreement using the results we indicate the optimal spatial range of application of each method depending on sediment texture and sedimentation rate the optimal spatial and temporal ranges differ for each method but show significant overlap a combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale

optical resetting in large drainage basins tests of zeroing assumptions using single aliquot procedures abstract optical resetting of previously accumulated charge populations at the time of transportation and burial of sediments remains a critical assumption in luminescence dating this is particularly the case for fluvial sediments we have tested the extent of bleaching in fluvial environments by applying the single aliquot regeneration sar equivalent dose d e determination technique coupled with tests for partial bleaching in the majority of cases samples collected from a point bar complex on the colorado river texas exhibit evidence of incomplete bleaching use of the minimum sar d e in these cases resulted in estimated residual ages of c 100 300 years in a similar examination of sar d e s for bed load samples collected along the loire river we observe a monotonic decrease in effective equivalent dose d e from values of 14 10 2 and 3 2 mean median and minimum gy at source down to 3 3 3 6 and 1 6 gy in the first kilometre of transportation down to levels of 0 3 0 2 and 0 0 gy for transportation distances exceeding 300 km we observed virtually no effect on the d e in the bed load of the introduction of trunk tributary streams and conclude that the likelihood of resetting of osl in large fluvial systems is somewhat more effective that previously considered when the recycling of sediments down the system are borne in mind

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

gravity models of the albuquerque basin rio grande rift new mexico as part of a hydrologic study gravity anomalies over the albuquerque basin were modeled to determine the thickness of the low density neogene basin deposits the major aquifer of the basin the models are constrained by substantial density log and well data fifteen two dimensional 2 d models include four major density units the upper unit density 2 20g cm3 corresponds to the neogene syn rifting deposits thickness of this unit averages about 1 5 km and exceeds 2 0 km in a strip along the east side of the basin the thickness is about 1 0 km between the albuquerque and the santo domingo basins at the south end of the albuquerque basin the thickness is less than 0 5 km the configuration of the remaining density units less constrained by well data raises significant questions about the history of the basin that cannot be answered with existing data

holocene incisions and flood activities of the keriya river nw margin of the tibetan plateau abstract fluvial terraces are common in the tectonically active western kunlun mountains wklm region northwestern margin of the tibetan plateau and incision of rivers since 5 ka was attributed to the accelerated uplift of the wklm however it is difficult to evaluate their tectonic or climatic origin without detailed chronology in this study optically stimulated luminescence osl dating was applied to fluvial flood and aeolian sediments on five lowest fluvial terraces 49 m in total along the keriya river and the 21 osl ages from 11 samples revealed the incision processes since the mid holocene especially the fast incision 3 5 0 ka 10 9 mm a during the late holocene this was supported by the climatic background i e increased precipitation from the westerlies and meltwater from frequent glacial advance retreat events as a response to the climatic changes large scale floods were frequent as well e g at 3 5 2 6 0 87 and 0 25 ka which were crucial for the fast incisions on the filling gravels additionally the flood also caused the river s further extension into the taklamakan desert and controlled the evolution of oases in the desert including ancient cities and cultures consequently we suggest that climatic change was important for the incision and formation of cut in fill terraces along the keriya river during the holocene and influences from other factors including surface uplift should not be ignored but were difficult to be evaluated

zeroing of the tl signal in sediment undergoing fluvioglacial transport an example from austerdalen western norway abstract study of levels of natural tl in sediment undergoing fluvial transport in austerdalen norway shows that bleaching does occur but does not follow a regular pattern this is attributed to the incorporation of older unbleached material into the sediment load as a result of riverbank erosion levels of residual tl are high enough to raise doubts about the accuracy of fine grain tl dating of fluvioglacial sediments which have experienced only a short distance of transport

use of floodplain sediment cores to investigate recent historical changes in overbank sedimentation rates and sediment sources in the catchment of the river ouse yorkshire uk abstract floodplain sediment cores collected from seven sites in the catchment of the river ouse in yorkshire uk have been used to provide information on recent historical changes in both rates of overbank sedimentation and sediment sources the environmental radionuclides 137 cs and unsupported 210 pb have been used to establish chronologies for each core and to estimate average sediment accumulation rates for the last ca 30 and 100 years respectively average sedimentation rates for the individual cores ranged from 0 11 to 1 04 g cm 2 yr 1 in all but one case the estimates of average sedimentation rate during the last ca 30 years for the individual cores are broadly similar to those for the last ca 100 years suggesting that overbank sedimentation rates have been essentially uniform over the longer time period composite fingerprints based on a combination of geochemical and mineral magnetic properties and a numerical mixing model have been used to investigate downcore changes in sediment source in the case of source type most of the cores reflect a primarily topsoil source although there have been periods with increased contributions from subsoil channel bank sources within the ouse basin in general the period commencing in the late 19th and early 20th century and extending through to the 1960s was characterised by increased contributions from topsoil sources however contributions from subsoil channel bank sources have increased over the last few decades the source tracing results relating to sediment contributions from the three main geological topographic zones are in broad agreement with the proportion of the area of the catchment underlain by each rock type temporal variations in the contributions from the three geological topographic zones vary from site to site but for the lower reaches of the river ouse contributions from areas underlain by permian and triassic rocks which mainly outcrop in the vale of york have increased since the turn of the century the changes in sediment source identified are probably a reflection of changes in land use and management and possibly changes in climate these results enable estimates of contemporary suspended sediment fluxes and sources to be placed into a historical context and provide a means of considering the likely impact of potential future changes in land use and climate in the study basin

deep structure of the northern rio grande rift beneath the san luis basin colorado from a seismic reflection survey implications for rift evolution abstract a seismic reflection survey by chevron across the san luis basin northern rio grande rift and san juan volcanic field of southern colorado is reprocessed with extended correlation to search for basement structure the trace of the main bounding fault of the basin a high angle normal fault against the sangre de cristo range can be correlated to a wide zone of dipping reflection fabric and soles out at lower crustal depths 26 28 km the deeper reflection fabric represent either broad extensional strain or pre existing structure such as a laramide thrust system the sangre de cristo bounding fault in san luis basin does not sole out at mid crustal depths but continues into the lower crust with a shallower dip the basin architecture in the northern rio grande rift san luis basin provides little if any evidence that the sangre de cristo bounding fault should flatten in a shallow listric fashion this fault geometry is quite similar to the high angle bounding fault in the espanola basin but contrasts with less deeply rooted faults in the albuquerque basin in the central rio grande rift deeper soling out of the sangre de cristo bounding fault could be due to less extension in the northern rio grande rift and or greater strength of the lithosphere compared to the central rio grande rift unequivocal moho reflections beneath the san luis basin cannot be identified probably due to limited signal penetration or a gradational nature of the moho the majority of rift related movement observed on the sangre de cristo bounding fault is post eocene either the western margin of the basin is marked by a tight monocline or a low angle normal fault

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

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

effectiveness of natural zeroing of the thermoluminescence in sediments the last exposure to sunlight of detrital feldspars and quartz grains in quaternary sediments can be dated directly by thermoluminescence tl methods but the effectiveness of zeroing of the light sensitive tl signal by daylight varies widely depending upon details of the sedimentation processes to document this variability and to help develop objective criteria useful for future sample selection and laboratory procedures i have collected and analyzed several eolian fluvial marine deltaic and lacustrine samples from less than 100 year old deposits as well as suspensions from sediment plumes also underwater spectral irradiance was measured during recovery of suspensions tl properties of polyrnineralic feldspar dominated 2 4 m and 4 11 m size fractions were measured in the laboratory unexpectedly some light sensitive tl was found to remain in all samples however the light sensitive tl was more effectively zeroed in some samples a proximal loess a sandy braided channel deposit tidal flat and upper delta front muds and thin clayey lacustrine laminae than in others a silty braided channel deposit submarine fan muds and silt bearing lacustrine laminae for the first group the minimum relict paleodose obtained with the partial bleach technique is about 0 4 0 3 gy corresponding to a relict tl age estimate of about 200 years for these sediments thus similar sediments excluding fluvial deposits as young as about 2 ka can be accurately and routinely dated without significant correction for a zero point error if one uses the partial bleach technique coupled with integrated optical bleaching energy fluences of less than 1 j cm 2 and wavelengths only above about 550 nm also some simple optical bleaching tests are reported that have a potential use in sample selection and in provenance studies of some surficial detrital sediments finally a simple mathematical model is presented that helps account for the gross features of the observed bleaching response curves

seismic reflection profiling of the cheyenne belt proterozoic suture in the medicine bow mountains southeastern wyoming a tie to geology the cheyenne belt a broad mylonitic shear zone marks a 1 8 1 7 ga suture between archean craton to the north and a proterozoic island arc to the south a university of wyoming seismic reflection profile across the medicine bow mountains imaged the steeply dipping cheyenne belt and crosscutting laramide faults by combining surface geology with reflection data from a line that trended oblique to the cheyenne belt suture and laramide thrusts the true subsurface geometry of both could be resolved the suture zone dips 60 southeast to depths of at least 9 to 14 km a 5 to 10 km thick zone of reflectors project to the surface near mapped laramide faults and have a true structural dip of 45 the imbricate thrusts within this zone cut the cheyenne belt near 9 km depth some weaker events persist throughout the archean lower crust however no clear moho is observed the subsurface geometry of the cheyenne belt suggests that archean crust is not absent at depth just south of the southernmost splay of the suture as nd isotope studies might suggest alternatively the proterozoic hanging wall thickens such that rising plutons may form completely within the hanging wall and show only proterozoic geochemical signatures the complex reflectivity pattern observed in the cheyenne belt line is caused mainly by the multideformational history of the area and the oblique orientation of the line ties between reflections and outcrop show that fault related lithologic contrasts and deformation within the fault zones generated reflectors at midcrustal depths during the proterozoic later laramide brittle faulting add to rather than erased the reflectivity

migration of noisy crustal seismic data a slowness weighted diffraction stack algorithm is used to migrate crustal seismic reflection data slowness information is extracted from the data using a slant stack beam forming process the main purpose of beam forming is to improve the signal to noise ratio over a given range of slowness values localized slant stacks are used to design a coherency weighted slowness filter in the time distance domain through migration the subsurface structure and the observed seismic data are tied via slownessselective diffraction curves a combination of nonlinear slowness filtering with migration gives excellent noise rejection and leads to an improved image in the depth domain the method which is particularly effective for migrating moderate to steep dipping events with low signal to noise ratio is applied to various poststack and prestack synthetic data examples and to a deep crustal data set collected on vancouver island as part of the canadian lithoprobe project

moho reflectivity and seismic signal penetration abstract analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by lithoprobe and the consortium for continental reflection profiling cocorp changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth results from montana show pronounced increases in average frequency and slight drops in amplitude associated with a die out of reflections at the moho results from quebec show amplitude and frequency change occurring through the moho on lines with and without moho reflections these contrasting observations confirm both adequate signal penetration and that the differences in moho reflectivity are representative of geologic differences

variables and potential models for the bleaching of luminescence signals in fluvial environments abstract luminescence dating of fluvial sediments rests on the assumption that sufficient sunlight is available to remove a previously obtained signal in a process deemed bleaching however luminescence signals obtained from sediment in the active channels of rivers often contain residual signals this paper explores and attempts to build theoretical models for the bleaching of luminescence signals in fluvial settings we present two models one for sediment transported in an episodic manner such as flood driven washes in arid environments and one for sediment transported in a continuous manner such as in large continental scale rivers the episodic flow model assumes that the majority of sediment is bleached while exposed to sunlight at the near surface between flood events and predicts a power law decay in luminescence signal with downstream transport distance the continuous flow model is developed by combining the beer lambert law for the attenuation of light through a water column with a general order kinetics equation to produce an equation with the form of a double negative exponential the inflection point of this equation is compared with the sediment concentration from a rouse profile to derive a non dimensional number capable of assessing the likely extent of bleaching for a given set of luminescence and fluvial parameters although these models are theoretically based and not yet necessarily applicable to real world fluvial systems we introduce these ideas to stimulate discussion and encourage the development of comprehensive bleaching models with predictive power

cenozoic normal faulting and the shallow structure of the rio grande rift near socorro new mexico migrated versions of a deep seismic section across the albuquerque basin in the southern rio grande rift cocorp abo pass line 1 and socorro line 1 a are used to develop a detailed interpretation of the shallow structure of the rift our interpretation which is consistent with nearby drill hole and gravity data suggests that listric faulting has been the dominant extensional style of faulting in this part of the rift during the early stages of rifting blocks of paleozoic and mesozoic prerift sedimentary strata overlying precambrian crystalline basement were offset and rotated along listric normal faults rooted in the basement syntectonic deposition of thick sections of mid tertiary sedimentary and volcanic rocks filled the developing fault bounded basins these units were continuously rotated during ongoing listric faulting and subsequently buried by later tertiary and quaternary basin fill present surface fault traces may reflect continued movement on the deeper listric faults or a more recent minor episode of shallow normal faulting in contrast to models of crustal extension proposed for nearby areas in the rio grande rift and basin and range province no clear evidence is seen for two time distinct stages of faulting characterized by early closely spaced faulting with large stratal tilts followed by wide spaced high angle faulting the large listric faults flatten into basement or converge upon an apparent detachment surface at a depth of about 5 km posing intriguing questions about the connection between shallow structure and deeper extension within the crust

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

wavefield extrapolation of body waves for 3 d imaging of earthquake sources summary reverse time imaging of earthquake source parameters is extended from two to three spatial dimensions and from two to three component recordings provided that the recording aperture is sufficiently large and the data are not spatially aliased source radiation patterns in both time and space can be reconstructed by elastic reverse time propagation of body waves such reconstructions are necessarily partial as only the energy that was recorded is available for reconstruction for a point source the origin time and the 3 d spatial location of an event can be reconstructed by extracting the time and position of the best focused energy from the backward propagating wavefield for a spatially and temporally extended source biased estimates of fault position and rupture slip time history can be estimated if the recording aperture is sufficient for smaller recording apertures the time history can still be estimated if the fault location and geometry are known a priori the latter is viable for a reasonable number 200 of three component recordings and so is potentially applicable to real data all these processes assume that a smoothed representation of the 3 d velocity distribution in the volume containing the source and receivers is available

elastic finite difference modelling and imaging for earthquake sources summary 2 d modelling and imaging of line double couple and spatially extended earthquake sources is demonstrated extrapolation of two component elastic wavefields is implemented using finite differences running the extrapolatioin forward in time produces synthetic seismograms running it backward in time produces an image of the source provided that the recording aperture is adequate the source location extent orientation and radiation pattern may be recovered from the data synthetic examples include a normal fault at the edge of a rift a reverse fault in a subduction zone and a line double couple in a cylindrical whole earth model

determining fluvial sediment virtual velocity on the mojave river using k feldspar irsl initial assessment abstract the mojave river of southern california was chosen as a field site to investigate the applicability of luminescence dating to sediment transport rate problems grains in the active channel of the river are expected to show signs of partial bleaching and this makes it difficult to determine time since deposition accurately a modification of the multiple elevated temperature post ir irsl metpirir procedure buylaert et al 2009 li and li 2011 was used for k feldspar grains 175 200 m at temperature increments of 50 95 140 185 and 230 c in order to provide more information about relative signal bleaching among samples the measurements show an exponential decrease in equivalent dose de with distance down the mojave river higher temperature pirir signals are bleached more slowly than lower temperature ones buylaert et al 2009 the de for samples at 50 c is roughly constant along the river these results suggest cyclical bleaching and burial as grains are transported downriver and higher energy deeper traps are vacated the pattern of de values for the mojave river can be used to constrain the sediment transport rate for this river by building a model of growth and bleach for each temperature increment a bleaching experiment was run with multiple aliquot samples for direct sunlight exposure times of 0 10 30 300 1000 3000 10 000 and 30 000 s the met pirir procedure was applied at each temperature increment for each exposure time aliquot and the results for all exposure times were fit to the general order kinetics equation using a non linear regression the bleaching parameters were used in conjunction with the sar growth curves to build a model of partial bleaching of grains during transport that is fitted with a 2 test to the pirir data from the mojave river this model is not a unique solution but can be used to assess the likelihood of various sediment transport regimes

proterozoic basin in the southern midcontinent of the united states revealed by cocorp deep seismic reflection profiling cocorp deep crustal seismic profiles in southwestern oklahoma show strong persistent continuous and undeformed layering in the basement over an area probably very much greater than 2 500 km 2 such layering is very unusual judging by cocorp experience with basement rocks elsewhere in the united states the data can be interpreted as representing a proterozoic basin filled with clastic sedimentary and felsic volcanic rocks 7 to 10 km thick whose base lies 10 to 13 km deep these rocks are believed on the basis of sparse evidence from regional geology to have been deposited or extruded about 1 200 to 1 400 m y ago and some of them may now be metamorphosed this basin lies on the south side of the wichita mountains under the paleozoic hardeman basin and is similar in depth to the paleozoic anadarko basin north of the mountains the deep basement layering is truncated on the south side of the wichita mountains probably by precambrian faults in conjunction with granitic intrusions pennsylvanian compression probably reactivated these precambrian trends extensive precambrian basin deposits in this area were unexpected on the basis of evidence from sparse well control and reports of other layered basement reflections elsewhere in the southern midcontinent suggest that precambrian basins may be an important feature of this region simple models for the evolution of southwestern oklahoma as an aulacogen must be reformulated in the light of these new data

three dimensional structure of low frequency pressure variations in the tropical atmosphere abstract results of a recent study show eastward propagation of information in the low frequency variations of the tropical sea level pressure slp field the current work extends that analysis to investigate the vertical structure of this signal it is found that the propagating signal exists to a height of at least 850 mb at 500 mb the signal is not so clear while at 200 mb there is little evidence of propagation of information instead the low frequency variations in 200 rnb height appear coherent for the tropical belt around the entire globe the analysis suggests that the anomalies discussed here appear first at the surface and later at higher levels in the atmosphere

reanalysis of the cocorp deep seismic reflection profile across the san andreas fault parkfield california in 1977 cocorp recorded a short seismic reflection test line across the san andreas fault through parkfield california where the salinian crustal block is juxtaposed against a contrasting terrane of franciscan and overlying material although analysis of this seismic line has been limited by crooked geometry lack of cross lines complex terrain possible recording noise and perhaps most of all its short length gross crustal features are discernible after extensive reprocessing and reanalysis the salinian block southwest of the san andreas consists of a maximum of 2 km of relatively underformed sediments overlying a reflective and inhomogeneous basement which exhibits deep subhorizontal layering at 8 to 10 km depth density and amplitude of reflections markedly decay perhaps corresponding to the base of a salinian granitic crust which overlies franciscan or other contrasting material northeast of the san andreas subhorizontal reflections suggest that franciscan material may extend as deeply as 13 km below which is a nonreflective zone which might represent relict oceanic crust reflections from moho depths differ across the san andreas fault beneath the salinian block is a thick zone of reflections beginning at about 22 km depth while less layered and weaker reflections seem to define the moho discontinuity at 29 km beneath the franciscan terrane the san andreas fault zone appears to be traceable as a distinct feature on the reflection section throughout the entire crust forming a near vertical zone about 3 km wide in the seismogenic upper crust down to 15 km the fault zone is marked by sharp structural truncations and short reflection or diffraction segments at depth it seems to be distinguished by a nonreflective column this vertical change in reflection character may correspond to the transition from a brittle to a ductile deformation regime

cocorp new perspectives on the deep crust summary relict sutures from colliding continents regions characterized by a young mono layering and faulting throughout the crust mid crustal magma traps and seismic bright spots which suggest deep crustal fluids are among recent cocorp findings in addition new studies of signal penetration noise mitigation recording geometry and coherency filtering have yielded better understanding of and substantial improvements in data quality amplitude anomalies or bright spots in the basin and range may be due to magma at mid crustal levels in one case a normal fault appears to link the deep magma with young surface volcanics another bright spot 15 km deep in southeastern georgia has a flat geometry that suggests a gas liquid interface perhaps within fluids underthrust along an appalachian suture the mono continues to appear relatively undisturbed in many regions of past deformation suggesting that its formation post dates these major tectonic episodes the diversity of reflection patterns from the u s cordillera casts further doubt on the generality of the common model of a reflective layered lower crust underlying a transparent upper crust