correlation and provenance of upper cretaceous campanian fluvial strata utah u s a from zircon u pb geochronology and petrography abstract detrital zircon and petrographic provenance analysis of upper cretaceous campanian strata corroborate correlation of the fluvial kaiparowits formation in southern utah with post castlegate strata of the mesaverde group in eastcentral utah sandstone samples of the kaiparowits formation in the kaiparowits plateau and mesaverde group neslen bluecastle tongue of castlegate farrer and tuscher formations in gray canyon north of green river utah contain six u pb detrital zircon age populations ranging in age from archean to late cretaceous thrust belt derived sublitharenite and quartzarenite of the lower part of the mesaverde group neslen and bluecastle lack or have only rare mesozoic grains but contain abundant archean proterozoic and paleozoic grains that record recycling of proterozoic paleozoic and jurassic sandstones exposed in uplifted thrust sheets to the west south and southwest derived feldspathic litharenite of the kaiparowits and farrer formations contains proterozoic grains and a diverse population of mesozoic grain ages archean grains are rare to absent maximum depositional ages interpreted from young zircon grains in two kaiparowits samples statistically overlap 40ar 39ar ages 76 74 ma from interbedded bentonitic tuffs and weighted mean ages of young grains from the farrer formation kolmogorov smirnoff statistics in conjunction with young grain ages and detrital age spectra corroborate petrographic evidence which indicates that the kaiparowits rivers were connected northward with a river system that deposited the farrer formation the young grain ages although imprecise indicate that the uppermost mesaverde group strata of the tuscher formation are younger than the kaiparowits formation and record partitioning of the former foreland basin by laramide uplifts

finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

effect of fault rupture characteristics on near fault strong ground motions the effect of fault rupture characteristics on near fault strong ground motions is investigated using a kinematic modeling approach in an attempt to identify physical processes that lead to specific ground motion patterns the shear stress distribution on the causative fault plane of four well documented seismic events i e 1979 imperial valley 1985 michoacan 1989 loma prieta and 1999 izmit is calculated based on fault slip models available in the literature using the methodology proposed by bouchon 1997 for stress field computations in order to associate the fault rupture characteristics i e slip rupture velocity state of stress of the investigated earthquakes with near fault ground motions generated by the events forward ground motion simulations are performed using the discrete wavenumber representation method and the concept of the s wave isochrones is exploited the results indicate that the seismic energy radiated from the high isochrone velocity region of the fault arrives at the receiver within a time interval that coincides with the time window of the long period ground motion pulse recorded at the site furthermore the near fault ground motion pulses are strongly correlated with large slip on the fault plane locally driven by high stress drop in addition the local rupture velocity seems to be inversely correlated to the spatial distribution of the strength excess over the fault plane confirming findings of previous studies for various events the area of the fault that contributes to the formation of the near fault pulse encompasses more than one patch of significant moment release subevent e g 1979 imperial valley 1989 loma prieta this observation explains why a dislocation model with average properties i e slip rise time etc reproduces successfully near fault ground motions for strike slip faults and for dip slip faults with intermediate to large earthquake magnitudes aki 1979 however for very large earthquakes such as megathrust events on subduction zones e g 1985 michoacan the fault region that contributes to the pulse formation encompasses individual subevents and consequently cracklike slip functions rather than dislocation models may be more appropriate for the simulation of the near fault ground motions

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

the pragmatic roots of american quaternary geology and geomorphology abstract h l fairchild s words from the 1904 geological society of america bulletin remain appropriate today geologists have been too generous in allowing other people to make their philosophy for them geologists have quietly followed a methodological trinity involving 1 inspiration by analogy 2 impartial and critical assessment of hypotheses and 3 skepticism of authority prevailing theoretical constraints or paradigms these methods are described in classical papers by quaternary geologists and geomorphologists mostly written a century ago in recent years these papers have all been criticized in modern philosophical terms with little appreciation for the late 19th century american philosophical tradition from which they arose recent scholarly research however has revealed some important aspects of that tradition giving it a coherence that has largely been underappreciated as 20th century philosophy of science pursued its successive fads of logical positivism critical rationalism relativism and deconstructivism for all of which science is synonymous with physics nearly all this ideology is geologically irrelevant as philosophy of science in the late 20th century has come to be identical with philosophy of analytical physics focused on explanations via ideal truths much of geology has remained true to its classical doctrines of commonsensism fallibilism and realism in contrast to the conceptualism and the reductionism of the analytical sciences geology has emphasized synthetic thinking the continuous activity of comparing connecting and putting together thoughts and perceptions the classical methodological studies of geological reasoning all concern the formulation and testing of hypotheses analysis does not serve to provide the ultimate answers for intellectual puzzles predefined by limiting assumptions imposed on the real world rather analysis in geology allows the investigator to consider the consequential effects of hypotheses the latter having been suggested by experience with nature itself rather than by our theories of nature these distinctions and methods were described in g k gilbert s papers on the inculcation of scientific method by example 1886 and the origin of hypotheses 1896 portions were elaborated in t c chamberlin s method of multiple working hypotheses 1890 and his method of the earth sciences 1904 in w m davis s value of outrageous geological hypotheses 1926 and in d johnson s role of analysis in scientific investigation 1933 american quaternary geology and geomorphology have their philosophical roots in the pragmatic tradition enunciated most clearly by c s peirce now recognized as the greatest american philosopher and considered by sir karl popper to be one of the greatest philosophers of all time quaternary geology and geomorphology afford numerous examples of peirce s method of science which might be termed the critical philosophy of common sense the most obvious influence of pragmatism in geology however has largely been conveyed by the tradition of its scientific community the elements of this tradition include a reverence for field work a humility before the facts of nature a continuing effort to discriminate the phenomena observed from the observer s inference in regard to them a propensity to pose hypotheses and a willingness to abandon them when their consequences are contradicted by reality

steady state near source models of the parkfield imperial valley and mexicali valley earthquakes some of the gross characteristics of the rupture processes for the 1966 parkfield 1979 imperial valley and 1980 mexicali valley earthquakes are determined by waveform inversion of near source data employing a steady state dislocation model in a layered half space the forward model involves a piecewise linear rupture front moving with a constant horizontal rupture velocity on a fault of infinite length and finite width the inferred shapes for the rupture front and for the distribution of slip as a function of depth are consistent with previous results obtained by use of more general models the results obtained show that a strong velocity pulse observed in the nearsource region can be modeled as the passage of the rupture front phase and that the supershear propagation of the rupture front in the sedimentary layers of the medium provides a mechanism for the generation of the observed large amplitudes

interactions between axial and transverse drainage systems in the late cretaceous cordilleran foreland basin evidence from detrital zircons in the straight cliffs formation southern utah usa new detrital zircon geochronologic data from the straight cliffs formation of southern utah provide insight into the controls on stratigraphic architecture of the western interior basin during turonian early campanian time detrital zircon ages n 40 n 3650 derived from linked fluvial and shallow marine depositional systems of the kaiparowits plateau indicate the majority of zircons in fluvial strata were derived from the mogollon highlands 1 25 1 90 ga 67 of fluvial zircons with subordinate contributions delivered from the sevier fold and thrust belt 265 1250 ma 17 and cordilleran magmatic sources 81 265 ma 16 integration of these data with fluvial facies distributions petrography clast counts and evidence of magmatic arc sources from the mohave region of california implies the presence of a northeast flowing axial fluvial system this system was fed by rivers draining the mogollon highlands to the south and by transverse drainages from the sevier fold and thrust belt to the west compared to the fluvial deposits shallow marine sandstones have a greater proportion of sevier fold and thrust belt derived zircons 42 which were delivered via longshore currents from the north shallow marine samples also contain less mogollon input 44 compared to contemporaneous fluvial systems and similar input from the magmatic arc 14 although proterozoic zircons associated with the mogollon highlands are also present in the sevier fold and thrust belt several lines of evidence argue for a distinct southerly source for the straight cliffs formation these include 1 moderate proportions of feldspar and angular quartz grains in fluvial sandstones which favor a felsic intrusive source and 2 prominent 1 4 and 1 7 ga zircon populations the 1 4 and 1 7 ga peaks are the only dominant proterozoic peaks in samples from the straight cliffs formation whereas samples derived more directly from the sevier fold and thrust belt tend to have a broader distribution of proterozoic age peaks up section architectural trends in the straight cliffs formation are linked to trends in detrital zircon geochronologic data underscoring the likelihood of common drivers and controls the axial system depositing straight cliffs fluvial strata was primarily fed by drainages originating in the mogollon highlands during a pulse of tectonic activity in the maria fold and thrust belt and generally high subsidence rates in the foreland basin turonian santonian over time activation of the paxton duplex in the sevier fold and thrust belt early campanian exhumed proximal foreland basin strata and enabled drainage systems from the sevier fold and thrust belt to feed into the basin more prominently the results presented here underscore the potential significance of axial fluvial systems and their complex interplay with transverse drainage networks in foreland basins

parametric study of near field ground motions for oblique slip and dip slip dislocation models the near field motion on the surface of a uniform half space for oblique slip and dip slip faults has been studied by the use of a dislocation model the fault is modeled by an infinitely long buried dislocation of finite width rupture propagates horizontally along the fault and past the observation points with a constant rupture velocity lower than the rayleigh wave velocity in addition to those parameters which control peak amplitudes near a vertical strike slip fault depth of the top of the fault horizontal rupture velocity the dip of the fault plays an important role the slip direction and the angle between the rupture front and the down dip direction of the fault also become increasingly important in determining amplitudes of peak ground motions as the dip of the fault decreases from vertical to shallow angles in some regions near a thrust fault peak amplitudes are significantly greater than the largest values near a vertical strike slip fault

strong motion modeling of the imperial valley earthquake of 1979 twelve three component strong motion displacement records are modeled for the 1979 imperial valley earthquake to recover the distribution of slip on the imperial fault plane the final model for which point source responses are calculated by a discrete wavenumber finite element technique uses a structure with gradients in material properties rather than layers the effects of a velocity gradient are investigated by comparing synthetics with a layer over a half space model using generalized rays it is shown that a uniform fault rupture model on a rectangular fault plane does not explain the data the preferred fault model has slip concentrated below 5 km in the basement material and between the epicenter 5 km south of the international border and highway 80 within this region there appears to be two localized areas of larger dislocations one just north of the border near bonds corner and a second under interstate 8 at meloland overpass a major arrival associated with large amplitude vertical accelerations up to 1 7 g is identified in the el centro array records this arrival has an s p time of approximately 2 3 sec at many of the array stations and is modeled as originating from a localized source 8 km to the south of the array the moment is estimated to be 5 0 10 25 dyne cm from the strong motion records which is consistent with teleseismic body wave estimates the preferred fault model is strike slip with a 90 dip the average strike is 143 however to explain vertical waveforms near the fault trace a corrugated or wiggly fault plane is introduced the average rupture velocity is in the range 2 5 to 2 7 km sec 0 8 to 0 9 times the basement shear wave velocity the preferred model has unilateral rupture propagation to the north although the data would allow a small amount of propagation to the south the estimated stress drop for the entire fault plane is only 5 to 10 bars however the stress drop over the more localized sources is about 200 bars the fault model is consistent with the pattern of seismicity and observations of aseismic creep in the imperial valley and suggests that the southern half of the imperial fault acts as a locked section which breaks periodically

integrating sandstone petrology and nonmarine sequence stratigraphy application to the late cretaceous fluvial systems of southwestern utah u s a abstract petrographic and dispersal data are essential to correct interpretation of mechanisms that create continental sequence stratigraphic architecture a case study from southern utah demonstrates that upper cretaceous upper santonian campanian alluvial successions in the southernmost part of the cordilleran foreland basin were deposited by fluvial systems of contrasting drainage directions and provenance and suggests that different mechanisms governed their sequence architecture most of the rivers flowed northeast subparallel to the basin foredeep less common fluvial systems flowed to the east southeast the fluvial sandstones fall naturally into four petrofacies 1 quartzofeldspatholithic mean qt61f19l20 2 feldspatholithic qt29f19l52 3 quartzolithic qt75f6l20 and 4 quartzose qt99f1l1 petrofacies 1 and 2 were derived from mixed supracrustal and basement sources to the southwest and south respectively whereas petrofacies 3 and 4 were derived from uplifted thrust sheets of the sevier orogenic belt to the southwest and west respectively only the east southeast flowing rivers transported the quartzose petrofacies the fluvial strata which include the uppermost straight cliffs wahweap and kaiparowits formations form two large scale stratigraphic successions typically interpreted as continental stratigraphic sequences hundreds of meters thick each succession begins with an amalgamated braided fluvial deposit grades to mudstone rich strata with low sandstone body connectivity and culminates in highly connected sandstone bodies with multistory stacking the basal amalgamated deposits of each succession are architecturally similar but their compositional and dispersal characteristics are different quartzofeldspatholithic quartzolithic and quartzose sandstones above the lower base level shift are variable but generally similar in compositional and dispersal characteristics to both underlying and overlying strata a phenomenon termed here congruence in contrast quartzose amalgamated fluvial sandstone above the upper base level shift differs sharply in composition and dispersal direction from underlying and overlying lithic rich strata the foredeep axis controlled the progradation direction of the congruent shift which was likely driven by climatically induced sediment influx a eustatic fall or both in the case of the incongruent shift increased sediment supply permitted the rivers to cross the foredeep temporal association of the upper amalgamated deposit with active structures in the thrust belt and foreland basin indicates that syntectonic thrust uplift not isostatic uplift or climate caused the influx of quartz

spatial and temporal evolution of coastal depositional systems and regional depositional process regimes campanian western interior seaway u s a abstract this paper provides a critical review and regional synthesis of late cretaceous shallow marine deposits along part of the western margin of the western interior seaway of north america which contains the most extensively documented outcrop based studies of siliciclastic coastal depositional systems in the world the results of this synthesis are presented in the form of paleogeographic maps covering present day new mexico utah colorado and wyoming usa for five timeslices in the campanian these maps are used to evaluate the spatial and temporal evolution of regional depositional process regimes along a large 1000 m stretch of coastline the evolution of regional depositional process regimes is linked to tectonic and paleoceanographic controls on the western interior seaway which enables the results of this synthesis to be applied to prediction of depositional process regimes in other less intensively studied basins six gross depositional environments have been mapped for each timeslice 1 alluvial to coastal plain sandstones 2 coastal plain coals mudstones and sandstones 3 shoreline sandstones 4 marine mudstones 5 gravity flow siltstones and sandstones and 6 marine marls and chalk shoreline sandstones in each timeslice are interpreted in further detail using documented evidence for the three principal classes of depositional process wave tidal and fluvial and published reconstructions of coastal morphology which is widely considered to reflect depositional process regime based on these interpretations shoreline sandstones are assigned to five categories of depositional process regime 1 regressive wave dominated shorefaces and delta fronts 2 regressive river dominated delta fronts 3 regressive mixed tide and wave influenced delta fronts 4 regressive tide dominated delta fronts and 5 transgressive barrier islands back barrier lagoons and estuaries the accuracy of and uncertainty in classification of depositional process regime are critically evaluated additionally stratal thickness and sediment routing pathways have been interpreted in order to assess the impact of tectonic and paleoceanographic controls on spatial and temporal changes in all of the evaluated shallow marine successions thin tide influenced intervals were deposited in back barrier and lagoon systems associated with net transgressive shorefaces and estuaries however it is notable that all preserved tide dominated and tide influenced regressive deltaic systems are located along the northern margin of the utah bight a tectonically related embayment that had a pronounced expression during the middle campanian the southern margin of the embayment and coastlines to the north are conspicuously more wave dominated which supports their exposure to a larger wave fetch in contrast the northern margin of the embayment was relatively wave protected it is concluded that tidal range was amplified due to resonance of the principal semidiurnal tide in the strongly embayed geometry of the middle campanian utah bight

sandstone detrital modes central utah foreland region stratigraphic record of cretaceous paleogene tectonic evolution abstract within a sedimentary basin the analysis of sandstone petrofacies defined by detrital modes of framework grains is a powerful means to detect areal or temporal changes in provenance related to evolving paleotectonics and paleogeography in the central utah foreland the regional tectonic transition from thin skinned sevier thrusting during the late cretaceous to deep seated laramide deformation during the paleogene is recorded by four contrasting petrofacies in uppermost cretaceous and paleogene sandstones of fluvial and deltaic facies coeval subaqueous environments were marine during the cretaceous and lacustrine during the paleogene the cretaceous quartzose petrofacies qm93 f1 lt6 of mainly campanian age was derived from sedime tary sources in the active sevier thrust belt and transported into a broad adjacent foreland basin by transverse paleocurrents the heterogeneous cretaceous quartzolithic petrofacies qm40 70 f5 20 lt25 40 of uppermost campanian age was derived from the sevier orogenic belt during the waning phases of sevier tectonism and was distributed by longitudinal paleocurrents within a foreland basin whose floor had begun to be deformed by incipient laramide tectonism the paleogene feldspathic petrofacies qm50 f35 lt15 in paleocene and eocene strata is composed of arkosic detritus that was derived principally from basement rocks then exposed in the extensive san luis uplift of laramide age in south central colorado the feldspathic sand was transported to the laramide uinta basin and vicinity in utah by a trunk river which flowed generally northwestward between monoclinal laramide uplifts of the colorado plateau the paleogene quartzolithic petrofacies qm75 f2 lt23 of dominantly eocene age was derived mainly from sedimentary sources in the laramide uinta uplift to the north but also in part from similar sources in the dormant sevier thrust belt to the west in the rocky mountain and intermountain regions more widespread and systematic use of petrofacies analysis in combination with standard paleocurrent and lithofacies analyses could lead to better definition of the timing of laramide uplift and improved understandi g of the evolution of complex laramide paleogeography

parametric study of near field ground motion for a strike slip dislocation model the near field motion on the surface of a uniform half space caused by the passage of the rupture on a vertical strike slip fault has been studied by the use of a dislocation model the fault is modeled by an infinitely long buried dislocation of finite width rupture propagates horizontally along this fault and past the observation points with a constant rupture velocity lower than the rayleigh wave velocity peak amplitudes caused by the passing rupture front are primarily controlled by the depth of the top of the fault and the rupture velocity when the slip on the fault and the rise time are held constant a nonvertical rupture front distorts the pulse shape but does not have an important effect on peak amplitudes one may interpret the motion caused by a finite fault with constant rupture velocity and slip rupturing past a site as consisting of a starting phase a phase associated with the passage of the rupture event and a stopping phase the motions caused by the infinite length fault dislocation model considered in this paper correspond to the rupture passage phase compared with empirical correlations and observations in imperial valley california the model yields peak amplitudes of acceleration which are typically a factor of 6 too small effects caused by layered structure in the earth which are absent from the dislocation model are the most likely source of this discrepancy

late cretaceous fluvial megafan and axial river systems in the southern cordilleran foreland basin drip tank member of straight cliffs formation and adjacent strata southern utah u s a abstract the upper cretaceous lower campanian drip tank member of the straight cliffs formation in the grand staircase region of southern utah is an upward coarsening sandstone rich low sinuosity fluvial deposit in the cordilleran foreland basin the drip tank member thins from 75 m to 30 m eastward across the basin foredeep approximately parallel to its dominant eastward sediment dispersal direction and thins southward along the foredeep to as little as 17 m near its southern outcrop extent along modern cliff lines it overlies and underlies mudstone rich fluvial strata of the john henry member of the straight cliffs formation and lower member of the wahweap formation respectively both of which were deposited by northeast flowing rivers the john henry drip tank contact lies in an interfingering progradational stratigraphic transition from medium grained lenticular sandstone bodies to tabular coarse grained amalgamated sandstone bodies the contact with the overlying wahweap formation is locally a prominent surface of erosion or sediment reworking sandstone petrography and detrital zircon age populations of the strata indicate detrital sources in basement south of the foreland basin thrust sheets to the west and magmatic arc rocks to the southwest the uppermost part of the john henry member contains as much as 10 feldspar and abundant 1 84 1 60 ga detrital zircons derived from yavapai mazatzal basement rocks in the mogollon highlands the drip tank member ranges from quartzarenite to chert bearing sublitharenite with grenville 1 2 1 1 ga neoproterozoic and early paleozoic detrital zircons derived from proterozoic through jurassic strata exposed in the sevier orogenic belt lower member wahweap litharenites contain abundant thrust belt derived detrital carbonate grains absent from compositionally mature drip tank sandstones as well as a broad range of zircon ages the drip tank member constitutes deposits of a sand rich fluvial megafan or distributive fluvial system that displaced a subjacent but partly time equivalent axial fluvial system eastward as the megafan driven in part by seasonally high discharge fed by monsoonal precipitation prograded across the basin foredeep a thick amalgamated fan axis succession present in northern exposures contrasts with a thinner fan flank succession in southern localities tabular sixth order lithosomes each containing channels on the order of 7 m deep define three individual fluvial fans or fan lobes in the megafan system younger wahweap fluvial channels eroded uppermost drip tank deposits of the fan axis and fan flank transverse fan progradation across the foredeep recorded falling accommodation sediment supply in an aggradational to progradational systems tract erosion and reworking of the upper fan surface record decreased accommodation sediment bypass and erosion prior to reoccupation of the foredeep by the wahweap axial fluvial system the upper contact of the drip tank member is thus the primary sequence boundary in this multi component fluvial system and the lower wahweap represents transitional systems tract deposits accumulated as accommodation sediment supply increased

measurements of coseismic deformation in southern california 1972 1982 since 1972 a number of static strain changes have been measured at pinon flat observatory southern california as a result of regional faulting as recorded by a collection of long baselength strainmeters and tiltmeters these signals are in good quantitative agreement with the deformation calculated for a dislocation in an elastic half space in contrast colocated short baselength sensors are found to produce spurious results whose size is much larger than expected theoretically and well correlated with peak site acceleration calculations show that the root mean square rms of the coseismic signals from several different instruments at one site or from several sites at known hypocentral distances provide a good estimate of the seismic moment a result confirmed by the observations static moment estimates for the four largest events in this period are imperial valley 1979 m0 9 0 1018nm mexicali valley 1980 m0 6 1 1018nm westmorland 1981 m0 5 3 1017nm and brawley 1979 m0 2 3 1017 nm these estimated moments generally exceed those determined seismically by about 25 which is as expected since they include some postseismic deformation results from the mexicali valley earthquake are particularly interesting indicating an average fault strike approximately 9 different from the historic fault trace several small to moderate earthquakes ml 3 7 5 2 were also well enough recorded to estimate source parameters for the only other detectable events in the period 1972 1982 a total of three others for which the strain change erms was greater than 0 4 ne high accelerations caused most of the instruments at the site to malfunction no precursory signals were evident for any of the earthquakes recorded at this site

correlation of fluvial strata in the subsurface a review abstract correlation of ancient fluvial strata in the subsurface is challenging due to complex facies architectures and the lateral discontinuity of facies over short distances 10 km the complexity of the strata is due to the interplay of allogenic factors that determine the rate of creation of accommodation sediment supply and ultimately fluvial architectures on regional scales the application of commonly used stratigraphic techniques in such successions has allowed for improved regional correlations and an enhanced understanding of the evolution of basin fills lithostratigraphy biostratigraphy chemostratigraphy magnetostratigraphy event stratigraphy sequence stratigraphy pedostratigraphy chronostratigraphy in particular geochronology cyclostratigraphy and provenance stratigraphy are assessed outlining their benefits drawbacks and application using several case studies lithostratigraphy is highly discouraged as facies boundaries are often time transgressive on the contrary chemostratigraphy event stratigraphy and provenance stratigraphy show excellent potential for correlating ancient fluvial strata as key signatures can be recognized between different localities over distances greater than 25 km the character and thickness of paleosols can also greatly enhance both sequence stratigraphic and pedostratigraphic correlations the most promising correlation technique involves the use of chronostratigraphy in particular geochronology the high precision radiometric dating of tuffs or precisely dating the youngest zircon population in a sandstone sample using u pb ca tims when available provides a precise and independent way to correlate fluvially dominated strata unambiguously over large distances 100 km the integration of several techniques such as sequence stratigraphy in conjunction with pedostratigraphy is also recommended to construct a comprehensive stratigraphic framework for fluvially dominated successions the most appropriate correlation technique is dependent on several factors including budget the purpose and scope of the study and whether the data come from outcrops or the subsurface improved correlations will enhance our understanding of ancient fluvial systems and their resource potential

twelve fallacies of uniformitarianism although uniformitarianism is widely recognized as the basic principle of geology the geological literature is riddled with false and misleading statements as to what uniformitarianism means these misconceptions must be eliminated so geologists feel free to propose any scientifically reasonable hypotheses and because they open geology to unwarranted attack from outside science the twelve specific fallacies identified herein are that uniformitarianism 1 is unique to geology 2 was originated by hutton 3 was named by lyell who established its current meaning 4 should be called actualism because it refers to real causes 5 holds that only currently acting processes operated during geologic time 6 holds that the rates of processes have been constant 7 holds that only gradual processes have acted and that catastrophes have not occurred during earth s past 8 holds that conditions on earth haven t changed much 9 holds that earth is very old 10 is a testable theory 11 is limited in both time and place and 12 holds that the laws governing nature have been constant through time geologists should abandon the terms uniformitarianism and actualism because they are fruitless confusing and inextricably associated with many fallacious concepts instead the fundamental philosophical approach of science should be recognized as basic to geology

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

the mesozoic mogollon highlands arizona an early cretaceous rift shoulder the mogollon highlands have long been considered the southern source terrane for much of the early to middle mesozoic fluvial sedimentary rocks of the southern colorado plateau because of the lack of mesozoic sedimentary strata in the mogollon region many workers have suggested these highlands were located in central arizona and new mexico new evidence suggests that during the triassic and most of the jurassic a large highland did not exist in central arizona new mexico but was present as a continental margin volcanic arc in southern arizona and northern mexico at this time central arizona would be more aptly called the mogollon slope early jurassic navajo equivalent sands were blown south from the colorado plateau area across the mogollon slope to become intercalated with volcanics along the northeast side of the arc the first good evidence for a structural and topographic highland in the mogollon area is in the early cretaceous when the uplifted northeast flank of the bisbee rift basin tilted

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

isotopic geochemical and temporal characterization of proterozoic basement rocks in the quitovac region northwestern sonora mexico implications for the reconstruction of the southwestern margin of laurentia a detailed geochemical characterization of 19 representative proterozoic basement rocks in the quitovac region in northwestern sonora mexico has identified two distinct paleoproterozoic basement blocks that coincide spatially with the previously proposed caborca and north america blocks new u pb zircon geochronology revises their age ranges the caborca 1 78 1 69 ga and north america 1 71 1 66 ga blocks at quitovac and precludes a simple age differentiation between them in addition grenvillian age granitoids ca 1 1 ga spatially associated with the caborca block have been identified at quitovac nd isotopes and major and trace element geochemistry support the distinction of these paleoproterozoic blocks granitoids of the north america block are characterized by depleted end values 3 4 3 9 and younger nd model ages 1800 1740 ma and have lower k 2 o y rb ba th ree and fe mg values than coeval rocks of the caborca block the caborca block granitoids are likewise characterized by slightly less depleted end 0 6 2 6 and older nd model ages 2070 1880 ma despite the subtle differences granitoids from both the caborca and north america blocks exhibit island arc like affinities we propose that the proterozoic basement rocks from the quitovac region are an extension of the proterozoic crustal provinces in the southwestern united states specifically rocks of the caborca block exhibit an affinity to rocks of either the yavapai province or the mojave yavapai transition zone whereas rocks of the north america block have signatures similar to those of the mazatzal province or possibly the yavapai province of arizona the new isotopic ages and geochemical data do not support the existence of the late jurassic mojave sonora megashear at quitovac as originally proposed however the quitovac region accounts only for a small fraction of the proterozoic basement in sonora so these findings do not eliminate the possibility of a megashear elsewhere in northern sonora our new data create the possibility of alternative hypotheses for the distribution of paleoproterozoic crustal provinces in southwestern north america that affect reconstructions of the original southwestern margin of laurentia and reduce uncertainties in the configuration timing and existence of the proterozoic supercontinent rodinia

analysis of near source static and dynamic measurements from the 1979 imperial valley earthquake gross features of the rupture mechanism of the 1979 imperial valley earthquake ml 6 6 are inferred from qualitative analysis of near source ground motion data and observed surface rupture a lower bound on the event s seismic moment of 2 5 1025 dyne cm is obtained by assuming that the average slip over the whole fault plane equals the average surface rupture 40 5 cm far field estimates of moment suggest an average slip over the fault plane of 105 cm from which a static stress drop of 11 bars is obtained an alternative slip model consistent with the far field moment has 40 5 cm of slip in the upper 5 km of the fault and 120 cm of slip in the lower 5 km this model suggests a static stress drop of 39 bars from the lower estimate of 11 bars an average strain drop of 32 strain is derived this strain drop is four times greater than the strain that could have accumulated since the 1940 el centro earthquake based on measured strain rates for the region hence a major portion of the strain released in the 1979 main shock had been accumulated prior to 1940 unusually large amplitude 500 to 600 cm sec2 vertical accelerations were recorded at stations e05 e06 e07 e08 eda of the ei centro array and the five stations of the differential array near eda although the peak acceleration of 1705 cm sec2 at e06 is probably amplified by a factor of 3 due to local site conditions these large amplitude vertical accelerations are unusual in that they are evident on only a few stations all of which are near the fault trace and at about the same epicentral range two possible explanations are considered first that they are due to a direct p wave generated from a region about 17 km north of the hypocenter or second that they are due to a pp phase that is unusually strong in the imperial valley due to the large p wave velocity gradient in the upper 5 km of the imperial valley based on the distribution of both the horizontal and vertical offsets it is likely that the rupture went beyond stations e06 and e07 during the main shock by exploiting the antisymmetry of the parallel components of particle velocity between e06 and e07 and by examining polarization diagrams of the particle velocity at e06 and e07 an average rupture velocity in the basement of 2 5 to 2 6 km sec between the hypocenter and station e06 is obtained in addition several lines of evidence suggest that the imperial fault dips about 75 to the ne

detrital zircon u pb ages provide provenance and chronostratigraphic information from eocene synorogenic deposits in northwestern argentina paleogene clastic sedimentary rocks in the puna plateau of northwestern argentina contain valuable information about the timing and location of early mountain building in the central andes because these rocks generally lack tuffaceous facies only paleontological ages have been available we present u pb ages from detrital zircons in the conglomeratic eocene geste formation of the central puna plateau the zircon ages indicate that the geste formation was derived from nearby high relief ranges composed of ordovician metasedimentary rocks a small population of ca 37 35 ma grains also confirms the late eocene stratigraphic age of the geste formation and suggests that u pb detrital zircon ages may provide a new tool for determining depositional ages and provenance of widespread paleogene deposits in the central andes

extraterrestrial geomorphology science and philosophy of earthlike planetary landscapes abstract newly discovered extraterrestrial landscapes afford an opportunity for adventitious change in the discipline of geomorphology this opportunity contrasts with the various programs of stipulative change to the discipline initiated over the past century and leading to the present systems process functional modeling spfm paradigm among anglo american geomorphologists planetary geomorphology has been largely irrelevant to this paradigm and has found its home almost exclusively within geology as exemplified by grove karl gilbert s classic 1893 study of the moon s surface planetary geomorphology affords a remarkable interplay of inference and observation in understanding nature the balanced approach to scientific understanding illustrated by gilbert s work has been lacking in some of the puzzle solving exercises of modern spfm geomorphology recent discoveries concerning mars and venus illustrate the role of extraterrestrial studies for enhancing the science of earthlike planets and particularly earth itself

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

sediment routing controls on sandstone bulk petrographic composition and texture across an ancient shelf example from cretaceous western interior basin utah and colorado u s a

depositional age and provenance of the wutai group evidence from zircon u pb and lu hf isotopes and whole rock geochemistry abstract the hengshan wutai fuping area is situated in the middle segment of the trans north china orogen and consists of the upper amphibolite to granulite facies hengshan and fuping complexes and the intervening greenschist to lower amphibolite facies wutai complex the wutai complex is composed of neoarchean to paleoproterozoic granitoids and low grade metamorphosed supracrustal rocks of which the latter are further divided into the wutai and hutuo groups geochemistry of the meta sedimentary samples from the wutai group indicates weak to moderate source weathering dominantly compositional variation trend and major felsic with few mafic source rocks on the tectonic discrimination plots most of the samples display continental island arc and active continental margin features u pb ages of detrital and xenocrystic zircons from the wutai group yield two major age populations of 2565 2465 ma and 2729 2676 ma and a subordinate age population of 2372 2331 ma the major 2522 ma and 2699 ma zircons were most likely derived from the wutai meta volcanic rocks the minor 2348 ma zircons are only found from the upper gaofan subgroup and most likely sourced from the 2360 2330 ma orthogneisses in the hengshan complex the maximum depositional ages of 2535 2516 ma from the meta sedimentary rocks are consistent with the eruption ages of 2548 2513 ma from meta volcanic rocks in the lower shizui and middle taihuai subgroups indicating coeval sedimentation and volcanism in a fore arc or intra arc basin this favors a tectonically juxtaposed contact rather than a layer cake model however the maximum depositional age of 2348 ma and the 2176 2161 ma felsic intrusions suggest a much younger formation age for the gaofan subgroup

andean shortening inversion and exhumation associated with thin and thick skinned deformation in southern peru a balanced cross section spanning the eastern cordillera and subandean zone of southern peru 13 15 s constrains 130 km 38 of cenozoic orogen normal sw ne andean deformation accommodated by thick and thin skinned retro arc fold thrust belt shortening that overprinted pre andean triassic normal faults zircon and apatite u th he ages demonstrate continuous oligocene to miocene cooling of the permo triassic coasa pluton in the eastern cordillera zircon u th he ages 34 18 ma are reset and define a steep age versus elevation relationship apatite u th he results reveal reset ages that define two spatially separated groups with ages of 30 26 ma and 17 11 ma detrital zircon u pb geochronologic results from cretaceous cenozoic siliciclastic rocks from the altiplano eastern cordillera record andean fold thrust belt and magmatic arc sediment sources correlative subandean zone rocks preserve a cratonic sediment contribution with minor andean sediment appearing in some cenozoic rocks we propose that earliest andean deformation and structural compartmentalization of the eastern cordillera was linked to selective inversion of inherited permo triassic basement involved normal faults that guided subsequent thick and thin skinned deformation provenance variations between the hinterland and foreland depocentres reveal competing eastern and western sediment sources reflecting an axial zone in the eastern cordillera that coincided with the inherited triassic graben and impeded sediment source mixing our zircon and apatite u th he ages are consistent with published constraints along strike and support pulses of eocene to late miocene exhumation that were likely driven by normal fault reactivation and protracted eastern cordillera deformation

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

provenance of upper cretaceous paleogene sandstones in the foreland basin system of the sierra madre oriental northeastern mexico and its bearing on fluvial dispersal systems of the mexican laramide province sandstone petrography and detrital zircon u pb analysis of upper cretaceous paleogene sandstones in the foreland basin of the sierra madre oriental in northeastern mexico indicate long distance sediment transport from arc basement and thrust belt sources lying to the west northwest and south the basin fill termed the difunta group consists of sublitharenites litharenites feldspathic litharenites and lithic arkoses derived from mixed sources that included sedimentary rocks magmatic arc rocks and subordinate basement rocks six age populations comprise the detrital zircon content of the sandstones proterozoic 1900 900 ma early paleozoic 500 400 ma late paleozoic early triassic 288 235 ma jurassic 180 151 ma early cretaceous 150 111 ma and late cretaceous paleogene 110 54 ma these grains were derived from several arc terranes ranging in age from permian to paleogene in western mexico and the southwestern united states from sedimentary rocks and possibly interbedded tuffs of the sierra madre oriental orogen and from basement sources or their derivative sandstones of the southwestern united states the petrographic and geochronologic provenance data corroborate existing models for derivation of much foreland detritus from arc sources to the west identify the sierra madre orogen itself as an important source for sediment and these data modify the late cretaceous paleogene paleogeography of mexico to include a long orogen parallel fluvial system with headwaters in the southwestern united states the difference in average ages of the youngest grains in the sandstones and their inferred depositional ages is 10 5 m y indicating that the initial coarse fill of the foreland basin was derived from early laramide uplift and eastward arc migration in northwestern mexico and the southwestern united states

mass balance constraints on stratigraphic interpretation of linked alluvial coastal shelfal deposits from source to sink example from cretaceous western interior basin utah and colorado u s a abstract experimental work suggests that the rate of upstream to downstream loss of sediment from an active depositional system to permanent storage exerts a fundamental control on stratigraphic architecture this rate of sediment mass loss is determined by the spatial distribution of tectonic subsidence and rate of sediment supply the character of input sediment grain size distribution and composition is the third parameter that affects stratigraphic architecture we apply this concept in a mass balance framework to linked alluvial coastal shelfal deposits of the upper cretaceous castlegate sandstone blackhawk formation star point sandstone and mancos shale western interior basin utah and colorado usa facies partitioning and sediment budgets are estimated for eight stratigraphic intervals in order to compare temporal dynamics of the sediment routing system from erosional source to depositional sink mapping of each stratigraphic interval and its constituent segments from upsystem to downsystem was achieved along a representative dip oriented 2d cross section over a distance of c 350 km using extensive outcrop exposure and densely spaced subsurface wells the cross section provides time averaged estimates of the spatial distribution of deposition grain size data show that there is limited downsystem fining of any particular facies within the castlegate sandstone but that the proportion of facies changes systematically downsystem to accommodate an overall fining trend therefore it is reasonable as a first approximation to use facies proportions as a textural replacement for grain size sediment supply characteristics for each of the eight stratigraphic intervals are constrained by total facies proportions in each interval for each stratigraphic interval we assess the level of interaction between alluvial and coastal to shelfal segments of the routing system comparison of the downsystem mass balance characteristics of the eight stratigraphic intervals suggests that there were depositional gains and losses of shallow marine shale in the five youngest intervals which can be attributed to along strike sediment transport this result is consistent with increased interaction through time with vigorous wave and tide driven circulation in the seaway as the sediment routing system advanced out of a sheltered embayment in response to decreasing tectonic subsidence in the youngest stratigraphic interval the upstream unconformable base of the castlegate sandstone is marked by a pronounced increase in the sand to gravel grade mass fraction of the fluvially supplied depositional volume this marked increase can be attributed to hinterland unroofing and or cannibalization of wedge top basins leading to import of coarse grained sediment into the castlegate fluvial system our results demonstrate the value of analyzing downsystem sediment loss i e downsystem mass extraction within a mass balance framework as a simple and practical tool to quantify the relationship between accommodation and sediment supply and thus to decode past external forcing mechanisms from stratigraphic architecture

evolution of late cretaceous and early tertiary basins of southwest utah based on clastic petrology abstract upper cretaceous to eocene clastic rocks of southwest utah record active sevier style thrust deformation cessation of thrusting and sedimentation related to laramide style folding the formations which record these tectonic events include from oldest to youngest the iron springs kaiparowits canaan peak grapevine wash grand castle informal name pine hollow and claron formations two petrographically distinct upward coarsening petrofacies occur in the foreland basin and are herein named for the formations they comprise the kaiparowits canaan peak and iron springs grand castle petrofacies the kaiparowits canaan peak petrofacies consists of volcanic and radiolarian chert lithic fragments derived from sources in southeastern california and southern nevada the finer grained kaiparowits formation was deposited during active thrusting in the middle to upper campanian while the conglomeratic canaan peak formation was deposited during cessation of thrust activity between the upper campanian and lower paleocene the iron springs grand castle petrofacies contains quartzite and carbonate clasts and sandstone compositions that suggest derivation from the wah wah and blue mountain thrust sheets in southwestern utah the iron springs formation represents upper cretaceous syn thrusting deposition whereas the grand castle conglomerates were deposited during lower paleocene rebound of these thrust sheets the upper cretaceous to paleocene grapevine wash formation is an areally restricted conglomeratic alluvial fan sequence composed of carbonate and sandstone clasts derived from paleozoic rocks in the upper plate of the square top mountain thrust in southwestern utah the pine hollow formation received sediment during development of laramide style folding of the sevier foreland basin during early paleocene to middle eocene time jurassic and cretaceous sedimentary units tilted during deformation shed detritus to the east and southwest into an intermontane basin the clastic basal part of the claron formation paleocene to eocene also received sediment from exposed highlands related to laramide structures the abundance of silty carbonate lithic grains suggests that much of the sedimentation in the claron basin was intraformational

hutton to horton views of sequence progression and equilibrium in geomorphology abstract the papers by strahler 1952 and chorley 1962 strongly advocated the adoption of a dynamic as opposed to an historical approach to geomorphology the opinion of some later workers notably simpson 1963 and mayr 1982 is however that any advance in the historical natural sciences depends upon the combined appreciation of immanent and configurational elements simpson s terminology and the view that events may have an essential historical or timebound component is now accepted even in experimental sciences such as chemistry prigogine 1978 in the light of these contrasting approaches to earth science an attempt is made to analyse the mjor lines of thought concerning change progression and equilibrium in the work of six leading precursors of modern geomorphology james hutton charles lyell charles darwin james dwight dana grove karl gilbert and robert e horton despite their perceived general adherence to the uniformitarian tradition it is suggested that the work of the six reveals two contrasting attitudes to ideas of change and of equilibrium it is argued that those authors lyell dana horton who are primarily concerned to demonstrate that the present state of the earth is in some sense the normal or optimum tend at the same time to accept the existence or desirability of some equilibrium state and paradoxically to overstate the role of unusual cataclysmic or catastrophic events in creating and sustaining this equilibrium the views of horton in particular lend themselves to the description punctuated equilibrium in contrast it is contended that hutton darwin and gilbert have no ideological commitment to the present state of the earth as anything other than one moment in time their ideas are considered to focus upon the entire sequence of changes which may be inferred to create the phenomena viewed at any time or place as a consequence the concept of equilibrium has only a minor role in the works of these authors who accept that earth history reveals a mix of events of all physically possible magnitudes and frequencies in the light of this analysis the distinction between dynamic and historical geomorphology advocated by strahler and chorley is felt to be unhelpful a truly historical study of the sequence of phenomena exemplified by chapter iv in gilbert s geology of the henry mountains 1877 necessarily requires consideration of all scales of dynamic processes

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

speculative tectonic model for the early mesozoic arc of the southwest cordilleran united states the early mesozoic continental arc of arizona california and western nevada is here interpreted to have occupied a continuous graben depression more than 1000 km long similar to the modern extensional are of central america high calculated rates of subsidence the presence of syndepositional normal faults and fault talus breccias the abundance of calderas and high level intrusions and interstratification of cale alkalic and alkalic volcanic rocks provide support for this model the arc graben depression acted as a long lived more than 40 m y trap for craton derived quartz sands which were funneled along the length and across the width of the depression

higher order interrelations between seismogenic structures and earthquake processes abstract aki k 1992 higher order interrelations between seismogenic structures and earthquake processes in t mikumo k aki m ohnaka l j ruff and p k p spudich guest editors earthquake source physics and earthquake precursors tectonophysics 211 1 12 to advance our understanding of higher order smaller scale interrelations between the fault zone structure and the earthquake process we start with the observation that the source parameters of major california earthquakes deviate systematically from the self similarity this departure from the self similarity is interpreted in terms of both the specific barrier model and the asperity model the asperity model however is found to be inconsistent with the following observations 1 sharp impulsive displacements waveform in the direction perpendicular to the fault trace 2 the complementary relationship between fault slip and aftershock and 3 existence of barriers in the creeping segment of the san andreas fault the application of the specific barrier model on the other hand leads to a predominant subevent crack size of a few hundred meters in the creeping segment this result is consistent with the negative correlation between coda q 1 and b value observed in the stone canyon area for 1 5 3 temporal increase in coda q 1 is sometimes accompanied by an increase in the rate of surface deformation but not always so this is attributed to the erratic coupling between the creeping brittle region and the overlying brittle region the applicability of the conclusions obtained from california earthquakes to other regions is investigated using the observed scaling laws of seismic source spectra and it is suggested that the crack size of the creeping brittle zone may vary regionally from a few hundred meters to a few kilometers as an alternative to the widely held view that the aseismic creep in the plate boundary may occur as a giant slow earthquake our result suggests that it may be occurring as small fractures with scale lengths from a few hundred meters to a few kilometers

u pb and 40ar 39ar ages for a tephra lens in the middle jurassic page sandstone first direct isotopic dating of a mesozoic eolianite on the colorado plateau abstract joint u pb and 40ar 39ar geochronology for pyroclastic zircon and biotite crystals from tephra lenses intercalated within eolianite of the middle jurassic page sandstone on the colorado plateau provides the first direct isotopic age for a jurassic eolianite on the colorado plateau and indicates a chronostratigraphic age of 171 5 169 5 ma for the page sandstone with stated uncertainties in the age boundaries of global stratigraphic stages taken into account the isotopic age of lower bajocian is coordinate with the inferred biostratigaphic age of upper bajocian from lateral correlation of page sandstone with equivalent marine strata comparative isotopic ages imply that the page sandstone was deposited during the same time frame as the temple cap formation inferred conventionally to be older and that the j1 and j2 surfaces of the colorado plateau are therefore not reliable chronostratigraphic marker horizons

towards a sequence stratigraphic solution set for autogenic processes and allogenic controls upper cretaceous strata book cliffs utah usa upper cretaceous strata exposed in the book cliffs of east central utah are widely used as an archetype for the sequence stratigraphy of marginal marine and shallow marine deposits their stratal architectures are classically interpreted in terms of accommodation controls that were external to the sediment routing system allogenic and that forced the formation of flooding surfaces sequence boundaries and parasequence and parasequence set stacking patterns processes internal to the sediment routing system autogenic and allogenic sediment supply controls provide alternatives that can plausibly explain aspects of the stratal architecture including the following 1 switching of wave dominated delta lobes expressed by the internal architecture of parasequences 2 river avulsion expressed by the internal architecture of multistorey fluvial sandbodies and related deposits 3 avulsion generated clustering of fluvial sandbodies in delta plain strata 4 autoretreat owing to increasing sediment storage on the delta plain as it lengthened during progradation expressed by progradational to aggradational stacking of parasequences 5 sediment supply control on the stacking of and sediment grain size fractionation within parasequence sets the various potential allogenic controls and autogenic processes are combined to form a sequence stratigraphic solution set this approach avoids anchoring of sequence stratigraphic interpretations on a specific control and acknowledges the non unique origin of stratal architectures

local magnitude and sediment amplification observations from earthquakes in the northern baja california southern california region abstract the data set that is analyzed in this study consists of strong motion and digital velocity records obtained during the last 6 yr for earthquakes along faults in baja california north and southern california from this set local magnitudes for 60 earthquakes recorded at stations on hard rock and sediments are calculated following the technique proposed by kanamori and jennings 1978 the calculated near source motion magnitudes m lsm are then compared with reported local magnitude values m l which are in the range from 3 to 6 6 from these comparisons it is found that for earthquakes with magnitude m l between 3 and 5 5 almost all the m lsm values estimated from data registered on sediments exceed the m l values by amounts which at the shorter distances less than about 10 km may be as high as an order of magnitude or even higher for a few cases the strong motion records associated with these high m lsm values have highly coherent pulse like s wave signals of very short period and large accelerations similar estimations of local magnitude from more limited data obtained on hard rock were comparable or lower than the standard m l values we conclude that the high m lsm values are the result of a combination of high frequency source spectra and a strong shear wave energy amplification due to the presence of sediments a sediment amplification factor of 3 2 is inferred from the data and is in good agreement with a theoretical factor of 3 4 estimated on the basis of the mcmechan and mooney 1980 velocity model for the imperial valley it is also found from the analyzed data that near source local magnitude decreases between 0 and 10 km indicating that for smaller earthquakes the attenuation function log a 0 richter 1958 used in local magnitude determinations needs to be modified to take into account the characteristics of the seismic energy radiated at short distances near source strong motion data for the 1979 imperial valley m l 6 6 and 1980 victoria m l 6 1 events do not show the strong discrepancy between m lsm and m l observed for the smaller magnitude events factors that might be responsible for this include nonlinear sediment attenuation for larger amplitude waves as well as complex near source spreading out of the waveform resulting from larger source size near source magnitude saturation we give a numerical example using small earthquakes as green9s functions to show that for a complex source function represented as a superposition of smaller earthquakes m lsm and m l for the 1980 victoria earthquake can be explained by the magnitude saturation phenomenon

high resolution correlation of the upper cretaceous stratigraphy between the book cliffs and the western henry mountains syncline utah u s a abstract exposures of upper cretaceous strata in the book cliffs have played a key role in the formulation of modern sequence stratigraphic concepts and yet correlations with time equivalent strata in nearby regions remain uncertain resulting in an incomplete understanding of regional paleogeography this study presents detailed high resolution correlations of the upper cretaceous stratigraphy between the book cliffs the wasatch plateau and the western henry mountains syncline in utah with emphasis on the campanian interval two newly acquired measured vertical sections at coal canyon near green river in the book cliffs and blind trail in the henry mountains inform the correlations which also incorporate the substantial body of previous research correlations are further constrained by vertical and lateral facies stacking patterns and changes in sediment dispersal directions at discrete stratigraphic horizons derived from paleocurrent measurements much of the campanian succession can be characterized as a single low frequency third order depositional sequence within which are nested numerous high frequency sequences while it is not possible to correlate every high frequency sequence recognized in individual outcrop areas we define four intervals numbered 1 4 in ascending stratigraphic order that we correlate across the entire region interval 1 basal campanian is represented by initially deltaic and later wave dominated shoreface strata of the star point sandstone and equivalent basal muley canyon sandstone interval 2 lower campanian comprises coastal plain facies passing eastward into shallow marine strata of the lower blackhawk formation and equivalent muley canyon sandstone interval 3 lower to mid middle campanian comprises a thick aggradational stack of coastal plain fluvial strata that pass eastward into coastal and shallow marine facies blackhawk formation and equivalent masuk formation interval 4 overlying a major facies dislocation third order sequence boundary comprises amalgamated sheet fluvial deposits of the lower castlegate sandstone and equivalent tarantula mesa sandstone during the early to middle campanian the northern wasatch plateau and western henry mountains regions were along depositional strike with respect to one another the book cliffs near green river were located obliquely down depositional dip from the western henry mountains region during the campanian these new correlations and the improved understanding of paleogeography that derive from them have the potential to inform basin scale stratigraphic interpretations to a greater extent than has been hitherto possible

multiproxy approaches to investigating palaeoecology and palaeohydrology in the upper cretaceous kaiparowits formation usa

charles lyell and modern changes of the earth the milledgeville gully abstract one major emphasis of lyell s principles of geology was on the abundance and efficacy of present or modern causes in producing geologic change he was also determined that the evidence given for the activity of such causes should be scientific and reliable to that end he expended tremendous efforts to make his own observations perhaps inevitably he stressed the operation of relatively high magnitude low frequency events storms floods and earthquakes especially those whose dates of onset were known a key example of lyell s interest in the modern was his examination of a 20 year old gully at milledgeville ga in january 1846 he explicitly related its ultimate cause to the laying bare of the clayey surface horizons as a result of recent forest clearance and its proximate formation to sunbaked cracks in the clay and erosion by rainfall induced runoff he also saw this example as merely one of hundreds in the region over the 140 years since lyell s visit the gully has been repeatedly revisited and even surveyed by h a ireland in 1937 its development shows continued growth followed by increasing stabilisation this seems to match evidence for the general behaviour of such landforms and the gully therefore may serve as a well documented exemplar for the nature of localised accelerated erosion regardless of the ultimate initiating factors lyell s interest in such short term erosional features as indicating the efficacy of modern causes stands in contrast to his successors focus on geological scale features such as niagara falls

early permian sediment provenance and paleogeographic reconstructions in southeastern gondwana using detrital zircon geochronology northern perth basin western australia abstract the perth basin western australia forms part of a north trending belt of intracratonic rift basins that initiated in southeastern gondwana in the late paleozoic existing provenance studies have highlighted the dominance of proterozoic zircons derived from southerly terranes such as the albany fraser orogen leeuwin complex and more distant provinces in east antarctica with limited input from the adjacent archean western australian craton new u pb geochronology data were acquired using a sensitive high resolution ion microprobe shrimp ii on detrital zircons from lower permian formations exposed in the northeastern perth basin in contrast to previous studies archean zircons showing fine oscillatory zoning dominate the age spectra with up to 90 grains ranging between 3 0 and 2 6 ga indicating that granitic parent rocks of the adjacent craton were a major sediment source during the early permian a minor paleoproterozoic age population from the gascoyne complex of the capricorn orogen was likely transported southward and dispersed in the basin by a combination of longshore drift and tidal currents meso and neoproterozoic detritus became prominent only from the middle artinskian and may relate to the establishment of large northward flowing river systems draining the albany fraser province the prydz leeuwin belt and or the wilkes land in east antarctica this study shows that integrating depositional processes and stratigraphic evolution of sedimentary formations with provenance analysis can enhance reconstruction of dispersal pathways to and within basins