a tectonic boundary for the southern colorado plateau abstract a tectonic boundary should be defined by changes in tectonic elements tectonic elements would include such parameters as structural style stress orientations volcanism heat flow seismicity and changes in crustal thickness examination of these tectonic elements for the southern colorado plateau suggests that the southwestern part of the physiographic plateau appears to be tectonically part of the basin and range province i therefore propose a boundary at 112 w longitude the locus of seismicity and recent volcanism and also the site of crustal thickening and a change of structural style the tectonic boundary of the southern colorado plateau is suggested to form part of the eastern boundary of the southern basin and range seismotectonic region this seismotectonic region is bounded on the north by the las vegas shear zone nevada seismic zone and has a different stress orientation than the northern basin and range from which it began to evolve separately 13 m y ago the present principal stress orientation of the southern basin and range of arizona appears to be the result of compound movements of the san andreas transform and garlock las vegas shear zone resulting in a shear couple affecting the crust in arizona and an associated ne sw least principal stress orientation

analysis of the 4 february 1976 chino valley arizona earthquake abstract this m b 4 9 earthquake was located about 20 km north of prescott in the transition zone between the basin and range and colorado plateau provinces several lines of approach were used to determine the focal mechanism p wave first motions surface wave amplitude and phase spectra and synthetic seismograms the preferred focal mechanism displays northwest trending nodal planes with predominantly normal movement the inferred fault plane strikes 120 dips 40 sw with a slip of 100 and a depth of 10 to 15 km the seismic moment is 1 0 10 23 dyne cm the azimuth of least horizontal stress is inferred to be 37 lithostatic stress generated by variations in elevation and density between the crusts of the southern basin and range and colorado plateau may contribute to the inferred stress orientations uplift and resultant extension in this zone of thin lithosphere and relatively high heat flow may also be important microearthquake studies in the chino valley during the summers of 1978 and 1979 indicate a microseismicity rate of 0 3 events per day this is an order of magnitude less than that observed in the intermountain seismic belt

contribution of nonlinear mechanisms in the persistence of multiple tidal inlet systems abstract multiple tidal inlet stability has been modeled following either a single inlet stability approach or a simple linear analytical approach and the results suggest that these systems are not stable however there is evidence that shallow multiple tidal inlet systems are strongly nonlinear and that they can be stable on a historical time scale based on numerical simulations of the hydrodynamics of ria formosa a shallow lagoon in the south of portugal in which multiple tidal inlets have persisted in a historical time scale the effects of tidal distortion and residual circulation on the persistence of multiple inlets in the system were analyzed numerical simulations using a 2 d vertically averaged finite element model served 1 to identify the existence of three hydrodynamically independent sub embayments in the system 2 to create realistic physical disturbances change in inlet cross sectional area and analyze the hydrodynamic response of the system to such disturbances 3 to identify and quantify the existing nonlinear hydrodynamic processes tidal distortion ebb flood dominance residual flow patterns and 4 to analyze the overall stability of the western sub embayment using the closure curve approach the results show the importance of the nonlinear processes in the hydrodynamics of multiple inlet systems and the need to consider them when analyzing inlet stability in such systems and suggest that specifically for ria formosa if the existing jetties at one of the inlets in the western sub embayment were removed this area of the system would reach natural stability and equilibrium with three inlets

long term morphological impacts of the opening of a new inlet on a multiple inlet system in this study the artificial opening of a new tidal inlet in an existing multiple inlet system is shown to significantly modify the adjacent nearshore and backbarrier morphology as well as both updrift and downdrift shorelines the study focuses on the dominant faro olhao and armona inlets in the ria formosa barrier island system of southern portugal the equilibrium state and future evolution of the system are inferred using a range of morphological and hydrodynamic indicators including the evolution of the inlet cross section changes in tidal prism and changes in the dimensions length and area of barrier islands the results reveal how the morphology of an interconnected two inlet bay system and the adjacent coastlines has evolved following the artificial opening and stabilization of faro olhao inlet since 1929 a clear relationship between barrier island size inlet cross section width and tidal prism is demonstrated decadal time scale changes in the tidal prism of the two interconnected inlets are shown to be the main mechanism responsible for morphological change and have resulted in the remobilization of ebb tidal delta sediments deposited during previous hydraulic configurations these changes in turn have contributed to a narrowing of armona inlet and an increase in the size of culatra island the work highlights the importance of ebb tidal deltas both as sand reservoirs and as conduits through which sand exchange between estuaries or lagoons and the open coast is regulated it also shows the pivotal role of ebb tidal deltas in trapping longshore transported sediment and releasing it again during periods of increased wave activity the findings have implications regarding the accurate assessment of the stability of multiple inlet systems copyright 2011 john wiley sons ltd

taphonomic modes on fluvial deposits of the monte hermoso formation early pliocene buenos aires province argentina abstract the sedimentary deposits of the monte hermoso formation outcropping at its type locality farola monte hermoso buenos aires province argentina accumulated through fluvial processes acting on highly sinuous rivers the monte hermoso formation comprises a rich vertebrate fauna corresponding to the montehermosan stage age early pliocene which includes numerous and diverse vertebrate remains comparative taphonomic analysis indicated that the faunal assemblages coming from two different fossiliferous levels fl 1 and fl 2 recognized in floodplain deposits and channel deposits respectively are composed of remains affected by diverse taphonomic processes and therefore representing distinct taphonomic histories the variations recorded between both fossiliferous levels with respect to their taphonomic attributes and sedimentary context allowed the recognition of two distinct taphonomic modes floodplain in the fl 1 and channel lag in the fl 2 these taphonomic modes represent different conditions within the range of taphonomic histories for the monte hermoso formation assemblage formed by attritional accumulation of remains at the moment continental systems linked to fluvial environments have been poorly analyzed for the south american neogene this study contributes new information on taphonomic and paleoenvironmental characteristics of this type of deposit

microseismicity of the kaibab plateau northern arizona and its tectonic implications abstract a microearthquake investigation of the kaibab plateau in northern arizona revealed swarms of small to very small magnitude earthquakes a 12 station network of portable seismographs recorded 296 events with magnitudes from 0 5 m l t axes from these fault plane solutions yields an average least principal horizontal stress direction of ene wsw the inferred orientation of principal stresses found in this study indicates the kaibab plateau lies within a region of predominantly normal faulting which extends from central arizona northward into southwestern utah although data are sparse in the region surrounding the kaibab plateau evidence for the location of a basin range colorado plateau stress boundary through the kaibab plateau is not supported by the results of this study

hydrodynamics and equilibrium of a multiple inlet system although it is generally acknowledged that most multiple inlets are unstable and cannot coexist there is evidence to suggest that such inlets can indeed be stable over decadal time scales multiple inlet systems servicing a single embayment in contrast to single inlet systems confer particular hydrodynamic characteristics such as the potential existence of residual discharges and currents between the inlets this paper presents detailed measurements of hydrodynamic variables obtained over complete spring and neap tidal cycles in a multiple inlet system in southern portugal ria formosa the results indicate that the two main interconnected inlets servicing an embayment can coexist at least over a time scale of several decades however their coexistence cannot be explained simply using empirical equilibrium relations or inlet hydraulics although residual flow between the inlets appears to play an important role enhancing their stability it is the availability of sediment stored in the ebb tidal deltas and its capacity to be carried into the inlets during storm events that ultimately dictates the overall equilibrium independently of the flushing capability provided by the hydraulics such movement of sediment leads to the obstruction of the inlet channels thereby affecting the hydraulic efficiency and eventually leading to inlet closure in the long term the results augment existing knowledge concerning multiple inlet systems and improve understanding of their short to medium term stability

a review of the regional geophysics of the arizona transition zone a review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the basin and range and colorado plateau occur in or directly adjacent to the region defined as the arizona transition zone although this region was designated on a physiographic basis studies indicate that it is also the geophysical transition between adjoining provinces the transition zone displays anomalous crustal and upper mantle seismic properties shallow curie isotherms high heat flow and steep down to the plateau bouguer gravity gradients seismic and gravity studies suggest that the change in crustal thickness from thin crust in the basin and range to thick crust in the colorado plateau may occur as a series of steps rather than a planar surface anomalous p wave velocities high heat flow shallow curie isotherms and results of gravity modeling suggest that the upper mantle is heterogeneous in this region a relatively shallow asthenosphere beneath the basin and range and transition zone contrasted with a thick lithosphere beneath the colorado plateau would be one explanation that would satisfy these geophysical observations

can multiple tidal inlets be stable abstract the gorge stability of inlets connecting the same bay to the ocean is analysed the basic premise is that when the tidal maximum of the bottom shear stress in an inlet is larger than the equilibrium shear the inlet is in a scouring mode and when smaller than the equilibrium shear stress the inlet is in a shoaling mode the tidal maximum of the bottom shear stress equals the equilibrium shear stress when the inlet is in equilibrium with the hydraulic environment an inlet is called stable when after a small change the cross sectional area returns to its equilibrium value using a linearized lumped parameter model for the flow in the inlets it is shown that for a two inlet bay system neither of the two inlets can be stable this implies that when assuming the inlets to be in equilibrium with the hydraulic environment a small change in the cross sectional area of one or both of the inlets will result in the closure of one or both of the inlets starting with the results for the two inlet bay system arguments are presented that suggest a similar conclusion holds for multiple inlets i e ultimately all inlets will close or at best one will remain open

mmi attenuation and historical earthquakes in the basin and range province of western north america earthquakes in central nevada 1932 1959 were used to develop a modified mercalli intensity mmi attenuation model for estimating moment mag nitude m for earthquakes in the basin and range province of interior western north america m is 7 4 7 5 for the 26 march 1872 owens valley california earthquake in agreement with beanland and clark s 1994 m 7 6 that was estimated from geologic field observations m is 7 5 for the 3 may 1887 sonora mexico earthquake in agreement with natali and sbar s 1982 m 7 4 and suter s 2006 m 7 5 both estimated from geologic field observations mmi at sites in california for earthquakes in the nevada basin and range appar ently are not much affected by the sierra nevada except at sites near the sierra nevada where mmi is reduced this reduction in mmi is consistent with a shadow zone produced by the root of the sierra nevada in contrast mmi assignments for earthquakes located in the eastern sierra nevada near the west margin of the basin and range are greater than predicted at sites in california these higher mmi values may result from critical reflections due to layering near the base of the sierra nevada

tidal and nontidal exchange at a subtropical inlet destin inlet northwest florida abstract a tidal cycle study at destin inlet northwest florida investigated intratidal and residual flow structures for the first time across the inlet underway current velocity profiles were combined with hydrographic station profiles at neap tides to document the appearance of tidal fronts the distribution of tidal currents across two cross sections and the residual or non tidal flow structure at the same cross sections intratidal variations of water density and velocity showed the presence of fronts both in 1 late ebb early flood and 2 late flood early ebb tidal stages late ebb early flood tidal intrusion fronts brought about depth independent changes in water density 10 kg m 3 in

a review of the discrete wavenumber method we present a review of the discrete wavenumber dwn method the method introduced by bouchon and art 1977 allows the simple and accurate calculation of the complete green s functions for many problems in elastodynamics

the effect of a topographic high on the morphological stability of a two inlet bay system abstract the cross sectional stability of two tidal inlets connecting the same back barrier lagoon to the ocean is investigated the condition for equilibrium is the cross sectional area tidal prism relationship in an earlier study van de kreeke j 1990 can multiple inlets be stable estuarine coastal and shelf science 30 261 273 using the same equilibrium condition it was concluded that where two inlets connect the same basin to the ocean ultimately one inlet will close one of the major assumptions in that study was that the water level in the basin fluctuated uniformly in hindsight this assumption might be too restrictive for example in the wadden sea the back barrier lagoon consists of a series of basins rather than one single basin separated by topographic highs these topographic highs limit but do not exclude the exchange of water between the sub basins for this reason in the present study a topographic high in the form of a weir was added separating the back barrier lagoon in two sub basins the water level in the sub basins rather than in the back barrier as a whole is assumed to fluctuate uniformly for this schematization the hydrodynamic equations are solved using a finite difference method the results together with the equilibrium condition yield the equilibrium flow curve for each of the inlets the intersections of the two equilibrium flow curves represent combinations of cross sectional areas for which both inlets are in equilibrium the stability of the equilibriums was investigated using a non linear stability analysis resulting in a flow diagram calculations were carried out for different inlet and weir characteristics sinussoidal tides were the same for both inlets the results show that for relatively large wetted cross sectional areas over the topographic high approaching the situation of a single basin there are no combinations of inlet cross sectional areas for which both inlets are in a stable equilibrium this supports the conclusion in the earlier study for relatively small wetted cross sectional areas over the topographic high there is one set of stable equilibriums in that case the two inlet bay system approaches that of two single inlet bay systems

delineation and interpretation of seismotectonic domains in western north america indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western north america the san andreas east central california ecc mendocino triple junction mtj and pacific northwest pnw domains are found to have similar stress fields although the style and rate of deformation in each is different we propose that all of these domains are generated by shear between the pacific and north american plates in the mtj and pnw domains this shear may be evidence for the initiation of a transform fault connection between the san andreas and the queen charlotte island fault systems the ecc a region including much of the sierra nevada appears to result from the relatively stiff nature of the san andreas fault system the big bend and region of relatively new faulting in northern california act to make the san andreas fault system appear stiff on a regional scale two other domains the northern basin and range nbr and the intermountain seismic belt isb show evidence of being controlled by more local forces the nbr has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the juan de fuca north america plate boundary the isb can be explained by readjustments between the colorado plateau and the basin and range province which may be caused by the lowering of the basin and range due to extension

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

seismic constraints on the nature of lower crustal reflectors beneath the extending southern transition zone of the colorado plateau arizona events maintain a constant amplitude with offset which is most consistent with a mafic layering model one exception is a high amplitude 10 db above background event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets the event has a pronounced s wave response which along with its negative avo trend is a possible indication of the presence of fluids in the lower crust the arizona transition zone is an active but weakly extended province which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers instead we favor horizontal basaltic intrusions as the primary origin of high impedance reflectors based on 1 the fact that most xenoliths in eruptive basalts of the transition zone are of mafic igneous composition 2 indications that a pulse of magmatic activity crossed the transition zone in the late tertiary period and 3 the high regional heat flow observed in the transition zone the apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion we present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust

effects of a shallow flood shoal and friction on hydrodynamics of a multiple inlet system prior studies have shown that frictional changes owing to evolving geometry of an inlet in a multiple inlet bay system can affect tidally driven circulation here a step between a relatively deep inlet and a shallow bay also is shown to affect tidal sea level fluctuations in a bay connected to multiple inlets to examine the relative importance of friction and a step a lumped element parameter model is used that includes tidal reflection from the step the model is applied to the two inlet system of katama inlet which connects katama bay on martha s vineyard ma to the atlantic ocean and edgartown channel which connects the bay to vineyard sound consistent with observations and previous numerical simulations the lumped element model suggests that the presence of a shallow flood shoal limits the influence of an inlet in addition the model suggests an increasing importance of friction relative to the importance of the step as an inlet shallows narrows and lengthens as observed at katama inlet from 2011 to 2014

contemporary seismicity faulting and the state of stress in the colorado plateau the contemporary seismicity of the colorado plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude and contrary to earlier views of a low to moderate rate of occurrence with earthquakes widely distributed concentrations of earthquakes have been observed in a few areas of the plateau the most seismically active area of the colorado plateau is the eastern wasatch plateau book cliffs where abundant small magnitude seismicity is induced by coal mining the largest earthquakes observed to date of estimated richter magnitude m l 5 6 have generally occurred in northern arizona although very few earthquakes can be associated with known geologic structures or tectonic features in the colorado plateau seismicity appears to be the result of the reactivation of pre existing faults lacking surficial expression but favorably oriented to the tectonic stress field the small to moderate size of the earthquakes and their widespread distribution are consistent with a highly faulted precambrian basement and upper crust and a moderate level of differential tectonic stress earthquakes in the plateau generally occur in the upper crust from the near surface to a depth of 15 20 km although events have been observed in both the lower crust and uppermost mantle in areas of low to normal heat flow the latter suggests that temperatures are sufficiently low at these depths that brittle failure and hence earthquakes are still possible the predominant mode of tectonic deformation within the plateau appears to be normal faulting on northwest to north northwest striking faults with some localized occurrences of strike slip faulting on north west or northeast striking planes at shallow depths the contemporary state of stress within the plateau is characterized by approximate northeast trending extension in contrast to the previous belief that the plateau was being subjected to east west tectonic compression one area of the plateau the eastern wasatch plateau and book cliffs may still be characterized by compressive stresses however the nature of these stresses is not well understood

rapid source mechanism analysis of the april 29 1993 cataract creek mw 5 3 northern arizona earthquake abstract the source mechanism of two moderate size normal faulting earthquakes located slightly south of the grand canyon in arizona are studied using several near real time seismic wave analysis procedures the events are found to have source depths near 10 to 15 km and the smaller of the two events m b 4 8 is used to provide empirical green9s functions to analyze the rupture directivity of the larger m w 5 3 mainshock these events indicate complexity in the crustal stress environment on the western margin of the colorado plateau