steady state fluid response in fractured rock a boundary element solution for a coupled discrete fracture continuum model a discrete fracture representation of a highly fractured groundwater aquifer is inappropriate as a modeling approach since the location of all fractures can never be explicitly specified however the alternative of exclusively employing a continuum description of the medium then neglects any geologic information that exists with regard to the location of specific fractures in such formations in order to take better advantage of geologic information in highly fractured aquifers a model is proposed which couples the discrete fracture and continuum conceptualizations in regions where discrete fractures are designated an effecient means of computing the steady state fluid responses is formulated with the use of the boundary element method this allows the consideration of highly complex fracture geometries since fluid responses in the porous rock are described with a set of linear equations written only in terms of the hydraulic head and fluid mass flux at the boundaries the porous rock shares with the fractures one dimensional equations of fluid movement in the fractures written in terms of the same variables make coupling of the two systems straight forward in this technique discretization only along the fractures is required thus the internal discretization of the host rock that would be characteristic in the use of finite difference and finite element methods is alleviated at the boundary of the region where specific fractures are identified a continuum conceptualization is coupled to the discrete fracture model steady state fluid responses in the continuum domain are defined using a semianalytic technique which is based on the discretization of green s third identity the resulting coupled model allows the discrete fracture and continuum conceptualizations to be employed in the modeling of those areas where they are judged to be most appropriate based on the availability of physical information

deformation of the oceanic crust between the north american and south american plates fracture zone trends and magnetic anomalies in the atlantic ocean indicate that the north american plate must have moved with respect to the south american plate during the opening of the atlantic a comparison of plate tectonic flow lines with fracture zones identified from geosat and seasat altimeter data suggests that the north american south american plate boundary migrated northward from die guinea demarara shear margin to the vema fracture zone before chron 34 84 ma to north of the doldrums fracture zone before chron 22 51 9 ma and to north of the mercurius fracture zone between chron 32 72 5 ma and chron 13 35 5 ma the paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area indicate that the triple junction may have been located between the mercurius and the fifteen twenty fracture zones after 67 ma chron 30 plate reconstructions indicate a late cretaceous phase of transtension followed by transpression in the tertiary for the tiburon barracuda ridge area south of the fifteen twenty fracture zone the ocean floor in this area is characterized by a series of ridges and troughs with large bouguer gravity anomalies up to 135 mgal we use smoothing spline estimation to invert bouguer anomalies for crustal layer structure our model results suggest that the moho is uplifted 2 4 km over short wavelengths 70 km at the barracuda and tiburon ridges and imply large anelastic strains the severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted we propose that the north south american plate boundary migrated to the latitude of the tiburon ridge bounded by the vema and marathon fracture zones before chron 34 84 ma post chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the vema marathon mercurius and fifteen twenty fracture zone troughs but reaching the fifteen twenty fracture zone and future barracuda ridge area only after chron 32 72 5 ma this interpretation concurs with our crustal structural model which shows stronger crustal thinning underneath the tiburon ridge than at the barracuda ridge subsequent transpression may have continued along the existing zones of weakness in the tertiary creating the presently observed crustal deformation and uplift of the moho accompanied by anelastic failure of the crust middle eocene upper oligocene turbidites on the slope of the tiburon ridge now located 800 m above the abyssal plain suggest that most of its uplift occurred at post oligocene times the unusually shallow moho underneath the tiburon and barracuda ridges represents an unstable density distribution which may indicate that compressive stresses are still present to maintain these anomalies and that the north american south american plate boundary may still be located in this area

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

paleomagnetically defined rotations from the precordillera of northern chile evidence of localized in situ fault controlled rotations a paleomagnetic study of paleocene cretaceous and jurassic units in the precordillera of northern chile 26 s to 27 s demonstrate a complex pattern of crustal rotations the region was selected to investigate the pattern of deformation associated with the sierra castillo agua amarga fault and its associated structural subdomains which form part of the domeyko fault system the fault system that controls much of the structure of the chilean precordillera paleocene lavas from the center of the study area pass a fold test indicating the primary nature of the remanence in these rocks a second group of lavas from the south has a similar formation mean inclination after tilt correction for a uniform dip which coupled with the presence of reversals suggests that these lavas also carry a primary remanence the data indicate clockwise rotations of 35 and 42 respectively for the two sampling localities three geographically separate areas of lower cretaceous red beds also reveal primary remanences based on the presence of sites with reversed polarity at two of the localities and an inclination only fold test of the three locality mean directions for this formation these sampling areas yield remanence directions that reveal 30 of counterclockwise and 31 of clockwise rotation as well as an area without statistically significant rotation these data include the first well constrained counterclockwise rotation to be recovered from rocks in the southern central andes although mapping suggests the block involved is of limited geographic extent jurassic strata failed to yield any reliable results overall the data indicate a greater variability in the rotation pattern than has previously been observed in most comparable areas south of the arica deflection the variation cannot be explained by large scale rotation mechanisms such as oroclinal bending or domino style block rotation by widely separated transandean faults instead the rotations are consistent spatially and temporally and in their sense and magnitude with the eocene transpressional deformation associated with the domeyko fault system the deformational event that generated most of the important structures in the studied area the data emphasize the significance of local tectonics in controlling rotations in this part of the south central andes and the importance of understanding the local structure of regions sampled for paleomagnetic study

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

paleomagnetism in the precordillera of northern chile 22 30 s implications for the history of tectonic rotations in the central andes abstract widespread clockwise rotations in mesozoic and lower tertiary rocks of northern chile have been interpreted as the sum of two rotational events separated in time an early rotation related to local deformation plus a late rotation related to wholesale rotation of northern chile linked to late cenozoic oroclinal bending in the central andes in this paper we report new paleomagnetic data from cretaceous upper oligocene and miocene sedimentary rocks in the precordillera of northern chile the results suggest that all these rocks acquired their remanence at or close to the time of deposition the lack of rotation in undeformed lower miocene strata clearly indicates that clockwise rotations found in underlying faulted and folded cretaceous rocks were completed before the late cenozoic results from nearby localities in deformed upper oligocene strata would argue for little 5 rotation since the late oligocene data from widely separated miocene localities covering an area of about 5000 km2 in the calama basin strongly suggest that northern chile did not undergo significant wholesale rotation during the late cenozoic this together with previous paleomagnetic evidence against neogene rigid body like rotation of the southern peruvian forearc suggests that the curved shape of the central andean forearc was not significantly enhanced during the late cenozoic by inference all of the rotation in most mesozoic and lower tertiary rocks of northern chile was accomplished in the cretaceous and or early cenozoic when the locus of deformation in the central andes was localized in the present forearc region

multiscale bed form interactions and their implications for the abruptness and stability of the downwind dune field margin at white sands new mexico usa the downwind margin of white sands dune field is an abrupt transition from mobile aeolian dunes to a dune free vegetated surface this margin is also relatively stable over the past 60 years it has migrated several times more slowly than the slowest dunes within the dune field resulting in a zone of dune coalescence aggradation and along most of the margin development of a dune complex i e dunes superimposed on draas repeat terrestrial laser scanning surveys conducted over a 3 month period demonstrate that sediment fluxes within the dune complex decrease on approach to the margin computational fluid dynamics modeling indicates that this decrease is due in part to a decrease in mean turbulent bed shear stress on the lee side of the dune complex as a result of flow line divergence or sheltering of the lee side dunes by the stoss side of the dune complex conservation of mass demands that this decrease in bed shear stress causes aggradation we speculate that aggradation on the lee side of the dune complex further enhances the sheltering effect in a positive feedback contributing to the growth and or maintenance of the dune complex and a relatively abrupt and stable dune field margin our model and data add to a growing body of evidence that aeolian dune field patterns are influenced by feedbacks that occur at scales larger than individual dunes

multi spatial analysis of aeolian dune field patterns abstract aeolian dune fields are composed of different spatial scales of bedform patterns that respond to changes in environmental boundary conditions over a wide range of time scales this study examines how variations in spatial scales of dune and ripple patterns found within dune fields are used in environmental reconstructions on earth mars and titan within a single bedform type different spatial scales of bedforms emerge as a pattern evolves from an initial state into a well organized pattern such as with the transition from protodunes to dunes additionally different types of bedforms such as ripples coarse grained ripples and dunes coexist at different spatial scales within a dune field analysis of dune field patterns at the intersection of different scales and types of bedforms at different stages of development provides a more comprehensive record of sediment supply and wind regime than analysis of a single scale and type of bedform interpretations of environmental conditions from any scale of bedform however are limited to environmental signals associated with the response time of that bedform large scale dune field patterns integrate signals over long term climate cycles and reveal little about short term variations in wind or sediment supply wind ripples respond instantly to changing conditions but reveal little about longer term variations in wind or sediment supply recognizing the response time scales across different spatial scales of bedforms maximizes environmental interpretations from dune field patterns

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

tectonic rotations within the bolivian altiplano implications for the geodynamic evolution of the central andes during the late tertiary paleomagnetic results from 61 sites in tertiary red beds and volcanic rocks from the bolivian andes allow quantification of tectonic rotations within the altiplano domain a total of 16 sites were also obtained in lower miocene ignimbrites that cover the forearc of the north chilean andes near arica in the southern altiplano lipez region a local clockwise rotation of up to 38 is recorded in lower miocene volcanic rocks farther north near the salar de uyuni there is no evidence of significant rotations counterclockwise rotations are observed in the northern altiplano the largest counterclockwise rotation 28 is found in eocene oligocene red beds tiwanaku formation along the eastern border of the corque syncline middle miocene sediments located within the center of the corque basin record a counterclockwise rotation of only 10 about 200 km north of the corque basin eocene oligocene sedimentary rocks near the locality of viacha record only 10 counterclockwise rotation paleomagnetic results in lower miocene ignimbrites from the forearc near the arica bend do not show evidence for late cenozoic rotation of the forearc these new results suggest that the 20 to 25 counterclockwise rotation of the southern peruvian forearc occurred prior to the miocene anisotropy of magnetic susceptibility ams data show a magnetic fabric mostly controlled by bedding but slight ams lineations are found almost parallel to the major regional structural trends such as fold axes the apparent relationship between tectonic rotations ams lineations and structural trends suggests that rotations occured after or during the final stage of folding paleomagnetic data obtained in paleozoic rocks show evidence of clockwise rotation of the southern sub andean ranges tectonic rotations during the neogene are mostly localized on the eastern side of the central andes the curvature of the occidental margin near arica was likely acquired prior to the last stage of andean deformation although the magnitude of the rotations may vary from one locality to the other there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south a large scale regional mechanism is needed to explain this pattern of tectonic rotations within the central andes we propose a model in which along strike segmentation of the andean foreland and indentation of the altiplano by a curved forearc are the major factors controlling tectonic rotations within the altiplano eastern cordillera and sub andean ranges

temporal variation of the stress field during the construction of the central andes constrains from the volcanic arc region 22 26 s western cordillera chile during the last 20 ma in order to understand the response of the stress field state to intrinsic processes during the construction of the andes such as thickening of the continental crust lithospheric delamination and or thermal weakening we investigate the stress field evolution of the arc region since the last 20 my in the central andes 22 26 5 s the 43 reduced paleostress tensors derived from inversion of 682 fault slip data reveal a complex pattern of stress states during the last episode of orogenic construction and topographic uplift we identify two geodynamic stages the first stage corresponds to the construction of the altiplano puna plateau and the second one to its gravitational collapse four stress states that have prevailed in the altiplano puna plateau since middle miocene times characterize the transition from one stage to the other along the study latitudes a spatiotemporal change in stress state is clearly observed which led to an understanding that a change in the stress field may be related not only to the boundary conditions but also to intrinsic factors associated with the construction of the andean orogeny our results suggest that ca 13 10 ma and ca 8 5 ma in the southern altiplano and northern puna and in the southern puna respectively regional elevation and crustal thicknesses reached threshold values necessary to generate the orogenic collapse

central andean rotation pattern evidence from paleomagnetic rotations of an anomalous domain in the forearc of northern chile we have reanalyzed the pattern of paleomagnetically detected rotations in the central andes the central andean rotation pattern in order to investigate the temporal and spatial distributions of rotations the dominant pattern of rotation is well known with counterclockwise rotations in the northern andes clockwise in the southern andes linked to neogene orogenesis and shortening however much of the rotation in the forearc of northern chile 23 30 s is distinctly anomalous because of markedly high rotations that appear to predate rotations observed elsewhere in the central andes we argue that the data define a domain located in the forearc of northern chile marked by major 25 clockwise crustal rotations related to late paleocene early eocene highly oblique convergence this rather diffuse and cryptic deformation style accommodated strong shortening perpendicular to the andean margin by vertical axis rotations rather than by conventional fold thrust belts or transpressional fault systems

salar de atacama basin a record of compressional tectonics in the central andes since the mid cretaceous the salar de atacama basin lies in the inner fore arc of northern chile topographically and structurally it is a first order feature of the central andes the sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid cretaceous we have studied good exposures along the western edge of the basin and have correlated them with seismic reflection sections and data from an exploration well throughout most of its history the basin developed in a foreland setting during periods of thin skinned and thick skinned thrusting growth strata provide evidence for coeval sedimentation and thrust motions during mid cretaceous paleogene and neogene times pre neogene deformation was significant in the basin and in surounding areas of the early central andes models that attempt to explain the current thickness of the central andes should consider late cretaceous and paleogene shortening as well as the more obvious neogene and quaternary shortening

paleomagnetic data from the precordillera of northern chile a multiphase rotation history related to a multiphase deformational history abstract one of the most conspicuous features of the andean chain is the change in its trajectory from nw se to n s at 18 s known as the bolivian orocline although the central andes rotation pattern carp agrees roughly with the geometry of the orocline large variations and greater than expected rotation magnitudes cannot be explained by a progressive oroclinal bending history alone in this work 621 oriented samples from mesozoic and cenozoic rocks in three different areas in the precordillera of northern chile north of calama were analyzed the samples were subjected to different demagnetization processes to isolate the characteristic magnetic remanence and calculate the tectonic rotations by sampling pretectonic syntectonic and post tectonic units the rotation history of different structural blocks was determined and related to specific deformational events the results obtained from lower cretaceous units intruded or overlain by upper cretaceous units not rotated or recording smaller rotations indicate the existence of 20 clockwise and 20 counterclockwise rotated sites related to the late cretaceous peruvian orogenic event data obtained from upper cretaceous lowermost paleocene rocks show areas slightly clockwise rotated 10 15 and not rotated during the early paleocene k t deformation event remanence data from upper cretaceous eocene units across the area indicate as much as 20 clockwise rotation in the hanging wall of the eocene chintoraste quetena rabonas jaspe reverse fault system whereas the footwall was unaffected this eocene rotation occurred during the early stages of the eocene oligocene incaic orogenic phase prior to emplacement of the 40 37 ma fortuna granodiorite complex current models propose that rotations in carp were entirely cenozoic and largely accomplished in a single paleogene event our data indicate that each of the peruvian k t and incaic deformation events contributed to carp with each event generating a non uniform pattern of rotations which was superimposed on prior rotations in a non uniform manner

paleomagnetism of jurassic and cretaceous rocks in central patagonia a key to constrain the timing of rotations during the breakup of southwestern gondwana abstract a paleomagnetic study in jurassic and cretaceous rocks from the canadon asfalto basin central patagonia indicates the occurrence of about 25 30 clockwise rotation in upper jurassic lowermost cretaceous rocks whereas the overlying mid cretaceous rocks do not show evidence of rotation this constrains the tectonic rotation to be related to a major regional unconformity in patagonia which in turn seems to be close in time with the early opening of the south atlantic ocean the sense and probably the timing of this rotation are similar to those of other paleomagnetically detected rotations in different areas of southwestern gondwana suggesting a possible relationship between these and major tectonic processes related with fragmentation of the supercontinent on the other hand the mid cretaceous rocks in the region yield a paleopole located at lat 87 south long 159 east a95 3 8 this pole position is consistent with coeval high quality paleopoles of other plates when transferred to south american coordinates implying it is an accurate determination of the aptian circa 116 ma geomagnetic field in south america

clockwise block rotations along the eastern border of the cordillera de domeyko northern chile 22 45 23 30 s abstract we report new paleomagnetic results from a study of 38 sites in cretaceous to early tertiary red beds and volcanic rocks belonging to the purilactis group which outcrop along the western border of the salar de atacama basin after detailed thermal demagnetization characteristic directions were determined for 32 units in most cases red bed sediments from the lower members of the purilactis group have a well defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement a large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre tectonic magnetization we interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the cretaceous normal polarity superchron large deviations of the paleomagnetic declinations from the expected ones for stable south america provides new evidence for clockwise tectonic rotations associated with tertiary deformation in the cordillera de domeyko these data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north chilean andes this study however indicates spatial variation in the magnitude of the rotation with rotations 60 in the cerro totola area these rotations have occurred in conjunction with transpressional deformation that affected large tracts of the cordillera de domeyko during eocene deformation the systematic observation of clockwise rotations contemporaneous with sinistral displacements in the cordillera de domeyko can be explained by shear traction at the base of the brittle crust

thermal and exhumation history of the coastal cordillera arc of northern chile revealed by thermochronological dating abstract the thermal and erosional history of convergent plate boundaries is important for understanding the links between subduction arc magmatism genesis of ore deposits topography and climate of orogenic belts unlike the continent continent collision that formed many of the largest orogenic belts known today the central andes of south america is a unique case where an oceanic continent collision has given rise to the earth s longest and second tallest orogenic belt over the last thirty years a plethora of models have been suggested in an attempt to explain how a plateau type orogen formed at the leading edge of western south america in the central andes most research have focussed attention on the study of the evolution of the arc and backarc since continuous subduction erosion of the forearc has left little trace of the interplate dynamics that initiated the orogenic belt in this article we present a new insight into the thermal and exhumation history of the forearc along the coastal cordillera of northern chile based on biotite k ar apatite fission track and apatite zircon u th he dating we collected diorite samples in a 2 km thick crustal section at the coastal cliff 22 s and a sea level isoelevation profile between 21 and 27 s results from all three dating methods show that the cooling of coastal cordillera took place shortly after emplacement during a period of rifting in jurassic times cooling took place in two episodes mainly in late jurassic early cretaceous 118 152 ma but also during late cretaceous 60 80 ma due to the resumption of compression rift closure arc uplift exhumation eastward migration of magmatic arc activity and thermal relaxation the youngest apatite u th he ages reveal a cooling event never reported previously between 40 and 50 ma eocene this thermal event affected a 500 km long and 1 km thick section of the coastal cordillera in northern chile rock cooling recorded in the eocene cannot be explained by the thermal effect of the magmatic arc we explore two scenarios that can provide an explanation for the observed cooling 1 forearc uplift and exhumation and 2 changes in plate subduction dynamics erosion rates of 0 24 to 0 36 km myr for a period of 10 myr are necessary to explain the cooling event alternatively the subduction of a ridge could explain significant cooling by flat slab dewatering based on recent sea floor spreading reconstruction we suggest the subduction of the farallon phoenix ridge as a possible candidate the subduction of this ridge can account for the cooling event at 40 50 ma but also for some important aspects of the eocene development of the central andes notably the onset of uplift an abrupt decrease in magmatic activity and porphyry copper mineralization

aeolian processes and features in cool climates this review concentrates on the characteristics of wind processes and products in cold lowland deserts its main aims are to show how difficult it can be to separate wind generated from water generated processes and features without detailed study of lamination types to suggest cold aeolian interpretations for some ancient pre quaternary rocks and to encourage students to revisit many ancient deposits with such interpretations in mind although upper quaternary and recent cold climate aeolian deposits have been studied for many years there are very few older examples described martian features are included since mars is now one of the most exciting areas of study

paleomagnetism of upper miocene ignimbrites at the puna an analysis of vertical axis rotations in the central andes the origin of the rotations detected paleomagnetically in the central andes is controversial tectonic models proposed to explain it involve late cenozoic oroclinal bending the bolivian orocline and or small block rotations driven by oblique subduction in this paper we report paleomagnetic data from upper miocene ignimbrites from the puna in the presumed southern limb of the orocline these rocks showed high unblocking temperature and high coercivity magnetization the resulting pole position latitude 85 7 s longitude 80 5 e a95 7 9 k 22 n 17 indicates that the study area has not been significantly affected by vertical axis rotation since late miocene onward this implies that the whole puna has not undergone significant regional rigid body rotation during the same time span a farther analysis based on neogene paleomagnetic data from 31 andean localities distributed from 11 s to 31 s shows the presence of rotated and unrotated areas systematic counterclockwise rotations in peru and northern bolivia are observed whereas clockwise rotations are present in southern bolivia northern chile and perhaps northwestern argentina no systematic rotations are found farther south the overall time spatial distributions of these neogene paleomagnetic data suggest that if orogenic bending occurred it must have taken place before the middle miocene thus the oroclinal hypothesis cannot explain the rotations observed in middle miocene rocks or younger ones which also show orocline like declination anomalies we suggest that small block rotations driven by distributed shear may be a single process that can account for the whole rotations in the central andes it is hypothesized that horizontal shear in the andean crust could be controlled by ancient structures

late cretaceous alkaline complexes southeastern brazil paleomagnetism and geochronology late cretaceous alkaline rocks from pocos de caldas itatiaia passa quatro and sao sebastiao island were studied with the main purpose of improving the south american apparent polar wander path apwp rbsr age determinations for passa quatro and alkaline stocks from sao sebastiao island are around 70 and 81 ma respectively the magnetic carriers in the rocks studied were maghemite and highly oxidized titanomagnetites the paleomagnetic poles obtained in this work are 320 1 e 83 2 s n 47 a95 2 7 ksc 2609 8 for pocos de caldas 360 0 e 79 5 s n 18 a95 5 7 ksc 607 4 for passa quatro and itatiaia and 331 9 e 79 4 s n 18 a95 4 9 ksc 862 4 for sao sebastiao island the island set

insight into the neogene tectonic history of the northern bolivian orocline from new paleomagnetic and geochronologic data a paleomagnetic study of 36 sites 315 samples of neogene strata from the peruvian altiplano and adjacent sub andean belt pilcopata area combined with 40ar 39ar dating of 13 lava flows provide new constraints on the cenozoic deformation history of the northern bolivian orocline in the peruvian altiplano 40ar 39ar dating brackets the age of the strata sampled for paleomagnetism between 26 5 and 12 1 ma for the huaccochullo basin 18 6 and 12 1 ma for the descanso yauri basin and 16 8 and 15 5 ma for the ayaviri basin sub andean sediments are paleontologically constrained as miocene yet pre 9 ma prefolding magnetizations isolated in the huaccochullo descanso yauri and pilcopata localities indicate significant counterclockwise vertical axis block rotations of 11 3 5 4 31 0 10 2 and 7 8 4 8 respectively the pattern of rotations together with mapped structures suggest that deformation of the northern altiplano was partitioned into large regions experiencing relatively minor rotation versus smaller isolated basins exhibiting high amplitude counterclockwise rotations that lie in a major left lateral shear zone our new results imply significant transpressional deformation occurred throughout the peruvian altiplano since circa 12 ma and are integrated in a model detailing the tectonic evolution of the northern bolivian orocline since 25 ma

effects of atmospheric conditions on wind profiles and aeolian sand transport with an example from white sands national monument quantifying aeolian sand transport rates relies upon the computation of the near surface shear velocity u determined from velocity profiles of the wind while it has been recognized that various conditions such as saltation surface roughness surface slope and atmospheric conditions have an effect on the velocity profile it is commonly assumed that measurements made above the surface will be representative of the near surface shear velocity airflow and temperature data collected over a flat substrate at white sands national monument in new mexico however show the significant effects that atmospheric conditions have on velocity profiles during the day when solar insolation is heating the surface atmospheric conditions in the lowest several metres become unstable resulting in enhanced convection and vertical mixing so that the velocity gradient changes little with height as a result the shear stress in this region of vertical mixing lessens while the near surface shear stress is increased because the higher wind speeds are now nearer the surface at night the near surface atmospheric conditions are stable thereby reducing convection and vertical mixing resulting in stratified airflow and increased shear velocity away from the surface unless this atmospheric effect is accounted for estimates of sediment transport rates may be in error by as much as a factor of 15 times when wind speeds are near threshold velocity at wind speeds approaching 10 ms1 at 5m above the surface this error in computing sediment transport is reduced to a factor of only two to three times and may be within the range of measurement error

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

a pure dipole analysis of the gondwana apparent polar wander path paleogeographic implications in the evolution of pangea the paleogeography of prebreakup pangea at the beginning of the atlantic spreading has been a subject of debate for the past 50 years reconciling this debate involves theoretical corrections that cast doubt on available data and paleomagnetism as an effective tool for performing paleoreconstructions this 50 year old debate focuses specifically on magnetic remanence and its ability to correctly record the inclination of the paleomagnetic field in this paper a selection of paleopoles was made to find the great circles containing the paleomagnetic pole and the respective sampling site the true dipole pole tdp was then calculated by intersecting these great circles effectively avoiding nondipolar contributions and inclination shallowing in an innovative method the great circle distance between each of these tdps and the paleomagnetic means show the accuracy of paleomagnetic determinations in the context of a dominantly geocentric axial and dipolar geomagnetic field the tdps calculated allowed a bootstrap analysis to be performed to further consider the flattening factor that should be applied to the sedimentary derived paleopoles it is argued that the application of a single theoretical correction factor for clastic sedimentary derived records could lead to a bias in the paleolatitude calculation and therefore to incorrect paleogeographic reconstructions the unbiased apwp makes it necessary to slide laurentia to the west in relation to gondwana in a b type pangea during the upper carboniferous later evolving during the early permian to reach the final a type pangea configuration of the upper permian 2017 american geophysical union all rights reserved

variations in wind velocity and sand transport on the windward flanks of desert sand dunes the magnitudes of increases in wind velocity or speed up factors have been measured on the windward flanks of transverse and linear dunes of varying height on transverse dunes velocity speed up varied with dune shape and height for linear dunes speed up factors varied principally with wind direction relative to the dune with dune shape and dune height the main effect of velocity speed up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas this is greatest for large dunes with winds of moderate velocity blowing at a large angle to the dune changing ratios of base to crest sand transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase on linear dunes the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi directional wind regime

on the mechanism of crustal block rotations in the central andes comment in this paper beck 1998 reviews the database of palaeomagnetically observed rotations in the andean margin of south america in particular with respect to the well known change in sense of rotation linked to the arica deflection bolivian orocline in the orogen the paper argues firstly for the latitudinal distribution of rotations supporting an interpretation that distributed shear combined with oroclinal bending is the best hypothesis to explain the observed pattern of rotations and secondly that this rotation deformation is time dependent while we essentially agree with the former conclusion we would define it more tightly in that the over arching control on the pattern of rotations is a combination of differential shortening as opposed to oroclinal bending along the orogen coupled with distributed shear across the orogen our main concern with this paper is that it fails to provide a convincing demonstration that there is a time dependent element to the magnitude of rotations primarily we will argue that it is far from clear from the palaeomagnetic data presented that there is a case for a continuous rate of rotation applicable to the southern limb of the orocline at least not withstanding this we would also argue that geologically the deformation cannot and should not be described by a continuous deformation rate over time as postulated we also make specific points about aspects of the paper dealing with the atacama fault zone and its relation to the observed rotations

paleogene clockwise tectonic rotations in the forearc of central andes antofagasta region northern chile 1 for the central valley of northern chile antofagasta region a paleomagnetic analysis of data from 108 sites mainly in mesozoic and paleogene volcanic rocks has yielded stable remanent magnetization directions for 86 sites from these data we infer clockwise tectonic rotations of up to 65 within the forearc domain of the central andes the apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the incaic orogenic event of eocene early oligocene age a few paleomagnetic results obtained in neogene rocks do not show evidence of clockwise rotations hence the development of the bolivian orocline during late neogene time cannot be explained by simple bending of the whole margin these results demonstrate that tectonic rotations within the forearc and pre cordillera are key elements of early andean deformation which should be taken into account by kinematic models of mountain building in the central andes

bending of the bolivian orocline and growth of the central andean plateau paleomagnetic and structural constraints from the eastern cordillera 22 24 s nw argentina 1 we report new paleomagnetic and structural data from late cretaceous to mio pliocene continental sandy silty sedimentary rocks from the eastern cordillera central andes here n s to nne ssw ridges hosting paleozoic basement and upper cretaceous continental red beds overthrust thick adjacent cenozoic basins pretilting and likely primary reliable directions gathered at 15 sites document 45 9 9 4 30 1 23 9 and 15 4 19 3 clockwise cw rotations with respect to south america occurring after the late cretaceous 80 ma oligo miocene 20 30 ma and late miocene pliocene 5 10 ma respectively conversely four upper cretaceous sites from the walls of a n s left lateral strike slip fault yavi abra pampa fault yield a null rotation about 20 km to the west flower structures and subvertical syntectonic strata dated at 14 26 0 19 ma are exposed along the subparallel abra moreta left lateral strike slip fault relying on data from the literature on the period when deformation began we suggest that since eo oligocene times 30 40 ma the eastern cordillera has undergone a regional cw rotation of 40 50 synchronous with crustal shortening and large scale bending of the andean salient the cw rotation is possibly still active today as documented by regional gps data from the andes since 15 ma ago the activity of n s left lateral strike slip faults induced counterclockwise rotations along the fault zone locally annulling the regional cw rotation in agreement with a previous model we speculate that mid miocene strike slip activity accommodated the progressive southward spreading of the altiplano puna high altitude plateau laterally migrating from the overthickened crustal region of the salient apex

the precordilleran fault system of chuquicamata northern chile evidence for reversals along arc parallel strike slip faults abstract the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile was the site of the andean magmatic arc from the late cretaceous to the eocene oligocene boundary magmatism came to an end during the incaic tectonic phase 38 ma which caused arc normal shortening and the development of longitudinal dextral strike slip faults precordilleran fault system this magmatic arc tectonism is also related to the formation of the chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the precordillera structural investigations in and around the chuqicamata open pit mine have shown that wrench tectonics determined the kinematics of the area the n s striking west fissure which separates a 35 ma old non mineralised pluton to the west from a central late paleozoic basement ridge containing the mineralization became a sinistral fault along with other parallel faults in the area the central part is separated from similar paleozoic rocks to the east by the messabi este fault and a narrow faulted and sheared syncline of mesozoic cenozoic sediments this fault bears structures indicating dextral movements which probably are of an age that is similar to the mylonites 34 8 ma in the western pluton the dextral movements preceded the sinistral shear thus the fault system of chuqicamata displays a reversal of arc parallel shear movements according to the orientation of quartz veins in the mineralized body it is presumed that the sense of displacement of these strike slip motions reversed when mineralization started at about 32 ma during this time the stress field must have changed fundamentally the incaic phase dextral transpression is supposed to have been induced by the oblique vector of plate motion the following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling as however the vector of plate motion remained unchanged during that time oblique subduction cannot be used as an argument for arc parallel sinistral shear movements

slow cenozoic uplift of the western andean cordillera indicated by cosmogenic 3he in alluvial boulders from the pacific planation surface to determine the mechanisms responsible for mountain belt growth it is important to accurately establish the timing of surface uplift here we exploit the altitude control on the production rate of in situ cosmogenic nuclides to test the hypothesis that the andes was uplifted in the late miocene high concentrations of in situ cosmogenic 3he 3hecos have previously been measured in alluvial boulders on the western flank of the central andes northern chile evenstar et al 2009 these are consistent with deposition soon after formation of the surface 13 14 ma we have modeled the accumulation of cosmogenic 3he in several different surface uplift scenarios and compared them to the measured concentrations the measured 3hecos concentrations are too high to be produced by late miocene uplift and imply that the western flank of the andean cordillera attained a substantial part of its current elevation prior to 14 myr ago

eocene paleomagnetic pole for south america northward continental motion in the cenozoic opening of drake passage and caribbean convergence 1 a paleomagnetic study of eocene volcanic rocks in patagonia yields high unblocking temperature and high coercivity magnetizations combining these results with those of a previous study on patagonian eocene basalts yields a high precision high quality pole located at latitude 81s longitude 337 4e a95 5 7 critically this paleopole is indistinguishable from that of the late cretaceous circa 85 65 ma pole position of south america indicating that the plate was essentially motionless with respect to the spin axis for a period of 45 m y the pole position places south america at higher 5 than present day latitudes during the eocene indicating that northward continental motion toward present day latitudes must have been accomplished sometime since the late eocene paleomagnetic and tectonic correlation admits the hypothesis that cenozoic northward drift was associated with oligocene miocene extension in the southern continental edge leading to the opening of the drake passage and it agrees with the timing of foredeep formation and development of fold thrust belts in the northern continental edge this positive correlation between the paleomagnetically predicted drift of a major continent with extension at its trailing edge and convergence at its leading edge during times for which seafloor tectonic fabric and the geological record are particularly well preserved illustrates the utility of paleomagnetism in constraining paleogeographic and tectonic reconstructions for pre cretaceous times

flow dynamics and sediment transport over a reversing barchan changli china abstract the flow and sediment transport over a reversing 6 9 m high barchan dune is examined on the changli gold coast in hebei province china wind velocity profiles are non logarithmic exhibit rapid accelerations up slope and display near surface jets at the dune crest and downwind lee slope from the windward lower slope to dune crest the wind speed at 5 cm height increased by 55 the windward slope is a slipface and therefore has a significantly greater gradient than most normal windward slopes and the magnitude of speed up is significantly greater speed up ratio of 5 25 the majority of sediment was transported in the 0 4 cm height above the bed range maximum total sediment flux occurred at the dune crest and was greater than the total flux from three other sites combined the leeward mid slope sedimentation rate is strongly influenced by the sediment plume streaming upwards and downwind from the actively reversing dune crest dunes in the process of reversing display quite different wind flow and sedimentation patterns than when the dune is in normal flow conditions

palaeomagnetism of the cordillera de la costa northern chile evidence for local forearc rotation abstract palaeomagnetic studies of lower jurassic to lower cretaceous volcanic plutonic and sedimentary rocks from the cordillera de la costa of northern chile indicate that there has been no significant latitudinal movement of northern chile since early cretaceous times a palaeomagnetic pole for the cordillera de la costa 63 s 187 e 95 7 7 was derived from combining the results of three studied units the la negra formation jurassic volcanics jurassic lower cretaceous granodioritic plutons and the e1 way formation hauterivian marine carbonates together with the coloso formation lower cretaceous red beds comparison with a cretaceous reference pole for the stable shield area of south america 87 s 235 e 95 2 4 indicates that 24 7 of clockwise rotation has taken place possible mechanisms for the rotation of the andean forearc in this area have been examined in relation to the kinetics of the subducting nazca and over riding south american plates the cordillera de la costa has been subjected to both compressional and extensional tectonics since the lower cretaceous major eastwardly directed thrusting in mid cretaceous and early tertiary times possibly associated with a period of oblique subduction of the nazca plate contributed to the accretion of the cordillera to the main forearc rotation may have occurred during this process in oligo miocene and later tertiary times extensional faulting possibly associated with roll back of the nazca plate led to the reactivation of the atacama fault zone and the formation of oblique extension faults in the cordillera de la costa structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results

150 million years of climatic stability evidence from the atacama desert northern chile the sedimentary succession in the atacama desert records deposition under an arid to semiarid climate from the late jurassic 150 ma to the present day palaeomagnetic data indicate no significant latitudinal movement of this area since the late jurassic the present day location of the atacama within the dry subtropical climate belt is the principal cause of aridity this situation is likely to have prevailed since the late jurassic supplemented by 1 the continentality effect enhanced by the gondwanan landmass and 2 the presence offshore of a cold upwelling current from at least the early cenozoic onwards and possibly earlier resulting in conditions promoting climatic stability and desert development rapid and extreme climatic fluctuations during the plio pleistocene were not sufficient to destabilize the climate within the atacama comparison with other long lived deserts e g sw usa namib sahara and australia suggests that the atacama is the oldest extant desert on earth

11 11 dune morphology and dynamics this chapter discusses the key types of sand dunes and how they can be classified and assesses the major controls on their development the processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune field scale rather than at the single dune scale

timescale dependence of aeolian sand flux observations under atmospheric turbulence 1 the transport of sand in saltation is driven by the persistently unsteady stresses exerted by turbulent winds based on coupled high frequency observations of wind velocity and sand flux on a desert dune during intermittent saltation we show here how observations of saltation by natural winds depend significantly on the timescale and method used for determining shear stress and sand flux the correlation between sand flux and excess shear stress stress above a threshold value systematically improves for longer averaging timescale t and is better for stress determined by the law of the wall versus the reynolds stress method fitting parameters for the stress flux relationship do not converge with increasing t which may be explained by the nonstationary nature of wind velocity statistics we show how it may be possible based on the scale dependent statistics of stress fluctuations to rescale saltation flux predictions for wind observations made at different timescales however our observations indicate hysteresis and time lags in thresholds for initiation and cessation of saltation which complicate threshold based approaches to predicting sediment transport at different timescales

tectonic implications of a paleomagnetic study of the sarmiento ophiolitic complex southern chile abstract a paleomagnetic study was carried out on the late jurassic sarmiento ophiolitic complex soc exposed in the magallanes fold and thrust belt in the southern patagonian andes southern chile this complex mainly consisting of a thick succession of pillow lavas sheeted dikes and gabbros is a seafloor remnant of the late jurassic to early cretaceous rocas verdes basin that developed along the south western margin of south america stepwise thermal and alternating field demagnetization permitted the isolation of a post folding characteristic remanence apparently carried by fine grain sd magnetite both in the pillow lavas and dikes the mean in situ direction for the soc is dec 286 9 inc 58 5 95 6 9 n 11 sites rock magnetic properties petrography and whole rock k ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the late cretaceous the mean remanence direction is anomalous relative to the expected late cretaceous direction from stable south america particularly a declination anomaly over 50 is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the jurassic el quemado fm located over 100 km north of the study area in argentina nevertheless a significant ccw rotation of the whole soc is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back arc basin sub parallel to the continental margin a rigid body 30 westward tilting of the soc block around a horizontal axis trending nnw is considered a much simpler explanation being consistent with geologic evidence this may have occurred as a consequence of inverse reactivation of old normal faults which limit both the soc exposures and the cordillera sarmiento to the east the age of tilting is unknown but it must postdate remanence acquisition in the late cretaceous two major orogenic events of the southern patagonian andes in the eocene ca 42 ma and middle miocene ca 12 ma respectively could have caused the proposed tilting

change in the aerodynamic roughness height by saltating grains experimental assessment test of theory and operational parameterization data from an experiment at owens lake provided an opportunity to verify the validity of raupach s formula that predicts the apparent roughness height at equilibrium with saltation during wind erosion episodes in addition to that verification a simplification of raupach s formula is presented which allows the computation of the wind friction velocity affected by saltating sand grains without the need to measure the threshold wind friction velocity and to do iterative calculations this method estimates the increase of the wind friction velocity u above the nonsaltating wind friction velocity caused by the saltating grains u 0 3 u ut 2 where u and ut are the wind speed and the threshold wind speed respectively at 10m

palaeomagnetism of the continental sector of the cameroon volcanic line west africa summary measurement of samples from 154 sites in the continental sector of the cameroon volcanic line yielded six palaeomagnetic poles at 243 6 e 84 6 n a 95 6 8 224 3 e 81 2 n a 95 8 4 176 1 e 82 0 n a 95 8 5 164 3 e 86 4 n a 95 3 4 169 4 e 82 6 n a 95 4 6 and 174 7 e 72 8 n a 95 9 5 belonging to rocks which have been dated by the k ar method at 0 4 0 9 ma 2 6 ma 6 5 11 ma 12 17 ma 20 24 ma and 28 31 ma respectively the results are in general agreement with other palaeomagnetic poles from oligocene to recent formations in africa the first three poles for rocks formed between 0 4 and 11 ma are not significantly diverent from the present geographical pole together with other african poles for the same period this suggests that the african continent has moved very little relative to the pole since 11 ma the other three poles for rocks dated between 12 and 31 ma are significantly diverent from the present geographical pole showing a 5 polar deviation from the present pole in the miocene and 13 in the middle oligocene

shear stress partitioning in sparsely vegetated desert canopies shear velocity ratios were derived for six neighbouring localities representing rangeland abandoned farmland and a playa in the semi arid region of the sonoran desert near surface and above canopy wind speeds mostly below saltation threshold were used to evaluate the partitioning of shear velocities and shear stresses between the plant canopy and the surface the technique utilized represents an alternative to the threshold shear velocity ratio rt approach of musick and gillette land degradation and rehabilitation 1990 2 87 94 and results demonstrate the need for partitioning over a range of wind speeds shear velocity ratios were variable over the playa and abandoned farmland while variance in near surface wind speeds was highest over shrub dominated sites the relationship between lateral cover lc and shear velocity r is comparable to previous findings utilizing rt although methodological differences result in mean shear velocity ratios less than shear velocity ratios derived for threshold conditions with respect to the model of raupach et al journal of geophysical research 1993 98 d2 3023 3029 this may be attributed to the difference between measured shear stresses and the average shear stress on the exposed surface at high lateral covers shear velocity ratios are lower than threshold shear velocity ratios due to increased wake interactions or decreased drag on the shrub elements at higher wind speeds

an equilibrium relationship for shear velocity and apparent roughness lenght in aeolian saltation abstract an equilibrium relationship between shear velocity and apparent surface roughness is established for aeolian saltation velocity profiles of wind over blowing sand were digitized from eleven published data sets the 106 profiles were analyzed to obtain estimates of shear velocity and apparent surface roughness lenghts these values were used to test charnock s relationship for suitability in aeolian environments the results indicate that z 0 0 0091 u 2 g modifying charnock s model to describe wind over an equilibrium saltation field yields z 0 2d 50 30 0 0252 u u t 2 g the latter result is used to demonstrate that the focal point concept proposed by bagnold stems from the increase in apparent roughness with increased shear velocity the results demonstrate an excellent example of threshold controlled equilibrium in dynamic systems

secondary airflow and sediment transport in the lee of a reversing dune lee side windspeed and sediment transport were measured over a small 1 2 m transverse ridge in the silver peak dunefield west central nevada usa using an intensive array of 25 cup anemometers and seven total flux traps during crest transverse and transporting flow conditions u0 3crest 8 4 m s 1 windspeed near the surface of the lee slope averaged half 48 per cent that of crest speeds dimensionless speeds in the separation zone ranged from 0 2 to 0 8 that of the outer flow u12 along the boundary of the separation cell windspeed increased by 10 per cent of the crest speed before separation equilibrium of upper and lower wake regions was not observed by the documented eight dune heights suggesting that wake recovery may not occur over closely spaced dunes sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs this suggests that a significant amount c 70 95 per cent of sediment transported over the crest moved as fallout for this data set flux was approximately proportional to the cube of the near surface windspeed u0 3 and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection beyond the interdune transport direction progressed from oblique to crest transverse this indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest transverse flows regarding the grain size and sorting properties of transported sediment there was no significant difference in mean grain size over the dune although in general particles were finer and more poorly sorted in the lee copyright 1999 john wiley sons ltd

a boundary element and particle tracking model for advective transport in zoned aquifers abstract one of the most crucial factors that affects both the accuracy and efficiency of a groundwater transport model is the flow simulation method which produces the velocity field this paper presents a boundary element technique that incorporates the zoning approach to allow aquifer heterogeneity first the accuracy of the flow model is verified by comparing it with analytic solutions for well pumping and injection problems the boundary element flow model is then coupled with a particle tracking procedure for advection based transport simulation the combined model also calculates streamlines travel times and breakthrough curves and is therefore a powerful tool for simulation and management of advection dominated contamination problems

kinky profiles effects of soil surface heating upon vertical dust concentration profiles in the channel country of western queensland australia 1 in rangelands unlike cultivated fields soil surface conditions and vegetation cover are far from uniform field measurements taken within rangeland environments show that the shape of vertical dust concentration profiles can vary significantly between wind erosion events regression fits used to analyze vertical dust concentration profiles are therefore better for some events than others such variability has usually been explained as experimental error or noise or as a result of the nonuniformity of surface conditions a computer simulation model dsism is used to examine this variability in terms of wind erosion processes analysis of these simulation results together with field observations shows that nonuniformity in surface conditions alone cannot explain variations observed in the vertical dust concentration profiles these same simulations suggest that much of the variability in vertical dust concentration profiles is the result of thermal heating of the soil surface

mesozoic fault systems deformation and fault block rotation in the andean forearc a crustal scale strike slip duplex in the coastal cordillera of northern chile abstract in this paper we discuss the evolution and tectonic significance of the mesozoic trench parallel fault systems which affected the coastal cordillera and their relation to magmatism and crustal rotation the oldest extensional fault system separates basement from rift related late triassic and younger sedimentary units this system i subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the coastal batholith during much of the jurassic to earliest cretaceous period this extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary during the early cretaceous c 132 125 ma the kinematics of this fault system changed to transtension ii and accommodated a major component of left lateral strike slip motion the principal fault being the atacama fault zone along which plutons continued to be emplaced the final phase of pluton emplacement within the coastal cordillera appears to be c 106 ma after which this magmatic arc and fault system was abandoned an late cretaceous arc and fault system iii developed some 20 ma later and located some 50 km to the east in what is now the central valley of northern chile this paper seeks to show that the coastal cordillera was deformed as a whole by this late cretaceous fault system iii which formed a crustal scale left lateral transpressional duplex during this deformation the thermally weakened crust was dissected into a series of large scale blocks bounded by nw trending left lateral strike slip faults which merge into a nne ssw fault zone which forms the eastern boundary to the duplex we term this eastern boundary zone the central valley fault zone cvfz and this together with the nw trending faults defines the duplex system which we refer to as a whole as the coastal cordillera fault system ccfs iii we have traced the ccfs duplex between 25 s and 29 s and suspect that it continues northward the timing of the deformation is constrained to be post 106 ma the age of coastal cordillera arc abandonment and pre tertiary based on the deformation and pluton emplacement in and along the central valley fault zone palaeomagnetic data from the fault bound blocks within the ccfs duplex indicate 35 45 of post early cretaceous clockwise rotation with no substantial latitudinal motion we suggest that the observed fault kinematics of the ccfs are consistent with this crustal scale duplex model where rotations would have occurred in response to left lateral transpression

andean uplift ocean cooling and atacama hyperaridity a climate modeling perspective abstract located along the west coast of subtropical south america the atacama desert features an extremely dry climate in sharp contrast with relatively wet conditions over the central andes and farther east the atacama s hyperaridity has been attributed to its subtropical location the cold waters over the adjacent southeast pacific ocean and the presence of the andes cordillera although geological evidence reveals less dry conditions in the remote past the timing of the arid to hyperarid transition is a matter of controversy several studies suggest that such transition occurred between 19 and 13 ma fostered by the concurrent uplift of the andes during the lower middle miocene other studies however suggest a much earlier 25 ma or later 2 1 ma transition associated with global or regional ocean cooling here we use plasim a simple global climate model to study the effect of the andean surface uplift and sea surface temperature changes upon atacama aridity in one set of experiments the continental topography was scaled by a factor ranging from 0 9 to 0 1 decreasing the height of the andes did not increase precipitation over the atacama region but rather decrease the precipitation over the central andes and the interior of the continent these results consistent with previous modeling studies suggest that the andean uplift was not an important ingredient in the onset of atacama hyperaridity even if both events overlapped in time a globally uniform sea surface warming does not result in a significant increase of precipitation over the atacama in contrast a regional warming over the subtropical southeast pacific very effectively increase precipitation along the west coast of south america by decreasing the subsidence and the dry advection at low levels it suggested that a reinforcement of the humboldt current since the late miocene and particularly during the pliocene pleistocene transition was crucial in the drying of the atacama desert that culminated with the present day hyperarid conditions

tectonic signatures in arid continental basins an example from the upper miocene pleistocene calama basin andean forearc northern chile abstract the calama basin a transtensional basin situated within the andean forearc and its extension to the pampa del tamarugal central depression is filled with 700 m of sediments detailed sedimentary logging and 40ar 39ar dates from interbedded volcanic deposits have established a lithostratigraphic and chronostratigraphic framework for the upper 95 m thick upper miocene pleistocene succession five facies associations fluvio lacustrine diatomite palustrine carbonate massive anhydrite fluvial and alluvial fan have been identified within this part of the basin fill the following events are recognised in the basin evolution 1 deposition of diatomaceous lacustrine sediments in two separate depocentres in the pampa del tamarugal and the central calama basin with alluvial fan accumulation along the basin margins at 8 ma 2 localised deformation within the central calama basin between 7 and 6 ma but continuous sedimentation elsewhere 3 palustrine carbonate deposition across the entire area from 6 to 3 ma except in the pampa del tamarugal where diatomaceous sedimentation continued until 5 ma before being replaced by palustrine carbonate deposition 4 widespread folding after 3 ma and entrenchment as the drainage system cut down to reach a new base level caused by the rio loa breaching the coastal cordillera and reaching the sea localised lacustrine deposition in topographic lows or tectonically created dams also occurred during this period comparison with similar age facies elsewhere in northern chile has shown changes such as these to be localised and diachronous indicating that tectonic controls on accommodation space and drainage patterns are the primary influence on sedimentation in this tectonic setting the lacustrine deposits do not appear to be a reliable recorder of low frequency climatic change this is exemplified by the change from lacustrine to palustrine deposition at 6 5 ma which would appear to record a drying of the climate however a synchronous change from ephemeral alluvial fan deposition to more permanent fluvial deposition at the basin margins indicates increased run off

evaluation of aeolian sand transport equations using intertidal zone measurements saunton sands england sand transport rates were measured using a vertical sand trap along the intertidal zone of a beach in north devon england together with simultaneous monitoring of the wind speed on the beach and moisture levels in the surface layers of sand the results of 88 sand trap samples in a wide range of wind speeds showed that moisture levels up to 14 in the top 1 2 mm of the beach sand have no discernible effect on the transport rates transport rates measured from areas of the beach where the moisture was below this level are compared with the rates predicted by seven expressions based on theoretical and wind tunnel research together with the empirical results of other published research measured transport rates range from 0 0001 to 0 22 kg m 1 s 1 the results indicate that expressions based on a power relation between the wind speed and the transport rate and which include a threshold velocity term provide the best estimates of the observed transport rates

a tale of two continents some tectonic contrasts between the central andes and the north american cordillera as illustrated by their paleomagnetic signatures comparison of patterns of paleomagnetic poles from orogenic belts with appropriate reference poles for the craton can help to delineate important large scale tectonic processes comparison of the paleomagnetic signatures of the western cordillera of north america and the central andes shows that the western edges of these belts have had profoundly different mesozoic and cenozoic histories specifically the north american cordilleran pattern shows strong evidence of post middle cretaceous relative northward displacement of outboard crustal blocks but there is almost no comparable evidence of margin parallel displacement in the andes we speculate that this may largely be a consequence of a simple difference in shape the convex westward western margin of north america facilitates margin parallel displacement as a response to oblique subduction whereas the concave westward margin of south america inhibits it the patterns of block rotations found along the western edge of the two orogens also are quite different nearly everywhere within the western north american cordillera crustal blocks have rotated clockwise since mid cretaceous time reflecting a pervading state of dextral shear within the western andes of peru and northern chile however mesozoic and cenozoic rocks in peru and northernmost chile are rotated strongly counterclockwise whereas rocks of the same age in the remainder of chile to about latitude 48 s are rotated clockwise a model combining oroclinal bending and block rotations driven by oblique subduction can account for the paleomagnetic observations

paleomagnetism of the patagonian plateau basalts southern chile and argentina a total of 505 paleomagnetic samples were collected from 65 sites volcanic flows of the patagonian plateau basalts at four locations in southern chile and argentina k ar analyses indicate that 38 flows from two locations form a late cretaceous group 64 79 ma while the remaining 27 flows are eocene in age 42 56 ma progressive demagnetization indicates that a characteristic remanent magnetization chrm has been successfully isolated from 59 flows rock magnetic properties and analysis of chrm directions within and between sites allow the secure inference that the chrm is a thermoremanent magnetization acquired during original cooling the eocene flows yield 15 independent samplings of the paleomagnetic field however these data fail the reversals test and probably do not adequately average geomagnetic secular variation the late cretaceous flows yield 18 independent samplings of the paleomagnetic field six of normal polarity and 12 of reversed polarity these data pass the reversals test and have angular dispersion of virtual geomagnetic poles vgps consistent with adequate sampling of geomagnetic secular variation the mean of these 18 vgps yields a late cretaceous paleomagnetic pole for south america latitude 78 7 s longitude 358 4 e a95 6 3 k 31 6 s 14 5 this pole is consistent with similar age poles from north america and africa when the atlantic is reconstructed to 70 ma contrary to previous indications the south american apparent polar wander path does not reach the present rotation axis until after late cretaceous time

paleomagnetic study of upper miocene rocks from northern chile implications for the origin of late miocene recent tectonic rotations in the southern central andes paleomagnetic studies in the southern central andes have shown the widespread presence of clockwise vertical axis rotations rock units sampled in these studies however are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern chile being from mesozoic and lower tertiary rocks whereas most data in the southern altiplano puna and cordillera oriental are from upper tertiary rocks in this paper we present the results of a paleomagnetic study on upper miocene sedimentary rocks and ignimbrites from the precordillera of northern chile at 22 s these rocks are coeval with the initiation of crustal shortening in the eastern foreland fold thrust belt which some tectonic models relate to oroclinal rotation of northern chile primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites suggesting the study area has acted as a single coherent block with respect to vertical axis rotational deformation although minor inadequate sampling of paleosecular variation can affect our data set its time averaged paleomagnetic direction is indistinguishable from the expected late miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 ma this suggests that late miocene recent oroclinal rotation of the northern chilean forearc if present must be either very low or nonuniform a direct implication of this result is that much of the unquestionable tectonic rotations detected from upper miocene rocks in the southern central andes is of local origin available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the andes

groundwater salinity as a control on development of eolian landscape an example from the white sands of new mexico abstract the white sands of southern new mexico form the largest field of gypsum dunes in the world the juxtaposition of several different types of dunes within the field has long been recognized and has generally been attributed to changes in the rate of sand transport across the dune field however steven fryberger fryberger s g 2003 geology of white sands national monument web page www2 nature nps gov geology parks whsa recently hypothesized however that a primary control is groundwater salinity the white sands dune field is underlain by saline waters fryberger proposed that parabolic dunes could form in topographic highs that accumulated a lens of fresher water derived from precipitation water with lower salinity allowed vegetation to grow and stabilized the sand except for the active noses of the parabolic dunes this study is a test of that hypothesis a study transect was established across the boundary between parabolic and barchan dunes groundwater and soil at six sites were sampled during december 2004 and march may and june of 2005 two sites were established in the parabolic field two in the barchans and two in the transition zone between them groundwater was found to be three times more saline in the barchan area conductivity and chloride decreased across the transition zone a gps topographic survey revealed two abrupt topographic steps one at the boundary between the barchans and the transition zone and a second at the edge of the parabolic field a second larger topographic step was found outside the study area defining a higher parabolic dune field these data indicate that fryberger s 2003 hypothesis is correct and groundwater is controlling the dune field morphology the dune field did not begin to form until the mid holocene 6500 years ago a compilation of dates shows that the parabolic dune field has probably been stable for the last 3500 years in contrast the barchan area has been recently deflated and exposed 2000 year old sediment near the surface

paleomagnetic tracking of mountain building in the peruvian andes since 10 ma 1 we report paleomagnetic data from 73 sites 628 samples of upper oligocene to pliocene rocks from central and northern peru the data indicate that the subandean zone has not experienced vertical axis rotation since the upper oligocene whereas the coast and the western cordillera record a coherent pattern of counterclockwise rotations emplaced in the last 10 ma this pattern can best be explained by two competing hypotheses 1 a propagation in rotations from the bolivian orocline toward the north or 2 a more punctual and widespread event linked to subduction of the nazca ridge that caused the rotations on the basis of the time space relationship of paleomagnetic rotations deformation magmatism and nazca south america plate convergence the latter appears more likely moreover because the rotations north of the abancay deflection 15 5 s are synchronous with deformation and exhumation they indicate that major mountain building in the peruvian andes has occurred since 10 ma finally we suggest that a succession of aseismic ridge subduction events played an important role in shaping the modern topography of the andean chain

velocity profiles measured on the windward slope of a transverse dune velocity gradients measured on the windward slope of a transverse dune show that flow acceleration strongly affects the turbulence structure of wind wind profiles measured below a height of 1 6m are nonlogarithmic and the altered nature of velocity profiles varies over the windward slope of a dune these findings suggest that calculated values of shear velocity are largely dependent upon the height and location of wind speed measurements as well as the strength of the prevailing wind moreover if bagnold type transport equations are used to calculate rates of sand transport on a dune wind speeds must be measured within 0 2 m of the dune surface if wind speeds are measured above this height the resulting errors in calculated shear velocity will manifest themselves as errors in calculated rates of sand transport if rates of sand transport are derived from wind speeds measured above 0 2 m the calculated rates may underestimate sand transport on the windward face of a dune as a result of flow acceleration in contrast if a dune has a separate crest upwind of the brink the expansion and subsequent deceleration of flow downwind from the crest may result in an overestimation of sand transport

widespread remagnetization of late proterozoic sedimentary units of uruguay and the apparent polar wander path for the rio de la plata craton summary a palaeomagnetic study was performed on three neoproterozoic sedimentary successions exposed in central and eastern uruguay in order to better constrain the palaeogeographic evolution of the rio de la plata craton by the latest proterozoic these units comprise the latest ediacaran to early cambrian calcareous cerro victoria fm exposed in the central nico perez terrane the late ediacaran clastic yerbal fm and the calcareous polanco fm exposed in the dom feliciano belt and the ediacaran clastic rocha fm exposed in the easternmost punta del este suspect terrane the study showed that none of the units carry the original remanence and that they were affected by remagnetizations the most widespread remagnetization is shown by the cerro victoria yerbal and rocha formations a dual polarity post folding remanence carried by hematite was isolated in all these units mean directions from them are cv cerro victoria dec 179 1 inc 59 0 95 2 9 n 79 samples y yerbal dec 182 4 inc 61 9 95 4 5 n 38 samples r rocha dec 4 2 inc 64 9 95 2 7 n 42 samples their corresponding pole positions are virtually identical cv 82 6 s 309 3 e a95 3 9 y 77 0 s 298 4 e a95 5 9 r 76 6 s 291 0 e a95 4 2 suggesting the same magnetization age and process comparison with the south american path suggests the late permian early triassic or the latest cretaceous palaeogene as the most likely times for its acquisition these poles are also identical to the pole of the late proterozoic la tinta fm which is interpreted as affected by the same remagnetization and therefore invalid for palaeogeographic reconstructions a different magnetic component also dual polarity and post folding was defined at two sites of the cerro victoria fm and other two of the polanco fm possibly carried by magnetite the mean geomagnetic poles from both units cvc 4 2 n 343 2 e a95 13 8 and p 3 2 n 325 8 e a95 15 2 fall on the middle cambrian to early ordovician segment of the gondwana apwp suggesting that age for the remagnetization this permits to establish the age of the tectonic event affecting the neoproterozoic units of the rio de la plata craton as early cambrian ca 525 ma the available palaeomagnetic poles for this craton permit to constrain its palaeogeographic evolution only since approximately 600 ma

tectonic rotations and internal structure of eocene plutons in chuquicamata northern chile abstract a paleomagnetic and ams study on eocene plutonic complexes in the calama area northern chile reveals high temperature high coercivity magnetizations of dominantly thermoremanent origin and magnetic fabrics controlled by magnetite the paleomagnetic results indicate that 43 ma plutons underwent clockwise tectonic rotation whereas adjacent 39 ma plutons did not undergo discernible rotation this points to a middle eocene age for the younger tectonic rotations associated with the central andean rotation pattern in the chuquicamata calama area the petrofabric in these rocks formed under conditions ranging from purely magmatic i e before full crystallization to low temperature solid state deformation ams and paleomagnetism suggest that the plutonic bodies were formed by progressive amalgamation of subvertical magma sheets spanning multiple magnetic polarity chrons the parallelism between magmatic and tectonic foliations suggests that regional tectonic stress controlled ascent emplacement and rock deformation during cooling in this context we suggest that magma ascent and emplacement in the upper crust likely exploited mesozoic structures which were locally reactivated in the eocene

on the mechanism of crustal block rotations in the central andes abstract the results of 62 paleomagnetic studies of jurassic and younger rock units from the andes of argentina bolivia chile and peru are examined for indications of the timing and mechanism of crustal block rotations rotations and their 95 confidence intervals are recalculated in pole space using a new south american reference apw path as observed by many investigators the arica deflection the change in topographic and tectonic trends located near arica northernmost chile divides the data into a counterclockwise rotated set north of the deflection and a clockwise rotated set to the south rotations range from 75 counterclockwise to nearly 50 clockwise and show substantial differences between neighboring crustal blocks south of arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin but no such correlation is observed north of arica rotation increases with age both north and south of arica for rock units of 90 ma for older rock units no correlation between age and amount of rotation exists unlike the western margin of north america no significant margin parallel displacements accompany the rotations several models for rotation are examined it is concluded that a combination of late cenozoic oroclinal bending and post mid cretaceous local block rotations in response to shear driven sub crustal ductile flow can explain most of the paleomagnetic and geologic observations

aeolian shear stress ratio measurements within mesquite dominated landscapes of the chihuahuan desert new mexico usa a field study was conducted to ascertain the amount of protection that mesquite dominated communities provide to the surface from wind erosion the dynamics of the locally accelerated evolution of a mesquite coppice dune landscape and the undetermined spatial dependence of potential erosion by wind from a shear stress partition model were investigated sediment transport and dust emission processes are governed by the amount of protection that can be provided by roughness elements although shear stress partition models exist that can describe this their accuracy has only been tested against a limited dataset because instrumentation has previously been unable to provide the necessary measurements this study combines the use of meteorological towers and surface shear stress measurements with irwin sensors to measure the partition of shear stress in situ the surface shear stress within preferentially aligned vegetation within coppice dune development exhibited highly skewed distributions while a more homogenous surface stress was recorded at a site with less developed coppice dunes above the vegetation the logarithmic velocity profile deduced roughness length based on 10 min averages exhibited a distinct correlation with compass direction for the site with vegetation preferentially aligned while the site with more homogenously distributed vegetation showed very little variation in the roughness length this distribution in roughness length within an area defines a distribution of a resolved shear stress partitioning model based on these measurements ultimately providing potential closure to a previously uncorrelated model parameter

paleomagnetism and tectonics of the southern atacama desert 25 28 s northern chile we report paleomagnetic results for 131 sites from the modern forearc of northern chile 25 s and 28 s remanent magnetization in volcanic and intrusive rocks is mostly primary while a secondary magnetization is observed in sedimentary rocks comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations from 7 3 21 6 counterclockwise to 52 7 17 clockwise jurassic to early cretaceous rocks from the coastal cordillera and cretaceous to paleocene rocks from the central depression show similar magnitude 30 clockwise rotations while more variable rotations occur in mesozoic to eocene rocks of the precordillera clockwise rotations in mesozoic and paleogene rocks occur in the chilean frontal cordillera south of 27 30 s paleomagnetic results in three large miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the precordillera and pre andean depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 ma early miocene and likely during and after the incaic tectonic event which affected large tracts of the central andes the postulated onset of rotations in the north chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the eastern cordillera in bolivia and northwestern argentina rotation of the chilean forearc enhancement of the curvature of the central andes and the formation of the bolivian orocline seem to be for the most part closely linked to the evolution of the eocene oligocene tectonics of the eastern cordillera

development of a continental forearc a cenozoic example from the central andes northern chile in order to understand the response of a continental forearc to changes in subduction zone geodynamics we constructed a high resolution chronostratigraphic cross section across the central andean forearc of northern chile 21 24 s the tectono stratigraphic development of the forearc differs from established models no relationship was found between changes in rate of relative plate convergence and amount and style of deformation forearc response to continual compression since the oligocene has been uplift and segmentation into discrete tectono stratigraphic zones from west to east these zones are the extensional coastal cordillera the extensional and or transtensional central depression and the transpressional and or compressional precordillera preandean depression each area has recorded almost continuous sedimentation from oligocene eocene time to the present day accommodation space has been generated by basin margin uplift rather than active subsidence we propose a model in which uplift of the leading edge of the south american plate is driven by subcrustal accretion of material removed at the trench by subduction erosion uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge the tectono stratigraphic complexity exhibited within the central andean forearc is likely to be representative of cordilleran type margins and would be difficult to recognize in an ancient continental forearc

magmatic arc tectonics in the central andes between 21 and 25 s abstract in northern chile four magmatic arc systems each displaced towards the east in relation to the previous one developed in sequence since the lias this migration of the magmatic arc from the coastal cordillera to the western cordillera probably was a consequence of tectonic erosion of the continental margin within each of these systems tectonic deformation took place contemporaneously to magmatic activity as the continental crust of the upper plate has been weakened in the area of the magmatic arc by the igneous processes it is relatively sensitive to the stresses introduced by plate convergence a very important factor for the deformation in the magmatic arc is the angle of convergence obliqueness which may cause shortening if obliqueness is small

paleomagnetic results from the mogollon datil volcanic field southwestern new mexico and a refined mid tertiary reference pole for north america miocene oligocene rocks of the mogollon datil volcanic field in southwestern new mexico have yielded paleomagnetic data giving 61 site mean unit vgps and a paleomagnetic pole located at 81 9 n 143 6 e a95 4 5 k 17 6 this pole is indistinguishable from other mid tertiary reference poles for north america combining the mogollon datil data with vgp data from other studies of mid tertiary 20 40 ma rocks yields a paleomagnetic pole at 81 5 n 147 3 e n 239 a95 2 4 k 15 4 when divided into normal and reversed polarity subsets the data yield paleomagnetic poles that are statistically different at the 95 confidence level mcfadden and lowes 1981 the paleopole determined from reversely magnetized units located at 78 6 n 138 5 e n 125 a95 3 4 k 14 5 is 6 8 on the far side of the paleopole determined from normally magnetized units positioned at 84 2 n 166 4 e n 114 a95 3 3 k 17 0 when viewed from a common site location i e the western united states similar pole positions an differences are also seen in 10 ma subsets of this data set incomplete removal of a normal overprint on the reversely magnetized units appears unlikely to explain these results rather we suggest that the differences are consistent with the concept that differing harmonics may be associated with the two polarity states over the long term merrill and mcelhinny 1977 merrill et al 1979

physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research abstract recently ultrasonic anemometers uas have become available for precise high frequency measurement of three dimensional velocity and turbulence properties except for a few wind tunnel and computational fluid dynamics cfd simulations advances in aeolian sediment transport and bedform research have been limited to field studies using instrumentation that is either incapable of measuring turbulence e g cup anemometers or unable to withstand sediment laden airflow e g hotfilms in contrast extensive progress has occurred in fluvial research where turbulence instrumentation has been available for some time this paper provides a pragmatic discussion on using uas in aeolian research recent advances using this technology are reviewed and key physical and logistical considerations for measuring airflow properties and near surface shear stress using uas over complex terrain are discussed physical considerations include limitations of applying boundary layer theory to flow over natural surfaces such as non logarithmic velocity profiles resulting from roughness and topographically induced effects and the inability of instrumentation to measure within the thin constant stress region these constraints hinder accurate shear velocity u shear stress and sand transport estimation uas allow measurement of turbulent reynolds stress rs that in theory should equal profile derived shear stress discrepancies often exist between these quantities however due to three dimensional spanwise flow components and rapid distortion effects i e unbalanced production and dissipation of turbulence common in flow over complex terrain while the rs approach yields information on turbulent contributions to near surface stress generation little evidence exists showing that rs is a better measure of forces responsible for sediment transport consequently predictive equations for sediment transport using rs do not exist there is also a need to identify the role of micro turbulent events e g burst sweep cycles and macro turbulent structures e g separation cells shear layers in aeolian dynamics in field settings to validate recent wind tunnel and cfd simulations a conundrum exists regarding whether velocity data should be rotated to correct for potential sensor misalignment effects in unsteady non uniform flow over complex terrain streamline angles vary spatially and temporally with height and location thus determination of and correction to true streamline coordinates is difficult caution should be exercised with correction methods that remove implicit vertical velocity trends as this may preclude detection of geomorphically important flow behaviour e g lift at a dune crest and may complicate interpretations of rs instruments should be aligned with the underlying surface and flow visualization should be used to adjust sensor measurement planes as close as possible to local streamlines logistical considerations include sensor design advantages and operational limitations data communication formats and field deployment strategies each can affect measurement accuracy and are easily overlooked sampling range frequency and period are also important as they limit the range of velocities and scales of turbulence that can be characterized ultrasonic anemometers offer a new sampling resolution to measure turbulent airflow properties in field settings with proper considerations of their limitations uas may allow researchers to close the gap between fluvial research and develop more robust models of aeolian processes and morphodynamics

methodology for reconstructing wind direction wind speed and duration of wind events from aeolian cross strata 1 a methodology for reconstructing wind direction speed and event duration from aeolian dune cross strata was developed from analysis of crescentic dunes at white sands new mexico during wind events dune lee faces were surveyed lee face deposits mapped deposition rates measured grain size sampled by stratification type and winds characterized from meteorological and field data the spatial distribution of lee face stratification styles is a function of the incidence angle formed between the wind and the brinkline with secondary controls by wind speed and dune sinuosity and height sets of wind ripple strata form at incidence angles of 25 40 grainfall grainflow foresets over wind ripple bottomsets at 40 70 and grainflow grainfall foresets at 70 90 erosional reactivation surfaces form at incidence angles up to 15 bypass surfaces up to 25 the total sediment load is fractionated within lee face stratification types wind speed can be reconstructed from relationships between grain size transport mode shear velocity and grain settling velocity where the full range of grain transport modes occurs and grain size is limited by shear stress the shear velocity and grain size range in each transport mode can be estimated by assuming the coarse fraction in grainflow strata traveled in creep and the coarse fraction in grainfall traveled in saltation the minimum duration of a wind event can be estimated using measures of shear velocity dune height and dune forward migration method limitations arise with source area control on grain size extremes in wind events and severe truncation of sets of cross strata

major crustal rotations in the andean margin paleomagnetic results from the coastal cordillera of northern chile paleomagnetic analyses of mesozoic lavas and dike swarms from the northern chilean coastal cordillera between 25 4 s and 26 4 s reveal a clockwise rotation of about 42 magnetizations from lava flows of andesitic basaltic composition of the middle jurassic la negra formation pass both fold and reversal tests and are interpreted as prefolding remanences five dike swarms of middle jurassic to early cretaceous age yield similar directions to that obtained from the la negra formation four of the five swarms have mixed polarity suggesting that they too carry a primary or very early remanence the structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence the clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid late cretaceous age post 100 ma associated with abandonment of the jurassic early cretaceous magmatic arc in this region and its eastward migration to form a new mid late cretaceous magmatic arc in the former back arc region this younger arc is located east of the coastal cordillera and lies in the central valley region the clockwise sense of rotation is consistent with other paleomagnetic data from northern chile and southern bolivia south of the arica deflection in the andean margin although it is the largest yet reported to the north of the arica deflection paleomagnetic studies report counterclockwise rotations and several large scale models have been proposed to explain the overall pattern of rotations models include oroclinal bending of an originally straight margin differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend in contrast to these models several workers have argued that rotation is better explained in terms of localized in situ rotations we review these models in light of our results and present a domino type model with blocks bounded by left lateral faults and rotating clockwise in response to mid late cretaceous transpression within a crustal scale shear zone this is consistent with the observed strike slip fault systems identified in the coastal cordillera

vertical axis rotations determined from paleomagnetism of mesozoic and cenozoic strata of the bolivian andes 1 thermal demagnetization and principal component analysis allowed determination of characteristic remanent magnetization chrm directions from 256 sites at 22 localities in mesozoic and cenozoic sedimentary strata of the bolivian altiplano and eastern cordillera an inclination only fold test of site mean chrm directions from cenozoic units principally the santa lucia formation indicates optimum unfolding at 97 1 unfolding consistent with a primary origin for the chrm for mesozoic strata optimum unfolding occurred at 89 2 perhaps indicating secondary remagnetization at some locations for cenozoic units comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations from 33 counterclockwise to 24 clockwise euler pole analysis of along strike variation in crustal shortening within the subandean and interandean zones indicates 18 clockwise rotation south of the axis of curvature of the bolivian andes and 6 counterclockwise rotation northwest of the axis during the past 10 m y along strike variation of shortening within the eastern cordillera indicates 8 clockwise rotation south of the axis and 8 counterclockwise rotation northwest of the axis from 35 to 10 ma these vertical axis rotations produced by along strike variations in crustal shortening during development of the bolivian fold thrust belt agree well with observed rotations determined from paleomagnetism of cenozoic rocks in the eastern cordillera and in the subandean and interandean zones however local rotations are required to account for complex rotations in the cochabamba basin and within the altiplano the curvature of the bolivian andes has been progressively enhanced during cenozoic fold thrust belt deformation

paleomagnetism and magnetic fabric of the eastern cordillera of colombia evidence for oblique convergence and nonrotational reactivation of a mesozoic intracontinental rift we report the paleomagnetic and magnetic fabric results of 58 sites from cretaceous miocene marine and continental strata from the eastern cordillera ec and the cucuta zone at the junction between the santander massif and the merida andes of colombia the ec is an intracontinental doubly vergent range inverting a triassic to early cretaceous rift zone twenty three sites reveal nonsystematic tectonic rotations including unrotated areas of the ec range with respect to stable south america our data show that the ec inverted a nne oriented rift zone and that the orientation of the mesozoic rift and the mountain chain roughly correspond interestingly magnetic lineations from anisotropy of magnetic susceptibility analysis do not trend parallel to the chain but rather are oblique to the main orogenic trend by also considering gps evidence of a 1 cm yr ene displacement of central western colombia accommodated by the ec we suggest that the miocene recent deformation event of this belt arises from ene oblique convergence reactivating a nne oriented rift zone oblique shortening was likely partitioned into pure dip slip shear characterizing thick skinned frontal thrust sheets well known along both chain fronts and by range parallel right lateral strike slip faults which have not been identified yet but likely exist in the axial part of the ec finally the 35 9 clockwise rotation observed in four post miocene magnetically overprinted sites from the cucuta zone reflects late cenozoic and ongoing right lateral strike slip displacement occurring along faults parallel to the bocono fault system possibly connected with the right lateral faults inferred to exist along the axial part of the ec

mean airflow patterns upwind of topographic obstacles and their implications for the formation of echo dunes a wind tunnel simulation of the effects of windward slope 1 secondary airflow plays an important role in dune formation and development in the case of the echo dunes that form upwind of an obstacle it introduces considerable complexity because both the initiation and development processes are influenced by airflow patterns in this study we measured the variations of airflow in front of obstacles with different windward stoss slope angles by means of particle image velocimetry in a series of wind tunnel tests the windward slope angle was the key factor in determining the secondary airflow patterns the horizontal velocities decreased and the vertical velocities increased as the airflow approached perpendicular to the obstacle and a vortex of reversed flow formed in front of obstacles with a stoss slope of 60 or steeper the positions of airflow separation and of the core of the reversed vortex were a function of the windward slope angle depending on the position of the reverse vortex and its significance for the formation of echo dunes the windward slope could be divided into three groups 65 or less 70 75 and 80 or more the horizontal velocity profiles deviated from a log linear distribution resulting in five airflow regions with different rates of change of horizontal velocity we discuss the significance of these velocity variations for sand accumulation and the effects of each airflow region on echo dune formation

paleomagnetic evidence for rotation in the precordillera of northern chile structural constraints and implications for the evolution of the andean forearc abstract palaeomagnetic results are reported for two late mesozoic to early cenozoic continental sedimentary sequences from the precordillera cordillera de domeyko of northern chile comparison of isolated components of magnetisation from the late cretaceous palaeocene purilactis formation and the unconformably overlying oligo miocene paciencia group with reference poles for south america indicate that significant post palaeocene and post lower miocene clockwise rotation has taken place a structural study of the area has revealed the presence of a number of thrust sheets that were periodically active throughout the cenozoic stratigraphie structural and palaeomagnetic data indicate that thrust sheet movement took place following deposition of both the purilactis formation lower eocene incaic orogeny and the paciencia group lowest miocene peheunche orogeny and resulted in the folding uplift and clockwise rotation of the two sequences differences in the amount of rotation between the two units reflect incremental movement of the thrust sheets 12 of rotation took place prior to deposition of the paciencia group followed by 17 of post lower miocene clockwise rotation this study demonstrates that palaeomagnetically detected rotation in the precordillera of northern chile 250 km inland of the peru chile trench is related to the periodic movement of an easterly propagating thin skinned thrust front between 22 30 and 23 30 s the north chilean forearc contains areas of non rotation thin skinned compressional related rotations and large block fault rotations indicating that the simplistic models previously presented to account for forearc rotation in the central andes should be modified in particular evidence implies that rotation could be related to a number of tectonic events and may not simply be due to one deformation event