the yanque prospect peru from polymetallic zn pb mineralization to a nonsulfide deposit the yanque nonsulfide pb zn deposit inferred resources 12 5 mt 3 7 pb and 3 5 zn is located in the andahuaylas yauri ore province cuzco southern peru the deposit occurs within a base metal mineralized district centered on the medium sized dolores porphyry copper a thorough geological mineralogical and geochemical study has carried out in order to define the relationships between the dolores cu porphyry ore and the yanque zn pb polymetallic mineralization and the characteristics of the economic nonsulfide concentrations both sedimentary and igneous rocks constitute the backbone of the yanque dolores area the sedimentary lithologies belong to the soraya mara and ferrobamba fms upper jurassic middle cretaceous the yanque orebody is hosted by the mara fm which prevailingly consists of a siliciclastic sedimentary breccia the original sulfide mineralization consisted of galena pyrite and sphalerite the host rock has been affected by a strong hydrothermal alteration characterized by prevailing sericite illite as in the typical porphyry related phyllic argillic alteration stage and by minor kaolinite dolomite and quartz minor element geochemistry characterized by sb as mn ag and locally also by cu points to magmatic hydrothermal related mineralizing fluids the pb isotopic compositions from dolores and yanque sulfides are similar and are typical of the tertiary magmatically derived ores in this part of peru the hydrothermally altered rocks at yanque have the same pb isotopic compositions as the sulfides thus confirming the hypothesis that the yanque primary zn pb mineralization may have been produced by hydrothermal circulation related to the emplacement of the dolores cu porphyry as it is the case of other porphyry cu systems associated with polymetallic mineralization elsewhere however no simple genetic model for the mineralization involving just one fluid circulation episode is able to explain the data the yanque economic nonsulfide ore association consists of sauconite hemimorphite smithsonite and cerussite which result from the weathering and alteration of the original sulfide mineralization zinc is allocated mainly in sauconite zn smectite rather than in carbonates a factor strictly related to the prevailing siliciclastic character of the host rock distinctive features of the yanque orebody are the comparable ore grades for both pb and zn 3 5 zn and 3 7 pb and the inverse supergene chemical zoning in fact contrary to other supergene ores of this type zinc prevails in the top zone of the yanque deposit whereas lead content increases with depth considering the different mobility of the two metals in solution it may be assumed that most of the primary zinc that was the source for the yanque nonsulfides was originally located far from the position occupied by the galena mineralization whose remnants have been observed on site zinc sulfides may have been originally contained in the eroded rock volumes that surrounded the actual deposit the zinc rich solutions have possibly migrated through the siliciclastic mara fm and precipitated the nonsulfide minerals by porosity filling and replacement processes in this sense the yanque secondary zn pb deposit could be considered as a special type of exotic mineralization

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

wave spectrum retrieval from airborne sunglitter images abstract reconstruction and evolution of two dimensional spectra of surface waves in the gulf of mexico are derived from airborne sun glitter imagery as the proposed method is based on a linear transfer function deduced from the shape of the sunglitter brightness the absolute wavenumber elevation spectrum does not require any additional assumption or information about sky brightness wind or wave energy the detailed description of the airborne image processing method is given as demonstrated retrieved spectra agree well with nearby ndbc buoy data both for spectrum shape level and energy angular distribution the 180 degree wave direction ambiguity inherent to image derived spectra is eliminated by using cross correlation analysis between two consecutive images a case study corresponding to the spectral evolution with increasing distance from shore in slanting fetch conditions is then considered energy level and peak position transformation are consistent with established approximations and laws of wind sea development the technical requirements flight altitude image resolution view angles etc and applicability of the suggested methodology are also discussed these results demonstrate the potential efficiency of high resolution sea state monitoring from drones or light aircrafts using sunglitter imagery

demonstration of two pulses of paleogene deformation in the andes of peru abstract new radiometric ages of about 25 m y on volcanic materials in a marine intercalation within clastic continental strata of the upper moquegua formation near caraveli southern peru together with an age of 25 3 0 4 m y obtained by tosdal et al from a locality about 300 km to the ese show that the formation contains strata of late oligocene as well as miocene age and demonstrate that the coastal region was at a low elevation during latest oligocene time because the unconformities between the upper moquegua formation and the underlying lower moquegua formation and between the lower moquegua formation and underlying paleocene rocks cannot both represent the same tectonic event two discrete paleogene events must be present in the andes of peru although the exact timing of these events is uncertain the unconformities are likely to be of paleocene and middle eocene age or possibly of middle eocene and oligocene age

multiple pulses of neogene compressive deformation in the ayacucho intermontane basin andes of central peru strata of the neogene ayacucho intermontane basin central peru can be subdivided into three tectonostratigraphic units the larampuquio volcanics consisting largely of intermediate lavas and coarse conglomerates of similar lithology the unconformably overlying huanta formation consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas tuffs and conglomerates and the unconformably overlying ayacucho formation consisting of complexly interfingering volcaniclastic sedimentary rocks silicic tuffs and lavas of intermediate to silicic composition radiometric ages of 17 3 0 2 m y and 18 3 0 6 m y have been obtained on tuffs from the upper and lower parts of the larampuquio volcanics determinations on 2 rocks from the lower member of the huanta formation indicate an age of 11 4 0 5 m y and a rock from the upper member is dated at 9 3 0 3 m y b p units of the ayacucho formation yield ages showing that it was deposited between 7 7 and 6 m y ago the intervening molinoyocc and puchcas volcanics were erupted between 9 9 and 7 4 m y ago these units may postdate the huanta formation or may be associated in part with the huanta and in part with the ayacucho formation coarse conglomerates of the larampuquio volcanics represent debris eroded from highlands uplifted during the first and most intense pulse of the late cenozoic quechuan phase of andean compressive tectonism that began in the early miocene folding of the larampuquio volcanics may have taken place during this or a subsequent pulse of compression a marked unconformity between the huanta and ayacucho formations demonstrates a period of deformation between 8 5 and 9 5 m y ago compression also was active during at least part of the deposition of the ayacucho formation as shown by inter fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation folded rocks of the ayacucho formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3 8 0 4 m y b p and 5 5 m y b p bracketing a well defined pulse of deformation slightly younger than 6 m y these subsequent episodes of compressive deformation represent recurrent pulses of the quechuan phase folding related to these late quechuan pulses is restricted in the andes to northwest trending belts corresponding to reactivated fault zones in the pre neogene substratum a system of north trending dikes in the huachocolpa district 70 km west of ayacucho suggests relaxation of compression in the pliocene and compression of quaternary age has been recognized in the huancayo area 170 km northwest of ayacucho taken together with the first pulse of quechuan deformation in the early and middle miocene these data indicate at least four pulses of crustal compression during the late cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension

ocean wave dispersion surface measured with airborne ir imaging system image sequences of the ocean have been collected at long range and low grazing angle with an airborne infrared system the images are geographically registered and 3d frequency wavenumber spectra are calculated and shown to have a strong 2d dispersion surface that is characteristic of wind waves and swell wave directions compare well with in situ measurements and their speeds are consistent with the water depth the authors conclude that temporal sequences of ir images acquired from long distance can provide information for extracting surface wave parameters

quaternary normal and reverse faulting and the state of stress in the central andes of south peru field studies in the andes of southern peru show that in the high andes and pacific lowlands quaternary and recent faults are normal this extensional tectonics postdates compressional deformations of pliocene early quaternary age in the sub andes the observed deformations are compressional they affect early quaternary deposits some of the faults separate quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects striations on the fault planes indicate n s trending extension in the high andes and pacific lowlands the total amount of crustal stretching is small probably of the order of 1 during the last 1 2 m y in the sub andes folds and faults affecting neogene and early quaternary deposits result from n s shortening nevertheless it is supposed that this n s shortening is of early quaternary age the present day compression probably strikes e w judging from focal mechanisms in the sub andes of central peru southern bolivia and northwest argentina data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the andes of southern peru the high andes and pacific lowlands subjected to n s trending extension are bounded by two zones of e w trending compression the sub andes to the east and the contact between the convergent nazca and south america plates to the west in our model the maximum horizontal compressive stress trajectory hmax is roughly parallel with the e w convergence between the two plates hmax corresponds to 1 in the sub andes and to 2 in the high andes the latter situation is caused by the elevated mass of the high andes where zz the vertical stress is inferred to be 1 thus the third principal stress axis being orthogonal to the other two axes it is oriented n s allowing extension to occur in that direction on the other hand in the sub andes zz is 3 and horizontal e w shortening occurs the state of stress in the andean continental crust above the 30 dipping slab appears to be different from that in the andes of central peru situated above the flat subducting segment in this region compressional deformantion affect a wider part of the cordillera

daylight imagery of ocean surface waves for wave spectra abstract both surface reflected daylight and upwelling light are modeled to second order to help provide a quantitative and physically interpretable description of daylight imagery of ocean surface waves for wave spectra an image recording of the slope modulated reflected and upwelled radiance can be optically analyzed to provide the surface slope spectrum as a function of wavenumber for a particular patch on the ocean surface sources of error involved in obtaining ocean spectral information from daylight imagery are outlined and methods of alleviating them discussed errors resulting from the nonlinear nature of the transfer function relating wave slope to radiance at the sensor are shown to be minimized using the second order theory in evaluating potential sensor geometries the directional sensitivity of daylight imagery to waves traveling in various directions is also evaluated a modified pierson wave spectrum was used to generate numerically a one dimensional wave slope profile this slope profi

structural styles of albian rafts in the espirito santo basin se brazil evidence for late raft compartmentalisation on a passive continental margin in recent years hydrocarbon exploration offshore se brazil has been focusing on lower cretaceous strata deformed by gravity gliding above aptian salt a three dimensional 3d seismic volume from the espirito santo basin se brazil is here used to a test the parameters considered to control raft tectonics on a margin tectonically reactivated in the cenozoic and b investigate the impact of prolonged halokinesis on raft deformation offshore espirito santo the combined effects of halokinesis and multiple andean tectonic phases are expressed by local collapse fault reactivation and late segmentation of albian rafts as a result of this deformation we observe four main raft geometries a rolled over rafts b tabular rafts c collapsed rafts and d folded and tilted rafts on the flanks of salt rollers this work shows that salt rollers formed buttresses to moving albian cenomanian rafts with withdrawal of salt from underneath some of the rafts leading to their collapse and welding onto pre salt strata this process occurred in the studied part of the espirito santo basin with minimum control of post raft overburden thickness on raft compartmentalisation salt withdrawal from underneath the rafts is an important phenomenon as it enhanced connectivity between pre salt and post salt units potentially promoting the migration of hydrocarbons from syn rift source units into post salt reservoirs

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

early to middle tertiary foreland basin development and the history of andean crustal shortening in bolivia a 2 5 km thick succession of tertiary strata in the eastern cordillera of southern central bolivia consists of predominantly fluvial and lacustrine deposits the age of the base of the succession is middle paleocene and its upper part which is erosionally truncated is probably late oligocene early miocene the lower 50 130 m of the succession consists of interbedded fluvial sandstone and mudstone including minor paleosols of the santa lucia formation these strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the santa lucia formation and lower part of the impora formation that may represent much of late paleocene eocene time above the paleosol zone is a thin zone 10 m of lacustrine carbonate and clastic rocks in the uppermost impora formation and this in turn is overlain by a 2000 m thick upward coarsening succession of clastic fluvial deposits in the cayara camargo and suticollo formations paleocurrent data indicate that fluvial channels that deposited the santa lucia impora and cayara formations flowed mainly westward whereas channels responsible for the camargo and suticollo formations flowed generally eastward modal sandstone petrographic data show a long term evolution from subarkosic during santa lucia deposition to quartz arenitic during impora and cayara deposition to sublitharenitic during camargo and suticollo deposition compositions we argue that the lithofacies and sediment accumulation history of this succession are most consistent with deposition in an eastward migrating foreland basin system e mail decelles geo arizona edu critical to this interpretation is the zone of extreme stratigraphic condensation in the lower impora formation the inferred upper paleocene eocene part of the succession the abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous postrift thermal subsidence resulted in a gradual continuous decay of early tertiary sediment accumulation rates stratigraphic condensation is however consistent with passage of the forebulge through the region an interpretation that also can be reconciled with the record of foreland basin development in the altiplano to the west and gross sediment accumulation patterns in the subandean zone and modern foreland basin to the east simple flexural modeling and palinspastic reconstruction of the complete cenozoic migration history of the foreland basin system suggest that 1000 km of foreland lithosphere has been underthrust westward beneath the andean orogenic belt at the latitude of southern bolivia the amount of middle and lower crust that would have been added to the andean infrastructure is sufficient to explain the present crustal thicknesses in bolivia

the south american retroarc foreland system the development of the bauru basin in the back bulge province abstract the aim of this research is to understand the tectonic setting of the bauru basin this basin in central eastern south america has been classified as intracratonic but the basin fill geometry the involved subsidence mechanisms and the age of the deposits are poorly understood in this work the ranges of the fossil taxa are analyzed and ages are proposed for the lithostratigraphic units isopach maps were used to reconstruct the stratigraphic intervals of the basin fill the stratigraphy of the bauru basin is compared with that of the adjacent basins and the data are integrated with the available information on south american geodynamics the fossil record indicates that sediment accumulated from the cenomanian to early paleocene beginning after the mochica phase of the andean orogeny the basin fill geometry demonstrates migration of the depocenter through time which occurred simultaneously with migration of the andean basin and immediately after the orogenic events of the peruvian phase we propose that the bauru basin is a component of a retroarc foreland system developed during the early stages of the andean evolution and that it was developed in the back bulge province of this system the andean basin constitutes the foredeep depozone of this foreland system including the potosi oriente acre and maranon basins in addition the upper cretaceous of the parecis and solimoes basins were likely also developed in the back bulge province the thickness of the bauru accumulation indicates that other mechanisms might have overlapped the flexural subsidence in this back bulge province

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

volcano stratigraphy and 40ar 39ar geochronology of the macusani ignimbrite field monitor of the miocene geodynamic evolution of the andes of southeast peru abstract a total of 1400 m of ignimbritic ash flow tuffs distributed in five cross sections and extending from the pre neogene base to the uppermost flows were studied in the miocene macusani ignimbrite basin southeast peru volcano stratigraphic interpolation and precise 40ar 39ar dating of sanidine biotite and rhyolitic glass from the ash flow tuffs give rise to a southwest northeast correlation chart subdivided into six volcanic cycles or fundamental cooling units i e 1 10 0 0 5 2 7 8 8 0 0 1 3 7 5 0 1 4 7 3 0 1 5 6 8 7 0 o 1 and 6 6 7 o 1 ma these delimit two brief e 10 1 and 7 1 ma which are sensibly synchronous with the quechua 2 and quechua 3 compressional events characterizing the tectonic regime in the central andes this close relationship between tectonic pulses and felsic magmatic activity adds a supplementary constraint to models of the miocene tectono magmatic evolution of the cordillera oriental the stratigraphic relationships of the uranium mineralization of the macusani field are defined for the first time the stratiform stratabound occurrences are restricted to three main cooling units dated at 7 8 7 5 and 6 9 6 8 ma which constitute a maximum age for uranium deposition

the karst hosted mina grande nonsulfide zinc deposit bongara district amazonas region peru the mina grande zinc nonsulfide deposit amazonas region peru consists of several accumulations in karst cavities of zn oxide minerals derived from weathering of a mississippi valley type deposit hosted in mesozoic carbonate rocks of the condorsinga formation pucara group the karst cavities hosting the nonsulfide ores were developed in the miocene along northwest southeast faults avg 230 dip 70 dip angle associated with regional structures and locally along stratification joints avg 8 strike 278 dip and 26 dip angle thus far the mina grande zn accumulations have been partially mined in two pits named fase a and fase b and explored in a third area named fase c in 2008 the resources were as follows fase a 160 000 metric tons t 21 2 zn fase b 36 400 t 28 9 zn and fase c 116 700 t 22 5 zn the nonsulfide mineral assemblage consisting mostly of hydrozincite smithsonite and hemimorphite is associated with few remnants of the hypogene ore pyrite and sphalerite mineralogical and petrographic studies revealed several texturally distinct smithsonite and hydrozincite generations which are characterized by extremely negative 13 c compositions pointing to the prevailing contribution of an isotopically light carbon component from the oxidation of organic matter derived carbon and different 18 o isotope compositions 26 9 to 27 2 and 26 0 to 26 3 18 o vsmow for smithsonite and averages 24 6 and 23 7 18 o vsmow for hydrozincite these distinct 18 o compositions indicate that smithsonite and hydrozincite precipitated at least after two depositional stages these stages were probably related to several periods of uplift that occurred in the bongara district since at least 10 ma when the transition from the pebas to acre systems affected the amazonas foreland basin in the miocene the karst activity to which the supergene mineralization is related was restricted to the late miocene and early pliocene periods climatic conditions reconstructed from the ecosystems persisting in the region from late tertiary to recent time suggest that the mina grande supergene mineralization was associated with several weathering episodes that occurred under a climate resembling the present day climatic conditions

extension and dynamics of the andes inferred from the 2016 parina huarichancara earthquake the mw 6 1 2016 parina earthquake led to extension of the south peruvian andes along a normal fault with evidence of holocene slip we use interferometric synthetic aperture radar seismology and field mapping to determine a source model for this event and show that extension at parina is oriented ne sw which is parallel to the shortening direction in the adjacent sub andean lowlands in addition we use earthquake source models and gps data to demonstrate that shortening within the sub andes is parallel to topographic gradients both observations imply that forces resulting from spatial variations in gravitational potential energy are important in controlling the geometry of the deformation in the andes we calculate the horizontal forces per unit length acting between the andes and south america due to these potential energy contrasts to be 4 8 1012 n m along strike of the mountain range normal faulting at parina implies that the andes in south peru have reached the maximum elevation that can be supported by the forces transmitted across the adjacent foreland which requires that the foreland faults have an effective coefficient of friction urn x wiley jgrb media jgrb53009 jgrb53009 math 00010 2 additionally the onset of extension in parts of the central andes following orogen wide compression in the late miocene suggests that there has been a change in the force balance within the mountains we propose that shortening on weak detachment faults within the andean foreland since 5 9 ma reduced the shear tractions acting along the base of the upper crust in the eastern andes leading to extension in the highest parts of the range

structural and surface features of arc shaped radar echoes along an outer tropical cyclone rainband this study uses ground based dual doppler radar and surface observations to document the structural and surface features of the arc shaped radar echoes asres evident along an outer rainband of typhoon longwang as it approached northern taiwan on 1 october 2005 the particular aim of this study is to explore the possible distinction between the present case previously documented tropical cyclone rainbands tcrs and squall lines the dual doppler derived fields show that the leading precipitation of the studied asres exhibited a convective nature with a sharp horizontal gradient of reflectivity and a significant vertical extent theregionsbehindtheleadingconvectionwerecharacterizedbyband relativerear to frontflowatlowlevels and were associated with a broader area of stratiform precipitation the deep layer of front to rear flow extending from the surface to the upper troposphere was generally present ahead of the asres this flow appearstobeliftedupwardatandimmediatelyaheadoftheleadingedgeofthelow levelrear to frontflowto form rearward tilting updrafts these airflow patterns are similar to those of the convective region of squall lines but differ fundamentally from those of previously documented tcrs that were located closer to the inner core of cyclones the detailed analyses of surface fluctuations during the passage of one of the studied asresfurthershowanabruptpressurerise 2mb atemperaturedrop 48c andapronounceddeceleration of inflow air coincident with the leading heavy precipitation the evaluation presented suggests that the convectively generated cold pool may be important in influencing the structures and propagation of the studied asres

long term erosion and exhumation of the altiplano antioqueno northern andes colombia from apatite u th he thermochronology abstract the antioqueno plateau ap in the northern cordillera central colombia is the largest high elevation erosional surface in the northern andes apatite u th he thermochronometry ahe of samples collected from two elevation profiles spanning 2 km of exhumed crustal sections reveal the long term erosional exhumation of the ap sample profiles exhibit ahe ages that increase with elevation from ca 22 ma 760 m at the bottom of regional scarps to ca 49 ma 2350 m on top of the ap a marked inflection point in age versus elevation data at ca 25 ma defines the bottom of the exhumed post oligocene he partial retention zone he prz elevation invariant ages below ca 25 ma record the onset of rapid exhumation and surface uplift of the ap that led to river incision a subtle change in slope within the he prz ca 41 ma is interpreted as a less intense exhumation related cooling episode these two exhumation pulses coincide with the proto andina and pre andina orogenic phases previously proposed for the colombian andes and are synchronous with tectonically driven exhumation events reported for the peruvian bolivian and argentinean andes and for some orogenic systems in the caribbean the pulses are correlated with variations in the rates of convergence between nazca farallon and south america documented for the middle eocene and the late oligocene suggesting continental scale controls on uplift and denudation throughout the andean range ahe data provide an average erosion rate of 0 04 mm yr for the last 25 million years erosion rates during the exhumation pulses were in the order of 0 2 0 4 mm yr similarity between ahe profiles indicates the whole ap was uplifted and exhumed as a coherent structural block corroborating previous structural evidence for the rigidity and coherence of this crustal block in the northern andes our results are in agreement with tectonostratigraphic data in the magdalena and cauca basins and with proposed scenarios for paleogeographic evolution in the northern andes

late cainozoic rotation of the peruvian western cordillera and the uplift of the central andes abstract a palaeomagnetic study was carried out in the region of lima peru on 55 sites 564 cores of middle cretaceous to late miocene age distributed along three northeast southwest transverses which cross the western cordillera of the central andes over the entire studied area the results consistently yield westerly declinations corresponding to post cretaceous and post eocene counterclockwise rotations with respect to stable south america of 16 6 and 13 7 8 1 respectively when compared to the ill defined reference poles of this continent these new results are in good agreement with previous ones from the central peruvian andes and support the model recently proposed for the uplift of the andean cordillera

undulatory silver rich polymetallic veins of the castrovirreyna district central peru fault growth and mineralization in a perturbed local stress field strongly undulatory silver veins of the castrovirreyna district central peru formed in a dilatant anastamosing low displacement left lateral fault system during late miocene east west directed compression the steeply dipping veins have multiple orders of undulations as seen in map view and underground mines expose strong dip reversals kinematic indicators from the east northeast to west northwest striking veins record mainly left lateral slip and lesser oblique reverse slip these striation data considered along with vein intersection angles and map patterns indicate that the veins did not form as a simple conjugate fracture set the preferred interpretation is that the curved fracture growth and complex slip distributions developed because of local stress field modifications by the propagating faults and dilating vein segments during regional east west directed compression variably oriented fault striations compound displacement profiles and multiple scales of vein curvature are all characteristics of stress interaction between subparallel veins veins possessing these features may be expected to host oreshoots with a complex geometry that depends not only on local accommodation space produced at dilant jogs but also on the proximity to other veins and changing slip directions

ocean wave spectra derived from shuttle imaging radar b imagery and surface measurements during the shuttle imaging radar b sir b mission in october 1984 synthetic aperture radar sar images were taken off the coast of japan wave spectra derived from sir b imagery were compared with simultaneous wave buoy observations waves at the time of flight were nearly perpendicular to the sar flight direction and were composed of two major systems a swell of about 200 m wavelength and wind waves with 70 m wavelength both the swell and wind wave peaks were identifiable in the sar image spectrum after stationary response correction peak frequencies derived from sir b images and from the wave gauge agreed satisfactorily however the relative magnitudes of the peaks are not in accordance with the surface measurements possible reasons for the disagreement are discussed

a numerical study of stratospheric gravity waves triggered by squall lines observed during the toga coare and copt 81 experiments a 3d mesoscale model is used to study the structure and intensity of stratospheric gravity waves generated by tropical convection two prototypical cases are examined a squall line observed during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare over the pacific warm pool and a west african squall line observed during the convection profonde tropicale 1981 copt 81 experiment gravity waves generated by these two squall lines are compared with those generated by a previously investigated event over northern australia although the individual squall lines vary in intensity the stratospheric gravity waves generated by the various storms have surprisingly similar amplitudes the similarity in the wave amplitudes arises because within each storm there is a positive correlation between the updraft intensity and the height of the level of neutral buoyancy when the level of neutral buoyancy is relatively high the atmospheric density in the region of wave generation is relatively low and this reduction in density tends to weaken the convectively triggered waves thereby compensating for the stronger updraft velocities in the more intense storms the sensitivity of the azimuthal distribution of the convectively generated gravity waves to the upper tropospheric and lower stratospheric wind profile is also examined in the absence of critical level absorption the motion of the storm relative to the stratospheric winds appears to be the single most important factor determining the azimuthal distribution of the waves stationary storm relative waves similar to mountain waves may also be generated when there is a strong storm relative stratospheric wind the interaction of the gravity waves with shear layers representative of the eastward and westward phases of the quasi biennial oscillation qbo is also examined an analysis of the domain averaged momentum budget supports the claim that the drag exerted by critical level absorption of convectively generated gravity waves plays a nontrivial role in the downward propagation of the qbo

fission track dating of granitic rocks from the eastern cordillera of peru evidence for late jurassic and cenozoic cooling apatite and zircon fission track ages of three samples of the huachon granite and one sample of the quiparacra granite from the eastern cordillera of the andes of central peru have been determined zircons from the huachon granite yielded two groups of ages an older group made up of zircons with the lowest uranium content gave an age of c 270 ma the zircons with high uranium content from this granite gave ages of c 160 ma the zircon ages from the huachon granite show no correlation with elevation the age of 270 ma is interpreted as the age of probable emplacement the 160 ma age is evidence of a mild cooling 200 c event that ended in late mid jurassic time this cooling correlates in central peru with thick conglomerates in the sub andes and strong exhumation in the eastern cordillera during bathonian to malm times zircons from the quiparaca granite which is a component of a composite batholith of possible precambrian to tertiary age show ages of c 30 ma which are interpreted as the emplacement age these zircons were totally annealed prior to the cooling in the oligocene this required temperatures 200 c apatite fission track dating of the huachon granite yielded only neogene ages which reflect the total annealing of the tracks present in the apatite before exhumation began c 22 ma apatite ages show a correlation with elevation and indicate two periods of cooling during the neogene at c 21 ma and between 12 ma and the present these young ages suggest approximately 4 to 6 km of erosion in the eastern cordillera of central peru in the past 30 ma these amounts are consistent with known geological data and fission track ages obtained from other parts of the eastern andes

detecting earliest shortening and deformation advance in thrust belt hinterlands example from the colombian andes low temperature thermochronometry and crosscutting relationships identified in newly released reflection seismic data reveal a previously unrecognized zone of early andean shortening in colombia apatite fission track data and thermal modeling help define a 60 50 ma onset of rapid exhumation along the present boundary between the magdalena valley hinterland basin and eastern cordillera thrust belt subsurface angular unconformities localized above fold thrust structures indicate paleogene deposition in a wedge top depozone containing doubly vergent reverse faults retrodeformation of a cross section based on interpreted seismic profiles and thermochronometric data indicates paleocene to early eocene shortening and exhumation occurred through simultaneous activation of east and west directed reverse faults across a broad orogenic front subsequent deformation focused along west directed inversion structures these relationships reveal that deformation operated in a disparate manner rather than following a systematic progression from hinterland to foreland the northern andes also exemplify the potential effects of hinterland sediment loading and fault strength on deformation advance in contractional orogens

numerical study of a sahelian synoptic weather system initiation and mature stages of convection and its interactions with the large scale dynamics summary a 24 h multi scale simulation of the life cycle of a sahelian squall line is presented the initial and coupling e elds have been taken from the european centre for medium range weather forecasts re analysis era 15 after humidity corrections to avoid spurious absolute and conditional convective instabilities comparisons of modelsynthetic and meteosat infra red radiances indicate that the simulated scenario of convection development and structure are realistic though differences are found concerning the size of the squall line with a delay of 2 3 h the simulated arc shaped structure and surface signature e t well with radar and ground station observations respectively the mature stage of the simulated squall line presents a cross line structure similar to conceptual models an intense rear to front e ow is generated below the trailing stratiform part an acceleration of the african easterly jet aej is simulated behind the system occurring across its full width this leads to the generation of a dipole of counter rotating vortices the resulting potential vorticity signature leads to local reinforcement of the barotropic instability on the north e ank of the aej this simulation is used to analyse the impact of the convection at synoptic scales and to compare it with era 15 it suggests that the convection contributes to reinforce the monsoon at least up to 500 km the monsoon ine ow moves eastward and westward with time scales shorter and longer than 24 h respectively the tropical easterly jet intensie es to the south west of the convection area up to 700 km away the aej is strongly intensie ed ahead of the trough where active convection occurs organized convection developing in the vicinity of the aej core induces weakening of the aej ahead of the squall line and strengthening behind it a common result in the simulation and in era 15 is that the aej core moves westward faster due to the convective activity

characterizing thunderstorm gust fronts near complex terrain abstractfire safety aviation wind energy and structural engineering operations are impacted by thunderstorm outflow boundaries or gust fronts gfs particularly when they occur in mountainous te

tectonic rotations and transcurrent deformation south of the abancay deflection in the andes of southern peru we report new paleomagnetic results from 55 out of 76 sites sampled at different localities along a transect from nazca to cuzco where the general structures of the peruvian andes are strongly offset across the abancay deflection nine new 39ar 40ar ages better constrain the timing of volcanism along the western edge of the western cordillera at the latitude of nazca a mean paleomagnetic result from 22 sites in the lower miocene volcanics does not show significant rotation r 2 3 7 7 of the western margin of the central andean plateau since the early miocene within the western cordillera we sampled three structural blocks bounded to the north by the abancay fault system in the westernmost block a large counterclockwise rotation r 65 0 11 1 is found in mesozoic limestones and paleocene eocene red beds magnitude of rotation decreases toward the east from r 35 6 12 8 in the central block to r 4 5 8 4 south of the town of cuzco the anisotropy of magnetic susceptibility ams recorded by the red beds sediments is the consequence of compaction and tectonic strain during the early stages of deformation we show that the magnetic lineations were also rotated counterclockwise as the remanent magnetizations the present study confirms results from the peruvian fore arc showing that rotations are not older than circa 40 ma and likely not younger than circa 20 ma the spatial variation in the amount of counterclockwise rotation suggests a large component of shear along the abancay deflection concomitant with a broad late eocene oligocene oroclinal deformation in southern peru

potential vorticity generation by west african squall lines abstractthe west african summer monsoon features multiple complex interactions between african easterly waves aews moist convection variable land surface properties dust aerosols and the diu

tertiary provenance history of the northern and central altiplano central andes bolivia a detrital record of plateau margin tectonics abstract the internally drained altiplano plateau in the high elevation hinterland of the central andes contains an extremely thick up to 12 km succession of tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins in this study temporal and spatial variations in detrital composition based on 113 sandstone thin sections and 31 conglomerate clast counts combined with sediment dispersal patterns and regional stratigraphic correlations help delineate the tertiary history of altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins stratigraphic and provenance data suggest limited subsidence and west directed sediment dispersal during the early tertiary history of the altiplano region by late eocene time development of a rapidly subsiding basin probable foredeep was underway as recorded by extensive east directed fluvial systems of the 3 6 5 km thick potoco formation potoco compositional data are compatible with sediment sources in the incipient western cordillera and regions farther west including 1 a probable thrust belt source terrane composed of paleozoic mesozoic strata and precambrian granitic gneissic basement and 2 a subordinate magmatic arc source terrane composed of tertiary volcanic rocks increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence due to either enhanced volcanism or eastward arc migration by late oligocene time west directed fluvial and alluvial fan systems of the upper potoco formation and the 3 km thick coniri formation reached the altiplano basin providing a minimum uplift age for the backthrust belt along the western flank of the eastern cordillera paleozoic clast conglomerate and relatively quartz rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly paleozoic strata in the backthrust belt continued late oligocene to quaternary accumulation in the altiplano reveals that the basin persisted as a depositional region during a well documented period of neogene shortening and uplift in the andean thrust belt farther east this intra orogenic hinterland deposition between eastern and western highs during ongoing shortening suggests that the altiplano basin existed as a crustal scale piggyback basin possibly in an internally drained plateau like geomorphic setting since 25 ma provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the altiplano although flexural subsidence due to thrust loading along both plateau margins initially in the west and subsequently in the east may explain most tertiary subsidence possible closed basin conditions due to structural damming could have promoted additional sediment accumulation from late oligocene to quaternary time

dynamical influence of microphysics in tropical squall lines a numerical study abstract based on environmental conditions of the 22 and 23 june squall lines during the convection profonde tropicale in 1981 copt81 experiment in west africa a series of numerical simulations are performed with a two dimensional nonhydrostatic cloud model to examine the dynamical effect of microphysics in tropical squall lines the role of ice phase microphysics strongly depends on ambient conditions for the environment of strong convective instability the ice phase is important regarding the system scale structure but is not important to the convective scale dynamics on the other hand the ice influence is crucial to the squall line convective system if the environment has a weak convective instability and is almost saturated at low levels

magnetostratigraphy of the miocene corque basin bolivia implications for the geodynamic evolution of the altiplano during the late tertiary a magnetostratigraphic study of a thick miocene continental red bed sequence totora formation located in the north central altiplano the corque syncline yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the altiplano paleomagnetic results obtained in 653 samples from a composite 4 5 km thick red bed sequence across the corque syncline indicate that most of the totora sequence was deposited in the middle miocene from 14 to 9 ma with high sedimentation rates up to 970 m myr from 12 to 9 ma the high rate of infill of the corque basin demonstrates active subsidence of the northern altiplano during the middle miocene deformation ceased in the northern altiplano domain only at the end of the miocene two major tuff beds within the sequence have been previously dated by 39 ar 40 ar of sanidine minerals marshall et al 1992 the excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by cande and kent 1995 the numerous paleomagnetic results obtained in middle miocene sediments within the corque syncline a structure which can be traced for more than 100 km along its axis demonstrate that the corque syncline rotated counterclockwise by 10 8 2 9 since 9 ma as a consequence of the internal deformation of the altiplano inclination flattening of 17 is recorded in these red bed sediments this result provides additional evidence that detrital magnetizations especially in red beds should be taken with caution when estimating paleolatitudes or long term variations of the geomagnetic field

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

erosion rates and mechanisms of knickzone retreat inferred from 10be measured across strong climate gradients on the northern and central andes western escarpment escarpment edge deep gorges and distinct knickzones in river profiles characterize the landscape on the western escarpment of the andes between 5 s and 18 s northern peru to northern chile th

geologic evolution of the caylloma epithermal vein district southern peru the silver and base metal rich caylloma epithermal district in the tertiary volcanic belt of southern peru has been worked intermittently since the incaic period and has produced over 100 million ounces moz of ag intermediate sulfidation mineralization is present in veins hosted by miocene andesitic volcanic and volcaniclastic rocks with minor ore in underlying folded jurassic sedimentary basement new 40 ar 39 ar dates give a host rock age of 20 30 0 11 ma andesitic volcanic matrix a hydrothermal alteration age of 18 35 0 17 ma adularia in vein wall rock and postmineralization ages of 11 8 0 8 and 12 25 0 07 ma sanidine and biotite from a rhyolite dome gangue minerals include quartz calcite rhodonite rhodochrosite pyrite and minor adularia illite barite and helvite ore minerals include sphalerite galena chalcopyrite and tetrahedrite hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks hydrothermal alteration types include silicification quartz adularia and quartz illite and propylitization chlorite calcite illite banded veins show four stages of mineral precipitation 1 early sugary quartz chalcedony pyrite 2 manganese minerals 3 quartz sulfides and 4 late calcite quartz cyclic bands in the manganese stage early sulfides coarse to medium grained quartz late rhodonite calcite chalcedony are a few millimeters to 5 cm thick and form ore bands up to 1 m thick veins occupy dextral normal faults 020 050 45 70 se and extension fractures 060 090 70 90 se and are 1 to 25 m wide and as much as several kilometers long the veins display complex and multi episodic filling with textures characteristic of open space precipitation such as crustiform banding symmetric banding vugs breccias and cockade and comb textures we present a structural model in which the principal veins formed in subparallel northeast striking dextral normal faults and related extension fractures within a northwest striking structural corridor bounded by sinistral regional faults second and third order veins formed by bookshelf sliding related to movement on higher order dominantly strike slip faults slickenlines have low rakes 20 50 sw and fault kinematic analysis suggests the extension axis was oriented 4 330 and the shortening axis 55 234 in agreement with the strain field commonly proposed for the early middle miocene of southern peru ore grade is discontinuous with high grade ore zones having strike lengths of tens to hundreds of meters and extending about 300 m downdip ore shoots in fault hosted veins are narrow subvertical zones a few tens of meters wide measured in the plane of the vein separated by lower grade ore zones ore shoots in extension fractures are shorter downdip and horizontally more continuous some ore shoots occur as half cymoids that form along the main vein only in the hanging wall of synmineral transverse faults mineral composition and metal content are zoned horizontally and vertically silver base metals and calcite and manganese minerals increase to the northeast and downward and sulfide content increases gradually with depth shallower levels of the hydrothermal system are exposed to the west and southwest in the district evidence from sulfur isotopes and high salinity fluid inclusions is consistent with a model in which magmatic fluids played a role in the hydrothermal system introduced through channels of enhanced structural permeability the cyclic nature of mineral precipitation with abrupt changes in mineralogy textures and fluid composition leads us to postulate that cyclic injections of magma derived hydrothermal fluids occurred economic mineralization is estimated to have formed at a depth of about 650 m below the paleowater table with a temperature of formation of 270 c

concept of continuum as opposed to periodic tectonism in the andes abstract results of a study of various andean sedimentary basins that are distinct in terms of age and geodynamic position allow us to postulate the idea of a compressional tectonic continuum as opposed to the short tectonic events which have so far been considered as one of the main features of the dynamic evolution of the andean belt in these basins the development of kilometre scale synsedimentary folded structures implies the existence of compressional stress over a period of several million years the existence of tectonic events of long duration allows us to modify the interpretation of the classic andean tectonic model the timing of the continuum tectonism is perhaps dependent on the variation in the convergence rate between the farallon nazca and south american plates

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

the west african squall line observed on 23 june 1981 during copt 81 kinematics and thermodynamics of the convective region abstract the squall line observed during the night of 23 24 june 1981 was one of the most intense events during the copt 81 convection profonde tropicale experiment conducted in may and june 1981 at korhogo in the northern ivory coast by french and ivorian research institutes the presquall environment possessed a very large convective instability with a stable layer in the lowest levels the presence of dry air in the midlevels which promoted the development of convective downdrafts and the unusually low altitude of the strong easterly winds which aided the development of a leading anvil above 6 km are two noteworthy characteristics of the environment of this squall line its mesoscale structure as deducted from radar data is similar to that previously observed for tropical squall lines convective updrafts and downdraft occur in the leading heavy precipitation region convective region and a mesoscale updraft and downdraft is found within and below the trailing mid to upper level anvil clouds

changes in the tectonic regime above a subduction zone of andean type the andes of peru and bolivia during the pliocene pleistocene this paper adresses the changes in the tectonic regime in the peruvian and bolivian andes that have occurred since the upper miocene when the present day elevation of the cordillera above sea level has been almost reached the stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes the cuzco fault system in southern peru is chosen as an example to illustrate the methodology used in this region striations on both active and holocene faults are in agreement with a n s extension but faults affecting early pleistocene deposits exhibit two families of striations the younger results from the previous n s extension the older involving reverse motions results from either an e w or a n s compression faults affecting pliocene formations often show an oldest family of striations resulting from a ne sw or an e w trending extension thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data 1 a pliocene extensional regime 2 a lower pleistocene compressional regime and 3 a mid pleistocene present day extensional regime similar analyses conducted in the pacific and sub andean lowlands allow sketching of the successive pliocene pleistocene stress patterns in the central andes the quaternary and present day stress pattern is characterized by a n s extension in the high andes and in the pacific lowlands and by an e w compression in the sub andean lowlands and at the contact between the nazca and south american plates this stress pattern is interpreted at a large wavelength 100 km as an effect of compensated topography this model supposes that the vertical lithospheric stress zz increases with the topography the crustal thickness and the low density mantle beneath and that the lithospheric maximum compressional horizontal stress hmax trending e w roughly parallel to the convergence is fairly constant on both edges of the andes the tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then the e w trending hmax is 2 and hmin trending n s is 3 allowing extension to occur in this direction the pliocene stress pattern was characterized by a ne sw or an e w trending extension in the high andes in the pacific lowlands and possibly in the sub andean lowlands this stress pattern was clearly different from the present day one because the e w trending stress was hmin this required a weak push or eventually tractional boundary forces acting on the south american lithosphere it is suggested that this might result from a strong slab pull due to a long steeply dipping slab which decreased the value of the xx stress transmitted to the overriding plate the early pleistocene state of stress was compressional since the elevation of the andes had not markedly decreased during this period this required an increase of the e w trending stress value this resulted from a strong coupling between the two lithospheric plates possibly due to a rupture of a long slab under its own weight other spatial changes in the stress pattern are related to the particular situation of the forearc to the subduction of the buoyant nazca ridge and to the different dips of the slab extension in the high andes is of small magnitude of the order of 1 during the last 1 2 m y in a few basins it may have attained 40 during the pliocene 5 3 m y

widening of the andes an interplay between subduction dynamics and crustal wedge tectonics shortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere we show that the locus of shortening in the western margin of south america has largely been controlled by the geometry of the slab numerical models confirm that horizontal subduction favors compression far from the trench above the asthenospheric wedge and steeply dipping segment of the subducting slab as a result a second crustal wedge grows in the hinterland of the continent and widens the andes in the bolivian orocline this wedge corresponds to the eastern cordillera whose growth was triggered by a major episode of horizontal subduction when the slab returned to a steeper dip angle shortening and uplift pursued facilitated by the structural and thermo chemical alteration of the continental lithosphere we review the successive episodes of horizontal subduction that have occurred beneath south america at different latitudes and show that they explain the diachronic widening of the andes we infer that the present day segmented physiography of the andes results from the latitudinally variable transient interplay between slab dynamics and upper plate tectonics over the cenozoic we emphasize that slab flattening or absence thereof is a major driving mechanism that sets the width of the andes at any latitude

mesoscale convective systems 1 mesoscale convective systems mcss have regions of both convective and stratiform precipitation and they develop mesoscale circulations as they mature the upward motion takes the form of a deep layer ascent drawn into the mcs in response to the latent heating and cooling in the convective region the ascending layer overturns as it rises but overall retains a coherent layer structure a middle level layer of inflow enters the stratiform region of the mcs from a direction determined by the large scale flow and descends in response to diabatic cooling at middle to low levels a middle level mesoscale convective vortex mcv develops in the stratiform region prolongs the mcs and may contribute to tropical cyclone development the propagation of an mcs may have a discrete component but may further be influenced by waves and disturbances generated both in response to the mcs and external to the mcs waves of a larger scale may affect the propagation velocity by phase locking with the mcs in a cooperative mode the horizontal scale of an mcs may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the rossby radius of the mcv the vertical redistribution of momentum by an mcs depends on the size of the stratiform region while the net vertical profile of heating of the large scale environment depends on the amount of stratiform rain regional variability of the stratiform rain from mcss affects the large scale circulation s response to mcs heating

comparison of airborne synthetic aperture radar imagery with in situ surface slope measurements across gulf stream slicks and a convergent front synthetic aperture radar sar imagery showing two types of features near the inshore edge of the gulf stream are compared with nearly simultaneous estimates of the sea surface slope field derived from optical shipboard measurements one class of feature consists of a set of narrow dark lines having radar signal modulations of about 10 db at l band these modulations are comparable to those observed in the in situ wave slope data over wavenumbers corresponding to the radar resonant bragg waves these modulations are also predicted by a simplified equilibrium wave spectrum model using a surface elasticity of about 25 mn m as determined from in situ measurements of surface tension and a wind friction velocity of about 10 cm s from buoy and shipboard measurements these results support lyzenga and marmorino s 1998 inference that the gulf stream dark lines represent relatively passive biogenically derived surfactant slicks that are advected and strained by the large scale surface flow field the second class of feature consists of a pair of bright dark signatures over the shallow continental shelf region near cape hatteras north carolina these features are characterized by signal modulations of about 4 and 4 db in both the l band sar and in situ data these modulations appear to be due to the interaction of surface waves with the current convergence and divergence regions associated with a rotary circulation in a shallow convergent front model calculations of this interaction by lyzenga 1998 indicate that the positive perturbations associated with current convergence should be relatively independent of the wavenumber which is in approximate agreement with the in situ measurements of the present paper the model also indicates that the negative perturbations associated with the region of current divergence should decrease for wavenumbers higher than l band that trend is observed in the sar data but not in the in situ data which show significant perturbations at higher wavenumbers it is conjectured that this is due to the effects of surfactants and either along front or time variability in the frontal dynamics which are not included in the model additional comparisons of this kind should be useful to further test and improve physical models of near surface phenomena

paleomagnetic evidence for rapid vertical axis rotation in the peruvian cordillera ca 8 ma paleomagnetic results from 31 neogene sites in the peruvian andes yield primary magnetizations as demonstrated by positive fold and reversal tests strata dated as 18 9 ma record a significant counterclockwise rotation 11 5 whereas unconformably overlying younger strata 7 6 ma are not rotated the age of rotation thus is between 9 and 7 ma a period that coincides with the widespread quechua 2 deformation phase moreover eight independent studies on 107 9 ma rocks from peru between 9 s and 15 s reveal similar and significant rotations 15 6 this suggests that the region rotated during a 2 m y period of deformation ca 8 ma when the andes underwent rapid uplift and important deformation commenced in the subandean zone

plio quaternary geodynamic evolution of a segment of the peruvian andean cordillera located above the change in the subduction geometry the cuzco region abstract the cuzco region which is located above a change in subduction geometry appears to be characterized by a variable plio quaternary tectono sedimentary evolution essentially located along the major fault system that separates the high plateaux from the eastern cordillera after the higher surface formation of the high plateaux a set of neogene basins were filled by miocene fluvio torrential series and by plio pleistocene fluvio lacustrine deposits the neogene series have been affected by compressional tectonic forces attributed to the late miocene this compression is followed by roughly e w trending syn sedimentary extensional tectonics attributed to the pliocene it is related to reactivation of the pre existing major faults basin evolution and volcanic activity concentrated along the faults in the early pleistocene fluvio lacustrine deposits are affected by syn and post sedimentary compressional tectonism it is characterized by shortening that trends both n s and e w and produces folding and faulting of the sedimentary cover extensional tectonism trending roughly n s has been taking place from the middle pleistocene to the present it is coeval with shoshonitic volcanic activity and with sedimentation of fluvio lacustrine terraces torrential fans and moraines quaternary and active normal faults due to this tectonism are located in a narrow zone more than 100 km long between the high plateaux and the eastern cordillera and two 15 km long fault sectors in the eastern cordillera characteristic pleistocene scarps 400 m or more high are due to the cumulative normal offset and there are also little scarps with heights ranging between 2 and 20 m which are related to holocene fault reactivations recent fault reactivation on the cuzco fault system during the april 5 1986 earthquake m b 5 3 is due to the n s trending extension this state of stress located at a mean elevation of roughly 3730 m is generally homogeneous to different scales the active cuzco normal faults may be a consequence of adjustment between the compensated western cordillera and the undercompensated eastern cordillera this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge the variation of the state of stress during the plio quaternary is in agreement with the variations of the compressional boundary forces it may be explained by variation of the convergence rate or by the variation of pull slab forces

climate and tectonic influences on continental erosion of tropical south america 0 13 ma is tectonic uplift or climate change the primary driving force behind continent scale erosion here we examine these two hypotheses for tropical south america by comparing a high resolution record of terrigenous sediment accumulation rates from the western tropical atlantic ocean with continental tectonic reconstructions and both low latitude and high latitude climate indices that span the past 13 m y over the full record terrigenous sedimentation rates most closely follow lowland aridity as recorded by abundance ratios of terrigenous oxide minerals an indication that basin integrated climate is an important control of erosion step like transitions in the flux and composition of terrigenous material near 8 and 4 5 ma however follow dated tectonic events which suggests progressive expansion of the drainage basin into less humid or less vegetated regions that were more prone to erosion on long time scales high latitude climate variability and sea level as recorded by 1 8 o do not closely track terrigenous sedimentation we conclude that regional climate change provides a mechanism for long term erosion of tropical south america tectonics plays a significant but secondary role in this area perhaps by shaping the drainage basin to include regions in which the climate is more favorable to erosion

high winds generated by bow echoes part ii the relationship between the mesovortices and damaging straight line winds airborne radar analysis of a mesovortex that developed near the apex of a bow echo is presented the mesovortex was shown to play a critical role in determining the location of intense straight line wind damage at the surface the perturbation pressure gradient force in natural coordinates along the parcel path accelerated the horizontal winds however intense mesovortices modified the low level outflow and largely determined the locations where the strongest winds occurred regions of maximum winds are accounted for as a superposition of the vortex and the flow in which it is embedded the strongest winds occur on the side of the vortex where translation and rotation effects are in the same direction this model explains the observed tongue of high wind speeds that were confined to the periphery of the mesovortex the origin of the mesovortex is also examined similarities and differences of this bow echo event with recent modeling studies are presented

testing global synchronism in peri pacific arc volcanism abstract episodicity of arc volcanism along the pacific rim is quantitatively analysed based on the record preserved by ash layers in deep sea sediments drilled by the deep sea drilling project and the ocean drilling program two major pulses of arc volcanism have been found one during middle miocene times 18 13 ma the other during pliocene quaternary times 5 0 ma these pulses are shown to relate to the tectonic stress regime of the arc lithosphere rather than to subduction rate these episodes of arc volcanism activity coincide with major tectonic events documented on circum pacific subduction zones as a consequence a global control of plate kinematics could influence both volcanic and tectonic evolution of pacific convergent margins

kinematics exhumation and sedimentation of the north central andes bolivia an integrated thermochronometer and thermokinematic modeling approach quantifying mountain building processes in convergent orogens requires determination of the timing and rate of deformation in the overriding plate in the central andes large discrepancies in both timing and rate of deformation prevent evaluating the shortening history in light of internal or external forcing factors geologic map patterns age and location of reset thermochronometer systems and synorogenic sediment distribution are all a function of the geometry kinematics and rate of deformation in a fold thrust belt foreland basin ftb fb system to determine the timing and rate of deformation in the northern bolivian andes we link thermokinematic modeling to a sequentially forward modeled balanced cross section isostatically accounting for thrust loads and erosion displacement vectors in 10 km increments are assigned variable ages to create velocity fields in a thermokinematic model for predicting thermochronometer ages we match both the pattern of predicted cooling ages with the across strike pattern of measured zircon fission track apatite fission track and apatite u th he cooling ages as well as the modeled age of fb formations to published sedimentary sections results indicate that northern bolivian ftb deformation started at 50 ma and may have begun as early as 55 ma acceptable rates of shortening permit either a constant rate of shortening 4 5 mm yr or varying shortening rates with faster rates 7 10 mm yr at 45 50 ma and 12 8 ma significantly slower rates 2 4 mm yr from 35 to 15 ma and indicate the northern bolivian subandes started deforming between 19 and 14 ma

polarimetric signatures and hydrometeor classification of west african squall lines during the amma campaign the doppler and polarimetric radar ronsard was deployed in kopargo benin in the present paper three squall lines sampled during that period are described it was the first time this type of system had been observed with a polarimetric radar in the west african region the strongest case occurred on 28 july 2006 and the others on 12 september and 30 june the polarimetric characteristics of these systems are described and a hydrometeor classification algorithm is applied in order to identify the microphysics of the systems finally the 3d wind field is retrieved for the 28 july event the convective part of the squall lines is generally characterized by maxima of reflectivity differential reflectivity zdr and specific differential phase kdp the results of the hydrometeor classification show that the convective region is composed of strong 30 mm h 1 to moderate rain 5 30 mm h 1 below the melting level and graupel above this zone seems to be characterized by relatively strong updraughts 5 10 m s 1 responsible both for the formation of graupel by riming and for the growth of drops due to coalescence to the rear the systems are characterized by light rain 5 mm h 1 below and snow or ice crystals above these regions present downward flow below the melting layer which prevents drop growth by condensation consistent with the light rain observed the stratiform part of these systems can be very non uniform due to the presence of old cells dissipating associated with moderate rain and thus stronger reflectivities

uplift of tibet from eclogites to granulites implications for the andean plateau and the variscan belt abstract using a simplified petrogenetic grid and a one dimensional thermal model to compute the evolution of the density of the crust with time the contribution to the uplift of tibet of metamorphic transitions in the lower crust is shown to be large the computations are made for a lower crust of either mafic or intermediate composition a mafic lower crust would have a mantle like density incompatible with seismological results this conclusion implies that any pre thickening gabbroic lower crust has been removed during the orogenic process a lower crust of intermediate composition would be affected by about 3 km of post thickening uplift over a few tens of millions of years this uplift is essentially the result of the eclogite to granulite transformation the resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere such a process would also explain why the neogene uplift of tibet to an average elevation of 5 km appears to have occurred without crustal shortening the same model may explain the uplift of the altiplano puna plateau which also occurred without surface shortening since about 10 ma a comparison is made with the now collapsed variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting

major neogene events of the southeastern pacific the chilean and peruvian record abstract according to the biochronostratigraphic model proposed the neogene marine deposits in chile and peru represent two independent transgressive sequences alloformations separated by a major regional hiatus the earlier sequence nseps i would stretch from the top of the early miocene up to the base of the late miocene ca 19 10 m y b p the upper sequence nseps ii would correspond in turn to the late pliocene ca 3 5 1 6 m y b p the regional hiatus separating both sequences would encompass the late miocene and the early pliocene ca 10 3 5 m y b p the planktonic foraminiferal events described suggest an age of terminal early miocene middle miocene and late pliocene respectively the paleoclimatic events recorded are three a warming event lasting from the terminal early miocene to the end of the middle miocene mmwe 18 11 5 m y b p a cooling event of the middle late pliocene lpce 2 5 0 2 m y b p and warming event spanning most of the late pliocene lpwe 3 5 2 2 m y b p restricted to the surface and intermediate sea waters of the middle and low latitudes the last paleoclimatic event not recorded as such so far is both synchronous and mutually exclusive in latitudinal terms with the second one three tectonic regional events are analyzed the quechuan 1 phase 19 1 5 m y b p the quechuan 2 phase 10 1 m y b p and the geographic rhodanian phase 3 5 0 5 m y b p in the studied area there is no late miocene marine sedimentary record to confirm the terminal tortonian or quechuan 3 tectonic phase ca 7 m y b p admitted to occur in the volcanoclastic continental sequences of northern chile and southern peru