geochronology geochemistry and tectonic evolution of the western and central cordilleras of colombia abstract autochthonous rocks of the pre cretaceous continental margin of nw south america the tahami terrane are juxtaposed against a series of para autochthonous rock units that assembled during the early cretaceous allochthonous oceanic crust of the caribbean large igneous province collided with and accreted onto the margin during the late cretaceous we present the first regional scale dataset of zircon u pb la icp ms ages for intrusive and metamorphic rocks of the autochthonous tahami terrane early cretaceous igneous para autochthonous rocks and accreted oceanic crust the u pb zircon data are complemented by multiphase 40ar 39ar crystallization and cooling ages the geochronological data are combined with whole rock major oxide trace element and ree data acquired from the same units to constrain the tectonic origin of the rock units and terranes exposed in the western cordillera cauca patia valley and the central cordillera of colombia the tahami terrane includes lower paleozoic orthogneisses 440 ma that may have erupted during the active margin stage of the rheic ocean basement gneisses were intruded by permian continental arc granites during the final assembly of pangea triassic sedimentary rocks were subsequently deposited in rift basins and partially melted during high t metamorphism associated with rifting of western pangea during 240 220 ma continental arc magmatism during 180 145 ma is preserved along the whole length of the central cordillera and was followed by an early cretaceous out board step of the arc axis and the inception of the quebradagrande arc that fringed the continental margin back stepping of the arc axis may have been caused by the collision of buoyant seamounts which were coeval with plateau rocks exposed in the nicoya peninsular of costa rica rapid westward drift of south america closed the quebradagrande basin in the late aptian and caused medium high p t metamorphic rocks of the arquia complex to exhume and obduct onto the continental margin subduction beneath hot spot derived rocks of the caribbean plateau 100 92 ma formed an intra oceanic arc 92 75 ma which collectively comprise the late cretaceous caribbean large igneous province the remnant ocean basin located between south america and the caribbean large igneous province was partly consumed via continental subduction forming the large antioquia batholith the caribbean large igneous province collided and accreted to south america during 75 70 ma along the cauca almaguer fault resulting in the cessation of both arcs and the paleocene onset of subduction beneath the accreted oceanic crust

a new look at evidence for a wadati benioff zone and active convergence at the north panama deformed belt the nature of the northern boundary of the panama microplate often referred to as the north panama deformed belt npdb has been the subject of much speculation previous studies have used a variety of data including teleseismic gravity bathymetric marine magnetic and field studies of uplift and tsunami deposits as well as modified mercalli intensity distributions from historic earthquakes to reveal the nature of the npdb data have been collected for over 30 yrs and yet the character of the npdb remains unclear current analyses and interpretation provide a number of mutually exclusive options and much controversy in this article we examine local and regional seismicity combined with teleseismic observations and historic earthquake data to present an alternative analysis for the npdb using small earthquakes recorded by a local network we image a well defined wadati benioff zone dipping southward beneath the panama microplate these data provide new evidence regarding the nature of panama microplate caribbean plate boundary kinematics and demonstrate the existence of an active subduction zone a more complete understanding of the nature of the seismicity and plate interactions along the npdb offshore panama including a subduction zone capable of producing earthquakes of m 7 should be included in future earthquake hazard assessments

kinematics of the western caribbean collision of the cocos ridge and upper plate deformation subduction of the cocos plate and collision of the cocos ridge have profound effects on the kinematics of the western caribbean including crustal shortening segmentation of the overriding plate and tectonic escape of the central american fore arc cafa tectonic models of the panama region pr have ranged from a rigid block to a deforming plate boundary zone recent expansion of gps networks in panama costa rica and colombia makes it possible to constrain the kinematics of the pr we present an improved kinematic block model for the western caribbean using this improved gps network to test a suite of tectonic models describing the kinematics of this region the best fit model predicts an euler vector for the counterclockwise rotation of the cafa relative to the caribbean plate at 89 10 w 7 74 n 1 193 ma 1 which is expressed as northwest directed relative block rates of 11 3 1 0 16 5 1 1 mm a 1 from northern costa rica to guatemala this model also predicts high coupling along the nicoya and osa segments of the middle american subduction zone our models demonstrate that the pr acts as a single tectonic block the panama block with a predicted euler vector of 107 65 w 26 50 n 0 133 ma 1 this rotation manifests as northeast migration of the panama block at rates of 6 9 4 0 7 8 4 8 mm a 1 from southern costa rica to eastern panama we interpret this motion as tectonic escape from cocos ridge collision redirected by collision with the north andes block which migrates to the northwest at 12 2 1 2 mm a 1

the progressive intensification of northern hemisphere glaciation as seen from the north pacific ocean drilling project odp site 882 50 22 n 167 36 e provides the first high resolution grape density magnetic susceptibility carbonate opal and foraminifera planktonic and benthic stable isotopes records between 3 2 and 2 4 ma in the northwest pacific we observed a dramatic increase in ice rafting debris at site 882 at 2 75 ma which is coeval with that found in the norwegian sea suggesting that the eurasian arctic and northeast asia were significantly glaciated from 2 75 ma onwards prior to 2 75 ma planktonic foraminifera 18o records indicate a warming or freshening trend of 4 c or 2 over 80 ka if this is interpreted as a warm pre glacial pliocene north pacific it may have provided the additional moisture required to initially build up the northern hemisphere continental ice sheet the dramatic drop in sea surface temperatures sst 7 5 c at 2 75 ma ended this suggested period of enhanced sst and thus the proposed moisture pump moreover at 2 79 and 2 73 ma opal mass accumulation rates mar decrease in two steps by five fold and is accompanied by a more gradual long term decrease in caco3 mars evidence from the southern ocean odp site 704 indicates that just prior to 2 6 ma there is a massive increase in opal mars the opposite to what is found in the north pacific this indicates that the intensification of northern hemisphere glaciation was accompanied by a major reorganisation of global oceanic chemical budget possibly caused by changes in deep ocean circulation the initiation of northern hemisphere glaciation occurred in the late miocene with a significant build up of ice on southern greenland however the progressive intensification did not occur until 3 5 3 ma when the greenland ice sheet expanded to include northern greenland following this stage we suggest that the eurasian arctic and northeast asia glaciated at 2 75 ma approximately 100 ka before the glaciation of alaska 2 65 ma and 200 ka before the glaciation of the north east american continent 2 54 ma

rock uplift and exhumation of continental margins by the collision accretion and subduction of buoyant and topographically prominent oceanic crust understanding the causes of rock and surface uplift is important because they control the location of mountain building depocenters and drainage characteristics and can influence climate here we combine previous thermochronological data with field observations to determine the amount of exhumation rock and surface uplift that occurs in the upper plate of central and south american subduction zones during the collision accretion and subduction of oceanic plateaus and aseismic ridges the collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 ma uplift appears to be an immediate response to collision and is generally independent of the slab dip the amount of rock uplift is controlled mainly by excess topography associated with the ridge ultimately linked to buoyancy and erosion while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel the quantity of exhumation is strongly dependant on climate induced erosion and the lifespan over which the topography is uplifted and supported sediment draining into the trench may leave the elevated ridge axis sediment starved increasing the shear stresses at the ridge subduction interface leading to positive feedback between ridge subduction rock uplift and exhumation trench parallel variations in exhumation have a direct impact on exploration paradigms for porphyry related metalliferous deposits and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates

inner forearc response to subduction of the panama fracture zone southern central america abstract subduction of the right lateral panama fracture zone along the convergent margin of central america creates abrupt lateral variations in convergence rate obliquity and subducting crustal thickness at its intersection with the middle america trench this intersection known as the panama co nz ca triple junction is migrating to the southeast at a rate of 55 mm yr and currently coincides with the lateral termination of the fila costena thrust belt in the inner forearc of the overriding plate mapping in the inner forearc in the area that straddles the subducting panama fracture zone reveals that cocos caribbean convergence west of the triple junction leads to the development of an inner forearc thrust belt inboard of the colliding cocos ridge while little deformation is evident inboard of nazca caribbean convergence east of the triple junction this results in the lateral termination of the fila costena thrust belt in the region of the forearc that projects over the panama fracture zone where four out of five mapped thrust faults tip out and are buried by lahars three new balanced cross sections indicate a steep gradient in shortening from the center of the thrust belt to its southeastern termination the short term history of the inner forearc recorded in the landscape and topography of the fila costena is consistent with the southeastward migration of the thrust belt and the panama triple junction throughout the past 3 ma with evidence for the growth of a new topographic divide and reorganization of stream channel networks

continental margin response to multiple arc continent collisions the northern andes caribbean margin abstract an integration of diverse strain markers into a palinspastic reconstruction of the northern andes provides a first order quantitative approximation to the response of a complex margin to two consecutive arc continent collisions a palinspastic reconstruction highlights margin parallel displacements with crustal blocks traveling 400 to 500 km northward while the margin normal shortening component excluding subduction is limited to 100 km the first arc continent collision closed a 500 km wide ocean basin where subduction of atlantic lithosphere took place under an intra oceanic arc 90 to 65 ma old originally 1100 km long and currently stretched 2200 km following collision eastward directed oblique subduction started at 65 ma with 800 km of proto caribbean lithosphere consumed under northwestern south america igniting a short lived magmatic arc along the post collisional margin two molasse deposits track deformation cooling exhumation and erosion of the margin the first heralds the arrival of the first intra oceanic arc to the southernmost northern andes in late campanian times younging northward as the collision propagated north this latest cretaceous molasse deposit records also the first altitudinal gain in the northern andean orogen along the central and real cordilleras the second molasse follows middle to late eocene magmatic arc shut down and regional exhumation and cooling of the margin formation rise and docking of the intra oceanic panama arc closed a 1200 km wide middle eocene seaway and renewed miocene magmatism in the northern andes as the nazca plate started subducting behind the docked arc

high resolution seismic imaging of faults beneath limon bay northern panama canal republic of panama high resolution seismic reflection profiles from limon bay republic of panama were acquired as part of a seismic hazard investigation of the northern panama canal region the seismic profiles image gently west and northwest dipping strata of upper miocene gatun formation unconformably overlain by a thin 20 m sequence of holocene muds numerous faults which have northeast trends where they can be correlated between seismic profiles break the upper miocene strata some of the faults have normal displacement but on many faults the amount and type of displacement cannot be determined the age of displacement is constrained to be late miocene or younger and regional geologic considerations suggest pliocene movement the faults may be part of a more extensive set of north to northeast trending faults and fractures in the canal region of central panama low topography and the faults in the canal area may be the result of the modern regional stress field bending of the isthmus of panama shearing in eastern panama or minor deformation of the panama block above the caribbean subduction zone for seismic hazard analysis of the northern canal area these faults led us to include a source zone of shallow faults proximal to northern canal facilities d 2003 elsevier b v all rights reserved

differentiating past events on a cone influenced by debris flow and snow avalanche activity a dendrogeomorphological approach dendrogeomorphology was used to investigate past events on a cone affected by both debris flows and snow avalanches we report on results of 520 cores from 251 injured larix decidua mill and picea abies l karst trees sampled on the birchbach cone swiss alps detailed analysis of tree ring sequences allowed dating of 561 growth disturbances in individual trees for a 252 yr period extending from 1750 to 2002 which could be attributed to 30 different event years we then localized the position of rows of traumatic resin ducts trds within the tree ring so as to assess the intra seasonal position of damage in agreement with data on the local growth period trds located at the beginning of the new growth ring were considered the result of avalanche impacts that occurred during the dormant season or in earliest earlywood between late october and early may in contrast trd found in late earlywood or within latewood were considered the result of periglacial debris flow activity as these layers of the tree ring are locally formed between july and early october for nine out of the 30 reconstructed event years the intra seasonal timing of trds indicated that reactions must be the result of past snow avalanche activity in 19 other event years trds showed that damage has been caused between july and early october and thus through debris flows in the birchbach torrent finally the spatial patterns of trees showing reactions as a result of particular events were assessed so as to approximate the extent of past debris flows and snow avalanches copyright 2006 john wiley sons ltd

formation of beach rock at siesta key florida and its influence on barrier island development seaward dipping strata of carbonate cemented shell debris located along the coast of siesta key on the gulf coast of the florida peninsula have long been interpreted to be beachrock equivalent in age to the pleistocene anastasia formation stage 5e of the east coast of florida detailed examination of thin sections along with radiometric dating and isotopic analyses demonstrates clearly that this is a holocene deposit that is not beachrock but was lithified in a meteoric environment whole rock dates dates from shells only and from cement only demonstrate that these beach deposits were in place by at least 1800 yr bp and might have been there as long ago as 4300 yr bp this means that some type of barrier island was in place at that time previous investigations have depicted siesta key as having a maximum age of 3000 yr with these deposits being located about 2 km landward of the beach deposits this suggests that the beach deposits might have been the site of the original position of siesta key these data also indicate that sea level must have been near its present position at the time that these foreshore beach deposits were deposited sometime between 1800 and 4300 yr ago this scenario indicates that sea level along this coastal reach probably reached its present level at least about 2000 yr ago

dendrogeomorphic analysis of flash floods in a small ungauged mountain catchment central spain flash floods represent one of the most significant natural hazards with serious death tolls and economic damage at a worldwide level in general and in mediterranean mountain catchments in particular in these environments systematic data is often lacking and analyses have to be based on alternative approaches such as dendrogeomorphology in this study we focus on the identification of flash floods based on growth disturbances gd observed in 98 heavily affected mediterranean pine trees pinus pinaster ait located in or next to the torrential channel of the pelayo river in the spanish central system flash floods are quite common in this catchment and are triggered by heavy storms with high discharge and debris transport rates favoured by high stream gradients comparison of the anomalies in tree morphology and the position of the trees in the channel showed that the intensity of the disturbance clearly depends on geomorphology the dating of past flash flood events was based on the number and intensity of gd observed in the tree ring series and on the spatial distribution of affected trees along the torrent thus allowing seven flash flood events during the last 50 years to be dated namely in 1963 1966 1973 1976 1996 2000 and 2005

climate change increases frequency of shallow spring landslides in the french alps in this contribution past process activity is reconstructed on seven landslide bodies of the riou bourdoux catchment southeastern french alps based on an unusually dense data set of 3036 tree ring series extracted from 759 conifers we provide evidence for 61 landslide reactivations since a d 1898 based on logistic regressions and threshold analyses of monthly rainfall data and temperature anomalies we determine that the combination of snow rich winters and positive temperature anomalies in spring enhanced snowmelt seems to have driven landslide reactivations in the past since the early 1990s however landslide reactivations clearly have been on the rise and thereby exhibit excessive and unprecedented rates of activity 12 5 events per 10 yr at the scale of the riou bourdoux catchment from the data evidence exists for a shift from snowmelt induced landslides controlled by winter precipitation to reactivations controlled by spring temperatures therefore this contribution also adds evidence to the hypothesis that climate change and related warmer springs could further enhance landslide activity in the course of the 21 st century

do the billecocha normal faults ecuador reveal extension due to lithospheric body forces in the northern andes active compressional and transcurrent structures are widely reported for the entire northern andes whereas there is no clear evidence of extensional tectonics in that region in contrast both active compressional and extensional structures occur extensively in the central andes extensional tectonism in the central andes occurs mainly in high plateaus of large wavelength 100 km and is interpreted to have resulted from the effect of compensated high topography at the lithospheric scale the observation of holocene normal faults on the low wavelength 15 km billecocha high plateau in the northern ecuadorian andes raises a problem could the lithospheric body forces be the cause of the normal faulting in a chain with a mean altitude of 2800 m compared to the central andes where normal faulting is widely distributed across the chain normal faulting on the billecocha plateau is concentrated in a restricted 4 km wide zone in addition normal faulting does not extend further than a few kilometres eastward where compressive structures have been observed considering the local character of the extension body forces at the lithospheric scale cannot be responsible for this normal faulting and thus we can preclude a significant lowering of the magnitude of hmax in the northern andes with respect to the central andes analysis of these normal faults shows that normal faulting occurred between 10 000 yr and 6000 yr b p in addition the relationship between the geometrical parameters vertical throw and rupture length of the major normal fault differs significantly from that given empirically by methods based on scale laws in such a case the billecocha normal faults are unlikely to have a tectonic origin various local processes e g gravity gliding sackung that may be possible are contemplated for triggering the normal faulting in addition a theoretical simple 2 d model is proposed in where local conditions peculiar to the plateau structure glaciation and the surrounding areas are taken into account in this model normal faulting occurs by shearing along the planes of weakness bedding planes in response to the elastic rebound of the fluid saturated plateau which prevailed after the last glaciation

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

palaeoceanography of organic rich rocks of the loma chumico formation of costa rica late cretaceous eastern pacific the late cretaceous late cenomanian campanian loma chumico formation consists of laminated to massive bedded siliceous shales cherts ash fall tuffs volcanic glass beds and volcaniclastic sandstones the loma chumico formation is at least 630 m thick core data in the tempisque basin and can be subdivided in the subsurface into organic rich and organic lean total organic carbon 1 facies rock eval total organic carbon toc data show that toc in the organic rich facies averages 15 5 with values as high as 33 inorganic geochemical petrological and sedimentological analyses indicate that all the lithologies of the loma chumico formation were deposited in middle shelf to outer slope environments adjacent to an active subaerial island arc the organic geochemistry and palaeontology of the loma chumico formation help constrain the factors that controlled biological productivity in the region during the late cretaceous biomarkers identified in organic rich facies by gas chromatography mass spectrometry gc ms show a high abundance of total steranes relative to hopanes in the morote 1 core indicating algal dominated organic input the hopane to sterane ratio of the extract from the manzanillo 1 core indicates a relatively high bacterial component to the organic matter upwelling and extrusive volcanism were the most important factors that controlled nutrient supply including silica flux and biological productivity within the loma chumico formation depositional system episodic radiolarian dinoflagellate or diatom blooms instigated by rapid infusions of volcanogenic silica may have resulted in expansion of the oxygen minimum zone omz enhancing the preservation of organic matter productivity appears to have been uniform even during deposition of the organic lean facies the findings of this study suggest that ocean wide anoxia in the eastern pacific was unlikely during the late cretaceous high toc contents of organic rich radiolarites from central caribbean deep sea drilling project dsdp sites may be related to upwelling produced in the eastern pacific by convergence of the easterly trade winds eastern pacific equatorial upwelling zone or may be part of a more complex pacific atlantic ocean interaction the end of deposition of organic rich rocks began near the end of the latest santonian earliest campanian in the central caribbean and costa rica palaeoceanographic models suggest that improved circulation and ventilation of bottom waters due to early opening of the north atlantic south atlantic seaway may explain this observation this may represent the influx of a previously unrecognized source of colder oxygenated intermediate bottom water proto antarctic intermediate water into the late cretaceous central atlantic and eastern pacific oceans

fore arc motion and cocos ridge collision in central america 1 we present the first regional surface velocity field for central america showing crustal response to interaction of the cocos and caribbean plates elastic half space models for interseismic strain accumulation on the dipping subduction plate boundary fit the gps data well and show strain accumulation offshore and beneath the nicoya and osa peninsulas in costa rica but not in nicaragua since large subduction zone earthquakes occur in nicaragua we suggest that interseismic locking in nicaragua and some other parts of central america occurs but is mainly shallow 20 km depth too far offshore to be detected by our on land gps measurements our data also show significant trench parallel motion for most of the region generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates however trench parallel motion is also observed in central costa rica where plate convergence is normal to the trench and in the nicaraguan fore arc where trench parallel motion is fast up to 9 mm a 1 but mechanical coupling is low a finite element model of collision as opposed to subduction involving the aseismic cocos ridge also fits the gps surface velocity field most significantly reproducing the pattern of trench parallel motion we infer that buoyant thickened cns 2 cocos ridge crust resists normal subduction and instead acts as an indenter to the caribbean plate driving crustal shortening in southern costa rica and contributing to trench parallel fore arc motion in costa rica and perhaps nicaragua as a type of tectonic escape

magmatic consequences of the transition from orthogonal to oblique subduction in panama the closure of the central american seaway is linked with tectonic and magmatic processes that have controlled the evolution of the isthmus of panama we focus on the terminal stages of arc activity in the central panama region and present new geochemical data from 9 ma explosive silicic volcanism preserved in three syngenetic tuff beds from the gatun formation the magmatic evolution of the gatun formation is controlled by a series of magma mushes where pyroxene is the dominant early forming mafic mineral with amphibole appearing only relatively late in the fractionation sequence our data shows gatun lavas exhibit a strong subduction signature consistent with plate reconstruction models showing arc normal subduction from costa rica to panama pre 8 5 ma however large ion lithophile elements are depleted in the gatun formation in comparison to other regional suites indicative of a lower flux of subduction fluid to the gatun formation mantle source which is explained by a shift towards magma generation by decompression following the collision of the arc with south america oblique subduction commencing 8 5 ma resulted in the shutdown of normal arc activity throughout panama we interpret subsequent regional quaternary adakitic volcanism as a response to this oblique subduction the now more refractory mantle wedge required greater fluid flux in order to melt the resultant volatile rich melts were more prone to deep fractionation of amphibole and garnet cumulates forming adakites deep fractionation was potentially enhanced by changing stress regimes on the upper plate caused by oblique subduction this article is protected by copyright all rights reserved

structural imprints at the front of the choco panama indenter field data from the north cauca valley basin central colombia abstract the northern andes are a complex area where tectonics is dominated by the interaction between three major plates and accessory blocks in particular the choco panama and northern andes blocks the studied cauca valley basin is located at the front of the choco panama indenter where the major romeral fault system active since the cretaceous changes its kinematics from right lateral in the south to left lateral in the north structural studies were performed at various scales dem observations in the central cordillera between 4 and 5 7 n aerial photograph analyses and field work in the folded oligo miocene rocks of the serrania de santa barbara and in the flat lying pleistocene quindio risaralda volcaniclastic sediments interfingering with the lacustrine to fluviatile sediments of the zarzal formation the data acquired allowed the detection of structures with a similar orientation at every scale and in all lithologies these families of structures are arranged similarly to riedel shears in a right lateral shear zone and are superimposed on the cretaceous romeral suture they appear in the central cordillera north of 4 5 n and define a broad zone where 060 oriented right lateral distributed shear strain affects the continental crust the romeral fault system stays active and strain partitioning occurs among both systems the southern limit of the distributed shear strain affecting the central cordillera corresponds to the e w trending garrapatas ibague shear zone constituted by several right stepping en echelon right lateral active faults and some lineaments north of this shear zone the romeral fault system strike changes from nne to n paleostress calculations gave a wnw ese trending maximum horizontal stress and 69 of compressive tensors the orientation of 1 is consistent with the orientation of the right lateral distributed shear strain and the compressive state characterizing the romeral fault system in the area it bisects the synthetic and antithetic riedels and is sub perpendicular to the active romeral fault system it is proposed that the continued movement of the choco panama indenter may be responsible for the 060 oriented right lateral distributed shear strain and may have closed the northern part of the cauca valley thereby forming the cauca valley basin conjugate extensional faults observed at surface in the flat lying sediments of the zarzal formation and quindio risaralda volcaniclastic fan are associatedwith soft sediment deformations these faults are attributed to lateral spreading of the superficial layers during earthquakes and testify to the continuous tectonic activity from pleistocene to present finally results presented here bring newinformation about the understanding of the seismic hazard in this area whereas the romeral fault systemwas so far thought to be themost likely source of earthquakes themore recent cross cutting fault systems described herein are another potential hazard to be considered

reconstruction of glacial lake outburst floods in northern tien shan implications for hazard assessment abstract glacier lake outburst floods glofs and related debris flows are among the most significant natural threats in the tien shan mountains of kyrgyzstan and have even caused the loss of life and damage to infrastructure in its capital bishkek an improved understanding of the occurrence of this process is essential so as to be able to design reliable disaster risk reduction strategies even more so in view of ongoing climate change and scenarios of future evolutions here we apply a dendrogeomorphic approach to reconstruct past debris flow activity on the aksay cone ala archa valley kyrgyz range where outbursting glacier lakes and intense rainfalls have triggered huge debris flows over the past decades a total of 96 picea abies l karst trees growing on the cone and along the main channel have been selected based on the evidence of past debris flow damage in their trunks these trees were then sampled using increment borers the dating of past events was based on the assessment of growth disturbances gd in the tree ring records and included the detection of injuries tangential rows of traumatic resin ducts reaction wood and abrupt growth changes in total 320 gd were identified in the tree ring samples in combination with aerial imagery and geomorphic recognition in the field reactions in trees and their position on the cone have allowed reconstruction of the main spatial patterns of past events on the aksay cone our findings suggest that at least 27 debris flows have occurred on the site between 1877 and 2015 and point to the occurrence of at least 17 events that were not documented prior to this study we also observe high process activity during the 1950s and 1960s with major events on the cone in 1950 1966 and 1968 coinciding with phases of slight glacier advance the spatial analyses of events also point to two different spatial patterns suggesting that quite dissimilar magnitudes probably occurred during glacier lake outburst floods and rainfall induced debris flow events the results presented here represent the longest annually resolved glof series in the region which in turn has key implications on risk assessment not just in the ala archa valley but also in the entire kyrgyz range northern tien shan

unhinging an indenter a new tectonic model for the internal deformation of panama 1 new paleoseismic results from panama conducted as part of the seismic hazard assessment for the expansion of the panama canal have led to a reevaluation of the tectonic framework and geologic history of the isthmus of central america we propose a soft block indenter model wherein the collision of central america and south america has resulted in significant internal deformation of the isthmus deformation is accommodated by both rapid slip on conjugate strike slip faults within the isthmus as well as the generally assumed flexure and northward buckling of panama the model is kinematically self consistent in that there are little or no space problems created with 3 ma of retrodeformation sparse gps velocity data are consistent to within uncertainties with the new geologically constrained block model supporting the rapid and extensive internal deformation of panama together the paleoseismologic and geodetic data suggest that central panama is an area of high risk due to earthquakes which is consistent with the historical occurrence of several moderate to large earthquakes in this region however this is generally counter to the current perception in central panama where most people live and where there have been no large damaging earthquakes for over 100 years

quaternary outer fore arc deformation and uplift inboard of the panama triple junction burica peninsula 1 the panama triple junction cocos nazca caribbean represents the point that abruptly separates the thick and rapidly subducting cocos plate to the northwest from the thin and obliquely subducting nazca plate to the southeast along the central american convergent margin new structural and geomorphic analyses on the burica peninsula an outer fore arc peninsula located only 100 km inboard the panama triple junction reveal that the peninsula is dominated primarily by contractional deformation along three listric thrust faults that root in the underlying plate boundary the geometry and spatial distribution of these thrusts indicate that this deformation occurs primarily in response to the change in crustal thickness occurring as a result of eastern migration of the flank of the cocos ridge coeval with migration of the panama triple junction at a rate of 55 mm yr to the southeast mapping and detailed elevation surveys reveal eight marine terraces on the peninsula with a distribution of inner edge elevations indicating that uplift is spatially uniform from north to south along strike in this area however terraces along the northwest part of the peninsula are offset across major thrust faults age control provided by 14c osl and soil chronosequences indicate that the terraces within the easternmost portion of the peninsula range in age from marine isotope stage mis 3 to holocene a result that indicates that this portion of the peninsula is younger than 60 ka time averaged uplift rates calculated from marine terraces and other quaternary marine deposits yield consistent uplift rates that range between 2 1 0 1 and 7 7 0 5 mm yr for samples older than 1 ka and between 6 9 1 0 and 19 3 8 0 mm yr for samples younger than 1 ka we interpret this temporal distribution in uplift rates to suggest that the eight terraces preserved on the peninsula are produced coseismically wherein the anomalously high uplift rates calculated from the youngest samples 1 ka are not yet averaged over a complete seismic cycle these observations combined with 1 shortening estimates from balanced cross sections indicating that minimum shortening decreases from northwest to southeast and 2 the observation of growth strata within the youngest marine units are consistent with a space for time model for triple junction migration these results indicate that both triple junction migration and the change in bathymetry occurring at the triple junction boundary are far more dominant factors in outer fore arc deformation than the change in rate and obliquity of subduction and basal tractions that also occur on either side of the triple junction

stratigraphic models for microtidal tidal deltas examples from the florida gulf coast extensive vibracoring of both flood and ebb tidal deltas along the central gulf coast of the florida peninsula reveals a strong overall similarity with subtle distinctions between flood and ebb varieties although the coast in question is microtidal the inlets range from tide dominated to distinctly wave dominated both types of tidal deltas overlie a muddy sand interpreted to have been deposited in a back barrier environment the sharp contact at the base of the tidal delta sequence is typically overlain by a thin shell gravel layer the ebb tidal delta sequence is characterized by fine quartz sand with shell gravel in various concentrations coarse and massive at the margins of the main ebb channel and finer and imbricated at the marginal flood channels the flood tidal deltas are characterized by the same facies but with a small amount of mud shelly facies on the channels on flood deltas are not as well developed as on the ebb deltas the combination of the stratigraphic sequence and the lithofacies make tidal deltas readily identifiable in the ancient record the differences between flood and ebb varieties are subtle but consistent

frequency and spread of debris floods on fans a dendrogeomorphic case study from a dolomite catchment in the austrian alps growth disturbances in tree ring series have been regularly used to date debris flow events in mountain environments in contrast no studies are available to date that have reconstructed debris floods by means of dendrogeomorphology therefore the aim of this study was to determine the event frequency and the spread of debris floods in the gratzental tyrol austria the analysis of growth disturbances in the tree ring series of 227 picea abies l karst and larix decidua mill allowed the reconstruction of 37 events for the last 200 years the lateral spread and preferable avulsion locations of reconstructed debris flood events were assessed based on the dating of the events and the spatial position of trees affected by an event on the fan results show that the gratzentalbach preferentially avulsed to the east but affected trees were evenly spread over the fan reconstructed data illustrates the high potential of dendrogeomorphology for hazard assessment of debris floods

is the cauca patia and romeral fault system left or rightlateral extrusion driven collision is controlled by strike slip faults of opposite senses whereas subduction related oblique convergence is accommodated by margin parallel faults of identical senses northern andes geodynamics has been interpreted either as resulting from the obliquity of convergence between nazca and south american plates or from the collision of the carnegie ridge with ecuadorian margin the sense of the cauca patia and romeral fault system cprfs has been interpreted in two opposite manners right or left lateral agreeing with oblique or collisional patterns respectively we analyze arguments which favor south to 4 n a right lateral motion along the cprfs that is consistent with obliquity driven geodynamics however north of 5 n the cprfs is actually left lateral these opposite motions between the northern and southern part of the cprfs have to be accommodated around 5 n by n s stretching and or shortening existing data support n s shortening around 5 n this buttressing could be due to the collision between the panama and andean blocks

hydrometeorological triggers of periglacial debris flows in the zermatt valley switzerland since 1864 1 the triggering of debris flows depends on a critical combination of available unconsolidated material and water supply in periglacial environments debris flows are generally triggered by liquefaction of loose material in a channel or by progressive erosion during a large release of water here we link an unusually dense and highly resolved database on periglacial debris flows with meteorological records dating back to ad 1864 to reconstruct 150 yr of rainstorms that triggered debris flows at high elevation sites source area elevations ranging from 2000 to 4545 m a s l in the swiss alps analysis is based on a tree ring derived frequency series of debris flows from eight torrents as well as on daily records from three meteorological stations and runoff data from four river gauging stations results show that the debris flow season at these high altitude sites now is much longer may to october than it used to be in the late nineteenth century when activity was limited to june september debris flows early in the season are generally triggered by lower rainstorm totals 20 mm day than those occurring later in the season because early season snowmelt adds considerable amounts of water to the system and therefore facilitates debris flow formation debris flows in may june july and august are triggered primarily by short duration high intensity rainstorms local thunderstorms whereas late season september october debris flows are commonly related to longer lasting advective rainstorms

latest pleistocene holocene tectonics of the ecuadorian andes abstract a structural study was carried out in the most tectonically active areas of the ecuadorian andes including part of the interandean valley the cordillera real and the subandean zone since the pleistocene the region has undergone complex wrench tectonics which are still active apparently two different stress states have coexisted within different parts of the andean range during the latest pleistocene and holocene 1 1 the external part of the chain including the cordillera real and the westernmost part of the subandean zone has been subject to a east northeast west southwest direction of greatest principal stress associated motions are partitioned between right lateral oblique thrusting and pure right lateral strike slip faulting along north south to north northeast trending structures as a whole these define a right lateral transpressive shear zone 2 2 in the interandean valley which lies above a major tertiary suture zone deformations are consistent with a north south direction of greatest horizontal principal stress associated with recent development of a left lateral transtensional shear zone in the first zone fault displacements are at least an order of magnitude larger than in the second zone the present tectonics of the area is explained by a kinematic model in which the wedge shaped cordillera real crustal block bounded to the east by a right lateral shear zone and to the west by a left lateral shear zone is being uplifted and extruded northward with respect to the cordillera occidental and to the stable south american foreland this motion is the result of oblique convergence between the nazca and south american plates

thermochronology and tectonics of the central and western cordilleras of colombia early cretaceous tertiary evolution of the northern andes abstract new thermochronological data record a complex cooling history in the central and western cordilleras of colombia that is a function of early cretaceous to late miocene tectonic events alkali feldspar 40ar 39ar cooling ages of 138 130 ma immediately post date the cessation of jurassic arc magmatism and a major unconformity within the retro foreland region of the northern andes we interpret these ages as cooling driven by exhumation in response to either compression driven by subduction of a seamount or extension and oceanward migration of the slab during the earliest cretaceous giving rise to the lower cretaceous quebradagrande arc sequence biotite and alkali feldspar 40ar 39ar data from the palaeocontinental margin reveal the presence of a younger cooling event at 117 107 ma which was contemporaneous with hornblende 40ar 39ar cooling ages obtained from medium high p t metamorphic relicts of a late jurassic early cretaceous subduction channel this cooling event is attributed to exhumation driven by the collision and accretion of a fringing arc against the continental margin and obduction of the subduction channel onto the forearc inverse modelling of zircon and apatite fission track and u th he data from throughout the central and western cordilleras reveals three periods of rapid cooling since the late cretaceous the earliest phase is recorded by jurassic and cretaceous granitoids that cooled rapidly during 75 65 ma we attribute cooling to exhumation of the continental margin during 75 70 ma 1 6 km my which was forced by the collision and accretion of the caribbean large igneous province in the campanian the central cordillera exhumed at moderate rates of 0 3 km my during 45 30 ma which are also observed over widely dispersed regions along the andean chain and were probably caused by an increase in continent ocean plate convergence rates exhumation rates drastically increased in the middle late miocene with the greatest amount occurring in southern colombia as a consequence of the collision and subduction of the buoyant carnegie ridge at 15 ma

spatiotemporal analysis of channel wall erosion in ephemeral torrents using tree roots an example from the patagonian andes this paper presents absolute ages for fl ash fl oods and related sediment entrainment in headwater catchments to construct a spatiotemporal framework of process dynamics and locations of major areas of channel wall erosion the most reliable method for dating erosion is through dendrogeomorphic studies of exposed tree roots based on the analysis of erosion signals in root ring records we documented a time series of channel wall erosion and successfully dated 21 erosive fl ash fl ood events since a d 1870 in an ephemeral gully in the patagonian andes the study was performed with roots from austrocedrus chilensis nothofagus dombeyi and pseudotsuga menziesii results demonstrate the potential of root analyses for the determination of major areas of sediment entrainment in addition we show that the position of damage within individual root rings allows inferences about the seasonal timing of fl ash fl ood effects and thus an assessment of possible meteorological triggers of erosive events short intense storms occurring primarily in austral fall and late winter in this case the approach presented adds signifi cantly to the documentation of sediment entrainment and facilitates identifi cation of areas of rapid erosion in small remote headwater catchments with ephemeral fl ash fl ood activity

neotectonics and paleoseismology of the limon and pedro miguel faults in panama earthquake hazard to the panama canal one of the sentences in the conclusions section of rockwell et al 2010 is incomplete and therefore

the geology of the darien panama and the late miocene pliocene collision of the panama arc with northwestern south america the geology of the darien province of eastern panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its upper cretaceous to upper miocene sediments the sequence of events inferred from the stratigraphic record includes the collision of the panama arc the southwestern margin of the caribbean plate and south american continent three tectonostratigraphic units underlie the darien region 1 precollisional upper cretaceous eocene crystalline basement rocks of the san blas complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the darien province these rocks formed part of a 20 m y submarine volcanic arc developed in a pacific setting distant from the continental margin of northwestern south america the northerly basement rocks are quartz diorites granodiorites and basaltic andesites through dacites to rhyolites indicating the presence of a magmatic arc the southerly basement rocks are an accreted suite of diabase pillow basalt and radiolarian chert deposited at abyssal depths precollisional arc related rocks of eocene to lower miocene age consist of 4000 m of pillow basalts and volcaniclastics and biogenic calcareous and siliceous deep water sediments they consist of the eocene oligocene darien formation the oligocene porcona formation and the lower middle miocene clarita formation postcollisional deposits are mostly coarse to fine grained siliciclastic sedimentary rocks and turbiditic sandstone of upper middle to latest miocene age this 3000 m thick sedimentary sequence is deformed as part of a complexly folded and faulted synclinorium that forms the central chucunaque tuira basin of the darien the sedimentary package reveals general shallowing of the basin from bathyal to inner neritic depths during the 12 8 7 1 ma collision of the panama arc with south america the sediments are divided into the upper middle miocene tapaliza formation the lower upper miocene tuira and membrillo formations the middle upper miocene yaviza formation and the middle to upper miocene chucunaque formation the precollisional open marine units of late cretaceous middle miocene age are separated from the overlying postcollisional sequence of middle to late miocene age by a regional unconformity at 14 8 12 8 ma this unconformity marks the disappearance of radiolarians the changeover of predominantly silica deposition from the atlantic to the pacific the initiation of the uplift of the isthmus of panama and the onset of shallowing upward coarser clastic deposition this pattern is also recorded from the southern limon basin of caribbean costa rica to the atrato basin of northwestern colombia by the middle late miocene neritic depths were widespread throughout the darien region and a regional unconformity suggests completion of the central american arc collision with south america by 7 1 ma no pliocene deposits are recorded from either the darien or the panama canal basin and no sediments younger than 4 8 ma have been identified in the atrato basin of colombia suggesting rapid uplift and extensive emergence of the central american isthmus in the latest miocene northward movement of the eastern segment of the panama arc along a now quiescent panama canal zone fault during eocene oligocene time may have dislocated the pre collision arc since collision the portion west of this fault chorotega block has remained stable without rotation to the east in the darien region compression has been accommodated through formation of a panama microplate with convergent boundaries to the north north panama deformed belt and south south panama deformed belt and suturing with south america along the atrato valley deformation within the microplate has been accommodated in the darien province by several major left lateral strike slip faults that were active until the early pliocene since when the plate has behaved rigidly

transition from the farallon plate subduction to the collision between south and central america geological evolution of the panama isthmus this paper presents new geological constraints on the collision of southern central america with south america and the resulting deformational episodes that have affected the panama isthmus since the late cretaceous the panama isthmus is located in southwestern central america and it represents the zone of contact between the two land masses central america and south america this collision event is still active today it has resulted in regional uplift since the late miocene pliocene and is responsible for the great american biotic interchange between south and north america depending on the methods of investigation used and due to the lack of data available the time when this collision began is still widely debated and poorly constrained to better constrain this age we have studied the rock formations and the tectonic deformations in central and eastern panama that have occurred since the late cretaceous this study presents new rock ages field work documentation and analyses and seismic line interpretations and it is complemented by spatial images for the eastern panama area during the middle eocene a number of changes suddenly appeared in the geological records that were synchronous with the break up of southern central america into two smaller blocks chorotega and choco our main results identify the prevalence of an extensional tectonic regime from the middle eocene to the middle miocene that caused the formation of horst and graben structures with thick sedimentary basin fills and a synchronous clockwise block rotation here we propose that these geologic events are associated with the initiation of the oblique collision of southern central america with south america the first contact of the southeastern extremity of central america occurred around 40 ma to 38 ma and then propagated northwestwards we describe here this long term collision episode in relation to the history of the panama isthmus

estimating flash flood discharge in an ungauged mountain catchment with 2d hydraulic models and dendrogeomorphic palaeostage indicators there is still wide uncertainty about past flash flood processes in mountain regions owing to the lack of systematic databases on former events this paper presents a methodology to reconstruct peak discharge of flash floods and illustrates a case in an ungauged catchment in the spanish central system the use of dendrogeomorphic evidence i e scars on trees together with the combined use of a two dimensional 2d numerical hydraulic model and a terrestrial laser scan tls has allowed estimation of peak discharge of a recent flash flood the size and height distribution of scars observed in the field have been used to define three hypothetical scenarios smin or minimum scenario smed or medium scenario and smax or maximum scenario thus illustrating the uncertainty involved in peak discharge estimation of flash floods in ungauged torrents all scars analysed with dendrogeomorphic techniques stem from a large flash flood which took place on 17 december 1997 on the basis of the scenarios peak discharge is estimated to 79 14 m3 s 1 the average deviation obtained between flood stage and expected scar height was 0 09 0 53 m from the data it becomes obvious that the geomorphic position of trees is the main factor controlling deviation rate in this sense scars with minimum deviation were located on trees growing in exposed locations especially on unruffled bedrock where the model predicts higher specific kinetic energy the approach used in this study demonstrates the potential of tree ring analysis in palaeohydrology and for flood risk assessment in catchments with vulnerable goods and infrastructure copyright 2010 john wiley sons ltd

quaternary state of stress in the northern andes and the restraining bend model for the ecuadorian andes inversion of shallow focal mechanisms in the northern andes together with a neotectonic analysis of the ecuadorian andes shows that the state of stress is homogeneous in most of the northern andes e w trending 1 however north of 5 n 1 is roughly nw se trending this difference in state of stress is due to the force balance between the nazca caribbean and south american plates south of 5 n the state of stress appears to be mainly controlled by the nazca south american plate interaction while north of 5 n it appears to be mainly controlled by the caribbean south american plate interaction this state of stress difference from south to north is consistent with dextral motions along faults trending parallel to the chain south of 5 n and sinistral motions along faults trending parallel to the chain north of 5 n this inversion of shallow focal mechanisms also illustrates the incomplete strain partitioning of the oblique convergence between the nazca and south american plates along the ecuadorian southern colombian trench the convergence obliquity that increases northward controls the active transcurrent fault slip rate in the upper plate finally this convergence obliquity is responsible for the location of the seismicity in the upper plate in addition the andean block is considered in this paper as a large triple zone under constriction with one of its borders caribbean behaving partly as a free border

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

the role of co2 decline for the onset of northern hemisphere glaciation the pliocene pleistocene transition ppt from around 3 2 to 2 5 million years ago ma represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern eurasia and north america paleo evidence indicates that the ppt was accompanied and possibly caused by a decrease in atmospheric co2 but the temporal resolution of co2 reconstructions is low for this period of time and uncertainties remain large therefore instead of applying existent co2 reconstructions we solved an inverse problem by finding a schematic co2 concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records to this end we performed an ensemble of transient simulations with an earth system model of intermediate complexity from which we derived a best guess transient co2 scenario for the interval from 3 2 to 2 4 ma that gives the best fit between the simulated and reconstructed benthic 18o and global sea surface temperature evolution our data constrained co2 scenarios are consistent with recent co2 reconstructions and suggest a gradual co2 decline from 375 425 to 275 300 ppm between 3 2 and 2 4 ma in addition to a gradual decline the best fit to paleoclimate data requires the existence of pronounced co2 variability coherent with the 41 kyr 1 kyr 1000 years obliquity cycle in our simulations the long term co2 decline is accompanied by a relatively abrupt intensification of northern hemisphere glaciation at around 2 7 ma this is the result of a threshold behaviour of the ice sheets response to gradual co2 decrease and orbital forcing the simulated northern hemisphere ice sheets during the early pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m both ice volume and benthic 18o are dominated by 41 kyr cyclicity our simulations suggest that before 2 7 ma greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation a fully glaciated greenland comparable to its present day ice volume is modelled only during glacial maxima after 2 7 ma and more continuously after 2 5 ma

model for late neogene deformation in panama previous workers mostly using earthquake data have postulated both discrete faults and complex diffuse zones for the caribbean nazca plate boundary in panama we have better resolved the location and character of late neogene plate boundary structures by using satellite imagery and a compilation of existing geologic map data on the basis of these results we propose that panama is moving northwestward away from a zone of active convergence along the continental margin of south america toward more easily subducted areas of the colombian basin movement is accommodated across a diffuse zone by a complex interaction of oroclinal bending left lateral strike slip faulting and subduction

current states of stress in the northern andes as indicated by focal mechanisms of earthquakes abstract systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern andes reveals relatively homogeneous patterns of crustal stress in three main regions the first region presently under the influence of the caribbean plate includes the northern segment of the eastern cordillera of colombia and the western flank of the central cordillera north of 4 n it is characterized by wnw ese compression of dominantly reverse type that deflects to nw se in the merida andes of venezuela where it becomes mainly strike slip in type a major bend of the eastern thrust front of the eastern cordillera near its junction with the merida andes coincides with a local deflection of the stress regime sw ne compression suggesting local accommodation of the thrust belt to a rigid indenter in this area the second region includes the sw pacific coast of colombia and ecuador currently under the influence of the nazca plate in this area approximately e w compression is mainly reverse in type it deflects to wsw ene in the northern andes south of 4 n where it is accommodated by right lateral displacement of the romeral fault complex and the eastern front of the northern andes the third and most complex region is the area of the triple junction between the south american nazca and caribbean plates it reveals two major stress regimes both mainly strike slip in type the first regime involves sw ne compression related to the interaction between the nazca and caribbean plates and the panama micro plate typically accommodated in an e w left lateral shear zone the second regime involves nw se compression mainly related to the interaction between the caribbean plate and the north andes block which induces left lateral displacement on the uramita and romeral faults north of 4 n deep seismicity about 150 170 km concentrates in the bucaramanga nest and cauca valley areas the inversion reveals a rather homogeneous attitude of the minimum stress axis which dips towards the e this extension is consistent with the present plunge of the nazca and caribbean slabs suggesting that a broken slab may be torn under gravitational stresses in the bucaramanga nest this model is compatible with current blocking of the subduction in the western northern andes inhibiting the eastward displacement of slabs which are forced to break and sink in to the asthenosphere under their own weight

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

exploring debris flow history and process dynamics using an integrative approach on a dolomitic cone in western austria the evolution of a debris flow cone depends on a multitude of factors in the hydrogeomorphic system investigations of debris flow history and cone dynamics in highly active catchments therefore require an integrative approach with a temporal and spatial resolution appropriate for the goals of the study we present the use of an orthophoto time series to augment standard dendrogeomorphic techniques to describe the spatio temporal dynamics of debris flows on a highly active cone in the western austrian alps analysis of seven orthophotos since 1951 revealed a migration of active deposition areas with a resulting severe loss of forest cover 80 and a mean tree loss per year of 10 4 range 1 3 16 6 trees per year analysis of 193 pinus mugo ssp uncinata trees allowed the identification of 161 growth disturbances corresponding to 16 debris flows since 1839 and an average decadal frequency of 0 9 events as a result of the severe loss of forest cover we speculate that 20 of the more recent events were actually captured in the tree ring record giving a decadal return interval of 7 5 events for a period of 60 years based on three annual field observations it is evident that this catchment the barenrufe produces very frequent 1 yr small in the order of a few 10 to 100 m3 debris flows with minor material relocation the specific challenges of tree ring analysis in this tree species and in highly active environments are explicitly addressed in the discussion and underline the necessity of employing complementary methods of analysis in an integrative manner copyright 2012 john wiley sons ltd

electrical resistivity in support of geological mapping along the panama canal abstract dredging and widening of the panama canal is currently being conducted to allow larger vessels to transit to and from the americas asia and europe dredging efficiency relies heavily on knowledge of the types and volumes of sediments and rocks beneath the waterway to ensure the right equipment is used for their removal to aid this process a waterborne streaming electrical resistivity survey was conducted along the entire length of the canal to provide information on its geology within the confines of the canal a total of 663 line kilometers of electrical resistivity data were acquired using the dipole dipole array the support of the survey data for dredging activities was realized by calibrating and qualitatively correlating the resistivity data with information obtained from nearby logged boreholes and geological maps the continuity of specific strata was determined in the resistivity sections by evaluating the continuity of similar ranges of resistivity values between boreholes it was evident that differing geological units and successions can have similar ranges of resistivity values for example quaternary sandy and gravelly alluvial fill from the former river channel of the chagres river had similar resistivity ranges generally from 40 to 250 m to those characteristic of late miocene basalt dikes from 100 to 400 m but for quite different reasons similarly competent marine based sedimentary rocks of the caimito formation were similar in resistivity values ranging from 0 7 to 10 m to sandstone conglomerate of the bohio formation consequently it would be difficult to use the resistivity data alone to extrapolate more complex geotechnical parameters such as the hardness or strength of the substrate a necessary component for such analyses requires detailed objective information regarding the specific context from which the geotechnical parameters were derived if these data from cored boreholes and detailed geological surveys are taken into account however then waterborne streaming resistivity surveying can be a powerful tool in this case it provided inexpensive and highly resolved quantitative information on the potential volume of loose suctionable material along the gamboa sub reach which could enable large cost savings to be made on a major engineering project involving modification of one of the most important navigable waterways in the world

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

intermediate depth seismicity in northern colombia and western venezuela and its relationship to caribbean plate subduction the recent intermediate depth seismicity in northern colombia and western venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern south america the area included in this study is located to the north and east of the bucaramanga nest which is a particular region beneath bucaramanga northern colombia that presents a high concentration of intermediate depth earthquakes to the north of the nest the seismicity of the area is sparse and most of the events are of low magnitude mb 5 l thus only 23 earthquakes were large enough to be investigated using teleseismic data the focal parameters of the two largest events mb 5 4 were obtained from the formal inversion of long period body waves recorded at teleseismic distances the focal mechanisms of 10 more events were determined from first motion data in total the focal mechanisms of 12 events were determined from both the inversion of p and s h waveforms and the polarities of first arrivals for the smaller earthquakes the focal depths were calculated by measuring the observed pp p interval time and comparing it to the theoretical travel time tables the isodepth curves reflect a slab striking in a nne ssw direction and dipping approximately at 25 32 to the southeast this observation is corroborated by the direction and plunge of the t axes of the focal mechanisms which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters these results indicate that the intermediate depth earthquakes in western venezuela and northern colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of colombia the two largest earthquakes located at a significant distance from the bucaramanga nest present similar fault plane solutions moreover they also agree with those of the two largest earthquakes reported inside the nest this similarity suggests that the bucaramanga nest lies on the same subducted slab where the other earthquakes occur there is not enough shallow seismicity to define the location where the caribbean lithosphere subducted beneath the south american plate however the extension of the slab toward the surface inferred from the intermediate depth seismicity suggests that the subducted lithosphere may still be attached to the caribbean plate

turbulent flow over a dune green river colorado detailed echo sounder and acoustic doppler velocimeter measurements are used to assess the temporal and spatial structure of turbulent flow over a mobile dune in a wide low gradient alluvial reach of the green river based on the geometric position of the sensor over the bedforms measurements were taken in the wake in transitional flow at the bedform crest and in the internal boundary layer spatial distributions of reynolds shear stress turbulent kinetic energy turbulence intensity and correlation coefficient are qualitatively consistent with those over fixed two dimensional bedforms in laboratory flows spectral and cospectral analysis demonstrates that energy levels in the lee of the crest i e wake are two to four times greater than over the crest itself with minima over the stoss slope within the developing internal boundary layer the frequency structure in the wake is sharply defined with single dominant peaks peak and total spectral and cross spectral energies vary over the bedform in a manner consistent with wave like perturbations that break or roll up into vortices that amalgamate grow in size and eventually diffuse as they are advected downstream fluid oscillations in the lee of the dune demonstrate strouhal similarity between laboratory and field environments and correspondence between the peak frequencies of these oscillations and the periodicity of surface boils was observed in the field copyright 2005 john wiley sons ltd

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

quantifying peak discharges for historical floods abstract it is usually advantageous to use information regarding historical floods if available to define the flood frequency relation for a stream peak stages can sometimes be determined for outstanding floods that occurred many years ago before systematic gaging of streams began in the united states this information is usually not available for more than 100 200 years but in countries with long cultural histories such as china historical flood data are available at some sites as far back as 2 000 years or more it is important in flood studies to be able to assign a maximum discharge rate and an associated error range to the historical flood this paper describes the significant characteristics and uncertainties of four commonly used methods for estimating the peak discharge of a flood these methods are 1 rating curve stage discharge relation extension 2 slope conveyance 3 slope area and 4 step backwater logarithmic extensions of rating curves are based on theoretical plotting techniques that results in straight line extensions provided that channel shape and roughness do not change significantly the slope conveyance and slope area methods are based on the manning equation which requires specific data on channel size shape and roughness as well as the water surface slope for one or more cross sections in a relatively straight reach of channel the slope conveyance method is used primarily for shaping and extending rating curves whereas the slope area method is used for specific floods the step backwater method also based on the manning equation requires more cross section data than the slope area ethod but has a water surface profile convergence characteristic that negates the need for known or estimated water surface slope uncertainties in calculating peak discharge for historical floods may be quite large various investigations have shown that errors in calculating peak discharges by the slope area method under ideal conditions for recent floods i e when flood elevations slope and channel characteristics are reasonably certain may be on the order of 10 25 under less than ideal conditions where streams are hydraulically steep and rough errors may be much larger the additional uncertainties for historical floods created by the passage of time may result in even larger errors of peak discharge

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

the paleogene arcs of the northern andes of colombia and panama insights on plate kinematic implications from new and existing geochemical geochronological and isotopic data abstract the northern andes of colombia comprise two non cogenetic paleogene arcs formed in contrasting geodynamic settings including continental and oceanic domains new whole rock geochemistry and isotopic constraints together with a review of 332 geochemical 76 isotopic and 204 geochronological data from paleocene to eocene volcanic and plutonic rocks exposed in central and western colombia and panama are used to evaluate cause effect relations between regional plate kinematics and the spatio temporal distribution of the circum caribbean magmatic arcs short lived 60 45 ma arc like magmatism in the central cordillera of colombia was emplaced in a thickened continental crust due to the oblique subduction of the caribbean oceanic plate underneath south america as suggested by the high sr y ratios conversely the panama arc and its poorly explored extension in the northwestern segment of the western cordillera of colombia document a major phase of tholeiitic to calc alkaline arc magmatism between 71 ma and 34 ma mostly derived from a hydrated mantle wedge and emplaced in an oceanic plateau crust this arc record a major phase of magmatic activity between 40 ma and 50 ma that can be associated to changes in the convergence direction and subduction rates of the farallon plate farther to the south in the pacific region of colombia eocene arc related rocks from the timbiqui complex show a geochemical signature that suggests a magmatic origin from melting of both a subduction modified mantle and lower tectonically thickened crust a possible interpretation for the pacific and western colombia panama arc systems and its analogous in ecuador recall an aleutian type convergent margin in which the continental arc laterally switches to a purely oceanic system along the trailing edge of the caribbean plate as consequence of eastward subduction of the farallon plate

reconstruction of caribbean sea surface temperature and salinity fluctuations in response to the pliocene closure of the central american gateway and radiative forcing using δ44 40ca δ18o and mg ca ratios the 18o values of planktonic foraminifera increased in the caribbean by about 0 5 relative to the equatorial east pacific values between 4 6 and 4 2 ma as a consequence of the closure of the central american gateway cag this increase in 18o can be interpreted either as an increase in caribbean sea surface mixed layer salinity sss or as a decrease in sea surface temperatures sst this problem represents an ideal situation to apply the recently developed paleotemperature proxy 44 40ca together with mg ca and 18o on the planktic foraminifer globigerinoides sacculifer from odp site 999 although differences in absolute temperature calibration of 44 40ca and mg ca exist the general pattern is similar indicating a sst decrease of about 2 3 c between 4 4 and 4 3 ma followed by an increase in the same order of magnitude between 4 3 and 4 0 ma correcting the 18o record for this temperature change and assuming that changes in global ice volume are negligible the salinity induced planktonic 18o signal decreased by about 0 4 between 4 4 and 4 3 ma and increased by about 0 9 between 4 3 and 4 0 ma in the caribbean the observed temperature and salinity trends are interpreted to reflect the restricted exchange of surface water between the caribbean and the pacific in response to the shoaling of the panamanian seaway possibly accompanied by a southward shift of the intertropical convergence zone itcz between 4 4 and 4 3 ma differences in mg ca and 44 40ca derived temperatures can be reconciled by corrections for secular variations of the marine mg casw and 44 40ca a salinity effect on the mg ca ratio and a constant temperature offset of 2 5 c between both sst proxy calibrations

central costa rica deformed belt kinematics of diffuse faulting across the western panama block fault kinematics seismicity and geodetic data across central costa rica reveal a diffuse fault zone here named the central costa rica deformed belt ccrdb the ccrdb defines the western margin of the panama block and links the north panama deformed belt npdb along the caribbean coast with the middle america trench mat along the pacific coast the junction of the ccrdb and the mat coincides with an abrupt transition from smooth to rough crust on the subducting cocos plate rough smooth boundary shallow subduction of rough thickened oceanic crust associated with the cocos ridge shifts active shortening into the volcanic arc along faults of the ccrdb variable fault kinematics along this zone may reflect three combined deformation mechanisms horizontal shortening and shear from oceanic ridge indentation basal traction from shallow subduction and localized block uplift from subducting seamount roughness within the forearc domain 1 mesoscale faults express transtension where steep ne striking regional scale faults intersect the pacific coast across the volcanic arc domain 2 mesoscale faults exhibit mostly sinistral and dextral slip on ne and nw striking conjugate faults respectively approaching the npdb in the back arc domain 3 transcurrent faulting is modified by transpression and crustal thickening fault kinematics are consistent with earthquake focal mechanisms and global positioning system gps measurements radiometric age constraints confirm that faulting postdates the late neogene onset of shallow subduction the ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active ccrdb within the forearc the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts

counting scars on tree stems to assess rockfall hazards a low effort approach but how reliable rockfall is a widespread and hazardous process in mountain environments but data on past events are only rarely available growth ring series from trees impacted by rockfall were successfully used in the past to overcome the lack of archival records dendrogeomorphic techniques have been demonstrated to allow very accurate dating and reconstruction of spatial and temporal rockfall activity but the approach has been cited to be labor intensive and time consuming in this study we present a simplified method to quantify rockfall processes on forested slopes requiring less time and efforts the approach is based on a counting of visible scars on the stem surface of common beech fagus sylvatica l data are presented from a site in the inn valley austria where rocks are frequently detached from an 200 m high south facing limestone cliff we compare results obtained from i the classical analysis of growth disturbances in the tree ring series of 33 norway spruces picea abies l karst and ii data obtained with a scar count on the stem surface of 50 f sylvatica trees a total of 277 rockfall events since a d 1819 could be reconstructed from tree ring records of p abies whereas 1140 scars were observed on the stem surface of f sylvatica absolute numbers of rockfalls and hence return intervals vary significantly between the approaches and the mean number of rockfalls observed on the stem surface of f sylvatica exceeds that of p abies by a factor of 2 7 on the other hand both methods yield comparable data on the spatial distribution of relative rockfall activity differences may be explained by a great portion of masked scars in p abies and the conservation of signs of impacts on the stem of f sylvatica besides data indicate that several scars on the bark of f sylvatica may stem from the same impact and thus lead to an overestimation of rockfall activity

dendrogeomorphic reconstruction of flash floods in the patagonian andes flash floods represent a significant natural hazard in small mountainous catchments of the patagonian andes and have repeatedly caused loss to life and infrastructure at the same time however documentary records of past events remain fairly scarce and highly fragmentary in most cases in this study we therefore reconstruct the spatiotemporal patterns of past flash flood activity along the los cipreses torrent neuquen argentina using dendrogeomorphic methods based on samples from austrocedrus chilensis pseudotsuga menziesii and nothofagus dombeyi we document 21 flash flood events covering the period a d 1890 2009 and reconstruct mean recurrence intervals of events at the level of individual trees being impacted which varies from 4 to 93 years results show that trees tend to be older younger in sectors of the torrent with gentler steeper slope gradients potential triggers of flash floods were analyzed using daily temperature and precipitation data from a nearby weather station weather conditions leading to flash floods are abundant precipitations during one to three consecutive days combined with temperatures above the rain snow threshold 2 c in the whole watershed