neogene and quaternary history of vegetation climate and plant diversity in amazonia the neotropical amazonian and andean plant diversity developed mainly during the tertiary in amazonia miocene oral diversity seems considerably higher than today during the neogene tropical taxa entered newly created montane area and montane taxa entered the tropical lowlands the general decrease of temperature during the upper neogene and especially during the quaternary glacial periods may have caused considerable extinctions in the lowlands representation of pollen of apparently principally montane taxa podocarpus hedyosmum in miocene pliocene and quaternary sediments of amazonia and surroundings is still di cult to interpret in terms of temperature decrease at low elevation changes in precipitation may have profound impact on the composition of vegetation communities ilex and melastomataceae increase signi cantly in many glacial pollen records increase of weinmannia in amazonian pollen records seems the best indicator of downward migration of montane vegetation belts a temperature lowering at sea level of 4 5 13c during the last glacial maximum lgm seems reasonable it may have caused a downslope migration of some 700 m of the lower montane vegetation belt lower montane arboreal species may have been able to grow in higher elevation areas 500 m of amazonia increasing background pollen values of montane taxa in the area di erence between a cool and wet middle pleniglacial 60 28 ka bp and a cold and dry upper pleniglacial 28 14 ka bp thus including the lgm is evident in andean and amazonian records statements about environmental conditions of the ice age amazon should be speci ed chronologically the middle pleniglacial is a time of accumulation of uvial sediments the upper pleniglacial is a time of incision of the rivers in their sediments sedimentation started again in the lateglacial since ca 13 ka bp and the holocene when lake levels rose again based on simpli ed considerations of precipitation changes and evaporation we estimate that lgm rainfall may have been reduced by values of ca 45 10 amazonian and cordilleran lakes dried up dry rain forest was locally replaced by savanna savanna forest or cerrado type vegetation dry rain forest savanna forest and pure savanna was locally replaced by extensive semi desert dune formations lower rio branco area in present day central amazonia the present day centers of higher rainfall 2500 mm surrounded by areas of lower rainfall are refuge areas of the very wet rain forest and of the very high plant diversity 300 plant species per 0 1 ha and they should have been that equally or more during the dry climate intervals plant diversity of drier forests is in the order of 100 150 species per 0 1 ha both extinction and speciation in isolation under precipitation and temperature stress may have taken place in these refugia 2000 elsevier science ltd all rights reserved

tracking exhumation of andean ranges bounding the middle magdalena valley basin colombia the shortening history of the andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of cenozoic climate however the timing of uplift in the northern andes is poorly constrained with estimates ranging from cretaceous to pliocene detrital zircon u pb ages from the middle magdalena valley basin in colombia reveal two provenance shifts during cenozoic time the fi rst shift occurs between early and late paleocene strata where u pb results show a switch from proterozoic dominated to phanerozoic dominated age spectra we attribute this change to uplift related exhumation of the central cordillera the second shift occurs between middle late eocene and late oligocene strata where increased grenville ages and diminished mesozoic ages can be linked to uplift of the eastern cordillera our results show that signifi cant pre neogene deformation affected the northern andes underscoring the potential importance of andean uplift on the dynamics of paleogene climate

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

astronomical tuning of long pollen records reveals the dynamic history of montane biomes and lake levels in the tropical high andes during the quaternary long pollen records from two sediment cores of the basin of bogota colombia are presented reflecting the dynamic history of environmental and vegetation changes in the tropical high andes during the quaternary an astronomically tuned age model has been developed by using the visual correlation of the temperature constrained record of alnus with the benthic 18o stacked record of odp site 846 and 849 for the last 1 million year ma in addition spectral analysis of the arboreal pollen percentages ap prior to 1 ma shows two consistent peaks at 7 6 and 9 5 m which could either be interpreted as reflecting the 19 and 23 kyr components of the precession cycle or the 41 kyr cycle of obliquity evidence for precession forcing comes from the sum of the filtered 7 6 and 9 5 m cycles revealing a long term modulation that can be linked to the short term 100 kyr and long term 400 kyr eccentricity cycles a precession forced scenario however is not consistent with zircon fission track ages and is difficult to explain in terms of climate forcing mechanisms on the contrary an obliquity control of the ap record is consistent with the fission track ages and with glacial interglacial bound temperature changes within the study region accordingly it appears that the funza09 record extends back to 2 25 ma during which four periods with distinct different depositional environments could be identified following a chronological poorly constrained interval of fluvial and fluvio lacustrine sediments from 2 25 to 1 47 ma sediment deposition occurred in an area of wetlands dissected by fluvial channels and swamps the strongest subsidence of the basin occurred between 1 47 and 1 23 ma when a proper lake developed lacustrine sediments accumulated in water up to 50 m deep between 1 23 and 0 86 ma hereafter water depth was generally lower than 50 m but fluctuated in conjunction with the 100 kyr dominated glacial interglacial variability during the middle and late pleistocene the evolutionary changes of the paramo and montane forest biomes are described in terms of five characteristic stages most of the pleistocene vegetation has no analogue to modern assemblages principally due to the late immigration events of alnus 1 01 ma and quercus 0 43 ma and we conclude that forest composition similar to modern was not established until the last interglacial however modern ecological constraints of suites of taxa that formed the vegetation of the pre mis 5 part of the record allow a reconstruction of environmental and climate change

tectonic reconstruction of the northern andean blocks oblique convergence and rotations derived from the kinematics of the piedras girardot area colombia abstract a detailed kinematic study in the piedras girardot area reveals that approximately 32 km of ene wsw oblique convergence is accommodated within a northeast trending transpressional shear zone with a shear strain of 0 8 and a convergence factor of 2 early campanian deformation is marked by the incipient propagation of northeast trending faults that uplifted gentle domes where the accumulation of sandy units did not take place maastrichtian unroofing of a metamorphic terrane to the west is documented by a conglomerate that was deformed shortly after deposition developing a conspicuous intragranular fabric of microscopic veins that accommodates less than 5 extension this extensional fabric distortion of fossil molds and a moderate cleavage accommodating less than 5 contraction developed concurrently but before large scale faulting and folding paleogene folding and southwestward thrust sheet propagation are recorded by syntectonic strata neogene deformation took place only in the western flank of this foldbelt the amount direction and timing of deformation documented here contradict current tectonic models for the cordillera oriental and demand a new tectonic framework to approach the study of the structure of the northern andes thus an alternative model was constructed by defining three continental blocks the maracaibo cordillera central and cordillera oriental blocks oblique deformation imposed by the relative eastward and northeastward motion of the caribbean plate was modeled as rigid body rotation and translation for rigid blocks derived from published paleomagnetic and kinematic data and as internal distortion and dilation for weak blocks derived from the piedras girardot area this model explains not only coeval dextral and sinistral transpression and transtension but also large clockwise rotation documented by paleomagnetic studies in the caribbean northern andean region

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

kinematic restoration of contractional basement structures using thermokinematic models a key tool for petroleum system modeling in this paper we demonstrate a workflow for constructing kinematic restorations in complex foothill areas devoid of growth strata and other indicators for the chronology of deformation our initial reconstructions utilize thermochronometric data a well documented structural geometry and a first order conversion of exhumation rates into tectonic rates we then utilize models obtained from the new in house developed software fetkin to build a first version of the thermokinematic restoration the fetkin approach is geared primarily toward testing and further calibration and refinement of the kinematic restoration based on the extent to which the model result agrees with thermochronometric data from the study area in the form of both discrete ages and inverse modeled time temperature envelopes this analysis also provides rates of shortening and time temperature paths throughout the model space that can be used to make first order predictions of when different source rocks entered the oil window these capabilities are demonstrated in a pilot case study along a cross section in the colombian eastern cordillera the improved confidence in the reconstruction that this technique provides allows us to show increasing shortening rates in this part of the andes during the neogene reaching up to 5 mm yr 0 20 in yr by the pliocene and constrain the timing of generation from the most important oil kitchens for the eastern cordillera llanos basin petroleum system this approach therefore proves to be a useful method for creating high resolution and high fidelity kinematic restorations

climate tectonics and the morphology of the andes large scale topographic analyses show that hemisphere scale climate variations are a first order control on the morphology of the andes zonal atmospheric circulation in the southern hemisphere creates strong latitudinal precipitation gradients that when incorporated in a generalized index of erosion intensity predict strong gradients in erosion rates both along and across the andes cross range asymmetry width hypsometry and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial glacial and tectonic processes and show that major morphologic features correlate with climatic regimes latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential indicating that like tectonics nonuniform erosion due to large scale climate patterns is a first order control on the topographic evolution of the andes

an environmental reconstruction of the palaeo amazon river system middle late miocene nw amazonia abstract new sedimentological and palynological data from the tertiary sediments in the upper amazon river area suggest that these sediments are fluvio lacustrine deposits of middle to late miocene age they were generated as a result of the uplift of the eastern cordillera andes and constitute possibly the oldest relics of the amazon river system the palaeoenvironment in which these sediments were deposited is characterized by extensive wetlands environments formed by swamps shallow lakes crevasse splay channels and crevasse delta lakes where the channel environment is poorly represented the palaeovegetation was dominated by palms e g mauritia and grimsdalea riverine taxa e g bombacaceae amanoa and alchornea ferns and fern allies e g polypodiaceae and selaginellaceae floating meadows gramineae and aquatic taxa ceratopteris botryococcus and azolla the relative abundance of gramineae and the occurrence of andean type pollen taxa is related to the andean origin of the fluvial system the varzeas of the present upper amazon river flood basin are probably the best analogue for the middle to late miocene environment intervals rich in marine palynomorphs mangrove pollen brackish tolerant molluscs and ostracods and ichnofossils of the thalassinoides teichichnus association suggest that the palaeoenvironment was characterized by brackish conditions and marine influence these marine incursions are possibly related to the langhian and the serravallian global sea level rise although in the middle miocene a global cooling is known to have occurred no indicators of a cooler climate have been observed in the miocene palynoflora of the upper amazon river area finally four new sporomorph species are described belonging to the form genera psilatriletes clavainaperturites and psilaperiporites

climatic control on rapid exhumation along the southern himalayan front abstract along the southern himalayan front shf areas with concentrated precipitation coincide with rapid exhumation as indicated by young mineral cooling ages twenty new young 80 of the annual precipitation aft cooling ages reveal a coincidence between rapid erosion and exhumation that is focused in a 50 70 km wide sector of the himalaya rather than encompassing the entire orogen assuming simplified constant exhumation rates the rocks of two age vs elevation transects were exhumed at 1 4 0 2 and 1 1 0 4 mm a with an average cooling rate of 40 50 c ma during pliocene quarternary time following other recently published hypotheses regarding the relation between tectonics and climate in the himalaya we suggest that this concentrated loss of material was accommodated by motion along a back stepping thrust to the south and a normal fault zone to the north as part of an extruding wedge climatically controlled erosional processes focus on this wedge and suggest that climatically controlled surface processes determine tectonic deformation in the internal part of the himalaya

kinematic evolution of andean fold thrust structures along the boundary between the eastern cordillera and middle magdalena valley basin colombia 1 surface and subsurface data support a kinematic reconstruction of cenozoic fold thrust deformation along the eastern cordillera magdalena valley transition in colombia the la salina fault lsf marks the boundary between west vergent eastern cordillera structures and hinterland deposits of the middle magdalena valley basin apatite fission track and u th he thermochronological results for the west directed lsf reveal initial hanging wall exhumation during middle eocene early oligocene 45 30 ma shortening renewed exhumation in the early middle miocene 18 12 ma and accelerated late miocene pliocene 12 3 ma exhumation vitrinite reflectance data suggest maximum burial of 4 6 km helping constrain cenozoic basin architecture mapping of the lsf reveals hanging wall cretaceous eocene rocks in a broad anticline syncline pair with limited faulting and footwall eocene quaternary basin fill in a complex series of tight thrust related folds limited displacement along the westernmost frontal thrust suggests that shortening is largely accommodated by east directed thrusting within a broader triangle zone of a passive roof duplex and probable minor strike slip deformation in the preferred kinematic restoration the most recent phase of shortening to transpressional deformation represents out of sequence reactivation of the lsf consistent with irregular crosscutting relationships among footwall structures earliest exhumation by 45 30 ma in the eastern cordillera fold thrust belt is correlated with increased sedimentary lithic fragments and high compositional maturity in sandstones of the adjacent magdalena valley basin exhumation since 15 ma coincided with decreased compositional maturity and elevated accumulation rates for the real group the compositional provenance shifts are attributed to westward advance of fold thrust deformation into the proximal eastern segments of the magdalena valley basin

controls on the strength of coupling among climate erosion and deformation in two sided frictional orogenic wedges at steady state 1 many important insights regarding the coupling among climate erosion and tectonics have come from numerical simulations using coupled tectonic and surface process models however analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered we present an approximate analytical solution for two sided orogenic wedges obeying a frictional rheology and in a condition of flux steady state that makes explicit the nature and sensitivity of the coupling between climate and deformation we make the simplifying assumption that the wedge grows in a self similar fashion consistent with airy isostasy such that topographic taper is invariant with orogen width tectonic influx rate and climate we illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen scale erosion rule we show that steady state orogen width crest elevation and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process remarkably we show that for most combinations of parameters in the erosion law rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux further assuming frontal accretion with no underplating the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro wedge and retro wedge and the time averaged trajectories of rocks through the orogen the restriction to invariant frictional properties is conservative in these respects systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate erosion and deformation than shown here

epicenter distribution and magnitude of earthquakes in fold thrust belts insights from sandbox models scaled analogue models are used to illustratethe eect of basal friction and erosion on fault activity andhence on epicentre distribution and magnitude of earth quakes in the sedimentary cover of active fold thrust belts model results suggest that in fold thrust belts shortenedabove low friction ductile decollements rock salt or over pressured mudstone low to moderate magnitude earth quakes mw 5 3 5 6 distributed over a wide area occuralong several long lived thrust faults conversely in areasshortened above high friction decollements large magnitudeearthquakes mw 6 6 6 8 distributed over a narrowzone are likely to occur along few short lived thrust ramps calculated magnitude of earthquakes from models and theirdistribution are in agreement with recorded earthquake pat tern from the zagros mountain belt which is partially short ened above a ductile decollement of hormuz salt formation model results also showed that erosion reactivates older in active thrusts and promotes formation of out of sequencethrusts

plateau reduction by drainage divide migration in the eastern cordillera of colombia defined by morphometry and 10be terrestrial cosmogenic nuclides catchment wide erosion rates were defined using 10be terrestrial cosmogenic nuclides for the eastern cordillera of the colombian andes to help determine the nature of drainage development and landscape evolution the eastern cordillera characterized by a smooth axial plateau bordered by steep flanks has a mean erosion rate of 11 1 mm ka across the plateau and 70 10 mm ka on its flanks with local high rates of 400 mm ka the erosional contrast between the plateau and its flanks was produced by the increase in the orogen regional slope derived from the progressive shortening and thickening of the eastern cordillera the erosion rates together with digital topographic analysis show that the drainage network is dynamic and confirms the view that drainage divides in the eastern cordillera are migrating towards the interior of the mountain belt resulting in progressive drainage reorganization from longitudinal to transverse dominated rivers and areal reduction of the sabana de bogota plateau this article is protected by copyright all rights reserved

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

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

the permian to cretaceous structural evolution of the western approaches basin melville sub basin uk summary the tectonic sedimentary evolution of the western part of the western approaches basin melville sub basin was initiated during early permian times when synrift clastic sedimentation and volcanism commenced within an ene trending half graben this sequence thickened to the south and onlapped a high to the north during the permian an early nnw sse basin extension was accommodated on an array of ene wsw trending normal faults linked by nnw sse orientated transfer faults this was followed by a period of ene wsw extension along the basin axis when the normal and transfer faults of the previous faulting episode swapped roles this was possibly related to early rifting in the bay of biscay both sets of faults appear to be controlled to some extent by variscan thrusts and strike slip faults within the basement during the upper triassic the ridge in the north subsided allowing a thick sequence of salt to be deposited where there had previously been a topographic high during jurassic times sedimentation extended across the permian triassic half graben into the french sector this was followed by an episode of rifting and wrench deformation and inversion during which the permian to jurassic basin was domed and uplifted the cretaceous to tertiary sedimentation which followed probably resulted from postrift thermal subsidence related to cooling of the continental platform adjacent to the opening atlantic ocean steep normal and or strike slip faults data from the earliest part of this postrifting stage

slab flattening and the rise of the eastern cordillera colombia abstract the topographic growth of a mountain belt is commonly attributed to isostatic balance in response to crustal and lithospheric thickening however deeper mantle processes may also influence the topography of the earth here we discuss the role of these processes in the eastern cordillera ec of colombia the ec is an active double vergent fold and thrust belt that formed during the cenozoic by the inversion of a mesozoic rift and topography there has risen up to 5 000 m cocuy sierra the belt is located 500 km away from the trench where two separate portions of the nazca plate subduct below the south american plate north of 5 n the ec rises above a flat slab subduction region volcanic arc migration implies slab shallowing by 10 ma and flattening up to the present day configuration at 6 ma the occurrence of a high vp vs anomaly and clustered seismicity below the belt at 160 km depth delineates the slab geometry and has been related to dehydration of the slab suggesting the presence of a hydrated mantle wedge we compiled thermochronologic data and inverted for the exhumation history of the chain over the last 20 ma using the age elevation relationship and the different closure temperatures of multiple thermochronologic systems results indicate that exhumation rates increased during the plio pleistocene at different wavelengths and amplitudes the small wavelength and large amplitude signals could be related to shallow crustal deformation whereas the source of the long wavelength and moderate amplitude signal has yet to be identified pulses of fast exhumation are found to be concomitant with the uplift that occurred from 7 ma to the present day previous studies suggested that the high topography of the chain cannot be achieved solely through isostatic adjustment the highest residual topography is centered on the highest elevations of the ec whereas the lowest residual topography corresponds to the magdalena valley following the regional slab geometry we propose that the recent uplift and exhumation events were triggered by the transition from regular to flat slab subduction along with the hydration of the mantle wedge above the slab we test the dynamic feasibility of our hypothesis with a series of numerical models for the present day state predicting the correct trends in elevation requires a flat slab geometry and a weak and buoyant mantle wedge

a fold and thrust belt along the western flank of the eastern cordillera of colombia style kinematics and timing constraints derived from seismic data and detailed surface mapping the eastern cordillera of colombia is considered to be the result of compression and uplift during the late miocene pliocene andean orogeny nevertheless detailed mapping carried out on a portion of the fold belt exposed along the western flank of the eastern cordillera suggests that deformation began in late paleocene early eocene time in the form of a west vergent forward propagating fold and thrust belt several structures related to this event remain concealed beneath the late paleocene late eocene unconformity and synkinematic sediments are preserved locally within some of these thrust sheets the relief generated during this deformational episode constrained the depositional axis of late eocene fluvial sediments to run along the present day crestal zone of the eastern cordillera onlapping basal miocene molassic sediments on tilted middle miocene fluvial deposits suggest that a series of seismically delineated intercutaneous wedges located at the thrust front formed in late miocene time these structures mark the onset of the andean deformational episode thrusting polarity reversed during the latter event that is toward the hinterland and paleogene structures were reactivated additionally the late paleocene early eocene unconformity surface was folded and incorporated into north plunging folds and cretaceous extensional faults were inverted as a result the eastern cordillera developed its present pop up doubly vergent fold belt geometry flanked by thick molassic sequences that document the miocene pliocene andean orogeny regionally the latest campanian maastrichtian uplift of the central cordillera paleocene early eocene thrusting in the middle magdalena valley and along the western flank of the eastern cordillera and ultimately inversion and major uplift of the entire eastern cordillera in late miocene pliocene time attests to the eastward propagation of deformation from the central toward the eastern cordillera kinematically the driving element of deformation would have been the obliquely accreting western terranes of the central and western cordilleras and the panama arc these transferred their easterly component of convergence to the eastern cordillera through a basal intracrustal detachment

an environmental reconstruction of the sediment infill of the bogota basin colombia during the last 3 million years from abiotic and biotic proxies abstract the lacustrine sediments of the intramontane basin of bogota 4 n 2550 m altitude were collected in a 586 m deep core funza 2 absolute datings show sediment infill started c 3 2 ma and continued almost without interruptions as a result of the balance between tectonic subsidence and sediment infill analysis of downcore changes in lithology grain size facies loss on ignition loi and hydrosere vegetation at 20 cm intervals along the core produced a 2200 sample record of basin dynamics and lake level changes with a temporal resolution from c 800 years during the last 1 5 ma to c 2000 years from 1 5 to 3 2 ma we recognized 11 discrete sedimentary facies that reflect 4 different depositional environments facies 1 and 2 correspond to lacustrine environment where the loi is 20 a fluvio lacustrine environment is represented by the facies 3 4 5 7 and 9 a fluvial environment is reflected by facies 6 8 and 10 basin infill started with accumulation in a fluvio lacustrine environment between 586 and 530 m below the present day sediment surface of the basin in the 530 to 325 m interval c 3 1 5 ma the basin contained mainly shallow water and swamps in combination with some fluvial activity the 325 5 m interval 1 5 0 028 ma shows almost uninterrupted lacustrine paleoenvironmental conditions the uppermost 5 m reflect a fluvial paleoenvironment deposited by the bogota river and its tributaries the downcore changing ratio between shallow water aquatics myriophyllum ludwigia polygonum cyperaceae and aquatics of deeper water isoetes is indicative of lake level changes isoetes as an indicator of high water levels is not consistent with fluvial and fluvio lacustrine depositional regimes several lithological discontinuities evidenced by the sharp transition from swamp deposits to fluvio lacustrine and fluvial deposits are present in the core in the interval from 530 to 325 m in particular it is plausible that swamp deposits have been eroded during episodes during which the water table quickly raised or during events of sudden coarse fluvial input into the basin

temporal and spatial distribution of tectonic events as deduced from reworked palynomorphs in the eastern northern andes in colombia palynology has been widely used as a biostratigraphic tool in oil exploration over the last two decades and as a result of these efforts an understanding of the chronostratigraphic range of thousands of palynomorph species is now available furthermore because of their relative resistance to physical and chemical degradation palynomorphs can often survive several tectonic erosive cycles allowing them to be used as unique tracers of long term sedimentological changes in this work we use the palynological record from wells and outcrops in the llanos foothills and the llanos basin of colombia to establish the intensity of cenozoic reworking and its relationship to the tectonic evolution of the colombian andes using this approach we were able to discern several tectonic episodes associated with the uplift of the eastern cordillera we documented three periods of either faster erosion in the hinterland or more widespread areas being eroded in the catchment areas late paleocene early eocene early to mid miocene and pliocene and two periods of tectonic quiescence mid eocene and mid late miocene these periods correlate well with the deposition of different elements of the petroleum systems in the llanos basin of colombia seals and reservoirs

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

transient slab flattening beneath colombia subduction of the nazca and caribbean plates beneath northwestern colombia is seen in two distinct wadati benioff zones one associated with a flat slab to the north and one associated with normal subduction south of 5 5 n the normal subduction region is characterized by an active arc whereas the flat slab region has no known holocene volcanism we analyze volcanic patterns over the past 14 ma to show that in the mid miocene a continuous arc extended up to 7 n indicating normal subduction of the nazca plate all along colombia s pacific margin however by 6 ma we find a complete cessation of this arc north of 3 n indicating the presence of a far more laterally extensive flat slab than at present volcanism did not resume between 3 n and 6 n until after 4 ma consistent with lateral tearing and re steepening of the southern portion of the colombian flat slab at that time

the pliocene and quaternary of the high plain of bogota colombia a history of tectonic uplift basin development and climatic change abstract the high plain of bogota in the eastern cordillera of colombia represents a large tectonic basin that is filled with hundreds of metres of lacustrine and fluvial sediments in the centre of the basin the sediment sequence attains a thickness of nearly 600 m and covers a time span of the last 3 ma late pliocene quaternary still older sediments of mainly fluvial origin cover the adjacent mountain slopes the age of these sediments reaches back into the late miocene to 6 ma and older the sediment sequence of the bogota area has been the subject of extensive stratigraphical and palynological studies for decades the results of these studies have provided a unique picture of the pliocene quaternary geological history of a tropical montane region three main factors have influenced the evolution of the landscape sedimentation and vegetation development these are the tectonic uplift of the andes between ca 5 and 3 ma the development of the large tectonic sedimentary basin of bogota after ca 3 5 ma and the quaternary climatic fluctuations and glaciations starting shortly after ca 2 7 ma

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

vertical tectonics at a continental crust oceanic plateau plate boundary zone fission track thermochronology of the sierra nevada de santa marta colombia 1 the topographically prominent sierra nevada de santa marta forms part of a faulted block of continental crust located along the northern boundary of the south american plate hosts the highest elevation in the world 5 75 km whose local base is at sea level and juxtaposes oceanic plateau rocks of the caribbean plate quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary and the driving forces of rock uplift along the active margin the sierra nevada province of the southernmost sierra nevada de santa marta exhumed at elevated rates 0 2 km my during 65 58 ma in response to the collision of the caribbean plateau with northwestern south america a second pulse of exhumation 0 32 km my during 50 40 ma was driven by underthrusting of the caribbean plate beneath northern south america subsequent exhumation at 40 25 ma 0 15 km my is recorded proximal to the santa marta bucaramanga fault more northerly regions of the sierra nevada province exhumed rapidly during 26 29 ma 0 7 km my further northward the santa marta province exhumed at elevated rates during 30 25 ma and 25 16 ma the highest exhumation rates within the sierra nevada de santa marta progressed toward the northwest via the propagation of nw verging thrusts exhumation is not recorded after 16 ma which is unexpected given the high elevation and high erosive power of the climate implying that rock and surface uplift that gave rise to the current topography was very recent i e 1 ma and there has been insufficient time to expose the fossil apatite partial annealing zone

tectonic inversion in the santa barbara system of the central andean foreland thrust belt northwestern argentina 1 the santa barbara system sbs of northern argentina is a 400 km long segment of the subandean foreland thrust belt it is characterized by predominantly west verging relatively high angle thrust faults many of these faults are reactivated normal faults from one branch of a complex cretaceous to paleogene rift system heteroaxial folding in parts of the sbs is probably due to a slight component of range parallel dextral strike slip motion acting on preexisting faults striking north and ne during inversion regional balanced cross sections along two transects across the northern sbs indicate that the major faults flatten into a detachment in the basement at about 10 km depth neogene e w contraction in the sbs is of the order of 21 26 km rift extension is not very well constrained but was probably less than 10 km the structural style of the sbs differs from the thin skinned subandean thrust belt to the north and from the large wavelength sierras pampeanas basement uplifts to the south the changes between the different styles are sharp and coincide with the northern and southern boundaries of the rift in the foreland suggesting that crustal or lithospheric heterogeneities exert an overriding control on foreland structural style

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

new thermo and geochronological constraints on the pliocene pleistocene eruption history of the paipa iza volcanic complex eastern cordillera colombia abstract the paipa iza volcanic complex is the only volcanic edifice in the eastern cordillera of colombia zircon fission track and u pb data from deposits of two eruption epochs presented in this study constrain the time of volcanic activity between 5 9 ma in the late miocene and at least 1 8 ma during the early pleistocene these ages correspond to 3 6 to 4 7 ma zircon fission track ages reported from tephra deposits of the tierra negra region to the southwest of the paipa iza volcanic complex in the eastern cordillera a 4 myr period of volcanic activity is longer than the previously documented 2 5 ma to 1 9 ma activity in the paipa iza area therefore our data show that the paipa iza volcanic complex was active during the entire pliocene phase of rapid surface uplift when the eastern cordillera reached its present day elevation unclear is if the magma chamber of the paipa iza volcanic complex was passively uplifted with the rest of the eastern cordillera or if magmatism contributed to surface uplift hydrothermal activity and steep thermal gradients in the study area today indicate that a magma chamber is still present at depth in both the zircon fission track and u pb data of this study the recycling of zircon from underlying mesozoic sedimentary rocks is evident in the wide age spectra the fission track data show that many recycled zircons were affected at least by partial annealing but further research is needed for constraining the thermal impact of the paipa iza volcanism on the eastern cordillera

climatic and ecologic changes during miocene surface uplift in the southern patagonian andes abstract the up to 4 km high southern patagonian andes form a pronounced topographic barrier to atmospheric circulation in the southern hemisphere westerlies and cause one of the most drastic orographic rain shadows on earth geologic data imply that this climatic pattern has been established or significantly enhanced during miocene surface uplift of this andean segment we report evidence for important climatic and ecologic changes in the eastern foreland of the patagonian andes that appear to be the result of this uplift to provide constraints on miocene plant ecosystems and precipitation in the eastern leeward foreland of the patagonian andes we determined carbon and oxygen isotope values of pedogenic carbonate nodules from a 500 m thick section of the continental santa cruz formation the age of these deposits was constrained by ar ar dating of intercalated tuffs which range from 22 to 14 ma at 16 5 ma the 13 c values increase by 3 the 18 o values decrease by 2 and the scatter in the oxygen isotope data increases significantly we interpret these changes as the consequence of 1 km surface uplift in this andean segment from the 18 o values and increased aridity to its east from the 13 c values and the increased scatter in the 18 o values sediments overlying the santa cruz formation are very limited in extent and volume and dominated by coarse conglomerates related to pleistocene and older glaciations it thus seems that by 14 ma deposition in the eastern foreland of the southern patagonian andes had essentially ceased as the result of rain shadow formation

montane forest evolution during the last 650 000 yr in colombia a multivariate approach based on pollen record funza i the core interval of 0 140 m of the 357 m long pollen record from site funza i was analysed using multivariate analysis this long continental record is from the high plain of bogota located at 2550 m altitude in the eastern cordillera of colombia time control is based on a visual match with core funza ii from the same location which includes absolute fission track ages and a match with the 18o record of odp site 677 we reassessed the time frame of funza i compared with previous papers and conclude that the period of ca 650 25 kyr bp is represented in this study 73 fossil taxa present in 4 of the 409 pollen assemblages were analysed using the divisional classification program twinspan we recognised 10 cluster types that form three main cluster zones w warm cw cool and c cold pollen zones based on previous visual inspection were compared with new cluster types which were graphed as a bar code along the pollen diagram the use of two pollen sums standard pollen sum and a zonal pollen sum appeared useful for identifying local effects on the forest composition modern pollen rain data for the study area were summarised we used the concept of indicator taxa that behave as differential taxa to reconstruct climatic change during the period of 650 330 kyr bp alnus hedyosmum weinmannia melastomataceae myrica and podocarpus were the most important taxa of the andean forest the new time frame indicates that the first scattered presence of quercus started 478 kyr bp 102 m core depth the first continuous presence was from 423 kyr bp 92 m and the presence of zonal quercus forest occurred from 330 kyr bp 72 m during the period of 330 135 kyr bp zonal quercus and weinmannia forest developed replacing partly podocarpus dominated forest in the uppermost andean forest belt as a consequence during the last 135 kyr bp a high representation of quercus on the high plain of bogota no longer coincides with warm climatic conditions but now occurs during cool to cold conditions the pollen record was correlated with the 18o record temperature drop during marine oxygen isotope stage 4 and the very first part of marine oxygen oxygen stage 2 is calculated as a minimum of 6 6 to 7 8 c excluding a potential effect of low glacial pco2 the inferred maximum amplitude of the upper forest line during the last 650 kyr was from ca 1900 to 3400 m cf 3200 m at the present day corresponding to a range of ca 1 2 c to 7 8 c compared with today copyright 2000 john wiley sons ltd

archean granite greenstone tectonics at kolar south india interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion 1 the structural study of the kolar greenstone belt and surrounding granite gneiss terrains combined with u pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern dharwar craton developed during a major magmatic accretion event between 2550 and 2530 ma the granite greenstone pattern resulted from the interference of gravity driven sagging of the greenstones i e diapirism e w bulk inhomogeneous shortening combined with horizontal n s stretching and syntectonic juvenile pluton emplacement bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive n s trending vertical foliation shallow stretching lineation and conjugate strike slip shear zone pattern within and outside the greenstone belt resulting in regional horizontal pure shear deformation the plutons around the greenstone belt record regional contraction by developing one set of strike slip c s fabrics of the shear zone pattern the development of the granite greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion melting and granulite metamorphism in the lower crust the kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain due to mechanical homogeneity and low viscosity provided by large scale melting during the accretion event this example further suggests specific boundary conditions to the craton that allowed e w inhomogeneous shortening to be accommodated by n s stretching and spreading of the crust without significant tectonic thickening such tectonoplutonic pattern is specific to the archean and may develop as a consequence of mantle plume activity in intracontinental settings

el nino s tropical climate and teleconnections as a blueprint for pre ice age climates 1 at 2 7 million years ago the warm equable climates of early and middle pliocene time used here to mean from 5 to 2 7 ma were replaced by recurring ice ages most attempts to explain the change appeal either to changes in co2 in the atmosphere or reduced heat transport by the atlantic ocean the sources of the strongest teleconnections in the current climate however lie in the tropics and such connections occur by transport of heat and moisture by the atmosphere the most prominent of these teleconnections link aberrations in sea surface temperatures in the equatorial pacific el nino southern oscillation enso variations with warm and dry or cool and wet anomalies in extratropical climates we show that in most cases early and middle pliocene climate both in equatorial regions and in the extratropics differ from present day climates with the same spatial pattern as that associated with enso for instance not only was canada warmer during early pliocene time than at present as it is during el nino but the region surrounding the gulf of mexico appears to have been cooler and a bit wetter as it commonly is during el nino a virtually permanent el nino like state appears to have characterized pre ice age climates suggesting that transport of heat by the atmosphere was the principal mechanism that maintained extratropical warmth accordingly cooling and the growth of recurring ice sheets in the northern hemisphere resulted from the development of a strong walker circulation and a weakening of the hadley circulation

drainage reorganization during mountain building in the river system of the eastern cordillera of the colombian andes the eastern cordillera of colombia is a thick skinned thrust fold belt that is characterized by two topographic domains 1 the axial zone a high altitude plateau the sabana de bogota 2500 masl with low local relief and dominated by longitudinal rivers and 2 the cordillera flanks where local relief exceeds 1000 m and transverse rivers dominate on the basis of an analysis of digital topography and river parameters combined with a review of paleodrainage data we show that the accumulation of shortening and crustal thickening during the andean orogeny triggered a process of fluvial reorganization in the cordillera owing to a progressive increase of the regional slope the drainage network evolves from longitudinal to transverse dominated a process that is still active at present this study provides the idea of progressive divide migration toward the inner part of the mountain belt by which the area of the sabana de bogota plateau is decreasing the flanks increase in area and ultimately transverse rivers will probably dominate the drainage of the cordillera

climatic forcing of asymmetric orogenic evolution in the eastern cordillera of colombia new apatite fission track data paleoelevation estimates from paleobotany and recently acquired geological data from the eastern cordillera of colombia document the onset of increased exhumation rates in the northeastern andes at ca 3 ma the eastern cordillera forms an efficient orographic barrier that intercepts moisture laden winds sourced in the amazon lowlands leading to high rainfall and erosion gradients across the eastern flank of the range in contrast the drier leeward western flank is characterized by lower rates of deformation and exhumation in light of the geological evolution of the eastern cordillera the combination of these data sets suggests that the orographic barrier reached a critical elevation between ca 6 and ca 3 ma which ultimately led to protracted yet more focused erosion along the eastern flank sequentially restored structural cross sections across the eastern flank of the eastern cordillera indicate that shortening rates also have increased during the past 3 ma from fission track and structural cross section balancing we infer that accelerated exhumation led to increasing tectonic rates on the eastern flank creating a pronounced topographic and structural asymmetry in the eastern cordillera the tectonic and climatic evolution of this orogen thus makes it a prime example of the importance of climatic forcing on tectonic processes

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

latitudinal variation of denudation in the evolution of the bolivian andes latitudinal gradients in topography relief climate and deformation have been used to suggest that climate driven erosion has exerted a first order control on the development of the central andes we synthesize the spatial and temporal variations in denudation across the eastern bolivian andes 14 22 s from new and existing estimates to test whether physical evidence exists to support the hypothesis that erosion influences thrust belt evolution basin morphometry channel network indices climate and longitudinal river profiles indicate a northward increase in relative relief fluvial incision and denudation short term denudation rate averages from landslide mapping and sediment flux data range from 1 to 9 mm yr in the north compared to 0 3 to 0 4 mm yr in the south long term denudation rate estimates from thermochronology cosmogenic radionuclides foreland basin sediment volumes stra tigraphy paleoerosion surface degradation and balanced cross sections range from 0 04 to 1 6 mm yr with rates up to more than twice as fast in the north when comparing estimates from the same method the shorter term denudation rates exhibit the greatest variance our denudation synthesis shows that an along strike disparity in denudation greater in the north has existed throughout the holocene and perhaps existed since as early as the late miocene our denudation synthesis also suggests that the disparity and denudation rates have increased to the present correlations between the thrust belt geology geometry geomorphology climate and kinematics of the orogenic wedge provides a case study in observing the regional scale interactions between uplift climate and erosion we conclude that the denudation history uplift history and tectonic geomorphic correlations suggest that models of the evolution of the bolivian andes should incorporate a latitudinal erosion gradient for the last 10 kyrs to perhaps 10 myrs

shortening structural relief and drainage evolution in inverted rifts insights from the atlas mountains the eastern cordillera of colombia and the pyrenees the atlas eastern cordillera and pyrenees are thick skinned thrust fold belts formed by tectonic inversion of rift basins in continental settings a comparison of shortening between them shows a gradation from 20 25 in the central high atlas to 25 30 in the eastern cordillera and c 40 in the pyrenees accordingly there is a structural variation from interior zones with low structural relief and isolated basement massifs in the first two cases to an axial culmination of stacked basement thrust sheets in the pyrenees this results in marked topographic and drainage variation the high atlas and eastern cordillera contain axial plateaus dominated by structure controlled longitudinal rivers and orogen flanks with slope controlled transverse rivers whereas the pyrenees show a two sided wedge profile dominated by transverse rivers in spite of singularities exhibited by each orogen we propose that this spatial variation can be understood as reflecting different degrees of evolution in mountain building rapidly incising transverse rivers are capturing earlier longitudinal streams of the atlas and eastern cordillera thus reducing their axial plateaux which will eventually disappear into a transverse dominated drainage this pattern of landscape evolution may be characteristic of inversion orogens as they develop from initial stages of inversion to full accretion

petroleum system modeling in the eastern cordillera of colombia using geochemistry and timing of thrusting and deformation in the present study stratigraphic data from cores and outcrop sections are integrated with data on thermal maturity organic facies and thermochronometric information to reconstruct the tectonic and associated petroleum system evolution of the eastern foothills thrust belt along the colombian eastern cordillera one of the most prolific hydrocarbon provinces in northern south america sedimentary and tectonic burial of the foreland autochthon caused maturation of the coniacian to santonian shallow marine chipaque formation resulting in successive and diachronous episodes of hydrocarbon migration and trapping one dimensional and two dimensional maturation modeling indicates that oil generation from the chipaque formation began at the paleocene eocene boundary 55 ma in the southern parts of what is now the eastern cordillera and progressed to the north by the late oligocene tectonic inversion of the eastern cordillera exhumed most of these kitchens terminating the oil generation from the chipaque formation kitchens migrated northward and eastward during the oligocene and early miocene because of the absence or subsequent erosion of traps it is likely that the southernmost source rocks expelled most of their oil without any appreciable accumulation our modeling indicates that there were two important kitchens during the cenozoic the larger of the two was located in the present day eastern cordillera and it was most productive in the late eocene early oligocene the second kitchen which generated oil throughout the neogene was located in the foredeep of the llanos basin adjacent to the mountain front considerable amounts of oil from this recent pulse have accumulated in both deep and shallow reservoirs along the eastern foothills the modeled reservoir charge history also explains the substantial biodegradation of oils in reservoirs that are today much too deep to support the process biodegradation must have occurred when the reservoirs were shallower and at cooler temperatures and they remained active until the reservoirs were buried to depths where temperatures were high enough to prevent further bacterial activity

geothermal regime and thermal history of the llanos basin colombia the llanos basin is a siliciclastic foreland sub andean sedimentary basin located in colombia between the cordillera oriental and the guyana precambrian shield data on bottom hole temperature lithology porosity and vitrinite reflectance from all 318 wells drilled in the central and southern parts of the basin were used to analyze its geothermal regime and thermal history average geothermal gradients in the llanos basin decrease generally with depth and westward toward the fold and thrust belt the geothermal regime is controlled by a moderate generally westward decreasing basement heat flow by depositional and compaction factors and in places by advection by formation waters compaction leads to increased thermal conductivity with depth whereas westward downdip flow in deep sandstone formations may exert a cooling effect in the central western part of the basin vitrinite reflectance variation with depth shows a major discontinuity at the pre cretaceous unconformity areally vitrinite reflectance increases southwestward in paleozoic strata and northwestward in post paleozoic strata these patterns indicate that the thermal history of the basin robably includes three thermal events that led to peaks in oil generation a paleozoic event in the southwest a failed cretaceous rifting event in the west and an early tertiary back arc event in the west rapid cooling since the last thermal event is possibly caused by subhorizontal subduction of cold oceanic lithospheric plate

assessing tectonic and climatic causal mechanisms in foreland basin stratal architecture insights from the alborz mountains northern iran the southern foreland basin of the alborz mountains of northern iran is characterized by an approximately 7 3 km thick sequence of miocene sedimentary rocks constituting three basin wde coarsening upward units spanning a period of 106 years we assess available magnetostratigraphy paleoclimatic reconstructions stratal architecture records of depositional environments and sediment provenance data to characterize the relationships between tectonically generated accommodation space a and sediment supply s our analysis allows an inversion of the stratigraphy for particular forcing mechanisms documenting causal relationships and providing a basis to decipher the relative contributions of tectonics and climate inferred changes in precipitation in controlling sediment supply to the foreland basin specifically a s 1 typical of each basal unit 17 5 16 0 13 8 13 1 and 10 3 9 6 ma is associated with sharp facies retrogradation and reflects substantial tectonic subsidence within these time intervals arid climatic conditions changes in sediment provenance and accelerated exhumation in the orogen suggest that sediment supply was most likely driven by high uplift rates conversely a s 1 13 8 and 13 8 11 ma units 1 and 2 reflects facies progradation during a sharp decline in tectonic subsidence caused by localized intra basinal uplift during these time intervals climate continued to be arid and exhumation active suggesting that sediment supply was again controlled by tectonics a s 1 at 11 10 3 ma and 9 6 7 6 ma and possibly 6 2 top of units 2 and 3 is also associated with two episodes of extensive progradation but during wetter phases the first episode appears to have been linked to a pulse in sediment supply driven by an increase in precipitation the second episode reflects a balance between a climatically induced increase in sediment supply and a reduction of subsidence through the incorporation of the proximal foreland into the orogenic wedge this in turn caused an expansion of the catchment and a consequent further increase in sediment supply copyright 2013 john wiley sons ltd

controls on architecture of the late cretaceous to cenozoic southern middle magdalena valley basin colombia the maastrichtian cenozoic southern middle magdalena valley basin of colombia contains a unique record of unconformities strata and structure from which we extract the histories of exhumation of the central cordillera to the west and evolution of the eastern cordillera fold and thrust belt to the east this study integrates field based analyses of stratigraphy laboratory analyses of provenance fission track thermochronology vitrinite reflectance data volcanic ash geochronology and studies of synorogenic geometries and structure displayed in seismic data a major unconformity the middle magdalena valley unconformity formed by eastward migration of central cordillera uplift during late cretaceous to early eocene time which transformed a maastrichtian marine basin into a paleocene depositional piedmont area this transformation is recorded by a coarsening upward sequence of marine shales to alluvial fan conglomerates which was partly eroded during further early eocene propagation of central cordillera deformation cessation of this phase of uplift led to formation of a pediment surface the middle magdalena valley unconformity which was buried by westward onlapping middle eocene to neogene alluvial deposits middle eocene to neogene facies paleoflow and unconformities were controlled by eastern cordillera deformation in the eastern cordillera foot hills growth strata and thermal history reveal two phases of folding of middle eocene oligocene and late miocene ages prior to intense pliocene pleistocene uplift two unconformities of early late miocene and pliocene pleistocene ages occur to the west of the eastern cordillera and record flexural tilting related to episodes of eastern cordillera loading

thick skin orogen foreland interactions and their controlling factors northern andes of colombia the study draws from reflection seismic in situ stress and low temperature geochronology data on the eastern cordillera llanos foreland basin system of colombia which is an example of the retro wedge of the orogen with an advancing subduction zone the system was obliquely converging during deposition of the lower oligocene lower miocene carbonera to recent guayabo formations recording it by the northeastward depocentre shift in the proximal llanos basin while the southern portion of the foreland did not flex and undergo flexural normal faulting after carbonera deposition the northern portion did the earthquake data indicate that the northern eastern cordillera undergoes strengthening by internal deformation while the southern segment is already strong enough to undergo significant displacements dominantly along its bounding fault systems furthermore the southern segment initiated detachment of the first thick skin blocks from the proximal llanos basin such co existence of different convergence maturity stages along the orogen strike allows unravelling of its development starting with internal deformation and foreland flexure followed by mountain building and large scale boundary fault displacements to the large scale foreland deformation the onset of new foreland accretion seems to be controlled by the resistance of the foreland plate to flexing given that the orogenic engine continues to drive convergence

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

mechanics and erosion of basement cored uplift provinces 1 basement cored uplift provinces are often characterized by high angle reverse faulting along preexisting crustal heterogeneities which may greatly affect the mechanics of deformation and the coupling between erosion and orogenic structure herein we construct a coupled deformation erosion model to understand the mechanics and erosion of mountain belts in which the spatial distribution of deformation is largely influenced by the presence of preexisting high angle faults in this case deformation is accommodated along and topography is built above these structures this topographic loading leads to increasing lithostatic stresses beneath these regions as a result active deformation may migrate to frictionally stronger structures in adjacent regions where lithostatic loading is lower the migration of deformation to such nearby structures depends on the hubbert rubey pore fluid pressure ratio of the crust the orientations of the frictionally weaker and stronger preexisting faults 1 and 2 respectively the friction coefficients b1 and b2 and hubbert rubey fluid pressure ratios b1 and b2 of these faults and the surface slope of the topography above the frictionally weaker structure assuming zero surface slope above the frictionally stronger structure in general we found that for a given and 1 as b1 increases b1 b2 increases and 2 decreases the value of b2 required to force deformation to migrate increases however as erosional processes lead to decreasing surface slopes deformation will be inhibited from migrating to frictionally stronger structures in adjacent regions our model results may help to explain some aspects of the deformation observed and the possible correlation between precipitation and the migration of deformation within these tectonic provinces

initiation and evolution of gullies along the shoreline of lake huron over one hundred gullies are deeply incised into algonquin bluff along the eastern shoreline of lake huron aerial photographs between 1930 and 1992 were used to measure temporal changes in area and length of a sample of forty four gullies regression analysis shows that gully area and length have increased in a linear fashion and that gully initiation occurred in the nineteenth century gully initiation coincides with deforestation and settlement of the area surface drainage was redirected into drainage ditches concentrating the flow and enhancing gully erosion natural processes associated with climatic fluctuations are probably insufficient to cause gully initiation but may contribute to ongoing gully expansion

a scenario for late neogene andean shortening transfer in the camisea subandean zone peru 12 s implications for growth of the northern andean plateau precise knowledge of the timing of deformation in the subandean zone of the andean plateau is a prerequisite for deciphering the late neogene growth of the andean plateau in this paper we report new apatite fission track aft and vitrinite reflectance ro data for a regional balanced cross section of the camisea basin in the central peruvian subandean zone adjacent to the northern andean plateau the balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust related anticlines the balanced cross section restoration shows 53 km 39 of total horizontal shortening we sampled paleozoic to cenozoic sedimentary strata for aft and ro analyses along the 4 km thick vertical profile of the mainique back thrust passive roof thrust the innermost preserved subandean structure young components of aft ages are spread between ca 6 ma and ca 24 ma a break in the slope in the aft ages determines the geometry of the miocene partial annealing zone and the exhumation of the mainique back thrust at ca 6 ma sequential restoration calibrated by aft and ro data indicates that the last 23 km horizontal shortening were accommodated by the camisea thrust system over the past 6 m y giving a mean shortening rate of 3 8 mm yr using this shortening rate for the first 30 km horizontal shortening we calculate that the andean shortening transfer into the peruvian subandean zone initially started at ca 14 ma this result suggests that the transfer of shortening from the northern andean plateau to the subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca 10 ma and ca 7 ma we rather propose that the late neogene growth of the northern andean plateau mostly resulted from a continuous crustal shortening combined with lower crustal flow

cenozoic contractional reactivation of mesozoic extensional structures in the eastern cordillera of colombia 1 the eastern cordillera of colombia is key to understanding the role of inherited basement anisotropies in the evolution of active noncollisional mountain belts in particular the rio blanco guatiquia region of the eastern cordillera is exemplary in displaying a variety of phenomena that document the importance of the orientation geometry and segmentation of preorogenic anisotropies we document the first unambiguous evidence that extensional basement structures played an important role in determining the locus of deformation during contractional reactivation in the eastern cordillera detailed structural field mapping and analysis of industry seismic reflection profiles have helped to identify the inherited san juanito naranjal and servita normal faults and associated transfer faults as important structures that were inverted during the cenozoic andean orogeny apparently the more internal faults in the former rift basin were not properly oriented for an efficient reactivation in contraction however these faults have a fundamental role as strain risers as folding is concentrated west of them in contrast reactivated normal faults such as the more external servita fault are responsible for uplifting the eastern flank of the eastern cordillera in addition these structures are adjacent and intimately linked to the development of thin skinned faults farther east in part the superimposed compression in this prestrained extensional region is compensated by lateral escape the dominant presence of basement involved buckling and thrusting and the restricted development of thin skinned thrusting in this inversion orogen makes the eastern cordillera a close analog to the intraplate atlas mountains of morocco and other inverted sectors of the andean orogen farther south

atlantic tropical systems of 1992 abstract a total of 69 tropical waves also known as african and easterly waves were counted in the atlantic basin during the 1992 hurricane season as was the case in 1991 the waves were in general relatively weak these waves led to the formation of only four tropical depressions in the atlantic hurricane basin of which one intensified into a tropical storm and another intensified into hurricane andrew andrew was the only 1992 atlantic hurricane to originate from a tropical wave there were five additional tropical depressions that were primarily initiated by systems of nontropical origin these produced three hurricanes and one tropical storm it appears that tropical waves led to the formation of practically all of the eastern pacific tropical cyclones in 1992

paleostress evolution of the northern andes eastern cordillera of colombia implications on plate kinematics of the south caribbean region 1 new tectonic field data and paleostress determinations in the eastern cordillera of colombia enable us to identify stress regimes that prevailed in the northern andes since late cretaceous times until present day these regimes were characterized by an e w to wsw ene active contraction from late cretaceous to late paleocene this direction subsequently changed to nw se and finally became wnw ese during the andean tectonic phase the characteristics and relative chronology of these regimes were deduced from field structures and stratigraphic criteria using stress inversion of fault slip data sets to evaluate our model we used the rotation poles of the south america caribbean and nazca plates relative to north america and paleogeographic reconstructions of the caribbean region to derive a kinematic model of the northern andes assuming a general correspondence between the regional convergence directions and the regional compression we built a theoretical model of the regional stress regimes based on kinematics of the northwestern south america since late cretaceous times this model reveals a major variation at the end of the paleocene when relative divergence shifted to convergence between the americas this is in agreement with our tectonic model based on independent tectonic field data inversion in the eastern cordillera this regional kinematic and tectonic change which affected the caribbean at the end of the paleocene was responsible for and consistent with the major change in stress patterns and structural style in the eastern cordillera it is also coeval with the development of a major regional unconformity in colombia

middle to late miocene middle eastern climate from stable oxygen and carbon isotope data southern alborz mountains n iran abstract the alborz mountains of northern iran intercept and divert the northern hemisphere westerlies carrying moisture from the mediterranean and black sea and form an orographic barrier to moisture sourced to the north in the caspian sea this implies that terrestrial deposits along the southern alborz mountains leeward side of northerly winds and windward side of westerlies with the respect to tibet and other mountainous terrain in the himalayan karakoram realm potentially track changes in past moisture and erosional regimes and mirror rainfall patterns here we present results of a stable isotope analysis and clay mineral study of the miocene ca 17 5 7 6 ma upper red formation in the foreland of the southern alborz mountains the changes recorded by stable oxygen and carbon isotope data from pedogenic and lacustrine palustrine carbonate in the southern alborz mountains suggest 1 an increase in aridity possibly related to the evolution of the alborz orographic rain shadow which became more efficient between 17 5 and 13 2 ma 2 a steady increase in precipitation between 13 2 and 10 3 ma with a significant increase in rainout along the southern slope of the alborz mountains between 11 and 10 3 ma possibly related to perturbations in atmospheric circulation pattern in the northern hemisphere as suggested by coeval wetter phases in southern europe and 3 a decrease in aridity from ca 9 6 to 7 6 ma possibly reflecting an increase in the seasonality of precipitation based on environmental and climatic changes observed across southern asia and india starting from ca 10 ma we speculate that the topographic evolution of the himalayan tibetan system might have affected the late miocene climate in middle east

processes of oscillatory basin filling and excavation in a tectonically active orogen quebrada del toro basin nw argentina intramontane basins may act as important sediment storage areas serve as recorders of the history of deformation record syntectonic deposition and document the evolution of climatic conditions during deposition we document the timing cyclicity and processes that led to the filling and reexcavation of the intramontane quebrada del toro basin in nw argentina geomorphic and geologic observations indicate that the basin was filled with sediment that has been subsequently excavated at least two times in the last 8 m y the last filling and excavation cycle occurred within the last 0 98 m y and has led to the deposition and removal of 61 4 km 3 of material from the basin leading to a basin wide averaged minimum denudation rate of 0 16 mm yr aggradation within the basin took place due to channel steepening of the downstream fluvial system that connects the intramontane basin to the foreland this portion of the fluvial system is actively incising through an uplifting bedrock zone we use observations within the toro to test a quasi physically based model of channel aggradation behind a rising base level that rises due to downstream channel steepening our work shows that the bedrock incision rate constant required to reproduce conditions observed within the toro basin is consistent with values measured independently in similar rock types therefore in intramontane basins that experience similar processes of filling and evacuation this model may be used to assess the relative importance of tectonic rock uplift bedrock resistance to fluvial incision and climate in determining the geomorphic and sedimentologic history of these basins

tectonic control on sedimentary facies pattern and sediment accumulation rates in the miocene foreland basin of the southern alborz mountains northern iran 1 the southern alborz mountains of northern iran are an integral part of the arabia eurasia collision zone a magnetostratigraphic and rock magnetic investigation of the eyvanekey stratigraphic section in the southern alborz mountains reveals the spatiotemporal character of sedimentary facies migration in the alborz foreland basin the section constitutes three coarsening upward units units 1 2 and 3 comprising the upper red and hezardarreh formations our data reveal that the upper red formation was deposited between 17 5 and 7 5 ma while the depositional age of the top of the hezardarreh formation can be extrapolated to 6 2 ma slow sediment accumulation rates correlate with sedimentary facies comprising prograding coarsening upward units this is likely the result of intraforeland uplift units 1 and 2 and basin inversion probably associated with a growth syncline located in the proximal foreland unit 3 in contrast fine grained strata at the bottom of each cycle are associated with faster sediment accumulation rates testifying to enhanced flexural basin subsidence in the course of thrust loading progradation of coarse grained facies also occurred during relatively fast sediment accumulation top of unit 2 suggesting that the influx of coarse grained sediment outpaced the storage capacity of the proximal foreland thus despite an overall southward propagation of deformation into the southern alborz foreland the locus of active deformation must have migrated back and forth on a time scale of circa 0 7 to 2 ma

late cenozoic increase in accumulation rates of terrestrial sediment how might climate change have affected erosion rates abstract accumulation rates of terrestrial sediment have increased in the past few million years both on and adjacent to continents although not everywhere apparently erosion has increased in elevated terrain regardless of when last tectonically active or what the present day climate in many regions sediment coarsened abruptly in late pliocene time sparser data suggest increased sedimentation rates at 15 ma approximately when oxygen isotopes in benthic foraminifera imply high latitude cooling if climate change effected accelerated erosion understanding how it did so remains the challenge some obvious candidates such as lowered sea level leading to erosion of continental shelves or increased glaciation account for increased sedimentation in some but not all areas perhaps stable climates that varied slowly allowed geomorphic processes to maintain a state of equilibrium with little erosion until 3 4 ma when large oscillations in climate with periods of 20 000 40 000 years developed and de

cenozoic paleogeography of the andean foreland and retroarc hinterland of colombia new biostratigraphic zonations core descriptions sandstone petrography facies analysis and seismic information are compared with published detrital and bedrock geo and thermochronology to build a cenozoic paleogeographic reconstruction of the andean retroarc region of colombia encompassing the ancestral central cordillera middle magdalena valley eastern cordillera and llanos basin we identify uplifted sediment source areas provenance domains depositional environments and thickness changes to propose a refined paleogeographic evolution of eastern colombia we conclude that cenozoic evolution of the northernmost andes includes 1 a period of contractional deformation focused in the central cordillera and middle magdalena valley that may have started by the late cretaceous although thermochronological data points to maximum shortening and exhumation during the late paleocene 2 a period of slower deformation rates or even tectonic quiescence during the middle eocene and 3 a renewed phase of contractional deformation from the late eocene to the pleistocene holocene expressed in provenance bedrock thermochronology and increased subsidence rates in the llanos foreland the sedimentary response in the llanos foreland basin is controlled by source area proximity exhumation and shortening rates relationships between accommodation and sediment supply as well as potential paleoclimate forcing this new reconstruction changes the picture of cenozoic basin evolution offered by previous reconstructions providing an updated chronology of deformation which is tied to a more precise understanding of basin evolution

stable isotope paleoaltimetry a critical review stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic in situ formed minerals to illustrate the usefulness as well as potential limitations of this method we review 1 processes controlling the isotopic composition of modern precipitation 2 stable isotope data from modern precipitation across regions of high topography and 3 stable isotope data from authigenic minerals that have been used to infer paleotopography from this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography with uncertainties that critically depend on a detailed understanding of local to global scale paleoclimate during the time interval of interest

basement balancing of rocky mountain foreland uplifts restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts local zones of shortening and extension occur in hinge zones above transitions in fault curvature high curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover 20 references 4 figures

water flow oil biodegradation and hydrodynamic traps in the llanos basin colombia in this study we provide new data to understand the groundwater flow patterns in the llanos basin and their impact on oil biodegradation and the geothermal regimes as well as how the structural styles and anthropogenic activities impact these patterns previous studies suggest an active flow of groundwater and variable salinities whose spatial pattern is apparently unrelated to topographically driven groundwater flow these observations have led to different hypotheses regarding the influence of groundwater flow on llanos basin geothermal gradients and oil biodegradation in this contribution we present data regarding the hydraulic heads salinities geothermal gradients and structural styles of the llanos basin to propose hypotheses explaining these observations structural cross sections and subsurface stratigraphic correlations allow us to suggest that the pattern of flow is best explained by a correlation between groundwater flow and structural styles a basement map of the llanos basin confirms that the most important factor controlling geothermal gradients is the type of basement whereas the factor of groundwater flow appears to be of secondary importance the evolution of the basin and the frequent absence of correlation between fresh water and the more biodegraded oils support the interpretation that biodegradation is controlled by an older flow of water that started as early as the oligocene finally mass balances suggest that the temporal scales and volumes of groundwater flow are much larger than the scales observed during the development of the oil fields

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

spatial controls on erosion in the three rivers region southeastern tibet and southwestern china abstract global data suggest that erosion rates variously scale with steepness or climate forcing precipitation or glacial excavation but the relative influence of these factors has proven difficult to assess without comparisons from a single location a new suite of detrital 10be data from the three rivers region se tibet is used to examine the relative importance of rainfall and relief in predicting patterns of erosion rates across a region with a strong gradient in exhumation the data reveal millennial erosion rates vary by two orders of magnitude from 0 01 to 8 mm yr across a regional gradient in exhumation rates inferred from previous thermochronology and cosmogenic nuclide data to the west and east of the study region the new millennial erosion rates mirror the pattern of decreasing exhumation rates from west to east across the region with the highest rates in the lower salween river drainage and the lowest rates in the yangtze river drainage erosion rates in the mekong and salween river drainages are correlated with mean local relief whereas in the yangtze river drainage they are correlated most strongly with mean annual rainfall the tectonic setting of this region with a strong west to east gradient in exhumation rates which we infer to mirror a gradient in rock uplift seems to exert a stronger control on erosion rate patterns than rainfall or relief

absolute chronology of the pliocene quaternary sediment sequence of the bogota area colombia abstract a revised geochronological framework is presented for the sequence of unconsolidated sediments present in the bogota area eastern cordillera colombia this is based on 11 fission track dates on zircons that were obtained both from exposed ash layers and from a series of ashes from the funza ii borehole which reached 586 m below the surface of the high plain the 3 dates obtained from the exposed ash layers provide control for the older part of the sequence 6 2 5 ma the 8 dates from the funza ii core give control for the younger part of the sequence 3 0 ma acceptance of these new fission track dates on volcanic zircons means that many of the tephra dates obtained from the funza i core that included fission track dates on glass shards and k ar dates using the mineral fractions in the ashes published in 1984 are rejected it also confirms earlier climate stratigraphical dating the neogene quaternary sediments of the bogota area span a period of at least the last 6 ma the fluvial lacustrine sediment record registers major tectonic uplift of the eastern cordillera for the period between 5 and 3 ma the development of the large sedimentary basin of bogota after 3 5 ma with an important phase of tectonic adjustment at about 1 ma and a long period of strong climatic fluctuations that started shortly after 2 7 ma

regional provenance from southwestern colombia fore arc and intra arc basins implications for middle to late miocene orogeny in the northern andes andean orogenic processes controlled the spatial and temporal distribution of the magmatic and sedimentary record this contribution integrates new u pb zircon ages heavy mineral analyses and biostratigraphic constraints from the neogene sedimentary record of the fore arc and intra arc basins and volcano plutonic rocks of southwestern colombia to reconstruct these orogenic processes the results reveal continuous arc magmatism since the late oligocene with a major post middle miocene magmatic peak and exhumation when integrated with other geological constraints the tectonic evolution of the margin includes eocene oligocene oblique convergence with limited magmatic activity followed by the initiation of a late oligocene early miocene arc that migrated to the east in the middle miocene when it experienced a major increase in magmatic activity crustal deformation exhumation and thickening this orogenic evolution is related to the shallowing of the slab dip due to the subduction of the neogene nazca plate

geometry and kinematic evolution of inversion structures positive inversion structures form by the compressional reactivation of preexisting extensional structures experimental models and observations of natural structures are used to develop quantitative models for the geometry and kinematic evolution of inversion structures in this paper i analyze two main formation mechanisms of inversion structures 1 fault propagation folding on planar faults and 2 fault bend folding on listric faults inversion structures formed by fault propagation folding occur in the southern north sea the central montana platform and the kangean basin during extension a broad fault propagation or drape fold develops above the master fault with the fault subsequently breaking through the fold synextensional growth units deposited in the hanging wall typically thicken into the basin compressional reactivation results in slip reversal on the master and secondary faults their rotation to shallower dips and the development of a compressional fault propagation fold inversion structures formed by fault bend folding on listric faults occur in the taranaki basin and possibly in the southern north sea rollover folding in the hanging wall occurs during extension possibly accompanied by a small component of fault propagation folding in the vicinity of the fault tip deformation is primarily localized along a system of antithetic faults synextensional growth sediments typically thicken into the fault but also show thinning in the immediate vicinity of the fault during compression the extensional fold is first unfolded and then folded into a compressional fault bend fold the characteristic variations in bed geometry and thickness provide predictive models for interpreting the subsurface geometries of these two classes of inversion structures in areas with poor seismic data these models are particularly useful in exploring for structural traps in the complex and relatively unexplored synextensional growth units

thrust geometry controlled by erosion and sedimentation a view from analogue models analogue sandbox models have been set up to study thrust systems formed above two decollements and affected by erosion and sedimentation the deformation was imaged by x ray scanner three erosion sedimentation conditions were tested for all the experiments a duplex develops between the two decollement levels for the experiment without erosion and without sedimentation ramp anticlines form in the upper series for the model with syntectonic sedimentation and without erosion the shortening in the upper series is first characterized by fault related folds and then by a forward propagation of the decollement that induces the development of a piggy back basin for the experiment with erosion and sedimentation a passive roof duplex is formed relationships between structural style and syntectonic superficial conditions in classical thin skinned thrust belts are similar to those found in the analogue models the ramp anticlines of the jura developed with low erosion and low sedimentation and the large sub andean thrust belt is controlled by syntectonic sedimentation whereas the passive roof duplex of pakistan is synchronized with intense erosion this study outlines the crucial effect of the superficial mass transport phenomena in the tectonic style of thrust belts

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

steady state erosion of critical coulomb wedges with applications to taiwan and the himalaya 1 orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion respectively we developed a coupled erosion deformation model for orogenic wedges that are in erosional steady state and deform at their coulomb failure limit erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock we found that the ultimate form of a wedge is controlled by the wedge mechanical properties sole out depth of the basal decollement erosional exponents basin geometry and the ratio of the added material flux to the erosional constant as this latter ratio is increased wedge width and surface slopes increase we applied these models to the taiwan and himalayan orogenic wedges and found that despite a higher flux of material entering the former the inferred ratio was larger for the latter calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the himalaya relative to taiwan these values are consistent with the lower precipitation regime in the himalaya relative to taiwan and the exposure of crystalline rocks within the himalayan orogenic wedge independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high grade metamorphic rocks exposed within taiwan and the himalaya respectively therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts

development of the colombian foreland basin system as a consequence of diachronous exhumation of the northern andes this study addresses multiple controls on foreland basin accommodation and contributes to enhanced understanding of the evolution of the northern andes the middle magdalena valley basin mmvb eastern cordillera and llanos basin are part of a late cretaceous cenozoic foreland basin system east of the colombian central cordillera mechanical modeling indicates that the primary control on complex distributions of sedimentary thicknesses facies and unconformities was lithospheric flexure in response to crustal loads from the central and eastern cordilleras shorter wavelength folding and paleoaltitude determined the local character of strata our mechanical modeling consists of the application of orogenic and sedimentary loads extracted from geologic data on a continuous elastic lithosphere the results validate two major basin configurations the first configuration was a maastrichtian early eocene foreland basin coupled with central cordillera uplift growth strata record continuous sedimentation in the eastern cordillera whereas regional unconformities in the llanos basin distal foreland basin reflect isostatic adjustments of the basin s amplitude and wavelength to central cordillera episodic uplift and tectonic quiescence the second major basin configuration was characterized by central cordillera erosion since middle eocene times recorded by a regional pediment surface in the absence of central cordillera effective loading loads from onlapping sediments and eastern cordillera piggyback sub basins provoked post middle eocene accommodation in the mmvb and llanos basin intensified eastern cordillera uplift during the neogene produced basinal tilting recorded by unconformities in the mmvb this study highlights the importance of assessing the causes of tectonic accommodation as a foundation for interpretation of the evolution of large foreland and intermontane basins