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

tem and xrd determination of crystallite size and lattice strain as a function of illite crystallinity in pelitic rocks average crystallite size and mean square strain of illite in rock specimens and clay separates were measured independently in tem images and by single line fourier voigt method profile analysis of the c 1 nm peak of xrd patterns for a prograde sequence of pelitic rocks illite crystallinity indices 0 17 0 58 d2h from the gaspepeninsula quebec the tem determined crystallite sizes in clay separates approxi mate those determined by fourier profile analyses and those calculated from illite crystallinity indices by the scherrer equation with the exception of the diagenetic sample the crystallite sizes and mean square strains of illite in rock samples exhibit a trend similar to that determined by profile analyses but the average crystallite sizes are up to five times larger than those measured for clay separates tem images show that all rock samples have a wide range of crystallite sizes and the proportions of larger crystallites increase with metamorphic grade the diagenetic illite is defect rich fine grained mean thickness by volume c 70 nm 1m d material anchizonal illite tends to occur as separate aggregates of small 1m d and larger 2m 1 crystals c 200 nm comprising arrays of subparallel coalescing packets the epizone sample has thick c 400 nm defect free crystals of muscovite occurring in stacks of parallel layers or subhedral crystals intergrown with large angle boundaries cleaved crystals that are free of intracrystalline layer terminations are dominant in clay separates of all samples having ranges of smaller sizes with volume average thicknesses of c 43 43 and 81 nm c 14 28 67 nm by the voigt method respectively for the three zones the results suggest that illite crystallinity indices do not provide a direct measure of a single microstruc tural state of illite in rocks although they yield consistent limits for average crystallite sizes for the anchizone 23 48 nm here therefore they serve as a general parameter of the degree of recrystallization on a relative basis in part because the contributions of all peak broadening variables mixed layering size and strain decrease regularly with prograde regional metamorphism of pelites the microstructural changes caused by rock disaggregation are probably a function of those variables as well the data collectively demonstrate a trend from metastable defect rich small crystals towards a stable assemblage of larger defect free crystals through dissolution of strained crystals and neocrystallization consistent with the ostwald step rule

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

structure of the eastern cordillera of colombia implications for trap styles and regional tectonics the northwest margin of south america has undergone multiple phases of compressional deformation beginning in the late cretaceous and culminating in the pliocene pleistocene the latest phases miocene to pleistocene of this deformation created the eastern cordillera of colombia compressional structures in the eastern cordillera formed traps that have petroleum exploration potential these are generally in the form of fault bend and fault propagation folds containing top sealed reservoir and mature source rocks trap delineation although difficult to achieve because of the poor seismic quality obtained in the region can be accomplished by supplementing the seismic data with geometric analysis of surficial structures the eastern cordillera is asymmetrical with the dominant tectonic transport direction towards the east west directed thrust faults are localized along the western flank of the eastern cordillera and are interpreted to be back thrusts splaying off a deep mid crustal detachment a regional retrodeformable cross section across the eastern cordillera shows that basement detached deformation of late miocene to pliocene age was followed by pliocene to holocene uplift of the cordillera on high angle basement involved reverse faults these two structural styles were superimposed on preexisting jurassic and lower cretaceous grabens whose normal faults were partially inverted during the later stages of compressional deformation a palinspastic restoration of this regional cross section indicates that approximately 150 km of shortening 40 shortening has occurred 105 km eastward directed towards the llanos basin and 45 km westward directed towards the middle magdalena basin this calculated shortening is very close to that calculated by summing microplate motions for the northwest corner of south america shortening in the sedimentary cover rocks exceeds that in the basement rocks by about three times this shortening can be regionally balanced with a gently dipping mid crustal detachment that extends beneath the middle magdalena basin and central cordillera and which roots in the benioff zone beneath the western cordillera the presence of a mid crustal detachment as postulated here provides a mechanism to transmit the shortening from the convergent plate margin in the pacific to the andean foreland and is interpreted to have provided the structural link between the three ranges of the colombian andes the detachment also provides a mechanism to explain the imbalance of shortening seen between eastern cordillera basement and the overlying strata and implies that the western and central colombian andes have been tectonically transported eastwards a distance of at least 150 km

isostasy dynamic topography and the elevation of the apennines of italy abstract the elevation of an orogenic belt is commonly related to crustal lithosphere thickening here we discuss the apennines as an example to show that topography at a plate margin may be controlled not only by isostatic adjustment but also by dynamic mantle driven processes using recent structural constraints for the crust and mantle we find that the expected crustal isostatic component explains only a fraction of the topography of the belt indicating positive residual topography in the central apennines and negative residual topography in the northern apennines and calabria the trend of the residual topography matches the mantle flow induced dynamic topography estimated from regional tomography models we infer that a large fraction of the apennines topography is related to mantle dynamics producing relative upwellings in the central apennines and downwellings in the northern apennines and calabria where subduction is still ongoing comparison between geodetic and geological data on vertical motions indicates that this dynamic process started in the early pleistocene and the resulting uplift appears related to the formation and enlargement of a slab window below the central apennines the case of the apennines shows that at convergent margins the elevation of a mountain belt may be significantly different from that predicted solely by crustal isostasy and that a large fraction of the elevation and its rate of change are dynamically controlled by mantle convection

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

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

petrochemistry of igneous rocks of the california vetas mining district santander colombia implications for northern andean tectonics and porphyry cu mo au metallogeny abstract porphyry mo and cu mineralization in the california vetas mining district is contemporaneous with 10 9 to 8 4 ma granodiorite porphyry stocks and overprinted by au ag mineralization of epithermal affinity mineralization is hosted by grenvillian aged paragneisses bucaramanga gneiss of the santander massif and late triassic to early jurassic granitic rocks all intrusive rocks are high k calc alkaline late triassic to early jurassic rocks include peraluminous granites with more than 70 wt sio2 as well as metaluminous diorites tonalites and granodiorites with sio2 between 54 9 and 60 4 wt late miocene rocks are weakly peraluminous granodiorite porphyries with sio2 between 61 and 67 wt sio2 late miocene rocks share some characteristics with adakite like rocks which are widely associated with porphyry and epithermal style mineralization elsewhere in the andes they have high ba 930 to 1500 ppm and high ba la 28 to 50 high sr 850 to 1100 ppm and sr y 48 78 and depleted middle rare earth elements mree compared to the mesozoic granites which have 400 to 700 ppm ba ba la 14 to 25 and 80 to 150 ppm sr sr y 2 5 to 14 and mesozoic diorites and tonalites which have 900 to 1200 ppm ba ba la 20 to 32 and 610 to 750 ppm sr sr y 22 to 25 miocene granodiorite porphyries in contrast to mesozoic intrusive rocks have only weak negative eu anomalies the miocene rocks have 87sr 86sr ratios of 0 7052 to 0 7067 and end of 1 9 to 5 4 and are significantly more isotopically primitive than all other rocks in the study area including the mesozoic diorites to tonalites 87sr 86sr 0 7082 and 0 7092 end 6 7 and 7 2 granites 87sr 86sr 0 730 n 2 end 8 2 and 8 3 and bucaramanga gneiss 0 718 to 0 743 end 10 8 to 14 1 lead isotope data are broadly consistent with the sr and nd isotope data and the miocene porphyries have the lowest 207pb 204pb ratios but overlap with the mesozoic diorites to tonalites in their 206pb 204pb and 208pb 204pb ratios the geochemical signature suggests that late miocene granodiorite porphyries represent oxidized and water rich melts however in contrast to many other porphyry districts in the andes they do not have low y

wisconsin glaciation of the sierra nevada 79 000 15 000 yr b p as recorded by rock flour in sediments of owens lake california chemical analyses of the clay sized fractions of 564 continuous sediment samples 200 yr resolution from composite core ol90 92 allow quantification of an abundance of glacial rock flour rock flour produced during glacier advances is represented by clay sized plagioclase k feldspar and biotite in homogeneous internal composition the abundance of rock flour is deemed proportional to the intensity of glacies advances age control for the composite section is provided by combining previously published radiocarbon dates on organics u th dates on ostracode shells and u th dates on saline minerals from nearby searles lake correlated to ol92 by pollen the rock flour record displays three levels of variability 1 a dominant one of about 20 000 yr related to summer insolation and precipitation 2 an intermediate one of 3000 5000 yr perhaps related to north atlantic heinrich events and 3 a minor one of 1000 2000 yr perhaps related to north atlantic thermohaline driven air temperature variation

an integrated analysis of an orogen sedimentary basin pair latest cretaceous cenozoic evolution of the linked eastern cordillera orogen and the llanos foreland basin of colombia the integration of restored basin geometry and internal features of syntectonic units e g stratal architecture thickness sandstone composition with fl exural modeling of the lithosphere constrains the evolution of a basin and its fl exural history related to orogenic growth spatial temporal loading confi guration using this approach we determined the maastrichtian cenozoic polyphase growth of the eastern cordillera of colombia an inverted mesozoic extensional basin the record of this growth occurs in an andean post middle miocene thrust belt the eastern cordillera and in adjacent foreland basins such as the llanos basin to the east this approach permitted the identifi cation of fitectono stratigraphic sequences in the foreland basin and fi ve phases of shortening for the eastern cordillera thermochronological and geochronological data support the spatial and temporal evolution of the orogen foreland basin pair tectono stratigraphic sequences were identifi ed in two restored cross sections one located at a salient and the other in a recess on the eastern fl ank of the eastern cordillera the lower two sequences correspond to late maastrichtian paleocene fl exural events and record the eastward migration of both tectonic loading and depositional zero in the llanos basin these sequences consist of amalgamated quartzarenites that abruptly grade upward to organic rich fi ne grained beds and to the top light colored mudstones interbedded with litharenites in isolated channels amalgamated conglomeratic quartzose sandstones of the third sequence record 15 m y of slow subsidence in the llanos basin and llanos foothills during early to middle eocene time while shortening was taking place farther west in the magdalena valley the fourth sequence of late eocene middle miocene age records a new episode of eastward migration of tectonic loads and depositional zero in the llanos basin this sequence begins with deposition of thick fi ne grained strata to the west whereas to the east in the llanos basin amalgamated quartzarenites unconformably overlie cretaceous and older rocks former forebulge apatite fi ssion tracks in the axial zone of the eastern cordillera growth strata in the

dynamic versus flexural controls of late cretaceous western interior basin usa the united states cretaceous western interior basin has long been considered a foreland basin driven by the sevier thrust and associated basin sediment loads however flexural studies demonstrate that this effect exists only within a narrow band in front of the thrust belt most of the basin appears to be due to mantle flow induced dynamic subsidence associated with farallon plate subduction here we show how the components of evolving long wavelength dynamic subsidence and flexural subsidence created the accommodation space and controlled the stratigraphy across the western united states based on a correlated stratigraphic section across central utah and colorado these backstripped subsidence data reveal the dynamic topography driven nature of the western interior basin the results seem to support that the depocenters track the trough of dynamic subsidence with ca 18 myr cycles through time and space and the stratigraphic patterns of large scale progradation eastward migration of depocenter and regional clinoform like downlap are related with the dynamic subsidence interpretation of these data also provides more insights into the repeated ca 2 to 6 myr cycles of thrust induced subsidence in front of the thrust belt which control the local eastward progradation of the sand bodies from the thrust belt the dynamic flexural subsidence and eustatic sea level changes interacted and controlled the timing and distribution of unconformities our work shows how the stratigraphy precisely records the timing patterns and position of dynamic versus flexural subsidences and that combination of such data leads to important geophysical discoveries and supplies strict constraints for geodynamic modeling

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

campanian miocene tectonostratigraphy depocenter evolution and basin development of colombia and western venezuela abstract the position of the central axis of deposition over colombian and venezuelan continental crust has varied markedly through time the axis migrated from west to east from late cretaceous to oligocene but at times secondary drainage divides were established by local uplift events in oligocene times with initial inversion of the eastern cordillera the central axis of deposition was divided into two main axes the proto magdalena and the proto orinoco systems the west to east migration of the central axis of deposition had a tectonic origin and it occurred in combination with tectonically driven changes in accommodation space depocenter evolution can be divided as follows 1 the axis of the campanian and early maastrichtian depocenter was located few km east of the present position of the central cordillera of colombia it migrated east with gradual uplift of the central cordillera 2 the central axis of late maastrichtian deposition is positioned approximately over the present day western foothills of the eastern cordillera possibly crosses the eastern cordillera over the santander massif and continues into the maracaibo lake in western venezuela accommodation space decreased from campanian to maastrichtian times in cretaceous tertiary boundary times parts of the eastern margin of the eastern cordillera were uplifted by an initial phase of inversion of deeply rooted jurassic and early cretaceous normal faults 3 in paleocene times the central axis of deposition was located along the spine of the eastern cordillera and extended into the maracaibo basin and accommodation space continued to decrease 4 in latest paleocene times the central axis of deposition shifted to eastern regions of the eastern cordillera and accommodation space decreased 5 the early eocene central axis of deposition was located along the present day eastern foothills of the eastern cordillera accommodation space continued to decrease and the regional middle eocene unconformity began to develop in middle eocene times a regional unconformity developed marking the climax of the pre andean orogeny deposition during these times was dominant in the maracaibo basin area where large amounts of sediment derived from vast exposed areas accumulated 6 the late eocene central axis of deposition was confined to the present position of the llanos foothills late eocene deposition reflects a regional increase in accommodation space in oligocene times the initial uplift of the eastern cordillera divided the main depocenter into two central axes accommodation space diminished in uplifted regions but continued to increase in the depocenters allowing sporadic marine ingressions into the present position of the llanos foothills as uplift of the eastern cordillera continued the eastern depocenter axis proto orinoco migrated east and the western depocenter axis proto magdalena migrated west this process continued through the rest of the cenozoic

chlorite illite muscovite interlayered and interstratified crystals a tem stem study phyllosilicates in rocks which are transitional from mudstone to slate from lehigh gap pa have been studied by a variety of techniques including high resolution transmission electron microscopy and analytical electron microscopy the principal minerals are white mica which is transitional from illite in mudstone to ordered twolayer mica in slate and chlorite 7a berthierine occurs more rarely dioctahedral and trioctahedral layers are shown to be interleaved in individual crystals at all scales between the following two end members 1 both random and regular 1 1 interlayering at the scale of individual layers as shown in part by lattice fringe images 2 packets of trioctahedral and dioctahedral layers up to a few thousand angstroms or microns in thickness detectable with ordinary optical techniques the complete range of intermediate structures is represented in samples which are in transition to slate bulk analytical empa x ray diffraction or other measurements are shown to result in averages over both kinds of layers when tem techniques are not used

generation and migration of hydrocarbons in the maracaibo basin venezuela an integrated basin study abstract the history of hydrocarbon generation and migration in the maracaibo basin venezuela is closely related to its stratigraphic structural and tectonic evolution during the cretaceous and cenozoic times geochemical evaluation of the possible source rocks within the cretaceous and the tertiary sequences indicates that the argillaceous limestones and calcareous shales of the la luna formation cenomanian coniacian are the most important source rocks in the basin these rocks contain organic carbon ranging from 1 5 to 9 6 c original toc 2 5 10 8 have fluorescent amorphous marine type ii organic matter oil prone and have in the immature stage a h index of 700 mg hc g toc and a h c ratio of 1 5 the beginning of the oil window in these rocks occurs at a relatively low thermal maturity lopatin tti about 10 and the peak of oil generation corresponds to a tti value of 35 geochemistry of oils from the different reservoirs cretaceous paleocene eocene and miocene beds and oil oil and oil source rock correlation studies based on biomarkers have identified three genetic types of oil in the basin a marine type essentially derived from the la luna source a terrestrial type possibly originating either from the cretaceous lisure shales or the paleocene shales and a third type derived from the mixing of the first two types the marine type oil is distributed extensively throughout the basin whereas the terrestrial as well as the mixed marine and terrestrial types are found only in the southwestern part of the basin the marine type oils have been further subdivided based on their alteration and maturity reconstruction of the hydrocarbon generating areas at different geological times using tti maturity diagrams indicates that the generation of oil and gas in the basin took place during two periods the first phase occured during the middle to upper eocene times in the northeastern part of the basin while the second phase occured during the upper miocene to recent times in the central western and southern parts of the basin based on organic geochemical and sedimentological characteristics it is proposed that primary migration of oil from the la luna source rocks took place by expulsion of hydrocarbons through abundant microfractures developed by the pore fluid overpressure during hydrocarbon generation mass balance considerations suggest that the expulsion probably occurred as a hydrocarbon single phase and the expulsion efficiency for oil in la luna source rocks should have been very high 30 50 taking into consideration the development of oil and gas kitchens through time the paleostructures at the time of hydrocarbon generation and the characteristics of oils the migration history of the oil and gas generated from the la luna source has been outlined mixing of oils derived at different times eocene and miocene recent from the la luna source of different areas appeared to have taken place in block iv block 11 lago lamar motatan and bolivar coastal fields

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

seismicity of the caribbean nazca boundary constraints on microplate tectonics of the panama region there are at least three kinds of models that have been used to explain plate motions and boundaries in the panama region these models suggest that the major caribbean nazca boundary is 1 a thrust fault along which the panama deformed belt has developed 2 a left lateral strike slip fault along the southern margin of panama or 3 a diffuse array of mostly thrust faults which cause internal deformation to occur throughout panama in this paper we use joint epicenter location methods to obtain improved locations for earthquakes that occurred in this region between 1964 and 1985 and we use waveform modeling to obtain more precise depths for five events hypocenters of 13 events beneath the panama deformed belt suggest that overthrusting of panama upon the caribbean plate causes active deformation which supports model 1 but does not rule out models 2 and 3 the nature of seismic faulting along the panama deformed belt is complex and supports the premise that slow north south convergence loads the cold thick caribbean lithosphere and causes it to fail seismically epicenters of 64 events along the panama fracture zone indicate that this north south trending rightlateral transform fault is splayed north of 6 n overall the seismicity throughout suggests that the panama arc can be treated as a microplate in accordance with models 1 or 2 similarly a possible second microplate may exist south of panama but if so it does not affect regional plate motions significantly the strongest evidence for model 3 occurs in the easternmost and westernmost portions of panama where the diffuse seismicity prevents us from resolving specific microplate triple junction boundaries

basin development and tectonic history of the llanos basin eastern cordillera and middle magdalena valley colombia the middle magdalena valley eastern cordillera and llanos basin constituted a major regional sedimentary basin from the triassic to the middle miocene basin development began during the triassic to the earliest cretaceous with a synrift megasequence related to the separation of north and south america in the proto caribbean the synrift megasequence began with deposition in a continental environment that became paralic and shallow marine in the early cretaceous basin development continued into the cretaceous in a back arc setting east of the andean subduction zone the back arc megasequence was dominated by shallow marine sedimentation and produced an excellent regional source rock during the turonian coniacian marine deposition was abruptly terminated during the ear y maastrichtian due to the final accretion of the western cordillera accretion of the western cordillera created the early pre andean foreland basin megasequence of late maastrichtian to early eocene age this depositional episode consists of coal rich alluvial plain coastal plain and estuarine deposits throughout the middle magdalena valley eastern cordillera and eastern llanos basin the megasequence was terminated by middle eocene deformation in the magdalena valley which ended sediment deposition throughout colombia loading effects of this deformation reestablished the basin in which the late pre andean foreland basin megasequence was deposited until the early miocene this megasequence also consists of alluvial plain coastal plain and estuarine deposits including the primary reservoir in the llanos foothills the upper eocene mirador fo mation the megasequence also includes a series of four major grossly coarsening upward cycles in the llanos basin these cycles correspond to changes in sea level sediment supply and foreland basin loading the mudstone in the lowermost of these cycles is the regional seal in the llanos basin and foothills the middle miocene onset of andean deformation in the eastern cordillera isolated the middle magdalena valley from the llanos basin the deformation was dominated by inversion of the basin controlling faults the resultant loading of the lithosphere created the accommodation space for the andean foreland basin megasequence a major transgression into the llanos basin coeval with this deformation caused deposition of marine mudstones in the lower part of the megasequence however the majority of the andean foreland basin megasequence consists of the guayabo formation a classic molasse sequence deposited in a high energy coarse grained bed load dominated fluvial system that was supplied by the developing mountains of the eastern cordillera

syntectonic cenozoic sedimentation in the northern middle magdalena valley basin of colombia and implications for exhumation of the northern andes development of the colombian middle magdalena valley basin mmvb was determined by late cretaceous early eocene uplift of the central cordillera to the west and subsequent transferal of deformation to the eastern cordillera to the east these phases are separated in the tectono stratigraphic record by a major unconformity the middle magdalena valley unconformity mmvu paleocene coastal to alluvial facies underneath the mmvu were deposited in a foreland basin coupled to central cordillera kilometer scale uplift the middle eocene to neogene continental strata that onlap the mmvu document transformation of the mmvb into an interior basin due to eastern cordillera deformation recorded by growth strata in seismic lines and changing provenance and paleoflow patterns exhumation histories of the mmvb bounding ranges are further constrained by apatit fission track and vitrinite reflectance thermochronology and volcanic ash chronology this study indicates that the mmvb is fundamentally related to evolution of the entire andean margin of south america

the pantelleria caldera geothermal system data from the hydrothermal minerals abstract this paper proposes on the basis of petrographic and mineralogic data on cutting and cores from two deep wells pantelleria 1 and pantelleria 2 the first model of the active hydrothermal system of the island of pantelleria phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes the results of these studies emphasize differences between the intracaldera and pericaldera areas of the island within the 45 ka caldera there is a high temperature 240 260 c at 600 800 m depth active hydrothermal system with five zones of characteristic alteration minerals with increasing depth rocks are unaltered to a depth of 200 m contain smectite and mixed layer chlorite smectite c s between 200 and 380 m chlorite illite chalcedony and quartz from 380 to 500 m albite adularia and saponite from 500 to 680 m mixed layer biotite vermiculite from 680 m to the depth drilled 1100 outside the caldera but near the rim a low temperature and low permeability

crustal shortening root spreading isostasy and the growth of orogenic belts a dimensional analysis scaling laws that describe the growth of mountain belts and the change in their regional topographic profiles with time t are obtained by physical arguments that account for crustal shortening isostasy and creeping flow at the mountain roots an average power law rheology characterized by an exponent n is assumed and different values of n are considered the model predicts the formation of a localized range characterized by a height h and a width a as crust on the sides of a range is compressed the root broadens due to buoyancy driven creep this process in conjunction with isostasy causes h to increase with time as t1 4 t1 3 and a as t3 4 t2 3 as n is varied from 1 newtonian rheology to plastic rheology accordingly the aspect ratio a h increases as t1 2 t1 3 the scaling laws depend on the rheology but rather weakly the aspect ratio increases with age but is nearly independent of the rate of shortening the deformation extends gradually inland with increasing time so that the age of tectonic deformation decreases with distance from the continental border uplift takes place inland from the crest while subsidence occurs at the opposite side the ratio a h of the andes and tibet is consistent with this model

spatial and temporal patterns of exhumation across the venezuelan andes implications for cenozoic caribbean geodynamics 1 the venezuelan andes formed by complex geodynamic interaction between the caribbean plate the panama arc the south american plate and the continental maracaibo block we study the spatial and temporal patterns of exhumation across the venezuelan andes using 47 new apatite fission track aft ages as well as topographic analyses this approach permits the identification of at least seven tectonic blocks escalante cerro azul trujillo caparo sierra nevada sierra la culata and el carmen blocks with contrasting exhumation and cooling histories the sierra nevada sierra la culata and el carmen blocks located in the central part of the venezuelan andes and separated by the bocono fault system cooled rapidly but diachronously during the late miocene pliocene major surface uplift and exhumation occurred in the sierra nevada block since before 8 ma a second phase of uplift and exhumation affected the el carmen and sierra la culata blocks to the north of the bocono fault during the late miocene pliocene the highest topography and steepest relief of the belt coincides with these blocks the caparo and trujillo blocks located at the northeastern and southwestern ends of the orogen cooled more slowly from the oligocene to the late miocene these blocks are characterized by significantly lower mean elevations and slightly lower mean slopes than the central blocks unraveling the cooling history of the individual blocks is important to better understand the control of preexisting faults and regional caribbean geodynamics on the evolution of the venezuelan andes our data indicate a strong control of major preexisting fault zones on exhumation patterns and temporal correlation between phases of rapid exhumation in different blocks with major tectonic events e g collision of the panama arc rotation of the maracaibo block

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

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

estimating exhumation rate and relief evolution by spectral analysis of age elevation datasets extracting independent information on mean exhumation rate and the rate of surface relief change from thermochronometric datasets is essential to improve our understanding of the complex coupling between tectonics and surface erosion i e the time scale over which landforms react to changes in uplift rate and or climate a new method based on spectral analysis of age elevation data collected along one dimensional profiles is presented that provides independent estimates of the mean exhumation rate and the relative change in surface relief the results are shown to be independent of the assumed geothermal gradient the method is applied to an existing age dataset from the sierra nevada california and provides constraints on the evolution of the present day relief the spectral analysis demonstrates how current sampling strategies should be modified to optimize the tectonic and geomorphic information that can be retrieved from a thermochronometric dataset

tectonic controls on sedimentation in an intermontane hinterland basin adjacent to inversion structures the nuevo mundo syncline middle magdalena valley colombia

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

palaeozoic to early jurassic history of the northwestern corner of gondwana and implications for the evolution of the iapetus rheic and pacific oceans abstract the palaeozoic to mesozoic igneous and metamorphic basement rocks exposed in the merida andes of venezuela and the santander massif of colombia are generally considered to define allochthonous terranes that accreted to the margin of gondwana during the ordovician and the carboniferous however terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains a general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the merida andes and the santander massif we present new zircon u pb icp ms data from 52 igneous and metamorphic rocks which we combine with whole rock geochemical and pb isotopic data to constrain the tectonic history of the precambrian to mesozoic basement of the merida andes and the santander massif these data show that the basement rocks of these massifs are autochthonous to gondwana and share a similar tectono magmatic history with the gondwanan margin of peru chile and argentina which evolved during the subduction of oceanic lithosphere of the iapetus ocean the oldest palaeozoic arc magmatism is recorded at 500 ma and was followed shortly by barrovian metamorphism peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at 477 ma and the intrusion of synkinematic granitoids until 472 ma subsequent retrogression resulted from localised back arc or intra arc extension at 453 ma when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement continental arc magmatism dwindled after 430 ma and terminated at 415 ma coevally with most of the western margin of gondwana after pangaea amalgamation in the late carboniferous to early permian a magmatic arc developed on its western margin at 294 ma as a result of subduction of oceanic crust of the palaeo pacific ocean intermittent arc magmatism recorded between 294 and 225 ma was followed by the onset of the andean subduction cycle at 213 ma in an extensional regime extension was accompanied by slab roll back which led to the migration of the arc axis into the central cordillera of colombia in the early jurassic

guidelines on the use of structure from motion photogrammetry in geomorphic research as a topographic modelling technique structure from motion sfm photogrammetry combines the utility of digital photogrammetry with a flexibility and ease of use derived from multi view computer vision methods in conjunction with the rapidly increasing availability of imagery particularly from unmanned aerial vehicles sfm photogrammetry represents a powerful tool for geomorphological research however to fully realize this potential its application must be carefully underpinned by photogrammetric considerations surveys should be reported in sufficient detail to be repeatable if practical and results appropriately assessed to understand fully the potential errors involved to deliver these goals robust survey and reporting must be supported through i using appropriate survey design ii applying suitable statistics to identify systematic error bias and to estimate precision within results and iii propagating uncertainty estimates into the final data products

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

active mountain building along the eastern colombian subandes a folding history from deformed terraces across the tame anticline llanos basin active crustal shortening and surface uplift are vividly expressed in the foothills of the eastern cordillera of colombia quaternary alluvial fan and terrace deposits are folded and uplifted tens to hundreds of meters above the local base level recording deformation and incision of the landscape we used geomorphic observations and structural analysis from a 300 km 2 three dimensional seismic reflection survey over an actively growing anticline to investigate the relationship between finite shortening and surface uplift and its effects on landscape evolution a quantitative description of finite shortening of the pregrowth strata from excess area and line length calculations were compared with uplift and shortening estimates obtained from geomorphic strain markers dated using in situ 10 be cosmogenic nuclide depth profiles this combined approach provides the first estimates of quaternary surface uplift and horizontal shortening rates along the foothills of the northeastern colombian andes finite shortening for the tame anticline was calculated using two different approaches excess area shortening varies from 530 36 m 2 at its southern end decreasing northward to 404 72 m 2 whereas line length shortening is typically an order of magnitude smaller using the maximum horizontal shortening our results for the tame anticline reveal a quaternary uplift rate of 2 8 1 5 mm yr and a shortening rate of 2 1 1 2 mm yr 2 with the older terraces located at the highest elevations and recording the highest rotation from the horizontal indicating growth by limb rotation based on these rates the initiation of folding is estimated to be 0 25 0 09 0 05 ma 2 we hypothesize that the discrepancy between shortening estimates from line length and excess area calculations can be reconciled by accounting for internal deformation likely developed during the initial stages of folding which can be represented by strain hardening for example the shortening rates we obtain are comparable to geodetically derived shortening rates but are slightly lower than long term geologic shortening rates slower shortening rates and the presence of a youthful structure at the thrust front suggest that the orogen has been migrating eastward since the late pleistocene probably due to reorganization of a supercritical thrust wedge farther west in the hinterland to attain critical taper

receiver functions and crustal structure of the northwestern andean region colombia we used the receiver function technique to deduce crustal thickness beneath the northwestern andean system using data from the permanent seismic network of colombia combined with some of the iris and ctbto stations in colombia and ecuador the estimation of crustal thickness was made using the primary p to s conversion and crustal reverberations the bulk crustal vp vs ratio was constrained using a crustal thickness versus vp vs stacking method in addition to estimations using a time to depth conversion technique based on results of a modified wadati diagram analysis we observed a wide range of crustal thicknesses including values around 17 km beneath the malpelo island on the pacific ocean 20 to 30 km at the coastal pacific and caribbean plains of colombia 25 to 40 km beneath the eastern plains and foothills 35 km beneath the western cordillera 45 km at the magdalena river intermountain valley 52 to 58 km under the northern central cordillera and reaching almost 60 km beneath some of the volcanoes of the southern cordilleran system of colombia crustal thickness can be slightly greater than 60 km beneath the plateau of the eastern cordillera the values of vp vs are particularly high for some of the stations on the volcanic centers reaching values above 1 79 probably related to the addition of mafic materials to the lower crust and in the plateau of the eastern cordillera near bogota where we speculate about the possibility of crustal seismic anisotropy associated with shear zones

episodic orogenic front migration in the northern andes constraints from low temperature thermochronology in the eastern cordillera colombia 1 new thermochronometric data from the eastern cordillera of the colombian andes reveal diachronous exhumation associated with cenozoic contractional deformation in this sector of the northern andes we present a comprehensive account of exhumation patterns along a 150 km long across strike transect between 4 and 6 n by integrating 29 new apatite fission track aft ages and 17 new zircon fission track zft ages with sparse published thermochronological data from this area our data reveal episodic eastward migration of the orogenic front at an average rate of 2 5 2 7 mm a during the late cretaceous cenozoic we identify three major stages of orogen propagation 1 slow propagation 0 5 3 1 mm a until early eocene 2 rapid orogenic advance 4 0 18 0 mm a during middle late eocene which accounts for 86 of the orogen s present width and 3 slow orogen propagation 1 2 2 1 mm a from oligocene to holocene times our data demonstrate that in the course of changes in plate kinematics the presence of inherited crustal anisotropies such as the former rift bounding faults of the eastern cordillera favor a nonsystematic progression of foreland basin deformation through time by preferentially concentrating accommodation of slip and thrust loading along these zones of weakness

asynchronous miocene pliocene exhumation of the central venezuelan andes constraining the uplift and exhumation history of the venezuelan andes is critical for understanding the complex geodynamic evolution of northern south america we present new apatite fission track aft data from the sierra nevada de merida in the central venezuelan andes and compare the results with previously published aft data for the sierra la culata el carmen block to the north the aft data are combined with three dimensional thermal modeling to constrain the exhumation and relief history of both blocks which show significant differences the sierra nevada indicates exhumation rates of 1 7 km m y from ca 10 ma until 4 ma decreasing to 0 4 km m y from 4 ma to present the el carmen block in contrast underwent much more rapid recent exhumation with rates of 1 4 km m y since 4 ma the data suggest 4 km of relative rock uplift of the sierra la culata with respect to the sierra nevada since 4 ma and support significant uplift of basement blocks along major preexisting discontinuities during transpression in the northern andes linked to accretion of the panama arc and indentation of the maracaibo block rapid middle late miocene exhumation in the sierra nevada resulted from surface uplift that caused the late miocene deflection of the orinoco river inferred from the neogene sedimentary record

radiation damage control on apatite u th he dates from the grand canyon region colorado plateau individual detrital apatite grains from the esplanade coconino and moenkopi formations in the grand canyon region of the colorado plateau yield u th he dates from 104 to 5 ma the range of dates within each unit far exceeds analytical uncertainty but correlates with both he concentration he and effective u concentration eu these dates are all signifi cantly younger than the sandstone units indicating partial to complete he loss following deposition recently published laboratory diffusion data suggest that he retentivity in apatite increases with radiation damage forward models predict that the consequences of this effect will be manifested most clearly as a correlation between u th he dates and the he and eu in suites of apatites that 1 are characterized by a large span of eu and 2 had thermal histories in which suffi cient time elapsed for the apatite he diffusion kinetics to diverge prior to reheating and partial resetting apatites in the sedimentary units investigated fi t these cri teria using geologically reasonable deposition burial and unroofi ng histories simulations that include the effect of radiation damage on apatite he retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations these results suggest that a span of u th he dates positively correlated with eu may provide important information regarding a sample s thermal history

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

some analytical methods for converting thermochronometric age to erosion rate 1 an analytical method is presented for converting thermochronometric ages to surface erosion or equivalently exhumation rate the method incorporates the two most important thermal processes during cooling by erosion the dependence of closure temperature on cooling rate and the advection of heat by rock motion toward the earth s surface two thermal models are considered 1 a steady state model valid for low erosion rates and 2 an eroding half space model which has no steady state but captures the transient increase of geothermal gradient with erosion in each case it is assumed that data consist of one or more thermochronometric ages present day surface geothermal gradient and topographic information including the elevation at which the age was obtained analytical solutions are provided to derive the erosion rate from these data either as an explicit expression for the steady case or as a root finding problem for the transient case a graphical method for plotting age against erosion rate and geothermal gradient is presented as a method for solving the root finding problem and for tracking analytical errors in observations of age and surface geothermal gradient the graphical method is also appropriate for comparing data from different elevations or from different thermochronometric systems examples are provided using synthetic data or published data from the literature

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

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

variation of illite muscovite 40 ar 39 ar age spectra during progressive low grade metamorphism an example from the us cordillera 40ar 39ar step heating data were collected from micron to submicron grain sizes of correlative illite and muscovite rich cambrian pelitic rocks from the western united states that range in metamorphic grade from the shallow diagenetic zone zeolite facies to the epizone greenschist facies with increasing metamorphic grade maximum ages from 40ar 39ar release spectra decrease as do total gas ages and retention ages previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation recrystallization of neo formed illite while recognizing the importance of these processes in evaluating our results we suggest that the inverse correlation between apparent age and metamorphic grade is controlled primarily by thermally activated volume diffusion analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments our results suggest that complete resetting of the illite muscovite ar thermochronometer occurs between the high anchizone and epizone or at roughly 300 c this empirical result is in agreement with previous calculations based on muscovite diffusion parameters which indicate that muscovite grains with radii of 0 05 2 m should have closure temperatures between 250 and 350 c at high anchizone conditions we observe a reversal in the age grain size relationship the finest grain size produces the oldest apparent age which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo formed illite muscovite crystallites typically surpass the size of detrital muscovite grains it is also approximately the stage at which neo formed illite muscovite crystallites develop sufficient ar retentivity to produce geologically meaningful 40ar 39ar ages results from our sampling transect of cambrian strata establish a framework for interpreting illite muscovite 40ar 39ar age spectra at different stages of low grade metamorphism and also illuminate the transformation of illite to muscovite at frenchman mtn nv where the cambrian bright angel formation is at zeolite facies conditions illite muscovite 40ar 39ar data suggest a detrital muscovite component with an apparent age 967 ma the correlative carrara fm is at anchizone conditions in the panamint and resting spring ranges of eastern california and in these locations illite muscovite 40ar 39ar data suggest an early permian episode of subgreenschist facies metamorphism the same type of data from equivalent strata at epizone conditions greenschist facies in the footwall of the bullfrog fluorspar canyon detachment in southern nevada reveals a period of slow to moderate late cretaceous cooling