Merida Andes Research Articles

Synthesis of geological data and comparative phylogeography of lowland tetrapods suggests recent dispersal through lowland portals crossing the Eastern Andean Cordillera.

Vicariance is the simplest explanation for divergence between sister lineages separated by a potential barrier, and the northern Andes would seem to provide an ideal example of a vicariant driver of divergence. We evaluated the potential role of the uplift of the Eastern Cordillera (EC) of the Colombian Andes and the Mérida Andes (MA) of Venezuela as drivers of vicariance between lowland populations co-distributed on both flanks. We synthesized published geological data and provided a new reconstruction showing that the EC-MA grew from north to south, reaching significant heights and separating drainages and changing sediment composition by 38–33 million years ago (Ma). A few lowland passes across the EC-MA may have reached their current heights (~1,900 m a.s.l.) at 3–5 Ma. We created a comparative phylogeographic data set for 37 lineages of lowland tetrapods. Based on molecular phylogenetic analyses, most divergences between sister populations or species across the EC-MA occurred during Pliocene and the Quaternary and a few during the latest Miocene, and coalescent simulations rejected synchronous divergence for most groups. Divergence times were on average slightly but significantly more recent in homeotherms relative to poikilotherms. Because divergence ages are mostly too recent relative to the geological history and too asynchronous relative to each other, divergence across the northern Andes may be better explained by organism-environment interactions concomitant with climate oscillations during the Pleistocene, and/or dispersal across portals through the Andes.

Crustal structure and tectonic implications of the southernmost Merida Andes, Venezuela, from wide-angle seismic data analysis

The crustal structure of the Merida Andes in southwestern Venezuela is crucial to our understanding of both, the complex regional geodynamics of the northern part of South America, as well as the development of the hydrocarbon basins located along its flanks. Symmetric and asymmetric models of the Merida Andes have been proposed based on geological and geophysical (principally gravimetric) surveys, without counting detailed structures from wide-angle seismic. This work examines the crustal structure of the southwestern part of the Merida Andes in Venezuela using controlled seismic sources along a 500 km wide-angle seismic profile (named ANDES SUR). Forward modeling is carried out to obtain a layer-based, P-wave, crustal velocity model. An alternative velocity model is obtained by applying a first arrival tomography for the upper 20 km depth, and a joint PmP and Pn inversion for the deeper crust. Important lateral P-wave velocity variations and low-velocity zones (LVZ) are observed within the upper crust. To the northwest, these lateral velocity variations are probably related to the deformation zones of Icotea and Tarra Faults, and the southeast to the Mantecal and Apure grabens. The integration of seismological data to the profile provides some clues about a possible slow-shear movement between the upper and the lower crust (brittle-ductile transition zone) at the NW end of the profile, as well as the possible northwestward incipient subduction of the South American Plate beneath the Maracaibo Triangle Block. We interpret the Boconó Fault's main trace as a take-off surface detaching from the interface between the upper and lower crust and dipping to the SE. A slight NW displaced crustal root of the Merida Andes is observed in all models, being the root ∼150 km wide with an approximately 9 km downward deflection (from ∼46 to 55 km depth) of the Moho. The crustal structure along the profile ANDES SUR, together with the observations along two profiles located further north, provide information about the tectonic implication along the strike of the Merida Andes, favoring an asymmetric model with possible NW directed South American lower crust and upper mantle underthrusting or incipient subduction beneath the Maracaibo Triangle Block beneath the center of the Merida Andes.

Middle-Late Triassic metamorphism of the Guajira Arch-basement: Insights from zircon U–Pb and Lu–Hf systematics

The Guajira Arch is located in the northernmost portion of South America and holds the record of several stages of magmatism and metamorphism. We performed LA-ICP-MS U–Pb zircon geochronology coupled with Lu–Hf isotope geochemistry on 11 samples from a high-grade gneissic basement hosting interleaved complexes of younger meta-granitoids and meta-sediments to unravel its tectono-metamorphic evolution. The oldest rocks correspond to a Rodinia-type basement at ca. 1100-880 Ma, affected by mafic magmatism during the Permian at ca. 272 Ma, anatexis during the Carnian at ca. 230 Ma, updoming and the formation of an extensional detachment fault during the Norian at ca. 224 Ma concluding with Jurassic magmatism since ca. 190 Ma. Trace element geochemistry shows that Triassic zircons are depleted in HREE and yield lower Th/U ratios than Jurassic zircons enriched both in HREE, Yb, and Y. Hf systematics indicates a Triassic to Jurassic evolution towards more radiogenic compositions. The lithological associations, structural setting, U–Pb ages, and Hf isotope geochemistry of the Triassic-Jurassic rocks at the Guajira Arch characterize an active margin setting of a hot orogenic belt in NW-Gondwana, comprising the Central Cordillera of Colombia, the Santa Marta Massif, the Merida Andes, the Perijá Range, the Santander Massif and the para-autochthonous terranes south of the Ouachita-Marathon Suture (Oaxaquia, Acatlán, and Maya), and thus vindicate the existence of a conjugate margin with Laurentia until the Late Triassic, preceding the breakup of western Pangaea. The Triassic anatectites and Jurassic granitoids from the Guajira Arch yield Proterozoic Hf TDMi ages which are indistinguishable from the autochthonous Proterozoic basement and thus render an allochthonous origin by terrane accretion highly improbable. Instead, these data point to melting of the continental crust by the incorporation of primitive material reflecting a two-stage process at the boundary of a large underlying mantle convection cell (i) Triassic slab steepening, subduction arrest, updoming anatexis at ca. 230-223 Ma and (ii) Early Jurassic arc-magmatism at ca. 190-165 Ma due renewed convergence. Furthermore, we propose that the structural setting, lithology, and isotope geochemistry of the Guajira Arch units, fit into the style of a cordilleran core-complex emplaced in a roll-back extensional setting preceding Pangaea break-up.

Permo–Triassic metamorphism in the Mérida Andes, Venezuela: new insights from geochronology, O-isotopes, and geothermobarometry

Although there is a consensus about Pangea assemblage in northwestern Gondwana spanning from the Late Carboniferous to the Early Permian, the tectonics of the Permo–Triassic period, including the onset of Pangea breakup, is still controversial. In this context, three regional tectonic features need to be considered: (1) The Ouachita-Marathon-Sonora suture, (2) east-dipping subduction of the Panthalassa oceanic crust beneath Gondwana, and (3) an extensional setting documented in Colombia, Ecuador, and Mexico suggesting Pangea breakup at ca. 240–220 Ma. A chemical, isotopic, and geochronological dataset is presented in this contribution to constrain the effects of these tectonic processes in the metamorphic basement of the Merida Andes in western Venezuela. U–Pb Secondary Ion Mass Spectrometry analyses on unpolished surfaces of zircons from orthogneisses yielded an average age of 251 ± 4 Ma. The corresponding δ18O values suggest metamorphic recrystallization of zircon instead of high-T fluid interaction. Rb–Sr and Sm–Nd geochronology in white mica and garnet yielded ages of 234 ± 3 Ma and 249 ± 2 Ma, respectively, whereas a younger Rb–Sr date of 197 ± 1 Ma was obtained from biotite. U–Pb and Sm–Nd ages constrain the latest Permian–earliest Triassic metamorphism in the Merida Andes at 251 ± 4 Ma. Geothermobarometry data suggest amphibolite-facies peak metamorphic conditions at ~ 685 °C and ~ 6.0 kbar. Metamorphism might be related to post-orogenic collapse, after the collision of Gondwana and Laurentia to form Pangea. Rb–Sr ages suggest retrogression and cooling, possibly caused by thermal relaxation of a tectonically overthickened crust and onset of extensional setting, followed by Pangea breakup during the Late Triassic–Early Jurassic.

New bird records from the arid Cúcuta Valley, north-east Colombia

Summary. Avian inventories of the poorly known tropical dry forest in the Cucuta Valley, north-east Colombia, yielded new distributional data for 15 bird species including several range extensions along the east slope of the East Andes, or filled distributional gaps between the Serrania de Perija and Merida Andes of Venezuela, and between the Maracaibo basin in Venezuela and eastern Llanos of Colombia. Some of the new records are of fairly common but easily overlooked species associated with dense habitats, whereas others concern open-country species whose spread has apparently been promoted by landscape transformation. Further work will probably yield additional new records in the region, especially in the transition from dry forest of the Cucuta Valley to more humid habitats in the Catatumbo and adjacent Andes of Colombia and Venezuela.

Ancient xenocrystic zircon in young volcanic rocks of the southern Lesser Antilles island arc

The Lesser Antilles arc is one of the best global examples in which to examine the effects of the involvement of subducted sediment and crustal assimilation in the generation of arc crust. Most of the zircon recovered in our study of igneous and volcaniclastic rocks from Grenada and Carriacou (part of the Grenadines chain) is younger than 2Ma. Within some late Paleogene to Neogene (~34–0.2Ma) lavas and volcaniclastic sediments however, there are Paleozoic to Paleoarchean (~250–3469Ma) xenocrysts, and Late Jurassic to Precambrian zircon (~158–2667Ma) are found in beach and river sands. The trace element characteristics of zircon clearly differentiate between different types of magmas generated in the southern Lesser Antilles through time. The zircon population from the younger arc (Miocene, ~22–19Ma, to Present) has minor negative Eu anomalies, well-defined positive Ce anomalies, and a marked enrichment in heavy rare earth elements (HREE), consistent with crystallization from very oxidized magmas in which Eu2+ was in low abundance. In contrast, zircon from the older arc (Eocene to mid-Oligocene, ~30–28Ma) has two different REE patterns: 1) slight enrichment in the light (L)REE, small to absent Ce anomalies, and negative Eu anomalies and 2) enriched High (H)REE, positive Ce anomalies and negative Eu anomalies (a similar pattern is observed in the xenocrystic zircon population). The combination of positive Ce and negative Eu anomalies in the zircon population of the older arc indicates crystallization from magmas that were variably, but considerably less oxidized than those of the younger arc. All the igneous zircon has positive εHf(t), reflecting derivation from a predominantly juvenile mantle source. However, the εHf(t) values vary significantly within samples, reflecting considerable Hf isotopic heterogeneity in the source.The presence of xenocrystic zircon in the southern Lesser Antilles is evidence for the assimilation of intra-arc crustal sediments and/or the recycling and incorporation of sediments into the magma sources in the mantle wedge. Most likely however, primitive magmas stalling and fractionating during their ascent through the Antilles crust entrained ancient zircon. This is evidence by the geochemistry of the study samples, which is inconsistent with any involvement of partially melted subducted sediment. Paleogeographic reconstructions show that the old zircon could derive from distant regions such as the Eastern Andean Cordillera of Colombia, the Merida Andes, and the northern Venezuela coastal ranges, transported for example by the Proto-Maracaibo River precursor of the Orinoco River.

Shear wave anisotropy in northwestern South America and its link to the Caribbean and Nazca subduction geodynamics

To investigate the subduction dynamics in northwestern South America, we measured SKS and slab-related local S splitting at 38 seismic stations. Comparison between the delay times of both phases shows that most of the SKS splitting is due to entrained mantle flow beneath the subducting Nazca and Caribbean slabs. On the other hand, the fast polarizations of local S-waves are consistently aligned with regional faults, which implies the existence of a lithosphere-confined anisotropy in the overriding plate, and that the mantle wedge is not contributing significantly to the splitting. Also, we identified a clear change in SKS fast directions at the trace of the Caldas Tear (∼5°N), which represents a variation in the subduction style. To the north of ∼5°N, fast directions are consistently parallel to the flat subduction of the Caribbean plate-Panama arc beneath South America, while to the south fast polarizations are subparallel to the Nazca-South America subduction direction. A new change in the SKS splitting pattern is detected at ∼2.8°N, which is related to another variation in the subduction geometry marked by the presence of a lithosphere-scale tearing structure, named here as Malpelo Tear; in this region, NE-SW-oriented SKS fast directions are consistent with the general dip direction of the underthrusting of the Carnegie Ridge beneath South America. Further inland, this NE-SW-trending mantle flow continues beneath the Eastern Cordillera of Colombia and Merida Andes of Venezuela. Finally, our results suggest that the subslab mantle flow in northwestern South America is strongly controlled by the presence of lithospheric tearing structures.

Tectonic evolution of the La González pull-apart basin in the Mérida Andes: combination of geological data and satellite radar interferometry (InSAR)

The 500-km-long Bocono strike-slip fault runs as a major active fault along the backbone of the Merida Andes fold-and-thrust belt. The recent right-lateral motion on the fault led to formation of numerous structures such as pull-apart basins which have formed in releasing bends and/or right-step offsets along the fault strands. The La Gonzalez pull-apart is the biggest basin generated as an extensional strike-slip duplex in the central part of the fault. This duplex is made up of two strands of the Bocono fault as master/first-order faults, while normal right-lateral faults which formed during evolution of the basin are second-order faults. The extension of the basin is associated with seismic activities and surface offsets along the Bocono fault. InSAR investigations over a 31-month period also support active deformation within the basin. These data indicate that the La Gonzalez basin is continuously being extended as a result of motion along the Bocono fault and formation of subsequent normal faults. In addition, the basin is being transversely shortened in NW–SE direction due to regional shortening across the Merida Andes range followed by convergence between the Maracaibo microplate and the Guyana shield.

A likelihood inference of historical biogeography in the world’s most diverse terrestrial vertebrate genus: Diversification of direct-developing frogs (Craugastoridae: Pristimantis) across the Neotropics

The geology of the northern Andean region has driven the evolutionary history of Neotropical fauna through the creation of barriers and connections that have resulted in speciation and dispersal events, respectively. One of the most conspicuous groups of anuran fauna in the Andes and surrounding areas is the direct-developing species of the genus Pristimantis. We investigated the molecular phylogenetic placement of 12 species from the montane Andes of Colombia in a broader geographical context with a new genus-level phylogeny in order to identify the role of Andean orogeny over the last 40million years and the effect of elevational differences in diversification of Pristimantis. We examined the biogeographic history of the genus using ancestral range reconstruction by biogeographic regions and elevational ranges. We recognized the middle elevational band (between 1000 and 3000m) in the Northwestern Andes region of Colombia and Ecuador as a focal point for the origin and radiation of Pristimantis species. Additionally, we found several Andean migrations toward new habitats in Central Andes and Merida Andes for some species groups. We suggest that the paleogeological changes in the Northwestern Andes were the main promoter of speciation in Pristimantis, and may have served as a corridor for the dispersion of lowland species.

Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela

We have combined surface wave tomography with Ps and Sp receiver-function images based on common-conversion-point (CCP) stacking to study the upper mantle velocity structure, particularly the lithosphere–asthenosphere boundary (LAB), beneath eastern and central Venezuela. Rayleigh phase velocities in the frequency range of 0.01–0.05 Hz (20–100 s in period) were measured using the two-plane-wave method and finite-frequency kernels, and then inverted on a 0.5°×0.5° grid. The phase velocity dispersion data at grid points were inverted for 1D shear velocity profiles using initial crust-mantle velocity models constructed from previous studies. The 3D velocity model and receiver-function images were interpreted jointly to determine the depth of the LAB and other upper mantle features. The tomographic images revealed two high velocity anomalies extending to more than ∼200 km depth. One corresponds to the top of the subducting Atlantic plate beneath the Serrania del Interior. The other anomaly is a highly localized feature beneath the Maturin Basin. The LAB depth varies significantly in the study region: It is located at ∼110 km depth beneath the Guayana Shield, and reaches ∼130 km at the northern edge of the Maturin Basin, which might be related to the downward flexural bending due to thrust loading of the Caribbean plate and pull from the subducting Atlantic plate. Immediately to the west, the lithosphere is thin (∼50–60 km) along the NE-SW trending Espino Graben from the Cariaco basin to the Orinoco River at the northern edge of the craton. The LAB in this region is the top of a pronounced low velocity zone. Westward, the lithosphere deepens to ∼80 km depth beneath the Barinas Apure Basin, and to ∼90 km beneath the Neogene Merida Andes and Maracaibo block. Both upper mantle velocity structure and lithosphere thickness correlate well with surface geology and are consistent with northern South American tectonics.

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 Boconó fault system, possibly connected with the right-lateral faults inferred to exist along the axial part of the EC.

Seismological observations in Northwestern South America: Evidence for two subduction segments, contrasting crustal thicknesses and upper mantle flow

The cause of tectonic deformation in northwestern South America and its link to upper mantle structure and flow are debated. We use a combination of broadband and short period travel time seismic data for P-waves to show that observations are consistent with the presence of two subduction segments in Colombia and contrasting values of crustal thickness. In Northern Colombia, at latitudes greater than 6°N, most of the seismic stations are associated with negative teleseismic travel time residuals, relative to a regional mean, suggesting that the upper mantle is seismically faster than predicted from global models. In particular, for the Caribbean coastal plains there are no signs of significant anomalies in the upper mantle, evidenced by the small magnitude of the travel time delays and subdued Pn speeds (~7.97km/s). To the southeast of such plains there is an increase in magnitude of the negative travel time residuals, including the Northern Eastern Cordillera, the Perija Range and the Merida Andes. An analysis of non-isostatic residual topography, based on a model of crustal thickness in northwestern South America, is consistent with a slab-associated upper mantle flow beneath the region just east of the Bucaramanga Nest. We interpret these results to indicate the presence of a Caribbean slab, initially flat beneath the Caribbean coastal plains, and steepening sharply in the southeast, including the area of Bucaramanga. For most of the western Andean region and the Pacific coast, south of 6°N, teleseismic differential travel time residuals are predominantly positive, indicating that the upper mantle is in general seismically slower than the reference model. Beneath the Central Cordillera, just to the east of this area, the residuals become smaller and predominantly negative; residual non-isostatic topography is negative as well. These features are probably related to the effect of the Nazca subduction developing an asthenospheric wedge.

U–Pb detrital zircon age patterns of Cenozoic clastic sedimentary rocks in Trinidad and its implications

The Cenozoic evolution of northern South America can be simplified as a diachronous, west-to-east change from north-facing passive margin, to active convergence and transcurrent plate margin. As the current eastern end of the Caribbean-South America plate boundary, the Trinidad area records the most recent tectonic regime transition. Documenting the provenance of Cenozoic clastic rocks in Trinidad provides insights into the spatial and temporal relationships between mountain range uplifting, sediment dispersal, and drainage system development along the eastern end of the Caribbean-South American plate boundary. Four Eocene to Pliocene age outcropping sandstone samples were collected from Trinidad and 545 detrital zircon grains were analyzed using the U–Pb LA-ICP-MS method. A total of 404 analyses with less than 10% discordance were used for the final interpretation. Results show that the age distribution of the Eocene to the Early Oligocene samples is very restricted and dominated by Precambrian age grains with age peaks at ~1400Ma, ~1800Ma, and ~2000Ma, typically derived from the Guyana Shield. In contrast, the Late Oligocene and younger samples show much broader and mixed age distribution that includes Mesozoic, Paleozoic, and Precambrian peaks. This age shift was interpreted as arriving of the Great Caribbean Arc and oblique collision between the South America and Caribbean plates. Continuing collision uplifted the Andes belts to the west. The Eastern Cordillera of Colombia, the Merida Andes, and the northern Venezuela coastal ranges then became the second primary sources. Offshore sedimentation switched from the passive margin with multiple small drainage systems to the active convergent and transcurrent plate margin with a single large river-dominated delivery system.

Paleoenvironmental trends in Venezuela during the last glacial cycle

This chapter summarizes and updates the main Quaternary paleoclimatic and sea-level trends recorded in Venezuela, and provides a paleoenvironmental framework for studies addressing biogeographical, ecological, and evolutionary topics. Venezuela has a number of localities that have provided paleoenvironmental sequences in the neotropical, circum-Caribbean domain, such as the Cariaco basin, Lake Valencia, the Merida Andes, and the Guayana region (e.g., Markgraf 2000). Venezuela encompasses a wide range of physiographic, climatic, and biogeographic conditions because of its geographic location. This spatial heterogeneity has led to complex, but interesting, paleoenvironmental reconstructions.

3D geological modeling of the Trujillo block: Insights for crustal escape models of the Venezuelan Andes

Abstract The Venezuelan Andes form a N50°E-trending mountain belt extending from the Colombian border in the SW to the Caribbean Sea in the NE. The belt began to rise since the Middle Miocene in response to the E–W collision between the Maracaibo block to the NW and the Guyana shield belonging to South America to the SE. This oblique collision led to strain partitioning with (1) shortening along opposite-vergent thrust fronts, (2) right-lateral slip along the Bocono fault crossing the belt more or less along-strike and (3) crustal escape of the Trujillo block moving towards the NE in between the Bocono fault and the N–S-striking left-lateral Valera fault. The geology of the Venezuelan Andes is well described at the surface, but its structure at depth remains hypothetic. We investigated the deep geometry of the Merida Andes by a 3D model newly developed from geological and geophysical data. The 3D fault model is restricted to the crust and is mainly based on the surface data of outcropping fault traces. The final model reveals the orogenic float concept where the mountain belt is decoupled from its underlying lithosphere over a horizontal decollement located either at the upper/lower crust boundary. The reconstruction of the Bocono and Valera faults results in a 3D shape of the Trujillo block, which floats over a mid-crustal decollement horizon emerging at the Bocono–Valera triple junction. Motion of the Trujillo block is accompanied by a widespread extension towards the NE accommodated by normal faults with listric geometries such as for the Motatan, Momboy and Tuname faults. Extension is explained by the gravitational spreading of the upper crust during the escape process.

Tectonics changes in NW South American Plate and their effect on the movement pattern of the Boconó Fault System during the Mérida Andes evolution

Abstract The NE–SW trending Merida–Andes fold-and-thrust belt, in the southern boundary of the Maracaibo Block, formed by a collisional event in Late Miocene at the boundary of the Maracaibo Block and Guyana Shield in the northwestern South American plate. The 500 km long, NE–SW striking right-lateral strike-slip Bocono Fault System lies in the Merida–Andes area. Mylonitic fault rocks along the Bocono Fault System developed during pre-Late Miocene-Early Pliocene. Microscopic and mesoscopic structures such as mica fish, asymmetrical porphyroclasts, S–C shear bands, and asymmetrical ductile folds in Bocono Fault mylonite indicate a sinistral movement along the fault. The movement is probably related to pre-collision extensional tectonics that began in the Late Triassic–Jurassic interval, and continued until the Late Cretaceous or Middle Eocene. Following the collision of the Panama Arc and the South American plate in the Late Miocene, and the change from extensional to contractional tectonic regime, the Bocono was reactivated as a dextral fault system.

A 3-D lithospheric model of the Caribbean-South American plate boundary

A 3-D structural model of the Caribbean-South American plate boundary was constructed by gravity modeling. The model was constrained by four wide-angle seismic refraction sections, Moho depth estimations from receiver functions, and additionally seismological hypocenters, surface geology, and geodynamic information. Density values were calculated from empirical velocity-density functions, and mineralogical-chemical composition considering specific P/T conditions. We tested different structural models for Western and Eastern Venezuela. In the final model, the fit of the measured and modeled gravity fields for a long Caribbean slab in Western Venezuela was better than the fit obtained for a short one. This interpretation is consistent with the constraining data. The slab is interpreted to extend further to the south beneath Northern Colombia and culminates in the area of the seismic cluster of the Bucaramanga nest. The modeling estimates a slab dip angle under Maracaibo and Merida Andes of 15°, which increases to 32° below 100 km depth. The dip direction of approx. N150°E ± 5 increases lightly eastward. In Eastern Venezuela, considering its short wavelength, lineaments analyzed from gravity data (by curvature methods and Euler deconvolution) seem to be related to shallow structures and density contrast in the Serrania del Interior and not from a deep detached slab beneath the continental crust. It is deduced from modeling results that this slab configuration has a very small influence on the gravity field. The slab was modeled according to the subduction-transform propagation model with purely westward subduction and a slab break off along a vertical dip-slip tear through the lithosphere.

Organic petrology of Paleocene Marcelina Formation coals, Paso Diablo mine, western Venezuela: Tectonic controls on coal type

Abstract About 7 Mt of high volatile bituminous coal are produced annually from the four coal zones of the Upper Paleocene Marcelina Formation at the Paso Diablo open-pit mine of western Venezuela. As part of an ongoing coal quality study, we have characterized twenty-two coal channel samples from the mine using organic petrology techniques. Samples also were analyzed for proximate–ultimate parameters, forms of sulfur, free swelling index, ash fusion temperatures, and calorific value. Six of the samples represent incremental benches across the 12–13 m thick No. 4 bed, the stratigraphically lowest mined coal, which is also mined at the 10 km distant Mina Norte open-pit. Organic content of the No. 4 bed indicates an upward increase of woody vegetation and/or greater preservation of organic material throughout the life of the original mire(s). An upward increase in telovitrinite and corresponding decrease in detrovitrinite and inertinite illustrate this trend. In contrast, stratigraphically higher coal groups generally exhibit a ‘dulling upward’ trend. The generally high inertinite content, and low ash yield and sulfur content, suggest that the Paso Diablo coals were deposited in rain-fed raised mires, protected from clastic input and subjected to frequent oxidation and/or moisture stress. However, the two thinnest coal beds (both 0.7 m thick) are each characterized by lower inertinite and higher telovitrinite content relative to the rest of Paso Diablo coal beds, indicative of less well-established raised mire environments prior to drowning. Foreland basin Paleocene coals of western Venezuela, including the Paso Diablo deposit and time-correlative coal deposits of the Tachira and Merida Andes, are characterized by high inertinite and consistently lower ash and sulfur relative to Eocene and younger coals of the area. We interpret these age-delimited coal quality characteristics to be due to water availability as a function of the tectonic control of subsidence rate. It is postulated that slower subsidence rates dominated during the Paleocene while greater foreland basin subsidence rates during the Eocene–Miocene resulted from the loading of nappe thrust sheets as part of the main construction phases of the Andean orogen. South-southeastward advance and emplacement of the Lara nappes during the oblique transpressive collision of the Caribbean and South American tectonic plates in the Paleocene was further removed from the sites of peat deposition, resulting in slower subsidence rates. Slower subsidence in the Paleocene may have favored the growth of raised mires, generating higher inertinite concentrations through more frequent moisture stress. Consistently low ash yield and sulfur content would be due to the protection from clastic input in raised mires, in addition to the leaching of mineral matter by rainfall and the development of acidic conditions preventing fixation of sulfur. In contrast, peat mires of Eocene–Miocene age encountered rapid subsidence due to the proximity of nappe emplacement, resulting in lower inertinite content, higher and more variable sulfur content, and higher ash yield.

Miocene tectonism and the separation of cis- and trans-Andean river basins: Evidence from Neotropical fishes

The fish fauna of trans-Andean river basins in northwestern South America is ancient and diverse, including 14% (558 of 4,085) of all Neotropical teleost species and representing 88% of the orders and 79% of the families. The evolutionary histories of these lineages provide many examples to test models of the tectonic uplift that isolated the trans-Andean basins. We report the results of two newly compiled data sets of phylogenetic and biogeographic information on the freshwater fishes of the region: (1) species-level phylogenies for 26 Neotropical freshwater teleost taxa, with a minimum of 37 cis-/trans-Andean clades and (2) species distributions for 641 genera of Neotropical freshwater teleosts, with a minimum of 140 cis-/trans-Andean clades. Although it provides only about one-quarter the total number of cis-/trans-Andean clades, species phylogeny preserves a more accurate record of the temporal sequence of basin isolation. Phylogenies using gene sequences also may provide estimates on the timing of lineage divergences. However, the great majority (70%) of available species phylogenies for Neotropical freshwater teleosts employ comparative morphology alone, partly because species-level sampling for most taxa requires collections over large spatial (10 3–10 4 km) scales, and collections of whole specimens for morphological study are readily available for many taxa from natural history museums. Fish species phylogenies are partially concordant with patterns of drainage basin isolation generated from geological data on the Miocene of northwestern South America, which associate the initial rise of the Eastern Cordillera (∼1 2 Ma) with the hydrological isolation of the Magdalena and Pacific Slope regions and the rise of the Merida Andes (∼8 Ma) with the isolation of the modern Maracaibo and Orinoco basins. Although some phylogenies unite taxa from the Maracaibo and cis-Andean Orinoco, a more common set of area relationships occurs between clades exclusive to the Maracaibo and trans-Andean Magdalena basins. The compound origin of the Maracaibo ichthyofauna may be due to partial extinction of cis-Andean taxa that resulted from a marine incursion in the Late Miocene and subsequent invasion by congeners from the adjacent Magdalena Basin. In combination, the pooled data on species phylogenies and distributions suggest that the origins of the trans-Andean freshwater fish fauna predate the Miocene tectonic events that dissected the landscape. Among families of freshwater teleosts from northwestern South America, species diversity is significantly correlated with a minimum number of cis-/ trans-Andean clades, which indicates that the relative species diversity and biogeographic distributions of Amazonian fishes were effectively modern by the late Middle Miocene. The diverse taxonomic composition of the trans-Andean ichthyofauna further suggests that Miocene tectonism fragmented the entire aquatic fauna of northwestern South America, leaving a clear signal on all major taxa.

Current states of stress in the northern Andes as indicated by focal mechanisms of earthquakes

Systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern Andes reveals relatively homogeneous patterns of crustal stress in three main regions. The first region, presently under the influence of the Caribbean plate, includes the northern segment of the Eastern Cordillera of Colombia and the western flank of the Central Cordillera (north of 4°N). It is characterized by WNW–ESE compression of dominantly reverse type that deflects to NW–SE in the Merida Andes of Venezuela, where it becomes mainly strike–slip in type. A major bend of the Eastern thrust front of the Eastern Cordillera, near its junction with the Merida Andes, coincides with a local deflection of the stress regime (SW–NE compression), suggesting local accommodation of the thrust belt to a rigid indenter in this area. The second region includes the SW Pacific coast of Colombia and Ecuador, currently under the influence of the Nazca plate. In this area, approximately E–W compression is mainly reverse in type. It deflects to WSW–ENE in the northern Andes south of 4°N, where it is accommodated by right-lateral displacement of the Romeral fault complex and the Eastern front of the northern Andes. The third, and most complex, region is the area of the triple junction between the South American, Nazca and Caribbean plates. It reveals two major stress regimes, both mainly strike–slip in type. The first regime involves SW–NE compression related to the interaction between the Nazca and Caribbean plates and the Panama micro-plate, typically accommodated in an E–W left-lateral shear zone. The second regime involves NW–SE compression, mainly related to the interaction between the Caribbean plate and the North Andes block which induces left-lateral displacement on the Uramita and Romeral faults north of 4°N. Deep seismicity (about 150–170 km) concentrates in the Bucaramanga nest and Cauca Valley areas. The inversion reveals a rather homogeneous attitude of the minimum stress axis, which dips towards the E. This extension is consistent with the present plunge of the Nazca and Caribbean slabs, suggesting that a broken slab may be torn under gravitational stresses in the Bucaramanga nest. This model is compatible with current blocking of the subduction in the western northern Andes, inhibiting the eastward displacement of slabs, which are forced to break and sink in to the asthenosphere under their own weight.