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A detailed sedimentological analysis of the so called “Chachil Limestones” at its type locality around the Mirador del Chachil area, southwestern Neuquen province, Argentina, is presented in this paper for the first time. It is based on a macro/microfacial analysis and their environmental interpretation by means on texture, fabric, bioclasts, intrabasinal and extrabasinal grain amounts, sedimentary structures, bioturbations and hydro-dynamism. Because of the recognition of different facies associations, but no pure limestones, it is more suitable to refer these sediments as the Chachil Formation. The depositional environment of this unit is interpreted to correspond to an internal platform dominated by tides, with carbonate sedimentation disturbed by repeated explosive volcanic episodes, which reduced the sedimentation space, causing retrogradation of the sedimentary system and coastal onlap. In addition, a new recalibration of the U-Pb zircon dating used for the geochronological analysis reveals a small change with regard to previous information that has been used to recalculate the data, is presented in this paper. Fil: Armella, Claudia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Leanza, Hector Armando. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: Corfu, Fernando. University of Oslo; Noruega

In this paper we present the results of an investigation on the electric crustal structures depicted by a magnetotelluric (MT) transect cutting through the easternmost sector of the Alta Floresta Gold Province, in the SW Amazon Craton, Brazil. The MT dataset was acquired along a 200 km NNE-SSW transect encompassing 35 broadband and 12 long-period stations, covering a period range of approximately 10−4 to 104 s. We adopted a data processing scheme based on robust estimators over the electromagnetic cross-spectra for combined single and remote reference stations in order to estimate horizontal and vertical transfer functions. The dataset was inverted using a 3-D MT based minimum structure algorithm, considering the data dimensionality, directionality analysis and the irregular station distribution. A set of inversions considering different model parameters have been performed to achieve a model with robust main features and to test the stability of the obtained solutions. A resolution study was also proposed in order to evaluate the average depth of model sensitivity. The electrical structures depicted can improve the understanding of the ancient tectonic features encrypted at the cratonic stable crust, contributing to elucidate the mechanisms that might have controlled its ore genesis. Conductors in the mid-crust correlate with shallow mafic enclave-rich granitic and foliated volcanic facies outcrops, linking the geoelectric anomalies to a possible mid-crustal source correlated to the Statherian magmatism, which is believed to be the main pulse related to gold metallogenesis at this province.

This paper shows the results obtained from a group of simple and basic procedures used to assess slope instability in terms of shallow landslides on data-scarce hillsides at the slope unit scale. The procedures are based on engineering geological mapping to obtain geological and geotechnical data and on an adapted Hovland 3D safety factor (SF3D) associated with a van Genuchten infiltration model to simulate the rainfall percolation. The procedures were applied on a data-scarce hillside in the Campos do Jordao municipality located in the Mantiqueira Mountains, Sao Paulo state, Brazil. This area was chosen because it is affected by shallow landslides due to a loss of shear strength when suction decreases or dissipates due to rainfall infiltration. The combined procedures permitted the total SF3Dto be obtained from the partial SF of each grid cell (considering 4 different rainfall events and quasi-saturation conditions) and the failure probability (SF3D All of these results verify that all the slopes and selected potential rupture surfaces are compatible with the occurrence of shallow landslides on December 31st, 1999/January 1st, 2000. Large-scale engineering geological mapping proved to be fundamental because it provided detailed mapping of the spatial arrangement and characterizations of the geological material, thereby permitting adequate definitions of the potential rupture surfaces and the adoption of geotechnical and hydraulic parameters to calculate the SF3D and the failure probability (SF

The distribution and characteristics of volcaniclastic deposits are the result of the interplay among numerous factors acting at different and multileveled scales of space and time. Such complexity requires multiple techniques for the study of volcanic regions. This approach provides a strong basis for understanding volcanic processes and their relationship with the resulting landforms or deposits and for reconstructing dynamic properties of flows. The aim of this paper is thus to understand the interplay between the geomorphologic environment and the origin and transport of large-magnitude debris flows. For this reason, we selected the eastern sector of the Citlaltepetl volcano and the Teteltzingo lahar event (Veracruz, Mexico). Information from geomorphological and morphometric analysis (including the linear features) was linked with field and shape-fabric data on selected outcrops. The study area was divided into three morphometric zones based on their elevation, slope, drainage network, and the trend in linear features. These zones are volcanic mountain zone, minor folded ranges and cumulative volcaniclastic plain. The geomorphological map enabled us to understand the origin of the landforms observed in each of the three morphometric zones. The characteristics of the Teteltzingo lahar deposit (i.e., geographic distribution, thickness and components) are associated with the morphologic and morphometric characteristics of the landscape. The particle shape-fabric analysis gave evidence that particle iso-orientation is consistent with the flow direction and orientation of some regional linear features, especially in the medial and distal zones. Moreover, the degree of iso-orientation was shown to be dependent on travel distance and particle diameter. The highest degree of iso-orientation was found in the coarse particles (gravel grain-size) throughout the entire deposit, being more developed in the distal zone. The evidence presented here suggests an interplay between geomorphological–structural controls and their spatial variability with the distribution of large-magnitude lahars. This highlights the importance of a multi-scale vision of mass movement phenomena that show how morphological and sedimentological characteristics are linked and could affect the generation and dynamics of future flows.

The Quequén Salado river basin has been the focus of several contributions since the first decades of the XX century, namely dealing with the general geological features of the deposits and with the vertebrate remains. In this paper, the Neogene geological history documented by the Quequén Salado river exposures is reconstructed by means of stratigraphic, sedimentological and paleomagnetic studies along with the paleontological analysis of vertebrate remains. The study area is a crucial setting not only to better understand the evolution of the southern Pampas basin during the late Miocene-early Pliocene interval, but also to test the validity of the biochronologic and biostratigraphic schemes, especially the “Irenense”. A geological model for the Quequén Salado river valley is proposed: a case of downcutting and headward erosion that contributes with a coherent interpretation to explain the spatial distribution of facies and fossil taxa: the younger in the distal sector of the Quequén Salado middle basin and the older in the lower basin. The sedimentary record is believed to represent the distal reaches of a distributary fluvial system that drained from the Ventania ranges. The stratigraphic section of Paso del Indio Rico results a key stratigraphic site to fully understand the stratigraphic nature of the boundary between the Miocene and the Pliocene (the Huayquerian and Montehermosan stages/ages). In this sense, two stratigraphically superposed range zones have been recognized in the area: Xenodontomys ellipticus Range Zone (latest Miocene-early Pliocene; late Huayquerian), and Eumysops laeviplicatus Range Zone (early Pliocene; Montehermosan). Taking into account the available geological and paleontological evidences, the “Irenense” would not represent a valid biostratigraphic unit, since, according to the geological model here proposed, it would be represented by elements of the Xenodontomys ellipticus Range Zone in the lower QS basin and by elements of the Eumysops laeviplicatus Range Zone in the middle QS basin. Fil: Beilinson, Elisa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina Fil: Gasparini, Germán Mariano. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Paleontología Vertebrados; Argentina Fil: Tomassini, Rodrigo Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; Argentina Fil: Zárate, Marcelo Arístides. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; Argentina Fil: Deschamps, Cecilia Marcela. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Paleontología Vertebrados; Argentina Fil: Barendregt, R.W.. University of Lethbridge; Canadá Fil: Rabassa, Jorge Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina

In the Merida Andes, a detailed deglaciation history reconstruction is difficult to achieve due to scattered deglaciation chronologies available. This paper contributes with 24 exposure ages of glacial landforms sampled in the Gavidia valley. Exposure ages were obtained based on terrestrial cosmogenic nuclide 10Be dating. Results indicate deglaciation mainly occurred between ∼21 ka and 16.5 ka and the complete deglaciation occurred at ∼16.0 ka. The glacier retreated in two different phases. The oldest one occurred since the LGM until middle OtD or the local climate event El Caballo Stadial. The youngest phase occurred at ages younger than ∼16.5 ka until complete deglaciation. A combination of topographic features and changes in the paleoclimate conditions at the end of the El Caballo Stadial seems leaded the fastest former glacier extinction. The topographic feature which seems contributed to the fastest glacier extinction was the low valley bottom slopes. In addition, exposure ages of the Gavidia valley were integrated with deglaciation chronologies from the central Merida Andes to compare deglaciation histories. Asynchronous deglaciation histories were observed. Local paleotemperatures and paleoprecipitations contrasts, different valleys aspects, insolation and catchments steepness could explain different deglaciation histories.

The integration of drainage analysis and topographic metrics provides excellent tools to assess the Quaternary tectonic activity modeling landscape evolution in evolving orogens. Furthermore, detecting active structures through the geomorphological analysis of landscapes helps to identify potentially seismogenic structures that could impact the socio-economic conditions in developing countries. Therefore, this paper presents a tectonic geomorphology study conducted on the Santander Massif (SM, Colombian Eastern Cordillera) and surrounding areas. The SM has structures with reported Quaternary activity such as the Bucaramanga, Morro Negro-Las Mercedes, and Chitaga faults, which summed to a poorly-constrained paleosismological history with significant shallow events is the prime incentive of this work. Our study is based on the acquisition of topographic data through swath profiles, local relief, slope variability, filtered topography, minimum bulk erosion, which along with geomorphic indices like the normalized concavity steepness (ksn), the hypsometric integral (HI), the ratio valley floor width to valley height (Vf) and normalized river profiles provided new insights on the recent landscape evolution in this part of the Northern Andes. We compared published apatite fission tracks (AFT) data for the study area with the uplift pattern deciphered from the geomorphic indices to detect the consistency in exhumation and denudation processes both in the long and short-term. Consequently, the central SM, as well as the faulted block between the Bucaramanga and Guamalito faults, evidence a highly-incised landscape with local relief values exceeding the 1500 m, then, responding with a sharp incision to recent surface uplift. Interestingly, this uplift pattern in central SM matches with published AFT data and is related to Neogene exhumation events controlled by secondary structures like the Surata Fault. We attribute this exhumation event, and accelerated denudation rates within the SM, to the influence exerted by the “collision” between the SM and the Pamplona Indenter, leading to the unroofing of basement rocks at the deformation front (e.g., the Vetas High) and the topographic building of the range. For the upthrown block (hanging wall) of the Bucaramanga Fault, the occurrence of poorly-graded river profiles with an average concavity factor of 12.01, as well as irregular hypsometric curves with convex lower reaches, reinforce the hypothesis that late Cenozoic topographic rejuvenation in the SM was also forced by the tectonic activity of the Bucaramanga Fault. We expect empirical uplift rates >0.08 mm/yr (very high to high tectonic activity) for the mountain front encompassed between La Esperanza and Bucaramanga localities. We state this assumption since the mountain front delimited by the structure mentioned above is quite straight, it preserves remarkable morphostructural features and displays V-shaped valleys with an average Vf index of 0.38. This uplift rate is in good agreement with recent thermochronological exhumation rates (0.1 to > 0.3 mm/yr) published by Siravo et al. (2019). Higher uplift rates associated with the Bucaramanga Fault were also constrained with the erosion proxy index, i.e., the ksn index. High ksn values (above 128) were observed following a linear pattern, which in turn seems controlled by the main trace of the Bucaramanga Fault. For instance, the river system draining the western piedmont of the SM, near Aguachica, records slope-break knickpoints that we interpret as evidence of an incisional wave migrating upstream and adjusting a late Neogene uplift event associated with the Bucaramanga Fault. Conversely, the northern SM presents variable tectonic activity with the inner part, east of the Guamalito Fault, presenting low tectonic activity with little surface uplift and dominant strike-slip kinematics. We recognize the Ocana-Abrego zone as a relict landscape with well-preserved non-consolidated deposits that may record the initial exhumation and denudational pulses of the SM during the Andean Orogeny.

Analysis of several morphotectonic tools (longitudinal river profiles, knickpoints, swath profiles, and geomorphic indices) together with geomorphological, lithological, and structural data provided objective and quantifiable elements to improve the knowledge of the Quaternary tectonic activity in a region. In this study, we analyzed and compared two large intermountain basins located in the Precordillera of the Central Andes of Argentina. Local distortions in longitudinal river profiles and river patterns may reflect resistant lithology, an increase in shear stress, or differential surface uplift. Both basins have similar shapes, although one mirrors the other producing a large-scale broom-shaped drainage pattern. The values of the Asymmetry Factor and Transverse Topographic Symmetry Factor show opposite signs and almost identical values, suggesting that both basins have mirror symmetry independently of order and scale. The asymmetry in both basins would be a result of the progressive migration of the Andean orogenic front towards the east. Knickpoints were mainly associated with resistant rocks, faults with the scarp facing upstream, or both. In the reaches where the rivers flow across areas with presumed higher rates of uplift, we observed a reduction in the width of the active channel belt by lateral confinement and river pattern changes. The results presented in this paper constitute one of the first attempts to apply geomorphic indices to large drainage basins in the Precordillera fold-and-thrust belt.

The Cordillera Blanca, located in the central zone of the Andes Mountains in Peru, has shown a retreat in its glaciers. This paper presents a trend analysis of the glacier area over the groups of Nevados Caullaraju-Pastoruri from 1975 to 2010 using Landsat-5 Thematic Mapper (TM) imagery. In the case of the Nevados Pastoruri/Tuco, the study period was extended back to 1957 by using an aerial photograph taken that year. The extent of clean glacier ice was estimated using Normalized Difference Snow Index (NDSI) thresholds. Moreover, the estimation of debris-covered glacier ice was retrieved by means of a decision tree classification method using NDSI, Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST). Area estimations derived from Landsat imagery were compared to the glacier ground-truth data in 1975 and 2010. Results show a statistically significant (p

The main goal of this paper is to analyze the late Quaternary alluvial record of the Andean piedmont between 33° and 34°S (Mendoza, Argentina) reconstructing the prevailing paleoenvironmental conditions and discussing their regional significance. The analysis was carried out along the outcrops of Arroyo La Estacada and its tributary Arroyo Anchayuyo, complementary sections were described at Arroyo Grande and Arroyo Yaucha. The sedimentological, stratigraphical and geochronological (radiocarbon and OSL dating) results as well as the paleoenvironmental interpretation are presented and discussed on the basis of the geomorphological units identified at the piedmont fluvial systems. Late Quaternary deposits of Arroyo La Estacada compose three main geomorphological units consisting of an extensive aggradational plain, a fill terrace and the present floodplain -which is not analyzed in this contribution- At the aggradational plain a distal alluvial fan lithofacial association was determined, mainly related to overbank sheet fluid overflows and probably temporary inactive channels of sandy-like braided streams between ∼50 ka BP and the early Holocene. The mid-late Holocene fining upward alluvial sequence of the fill terrace corresponds to a sinuous fluvial system lithofacial association. The mid-to late Holocene alluvial sequence exposed at Arroyo Grande banks shows a fining upward sequence probably related to a sinuous fluvial system lithofacial association. Finally, in the Arroyo Yaucha the upper and lower terraces analyzed in this study record a Late Glacial and Holocene fining upward alluvial sequence of a sinuous fluvial system lithofacial association. The late Pleistocene - early Holocene dynamic of the Andean piedmont documents an interval of alluvial aggradation characterized by the occurrence of sandy systems similar to braided rivers in the distal fan environments. The mid-late Holocene is marked by aggradation in sinuous fluvial systems and by two major episodes of degradation registered across the fluvial systems of the study area. This paleoenvironmental reconstruction allowed us to propose an evolutionary scheme and chronology of the major fluvial landscape landforms contributing to the general understanding of the sedimentological and geomorphological dynamic during the late Quaternary. Fil: Mehl, Adriana Ester. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; Argentina Fil: Zárate, Marcelo Arístides. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; Argentina

This paper presents the results of a new investigation of the Guarani Aquifer System (SAG) in Sao Paulo state. New data were acquired about sedimentary framework, flow pattern, and hydrogeochemistry. The flow direction in the north of the state is towards the southwest and not towards the west as expected previously. This is linked to the absence of SAG outcrop in the northeast of Sao Paulo state. Both the underlying Piramboia Formation and the overlying Botucatu Formation possess high porosity (18.9% and 19.5%, respectively), which was not modified significantly by diagenetic changes. Investigation of sediments confirmed a zone of chalcedony cement close to the SAG outcrop and a zone of calcite cement in the deep confined zone. The main events in the SAG post-sedimentary history were: (1) adhesion of ferrugineous coatings on grains, (2) infiltration of clays in eodiagenetic stage, (3) regeneration of coatings with formation of smectites, (4) authigenic overgrowth of quartz and K-feldspar in advanced eodiagenetic stage, (5) bitumen cementation of Piramboia Formation in mesodiagenetic stage, (6) cementation by calcite in mesodiagenetic and telodiagenetic stages in Piramboia Formation, (7) formation of secondary porosity by dissolution of unstable minerals after appearance of hydraulic gradient and penetration of the meteoric water caused by the uplift of the Serra do Mar coastal range in the Late Cretaceous, (8) authigenesis of kaolinite and amorphous silica in unconfined zone of the SAG and cation exchange coupled with the dissolution of calcite at the transition between unconfined and confined zone, and (9) authigenesis of analcime in the confined SAG zone. The last two processes are still under operation. The deep zone of the SAG comprises an alkaline pH, Na–HCO3 groundwater type with old water and enriched δ13C values ( −18.8) close to the SAG outcrop. This is consistent with a conceptual geochemical model of the SAG, suggesting dissolution of calcite driven by cation exchange, which occurs at a relatively narrow front recently moving downgradient at much slower rate compared to groundwater flow. More depleted values of δ18O in the deep confined zone close to the Parana River compared to values of relative recent recharged water indicate recharge occur during a period of cold climate. The SAG is a “storage-dominated” type of aquifer which has to be managed properly to avoid its overexploitation.

A new record of the Marine Isotopic Stage 5, the last Interglacial Stage before present is presented in this paper. Sedimentological, micromorphological, trace elements analyses (Rb-Sr) and magnetic polarity determination were performed on Buenos Aires and Ensenada Formation (Late Cenozoic) deposits in the southern Chaco-Pampean Basin (Argentina). This work aims to unravel paleoclimatic and paleoenvironmental information from the analyzed data. The studied deposits encompass a complex and cyclic 8 m-thick sedimentary-pedogenetic sequence formed by loessic sediments and paleosols with volcano-pyroclastic provenance. Four tabular units, with net base and top, were defined from erosion surfaces. An OSL age >126 kyr was obtained from the upper middle part of unit B, which suggests that this unit as well as unit C, could have developed during the latest interglacial stage, equivalent to MIS 5. The occurrence of calcretes indicates periods of little clastic supply and seasonal arid or semiarid climate while iron oxides, smectites and illite-bearing pedogenetic calcretes point to annual rain rates between 100 and 500 mm. No calcretes of any origin occur in present soils of the same zone. According to our proposed interpretation of the available data, climate during MIS 5 was drier than today. The drier conditions may have been related to lower temperatures during summer. Fil: Tofalo, Ofelia Rita de Las Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología; Argentina Fil: Orgeira, Maria Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología; Argentina Fil: Compagnucci, Rosa Hilda. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina Fil: Alonso, Maria Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología; Argentina Fil: Ramos, Adriana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotópica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina

This paper is a contribution to the knowledge of the sedimentation of Neoproterozoic sequences, known as the Jequitai Formation and Macaubas Group. These sequences are present along the transitional zone between the Sao Francisco Craton and the Brasiliano (≌600 Ma) Aracuai fold belt in Minas Gerais, Brazil. A sedimentological study of these Neoproterozoic sequences enables us to distinguish between true continental and marine glacial facies and glacial material reworked by various subaqueous gravitational processes. The cratonic Jequitai Formation consists of massive and stratified diamictites up to 100 m thick. This diamictite association is tentatively interpreted as glaciomarine in origin. It continues eastward, in the Aracuai fold belt, as the metasedimentary Macaubas Group, which is composed of metadiamictites, quartzites and schists from 5–12(?)km thick. The Macaubas Group consists of resedimented glacial material deposited by subaqueous debris flows and turbidity currents. A depositional model is proposed for the Jequitai–Macaubas glacial/gravitational sequence. From west to east, a glaciomarine sequence, possibly deposited from an ice-sheet and slightly reworked by gravitational processes, was reworked along the Sao Francisco cratonic border and generated a slope apron system made up of diamictites associated with turbidites and rhythmites.

Brecciated lower Cretaceous carbonates on a collapsed margin in Guerrero add support to the growing evidence for the Chortis block originating from, and later detaching from, southern Mexico. Fault blocks with over 2000 m of breccia have been measured, suggesting breakup of a carbonate platform due first to magmatic uplift and later due to transcurrent faulting during detachment. There is less breccia on the Chortis block; however, magmatism may have been responsible for extensive block faulting which resulted in thick sections of well rounded limestone conglomerate that have been observed in Honduras. Mylonite scars may be evidence of basement rifting and are the basis for dividing the Xolapa complex, created by early Paleogene magmatism, into three sub-blocks. At about the same time Chortis may also have been broken into three sub-blocks by sinistral forces. A tectonic history is proposed that is based on published work by others but supported by data presented in this paper.

Summarization: The Eastern Cordillera of southern Bolivia is situated between the Altiplano high plateau in the west and the thin-skinned Subandean thrust belt in the east. It is mainly built of Ordovician anchimetamorphic sedimentary rocks with a discontinuous, unconformable cover of Cretaceous to Neogene, mostly continental sediments. The eastern margin of the Eastern Cordillera exhibits E-verging basement-involved thrusts linked to the Subandean belt. Its central and western parts, discussed in this paper, represent a province of dominantly W-verging, basement-involved thrusts of Tertiary age superimposed on a deeply eroded pre-Cretaceous fold belt. Several well-preserved remnants of the pre-Cretaceous fold belt show kilometer-scale, W-verging folds with axial plane slaty cleavage. The main activity of Andean (post-Cretaceous) thrusts is bracketed between about 25 and 12 Ma. However, some Andean contractional deformation occurred earlier. The exposed major Andean thrusts dip steep to intermediately at the surface. Cross-section balancing suggests that they merge into a detachment at maximum depths of 9 to 17 km. Neogene shortening is concentrated in an imbricated thrust system at the border of the Eastern Cordillera with the Altiplano in the west, but is relatively moderate (about 10-15%) within the Eastern Cordillera itself. W-verging thrusts in the Eastern Cordillera predate the E-verging thrust belt which underlies the eastern flank of the Andes. Strain rates are comparatively low during these first stages of deformation, but increase markedly at the onset of thin-skinned thrusting farther east. Balancing of a regional cross-section of 150 km present length indicates that the western and central Eastern Cordillera has shortened by about 55-80 km since Cretaceous time. Overall Tertiary tectonic shortening between the deformation front and the magmatic arc is about 215-250 km. This is insufficient to build the entire crustal root of the Andes by thrust thickening alone. Presented on: Journal of South American Earth Sciences

Surface geology and heophysical data, supplemented by regional structural interpretations, indicate that the Valle del Cauca basin and adjacent areas in west-central Colombia form a west-vergent, basement-involved fold and thrust belt. This belt is part of a Cenozoic orogen developed along the west side of the Romeral fault system. Structural analysis and geometrical constraints show that the Mesozoic ophiolitic basement and its Cenozoic sedimentary cover are involved in a “thick-skinned” west-vergent foreland style deformation. The rocks are transported and shortened by deeply rooted thrust faults and stacked in imbricate fashion. The faults have a NESW regional trend, are listric in shape, developed as splay faults which are interpreted as joining a common detachment at over 10 km depth. The faults carry Paleogene sedimentary strata and Cretaceous basement rocks westward over Miocene strata of the Valle del Cauca Basin. Fold axes trend parallel or sub parallel to the thrust faults. The folds are westwardly asymmetrical with parallel to kink geometry, and are interpreted to be fault-propagation folds stacked in an imbricate thrust system. Stratigraphic evidence suggests that the Valle del Cauca basin was deformed between Oligocene and upper Miocene time. The kinematic history outlined above is consistent with an oblique convergence between the Panama and South American plates during the Cenozoic. A negative residual Bouguer anomaly of 20–70 mgls in the central part of the Valle del Cauca basin indicates that a substantial volume of low density sedimentary rocks is concealed beneath the thrust sheets exposed at the land surface. The hydrocarbon potential of the Valle del Cauca should be reevaluated in light of the structural interpretations presented in this paper.